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Building Practical Descriptors for Defect Engineering of Electrocatalytic Materials

Cite this: ACS Catal. 2020, 10, 16, 9046–9056
Publication Date (Web):July 13, 2020
https://doi.org/10.1021/acscatal.0c02144
Copyright © 2020 American Chemical Society
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Supporting Info (1)»
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Defect engineering has recently gained considerable interest in electrocatalysis by extending the portfolio of materials’ design strategies toward more efficient and sustainable nanocatalysts. (1) This approach is highly versatile as the term “defect” in crystallography refers to any interruption of the regular pattern defining a perfect crystal. As such, point defects (atomic vacancies, interstitial atoms, antisite defects), line defects (dislocations, disclinations), planar defects (grain boundaries, stacking faults, twinning, steps), or bulk defects (voids, pores, cracks) are a nonexhaustive list of structural parameters to be possibly tailored to meet a catalytic material’s functional specifications. In contrast to former approaches in electrocatalysis, within the defects’ engineering strategies, electrocatalytic properties are tuned locally not globally. Indeed, defects produce (simultaneous) local changes in key structural parameters relative to catalytic performance, such as site coordination, strain, or chemical composition.
Historically, density functional theory (DFT) calculations, molecular dynamics, and kinetic Monte Carlo simulations have been extensively used to connect the catalytic site local environment to its catalytic properties from computational screening over the trial-and-error approach. (2) These methods rely on the determination of catalytic site electronic structure that can be further bridged to the catalytic performance through the adsorption strength of reaction intermediates. Especially, for a given reaction mechanism, focusing on the “limiting” reaction step(s) in the frame of the Sabatier principle represents a remarkably powerful and efficient route toward the reduction of numerous system variables to a few “key” reactivity descriptors. (3) In the case of the sluggish oxygen reduction reaction (ORR) on Pt, which restricts the development of proton exchange membrane fuel cells (PEMFC), the existence of so-called scaling relations between the *O, *OH, and *OOH intermediates’ adsorption strengths allows focusing on a single of these adsorbates. In that frame, calculations predict the optimal ORR rate is obtained by decreasing the adsorption energy of *O (or by extension *OH and *OOH) by ∼0.2 eV (or ∼ 0.1 eV) compared with Pt(111). (4) The pivotal correlation between the adsorption trends of the reaction intermediates and the electronic structure of the catalytic sites is described by the d-band model introduced by Hammer and Nørskov. (5,6) Since from this principle, the catalytic properties of a site can be determined completely by its electronic structure, descriptors of the d-band ultimately become (electronic) reactivity descriptors.
Figure 1a illustrates how the d-band theory applies to a model Pt(111) surface, where all sites are equivalent (uniform coordination, interatomic distances, and chemical environment). The surface thus features a Dirac type *OH adsorption free energy (ΔG*OH) distribution, centered on the value expected for a single Pt(111) site, and its global activity (star symbol) can thus be virtually described by that of a single catalytic site (circle symbol). In contrast, Figure 1b displays the situation when structural disorder is present at the surface (defect-engineered Pt(111) surface). Here, the coexistence of sites with various local environments results in a broad, near-continuous distribution of chemisorption properties. (7,8) Supposing the scaling relations still hold, this implies that the global catalytic activity of the surface can no longer be described by that of a single site, but rather as the average catalytic activity of each site. Consequently, whereas the ORR activity of flat Pt(111) is limited by the overall too strong affinity with *OH species (left branch of the volcano), a minority of catalytic sites with close-to-optimal structural configuration, but exponential contribution to the global activity (Butler–Volmer kinetics), ensures enhanced ORR activity for the defect-engineered Pt(111) surface.

Figure 1

Figure 1. Defect engineering approach. Schematic representation principle of the narrow vs near-continuous broad distributions of catalytic sites configurations and electrocatalytic performance for (a) ordered, model vs (b) defective Pt(111) surfaces.

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However, structure-sensitive, electronic descriptors alone (such as the d-band center, εd) do not provide any clear-cut reverse structural information about the catalytic site local environment that could guide material design. Also, even if computational methods from model structures can correlate a fine local atomic motif and electronic properties (see coordination-based descriptors such as CN, (9) CNα, (10) or CNsd (11)), ensuring the successful implementation of such fine atomic motif onto practical nanocatalysts surface remains extremely difficult. That is because (i) it requires an atomic scale control of the surface structure design (other than atomically flat) and (ii) characterization techniques are limited in describing such fine structural atomic motifs on “real-life” nanocatalysts. This apparent dichotomy between extremely fine theoretical insights on well-defined systems and the extreme complexity of poorly defined practical systems is at the heart of the defect engineering current challenge. Whereas the uniformity of the catalytic sites was a convenient pillar of surface science, here each surface atom has its own configuration and should be considered individually (Figure 1). In that frame, practical descriptors able to bridge surface heterogeneous structure and catalytic performance experimentally are of primary importance.
This Viewpoint article intends to comment on the establishment of practical descriptors able to guide the defect engineering of practical nanocatalysts. We first discuss the case of the local strain and especially how an explicit bulk physical parameter can be turned to a surface structural descriptor: the surface distortion (SD). Then, we show how cyclic voltammetry experiments analyzed in light of SD can reveal electrochemical adsorption fingerprint of structural disorder, thus providing a versatile method to shortcut in-depth structural characterization. Importantly, we emphasize that, because structural or adsorption properties are averaged over the whole catalyst (i.e., the spatial resolution is lost), such descriptors do not reveal the nature of the active site but mostly provide statistical insights on their emergence from disorder. Consequently, we finally discuss the opportunities offered by emerging analytical techniques, which we anticipate will advance our understanding of defect characterization toward identification of electrocatalytically active and stable motifs. Whereas Pt-based catalysts for PEM fuel cell served as a discovery platform of the presented methods, all results (except the cyclic voltammetry techniques) apply to other types of nanocatalysts.

1. Physical and Electrochemical Assessments of Local Strain to Describe Surface Disorder

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Microstrain Line Broadening in Powder X-ray Diffraction

Many phenomena synergistically contribute to the diffractograms profile observed in powder X-ray diffraction (PXRD) experiments. As PXRD (especially from laboratory sources) has become a routine technique in numerous fields of research, simple analytical softwares have been developed to extract sample structural information from PXRD data. However, extracting reliable information related to the quantification of structural disorder requires a rigorous methodology to minimize the impact of errors related to both experimental and interpretation artifacts. Numerous analytical approaches such as the Williamson-Hall method, the Warren-Averbach method, the Fourier method, or full profile refinement (Rietveld, Le Bail) methods can be used to investigate the microstructure of a powder sample from PXRD data. In the case of nanocrystals with sizes < 10 nm, extensive peak broadening and overlap makes the methods based on the analysis of few individual reflections inaccurate and full profile refinement technique of PXRD patterns extending over a large Q-range are preferred (see Figure 2a). This reason, together with the versatility offered by the full profile refinement methods in terms of the peak shape description and possible anisotropy consideration, make the Rietveld method (12) highly attractive, despite its complexity due to the large number of control parameters.

Figure 2

Figure 2. Powder X-ray diffraction (PXRD) patterns’ broadening and sources of microstrain in PtM nanoparticles (M being an early or late transition metal element). (a) Typical experimental PXRD patterns measured at the high-energy ID31 beamline of the European Synchrotron Radiation facility on structurally disordered and structurally ordered catalysts (here hollow PtNi/C and cube Pt/C nanoparticles, respectively) plotted as a function of the momentum transfer Q. (b) Possible sources of microstrain in PtM nanocatalysts (grain boundaries, inhomogeneous alloying, or (electro)chemical surface dealloying). The inset in (a) shows the influence of macrostrain, instrumental contribution, and microstrain on the position and the broadening of the PXRD reflections. Adapted from ref (13). Copyright 2018 Springer Nature.

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It is generally admitted that the Bragg peak profile (shape and width) can be described as the convolution of three contributions: (i) the incident X-ray beam monochromaticity, (ii) the instrument optics (slit gaps and positions, detector pixel size, etc.), and (iii) sample-related broadening effects. The latter is the consequence of two microstructural properties of the sample investigated, namely, the finite size of the coherent domains (or grain size) and the presence of microstrain (or local strain, the parameter of interest here), which reflects the local variations of inter interplanar distances caused by some sort of stress (disorder, stacking fault, twins, grain boundaries, inhomogeneous alloying, etc.; see Figure 2b). As described in the Supporting Information, the Rietveld method can disentangle microstrain broadening from other contributions, thus providing a physical parameter that is representative of local strain originating from structural disorder.
Rietveld refinement of powder X-ray diffractograms becomes limited when the nanoparticle size is substantially smaller than the beam coherence length, (i.e., typically a few nanometers (14)) and a periodic crystal-type description of the atomic arrangement breaks down, as does the Bragg’s law. In such case, pair distribution function (PDF) analysis of total scattering data can provide quantitative information on the structural and microstructural properties of nanocatalysts. (15) Experimentally, the PDF is obtained from a PXRD pattern by realizing the Fourier transform as:
(1)
where ρ(r) is the microscopic pair density and Q = 4π·sin(θ)/λ. S(Q) is the total structure function, that is, the normalized coherent scattered intensity obtained from the PXRD pattern after subtraction of the environment and inelastic/incoherent scattering and normalization. It is worth noting that the whole diffraction pattern is used to obtain G(r), not only the Bragg peak intensities as for Rietveld refinement. Therefore, the effects of disorder, defects, and so on, which contribute to the pattern through diffuse scattering outside of the Bragg peaks, will also be included in G(r). The PDF yields the probability of finding a pair of atoms separated by a distance r. It will oscillate in a way that is characteristic of a given structure, about the average pair density that would result from a completely unordered atomic arrangement. Thus, it can be calculated from a structural model describing the distribution of atoms in a sample. The calculated PDF can be fitted to the observed one, as in a “direct space Rietveld refinement”. The fit is carried out in a given interatomic distance range, yielding accurate structural parameters relative to this range. Comparing refinements for very small or longer distances allows revealing the presence of local structural distortions with respect to an average long-range atomic arrangement. In addition, the PDF signal vanishes for distances longer than the structural coherence length (or domain size), which provides a simple way to determine the average nanoparticle size in a sample. On the other hand, the effect of microstrain will be to broaden the distribution of interatomic distances about their average values, which will result in a broadening of the corresponding peaks of the PDF. The PDF is thus sensitive to microstrain; however, its effect is combined to thermal displacement and instrumental broadening, from which it is difficult to extract. The PDF can also be used with Reverse Monte Carlo minimization of large atomic assemblies, which allows a statistical description of samples from liquids and amorphous to crystalline nanoparticles. (16)

From Bulk Microstrain to Surface Distortion

The main reason to correct microstrain values is related to the definition of microstrain itself, which links a local variation of the interplanar distance d between d–Δd and d+Δd according to:
(2)
In practice, there is a distribution of Δd/d within and across the coherent domains. The refined ε value thus corresponds to the root-mean-square <ε> , averaged over all crystallites in the sample. Consequently, microstrain is a global and relative estimation of structural disorder, since the even perfectly ordered domains (Δd = 0) also contribute to the average value. To build a consistent structure–activity relationship in heterogeneous catalysis, a surface-specific and quantitative structural descriptor is needed. To fulfill these requirements, two complementary “corrections” of the as-measured microstrain values were proposed. (13,17) These corrections derive from experimental (electron microscopy observations (18−23)) and theoretical studies, (8,13,24) which suggest that microstrain can be decomposed into two main and independent contributions:

  • The surface microstrain mostly associated with the presence of grain boundaries and/or arising from dealloying. Despite being surface bound, such microstrain values are relative and need to be corrected from the crystallite size;

  • The microstrain of the whole crystallite volume (mostly arising from heterogeneous alloying and hereafter referred to as “chemical disorder” in what follows). Such microstrain is not representative of the surface and must be estimated and subtracted.

As detailed in the Supporting Information, for PtM bimetallic catalysts, these hypotheses allow to decompose the measured microstrain as a sum of bulk chemical disorder (f(%M), estimated experimentally or from DFT calculations) and surface defectiveness (SD), weighted by the surface-to-volume ratio (D, calculated using Montejano-Carrizales et al.’s model (25)):
(3)
Figure 3 shows how the SD descriptor can be used to unveil the “operating mode” of a library of Pt and PtNi nanocatalysts. The ORR specific activity-SD plot in Figure 3k indicates that enhanced ORR activity arises from two distinct approaches:

  • The global surface tuning approach (null and low SD values), derived from DFT calculations and single crystal studies, which aims at enhancing the ORR rate by synthetically tailoring and maximizing the density of catalytic sites that weakly chemisorb the ORR intermediates (in this category, octahedral PtNi nanoparticles feature the highest ORR activity),

  • The local surface tuning, or defect-engineering approach (high SD values), in which the ORR rate is controlled by the fraction of catalytic sites with close-to-optimal affinity to the ORR intermediates.

Figure 3

Figure 3. Using the SD descriptor to unveil the “operating mode” of a variety of ORR nanocatalysts. (a–j) TEM images of the various nanostructures investigated. The insets show higher magnification TEM or STEM/X-EDS elemental maps. (k) ORR activity measured at 0.95 V vs. RHE plotted as a function of the SD descriptor. The activity–SD plot quantitatively confirms that high ORR activity can be reached through the two approaches presented in Figure 1. Panels (a–j) are reprinted with permission from ref (26). Copyright 2020 American Chemical Society. Panel (k) is adapted with permission from ref (13). Copyright 2018 Springer Nature.

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It is noteworthy that as SD values are derived from microstrain values, they are only sensitive to the local strain caused by defects and do not take into account other ORR-relevant fundamental parameters such as the average lattice contraction (global strain), the near surface chemical composition, or the site coordination (that is why multiple ORR activities can be found at low SD values). However, the fact that the ORR activities increase almost linearly with the SD values (Figure 3k) is clear evidence that (i) similar to the “classical” strain and ligand effects, surface disorder plays a key role in ORR electrocatalysis; and (ii) local strain alone is one of the most, if not the most, influential facets of disorder on catalytic performance.

Quantifying Surface Distortion from Cyclic Voltammetry

As discussed previously, the fundamental and direct consequence of structural disorder is the emergence of a wide variety of catalytic site configurations on a given electrocatalytic surface (Figure 1b). DFT calculations by Le Bacq et al. (8,13) have revealed a ±1 eV distribution width in *OH binding energy on structurally disordered Pt surfaces. Whereas a fraction of these catalytic sites is statistically close to the optimal configuration for a given catalytic reaction (∼0.1 eV weaker chemisorption of *OH in the case of the ORR), another fraction is in the optimal configuration for a different reaction. Indeed, highly aggregated/polycristalline nanocatalysts have demonstrated desirable catalytic performances for a wide range of electrochemical reactions including the ORR, (17,27−35) the CO electrooxidation, (17,36) the methanol or ethanol electrooxidation, (8,37−39) the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). (40−43) Consequently, surface disorder and catalytic polyvalence must be somehow linked, and a similar SD–activity plot should be found for almost any reaction, at least involving *OH species.
In that frame, COads stripping (here referred to COOR) can be chosen as a complementary reaction to the ORR to investigate surface catalytic polyvalence because the prerequisite for optimal COOR are different regarding *OH adsorption strength (increased *OH binding by ∼ 0.4 eV relative to Pt(111) according to Calle-Vallejo et al.). (44) Simultaneous improvement in ORR and COOR specific activity should consequently evidence catalytic polyvalence and thus surface defectiveness. The COOR activity (or CO tolerance) of a Pt-based nanocatalyst can be quantified by the average COads oxidation potential (μ1CO, or the first moment of the potential weight in the COads stripping voltammograms, see details in ref (17) and examples in Figure 4a).

Figure 4

Figure 4. Probing catalytic sites diversity and oxophilicity. (a) Background-subtracted COads stripping voltammograms recorded on Pt and PtNi nanomaterials featuring increasing SD values and their associated average potential μ1CO (dashed lines), (b) SD plotted as a function of the COOR activity (μ1CO) for a wide range of PtNi materials with different shape, size, or Ni content. (c) Background-subtracted Hads desorption curves (so-called-HUPD region) of materials featuring increasing SD values pointing an extra electrochemical process at high potential ascribed to *H displacement by *OH on extremely oxophilic sites and (d) SD plotted as a function of the ratio QCO/2QH for a wide range of PtNi materials with different shape, size, or Ni content. In (b), the numbers displayed close to the experimental points are the SD values associated to these catalysts. In both (a) and (c), the curves are normalized by the Pt surface area measured by COads stripping. Panel (d) is adapted with permission from ref (26). Copyright 2020 American Chemical Society.

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As shown in Figure 4b, a drawback of using complementary reactions to probe surface catalytic polyvalence is that it is not generally applicable. If structural disorder must induce catalytic polyvalence (μ1CO generally decreases with increasing SD), the inverse is not necessarily true. Indeed, the COOR overpotential is not strictly governed only by *OH adsorption strength but by an appropriate balance between the formation of *OH species and their recombination with diffusing (45) *CO species (Langmuir–Hinshelwood mechanism). In practice, not only structural disorder but also crystallite size, surface chemical composition, and site coordination influence the *CO binding energy and thus the μ1CO values. A very good example of this was provided by Stamenkovic et al., (46) who reported a simultaneous negative shift of the COads stripping profile and tremendous ORR activity for the Pt3Ni(111)-skin surface, in agreement with results from shape-controlled nanoparticles displayed in Figure 5b, especially regarding PtNi octahedra. A method relying only on *OH bond strength is consequently needed.

Figure 5

Figure 5. Determining the nature and structure of the catalytically active sites. (a) 3D atomic displacements measured in a FePt nanoparticle using GENFIRE algorithm from AC-STEM images; (b) Atomic resolution STEM images, with atomic displacement and strain maps within two crystallographic planes, as indicated in the Pt nanoparticle; (c) APT data showing the atom map from a Ag@Pd nanoparticle; (d) Sketch of the BCDI setup, 2D slice of the strain field through the center of a reconstructed Pt nanoparticle under Ar/CO flow and associated probability density of the strain field; (e) Scheme explaining the concept of using EC-STM noise analysis to reveal the catalytic sites under reaction conditions. Panel (a) is adapted with permission from ref (22). Copyright 2017 Springer Nature. Panel (b) is adapted with permission from ref (21). Copyright 2018 Springer Nature. Panel (c) is adapted with permission from ref (63). Copyright 2011 Springer Nature. Panel (d) is adapted with permission from ref (68). Copyright 2019 American Chemical Society. Panel (e) is adapted with permission from ref (80). Copyright 2017 Springer Nature.

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It is deduced from the extremely low COOR onset potential observed in the case of the highly defective sponge PtNi/C in Figure 4a (∼0.4 V vs. RHE) that *OH species are adsorbed on the surface at such low electrode potentials. The idea of concomitant adsorption of *OH and under-potentially deposited hydrogen (HUPD) has been discussed many years ago by Marichev et al., (47) Van Der Niet et al., (48) and more recently by Janik et al. (49) The results of infrared spectroscopy (IRAS) (50) and CO displacement measurements (51) on extended surfaces, as well as DFT calculations, have suggested that *H species adsorbed on extremely oxophilic sites can be displaced by *OH species, following an electrochemical reaction of the type:
(4)
The displacement process described by eq 4 creates an extra charge compared with the charge required just to desorb under-potentially deposited H atoms (QH). Consequently, an increase in QH compared to the COads stripping charge (QCO), as experimentally observed in Figure 4c, supports the presence of highly oxophilic sites on Pt-based surfaces. (26)
Most importantly, as displayed in Figure 4d, contrary to μ1CO, the QCO/2QH ratio measured on a wide range of nanocatalysts decreases almost linearly only with strictly positive values of SD, independently from other structural and chemical parameters. This suggests that such highly oxophilic sites are possibly generated only from structural disorder; that is, there are not the “simple” undercoordinated sites found at nanoparticles surface edges, steps, or kinks (otherwise, low QCO/2QH ratio would be systematically found on nanocatalysts). In other words, the QCO/2QH ratio quantitatively probes the fraction of highly oxophilic sites, which catalyze the process described by eq 4. Because such extreme *OH adsorption energies are more likely reached via the broadening of the *OH adsorption energies distribution than its simple uniform shift, the ratio indirectly quantifies the structural disorder responsible for this broadening. This hypothesis is supported by the SD-QCO/2QH plot in Figure 4d and the ORR activity-QCO/2QH plot in Figure S2 of the Supporting Information, which show that, counter-intuitively, the ORR activity increases with increasing fraction of highly oxophilic sites. This result is of primary importance since the QCO/2QH ratio can thus be used to estimate the degree of surface defectiveness of a given catalyst, just as the synchrotron X-ray based SD descriptor does, yet in a much more facile and accessible way. Besides, this method is by definition truly surface sensitive, likely incorporating the effects of coordination and/or chemical composition variations as well as local strain. The method can also be employed operando (i.e., in a PEMFC device). (26) Nevertheless, although these descriptors (SD and/or QCO/2QH) have a great advantage to provide insights on the statistical existence of active atomic motifs over the whole catalyst surface, the exact structure of these motifs and their location on the surface remain largely unknown.

2. Perspectives toward the Identification of the Active Structural Motifs

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In the previous section, we showed how the local strain averaged over the whole catalyst surface could serve as a powerful structural descriptor to rationalize the unique electrochemical behavior of practical disordered surfaces. The SD descriptor, in addition to the QCO/2QH ratio, suggests that increasing surface disorder promotes the emergence of only a fraction of active catalytic sites, in detriment of another fraction. Further insights on the nature of these active sites (such as their location, structural environment, or which kind of defect generates them) would certainly guide the optimization of defective nanocatalysts. In the following section, we review several emerging analytical techniques that we believe will play a key role in that purpose.

Electron Microscopy Tomography

Recent advances in transmission electron microscopy (TEM) methodology regarding spherical and chromatic aberration correction and improvements of cameras, energy resolution of electron guns, and processing algorithms (52) now allow the imaging and/or 3D reconstruction of nanoparticles at the atomic scale. For example, using aberration corrected scanning transmission electron microscopy (AC-STEM) operated in annular dark field mode (STEM-ADF) tomography coupled with a generalized Fourier iterative reconstruction (GENFIRE) algorithm, Yang et al. (22) determined the coordinates of each atom composing a face-centered tetragonal (L10) PtFe nanoparticle (Figure 5a). Atomic coordinate determination represents the ultimate structural characterization one can expect, and the results showed that a nominally ordered particle was in fact composed of 8 individual grains, each featuring a different phase, including ordered L10 and L20 or disordered face-centered cubic (fcc) phases. Each surface atom coordination, local displacement compared with the perfect crystal structure and local chemical environment can be determined from such measurements. Similarly, an increasing number of studies (18−23) have successfully mapped the distribution of strain in extended surfaces or mono- and bimetallic nanoparticles. Using high-resolution scanning transmission electron microscopy (HRSTEM), Nilsson et al. (21) have reported that the bulk of Pt nanoparticles supported on alumina or ceria is (“rather”, as employed by the authors) deformation-free. Interestingly, local strain was systematically detected close to twin boundaries, at high surface curvature (convex edge sites or concavities) and at the nanoparticle–support interface (see in Figure 5b), in agreement with other TEM studies. (22,23,53,54) In addition, the “true” surface strain distribution was directly deduced from the data, which constitutes an improvement compared with SD that requires numerous hypotheses. This direct strain–defect type correlation is precious for the defect engineering approach. It also evidences that coordination and strain are strongly linked.
However, these measurements are extremely resource demanding and are usually limited to a limited number of nanoparticles, which are supposed to be representative of the whole catalyst. In addition, identical-location-aberration-corrected scanning STEM (IL-AC-STEM) measurements (23) evidenced that atomic coordinates measured under TEM vacuum conditions change upon exposure to a PEMFC-relevant electrochemical environment, which compromises the straightforward establishment of valid structure–activity relationships from ex situ TEM imaging. Moreover, while environmental in situ TEM methods are also progressing, (52) the dramatic impact of electron-irradiation on nanomaterials’ structure and chemistry in liquid cells is a major issue that cannot be neglected. (55)

Atom Probe Tomography (APT)

Kelly et al. (56) have stated that “The ideal microscope would reveal the position and identity of all atoms in a specimen for as far as the user wishes to see” and have added that “atom probes approach this ideal”. APT is based on the evaporation of a specimen atom by atom, layer by layer, from its surface by the field effect, the ions being further projected onto a position sensitive detector. The detector allows the simultaneous determination of position and the element evaporated. The position and the order of arrival of the ion impact on the detector allows one to reconstruct the original position of the atoms on the initial tip and the time-of-flight of the ions contains their elemental identity via the mass to charge ratio. The sample must be in the form of a very sharp or a needle-shape tip. This is critical because the atoms are progressively removed from the tip allowing the reconstruction of a 3D image of the sample at the atomic scale. In relation with the concerns of the present paper, the combination of highly sensitive composition measurement and three-dimensional microstructural characterization represents a useful tool for the characterization of structural defects. Indeed, APT has the unique capacity to study chemical segregation to crystalline defects such as dislocations, stacking faults, and grain boundaries. (57)
Recently, APT has met fundamental electrocatalysis. The oxidation/reduction behavior of Pt-based alloys has been studied at different temperatures to show preferential segregation effect of one metal. The oxidation of PtRhRu alloy was dynamically monitored, and it was shown that oxidation is initiated at grain boundaries and propagates to form a Ru and Rh-rich oxide. (58) APT also revealed the interplay between surface structure/composition, activity for OER, and stability of electrochemical Ir oxide. (59) At first stage of the metallic Ir oxidation by anodic polarization, the high OER activity was rationalized by the formation of nonstoichiometric Ir oxide (IrOx with x < 2) clusters composed of a very limited number of atoms (only few tens) predominantly located at grain boundaries. This structure gradually evolves toward more stable but less active IrO2. By combining isotope labeling, APT, and inductively coupled plasma mass spectrometry, the same group has demonstrated that oxygen from the hydrated Ir oxide participates in the formation of molecular oxygen, and that this mechanism accelerates Ir dissolution. (59) Lastly, spatially resolved elemental compositions achieved by APT have also demonstrated surface compositional changes of NiP catalysts used to electrocatalyze the OER upon aging in alkaline medium. The APT results provided evidence of the complete depletion of P along with the presence of amorphous and resistive NiO at the near-surface layers of the catalyst, and these results were used to rationalize the OER activity decay. (60)
Nevertheless, APT sample preparation remains an issue to envision the use of this technique in the (electro)catalysis field. The specific geometry of the APT tip with an end-tip radius of 30–100 nm is hardly compatible with powder samples such as supported or even unsupported nanoparticles. (61) Similarly, analysis of porous materials usually designed by dealloying processes requires a specific preparation. Pores and voids have to be previously filled to avoid overlapping ion trajectories and distorted reconstructions induced by porosity. (62) Nevertheless, these recent results show that APT is a growing analytical tool in (electro)catalysis and its emergence provides hope for detailed characterization of the topmost atomic surface layers of nanostructured electrocatalysts, keeping in mind that APT overcomes the main limitations of regular TEM. For example, Tedsree et al. (63) successfully used APT to reveal the internal structure of an Ag@Pd core@shell nanoparticle (Figure 5c), where the close proximity of Ag and Pd elements in the periodic table makes the imaging contrast extremely challenging in TEM.

