Surface and Subsurface Structures of the Pt-Fe Surface Alloy on Pt

Jun 13, 2019 - Pt-Fe bimetallic alloys are important model catalysts for a number of catalytic reactions. Combining scanning tunneling microscopy (STM...
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Surface and Subsurface Structures of the Pt−Fe Surface Alloy on Pt(111) Hao Chen,†,§ Rui Wang,‡ Rong Huang,‡ Changbao Zhao,‡ Yangsheng Li,†,§ Zhongmiao Gong,‡ Yunxi Yao,∥ Yi Cui,*,‡ Fan Yang,*,† and Xinhe Bao†

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State Key Laboratory of Catalysis, Collaborative Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ‡ Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou 215123, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ Institute of Materials, China Academy of Engineering Physics, Mianyang 621907, China S Supporting Information *

ABSTRACT: Pt−Fe bimetallic alloys are important model catalysts for a number of catalytic reactions. Combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we have studied the structures of Pt−Fe surface alloys prepared on Pt(111) under a variety of conditions. Although the surface and subsurface structures of the Pt−Fe surface alloy could be varied with the deposition amount of Fe atoms and the annealing temperature, a characteristic alloy surface with a bright striped pattern could be identified, which consists of a Pt-dominant surface layer with a small percentage of Fe atoms in the form of isolated atoms or clusters in the surface lattice and a subsurface layer with an ordered Pt3Fe alloy structure. The bright stripes observed in STM were surface dislocations caused by stress relaxation owing to the lattice mismatch between the surface and subsurface layers. This characteristic alloy surface could be prepared on Pt(111) by depositing sub-monolayer Fe at ∼460 K to facilitate Fe diffusion in the nearsurface region, or annealing multilayer Fe at ∼700 K, to enhance bulk diffusion of Fe atoms. The synthesis of this Pt−Fe alloy surface with well-defined structures could allow for further model catalytic studies.

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atomic arrangement in the subsurface layer.15 For the Pt−Ni alloy, the topmost layer of Pt3Ni(111) was found to consist entirely of Pt, whereas the second atomic layer was found to be enriched by Ni.6 Similarly, the structure of a Pt skin layer and a Co-enriched subsurface layer has also been suggested for Pt3Co alloy catalysts.12 Thus, the Pt-dominant surface layer has been generally proposed as the surface structure for the Pt3M (M = Fe, Co, Ni) alloy catalysts. However, the structure of the layer underneath the Pt skin layer does not appear consistent for Pt3M alloy catalysts among the reported studies and remains to be explored. For Pt80Co20(111), the Co concentration in the subsurface layer was reported to be ∼50%, 16 whereas the Co concentration in the subsurface layer of Pt3Co particles was proposed to be ∼75%.12 Likewise, an LEED study on Pt78Ni22(111) suggested that the Ni concentration in the subsurface layer could reach ∼70%.17 However, in situ studies on Pt3Ni(111) under electrochemical conditions showed that the subsurface layer consisted of ∼52% Ni atoms.6 The concentration profile of Co or Ni has been found to be

INTRODUCTION Bimetallic alloys, as a major class of metal catalysts, have attracted a lot of attention for model catalysis research. Particularly, Pt-based alloy catalysts are of tremendous interest for both heterogeneous catalysis and electrocatalysis.1 Alloys between Pt and M (M = Fe, Co, Ni) have been used for catalyzing reactions, such as the preferential oxidation of CO in excess H2,2−4 selective dehydrogenation reaction,5 and the electrocatalytic oxygen reduction reaction (ORR).6 For ORR, Pt3M alloy single crystals6−11 were often used as model catalysts, and previous studies have found that surface segregation of Pt atoms could lead to the formation of a Pt skin layer on the alloy surfaces. A cheap-metal ingredient, M, was proposed to enrich the layer underneath the Pt skin layer12,13 (the subsurface layer), tune the surface electronic structure,14 and subsequently enhance the catalytic activity.12 However, atomic-scale studies on the surface and subsurface structures of the Pt−M surface alloy have been limited, especially for the Pt−Fe surface alloy. Beccat et al.13 found that in the top three layers of Pt80Fe20(111), the concentration profile of Pt decreased monotonically in the near-surface region. A (2 × 2) superstructure was observed on Pt80Fe20(111) by low-energy electron diffraction (LEED)13 and attributed as an ordered © 2019 American Chemical Society

