Ultrahigh Vacuum Synthesis of Strain-Controlled Model Pt(111)-Shell

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Letter

Ultra-High Vacuum Synthesis of Strain-Controlled Model Pt(111)Shell Layers: Surface Strain and Oxygen Reduction Reaction Activity Soma Kaneko, Rikiya Myochi, Shuntaro Takahashi, Naoto Todoroki, Toshimasa Wadayama, and Tadao Tanabe J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02525 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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The Journal of Physical Chemistry Letters

Ultra-High Vacuum Synthesis of Strain-Controlled Model Pt(111)-Shell Layers: Surface Strain and Oxygen Reduction Reaction Activity Soma Kaneko1, Rikiya Myochi1, Shuntaro Takahashi1, Naoto Todoroki1*, Toshimasa Wadayama1, Tadao Tanabe2 1

Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan 2

Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan

*Corresponding_author: [email protected]

ABSTRACT

In this study, we perform ultra-high vacuum (UHV) and arc-plasma synthesis of strain-controlled Pt(111) model shells on Pt–Co(111) layers with various atomic ratios of Pt/Co and an oxygen reduction reaction (ORR) activity enhancement trend against the surface strain induced by lattice mismatch between the Pt shell and Pt–Co alloy-core interface structures was observed. The results showed that the Pt(111)-shell with 2.0% compressive surface strain vs. intrinsic Pt(111)

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lattice gave rise to a maximum activity enhancement, ca. 13-fold higher activity than that of clean Pt(111). This study clearly demonstrates that the UHV-synthesized, strain-controlled Pt shells

furnish

useful

surface

templates

for

electro

catalysis.

TOC GRAPHICS

Well-defined bimetallic surfaces have been prepared for designing new materials with unique surface properties.1,2 Constructing surface nanostructures with specific atomic arrangements and


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alloy compositions is crucial not only for investigating the physical, chemical, and electronic properties of bimetallic surfaces but also for innovating industrial processes, particularly in the fields of semiconductors,3–5 magnetic random access memories,6,7 and heterogeneous catalysis.8– 10

Pt–M (M = 3d transition metals) bimetallic surface systems are particularly important. For

example, precise control of Pt/Fe hetero-structures is indispensable for discussing perpendicular magnetic anisotropy.11,12 From a catalytic perspective, numerous studies have been performed on Pt-based alloys to synthesize highly active catalysts with a low noble–metal content oxygen reduction reaction (ORR) catalysts for cathodes of proton exchange membrane fuel cells (PEMFCs) to enhance the slow reaction kinetics.13–16 Therefore, Pt-M alloy nanoparticles (NPs) have been intensively studied, primarily for their superior activity toward ORR.17 The activity enhancements of Pt–M alloy catalysts can be explained by the lattice strain (geometric) and ligand (electronic) effects18 of the surface Pt atoms of the Pt–M NPs (Pt shell), which are induced by the underlying Pt-based alloy core. Therefore, surface atomic arrangements and electronic properties for Pt atoms located in the vicinity of the surface determine the ORR mechanism and are crucial to develop novel electrode catalysts. In order to investigate the ORR mechanisms, the nano-architecture of Pt shell/Pt–M core catalysts can be modeled as surface templates. The arc-plasma deposition (APD) method is a physical vapor deposition technique applicable to synthesize mono-dispersed metal NPs without introducing organic impurities.19 Because the arc–voltage and pulse repetitions of the APD enable us to precisely control the amount of deposition materials, the APD should also be applicable to synthesize hetero-layered structures of Pt and M with ultra-clean surfaces. In the aforementioned context, we fabricate Pt/Co hetero-layered nanostructures on a clean Pt(111) substrate as a model of the Pt shell/Pt–M-core NPs using a cathode catalyst through alternate


