Near-Surface Alloys of PtRh on Rh(111) and Pt(111) Characterized by STM

: The use of Pt − Rh catalysts is central in a number of industrial processes, and to explain their performance it is essential to have a solid understanding of their nanoscopic surface structure. Here we use scanning tunneling microscopy to investigate the growth, elemental nanostructuring, and reconstruction behavior of various Pt − Rh near-surface alloys (NSA) on fcc Rh(111) and Pt(111). We document the formation of a novel lamellar island reconstruction atop Pt/Rh(111) islands as well as network reconstructions on Rh/Pt(111), similar to those previously observed for Pt/Pt(111). The extended roadmap for preparation of PtRh NSAs allows comparison with other homo/heteroepitaxial metal/fcc surfaces and provides a facile guideline for producing tailor-made model catalytic surfaces.


■ INTRODUCTION
PtRh alloys are active catalysts for high-temperature (1100− 1500 K) industrial processes (HNO 3 and HCN production) and for NO x abatement at milder process conditions (<750 K).On the basis of catalytic performance (activity, product selectivity), alloyed metals are frequently the preferred catalytic material over the individual metal constituents. 1Thus, atomistic details about elemental mixing of Pt and Rh at the surface is a prerequisite for explaining their structure− performance relationship.
The mixing and segregation behavior of PtRh alloys was investigated on the basis of fcc Pt(111) and Rh(111) surfaces.In particular, both computational modeling and experimental data indicate a lower surface energy for Pt(111) in comparison with Rh(111), namely, 2.299 versus 2.472 J/m 2 according to modeling and 2.482 versus 2.680 J/m 2 found experimentally. 2or bimetallic surface alloys, calculations further point to surface segregation for Pt in a Rh(111) host, but Pt−Rh mixing in a Pt(111) host. 3 When a solute metal is present at the surface or near the surface of a host metal in concentrations different from the bulk, the obtained composition constitutes respectively a surface or a near-surface alloy (NSA). 1,4ssentially, whether the metal adatoms reside immediately at the surface of a host or just below the first surface layer(s) determines if a surface alloy or a NSA is formed.In turn, the preference for surface/subsurface residency and mixing/ segregation behavior results from the differences in host and adatom surface free energy, segregation energy, etc.
Preparation of NSAs can be done through a deposition of metal on metal substrates.The obtained morphology and elemental mixing are highly dependent on the host crystal and its temperature during deposition.−7 In particular, recently we summarized a roadmap of the heteroepitaxy, growth, and mixing of Rh on Pt(111). 7Key findings are that Rh, similarly to Pt/Pt(111), follows a 2D growth mode in a broad temperature range, where lower temperature (300−450 K) deposition leads to triangular islands, with a transition to hexagonal islands closer to 500 K.According to the theoretic prediction, Rh mixing with Pt(111) is expected, 3 and therefore, a Rh subsurface location within PtRh NSA on Rh/Pt(111) is thermodynamically favorable.However, as showed by temperature-resolved and photon-energy-dependent X-ray photoelectron spectroscopy studies, 7,8 a temperature above 550− 600 K is required to allow significant Rh diffusion subsurface, accompanied by Pt diffusion onto the surface.Lower temperatures enable Rh ad-islands flattening, growth, and coalescence.Higher temperatures (≥650 K) are therefore required for the preparation of PtRh islands highly enriched with Pt.
Due to the reduced coordination available to the surface atoms of metals, bare metallic surfaces as well as epitaxial layers deposited on single-crystal surfaces generally are strained because of their lattice mismatch with the substrate. 9To minimize the strain, surface reconstructions form even on bare surfaces of Au (herringbone) and Pt (network, see below), leading to the creation of transition regions, in which the tensile stress is accommodated by a combination of lattice strain and misfit dislocations. 10In some cases of heteroepitaxial films, the driving force to resolve surface strain can lead to nanostructured alloying, as was shown for Cu and Ag on Ru(0001). 10hile no surface reconstruction was reported for Rh/ Pt(111) and Pt/Rh(111), bare Pt(111) was shown to reconstruct at high temperature or in the presence of supersaturated Pt vapor in the temperature range 400−700 K. 11 This reconstruction produces a uniform network of features like U-endings, dark stars with three arms, bright rotors, and elbows, all of which are formed by merging, bending, or terminating double lines that propagate along the [112], [121], and [211] directions. 11To compensate for the lack of surface atom coordination, insertion of additional atoms into the surface layer produces the reconstruction with extra coverage of ∼5% and atomic rows with elevated height appearing as bright lines. 11At the atomic scale, each bright line corresponds to seven or eight atoms in the bridge position (Figure 1), separating unreconstructed fcc regions outside of the double line and narrow purely hcp regions (ca. 2 atoms) inside of it, similarly to the herringbone reconstruction observed on Au(111). 12The double lines have a separation of 2.5 ± 0.2 nm and a height of 0.025 ± 0.005 nm above fcc domains. 13In a computational study, Pushpa and Narasimhan concluded that unreconstructed Pt(111) is at the edge of instability, and they predicted that the formation of the network reconstruction on Pt(111) is highly sensitive to changes in the environment. 14Essentially, if the chemical potential of the surface is altered by the presence of other adatoms (for example, Pt or Rh), Pt(111) will easily reconstruct.
Observation of a rich spectrum of morphologies and alloying for Rh/Pt(111) raised the question whether the reverse system Pt/Rh(111), for which the Pt surface segregation is predicted, can be induced to form an alloy.Furthermore, will the Pt and Rh lattice mismatch lead to surface reconstructions?In this paper, a direct comparison of the structure of near-surface alloys of PtRh on Rh(111) and Pt(111) is conducted using ultrahigh vacuum scanning tunneling microscopy (UHV STM).