Received: February 19, 2019 Revised: June 5, 2019 Published: June 13, 2019 17225

DOI: 10.1021/acs.jpcc.9b01626 J. Phys. Chem. C 2019, 123, 17225−17231

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The Journal of Physical Chemistry C oscillatory in the top three layers for these Pt3M single crystals, which is in contrast to the monotonic concentration profile of Pt3Fe.16 The subsurface layer of Pt80Fe20(111) was suggested as Pt-rich, with an Fe concentration of ∼12%.13 Thus, the structural difference between Pt3Fe and Pt3Co/Pt3Ni alloys requires detailed investigation. Particularly, since previous studies have used mainly ensemble-average spectroscopic techniques, microscopic studies on the Pt−Fe alloy surfaces are necessary to obtain an insight into their (sub)surface structure. In this study, we have employed STM, XPS, ion-scattering spectroscopy (ISS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to study the Pt−Fe surface alloy prepared by depositing Fe atoms onto Pt(111). The surface concentration of Fe atoms was varied by controlling the Fe deposition time and the annealing temperature. Our studies suggested the presence of a characteristic Pt−Fe alloy surface structure, which could be prepared on Pt(111) and allows for further model catalytic studies.

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EXPERIMENTAL SECTION All experiments were performed in three combined ultrahigh vacuum (UHV) systems. STM and XPS experiments were carried out in a customized SPECS system, equipped with an STM chamber (base pressure < 3 × 10−10 mbar), an XPS chamber with a monochromated Al Kα source (base pressure < 5 × 10−10 mbar), and a preparation chamber with cleaning facilities. ISS experiments were conducted in a customized Omicron system, equipped with low-energy ISS, XPS, and a molecular beam epitaxy chamber. ISS measurements use a 1000 eV He+-ion beam (∼5.0 × 10−8 mbar), and the ion incidence was set at the angle of 50° with respect to the sample surface. These conditions were chosen to ensure no influences of geometric effects and no changes in the spectral intensity during measurements. Measurements of TOF-SIMS were conducted on a TOF-SIMS 5−100 instrument (ION-TOF GmbH), which was connected with a UHV preparation chamber. A sputter-ion source of O2+ with ion energy at 500 eV, a beam current of 10 nA, and a raster size of 300 μm was used for the depth profile. The analyses were carried out with a Bi+ primary gun with a beam energy of 15 kV, a beam current of 0.6 pA, and a raster size of 100 μm. The Pt(111) single crystal (Mateck) was cleaned by cycles of Ar ion sputtering (2 keV, 10 μA) and annealing at ∼1200 K. The Pt−Fe surface alloy was prepared by evaporating Fe atoms onto Pt(111) in UHV at various temperatures. The subsequent annealing at higher temperatures led to the formation of the Pt−Fe surface alloy. The amount of deposited Fe atoms was controlled by varying the deposition flux and time using an ebeam evaporator. The Fe coverage in the unit of monolayer (ML) was estimated from STM measurements and the deposition time. STM images were acquired in the constantcurrent mode, using Pt−Ir tips, with the bias voltage (Vs) applied to the sample. STM images were processed with SPIP (Image Metrology, Denmark).

Figure 1. Growth of Fe layers on Pt(111) at 300 K. (a, b) Large-scale STM images of Fe islands deposited on Pt(111) at 300 K. Blue and green arrows mark the first- and second-layer Fe islands, respectively. During the growth of sub-ML Fe, second-layer Fe islands could be observed both near the step (a) and on the terrace (b). (c, d) Atomicresolution STM image of the first-layer Fe island (c) and its reverse contrast image (d). (e) Large-scale STM images of 5.2 ML Fe deposited on Pt(111) at 300 K.