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deposition of Pt and Co in UHV (~10−8 Pa) using the APD method. The hetro-layered nanostructures furnish us a useful model catalyst to clarify the structural factors on ORR properties of the core-shell NPs. The surface strain of the Pt shell induced by lattice mismatch between the Pt shell and the underlying Pt–Co alloy layer can be tuned experimentally by modifying the underlying Co and/or Pt–Co alloy thickness on single crystal substrates, while previous results for polycrystalline thin films20 and nanoparticles catalysts21 have been unclear on the correlation between the lattice strain and the activity, due to the structural inhomogeneity, such as surface atomic arrangements and particle sizes. To our best knowledge, this experimental study is the first UHV synthesis and experimental strain evaluation by in-plane XRD of a surface strain-controlled and well-defined Pt(111) model shell layer. Furthermore, ORR activity evaluation shows that a Pt(111) shell with ca. 2.0% compressive surface strain exhibits provides the maximum ORR activity enhancement, revealing that the strain-controlled Pt(111)-shell is a useful surface template for electro catalysis. The Co and Pt deposition sequences for synthesizing the Pt/Co/Pt(111) model catalysts are schematically shown in Figure 1. The total thicknesses of the catalyst layers, the first layer Co, the fourth layer Pt (model shell), and the sum of the thicknesses of the second layer Pt and the third layer Co were fixed to 6.0 nm, 0.4 nm, 1.6 nm, and 4.0 nm, respectively. To control the Pt-shell layer surface strain, the ratio of the second Pt layer and the third Co layer thicknesses were used: the corresponding model catalysts are denoted as the third Co layer thickness; Coxnm (x = 0.4, 0.8, 1.6, and 3.2)


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Figure 1 Schematic deposition sequences of Co and Pt on Pt(111) The XRD patterns for the model catalysts were obtained on a Smart Lab (RIGAKU) diffractometer using Cu Kα radiation (λ = 0.1542 nm) in the in-plane layout. The detailed layout of the catalysts substrate and the detector were described in the literature.22 The in-plane XRD results for the UHV-prepared Pt/Co/Pt(111) are presented in Figure 2 for 2θχ angles from 60° to 80°.


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Figure 2 In-plane XRD patterns of the UHV-synthesize Pt/Co/Pt(111) model catalysts. The angle of incidence is 0.1°. A schematic of the measurements is depicted at the top of the figure. Insets: (top) enlarged diffraction of the Co3.2nm (brown) between 75° and 77° of 2θχ; (bottom) the deconvoluted diffraction peak for Pt(220) of Co0.4nm between 66° and 70° of 2θχ. In the measurements, the angle of incidence is 0.1° and thus, the X-ray penetration depths for Pt and Co were ca. 2 and 3 nm, respectively.23 New diffraction peaks emerged at high diffraction angles of the substrate (220) peak (67.52°) at 67.89° for Co0.4nm, 68.83° for Co0.8nm, 69.43° for Co1.6nm, and 70.47° for Co3.2nm. In-plane XRD provides the distance between the lattice planes oriented normally to the substrate Pt(111). In this diffraction region, the in-plane lattice distance in the 220 direction of the Pt(111) lattice can be discussed. The peak shifts


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relative to the substrate Pt(220) diffraction should reflect the in-plane Pt(111) lattice compression within ca. 2 nm from each topmost surfaces. Because the Pt and Co atoms differ in size by ca. 9%, the underlying Co atoms induce compressive strain on the surface Pt(111) lattice. In this study, the in-plane surface strain (εPt(111)-shell (%)) of the Pt(111)-shell layer is defined based on the difference in lattice parameter d of the direction of the substrate Pt(111) and the surface Pt(111)-shell:

% =

× 100 1

where dPt(111)-shell and dPt(111)substrate are estimated using the diffraction angles on the in-plane XRD patterns and the Bragg condition 2dsinθ = nλ. The estimated εPt(111)-shell (%) of the model Pt(111) shells are summarized in Table 1. Table 1. Diffraction angles due to the surface Pt(220) and εPt(111)-shell (%) estimated from (1).

Pt : Co

Pt(220) diffraction angle (2θ / degree)

Surface strain (%)

Co0.4nm

67.89±0.01

−0.45±0.02

Co0.8nm

68.83±0.01

−1.66±0.02

Co1.6nm

69.43±0.01

−2.48±0.02

Co3.2nm

70.47±0.01

−3.65±0.02

With increasing 3rd-layer Co thickness, the estimated strains increase, providing evidence for the strained Pt(111)-shell lattice induced by the underlying Co atoms. A slight diffraction