■ EXPERIMENTAL DETAILS
Sample Preparation and Characterization.The sample preparation and characterization were performed using a commercial Reactor STM system at the University of Oslo. 15t(111) and Rh(111) single crystals (Surface Preparation Laboratory) were used for the preparation of nanostructures.Rh(111) was cleaned in multiple 15 min cycles of Ar + (5.0, Westfalen) sputtering, using a SPECS IQE 11/35 with 1.5 kV energy and a sample current of ∼10 μA/cm 2 .Subsequent annealing was performed in UHV at 1100−1200 K for 10 min.Every second cleaning cycle included oxidation in 1 × 10 −7 mbar O 2 (5.0, Westfalen) at 923 K for 5 min followed by UHV annealing for 10 min.Pt(111) was prepared similarly, except the oxidation step, which was conducted at 1150 K for 2 min, followed by 8 min of annealing at the same temperature in UHV.To ensure a reproducible readout of sample temperature (above 900 K), a K-type thermocouple was used in combination with a Micro-Epsilon TIM 1M infrared thermal imaging camera.Emissivity values of 0.8 and 0.4 were used for the polished Pt and Rh surfaces, respectively.The cleanness and crystallinity of the surface were verified by means of low energy electron diffraction (LEED, ErLEED, SPECS) and STM.Between experiments, the crystal was cleaned in three cycles, including oxidation during the second cycle.After deposition of thicker films of Pt on Rh(111), sputtering was longer and more aggressive, that is, with the same ion energy but increased sample current, between 12 and 20 μA/cm 2 .Deposition of Pt (99.95%,Goodfellow) and Rh (99.9%,Goodfellow) was performed at a base pressure of 5 × 10 −9 mbar, using a SPECS four-pocket electron beam evaporator (EBE-4).The deposition was performed with 1.5 kV voltage at a rate of approximately 0.04 monolayers (ML) min −1 , as determined from the coverage found in the STM images.Sample temperature during deposition was kept in the range 300−700 K for 5−100 min.Subsequent postannealing was performed at 600−700 K for 5−10 min.
STM Measurements.STM measurements were conducted with manually cut Pt 80 It 20 0.25 mm diameter tips (Goodfellow).The CAMERA 4.3 software package developed at Leiden University was used for data recording.Images were recorded in constant current mode, with a sample bias between −0.05 and −0.5 V.All measurements were conducted at room temperature (RT), with a base pressure of 1 × 10 −9 mbar.Analysis of STM images was performed in Gwyddion 2.55. 16mages were corrected for horizontal scars and a polynomial (quadratic) background.Ad-layer coverage was estimated away from the steps, by shading the islands with the threshold height method.Due to the similarity of Pt and Rh surface atom densities, the overlayer coverage is defined by the projected area of the layers and was obtained by averaging the coverage from several distant frames.WSxM 5.0 software was used for analysis of height profiles and for export of the STM images. 17