ML Fe, although the Pt substrate was only partially covered by Fe, overlayer Fe islands could be observed on the first Fe layer (Figure 1b). The surface of the first Fe layer did not display a rigorous hexagonal lattice, with the lattice spacing ranging between ∼2.6 and 3.0 Å (Figure 1c,d), which probably indicated a pseudomorphological growth of the first Fe layer on Pt(111). In contrast, the deposition of multilayer Fe on Pt(111) led to the formation of highly corrugated Fe islands (Figure 1e), instead of planar Fe thin films, as often expected in the growth of metal thin films on metal single crystals.18 Previous studies on the growth of ultrathin Fe films have suggested the formation of Fe(110) thin films adopting the body-centered cubic (BCC) structure, even when Fe was deposited onto face-centered cubic (FCC) substrates, such as Ag(111)19 and Cu(111).20 BCC-Fe was found to be thermodynamically more stable than FCC-Fe at ambient temperatures.21 Typically, Fe atoms on BCC-Fe(110) form an oblique lattice with an angle of ∼71° between the two directions of atoms. Thus, our results suggest that roomtemperature deposition of Fe atoms on Pt(111) could lead to the formation of BCC-Fe(110) layers. Growth of Fe Layers on Pt(111) at Elevated Temperatures. In contrast to the island growth mode at 300 K, Fe atoms deposited at temperatures above 400 K (Figure 2) in UHV form flat surface layers with large terraces on Pt(111). At sub-ML Fe coverage, Fe islands of monolayer thickness and an irregular shape were found to grow preferentially from the step edges of Pt(111) (Figure S1a). The temperature-dependent surface morphologies of the Pt−Fe alloy surfaces are compared in Figure 2. When sub-ML Fe was deposited on Pt(111) at ∼460 K (Figure 2a), bright stripes could be observed on the surface with a pattern similar to the misfit dislocations often

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RESULTS AND DISCUSSION Growth of Fe Layers on Pt(111) at 300 K. Figure 1 displays the structure and morphology of Fe layers deposited by evaporating Fe atoms onto Pt(111) in UHV at 300 K. SubML Fe was found to grow both on the surface terraces and at the steps of Pt(111) (Figure 1a). Upon the deposition of ∼0.7 17226

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Figure 2. STM images and XPS spectra of (a−d) sub-ML Fe and (e−h) ∼3.7 ML Fe deposited on Pt(111) at ∼460 K and annealed at various temperatures. (a−c) STM images of (a) sub-ML Fe deposited on Pt(111) at ∼460 K, (b) after the annealing at ∼600 K in UHV for 5 min, and (c) after the annealing at ∼700 K in UHV for 5 min. (d) The corresponding Fe 2p3/2 spectra of sub-ML Fe/Pt(111) upon deposition and after UHV annealing. (e−g) STM images of (e) 3.7 ML Fe deposited on Pt(111) at ∼460 K, (f) after the annealing at ∼600 K in UHV for 5 min, and (g) after the annealing at ∼700 K in UHV for 5 min. (h) Corresponding Fe 2p3/2 spectra of 3.7 ML Fe/Pt(111) upon deposition and after UHV annealing.

Figure 3. Growth of sub-ML Fe on Pt(111) at ∼460 K. (a) Large-scale STM image of an Fe layer deposited on Pt(111) at ∼460 K (θFe ∼ 0.9 ML), showing bright stripes on the surface. The squared area is magnified in (b) and shows dark holes dispersed randomly across the surface. The structural model for the formation of surface stripes is illustrated in (c). Yellow circles mark the bridge-site Pt atoms, which form bright stripes in STM. (d) ISS spectra of 0.25 ML Fe deposited on Pt(111) at 300 K, which was followed by the annealing at 450 and 500 K sequentially. (e) Atomresolved STM image of the sub-ML Fe/Pt(111) surface, showing a hexagonal lattice with the inset showing its fast Fourier transform (FFT) image. The square and circle in the inset mark the surface and subsurface lattice, respectively. A (2 × 2) superstructure could be observed and is indicated by the rhombus. The surface Fe atom (orange circle) appears as a dark hole and the subsurface Fe atom (white open circle) appears as a dark depression between bright atomic dots. (f) TOF-SIMS spectra of ∼1 ML Fe deposited on Pt(111) at ∼460 K (red: high signal ratio of Fe to Pt; blue: low signal ratio of Fe to Pt). Scanning parameters: (e) Vs = 0.013 V, It = 0.35 nA.