Advanced (Coherent) X-ray Methods

Beyond the averaged microstrain values extracted from PXRD, Bragg coherent diffraction imaging (BCDI) is an emerging technique designed specifically for resolving the 3D strain field inside individual nanocrystals using nanofocused X-rays (64) (see Figure 5d). Strain fields inside a nanocrystal create phase shifts in the diffracted beam, producing an interference pattern at the detector. A single 3D rocking curve coherent diffraction pattern can recover 3D images of the electron density and strain inside Pt nanoparticles. Complete data sets can be acquired on the order of minutes per nanocrystal. (65) Strain maps of single nanocrystals allow the structural influence of individual crystal defects to be resolved. The strain field present at the surface of the particle can be quantitatively determined, and different types of strain-inducing defect structures can be visualized (e.g., vacancies from alloy leaching vs dislocation loops). (66)
The strain and real space resolutions of BCDI have benefited from ongoing improvements in synchrotron brightness and coherent flux and now approach those of electron microscopy. The most significant advantage of X-ray based techniques is that they can be performed in situ (67,68) with much less beam damage artifacts and easier electrochemical control compared with TEM methods. The extremely small size of commercial fuel cell Pt catalyst particles (<5 nm) remains, however, below the present spatial resolution limit of BCDI, but shape-controlled model systems and other electrocatalysts are already within reach. (69) The phase sensitivity and strain resolution in BCDI vastly exceeds the real-space electron density resolution of the nanocrystal. More specifically, the presence of point defects can be detected, even when they cannot be atomically resolved, because the locally induced strain field propagates several unit cells away. The increased coherence of X-ray (nano)beams at fourth generation synchrotron sources, such as the Extremely Brilliant Source (EBS) at the European Synchrotron Radiation Facility (ESRF) are expectedly to rapidly advance the ease and applicability of BCDI for in situ electrocatalyst strain imaging at the single particle level, under the environment of a PEMFC device.
Complementarily, if one of the major advantages of high energy X-ray based techniques over other methods is their ability to probe practical catalysts in situ or even operando, the coherent diffraction imaging principles can be also applied to “revisit” model surfaces. The advantage over experiments on practical nanoparticle systems is that specific defects can be introduced on single crystal surfaces in a well-controlled manner, using techniques such as ion irradiation, (70) cathodic atomization, (71) and plasma or laser etching. (72) The structure of such model-defect-engineered surfaces can be resolved in situ from surface X-ray diffraction (73) and electrochemical scanning tunneling microscopy (EC-STM). (74,75) Coherent X-ray diffraction can also be used to reconstruct the local strain fields on extended surfaces. Although experimental protocols and X-ray methodology still require further development, the first steps in this direction have already been taken. (76,77)
One of the ultimate goals of nanofocused X-ray beam techniques is to study nanostructured materials in situ and operando conditions utilizing the X-ray equivalent of a TEM. The main advantage of direct, real space nanofocused beams over reciprocal space imaging techniques is the ease of use and significantly simpler data analysis, due to the lack of the diffraction phase problem. Currently, dedicated dark-field hard X-ray microscopy (DFXRM) instruments are being developed at ESRF. (78) The first imaging results are encouraging, but the instrument still lacks the necessary resolution to study catalyst particles in nanometre range. The spatial resolution ultimately depends on the quality of the imaging lens, which is the current bottleneck. However, recent developments in multilayer Laue lenses give hope that this obstacle will be soon surpassed and imaging at the few-nanometre scale can be achieved in the near future. (79)

Electrochemical Scanning Tunneling Microscopy Noise Measurements

Whereas the various methods described above shed light on the complex local structure of disordered surfaces, identification of the electrocatalytically active sites/motifs is pivotal to the further development of the defect-engineering approach. In that frame, it is worth pointing electrochemical scanning tunneling microscopy noise measurements (n-ECSTM) first introduced by Pfisterer et al. in 2017. (80) This technique is based on the energy barrier difference for electrons tunneling from a sample to a STM tip in aqueous electrolyte. The tunneling barrier is sensitive to all the molecules present along the electrons’ path, such as those adsorbed on the electrocatalytic surface (81) and those diffusing to/from the electrochemical double layer. (82,83) Local modifications of the tunneling barrier in turn allow identification of the most active sites for a given reaction (Figure 5e). The n-ECSTM technique revealed that Pt(111) terraces best electrocatalyze the ORR in alkaline media (84) and that step sites and concave sites are the most active in acidic media. (80,84) Besides ORR, this technique revealed that the edges of single MoS2 and MoSe2 flakes are more active than basal planes for the HER, but that the latter can be activated upon bombarding with He-ions. (85)

Conclusions

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In summary, the defect engineering approach has the particularity to touch the extreme complexity of nanoscale intrinsic structural disorder. Contrary to the “classical” surface science approach, the connection between theoretical computations (and their necessary simplifying assumptions) and experimental measurements on practical systems is not yet established. Numerous theoretical descriptors have nonetheless been applied to complex surfaces. For example, the generalized coordination number (CN) theory predicts the benefits of locally concave surface curvature but without considering the local strain possibly generated by surface roughness. On the contrary, experimental observations (electron microscopy, X-ray diffraction) reveal significant local strain in the vicinity of structural defects. From this factual result, local strain alone (without considering the coordination effect) is a desirable experimental structural descriptor of surface defectiveness that is able to rationalize the electrochemical adsorption behavior of disordered nanocatalysts, whereas each surface CN remains hardly accessible in practice.
Consequently, there are currently no clear theoretical guidelines driving the “defective-by-design” engineering of catalytic materials, and computational models must be adapted to better describe experimental observations (and not the other way around). Practical descriptors, however, suggest that maximizing surface distortion is an efficient approach to boost catalytic performance, and the introduction of grain boundaries or surface dealloying was identified as an experimental lever to implement such distortion. If practical structural descriptors inherit the limitations of their characterization technique(s) (resolution, artifacts, (un)realistic sample environment, etc.), the constant technical development and fast progresses of instrumental tools predict an ongoing improvement of their descriptive power.
Finally, one could reasonably think that the ultimate goal in the defect engineering approach is to identify the structure of the most catalytically active site and to maximize its implementation on the nanocatalyst surface. This would be missing an important subtlety of this approach: the most active site is not necessarily desirable. It depends on the compromise between the resulting site activity and the number of (neighboring) sites that are “sacrificed” for this purpose. This adds an extra layer of complexity above the previously mentioned. One thing is certain: there is still a lot to uncover.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.0c02144.

  • Discussion on the extraction of the microstrain parameter from powder X-ray diffraction experiments with the Rietveld method; discussion on the correction of the as-measured microstrain to describe surface defectiveness; and a plot bridging the specific activity for the ORR and the QCO/2QH ratio (PDF)

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Author Information

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Acknowledgments

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This work was performed within the framework of the Centre of Excellence of Multifunctional Architectured Materials “CEMAM” (grant number ANR-10-LABX-44-01). The authors acknowledge financial support from the French National Research Agency through the BRIDGE project (grant number ANR-19-ENER-0008-01).