■ RESULTS
Growth, Morphology, and Reconstructions of Pt/ Rh(111).To get an overview of how Rh(111) substrate temperature affects ad-metal surface mobility and results in different morphology, 0.2 ML of Pt was deposited on Rh(111) kept at 300−600 K and subsequently imaged at 300 K (Figure 2).Similarity in the growth patterns allowed dividing the temperature range into four regions, with comparable features: 300, 335, 385−500, and above 600 K.The Journal of Physical Chemistry C STM of surfaces prepared in the two lowest temperature ranges of 300 and 335 K shows that islands with an irregular dendritic shape formed both on terraces and along step edges.However, at 300 K the islands are 2 orders of magnitude smaller compared to 335 K results.Similar dendritic growth is reported for Pt/Pt(111), 5,18 Rh/Rh(111), 19 Co/Re(111), 20 and Au/Ru(0001). 21It can be observed that the islands grow along three common directions.The islands' average step height of 0.25 ± 0.02 nm compares with one atomic step of Pt.The presence of islands with only one layer contrasts Rh/ Pt(111) in this temperature range, where pyramidal islands with three layers were observed. 7bove 335 K, a transition to compact island growth occurs.While at 400 K the islands still have an ill-defined triangular shape, at 425 K the presence of islands with six facets is clearly visible.A further increase in the temperature to 450 and 500 K results in larger, truncated triangular islands.A similar transition from triangular through hexagonal and again to triangular islands has been reported for Pt/Pt(111), 5 Ru/ Pt(111), 22 and Rh/Pt(111). 7In this temperature range, the Pt islands remain monolayered, indicating a 2D growth mode, with nucleation occurring both on the Rh terraces and along the step edges.As deposited coverage increases [see 0.7−2.3ML in Figure S1 of the Supporting Information (SI)], the islands start to coalesce into larger islands.Raising the surface temperature to 500 K leads to an increase in island size from 8 to 25 nm on average.No apparent change in islands coverage was found for the surfaces prepared at different temperatures, despite the noticeable variation in islands' size and quantity.This observation is consistent with Pt surface location in Pt/ Rh(111), supporting an earlier experimental finding that subsurface diffusion is not favored below 600 K. 23 A clear contrast between the upper Rh terrace and Pt islands that joined step edges also indicates the absence of Pt−Rh mixing.The LEED obtained for the surface prepared at 425 K (Figure 2) displays a hexagonal diffraction pattern with bright, sharp spots indicating epitaxial growth and crystalline Pt on the Rh(111) substrate.This LEED pattern is identical with the LEED results for the surfaces prepared at different temperatures and with higher coverage (Figures S1 and S6, SI).
By increasing the deposition temperature to 600 K, the Pt islands lose their clear (truncated) triangular shapes, and instead form elongated 20−60 nm branch-like structures with a clear preference for step decoration.The shape of the branches resemble hexagons elongated along three equivalent crystallographic directions.We note that Pt deposition at 600 K induced uniform, disordered, dendritic corrugations all over the terraces.This could be interpreted as an onset of Pt alloying into Rh(111) terraces, but requires confirmation using a dedicated atomically resolved imaging, as was shown for deposition at 700 K previously. 23loser inspection of the Pt/Rh(111) islands prepared above 400 K reveals a lamellar domain structure formed on ad-islands located on terraces and at the step edges (Figure 3a).The structure consists of rows that developed along the three different crystallographic orientations.The directions, indicated as a, b, and c in Figure 3b, are parallel to the facets of the ad-islands.The domains with distinct striped orientation are separated by 0.7 ± 0.1 nm wide boundaries.It appears that the rows do not have arbitrary length, and they span for ca.3.5 nm on average.The majority of the reconstructed domains feature a darker stripe that formed across multiple rows approximately in their center [Figures 3b,c and S4d (SI)], but some diagonal and double-crossing ones were also observed.
