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Elucidating the Role of Reduction Kinetics in the Phase-Controlled Growth on Preformed Nanocrystal Seeds: A Case Study of Ru
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  • Quynh N. Nguyen
    Quynh N. Nguyen
    School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
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    Orcidhttps://orcid.org/0000-0003-1544-6139
  • Eun Mi Kim
    Eun Mi Kim
    Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16803, United States
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  • Yong Ding
    Yong Ding
    School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
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  • Annemieke Janssen
    Annemieke Janssen
    School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
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    Orcidhttps://orcid.org/0000-0002-7721-597X
  • Chenxiao Wang
    Chenxiao Wang
    School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
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    Orcidhttps://orcid.org/0000-0001-7815-6062
  • Kei Kwan Li
    Kei Kwan Li
    School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
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  • Junseok Kim
    Junseok Kim
    Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16803, United States
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  • Kristen A. Fichthorn*
    Kristen A. Fichthorn
    Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16803, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-4256-714X
  • Younan Xia*
    Younan Xia
    School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
    The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-2431-7048
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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2024, 146, 17, 12040–12052
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https://doi.org/10.1021/jacs.4c01725
Published March 30, 2024

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Abstract

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This study demonstrates the crucial role of reduction kinetics in phase-controlled synthesis of noble-metal nanocrystals using Ru nanocrystals as a case study. We found that the reduction kinetics played a more important role than the templating effect from the preformed seed in dictating the crystal structure of the deposited overlayers despite their intertwined effects on successful epitaxial growth. By employing two different polyols, a series of Ru nanocrystals with tunable sizes of 3–7 nm and distinct patterns of crystal phase were synthesized by incorporating different types of Ru seeds. Notably, the use of ethylene glycol and triethylene glycol consistently resulted in the formation of Ru shell in natural hexagonal close-packed (hcp) and metastable face-centered cubic (fcc) phases, respectively, regardless of the size and phase of the seed. Quantitative measurements and theoretical calculations suggested that this trend was a manifestation of the different reduction kinetics associated with the precursor and the chosen polyol, which, in turn, affected the reduction pathway (solution versus surface) and packing sequence of the deposited Ru atoms. This work not only underscores the essential role of reduction kinetics in controlling the packing of atoms and thus the phase taken by Ru nanocrystals but also suggests a potential extension to other noble-metal systems.

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Introduction

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The properties of noble-metal nanocrystals for specific applications have been traditionally tuned by tailoring parameters such as size, shape, and internal structure. (1−4) Exploring the polymorphism of nanocrystals, specifically controlling their crystal structures or phases, represents a nascent research area with many mechanistic details yet to be elucidated, reminiscent of the early days of shape-controlled synthesis. (5−8) While shape control only involves surface atoms or a few internal atoms to generate twin boundaries and stacking faults, the modulation of crystal structure requires the rearrangement of essentially all atoms in order to form different phases. (7) In recent years, seed-mediated growth has been established as a simple and versatile method for achieving phase-controlled synthesis. (9−13) Under optimal conditions, the deposited shell can replicate both the crystal and surface structures of the preformed seed, leading to the formation of a metastable phase. With the ability to simultaneously maneuver thermodynamic and kinetic factors in a colloidal synthesis, such phase-controlled growth allows for good reproducibility between batches, tight controls over the size and shape, and a systematic examination of parameters responsible for phase evolution. (14,15) Although many studies have delved into the effects of overlayer thickness, template shape (surface structure), and size (bulk versus surface energy) on phase evolution, (16−20) creating nanocrystals in a metastable phase of more than six atomic layers in thickness via epitaxial growth still presents a major challenge. The explicit role(s) played by reaction kinetics in affecting the packing of atoms in the bulk is yet to be resolved.
Unlike other noble metals, such as Ag, Au, Pd, and Pt, research on the colloidal synthesis of Ru nanocrystals is relatively scarce. Most studies have focused on their thermodynamically stable hexagonal close-packed (hcp) phase and the metastable face-centered cubic (fcc) phase. (7) The difference between these phases lies in the arrangement of the third atomic layer (ABCABC for fcc versus ABABAB for hcp), a subtle variation that significantly influences catalytic performance by altering interactions between reaction intermediates and surface atoms. (16,20−23) As reported in the literature, various factors can impact the packing of Ru atoms. For the most robust polyol synthesis involving homogeneous nucleation, chemical selection was crucial for determining the crystal phase. The first report of fcc-Ru nanocrystals utilized polyol reduction with an appropriate pair of precursor and reductant. (24) Specifically, Ru nanocrystals derived from RuCl3 and ethylene glycol (EG) adopted the conventional hcp phase, while those from Ru(acac)3 and triethylene glycol (TEG) favored the fcc phase. It was hypothesized that the stabilization of Ru(III) ions by the acac– ligand decelerated the reduction kinetics, causing atypical atomic packing. (25) This trend was also attributed to the similarity in distance between the oxygen atoms in acac– and the Ru atoms on fcc facets, suggesting preferential ligand-fcc facet binding to facilitate their formation. However, one study also successfully prepared fcc-Ru nanocrystals in TEG and hcp-Ru nanocrystals in EG using either RuCl3 or Ru(acac)3 as the precursor. (26) This finding highlights the influence of the chemical species on phase control; yet, it is debatable whether this effect was actually exerted through the reduction kinetics. A recent study shed light on this issue by elucidating a quantitative correlation between the crystal phase of seeds formed during nucleation and the initial reduction rate of the precursor. (27) While the authors were able to obtain Ru nanocrystals with varying percentages of the metastable fcc phase, decoupling the effect of reaction kinetics from the nucleation process would lead to a more precise control over the crystal phase and thus enhance the purity of the products.
With regard to the seed-mediated growth, prior studies have mainly harnessed the templating effect from the seeds to produce Ru-based nanocrystals featuring fcc structure and well-controlled shapes. (16−20,28−30) It is still unclear if the reduction kinetics, as controlled by the type or concentration of the precursor and reducing agent, reaction temperature, and injection rate, also affect the nucleation and growth modes, ultimately determining the packing of Ru atoms. Some compelling evidence can be found in the deposition of Ru atoms on Pd nanocubes. (17,20) When the amount of Ru precursor injected into the reaction was increased, the deposition of Ru atoms was switched from layer-by-layer to layer-plus-island and further to island mode as a result of faster reduction kinetics and thus self-nucleation. In conjunction with the variation in growth mode, the packing of Ru atoms was changed accordingly from fcc, through a mix of fcc and hcp, to hcp. This observation accentuates how the reduction kinetics of the precursor, even in the presence of the templating effect from seeds, can drastically alter the packing of Ru atoms.
The intricate roles played by reaction kinetics in phase-selective epitaxial growth have also been manifested in other noble metals and II–IV semiconductor systems. For instance, utilizing fcc-Au nanospheres as seeds enabled the formation of metastable hcp-Au hexagonal stars when involving ethylenediaminetetraacetic acid to complex with Au(III) and 2-phospho-l-ascorbic acid trisodium salt (Asc-2P) as a reducing agent to maneuver the reduction kinetics. (31) The difference in atomic arrangement was linked to the interaction between the phosphate groups of Asc-2P and Au atoms. It should be pointed out that certain chemical species can not only affect the reduction kinetics but also bind to the metal surface to dictate atomic packing and thus the crystal phase of the resultant nanocrystals. (32−35) In semiconductor nanocrystals, cadmium phosphonate with a long hydrocarbon chain was found to exclusively promote the formation of wurtzite phase, irrespective of whether the initial CdSe seeds took zinc blend or wurtzite phase. (36) This result highlighted the predominant influence of the chemical environment on regulating the phase of CdSe nanocrystals. Nevertheless, no kinetic measurement was conducted, while the explicit mechanism of ligand-surface coordination remains elusive, necessitating additional studies for a conclusive interpretation. All of these findings collectively attest that a successful phase-controlled synthesis of metal nanocrystals may rely on the synergy arising from the templating effect and the chemical environment due to their specific impacts on reduction kinetics and ligand-surface interactions.
Intrigued by varied outcomes in prior syntheses, even with the use of seeds in desired phases, this study aims to discern which factor─the reduction kinetics or the templating effect from the seed, exerts a greater impact on dictating the crystal phase of the deposited overlayers. With Ru as a model system, we demonstrated a correlation between the phase and the initial reduction rate and revealed the explicit role played by reduction kinetics in phase-controlled growth on preformed seeds. By leveraging polyols with distinct reducing powers and Ru nanocrystals of varying phases as templates, a series of Ru nanocrystals with tunable sizes of 3–7 nm and different patterns of crystal phases were synthesized. Our findings suggest that the reduction kinetics played a more dominant role than the templating effect of the seeds in dictating the crystal phase of the deposited Ru, despite their intertwined effects on successful epitaxial growth. Specifically, EG consistently favored the formation of hcp-Ru overlayers, while TEG led to metastable fcc-Ru, regardless of the size and crystal phase of the preformed seeds. Based on the findings in Density Functional Theory (DFT) total energy calculations, this trend was attributed to the different reduction kinetics of the precursor in the respective polyol, influencing the reduction pathway (solution versus surface reduction) and thereby the packing of Ru atoms. Explicating the critical role of the chemical environment, this work highlights the intricate balance between the templating effect and reaction kinetics in generating the desired phase during nanocrystal growth.

Results and Discussion

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Selection and Rationale of a Model System

The selection of monometallic Ru nanocrystals as a model system in this study has several merits. First, Ru is one of the few noble metals that can be prepared as nanocrystals purely in hcp or fcc phase by altering the polyol employed in a one-pot synthesis (24,26) or by leveraging seeds with an intrinsic fcc structure. (28−30) These robust protocols offer a framework to fine-tune specific parameters to attain Ru nanocrystals in the specific phases, establishing a well-controlled platform to systematically probe the influence of reduction kinetics. Second, employing a monometallic system enables us to exclusively attribute the observed variations in phase to the reduction kinetics, avoiding complications introduced by the lattice mismatch intrinsic to a bimetallic system. As a factor inherently encoded in the seed, a large lattice mismatch can force the deposited shell to adopt the native phase and thus minimize the total surface energy, while the templating effect from the seed tends to prevail over other factors when the lattice mismatch is minimal. (9,37,38) For instance, an fcc-Ru shell could be generated in a synthesis based on EG and fcc-Pd nanocrystal seeds due to the prominent templating effect stemming from the small lattice mismatch between Pd and fcc-Ru (1.8%, 3.89 versus 3.82 Å), when coupled with the symmetry alignment across different facets for the balance of surface and bulk energies. (18−20) However, the templating effect in the Pd–Ru system would gradually vanish when the Ru shell was beyond ca. 5 atomic layers, (16,17) possibly due to the dominance of other effects (i.e., growth kinetics) in dictating phase evolution. Third, incorporating a second metal inevitably introduces impurities into the Ru shell due to interdiffusion, (16,29) potentially altering the fcc crystallization of Ru and thus overshadowing the effects from other factors or parameters.
Although many compounds have been documented as reducing agents and/or solvents for Ru nanocrystal synthesis, (21,39−41) we opted for polyols, specifically EG and TEG. They stand out among other candidates due to their dual functionalities in a colloidal synthesis, systematic variation in molecular structure, and different reduction potentials to enable distinct reduction kinetics. (42) Overall, all of the experiments in this study involved the reduction of Ru(acac)3 by EG or TEG at 180 °C in the presence of poly(vinylpyrrolidone) (PVP) as a colloidal stabilizer. No capping agent was added to ensure direct interactions between the polyol molecules and nanocrystal surface while avoiding its kinetic and thermodynamic alterations to the evolution of crystal phase. Figure 1 shows a summary of the two rounds of epitaxial overgrowth involved in our study, distinguished solely by the employment of EG or TEG to reduce the precursor.

Figure 1

Figure 1. Schematic illustration of four synthetic routes to the preparation of Ru nanocrystals with different patterns of phases by simply switching the polyol used in the overgrowth process.

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Synthesis and Characterizations of the hcp-Ru Seeds

The synthesis started with the preparation of 3.1 nm Ruhcp nanocrystals using a slightly modified protocol from the literature that involves the reduction of Ru(acac)3 by EG at 180 °C in the presence of PVP. (24) Figure S1A shows a typical transmission electron microscopy (TEM) image of the as-obtained nanocrystals, which had an average size of 3.1 ± 0.5 nm and a quasi-spherical shape. The phase of the product was characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) to ensure that any structural transformation or preservation in the next step(s) could be attributed to experimental conditions rather than the inherent characteristics of the seeds. The XRD pattern shows a broad peak at 2θ of 42.1° due to an overlap of the diffractions from (101̅0), (0002), and (101̅1) planes of hcp-Ru (Figure S1B). The overlapping XRD signal is largely attributed to the broadening of the three distinctive peaks of hcp-Ru in the region of 38–45° as a result of small crystallite size of the nanocrystals. (24,26,43) The HRTEM image from an individual Ru nanocrystal confirmed the characteristic lattice spacings of hcp-Ru (Figure S1C), which is in agreement with the XRD pattern. Meanwhile, the corresponding fast Fourier transform (FFT) pattern also supported an hcp structure viewed along the [12̅13̅] zone axis (Figure S1D, E). The as-obtained hcp-Ru nanocrystals (denoted as Ruhcp seeds thereafter) were directly used for the first round of overgrowth in EG without additional treatment. For subsequent procedures involving TEG, thorough washing with a mixture of acetone and ethanol was carried out to ensure the complete removal of residual EG and other impurities, mitigating the influence of undesired chemicals in the subsequent phase evolution.

Growth of Ru Overlayers on Ru Seeds in Different Polyols

The first round of overgrowth was initiated by introducing the Ru(acac)3 precursor solution into a suspension of the 3.1 nm Ruhcp seeds in EG or TEG to initiate the overgrowth of Ru. Depending on the experimental conditions, the deposition of Ru can proceed via two pathways in the presence of seeds: epitaxial layer-by-layer growth through heterogeneous nucleation and island/attachment growth through homogeneous (or self-) nucleation. (14,44) For the former, Ru atoms are generated via solution or surface reduction, followed by their heterogeneous nucleation and deposition on the seed to produce a smooth Ru shell. (45−47) Conversely, Ru atoms are produced in the solution, followed by homogeneous nucleation of small nuclei, which then aggregate or attach to the seed. (2) In this case, the final product usually comprises two distinct populations, one of which contains Ru particles with significantly different morphologies or much smaller sizes than the original seeds. Since the polyol has been found to influence the phase evolution of Ru during homogeneous nucleation, (24−26) the occurrence of both nucleation events would overshadow the templating effect, complicating the differentiation of the individual contribution from solvent versus template in a seed-mediated synthesis. To discern their synergic or predominant roles in inducing the observed changes in the crystal phase, it is vital to ensure the dominance of heterogeneous nucleation and subsequent layer-by-layer growth.
In principal, successful synthesis of nanocrystals in the phase of the template relies on the achievement of slow reduction kinetics and a faster atomic diffusion rate compared to the deposition rate to facilitate heterogeneous nucleation and layer-by-layer growth, respectively. (15,48) In contrast, both homogeneous nucleation and island growth mode usually favor the formation of the thermodynamically stable phase. (20,49) We examined various experimental parameters that could affect these kinetic factors, including the injection rate, reaction temperature, and speciation of the precursor. In the case of EG, we found that a relatively slow injection rate of 0.5 mL h–1, a high temperature of 180 °C, and the use of halide-free precursor offered an optimal combination of conditions to simultaneously achieve these goals. First, the precursor had to be introduced at a sufficiently slow rate to ensure a low and stable concentration of Ru atoms in the reaction mixture, thereby avoiding supersaturation and subsequent homogeneous nucleation. (48,50) With a reduced supply of atoms, a slow injection rate directly decelerated the atom deposition rate, promoting layer-by-layer growth. When the injection rate was carefully controlled at a relatively slow rate of 0.5 mL h–1 using a syringe pump, Ru nanocrystals with a well-retained quasi-spherical shape and enlarged sizes were obtained (Figure 2A). No second population of particles with a much smaller size was observed, confirming the dominance of heterogeneous nucleation and uniform growth on the surface of the 3.1 nm Ruhcp seeds to produce a smooth Ru shell. In comparison, If the injection rate was increased to 4 mL h–1, the surface of each Ru seed became very rough and was covered by a high density of small bumps, indicating the prevalence of homogeneous nucleation and island growth (Figure S2A). In this case, due to a higher concentration and thus faster reduction of the precursor, Ru atoms would self-nucleate in the solution and then aggregate into small particles, which subsequently attached to the surface of Ru seeds, driven by a desire to minimize the total surface energy.

Figure 2

Figure 2. Ru@Ru nanocrystals with an hcp or fcc phase in the shell obtained through the first round of overgrowth. (A, B) TEM images of the Ru@Ru nanocrystals synthesized from the Ruhcp seeds using (A) EG and (B) TEG, respectively. (C) XRD patterns of Ruhcp@Ruhcp and Ruhcp@Rufcc nanocrystals produced in EG and TEG, respectively, together with that of the Ruhcp seeds for comparison. The insets show the corresponding atomic model of the nanocrystals in a cross-sectional view. The red and blue lines correspond to the characteristic peaks of fcc-Ru (JCPDS No. 01–088–2333) and hcp-Ru (JCPDS No. 06–0663), respectively.

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The reaction temperature should also be taken into consideration since it affects not only the surface diffusion rate of adatoms but also the reduction kinetics, which will in turn influence the deposition rate of the Ru atoms. (14,20) When the reaction was conducted at a higher temperature of 200 °C, homogeneous nucleation was triggered as the concentration of the newly formed Ru atoms was elevated while island growth was initiated as a result of the fast deposition rate, producing Ru nanocrystals with a high density of branched arms due to the attachment of small particles (Figure S2B). In contrast, an optimal temperature of 180 °C led to products with uniform size and smooth shell since both deposition and diffusion rates of the newly generated Ru atoms were adequate (Figure 2A). Further reducing the temperature to 160 °C caused no significant change in size to the final products because of the weakened reducing power of EG at this temperature and, thus, insufficient reduction of the precursor to ensure an adequate supply of Ru atoms (Figure S2C). Regarding the type of precursor, Ru(acac)3 was used to produce Ru atoms while avoiding possible oxidative etching and preferential capping to fcc facets commonly associated with halide ions. (8,49,51) Moreover, the strong coordination of acac– to Ru(III) ions significantly slowed down the reduction kinetics to help promote heterogeneous nucleation for the epitaxial growth of Ru. (25) Indeed, when RuCl3 was used as a precursor, we obtained a polydisperse sample with the formation of tiny Ru particles in addition to a Ru shell due to the fast reduction rate and thus self-nucleation (Figure S2D). Taken together, all of the reaction parameters must be optimized to promote heterogeneous nucleation and layer-by-layer growth for the generation of a smooth, conformal shell while suppressing the formation of small Ru particles with phases determined solely by the polyol.
Figure 2A, B shows typical TEM images of the Ru nanocrystals obtained from the first round of overgrowth on the 3.1 nm Ruhcp seeds using the standard protocol based on EG and TEG, respectively. Both products exhibited a relatively uniform size distribution, with average diameters of 5.0 ± 0.7 nm and 5.1 ± 1.2 nm, respectively. The increase in size from ca. 3 to 5 nm confirmed the successful deposition of Ru overlayers on the seeds. The crystal phase taken by the deposited Ru shell was determined by analyzing the XRD pattern of the obtained sample (Figure 2C). The XRD pattern of the sample produced in EG was similar to that of the Ruhcp seeds, except for better separation of the three characteristic peaks of hcp-Ru in the region of 38–45° due to the increase in crystallite size. However, for the sample produced in TEG, the position of the main XRD peak shifted to 41.4°, between the reference peaks of fcc-Ru(111) and hcp-Ru(0002). Additionally, a broad shoulder peak appeared at 2θ of 47.4°. This peak could be assigned to the diffraction from the (200) planes of fcc-Ru, suggesting the existence of the fcc phase in the particles. The weak intensity of the shoulder peak could be attributed to the relatively small contribution from the fcc phase as a result of the thin fcc-Ru shell over the hcp-Ru core. From the XRD patterns, it can be concluded that even in the presence of a templating effect from the Ruhcp seeds, the Ru shell adopted the native hcp phase when synthesized in EG while TEG favored the formation of metastable fcc-Ru shell, leading to the formation of homophased Ruhcp@Ruhcp and heterophased Ruhcp@Rufcc nanocrystals, respectively.
One might argue that the crystal structure of the template could have been altered due to the interactions with TEG during the reaction, which might have led to the observed phase changes in the Ru shell. To address this issue, a control experiment was carried out, wherein the Ruhcp seeds were mixed with PVP and heated in TEG at 180 °C for 22 h without introducing the Ru(III) precursor. From the TEM image and XRD pattern (Figure S3), we observed no changes to the size, morphology, or phase, reinforcing the argument that TEG influenced the deposition of Ru in a phase different from that of the seed rather than causing a phase change to the seed itself. This observation is consistent with the result from DFT calculation in that EG or TEG adsorption on the surface did not change the arrangement of Ru adatoms on the hcp-Ru(0001) template, which will be further discussed in the computational section.
We further confirmed the phase compositions of Ruhcp@Ruhcp and Ruhcp@Rufcc nanocrystals by HRTEM. The HRTEM image of an individual Ruhcp@Ruhcp nanocrystal and the corresponding magnified image of the surface region clearly show the distinctive “ABABAB” atomic stacking sequence of the hcp structure along the close-packing direction (Figure 3A, B). The lattice fringe spacings of 2.1, 2.3, and 2.0 Å correspond to the (0002), (101̅0), and (101̅1) planes of hcp-Ru. The FFT pattern obtained from the surface region, marked by a blue box, fits the hcp lattice viewed along the [21̅10] zone axis (Figure 3C). This result is also in agreement with the projected atomic model of hcp-Ru from the same perspective (Figure 3D). On the other hand, the HRTEM image and the corresponding magnified image of an individual Ruhcp@Rufcc nanocrystal revealed a plate-like morphology and an incomplete fcc-Ru shell, demonstrating the existence of an hcp/fcc heterophased interface (Figure 3E, F). The distinct atomic arrangements in the hcp phase (ABABAB) and fcc phase (ABCABC) could be clearly resolved along the [21̅10] and [011] zone axes, respectively, perpendicular to the basal plane of the nanoplate. The interface between the hcp and fcc phases, indicated by a stacking fault, could be formed due to the symmetry alignment (C3) of the fcc-{111} and hcp-{0001} facets. (18,20,27) The FFT pattern taken from the middle area of the core (blue boxed) matches the diffraction pattern of the hcp phase along the [21̅10] zone axis, while the FFT pattern acquired from the shell region (red boxed) is consistent with the diffraction pattern of the fcc phase along the [011] zone axis (Figure 3G, H). The deposition of fcc-Ru overlayers predominantly occurred on the top half of the Ruhcp seed, and this could be attributed to an asymmetrical growth mode initiated following the formation of the fcc phase in the first few layers. This asymmetrical deposition of the fcc-Ru shell likely resulted from a combination of factors, including the preferential alignment of crystal facets at the hcp/fcc interface, the high surface energy introduced by stacking faults, and the limited supply of the precursor. Taken together, the type of polyol still plays an essential role in determining the phase of the deposited Ru even in the case of seed-mediated growth.

Figure 3

Figure 3. (A–D) Characterizations of Ruhcp@Ruhcp nanocrystals: (A) HRTEM image, (B) atomic resolution HRTEM image, (C) the corresponding FFT pattern of the blue boxed region in A, and (D) a projected model of Ru atoms in the hcp structure viewed along the [21̅10] zone axis. (E–H) Characterizations of Ruhcp@Rufcc nanocrystals: (E) HRTEM image, (F) atomic resolution HRTEM image, and (G,H) the corresponding FFT patterns of the red and blue boxed regions in E, respectively. The inset in panel E shows a schematic illustration of the Ruhcp@Rufcc nanocrystal with a plate-like morphology.

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To further validate the observed effects of polyol on the crystal phase taken by the Ru shell, we conducted a second round of overgrowth from the as-obtained Ruhcp@Ruhcp and Ruhcp@Rufcc nanocrystals. This study aimed to discern whether the dominance of the polyol over the template was somewhat related to the small size (3.1 nm) of the initial seed. The second round of overgrowth was conducted by reducing the amount of Ru(acac)3 precursor from 5 to 2.5 mg while keeping all other parameters consistent with the protocol for the first round. Figure S4 shows typical TEM images of a series of nanocrystals obtained by depositing an additional Ru shell on Ruhcp@Ruhcp or Ruhcp@Rufcc nanocrystals in the cases of EG and TEG, respectively. All products exhibited similar sizes of ca. 7 nm, which, compared to the seed size of ca. 5 nm, suggested the additional deposition of a Ru shell. The absence of particles smaller than the seeds or with significantly different morphologies implied the dominance of heterogeneous nucleation and layer-by-layer growth of the Ru atoms.
The homo- or heterophased structure of the 7 nm Ru nanocrystals was validated by analyzing their XRD patterns. For the sample synthesized in EG from the Ruhcp@Ruhcp template (referred to as Ruhcp@Ruhcp@Ruhcp), the XRD pattern displayed three more resolved peaks at 2θ of 38.4°, 42.1°, and 44.0°, corresponding to (101̅0), (0002), and (101̅1) planes of hcp-Ru, respectively. Although the peaks were not sharply defined, their higher degree of separation is indicative of a high crystallinity and a consistent homophase in the entire particle (Figure 4A). Conversely, the XRD pattern of the products from the Ruhcp@Ruhcp seed and TEG (denoted Ruhcp@Ruhcp@Rufcc) presents an intriguing mix of the hcp and fcc phases. Besides the stronger intensity of the characteristic peaks of hcp-Ru (i.e., the (101̅0) peak at 38.6°) due to the size increase, a weak peak was observed around 47.4°, which can be assigned to the (200) diffraction of fcc-Ru (Figure 4B). Additionally, the main peak slightly shifted to 2θ of 41.4°, situated between the reference peaks of fcc-Ru(111) and hcp-Ru(0002). These results suggested that the interior maintained its initial hcp structure while the outermost shell adopted the fcc structure as directed by TEG.