References

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    The application of the Rietveld refinement technique to synchrotron x-ray data collected from a capillary sample of Al2O3 in Debye-Scherrer geometry is described. The data were obtained at a high-energy synchrotron source with a Si(111) double-crystal monochromator and a Ge(111) crystal analyzer. Fits to a no. of well-resolved individual peaks demonstrate that the peak shapes are very well described by the pseudo-Voigt function, which is a simple approxn. to the convolution of Gaussian and Lorentzian functions. The variation of the Gaussian and Lorentzian half widths, ΓG and ΓL, with Bragg angle can be approximated quite closely by the functions V tanθ and X/cosθ which represent the contributions from instrumental resoln. and particle-size broadening resp. Rietveld refinement based on this model yields generally satisfactory results. The refined values of V and X are consistent with the expected vertical divergence (∼0.1 mrad) and and nominal particle size (∼0.3 μm). In particular, the use of a capillary specimen virtually eliminates preferred orientation effects, which are highly significant in flat-plate samples of this material.
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  16. 16
    Harada, M.; Ikegami, R.; Kumara, L. S. R.; Kohara, S.; Sakata, O. Reverse Monte Carlo Modeling for Local Structures of Noble Metal Nanoparticles Using High-Energy XRD and EXAFS. RSC Adv. 2019, 9 (51), 29511– 29521,  DOI: 10.1039/C9RA06519A
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    16
    Reverse Monte Carlo modeling for local structures of noble metal nanoparticles using high-energy XRD and EXAFS
    Harada, Masafumi; Ikegami, Risa; Kumara, Loku Singgappulige Rosantha; Kohara, Shinji; Sakata, Osami
    RSC Advances (2019), 9 (51), 29511-29521CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Reverse Monte Carlo (RMC) modeling based on the total structure factor S(Q) obtained from high-energy X-ray diffraction (HEXRD) and the k3χ(k) obtained from extended X-ray absorption fine structure (EXAFS) measurements was employed to det. the 3-dimensional (3D) at.-scale structure of Pt, Pd, and Rh nanoparticles, with sizes less than 5 nm, synthesized by photoredn. The total structure factor and Fourier-transformed PDF showed that the first nearest neighbor peak is in accordance with that obtained from conventional EXAFS anal. RMC constructed 3D models were analyzed in terms of prime structural characteristics such as metal-to-metal bond lengths, first-shell coordination nos. and bond angle distributions. The first-shell coordination nos. and bond angle distributions for the RMC-simulated metal nanoparticles indicated a face-centered cubic (fcc) structure with appropriate no. d. Modeling disorder effects in these RMC-simulated metal nanoparticles also revealed substantial differences in bond-length distributions for resp. nanoparticles.
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  17. 17
    Chattot, R.; Asset, T.; Bordet, P.; Drnec, J.; Dubau, L.; Maillard, F. Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures. ACS Catal. 2017, 7, 398– 408,  DOI: 10.1021/acscatal.6b02356
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    17
    Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures
    Chattot, Raphael; Asset, Tristan; Bordet, Pierre; Drnec, Jakub; Dubau, Laetitia; Maillard, Frederic
    ACS Catalysis (2017), 7 (1), 398-408CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The elec. performance of a proton exchange membrane fuel cell (PEMFC) is limited by the slow oxygen redn. reaction (ORR) kinetics. Catalytic improvements for the ORR were obtained on alloyed PtM/C or M-rich core@Pt-rich shell catalysts (where M is an early or late transition metal) compared to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure on the ORR kinetics remains under investigated. In this study, nanometer-sized PtNi/C electrocatalysts with low Ni content (∼15 at. %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow or sea sponge PtNi/C nanoalloys, and solid Ni-core@Pt-shell/C nanoparticles. These nanostructures were characterized by transmission and scanning-transmission electron microscopy (TEM and STEM, resp.), X-ray energy dispersive spectroscopy (X-EDS) Synchrotron wide-angle X-ray scattering (WAXS), at. absorption spectroscopy (AAS) and electrochem. techniques. Their electrocatalytic activities for the ORR were detd., and structure-activity relationships established. The results showed that (i) the compression of the Pt lattice by ca. 15 at. % Ni provides mild ORR activity enhancement compared to pure Pt/C, (ii) highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a com. Pt/C material with similar crystallite size, (iii) the enhancement in ORR catalytic activity can be ascribed to the presence of structural defects as shown by two independent parameters: the microstrain detd. from WAXS, and the av. COads electrooxidn. potential detd. from COads stripping measurements. This work indicates that, at fixed Ni at. %, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono or bimetallic nanocatalysts.
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  18. 18
    Sneed, B. T.; Young, A. P.; Tsung, C. K. Building up Strain in Colloidal Metal Nanoparticle Catalysts. Nanoscale 2015, 7 (29), 12248– 12265,  DOI: 10.1039/C5NR02529J
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    18
    Building up strain in colloidal metal nanoparticle catalysts
    Sneed, Brian T.; Young, Allison P.; Tsung, Chia-Kuang
    Nanoscale (2015), 7 (29), 12248-12265CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The focus on surface lattice strain in nanostructures as a fundamental research topic has gained momentum in recent years as scientists investigated its significant impact on the surface electronic structure and catalytic properties of nanomaterials. Researchers have begun to tell a more complete story of catalysis from a perspective which brings this concept to the forefront of the discussion. The nano-'realm' makes the effects of surface lattice strain, which acts on the same spatial scales, more pronounced due to a higher ratio of surface to bulk atoms. This is esp. evident in the field of metal nanoparticle catalysis, where displacement of atoms on surfaces can significantly alter the sorption properties of mols. In part, the concept of strain-engineering for catalysis opened up due to the achievements that were made in the synthesis of a more sophisticated nanoparticle library from an ever-expanding set of methodologies. Developing synthesis methods for metal nanoparticles with well-defined and strained architectures is a worthy goal that, if reached, will have considerable impact in the search for catalysts. In this review, we summarize the recent accomplishments in the area of surface lattice-strained metal nanoparticle synthesis, framing the discussion from the important perspective of surface lattice strain effects in catalysis.
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  19. 19
    Gan, L.; Yu, R.; Luo, J.; Cheng, Z.; Zhu, J. Lattice Strain Distributions in Individual Dealloyed Pt-Fe Catalyst Nanoparticles. J. Phys. Chem. Lett. 2012, 3 (7), 934– 938,  DOI: 10.1021/jz300192b
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    19
    Lattice Strain Distributions in Individual Dealloyed Pt-Fe Catalyst Nanoparticles
    Gan, Lin; Yu, Rong; Luo, Jun; Cheng, Zhiying; Zhu, Jing
    Journal of Physical Chemistry Letters (2012), 3 (7), 934-938CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Lattice strain is considered to play an important role in the O redn. catalysis on Pt-based catalysts. However, so far, direct evidence of the lattice strain in the catalyst nanoparticles was not achieved. By using aberration-cor. high-resoln. TEM combined with image simulations, a unique core-shell structure, i.e., a percolated lattice-contracted Pt-Fe alloy core and a Pt-rich surface with a gradient compressive strain, was directly demonstrated within individual dealloyed Pt-Fe nanoparticles and thus provides direct evidence for the strain effect on their enhanced O redn. activity.
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  20. 20
    Suzana, A. F.; Rochet, A.; Passos, A. R.; Castro Zerba, J. P.; Polo, C. C.; Santilli, C. V.; Pulcinelli, S. H.; Berenguer, F.; Harder, R.; Maxey, E.; Meneau, F. In Situ Three-Dimensional Imaging of Strain in Gold Nanocrystals during Catalytic Oxidation. Nanoscale Adv. 2019, 1 (8), 3009– 3014,  DOI: 10.1039/C9NA00231F
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    20
    In situ three-dimensional imaging of strain in gold nanocrystals during catalytic oxidation
    Suzana, Ana Flavia; Rochet, Amelie; Passos, Aline Ribeiro; Castro Zerba, Joao Paulo; Polo, Carla Cristina; Santilli, Celso Valentim; Pulcinelli, Sandra Helena; Berenguer, Felisa; Harder, Ross; Maxey, Evan; Meneau, Florian
    Nanoscale Advances (2019), 1 (8), 3009-3014CODEN: NAADAI; ISSN:2516-0230. (Royal Society of Chemistry)
    The chem. properties of materials are dependent on dynamic changes in their three-dimensional (3D) structure as well as on the reactive environment. We report an in situ 3D imaging study of defect dynamics of a single gold nanocrystal. Our findings offer an insight into its dynamic nanostructure and unravel the formation of a nanotwin network under CO oxidn. conditions. In situ/operando defect dynamics imaging paves the way to elucidate chem. processes at the single nano-object level towards defect-engineered nanomaterials.
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    Nilsson Pingel, T.; Jørgensen, M.; Yankovich, A. B.; Grönbeck, H.; Olsson, E. Influence of Atomic Site-Specific Strain on Catalytic Activity of Supported Nanoparticles. Nat. Commun. 2018, 9, 2722,  DOI: 10.1038/s41467-018-05055-1
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    21
    Influence of atomic site-specific strain on catalytic activity of supported nanoparticles
    Nilsson Pingel Torben; Jorgensen Mikkel; Yankovich Andrew B; Gronbeck Henrik; Olsson Eva; Nilsson Pingel Torben; Jorgensen Mikkel; Gronbeck Henrik; Olsson Eva
    Nature communications (2018), 9 (1), 2722 ISSN:.
    Heterogeneous catalysis is an enabling technology that utilises transition metal nanoparticles (NPs) supported on oxides to promote chemical reactions. Structural mismatch at the NP-support interface generates lattice strain that could affect catalytic properties. However, detailed knowledge about strain in supported NPs remains elusive. We experimentally measure the strain at interfaces, surfaces and defects in Pt NPs supported on alumina and ceria with atomic resolution using high-precision scanning transmission electron microscopy. The largest strains are observed at the interfaces and are predominantly compressive. Atomic models of Pt NPs with experimentally measured strain distributions are used for first-principles kinetic Monte Carlo simulations of the CO oxidation reaction. The presence of only a fraction of strained surface atoms is found to affect the turnover frequency. These results provide a quantitative understanding of the relationship between strain and catalytic function and demonstrate that strain engineering can potentially be used for catalyst design.
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  22. 22
    Yang, Y.; Chen, C. C.; Scott, M. C.; Ophus, C.; Xu, R.; Pryor, A.; Wu, L.; Sun, F.; Theis, W.; Zhou, J.; Eisenbach, M.; Kent, P. R. C.; Sabirianov, R. F.; Zeng, H.; Ercius, P.; Miao, J. Deciphering Chemical Order/Disorder and Material Properties at the Single-Atom Level. Nature 2017, 542 (7639), 75– 79,  DOI: 10.1038/nature21042
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    Deciphering chemical order/disorder and material properties at the single-atom level
    Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.; Ophus, Colin; Xu, Rui; Pryor, Alan; Wu, Li; Sun, Fan; Theis, Wolfgang; Zhou, Jihan; Eisenbach, Markus; Kent, Paul R. C.; Sabirianov, Renat F.; Zeng, Hao; Ercius, Peter; Miao, Jianwei
    Nature (London, United Kingdom) (2017), 542 (7639), 75-79CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Perfect crystals are rare in nature. Real materials often contain crystal defects and chem. order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quant. material characterization methods, correlating three-dimensional (3D) at. arrangements of chem. order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calcns. such as d. functional theory (DFT) have progressed from the modeling of ideal bulk systems to modeling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calcns. rely heavily on av. at. models extd. from crystallog. To improve the predictive power of first-principles calcns., there is a pressing need to use at. coordinates of real systems beyond av. crystallog. measurements. Here we det. the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chem. order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including at. compn., grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the exptl. measured coordinates and chem. species with 22 pm precision can be used as direct input for DFT calcns. of material properties such as at. spin and orbital magnetic moments and local magnetocryst. anisotropy. This work combines 3D at. structure detn. of crystal defects with DFT calcns., which is expected to advance our understanding of structure-property relationships at the fundamental level.
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    Ruiz-Zepeda, F.; Gatalo, M.; Pavlišič, A.; Dražić, G.; Jovanovič, P.; Bele, M.; Gaberšček, M.; Hodnik, N. Atomically Resolved Anisotropic Electrochemical Shaping of Nano-Electrocatalyst. Nano Lett. 2019, 19 (8), 4919– 4927,  DOI: 10.1021/acs.nanolett.9b00918
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    Atomically Resolved Anisotropic Electrochemical Shaping of Nano-electrocatalyst
    Ruiz-Zepeda, Francisco; Gatalo, Matija; Pavlisic, Andraz; Drazic, Goran; Jovanovic, Primoz; Bele, Marjan; Gaberscek, Miran; Hodnik, Nejc
    Nano Letters (2019), 19 (8), 4919-4927CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Catalytic properties of advanced functional materials are detd. by their surface and near-surface at. structure, compn., morphol., defects, compressive and tensile stresses, etc.; also known as a structure-activity relationship. The catalysts structural properties are dynamically changing as they perform via complex phenomenon dependent on the reaction conditions. In turn, not just the structural features but even more importantly, catalytic characteristics of nanoparticles get altered. Definitive conclusions about these phenomena are not possible with imaging of random nanoparticles with unknown at. structure history. Using a contemporary PtCu-alloy electrocatalyst as a model system, a unique approach allowing unprecedented insight into the morphol. dynamics on the at.-scale caused by the process of dealloying is presented. Observing the detailed structure and morphol. of the same nanoparticle at different stages of electrochem. treatment reveals new insights into at.-scale processes such as size, faceting, strain and porosity development. Furthermore, based on precise atomically resolved microscopy data, Kinetic Monte Carlo (KMC) simulations provide further feedback into the phys. parameters governing electrochem. induced structural dynamics. This work introduces a unique approach toward observation and understanding of nanoparticles dynamic changes on the at. level and paves the way for an understanding of the structure-stability relationship.
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    Stukowski, A.; Markmann, J.; Weissmüller, J.; Albe, K. Atomistic Origin of Microstrain Broadening in Diffraction Data of Nanocrystalline Solids. Acta Mater. 2009, 57 (5), 1648– 1654,  DOI: 10.1016/j.actamat.2008.12.011
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    Atomistic origin of microstrain broadening in diffraction data of nanocrystalline solids
    Stukowski, A.; Markmann, J.; Weissmueller, J.; Albe, K.
    Acta Materialia (2009), 57 (5), 1648-1654CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
    The origin of microstrain broadening in x-ray diffraction patterns of nanocryst. metals is investigated by comparing data obtained from virtual diffractograms and from direct anal. of computer-generated samples. A new method is introduced that allows the local deformation gradient to be calcd. for each lattice site in the microstructure from at. coordinates obtained by mol. dynamics simulations. Microstrain broadening in undeformed specimens cannot be attributed to lattice dislocations or strain fields near grain boundaries. The broadening arises, instead, from long-range correlated displacement fields that extend throughout the grains. The microstrain therefore provides a quant. measure for distortions far from grain boundaries. Evidently diffraction-based strategies for inferring the dislocation d. in ultrafine-grained metals do not necessarily apply to nanocryst. materials.
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    Montejano-Carrizales, J. M.; Morán-López, J. L. Geometrical Characteristics of Compact Nanoclusters. Nanostruct. Mater. 1992, 1 (5), 397– 409,  DOI: 10.1016/0965-9773(92)90090-K
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    Geometrical characteristics of compact nanoclusters
    Montejano-Carrizales, J. M.; Moran-Lopez, J. L.
    Nanostructured Materials (1992), 1 (5), 397-409CODEN: NMAEE7; ISSN:0965-9773.
    The geometrical characteristics of icosahedral and cubo-octahedral clusters were studied. The various shells around the central atom that form the cluster, the no. of atoms forming each shell, the no. of nearest neighbors to each atom, the various surface sites and the total no. of bonds were analyzed. General expressions are given for the total no. of atoms, the total no. of surface atoms and their distribution, and the total no. of bonds. The cluster shape stability is discussed.
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    Chattot, R.; Martens, I.; Scohy, M.; Herranz, J.; Drnec, J.; Maillard, F.; Dubau, L. Disclosing Pt-Bimetallic Alloy Nanoparticle Surface Lattice Distortion with Electrochemical Probes. ACS Energy Lett. 2020, 5 (1), 162– 169,  DOI: 10.1021/acsenergylett.9b02287
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    Disclosing Pt-Bimetallic Alloy Nanoparticle Surface Lattice Distortion with Electrochemical Probes
    Chattot, Raphael; Martens, Isaac; Scohy, Marion; Herranz, Juan; Drnec, Jakub; Maillard, Frederic; Dubau, Laetitia
    ACS Energy Letters (2020), 5 (1), 162-169CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Structural defects are of significant importance in (electro)catalysis, as they provide sites of unusually high activity that find applications in many key electrochem. processes. However, tools to characterize surface defects remain scarce and complex, esp. for nanocatalysts where classical methods such as TEM or x-ray scattering are limited in their ability to probe the structure and distribution of the active surface sites. Herein, the ratio between the COads stripping charge (QCO) and the charge required to desorb under-potentially deposited H atoms (QH) in structurally-disordered Pt-based nanocatalysts scales almost linearly with the Surface Distortion descriptor obtained via advanced phys. methods. This trend is valid in both rotating disk electrode configuration and in a real fuel cell device, thus providing the scientific community with a powerful and versatile approach for semi-quant. estn. of the surface lattice distortion in Pt-based catalysts without the need for exhaustive structural characterization.
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  27. 27
    Henning, S.; Kühn, L.; Herranz, J.; Durst, J.; Binninger, T.; Nachtegaal, M.; Werheid, M.; Liu, W.; Adam, M.; Kaskel, S.; Eychmüller, A.; Schmidt, T. J. Pt-Ni Aerogels as Unsupported Electrocatalysts for the Oxygen Reduction Reaction. J. Electrochem. Soc. 2016, 163 (9), F998– F1003,  DOI: 10.1149/2.0251609jes
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    Pt-Ni Aerogels as Unsupported Electrocatalysts for the Oxygen Reduction Reaction
    Henning, Sebastian; Kuhn, Laura; Herranz, Juan; Durst, Julien; Binninger, Tobias; Nachtegaal, Maarten; Werheid, Matthias; Liu, Wei; Adam, Marion; Kaskel, Stefan; Eychmuller, Alexander; Schmidt, Thomas J.
    Journal of the Electrochemical Society (2016), 163 (9), F998-F1003CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The com. feasibility of polymer electrolyte fuel cells (PEFCs) passes by the development of Pt-based, O2-redn. catalysts with greater activities and/or lower Pt-contents, as well as an improved stability. In an effort to tackle these requirements, unsupported bimetallic Pt-Ni nanoparticles (NPs) interconnected in the shape of nanochain networks (aerogels) were synthesized using a simple one-step redn. and gel formation process in aq. soln. The products of this novel synthetic route were characterized by X-ray absorption spectroscopy to elucidate the materials' structure. Using electrochem. expts., we probed the surface compn. of the as-synthesized aerogels and of equiv. materials exposed to acid, and concluded that a Ni-(hydr)oxide side phase is present in the aerogel with a larger Ni-concn. Regardless of this initial surface compn., the Pt-Ni aerogels feature a ≈3-fold increase of surface-specific ORR activity when compared to a com. platinum-on-carbon catalyst, reaching the mass-specific requirement for application in automotive PEFCs.
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    Henning, S.; Herranz, J.; Ishikawa, H.; Kim, B. J.; Abbott, D.; Kühn, L.; Eychmüller, A.; Schmidt, T. J. Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes. J. Electrochem. Soc. 2017, 164 (12), F1136– F1141,  DOI: 10.1149/2.0131712jes
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    Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes
    Henning, Sebastian; Herranz, Juan; Ishikawa, Hiroshi; Kim, Bae Jung; Abbott, Daniel; Kuhn, Laura; Eychmuller, Alexander; Schmidt, Thomas J.
    Journal of the Electrochemical Society (2017), 164 (12), F1136-F1141CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The com. success of polymer electrolyte fuel cells (PEFCs) depends on the development of Pt-based oxygen redn. reaction (ORR) catalysts with greater activity and stability to reduce the amt. of expensive noble metal per device. To advance toward this goal, we have tested a novel class of unsupported bimetallic alloy catalysts (aerogels) as the cathode material in PEFCs under two accelerated stress test conditions and compared it to a state-of-the-art carbon-supported benchmark (Pt/C). The investigated Pt3Ni aerogel shows little degrdn. under high potential conditions (> 1.0 V) which can occur during fuel starvation and start-up/shut-down of the cell. If tested under the same conditions, the Pt/C benchmark displays significant losses of electrochem. surface area and ORR activity due to carbon support corrosion as obsd. in cross section and transmission electron microscopy anal. When testing the durability upon extended load cycling (0.6-1.0 V), Pt3Ni aerogel demonstrates less stability than Pt/C which is related to the severe Ni leaching from the alloy under such conditions. These findings highlight the advantages of using unsupported ORR catalysts in PEFCs and point to the redn. of non-noble metal dissoln. as the next development step.
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    Henning, S.; Ishikawa, H.; Kühn, L.; Herranz, J.; Müller, E.; Eychmüller, A.; Schmidt, T. J. Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes. Angew. Chem., Int. Ed. 2017, 56 (36), 10707– 10710,  DOI: 10.1002/anie.201704253
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    29
    Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes
    Henning, Sebastian; Ishikawa, Hiroshi; Kuehn, Laura; Herranz, Juan; Mueller, Elisabeth; Eychmueller, Alexander; Schmidt, Thomas J.
    Angewandte Chemie, International Edition (2017), 56 (36), 10707-10710CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Highly active and durable oxygen redn. catalysts are needed to reduce the costs and enhance the service life of polymer electrolyte fuel cells (PEFCs). This can be accomplished by alloying Pt with a transition metal (for example Ni) and by eliminating the corrodible, carbon-based catalyst support. However, materials combining both approaches have seldom been implemented in PEFC cathodes. In this work, an unsupported Pt-Ni alloy nanochain ensemble (aerogel) demonstrates high current PEFC performance commensurate with that of a carbon-supported benchmark (Pt/C) following optimization of the aerogel's catalyst layer (CL) structure. The latter is accomplished using a sol. filler to shift the CL's pore size distribution towards larger pores which improves reactant and product transport. Chiefly, the optimized PEFC aerogel cathodes display a circa 2.5-fold larger surface-specific ORR activity than Pt/C and maintain 90 % of the initial activity after an accelerated stress test (vs. 40 % for Pt/C).
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    Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C. Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; Zhu, E.; Yu, T.; Jia, Q.; Guo, J.; Zhang, L.; Goddard, W. A.; Huang, Y.; Duan, X. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354 (6318), 1414– 1419,  DOI: 10.1126/science.aaf9050
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    30
    Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction
    Li, Mufan; Zhao, Zipeng; Cheng, Tao; Fortunelli, Alessandro; Chen, Chih-Yen; Yu, Rong; Zhang, Qinghua; Gu, Lin; Merinov, Boris V.; Lin, Zhaoyang; Zhu, Enbo; Yu, Ted; Jia, Qingying; Guo, Jinghua; Zhang, Liang; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng
    Science (Washington, DC, United States) (2016), 354 (6318), 1414-1419CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Improving the platinum (Pt) mass activity for the oxygen redn. reaction (ORR) requires optimization of both the specific activity and the electrochem. active surface area (ECSA). We found that soln.-synthesized Pt/NiO core/shell nanowires can be converted into PtNi alloy nanowires through a thermal annealing process and then transformed into jagged Pt nanowires via electrochem. dealloying. The jagged nanowires exhibit an ECSA of 118 square meters per g of Pt and a specific activity of 11.5 mA per square centimeter for ORR (at 0.9 V vs. reversible hydrogen electrode), yielding a mass activity of 13.6 A per mg of Pt, nearly double previously reported best values. Reactive mol. dynamics simulations suggest that highly stressed, undercoordinated rhombus-rich surface configurations of the jagged nanowires enhance ORR activity vs. more relaxed surfaces.
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  31. 31
    Alia, S. M.; Ngo, C.; Shulda, S.; Ha, M. A.; Dameron, A. A.; Weker, J. N.; Neyerlin, K. C.; Kocha, S. S.; Pylypenko, S.; Pivovar, B. S. Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum-Nickel Nanowires through Synthesis and Post-Treatment Optimization. ACS Omega 2017, 2 (4), 1408– 1418,  DOI: 10.1021/acsomega.7b00054
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    31
    Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum-Nickel Nanowires through Synthesis and Post-Treatment Optimization
    Alia, Shaun M.; Ngo, Chilan; Shulda, Sarah; Ha, Mai-Anh; Dameron, Arrelaine A.; Weker, Johanna Nelson; Neyerlin, Kenneth C.; Kocha, Shyam S.; Pylypenko, Svitlana; Pivovar, Bryan S.
    ACS Omega (2017), 2 (4), 1408-1418CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cmPt-2 at 0.9 V) and high surface areas (>90 m2 gPt-1). Platinum-nickel (Pt-Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt-1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochem. test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt-Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.
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  32. 32
    Van Der Vliet, D. F.; Wang, C.; Tripkovic, D.; Strmcnik, D.; Zhang, X. F.; Debe, M. K.; Atanasoski, R. T.; Markovic, N. M.; Stamenkovic, V. R. Mesostructured Thin Films as Electrocatalysts with Tunable Composition and Surface Morphology. Nat. Mater. 2012, 11 (12), 1051– 1058,  DOI: 10.1038/nmat3457
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    32
    Mesostructured thin films as electrocatalysts with tunable composition and surface morphology
    van der Vliet, Dennis F.; Wang, Chao; Tripkovic, Dusan; Strmcnik, Dusan; Zhang, Xiao Feng; Debe, Mark K.; Atanasoski, Radoslav T.; Markovic, Nenad M.; Stamenkovic, Vojislav R.
    Nature Materials (2012), 11 (12), 1051-1058CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Among the most challenging issues in technologies for electrochem. energy conversion are the insufficient activity of the catalysts for the oxygen redn. reaction, catalyst degrdn. and carbon-support corrosion. In an effort to address these barriers, carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored phys. properties are considered. Here a new class of metallic materials with tunable near-surface compn., morphol. and structure that have led to greatly improved affinity are presented for the electrochem. redn. of oxygen. The level of activity for the oxygen redn. reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk polycryst. Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.
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  33. 33
    Bae, S. J.; Yoo, S. J.; Lim, Y.; Kim, S.; Lim, Y.; Choi, J.; Nahm, K. S.; Hwang, S. J.; Lim, T. H.; Kim, S. K.; Kim, P. Facile Preparation of Carbon-Supported PtNi Hollow Nanoparticles with High Electrochemical Performance. J. Mater. Chem. 2012, 22 (18), 8820– 8825,  DOI: 10.1039/c2jm16827h
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    Facile preparation of carbon-supported PtNi hollow nanoparticles with high electrochemical performance
    Bae, Sung Jong; Yoo, Sung Jong; Lim, Yuntaek; Kim, Sojeong; Lim, Yirang; Choi, Junghun; Nahm, Kee Suk; Hwang, Seung Jun; Lim, Tae-Hoon; Kim, Soo-Kil; Kim, Pil
    Journal of Materials Chemistry (2012), 22 (18), 8820-8825CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    To design Pt-based materials with a hollow structure via a galvanic reaction would be one of the effective ways to prep. electro- catalysts with high activity. The galvanic reaction between Pt ions and metal template is usually conducted under limited conditions, which makes the prepn. of Pt hollow nanoparticles laborious. Here, we introduce a one-step and one-pot synthetic approach for the prepn. of carbon-supported PtNi alloy hollow nanoparticles with a narrow size distribution. Prepd. PtNi alloys were characterized by a nonporous shell consisting of a Pt-enriched surface layer and an inner alloy layer of Pt and Ni. Due to its unique structural advantages, this material showed excellent electrocatalytic performance for oxygen redn. (3.3- and 7.8-fold enhanced mass and specific activities compared to those of a com. carbon-supported Pt nanoparticle). A possible mechanism for the formation of PtNi hollow structure is suggested.
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  34. 34
    Snyder, J.; Fujita, T.; Chen, M. W.; Erlebacher, J. Oxygen Reduction in Nanoporous Metal-Ionic Liquid Composite Electrocatalysts. Nat. Mater. 2010, 9 (11), 904– 907,  DOI: 10.1038/nmat2878
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    Oxygen reduction in nanoporous metal-ionic liquid composite electrocatalysts
    Snyder, J.; Fujita, T.; Chen, M. W.; Erlebacher, J.
    Nature Materials (2010), 9 (11), 904-907CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The improvement of catalysts for the 4-electron redn. reaction (ORR; O2+4H++4e-→2H2O) remains a crit. challenge for fuel cells and other electrochem.-energy technologies. Recent attention in this area has centered on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles. For instance, with a outer surface and Ni-rich 2nd at. layer, Pt3Ni(111) is one of the most active surfaces for the ORR, owing to a shift in the d-band center of the surface atoms that results in a weakened interaction between and intermediate oxide species, freeing more active sites for O2 adsorption. However, enhancements due solely to alloy structure and compn. may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel cell commercialization, esp. as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. A tailored geometric and chem. materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous alloy impregnated with a hydrophobic, high O soly. and protic ionic liq. has extremely high mass activity. The results are consistent with an engineered chem. bias within a catalytically active nanoporous framework that pushes the ORR towards completion.
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  35. 35
    Luo, M.; Sun, Y.; Zhang, X.; Qin, Y.; Li, M.; Li, Y.; Li, C.; Yang, Y.; Wang, L.; Gao, P.; Lu, G.; Guo, S. Stable High-Index Faceted Pt Skin on Zigzag-Like PtFe Nanowires Enhances Oxygen Reduction Catalysis. Adv. Mater. 2018, 30 (10), 1705515,  DOI: 10.1002/adma.201705515
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    Dubau, L.; Nelayah, J.; Moldovan, S.; Ersen, O.; Bordet, P.; Drnec, J.; Asset, T.; Chattot, R.; Maillard, F. Defects Do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles. ACS Catal. 2016, 6 (7), 4673– 4684,  DOI: 10.1021/acscatal.6b01106
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    36
    Defects do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles
    Dubau, Laetitia; Nelayah, Jaysen; Moldovan, Simona; Ersen, Ovidiu; Bordet, Pierre; Drnec, Jakub; Asset, Tristan; Chattot, Raphael; Maillard, Frederic
    ACS Catalysis (2016), 6 (7), 4673-4684CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The catalytic performance of extended and nanometer-sized surfaces strongly depends on the amt. and the nature of structural defects that they exhibit. However, whereas the effect of steps or adatoms may be unraveled with single crystals ("surface science approach"), implementing reproducibly in a controlled manner structural defects on nanomaterials remains hardly feasible. A case that deserves particular attention is that of bimetallic nanomaterials, which are used to catalyze the oxygen redn. reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Point defects (vacancies), planar defects (dislocations and grain boundaries), and bulk defects (voids, pores) are likely to be generated in alloy or core@shell nanomaterials based on Pt and a transition metal due to the high lattice mismatch between the two elements. Here, we report the morphol. and structural trajectories of hollow PtNi/C nanoparticles during thermal annealing under vacuum, N2, H2, or air atm. by in situ transmission electron microscopy and synchrotron X-ray diffraction. We evidence atm.-dependent restructuring kinetics, which enabled us to synthesize a set of catalysts with identical chem. compns. and elemental distributions but different morphologies, crystallite sizes, and lattice strain. By combining the results of Rietveld and pair-distribution function analyses and electrochem. measurements, we demonstrate that the structurally disordered areas located at the interface between individual crystallites are highly active for two reactions of interest for PEMFC devices: the electrochem. COads oxidn. and the ORR. These results shed fundamental light on the effect of structural defects on the catalytic performance of bimetallic nanomaterials and should aid in the rational design of more efficient ORR electrocatalysts.
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  37. 37
    Alia, S. M.; Pylypenko, S.; Neyerlin, K. C.; Kocha, S. S.; Pivovar, B. S. Nickel Nanowire Oxidation and Its Effect on Platinum Galvanic Displacement and Methanol Oxidation. ECS Trans. 2014, 64 (3), 89– 95,  DOI: 10.1149/06403.0089ecst
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    Nickel nanowire oxidation and its effect on platinum galvanic displacement and methanol oxidation
    Alia, Shaun M.; Pylypenko, Svitlana; Neyerlin, K. C.; Kocha, Shyam S.; Pivovar, Bryan S.
    ECS Transactions (2014), 64 (3, Polymer Electrolyte Fuel Cells 14 (PEFC 14)), 89-95CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
    The heat treatment of nickel (Ni) nanowires (NiNWs) in oxygen is examd. for its effect on platinum (Pt) galvanic displacement and the activity of Pt-coated NiNWs (PtNiNWs) in the methanol oxidn. reaction (MOR). Annealing of NiNWs in oxygen is found to reduce Pt displacement and increase the resistance of surface Ni to potential-driven dissoln. (during electrochem. break-in). By reducing Ni dissoln., oxophilic species are provided in close proximity to Pt sites and the MOR activity of PtNiNWs improves, particularly at low overpotential in rotating disk electrode half-cells.
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  38. 38
    Xiong, Z.; Li, S.; Xu, H.; Zhang, K.; Yan, B.; Du, Y. Newly Designed Ternary Metallic PtPdBi Hollow Catalyst with High Performance for Methanol and Ethanol Oxidation. Catalysts 2017, 7, 208,  DOI: 10.3390/catal7070208
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    Newly designed ternary metallic PtPdBi hollow catalyst with high performance for methanol and ethanol oxidation
    Xiong, Zhiping; Li, Shumin; Xu, Hui; Zhang, Ke; Yan, Bo; Du, Yukou
    Catalysts (2017), 7 (7), 208/1-208/10CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
    This paper reported the fabrication of ternary metallic PtPdBi hollow nanocatalyst through a facile, one-pot, wet-chem. method by adopting sodium borohydride and polyvinylpyrrolidone as reducing agent and surfactant directing agent, resp. The hollow structure offers novel morphol. and large surface areas, which are conducive to enhancing the electrocatalytic activity. The electrocatalytic properties of hollow PtPdBi nanocatalyst were investigated systematically in alk. media through cyclic voltammetry and the as-prepd. PtPdBi nanocatalyst displays greatly enhanced electrocatalytic activities towards methanol and ethanol oxidn. The calcd. mass activities of PtPdBi electrocatalyst are 2.133 A mgPtPd-1 for methanol oxidn. reaction and 5.256 A mgPdPt-1 for ethanol oxidn. reaction, which are much better than that of com. Pt/C and com. Pd/C. The as-prepd. hollow nanocatalyst may be a potential promising electrocatalyst in fuel cells and also may be extended to the applications of other desirable functions.
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  39. 39
    Mao, J.; Chen, W.; He, D.; Wan, J.; Pei, J.; Dong, J.; Wang, Y.; An, P.; Jin, Z.; Xing, W.; Tang, H.; Zhuang, Z.; Liang, X.; Huang, Y.; Zhou, G.; Wang, L.; Wang, D.; Li, Y. Design of Ultrathin Pt-Mo-Ni Nanowire Catalysts for Ethanol Electrooxidation. Sci. Adv. 2017, 3, e1603068  DOI: 10.1126/sciadv.1603068
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    Design of ultrathin Pt-Mo-Ni nanowire catalysts for ethanol electrooxidation
    Mao, Junjie; Chen, Wenxing; He, Dongsheng; Wan, Jiawei; Pei, Jiajing; Dong, Juncai; Wang, Yu; An, Pengfei; Jin, Zhao; Xing, Wei; Tang, Haolin; Zhuang, Zhongbin; Liang, Xin; Huang, Yu; Zhou, Gang; Wang, Leyu; Wang, Dingsheng; Li, Yadong
    Science Advances (2017), 3 (8), e1603068/1-e1603068/9CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
    Developing cost-effective, active, and durable electrocatalysts is one of the most important issues for the com.-ization of fuel cells. Ultrathin Pt-Mo-Ni nanowires (NWs) with a diam. of ∼2.5 nm and lengths of up to several micro-meters were synthesized via a H2 -assisted soln. route (HASR). This catalyst was designed on the basis of thefollowing three points: (i) ultrathin NWs with high nos. of surface atoms can increase the at. efficiency ofPt and thus decrease the catalyst cost; (ii) the incorporation of Ni can isolate Pt atoms on the surface and producesurface defects, leading to high catalytic activity (the unique structure and superior activity were confirmed by spheri-cal aberration-cor. electron microscopy measurements and ethanol oxidn. tests, resp.); and (iii) theincorporation of Mo can stabilize both Ni and Pt atoms, leading to high catalytic stability, which was confirmed byexperiments and d. functional theory calcns. Furthermore, the developed HASR strategy can be extendedto synthesize a series of Pt-Mo-M (M = Fe, Co, Mn, Ru, etc.) NWs. These multimetallic NWs would open up new oppor-tunities for practical fuel cell applications.
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  40. 40
    Zhao, S.; Li, M.; Han, M.; Xu, D.; Yang, J.; Lin, Y.; Shi, N. E.; Lu, Y.; Yang, R.; Liu, B.; Dai, Z.; Bao, J. Defect-Rich Ni3FeN Nanocrystals Anchored on N-Doped Graphene for Enhanced Electrocatalytic Oxygen Evolution. Adv. Funct. Mater. 2018, 28 (18), 1706018,  DOI: 10.1002/adfm.201706018
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    Wang, P.; Jiang, K.; Wang, G.; Yao, J.; Huang, X. Phase and Interface Engineering of Platinum-Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55 (41), 12859– 12863,  DOI: 10.1002/anie.201606290
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    Phase and Interface Engineering of Platinum-Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution
    Wang, Pengtang; Jiang, Kezhu; Wang, Gongming; Yao, Jianlin; Huang, Xiaoqing
    Angewandte Chemie, International Edition (2016), 55 (41), 12859-12863CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The design of high-performance electrocatalysts for the alk. hydrogen evolution reaction (HER) is highly desirable for the development of alk. water electrolysis. Phase- and interface-engineered platinum-nickel nanowires (Pt-Ni NWs) are highly efficient electrocatalysts for alk. HER. The phase and interface engineering is achieved by simply annealing the pristine Pt-Ni NWs under a controlled atm. Impressively, the newly generated nanomaterials exhibit superior activity for the alk. HER, outperforming the pristine Pt-Ni NWs and com. Pt/C, and also represent the best alk. HER catalysts to date. The enhanced HER activities are attributed to the superior phase and interface structures in the engineered Pt-Ni NWs.
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    Dickens, C. F.; Nørskov, J. K. A Theoretical Investigation into the Role of Surface Defects for Oxygen Evolution on RuO2. J. Phys. Chem. C 2017, 121 (34), 18516– 18524,  DOI: 10.1021/acs.jpcc.7b03481
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    A Theoretical Investigation into the Role of Surface Defects for Oxygen Evolution on RuO2
    Dickens, Colin F.; Noerskov, Jens K.
    Journal of Physical Chemistry C (2017), 121 (34), 18516-18524CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The inability of conventional theor. models to corroborate the known exptl. activity of RuO2 for the O evolution reaction (OER) has recently been the subject of numerous research efforts. The authors use d. functional theory calcns. to study the possibility that surface defects formed during dissoln. are responsible for the OER activity of RuO2. It was well-established exptl. that RuO2 undergoes dissoln. during OER, yet little is known about the at. structure or catalytic reactivity of the resulting defect sites. Through simulation of point defects, steps, and kinks derived from the RuO2 (110) surface, the authors discover a 0.7 eV range in the primary descriptor for OER activity, with the most active sites outperforming those at the ideal (110) surface by nearly 0.5 eV. The authors postulate that these variations in reactivity are due to differences in the local electronic structure, and the authors study in more detail the electronic structure of two singly coordinated sites at the same Ru atom located at a kink. Finally, the authors study possible dissoln. pathways that may proceed at the RuO2 surface under OER conditions.
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    Shi, Q.; Zhu, C.; Zhong, H.; Su, D.; Li, N.; Engelhard, M. H.; Xia, H.; Zhang, Q.; Feng, S.; Beckman, S. P.; Du, D.; Lin, Y. Nanovoid Incorporated IrxCu Metallic Aerogels for Oxygen Evolution Reaction Catalysis. ACS Energy Lett. 2018, 3 (9), 2038– 2044,  DOI: 10.1021/acsenergylett.8b01338
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    Nanovoid Incorporated IrxCu Metallic Aerogels for Oxygen Evolution Reaction Catalysis
    Shi, Qiurong; Zhu, Chengzhou; Zhong, Hong; Su, Dong; Li, Na; Engelhard, Mark H.; Xia, Haibing; Zhang, Qiang; Feng, Shuo; Beckman, Scott P.; Du, Dan; Lin, Yuehe
    ACS Energy Letters (2018), 3 (9), 2038-2044CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    IR-based nanomaterials are regarded as state-of-the-art cathode electrocatalysts in proton exchange membrane water electrolyzers (PEMWEs). Engineering the morphol. of Ir-based three-dimensional architectures as electrocatalysts toward oxygen evolution reaction (OER) was rarely studied. Here, the authors report the gelation of IrxCu metallic hydrogels self-assembled with ultrafine and nanovoid incorporated building blocks for enhancing the electrocatalytic performance toward OER. The compn.-optimized Ir3Cu metallic aerogels exhibited improved catalytic activity and durability toward OER. The mesosized voids generated through the in situ galvanic replacement reaction and the macrosized porous systems make a great contribution to the increased no. of active sites. First-principle calcns. revealed the intrinsic optimized binding energy of Ir by alloying with Cu. The best catalytic performance necessities a balance of the adsorption and desorption energy. The well-defined morphol. and enhanced OER electrochem. performances of nanovoid incorporated IrxCu metallic aerogels hold great promise in further applications in PEMWEs.
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    Calle-Vallejo, F.; Pohl, M. D.; Bandarenka, A. S. Quantitative Coordination-Activity Relations for the Design of Enhanced Pt Catalysts for CO Electro-Oxidation. ACS Catal. 2017, 7 (7), 4355– 4359,  DOI: 10.1021/acscatal.7b01105
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    Quantitative Coordination-Activity Relations for the Design of Enhanced Pt Catalysts for CO Electro-oxidation
    Calle-Vallejo, Federico; Pohl, Marcus D.; Bandarenka, Aliaksandr S.
    ACS Catalysis (2017), 7 (7), 4355-4359CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Efficient redox transformations of CO are vital for remediating the imbalance of the biogeochem. cycle of carbon and also for the development of next-generation energy technologies such as fuel cells. For instance, CO oxidn. is ubiquitous in hydrocarbon-based fuel cells and dets. to a large extent their efficiency. As Pt is used in these cells, minimal catalyst loadings that do not compromise the activity are needed. In view of that, we present here a "coordination-activity plot" and electrochem. expts. on electro-oxidn. of adsorbed CO to CO2 to establish quant. structure-activity relations for various Pt electrodes. We predict theor. and verify exptl. that catalytic CO oxidn. is enhanced by creating surface defects with optimal coordination, without alloying. Both largely overcoordinated and undercoordinated defects on Pt (at rough surfaces with cavities and metal adatoms, resp.) hinder the reaction, so that the maximal activity is reached on sites with a generalized coordination of ‾C‾N = 5.4. Importantly, this moderate coordination is found at defects such as step edges of electrodes with relatively short terrace lengths of 3-4 metal atoms.
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    Maillard, F.; Eikerling, M.; Cherstiouk, O. V; Schreier, S.; Savinova, E.; Stimming, U. Size Effects on Reactivity of Pt Nanoparticles in CO Monolayer Oxidation: The Role of Surface Mobility. Faraday Discuss. 2004, 125, 357– 377,  DOI: 10.1039/b303911k
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    Size effects on reactivity of Pt nanoparticles in CO monolayer oxidation: the role of surface mobility
    Maillard F; Eikerling M; Cherstiouk O V; Schreier S; Savinova E; Stimming U
    Faraday discussions (2004), 125 (), 357-77; discussion 391-407 ISSN:1359-6640.
    In the present paper we study the reactivity of model Pt nanoparticles supported on glassy carbon. The particle size effect is rationalized for CO monolayer oxidation exploring electrochemical methods (stripping voltammetry and chronoamperometry) and modelling. Significant size effects are observed in the particle size interval from ca. 1 to 4 nm, including the positive shift of the CO stripping peak with decreasing particle size and a pronounced asymmetry of the current transients at constant potential. The latter go through a maximum at low COads conversion and exhibit tailing, which is the longer the smaller the particle size. Neither mean field nor nucleation & growth models give a coherent explanation of these experimental findings. We, therefore, suggest a basic model employing the active site concept. With a number of reasonable simplifications a full analytical solution is obtained, which allows a straightforward comparison of the theory with the experimental data. A good correspondence between experiment and theory is demonstrated. The model suggests restricted COads mobility at Pt nanoparticles below ca. 2 nm size, with the diffusion coefficient strongly dependent on the particle size, and indicates a transition towards fast diffusion when the particle size exceeds ca. 3 nm. Estimates of relevant kinetic parameters, including diffusion coefficient, reaction constant etc. are obtained and compared to the literature data for extended Pt surfaces.
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  46. 46
    Van Der Vliet, D. F.; Wang, C.; Li, D.; Paulikas, A. P.; Greeley, J.; Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.; Stamenkovic, V. R. Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces. Angew. Chem., Int. Ed. 2012, 51 (13), 3139– 3142,  DOI: 10.1002/anie.201107668
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    Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces
    van der Vliet, Dennis F.; Wang, Chao; Li, Dongguo; Paulikas, Arvydas P.; Greeley, Jeffrey; Rankin, Rees B.; Strmcnik, Dusan; Tripkovic, Dusan; Markovic, Nenad M.; Stamenkovic, Vojislav R.