The average inter-row spacing (period) is ca.0.44 ± 0.04 nm (4.44 nm/10 rows in the line profile KL, Figure 3c).The line profiles in Figure 3 show that the apparent height of the rows varies within a wide range of 0.01−0.3nm, depending on the location and temperature treatment.The most shallow height modulation of 0.01 nm appears on the as-prepared adislands (KL profile in Figure 3c).The island edges and steps exhibit 0.02−0.04nm features with the vacancies down to 0.08−0.10nm (EF profile in Figure 3b).Remarkably, the postannealing to 600 K induces deepening of the modulation to 0.09−0.14nm separated by trenches as deep as 0.2−0.3nm (XZ profile in Figure 3d).The Journal of Physical Chemistry C At the step edges (Figure 3b), similar row directions (a, b, c) are observed, although there is a strong preference for the orientation perpendicular to the step edge (a).In a tendency to minimize the rows parallel to the step edge (c), inclined orientations (d, e) are also observed.
To explain the origin of the island restructuring we considered two possibilities: (i) an ordered formation of impurities, segregated from Rh(111) bulk or adsorbed from UHV and (ii) a surface strain accompanied by the discrepancy of surface lattices of Pt and Rh (0.392 and 0.380 nm, respectively). 24On the one hand, the regular lamellar features observed in STM of Pt ad-islands, namely, symmetry, row length, and periodicity, resemble a description of hydrocarbon chains that formed along the three equivalent close-packed crystallographic directions of Co(0001) during high pressure (1 bar) dosing of CO + H 2 at 494 K. 25 This hypothesis was discarded due to incomparable experimental conditions, namely, UHV versus high pressure.Other alternatives such as hydrogen-induced surface reconstruction previously found on Cu surfaces 26 was excluded on the basis of LEED data, by performing deposition with the ion gauge off, and from the absence of an ordered structure on pure Rh(111), Rh/ Rh(111), and PtRh/Rh(111) [see Figures 7 and S2 (SI)].
The likely origin of the observed features is due to straininduced surface reconstruction.Indeed, since unreconstructed bare metallic surfaces are at the limit of their stability, the presence of surface species with a distinct (larger) surface lattice is capable of inducing surface and island reconstruction.In the considered scenario, the narrower space available for the Pt−Pt lattice on top of the Rh(111) lattice can cause rows of Pt to be slightly (0.01−0.09 nm) shifted upward, resulting in a lamellar domain structure (see details visually summarized in Figure 4).A simplistic estimation can be done by assuming that the origin of the observed contrast is from the purely morphologic effects due to elevation of Pt−Pt species to be able to fit into a smaller Rh−Rh lattice.In such a case, to ensure a Pt−Pt spacing of 0.392 nm within 0.380 nm defined by the Rh−Rh lattice underneath, a Pt atom should be lifted by 0.096 nm.Comparable height features were indeed observed within the ad-islands after postannealing (see the XZ profile in Figure 3d).The lower height modulations observed in the asprepared islands may hint at nonequilibrium island morphology.
To the best of our knowledge, this specific type of reconstruction has not been observed in this or other systems previously.Notably, reconstruction of the top of an ad-island of Pt homoepitaxially grown on Pt(111) was reported as practically impossible, 27 and the more facile way to release the surface strain of Pt/Pt(111) is via network reconstructions, as mentioned in the Introduction.
The large temperature window in which the reconstruction is observed (400−600 K) suggests a high temperature stability, even for higher coverage surfaces (see 0.8 ML in Figure S3, SI) and for postannealing to 600 K [Figures 3d and S4 (SI)].
For the surfaces prepared at 300−500 K, only limited Pt−Rh interdiffusion occurs; therefore, to promote alloying, the surfaces prepared at lower temperature were postannealed to 600 K (Figure 5).