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peak due to Co(220) (around 2θχ = 75.7°) is only detected for the Co3.2nm model catalyst in this in-plane XRD, suggesting that most of the deposited Co is present not as a Co epitaxial layer but as Pt–Co alloys in this study. The in-plane XRD results clearly indicate that the surface strain of the Pt(111)-shells can be controlled by the thicknesses ratio of the second Pt layer and the third Co layer. The cross-sectional HAADF-STEM images and the corresponding energy-dispersive Xray spectrometry (EDS) line profiles of Pt and Co for the Co3.2nm and Co1.6nm model catalysts are presented in Figure 3. The STEM images were collected from the direction ((110) crosssection). As shown in the image of Co1.6nm (left), the Z contrast of Pt (bright) and Co (dark) near the surface in the dark-field STEM images is obscure, indicating that the interface between the surface Pt shell and the underlying Co (Pt–Co alloy) layers is not abrupt. As for the image of Co3.2nm, the Z contrast is relatively clear although the interface is incoherent. The results suggest that the atomic near-surface structures of Co1.6nm and Co3.2nm are not composed of altered abrupt layers of Pt and Co but of Pt–Co surface alloy. The intermixing of Co and Pt should occur through the thermal annealing process (at the substrate Pt(111) temperature of 573 K) aimed at flattening the Pt and Co layer surfaces, resulting in obscure (incoherent) interfaces of the topmost Pt and underlying Co (Pt–Co) layers. The EDS line profiles of Pt and Co for both model catalysts show only the signal due to Pt increasing at the topmost surfaces, whereas the Co signal is recorded from ca. 0.5 nm to 5.0 nm from the topmost surface, indicating that a ca. 0.5-nm-thickPt(111)-shell with a ca. 4 nm thickness is generated on the Pt–Co(111) surface alloy of both model catalysts. Furthermore, the Co3.2nm intensity is ca. twice that of Co1.6nm, suggesting that the Pt/Co atomic ratios of the underlying Pt–Co layers can be correlated with 3rd-layer Co thickness. In any case, the STEM images and the corresponding EDS profiles of the model catalysts clearly


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show that the ca. 0.5-nm-thick-Pt(111) shells can be synthesized on ca. 4-nm-thick-Pt–Co(111) alloy layers with various atomic ratios of Pt and Co by the APD sequences in this study (Figure 1).

Figure 3 Cross-sectional STEM images and corresponding EDS line profiles of the Co3.2nm (left) and Co1.6nm (right) model catalysts Average atomic distances in the direction in the surface vicinity and in the Pt(111) substrate regions can be estimated by close inspection of the STEM images; a detailed estimation method is described in Supporting Information (SI) (Figure S2). The calculated respective estimated surface strain values for Co1.6nm and Co3.2nm are −2.3% and −3.1%, respectively; these values agree with the values estimated from the in-plane XRD patterns (Figure 2). Figure 4 presents the Cyclic voltammetry (CV) curves of a series of the Pt/Co/Pt(111) model catalysts compared with clean Pt(111), which are shown at the bottom. The CV features of the model catalysts for hydrogen adsorption and desorption regions (0.05−0.4 V) clearly show a decrease in the adsorption charges (QHads) and lower shift of the onset potentials of the hydrogen adsorption reaction. Moreover, with decreasing 3rd-layer Co thicknesses, QHads tends to decrease.


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Furthermore, in the oxidation and reduction region of Pt (above 0.6 V), a positive shift of the onset potentials is obvious. The reduction in QHads and the onset potential positive shift are typical features for highly active well-defined Pt-based bimetallic surfaces reported to date.24–29 The CV results shown in Figure 4 indicate that 3rd-layer Co thickness strongly modifies the EC properties of the Pt(111)-shell surface. Indeed, the corresponding linear sweep voltammetry (LSV) curves for the model catalysts show positive shifts in half-wave potential, indicating marked ORR activity enhancement vs. clean Pt(111). (SI; Figure S3)

0.01

i / mA

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0.00 Co0.4nm Co0.8nm Co1.6nm Co3.2nm Pt(111)

-0.01

-0.02 0.0

0.2

0.4

0.6

0.8

1.0

E / V (vs. RHE)

Figure 4 CV curves for the Pt/Co/Pt(111) model catalysts recorded in N2-purged 0.1 M HClO4