    Angewandte Chemie, International Edition (2012), 51 (13), 3139-3142, S3139/1-S3139/5CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Authors demonstrated that by alloying platinum with a non-noble second metal and inducing a Pt-skin-type structure, the adsorption properties of formed structures are significantly changed. The adsorption of hydrogen and oxide species is shifted in potential and reduced in magnitude compared to platinum. The LEIS data showed that the surface consists only of platinum. The suppression of hydrogen adatoms on platinum underestimate the surface area and overrates the specific activity. The Co stripping can be used alongside with the adsorbed hydrogen charge for the detn. of the electrochem. active surface area of platinum-alloy catalysts.
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    Marichev, V. A. Reversibility of Platinum Voltammograms in Aqueous Electrolytes and Ionic Product of Water. Electrochim. Acta 2008, 53 (27), 7952– 7960,  DOI: 10.1016/j.electacta.2008.05.076
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    Reversibility of platinum voltammograms in aqueous electrolytes and ionic product of water
    Marichev, V. A.
    Electrochimica Acta (2008), 53 (27), 7952-7960CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
    Known reversibility of Pt voltammograms in supporting aq. electrolytes formally contradicts the equil. defined by the ionic product of H2O: Kw = [H+]·[OH-] ≃ 1 × 10-14, since at any pH value, concn. of one of these ions in soln. should be too low (≤10-7 M) to ensure reversibility of voltammograms in the whole potential region of study at usual potential scan rates. This contradiction could be overcome by allowing dissociative H2O adsorption. There are no direct exptl. proofs of this process on Pt, however all available relevant data obtained by in situ phys. methods appear to provide indirect evidences for this hypothesis.
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    Van Der Niet, M. J. T. C.; Garcia-Araez, N.; Hernández, J.; Feliu, J. M.; Koper, M. T. M. Water Dissociation on Well-Defined Platinum Surfaces: The Electrochemical Perspective. Catal. Today 2013, 202, 105– 113,  DOI: 10.1016/j.cattod.2012.04.059
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    Water dissociation on well-defined platinum surfaces: The electrochemical perspective
    van der Niet, Maria J. T. C.; Garcia-Araez, Nuria; Hernandez, Javier; Feliu, Juan M.; Koper, Marc T. M.
    Catalysis Today (2013), 202 (), 105-113CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    This paper discusses three important discrepancies in the current interpretation of the role of water dissocn. on the blank cyclic voltammetry of well-defined single-cryst. stepped platinum surfaces. First, for H adsorption both H-terrace and H-step contributions were identified, whereas for OH adsorption only OH-terrace was identified. Second, different shapes (broad vs. sharp) of the H-terrace and H-step voltammetric peaks imply different lateral interactions between hydrogen adatoms at terraces and steps, i.e. repulsive vs. attractive interactions. Third, the H-step peak exhibits an unusual pH-dependent shift of 50 mVNHE/pH unit. We propose here a model that can explain all these observations. In the model, the H-step peak is not due to only ad- and desorption of hydrogen, but to the replacement of H with O and/or OH. The O:OH ratio in the step varies with step geometry, step d. and medium. In alk. media relatively more OH is adsorbed in (or on) the step than in acidic media, under which conditions more O is adsorbed in (or on) the step. This would explain the anomalous pH dependence and would provide a possible explanation for the higher catalytic activity of alk. media for electro-oxidn. reactions. Although the model certainly still contains speculative elements, we believe it provides the most consistent interpretation of platinum single-crystal electrochem. currently available, and presents an important and significant improvement over previous interpretations. In situ spectroscopic data are ultimately needed to confirm or disprove some of the assumptions of the model.
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    Janik, M. J.; McCrum, I. T.; Koper, M. T. M. On the Presence of Surface Bound Hydroxyl Species on Polycrystalline Pt Electrodes in the “Hydrogen Potential Region” (0 to 0.4 V-RHE). J. Catal. 2018, 367, 332– 337,  DOI: 10.1016/j.jcat.2018.09.031
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    On the presence of surface bound hydroxyl species on polycrystalline Pt electrodes in the "hydrogen potential region" (0-0.4 V-RHE)
    Janik, Michael J.; McCrum, Ian T.; Koper, Marc T. M.
    Journal of Catalysis (2018), 367 (), 332-337CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    A review is given on an effect of surface bound hydroxyl species on polycryst. platinum electrodes is given. The hydrogen binding energy, surface area, adsorption-induced restructuring, hydrogen oxidn. kinetics and carbon monoxide stripping are briefly discussed.
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    Tanaka, H.; Sugawara, S.; Shinohara, K.; Ueno, T.; Suzuki, S.; Hoshi, N.; Nakamura, M. Infrared Reflection Absorption Spectroscopy of OH Adsorption on the Low Index Planes of Pt. Electrocatalysis 2015, 6 (3), 295– 299,  DOI: 10.1007/s12678-014-0245-7
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    Infrared Reflection Absorption Spectroscopy of OH Adsorption on the Low Index Planes of Pt
    Tanaka, Hiroyuki; Sugawara, Seiho; Shinohara, Kazuhiko; Ueno, Takahiro; Suzuki, Shunsuke; Hoshi, Nagahiro; Nakamura, Masashi
    Electrocatalysis (2015), 6 (3), 295-299CODEN: ELECCF; ISSN:1868-2529. (Springer)
    The adsorption of hydroxide (OHad) on Pt has been studied on the low index planes of Pt using IR reflection absorption spectroscopy (IRAS) in electrochem. environments. We discuss the correlation between the integrated band intensity of the bending mode of OH of Pt-OH (δPtOH) and the charge d. of the oxide formation of Pt. The band of δPtOH is obsd. around 1100 cm-1, and the onset potential depends on the surface structure. The onset potential of δPtOH on Pt(110) and Pt(100) overlaps with the hydrogen adsorption/desorption potential region. The order of the integrated band intensity of δPtOH at 0.9 V vs RHE is opposite to the order of the oxygen redn. reaction (ORR) activity. This finding supports that the OHad is one of the species deactivating the ORR.
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    Garcia-Araez, N.; Climent, V.; Feliu, J. M. Analysis of Temperature Effects on Hydrogen and OH Adsorption on Pt(111), Pt(100) and Pt(110) by Means of Gibbs Thermodynamics. J. Electroanal. Chem. 2010, 649 (1–2), 69– 82,  DOI: 10.1016/j.jelechem.2010.01.024
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    Analysis of temperature effects on hydrogen and OH adsorption on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) by means of Gibbs thermodynamics
    Garcia-Araez, Nuria; Climent, Victor; Feliu, Juan M.
    Journal of Electroanalytical Chemistry (2010), 649 (1-2), 69-82CODEN: JECHES; ISSN:1572-6657. (Elsevier B.V.)
    Gibbs thermodn. in the presence of charge-transfer processes were applied to characterize temp. effects on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) electrodes in 0.1 M HClO4 solns. The present anal. allows the evaluation of the entropy of formation of the interphase for the three Pt basal planes, which is structure-sensitive. These results were used to test the suitability of the more common approach to characterize temp. effects, based on the application of a generalized isotherm. Besides, the comparison of both approaches allows the characterization of the different components involved in the entropy of H and OH adsorption, namely, the std. entropy of adsorption, the entropy due to lateral interactions, and the configurational entropy. Finally, the comparison with statistical mech. calcns. demonstrates that adsorbed H on Pt(1 1 1) and Pt(1 0 0) is rather mobile, while adsorbed OH is rather immobile. Adsorbed H on Pt(1 1 0) exhibits an unexpected behavior, since it appears to be rather immobile at low coverages, but it becomes very mobile at near satn. coverages.
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    Zhang, C.; Firestein, K. L.; Fernando, J. F. S.; Siriwardena, D.; von Treifeldt, J. E.; Golberg, D. Recent Progress of In Situ Transmission Electron Microscopy for Energy Materials. Adv. Mater. 2020, 32 (18), 1904094,  DOI: 10.1002/adma.201904094
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    Recent progress of in situ transmission electron microscopy for energy materials
    Zhang, Chao; Firestein, Konstantin L.; Fernando, Joseph F. S.; Siriwardena, Dumindu; von Treifeldt, Joel E.; Golberg, Dmitri
    Advanced Materials (Weinheim, Germany) (2020), 32 (18), 1904094CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. In situ transmission electron microscopy (TEM) is one of the most powerful approaches for revealing phys. and chem. process dynamics at at. resolns. The most recent developments for in situ TEM techniques are summarized; in particular, how they enable visualization of various events, measure properties, and solve problems in the field of energy by revealing detailed mechanisms at the nanoscale. Related applications include rechargeable batteries such as Li-ion, Na-ion, Li-O2, Na-O2, Li-S, etc., fuel cells, thermoelecs., photovoltaics, and photocatalysis. To promote various applications, the methods of introducing the in situ stimuli of heating, cooling, elec. biasing, light illumination, and liq. and gas environments are discussed. The progress of recent in situ TEM in energy applications should inspire future research on new energy materials in diverse energy-related areas.
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    Daio, T.; Staykov, A.; Guo, L.; Liu, J.; Tanaka, M.; Matthew Lyth, S.; Sasaki, K. Lattice Strain Mapping of Platinum Nanoparticles on Carbon and SnO2 Supports. Sci. Rep. 2015, 5, 13126,  DOI: 10.1038/srep13126
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    Lattice Strain Mapping of Platinum Nanoparticles on Carbon and SnO2 Supports
    Daio, Takeshi; Staykov, Aleksandar; Guo, Limin; Liu, Jianfeng; Tanaka, Masaki; Matthew Lyth, Stephen; Sasaki, Kazunari
    Scientific Reports (2015), 5 (), 13126CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    It is extremely important to understand the properties of supported metal nanoparticles at the at. scale. In particular, visualizing the interaction between nanoparticle and support, as well as the strain distribution within the particle is highly desirable. Lattice strain can affect catalytic activity, and therefore strain engineering via e.g. synthesis of core-shell nanoparticles or compositional segregation has been intensively studied. However, substrate-induced lattice strain has yet to be visualized directly. In this study, platinum nanoparticles decorated on graphitized carbon or tin oxide supports are investigated using spherical aberration-cor. scanning transmission electron microscopy (Cs-cor. STEM) coupled with geometric phase anal. (GPA). Local changes in lattice parameter are obsd. within the Pt nanoparticles and the strain distribution is mapped. This reveals that Pt nanoparticles on SnO2 are more highly strained than on carbon, esp. in the region of at. steps in the SnO2 lattice. These substrate-induced strain effects are also reproduced in d. functional theory simulations, and related to catalytic oxygen redn. reaction activity. This study suggests that tailoring the catalytic activity of electrocatalyst nanoparticles via the strong metal-support interaction (SMSI) is possible. This technique also provides an exptl. platform for improving our understanding of nanoparticles at the at. scale.
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    Wang, Y.; Zhang, W. Mapping the Strain Distribution within Embedded Nanoparticles via Geometrical Phase Analysis. Micron 2019, 125, 102715,  DOI: 10.1016/j.micron.2019.102715
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    Mapping the strain distribution within embedded nanoparticles via geometrical phase analysis
    Wang, Yan; Zhang, Wei
    Micron (2019), 125 (), 102715CODEN: MCONEN; ISSN:0968-4328. (Elsevier Ltd.)
    Strain variation within a nanoparticle plays a crucial role in tuning its properties. Geometrical phase anal. (GPA) is typically a powerful tool to investigate the strain in high-resoln. transmission electron microscopy (HRTEM) images. It is known that the traditional GPA method measuring the displacement of lattice fringes directly in an HRTEM image is inapplicable to strain measurements on nanoparticles embedded in a matrix, where lattice fringes of nanoparticles are invisible, instead Moire´ fringes are present. Furthermore, considering the small size of embedded nanoparticles, generally a few nanometers, no ref. region can be chosen and utilized to calc. the relative displacement by GPA. Hence advanced methods need to be developed to break through the barriers of invisible lattice fringes and lack of a ref. region. In this work, using α-Fe nanoparticles embedded in sapphire as a test object, we illustrate a GPA method dedicated to embedded nanoparticles. Both the Fourier filter method and the inverse Moire´ fringes method were used to reconstruct the invisible lattice fringes of α-Fe nanoparticles. Then a computer-generated image corresponding to an unstrained α-Fe lattice was used as the ref. during GPA. The GPA results indicate that there exists a compressive strain in the range of 1.5∼2% within the α-Fe nanoparticles. Our work presents an effective approach to revealing the strain distributions within embedded nanoparticles.
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    Ahmad, N.; Le Bouar, Y.; Ricolleau, C.; Alloyeau, D. Growth of Dendritic Nanostructures by Liquid-Cell Transmission Electron Microscopy: A Reflection of the Electron-Irradiation History. Adv. Struct. Chem. Imaging 2016, 2, 9,  DOI: 10.1186/s40679-016-0023-0
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    Growth of dendritic nanostructures by liquid-cell transmission electron microscopy: a reflection of the electron-irradiation history
    Ahmad, Nabeel; Le Bouar, Yann; Ricolleau, Christian; Alloyeau, Damien
    Advanced Structural and Chemical Imaging (2016), 2 (), 9/1-9/10CODEN: ASCICS; ISSN:2198-0926. (Springer International Publishing AG)
    Studying dynamical processes by transmission electron microscopy (TEM) requires considering the electron-irradn. history, including the instantaneous dose rate and the cumulative dose delivered to the sample. Here, we have exploited liq.-cell TEM to study the effects of the electron-irradn. history on the radiochem. growth of dendritic Au nanostructures. Besides the well-established direct link between the dose rate and the growth rate of the nanostructures, we demonstrate that the cumulative dose in the irradiated area can also induce important transitions in the growth mode of the nanostructures. By comparing in situ observations with an extended diffusion-limited aggregation model, we reveal how the shape of the nanostructures is severely affected by the local lack of metal precursors and the resulting restricted accessibility of gold atoms to the nanostructures. This study highlights the effects of electron irradn. on the soln. chem. in the irradiated area and in the whole liq. cell that are of primary importance to ext. quant. information on nanoscale processes.
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    Kelly, T. F.; Miller, M. K. Atom Probe Tomography. Rev. Sci. Instrum. 2007, 78, 031101,  DOI: 10.1063/1.2709758
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    Invited review article: atom probe tomography
    Kelly, Thomas F.; Miller, Michael K.
    Review of Scientific Instruments (2007), 78 (3), 031101/1-031101/20CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
    A review. The technique of atom probe tomog. (APT) is reviewed with an emphasis on illustrating what is possible with the technique both now and in the future. APT delivers the highest spatial resoln. (sub-0.3-nm) three-dimensional compositional information of any microscopy technique. Recently, APT has changed dramatically with new hardware configurations that greatly simplify the technique and improve the rate of data acquisition. New methods were developed to fabricate suitable specimens from new classes of materials. Applications of APT have expanded from structural metals and alloys to thin multilayer films on planar substrates, dielec. films, semiconducting structures and devices, and ceramic materials. This trend toward a broader range of materials and applications is likely to continue.
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    Gault, B.; Breen, A. J.; Chang, Y.; He, J.; Jägle, E. A.; Kontis, P.; Kürnsteiner, P.; Kwiatkowski Da Silva, A.; Makineni, S. K.; Mouton, I.; Peng, Z.; Ponge, D.; Schwarz, T.; Stephenson, L. T.; Szczepaniak, A.; Zhao, H.; Raabe, D. Interfaces and Defect Composition at the Near-Atomic Scale through Atom Probe Tomography Investigations. J. Mater. Res. 2018, 33 (23), 4018– 4030,  DOI: 10.1557/jmr.2018.375
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    Interfaces and defect composition at the near-atomic scale through atom probe tomography investigations
    Gault, Baptiste; Breen, Andrew J.; Chang, Yanhong; He, Junyang; Jaegle, Eric A.; Kontis, Paraskevas; Kuernsteiner, Philipp; Kwiatkowski da Silva, Alisson; Makineni, Surendra Kumar; Mouton, Isabelle; Peng, Zirong; Ponge, Dirk; Schwarz, Torsten; Stephenson, Leigh T.; Szczepaniak, Agnieszka; Zhao, Huan; Raabe, Dierk
    Journal of Materials Research (2018), 33 (23), 4018-4030CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
    Atom probe tomog. (APT) is rising in influence across many parts of materials science and engineering thanks to its unique combination of highly sensitive compn. measurement and three-dimensional microstructural characterization. In this invited article, we have selected a few recent applications that showcase the unique capacity of APT to measure the local compn. at structural defects. Whether we consider dislocations, stacking faults, or grain boundary, the detailed compositional measurements tend to indicate specific partitioning behaviors for the different solutes in both complex engineering and model alloys we investigated.
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    Li, T.; Bagot, P. A. J.; Marquis, E. A.; Tsang, S. C. E.; Smith, G. D. W. Characterization of Oxidation and Reduction of Pt-Ru and Pt-Rh-Ru Alloys by Atom Probe Tomography and Comparison with Pt-Rh. J. Phys. Chem. C 2012, 116 (33), 17633– 17640,  DOI: 10.1021/jp304359m
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    Characterization of Oxidation and Reduction of Pt-Ru and Pt-Rh-Ru Alloys by Atom Probe Tomography and Comparison with Pt-Rh
    Li, Tong; Bagot, Paul A. J.; Marquis, Emmanuelle A.; Tsang, S. C. Edman; Smith, George D. W.
    Journal of Physical Chemistry C (2012), 116 (33), 17633-17640CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Pt-based alloys contg. Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temp. fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidn. of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidn. and oxidn./redn. behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomog. The surface of the Pt-Ru alloy is enriched with Ru after oxidn. at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temps., leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidn. initiates from the grain boundaries, forming an oxide with a stoichiometry of MO2. As the oxidn. time increases, this oxide evolves into a two-phase nanostructure, involving a Rh-rich oxide phase (Rh, Ru)2O3 and a Ru-rich oxide phase (Ru, Rh)O2. When this two-phase oxide is reduced in hydrogen at low temps., sep. Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts.
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    Li, T.; Kasian, O.; Cherevko, S.; Zhang, S.; Geiger, S.; Scheu, C.; Felfer, P.; Raabe, D.; Gault, B.; Mayrhofer, K. J. J. Atomic-Scale Insights into Surface Species of Electrocatalysts in Three Dimensions. Nat. Catal. 2018, 1 (4), 300– 305,  DOI: 10.1038/s41929-018-0043-3
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    Wilde, P.; Dieckhöfer, S.; Quast, T.; Xiang, W.; Bhatt, A.; Chen, Y. T.; Seisel, S.; Barwe, S.; Andronescu, C.; Li, T.; Schuhmann, W.; Masa, J. Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiOx Core-Shell Heterostructures for the Oxygen Evolution Reaction. ACS Appl. Energy Mater. 2020, 3 (3), 2304– 2309,  DOI: 10.1021/acsaem.9b02481
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    Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiOx Core-Shell Heterostructures for the Oxygen Evolution Reaction
    Wilde, Patrick; Dieckhoefer, Stefan; Quast, Thomas; Xiang, Weikai; Bhatt, Anjali; Chen, Yen-Ting; Seisel, Sabine; Barwe, Stefan; Andronescu, Corina; Li, Tong; Schuhmann, Wolfgang; Masa, Justus
    ACS Applied Energy Materials (2020), 3 (3), 2304-2309CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    NiyP emerged as a highly active precatalyst for the alk. oxygen evolution reaction where structural changes play a crucial role for its catalytic performance. We probed the chem. stability of NiyP in 1 M KOH at 80°C and examd. how exposure up to 168 h affects its structure and catalytic performance. We obsd. selective P-leaching and formation of NiyP/NiOx core-shell heterostructures, where shell thickness increases with ageing time, which is detrimental for the activity. By tuning the particle size, we demonstrate that prevention of complete catalyst oxidn. is essential to preserve the outstanding electrochem. performance of NiyP in alk. media.
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    Barroo, C.; Akey, A. J.; Bell, D. C. Atom Probe Tomography for Catalysis Applications: A Review. Appl. Sci. 2019, 9 (13), 2721,  DOI: 10.3390/app9132721
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    Atom probe tomography for catalysis applications: a review
    Barroo, Cedric; Akey, Austin J.; Bell, David C.
    Applied Sciences (2019), 9 (13), 2721CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
    Atom probe tomog. is a well-established anal. instrument for imaging the 3D structure and compn. of materials with high mass resoln., sub-nanometer spatial resoln. and ppm elemental sensitivity. Thanks to recent hardware developments in Atom Probe Tomog. (APT), combined with progress on site-specific focused ion beam (FIB)-based sample prepn. methods and improved data treatment software, complex materials can now be routinely investigated. From model samples to complex, usable porous structures, there is currently a growing interest in the anal. of catalytic materials. APT is able to probe the end state of at.-scale processes, providing information needed to improve the synthesis of catalysts and to unravel structure/compn./reactivity relationships. This review focuses on the study of catalytic materials with increasing complexity (tip-sample, unsupported and supported nanoparticles, powders, self-supported catalysts and zeolites), as well as sample prepn. methods developed to obtain suitable specimens for APT expts.
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    Pfeiffer, B.; Erichsen, T.; Epler, E.; Volkert, C. A.; Trompenaars, P.; Nowak, C. Characterization of Nanoporous Materials with Atom Probe Tomography. Microsc. Microanal. 2015, 21 (3), 557– 563,  DOI: 10.1017/S1431927615000501
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    Characterization of Nanoporous Materials with Atom Probe Tomography
    Pfeiffer, Bjoern; Erichsen, Torben; Epler, Eike; Volkert, Cynthia A.; Trompenaars, Piet; Nowak, Carsten
    Microscopy and Microanalysis (2015), 21 (3), 557-563CODEN: MIMIF7; ISSN:1431-9276. (Cambridge University Press)
    A method to characterize open-cell nanoporous materials with atom probe tomog. (APT) has been developed. For this, open-cell nanoporous gold with pore diams. of around 50 nm was used as a model system, and filled by electron beam-induced deposition (EBID) to obtain a compact material. Two different EBID precursors were successfully tested-dicobalt octacarbonyl [Co2(CO)8] and diiron nonacarbonyl [Fe2(CO)9]. Penetration and filling depth are sufficient for focused ion beam-based APT sample prepn. With this approach, stable APT anal. of the nanoporous material can be performed. Reconstruction reveals the compn. of the deposited precursor and the nanoporous material, as well as chem. information of the interfaces between them. Thus, it is shown that, using an appropriate EBID process, local chem. information in three dimensions with sub-nanometer resoln. can be obtained from nanoporous materials using APT.
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    Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Hydrogen Production from Formic Acid Decomposition at Room Temperature Using a Ag-Pd Core-Shell Nanocatalyst. Nat. Nanotechnol. 2011, 6 (5), 302– 307,  DOI: 10.1038/nnano.2011.42
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    Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst
    Tedsree, Karaked; Li, Tong; Jones, Simon; Chan, Chun Wong Aaron; Yu, Kai Man Kerry; Bagot, Paul A. J.; Marquis, Emmanuelle A.; Smith, George D. W.; Tsang, Shik Chi Edman
    Nature Nanotechnology (2011), 6 (5), 302-307CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Formic acid has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy d., is nontoxic and can be safely handled in aq. soln. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen prodn. from formic acid at room temp. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the prodn. of H2 from formic acid at ambient temp. Atom probe tomog. confirmed that the nanoparticles have a core-shell configuration, with the shell contg. between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions.
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    Abbey, B. From Grain Boundaries to Single Defects: A Review of Coherent Methods for Materials Imaging in the X-Ray Sciences. JOM 2013, 65 (9), 1183– 1201,  DOI: 10.1007/s11837-013-0702-4
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    From Grain Boundaries to Single Defects: A Review of Coherent Methods for Materials Imaging in the X-ray Sciences
    Abbey, Brian
    JOM (2013), 65 (9), 1183-1201CODEN: JOMMER; ISSN:1047-4838. (Springer)
    This review summarizes the literature describing recent advances in the coherent x-ray sciences for the high-resoln. characterization of materials. The principles and some of the main exptl. techniques as well as their applications are discussed. The advantages of x-ray methods for characterizing 3D microstructures as well as for characterizing plasticity in the bulk become clear from the examples presented. Materials that exhibit size effects within the 0.1-10-μm range benefit enormously from these techniques, and development of the relevant x-ray methods will add to our fundamental understanding of these phenomena. Many of the ideas that have developed in the coherent x-ray science literature have been enabled through advances in x-ray source and detection technol., which has occurred over the past 10 years or so. It is a topic of considerable importance to consider how these techniques, which have matured rapidly, may be best applied to materials imaging in order to meet the growing needs of the community. As coherent x-ray methods for characterizing materials at multiple length scales have developed, several key applications for these techniques have emerged. The key breakthroughs that have been enabled by these new methods are discussed throughout this review, together with an examn. of some of the problems that will be addressed by these techniques within the next few years.
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    Ulvestad, A.; Tripathi, A.; Hruszkewycz, S. O.; Cha, W.; Wild, S. M.; Stephenson, G. B.; Fuoss, P. H. Coherent Diffractive Imaging of Time-Evolving Samples with Improved Temporal Resolution. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93 (18), 184105,  DOI: 10.1103/PhysRevB.93.184105
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    Carnis, J.; Gao, L.; Labat, S.; Kim, Y. Y.; Hofmann, J. P.; Leake, S. J.; Schülli, T. U.; Hensen, E. J. M.; Thomas, O.; Richard, M.-I. Towards a Quantitative Determination of Strain in Bragg Coherent X-Ray Diffraction Imaging: Artefacts and Sign Convention in Reconstructions. Sci. Rep. 2019, 9, 17357,  DOI: 10.1038/s41598-019-53774-2
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    Towards a quantitative determination of strain in Bragg Coherent X-ray Diffraction Imaging: artefacts and sign convention in reconstructions
    Carnis Jerome; Labat Stephane; Thomas Olivier; Richard Marie-Ingrid; Carnis Jerome; Leake Steven J; Schulli Tobias U; Richard Marie-Ingrid; Gao Lu; Hofmann Jan P; Hensen Emiel J M; Kim Young Yong
    Scientific reports (2019), 9 (1), 17357 ISSN:.
    Bragg coherent X-ray diffraction imaging (BCDI) has emerged as a powerful technique to image the local displacement field and strain in nanocrystals, in three dimensions with nanometric spatial resolution. However, BCDI relies on both dataset collection and phase retrieval algorithms that can induce artefacts in the reconstruction. Phase retrieval algorithms are based on the fast Fourier transform (FFT). We demonstrate how to calculate the displacement field inside a nanocrystal from its reconstructed phase depending on the mathematical convention used for the FFT. We use numerical simulations to quantify the influence of experimentally unavoidable detector deficiencies such as blind areas or limited dynamic range as well as post-processing filtering on the reconstruction. We also propose a criterion for the isosurface determination of the object, based on the histogram of the reconstructed modulus. Finally, we study the capability of the phasing algorithm to quantitatively retrieve the surface strain (i.e., the strain of the surface voxels). This work emphasizes many aspects that have been neglected so far in BCDI, which need to be understood for a quantitative analysis of displacement and strain based on this technique. It concludes with the optimization of experimental parameters to improve throughput and to establish BCDI as a reliable 3D nano-imaging technique.
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    Ulvestad, U.; Singer, A.; Clark, J. N.; Cho, H. M.; Kim, J. W.; Harder, R.; Maser, J.; Meng, Y. S.; Shpyrko, O. G. Topological Defect Dynamics in Operando Battery Nanoparticles. Science 2015, 348 (6241), 1344– 1347,  DOI: 10.1126/science.aaa1313
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    Topological defect dynamics in operando battery nanoparticles
    Ulvestad, A.; Singer, A.; Clark, J. N.; Cho, H. M.; Kim, J. W.; Harder, R.; Maser, J.; Meng, Y. S.; Shpyrko, O. G.
    Science (Washington, DC, United States) (2015), 348 (6241), 1344-1347CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Topol. defects can markedly alter nanomaterial properties. This presents opportunities for defect engineering, where desired functionalities are generated through defect manipulation. However, imaging defects in working devices with nanoscale resoln. remains elusive. The authors report 3-dimensional imaging of dislocation dynamics in individual battery cathode nanoparticles under operando conditions using Bragg coherent diffractive imaging. Dislocations are static at room temp. and mobile during charge transport. During the structural phase transformation, the Li-rich phase nucleates near the dislocation and spreads inhomogeneously. The dislocation field is a local probe of elastic properties, and a region of the material exhibits a neg. Poisson's ratio at high voltage. Operando dislocation imaging thus opens a powerful avenue for facilitating improvement and rational design of nanostructured materials.
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    Abuin, M.; Kim, Y. Y.; Runge, H.; Kulkarni, S.; Maier, S.; Dzhigaev, D.; Lazarev, S.; Gelisio, L.; Seitz, C.; Richard, M.-I.; Zhou, T.; Vonk, V.; Keller, T. F.; Vartanyants, I. A.; Stierle, A. Coherent X-Ray Imaging of CO-Adsorption-Induced Structural Changes in Pt Nanoparticles: Implications for Catalysis. ACS Appl. Nano Mater. 2019, 2 (8), 4818– 4824,  DOI: 10.1021/acsanm.9b00764
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    Coherent X-ray Imaging of CO-Adsorption-Induced Structural Changes in Pt Nanoparticles: Implications for Catalysis
    Abuin, Manuel; Kim, Young Yong; Runge, Henning; Kulkarni, Satishkumar; Maier, Simon; Dzhigaev, Dmitry; Lazarev, Sergey; Gelisio, Luca; Seitz, Christoph; Richard, Marie-Ingrid; Zhou, Tao; Vonk, Vedran; Keller, Thomas F.; Vartanyants, Ivan A.; Stierle, Andreas
    ACS Applied Nano Materials (2019), 2 (8), 4818-4824CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    Using coherent X-ray diffraction imaging (CXDI) as an in situ tool, we detd. the shape and strain state of a platinum nanoparticle with ≈160 nm diam. supported by a strontium titanate substrate. The expt. was performed at a temp. of 400 K under continuous gas flow conditions of pure Ar and Ar/CO mixts. The nanoparticle was preselected by SEM (SEM) and postanalyzed by at. force microscopy (AFM). We obtain a very good agreement between the overall nanoparticle size, shape, and defect structure as detd. by CXDI and AFM. In addn., we compare the strain state in the nanoparticle near surface region and its bulk: For pure Ar flow, we find a slight compressive strain in the nanoparticle bulk compared to an expansion in the near surface region. We ascribe the latter to the presence of high index vicinal surfaces. Our anal. suggests that under mixed Ar/CO flow at 400 K reshaping of the nanoparticle occurred. New high index facets developed, leading to a stronger lattice expansion, also propagating into the nanoparticle bulk. Our high-resoln. expts. pave the way for future CXDI expts. under operando catalytic reaction conditions.
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    Kim, D.; Chung, M.; Kim, S.; Yun, K.; Cha, W.; Harder, R.; Kim, H. Defect Dynamics at a Single Pt Nanoparticle during Catalytic Oxidation. Nano Lett. 2019, 19 (8), 5044– 5052,  DOI: 10.1021/acs.nanolett.9b01332
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    Defect Dynamics at a Single Pt Nanoparticle during Catalytic Oxidation
    Kim, Dongjin; Chung, Myungwoo; Kim, Sungwon; Yun, Kyuseok; Cha, Wonsuk; Harder, Ross; Kim, Hyunjung
    Nano Letters (2019), 19 (8), 5044-5052CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Defects can affect all aspects of a material by altering its electronic properties and controlling its chem. reactivity. At defect sites, preferential adsorption of reactants and/or formation of chem. species at active sites are obsd. in heterogeneous catalysis. Understanding the structural response at defect sites during catalytic reactions provides a unique opportunity to exploit defect control of nanoparticle-based catalysts. However, it remains difficult to characterize the strain and defect evolution for a single nanocrystal catalyst in situ. Here, we report Bragg coherent X-ray diffraction imaging of defect dynamics in an individual Pt nanoparticle during catalytic methane oxidn. We obsd. that the initially tensile strained regions of the crystal became seed points for the development of further strain and subsequent disappearance of diffraction d. during oxidn. reactions. Our detailed understanding of the catalytically induced deformation at the defect sites and obsd. reversibility during the relevant steps of the catalytic oxidn. process provide important insights of defect control and engineering of heterogeneous catalysts.
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    Sun, Y.; Liang, Y.; Luo, M.; Lv, F.; Qin, Y.; Wang, L.; Xu, C.; Fu, E.; Guo, S. Defects and Interfaces on PtPb Nanoplates Boost Fuel Cell Electrocatalysis. Small 2018, 14 (3), 1702259,  DOI: 10.1002/smll.201702259
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    Fichtner, J.; Watzele, S.; Garlyyev, B.; Kluge, R. M.; Haimerl, F.; El-Sayed, H. A.; Li, W. J.; Maillard, F. M.; Dubau, L.; Chattot, R.; Michalička, J.; MacAk, J. M.; Wang, W.; Wang, D.; Gigl, T.; Hugenschmidt, C.; Bandarenka, A. S. Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach. ACS Catal. 2020, 10 (5), 3131– 3142,  DOI: 10.1021/acscatal.9b04974
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    Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach
    Fichtner, Johannes; Watzele, Sebastian; Garlyyev, Batyr; Kluge, Regina M.; Haimerl, Felix; El-Sayed, Hany A.; Li, Wei-Jin; Maillard, Frederic M.; Dubau, Laetitia; Chattot, Raphael; Michalicka, Jan; Macak, Jan M.; Wang, Wu; Wang, Di; Gigl, Thomas; Hugenschmidt, Christoph; Bandarenka, Aliaksandr S.
    ACS Catalysis (2020), 10 (5), 3131-3142CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Results from Pt model catalyst surfaces have demonstrated that surface defects, in particular surface concavities, can improve the oxygen redn. reaction (ORR) kinetics. It is, however, a challenging task to synthesize nanostructured catalysts with such defective surfaces. Hence, we present a one-step and upscalable top-down approach to produce a Pt/C catalyst (with ∼3 nm Pt nanoparticle diam.). Using high-resoln. transmission electron microscopy and tomog., electrochem. techniques, high-energy X-ray measurements, and positron annihilation spectroscopy, we provide evidence of a high d. of surface defects (including surface concavities). The ORR activity of the developed catalyst exceeds that of a com. Pt/C catalyst, at least 2.7 times in terms of specific activity (∼1.62 mA/cm2Pt at 0.9 V vs the reversible hydrogen electrode) and at least 1.7 times in terms of mass activity (∼712 mA/mgPt), which can be correlated to the enhanced amt. of surface defects. In addn., the technique used here reduces the complexity of the synthesis (and therefore prodn. costs) in comparison to state of the art bottom-up techniques.
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    Dou, S.; Tao, L.; Wang, R.; El Hankari, S.; Chen, R.; Wang, S. Plasma-Assisted Synthesis and Surface Modification of Electrode Materials for Renewable Energy. Adv. Mater. 2018, 30 (21), 1705850,  DOI: 10.1002/adma.201705850
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    Ruge, M.; Drnec, J.; Rahn, B.; Reikowski, F.; Harrington, D. A.; Carlà, F.; Felici, R.; Stettner, J.; Magnussen, O. M. Structural Reorganization of Pt(111) Electrodes by Electrochemical Oxidation and Reduction. J. Am. Chem. Soc. 2017, 139 (12), 4532– 4539,  DOI: 10.1021/jacs.7b01039
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    Structural Reorganization of Pt(111) Electrodes by Electrochemical Oxidation and Reduction
    Ruge, Martin; Drnec, Jakub; Rahn, Bjoern; Reikowski, Finn; Harrington, David A.; Carla, Francesco; Felici, Roberto; Stettner, Jochim; Magnussen, Olaf M.
    Journal of the American Chemical Society (2017), 139 (12), 4532-4539CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The surface restructuring of Pt(111) electrodes upon electrochem. oxidn./redn. in 0.1M HClO4 was studied by in situ grazing-incidence small-angle x-ray scattering and complementary scanning tunneling microscopy measurements. These methods allow quant. detn. of the formation and structural evolution of nanoscale Pt islands during potential cycles into the oxidn. region. A characteristic ripening behavior is obsd., where these islands become more prominent and homogeneous in size with increasing no. of cycles. Their characteristic lateral dimensions primarily depend on the upper potential limit of the cycle and only slightly increase with cycle no. The structural evolution of the Pt surface morphol. strongly resembles that found in studies of Pt(111) homoepitaxial growth and ion erosion in ultrahigh vacuum. It can be fully explained by a microscopic model based on the known surface dynamic behavior under vacuum conditions, indicating that the same dynamics also describe the structural evolution of Pt in the electrochem. environment.
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    Jacobse, L.; Huang, Y. F.; Koper, M. T. M.; Rost, M. J. Correlation of Surface Site Formation to Nanoisland Growth in the Electrochemical Roughening of Pt(111). Nat. Mater. 2018, 17 (3), 277– 282,  DOI: 10.1038/s41563-017-0015-z
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    Correlation of surface site formation to nanoisland growth in the electrochemical roughening of Pt(111)
    Jacobse, Leon; Huang, Yi-Fan; Koper, Marc T. M.; Rost, Marcel J.
    Nature Materials (2018), 17 (3), 277-282CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    Pt plays a central role in a variety of electrochem. devices and its practical use depends on preventing electrode degrdn.; however, understanding the underlying at. processes under repeated oxidn. and redn. conditions which induce irreversible surface structure changes, is challenging. This work assessed the correlation between the evolution of the Pt(111) electrochem. signal and its surface roughening by simultaneously performing cyclic voltammetry and in-situ electrochem. scanning tunneling microscopy (EC-STM). A nucleation and early growth regime of nano-island formation, and a late growth regime following island coalescence which continues up to at least 170 cycles, were identified. A correlation anal. showed that each created step site in the late growth regime contributes equally, strongly to electrochem. and roughness evolutions. In the nucleation and early growth regime, created step sites contribute to roughness but not to the electrochem. signal.
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    Jacobse, L.; Rost, M. J.; Koper, M. T. M. Atomic-Scale Identification of the Electrochemical Roughening of Platinum. ACS Cent. Sci. 2019, 5 (12), 1920– 1928,  DOI: 10.1021/acscentsci.9b00782
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    Atomic-Scale Identification of the Electrochemical Roughening of Platinum
    Jacobse, Leon; Rost, Marcel J.; Koper, Marc T. M.
    ACS Central Science (2019), 5 (12), 1920-1928CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Electrode degrdn. under oxidizing conditions is a major drawback for large scale applications of Pt electrocatalysts. Subjecting Pt(111) to oxidn.-redn. cycles is known to lead to the growth of nanoislands. The authors study this phenomenon using a combination of simultaneous in situ electrochem. scanning tunneling microscopy and cyclic voltammetry. Here, the authors present a detailed anal. of the formed islands, deriving the (evolution of the) av. island growth shape. From the island shapes, the authors det. the densities of at.-scale defect sites, e.g. steps and facets, which show an excellent correlation with the different voltammetric H adsorption peaks. Based on this combination of EC-STM and CV data, the authors derive a detailed atomistic picture of nanoisland evolution during potential cycling, delivering new insights into the initial stages of Pt electrode degrdn. EC-STM images and av. nanoisland structures: (A) parts of EC-STM images after 0, 4, 8, 15, 28, 50, and 170 ORCs (from left to right) illustrating the roughening of a single surface area. The original, individual images are 230 × 230 nm2, recorded with Utip = 0.45 V, Usample = 0.4 V, and Itunneling < 300 pA. Note that the total contrast is the same for the different images. The full set of EC-STM images is available in ref. (B); the av. at.-scale growth structure of the islands as detd. from the EC-STM images for the same situations as shown in (A). The different colors indicate the different at. layers in the island. The full image sequence is provided in the Supporting Information. Unit cells and time evolution of defect site densities derived from av. nanoisland structures: (A-C) Representative unit cells for the {111}-, {112}-, and {100}-side steps/facets, resp. The structures range from sepd. steps (top) to low index facets (bottom). All counting rules and unit cells for other possible structures, as well as all individual site densities are provided in the Supporting Information. (D) Site densities for the sepd. steps & wide facets (top), and narrow facets & low index facets (bottom) for both the {111}- and {100}-side (solid and dashed lines, resp.). Detailed anal. of in situ EC-STM images and cyclic voltammetry data resolves the evolution of surface site densities during the roughening of Pt(111) by potential cycling.
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    Livet, F.; Beutier, G.; De Boissieu, M.; Ravy, S.; Picca, F.; Le Bolloc’h, D.; Jacques, V. Coherent Scattering from Silicon Monocrystal Surface. Surf. Sci. 2011, 605, 390– 395,  DOI: 10.1016/j.susc.2010.11.006
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    Coherent scattering from silicon monocrystal surface
    Livet, F.; Beutier, G.; de Boissieu, M.; Ravy, S.; Picca, F.; Le Bolloc'h, D.; Jacques, V.
    Surface Science (2011), 605 (3-4), 390-395CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    Using coherent x-ray scattering, at. step roughness was evidenced at the (111) vicinal surface of a Si single crystal of 0.05° miscut. Close to the 1/2 1/2 1/2 anti-Bragg position of the reciprocal space which is particularly sensitive to the (111) surface, the truncation rod exhibits a contrasted speckle pattern that merges into a single peak closer to the 111 Bragg peak of the bulk. The elongated shape of the speckles along the (111) direction confirms the monoat. step sensitivity of the technique. This expt. opens the way towards studies of step dynamics on cryst. surfaces.
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    Nicolas, J. D.; Reusch, T.; Osterhoff, M.; Sprung, M.; Schülein, F. J. R.; Krenner, H. J.; Wixforth, A.; Salditt, T. Time-Resolved Coherent X-Ray Diffraction Imaging of Surface Acoustic Waves. J. Appl. Crystallogr. 2014, 47 (5), 1596– 1605,  DOI: 10.1107/S1600576714016896
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    Time-resolved coherent X-ray diffraction imaging of surface acoustic waves
    Nicolas, Jan-David; Reusch, Tobias; Osterhoff, Markus; Sprung, Michael; Schuelein, Florian J. R.; Krenner, Hubert J.; Wixforth, Achim; Salditt, Tim
    Journal of Applied Crystallography (2014), 47 (5), 1596-1605CODEN: JACGAR; ISSN:1600-5767. (International Union of Crystallography)
    Time-resolved coherent X-ray diffraction expts. of standing surface acoustic waves, illuminated under grazing incidence by a nanofocused synchrotron beam, are reported. The data have been recorded in stroboscopic mode at controlled and varied phase between the acoustic frequency generator and the synchrotron bunch train. At each time delay (phase angle), the coherent far-field diffraction pattern in the small-angle regime is inverted by an iterative algorithm to yield the local instantaneous surface height profile along the optical axis. The results show that periodic nanoscale dynamics can be imaged at high temporal resoln. in the range of 50 ps (pulse length).
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    Kutsal, M.; Bernard, P.; Berruyer, G.; Cook, P. K.; Hino, R. The ESRF Dark-Field x-Ray Microscope at ID06. IOP Conf. Ser.: Mater. Sci. Eng. 2019, 580, 012007,  DOI: 10.1088/1757-899X/580/1/012007
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    The ESRF dark-field x-ray microscope at ID06
    Kutsal, M.; Bernard, P.; Berruyer, G.; Cook, P. K.; Hino, R.; Jakobsen, A. C.; Ludwig, W.; Ormstrup, J.; Roth, T.; Simons, H.; Smets, K.; Sierra, J. X.; Wade, J.; Wattecamps, P.; Yildirim, C.; Poulsen, H. F.; Detlefs, C.
    IOP Conference Series: Materials Science and Engineering (2019), 580 (40th Riso International Symposium on Materials Science: Metal Microstructures in 2D, 3D and 4D, 2019), 12007CODEN: ICSMGW; ISSN:1757-899X. (IOP Publishing Ltd.)
    We present an instrument for dark-field x-ray microscopy installed on beamline ID06 of the ESRF - the first of its kind. Dark-field x-ray microscopy uses full field illumination of the sample and provides three-dimensional (3D) mapping of micro-structure and lattice strain in cryst. matter. It is analogous to dark-field electron microscopy in that an objective lens magnifies diffracting features of the sample. The use of high-energy synchrotron x-rays, however, means that these features can be large and deeply embedded. 3D movies can be acquired with a time resoln. of seconds to minutes. The field of view and spatial resoln. can be adapted by simple reconfiguration of the x-ray objective lens, reaching spatial and angular resoln. of 30-100 nm and 0.001°, resp. The instrument furthermore allows pre-characterization of samples at larger length scales using 3DXRD or DCT, such that a region of interest (e.g. a single grain) can be selected for high-resoln. studies without the need to dismount the sample. As examples of applications we show work on mapping the subgrains in plastically deformed iron and aluminum alloys, mapping domains and strain fields in ferroelec. crystals, and studies of biominerals. This ability to directly characterize complex, multi-scale phenomena in-situ is a key step towards formulating and validating multi-scale models that account for the entire heterogeneity of materials. As an outlook, we discuss future prospects for such multi-scale characterization by combining DFXM with 3DXRD/DCT, and coherent x-ray methods for coarser and finer length-scales, resp.
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    Morgan, A. J.; Prasciolu, M.; Andrejczuk, A.; Krzywinski, J.; Meents, A.; Pennicard, D.; Graafsma, H.; Barty, A.; Bean, R. J.; Barthelmess, M.; Oberthuer, D.; Yefanov, O.; Aquila, A.; Chapman, H. N.; Bajt, S. High Numerical Aperture Multilayer Laue Lenses. Sci. Rep. 2015, 5, 9892,  DOI: 10.1038/srep09892
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    High numerical aperture multilayer Laue lenses
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  • Abstract