Comparison of as-prepared morphology in Figure 2 and the island shapes after postannealing (Figure 5) indicates that the resulting features are moderately affected by heating to 600 K.In general, postannealing leads to a mild reshaping of the islands and smoothening of sharp edges, resulting in more rounded islands.For the lower initial deposition temperature (335−400 K), initially producing dendrites, the reshaping is pronouncedly stronger (not shown).There was no apparent  The Journal of Physical Chemistry C change in the total coverage for the surfaces that had been postannealed to 600 K, in contrast to some coverage loss reported for the reverse system, 7 albeit postannealed to 700 K.The coverage stability was also confirmed by LEED (Figure S6, SI), featuring identical hexagonal patterns, indicating that the epitaxy and crystallinity are retained.
Although the compact islands exhibit thermal stability, the dendritic islands formed at lower deposition temperature of   The Journal of Physical Chemistry C upon postannealing to 700 K. Hence, in order to form thermally stable islands of Pt on Rh(111), the initial deposition should be performed at sufficiently high temperatures, for example above 385 K.
Reconstruction of a Mirror Rh/Pt(111) Surface.As previously reported by our group, 7 deposition of Rh on Pt(111) at 300−650 K results in a variety of triangular and hexagonal islands without apparent surface reconstructions.Inspired by reconstructions seen in Pt/Rh(111) around 425 K, we revisited the surface morphology formed on Rh/Pt(111) around 450−550 K (Figure 6).STM for 0.2 ML of Rh/ Pt(111) prepared at 450 K reveals small nuclei aligned almost in straight lines on top of the surface, which appear embedded into the surface at 500 K and eventually induce double-line surface network reconstruction at 550 K. Formations of stars (S), elbows (E), and clockwise and counterclockwise rotors (Rc and Rcc) in Figure 6 are clearly visible for preparation at 550 K and are visually identical to the network reconstruction of Pt(111).The change in apparent height of the nuclei is captured in the height profiles below the STM images.Initially, at 450 K nuclei exhibit a height of 0.18 nm, close to the Rh islands' height (ABC profile, Figure 6), decreasing to about 0.1 nm at 500−525 K (EFGH and KLMN profiles, Figure 6).The reconstruction appears complete at 550 K, forming a network of double lines spaced by 2.5 ± 0.5 nm and with a height of 0.02 nm (XYZ profile, Figure 6), branching and surrounding the Rh islands as well as decorating empty terraces with a hexagonal mesh.The height and spacing closely resemble network reconstructions of Pt(111), as described in the Introduction. 13,14,28,29A notable feature captured at 450−500 K is multiple small nuclei, aligned along the same directions as the fully developed Rh islands.Because such reconstructions help to relieve surface strain, it can be hypothesized that the nuclei tend to form along the preexisting surface strain directions, and in turn, strain lines serve as preferred nucleation sites.Surprisingly, contrasting results were achieved for the two codepositions: the enhancement of network reconstruction around the islands was found for PtRh/Pt(111) and an absence of reconstruction within PtRh/Rh(111).One of the distinctions in these two systems is the tendency of adatoms of Rh to diffuse subsurface on Pt(111), but they remain at the surface of Rh(111).Thus, the diffusion of Rh into the surface layer could be linked to the formation of a reconstruction network surrounding islands of Rh/Pt(111).Another common denominator of the reconstructed surfaces is that the reconstructions only form on Pt, either inside the Pt islands [on Rh(111)] or on the Pt(111) substrate [as in PtRh/ Pt(111) and Rh/Pt(111)].Notably, out of the four bimetallic surfaces, the only surface without reconstructions was the surface with the highest Rh content, PtRh/Rh(111), for which surface Pt could help to stabilize Rh at the surface, preventing subsurface diffusion.This likely leads to highly mixed Rh and Pt ad-islands, however without a lamellar domain structure.The absence of reconstruction atop of PtRh/Rh(111) allows one to hypothesize that the island reconstruction on Pt/ Rh(111) is not due to mixed and elevated PtRh rows.The Journal of Physical Chemistry C