observed in the growth of metal thin films on Pt(111)22,23 or the surface herringbones on Au(111).24 The separation distance between these stripes became larger at the surface after the annealing at ∼600 K (Figure 2b). When the Pt−Fe

surface alloy was annealed further at ∼700 K, the bright stripes were found to disappear from the surface, whereas vacancy islands of tens of nanometers in width and a monolayer depth could be observed, occupying ∼20% of the surface area (Figure 17227

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Figure 4. Growth of sub-ML Fe on Pt(111) at ∼440−460 K. (a) Large-scale STM image of the Fe layer deposited on Pt(111) at ∼440 K. The squared area is magnified in (b) and shows a hexagonal lattice with an inhomogeneous apparent height at the atomic scale. (c) Large-scale STM image of the surface of (a) after the annealing at 460 K in UHV for 10 min. Bright stripes could be observed on the surface terrace. The squared area is magnified in (d), showing that the annealed surface was still not homogeneous atomically. The green and blue squares, marking the planar surface terrace and the bright striped region, are magnified in (e) and (f) to show their atomic lattices, respectively. In (f), the stripe is marked by white arrows. Scanning parameters: (b) Vs = 0.074 V, It = 0.27 nA; (e) Vs = 0.016 V, It = 0.71 nA; (f) Vs = 0.010V, It = 0.88 nA.

which is much smaller than the step height of Pt(111) (2.27 Å). Also, atom-size vacancies are likely not stable on metal surfaces at elevated temperatures. These dark holes could thus be assigned as Fe atoms or clusters in the topmost layer. Due to the electron transfer from Fe to Pt,25,26 Fe atoms appear dark in the STM image, whereas Pt atoms were resolved with higher apparent heights. The formation of a Pt-rich surface layer could be expected, since Pt has a lower surface energy than Fe27−29 and Pt atoms prefer to segregate to the surface thermodynamically. A previous infrared reflection adsorption spectroscopy (IRRAS) study30 has also reported the dominance of surface Pt sites and the presence of CO adsorption on both surface Pt and Fe when 1 ML Fe atoms were deposited at 473 K onto Pt(111). The main CO peak in IRRAS was assigned to CO adsorption on the top sites of Pt and demonstrated a Pt-dominant surface layer. From STM, the bright stripes appeared ∼0.2 Å above the neighboring Pt surface plane (Figure S1), which suggests the formation of surface dislocations caused by the misfit between the Pt−Fe surface alloy and the Pt bulk or between the Ptdominant surface layer and the alloy layers underneath23 (Figure 3c). Nevertheless, the Pt-dominant surface layer could be confirmed by ISS spectra, which showed a small percentage of Fe concentration at an annealing temperature between 450 and 500 K (Figure 3d). Relaxation-induced surface dislocations have been typical for the Au(111) surface, where Au atoms in the compact surface layer are located sequentially in the FCC and hexagonal closed-packed (HCP) sites of the substrate. In the transition region, Au atoms are located at the bridging sites, forming striped patterns between the FCC and HCP domains of Au(111) in STM.24 For the Cu layer on Au(111), surface dislocations were also observed, which was caused by the diffusion of Cu atoms into the subsurface.31 Since Cu(111) has a smaller atomic spacing than Au(111), surface Au atoms would form a compact layer and occupy different sites on subsurface Cu atoms, rendering the formation of surface dislocations. Similarly, due to the penetration of Fe atoms into