The enhanced ORR activity in Pt-based alloy catalysts can be explained by ligand and/or strain effects.30 The former ligand effect can be effective for modifying the electronic properties of the topmost surface Pt shell when the surface Pt shell thickness is just one atomic layer of Pt.31 Moreover, compressive strain on the Pt-rich shell surface is well-known to increase with decreasing amount of oxygen-related species adsorbed onto the surface of the Pt atoms, resulting in ORR activity enhancement.32,33 These ORR enhancement mechanisms may be distinguished


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by the location of the alloying elements. In the former ligand effect, the Co atoms located just next to the topmost surface Pt atoms can contribute to the activity enhancement. In contrast, in the latter strain-induced effect, even the Co atoms located away from the topmost Pt shell surface (few monolayers below the topmost Pt(111) layer) may enhance the activity. Temmel et al. estimated the Pt strain of the Pt epitaxial films on the SrTiO3(111) substrate and discussed the strain-induced activity enhancements.34 They showed that rather low in-plane compressive strain of 0.3% for the 12-nm-thick Pt film increased eight-fold the activity as compared to the nonstrained Pt surface although their atomic force microscopy and secondary electron microscopy images of the samples showed island-like film structures. In contrast, the present results demonstrate a direct influence on the topmost Pt(111) shells of the underlying Pt–Co(111) layers that is generated through the intermixing of the second layer Pt and third layer Co. The in-plane XRD results (Figure 2) provide strong evidence that increasing 3rd-layer Co thickness induces more compressive strain on the Pt(111) shells. Therefore, correlation between the induced compressive strains of the Pt(111) shell and the activity enhancements can be discussed quantitatively in the series of the UHV-synthesized Pt/Co/Pt(111) model catalysts. The surface strain in the shell should modify adsorption behavior for OH and/or O species on the topmost surface responsible to the ORR34. Actually, OH adsorption properties in CV curves (Fig.4) dramatically change depending on the 3nd layer Co thickness, i.e. induced compressive strain of the shell. Electrochemical properties of the topmost surface (1st atomic) Pt layer of the each multilayer samples should be influenced by the different lattice strains of Pt(111) shells. At any rate, CV changes shown in Figure 4 clearly demonstrate that electrochemical properties of the topmost surfaces of the shells are dominated by the estimated surface strain (Table 1) that induced by the lattice mismatches of the shells and underlying alloy layers.


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The ORR activities of the model catalysts were estimated using Koutecky–Levich equation.35 The estimated activity enhancement factors vs. clean Pt(111) are 4-fold for Co0.4nm, 11-fold for Co0.8nm, 13-fold for Co1.6nm, and 11-fold for Co3.2nm, indicating strong correlation between the activity enhancements and the 3rd-layer Co thickness, i.e., the in-plane surface strain of the Pt(111)-shells. Figure 5 presents the relation between the logarithmic jk values at 0.9 V and inplane surface strain against pristine Pt(111) estimated by the in-plane XRD (Figure 2). Strasser et al.21 predicted through theoretical calculation that the surface strain of −2.0 % can be the most beneficial to enhance the ORR activity although the volcano-like trends against the strain are obscure for practical Pt-Cu NPs. According to the modified DFT calculations by Stephens et al.,36 the −2.0%-strained Pt layer for the Pt-based bimetallic systems could present highest ORR activity enhancement. Indeed, our experimental study for the various-thick Pt monolayers on Pt25Ni75(111) substrate28 shows that the enhancement factors of the Pt(111)-shell layers having compressive strain less than −1.7% correspond well to the aforementioned theoretical predictions. For Co0.4nm and Co0.8nm, the direct electronic influence of the Co atoms to the surface Pt atoms should be less effective because of the low Co/Pt atomic ratios of the underlying Pt–Co surface alloys (Table 1). Thus, the ORR activity enhancements should be dominated by the strain effect. As a result, the estimated strain less than ca. −2% (right hand side of Figure 5) reveals a steep increase in the jk value with increasing surface strain. In contrast, the activity enhancement of the Co3.2nm model catalyst clearly deviates from the theoretical predictions,21,36 in which the ORR activity enhancements markedly decrease with a strain above −2 %. In this study, the deposition thickness of the fourth layer Pt is fixed at 1.6 nm. However, the effective Pt(111)-shell layer thickness might be less than the deposition thickness (1.6 nm) because inter-diffusion of the third layer Co and the fourth layer Pt is unavoidable during the UHV synthesis process. Therefore, the