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    Figure 1

    Figure 1. Defect engineering approach. Schematic representation principle of the narrow vs near-continuous broad distributions of catalytic sites configurations and electrocatalytic performance for (a) ordered, model vs (b) defective Pt(111) surfaces.

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    Figure 2

    Figure 2. Powder X-ray diffraction (PXRD) patterns’ broadening and sources of microstrain in PtM nanoparticles (M being an early or late transition metal element). (a) Typical experimental PXRD patterns measured at the high-energy ID31 beamline of the European Synchrotron Radiation facility on structurally disordered and structurally ordered catalysts (here hollow PtNi/C and cube Pt/C nanoparticles, respectively) plotted as a function of the momentum transfer Q. (b) Possible sources of microstrain in PtM nanocatalysts (grain boundaries, inhomogeneous alloying, or (electro)chemical surface dealloying). The inset in (a) shows the influence of macrostrain, instrumental contribution, and microstrain on the position and the broadening of the PXRD reflections. Adapted from ref (13). Copyright 2018 Springer Nature.

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    Figure 3

    Figure 3. Using the SD descriptor to unveil the “operating mode” of a variety of ORR nanocatalysts. (a–j) TEM images of the various nanostructures investigated. The insets show higher magnification TEM or STEM/X-EDS elemental maps. (k) ORR activity measured at 0.95 V vs. RHE plotted as a function of the SD descriptor. The activity–SD plot quantitatively confirms that high ORR activity can be reached through the two approaches presented in Figure 1. Panels (a–j) are reprinted with permission from ref (26). Copyright 2020 American Chemical Society. Panel (k) is adapted with permission from ref (13). Copyright 2018 Springer Nature.

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    Figure 4

    Figure 4. Probing catalytic sites diversity and oxophilicity. (a) Background-subtracted COads stripping voltammograms recorded on Pt and PtNi nanomaterials featuring increasing SD values and their associated average potential μ1CO (dashed lines), (b) SD plotted as a function of the COOR activity (μ1CO) for a wide range of PtNi materials with different shape, size, or Ni content. (c) Background-subtracted Hads desorption curves (so-called-HUPD region) of materials featuring increasing SD values pointing an extra electrochemical process at high potential ascribed to *H displacement by *OH on extremely oxophilic sites and (d) SD plotted as a function of the ratio QCO/2QH for a wide range of PtNi materials with different shape, size, or Ni content. In (b), the numbers displayed close to the experimental points are the SD values associated to these catalysts. In both (a) and (c), the curves are normalized by the Pt surface area measured by COads stripping. Panel (d) is adapted with permission from ref (26). Copyright 2020 American Chemical Society.

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    Figure 5

    Figure 5. Determining the nature and structure of the catalytically active sites. (a) 3D atomic displacements measured in a FePt nanoparticle using GENFIRE algorithm from AC-STEM images; (b) Atomic resolution STEM images, with atomic displacement and strain maps within two crystallographic planes, as indicated in the Pt nanoparticle; (c) APT data showing the atom map from a Ag@Pd nanoparticle; (d) Sketch of the BCDI setup, 2D slice of the strain field through the center of a reconstructed Pt nanoparticle under Ar/CO flow and associated probability density of the strain field; (e) Scheme explaining the concept of using EC-STM noise analysis to reveal the catalytic sites under reaction conditions. Panel (a) is adapted with permission from ref (22). Copyright 2017 Springer Nature. Panel (b) is adapted with permission from ref (21). Copyright 2018 Springer Nature. Panel (c) is adapted with permission from ref (63). Copyright 2011 Springer Nature. Panel (d) is adapted with permission from ref (68). Copyright 2019 American Chemical Society. Panel (e) is adapted with permission from ref (80). Copyright 2017 Springer Nature.

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    This article references 85 other publications.