2c). Accordingly, XPS measurements showed a sharp decrease of the Fe 2p peak at ∼700 K (Figure 2d), suggesting a significant bulk diffusion of Fe atoms at this temperature. At 600 K or below, the intensity of the Fe 2p peak did not show an obvious change, whereas the increased separation of surface stripes indicates that the diffusion of Fe atoms remained within the near-surface region. When ∼3.7 ML Fe were deposited onto Pt(111) at ∼460 K, a planar thin film surface was observed (Figure 2e), similar to the growth of sub-ML Fe at this temperature. Since multilayer Fe deposited at 300 K appeared as corrugated islands with an elongated shape on Pt(111) (Figure 1e), the surface of the multilayer Fe deposited at 460 K is expected to be a Pt−Fe surface alloy layer. When the 3.7 ML Fe/Pt(111) surface was annealed at ∼600 K, the surface morphology did not show an obvious change in STM (Figure 2f). However, when the surface was annealed further at ∼700 K, a high density of bright stripes emerged at the surface (Figure 2g). Accordingly, XPS measurements showed that the intensity of the Fe 2p peak dropped by nearly half when the alloy surface was annealed at ∼700 K (Figure 2h), owing to the facilitated bulk diffusion of Fe atoms. Consequently, the surface and subsurface structures of the Pt−Fe alloy on Pt(111) are dependent on the amount of deposited Fe atoms as well as the growth or annealing temperature. A typical Pt−Fe alloy surface with the formation of surface stripes appeared common among the different growth conditions we studied and will be discussed below with respect to its surface and subsurface structures. Structures of a Pt−Fe Surface Alloy on Pt(111). The structure of the Pt−Fe alloy prepared by depositing sub-ML Fe on Pt(111) at ∼460 K, which is the characteristic of bright striped patterns (Figure 3a), was analyzed further with highresolution STM images. Figure 3b shows that dark holes could be observed near the bright stripes on the as-deposited surface. These dark holes could be attributed to vacancies (clusters) or atoms of different chemical contrast.8,25 From STM, the depth of the dark holes was found to be only ∼0.5 Å (Figure S1), 17228

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Figure 5. Structure of ∼3.7 ML Fe deposited on Pt(111) at ∼460 K and after the UHV annealing at ∼700 K for 5 min. (a) Large-scale STM image. (b) High-resolution STM image showing a hexagonal lattice, with the lattice spacing at ∼5.5 Å. The same area was resolved atomically with another hexagonal lattice (due to a tip change), with the lattice spacing at ∼2.8 Å in (c). Inset of (c) shows its FFT image with square and circle marking the surface and subsurface lattices, respectively. The same areas in (b) and (c) are marked by red circles and white arrows. (d) Atom-resolved STM image with the grid lines marking the positions of surface Pt atoms. Orange spheres mark dark spots in the Pt lattice, which could be attributed to surface Fe atoms. (e) Atom-resolved STM image with the grid lines marking the Pt surface lattice and open circles marking the depressions at the hollow sites of the Pt lattice. These depressions could be attributed to subsurface Fe atoms, which form the Pt3Fe subsurface alloy and the (2 × 2) superstructure. (f) The structural model of the Pt-skin surface layer and the Pt3Fe subsurface alloy (blue and green, Pt; brown, Fe). Scanning parameters: (b) Vs = 1.3 V, It = 0.24 nA; (c) Vs = 0.015 V, It = 0.34 nA; (d) Vs = 0.015 V, It = 0.38 nA; (e) Vs = 0.015 V, It = 0.38 nA.

of surface dislocations, could consist of a Pt-dominant surface layer (with the atomic concentration of Fe atoms at a small percentage) and an ordered Pt3Fe layer at the subsurface. Note that, prior to the formation of surface dislocations, the formation of the Pt−Fe surface alloy could be readily observed at temperatures above 400 K. For instance, when sub-ML Fe was deposited onto Pt(111) at ∼440 K (Figure 4a), a high concentration of bright atoms was observed at the surface, which could be attributed to the intermixing of Pt and Fe atoms at this temperature (Figure 4b). The assignment of atoms with a lower apparent height in Figure 4b has been difficult in this study since their apparent heights appeared varying. These atoms could be assigned as surface Pt atoms sitting on the subsurface Fe atoms or as surface Fe atoms depending on their apparent heights. A similar height contrast on alloy surfaces has been observed previously31,32 and attributed to the structural differences in the subsurface. Nevertheless, their atomic spacing (∼2.8 Å) suggested that all atoms in the surface lattice grew according to the lattice of Pt(111). Meanwhile, the (2 × 2) superstructure has also started to emerge, indicating the formation of the Pt3Fe structure at the subsurface. Therefore, the Pt3Fe structure is ready to form at the subsurface when the interdiffusion of Fe atoms has been initiated. Misfit dislocations were formed subsequently when the Pt− Fe alloy surface was annealed further at ∼460 K (Figure 4c). Surface segregation of Pt atoms and interdiffusion of Fe atoms led to the formation of surface dislocations, caused by the structural relaxation of the Pt-dominant surface layer23 (Figure 4c,d). The surface plane of the Pt−Fe alloy still appeared inhomogeneous at the atomic level, indicating a lattice mismatch between the Pt-dominant surface layer and the Pt−Fe alloy subsurface layer and the lack of long-range order at the subsurface (Figure 4e). Consequently, surface Pt atoms