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ligand effect induced by the Co atoms located just below the topmost Pt(111) lattice would also be operative, particularly for the activity enhancement of Co3.2nm. Actually, as shown in crosssectional STEM images and EDS profiles (Figure 3), the estimated Pt(111)-shell thickness for Co3.2nm is ca. 0.5 nm. Considering the atomic size of the Pt atoms (ca. 0.273 nm), the effective shell thickness is probably less than two Pt monolayers. As a result, the volcano slope shown in Figure 5, especially the strain above −2 %, becomes gentle to the region less than −2 %. Similar phenomenon was observed on recent published results for strain-controlled Pt/Pt25Ni75(111) systems.28 Therefore, we conclude that homogeneous, in-plane surface strain of ca. −2 % of the Pt(111)-shell vs. the intrinsic Pt(111) lattice gives the maximum ORR activity enhancement.

100

jk / mAcm -2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 Pt(111) Co0.4nm

1

Co0.8nm Co1.6nm Co3.2nm

0.1 -4

-3

-2

-1

0

Surface Strain (%) vs. Pt(111) Figure 5 Logarithmic jk at 0.9 V vs estimated surface strain of the model catalysts. In summary, we can successfully synthesize the strain-controlled model Pt(111) shell on the Pt– Co(111) surface alloy layers using alternative APDs of Co and Pt on clean Pt(111) substrate in UHV. In-plane XRD and cross-sectional STEM images with EDS line profiles of the model catalysts gave direct experimental evidence for UHV synthesis of the strain-controlled, welldefined Pt(111)-shell surface. The ORR activity enhancement trend of the synthesized model


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catalysts against the in-plane compressive surface strain of the Pt(111) shell was correlated with the underlying Pt–Co(111) layers. The ORR activity enhancement trend clearly demonstrated that the Pt(111) shell with the in-plane surface strain of ca. −2% vs. the intrinsic Pt(111) lattice would provide the maximum ORR activity enhancement for the practical Pt shell with (111) surface atomic arrangement. The aforementioned results demonstrate that the UHV-synthesized, strain-controlled Pt shell in this study furnishes useful surface templates for electrocatalysis.

Experimental Methods The experimental equipment used in this study has been described elsewhere.37,38 The Pt(111) (< 0.1° miscut) crystal surface was cleaned by repeated Ar+ sputtering and annealing at 1000 K under UHV conditions. Co and Pt layers were alternately deposited onto the clean Pt(111) substrate by two APD sources (ULVAC-RIKO ARL-300). The substrate temperature during the APDs was maintained at 573 K to flatten the model Pt(111)-shell surfaces. The APD parameters and detailed deposition sequences of the Co and Pt are described in Table S1 in the SI. Structural analysis for the APD-synthesized model catalysts is performed by in-plane XRD (Rigaku, Smart Lab), cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and EDS (JEOL, JEM-ARM200F). The UHV-synthesized Pt/Co/Pt(111) model catalysts were transferred using the transfer vessel without air exposure to the N2-purged glove-box.24,26 EC measurements were conducted in a conventional electrochemical cell that included a platinum counter electrode and a reversible hydrogen electrode (RHE) with flowing H2 gas in the N2-purged glove-box; all potentials described have been presented with respect to RHE. CV (scan rate = 50 mV s-1 without disk rotation) and LSV (scan rate = 10 mV s-1; disk


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rotations = 400–2500 rpm) were conducted in N2-purged and O2-saturated 0.1 M HClO4 (Ultrapure, Kanto Chemical).

ASSOCIATED CONTENT The SI is available free of charge on the ACS Publications website at DOI: More detailed model catalysts fabrication sequences, XPS spectra, evaluations of average atomic distances for cross-sectional HAADF-STEM image, and linear-sweep voltammograms recorded in O2-saturated 0.1 M HClO4 for the model catalysts.

ACKNOWLEDGMENT This study was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and by a Grant-in-Aid for Challenging Exploratory Research from the Japan Society for the Promotion of Science (T. W.).

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Ultrahigh Vacuum Synthesis of Strain-Controlled Model Pt(111)-Shell

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