Figure 4

Figure 4. XRD patterns of the 7 nm Ru nanocrystals with an hcp or fcc phase in the outermost shell obtained from additional overgrowth on (A, B) Ruhcp@Ruhcp and (C, D) Ruhcp@Rufcc seeds in different types of polyols: (A, C) EG and (B, D) TEG. The red and blue lines correspond to the characteristic peaks of fcc-Ru (JCPDS No. 01–088–2333) and hcp-Ru (JCPDS No. 06–0663), respectively. The insets in panels A and D are the corresponding atomic models.

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The sample prepared from the Ruhcp@Rufcc seed in EG (denoted Ruhcp@Rufcc@Ruhcp) offered yet another layer of complexity in the XRD analysis. The three characteristic peaks in the region of 38–45° reveal the dominance of the hcp phase, attributed to the outermost shell and the core (Figure 4C). However, the shoulder fcc-Ru(200) peak at 2θ of 47.4°, coupled with the shift of the main peak to the region between fcc-Ru(111) at 40.8° and hcp-Ru(0002) at 42.1°, confirmed the existence of the fcc phase in the intermediate shell, albeit at a low intensity due to its minor contribution to the entire particle. As for the sample prepared from the Ruhcp@Rufcc seeds in TEG (denoted Ruhcp@Rufcc@Rufcc), the XRD pattern displayed dominant fcc-Ru characteristics with sharper (111) and (200) peaks due to the increased crystallite size and larger contribution from the fcc phase (Figure 4D). The significant shift of the primary peaks toward the characteristic peak positions of fcc-Ru(111) and fcc-Ru(200) diffractions is indicative of the transformative effect of TEG to favor the formation of fcc-Ru. The residual hcp peaks from the core could still be observed but notably diminished, further underlining the overpowering influence of the polyol on the crystal phase, even in the presence of a Ruhcp seed as the template.
To support the XRD data, we employed HRTEM to confirm the crystal phases of the products obtained from the second round of overgrowth. Given the emphasis on understanding the influence of the polyol and the need for clarity, we prioritized the analysis of two representative samples: Ruhcp@Ruhcp@Ruhcp exemplifying a homophased structure and Ruhcp@Rufcc@Rufcc with the metastable fcc phase in dominance to highlight the polyol-induced phase transitions. Figure 5A, B shows the HRTEM image and the corresponding atomic resolution image of Ruhcp@Ruhcp@Ruhcp nanocrystals. The lattice fringe spacings align well with the expected interplanar spacings for the hcp-Ru plane. A magnified view of the surface region reveals continuous ABABAB atomic packing indicative of the hcp phase throughout the particle. This observation is in accord with our XRD findings, reaffirming the homophased structure. The FFT analysis of this region yields a diffraction pattern that corresponds to the hcp crystal structure viewed along the [21̅10] zone axis (Figure 5C, D). In contrast, the HRTEM image of a Ruhcp@Rufcc@Rufcc nanocrystal shows a 5-fold twinned structure typically observed in fcc nanocrystals (Figure 5E). The hcp-Ru core could not be clearly resolved in the HRTEM image due to its small size and the dominance of the fcc-Ru shell in the second round of overgrowth. Besides the ABCABC atomic packing, the lattice fringes aligning with fcc-Ru interplanar spacings also dominate each tetrahedral subunit of the multiply twinned nanocrystal (Figure 5F). The FFT pattern of one tetrahedral subunit also exhibits diffraction spots corresponding to the fcc-Ru phase viewed along the [011] direction (Figure 5G, H). Collectively, these findings attest to the dominant role played by the polyol in dictating the crystal phase of the additionally deposited Ru. Choosing EG or TEG consistently resulted in the formation of a Ru shell with the native hcp and metastable fcc phases, respectively. This outcome was observed regardless of the size and initial phase of the Ru nanocrystal seeds.

Figure 5

Figure 5. (A–D) Characterizations of Ruhcp@Ruhcp@Ruhcp nanocrystals: (A) HRTEM image, (B) atomic resolution HRTEM image, (C) the corresponding FFT pattern of the blue boxed region in A, and (D) a simulated model for the atomic arrangement of hcp structure viewed along the [21̅10] direction. (E–H) Characterizations of Ruhcp@Rufcc@Rufcc nanocrystals: (E) HRTEM image, (F) atomic resolution HRTEM image, (G–H) the corresponding FFT patterns of the red boxed region in E, and (D) a projected model of Ru atoms in the fcc structure viewed along the [011] direction. In panel E, the white lines indicate twin boundaries while the inset shows a schematic of the Ruhcp@Rufcc@Rufcc nanocrystal with a 5-fold twin structure.

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Despite the profound influence of the polyol, it is worth noting that the seed still plays a pivotal role in the synthesis. As a noble metal high in cohesive energy, Ru nanoparticles tended to show very small sizes (typically, < 5 nm) in previous studies that employed a one-pot, solution-phase synthesis. (24,27,52) The presence of seeds herein served as catalysts to drive heterogeneous nucleation and further growth for the formation of Ru nanocrystals with larger sizes. The appropriate size of the seeds would also ensure the balance between the bulk and surface energies to favor the metastable fcc-Ru phase when TEG was used as the polyol. Overall, our experimental results highlight the intricate interplay among the polyol, seed, and deposition conditions in determining the crystal phase taken by the final products. Polyol selection has emerged as a key factor in the phase-controlled synthesis of Ru nanocrystals. Several plausible mechanisms might underpin this phase dependence on the chemical environment, such as the preferential surface capping of EG or TEG toward hcp or fcc facets, the decrease in reduction kinetics of metal precursor due to reduced electron transfer efficiency and thus weaker reducing power of long-chain polyols, or other types of molecular interactions. (16,27,35,53) While these hypotheses provide avenues for exploration, a more comprehensive understanding at the atomic and/or molecular level is necessary to achieve rational synthesis of Ru nanocrystals with the desired phases in high purity.

Quantitative Analysis of the Reduction Kinetics and Pathways Involved

The formation of a nanocrystal with a specific phase is intrinsically linked to its nucleation and growth details, which are in turn determined by the reduction and deposition kinetics of the metal precursor. (8,49,53) To gain quantitative insights into the transition from hcp-Ru core to fcc-Ru shell when substituting EG with TEG as the polyol, we conducted a kinetic measurement of the reduction rate constants of Ru(acac)3 precursor at 180 °C, under different reaction conditions, and in the presence of the 3.1 nm Ruhcp seeds. Figure S5A shows plots of the concentrations of the remaining precursor as a function of reaction time for one-shot syntheses involving EG and TEG, respectively, which were measured using inductively-coupled plasma mass spectrometry (ICP-MS). Even when initiated at the same concentration, the precursor was quickly depleted within the first 10 min in the EG-based synthesis, whereas nearly 50% of the precursor still remained after 50 min in the TEG system, suggesting a large difference in reduction kinetics.
By approximating the reaction kinetics as a pseudo-first-order reaction and calculating the slopes of the linear regression lines, the reduction rate constants (k) were determined as 0.18 s–1 and 0.015 s–1, respectively (Figure S5B). Combining with the initial concentration of Ru(acac)3 (1.14 mM), we deduced the initial reaction rates as 2.1 × 10–4 M s–1 for EG and 1.7 × 10–5 M s–1 for TEG, corresponding to the formation of hcp- and fcc-Ru overlayers, respectively. This substantial difference in initial reduction rate, spanning over 1 order of magnitude, further corroborates the more rapid reduction process in EG compared to TEG. On the basis of k values derived from one-shot injections, we further simulated the instantaneous concentrations and thus reduction rates of Ru(acac)3 precursor under dropwise syntheses as described in the standard protocol (Figure S5C). In the EG-based system, the reduction rate of the precursor reached a maximum level of 10.6 × 10–6 M min–1 during the first 15 min, which then transitioned to a steady state as the reaction proceeded. In contrast, the reduction in TEG proceeded at a markedly slower rate without the sharp initial peak or high-frequency oscillations seen in EG, implying a more gradual increase in the reduction rate. From the kinetic measurements for one-shot and dropwise injections, the precursor consistently exhibited a faster reduction in EG compared to TEG.
Our prior studies have established that the reduction pathway undertaken by a salt precursor is contingent upon the reduction kinetics involved. (47) Fast kinetics typically leads to solution reduction, while slow kinetics favor reduction on the surface of existing seeds through an autocatalytic process. Based on the different kinetic profiles observed in the two systems, we postulated that the conversion from precursor to atoms predominantly occurs via two distinct pathways: solution reduction in EG versus surface reduction in TEG (Figure S5D). In the EG-based synthesis, the solution reduction manifests as an initial spike in reduction rate, followed by a gradual plateau after 15 min, indicative of a fast conversion to Ru atoms in the solution that transitions to a steady state, where the rate of atomic formation is balanced by the rate of atomic addition onto the surface of existing seeds through heterogeneous nucleation. However, the slower initial reduction rate observed in TEG suggests a surface reduction pathway, where the precursor is first adsorbed onto the seed and then reduced to atoms for the continuous growth of the particle. The gradual increase in the reduction rate over the course of TEG-based synthesis before reaching a steady state could be ascribed to the continuous growth of the seed and thus availability of a larger surface serving as a catalyst for the reduction process. Combining the synthetic and kinetic results, the fast reduction kinetics in EG corresponds to solution reduction, leading to the formation of the hcp-Ru shell, while a slow reduction kinetics in TEG favors surface reduction and the formation of the hcp-Ru shell.

Mechanistic Investigation by First-Principles DFT Calculations

We used DFT-based calculations to elucidate the atomic-scale interactions that govern the phase-selective growth observed experimentally. Previous studies indicate that RuO2 is formed as an intermediate during the early stages of Ru(acac)3 decomposition in the presence of oxygen or oxygen-containing species, (54−57) serving as a transition point between the Ru(III) precursor and Ru atoms. As such, it is not unreasonable to focus our analyses on interactions involving RuO2 rather than Ru(acac)3. In our DFT model, we assume that the polyol is not directly involved in the decomposition of Ru(acac)3, but that it participates in the subsequent reduction of RuO2. Significantly, the reduction kinetics of the precursor play a vital role in dictating the formation of different phases of Ru overlayers on the Ru template.
We first investigated the solution-phase reduction of a single RuO2 by EG and TEG. We studied a variety of solution temperatures (see the Experimental Section) and found that reduction of RuO2 by EG occurred over the Ab Initio Molecular Dynamics (AIMD) time scale at 2000 K─the highest temperature examined for which pyrolysis did not occur. Figure 6 outlines the observed reduction mechanism. In the first step (Figure 6A), EG donated a hydrogen atom (from a carbon) to RuO2, creating a hydroxyl group associated with the Ru atom (O–Ru–OH) and ethane-1,2-diol. The Ru–O bond length in the hydroxyl group increased from 1.68 Å in RuO2 to 2.16 Å and the increased bond length could be interpreted as RuO2 becoming partially reduced by EG. In Figure 6B, ethane-1,2-diol was reduced to glycolaldehyde with the loss of another hydrogen atom from the OH group. The hydrogen atom created a water molecule by making a bond with the hydroxyl group associated with the Ru atom. The newly created water left the Ru atom with a Ru–O distance of ca. 2.5 Å. In Figure 6C, another EG molecule donated a hydrogen atom, creating a hydroxyl group around the Ru atom in RuO and ethane-1,2-diol. In Figure 6D, the new ethane-1,2-diol surrounding Ru was reduced to glycolaldehyde with the loss of another hydrogen atom from the OH group, creating another water molecule. On the other hand, RuO2 remained intact during our AIMD simulation in TEG at the same temperature over the same time scale (Figure S7).

Figure 6

Figure 6. Snapshots from an AIMD simulation of the reduction of RuO2 by EG. The blue box depicts the region where Ru reduction occurs and (A–D) show the progress of RuO2 reduction, together with the OS of Ru. Yellow: Ru in EG; pink: oxygen; gray: carbon; white: hydrogen.

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To quantify the reduction of RuO2 by EG, we created a correlation between the oxidation state (OS) and the Bader charge of Ru using known Ru compounds. In Figure S8, we plot this correlation. After our AIMD simulation at 2000 K, the RuO2 molecule stayed intact in TEG solution, showing OS = 4.06 and OS = 4.18 for Figure S7A and B, respectively, which is similar to an RuO2 molecule in the gas phase (OS = 4.43). On the other hand, the final Ru species was partially reduced by EG. In Figure 6A, the OS was partially reduced to 3.26 as a hydrogen atom in EG adsorbed on RuO2, creating a hydroxyl group associated with the Ru atom (O–Ru–OH) and ethane-1,2-diol. In Figure 6B, the OS was further reduced to 2.45 by donating another hydrogen from EG and creating another water surrounding Ru. In Figure 6C, the OS decreased to 1.74 by another EG donating hydrogen, creating another hydroxyl group associated with the Ru atom. Finally in Figure 6D, the OS decreased to 0.98 by creating another water molecule while making a bond with the hydroxyl group associated with the Ru atom. The OS seemed to be affected by the resulting molecules surrounding the Ru atom in solution. For example, the OS decreased from 2.45 to 1.74 when the distance between Ru atom and resultant molecules in solution was increased (Figure 6B, C). However, the OS did not fully decrease to zero when both oxygen atoms left Ru (Figure 6D). This may be due to limitations of the semilocal PBE functional employed in our DFT calculations (58) or charge self-regulation. (59) Nevertheless, it is evident in the AIMD simulations that EG can remove oxygen atoms from RuO2, resulting in solution-phase reduction.
When RuO2 is completely reduced by EG, the resulting Ru atoms will prefer hcp binding sites on hcp-Ru(0001) in the presence of EG. As a check of the hypothesis, we ran various DFT calculations to probe the influence of EG on Ru binding to Ru(0001), and it was found that the binding preference of the Ru atom in EG was the same as that in vacuum (see Figures S9).
Since TEG did not reduce RuO2 in solution (Figure S7), we argued that RuO2 would be deposited on the surface of an hcp-Ru seed in the case of TEG. Using evidence from a previous DFT study, (60) we proposed that the O in RuO2 would affect the binding site of Ru. We investigated the preferred site and binding configuration of RuO2 using different structures in DFT. First, we calculated ΔERuNOM using Equation S1, see the Supporting Information, for four configurations of a single RuO2 on hcp-Ru(0001), with the Ru atom residing on the fcc and hcp sites, respectively. These configurations were optimized to four configurations, as shown in Figure S10. For all configurations, the results indicated that a single RuO2 preferred to bind to the fcc site on hcp-Ru (0001) (negative ΔERuNOM from Equation S2). Figure 7A shows the preferred binding site and configuration for RuO2 on the fcc and hcp sites of hcp-Ru(0001).

Figure 7

Figure 7. Top and side view of RuNOM at fcc and hcp site on hcp-Ru(0001). RuNOM, given by Equation S1, is indicated. (A) one RuO2 deposited with Ru at fcc and hcp sites; (B) RuO2 with a coverage (θ) of 3/8 at fcc and hcp sites; (C) RuNOM where Ru atoms are at fcc (left) and hcp (right) and oxygen atoms are at hcp (left) and fcc (right) sites on hcp-Ru (0001). Sky and blue: hcp-Ru; red: fcc-Ru; pink: oxygen.

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We subsequently probed four different RuO2 coverages (θ, where θ = the number of Ru atoms divided by the number of Ru atoms in the top surface layer), with RuO2 initially on fcc and hcp binding sites on Ru(0001). In these calculations, we observed that RuO2 became progressively more disordered on the Ru(0001) template as the coverage increased (Figure S11), with O atoms leaving the original RuO2 to assume various binding configurations on the surface. We were able to obtain a maximum coverage of RuO2 at θ = 3/8, as steric constraints occurred at higher coverages. For all of these studied coverages, the fcc sites were energetically preferred over hcp sites for the Ru atoms (Figure S11). Figure 7B shows a snapshot for θ = 3/8. In calculations with these disordered layers, we found that TEG was able to spontaneously remove the adsorbed oxygen during DFT optimization, as we discuss below. Thus, we considered that additional RuO2 could adsorb to the point at which we achieved a single fcc layer of Ru on hcp-Ru(0001) with a complete layer of adsorbed O, as shown in Figure 7C. Our results also indicate that the fcc-Ru layer is preferred over the hcp-Ru layer for this configuration.
Though our calculations indicate that TEG can reduce any of the RuO2 layers shown in Figure S11, it is advantageous to investigate the reduction of the high-symmetry layer in Figure 7C, where there is a one-to-one ratio of Ru to O. First, TEG chemisorbed onto the layer in Figure 8 with its long axis perpendicular to the surface─when TEG was placed in a configuration with its long axis parallel to the surface, it was physically adsorbed. We considered four different locations for the terminal OH group in TEG with respect to the surface, and these are shown in Figure S12A. After geometry optimization, we found TEG could remove the oxygen atoms from RuNOM through the hydrogens spontaneously leaving the TEG molecule (Figure 8 and Figure S12B).

Figure 8

Figure 8. Snapshot of final configurations and the detailed mechanism involving the removal of hydrogen atoms from TEG. The green circles indicate the created hydroxyl groups by removal of hydrogen atoms from TEG. Sky and blue: hcp-Ru seed; red: fcc-Ru; pink: oxygen; gray: carbon; white: hydrogen.

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For each of the four different locations of TEG adsorption in Figure S12A, hydrogen atoms on both the terminal OH and a neighboring terminal C in TEG left spontaneously during structural optimization in DFT at zero K and created two adsorbed hydroxyl groups with O atoms on the surface by oxidizing TEG and creating [2-(2-hydroxyethoxy) ethoxy] acetaldehyde. The resulting hydroxyl groups could also be spontaneously removed by an 8-hydroxyl-3,6-dioxaoctanal molecule, an oxidized TEG molecule that was observed in our AIMD simulation for TEG (Figure S13). We argue that this sequence of hydroxylation and dehydroxylation would continue until all the oxygen atoms were removed from the Ru surface. Unlike TEG, EG was not chemisorbed to the surface during DFT optimization. In Figure S14, we decomposed Ebind (binding energy of EG to the RuNOM surface) into two contributions: a short-range contribution (Eelectronic) and a long-range vdW interaction (EvdW). Based on our binding energy calculations, the contribution from EvdW was 80–90% of the total Ebind, indicating that EG was weakly bound to the surface. We also observed that EG did not lose its hydrogen spontaneously during geometry optimization, unlike TEG. Therefore, our calculations suggested that EG did not readily assist in the removal of oxygen atoms from the Ru surface.
After the oxygen atoms had been removed by TEG, fcc-Ru remained metastable during the DFT geometry optimization. We investigated the growth of a second layer of Ru, assuming all of the oxygen atoms in the first fcc-RuO2 overlayer had been removed during surface reduction by TEG. First, we deposited one RuO2 on the first fcc-Ru overlayer (Figure S15). We started with six different initial configurations for RuO2. After geometry optimization, we compared the binding energies of RuO2 on the fcc-Ru overlayer. Figure S15 shows the lowest energy configuration, with RuO2 residing at an fcc or hcp site. Our results indicated that fcc-RuO2 was preferred over hcp-RuO2 by ca. 0.03 eV.
We also deposited different coverages (θ) of RuNOM as a second Ru overlayer (Figure S16). As for the first Ru layer, we obtained a maximum coverage of RuO2 at θ = 3/8. Based on relative binding energy calculations using Equation S1, fcc-RuNOM was preferred over hcp-RuNOM. At the maximum coverage (θ) of RuO2, a second fcc-RuO2 overlayer was preferred over the hcp-RuO2 overlayer by ca. 0.38 eV (Figure S16), compared to a first fcc-RuO2 overlayer preference over hcp-RuO2 only by ca. 0.07 eV (Figure S11). Next, we deposited more RuNOM, with oxygen adsorption occurring exclusively at hcp or fcc sites (Figure S17). The fcc-RuO2 was still preferred over hcp sites by ca. 0.02 eV. When we deposited Ru instead of RuO2, Ru preferred hcp sites over fcc by ca = 0.19 eV (Figure S18). Altogether, these results suggested that RuO2 overlayers could be subsequently deposited on the fcc-Ru seed, following surface reduction by TEG. This is consistent with the experimental finding that a second overlayer of Ru would also take the fcc phase when growth occurs in TEG.
Collectively, our calculations support the experimental findings that the different reduction kinetics of the precursor with the chosen polyol play a dominant role in dictating the Ru crystal structure via distinct reduction pathways (Figure 9). Specifically, TEG reduces the precursor by removing the oxygen atoms in fcc-RuO2 at a slow reduction rate, leading to the fcc-Ru crystal phase. In contrast, EG has the capability to reduce the precursor in the solution at a fast reduction rate, favoring the formation of the hcp-Ru phase.

Figure 9

Figure 9. Mechanism of the effect of reduction kinetics on the phase-selective epitaxial growth of Ru overlayers based on both computational and experimental results.

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Conclusion

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In summary, we have systematically investigated the use of seed-mediated growth to fabricate Ru nanocrystals with tunable sizes of 3–7 nm and distinct patterns (either homo- or heterophased) of crystal phases by manipulating the polyol serving as a solvent and a reducing agent in each round of growth. By keeping all experimental conditions except the type of polyol fixed, we found that EG exclusively yielded the thermodynamic hcp phase, whereas TEG favored the formation of the metastable fcc phase, regardless of the size and crystal structure of the seeds involved. While both the chemical environment and template could affect the outcome of a phase-selective epitaxial growth, the chemical environment tended to play a more important role in controlling the reduction kinetics of the precursor and thereby dictating the polymorphism (hcp versus fcc) of the Ru overlayers.
Our theoretical analyses, based on both DFT and AIMD, supported the experimental findings by attributing the polyol-based phase control to its different interactions with both the seed surface and the precursor, which directly influenced the reduction pathway. Specifically, the reduction could follow two different pathways, resulting in distinct packing patterns for the Ru atoms. Under fast reduction kinetics due to the strong reducing power of EG, solution reduction dominated, where different Ru-based complexes were formed as intermediates and then reduced to Ru atoms in solution, which were preferentially deposited on the hcp sites of the hcp-Ru seeds. In contrast, a slower reduction kinetics due to a weaker reducing power of TEG favored the surface reduction pathway, initiating with the formation of a RuO2 layer that preferentially adsorbed on the fcc sites, followed by the removal of oxygen by TEG to generate fcc-Ru overlayers. The insights into the mechanisms of reduction kinetics in different polyols not only offer guidelines for achieving Ru nanocrystals with a desired phase in high purity but also shed light on the rational synthesis of phase-controlled metal nanocrystals. Future studies are encouraged to extend this work to other metals, validating the universality and potential of reduction kinetics as a pivotal knob to maneuver the crystal phases of various types of nanocrystals.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c01725.

  • Descriptions of synthetic protocols, computational details, additional TEM and HRTEM images, and XRD patterns of the initial Ru seeds and samples prepared under different conditions, as well as results from additional computational analyses (PDF)

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Author Information

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  • Corresponding Authors
    • Kristen A. Fichthorn - Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16803, United States;  Orcidhttps://orcid.org/0000-0002-4256-714X;  Email: [email protected]
    • Younan Xia - School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States;  The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States;  Orcidhttps://orcid.org/0000-0003-2431-7048;  Email: [email protected]
  • Authors
    • Quynh N. Nguyen - School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States;  Orcidhttps://orcid.org/0000-0003-1544-6139
    • Eun Mi Kim - Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16803, United States
    • Yong Ding - School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
    • Annemieke Janssen - School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States;  Orcidhttps://orcid.org/0000-0002-7721-597X
    • Chenxiao Wang - School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States;  Orcidhttps://orcid.org/0000-0001-7815-6062
    • Kei Kwan Li - School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
    • Junseok Kim - Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16803, United States
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported in part by a grant from the National Science Foundation (CHE-2105602) and start-up funds from the Georgia Institute of Technology. Q.N.N. acknowledges the support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-2039655. TEM imaging was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-2025462). The theoretical work was funded by the Department of Energy, Office of Basic Energy Sciences, Materials Science Division, Grant DE-FG02-07ER46414 (EK, JK, and KF). This work used Bridges-2 at the Pittsburgh Supercomputing Center through allocation DMR110061 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296.