the subsurface, surface Pt atoms are often located above the subsurface Fe atoms. Because Fe atoms are smaller than Pt atoms, the surface Pt layer becomes strained and the structural relaxation causes the formation of surface dislocations. From the atomic-resolution STM image (Figure 3e), a hexagonal atomic lattice with the lattice distance of ∼2.8 Å could be resolved, in accordance with the lattice spacing of Pt(111). In addition, a (2 × 2) superstructure could also be observed in Figure 3e with dark depressions located at each corner of the (2 × 2) unit cell, which could be evident from the corresponding FFT image (inset). Since these dark depressions did not appear at the lattice sites of the surface layer, the (2 × 2) superstructure could be derived as a subsurface structure. A previous study on Pt80Fe20(111) has also shown a distinct (2 × 2) LEED pattern and suggested the formation of an ordered Pt3Fe subsurface structure.13 If these dark depressions were assigned as subsurface Fe atoms and other atoms in the subsurface layer were assumed to be Pt atoms, a Pt3Fe subsurface phase could be reached, which explains the image contrast and (2 × 2) superstructure in Figure 3d. The phase diagram for the Pt−Fe alloy showed that the Pt3Fe phase is stable at temperatures below 1600 K when the Pt concentration is close to 75% in the alloy.15 Our study showed that during the interdiffusion of Fe atoms in Pt(111), the formation of an ordered Pt3Fe phase at the subsurface is also a favorable process. Correspondingly, TOF-SIMS measurement of the Pt−Fe alloy surface (Figure 3f), prepared by depositing ∼1 ML Fe onto Pt(111) at ∼460 K, showed that the concentration profile of Fe increased first and then decreased with the depth of Pt(111), indicating the enrichment of Fe atoms in the subsurface region. The extension of the Fe concentration a few layers into the bulk suggested the formation of the Pt3Fe phase, rather than PtFe3, in the subsurface layer. Thus, the Pt−Fe alloy surface, characteristic 17229

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The Journal of Physical Chemistry C could locate at different sites of the subsurface layer, such that varying apparent heights were observed. The stripe dislocations also appeared corrugated at the atomic scale (Figure 4f). The formation of misfit dislocations on the surface of 3.7 ML Fe deposited at ∼460 K on Pt(111) requires further annealing at ∼700 K in UHV (Figures 2g and 5a). The atomic structure of this annealed Pt−Fe alloy surface is displayed in Figure 5, which is similar to the surface of sub-ML Fe deposited on Pt(111) at ∼460 K. Depending on the tip state, a hexagonal lattice with the lattice spacing at ∼5.5 Å (Figure 5b) could be resolved. An atom-resolved STM image of exactly the same area further showed a hexagonal lattice with the lattice spacing at ∼2.8 Å (Figure 5c), the same as the structure observed in Figure 3e. The inset of Figure 5c showed clearly a (2 × 2) superstructure in addition to the hexagonal atomic lattice. No missing atoms were observed in the surface lattice of 3.7 ML Fe/Pt(111), although Fe atoms exhibited a lower apparent height than Pt atoms in the hexagonal lattice. Note that the agglomeration of three or four iron atoms in the surface lattice of this Pt-rich surface could also be observed (Figure 5d), whereas the majority of the surface Fe atoms appeared isolated. This (2 × 2) superlattice was caused by dark depressions appearing periodically at the threefold hollow sites of the surface lattice (Figure 5e). Assuming that each dark depression is a subsurface Fe atom, the subsurface structure could be suggested as a Pt3Fe layer (Figure 5f). Compared with the Pt−Fe alloy surface prepared by depositing ∼1 ML Fe on Pt(111) at ∼460 K, TOF-SIMS measurement on the 3.7 ML Fe/Pt(111) surface (Figure S2) showed a further extended concentration profile of Fe into the bulk of Pt(111), whereas the Fe concentration at the subsurface region appeared similar. From STM, both Pt−Fe alloy surfaces exhibit the same surface and subsurface structures, that is, a Pt-dominant surface layer and an ordered Pt3Fe subsurface layer. Although alloy single crystals, such as Pt3M(111) (M = Fe, Co, Ni),6,12 have been typically used as model catalysts to understand the catalysis by Pt-based alloys, our study showed that the growth of alloy layers on Pt(111) could also provide a well-defined model system, with surface and subsurface structures similar to those of the alloy single crystal. The Ptdominant surface structure and the (2 × 2) subsurface structure proposed for Pt80Fe20(111)13,15 could also be observed in the Pt−Fe alloy surfaces prepared in our study. Although the surface and subsurface structures of Fe layers deposited onto Pt(111) are dependent on the deposition amount and the annealing temperature, we demonstrated the presence of a characteristic Pt−Fe alloy surface with bright striped patterns. This alloy surface consists of a Pt-dominant surface layer and an ordered Pt3Fe subsurface layer. For subML Fe deposition, such structures could be obtained, via facilitating interdiffusion of Fe atoms at the near-surface region, by depositing Fe on Pt(111) at ∼460 K. For multilayer Fe deposition, the annealing temperature has to reach ∼700 K to enable bulk diffusion of Fe atoms, and as such, to obtain the above surface structures.