Figure 1 .
Figure 1.Schematic structure of the double-line network reconstruction of the Pt(111) surface.Bright parallel lines (height +0.025 nm) encompass hcp regions on the fcc surface (adapted from ref 11).

Figure 3 .
Figure 3. (a) Overview STM image of 0.2 ML of Pt/Rh(111) prepared at 425 K. (b) Zoom into the ad-island and step reconstructions showing possible row directions (a, b, c, d, e).STM image of another reconstructed island as-prepared (425 K) (c) and after postannealing to 600 K (d).Line profiles across the reconstruction presented below each image show the heights of islands and reconstruction features.Measured with U t = −0.5 V, I t = −0.1 nA.

Figure 4 .
Figure 4. Summary of the lateral features of the observed reconstruction, schematically showing three equivalent close-packed directions, rows periodicity (ca.0.44 nm), typical length (3.5 nm), and width of the domain boundary (0.7 nm).

Figure 6 .
Figure 6.STM morphology of 0.2 ML of Rh/Pt(111) prepared at 450−550 K, measured at RT with I t = −0.2nA and U t = −0.25 V. Line profiles below the images show gradual integration of small nuclei into the surface layer, with a height decrease from 0.18 to 0.02 nm.Dark dotted lines indicate three directions of islands' growth, which match with the three directions along which nuclei form.The STM image of the surface prepared at 450 K was originally published in ref 7 (Copyright 2018 American Chemical Society).

Figure 7 .
Figure 7.Comparison of single metal depositions (left and right panels) of Rh or Pt on Pt(111) and Rh(111) (top and bottom, respectively) with coevaporation of Pt + Rh (middle), all at 450 Network reconstruction is visible on PtRh/Pt(111) and Rh/Pt(111) surfaces.Island reconstruction forms only for Pt/Rh(111).
Preparation of Bimetallic Surfaces by Codeposition.Another route, alternative to postannealing, allows preparation of a mixed PtRh surface in one step by codeposition of Rh and Pt on either Pt(111) or Rh(111) at elevated temperature.

Figure 7
compares STM images for deposition of a single metal (Pt or Rh) on Pt(111) and Rh(111), and Pt + Rh coevaporated on the same surfaces at 450 K.In line with previous literature, the homoepitaxy of Pt/Pt(111) and Rh/ Rh(111) produce large triangular islands.The absence of surface reconstructions for Pt/Pt(111) at this temperature is in agreement with the lower nucleation probability below 500 K 11 and our previous experimental results [see also Figure S5 (SI) for reconstruction of Pt/Pt(111) formed at 525 K]. 7 In agreement with Figures 3 and 6, heteroepitaxy produces either island reconstruction [Pt/Rh(111)] or network reconstruction around the islands [Rh/Pt(111)].

Figure 8 .
Figure 8. Schematic diagram for the morphology and surface mixing found for PtRh alloys prepared on Pt(111) and Rh(111).Due to preference of Pt to reside on the surface at UHV conditions, surface Pt enrichment (gray) is obtained naturally for Pt/Rh(111) ad-islands in a wide temperature range.Rh enrichment (blue) in Rh/Pt(111) ad-islands can be induced to form a surface alloy with Pt at moderate temperatures and a near-surface alloy at higher temperatures (light blue shading).Close-up of the reconstruction on the left side shows the novel atop reconstruction of Pt islands on Rh(111), which are present in the temperature range 385−600 K. Close-up of the reconstruction on the right side shows network reconstruction on Pt(111) surrounding Rh islands.Pt/Pt(111) results are adapted from refs 5, 19, and 30.Rh/Pt(111) is described in detail in ref 7, and the surface alloy of Pt/Rh(111) prepared at 700 K is from ref 23.