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    Accounts of Chemical Research (2019), 52 (7), 2015-2025CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Intermetallic nanoparticles (NPs) described in this Account are a class of metallic alloy NPs within which metal atoms are bonded via strong d-orbital interaction and ordered anisotropically in a specific crystallog. direction. Compared to the common metallic alloy NPs with solid soln. structure, intermetallic NPs are generally more stable against chem. oxidn. and etching. The strict stoichiometry requirement, well-defined atom binding environment and layered at. arrangement also make intermetallic NPs an ideal model for understanding their phys. and catalytic properties. This account summarizes the synthetic principles and strategies developed to obtain monodisperse intermetallic NPs, esp. tetragonal L10-NPs. The thermodn. and kinetics involved in the conversion between disordered and ordered structures are briefly discussed. The synthetic methods are grouped into two slightly different categories: soln.-phase synthesis followed by solid state annealing and direct soln.-phase synthesis. In the former method, high-surface-area supports are often needed to disperse NPs and to prevent them from aggregation, while in the latter method such supports are not required since the structure conversion temp. is lowered to a level that the conversion can proceed in the soln. reaction condition. In any of these two synthetic approaches, various factors influencing intermetallic structure formation should be carefully controlled to ensure more complete structural transition within NPs. Using representative synthetic examples, we highlight the strategies explored to facilitate the formation of intermetallic structure, including the introduction of vacancies/defects within NP structures and the control of atom addn. rate/seed-mediated diffusion to lower the energy barrier. These strategies illustrate how the concept of thermodn. and kinetics can be used to design the synthesis of intermetallic NPs. Addnl., to correlate NP structure and catalysis, we introduce briefly the d-band theory to explain how the electronic, strain and ensemble effects can be used to tune NP catalysis. We focus specifically on Pt-, Pd-, and Au-based L10-NPs and demonstrate how these L10-NPs could be prepd. to show much enhanced catalysis for electrochem. reactions, including oxygen redn. reaction (ORR), hydrogen evolution reaction (HER), formic acid oxidn. reaction (FAOR), and thermo-oxidn. reaction of CO. Due to the enhanced metal atom stability in the "sandwich"-type structure, the roles of the first-row transition metal atoms in catalysis are better understood to achieve catalysis optimization. This concept can be extended to other alloy NPs, demonstrating great potentials in using intermetallic structures to control NP redn. and oxidn. catalysis for important chem. and energy applications.
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    Chen, Y.; Lai, Z.; Zhang, X.; Fan, Z.; He, Q.; Tan, C.; Zhang, H. Phase engineering of nanomaterials. Nat. Rev. Chem. 2020, 4, 243– 256,  DOI: 10.1038/s41570-020-0173-4
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    Phase engineering of nanomaterials
    Chen, Ye; Lai, Zhuangchai; Zhang, Xiao; Fan, Zhanxi; He, Qiyuan; Tan, Chaoliang; Zhang, Hua
    Nature Reviews Chemistry (2020), 4 (5), 243-256CODEN: NRCAF7; ISSN:2397-3358. (Nature Research)
    A review. Abstr.: Phase has emerged as an important structural parameter - in addn. to compn., morphol., architecture, facet, size and dimensionality - that dets. the properties and functionalities of nanomaterials. In particular, unconventional phases in nanomaterials that are unattainable in the bulk state can potentially endow nanomaterials with intriguing properties and innovative applications. Great progress has been made in the phase engineering of nanomaterials (PEN), including synthesis of nanomaterials with unconventional phases and phase transformation of nanomaterials. This Review provides an overview on the recent progress in PEN. We discuss various strategies used to synthesize nanomaterials with unconventional phases and induce phase transformation of nanomaterials, by taking noble metals and layered transition metal dichalcogenides as typical examples. Moreover, we also highlight recent advances in the prepn. of amorphous nanomaterials, amorphous-cryst. and crystal phase-based hetero-nanostructures. We also provide personal perspectives on challenges and opportunities in this emerging field, including exploration of phase-dependent properties and applications, rational design of phase-based heterostructures and extension of the concept of phase engineering to a wider range of materials.
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    Yao, Q.; Yu, Z.; Li, L.; Huang, X. Strain and surface engineering of multicomponent metallic nanomaterials with unconventional phases. Chem. Rev. 2023, 123, 9676– 9717,  DOI: 10.1021/acs.chemrev.3c00252
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    Strain and Surface Engineering of Multicomponent Metallic Nanomaterials with Unconventional Phases
    Yao, Qing; Yu, Zhiyong; Li, Leigang; Huang, Xiaoqing
    Chemical Reviews (Washington, DC, United States) (2023), 123 (15), 9676-9717CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Multicomponent metallic nanomaterials with unconventional phases show great prospects in electrochem. energy storage and conversion, owing to unique crystal structures and abundant structural effects. In this review, we emphasize the progress in the strain and surface engineering of these novel nanomaterials. We start with a brief introduction of the structural configurations of these materials, based on the interaction types between the components. Next, the fundamentals of strain, strain effect in relevant metallic nanomaterials with unconventional phases, and their formation mechanisms are discussed. Then the progress in surface engineering of these multicomponent metallic nanomaterials is demonstrated from the aspects of morphol. control, crystallinity control, surface modification, and surface reconstruction. Moreover, the applications of the strain- and surface-engineered unconventional nanomaterials mainly in electrocatalysis are also introduced, where in addtion to the catalytic performance, the structure-performance correlations are highlighted. Finally, the challenges and opportunities in this promising field are prospected.
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    Zhao, M.; Xia, Y. Crystal-phase and surface-structure engineering of ruthenium nanocrystals. Nat. Rev. Mater. 2020, 5, 440– 459,  DOI: 10.1038/s41578-020-0183-3
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    7
    Crystal-phase and surface-structure engineering of ruthenium nanocrystals
    Zhao, Ming; Xia, Younan
    Nature Reviews Materials (2020), 5 (6), 440-459CODEN: NRMADL; ISSN:2058-8437. (Nature Research)
    A review. Metal nanocrystals with controlled shapes or surface structures have received increasing attention, owing to their desirable properties for applications ranging from catalysis to photonics, energy and biomedicine. Most studies, however, have been limited to nanocrystals with the same crystal phase as the bulk material. Engineering the phase of metal nanocrystals while simultaneously attaining shape-controlled synthesis has recently emerged as a new frontier of research. Here, we use Ru as an example to evaluate recent progress in the synthesis of metal nanocrystals featuring different crystal phases and well-controlled shapes. We first discuss synthetic strategies for controlling the crystal phase and shape of Ru nanocrystals, with a focus on new mechanistic insights. We then highlight the major factors that affect the packing of Ru atoms and, thus, the crystal phase, followed by an examn. of the thermal stability of Ru nanocrystals in terms of both crystal phase and shape. Next, we showcase the successful implementation of these Ru nanocrystals in various catalytic applications. Finally, we end with a discussion of the challenges and opportunities in the field, including leveraging the lessons learned from Ru to engineer the crystal phase and surface structure of other metals.
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    Janssen, A.; Nguyen, Q. N.; Xia, Y. Colloidal metal nanocrystals with metastable crystal structures. Angew. Chem., Int. Ed. 2021, 60, 12192– 12203,  DOI: 10.1002/anie.202017076
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    8
    Colloidal Metal Nanocrystals with Metastable Crystal Structures
    Janssen, Annemieke; Nguyen, Quynh N.; Xia, Younan
    Angewandte Chemie, International Edition (2021), 60 (22), 12192-12203CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. In addn. to the conventional knobs such as compn., size, shape, and defect structure, the crystal structure (or phase) of metal nanocrystals offers a new avenue for engineering their properties. Various strategies have recently been developed for the fabrication of colloidal metal nanocrystals in metastable phases different from their bulk counterparts. With a focus on noble metals, we begin with a brief introduction to their at. packing, followed by a discussion about five major synthetic approaches to their colloidal nanocrystals in unconventional phases. We then highlight the success of synthesis in terms of mechanistic insights and exptl. controls, as well as the enhanced catalytic properties. We end this Minireview with perspectives on the remaining issues and future opportunities.
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  9. 9
    Fan, Z.; Zhang, H. Template synthesis of noble metal nanocrystals with unusual crystal structures and their catalytic applications. Acc. Chem. Res. 2016, 49, 2841– 2850,  DOI: 10.1021/acs.accounts.6b00527
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    9
    Template Synthesis of Noble Metal Nanocrystals with Unusual Crystal Structures and Their Catalytic Applications
    Fan, Zhanxi; Zhang, Hua
    Accounts of Chemical Research (2016), 49 (12), 2841-2850CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. The authors review the recent research progress on the crystal structure control of noble metal nanocrystals with a template synthetic approach and their crystal structure-dependent catalytic properties. The authors describe the template synthetic methods, such as epitaxial growth and galvanic replacement reaction methods, in which a presynthesized noble metal nanocrystal with either new or common crystal structure is used as the template to direct the growth of unusual crystal structures of other noble metals. The template synthetic strategy described here provides an efficient, simple and straightforward way to synthesize unusual crystal structures of noble metal nanocrystals, which might not be easily synthesized by commonly used chem. synthesis. To be specific, by using the epitaxial growth method, a series of noble metal nanocrystals with unusual crystal structures has been obtained, such as hcp. Ag, 4H Ag, Pd, Pt, Ir, Rh, Os, and Ru, and face-centered cubic Ru nanostructures. Meanwhile, the galvanic replacement reaction method offers an efficient way to synthesize noble metal alloy nanocrystals with unusual crystal structures, such as 4H PdAg, PtAg, and PtPdAg nanostructures. The stability of noble metal nanocrystals with unusual crystal structures is discussed. The authors demonstrate the catalytic applications of the resultant noble metal nanocrystals with unusual crystal structures toward different chem. reactions like hydrogen evolution reaction, hydrogen oxidn. reaction and org. reactions. The relationship between crystal structures of noble metal nanocrystals and their catalytic performances is discussed.
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  10. 10
    Fan, Z.; Chen, Y.; Zhu, Y.; Wang, J.; Li, B.; Zong, Y.; Han, Y.; Zhang, H. Epitaxial growth of unusual 4H hexagonal Ir, Rh, Os, Ru, and Cu nanostructures on 4H Au nanoribbons. Chem. Sci. 2017, 8, 795– 799,  DOI: 10.1039/C6SC02953A
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    There is no corresponding record for this reference.
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    Ge, Y.; Huang, Z.; Ling, C.; Chen, B.; Liu, G.; Zhou, M.; Liu, J.; Zhang, X.; Cheng, H.; Liu, G. Phase-selective epitaxial growth of heterophase nanostructures on unconventional 2H-Pd nanoparticles. J. Am. Chem. Soc. 2020, 142, 18971– 18980,  DOI: 10.1021/jacs.0c09461
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    11
    Phase-Selective Epitaxial Growth of Heterophase Nanostructures on Unconventional 2H-Pd Nanoparticles
    Ge, Yiyao; Huang, Zhiqi; Ling, Chongyi; Chen, Bo; Liu, Guigao; Zhou, Ming; Liu, Jiawei; Zhang, Xiao; Cheng, Hongfei; Liu, Guanghua; Du, Yonghua; Sun, Cheng-Jun; Tan, Chaoliang; Huang, Jingtao; Yin, Pengfei; Fan, Zhanxi; Chen, Ye; Yang, Nailiang; Zhang, Hua
    Journal of the American Chemical Society (2020), 142 (44), 18971-18980CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Heterostructured, including heterophase, noble-metal nanomaterials have attracted much interest due to their promising applications in diverse fields. However, great challenges still remain in the rational synthesis of well-defined noble-metal heterophase nanostructures. Herein, we report the prepn. of Pd nanoparticles with an unconventional hcp. (2H type) phase, referred to as 2H-Pd nanoparticles, via a controlled phase transformation of amorphous Pd nanoparticles. Impressively, by using the 2H-Pd nanoparticles as seeds, Au nanomaterials with different crystal phases epitaxially grow on the specific exposed facets of the 2H-Pd, i.e., face-centered cubic (fcc) Au (fcc-Au) on the (002)h facets of 2H-Pd while 2H-Au on the other exposed facets, to achieve well-defined fcc-2H-fcc heterophase Pd@Au core-shell nanorods. Moreover, through such unique facet-directed crystal-phase-selective epitaxial growth, a series of unconventional fcc-2H-fcc heterophase core-shell nanostructures, including Pd@Ag, Pd@Pt, Pd@PtNi, and Pd@PtCo, have also been prepd. Impressively, the fcc-2H-fcc heterophase Pd@Au nanorods show excellent performance toward the electrochem. carbon dioxide redn. reaction (CO2RR) for prodn. of carbon monoxide with Faradaic efficiencies of over 90% in an exceptionally wide applied potential window from -0.9 to -0.4 V (vs. the reversible hydrogen electrode), which is among the best reported CO2RR catalysts in H-type electrochem. cells.
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  12. 12
    Zhou, X.; Ma, Y.; Ge, Y.; Zhu, S.; Cui, Y.; Chen, B.; Liao, L.; Yun, Q.; He, Z.; Long, H. Preparation of Au@Pd core-shell nanorods with fcc-2H-fcc heterophase for highly efficient electrocatalytic alcohol oxidation. J. Am. Chem. Soc. 2022, 144, 547– 555,  DOI: 10.1021/jacs.1c11313
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    12
    Preparation of Au@Pd Core-Shell Nanorods with fcc-2H-fcc. Heterophase for Highly Efficient Electrocatalytic Alcohol Oxidation
    Zhou, Xichen; Ma, Yangbo; Ge, Yiyao; Zhu, Shangqian; Cui, Yu; Chen, Bo; Liao, Lingwen; Yun, Qinbai; He, Zhen; Long, Huiwu; Li, Lujiang; Huang, Biao; Luo, Qinxin; Zhai, Li; Wang, Xixi; Bai, Licheng; Wang, Gang; Guan, Zhiqiang; Chen, Ye; Lee, Chun-Sing; Wang, Jinlan; Ling, Chongyi; Shao, Minhua; Fan, Zhanxi; Zhang, Hua
    Journal of the American Chemical Society (2022), 144 (1), 547-555CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Controlled construction of bimetallic nanostructures with a well-defined heterophase is of great significance for developing highly efficient nanocatalysts and studying the structure-dependent catalytic performance. Here, a wet-chem. synthesis method was used to prep. Au@Pd core-shell nanorods with a unique fcc.-2H-fcc. heterophase (fcc.: fcc.; 2H: hcp. with a stacking sequence of AB). The obtained fcc.-2H-fcc. heterophase Au@Pd core-shell nanorods exhibit superior electrocatalytic EtOH oxidn. performance with a mass activity ≤6.82 A mgPd-1, which is 2.44, 6.96, and 6.43 times those of 2H-Pd nanoparticles, fcc.-Pd nanoparticles, and com. Pd/C, resp. The operando IR reflection absorption spectroscopy reveals a C2 pathway with fast reaction kinetics for the EtOH oxidn. on the prepd. heterophase Au@Pd nanorods. The authors' exptl. results together with d. functional theory calcns. indicate that the enhanced performance of heterophase Au@Pd nanorods can be attributed to the unconventional 2H phase, the 2H/fcc. phase boundary, and the lattice expansion of the Pd shell. Also, the heterophase Au@Pd nanorods can also serve as an efficient catalyst for the electrochem. oxidn. of MeOH, ethylene glycol, and glycerol. The authors' work in the area of phase engineering of nanomaterials (PENs) opens the way for developing high-performance electrocatalysts toward future practical applications.
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    Tan, X.; Geng, S.; Ji, Y.; Shao, Q.; Zhu, T.; Wang, P.; Li, Y.; Huang, X. Closest packing polymorphism interfaced metastable transition metal for efficient hydrogen evolution. Adv. Mater. 2020, 32, 2002857  DOI: 10.1002/adma.202002857
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    13
    Closest Packing Polymorphism Interfaced Metastable Transition Metal for Efficient Hydrogen Evolution
    Tan, Xinyue; Geng, Shize; Ji, Yujin; Shao, Qi; Zhu, Ting; Wang, Pengtang; Li, Youyong; Huang, Xiaoqing
    Advanced Materials (Weinheim, Germany) (2020), 32 (40), 2002857CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Metastable materials are promising because of their catalytic properties, high-energy structure, and unique electronic environment. However, the unstable nature inherited from the metastability hinders further performance improvement and practical applications of these materials. Herein, this limitation is successfully addressed by constructing an in situ polymorphism interface (inf) between the metastable hexagonal-close-packed (hcp) phase and its stable counterpart (face-centered cubic, fcc) in cobalt-nickel (CoNi) alloy. Calcns. reveal that the interfacial synergism derived from the hcp and fcc phases lowers the formation energy and enhances stability. Consequently, the optimized CoNi-inf exhibits an exceptionally low potential of 72 mV at 10 mA cm-2 and a Tafel slope of 57 mV dec-1 for the hydrogen evolution reaction (HER) in 1.0 M KOH. Furthermore, it is superior to most state-of-the-art non-noble-metal-based HER catalysts. No noticeable activity decay or structural changes are obsd. even over 14 h of catalysis. The computational simulation further rationalizes that the interface of CoNi-inf with a suitable d-band center provides uniform sites for hydrogen adsorption, leading to a distinguished HER catalytic activity. This work, therefore, presents a new route for designing metastable catalysts for potential energy conversion.
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    Xia, Y.; Gilroy, K. D.; Peng, H. C.; Xia, X. Seed-mediated growth of colloidal metal nanocrystals. Angew. Chem., Int. Ed. 2017, 56, 60– 95,  DOI: 10.1002/anie.201604731
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    14
    Seed-Mediated Growth of Colloidal Metal Nanocrystals
    Xia, Younan; Gilroy, Kyle D.; Peng, Hsin-Chieh; Xia, Xiaohu
    Angewandte Chemie, International Edition (2017), 56 (1), 60-95CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, compn., and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also det. their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a no. of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and redn. kinetics. New capabilities and future directions are also highlighted.
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    Gilroy, K. D.; Yang, X.; Xie, S.; Zhao, M.; Qin, D.; Xia, Y. Shape-controlled synthesis of colloidal metal nanocrystals by replicating the surface atomic structure on the seed. Adv. Mater. 2018, 30, 1706312  DOI: 10.1002/adma.201706312
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    Zhao, M.; Xu, L.; Vara, M.; Elnabawy, A. O.; Gilroy, K. D.; Hood, Z. D.; Zhou, S.; Figueroa-Cosme, L.; Chi, M.; Mavrikakis, M.; Xia, Y. Synthesis of Ru icosahedral nanocages with a face-centered-cubic structure and evaluation of their catalytic properties. ACS Catal. 2018, 8, 6948– 6960,  DOI: 10.1021/acscatal.8b00910
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    16
    Synthesis of Ru Icosahedral Nanocages with a Face-Centered-Cubic Structure and Evaluation of Their Catalytic Properties
    Zhao, Ming; Xu, Lang; Vara, Madeline; Elnabawy, Ahmed O.; Gilroy, Kyle D.; Hood, Zachary D.; Zhou, Shan; Figueroa-Cosme, Legna; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
    ACS Catalysis (2018), 8 (8), 6948-6960CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Owing to the presence of {111} facets, twin boundaries, and strain fields on the surface, noble-metal nanocrystals with an icosahedral shape have been reported with stellar performance toward an array of catalytic reactions. Here, we report the successful synthesis of Ru icosahedral nanocages with a face-centered cubic (fcc) structure by conformally coating Pd icosahedral seeds with ultrathin Ru shells, followed by selective removal of the Pd cores via chem. etching. We discovered that the presence of bromide ions was crit. to the layer-by-layer deposition of Ru atoms. According to in situ XRD, the fcc structure in the Ru nanocages could be retained up to 300 °C before it was transformed into the conventional hcp. (hcp) structure. Addnl., the icosahedral shape of the Ru nanocages could be largely preserved up to 300 °C. The Ru icosahedral nanocages with twin boundaries on the surface exhibited greatly enhanced activities toward both the redn. of 4-nitrophenol and decompn. of hydrazine than their cubic and octahedral counterparts. When benchmarked against the parental Pd@Ru core-shell nanocrystals, all the Ru nanocages displayed superior catalytic activities. First-principles d. functional theory calcns. also suggest that the fcc-Ru icosahedral nanocages contg. residual Pd atoms are more promising than the conventional hcp-Ru solid nanoparticles in catalyzing nitrogen redn. for ammonia synthesis. With the subsurface impurities of Pd, the twin boundary regions of the icosahedral nanocages are able to stabilize the N2 dissocn. transition state, reducing the overall reaction barrier and promoting the competition with the N2 desorption process.
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    Zhao, M.; Figueroa-Cosme, L.; Elnabawy, A. O.; Vara, M.; Yang, X.; Roling, L. T.; Chi, M.; Mavrikakis, M.; Xia, Y. Synthesis and characterization of Ru cubic nanocages with a face-centered cubic structure by templating with Pd nanocubes. Nano Lett. 2016, 16, 5310– 5317,  DOI: 10.1021/acs.nanolett.6b02795
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    17
    Synthesis and Characterization of Ru Cubic Nanocages with a Face-Centered Cubic Structure by Templating with Pd Nanocubes
    Zhao, Ming; Figueroa-Cosme, Legna; Elnabawy, Ahmed O.; Vara, Madeline; Yang, Xuan; Roling, Luke T.; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
    Nano Letters (2016), 16 (8), 5310-5317CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Ru cubic nanocages with ultrathin walls, in which the atoms are crystd. in fcc. rather than hcp. structure, have been prepd. The key to the success of this synthesis was to ensure layer-by-layer deposition of Ru atoms on the surface of Pd cubic seeds by controlling the reaction temp. and the injection rate of a Ru(III) precursor. By selectively etching away the Pd from the Pd@Ru core-shell nanocubes, the authors obtained Ru nanocages with an av. wall thickness of 1.1 nm or about six at. layers. The Ru nanocages adopted an fcc. crystal structure rather than the hcp. structure obsd. in bulk Ru. The synthesis was applied to Pd cubic seeds with different edge lengths in the range of 6-18 nm, with smaller seeds being more favorable for the formation of Ru shells with a flat, smooth surface due to shorter distance for the surface diffusion of the Ru adatoms. Self-consistent d. functional theory calcns. indicated that these unique fcc.-structured Ru nanocages might possess promising catalytic properties for ammonia synthesis compared to hcp. Ru(0001), on the basis of strengthened binding of at. N and substantially reduced activation energies for N2 dissocn., which is the rate-detg. step for ammonia synthesis on hcp. Ru catalysts.
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    Gu, J.; Guo, Y.; Jiang, Y.-Y.; Zhu, W.; Xu, Y.-S.; Zhao, Z.-Q.; Liu, J.-X.; Li, W.-X.; Jin, C.-H.; Yan, C.-H.; Zhang, Y. W. Robust phase control through hetero-seeded epitaxial growth for face-centered cubic Pt@Ru nanotetrahedrons with superior hydrogen electro-oxidation activity. J. Phys. Chem. C 2015, 119, 17697– 17706,  DOI: 10.1021/acs.jpcc.5b04587
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    18
    Robust Phase Control through Hetero-Seeded Epitaxial Growth for Face-Centered Cubic Pt@Ru Nanotetrahedrons with Superior Hydrogen Electro-Oxidation Activity
    Gu, Jun; Guo, Yu; Jiang, Ying-Ying; Zhu, Wei; Xu, Yan-Shuang; Zhao, Ze-Qiong; Liu, Jin-Xun; Li, Wei-Xue; Jin, Chuan-Hong; Yan, Chun-Hua; Zhang, Ya-Wen
    Journal of Physical Chemistry C (2015), 119 (31), 17697-17706CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Controllable synthesis of metallic nanocrystals (NCs) with tunable phase, uniform shape, and size is of multidisciplinary interests but has still remained challenging. Herein, a robust phase control strategy is developed, in which seeds with a given phase are added to guide the epitaxial growth of the target metal to inherit the seeds' phase. Through this strategy, M@Ru (M = Pt, Pd) NCs in the fcc. phase, a metastable phase for Ru under ambient conditions, were synthesized with the hydrothermal method. The Pt@Ru NCs showed not only the pure fcc. phase but also high morphol. selectivity to tetrahedrons surrounded by {111} facets. As revealed by d. function theory (DFT) calcns., the preferentially epitaxial growth of Ru atom layers on the nonclosest-packed facets of hetero fcc. metal seeds gave fcc. Ru shells. Also, the fcc. Pt@Ru tetrahedrons/C showed electrocatalytic activity enhancement with more than an order of magnitude toward H oxidn. reaction (HOR) in acidic electrolyte compared with hydrothermally synthesized Ru/C. Electrochem. measurement combined with DFT calcns. revealed that the optimum HOR activity should be achieved on well-crystd. fcc. Ru catalysts exposing max. {111} facets.
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    Janssen, A.; Lyu, Z.; Figueras-Valls, M.; Chao, H.-Y.; Shi, Y.; Pawlik, V.; Chi, M.; Mavrikakis, M.; Xia, Y. Phase-controlled synthesis of Ru nanocrystals via template-directed growth: surface energy versus bulk energy. Nano Lett. 2022, 22, 3591– 3597,  DOI: 10.1021/acs.nanolett.1c05009
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    Phase-Controlled Synthesis of Ru Nanocrystals via Template-Directed Growth: Surface Energy versus Bulk Energy
    Janssen, Annemieke; Lyu, Zhiheng; Figueras-Valls, Marc; Chao, Hsin-Yun; Shi, Yifeng; Pawlik, Veronica; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
    Nano Letters (2022), 22 (9), 3591-3597CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Despite the successful control of crystal phase using template-directed growth, much remains unknown about the underlying mechanisms. Here we demonstrate that the crystal phase taken by the deposited metal depends on the lateral size of fcc.-Pd nanoplate templates, with 12 nm plates giving fcc.-Ru while 18-26 nm plates result in hcp.-Ru. Although Ru overlayers with a metastable fcc.- (high in bulk energy) or stable hcp.-phase (low in bulk energy) can be epitaxially deposited on the basal planes, the lattice mismatch will lead to jagged hcp.- (high in surface energy) and smooth fcc.-facets (low in surface energy), resp., on the side faces. As the proportion of basal and side faces on the nanoplates varies with lateral size, the crystal phase will change depending on the relative contributions from the surface and bulk energies. The [email protected] outperform the [email protected] nanoplates toward ethylene glycol and glycerol oxidn. reactions.
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    Janssen, A.; Nguyen, Q. N.; Lyu, Z.; Pawlik, V.; Wang, C.; Xia, Y. Phase-controlled deposition of Ru on Pd nanocrystal templates: effects of particle shape and size. J. Phys. Chem. C 2023, 127, 1280– 1291,  DOI: 10.1021/acs.jpcc.2c08825
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    Joo, S. H.; Park, J. Y.; Renzas, J. R.; Butcher, D. R.; Huang, W.; Somorjai, G. A. Size effect of ruthenium nanoparticles in catalytic carbon monoxide oxidation. Nano Lett. 2010, 10, 2709– 2713,  DOI: 10.1021/nl101700j
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    Size Effect of Ruthenium Nanoparticles in Catalytic Carbon Monoxide Oxidation
    Joo, Sang Hoon; Park, Jeong Y.; Renzas, J. Russell; Butcher, Derek R.; Huang, Wenyu; Somorjai, Gabor A.
    Nano Letters (2010), 10 (7), 2709-2713CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    CO oxidn. over Ru catalysts exhibits an unusual catalytic behavior. This work discusses particle size effect on CO oxidn. over Ru nano-particle (NP) catalysts. Uniform Ru NP with a tunable particle size of 2-6 nm were synthesized by a polyol redn. of ruthenium acetylacetonate precursor in the presence of a poly(vinylpyrrolidone) stabilizer. Catalyst activity measurement for CO oxidn. over 2-dimensional Ru NP arrays under oxidizing reaction conditions (40 Torr CO, 100 Torr O2) showed activity depended on Ru NP size. CO oxidn. activity increased with NP size; the 6 nm Ru NP catalyst exhibited 8-fold higher activity than 2 nm catalysts. Results provide the scientific basis for future design of Ru-based oxidn. catalysts.
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  22. 22
    Li, W. Z.; Liu, J. X.; Gu, J.; Zhou, W.; Yao, S. Y.; Si, R.; Guo, Y.; Su, H. Y.; Yan, C. H.; Li, W. X. Chemical insights into the design and development of face-centered cubic ruthenium catalysts for Fischer–Tropsch synthesis. J. Am. Chem. Soc. 2017, 139, 2267– 2276,  DOI: 10.1021/jacs.6b10375
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    Chemical Insights into the Design and Development of Face-Centered Cubic Ruthenium Catalysts for Fischer-Tropsch Synthesis
    Li, Wei-Zhen; Liu, Jin-Xun; Gu, Jun; Zhou, Wu; Yao, Si-Yu; Si, Rui; Guo, Yu; Su, Hai-Yan; Yan, Chun-Hua; Li, Wei-Xue; Zhang, Ya-Wen; Ma, Ding
    Journal of the American Chemical Society (2017), 139 (6), 2267-2276CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Ruthenium is a promising low-temp. catalyst for Fischer-Tropsch synthesis (FTS). However, its scarcity and modest specific activity limit its widespread industrialization. The authors demonstrate here a strategy for tuning the crystal phase of catalysts to expose denser and active sites for a higher mass-specific activity. D. functional theory calcns. show that upon CO dissocn. there are a no. of open facets with modest barrier available on the fcc. Ru but only a few step edges with a lower barrier on conventional hcp. Ru. Guided by theor. calcns., water-dispersible fcc. Ru catalysts contg. abundant open facets were synthesized and showed an unprecedented mass-specific activity in the aq.-phase FTS, 37.8 molCO·molRu-1·h-1 at 433 K. The mass-specific activity of the fcc. Ru catalysts with an av. size of 6.8 nm is about three times larger than the previous best hcp. catalyst with a smaller size of 1.9 nm and a higher sp. surface area. The origin of the higher mass-specific activity of the fcc. Ru catalysts is identified exptl. from the 2 orders of magnitude higher d. of the active sites, despite its slightly higher apparent barrier. Exptl. results are in excellent agreement with prediction of theory. The great influence of the crystal phases on site distribution and their intrinsic activities revealed here provides a rationale design of catalysts for higher mass-specific activity without decrease of the particle size.
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    Li, L.; Liu, C.; Liu, S.; Wang, J.; Han, J.; Chan, T. S.; Li, Y.; Hu, Z.; Shao, Q.; Zhang, Q.; Huang, X. Phase engineering of a ruthenium nanostructure toward high-performance bifunctional hydrogen catalysis. ACS Nano 2022, 16, 14885– 14894,  DOI: 10.1021/acsnano.2c05776
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    Phase Engineering of a Ruthenium Nanostructure toward High-Performance Bifunctional Hydrogen Catalysis
    Li, Leigang; Liu, Cheng; Liu, Shangheng; Wang, Juan; Han, Jiajia; Chan, Ting-Shan; Li, Youyong; Hu, Zhiwei; Shao, Qi; Zhang, Qiaobao; Huang, Xiaoqing
    ACS Nano (2022), 16 (9), 14885-14894CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The physicochem. properties and catalytic performance of transition metals are highly phase-dependent. Ru-based nanomaterials are superior catalysts toward hydrogen evolution reaction (HER) and hydrogen oxidn. reaction (HOR), but studies are mostly limited to conventional hexagonal-close-packed (hcp) Ru, mainly arising from the difficulty in synthesizing Ru with pure face-centered-cubic (fcc) phase. Herein, we report a crystal-phase-dependent catalytic study of MoOx-modified Ru (MoOx-Ru fcc and MoOx-Ru hcp) for bifunctional HER and HOR. MoOx-Ru fcc is proven to outperform MoOx-Ru hcp in catalyzing both HER and HOR with much higher catalytic activity and more durable stability. The modification effect of MoOx gives rise to optimal adsorption of H and OH esp. on fcc Ru, which thus has resulted in the superior catalytic performance. This work highlights the significance of phase engineering in constructing superior electrocatalysts and may stimulate more efforts on phase engineering of other metal-based materials for diversified applications.
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    Kusada, K.; Kobayashi, H.; Yamamoto, T.; Matsumura, S.; Sumi, N.; Sato, K.; Nagaoka, K.; Kubota, Y.; Kitagawa, H. Discovery of face-centered-cubic ruthenium nanoparticles: facile size-controlled synthesis using the chemical reduction method. J. Am. Chem. Soc. 2013, 135, 5493– 5496,  DOI: 10.1021/ja311261s
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    24
    Discovery of Face-Centered-Cubic Ruthenium Nanoparticles: Facile Size-Controlled Synthesis Using the Chemical Reduction Method
    Kusada, Kohei; Kobayashi, Hirokazu; Yamamoto, Tomokazu; Matsumura, Syo; Sumi, Naoya; Sato, Katsutoshi; Nagaoka, Katsutoshi; Kubota, Yoshiki; Kitagawa, Hiroshi
    Journal of the American Chemical Society (2013), 135 (15), 5493-5496CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report the first discovery of pure face-centered-cubic (fcc) Ru nanoparticles. Although the fcc structure does not exist in the bulk Ru phase diagram, fcc Ru was obtained at room temp. because of the nanosize effect. We succeeded in sep. synthesizing uniformly sized nanoparticles of both fcc and hcp Ru having diams. of 2-5.5 nm by simple chem. redn. methods with different metal precursors. The prepd. fcc and hcp nanoparticles were both supported on γ-Al2O3, and their catalytic activities in CO oxidn. were investigated and found to depend on their structure and size.
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    Araki, N.; Kusada, K.; Yoshioka, S.; Sugiyama, T.; Ina, T.; Kitagawa, H. Observation of the formation processes of hexagonal close-packed and face-centered cubic Ru nanoparticles. Chem. Lett. 2019, 48, 1062– 1064,  DOI: 10.1246/cl.190338
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    Observation of the Formation Processes of Hexagonal Close-packed and Face-centered Cubic Ru Nanoparticles
    Araki, Naoki; Kusada, Kohei; Yoshioka, Satoru; Sugiyama, Takeharu; Ina, Toshiaki; Kitagawa, Hiroshi
    Chemistry Letters (2019), 48 (9), 1062-1064CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
    Face-centered cubic (fcc) Ru nanoparticles (NPs) have been recently synthesized by reducing Ru(III) acetylacetonate, despite bulk Ru having a hcp. (hcp) structure. We obsd. the formation processes of fcc and hcp Ru NPs using X-ray diffraction and X-ray absorption fine structure. We concluded that the strong Ru-O bond between Ru and the acetylacetonate ligand leads to the formation of the fcc structure.
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    Zhao, M.; Hood, Z. D.; Vara, M.; Gilroy, K. D.; Chi, M.; Xia, Y. Ruthenium nanoframes in the face-centered cubic phase: facile synthesis and their enhanced catalytic performance. ACS Nano 2019, 13, 7241– 7251,  DOI: 10.1021/acsnano.9b02890
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    Ruthenium Nanoframes in the Face-Centered Cubic Phase: Facile Synthesis and Their Enhanced Catalytic Performance
    Zhao, Ming; Hood, Zachary D.; Vara, Madeline; Gilroy, Kyle D.; Chi, Miaofang; Xia, Younan
    ACS Nano (2019), 13 (6), 7241-7251CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Owing to their highly open structure and a large no. of low-coordination sites on the surface, noble-metal nanoframes are intriguing for catalytic applications. Here, we demonstrate the rational synthesis of Ru cuboctahedral nanoframes with enhanced catalytic performance toward hydrazine decompn. The synthesis starts from Pd nanocubes, which quickly undergo truncation at the corners as a consequence of oxidative etching caused by Br- ions. Afterward, the galvanic replacement reaction between Pd and Ru(III) ions dominates, leading to the selective deposition of Ru atoms on the corners and edges and thereby the fabrication of Pd@Ru core-frame cuboctahedra. Significantly, the deposited Ru atoms are crystd. in a face-centered cubic (fcc) phase instead of the hcp. (hcp) structure typical of bulk Ru. Upon the removal of Pd remaining in the core via chem. etching, we obtain Ru cuboctahedral nanoframes. By varying the amt. of the Ru(III) precursor, the ridge thickness of the nanoframes can be tuned from a few at. layers up to 10. Both the frame structure and fcc crystal phase of the Ru cuboctahedral nanoframes can be well preserved up to 300°C. When compared with hcp-Ru nanoparticles, the fcc-Ru nanoframes displayed substantial enhancement in terms of H2 selectivity toward hydrazine decompn. This work offers the opportunity to engineer both the morphol. and crystal phase of Ru nanocrystals for catalytic applications.