corrugated Fe film surface, with elongated islands of multilayer thickness. The growth of Fe layers at temperatures above 400 K led to the formation of planar alloy surfaces. Since Pt has a lower surface energy than Fe, the atomic exchange process at the near-surface region starts at elevated temperatures, where Fe atoms tend to diffuse into the subsurface whereas Pt atoms underneath tend to segregate onto the surface. At 700 K or above, bulk diffusion of Fe atoms is facilitated. For sub-ML Fe deposition, the annealing at ∼700 K led to the formation of vacancy islands on Pt(111) due to bulk diffusion of Fe atoms. Although the surface and subsurface structures could be varied with the deposition amount of Fe atoms and the annealing temperature, a characteristic alloy surface with bright striped patterns was identified, which consists of the Pt-dominant surface layer and an ordered Pt3Fe layer at the subsurface. The Pt-dominant surface layer exhibited a hexagonal lattice with the atomic spacing at ∼2.8 Å and contained a small percentage of Fe atoms, which appear isolated or form clusters in the surface lattice. The Pt3Fe subsurface layer was identified in STM by the presence of a (2 × 2) depression structure. This characteristic alloy surface could be prepared through the deposition of sub-ML Fe at ∼460 K or the deposition of multilayer Fe at ∼460 K, followed by the UHV annealing at ∼700 K. Our study has thus provided insight into the surface and subsurface structures of the Pt−Fe surface alloy and a method for preparing model systems for further catalytic studies.

SUMMARY Overall, the surface and subsurface structures of Fe layers deposited onto Pt(111) have been explored. The Pt−Fe alloy surface was prepared by varying the deposition amount of Fe atoms and the growth and annealing temperatures. At 300 K, the deposition of Fe layers initially formed two-dimensional islands on Pt(111) and continuous deposition led to a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b01626.

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STM images of sub-ML Fe deposited on Pt(111) at ∼460 K; TOF-SIMS spectra of ∼3.7 ML Fe deposited on Pt(111) at ∼460 K and after the UHV annealing at ∼700 K (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected] (F.Y.). ORCID

Yi Cui: 0000-0002-9182-9038 Fan Yang: 0000-0002-1406-9717 Xinhe Bao: 0000-0001-9404-6429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Ministry of Science and Technology of China (2017YFB0602205, 2016YFA0202803, 2017YFA0303104), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200), and National Science Foundation of China (91545204).

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REFERENCES

(1) Yu, W.; Porosoff, M. D.; Chen, J. G. Review of Pt-Based Bimetallic Catalysis: From Model Surfaces to Supported Catalysts. Chem. Rev. 2012, 112, 5780−817. (2) Kim, J.; Park, W. H.; Doh, W. H.; Lee, S. W.; Noh, M. C.; Gallet, J. J.; Bournel, F.; Kondoh, H.; Mase, K.; Jung, Y.; et al. Adsorbate-

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DOI: 10.1021/acs.jpcc.9b01626 J. Phys. Chem. C 2019, 123, 17225−17231

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DOI: 10.1021/acs.jpcc.9b01626 J. Phys. Chem. C 2019, 123, 17225−17231