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      Hammer, B.; Norskov, J. K. Why Gold Is the Noblest of All the Metals. Nature 1995, 376, 238– 240,  DOI: 10.1038/376238a0
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      Why gold is the noblest of all the metals
      Hammer, B.; Norskov, J. K.
      Nature (London) (1995), 376 (6537), 238-40CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
      The unique role that gold plays in society is to a large extent related to the fact that it is the most noble of all metals: it is the least reactive metal towards atoms or mols. at the interface with a gas or a liq. The inertness of gold does not reflect a general inability to form chem. bonds, however-gold forms stable alloys with many other metals. To understand the nobleness of gold, we have studied a simple surface reaction, the dissocn. of H2 on the surface of gold and of three other metals (copper, nickel and platinum) that lie close to it in the periodic table. We present self-consistent d.-functional calcns. of the activation barriers and chemisorption energies which clearly illustrate that nobleness is related to two factors: the degree of filling of the anti-bonding states on adsorption, and the degree of orbital overlap with the adsorbate. These two factors, which det. both the strength of the adsorbate-metal interaction and the energy barrier for dissocn., operate together to the maximal detriment of adsorbate binding and subsequent reactivity on gold.
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    6. 6
      Hammer, B.; Nørskov, J. K. Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343 (3), 211– 220,  DOI: 10.1016/0039-6028(96)80007-0
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      Electronic factors determining the reactivity of metal surfaces
      Hammer, B.; Noerskov, J. K.
      Surface Science (1995), 343 (3), 211-20CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
      Based on d. functional theory calcns. of H2 dissocn. on Al(111), Cu(111), Pt(111) and Cu3Pt(111) we present a consistent picture of some key phys. properties detg. the reactivity of metal and alloy surfaces. The four metal surfaces are chosen to represent metals with no d-bands, with filled d-bands and with d-states at the Fermi level. We show that electronic states in the entire valence band of the metal surface are responsible for the reactivity, which consequently cannot be understood solely in terms of the d. of states at the Fermi level nor in terms of the empty d-states above it. Rather we suggest that trends in reactivities can be understood in terms of the hybridization energy between the bonding and anti-bonding adsorbate states and the metal d-bands (when present), and we demonstrate that a simple frozen potential based est. of the hybridization energy correlates well with the calcd. variation of the barrier height for the different metal surfaces.
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    7. 7
      Batchelor, T. A. A.; Pedersen, J. K.; Winther, S. H.; Castelli, I. E.; Jacobsen, K. W.; Rossmeisl, J. High-Entropy Alloys as a Discovery Platform for Electrocatalysis. Joule 2019, 3 (3), 834– 845,  DOI: 10.1016/j.joule.2018.12.015
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      7
      High-Entropy Alloys as a Discovery Platform for Electrocatalysis
      Batchelor, Thomas A. A.; Pedersen, Jack K.; Winther, Simon H.; Castelli, Ivano E.; Jacobsen, Karsten W.; Rossmeisl, Jan
      Joule (2019), 3 (3), 834-845CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      High-entropy alloys (HEAs) provide a near-continuous distribution of adsorption energies. With a minority of sites having optimal properties that dominate the catalysis, the overall catalytic activity can increase. In this article, we focus on the oxygen redn. reaction (ORR). We present d. functional theory (DFT) calcd. *OH and *O adsorption energies on a random subset of available binding sites on the surface of the HEA IrPdPtRhRu. Employing a simple machine learning algorithm, we predict remaining adsorption energies, finding good agreement between calcd. and predicted values. With a full catalog of available adsorption energies, an appropriate expression for predicting catalytic activity was used to optimize the HEA compn. The HEA then becomes a design platform for the unbiased discovery of new alloys by promoting sites with exceptional catalytic activity. Setting different optimization constraints led to a new HEA compn. and binary alloy IrPt, showing significant enhancements over pure Pt(111).
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    8. 8
      Le Bacq, O.; Pasturel, A.; Chattot, R.; Previdello, B.; Nelayah, J.; Asset, T.; Dubau, L.; Maillard, F. Effect of Atomic Vacancies on the Structure and the Electrocatalytic Activity of Pt-Rich/C Nanoparticles: A Combined Experimental and Density Functional Theory Study. ChemCatChem 2017, 9 (12), 2324– 2338,  DOI: 10.1002/cctc.201601672
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      8
      Effect of Atomic Vacancies on the Structure and the Electrocatalytic Activity of Pt-rich/C Nanoparticles: A Combined Experimental and Density Functional Theory Study
      Le Bacq, Olivier; Pasturel, Alain; Chattot, Raphael; Previdello, Bruno; Nelayah, Jaysen; Asset, Tristan; Dubau, Laetitia; Maillard, Frederic
      ChemCatChem (2017), 9 (12), 2324-2338CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors present a joint exptl. and d. functional theory (DFT) study on the effect of at. vacancies on the restructuring of Pt-transition metal alloy nanocatalysts and the assocd. changes in electrocatalytic activity. Atomic vacancies were introduced into slabs composed of pure Pt monolayers, and the structures were relaxed using the Vienna ab initio simulation package code. Effects of (i) the concn. and (ii) the spatial distribution of at. vacancies in the slabs on surface and bulk restructuring were studied. Highly disordered nanostructures featuring large variations of the in-plane and out-of-plane nearest-neighbor distances around the mean were obsd. upon relaxation. These findings were confirmed exptl. by using hollow PtNi/C nanoparticles synthesized by a combination of galvanic replacement and the nanoscale Kirkendall effect (a vacancy-mediated interdiffusion mechanism). Also hollow PtNi/C nanoparticles feature a combination of oxophilic and oxophobic catalytic sites on their surface and are thus highly active both for electrochem. oxidn. and redn. reactions.
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    9. 9
      Calle-Vallejo, F.; Tymoczko, J.; Colic, V.; Vu, Q. H.; Pohl, M. D.; Morgenstern, K.; Loffreda, D.; Sautet, P.; Schuhmann, W.; Bandarenka, A. S. Finding Optimal Surface Sites on Heterogeneous Catalysts by Counting Nearest Neighbors. Science 2015, 350 (6527), 185– 189,  DOI: 10.1126/science.aab3501
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      9
      Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors
      Calle-Vallejo, Federico; Tymoczko, Jakub; Colic, Viktor; Vu, Quang Huy; Pohl, Marcus D.; Morgenstern, Karina; Loffreda, David; Sautet, Philippe; Schuhmann, Wolfgang; Bandarenka, Aliaksandr S.
      Science (Washington, DC, United States) (2015), 350 (6257), 185-189CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A good heterogeneous catalyst for a given chem. reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots det. optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen redn. reaction catalyzed by platinum. Sites with the same no. of first-nearest neighbors as (111) terraces but with an increased no. of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable exptl. methods.
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    10. 10
      Ma, X.; Xin, H. Orbitalwise Coordination Number for Predicting Adsorption Properties of Metal Nanocatalysts. Phys. Rev. Lett. 2017, 118 (3), 036101,  DOI: 10.1103/PhysRevLett.118.036101
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      Orbitalwise coordination number for predicting adsorption properties of metal nanocatalysts
      Ma, Xianfeng; Xin, Hongliang
      Physical Review Letters (2017), 118 (3), 036101/1-036101/5CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      We present the orbital wise coordination no. CNα (α = s or d) as a reactivity descriptor for metal nanocatalysts. With the noble metal Au (5d106s1) as a specific case, the CNs computed using the two-center s-electron hopping integrals to neighboring atoms provides an accurate and robust description of the trends in CO and O adsorption energies on extended surfaces terminated with different facets and nanoparticles of varying size and shape, outperforming existing bond-counting methods. Importantly, the CNs has a solid physiochem. basis via a direct connection to the moment characteristics of the projected d. of states onto the s orbital of a Au adsorption site. Furthermore, the CNs shows promise as a viable descriptor for predicting adsorption properties of Au alloy nanoparticles with size-dependent lattice strains and coinage metal ligands.
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    11. 11
      Wu, D.; Dong, C.; Zhan, H.; Du, X. Bond-Energy-Integrated Descriptor for Oxygen Electrocatalysis of Transition Metal Oxides. J. Phys. Chem. Lett. 2018, 9, 3387– 3391,  DOI: 10.1021/acs.jpclett.8b01493
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      11
      Bond-Energy-Integrated Descriptor for Oxygen Electrocatalysis of Transition Metal Oxides
      Wu, Deyao; Dong, Cunku; Zhan, Hongbing; Du, Xi-Wen
      Journal of Physical Chemistry Letters (2018), 9 (12), 3387-3391CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Unraveling a descriptor of catalytic reactivity is essential for fast screening catalysts for a given reaction. Transition metal (TM) compds. have been widely used for oxygen electrocatalysis. Nevertheless, there is a lack of an exact descriptor to predict their catalytic behavior so far. Herein, we propose that the bond-energy-integrated orbitalwise coordination no. (‾C‾Nsd), which takes into account both geometrical and electronic structures around the active site, can serve as a simple and accurate descriptor for catalysts consisting of TM oxides (TMOs) as well as avoid excessive computation burden. This descriptor exhibits a strong scaling relation with the activity in oxygen electrocatalysis, with a goodness of fit higher than those of the usual coordination no. (cn), the generalized coordination no. (‾C‾N), and the orbitalwise coordination no. (CNα). Esp., the theor. prediction made by the ‾C‾Nsd descriptor is very consistent with exptl. results and universal for various TMOs (e.g., MnOx and RuO2), enabling the rational design of novel catalysts.
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    12. 12
      Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2 (2), 65– 71,  DOI: 10.1107/S0021889869006558
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      12
      Profile refinement method for nuclear and magnetic structures
      Rietveld, H. M.
      Journal of Applied Crystallography (1969), 2 (Pt. 2), 65-71CODEN: JACGAR; ISSN:0021-8898.
      A structural refinement method for neutron diffraction is presented which makes direct use of the profile intensities obtained from the powder diagram. It is applicable to nuclear structures and to magnetic structures which can be described on the nuclear unit cell or a multiple thereof. Equations for the measured profiles to be used in the least sqs. treatment are cor. for asymmetry and preferred orientation; the angular dependence of the half widths of the peaks is given by the formula of Caglioti, et al. (1958). The magnetic contribution to the profile equation is expressed by calcg. only one av. cross section for each set of equiv. reflections. It is possible to introduce constraint functions, linear or quadratic, between parameters used in the least sqs. treatment. Results of the use of this method are given for a series of compds. In all instances it has proved superior to any other method involving integrated neutron powder intensities, single or overlapping.
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    13. 13
      Chattot, R.; Le Bacq, O.; Beermann, V.; Kühl, S.; Herranz, J.; Henning, S.; Kühn, L.; Asset, T.; Guétaz, L.; Renou, G.; Drnec, J.; Bordet, P.; Pasturel, A.; Eychmüller, A.; Schmidt, T. J.; Strasser, P.; Dubau, L.; Maillard, F. Surface Distortion as a Unifying Concept and Descriptor in Oxygen Reduction Reaction Electrocatalysis. Nat. Mater. 2018, 17 (9), 827– 833,  DOI: 10.1038/s41563-018-0133-2
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      13
      Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis
      Chattot, Raphael; Le Bacq, Olivier; Beermann, Vera; Kuhl, Stefanie; Herranz, Juan; Henning, Sebastian; Kuhn, Laura; Asset, Tristan; Guetaz, Laure; Renou, Gilles; Drnec, Jakub; Bordet, Pierre; Pasturel, Alain; Eychmuller, Alexander; Schmidt, Thomas J.; Strasser, Peter; Dubau, Laetitia; Maillard, Frederic
      Nature Materials (2018), 17 (9), 827-833CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Tuning the surface structure at the at. level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temp. proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory exptl. observations in regards to the electrocatalytic oxygen redn. reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct at. compn., size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.
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    14. 14
      Billinge, S. J. L. The Rise of the X-Ray Atomic Pair Distribution Function Method: A Series of Fortunate Events. Philos. Trans. R. Soc., A 2019, 377 (2147), 20180413,  DOI: 10.1098/rsta.2018.0413
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      The rise of the X-ray atomic pair distribution function method: a series of fortunate events
      Billinge, Simon J. L.
      Philosophical Transactions of the Royal Society, A: Mathematical, Physical & Engineering Sciences (2019), 377 (2147), 20180413CODEN: PTRMAD; ISSN:1364-503X. (Royal Society)
      A review. The at. pair distribution function (PDF) technique is a powerful approach to gain quant. insight into the structure of materials where the structural coherence extends only over a few nanometers. In this paper, I focus on PDF from synchrotron X-rays and describe what is the PDF and where it came from, as well as key moments on the journey that have contributed to its enormous recent growth and expanding impact in materials science today. Synchrotron X-ray sources played a starring role in this story. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.
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    15. 15
      Thompson, P.; Cox, D. E.; Hastings, J. B. Rietveld Refinement of Debye-Scherrer Synchrotron X-Ray Data from Al2O3. J. Appl. Crystallogr. 1987, 20 (2), 79– 83,  DOI: 10.1107/S0021889887087090
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      15
      Rietveld refinement of Debye-Scherrer synchrotron x-ray data from alumina
      Thompson, P.; Cox, D. E.; Hastings, J. B.
      Journal of Applied Crystallography (1987), 20 (2), 79-83CODEN: JACGAR; ISSN:0021-8898.
      The application of the Rietveld refinement technique to synchrotron x-ray data collected from a capillary sample of Al2O3 in Debye-Scherrer geometry is described. The data were obtained at a high-energy synchrotron source with a Si(111) double-crystal monochromator and a Ge(111) crystal analyzer. Fits to a no. of well-resolved individual peaks demonstrate that the peak shapes are very well described by the pseudo-Voigt function, which is a simple approxn. to the convolution of Gaussian and Lorentzian functions. The variation of the Gaussian and Lorentzian half widths, ΓG and ΓL, with Bragg angle can be approximated quite closely by the functions V tanθ and X/cosθ which represent the contributions from instrumental resoln. and particle-size broadening resp. Rietveld refinement based on this model yields generally satisfactory results. The refined values of V and X are consistent with the expected vertical divergence (∼0.1 mrad) and and nominal particle size (∼0.3 μm). In particular, the use of a capillary specimen virtually eliminates preferred orientation effects, which are highly significant in flat-plate samples of this material.
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    16. 16
      Harada, M.; Ikegami, R.; Kumara, L. S. R.; Kohara, S.; Sakata, O. Reverse Monte Carlo Modeling for Local Structures of Noble Metal Nanoparticles Using High-Energy XRD and EXAFS. RSC Adv. 2019, 9 (51), 29511– 29521,  DOI: 10.1039/C9RA06519A
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      16
      Reverse Monte Carlo modeling for local structures of noble metal nanoparticles using high-energy XRD and EXAFS
      Harada, Masafumi; Ikegami, Risa; Kumara, Loku Singgappulige Rosantha; Kohara, Shinji; Sakata, Osami
      RSC Advances (2019), 9 (51), 29511-29521CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Reverse Monte Carlo (RMC) modeling based on the total structure factor S(Q) obtained from high-energy X-ray diffraction (HEXRD) and the k3χ(k) obtained from extended X-ray absorption fine structure (EXAFS) measurements was employed to det. the 3-dimensional (3D) at.-scale structure of Pt, Pd, and Rh nanoparticles, with sizes less than 5 nm, synthesized by photoredn. The total structure factor and Fourier-transformed PDF showed that the first nearest neighbor peak is in accordance with that obtained from conventional EXAFS anal. RMC constructed 3D models were analyzed in terms of prime structural characteristics such as metal-to-metal bond lengths, first-shell coordination nos. and bond angle distributions. The first-shell coordination nos. and bond angle distributions for the RMC-simulated metal nanoparticles indicated a face-centered cubic (fcc) structure with appropriate no. d. Modeling disorder effects in these RMC-simulated metal nanoparticles also revealed substantial differences in bond-length distributions for resp. nanoparticles.
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    17. 17
      Chattot, R.; Asset, T.; Bordet, P.; Drnec, J.; Dubau, L.; Maillard, F. Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures. ACS Catal. 2017, 7, 398– 408,  DOI: 10.1021/acscatal.6b02356
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      17
      Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures
      Chattot, Raphael; Asset, Tristan; Bordet, Pierre; Drnec, Jakub; Dubau, Laetitia; Maillard, Frederic
      ACS Catalysis (2017), 7 (1), 398-408CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The elec. performance of a proton exchange membrane fuel cell (PEMFC) is limited by the slow oxygen redn. reaction (ORR) kinetics. Catalytic improvements for the ORR were obtained on alloyed PtM/C or M-rich core@Pt-rich shell catalysts (where M is an early or late transition metal) compared to pure Pt/C, due to a combination of strain and ligand effects. However, the effect of the fine nanostructure on the ORR kinetics remains under investigated. In this study, nanometer-sized PtNi/C electrocatalysts with low Ni content (∼15 at. %) but different nanostructures and different densities of grain boundary were synthesized: solid, hollow or sea sponge PtNi/C nanoalloys, and solid Ni-core@Pt-shell/C nanoparticles. These nanostructures were characterized by transmission and scanning-transmission electron microscopy (TEM and STEM, resp.), X-ray energy dispersive spectroscopy (X-EDS) Synchrotron wide-angle X-ray scattering (WAXS), at. absorption spectroscopy (AAS) and electrochem. techniques. Their electrocatalytic activities for the ORR were detd., and structure-activity relationships established. The results showed that (i) the compression of the Pt lattice by ca. 15 at. % Ni provides mild ORR activity enhancement compared to pure Pt/C, (ii) highly defective PtNi/C nanostructures feature up to 9.3-fold enhancement of the ORR specific activity over a com. Pt/C material with similar crystallite size, (iii) the enhancement in ORR catalytic activity can be ascribed to the presence of structural defects as shown by two independent parameters: the microstrain detd. from WAXS, and the av. COads electrooxidn. potential detd. from COads stripping measurements. This work indicates that, at fixed Ni at. %, ORR activity can be tuned by nanostructuring and suggests that targeting structural disorder is a promising approach to improve the electrocatalytic properties of mono or bimetallic nanocatalysts.
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      Sneed, B. T.; Young, A. P.; Tsung, C. K. Building up Strain in Colloidal Metal Nanoparticle Catalysts. Nanoscale 2015, 7 (29), 12248– 12265,  DOI: 10.1039/C5NR02529J
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      Building up strain in colloidal metal nanoparticle catalysts
      Sneed, Brian T.; Young, Allison P.; Tsung, Chia-Kuang
      Nanoscale (2015), 7 (29), 12248-12265CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The focus on surface lattice strain in nanostructures as a fundamental research topic has gained momentum in recent years as scientists investigated its significant impact on the surface electronic structure and catalytic properties of nanomaterials. Researchers have begun to tell a more complete story of catalysis from a perspective which brings this concept to the forefront of the discussion. The nano-'realm' makes the effects of surface lattice strain, which acts on the same spatial scales, more pronounced due to a higher ratio of surface to bulk atoms. This is esp. evident in the field of metal nanoparticle catalysis, where displacement of atoms on surfaces can significantly alter the sorption properties of mols. In part, the concept of strain-engineering for catalysis opened up due to the achievements that were made in the synthesis of a more sophisticated nanoparticle library from an ever-expanding set of methodologies. Developing synthesis methods for metal nanoparticles with well-defined and strained architectures is a worthy goal that, if reached, will have considerable impact in the search for catalysts. In this review, we summarize the recent accomplishments in the area of surface lattice-strained metal nanoparticle synthesis, framing the discussion from the important perspective of surface lattice strain effects in catalysis.
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    19. 19
      Gan, L.; Yu, R.; Luo, J.; Cheng, Z.; Zhu, J. Lattice Strain Distributions in Individual Dealloyed Pt-Fe Catalyst Nanoparticles. J. Phys. Chem. Lett. 2012, 3 (7), 934– 938,  DOI: 10.1021/jz300192b
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      19
      Lattice Strain Distributions in Individual Dealloyed Pt-Fe Catalyst Nanoparticles
      Gan, Lin; Yu, Rong; Luo, Jun; Cheng, Zhiying; Zhu, Jing
      Journal of Physical Chemistry Letters (2012), 3 (7), 934-938CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Lattice strain is considered to play an important role in the O redn. catalysis on Pt-based catalysts. However, so far, direct evidence of the lattice strain in the catalyst nanoparticles was not achieved. By using aberration-cor. high-resoln. TEM combined with image simulations, a unique core-shell structure, i.e., a percolated lattice-contracted Pt-Fe alloy core and a Pt-rich surface with a gradient compressive strain, was directly demonstrated within individual dealloyed Pt-Fe nanoparticles and thus provides direct evidence for the strain effect on their enhanced O redn. activity.
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    20. 20
      Suzana, A. F.; Rochet, A.; Passos, A. R.; Castro Zerba, J. P.; Polo, C. C.; Santilli, C. V.; Pulcinelli, S. H.; Berenguer, F.; Harder, R.; Maxey, E.; Meneau, F. In Situ Three-Dimensional Imaging of Strain in Gold Nanocrystals during Catalytic Oxidation. Nanoscale Adv. 2019, 1 (8), 3009– 3014,  DOI: 10.1039/C9NA00231F
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      20
      In situ three-dimensional imaging of strain in gold nanocrystals during catalytic oxidation
      Suzana, Ana Flavia; Rochet, Amelie; Passos, Aline Ribeiro; Castro Zerba, Joao Paulo; Polo, Carla Cristina; Santilli, Celso Valentim; Pulcinelli, Sandra Helena; Berenguer, Felisa; Harder, Ross; Maxey, Evan; Meneau, Florian
      Nanoscale Advances (2019), 1 (8), 3009-3014CODEN: NAADAI; ISSN:2516-0230. (Royal Society of Chemistry)
      The chem. properties of materials are dependent on dynamic changes in their three-dimensional (3D) structure as well as on the reactive environment. We report an in situ 3D imaging study of defect dynamics of a single gold nanocrystal. Our findings offer an insight into its dynamic nanostructure and unravel the formation of a nanotwin network under CO oxidn. conditions. In situ/operando defect dynamics imaging paves the way to elucidate chem. processes at the single nano-object level towards defect-engineered nanomaterials.
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    21. 21
      Nilsson Pingel, T.; Jørgensen, M.; Yankovich, A. B.; Grönbeck, H.; Olsson, E. Influence of Atomic Site-Specific Strain on Catalytic Activity of Supported Nanoparticles. Nat. Commun. 2018, 9, 2722,  DOI: 10.1038/s41467-018-05055-1
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      Influence of atomic site-specific strain on catalytic activity of supported nanoparticles
      Nilsson Pingel Torben; Jorgensen Mikkel; Yankovich Andrew B; Gronbeck Henrik; Olsson Eva; Nilsson Pingel Torben; Jorgensen Mikkel; Gronbeck Henrik; Olsson Eva
      Nature communications (2018), 9 (1), 2722 ISSN:.
      Heterogeneous catalysis is an enabling technology that utilises transition metal nanoparticles (NPs) supported on oxides to promote chemical reactions. Structural mismatch at the NP-support interface generates lattice strain that could affect catalytic properties. However, detailed knowledge about strain in supported NPs remains elusive. We experimentally measure the strain at interfaces, surfaces and defects in Pt NPs supported on alumina and ceria with atomic resolution using high-precision scanning transmission electron microscopy. The largest strains are observed at the interfaces and are predominantly compressive. Atomic models of Pt NPs with experimentally measured strain distributions are used for first-principles kinetic Monte Carlo simulations of the CO oxidation reaction. The presence of only a fraction of strained surface atoms is found to affect the turnover frequency. These results provide a quantitative understanding of the relationship between strain and catalytic function and demonstrate that strain engineering can potentially be used for catalyst design.
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    22. 22
      Yang, Y.; Chen, C. C.; Scott, M. C.; Ophus, C.; Xu, R.; Pryor, A.; Wu, L.; Sun, F.; Theis, W.; Zhou, J.; Eisenbach, M.; Kent, P. R. C.; Sabirianov, R. F.; Zeng, H.; Ercius, P.; Miao, J. Deciphering Chemical Order/Disorder and Material Properties at the Single-Atom Level. Nature 2017, 542 (7639), 75– 79,  DOI: 10.1038/nature21042
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      22
      Deciphering chemical order/disorder and material properties at the single-atom level
      Yang, Yongsoo; Chen, Chien-Chun; Scott, M. C.; Ophus, Colin; Xu, Rui; Pryor, Alan; Wu, Li; Sun, Fan; Theis, Wolfgang; Zhou, Jihan; Eisenbach, Markus; Kent, Paul R. C.; Sabirianov, Renat F.; Zeng, Hao; Ercius, Peter; Miao, Jianwei
      Nature (London, United Kingdom) (2017), 542 (7639), 75-79CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Perfect crystals are rare in nature. Real materials often contain crystal defects and chem. order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quant. material characterization methods, correlating three-dimensional (3D) at. arrangements of chem. order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calcns. such as d. functional theory (DFT) have progressed from the modeling of ideal bulk systems to modeling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calcns. rely heavily on av. at. models extd. from crystallog. To improve the predictive power of first-principles calcns., there is a pressing need to use at. coordinates of real systems beyond av. crystallog. measurements. Here we det. the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chem. order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including at. compn., grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the exptl. measured coordinates and chem. species with 22 pm precision can be used as direct input for DFT calcns. of material properties such as at. spin and orbital magnetic moments and local magnetocryst. anisotropy. This work combines 3D at. structure detn. of crystal defects with DFT calcns., which is expected to advance our understanding of structure-property relationships at the fundamental level.
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    23. 23
      Ruiz-Zepeda, F.; Gatalo, M.; Pavlišič, A.; Dražić, G.; Jovanovič, P.; Bele, M.; Gaberšček, M.; Hodnik, N. Atomically Resolved Anisotropic Electrochemical Shaping of Nano-Electrocatalyst. Nano Lett. 2019, 19 (8), 4919– 4927,  DOI: 10.1021/acs.nanolett.9b00918
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      23
      Atomically Resolved Anisotropic Electrochemical Shaping of Nano-electrocatalyst
      Ruiz-Zepeda, Francisco; Gatalo, Matija; Pavlisic, Andraz; Drazic, Goran; Jovanovic, Primoz; Bele, Marjan; Gaberscek, Miran; Hodnik, Nejc
      Nano Letters (2019), 19 (8), 4919-4927CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Catalytic properties of advanced functional materials are detd. by their surface and near-surface at. structure, compn., morphol., defects, compressive and tensile stresses, etc.; also known as a structure-activity relationship. The catalysts structural properties are dynamically changing as they perform via complex phenomenon dependent on the reaction conditions. In turn, not just the structural features but even more importantly, catalytic characteristics of nanoparticles get altered. Definitive conclusions about these phenomena are not possible with imaging of random nanoparticles with unknown at. structure history. Using a contemporary PtCu-alloy electrocatalyst as a model system, a unique approach allowing unprecedented insight into the morphol. dynamics on the at.-scale caused by the process of dealloying is presented. Observing the detailed structure and morphol. of the same nanoparticle at different stages of electrochem. treatment reveals new insights into at.-scale processes such as size, faceting, strain and porosity development. Furthermore, based on precise atomically resolved microscopy data, Kinetic Monte Carlo (KMC) simulations provide further feedback into the phys. parameters governing electrochem. induced structural dynamics. This work introduces a unique approach toward observation and understanding of nanoparticles dynamic changes on the at. level and paves the way for an understanding of the structure-stability relationship.
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      Stukowski, A.; Markmann, J.; Weissmüller, J.; Albe, K. Atomistic Origin of Microstrain Broadening in Diffraction Data of Nanocrystalline Solids. Acta Mater. 2009, 57 (5), 1648– 1654,  DOI: 10.1016/j.actamat.2008.12.011
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      24
      Atomistic origin of microstrain broadening in diffraction data of nanocrystalline solids
      Stukowski, A.; Markmann, J.; Weissmueller, J.; Albe, K.
      Acta Materialia (2009), 57 (5), 1648-1654CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
      The origin of microstrain broadening in x-ray diffraction patterns of nanocryst. metals is investigated by comparing data obtained from virtual diffractograms and from direct anal. of computer-generated samples. A new method is introduced that allows the local deformation gradient to be calcd. for each lattice site in the microstructure from at. coordinates obtained by mol. dynamics simulations. Microstrain broadening in undeformed specimens cannot be attributed to lattice dislocations or strain fields near grain boundaries. The broadening arises, instead, from long-range correlated displacement fields that extend throughout the grains. The microstrain therefore provides a quant. measure for distortions far from grain boundaries. Evidently diffraction-based strategies for inferring the dislocation d. in ultrafine-grained metals do not necessarily apply to nanocryst. materials.
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      Montejano-Carrizales, J. M.; Morán-López, J. L. Geometrical Characteristics of Compact Nanoclusters. Nanostruct. Mater. 1992, 1 (5), 397– 409,  DOI: 10.1016/0965-9773(92)90090-K
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      Geometrical characteristics of compact nanoclusters
      Montejano-Carrizales, J. M.; Moran-Lopez, J. L.
      Nanostructured Materials (1992), 1 (5), 397-409CODEN: NMAEE7; ISSN:0965-9773.
      The geometrical characteristics of icosahedral and cubo-octahedral clusters were studied. The various shells around the central atom that form the cluster, the no. of atoms forming each shell, the no. of nearest neighbors to each atom, the various surface sites and the total no. of bonds were analyzed. General expressions are given for the total no. of atoms, the total no. of surface atoms and their distribution, and the total no. of bonds. The cluster shape stability is discussed.
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    26. 26
      Chattot, R.; Martens, I.; Scohy, M.; Herranz, J.; Drnec, J.; Maillard, F.; Dubau, L. Disclosing Pt-Bimetallic Alloy Nanoparticle Surface Lattice Distortion with Electrochemical Probes. ACS Energy Lett. 2020, 5 (1), 162– 169,  DOI: 10.1021/acsenergylett.9b02287
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      26
      Disclosing Pt-Bimetallic Alloy Nanoparticle Surface Lattice Distortion with Electrochemical Probes
      Chattot, Raphael; Martens, Isaac; Scohy, Marion; Herranz, Juan; Drnec, Jakub; Maillard, Frederic; Dubau, Laetitia
      ACS Energy Letters (2020), 5 (1), 162-169CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Structural defects are of significant importance in (electro)catalysis, as they provide sites of unusually high activity that find applications in many key electrochem. processes. However, tools to characterize surface defects remain scarce and complex, esp. for nanocatalysts where classical methods such as TEM or x-ray scattering are limited in their ability to probe the structure and distribution of the active surface sites. Herein, the ratio between the COads stripping charge (QCO) and the charge required to desorb under-potentially deposited H atoms (QH) in structurally-disordered Pt-based nanocatalysts scales almost linearly with the Surface Distortion descriptor obtained via advanced phys. methods. This trend is valid in both rotating disk electrode configuration and in a real fuel cell device, thus providing the scientific community with a powerful and versatile approach for semi-quant. estn. of the surface lattice distortion in Pt-based catalysts without the need for exhaustive structural characterization.
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    27. 27
      Henning, S.; Kühn, L.; Herranz, J.; Durst, J.; Binninger, T.; Nachtegaal, M.; Werheid, M.; Liu, W.; Adam, M.; Kaskel, S.; Eychmüller, A.; Schmidt, T. J. Pt-Ni Aerogels as Unsupported Electrocatalysts for the Oxygen Reduction Reaction. J. Electrochem. Soc. 2016, 163 (9), F998– F1003,  DOI: 10.1149/2.0251609jes
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      Pt-Ni Aerogels as Unsupported Electrocatalysts for the Oxygen Reduction Reaction
      Henning, Sebastian; Kuhn, Laura; Herranz, Juan; Durst, Julien; Binninger, Tobias; Nachtegaal, Maarten; Werheid, Matthias; Liu, Wei; Adam, Marion; Kaskel, Stefan; Eychmuller, Alexander; Schmidt, Thomas J.
      Journal of the Electrochemical Society (2016), 163 (9), F998-F1003CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The com. feasibility of polymer electrolyte fuel cells (PEFCs) passes by the development of Pt-based, O2-redn. catalysts with greater activities and/or lower Pt-contents, as well as an improved stability. In an effort to tackle these requirements, unsupported bimetallic Pt-Ni nanoparticles (NPs) interconnected in the shape of nanochain networks (aerogels) were synthesized using a simple one-step redn. and gel formation process in aq. soln. The products of this novel synthetic route were characterized by X-ray absorption spectroscopy to elucidate the materials' structure. Using electrochem. expts., we probed the surface compn. of the as-synthesized aerogels and of equiv. materials exposed to acid, and concluded that a Ni-(hydr)oxide side phase is present in the aerogel with a larger Ni-concn. Regardless of this initial surface compn., the Pt-Ni aerogels feature a ≈3-fold increase of surface-specific ORR activity when compared to a com. platinum-on-carbon catalyst, reaching the mass-specific requirement for application in automotive PEFCs.
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    28. 28
      Henning, S.; Herranz, J.; Ishikawa, H.; Kim, B. J.; Abbott, D.; Kühn, L.; Eychmüller, A.; Schmidt, T. J. Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes. J. Electrochem. Soc. 2017, 164 (12), F1136– F1141,  DOI: 10.1149/2.0131712jes
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      Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes
      Henning, Sebastian; Herranz, Juan; Ishikawa, Hiroshi; Kim, Bae Jung; Abbott, Daniel; Kuhn, Laura; Eychmuller, Alexander; Schmidt, Thomas J.
      Journal of the Electrochemical Society (2017), 164 (12), F1136-F1141CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The com. success of polymer electrolyte fuel cells (PEFCs) depends on the development of Pt-based oxygen redn. reaction (ORR) catalysts with greater activity and stability to reduce the amt. of expensive noble metal per device. To advance toward this goal, we have tested a novel class of unsupported bimetallic alloy catalysts (aerogels) as the cathode material in PEFCs under two accelerated stress test conditions and compared it to a state-of-the-art carbon-supported benchmark (Pt/C). The investigated Pt3Ni aerogel shows little degrdn. under high potential conditions (> 1.0 V) which can occur during fuel starvation and start-up/shut-down of the cell. If tested under the same conditions, the Pt/C benchmark displays significant losses of electrochem. surface area and ORR activity due to carbon support corrosion as obsd. in cross section and transmission electron microscopy anal. When testing the durability upon extended load cycling (0.6-1.0 V), Pt3Ni aerogel demonstrates less stability than Pt/C which is related to the severe Ni leaching from the alloy under such conditions. These findings highlight the advantages of using unsupported ORR catalysts in PEFCs and point to the redn. of non-noble metal dissoln. as the next development step.
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    29. 29
      Henning, S.; Ishikawa, H.; Kühn, L.; Herranz, J.; Müller, E.; Eychmüller, A.; Schmidt, T. J. Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes. Angew. Chem., Int. Ed. 2017, 56 (36), 10707– 10710,  DOI: 10.1002/anie.201704253
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      Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes
      Henning, Sebastian; Ishikawa, Hiroshi; Kuehn, Laura; Herranz, Juan; Mueller, Elisabeth; Eychmueller, Alexander; Schmidt, Thomas J.
      Angewandte Chemie, International Edition (2017), 56 (36), 10707-10710CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Highly active and durable oxygen redn. catalysts are needed to reduce the costs and enhance the service life of polymer electrolyte fuel cells (PEFCs). This can be accomplished by alloying Pt with a transition metal (for example Ni) and by eliminating the corrodible, carbon-based catalyst support. However, materials combining both approaches have seldom been implemented in PEFC cathodes. In this work, an unsupported Pt-Ni alloy nanochain ensemble (aerogel) demonstrates high current PEFC performance commensurate with that of a carbon-supported benchmark (Pt/C) following optimization of the aerogel's catalyst layer (CL) structure. The latter is accomplished using a sol. filler to shift the CL's pore size distribution towards larger pores which improves reactant and product transport. Chiefly, the optimized PEFC aerogel cathodes display a circa 2.5-fold larger surface-specific ORR activity than Pt/C and maintain 90 % of the initial activity after an accelerated stress test (vs. 40 % for Pt/C).
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    30. 30
      Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C. Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; Zhu, E.; Yu, T.; Jia, Q.; Guo, J.; Zhang, L.; Goddard, W. A.; Huang, Y.; Duan, X. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354 (6318), 1414– 1419,  DOI: 10.1126/science.aaf9050
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      Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction
      Li, Mufan; Zhao, Zipeng; Cheng, Tao; Fortunelli, Alessandro; Chen, Chih-Yen; Yu, Rong; Zhang, Qinghua; Gu, Lin; Merinov, Boris V.; Lin, Zhaoyang; Zhu, Enbo; Yu, Ted; Jia, Qingying; Guo, Jinghua; Zhang, Liang; Goddard, William A., III; Huang, Yu; Duan, Xiangfeng