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  27. 27
    Lin, J.-T.; Liu, Y.-H.; Tsao, C.-Y.; Wu, C.-Y.; Hsieh, C.-J.; Chen, M.-Z.; Chang, C.-W.; Hsiao, Y.-C.; Chen, H.-L.; Yang, T.-H. Toward a quantitative understanding of crystal-phase engineering of Ru nanocrystals. Chem. Mater. 2023, 35, 4276– 4285,  DOI: 10.1021/acs.chemmater.3c00326
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    Toward a Quantitative Understanding of Crystal-Phase Engineering of Ru Nanocrystals
    Lin, Jui-Tai; Liu, Yi-Hong; Tsao, Chi-Yen; Wu, Cheng-Yu; Hsieh, Chia-Jui; Chen, Meng-Zhe; Chang, Chun-Wei; Hsiao, Yueh-Chun; Chen, Hsin-Lung; Yang, Tung-Han
    Chemistry of Materials (2023), 35 (11), 4276-4285CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The crystal phase with a specific stacking sequence of atoms largely affects the catalytic performance of metal nanocrystals. Since the control of the phase at the same compn. is extremely difficult, the phase-dependent performance of metal nanocrystals is studied rarely. Here, we show the synthesis of Ru nanocrystals with different percentages of face-centered cubic (FCC) and hcp. (HCP) phases via kinetic control, further revealing a quant. correlation between the phase percentage of Ru nanocrystals and the initial redn. rate of Ru(III) precursors. Specifically, we manipulate the single parameter-initial redn. rate by controlling the Ru(III) injection rate into the dropwise synthesis at a fixed reaction temp. and correlate the kinetic data with the Ru phase percentage analyzed by at.-resoln. electron microscopy and synchrotron X-ray scattering. Based on the quant. anal., the ranges of initial redn. rates of Ru precursors can be detd. for synthesizing Ru nanocrystals with the percentages of unusual FCC phase from 9.0 to 55.1%. We demonstrate that a low initial redn. rate corresponds to the crystn. of the Ru HCP phase, while a high initial redn. rate favors the crystn. of the FCC lattice. Furthermore, we also systematically examine the catalytic performance of Ru nanocrystals with different phases.
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    Ye, H.; Wang, Q.; Catalano, M.; Lu, N.; Vermeylen, J.; Kim, M. J.; Liu, Y.; Sun, Y.; Xia, X. Ru nanoframes with an fcc structure and enhanced catalytic properties. Nano Lett. 2016, 16, 2812– 2817,  DOI: 10.1021/acs.nanolett.6b00607
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    Ru Nanoframes with an fcc Structure and Enhanced Catalytic Properties
    Ye, Haihang; Wang, Qingxiao; Catalano, Massimo; Lu, Ning; Vermeylen, Joseph; Kim, Moon J.; Liu, Yuzi; Sun, Yugang; Xia, Xiaohu
    Nano Letters (2016), 16 (4), 2812-2817CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Noble-metal nanoframes are of great interest to many applications due to their unique open structures. Among various noble metals, Ru has never been made into nanoframes. In this study, we report for the first time an effective method based on seeded growth and chem. etching for the facile synthesis of Ru nanoframes with high purity. The essence of this approach is to induce the preferential growth of Ru on the corners and edges of Pd truncated octahedra as the seeds by kinetic control. The resultant Pd-Ru core-frame octahedra could be easily converted to Ru octahedral nanoframes of ∼2 nm in thickness by selectively removing the Pd cores through chem. etching. Most importantly, in this approach the face-centered cubic (fcc) crystal structure of Pd seeds was faithfully replicated by Ru that usually takes an hcp structure. The fcc Ru nanoframes showed higher catalytic activities toward the redn. of p-nitrophenol by NaBH4 and the dehydrogenation of ammonia borane compared with hcp Ru nanowires with roughly the same thickness.
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  29. 29
    Zhao, M.; Elnabawy, A. O.; Vara, M.; Xu, L.; Hood, Z. D.; Yang, X.; Gilroy, K. D.; Figueroa-Cosme, L.; Chi, M.; Mavrikakis, M.; Xia, Y. Facile synthesis of Ru-based octahedral nanocages with ultrathin walls in a face-centered cubic structure. Chem. Mater. 2017, 29, 9227– 9237,  DOI: 10.1021/acs.chemmater.7b03092
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    29
    Facile synthesis of Ru-based octahedral nanocages with ultrathin walls in face-centered cubic structure
    Zhao, Ming; Elnabawy, Ahmed O.; Vara, Madeline; Xu, Lang; Hood, Zachary D.; Yang, Xuan; Gilroy, Kyle D.; Figueroa-Cosme, Legna; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
    Chemistry of Materials (2017), 29 (21), 9227-9237CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hcp. (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temp. were all instrumental to the formation of Pd@Ru core-shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five at. layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent d. functional theory calcns. to study the adsorption and dissocn. of N2 as a means to predict the catalytic performance toward ammonia synthesis. Our results suggested that the small proportions of Pd atoms left behind in the walls during etching could play a key role in stabilizing the adsorption of N2 as well as in reducing the activation energy barrier to N2 dissocn.
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    Zhao, M.; Chen, Z.; Lyu, Z.; Hood, Z. D.; Xie, M.; Vara, M.; Chi, M.; Xia, Y. Ru octahedral nanocrystals with a face-centered cubic structure, {111} facets, thermal stability up to 400 °C, and enhanced catalytic activity. J. Am. Chem. Soc. 2019, 141, 7028– 7036,  DOI: 10.1021/jacs.9b01640
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    Ru Octahedral Nanocrystals with a Face-Centered Cubic Structure, {111} Facets, Thermal Stability up to 400°C, and Enhanced Catalytic Activity
    Zhao, Ming; Chen, Zitao; Lyu, Zhiheng; Hood, Zachary D.; Xie, Minghao; Vara, Madeline; Chi, Miaofang; Xia, Younan
    Journal of the American Chemical Society (2019), 141 (17), 7028-7036CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Ruthenium nanocrystals with both a face-centered cubic (fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepd. as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the at. packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hcp. (hcp) lattice assocd. with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400°C, which is more than 100°C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.
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    Huo, D.; Cao, Z.; Li, J.; Xie, M.; Tao, J.; Xia, Y. Seed-mediated growth of Au nanospheres into hexagonal stars and the emergence of a hexagonal close-packed phase. Nano Lett. 2019, 19, 3115– 3121,  DOI: 10.1021/acs.nanolett.9b00534
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    Seed-Mediated Growth of Au Nanospheres into Hexagonal Stars and the Emergence of a Hexagonal Close-Packed Phase
    Huo, Da; Cao, Zhenming; Li, Jun; Xie, Minghao; Tao, Jing; Xia, Younan
    Nano Letters (2019), 19 (5), 3115-3121CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Gold (Au) typically crystallizes in a cubic close-packed (ccp) structure to present a face-centered cubic (fcc) lattice or crystal phase. Herein, we demonstrate that Au nanoscale hexagonal stars featuring a hcp. (hcp) structure can be synthesized in an aq. system in the presence of fcc-Au nanospheres as the seeds. The success of this synthesis critically relies on the use of EDTA to complex with Au3+ ions (the precursor) and the introduction of 2-phospho-L-ascorbic acid trisodium salt (Asc-2P) as a novel reducing agent to maneuver the redn. kinetics. The use of Asc-2P favorably promotes the formation of hexagonal stars with uneven surfaces at the top and bottom faces, together with concave side faces around the edges. By varying the amt. of Asc-2P to fine-tune the redn. kinetics, we can adjust the concaveness of the side faces, with a faster redn. rate favoring greater concaveness and a red shift of the plasmon resonance peak to the near-IR. For the first time, our results suggest that the phosphate and hydroxyl groups can act synergistically in controlling the morphol. of Au nanocrystals. Most significantly, the newly deposited Au atoms can also crystallize in an hcp structure, leading to the observation of a phase transition from fcc to hcp along the growth direction. This new protocol based upon kinetic control can be potentially extended to other noble metals for the facile synthesis of nanocrystals featuring unprecedented structures or phases.
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    Gloag, L.; Benedetti, T. M.; Cheong, S.; Marjo, C. E.; Gooding, J. J.; Tilley, R. D. Cubic-core hexagonal-branch mechanism to synthesize bimetallic branched and faceted Pd–Ru nanoparticles for oxygen evolution reaction electrocatalysis. J. Am. Chem. Soc. 2018, 140, 12760– 12764,  DOI: 10.1021/jacs.8b09402
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    Cubic-Core Hexagonal-Branch Mechanism To Synthesize Bimetallic Branched and Faceted Pd-Ru Nanoparticles for Oxygen Evolution Reaction Electrocatalysis
    Gloag, Lucy; Benedetti, Tania M.; Cheong, Soshan; Marjo, Christopher E.; Gooding, J. Justin; Tilley, Richard D.
    Journal of the American Chemical Society (2018), 140 (40), 12760-12764CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A major synthetic challenge is to make metal nanoparticles with nanosized branches and well-defined facets for high-performance catalysts. Here, we introduce a mechanism that uses the growth of hexagonal crystal structured branches off cubic crystal structured core nanoparticles. We control the growth to form Pd-core Ru-branch nanoparticles that have nanosized branches with low index Ru facets. We demonstrate that the branched and faceted structural features of the Pd-Ru nanoparticles retain high catalytic activity while also achieving high stability for the O evolution reaction.
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    Yun, Q.; Ge, Y.; Huang, B.; Wa, Q.; Zhang, H. Ligand-assisted phase engineering of nanomaterials. Acc. Chem. Res. 2023, 56, 1780– 1790,  DOI: 10.1021/acs.accounts.3c00121
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    Ligand-Assisted Phase Engineering of Nanomaterials
    Yun, Qinbai; Ge, Yiyao; Huang, Biao; Wa, Qingbo; Zhang, Hua
    Accounts of Chemical Research (2023), 56 (13), 1780-1790CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    The synthesis of monodisperse colloidal nanomaterials with well-defined structures is important for both fundamental research and practical application. To achieve it, wet-chem. methods with the usage of various ligands have been extensively explored to finely control the structure of nanomaterials. During the synthesis, ligands cap the surface and thus modulate the size, shape, and stability of nanomaterials in solvents. Besides these widely investigated roles of ligands, it has been recently discovered that ligands can affect the phase of nanomaterials, i.e., their at. arrangement, providing an effective strategy to realize the phase engineering of nanomaterials (PEN) by selecting appropriate ligands. Nanomaterials normally exist in the phases that are thermodynamically stable in their bulk states. Previous studies have shown that under high temp. or high pressure, nanomaterials can exist in unconventional phases which are unattainable in the bulks. Importantly, nanomaterials with unconventional phases exhibit unique properties and functions different from conventional-phase ones. Consequently, it is feasible to utilize the PEN to tune the physicochem. properties and application performance of nanomaterials. During wet-chem. synthesis, ligands binding to the surface of nanomaterials can modify their surface energy, which could significantly affect the Gibbs free energy of nanomaterials and thus det. the stability of different phases, making it possible to obtain nanomaterials with unconventional phases at mild reaction conditions. For instance, a series of Au nanomaterials with unconventional hexagonal phases have been prepd. with the assistance of oleylamine. Therefore, the rational design and selection of different ligands and deep understanding of their effect on the phase of nanomaterials would significantly accelerate the development of PEN and the discovery of novel functional nanomaterials for diverse applications. In this Account, we briefly summarize the recent progress in ligand-assisted PEN, elaborating the important roles of different ligands in the direct synthesis of nanomaterials with unconventional crystal phases and amorphous phase as well as the phase transformation of nanomaterials. We first introduce the background of this research topic, highlighting the concept of PEN and why ligands can modulate the phase of nanomaterials. Then we discuss the usage of four kinds of ligands, i.e., amines, fatty acids, sulfur-contg. ligands, and phosphorus-contg. ligands, in phase engineering of different nanomaterials, esp. metal, metal chalcogenide, and metal oxide nanomaterials. Finally, we provide our personal views of the challenges and future promising research directions in this exciting field.
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    Nguyen, Q. N.; Chen, R.; Lyu, Z.; Xia, Y. Using reduction kinetics to control and predict the outcome of a colloidal synthesis of noble-metal nanocrystals. Inorg. Chem. 2021, 60, 4182– 4197,  DOI: 10.1021/acs.inorgchem.0c03576
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    Using reduction kinetics to control and predict outcome of colloidal synthesis of noble-metal nanocrystals
    Nguyen, Quynh N.; Chen, Ruhui; Lyu, Zhiheng; Xia, Younan
    Inorganic Chemistry (2021), 60 (7), 4182-4197CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Improving the performance of noble-metal nanocrystals in various applications critically depends on our ability to manipulate their synthesis in a rational, robust, and controllable fashion. Different from a conventional trial-and-error approach, the redn. kinetics of a colloidal synthesis has recently been demonstrated as a reliable knob for controlling the synthesis of noble-metal nanocrystals in a deterministic and predictable manner. Here we present a brief viewpoint on the recent progress in leveraging redn. kinetics for controlling and predicting the outcome of a synthesis of noble-metal nanocrystals. With a focus on Pd nanocrystals, we first offer a discussion on the correlation between the initial redn. rate and the internal structure of the resultant seeds. The kinetic approaches for controlling both nucleation and growth in a one-pot setting are then introduced with an emphasis on manipulation of the redn. pathways taken by the precursor. We then illustrate how to extend the strategy into a bimetallic system for the prepn. of nanocrystals with different shapes and elemental distributions. Finally, the influence of speciation of the precursor on redn. kinetics is highlighted, followed by our perspectives on the challenges and future endeavors in achieving a controllable and predictable synthesis of noble-metal nanocrystals.
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    Huang, X.; Li, S.; Huang, Y.; Wu, S.; Zhou, X.; Li, S.; Gan, C. L.; Boey, F.; Mirkin, C. A.; Zhang, H. Synthesis of hexagonal close-packed gold nanostructures. Nat. Commun. 2011, 2, 292,  DOI: 10.1038/ncomms1291
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    Synthesis of hexagonal close-packed gold nanostructures
    Huang Xiao; Li Shaozhou; Huang Yizhong; Wu Shixin; Zhou Xiaozhu; Li Shuzhou; Gan Chee Lip; Boey Freddy; Mirkin Chad A; Zhang Hua
    Nature communications (2011), 2 (), 292 ISSN:.
    Solid gold is usually most stable as a face-centred cubic (fcc) structure. To date, no one has synthesized a colloidal form of Au that is exclusively hexagonal close-packed (hcp) and stable under ambient conditions. Here we report the first in situ synthesis of dispersible hcp Au square sheets on graphene oxide sheets, which exhibit an edge length of 200-500 nm and a thickness of ~ 2.4 nm (~ 16 Au atomic layers). Interestingly, the Au square sheet transforms from hcp to a fcc structure on exposure to an electron beam during transmission electron microscopy analysis. In addition, as the square sheet grows thicker (from ~ 2.4 to 6 nm), fcc segments begin to appear. A detailed experimental analysis of these structures shows that for structures with ultrasmall dimensions (for example, <~ 6 nm thickness for the square sheets), the previously unobserved pure hcp structure becomes stable and isolable.
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    Gao, Y.; Peng, X. Crystal structure control of CdSe nanocrystals in growth and nucleation: dominating effects of surface versus interior structure. J. Am. Chem. Soc. 2014, 136, 6724– 6732,  DOI: 10.1021/ja5020025
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    Crystal Structure Control of CdSe Nanocrystals in Growth and Nucleation: Dominating Effects of Surface versus Interior Structure
    Gao, Yuan; Peng, Xiaogang
    Journal of the American Chemical Society (2014), 136 (18), 6724-6732CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    For the most studied nanocrystal system in the literature, exptl. results in this paper revealed that formation of either Zn blende or wurtzite CdSe nanocrystals was dominated by the ligand-surface interaction, instead of the interior structure difference. This conclusion was considered to be reasonable, given the very small energy difference between wurtzite and Zn blende CdSe (only 1.4 meV per CdSe unit and ∼1000 times smaller than the energy of a single Cd-ligand bond). Cd carboxylate ligands as Cd fatty acid salts promoted formation of the Zn blende structure. Conversely, Cd phosphonate ligands with a long hydrocarbon chain favored the formation of the wurtzite structure. The effects of either Cd carboxylate or Cd phosphonate ligands play a detg. role during both nucleation and growth. Different from the authors' expectation, fatty amine is only a secondary factor for crystal structure detn. With an appropriate choice of capping ligands, it was possible to achieve precise control of the crystal structure of the CdSe nanocrystals in both nucleation and growth for either the Zn blende or wurtzite structure.
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    Yao, Y.; He, D.; Lin, Y.; Feng, X.; Wang, X.; Yin, P.; Hong, X.; Zhou, G.; Wu, Y.; Li, Y. Modulating fcc and hcp ruthenium on the surface of palladium-copper alloy through tunable lattice mismatch. Angew. Chem., Int. Ed. 2016, 55, 5501– 5505,  DOI: 10.1002/anie.201601016
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    Modulating fcc and hcp Ruthenium on the Surface of Palladium-Copper Alloy through Tunable Lattice Mismatch
    Yao, Yancai; He, Dong Sheng; Lin, Yue; Feng, Xiaoqian; Wang, Xin; Yin, Peiqun; Hong, Xun; Zhou, Gang; Wu, Yuen; Li, Yadong
    Angewandte Chemie, International Edition (2016), 55 (18), 5501-5505CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein, we report an epitaxial-growth-mediated method to grow fcc. Ru, which is thermodynamically unfavorable in the bulk form, on the surface of Pd-Cu alloy. Induced by the galvanic replacement between Ru and Pd-Cu alloy, a shape transformation from a Pd-Cu@Ru core-shell to a yolk-shell structure was obsd. during the epitaxial growth. The successful coating of the unconventional crystallog. structure is critically dependent on the moderate lattice mismatch between the fcc. Ru overlayer and PdCu3 alloy substrate. Further, both fcc. and hcp. Ru can be selectively grown through varying the lattice spacing of the Pd-Cu substrate. The presented findings provide a new synthetic pathway to control the crystallog. structure of metal nanomaterials.
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    Gloag, L.; Benedetti, T. M.; Cheong, S.; Li, Y.; Chan, X.-H.; Lacroix, L.-M.; Chang, S. L. Y.; Arenal, R.; Florea, I.; Barron, H.; Barnard, A. S.; Henning, A. M.; Zhao, C.; Schuhmann, W.; Gooding, J. J.; Tilley, R. D. Three-dimensional branched and faceted gold-ruthenium nanoparticles: using nanostructure to improve stability in oxygen evolution electrocatalysis. Angew. Chem., Int. Ed. 2018, 57, 10241– 10245,  DOI: 10.1002/anie.201806300
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    Three-Dimensional Branched and Faceted Gold-Ruthenium Nanoparticles: Using Nanostructure to Improve Stability in Oxygen Evolution Electrocatalysis
    Gloag, Lucy; Benedetti, Tania M.; Cheong, Soshan; Li, Yibing; Chan, Xuan-Hao; Lacroix, Lise-Marie; Chang, Shery L. Y.; Arenal, Raul; Florea, Ileana; Barron, Hector; Barnard, Amanda S.; Henning, Anna M.; Zhao, Chuan; Schuhmann, Wolfgang; Gooding, J. Justin; Tilley, Richard D.
    Angewandte Chemie, International Edition (2018), 57 (32), 10241-10245CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Achieving stability with highly active Ru nanoparticles for electrocatalysis is a major challenge for the O evolution reaction. As improved stability of Ru catalysts was shown for bulk surfaces with low-index facets, there is an opportunity to incorporate these stable facets into Ru nanoparticles. Now, a new soln. synthesis is presented in which hcp. structured Ru is grown on Au to form nanoparticles with 3-dimensional branches. Exposing low-index facets on these 3-dimensional branches creates stable reaction kinetics to achieve high activity and the highest stability obsd. for Ru nanoparticle O evolution reaction catalysts. These design principles provide a synthetic strategy to achieve stable and active electrocatalysts.
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    Koenigsmann, C.; Semple, D. B.; Sutter, E.; Tobierre, S. E.; Wong, S. S. Ambient synthesis of high-quality ruthenium nanowires and the morphology-dependent electrocatalytic performance of platinum-decorated ruthenium nanowires and nanoparticles in the methanol oxidation reaction. ACS Appl. Mater. Interfaces 2013, 5, 5518– 5530,  DOI: 10.1021/am4007462
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    Ambient Synthesis of High-Quality Ruthenium Nanowires and the Morphology-Dependent Electrocatalytic Performance of Platinum-Decorated Ruthenium Nanowires and Nanoparticles in the Methanol Oxidation Reaction
    Koenigsmann, Christopher; Semple, Dara Bobb; Sutter, Eli; Tobierre, Sybil E.; Wong, Stanislaus S.
    ACS Applied Materials & Interfaces (2013), 5 (12), 5518-5530CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The synthesis is reported for first time of elemental ruthenium nanowires (Ru NWs), method for modifying their surfaces with platinum (Pt), and the morphol.-dependent methanol oxidn. reaction (MOR) performance of high-quality Pt-modified Ru NW electrocatalysts. The synthesis of the elemental Ru NWs was accomplished utilizing a template-based method under ambient conditions. As-prepd. Ru NWs are cryst. and elementally pure, maintain electrochem. properties analogous to elemental Ru, and can be generated with av. diams. ranging from 44-280 nm. The morphol.-dependent performance is examd. of the Ru NWs by comparison with com. Ru nanoparticle (NP)/carbon (C) systems after decorating the surfaces of these structures with Pt. It is demonstrated that the deposition of Pt onto the Ru NWs (Pt∼Ru NWs) results in a unique hierarchical structure, wherein the deposited Pt exists as discrete clusters on the surface. By contrast, it is found that the Pt-decorated com. Ru NP/C (Pt∼Ru NP/C) results in the formation of an alloy-type NP. The Pt∼Ru NPs (0.61 A/mg of Pt) possess nearly 2-fold higher Pt mass activity than analogous Pt∼Ru NW electrocatalysts (0.36 A/mg of Pt). On the basis of a long-term durability test, it is apparent that both catalysts undergo significant declines in performance, potentially resulting from aggregation and ripening in the case of Pt∼Ru NP/C and the effects of catalyst poisoning in the Pt∼Ru NWs. Both catalysts maintain comparable performance, despite a slightly enhanced performance in Pt∼Ru NP/C. In addn., the measured mass-normalized MOR activity of the Pt∼Ru NWs (0.36 A/mg of Pt) was significantly enhanced as compared with supported elemental Pt (Pt NP/C, 0.09 A/mg of Pt) and alloy-type PtRu (PtRu NP/C, 0.24 A/mg of Pt) NPs, both serving as com. stds.
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    Yin, A. X.; Liu, W. C.; Ke, J.; Zhu, W.; Gu, J.; Zhang, Y. W.; Yan, C. H. Ru nanocrystals with shape-dependent surface-enhanced Raman spectra and catalytic properties: controlled synthesis and DFT calculations. J. Am. Chem. Soc. 2012, 134, 20479– 20489,  DOI: 10.1021/ja3090934
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    Ru Nanocrystals with Shape-Dependent Surface-Enhanced Raman Spectra and Catalytic Properties: Controlled Synthesis and DFT Calculations
    Yin, An-Xiang; Liu, Wen-Chi; Ke, Jun; Zhu, Wei; Gu, Jun; Zhang, Ya-Wen; Yan, Chun-Hua
    Journal of the American Chemical Society (2012), 134 (50), 20479-20489CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Despite its multidisciplinary interests and technol. importance, the shape control of Ru nanocrystals still remains a great challenge. In this article, we demonstrated a facile hydrothermal approach toward the controlled synthesis of Ru nanocrystals with the assistance of first-principles calcns. For the first time, Ru triangular and irregular nanoplates as well as capped columns with tunable sizes were prepd. with high shape selectivity. In consistency with the exptl. observations and d. functional theory (DFT) calcns. confirmed that both the intrinsic characteristics of Ru crystals and the adsorption of certain reaction species were responsible for the shape control of Ru nanocrystals. Ultrathin Ru nanoplates exposed a large portion of (0001) facets due to the lower surface energy of Ru(0001). The selective adsorption of oxalate species on Ru(10-10) would retard the growth of the side planes of the Ru nanocrystals, while the gradual thermolysis of the oxalate species would eliminate their adsorption effects, leading to the shape evolution of Ru nanocrystals from prisms to capped columns. The surface-enhanced Raman spectra (SERS) signals of these Ru nanocrystals with 4-mercaptopyridine as mol. probes showed an enhancement sequence of capped columns > triangle nanoplates > nanospheres, probably due to the sharp corners and edges in the capped columns and nanoplates as well as the shrunk interparticle distance in their assemblies. CO-selective methanation tests on these Ru nanocrystals indicated that the nanoplates and nanospheres had comparable activities, but the former has much better CO selectivity than the latter.
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    Watt, J.; Yu, C.; Chang, S. L. Y.; Cheong, S.; Tilley, R. D. Shape control from thermodynamic growth conditions: the case of hcp ruthenium hourglass nanocrystals. J. Am. Chem. Soc. 2013, 135, 606– 609,  DOI: 10.1021/ja311366k
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    Shape Control from Thermodynamic Growth Conditions: The Case of hcp Ruthenium Hourglass Nanocrystals
    Watt, John; Yu, Chenlong; Chang, Shery L. Y.; Cheong, Soshan; Tilley, Richard D.
    Journal of the American Chemical Society (2013), 135 (2), 606-609CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Recent successes in forming different shaped fcc. (fcc) metal nanostructures has enabled a greater understanding of nanocrystal growth mechanisms. Here we extend this understanding to the synthesis of hexagonally close packed (hcp) metal nanostructures, to form uniquely faceted ruthenium nanocrystals with a well-defined hourglass shape. The hourglass nanocrystals are formed in a three-step thermodn. growth process with dodecylamine as the org. stabilizer. The hourglass nanocrystals are then shown to readily self-assemble to form a new type of nanocrystal superlattice.
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    Rodrigues, T. S.; Zhao, M.; Yang, T.-H.; Gilroy, K. D.; da Silva, A. G. M.; Camargo, P. H. C.; Xia, Y. Synthesis of colloidal metal nanocrystals: a comprehensive review on the reductants. Chem.─Eur. J. 2018, 24, 16944– 16963,  DOI: 10.1002/chem.201802194
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    Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants
    Rodrigues, Thenner S.; Zhao, Ming; Yang, Tung-Han; Gilroy, Kyle D.; da Silva, Anderson G. M.; Camargo, Pedro H. C.; Xia, Younan
    Chemistry - A European Journal (2018), 24 (64), 16944-16963CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chem. redn.: the reductants. We start with a brief introduction to the compds. commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidn. half-reactions and the corresponding oxidn. potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion redn. based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, redn. mechanisms, and addnl. effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.
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    Holder, C. F.; Schaak, R. E. Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials. ACS Nano 2019, 13, 7359– 7365,  DOI: 10.1021/acsnano.9b05157
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    Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials
    Holder, Cameron F.; Schaak, Raymond E.
    ACS Nano (2019), 13 (7), 7359-7365CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. The authors provide the broad nanoscience and nanotechnol. communities with a brief tutorial on some of the key aspects of powder XRD data that are often encountered when analyzing samples of nanoscale materials, with an emphasis on inorg. nanoparticles of various sizes, shapes, and dimensionalities.
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    Nguyen, Q. N.; Wang, C.; Shang, Y.; Janssen, A.; Xia, Y. Colloidal synthesis of metal nanocrystals: from asymmetrical growth to symmetry breaking. Chem. Rev. 2023, 123, 3693– 3760,  DOI: 10.1021/acs.chemrev.2c00468
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    Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking
    Nguyen, Quynh N.; Wang, Chenxiao; Shang, Yuxin; Janssen, Annemieke; Xia, Younan
    Chemical Reviews (Washington, DC, United States) (2023), 123 (7), 3693-3760CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Nanocrystals offer a unique platform for tailoring the physicochem. properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around sym. growth, the introduction of asym. growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies, as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asym. growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a no. of methods capable of generating seeds with diverse symmetry while achieving asym. growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research in understanding and controlling the symmetry breaking process.
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    Yin, X.; Shi, M.; Wu, J.; Pan, Y.-T.; Gray, D. L.; Bertke, J. A.; Yang, H. Quantitative analysis of different formation modes of platinum nanocrystals controlled by ligand chemistry. Nano Lett. 2017, 17, 6146– 6150,  DOI: 10.1021/acs.nanolett.7b02751
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    Quantitative Analysis of Different Formation Modes of Platinum Nanocrystals Controlled by Ligand Chemistry
    Yin, Xi; Shi, Miao; Wu, Jianbo; Pan, Yung-Tin; Gray, Danielle L.; Bertke, Jeffery A.; Yang, Hong
    Nano Letters (2017), 17 (10), 6146-6150CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors report that spontaneous ligand replacement and anion exchange control the form of coordinated Pt-ligand intermediates in the system of platinum acetylacetonate [Pt(acac)2], primary aliph. amine, and carboxylic acid ligands. The formed intermediates govern the formation mode of Pt nanocrystals, leading to either a pseudo two-step or a one-step mechanism by switching on or off an autocatalytic surface growth. This finding shows the importance of metal-ligand complexation at the prenucleation stage and represents a crit. step forward for the designed synthesis of nanocrystal-based materials.
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    Yang, T.-H.; Zhou, S.; Gilroy, K. D.; Figueroa-Cosme, L.; Lee, Y.-H.; Wu, J.-M.; Xia, Y. Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 13619– 13624,  DOI: 10.1073/pnas.1713907114
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    Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals
    Yang, Tung-Han; Zhou, Shan; Gilroy, Kyle D.; Figueroa-Cosme, Legna; Lee, Yi-Hsien; Wu, Jenn-Ming; Xia, Younan
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (52), 13619-13624CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The growth of colloidal metal nanocrystals typically involves an autocatalytic process, where the salt precursor is adsorbed on a growing nanocrystal surface, followed by chem. redn. to atoms for their incorporation into the nanocrystal. Despite its universal role in colloidal nanocrystal synthesis, it is still poorly understood and controlled in terms of kinetics. Using well-defined nanocrystals as seeds (including those with different types of facets, sizes, internal twin structure) this work quant. analyzed the kinetics of autocatalytic surface redn. to control the evolution of nanocrystals as predictable shapes. Kinetic measurements demonstrated the activation energy barrier to autocatalytic surface redn. highly depended on type of facet and presence of twin boundary, corresponding to distinctive growth patterns and products. The autocatalytic process effectively eliminates homogeneous nucleation and activates and sustains octahedral nanocrystal growth. This work was a major step toward quant. understanding and controlling the autocatalytic process involved in colloidal metal nanocrystal synthesis.
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    Yang, T.-H.; Peng, H.-C.; Zhou, S.; Lee, C.-T.; Bao, S.; Lee, Y.-H.; Wu, J.-M.; Xia, Y. Toward a quantitative understanding of the reduction pathways of a salt precursor in the synthesis of metal nanocrystals. Nano Lett. 2017, 17, 334– 340,  DOI: 10.1021/acs.nanolett.6b04151
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    Toward a Quantitative Understanding of the Reduction Pathways of a Salt Precursor in the Synthesis of Metal Nanocrystals
    Yang, Tung-Han; Peng, Hsin-Chieh; Zhou, Shan; Lee, Chi-Ta; Bao, Shixiong; Lee, Yi-Hsien; Wu, Jenn-Ming; Xia, Younan
    Nano Letters (2017), 17 (1), 334-340CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Despite the pivotal role played by the redn. of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the soln. phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by redn. through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the redn. kinetics involved. Quant. analyses of the redn. kinetics of PdCl42- and PdBr42- by ascorbic acid at room temp. in the absence and presence of Pd nanocubes, resp., suggest that PdCl42- was reduced in the soln. phase while PdBr42- was reduced on the surface of a growing nanocrystal. These results demonstrate that the redn. pathway of PdBr42- by ascorbic acid could be switched from surface to soln. by raising the reaction temp.
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    Xia, X.; Xie, S.; Liu, M.; Peng, H. C.; Lu, N.; Wang, J.; Kim, M. J.; Xia, Y. On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals. Proc. Natl. Acad. Sci. U.S.A 2013, 110, 6669– 6673,  DOI: 10.1073/pnas.1222109110
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    On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals
    Xia, Xiaohu; Xie, Shuifen; Liu, Maochang; Peng, Hsin-Chieh; Lu, Ning; Wang, Jinguo; Kim, Moon J.; Xia, Younan
    Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (17), 6669-6673, S6669/1-S6669/6CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Controlling the shape or morphol. of metal nanocrystals is central to the realization of their many applications in catalysis, plasmonics, and electronics. In one of the approaches, the metal nanocrystals are grown from seeds of certain crystallinity through the addn. of at. species. In this case, manipulating the rates at which the at. species are added onto different crystallog. planes of a seed has been actively explored to control the growth pattern of a seed and thereby the shape or morphol. taken by the final product. Upon deposition, however, the adsorbed atoms (adatoms) may not stay at the same sites where the depositions occur. Instead, they can migrate to other sites on the seed owing to the involvement of surface diffusion, and this could lead to unexpected deviations from a desired growth pathway. The authors demonstrate that the growth pathway of a seed is indeed detd. by the ratio between the rates for atom deposition and surface diffusion. Surface diffusion needs to be taken into account when controlling the shape or morphol. of metal nanocrystals.
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    Janssen, A.; Pawlik, V.; von Rueden, A. D.; Xu, L.; Wang, C.; Mavrikakis, M.; Xia, Y. Facile synthesis of palladium-based nanocrystals with different crystal phases and a comparison of their catalytic properties. Adv. Mater. 2021, 33, 2103801  DOI: 10.1002/adma.202103801
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  1. Jiayi Li, Jilan Zeng, Fuwei Zhao, Xinran Sun, Sibo Wang, Xue Feng Lu. A Review on Highly Efficient Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction. Energy & Fuels 2024, 38 (13) , 11521-11540. https://doi.org/10.1021/acs.energyfuels.4c02080
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  • Abstract

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    Figure 1

    Figure 1. Schematic illustration of four synthetic routes to the preparation of Ru nanocrystals with different patterns of phases by simply switching the polyol used in the overgrowth process.

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    Figure 2

    Figure 2. Ru@Ru nanocrystals with an hcp or fcc phase in the shell obtained through the first round of overgrowth. (A, B) TEM images of the Ru@Ru nanocrystals synthesized from the Ruhcp seeds using (A) EG and (B) TEG, respectively. (C) XRD patterns of Ruhcp@Ruhcp and Ruhcp@Rufcc nanocrystals produced in EG and TEG, respectively, together with that of the Ruhcp seeds for comparison. The insets show the corresponding atomic model of the nanocrystals in a cross-sectional view. The red and blue lines correspond to the characteristic peaks of fcc-Ru (JCPDS No. 01–088–2333) and hcp-Ru (JCPDS No. 06–0663), respectively.

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    Figure 3

    Figure 3. (A–D) Characterizations of Ruhcp@Ruhcp nanocrystals: (A) HRTEM image, (B) atomic resolution HRTEM image, (C) the corresponding FFT pattern of the blue boxed region in A, and (D) a projected model of Ru atoms in the hcp structure viewed along the [21̅10] zone axis. (E–H) Characterizations of Ruhcp@Rufcc nanocrystals: (E) HRTEM image, (F) atomic resolution HRTEM image, and (G,H) the corresponding FFT patterns of the red and blue boxed regions in E, respectively. The inset in panel E shows a schematic illustration of the Ruhcp@Rufcc nanocrystal with a plate-like morphology.

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    Figure 4

    Figure 4. XRD patterns of the 7 nm Ru nanocrystals with an hcp or fcc phase in the outermost shell obtained from additional overgrowth on (A, B) Ruhcp@Ruhcp and (C, D) Ruhcp@Rufcc seeds in different types of polyols: (A, C) EG and (B, D) TEG. The red and blue lines correspond to the characteristic peaks of fcc-Ru (JCPDS No. 01–088–2333) and hcp-Ru (JCPDS No. 06–0663), respectively. The insets in panels A and D are the corresponding atomic models.

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    Figure 5

    Figure 5. (A–D) Characterizations of Ruhcp@Ruhcp@Ruhcp nanocrystals: (A) HRTEM image, (B) atomic resolution HRTEM image, (C) the corresponding FFT pattern of the blue boxed region in A, and (D) a simulated model for the atomic arrangement of hcp structure viewed along the [21̅10] direction. (E–H) Characterizations of Ruhcp@Rufcc@Rufcc nanocrystals: (E) HRTEM image, (F) atomic resolution HRTEM image, (G–H) the corresponding FFT patterns of the red boxed region in E, and (D) a projected model of Ru atoms in the fcc structure viewed along the [011] direction. In panel E, the white lines indicate twin boundaries while the inset shows a schematic of the Ruhcp@Rufcc@Rufcc nanocrystal with a 5-fold twin structure.

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    Figure 6

    Figure 6. Snapshots from an AIMD simulation of the reduction of RuO2 by EG. The blue box depicts the region where Ru reduction occurs and (A–D) show the progress of RuO2 reduction, together with the OS of Ru. Yellow: Ru in EG; pink: oxygen; gray: carbon; white: hydrogen.

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    Figure 7

    Figure 7. Top and side view of RuNOM at fcc and hcp site on hcp-Ru(0001). RuNOM, given by Equation S1, is indicated. (A) one RuO2 deposited with Ru at fcc and hcp sites; (B) RuO2 with a coverage (θ) of 3/8 at fcc and hcp sites; (C) RuNOM where Ru atoms are at fcc (left) and hcp (right) and oxygen atoms are at hcp (left) and fcc (right) sites on hcp-Ru (0001). Sky and blue: hcp-Ru; red: fcc-Ru; pink: oxygen.

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    Figure 8

    Figure 8. Snapshot of final configurations and the detailed mechanism involving the removal of hydrogen atoms from TEG. The green circles indicate the created hydroxyl groups by removal of hydrogen atoms from TEG. Sky and blue: hcp-Ru seed; red: fcc-Ru; pink: oxygen; gray: carbon; white: hydrogen.

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    Figure 9

    Figure 9. Mechanism of the effect of reduction kinetics on the phase-selective epitaxial growth of Ru overlayers based on both computational and experimental results.