      Science (Washington, DC, United States) (2016), 354 (6318), 1414-1419CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Improving the platinum (Pt) mass activity for the oxygen redn. reaction (ORR) requires optimization of both the specific activity and the electrochem. active surface area (ECSA). We found that soln.-synthesized Pt/NiO core/shell nanowires can be converted into PtNi alloy nanowires through a thermal annealing process and then transformed into jagged Pt nanowires via electrochem. dealloying. The jagged nanowires exhibit an ECSA of 118 square meters per g of Pt and a specific activity of 11.5 mA per square centimeter for ORR (at 0.9 V vs. reversible hydrogen electrode), yielding a mass activity of 13.6 A per mg of Pt, nearly double previously reported best values. Reactive mol. dynamics simulations suggest that highly stressed, undercoordinated rhombus-rich surface configurations of the jagged nanowires enhance ORR activity vs. more relaxed surfaces.
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    31. 31
      Alia, S. M.; Ngo, C.; Shulda, S.; Ha, M. A.; Dameron, A. A.; Weker, J. N.; Neyerlin, K. C.; Kocha, S. S.; Pylypenko, S.; Pivovar, B. S. Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum-Nickel Nanowires through Synthesis and Post-Treatment Optimization. ACS Omega 2017, 2 (4), 1408– 1418,  DOI: 10.1021/acsomega.7b00054
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      Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum-Nickel Nanowires through Synthesis and Post-Treatment Optimization
      Alia, Shaun M.; Ngo, Chilan; Shulda, Sarah; Ha, Mai-Anh; Dameron, Arrelaine A.; Weker, Johanna Nelson; Neyerlin, Kenneth C.; Kocha, Shyam S.; Pylypenko, Svitlana; Pivovar, Bryan S.
      ACS Omega (2017), 2 (4), 1408-1418CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 μA cmPt-2 at 0.9 V) and high surface areas (>90 m2 gPt-1). Platinum-nickel (Pt-Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt-1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochem. test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt-Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.
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    32. 32
      Van Der Vliet, D. F.; Wang, C.; Tripkovic, D.; Strmcnik, D.; Zhang, X. F.; Debe, M. K.; Atanasoski, R. T.; Markovic, N. M.; Stamenkovic, V. R. Mesostructured Thin Films as Electrocatalysts with Tunable Composition and Surface Morphology. Nat. Mater. 2012, 11 (12), 1051– 1058,  DOI: 10.1038/nmat3457
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      Mesostructured thin films as electrocatalysts with tunable composition and surface morphology
      van der Vliet, Dennis F.; Wang, Chao; Tripkovic, Dusan; Strmcnik, Dusan; Zhang, Xiao Feng; Debe, Mark K.; Atanasoski, Radoslav T.; Markovic, Nenad M.; Stamenkovic, Vojislav R.
      Nature Materials (2012), 11 (12), 1051-1058CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Among the most challenging issues in technologies for electrochem. energy conversion are the insufficient activity of the catalysts for the oxygen redn. reaction, catalyst degrdn. and carbon-support corrosion. In an effort to address these barriers, carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored phys. properties are considered. Here a new class of metallic materials with tunable near-surface compn., morphol. and structure that have led to greatly improved affinity are presented for the electrochem. redn. of oxygen. The level of activity for the oxygen redn. reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk polycryst. Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.
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    33. 33
      Bae, S. J.; Yoo, S. J.; Lim, Y.; Kim, S.; Lim, Y.; Choi, J.; Nahm, K. S.; Hwang, S. J.; Lim, T. H.; Kim, S. K.; Kim, P. Facile Preparation of Carbon-Supported PtNi Hollow Nanoparticles with High Electrochemical Performance. J. Mater. Chem. 2012, 22 (18), 8820– 8825,  DOI: 10.1039/c2jm16827h
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      Facile preparation of carbon-supported PtNi hollow nanoparticles with high electrochemical performance
      Bae, Sung Jong; Yoo, Sung Jong; Lim, Yuntaek; Kim, Sojeong; Lim, Yirang; Choi, Junghun; Nahm, Kee Suk; Hwang, Seung Jun; Lim, Tae-Hoon; Kim, Soo-Kil; Kim, Pil
      Journal of Materials Chemistry (2012), 22 (18), 8820-8825CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      To design Pt-based materials with a hollow structure via a galvanic reaction would be one of the effective ways to prep. electro- catalysts with high activity. The galvanic reaction between Pt ions and metal template is usually conducted under limited conditions, which makes the prepn. of Pt hollow nanoparticles laborious. Here, we introduce a one-step and one-pot synthetic approach for the prepn. of carbon-supported PtNi alloy hollow nanoparticles with a narrow size distribution. Prepd. PtNi alloys were characterized by a nonporous shell consisting of a Pt-enriched surface layer and an inner alloy layer of Pt and Ni. Due to its unique structural advantages, this material showed excellent electrocatalytic performance for oxygen redn. (3.3- and 7.8-fold enhanced mass and specific activities compared to those of a com. carbon-supported Pt nanoparticle). A possible mechanism for the formation of PtNi hollow structure is suggested.
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      Snyder, J.; Fujita, T.; Chen, M. W.; Erlebacher, J. Oxygen Reduction in Nanoporous Metal-Ionic Liquid Composite Electrocatalysts. Nat. Mater. 2010, 9 (11), 904– 907,  DOI: 10.1038/nmat2878
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      Oxygen reduction in nanoporous metal-ionic liquid composite electrocatalysts
      Snyder, J.; Fujita, T.; Chen, M. W.; Erlebacher, J.
      Nature Materials (2010), 9 (11), 904-907CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The improvement of catalysts for the 4-electron redn. reaction (ORR; O2+4H++4e-→2H2O) remains a crit. challenge for fuel cells and other electrochem.-energy technologies. Recent attention in this area has centered on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles. For instance, with a outer surface and Ni-rich 2nd at. layer, Pt3Ni(111) is one of the most active surfaces for the ORR, owing to a shift in the d-band center of the surface atoms that results in a weakened interaction between and intermediate oxide species, freeing more active sites for O2 adsorption. However, enhancements due solely to alloy structure and compn. may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel cell commercialization, esp. as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. A tailored geometric and chem. materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous alloy impregnated with a hydrophobic, high O soly. and protic ionic liq. has extremely high mass activity. The results are consistent with an engineered chem. bias within a catalytically active nanoporous framework that pushes the ORR towards completion.
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      Luo, M.; Sun, Y.; Zhang, X.; Qin, Y.; Li, M.; Li, Y.; Li, C.; Yang, Y.; Wang, L.; Gao, P.; Lu, G.; Guo, S. Stable High-Index Faceted Pt Skin on Zigzag-Like PtFe Nanowires Enhances Oxygen Reduction Catalysis. Adv. Mater. 2018, 30 (10), 1705515,  DOI: 10.1002/adma.201705515
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      Dubau, L.; Nelayah, J.; Moldovan, S.; Ersen, O.; Bordet, P.; Drnec, J.; Asset, T.; Chattot, R.; Maillard, F. Defects Do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles. ACS Catal. 2016, 6 (7), 4673– 4684,  DOI: 10.1021/acscatal.6b01106
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      Defects do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles
      Dubau, Laetitia; Nelayah, Jaysen; Moldovan, Simona; Ersen, Ovidiu; Bordet, Pierre; Drnec, Jakub; Asset, Tristan; Chattot, Raphael; Maillard, Frederic
      ACS Catalysis (2016), 6 (7), 4673-4684CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The catalytic performance of extended and nanometer-sized surfaces strongly depends on the amt. and the nature of structural defects that they exhibit. However, whereas the effect of steps or adatoms may be unraveled with single crystals ("surface science approach"), implementing reproducibly in a controlled manner structural defects on nanomaterials remains hardly feasible. A case that deserves particular attention is that of bimetallic nanomaterials, which are used to catalyze the oxygen redn. reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Point defects (vacancies), planar defects (dislocations and grain boundaries), and bulk defects (voids, pores) are likely to be generated in alloy or core@shell nanomaterials based on Pt and a transition metal due to the high lattice mismatch between the two elements. Here, we report the morphol. and structural trajectories of hollow PtNi/C nanoparticles during thermal annealing under vacuum, N2, H2, or air atm. by in situ transmission electron microscopy and synchrotron X-ray diffraction. We evidence atm.-dependent restructuring kinetics, which enabled us to synthesize a set of catalysts with identical chem. compns. and elemental distributions but different morphologies, crystallite sizes, and lattice strain. By combining the results of Rietveld and pair-distribution function analyses and electrochem. measurements, we demonstrate that the structurally disordered areas located at the interface between individual crystallites are highly active for two reactions of interest for PEMFC devices: the electrochem. COads oxidn. and the ORR. These results shed fundamental light on the effect of structural defects on the catalytic performance of bimetallic nanomaterials and should aid in the rational design of more efficient ORR electrocatalysts.
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    37. 37
      Alia, S. M.; Pylypenko, S.; Neyerlin, K. C.; Kocha, S. S.; Pivovar, B. S. Nickel Nanowire Oxidation and Its Effect on Platinum Galvanic Displacement and Methanol Oxidation. ECS Trans. 2014, 64 (3), 89– 95,  DOI: 10.1149/06403.0089ecst
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      37
      Nickel nanowire oxidation and its effect on platinum galvanic displacement and methanol oxidation
      Alia, Shaun M.; Pylypenko, Svitlana; Neyerlin, K. C.; Kocha, Shyam S.; Pivovar, Bryan S.
      ECS Transactions (2014), 64 (3, Polymer Electrolyte Fuel Cells 14 (PEFC 14)), 89-95CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
      The heat treatment of nickel (Ni) nanowires (NiNWs) in oxygen is examd. for its effect on platinum (Pt) galvanic displacement and the activity of Pt-coated NiNWs (PtNiNWs) in the methanol oxidn. reaction (MOR). Annealing of NiNWs in oxygen is found to reduce Pt displacement and increase the resistance of surface Ni to potential-driven dissoln. (during electrochem. break-in). By reducing Ni dissoln., oxophilic species are provided in close proximity to Pt sites and the MOR activity of PtNiNWs improves, particularly at low overpotential in rotating disk electrode half-cells.
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    38. 38
      Xiong, Z.; Li, S.; Xu, H.; Zhang, K.; Yan, B.; Du, Y. Newly Designed Ternary Metallic PtPdBi Hollow Catalyst with High Performance for Methanol and Ethanol Oxidation. Catalysts 2017, 7, 208,  DOI: 10.3390/catal7070208
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      Newly designed ternary metallic PtPdBi hollow catalyst with high performance for methanol and ethanol oxidation
      Xiong, Zhiping; Li, Shumin; Xu, Hui; Zhang, Ke; Yan, Bo; Du, Yukou
      Catalysts (2017), 7 (7), 208/1-208/10CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
      This paper reported the fabrication of ternary metallic PtPdBi hollow nanocatalyst through a facile, one-pot, wet-chem. method by adopting sodium borohydride and polyvinylpyrrolidone as reducing agent and surfactant directing agent, resp. The hollow structure offers novel morphol. and large surface areas, which are conducive to enhancing the electrocatalytic activity. The electrocatalytic properties of hollow PtPdBi nanocatalyst were investigated systematically in alk. media through cyclic voltammetry and the as-prepd. PtPdBi nanocatalyst displays greatly enhanced electrocatalytic activities towards methanol and ethanol oxidn. The calcd. mass activities of PtPdBi electrocatalyst are 2.133 A mgPtPd-1 for methanol oxidn. reaction and 5.256 A mgPdPt-1 for ethanol oxidn. reaction, which are much better than that of com. Pt/C and com. Pd/C. The as-prepd. hollow nanocatalyst may be a potential promising electrocatalyst in fuel cells and also may be extended to the applications of other desirable functions.
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    39. 39
      Mao, J.; Chen, W.; He, D.; Wan, J.; Pei, J.; Dong, J.; Wang, Y.; An, P.; Jin, Z.; Xing, W.; Tang, H.; Zhuang, Z.; Liang, X.; Huang, Y.; Zhou, G.; Wang, L.; Wang, D.; Li, Y. Design of Ultrathin Pt-Mo-Ni Nanowire Catalysts for Ethanol Electrooxidation. Sci. Adv. 2017, 3, e1603068  DOI: 10.1126/sciadv.1603068
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      Design of ultrathin Pt-Mo-Ni nanowire catalysts for ethanol electrooxidation
      Mao, Junjie; Chen, Wenxing; He, Dongsheng; Wan, Jiawei; Pei, Jiajing; Dong, Juncai; Wang, Yu; An, Pengfei; Jin, Zhao; Xing, Wei; Tang, Haolin; Zhuang, Zhongbin; Liang, Xin; Huang, Yu; Zhou, Gang; Wang, Leyu; Wang, Dingsheng; Li, Yadong
      Science Advances (2017), 3 (8), e1603068/1-e1603068/9CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
      Developing cost-effective, active, and durable electrocatalysts is one of the most important issues for the com.-ization of fuel cells. Ultrathin Pt-Mo-Ni nanowires (NWs) with a diam. of ∼2.5 nm and lengths of up to several micro-meters were synthesized via a H2 -assisted soln. route (HASR). This catalyst was designed on the basis of thefollowing three points: (i) ultrathin NWs with high nos. of surface atoms can increase the at. efficiency ofPt and thus decrease the catalyst cost; (ii) the incorporation of Ni can isolate Pt atoms on the surface and producesurface defects, leading to high catalytic activity (the unique structure and superior activity were confirmed by spheri-cal aberration-cor. electron microscopy measurements and ethanol oxidn. tests, resp.); and (iii) theincorporation of Mo can stabilize both Ni and Pt atoms, leading to high catalytic stability, which was confirmed byexperiments and d. functional theory calcns. Furthermore, the developed HASR strategy can be extendedto synthesize a series of Pt-Mo-M (M = Fe, Co, Mn, Ru, etc.) NWs. These multimetallic NWs would open up new oppor-tunities for practical fuel cell applications.
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    40. 40
      Zhao, S.; Li, M.; Han, M.; Xu, D.; Yang, J.; Lin, Y.; Shi, N. E.; Lu, Y.; Yang, R.; Liu, B.; Dai, Z.; Bao, J. Defect-Rich Ni3FeN Nanocrystals Anchored on N-Doped Graphene for Enhanced Electrocatalytic Oxygen Evolution. Adv. Funct. Mater. 2018, 28 (18), 1706018,  DOI: 10.1002/adfm.201706018
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      Wang, P.; Jiang, K.; Wang, G.; Yao, J.; Huang, X. Phase and Interface Engineering of Platinum-Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55 (41), 12859– 12863,  DOI: 10.1002/anie.201606290
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      Phase and Interface Engineering of Platinum-Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution
      Wang, Pengtang; Jiang, Kezhu; Wang, Gongming; Yao, Jianlin; Huang, Xiaoqing
      Angewandte Chemie, International Edition (2016), 55 (41), 12859-12863CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The design of high-performance electrocatalysts for the alk. hydrogen evolution reaction (HER) is highly desirable for the development of alk. water electrolysis. Phase- and interface-engineered platinum-nickel nanowires (Pt-Ni NWs) are highly efficient electrocatalysts for alk. HER. The phase and interface engineering is achieved by simply annealing the pristine Pt-Ni NWs under a controlled atm. Impressively, the newly generated nanomaterials exhibit superior activity for the alk. HER, outperforming the pristine Pt-Ni NWs and com. Pt/C, and also represent the best alk. HER catalysts to date. The enhanced HER activities are attributed to the superior phase and interface structures in the engineered Pt-Ni NWs.
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      Dickens, C. F.; Nørskov, J. K. A Theoretical Investigation into the Role of Surface Defects for Oxygen Evolution on RuO2. J. Phys. Chem. C 2017, 121 (34), 18516– 18524,  DOI: 10.1021/acs.jpcc.7b03481
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      A Theoretical Investigation into the Role of Surface Defects for Oxygen Evolution on RuO2
      Dickens, Colin F.; Noerskov, Jens K.
      Journal of Physical Chemistry C (2017), 121 (34), 18516-18524CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The inability of conventional theor. models to corroborate the known exptl. activity of RuO2 for the O evolution reaction (OER) has recently been the subject of numerous research efforts. The authors use d. functional theory calcns. to study the possibility that surface defects formed during dissoln. are responsible for the OER activity of RuO2. It was well-established exptl. that RuO2 undergoes dissoln. during OER, yet little is known about the at. structure or catalytic reactivity of the resulting defect sites. Through simulation of point defects, steps, and kinks derived from the RuO2 (110) surface, the authors discover a 0.7 eV range in the primary descriptor for OER activity, with the most active sites outperforming those at the ideal (110) surface by nearly 0.5 eV. The authors postulate that these variations in reactivity are due to differences in the local electronic structure, and the authors study in more detail the electronic structure of two singly coordinated sites at the same Ru atom located at a kink. Finally, the authors study possible dissoln. pathways that may proceed at the RuO2 surface under OER conditions.
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    43. 43
      Shi, Q.; Zhu, C.; Zhong, H.; Su, D.; Li, N.; Engelhard, M. H.; Xia, H.; Zhang, Q.; Feng, S.; Beckman, S. P.; Du, D.; Lin, Y. Nanovoid Incorporated IrxCu Metallic Aerogels for Oxygen Evolution Reaction Catalysis. ACS Energy Lett. 2018, 3 (9), 2038– 2044,  DOI: 10.1021/acsenergylett.8b01338
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      Nanovoid Incorporated IrxCu Metallic Aerogels for Oxygen Evolution Reaction Catalysis
      Shi, Qiurong; Zhu, Chengzhou; Zhong, Hong; Su, Dong; Li, Na; Engelhard, Mark H.; Xia, Haibing; Zhang, Qiang; Feng, Shuo; Beckman, Scott P.; Du, Dan; Lin, Yuehe
      ACS Energy Letters (2018), 3 (9), 2038-2044CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      IR-based nanomaterials are regarded as state-of-the-art cathode electrocatalysts in proton exchange membrane water electrolyzers (PEMWEs). Engineering the morphol. of Ir-based three-dimensional architectures as electrocatalysts toward oxygen evolution reaction (OER) was rarely studied. Here, the authors report the gelation of IrxCu metallic hydrogels self-assembled with ultrafine and nanovoid incorporated building blocks for enhancing the electrocatalytic performance toward OER. The compn.-optimized Ir3Cu metallic aerogels exhibited improved catalytic activity and durability toward OER. The mesosized voids generated through the in situ galvanic replacement reaction and the macrosized porous systems make a great contribution to the increased no. of active sites. First-principle calcns. revealed the intrinsic optimized binding energy of Ir by alloying with Cu. The best catalytic performance necessities a balance of the adsorption and desorption energy. The well-defined morphol. and enhanced OER electrochem. performances of nanovoid incorporated IrxCu metallic aerogels hold great promise in further applications in PEMWEs.
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    44. 44
      Calle-Vallejo, F.; Pohl, M. D.; Bandarenka, A. S. Quantitative Coordination-Activity Relations for the Design of Enhanced Pt Catalysts for CO Electro-Oxidation. ACS Catal. 2017, 7 (7), 4355– 4359,  DOI: 10.1021/acscatal.7b01105
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      Quantitative Coordination-Activity Relations for the Design of Enhanced Pt Catalysts for CO Electro-oxidation
      Calle-Vallejo, Federico; Pohl, Marcus D.; Bandarenka, Aliaksandr S.
      ACS Catalysis (2017), 7 (7), 4355-4359CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Efficient redox transformations of CO are vital for remediating the imbalance of the biogeochem. cycle of carbon and also for the development of next-generation energy technologies such as fuel cells. For instance, CO oxidn. is ubiquitous in hydrocarbon-based fuel cells and dets. to a large extent their efficiency. As Pt is used in these cells, minimal catalyst loadings that do not compromise the activity are needed. In view of that, we present here a "coordination-activity plot" and electrochem. expts. on electro-oxidn. of adsorbed CO to CO2 to establish quant. structure-activity relations for various Pt electrodes. We predict theor. and verify exptl. that catalytic CO oxidn. is enhanced by creating surface defects with optimal coordination, without alloying. Both largely overcoordinated and undercoordinated defects on Pt (at rough surfaces with cavities and metal adatoms, resp.) hinder the reaction, so that the maximal activity is reached on sites with a generalized coordination of ‾C‾N = 5.4. Importantly, this moderate coordination is found at defects such as step edges of electrodes with relatively short terrace lengths of 3-4 metal atoms.
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    45. 45
      Maillard, F.; Eikerling, M.; Cherstiouk, O. V; Schreier, S.; Savinova, E.; Stimming, U. Size Effects on Reactivity of Pt Nanoparticles in CO Monolayer Oxidation: The Role of Surface Mobility. Faraday Discuss. 2004, 125, 357– 377,  DOI: 10.1039/b303911k
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      Size effects on reactivity of Pt nanoparticles in CO monolayer oxidation: the role of surface mobility
      Maillard F; Eikerling M; Cherstiouk O V; Schreier S; Savinova E; Stimming U
      Faraday discussions (2004), 125 (), 357-77; discussion 391-407 ISSN:1359-6640.
      In the present paper we study the reactivity of model Pt nanoparticles supported on glassy carbon. The particle size effect is rationalized for CO monolayer oxidation exploring electrochemical methods (stripping voltammetry and chronoamperometry) and modelling. Significant size effects are observed in the particle size interval from ca. 1 to 4 nm, including the positive shift of the CO stripping peak with decreasing particle size and a pronounced asymmetry of the current transients at constant potential. The latter go through a maximum at low COads conversion and exhibit tailing, which is the longer the smaller the particle size. Neither mean field nor nucleation & growth models give a coherent explanation of these experimental findings. We, therefore, suggest a basic model employing the active site concept. With a number of reasonable simplifications a full analytical solution is obtained, which allows a straightforward comparison of the theory with the experimental data. A good correspondence between experiment and theory is demonstrated. The model suggests restricted COads mobility at Pt nanoparticles below ca. 2 nm size, with the diffusion coefficient strongly dependent on the particle size, and indicates a transition towards fast diffusion when the particle size exceeds ca. 3 nm. Estimates of relevant kinetic parameters, including diffusion coefficient, reaction constant etc. are obtained and compared to the literature data for extended Pt surfaces.
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    46. 46
      Van Der Vliet, D. F.; Wang, C.; Li, D.; Paulikas, A. P.; Greeley, J.; Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.; Stamenkovic, V. R. Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces. Angew. Chem., Int. Ed. 2012, 51 (13), 3139– 3142,  DOI: 10.1002/anie.201107668
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      Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces
      van der Vliet, Dennis F.; Wang, Chao; Li, Dongguo; Paulikas, Arvydas P.; Greeley, Jeffrey; Rankin, Rees B.; Strmcnik, Dusan; Tripkovic, Dusan; Markovic, Nenad M.; Stamenkovic, Vojislav R.
      Angewandte Chemie, International Edition (2012), 51 (13), 3139-3142, S3139/1-S3139/5CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Authors demonstrated that by alloying platinum with a non-noble second metal and inducing a Pt-skin-type structure, the adsorption properties of formed structures are significantly changed. The adsorption of hydrogen and oxide species is shifted in potential and reduced in magnitude compared to platinum. The LEIS data showed that the surface consists only of platinum. The suppression of hydrogen adatoms on platinum underestimate the surface area and overrates the specific activity. The Co stripping can be used alongside with the adsorbed hydrogen charge for the detn. of the electrochem. active surface area of platinum-alloy catalysts.
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    47. 47
      Marichev, V. A. Reversibility of Platinum Voltammograms in Aqueous Electrolytes and Ionic Product of Water. Electrochim. Acta 2008, 53 (27), 7952– 7960,  DOI: 10.1016/j.electacta.2008.05.076
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      Reversibility of platinum voltammograms in aqueous electrolytes and ionic product of water
      Marichev, V. A.
      Electrochimica Acta (2008), 53 (27), 7952-7960CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
      Known reversibility of Pt voltammograms in supporting aq. electrolytes formally contradicts the equil. defined by the ionic product of H2O: Kw = [H+]·[OH-] ≃ 1 × 10-14, since at any pH value, concn. of one of these ions in soln. should be too low (≤10-7 M) to ensure reversibility of voltammograms in the whole potential region of study at usual potential scan rates. This contradiction could be overcome by allowing dissociative H2O adsorption. There are no direct exptl. proofs of this process on Pt, however all available relevant data obtained by in situ phys. methods appear to provide indirect evidences for this hypothesis.
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      Van Der Niet, M. J. T. C.; Garcia-Araez, N.; Hernández, J.; Feliu, J. M.; Koper, M. T. M. Water Dissociation on Well-Defined Platinum Surfaces: The Electrochemical Perspective. Catal. Today 2013, 202, 105– 113,  DOI: 10.1016/j.cattod.2012.04.059
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      Water dissociation on well-defined platinum surfaces: The electrochemical perspective
      van der Niet, Maria J. T. C.; Garcia-Araez, Nuria; Hernandez, Javier; Feliu, Juan M.; Koper, Marc T. M.
      Catalysis Today (2013), 202 (), 105-113CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      This paper discusses three important discrepancies in the current interpretation of the role of water dissocn. on the blank cyclic voltammetry of well-defined single-cryst. stepped platinum surfaces. First, for H adsorption both H-terrace and H-step contributions were identified, whereas for OH adsorption only OH-terrace was identified. Second, different shapes (broad vs. sharp) of the H-terrace and H-step voltammetric peaks imply different lateral interactions between hydrogen adatoms at terraces and steps, i.e. repulsive vs. attractive interactions. Third, the H-step peak exhibits an unusual pH-dependent shift of 50 mVNHE/pH unit. We propose here a model that can explain all these observations. In the model, the H-step peak is not due to only ad- and desorption of hydrogen, but to the replacement of H with O and/or OH. The O:OH ratio in the step varies with step geometry, step d. and medium. In alk. media relatively more OH is adsorbed in (or on) the step than in acidic media, under which conditions more O is adsorbed in (or on) the step. This would explain the anomalous pH dependence and would provide a possible explanation for the higher catalytic activity of alk. media for electro-oxidn. reactions. Although the model certainly still contains speculative elements, we believe it provides the most consistent interpretation of platinum single-crystal electrochem. currently available, and presents an important and significant improvement over previous interpretations. In situ spectroscopic data are ultimately needed to confirm or disprove some of the assumptions of the model.
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    49. 49
      Janik, M. J.; McCrum, I. T.; Koper, M. T. M. On the Presence of Surface Bound Hydroxyl Species on Polycrystalline Pt Electrodes in the “Hydrogen Potential Region” (0 to 0.4 V-RHE). J. Catal. 2018, 367, 332– 337,  DOI: 10.1016/j.jcat.2018.09.031
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      On the presence of surface bound hydroxyl species on polycrystalline Pt electrodes in the "hydrogen potential region" (0-0.4 V-RHE)
      Janik, Michael J.; McCrum, Ian T.; Koper, Marc T. M.
      Journal of Catalysis (2018), 367 (), 332-337CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      A review is given on an effect of surface bound hydroxyl species on polycryst. platinum electrodes is given. The hydrogen binding energy, surface area, adsorption-induced restructuring, hydrogen oxidn. kinetics and carbon monoxide stripping are briefly discussed.
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      Tanaka, H.; Sugawara, S.; Shinohara, K.; Ueno, T.; Suzuki, S.; Hoshi, N.; Nakamura, M. Infrared Reflection Absorption Spectroscopy of OH Adsorption on the Low Index Planes of Pt. Electrocatalysis 2015, 6 (3), 295– 299,  DOI: 10.1007/s12678-014-0245-7
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      Infrared Reflection Absorption Spectroscopy of OH Adsorption on the Low Index Planes of Pt
      Tanaka, Hiroyuki; Sugawara, Seiho; Shinohara, Kazuhiko; Ueno, Takahiro; Suzuki, Shunsuke; Hoshi, Nagahiro; Nakamura, Masashi
      Electrocatalysis (2015), 6 (3), 295-299CODEN: ELECCF; ISSN:1868-2529. (Springer)
      The adsorption of hydroxide (OHad) on Pt has been studied on the low index planes of Pt using IR reflection absorption spectroscopy (IRAS) in electrochem. environments. We discuss the correlation between the integrated band intensity of the bending mode of OH of Pt-OH (δPtOH) and the charge d. of the oxide formation of Pt. The band of δPtOH is obsd. around 1100 cm-1, and the onset potential depends on the surface structure. The onset potential of δPtOH on Pt(110) and Pt(100) overlaps with the hydrogen adsorption/desorption potential region. The order of the integrated band intensity of δPtOH at 0.9 V vs RHE is opposite to the order of the oxygen redn. reaction (ORR) activity. This finding supports that the OHad is one of the species deactivating the ORR.
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      Garcia-Araez, N.; Climent, V.; Feliu, J. M. Analysis of Temperature Effects on Hydrogen and OH Adsorption on Pt(111), Pt(100) and Pt(110) by Means of Gibbs Thermodynamics. J. Electroanal. Chem. 2010, 649 (1–2), 69– 82,  DOI: 10.1016/j.jelechem.2010.01.024
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      Analysis of temperature effects on hydrogen and OH adsorption on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) by means of Gibbs thermodynamics
      Garcia-Araez, Nuria; Climent, Victor; Feliu, Juan M.
      Journal of Electroanalytical Chemistry (2010), 649 (1-2), 69-82CODEN: JECHES; ISSN:1572-6657. (Elsevier B.V.)
      Gibbs thermodn. in the presence of charge-transfer processes were applied to characterize temp. effects on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) electrodes in 0.1 M HClO4 solns. The present anal. allows the evaluation of the entropy of formation of the interphase for the three Pt basal planes, which is structure-sensitive. These results were used to test the suitability of the more common approach to characterize temp. effects, based on the application of a generalized isotherm. Besides, the comparison of both approaches allows the characterization of the different components involved in the entropy of H and OH adsorption, namely, the std. entropy of adsorption, the entropy due to lateral interactions, and the configurational entropy. Finally, the comparison with statistical mech. calcns. demonstrates that adsorbed H on Pt(1 1 1) and Pt(1 0 0) is rather mobile, while adsorbed OH is rather immobile. Adsorbed H on Pt(1 1 0) exhibits an unexpected behavior, since it appears to be rather immobile at low coverages, but it becomes very mobile at near satn. coverages.
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      Zhang, C.; Firestein, K. L.; Fernando, J. F. S.; Siriwardena, D.; von Treifeldt, J. E.; Golberg, D. Recent Progress of In Situ Transmission Electron Microscopy for Energy Materials. Adv. Mater. 2020, 32 (18), 1904094,  DOI: 10.1002/adma.201904094
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      Recent progress of in situ transmission electron microscopy for energy materials
      Zhang, Chao; Firestein, Konstantin L.; Fernando, Joseph F. S.; Siriwardena, Dumindu; von Treifeldt, Joel E.; Golberg, Dmitri
      Advanced Materials (Weinheim, Germany) (2020), 32 (18), 1904094CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. In situ transmission electron microscopy (TEM) is one of the most powerful approaches for revealing phys. and chem. process dynamics at at. resolns. The most recent developments for in situ TEM techniques are summarized; in particular, how they enable visualization of various events, measure properties, and solve problems in the field of energy by revealing detailed mechanisms at the nanoscale. Related applications include rechargeable batteries such as Li-ion, Na-ion, Li-O2, Na-O2, Li-S, etc., fuel cells, thermoelecs., photovoltaics, and photocatalysis. To promote various applications, the methods of introducing the in situ stimuli of heating, cooling, elec. biasing, light illumination, and liq. and gas environments are discussed. The progress of recent in situ TEM in energy applications should inspire future research on new energy materials in diverse energy-related areas.
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    53. 53
      Daio, T.; Staykov, A.; Guo, L.; Liu, J.; Tanaka, M.; Matthew Lyth, S.; Sasaki, K. Lattice Strain Mapping of Platinum Nanoparticles on Carbon and SnO2 Supports. Sci. Rep. 2015, 5, 13126,  DOI: 10.1038/srep13126
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      Lattice Strain Mapping of Platinum Nanoparticles on Carbon and SnO2 Supports
      Daio, Takeshi; Staykov, Aleksandar; Guo, Limin; Liu, Jianfeng; Tanaka, Masaki; Matthew Lyth, Stephen; Sasaki, Kazunari
      Scientific Reports (2015), 5 (), 13126CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      It is extremely important to understand the properties of supported metal nanoparticles at the at. scale. In particular, visualizing the interaction between nanoparticle and support, as well as the strain distribution within the particle is highly desirable. Lattice strain can affect catalytic activity, and therefore strain engineering via e.g. synthesis of core-shell nanoparticles or compositional segregation has been intensively studied. However, substrate-induced lattice strain has yet to be visualized directly. In this study, platinum nanoparticles decorated on graphitized carbon or tin oxide supports are investigated using spherical aberration-cor. scanning transmission electron microscopy (Cs-cor. STEM) coupled with geometric phase anal. (GPA). Local changes in lattice parameter are obsd. within the Pt nanoparticles and the strain distribution is mapped. This reveals that Pt nanoparticles on SnO2 are more highly strained than on carbon, esp. in the region of at. steps in the SnO2 lattice. These substrate-induced strain effects are also reproduced in d. functional theory simulations, and related to catalytic oxygen redn. reaction activity. This study suggests that tailoring the catalytic activity of electrocatalyst nanoparticles via the strong metal-support interaction (SMSI) is possible. This technique also provides an exptl. platform for improving our understanding of nanoparticles at the at. scale.
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      Wang, Y.; Zhang, W. Mapping the Strain Distribution within Embedded Nanoparticles via Geometrical Phase Analysis. Micron 2019, 125, 102715,  DOI: 10.1016/j.micron.2019.102715
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      Mapping the strain distribution within embedded nanoparticles via geometrical phase analysis
      Wang, Yan; Zhang, Wei
      Micron (2019), 125 (), 102715CODEN: MCONEN; ISSN:0968-4328. (Elsevier Ltd.)
      Strain variation within a nanoparticle plays a crucial role in tuning its properties. Geometrical phase anal. (GPA) is typically a powerful tool to investigate the strain in high-resoln. transmission electron microscopy (HRTEM) images. It is known that the traditional GPA method measuring the displacement of lattice fringes directly in an HRTEM image is inapplicable to strain measurements on nanoparticles embedded in a matrix, where lattice fringes of nanoparticles are invisible, instead Moire´ fringes are present. Furthermore, considering the small size of embedded nanoparticles, generally a few nanometers, no ref. region can be chosen and utilized to calc. the relative displacement by GPA. Hence advanced methods need to be developed to break through the barriers of invisible lattice fringes and lack of a ref. region. In this work, using α-Fe nanoparticles embedded in sapphire as a test object, we illustrate a GPA method dedicated to embedded nanoparticles. Both the Fourier filter method and the inverse Moire´ fringes method were used to reconstruct the invisible lattice fringes of α-Fe nanoparticles. Then a computer-generated image corresponding to an unstrained α-Fe lattice was used as the ref. during GPA. The GPA results indicate that there exists a compressive strain in the range of 1.5∼2% within the α-Fe nanoparticles. Our work presents an effective approach to revealing the strain distributions within embedded nanoparticles.
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    55. 55
      Ahmad, N.; Le Bouar, Y.; Ricolleau, C.; Alloyeau, D. Growth of Dendritic Nanostructures by Liquid-Cell Transmission Electron Microscopy: A Reflection of the Electron-Irradiation History. Adv. Struct. Chem. Imaging 2016, 2, 9,  DOI: 10.1186/s40679-016-0023-0
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      Growth of dendritic nanostructures by liquid-cell transmission electron microscopy: a reflection of the electron-irradiation history
      Ahmad, Nabeel; Le Bouar, Yann; Ricolleau, Christian; Alloyeau, Damien
      Advanced Structural and Chemical Imaging (2016), 2 (), 9/1-9/10CODEN: ASCICS; ISSN:2198-0926. (Springer International Publishing AG)
      Studying dynamical processes by transmission electron microscopy (TEM) requires considering the electron-irradn. history, including the instantaneous dose rate and the cumulative dose delivered to the sample. Here, we have exploited liq.-cell TEM to study the effects of the electron-irradn. history on the radiochem. growth of dendritic Au nanostructures. Besides the well-established direct link between the dose rate and the growth rate of the nanostructures, we demonstrate that the cumulative dose in the irradiated area can also induce important transitions in the growth mode of the nanostructures. By comparing in situ observations with an extended diffusion-limited aggregation model, we reveal how the shape of the nanostructures is severely affected by the local lack of metal precursors and the resulting restricted accessibility of gold atoms to the nanostructures. This study highlights the effects of electron irradn. on the soln. chem. in the irradiated area and in the whole liq. cell that are of primary importance to ext. quant. information on nanoscale processes.
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      Kelly, T. F.; Miller, M. K. Atom Probe Tomography. Rev. Sci. Instrum. 2007, 78, 031101,  DOI: 10.1063/1.2709758
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      Invited review article: atom probe tomography
      Kelly, Thomas F.; Miller, Michael K.
      Review of Scientific Instruments (2007), 78 (3), 031101/1-031101/20CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      A review. The technique of atom probe tomog. (APT) is reviewed with an emphasis on illustrating what is possible with the technique both now and in the future. APT delivers the highest spatial resoln. (sub-0.3-nm) three-dimensional compositional information of any microscopy technique. Recently, APT has changed dramatically with new hardware configurations that greatly simplify the technique and improve the rate of data acquisition. New methods were developed to fabricate suitable specimens from new classes of materials. Applications of APT have expanded from structural metals and alloys to thin multilayer films on planar substrates, dielec. films, semiconducting structures and devices, and ceramic materials. This trend toward a broader range of materials and applications is likely to continue.