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      Chen, Ye; Lai, Zhuangchai; Zhang, Xiao; Fan, Zhanxi; He, Qiyuan; Tan, Chaoliang; Zhang, Hua
      Nature Reviews Chemistry (2020), 4 (5), 243-256CODEN: NRCAF7; ISSN:2397-3358. (Nature Research)
      A review. Abstr.: Phase has emerged as an important structural parameter - in addn. to compn., morphol., architecture, facet, size and dimensionality - that dets. the properties and functionalities of nanomaterials. In particular, unconventional phases in nanomaterials that are unattainable in the bulk state can potentially endow nanomaterials with intriguing properties and innovative applications. Great progress has been made in the phase engineering of nanomaterials (PEN), including synthesis of nanomaterials with unconventional phases and phase transformation of nanomaterials. This Review provides an overview on the recent progress in PEN. We discuss various strategies used to synthesize nanomaterials with unconventional phases and induce phase transformation of nanomaterials, by taking noble metals and layered transition metal dichalcogenides as typical examples. Moreover, we also highlight recent advances in the prepn. of amorphous nanomaterials, amorphous-cryst. and crystal phase-based hetero-nanostructures. We also provide personal perspectives on challenges and opportunities in this emerging field, including exploration of phase-dependent properties and applications, rational design of phase-based heterostructures and extension of the concept of phase engineering to a wider range of materials.
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    6. 6
      Yao, Q.; Yu, Z.; Li, L.; Huang, X. Strain and surface engineering of multicomponent metallic nanomaterials with unconventional phases. Chem. Rev. 2023, 123, 9676– 9717,  DOI: 10.1021/acs.chemrev.3c00252
      6
      Strain and Surface Engineering of Multicomponent Metallic Nanomaterials with Unconventional Phases
      Yao, Qing; Yu, Zhiyong; Li, Leigang; Huang, Xiaoqing
      Chemical Reviews (Washington, DC, United States) (2023), 123 (15), 9676-9717CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Multicomponent metallic nanomaterials with unconventional phases show great prospects in electrochem. energy storage and conversion, owing to unique crystal structures and abundant structural effects. In this review, we emphasize the progress in the strain and surface engineering of these novel nanomaterials. We start with a brief introduction of the structural configurations of these materials, based on the interaction types between the components. Next, the fundamentals of strain, strain effect in relevant metallic nanomaterials with unconventional phases, and their formation mechanisms are discussed. Then the progress in surface engineering of these multicomponent metallic nanomaterials is demonstrated from the aspects of morphol. control, crystallinity control, surface modification, and surface reconstruction. Moreover, the applications of the strain- and surface-engineered unconventional nanomaterials mainly in electrocatalysis are also introduced, where in addtion to the catalytic performance, the structure-performance correlations are highlighted. Finally, the challenges and opportunities in this promising field are prospected.
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    7. 7
      Zhao, M.; Xia, Y. Crystal-phase and surface-structure engineering of ruthenium nanocrystals. Nat. Rev. Mater. 2020, 5, 440– 459,  DOI: 10.1038/s41578-020-0183-3
      7
      Crystal-phase and surface-structure engineering of ruthenium nanocrystals
      Zhao, Ming; Xia, Younan
      Nature Reviews Materials (2020), 5 (6), 440-459CODEN: NRMADL; ISSN:2058-8437. (Nature Research)
      A review. Metal nanocrystals with controlled shapes or surface structures have received increasing attention, owing to their desirable properties for applications ranging from catalysis to photonics, energy and biomedicine. Most studies, however, have been limited to nanocrystals with the same crystal phase as the bulk material. Engineering the phase of metal nanocrystals while simultaneously attaining shape-controlled synthesis has recently emerged as a new frontier of research. Here, we use Ru as an example to evaluate recent progress in the synthesis of metal nanocrystals featuring different crystal phases and well-controlled shapes. We first discuss synthetic strategies for controlling the crystal phase and shape of Ru nanocrystals, with a focus on new mechanistic insights. We then highlight the major factors that affect the packing of Ru atoms and, thus, the crystal phase, followed by an examn. of the thermal stability of Ru nanocrystals in terms of both crystal phase and shape. Next, we showcase the successful implementation of these Ru nanocrystals in various catalytic applications. Finally, we end with a discussion of the challenges and opportunities in the field, including leveraging the lessons learned from Ru to engineer the crystal phase and surface structure of other metals.
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    8. 8
      Janssen, A.; Nguyen, Q. N.; Xia, Y. Colloidal metal nanocrystals with metastable crystal structures. Angew. Chem., Int. Ed. 2021, 60, 12192– 12203,  DOI: 10.1002/anie.202017076
      8
      Colloidal Metal Nanocrystals with Metastable Crystal Structures
      Janssen, Annemieke; Nguyen, Quynh N.; Xia, Younan
      Angewandte Chemie, International Edition (2021), 60 (22), 12192-12203CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. In addn. to the conventional knobs such as compn., size, shape, and defect structure, the crystal structure (or phase) of metal nanocrystals offers a new avenue for engineering their properties. Various strategies have recently been developed for the fabrication of colloidal metal nanocrystals in metastable phases different from their bulk counterparts. With a focus on noble metals, we begin with a brief introduction to their at. packing, followed by a discussion about five major synthetic approaches to their colloidal nanocrystals in unconventional phases. We then highlight the success of synthesis in terms of mechanistic insights and exptl. controls, as well as the enhanced catalytic properties. We end this Minireview with perspectives on the remaining issues and future opportunities.
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    9. 9
      Fan, Z.; Zhang, H. Template synthesis of noble metal nanocrystals with unusual crystal structures and their catalytic applications. Acc. Chem. Res. 2016, 49, 2841– 2850,  DOI: 10.1021/acs.accounts.6b00527
      9
      Template Synthesis of Noble Metal Nanocrystals with Unusual Crystal Structures and Their Catalytic Applications
      Fan, Zhanxi; Zhang, Hua
      Accounts of Chemical Research (2016), 49 (12), 2841-2850CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. The authors review the recent research progress on the crystal structure control of noble metal nanocrystals with a template synthetic approach and their crystal structure-dependent catalytic properties. The authors describe the template synthetic methods, such as epitaxial growth and galvanic replacement reaction methods, in which a presynthesized noble metal nanocrystal with either new or common crystal structure is used as the template to direct the growth of unusual crystal structures of other noble metals. The template synthetic strategy described here provides an efficient, simple and straightforward way to synthesize unusual crystal structures of noble metal nanocrystals, which might not be easily synthesized by commonly used chem. synthesis. To be specific, by using the epitaxial growth method, a series of noble metal nanocrystals with unusual crystal structures has been obtained, such as hcp. Ag, 4H Ag, Pd, Pt, Ir, Rh, Os, and Ru, and face-centered cubic Ru nanostructures. Meanwhile, the galvanic replacement reaction method offers an efficient way to synthesize noble metal alloy nanocrystals with unusual crystal structures, such as 4H PdAg, PtAg, and PtPdAg nanostructures. The stability of noble metal nanocrystals with unusual crystal structures is discussed. The authors demonstrate the catalytic applications of the resultant noble metal nanocrystals with unusual crystal structures toward different chem. reactions like hydrogen evolution reaction, hydrogen oxidn. reaction and org. reactions. The relationship between crystal structures of noble metal nanocrystals and their catalytic performances is discussed.
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    10. 10
      Fan, Z.; Chen, Y.; Zhu, Y.; Wang, J.; Li, B.; Zong, Y.; Han, Y.; Zhang, H. Epitaxial growth of unusual 4H hexagonal Ir, Rh, Os, Ru, and Cu nanostructures on 4H Au nanoribbons. Chem. Sci. 2017, 8, 795– 799,  DOI: 10.1039/C6SC02953A
      There is no corresponding record for this reference.
    11. 11
      Ge, Y.; Huang, Z.; Ling, C.; Chen, B.; Liu, G.; Zhou, M.; Liu, J.; Zhang, X.; Cheng, H.; Liu, G. Phase-selective epitaxial growth of heterophase nanostructures on unconventional 2H-Pd nanoparticles. J. Am. Chem. Soc. 2020, 142, 18971– 18980,  DOI: 10.1021/jacs.0c09461
      11
      Phase-Selective Epitaxial Growth of Heterophase Nanostructures on Unconventional 2H-Pd Nanoparticles
      Ge, Yiyao; Huang, Zhiqi; Ling, Chongyi; Chen, Bo; Liu, Guigao; Zhou, Ming; Liu, Jiawei; Zhang, Xiao; Cheng, Hongfei; Liu, Guanghua; Du, Yonghua; Sun, Cheng-Jun; Tan, Chaoliang; Huang, Jingtao; Yin, Pengfei; Fan, Zhanxi; Chen, Ye; Yang, Nailiang; Zhang, Hua
      Journal of the American Chemical Society (2020), 142 (44), 18971-18980CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Heterostructured, including heterophase, noble-metal nanomaterials have attracted much interest due to their promising applications in diverse fields. However, great challenges still remain in the rational synthesis of well-defined noble-metal heterophase nanostructures. Herein, we report the prepn. of Pd nanoparticles with an unconventional hcp. (2H type) phase, referred to as 2H-Pd nanoparticles, via a controlled phase transformation of amorphous Pd nanoparticles. Impressively, by using the 2H-Pd nanoparticles as seeds, Au nanomaterials with different crystal phases epitaxially grow on the specific exposed facets of the 2H-Pd, i.e., face-centered cubic (fcc) Au (fcc-Au) on the (002)h facets of 2H-Pd while 2H-Au on the other exposed facets, to achieve well-defined fcc-2H-fcc heterophase Pd@Au core-shell nanorods. Moreover, through such unique facet-directed crystal-phase-selective epitaxial growth, a series of unconventional fcc-2H-fcc heterophase core-shell nanostructures, including Pd@Ag, Pd@Pt, Pd@PtNi, and Pd@PtCo, have also been prepd. Impressively, the fcc-2H-fcc heterophase Pd@Au nanorods show excellent performance toward the electrochem. carbon dioxide redn. reaction (CO2RR) for prodn. of carbon monoxide with Faradaic efficiencies of over 90% in an exceptionally wide applied potential window from -0.9 to -0.4 V (vs. the reversible hydrogen electrode), which is among the best reported CO2RR catalysts in H-type electrochem. cells.
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    12. 12
      Zhou, X.; Ma, Y.; Ge, Y.; Zhu, S.; Cui, Y.; Chen, B.; Liao, L.; Yun, Q.; He, Z.; Long, H. Preparation of Au@Pd core-shell nanorods with fcc-2H-fcc heterophase for highly efficient electrocatalytic alcohol oxidation. J. Am. Chem. Soc. 2022, 144, 547– 555,  DOI: 10.1021/jacs.1c11313
      12
      Preparation of Au@Pd Core-Shell Nanorods with fcc-2H-fcc. Heterophase for Highly Efficient Electrocatalytic Alcohol Oxidation
      Zhou, Xichen; Ma, Yangbo; Ge, Yiyao; Zhu, Shangqian; Cui, Yu; Chen, Bo; Liao, Lingwen; Yun, Qinbai; He, Zhen; Long, Huiwu; Li, Lujiang; Huang, Biao; Luo, Qinxin; Zhai, Li; Wang, Xixi; Bai, Licheng; Wang, Gang; Guan, Zhiqiang; Chen, Ye; Lee, Chun-Sing; Wang, Jinlan; Ling, Chongyi; Shao, Minhua; Fan, Zhanxi; Zhang, Hua
      Journal of the American Chemical Society (2022), 144 (1), 547-555CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Controlled construction of bimetallic nanostructures with a well-defined heterophase is of great significance for developing highly efficient nanocatalysts and studying the structure-dependent catalytic performance. Here, a wet-chem. synthesis method was used to prep. Au@Pd core-shell nanorods with a unique fcc.-2H-fcc. heterophase (fcc.: fcc.; 2H: hcp. with a stacking sequence of AB). The obtained fcc.-2H-fcc. heterophase Au@Pd core-shell nanorods exhibit superior electrocatalytic EtOH oxidn. performance with a mass activity ≤6.82 A mgPd-1, which is 2.44, 6.96, and 6.43 times those of 2H-Pd nanoparticles, fcc.-Pd nanoparticles, and com. Pd/C, resp. The operando IR reflection absorption spectroscopy reveals a C2 pathway with fast reaction kinetics for the EtOH oxidn. on the prepd. heterophase Au@Pd nanorods. The authors' exptl. results together with d. functional theory calcns. indicate that the enhanced performance of heterophase Au@Pd nanorods can be attributed to the unconventional 2H phase, the 2H/fcc. phase boundary, and the lattice expansion of the Pd shell. Also, the heterophase Au@Pd nanorods can also serve as an efficient catalyst for the electrochem. oxidn. of MeOH, ethylene glycol, and glycerol. The authors' work in the area of phase engineering of nanomaterials (PENs) opens the way for developing high-performance electrocatalysts toward future practical applications.
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    13. 13
      Tan, X.; Geng, S.; Ji, Y.; Shao, Q.; Zhu, T.; Wang, P.; Li, Y.; Huang, X. Closest packing polymorphism interfaced metastable transition metal for efficient hydrogen evolution. Adv. Mater. 2020, 32, 2002857  DOI: 10.1002/adma.202002857
      13
      Closest Packing Polymorphism Interfaced Metastable Transition Metal for Efficient Hydrogen Evolution
      Tan, Xinyue; Geng, Shize; Ji, Yujin; Shao, Qi; Zhu, Ting; Wang, Pengtang; Li, Youyong; Huang, Xiaoqing
      Advanced Materials (Weinheim, Germany) (2020), 32 (40), 2002857CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Metastable materials are promising because of their catalytic properties, high-energy structure, and unique electronic environment. However, the unstable nature inherited from the metastability hinders further performance improvement and practical applications of these materials. Herein, this limitation is successfully addressed by constructing an in situ polymorphism interface (inf) between the metastable hexagonal-close-packed (hcp) phase and its stable counterpart (face-centered cubic, fcc) in cobalt-nickel (CoNi) alloy. Calcns. reveal that the interfacial synergism derived from the hcp and fcc phases lowers the formation energy and enhances stability. Consequently, the optimized CoNi-inf exhibits an exceptionally low potential of 72 mV at 10 mA cm-2 and a Tafel slope of 57 mV dec-1 for the hydrogen evolution reaction (HER) in 1.0 M KOH. Furthermore, it is superior to most state-of-the-art non-noble-metal-based HER catalysts. No noticeable activity decay or structural changes are obsd. even over 14 h of catalysis. The computational simulation further rationalizes that the interface of CoNi-inf with a suitable d-band center provides uniform sites for hydrogen adsorption, leading to a distinguished HER catalytic activity. This work, therefore, presents a new route for designing metastable catalysts for potential energy conversion.
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    14. 14
      Xia, Y.; Gilroy, K. D.; Peng, H. C.; Xia, X. Seed-mediated growth of colloidal metal nanocrystals. Angew. Chem., Int. Ed. 2017, 56, 60– 95,  DOI: 10.1002/anie.201604731
      14
      Seed-Mediated Growth of Colloidal Metal Nanocrystals
      Xia, Younan; Gilroy, Kyle D.; Peng, Hsin-Chieh; Xia, Xiaohu
      Angewandte Chemie, International Edition (2017), 56 (1), 60-95CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, compn., and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also det. their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a no. of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and redn. kinetics. New capabilities and future directions are also highlighted.
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    15. 15
      Gilroy, K. D.; Yang, X.; Xie, S.; Zhao, M.; Qin, D.; Xia, Y. Shape-controlled synthesis of colloidal metal nanocrystals by replicating the surface atomic structure on the seed. Adv. Mater. 2018, 30, 1706312  DOI: 10.1002/adma.201706312
      There is no corresponding record for this reference.
    16. 16
      Zhao, M.; Xu, L.; Vara, M.; Elnabawy, A. O.; Gilroy, K. D.; Hood, Z. D.; Zhou, S.; Figueroa-Cosme, L.; Chi, M.; Mavrikakis, M.; Xia, Y. Synthesis of Ru icosahedral nanocages with a face-centered-cubic structure and evaluation of their catalytic properties. ACS Catal. 2018, 8, 6948– 6960,  DOI: 10.1021/acscatal.8b00910
      16
      Synthesis of Ru Icosahedral Nanocages with a Face-Centered-Cubic Structure and Evaluation of Their Catalytic Properties
      Zhao, Ming; Xu, Lang; Vara, Madeline; Elnabawy, Ahmed O.; Gilroy, Kyle D.; Hood, Zachary D.; Zhou, Shan; Figueroa-Cosme, Legna; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
      ACS Catalysis (2018), 8 (8), 6948-6960CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Owing to the presence of {111} facets, twin boundaries, and strain fields on the surface, noble-metal nanocrystals with an icosahedral shape have been reported with stellar performance toward an array of catalytic reactions. Here, we report the successful synthesis of Ru icosahedral nanocages with a face-centered cubic (fcc) structure by conformally coating Pd icosahedral seeds with ultrathin Ru shells, followed by selective removal of the Pd cores via chem. etching. We discovered that the presence of bromide ions was crit. to the layer-by-layer deposition of Ru atoms. According to in situ XRD, the fcc structure in the Ru nanocages could be retained up to 300 °C before it was transformed into the conventional hcp. (hcp) structure. Addnl., the icosahedral shape of the Ru nanocages could be largely preserved up to 300 °C. The Ru icosahedral nanocages with twin boundaries on the surface exhibited greatly enhanced activities toward both the redn. of 4-nitrophenol and decompn. of hydrazine than their cubic and octahedral counterparts. When benchmarked against the parental Pd@Ru core-shell nanocrystals, all the Ru nanocages displayed superior catalytic activities. First-principles d. functional theory calcns. also suggest that the fcc-Ru icosahedral nanocages contg. residual Pd atoms are more promising than the conventional hcp-Ru solid nanoparticles in catalyzing nitrogen redn. for ammonia synthesis. With the subsurface impurities of Pd, the twin boundary regions of the icosahedral nanocages are able to stabilize the N2 dissocn. transition state, reducing the overall reaction barrier and promoting the competition with the N2 desorption process.
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    17. 17
      Zhao, M.; Figueroa-Cosme, L.; Elnabawy, A. O.; Vara, M.; Yang, X.; Roling, L. T.; Chi, M.; Mavrikakis, M.; Xia, Y. Synthesis and characterization of Ru cubic nanocages with a face-centered cubic structure by templating with Pd nanocubes. Nano Lett. 2016, 16, 5310– 5317,  DOI: 10.1021/acs.nanolett.6b02795
      17
      Synthesis and Characterization of Ru Cubic Nanocages with a Face-Centered Cubic Structure by Templating with Pd Nanocubes
      Zhao, Ming; Figueroa-Cosme, Legna; Elnabawy, Ahmed O.; Vara, Madeline; Yang, Xuan; Roling, Luke T.; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
      Nano Letters (2016), 16 (8), 5310-5317CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Ru cubic nanocages with ultrathin walls, in which the atoms are crystd. in fcc. rather than hcp. structure, have been prepd. The key to the success of this synthesis was to ensure layer-by-layer deposition of Ru atoms on the surface of Pd cubic seeds by controlling the reaction temp. and the injection rate of a Ru(III) precursor. By selectively etching away the Pd from the Pd@Ru core-shell nanocubes, the authors obtained Ru nanocages with an av. wall thickness of 1.1 nm or about six at. layers. The Ru nanocages adopted an fcc. crystal structure rather than the hcp. structure obsd. in bulk Ru. The synthesis was applied to Pd cubic seeds with different edge lengths in the range of 6-18 nm, with smaller seeds being more favorable for the formation of Ru shells with a flat, smooth surface due to shorter distance for the surface diffusion of the Ru adatoms. Self-consistent d. functional theory calcns. indicated that these unique fcc.-structured Ru nanocages might possess promising catalytic properties for ammonia synthesis compared to hcp. Ru(0001), on the basis of strengthened binding of at. N and substantially reduced activation energies for N2 dissocn., which is the rate-detg. step for ammonia synthesis on hcp. Ru catalysts.
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    18. 18
      Gu, J.; Guo, Y.; Jiang, Y.-Y.; Zhu, W.; Xu, Y.-S.; Zhao, Z.-Q.; Liu, J.-X.; Li, W.-X.; Jin, C.-H.; Yan, C.-H.; Zhang, Y. W. Robust phase control through hetero-seeded epitaxial growth for face-centered cubic Pt@Ru nanotetrahedrons with superior hydrogen electro-oxidation activity. J. Phys. Chem. C 2015, 119, 17697– 17706,  DOI: 10.1021/acs.jpcc.5b04587
      18
      Robust Phase Control through Hetero-Seeded Epitaxial Growth for Face-Centered Cubic Pt@Ru Nanotetrahedrons with Superior Hydrogen Electro-Oxidation Activity
      Gu, Jun; Guo, Yu; Jiang, Ying-Ying; Zhu, Wei; Xu, Yan-Shuang; Zhao, Ze-Qiong; Liu, Jin-Xun; Li, Wei-Xue; Jin, Chuan-Hong; Yan, Chun-Hua; Zhang, Ya-Wen
      Journal of Physical Chemistry C (2015), 119 (31), 17697-17706CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Controllable synthesis of metallic nanocrystals (NCs) with tunable phase, uniform shape, and size is of multidisciplinary interests but has still remained challenging. Herein, a robust phase control strategy is developed, in which seeds with a given phase are added to guide the epitaxial growth of the target metal to inherit the seeds' phase. Through this strategy, M@Ru (M = Pt, Pd) NCs in the fcc. phase, a metastable phase for Ru under ambient conditions, were synthesized with the hydrothermal method. The Pt@Ru NCs showed not only the pure fcc. phase but also high morphol. selectivity to tetrahedrons surrounded by {111} facets. As revealed by d. function theory (DFT) calcns., the preferentially epitaxial growth of Ru atom layers on the nonclosest-packed facets of hetero fcc. metal seeds gave fcc. Ru shells. Also, the fcc. Pt@Ru tetrahedrons/C showed electrocatalytic activity enhancement with more than an order of magnitude toward H oxidn. reaction (HOR) in acidic electrolyte compared with hydrothermally synthesized Ru/C. Electrochem. measurement combined with DFT calcns. revealed that the optimum HOR activity should be achieved on well-crystd. fcc. Ru catalysts exposing max. {111} facets.
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    19. 19
      Janssen, A.; Lyu, Z.; Figueras-Valls, M.; Chao, H.-Y.; Shi, Y.; Pawlik, V.; Chi, M.; Mavrikakis, M.; Xia, Y. Phase-controlled synthesis of Ru nanocrystals via template-directed growth: surface energy versus bulk energy. Nano Lett. 2022, 22, 3591– 3597,  DOI: 10.1021/acs.nanolett.1c05009
      19
      Phase-Controlled Synthesis of Ru Nanocrystals via Template-Directed Growth: Surface Energy versus Bulk Energy
      Janssen, Annemieke; Lyu, Zhiheng; Figueras-Valls, Marc; Chao, Hsin-Yun; Shi, Yifeng; Pawlik, Veronica; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
      Nano Letters (2022), 22 (9), 3591-3597CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Despite the successful control of crystal phase using template-directed growth, much remains unknown about the underlying mechanisms. Here we demonstrate that the crystal phase taken by the deposited metal depends on the lateral size of fcc.-Pd nanoplate templates, with 12 nm plates giving fcc.-Ru while 18-26 nm plates result in hcp.-Ru. Although Ru overlayers with a metastable fcc.- (high in bulk energy) or stable hcp.-phase (low in bulk energy) can be epitaxially deposited on the basal planes, the lattice mismatch will lead to jagged hcp.- (high in surface energy) and smooth fcc.-facets (low in surface energy), resp., on the side faces. As the proportion of basal and side faces on the nanoplates varies with lateral size, the crystal phase will change depending on the relative contributions from the surface and bulk energies. The [email protected] outperform the [email protected] nanoplates toward ethylene glycol and glycerol oxidn. reactions.
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    20. 20
      Janssen, A.; Nguyen, Q. N.; Lyu, Z.; Pawlik, V.; Wang, C.; Xia, Y. Phase-controlled deposition of Ru on Pd nanocrystal templates: effects of particle shape and size. J. Phys. Chem. C 2023, 127, 1280– 1291,  DOI: 10.1021/acs.jpcc.2c08825
      There is no corresponding record for this reference.
    21. 21
      Joo, S. H.; Park, J. Y.; Renzas, J. R.; Butcher, D. R.; Huang, W.; Somorjai, G. A. Size effect of ruthenium nanoparticles in catalytic carbon monoxide oxidation. Nano Lett. 2010, 10, 2709– 2713,  DOI: 10.1021/nl101700j
      21
      Size Effect of Ruthenium Nanoparticles in Catalytic Carbon Monoxide Oxidation
      Joo, Sang Hoon; Park, Jeong Y.; Renzas, J. Russell; Butcher, Derek R.; Huang, Wenyu; Somorjai, Gabor A.
      Nano Letters (2010), 10 (7), 2709-2713CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      CO oxidn. over Ru catalysts exhibits an unusual catalytic behavior. This work discusses particle size effect on CO oxidn. over Ru nano-particle (NP) catalysts. Uniform Ru NP with a tunable particle size of 2-6 nm were synthesized by a polyol redn. of ruthenium acetylacetonate precursor in the presence of a poly(vinylpyrrolidone) stabilizer. Catalyst activity measurement for CO oxidn. over 2-dimensional Ru NP arrays under oxidizing reaction conditions (40 Torr CO, 100 Torr O2) showed activity depended on Ru NP size. CO oxidn. activity increased with NP size; the 6 nm Ru NP catalyst exhibited 8-fold higher activity than 2 nm catalysts. Results provide the scientific basis for future design of Ru-based oxidn. catalysts.
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    22. 22
      Li, W. Z.; Liu, J. X.; Gu, J.; Zhou, W.; Yao, S. Y.; Si, R.; Guo, Y.; Su, H. Y.; Yan, C. H.; Li, W. X. Chemical insights into the design and development of face-centered cubic ruthenium catalysts for Fischer–Tropsch synthesis. J. Am. Chem. Soc. 2017, 139, 2267– 2276,  DOI: 10.1021/jacs.6b10375
      22
      Chemical Insights into the Design and Development of Face-Centered Cubic Ruthenium Catalysts for Fischer-Tropsch Synthesis
      Li, Wei-Zhen; Liu, Jin-Xun; Gu, Jun; Zhou, Wu; Yao, Si-Yu; Si, Rui; Guo, Yu; Su, Hai-Yan; Yan, Chun-Hua; Li, Wei-Xue; Zhang, Ya-Wen; Ma, Ding
      Journal of the American Chemical Society (2017), 139 (6), 2267-2276CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Ruthenium is a promising low-temp. catalyst for Fischer-Tropsch synthesis (FTS). However, its scarcity and modest specific activity limit its widespread industrialization. The authors demonstrate here a strategy for tuning the crystal phase of catalysts to expose denser and active sites for a higher mass-specific activity. D. functional theory calcns. show that upon CO dissocn. there are a no. of open facets with modest barrier available on the fcc. Ru but only a few step edges with a lower barrier on conventional hcp. Ru. Guided by theor. calcns., water-dispersible fcc. Ru catalysts contg. abundant open facets were synthesized and showed an unprecedented mass-specific activity in the aq.-phase FTS, 37.8 molCO·molRu-1·h-1 at 433 K. The mass-specific activity of the fcc. Ru catalysts with an av. size of 6.8 nm is about three times larger than the previous best hcp. catalyst with a smaller size of 1.9 nm and a higher sp. surface area. The origin of the higher mass-specific activity of the fcc. Ru catalysts is identified exptl. from the 2 orders of magnitude higher d. of the active sites, despite its slightly higher apparent barrier. Exptl. results are in excellent agreement with prediction of theory. The great influence of the crystal phases on site distribution and their intrinsic activities revealed here provides a rationale design of catalysts for higher mass-specific activity without decrease of the particle size.
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    23. 23
      Li, L.; Liu, C.; Liu, S.; Wang, J.; Han, J.; Chan, T. S.; Li, Y.; Hu, Z.; Shao, Q.; Zhang, Q.; Huang, X. Phase engineering of a ruthenium nanostructure toward high-performance bifunctional hydrogen catalysis. ACS Nano 2022, 16, 14885– 14894,  DOI: 10.1021/acsnano.2c05776
      23
      Phase Engineering of a Ruthenium Nanostructure toward High-Performance Bifunctional Hydrogen Catalysis
      Li, Leigang; Liu, Cheng; Liu, Shangheng; Wang, Juan; Han, Jiajia; Chan, Ting-Shan; Li, Youyong; Hu, Zhiwei; Shao, Qi; Zhang, Qiaobao; Huang, Xiaoqing
      ACS Nano (2022), 16 (9), 14885-14894CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The physicochem. properties and catalytic performance of transition metals are highly phase-dependent. Ru-based nanomaterials are superior catalysts toward hydrogen evolution reaction (HER) and hydrogen oxidn. reaction (HOR), but studies are mostly limited to conventional hexagonal-close-packed (hcp) Ru, mainly arising from the difficulty in synthesizing Ru with pure face-centered-cubic (fcc) phase. Herein, we report a crystal-phase-dependent catalytic study of MoOx-modified Ru (MoOx-Ru fcc and MoOx-Ru hcp) for bifunctional HER and HOR. MoOx-Ru fcc is proven to outperform MoOx-Ru hcp in catalyzing both HER and HOR with much higher catalytic activity and more durable stability. The modification effect of MoOx gives rise to optimal adsorption of H and OH esp. on fcc Ru, which thus has resulted in the superior catalytic performance. This work highlights the significance of phase engineering in constructing superior electrocatalysts and may stimulate more efforts on phase engineering of other metal-based materials for diversified applications.
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    24. 24
      Kusada, K.; Kobayashi, H.; Yamamoto, T.; Matsumura, S.; Sumi, N.; Sato, K.; Nagaoka, K.; Kubota, Y.; Kitagawa, H. Discovery of face-centered-cubic ruthenium nanoparticles: facile size-controlled synthesis using the chemical reduction method. J. Am. Chem. Soc. 2013, 135, 5493– 5496,  DOI: 10.1021/ja311261s
      24
      Discovery of Face-Centered-Cubic Ruthenium Nanoparticles: Facile Size-Controlled Synthesis Using the Chemical Reduction Method
      Kusada, Kohei; Kobayashi, Hirokazu; Yamamoto, Tomokazu; Matsumura, Syo; Sumi, Naoya; Sato, Katsutoshi; Nagaoka, Katsutoshi; Kubota, Yoshiki; Kitagawa, Hiroshi
      Journal of the American Chemical Society (2013), 135 (15), 5493-5496CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report the first discovery of pure face-centered-cubic (fcc) Ru nanoparticles. Although the fcc structure does not exist in the bulk Ru phase diagram, fcc Ru was obtained at room temp. because of the nanosize effect. We succeeded in sep. synthesizing uniformly sized nanoparticles of both fcc and hcp Ru having diams. of 2-5.5 nm by simple chem. redn. methods with different metal precursors. The prepd. fcc and hcp nanoparticles were both supported on γ-Al2O3, and their catalytic activities in CO oxidn. were investigated and found to depend on their structure and size.
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    25. 25
      Araki, N.; Kusada, K.; Yoshioka, S.; Sugiyama, T.; Ina, T.; Kitagawa, H. Observation of the formation processes of hexagonal close-packed and face-centered cubic Ru nanoparticles. Chem. Lett. 2019, 48, 1062– 1064,  DOI: 10.1246/cl.190338
      25
      Observation of the Formation Processes of Hexagonal Close-packed and Face-centered Cubic Ru Nanoparticles
      Araki, Naoki; Kusada, Kohei; Yoshioka, Satoru; Sugiyama, Takeharu; Ina, Toshiaki; Kitagawa, Hiroshi
      Chemistry Letters (2019), 48 (9), 1062-1064CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
      Face-centered cubic (fcc) Ru nanoparticles (NPs) have been recently synthesized by reducing Ru(III) acetylacetonate, despite bulk Ru having a hcp. (hcp) structure. We obsd. the formation processes of fcc and hcp Ru NPs using X-ray diffraction and X-ray absorption fine structure. We concluded that the strong Ru-O bond between Ru and the acetylacetonate ligand leads to the formation of the fcc structure.
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    26. 26
      Zhao, M.; Hood, Z. D.; Vara, M.; Gilroy, K. D.; Chi, M.; Xia, Y. Ruthenium nanoframes in the face-centered cubic phase: facile synthesis and their enhanced catalytic performance. ACS Nano 2019, 13, 7241– 7251,  DOI: 10.1021/acsnano.9b02890
      26
      Ruthenium Nanoframes in the Face-Centered Cubic Phase: Facile Synthesis and Their Enhanced Catalytic Performance
      Zhao, Ming; Hood, Zachary D.; Vara, Madeline; Gilroy, Kyle D.; Chi, Miaofang; Xia, Younan
      ACS Nano (2019), 13 (6), 7241-7251CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Owing to their highly open structure and a large no. of low-coordination sites on the surface, noble-metal nanoframes are intriguing for catalytic applications. Here, we demonstrate the rational synthesis of Ru cuboctahedral nanoframes with enhanced catalytic performance toward hydrazine decompn. The synthesis starts from Pd nanocubes, which quickly undergo truncation at the corners as a consequence of oxidative etching caused by Br- ions. Afterward, the galvanic replacement reaction between Pd and Ru(III) ions dominates, leading to the selective deposition of Ru atoms on the corners and edges and thereby the fabrication of Pd@Ru core-frame cuboctahedra. Significantly, the deposited Ru atoms are crystd. in a face-centered cubic (fcc) phase instead of the hcp. (hcp) structure typical of bulk Ru. Upon the removal of Pd remaining in the core via chem. etching, we obtain Ru cuboctahedral nanoframes. By varying the amt. of the Ru(III) precursor, the ridge thickness of the nanoframes can be tuned from a few at. layers up to 10. Both the frame structure and fcc crystal phase of the Ru cuboctahedral nanoframes can be well preserved up to 300°C. When compared with hcp-Ru nanoparticles, the fcc-Ru nanoframes displayed substantial enhancement in terms of H2 selectivity toward hydrazine decompn. This work offers the opportunity to engineer both the morphol. and crystal phase of Ru nanocrystals for catalytic applications.
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    27. 27
      Lin, J.-T.; Liu, Y.-H.; Tsao, C.-Y.; Wu, C.-Y.; Hsieh, C.-J.; Chen, M.-Z.; Chang, C.-W.; Hsiao, Y.-C.; Chen, H.-L.; Yang, T.-H. Toward a quantitative understanding of crystal-phase engineering of Ru nanocrystals. Chem. Mater. 2023, 35, 4276– 4285,  DOI: 10.1021/acs.chemmater.3c00326
      27
      Toward a Quantitative Understanding of Crystal-Phase Engineering of Ru Nanocrystals
      Lin, Jui-Tai; Liu, Yi-Hong; Tsao, Chi-Yen; Wu, Cheng-Yu; Hsieh, Chia-Jui; Chen, Meng-Zhe; Chang, Chun-Wei; Hsiao, Yueh-Chun; Chen, Hsin-Lung; Yang, Tung-Han
      Chemistry of Materials (2023), 35 (11), 4276-4285CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The crystal phase with a specific stacking sequence of atoms largely affects the catalytic performance of metal nanocrystals. Since the control of the phase at the same compn. is extremely difficult, the phase-dependent performance of metal nanocrystals is studied rarely. Here, we show the synthesis of Ru nanocrystals with different percentages of face-centered cubic (FCC) and hcp. (HCP) phases via kinetic control, further revealing a quant. correlation between the phase percentage of Ru nanocrystals and the initial redn. rate of Ru(III) precursors. Specifically, we manipulate the single parameter-initial redn. rate by controlling the Ru(III) injection rate into the dropwise synthesis at a fixed reaction temp. and correlate the kinetic data with the Ru phase percentage analyzed by at.-resoln. electron microscopy and synchrotron X-ray scattering. Based on the quant. anal., the ranges of initial redn. rates of Ru precursors can be detd. for synthesizing Ru nanocrystals with the percentages of unusual FCC phase from 9.0 to 55.1%. We demonstrate that a low initial redn. rate corresponds to the crystn. of the Ru HCP phase, while a high initial redn. rate favors the crystn. of the FCC lattice. Furthermore, we also systematically examine the catalytic performance of Ru nanocrystals with different phases.
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    28. 28
      Ye, H.; Wang, Q.; Catalano, M.; Lu, N.; Vermeylen, J.; Kim, M. J.; Liu, Y.; Sun, Y.; Xia, X. Ru nanoframes with an fcc structure and enhanced catalytic properties. Nano Lett. 2016, 16, 2812– 2817,  DOI: 10.1021/acs.nanolett.6b00607
      28
      Ru Nanoframes with an fcc Structure and Enhanced Catalytic Properties
      Ye, Haihang; Wang, Qingxiao; Catalano, Massimo; Lu, Ning; Vermeylen, Joseph; Kim, Moon J.; Liu, Yuzi; Sun, Yugang; Xia, Xiaohu
      Nano Letters (2016), 16 (4), 2812-2817CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Noble-metal nanoframes are of great interest to many applications due to their unique open structures. Among various noble metals, Ru has never been made into nanoframes. In this study, we report for the first time an effective method based on seeded growth and chem. etching for the facile synthesis of Ru nanoframes with high purity. The essence of this approach is to induce the preferential growth of Ru on the corners and edges of Pd truncated octahedra as the seeds by kinetic control. The resultant Pd-Ru core-frame octahedra could be easily converted to Ru octahedral nanoframes of ∼2 nm in thickness by selectively removing the Pd cores through chem. etching. Most importantly, in this approach the face-centered cubic (fcc) crystal structure of Pd seeds was faithfully replicated by Ru that usually takes an hcp structure. The fcc Ru nanoframes showed higher catalytic activities toward the redn. of p-nitrophenol by NaBH4 and the dehydrogenation of ammonia borane compared with hcp Ru nanowires with roughly the same thickness.
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    29. 29
      Zhao, M.; Elnabawy, A. O.; Vara, M.; Xu, L.; Hood, Z. D.; Yang, X.; Gilroy, K. D.; Figueroa-Cosme, L.; Chi, M.; Mavrikakis, M.; Xia, Y. Facile synthesis of Ru-based octahedral nanocages with ultrathin walls in a face-centered cubic structure. Chem. Mater. 2017, 29, 9227– 9237,  DOI: 10.1021/acs.chemmater.7b03092
      29
      Facile synthesis of Ru-based octahedral nanocages with ultrathin walls in face-centered cubic structure
      Zhao, Ming; Elnabawy, Ahmed O.; Vara, Madeline; Xu, Lang; Hood, Zachary D.; Yang, Xuan; Gilroy, Kyle D.; Figueroa-Cosme, Legna; Chi, Miaofang; Mavrikakis, Manos; Xia, Younan
      Chemistry of Materials (2017), 29 (21), 9227-9237CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Noble-metal nanocages with ultrathin (less than 2 nm) walls and well-defined facets have received great interest owing to their remarkable utilization efficiency of atoms and facet-dependent catalytic activities toward various reactions. Here, we report the synthesis of Ru-based octahedral nanocages covered by {111} facets, together with ultrathin walls in a face-centered cubic (fcc) structure rather than the hcp. (hcp) of bulk Ru. The involvement of slow injection for the Ru(III) precursor, the introduction of KBr, and the use of elevated temp. were all instrumental to the formation of Pd@Ru core-shell octahedra with a conformal, uniform shell and a smooth surface. The {111} facets were well preserved during the selective removal of the Pd cores via wet etching, even when the Ru walls were only five at. layers in thickness. Through in situ XRD, we demonstrated that the fcc structure of the Ru nanocages was stable up to 300 °C. We also used first-principles, self-consistent d. functional theory calcns. to study the adsorption and dissocn. of N2 as a means to predict the catalytic performance toward ammonia synthesis. Our results suggested that the small proportions of Pd atoms left behind in the walls during etching could play a key role in stabilizing the adsorption of N2 as well as in reducing the activation energy barrier to N2 dissocn.
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    30. 30
      Zhao, M.; Chen, Z.; Lyu, Z.; Hood, Z. D.; Xie, M.; Vara, M.; Chi, M.; Xia, Y. Ru octahedral nanocrystals with a face-centered cubic structure, {111} facets, thermal stability up to 400 °C, and enhanced catalytic activity. J. Am. Chem. Soc. 2019, 141, 7028– 7036,  DOI: 10.1021/jacs.9b01640
      30
      Ru Octahedral Nanocrystals with a Face-Centered Cubic Structure, {111} Facets, Thermal Stability up to 400°C, and Enhanced Catalytic Activity
      Zhao, Ming; Chen, Zitao; Lyu, Zhiheng; Hood, Zachary D.; Xie, Minghao; Vara, Madeline; Chi, Miaofang; Xia, Younan
      Journal of the American Chemical Society (2019), 141 (17), 7028-7036CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Ruthenium nanocrystals with both a face-centered cubic (fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepd. as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the at. packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hcp. (hcp) lattice assocd. with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400°C, which is more than 100°C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.
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    31. 31
      Huo, D.; Cao, Z.; Li, J.; Xie, M.; Tao, J.; Xia, Y. Seed-mediated growth of Au nanospheres into hexagonal stars and the emergence of a hexagonal close-packed phase. Nano Lett. 2019, 19, 3115– 3121,  DOI: 10.1021/acs.nanolett.9b00534
      31
      Seed-Mediated Growth of Au Nanospheres into Hexagonal Stars and the Emergence of a Hexagonal Close-Packed Phase
      Huo, Da; Cao, Zhenming; Li, Jun; Xie, Minghao; Tao, Jing; Xia, Younan
      Nano Letters (2019), 19 (5), 3115-3121CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Gold (Au) typically crystallizes in a cubic close-packed (ccp) structure to present a face-centered cubic (fcc) lattice or crystal phase. Herein, we demonstrate that Au nanoscale hexagonal stars featuring a hcp. (hcp) structure can be synthesized in an aq. system in the presence of fcc-Au nanospheres as the seeds. The success of this synthesis critically relies on the use of EDTA to complex with Au3+ ions (the precursor) and the introduction of 2-phospho-L-ascorbic acid trisodium salt (Asc-2P) as a novel reducing agent to maneuver the redn. kinetics. The use of Asc-2P favorably promotes the formation of hexagonal stars with uneven surfaces at the top and bottom faces, together with concave side faces around the edges. By varying the amt. of Asc-2P to fine-tune the redn. kinetics, we can adjust the concaveness of the side faces, with a faster redn. rate favoring greater concaveness and a red shift of the plasmon resonance peak to the near-IR. For the first time, our results suggest that the phosphate and hydroxyl groups can act synergistically in controlling the morphol. of Au nanocrystals. Most significantly, the newly deposited Au atoms can also crystallize in an hcp structure, leading to the observation of a phase transition from fcc to hcp along the growth direction. This new protocol based upon kinetic control can be potentially extended to other noble metals for the facile synthesis of nanocrystals featuring unprecedented structures or phases.
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    32. 32
      Gloag, L.; Benedetti, T. M.; Cheong, S.; Marjo, C. E.; Gooding, J. J.; Tilley, R. D. Cubic-core hexagonal-branch mechanism to synthesize bimetallic branched and faceted Pd–Ru nanoparticles for oxygen evolution reaction electrocatalysis. J. Am. Chem. Soc. 2018, 140, 12760– 12764,  DOI: 10.1021/jacs.8b09402
      32
      Cubic-Core Hexagonal-Branch Mechanism To Synthesize Bimetallic Branched and Faceted Pd-Ru Nanoparticles for Oxygen Evolution Reaction Electrocatalysis
      Gloag, Lucy; Benedetti, Tania M.; Cheong, Soshan; Marjo, Christopher E.; Gooding, J. Justin; Tilley, Richard D.
      Journal of the American Chemical Society (2018), 140 (40), 12760-12764CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A major synthetic challenge is to make metal nanoparticles with nanosized branches and well-defined facets for high-performance catalysts. Here, we introduce a mechanism that uses the growth of hexagonal crystal structured branches off cubic crystal structured core nanoparticles. We control the growth to form Pd-core Ru-branch nanoparticles that have nanosized branches with low index Ru facets. We demonstrate that the branched and faceted structural features of the Pd-Ru nanoparticles retain high catalytic activity while also achieving high stability for the O evolution reaction.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhvVeisrfI&md5=75d644b447b050d3936cdc8b4225929b
    33. 33
      Yun, Q.; Ge, Y.; Huang, B.; Wa, Q.; Zhang, H. Ligand-assisted phase engineering of nanomaterials. Acc. Chem. Res. 2023, 56, 1780– 1790,  DOI: 10.1021/acs.accounts.3c00121
      33
      Ligand-Assisted Phase Engineering of Nanomaterials
      Yun, Qinbai; Ge, Yiyao; Huang, Biao; Wa, Qingbo; Zhang, Hua
      Accounts of Chemical Research (2023), 56 (13), 1780-1790CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      The synthesis of monodisperse colloidal nanomaterials with well-defined structures is important for both fundamental research and practical application. To achieve it, wet-chem. methods with the usage of various ligands have been extensively explored to finely control the structure of nanomaterials. During the synthesis, ligands cap the surface and thus modulate the size, shape, and stability of nanomaterials in solvents. Besides these widely investigated roles of ligands, it has been recently discovered that ligands can affect the phase of nanomaterials, i.e., their at. arrangement, providing an effective strategy to realize the phase engineering of nanomaterials (PEN) by selecting appropriate ligands. Nanomaterials normally exist in the phases that are thermodynamically stable in their bulk states. Previous studies have shown that under high temp. or high pressure, nanomaterials can exist in unconventional phases which are unattainable in the bulks. Importantly, nanomaterials with unconventional phases exhibit unique properties and functions different from conventional-phase ones. Consequently, it is feasible to utilize the PEN to tune the physicochem. properties and application performance of nanomaterials. During wet-chem. synthesis, ligands binding to the surface of nanomaterials can modify their surface energy, which could significantly affect the Gibbs free energy of nanomaterials and thus det. the stability of different phases, making it possible to obtain nanomaterials with unconventional phases at mild reaction conditions. For instance, a series of Au nanomaterials with unconventional hexagonal phases have been prepd. with the assistance of oleylamine. Therefore, the rational design and selection of different ligands and deep understanding of their effect on the phase of nanomaterials would significantly accelerate the development of PEN and the discovery of novel functional nanomaterials for diverse applications. In this Account, we briefly summarize the recent progress in ligand-assisted PEN, elaborating the important roles of different ligands in the direct synthesis of nanomaterials with unconventional crystal phases and amorphous phase as well as the phase transformation of nanomaterials. We first introduce the background of this research topic, highlighting the concept of PEN and why ligands can modulate the phase of nanomaterials. Then we discuss the usage of four kinds of ligands, i.e., amines, fatty acids, sulfur-contg. ligands, and phosphorus-contg. ligands, in phase engineering of different nanomaterials, esp. metal, metal chalcogenide, and metal oxide nanomaterials. Finally, we provide our personal views of the challenges and future promising research directions in this exciting field.
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    34. 34
      Nguyen, Q. N.; Chen, R.; Lyu, Z.; Xia, Y. Using reduction kinetics to control and predict the outcome of a colloidal synthesis of noble-metal nanocrystals. Inorg. Chem. 2021, 60, 4182– 4197,  DOI: 10.1021/acs.inorgchem.0c03576
      34
      Using reduction kinetics to control and predict outcome of colloidal synthesis of noble-metal nanocrystals
      Nguyen, Quynh N.; Chen, Ruhui; Lyu, Zhiheng; Xia, Younan
      Inorganic Chemistry (2021), 60 (7), 4182-4197CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Improving the performance of noble-metal nanocrystals in various applications critically depends on our ability to manipulate their synthesis in a rational, robust, and controllable fashion. Different from a conventional trial-and-error approach, the redn. kinetics of a colloidal synthesis has recently been demonstrated as a reliable knob for controlling the synthesis of noble-metal nanocrystals in a deterministic and predictable manner. Here we present a brief viewpoint on the recent progress in leveraging redn. kinetics for controlling and predicting the outcome of a synthesis of noble-metal nanocrystals. With a focus on Pd nanocrystals, we first offer a discussion on the correlation between the initial redn. rate and the internal structure of the resultant seeds. The kinetic approaches for controlling both nucleation and growth in a one-pot setting are then introduced with an emphasis on manipulation of the redn. pathways taken by the precursor. We then illustrate how to extend the strategy into a bimetallic system for the prepn. of nanocrystals with different shapes and elemental distributions. Finally, the influence of speciation of the precursor on redn. kinetics is highlighted, followed by our perspectives on the challenges and future endeavors in achieving a controllable and predictable synthesis of noble-metal nanocrystals.
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    35. 35
      Huang, X.; Li, S.; Huang, Y.; Wu, S.; Zhou, X.; Li, S.; Gan, C. L.; Boey, F.; Mirkin, C. A.; Zhang, H. Synthesis of hexagonal close-packed gold nanostructures. Nat. Commun. 2011, 2, 292,  DOI: 10.1038/ncomms1291
      35
      Synthesis of hexagonal close-packed gold nanostructures
      Huang Xiao; Li Shaozhou; Huang Yizhong; Wu Shixin; Zhou Xiaozhu; Li Shuzhou; Gan Chee Lip; Boey Freddy; Mirkin Chad A; Zhang Hua
      Nature communications (2011), 2 (), 292 ISSN:.
      Solid gold is usually most stable as a face-centred cubic (fcc) structure. To date, no one has synthesized a colloidal form of Au that is exclusively hexagonal close-packed (hcp) and stable under ambient conditions. Here we report the first in situ synthesis of dispersible hcp Au square sheets on graphene oxide sheets, which exhibit an edge length of 200-500 nm and a thickness of ~ 2.4 nm (~ 16 Au atomic layers). Interestingly, the Au square sheet transforms from hcp to a fcc structure on exposure to an electron beam during transmission electron microscopy analysis. In addition, as the square sheet grows thicker (from ~ 2.4 to 6 nm), fcc segments begin to appear. A detailed experimental analysis of these structures shows that for structures with ultrasmall dimensions (for example, <~ 6 nm thickness for the square sheets), the previously unobserved pure hcp structure becomes stable and isolable.
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    36. 36
      Gao, Y.; Peng, X. Crystal structure control of CdSe nanocrystals in growth and nucleation: dominating effects of surface versus interior structure. J. Am. Chem. Soc. 2014, 136, 6724– 6732,  DOI: 10.1021/ja5020025
      36
      Crystal Structure Control of CdSe Nanocrystals in Growth and Nucleation: Dominating Effects of Surface versus Interior Structure
      Gao, Yuan; Peng, Xiaogang
      Journal of the American Chemical Society (2014), 136 (18), 6724-6732CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      For the most studied nanocrystal system in the literature, exptl. results in this paper revealed that formation of either Zn blende or wurtzite CdSe nanocrystals was dominated by the ligand-surface interaction, instead of the interior structure difference. This conclusion was considered to be reasonable, given the very small energy difference between wurtzite and Zn blende CdSe (only 1.4 meV per CdSe unit and ∼1000 times smaller than the energy of a single Cd-ligand bond). Cd carboxylate ligands as Cd fatty acid salts promoted formation of the Zn blende structure. Conversely, Cd phosphonate ligands with a long hydrocarbon chain favored the formation of the wurtzite structure. The effects of either Cd carboxylate or Cd phosphonate ligands play a detg. role during both nucleation and growth. Different from the authors' expectation, fatty amine is only a secondary factor for crystal structure detn. With an appropriate choice of capping ligands, it was possible to achieve precise control of the crystal structure of the CdSe nanocrystals in both nucleation and growth for either the Zn blende or wurtzite structure.
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    37. 37
      Yao, Y.; He, D.; Lin, Y.; Feng, X.; Wang, X.; Yin, P.; Hong, X.; Zhou, G.; Wu, Y.; Li, Y. Modulating fcc and hcp ruthenium on the surface of palladium-copper alloy through tunable lattice mismatch. Angew. Chem., Int. Ed. 2016, 55, 5501– 5505,  DOI: 10.1002/anie.201601016
      37
      Modulating fcc and hcp Ruthenium on the Surface of Palladium-Copper Alloy through Tunable Lattice Mismatch
      Yao, Yancai; He, Dong Sheng; Lin, Yue; Feng, Xiaoqian; Wang, Xin; Yin, Peiqun; Hong, Xun; Zhou, Gang; Wu, Yuen; Li, Yadong
      Angewandte Chemie, International Edition (2016), 55 (18), 5501-5505CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein, we report an epitaxial-growth-mediated method to grow fcc. Ru, which is thermodynamically unfavorable in the bulk form, on the surface of Pd-Cu alloy. Induced by the galvanic replacement between Ru and Pd-Cu alloy, a shape transformation from a Pd-Cu@Ru core-shell to a yolk-shell structure was obsd. during the epitaxial growth. The successful coating of the unconventional crystallog. structure is critically dependent on the moderate lattice mismatch between the fcc. Ru overlayer and PdCu3 alloy substrate. Further, both fcc. and hcp. Ru can be selectively grown through varying the lattice spacing of the Pd-Cu substrate. The presented findings provide a new synthetic pathway to control the crystallog. structure of metal nanomaterials.
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    38. 38
      Gloag, L.; Benedetti, T. M.; Cheong, S.; Li, Y.; Chan, X.-H.; Lacroix, L.-M.; Chang, S. L. Y.; Arenal, R.; Florea, I.; Barron, H.; Barnard, A. S.; Henning, A. M.; Zhao, C.; Schuhmann, W.; Gooding, J. J.; Tilley, R. D. Three-dimensional branched and faceted gold-ruthenium nanoparticles: using nanostructure to improve stability in oxygen evolution electrocatalysis. Angew. Chem., Int. Ed. 2018, 57, 10241– 10245,  DOI: 10.1002/anie.201806300
      38
      Three-Dimensional Branched and Faceted Gold-Ruthenium Nanoparticles: Using Nanostructure to Improve Stability in Oxygen Evolution Electrocatalysis
      Gloag, Lucy; Benedetti, Tania M.; Cheong, Soshan; Li, Yibing; Chan, Xuan-Hao; Lacroix, Lise-Marie; Chang, Shery L. Y.; Arenal, Raul; Florea, Ileana; Barron, Hector; Barnard, Amanda S.; Henning, Anna M.; Zhao, Chuan; Schuhmann, Wolfgang; Gooding, J. Justin; Tilley, Richard D.