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      Gault, B.; Breen, A. J.; Chang, Y.; He, J.; Jägle, E. A.; Kontis, P.; Kürnsteiner, P.; Kwiatkowski Da Silva, A.; Makineni, S. K.; Mouton, I.; Peng, Z.; Ponge, D.; Schwarz, T.; Stephenson, L. T.; Szczepaniak, A.; Zhao, H.; Raabe, D. Interfaces and Defect Composition at the Near-Atomic Scale through Atom Probe Tomography Investigations. J. Mater. Res. 2018, 33 (23), 4018– 4030,  DOI: 10.1557/jmr.2018.375
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      Interfaces and defect composition at the near-atomic scale through atom probe tomography investigations
      Gault, Baptiste; Breen, Andrew J.; Chang, Yanhong; He, Junyang; Jaegle, Eric A.; Kontis, Paraskevas; Kuernsteiner, Philipp; Kwiatkowski da Silva, Alisson; Makineni, Surendra Kumar; Mouton, Isabelle; Peng, Zirong; Ponge, Dirk; Schwarz, Torsten; Stephenson, Leigh T.; Szczepaniak, Agnieszka; Zhao, Huan; Raabe, Dierk
      Journal of Materials Research (2018), 33 (23), 4018-4030CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
      Atom probe tomog. (APT) is rising in influence across many parts of materials science and engineering thanks to its unique combination of highly sensitive compn. measurement and three-dimensional microstructural characterization. In this invited article, we have selected a few recent applications that showcase the unique capacity of APT to measure the local compn. at structural defects. Whether we consider dislocations, stacking faults, or grain boundary, the detailed compositional measurements tend to indicate specific partitioning behaviors for the different solutes in both complex engineering and model alloys we investigated.
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      Li, T.; Bagot, P. A. J.; Marquis, E. A.; Tsang, S. C. E.; Smith, G. D. W. Characterization of Oxidation and Reduction of Pt-Ru and Pt-Rh-Ru Alloys by Atom Probe Tomography and Comparison with Pt-Rh. J. Phys. Chem. C 2012, 116 (33), 17633– 17640,  DOI: 10.1021/jp304359m
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      Characterization of Oxidation and Reduction of Pt-Ru and Pt-Rh-Ru Alloys by Atom Probe Tomography and Comparison with Pt-Rh
      Li, Tong; Bagot, Paul A. J.; Marquis, Emmanuelle A.; Tsang, S. C. Edman; Smith, George D. W.
      Journal of Physical Chemistry C (2012), 116 (33), 17633-17640CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Pt-based alloys contg. Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temp. fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidn. of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidn. and oxidn./redn. behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomog. The surface of the Pt-Ru alloy is enriched with Ru after oxidn. at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temps., leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidn. initiates from the grain boundaries, forming an oxide with a stoichiometry of MO2. As the oxidn. time increases, this oxide evolves into a two-phase nanostructure, involving a Rh-rich oxide phase (Rh, Ru)2O3 and a Ru-rich oxide phase (Ru, Rh)O2. When this two-phase oxide is reduced in hydrogen at low temps., sep. Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts.
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      Li, T.; Kasian, O.; Cherevko, S.; Zhang, S.; Geiger, S.; Scheu, C.; Felfer, P.; Raabe, D.; Gault, B.; Mayrhofer, K. J. J. Atomic-Scale Insights into Surface Species of Electrocatalysts in Three Dimensions. Nat. Catal. 2018, 1 (4), 300– 305,  DOI: 10.1038/s41929-018-0043-3
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      Wilde, P.; Dieckhöfer, S.; Quast, T.; Xiang, W.; Bhatt, A.; Chen, Y. T.; Seisel, S.; Barwe, S.; Andronescu, C.; Li, T.; Schuhmann, W.; Masa, J. Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiOx Core-Shell Heterostructures for the Oxygen Evolution Reaction. ACS Appl. Energy Mater. 2020, 3 (3), 2304– 2309,  DOI: 10.1021/acsaem.9b02481
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      Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiOx Core-Shell Heterostructures for the Oxygen Evolution Reaction
      Wilde, Patrick; Dieckhoefer, Stefan; Quast, Thomas; Xiang, Weikai; Bhatt, Anjali; Chen, Yen-Ting; Seisel, Sabine; Barwe, Stefan; Andronescu, Corina; Li, Tong; Schuhmann, Wolfgang; Masa, Justus
      ACS Applied Energy Materials (2020), 3 (3), 2304-2309CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      NiyP emerged as a highly active precatalyst for the alk. oxygen evolution reaction where structural changes play a crucial role for its catalytic performance. We probed the chem. stability of NiyP in 1 M KOH at 80°C and examd. how exposure up to 168 h affects its structure and catalytic performance. We obsd. selective P-leaching and formation of NiyP/NiOx core-shell heterostructures, where shell thickness increases with ageing time, which is detrimental for the activity. By tuning the particle size, we demonstrate that prevention of complete catalyst oxidn. is essential to preserve the outstanding electrochem. performance of NiyP in alk. media.
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      Barroo, C.; Akey, A. J.; Bell, D. C. Atom Probe Tomography for Catalysis Applications: A Review. Appl. Sci. 2019, 9 (13), 2721,  DOI: 10.3390/app9132721
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      Atom probe tomography for catalysis applications: a review
      Barroo, Cedric; Akey, Austin J.; Bell, David C.
      Applied Sciences (2019), 9 (13), 2721CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
      Atom probe tomog. is a well-established anal. instrument for imaging the 3D structure and compn. of materials with high mass resoln., sub-nanometer spatial resoln. and ppm elemental sensitivity. Thanks to recent hardware developments in Atom Probe Tomog. (APT), combined with progress on site-specific focused ion beam (FIB)-based sample prepn. methods and improved data treatment software, complex materials can now be routinely investigated. From model samples to complex, usable porous structures, there is currently a growing interest in the anal. of catalytic materials. APT is able to probe the end state of at.-scale processes, providing information needed to improve the synthesis of catalysts and to unravel structure/compn./reactivity relationships. This review focuses on the study of catalytic materials with increasing complexity (tip-sample, unsupported and supported nanoparticles, powders, self-supported catalysts and zeolites), as well as sample prepn. methods developed to obtain suitable specimens for APT expts.
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      Pfeiffer, B.; Erichsen, T.; Epler, E.; Volkert, C. A.; Trompenaars, P.; Nowak, C. Characterization of Nanoporous Materials with Atom Probe Tomography. Microsc. Microanal. 2015, 21 (3), 557– 563,  DOI: 10.1017/S1431927615000501
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      Characterization of Nanoporous Materials with Atom Probe Tomography
      Pfeiffer, Bjoern; Erichsen, Torben; Epler, Eike; Volkert, Cynthia A.; Trompenaars, Piet; Nowak, Carsten
      Microscopy and Microanalysis (2015), 21 (3), 557-563CODEN: MIMIF7; ISSN:1431-9276. (Cambridge University Press)
      A method to characterize open-cell nanoporous materials with atom probe tomog. (APT) has been developed. For this, open-cell nanoporous gold with pore diams. of around 50 nm was used as a model system, and filled by electron beam-induced deposition (EBID) to obtain a compact material. Two different EBID precursors were successfully tested-dicobalt octacarbonyl [Co2(CO)8] and diiron nonacarbonyl [Fe2(CO)9]. Penetration and filling depth are sufficient for focused ion beam-based APT sample prepn. With this approach, stable APT anal. of the nanoporous material can be performed. Reconstruction reveals the compn. of the deposited precursor and the nanoporous material, as well as chem. information of the interfaces between them. Thus, it is shown that, using an appropriate EBID process, local chem. information in three dimensions with sub-nanometer resoln. can be obtained from nanoporous materials using APT.
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      Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Hydrogen Production from Formic Acid Decomposition at Room Temperature Using a Ag-Pd Core-Shell Nanocatalyst. Nat. Nanotechnol. 2011, 6 (5), 302– 307,  DOI: 10.1038/nnano.2011.42
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      Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst
      Tedsree, Karaked; Li, Tong; Jones, Simon; Chan, Chun Wong Aaron; Yu, Kai Man Kerry; Bagot, Paul A. J.; Marquis, Emmanuelle A.; Smith, George D. W.; Tsang, Shik Chi Edman
      Nature Nanotechnology (2011), 6 (5), 302-307CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Formic acid has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy d., is nontoxic and can be safely handled in aq. soln. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen prodn. from formic acid at room temp. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the prodn. of H2 from formic acid at ambient temp. Atom probe tomog. confirmed that the nanoparticles have a core-shell configuration, with the shell contg. between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions.
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      Abbey, B. From Grain Boundaries to Single Defects: A Review of Coherent Methods for Materials Imaging in the X-Ray Sciences. JOM 2013, 65 (9), 1183– 1201,  DOI: 10.1007/s11837-013-0702-4
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      From Grain Boundaries to Single Defects: A Review of Coherent Methods for Materials Imaging in the X-ray Sciences
      Abbey, Brian
      JOM (2013), 65 (9), 1183-1201CODEN: JOMMER; ISSN:1047-4838. (Springer)
      This review summarizes the literature describing recent advances in the coherent x-ray sciences for the high-resoln. characterization of materials. The principles and some of the main exptl. techniques as well as their applications are discussed. The advantages of x-ray methods for characterizing 3D microstructures as well as for characterizing plasticity in the bulk become clear from the examples presented. Materials that exhibit size effects within the 0.1-10-μm range benefit enormously from these techniques, and development of the relevant x-ray methods will add to our fundamental understanding of these phenomena. Many of the ideas that have developed in the coherent x-ray science literature have been enabled through advances in x-ray source and detection technol., which has occurred over the past 10 years or so. It is a topic of considerable importance to consider how these techniques, which have matured rapidly, may be best applied to materials imaging in order to meet the growing needs of the community. As coherent x-ray methods for characterizing materials at multiple length scales have developed, several key applications for these techniques have emerged. The key breakthroughs that have been enabled by these new methods are discussed throughout this review, together with an examn. of some of the problems that will be addressed by these techniques within the next few years.
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      Ulvestad, A.; Tripathi, A.; Hruszkewycz, S. O.; Cha, W.; Wild, S. M.; Stephenson, G. B.; Fuoss, P. H. Coherent Diffractive Imaging of Time-Evolving Samples with Improved Temporal Resolution. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93 (18), 184105,  DOI: 10.1103/PhysRevB.93.184105
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      Carnis, J.; Gao, L.; Labat, S.; Kim, Y. Y.; Hofmann, J. P.; Leake, S. J.; Schülli, T. U.; Hensen, E. J. M.; Thomas, O.; Richard, M.-I. Towards a Quantitative Determination of Strain in Bragg Coherent X-Ray Diffraction Imaging: Artefacts and Sign Convention in Reconstructions. Sci. Rep. 2019, 9, 17357,  DOI: 10.1038/s41598-019-53774-2
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      Towards a quantitative determination of strain in Bragg Coherent X-ray Diffraction Imaging: artefacts and sign convention in reconstructions
      Carnis Jerome; Labat Stephane; Thomas Olivier; Richard Marie-Ingrid; Carnis Jerome; Leake Steven J; Schulli Tobias U; Richard Marie-Ingrid; Gao Lu; Hofmann Jan P; Hensen Emiel J M; Kim Young Yong
      Scientific reports (2019), 9 (1), 17357 ISSN:.
      Bragg coherent X-ray diffraction imaging (BCDI) has emerged as a powerful technique to image the local displacement field and strain in nanocrystals, in three dimensions with nanometric spatial resolution. However, BCDI relies on both dataset collection and phase retrieval algorithms that can induce artefacts in the reconstruction. Phase retrieval algorithms are based on the fast Fourier transform (FFT). We demonstrate how to calculate the displacement field inside a nanocrystal from its reconstructed phase depending on the mathematical convention used for the FFT. We use numerical simulations to quantify the influence of experimentally unavoidable detector deficiencies such as blind areas or limited dynamic range as well as post-processing filtering on the reconstruction. We also propose a criterion for the isosurface determination of the object, based on the histogram of the reconstructed modulus. Finally, we study the capability of the phasing algorithm to quantitatively retrieve the surface strain (i.e., the strain of the surface voxels). This work emphasizes many aspects that have been neglected so far in BCDI, which need to be understood for a quantitative analysis of displacement and strain based on this technique. It concludes with the optimization of experimental parameters to improve throughput and to establish BCDI as a reliable 3D nano-imaging technique.
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      Ulvestad, U.; Singer, A.; Clark, J. N.; Cho, H. M.; Kim, J. W.; Harder, R.; Maser, J.; Meng, Y. S.; Shpyrko, O. G. Topological Defect Dynamics in Operando Battery Nanoparticles. Science 2015, 348 (6241), 1344– 1347,  DOI: 10.1126/science.aaa1313
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      Topological defect dynamics in operando battery nanoparticles
      Ulvestad, A.; Singer, A.; Clark, J. N.; Cho, H. M.; Kim, J. W.; Harder, R.; Maser, J.; Meng, Y. S.; Shpyrko, O. G.
      Science (Washington, DC, United States) (2015), 348 (6241), 1344-1347CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Topol. defects can markedly alter nanomaterial properties. This presents opportunities for defect engineering, where desired functionalities are generated through defect manipulation. However, imaging defects in working devices with nanoscale resoln. remains elusive. The authors report 3-dimensional imaging of dislocation dynamics in individual battery cathode nanoparticles under operando conditions using Bragg coherent diffractive imaging. Dislocations are static at room temp. and mobile during charge transport. During the structural phase transformation, the Li-rich phase nucleates near the dislocation and spreads inhomogeneously. The dislocation field is a local probe of elastic properties, and a region of the material exhibits a neg. Poisson's ratio at high voltage. Operando dislocation imaging thus opens a powerful avenue for facilitating improvement and rational design of nanostructured materials.
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    68. 68
      Abuin, M.; Kim, Y. Y.; Runge, H.; Kulkarni, S.; Maier, S.; Dzhigaev, D.; Lazarev, S.; Gelisio, L.; Seitz, C.; Richard, M.-I.; Zhou, T.; Vonk, V.; Keller, T. F.; Vartanyants, I. A.; Stierle, A. Coherent X-Ray Imaging of CO-Adsorption-Induced Structural Changes in Pt Nanoparticles: Implications for Catalysis. ACS Appl. Nano Mater. 2019, 2 (8), 4818– 4824,  DOI: 10.1021/acsanm.9b00764
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      Coherent X-ray Imaging of CO-Adsorption-Induced Structural Changes in Pt Nanoparticles: Implications for Catalysis
      Abuin, Manuel; Kim, Young Yong; Runge, Henning; Kulkarni, Satishkumar; Maier, Simon; Dzhigaev, Dmitry; Lazarev, Sergey; Gelisio, Luca; Seitz, Christoph; Richard, Marie-Ingrid; Zhou, Tao; Vonk, Vedran; Keller, Thomas F.; Vartanyants, Ivan A.; Stierle, Andreas
      ACS Applied Nano Materials (2019), 2 (8), 4818-4824CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      Using coherent X-ray diffraction imaging (CXDI) as an in situ tool, we detd. the shape and strain state of a platinum nanoparticle with ≈160 nm diam. supported by a strontium titanate substrate. The expt. was performed at a temp. of 400 K under continuous gas flow conditions of pure Ar and Ar/CO mixts. The nanoparticle was preselected by SEM (SEM) and postanalyzed by at. force microscopy (AFM). We obtain a very good agreement between the overall nanoparticle size, shape, and defect structure as detd. by CXDI and AFM. In addn., we compare the strain state in the nanoparticle near surface region and its bulk: For pure Ar flow, we find a slight compressive strain in the nanoparticle bulk compared to an expansion in the near surface region. We ascribe the latter to the presence of high index vicinal surfaces. Our anal. suggests that under mixed Ar/CO flow at 400 K reshaping of the nanoparticle occurred. New high index facets developed, leading to a stronger lattice expansion, also propagating into the nanoparticle bulk. Our high-resoln. expts. pave the way for future CXDI expts. under operando catalytic reaction conditions.
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      Kim, D.; Chung, M.; Kim, S.; Yun, K.; Cha, W.; Harder, R.; Kim, H. Defect Dynamics at a Single Pt Nanoparticle during Catalytic Oxidation. Nano Lett. 2019, 19 (8), 5044– 5052,  DOI: 10.1021/acs.nanolett.9b01332
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      Defect Dynamics at a Single Pt Nanoparticle during Catalytic Oxidation
      Kim, Dongjin; Chung, Myungwoo; Kim, Sungwon; Yun, Kyuseok; Cha, Wonsuk; Harder, Ross; Kim, Hyunjung
      Nano Letters (2019), 19 (8), 5044-5052CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Defects can affect all aspects of a material by altering its electronic properties and controlling its chem. reactivity. At defect sites, preferential adsorption of reactants and/or formation of chem. species at active sites are obsd. in heterogeneous catalysis. Understanding the structural response at defect sites during catalytic reactions provides a unique opportunity to exploit defect control of nanoparticle-based catalysts. However, it remains difficult to characterize the strain and defect evolution for a single nanocrystal catalyst in situ. Here, we report Bragg coherent X-ray diffraction imaging of defect dynamics in an individual Pt nanoparticle during catalytic methane oxidn. We obsd. that the initially tensile strained regions of the crystal became seed points for the development of further strain and subsequent disappearance of diffraction d. during oxidn. reactions. Our detailed understanding of the catalytically induced deformation at the defect sites and obsd. reversibility during the relevant steps of the catalytic oxidn. process provide important insights of defect control and engineering of heterogeneous catalysts.
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      Sun, Y.; Liang, Y.; Luo, M.; Lv, F.; Qin, Y.; Wang, L.; Xu, C.; Fu, E.; Guo, S. Defects and Interfaces on PtPb Nanoplates Boost Fuel Cell Electrocatalysis. Small 2018, 14 (3), 1702259,  DOI: 10.1002/smll.201702259
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      Fichtner, J.; Watzele, S.; Garlyyev, B.; Kluge, R. M.; Haimerl, F.; El-Sayed, H. A.; Li, W. J.; Maillard, F. M.; Dubau, L.; Chattot, R.; Michalička, J.; MacAk, J. M.; Wang, W.; Wang, D.; Gigl, T.; Hugenschmidt, C.; Bandarenka, A. S. Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach. ACS Catal. 2020, 10 (5), 3131– 3142,  DOI: 10.1021/acscatal.9b04974
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      Tailoring the Oxygen Reduction Activity of Pt Nanoparticles through Surface Defects: A Simple Top-Down Approach
      Fichtner, Johannes; Watzele, Sebastian; Garlyyev, Batyr; Kluge, Regina M.; Haimerl, Felix; El-Sayed, Hany A.; Li, Wei-Jin; Maillard, Frederic M.; Dubau, Laetitia; Chattot, Raphael; Michalicka, Jan; Macak, Jan M.; Wang, Wu; Wang, Di; Gigl, Thomas; Hugenschmidt, Christoph; Bandarenka, Aliaksandr S.
      ACS Catalysis (2020), 10 (5), 3131-3142CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Results from Pt model catalyst surfaces have demonstrated that surface defects, in particular surface concavities, can improve the oxygen redn. reaction (ORR) kinetics. It is, however, a challenging task to synthesize nanostructured catalysts with such defective surfaces. Hence, we present a one-step and upscalable top-down approach to produce a Pt/C catalyst (with ∼3 nm Pt nanoparticle diam.). Using high-resoln. transmission electron microscopy and tomog., electrochem. techniques, high-energy X-ray measurements, and positron annihilation spectroscopy, we provide evidence of a high d. of surface defects (including surface concavities). The ORR activity of the developed catalyst exceeds that of a com. Pt/C catalyst, at least 2.7 times in terms of specific activity (∼1.62 mA/cm2Pt at 0.9 V vs the reversible hydrogen electrode) and at least 1.7 times in terms of mass activity (∼712 mA/mgPt), which can be correlated to the enhanced amt. of surface defects. In addn., the technique used here reduces the complexity of the synthesis (and therefore prodn. costs) in comparison to state of the art bottom-up techniques.
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      Dou, S.; Tao, L.; Wang, R.; El Hankari, S.; Chen, R.; Wang, S. Plasma-Assisted Synthesis and Surface Modification of Electrode Materials for Renewable Energy. Adv. Mater. 2018, 30 (21), 1705850,  DOI: 10.1002/adma.201705850
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      Ruge, M.; Drnec, J.; Rahn, B.; Reikowski, F.; Harrington, D. A.; Carlà, F.; Felici, R.; Stettner, J.; Magnussen, O. M. Structural Reorganization of Pt(111) Electrodes by Electrochemical Oxidation and Reduction. J. Am. Chem. Soc. 2017, 139 (12), 4532– 4539,  DOI: 10.1021/jacs.7b01039
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      Structural Reorganization of Pt(111) Electrodes by Electrochemical Oxidation and Reduction
      Ruge, Martin; Drnec, Jakub; Rahn, Bjoern; Reikowski, Finn; Harrington, David A.; Carla, Francesco; Felici, Roberto; Stettner, Jochim; Magnussen, Olaf M.
      Journal of the American Chemical Society (2017), 139 (12), 4532-4539CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The surface restructuring of Pt(111) electrodes upon electrochem. oxidn./redn. in 0.1M HClO4 was studied by in situ grazing-incidence small-angle x-ray scattering and complementary scanning tunneling microscopy measurements. These methods allow quant. detn. of the formation and structural evolution of nanoscale Pt islands during potential cycles into the oxidn. region. A characteristic ripening behavior is obsd., where these islands become more prominent and homogeneous in size with increasing no. of cycles. Their characteristic lateral dimensions primarily depend on the upper potential limit of the cycle and only slightly increase with cycle no. The structural evolution of the Pt surface morphol. strongly resembles that found in studies of Pt(111) homoepitaxial growth and ion erosion in ultrahigh vacuum. It can be fully explained by a microscopic model based on the known surface dynamic behavior under vacuum conditions, indicating that the same dynamics also describe the structural evolution of Pt in the electrochem. environment.
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      Jacobse, L.; Huang, Y. F.; Koper, M. T. M.; Rost, M. J. Correlation of Surface Site Formation to Nanoisland Growth in the Electrochemical Roughening of Pt(111). Nat. Mater. 2018, 17 (3), 277– 282,  DOI: 10.1038/s41563-017-0015-z
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      Correlation of surface site formation to nanoisland growth in the electrochemical roughening of Pt(111)
      Jacobse, Leon; Huang, Yi-Fan; Koper, Marc T. M.; Rost, Marcel J.
      Nature Materials (2018), 17 (3), 277-282CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Pt plays a central role in a variety of electrochem. devices and its practical use depends on preventing electrode degrdn.; however, understanding the underlying at. processes under repeated oxidn. and redn. conditions which induce irreversible surface structure changes, is challenging. This work assessed the correlation between the evolution of the Pt(111) electrochem. signal and its surface roughening by simultaneously performing cyclic voltammetry and in-situ electrochem. scanning tunneling microscopy (EC-STM). A nucleation and early growth regime of nano-island formation, and a late growth regime following island coalescence which continues up to at least 170 cycles, were identified. A correlation anal. showed that each created step site in the late growth regime contributes equally, strongly to electrochem. and roughness evolutions. In the nucleation and early growth regime, created step sites contribute to roughness but not to the electrochem. signal.
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      Jacobse, L.; Rost, M. J.; Koper, M. T. M. Atomic-Scale Identification of the Electrochemical Roughening of Platinum. ACS Cent. Sci. 2019, 5 (12), 1920– 1928,  DOI: 10.1021/acscentsci.9b00782
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      Atomic-Scale Identification of the Electrochemical Roughening of Platinum
      Jacobse, Leon; Rost, Marcel J.; Koper, Marc T. M.
      ACS Central Science (2019), 5 (12), 1920-1928CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Electrode degrdn. under oxidizing conditions is a major drawback for large scale applications of Pt electrocatalysts. Subjecting Pt(111) to oxidn.-redn. cycles is known to lead to the growth of nanoislands. The authors study this phenomenon using a combination of simultaneous in situ electrochem. scanning tunneling microscopy and cyclic voltammetry. Here, the authors present a detailed anal. of the formed islands, deriving the (evolution of the) av. island growth shape. From the island shapes, the authors det. the densities of at.-scale defect sites, e.g. steps and facets, which show an excellent correlation with the different voltammetric H adsorption peaks. Based on this combination of EC-STM and CV data, the authors derive a detailed atomistic picture of nanoisland evolution during potential cycling, delivering new insights into the initial stages of Pt electrode degrdn. EC-STM images and av. nanoisland structures: (A) parts of EC-STM images after 0, 4, 8, 15, 28, 50, and 170 ORCs (from left to right) illustrating the roughening of a single surface area. The original, individual images are 230 × 230 nm2, recorded with Utip = 0.45 V, Usample = 0.4 V, and Itunneling < 300 pA. Note that the total contrast is the same for the different images. The full set of EC-STM images is available in ref. (B); the av. at.-scale growth structure of the islands as detd. from the EC-STM images for the same situations as shown in (A). The different colors indicate the different at. layers in the island. The full image sequence is provided in the Supporting Information. Unit cells and time evolution of defect site densities derived from av. nanoisland structures: (A-C) Representative unit cells for the {111}-, {112}-, and {100}-side steps/facets, resp. The structures range from sepd. steps (top) to low index facets (bottom). All counting rules and unit cells for other possible structures, as well as all individual site densities are provided in the Supporting Information. (D) Site densities for the sepd. steps & wide facets (top), and narrow facets & low index facets (bottom) for both the {111}- and {100}-side (solid and dashed lines, resp.). Detailed anal. of in situ EC-STM images and cyclic voltammetry data resolves the evolution of surface site densities during the roughening of Pt(111) by potential cycling.
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      Livet, F.; Beutier, G.; De Boissieu, M.; Ravy, S.; Picca, F.; Le Bolloc’h, D.; Jacques, V. Coherent Scattering from Silicon Monocrystal Surface. Surf. Sci. 2011, 605, 390– 395,  DOI: 10.1016/j.susc.2010.11.006
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      Coherent scattering from silicon monocrystal surface
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      Surface Science (2011), 605 (3-4), 390-395CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      Using coherent x-ray scattering, at. step roughness was evidenced at the (111) vicinal surface of a Si single crystal of 0.05° miscut. Close to the 1/2 1/2 1/2 anti-Bragg position of the reciprocal space which is particularly sensitive to the (111) surface, the truncation rod exhibits a contrasted speckle pattern that merges into a single peak closer to the 111 Bragg peak of the bulk. The elongated shape of the speckles along the (111) direction confirms the monoat. step sensitivity of the technique. This expt. opens the way towards studies of step dynamics on cryst. surfaces.
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      Nicolas, J. D.; Reusch, T.; Osterhoff, M.; Sprung, M.; Schülein, F. J. R.; Krenner, H. J.; Wixforth, A.; Salditt, T. Time-Resolved Coherent X-Ray Diffraction Imaging of Surface Acoustic Waves. J. Appl. Crystallogr. 2014, 47 (5), 1596– 1605,  DOI: 10.1107/S1600576714016896
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      Time-resolved coherent X-ray diffraction imaging of surface acoustic waves
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      Journal of Applied Crystallography (2014), 47 (5), 1596-1605CODEN: JACGAR; ISSN:1600-5767. (International Union of Crystallography)
      Time-resolved coherent X-ray diffraction expts. of standing surface acoustic waves, illuminated under grazing incidence by a nanofocused synchrotron beam, are reported. The data have been recorded in stroboscopic mode at controlled and varied phase between the acoustic frequency generator and the synchrotron bunch train. At each time delay (phase angle), the coherent far-field diffraction pattern in the small-angle regime is inverted by an iterative algorithm to yield the local instantaneous surface height profile along the optical axis. The results show that periodic nanoscale dynamics can be imaged at high temporal resoln. in the range of 50 ps (pulse length).
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      Kutsal, M.; Bernard, P.; Berruyer, G.; Cook, P. K.; Hino, R. The ESRF Dark-Field x-Ray Microscope at ID06. IOP Conf. Ser.: Mater. Sci. Eng. 2019, 580, 012007,  DOI: 10.1088/1757-899X/580/1/012007
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      The ESRF dark-field x-ray microscope at ID06
      Kutsal, M.; Bernard, P.; Berruyer, G.; Cook, P. K.; Hino, R.; Jakobsen, A. C.; Ludwig, W.; Ormstrup, J.; Roth, T.; Simons, H.; Smets, K.; Sierra, J. X.; Wade, J.; Wattecamps, P.; Yildirim, C.; Poulsen, H. F.; Detlefs, C.
      IOP Conference Series: Materials Science and Engineering (2019), 580 (40th Riso International Symposium on Materials Science: Metal Microstructures in 2D, 3D and 4D, 2019), 12007CODEN: ICSMGW; ISSN:1757-899X. (IOP Publishing Ltd.)
      We present an instrument for dark-field x-ray microscopy installed on beamline ID06 of the ESRF - the first of its kind. Dark-field x-ray microscopy uses full field illumination of the sample and provides three-dimensional (3D) mapping of micro-structure and lattice strain in cryst. matter. It is analogous to dark-field electron microscopy in that an objective lens magnifies diffracting features of the sample. The use of high-energy synchrotron x-rays, however, means that these features can be large and deeply embedded. 3D movies can be acquired with a time resoln. of seconds to minutes. The field of view and spatial resoln. can be adapted by simple reconfiguration of the x-ray objective lens, reaching spatial and angular resoln. of 30-100 nm and 0.001°, resp. The instrument furthermore allows pre-characterization of samples at larger length scales using 3DXRD or DCT, such that a region of interest (e.g. a single grain) can be selected for high-resoln. studies without the need to dismount the sample. As examples of applications we show work on mapping the subgrains in plastically deformed iron and aluminum alloys, mapping domains and strain fields in ferroelec. crystals, and studies of biominerals. This ability to directly characterize complex, multi-scale phenomena in-situ is a key step towards formulating and validating multi-scale models that account for the entire heterogeneity of materials. As an outlook, we discuss future prospects for such multi-scale characterization by combining DFXM with 3DXRD/DCT, and coherent x-ray methods for coarser and finer length-scales, resp.
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      Morgan, A. J.; Prasciolu, M.; Andrejczuk, A.; Krzywinski, J.; Meents, A.; Pennicard, D.; Graafsma, H.; Barty, A.; Bean, R. J.; Barthelmess, M.; Oberthuer, D.; Yefanov, O.; Aquila, A.; Chapman, H. N.; Bajt, S. High Numerical Aperture Multilayer Laue Lenses. Sci. Rep. 2015, 5, 9892,  DOI: 10.1038/srep09892
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      Scientific Reports (2015), 5 (), 9892CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      The ever-increasing brightness of synchrotron radiation sources demands improved X-ray optics to utilize their capability for imaging and probing biol. cells, nanodevices, and functional matter on the nanometer scale with chem. sensitivity. Here we demonstrate focusing a hard X-ray beam to an 8 nm focus using a vol. zone plate (also referred to as a wedged multilayer Laue lens). This lens was constructed using a new deposition technique that enabled the independent control of the angle and thickness of diffracting layers to microradian and nanometer precision, resp. This ensured that the Bragg condition is satisfied at each point along the lens, leading to a high numerical aperture that is limited only by its extent. We developed a phase-shifting interferometric method based on ptychog. to characterize the lens focus. The precision of the fabrication and characterization demonstrated here provides the path to efficient X-ray optics for imaging at 1 nm resoln.
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      Pfisterer, J. H. K.; Liang, Y.; Schneider, O.; Bandarenka, A. S. Direct Instrumental Identification of Catalytically Active Surface Sites. Nature 2017, 549 (7670), 74– 77,  DOI: 10.1038/nature23661
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      Direct instrumental identification of catalytically active surface sites
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      The activity of heterogeneous catalysts-which are involved in some 80 per cent of processes in the chem. and energy industries-is detd. by the electronic structure of sp. surface sites that offer optimal binding of reaction intermediates. Directly identifying and monitoring these sites during a reaction should therefore provide insight that might aid the targeted development of heterogeneous catalysts and electrocatalysts (those that participate in electrochem. reactions) for practical applications. The invention of the scanning tunnelling microscope (STM) and the electrochem. STM promised to deliver such imaging capabilities, and both have indeed contributed greatly to our atomistic understanding of heterogeneous catalysis. But although the STM has been used to probe and initiate surface reactions, and has even enabled local measurements of reactivity in some systems, it is not generally thought to be suited to the direct identification of catalytically active surface sites under reaction conditions. Here we demonstrate, however, that common STMs can readily map the catalytic activity of surfaces with high spatial resoln.: we show that by monitoring relative changes in the tunnelling current noise, active sites can be distinguished in an almost quant. fashion according to their ability to catalyze the hydrogen-evolution reaction or the oxygen-redn. reaction. These data allow us to evaluate directly the importance and relative contribution to overall catalyst activity of different defects and sites at the boundaries between two materials. With its ability to deliver such information and its ready applicability to different systems, we anticipate that our method will aid the rational design of heterogeneous catalysts.
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      Sumetskii, M.; Kornyshev, A. A.; Stimming, U. Adatom Diffusion Characteristics from STM Noise: Theory. Surf. Sci. 1994, 307–309, 23– 27,  DOI: 10.1016/0039-6028(94)90364-6
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      Adatoms moving along the substrate surface under an STM tip cause a random time dependence of the current, J(t). The authors obtained simple expressions for the av. current 〈J(t)〉 and the temporary current correlation function 〈J(t+τ)J(τ)〉τ-〈J(t)〉2 through the diffusion coeffs. of the adatoms and a few parameters of the tunneling gap. The results offer a way to study the local diffusion of adatoms by treatment of exptl. measured J(t) dependencies.
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      Sumetskii, M.; Kornyshev, A. A. Noise in STM Due to Atoms Moving in the Tunneling Space. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48 (23), 17493– 17506,  DOI: 10.1103/PhysRevB.48.17493
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      Noise in STM due to atoms moving in the tunneling space
      Sumetskii; Kornyshev
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      There is no expanded citation for this reference.
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      The tunneling probability at the solid/liq. interface has been probed in real space at Au(111) surfaces using high resoln. in situ distance tunneling spectroscopy (DTS). The exponential decay of the tunneling current with the gap width is modulated with oscillations of a period of 0.35 nm. This period coincides with the theor. predicted spacing of the interfacial water layers at the charged solid/liq. interface.
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      Liang, Y.; McLaughlin, D.; Csoklich, C.; Schneider, O.; Bandarenka, A. S. The Nature of Active Centers Catalyzing Oxygen Electro-Reduction at Platinum Surfaces in Alkaline Media. Energy Environ. Sci. 2019, 12 (1), 351– 357,  DOI: 10.1039/C8EE03228A
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      The nature of active centers catalyzing oxygen electro-reduction at platinum surfaces in alkaline media
      Liang, Yunchang; McLaughlin, David; Csoklich, Christoph; Schneider, Oliver; Bandarenka, Aliaksandr S.
      Energy & Environmental Science (2019), 12 (1), 351-357CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Energy conversion devices that use OH-conducting electrolytes are becoming more and more attractive with recent substantial progress in the development and commercialization of relevant membrane materials and systems. However, the activities of numerous catalysts towards oxygen electro-redn. taking place at the cathodes of such devices in alk. media are surprisingly different from those in acidic electrolytes. This is for instance the case for Pt and Pt-alloy electrodes, which demonstrate unexpected and drastic variations in activity depending on the electrolyte pH and surface structure. Here we apply recently introduced electrochem. scanning tunnelling microscopy noise measurements to directly identify active centers at Pt(111)-based surfaces in three alk. electrolytes (LiOH, KOH and CsOH) under reaction conditions. For all three solns. it was found that the most active sites are located on the Pt(111) terraces, in contrast to the acidic media, where concave defects significantly increase the oxygen redn. activity. These defect-centers are to all practical purposes deactivated in alk. media. This not only explains the above-mentioned activity differences between acidic and alk. electrolytes but also suggests strategies to design nanostructured Pt-electrocatalysts for applications at high pH values.
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      Mitterreiter, E.; Liang, Y.; Golibrzuch, M.; McLaughlin, D.; Csoklich, C.; Bartl, J. D.; Holleitner, A.; Wurstbauer, U.; Bandarenka, A. S. In-Situ Visualization of Hydrogen Evolution Sites on Helium Ion Treated Molybdenum Dichalcogenides under Reaction Conditions. npj 2D Mater. Appl. 2019, 3, 25,  DOI: 10.1038/s41699-019-0107-5
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  • Supporting Information

    Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.0c02144.

    • Discussion on the extraction of the microstrain parameter from powder X-ray diffraction experiments with the Rietveld method; discussion on the correction of the as-measured microstrain to describe surface defectiveness; and a plot bridging the specific activity for the ORR and the QCO/2QH ratio (PDF)


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