      Angewandte Chemie, International Edition (2018), 57 (32), 10241-10245CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Achieving stability with highly active Ru nanoparticles for electrocatalysis is a major challenge for the O evolution reaction. As improved stability of Ru catalysts was shown for bulk surfaces with low-index facets, there is an opportunity to incorporate these stable facets into Ru nanoparticles. Now, a new soln. synthesis is presented in which hcp. structured Ru is grown on Au to form nanoparticles with 3-dimensional branches. Exposing low-index facets on these 3-dimensional branches creates stable reaction kinetics to achieve high activity and the highest stability obsd. for Ru nanoparticle O evolution reaction catalysts. These design principles provide a synthetic strategy to achieve stable and active electrocatalysts.
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    39. 39
      Koenigsmann, C.; Semple, D. B.; Sutter, E.; Tobierre, S. E.; Wong, S. S. Ambient synthesis of high-quality ruthenium nanowires and the morphology-dependent electrocatalytic performance of platinum-decorated ruthenium nanowires and nanoparticles in the methanol oxidation reaction. ACS Appl. Mater. Interfaces 2013, 5, 5518– 5530,  DOI: 10.1021/am4007462
      39
      Ambient Synthesis of High-Quality Ruthenium Nanowires and the Morphology-Dependent Electrocatalytic Performance of Platinum-Decorated Ruthenium Nanowires and Nanoparticles in the Methanol Oxidation Reaction
      Koenigsmann, Christopher; Semple, Dara Bobb; Sutter, Eli; Tobierre, Sybil E.; Wong, Stanislaus S.
      ACS Applied Materials & Interfaces (2013), 5 (12), 5518-5530CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The synthesis is reported for first time of elemental ruthenium nanowires (Ru NWs), method for modifying their surfaces with platinum (Pt), and the morphol.-dependent methanol oxidn. reaction (MOR) performance of high-quality Pt-modified Ru NW electrocatalysts. The synthesis of the elemental Ru NWs was accomplished utilizing a template-based method under ambient conditions. As-prepd. Ru NWs are cryst. and elementally pure, maintain electrochem. properties analogous to elemental Ru, and can be generated with av. diams. ranging from 44-280 nm. The morphol.-dependent performance is examd. of the Ru NWs by comparison with com. Ru nanoparticle (NP)/carbon (C) systems after decorating the surfaces of these structures with Pt. It is demonstrated that the deposition of Pt onto the Ru NWs (Pt∼Ru NWs) results in a unique hierarchical structure, wherein the deposited Pt exists as discrete clusters on the surface. By contrast, it is found that the Pt-decorated com. Ru NP/C (Pt∼Ru NP/C) results in the formation of an alloy-type NP. The Pt∼Ru NPs (0.61 A/mg of Pt) possess nearly 2-fold higher Pt mass activity than analogous Pt∼Ru NW electrocatalysts (0.36 A/mg of Pt). On the basis of a long-term durability test, it is apparent that both catalysts undergo significant declines in performance, potentially resulting from aggregation and ripening in the case of Pt∼Ru NP/C and the effects of catalyst poisoning in the Pt∼Ru NWs. Both catalysts maintain comparable performance, despite a slightly enhanced performance in Pt∼Ru NP/C. In addn., the measured mass-normalized MOR activity of the Pt∼Ru NWs (0.36 A/mg of Pt) was significantly enhanced as compared with supported elemental Pt (Pt NP/C, 0.09 A/mg of Pt) and alloy-type PtRu (PtRu NP/C, 0.24 A/mg of Pt) NPs, both serving as com. stds.
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    40. 40
      Yin, A. X.; Liu, W. C.; Ke, J.; Zhu, W.; Gu, J.; Zhang, Y. W.; Yan, C. H. Ru nanocrystals with shape-dependent surface-enhanced Raman spectra and catalytic properties: controlled synthesis and DFT calculations. J. Am. Chem. Soc. 2012, 134, 20479– 20489,  DOI: 10.1021/ja3090934
      40
      Ru Nanocrystals with Shape-Dependent Surface-Enhanced Raman Spectra and Catalytic Properties: Controlled Synthesis and DFT Calculations
      Yin, An-Xiang; Liu, Wen-Chi; Ke, Jun; Zhu, Wei; Gu, Jun; Zhang, Ya-Wen; Yan, Chun-Hua
      Journal of the American Chemical Society (2012), 134 (50), 20479-20489CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Despite its multidisciplinary interests and technol. importance, the shape control of Ru nanocrystals still remains a great challenge. In this article, we demonstrated a facile hydrothermal approach toward the controlled synthesis of Ru nanocrystals with the assistance of first-principles calcns. For the first time, Ru triangular and irregular nanoplates as well as capped columns with tunable sizes were prepd. with high shape selectivity. In consistency with the exptl. observations and d. functional theory (DFT) calcns. confirmed that both the intrinsic characteristics of Ru crystals and the adsorption of certain reaction species were responsible for the shape control of Ru nanocrystals. Ultrathin Ru nanoplates exposed a large portion of (0001) facets due to the lower surface energy of Ru(0001). The selective adsorption of oxalate species on Ru(10-10) would retard the growth of the side planes of the Ru nanocrystals, while the gradual thermolysis of the oxalate species would eliminate their adsorption effects, leading to the shape evolution of Ru nanocrystals from prisms to capped columns. The surface-enhanced Raman spectra (SERS) signals of these Ru nanocrystals with 4-mercaptopyridine as mol. probes showed an enhancement sequence of capped columns > triangle nanoplates > nanospheres, probably due to the sharp corners and edges in the capped columns and nanoplates as well as the shrunk interparticle distance in their assemblies. CO-selective methanation tests on these Ru nanocrystals indicated that the nanoplates and nanospheres had comparable activities, but the former has much better CO selectivity than the latter.
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    41. 41
      Watt, J.; Yu, C.; Chang, S. L. Y.; Cheong, S.; Tilley, R. D. Shape control from thermodynamic growth conditions: the case of hcp ruthenium hourglass nanocrystals. J. Am. Chem. Soc. 2013, 135, 606– 609,  DOI: 10.1021/ja311366k
      41
      Shape Control from Thermodynamic Growth Conditions: The Case of hcp Ruthenium Hourglass Nanocrystals
      Watt, John; Yu, Chenlong; Chang, Shery L. Y.; Cheong, Soshan; Tilley, Richard D.
      Journal of the American Chemical Society (2013), 135 (2), 606-609CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Recent successes in forming different shaped fcc. (fcc) metal nanostructures has enabled a greater understanding of nanocrystal growth mechanisms. Here we extend this understanding to the synthesis of hexagonally close packed (hcp) metal nanostructures, to form uniquely faceted ruthenium nanocrystals with a well-defined hourglass shape. The hourglass nanocrystals are formed in a three-step thermodn. growth process with dodecylamine as the org. stabilizer. The hourglass nanocrystals are then shown to readily self-assemble to form a new type of nanocrystal superlattice.
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    42. 42
      Rodrigues, T. S.; Zhao, M.; Yang, T.-H.; Gilroy, K. D.; da Silva, A. G. M.; Camargo, P. H. C.; Xia, Y. Synthesis of colloidal metal nanocrystals: a comprehensive review on the reductants. Chem.─Eur. J. 2018, 24, 16944– 16963,  DOI: 10.1002/chem.201802194
      42
      Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants
      Rodrigues, Thenner S.; Zhao, Ming; Yang, Tung-Han; Gilroy, Kyle D.; da Silva, Anderson G. M.; Camargo, Pedro H. C.; Xia, Younan
      Chemistry - A European Journal (2018), 24 (64), 16944-16963CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chem. redn.: the reductants. We start with a brief introduction to the compds. commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidn. half-reactions and the corresponding oxidn. potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion redn. based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, redn. mechanisms, and addnl. effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.
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    43. 43
      Holder, C. F.; Schaak, R. E. Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials. ACS Nano 2019, 13, 7359– 7365,  DOI: 10.1021/acsnano.9b05157
      43
      Tutorial on Powder X-ray Diffraction for Characterizing Nanoscale Materials
      Holder, Cameron F.; Schaak, Raymond E.
      ACS Nano (2019), 13 (7), 7359-7365CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. The authors provide the broad nanoscience and nanotechnol. communities with a brief tutorial on some of the key aspects of powder XRD data that are often encountered when analyzing samples of nanoscale materials, with an emphasis on inorg. nanoparticles of various sizes, shapes, and dimensionalities.
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    44. 44
      Nguyen, Q. N.; Wang, C.; Shang, Y.; Janssen, A.; Xia, Y. Colloidal synthesis of metal nanocrystals: from asymmetrical growth to symmetry breaking. Chem. Rev. 2023, 123, 3693– 3760,  DOI: 10.1021/acs.chemrev.2c00468
      44
      Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking
      Nguyen, Quynh N.; Wang, Chenxiao; Shang, Yuxin; Janssen, Annemieke; Xia, Younan
      Chemical Reviews (Washington, DC, United States) (2023), 123 (7), 3693-3760CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Nanocrystals offer a unique platform for tailoring the physicochem. properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around sym. growth, the introduction of asym. growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies, as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asym. growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a no. of methods capable of generating seeds with diverse symmetry while achieving asym. growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research in understanding and controlling the symmetry breaking process.
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    45. 45
      Yin, X.; Shi, M.; Wu, J.; Pan, Y.-T.; Gray, D. L.; Bertke, J. A.; Yang, H. Quantitative analysis of different formation modes of platinum nanocrystals controlled by ligand chemistry. Nano Lett. 2017, 17, 6146– 6150,  DOI: 10.1021/acs.nanolett.7b02751
      45
      Quantitative Analysis of Different Formation Modes of Platinum Nanocrystals Controlled by Ligand Chemistry
      Yin, Xi; Shi, Miao; Wu, Jianbo; Pan, Yung-Tin; Gray, Danielle L.; Bertke, Jeffery A.; Yang, Hong
      Nano Letters (2017), 17 (10), 6146-6150CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors report that spontaneous ligand replacement and anion exchange control the form of coordinated Pt-ligand intermediates in the system of platinum acetylacetonate [Pt(acac)2], primary aliph. amine, and carboxylic acid ligands. The formed intermediates govern the formation mode of Pt nanocrystals, leading to either a pseudo two-step or a one-step mechanism by switching on or off an autocatalytic surface growth. This finding shows the importance of metal-ligand complexation at the prenucleation stage and represents a crit. step forward for the designed synthesis of nanocrystal-based materials.
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    46. 46
      Yang, T.-H.; Zhou, S.; Gilroy, K. D.; Figueroa-Cosme, L.; Lee, Y.-H.; Wu, J.-M.; Xia, Y. Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 13619– 13624,  DOI: 10.1073/pnas.1713907114
      46
      Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals
      Yang, Tung-Han; Zhou, Shan; Gilroy, Kyle D.; Figueroa-Cosme, Legna; Lee, Yi-Hsien; Wu, Jenn-Ming; Xia, Younan
      Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (52), 13619-13624CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The growth of colloidal metal nanocrystals typically involves an autocatalytic process, where the salt precursor is adsorbed on a growing nanocrystal surface, followed by chem. redn. to atoms for their incorporation into the nanocrystal. Despite its universal role in colloidal nanocrystal synthesis, it is still poorly understood and controlled in terms of kinetics. Using well-defined nanocrystals as seeds (including those with different types of facets, sizes, internal twin structure) this work quant. analyzed the kinetics of autocatalytic surface redn. to control the evolution of nanocrystals as predictable shapes. Kinetic measurements demonstrated the activation energy barrier to autocatalytic surface redn. highly depended on type of facet and presence of twin boundary, corresponding to distinctive growth patterns and products. The autocatalytic process effectively eliminates homogeneous nucleation and activates and sustains octahedral nanocrystal growth. This work was a major step toward quant. understanding and controlling the autocatalytic process involved in colloidal metal nanocrystal synthesis.
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    47. 47
      Yang, T.-H.; Peng, H.-C.; Zhou, S.; Lee, C.-T.; Bao, S.; Lee, Y.-H.; Wu, J.-M.; Xia, Y. Toward a quantitative understanding of the reduction pathways of a salt precursor in the synthesis of metal nanocrystals. Nano Lett. 2017, 17, 334– 340,  DOI: 10.1021/acs.nanolett.6b04151
      47
      Toward a Quantitative Understanding of the Reduction Pathways of a Salt Precursor in the Synthesis of Metal Nanocrystals
      Yang, Tung-Han; Peng, Hsin-Chieh; Zhou, Shan; Lee, Chi-Ta; Bao, Shixiong; Lee, Yi-Hsien; Wu, Jenn-Ming; Xia, Younan
      Nano Letters (2017), 17 (1), 334-340CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Despite the pivotal role played by the redn. of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the soln. phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by redn. through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the redn. kinetics involved. Quant. analyses of the redn. kinetics of PdCl42- and PdBr42- by ascorbic acid at room temp. in the absence and presence of Pd nanocubes, resp., suggest that PdCl42- was reduced in the soln. phase while PdBr42- was reduced on the surface of a growing nanocrystal. These results demonstrate that the redn. pathway of PdBr42- by ascorbic acid could be switched from surface to soln. by raising the reaction temp.
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    48. 48
      Xia, X.; Xie, S.; Liu, M.; Peng, H. C.; Lu, N.; Wang, J.; Kim, M. J.; Xia, Y. On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals. Proc. Natl. Acad. Sci. U.S.A 2013, 110, 6669– 6673,  DOI: 10.1073/pnas.1222109110
      48
      On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals
      Xia, Xiaohu; Xie, Shuifen; Liu, Maochang; Peng, Hsin-Chieh; Lu, Ning; Wang, Jinguo; Kim, Moon J.; Xia, Younan
      Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (17), 6669-6673, S6669/1-S6669/6CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Controlling the shape or morphol. of metal nanocrystals is central to the realization of their many applications in catalysis, plasmonics, and electronics. In one of the approaches, the metal nanocrystals are grown from seeds of certain crystallinity through the addn. of at. species. In this case, manipulating the rates at which the at. species are added onto different crystallog. planes of a seed has been actively explored to control the growth pattern of a seed and thereby the shape or morphol. taken by the final product. Upon deposition, however, the adsorbed atoms (adatoms) may not stay at the same sites where the depositions occur. Instead, they can migrate to other sites on the seed owing to the involvement of surface diffusion, and this could lead to unexpected deviations from a desired growth pathway. The authors demonstrate that the growth pathway of a seed is indeed detd. by the ratio between the rates for atom deposition and surface diffusion. Surface diffusion needs to be taken into account when controlling the shape or morphol. of metal nanocrystals.
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    49. 49
      Janssen, A.; Pawlik, V.; von Rueden, A. D.; Xu, L.; Wang, C.; Mavrikakis, M.; Xia, Y. Facile synthesis of palladium-based nanocrystals with different crystal phases and a comparison of their catalytic properties. Adv. Mater. 2021, 33, 2103801  DOI: 10.1002/adma.202103801
      49
      Facile synthesis of palladium-based nanocrystals with different crystal phases and comparison of their catalytic properties
      Janssen, Annemieke; Pawlik, Veronica; von Rueden, Alexander D.; Xu, Lang; Wang, Chenxiao; Mavrikakis, Manos; Xia, Younan
      Advanced Materials (Weinheim, Germany) (2021), 33 (49), 2103801CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A relatively unexplored aspect of noble-metal nanomaterials is polymorphism, or their ability to crystallize in different crystal phases. Here, a method is reported for the facile synthesis of Ru@Pd core-shell nanocrystals featuring polymorphism, with the core made of hexagonally close-packed (hcp)-Ru while the Pd shell takes either an hcp or face-centered cubic (fcc) phase. The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking the hcp phase whereas the thicker ones revert to fcc. The injection rate provides an exptl. knob for controlling the phase, with one-shot and drop-wise injection of the Pd precursor corresponding to fcc-Pd and hcp-Pd shells, resp. When these nanocrystals are tested as catalysts toward formic acid oxidn., the Ru@Pdhcp nanocrystals outperform Ru@Pdfcc in terms of both specific activity and peak potential. D. functional theory calcns. are also performed to elucidate the origin of this performance enhancement.
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    50. 50
      Zhang, H.; Li, W.; Jin, M.; Zeng, J.; Yu, T.; Yang, D.; Xia, Y. Controlling the morphology of rhodium nanocrystals by manipulating the growth kinetics with a syringe pump. Nano Lett. 2011, 11, 898– 903,  DOI: 10.1021/nl104347j
      50
      Controlling the Morphology of Rhodium Nanocrystals by Manipulating the Growth Kinetics with a Syringe Pump
      Zhang, Hui; Li, Weiyang; Jin, Mingshang; Zeng, Jie; Yu, Taekyung; Yang, Deren; Xia, Younan
      Nano Letters (2011), 11 (2), 898-903CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Noble-metal nanocrystals with well-defined and controllable morphologies are of great importance to applications in catalysis, plasmonics, and surface-enhanced spectroscopy. Many synthetic approaches have been demonstrated for controlling the growth habit and thus morphol. of metal nanocrystals, but most of them are based on a thermodn. approach, including the use of a capping agent. While thermodn. control has shown its power in generating nanocrystals with a myriad of different morphologies, it is ultimately limited by the obligation to minimize the surface energy of a system. As a result, it is impractical to use thermodn. control to generate nanocrystals having high-energy facets and/or a neg. curvature. Using rhodium as an example, here we demonstrate a general method based on kinetic control with a syringe pump that can be potentially extended to other noble metals and even other solid materials. For the first time, we were able to produce concave nanocubes with a large fraction of {110} facets and octapods with a cubic symmetry in high yields by simply controlling the injection rate at which the precursor was added into the reaction soln. The concave nanocubes with {110} facets and a unique cavity structure on the surface are important for a variety of applications.
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      Zhao, M.; Chen, Z.; Shi, Y.; Hood, Z. D.; Lyu, Z.; Xie, M.; Chi, M.; Xia, Y. Kinetically controlled synthesis of rhodium nanocrystals with different shapes and a comparison study of their thermal and catalytic Properties. J. Am. Chem. Soc. 2021, 143, 6293– 6302,  DOI: 10.1021/jacs.1c02734
      51
      Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties
      Zhao, Ming; Chen, Zitao; Shi, Yifeng; Hood, Zachary D.; Lyu, Zhiheng; Xie, Minghao; Chi, Miaofang; Xia, Younan
      Journal of the American Chemical Society (2021), 143 (16), 6293-6302CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable redn. kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temp. to remove Br- ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from in situ electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500°, resp. When tested as catalysts for hydrazine decompn., the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H2 selectivity relative to the cubic counterpart. As for EtOH oxidn., the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.
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      Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. The surface energy of metals. Surf. Sci. 1998, 411, 186– 202,  DOI: 10.1016/S0039-6028(98)00363-X
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      Surface Science (1998), 411 (1/2), 186-202CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
      The authors used d. functional theory to establish a database of surface energies for low index surfaces of 60 metals in the periodic table. The data may be used as a consistent starting point for models of surface science phenomena. The accuracy of the database is established in a comparison with other d. functional theory results and the calcd. surface energy anisotropies are applied in a detn. of the equil. shape of nano-crystals of Fe, Cu, Mo, Ta, Pt and Pb.
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      Wang, Y.; Peng, H.-C.; Liu, J.; Huang, C. Z.; Xia, Y. Use of reduction rate as a quantitative knob for controlling the twin structure and shape of palladium nanocrystals. Nano Lett. 2015, 15, 1445– 1450,  DOI: 10.1021/acs.nanolett.5b00158
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      Use of Reduction Rate as a Quantitative Knob for Controlling the Twin Structure and Shape of Palladium Nanocrystals
      Wang, Yi; Peng, Hsin-Chieh; Liu, Jingyue; Huang, Cheng Zhi; Xia, Younan
      Nano Letters (2015), 15 (2), 1445-1450CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Kinetic control is a powerful means for maneuvering the twin structure and shape of metal nanocrystals and thus optimizing their performance in a variety of applications. However, there is only a vague understanding of the explicit roles played by reaction kinetics due to the lack of quant. information about the kinetic parameters. With Pd as an example, here we demonstrate that kinetic parameters, including rate const. and activation energy, can be derived from spectroscopic measurements and then used to calc. the initial redn. rate and further have this parameter quant. correlated with the twin structure of a seed and nanocrystal. On a quant. basis, we were able to det. the ranges of initial redn. rates required for the formation of nanocrystals with a specific twin structure, including single-crystal, multiply twinned, and stacking fault-lined. This work represents a major step forward toward the deterministic syntheses of colloidal noble-metal nanocrystals with specific twin structures and shapes.
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      Plyuto, Y. V.; Babich, I. V.; Sharanda, L. F.; Marco De Wit, A.; Mol, J. C. Thermolysis of Ru(acac)3 supported on silica and alumina. Thermochim. Acta 1999, 335, 87– 89,  DOI: 10.1016/S0040-6031(99)00148-3
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      Morozova, N. B.; Gelfond, N. V.; Semyannikov, P. P.; Trubin, S. V.; Igumenov, I. K.; Gutakovskii, A. K.; Latyshev, A. V. Preparation of thin films of platinum group metals by pulsed MOCVD. II. Deposition of Ru layers. J. Struct. Chem. 2012, 53, 725– 733,  DOI: 10.1134/S0022476612040154
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      Mahfouz, R. M.; Siddiqui, M. R. H.; Al-Ahmari, S. A.; Alkayali, W. Z. Kinetic analysis of thermal decomposition of unirradiated and γ-irradiated tris(acetylacetonato)-ruthenium(III) [Ru(acac)3]. Prog. React. Kinet. Mech. 2007, 32, 1– 27,  DOI: 10.3184/146867807X217337
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      Kucharyson, J. F.; Gaudet, J. R.; Wyvratt, B. M.; Thompson, L. T. Characterization of structural and electronic transitions during reduction and oxidation of Ru(acac)3 flow battery electrolytes by using X-ray absorption spectroscopy. ChemElectroChem. 2016, 3, 1875– 1883,  DOI: 10.1002/celc.201600360
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      Vydrov, O. A.; Scuseria, G. E.; Perdew, J. P. Tests of functionals for systems with fractional electron number. J. Chem. Phys. 2007, 126, 154109  DOI: 10.1063/1.2723119
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      Tests of functionals for systems with fractional electron number
      Vydrov, Oleg A.; Scuseria, Gustavo E.; Perdew, John P.
      Journal of Chemical Physics (2007), 126 (15), 154109/1-154109/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      In the exact theory, the ground state energy of an open system varies linearly when the electron no. is changed between two adjacent integers. This linear dependence is not reproduced by common approx. d. functionals. Deviation from linearity in this dependence has been suggested as a basis for the concept of many-electron self-interaction error (SIE). In this paper, we quantify many-electron SIE of a no. of approxns. by performing calcns. on fractionally charged atoms. We demonstrate the direct relevance of these studies to such problems of common approx. functionals as instabilities of anions, spurious fractional charges on dissocd. atoms, and poor description of charge transfer. Semilocal approxns. have the largest many-electron SIE, which is only slightly reduced in typical global hybrids. In these approxns. the energy vs. fractional electron no. curves upward, while in Hartree-Fock theory the energy curves downward. Perdew-Zunger self-interaction correction [Phys. Rev. B 23, 5048 (1981)] significantly reduces the many-electron SIE of semilocal functionals but impairs their accuracy for equil. properties. In contrast, a long-range cor. hybrid functional can be nearly many-electron SIE-free in many cases (for reasons we discuss) and at the same time performs remarkably well for many mol. properties.
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      Raebiger, H.; Lany, S.; Zunger, A. Charge self-regulation upon changing the oxidation state of transition metals in insulators. Nature 2008, 453, 763– 766,  DOI: 10.1038/nature07009
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      Charge self-regulation upon changing the oxidation state of transition metals in insulators
      Raebiger, Hannes; Lany, Stephan; Zunger, Alex
      Nature (London, United Kingdom) (2008), 453 (7196), 763-766CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidn. states that have distinct signatures in X-ray photoemission spectroscopy and ionic radii' which vary with the oxidn. state of the atom. We explain this peculiar tendency of transition-metal atoms to maintain a const. local charge under external perturbations in terms of an inherent, homeostasis-like neg. feedback. We show that signatures of oxidn. states and multivalence-such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization-that have often been interpreted as literal charge transfer are instead a consequence of the neg.-feedback charge regulation.
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      Stampfl, C.; Scheffler, M. Theoretical study of O adlayers on Ru(0001). Phys. Rev. B 1996, 54, 2868– 2872,  DOI: 10.1103/PhysRevB.54.2868
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c01725.

    • Descriptions of synthetic protocols, computational details, additional TEM and HRTEM images, and XRD patterns of the initial Ru seeds and samples prepared under different conditions, as well as results from additional computational analyses (PDF)


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