Ultra-High Vacuum Synthesis of Strain-Controlled Model. Pt(111)-Shell Layers: Surface Strain and Oxygen Reduction

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1 Supporting Information Ultra-High Vacuum Synthesis of Strain-Controlled Model Pt(111)-Shell Layers: Surface Strain and Oxygen Reduction Reaction Activity Soma Kaneko 1, Rikiya Myochi 1, Shuntaro Takahashi 1, Naoto Todoroki 1*, Toshimasa Wadayama 1, Tadao Tanabe 2 1 Graduate School of Environmental Studies, Tohoku University, Sendai , Japan 2 Graduate School of Engineering, Tohoku University, Sendai , Japan TEL: *Corresponding_author: n-todoroki@material.tohoku.ac.jp 1. Detailed Pt/Co/Pt(111) model catalyst fabrication sequences Following deposition sequences are performed for each model catalysts: (1) Pt(111) substrate surface was cleaned by a conventional surface cleaning method (repeated Ar + sputtering at room temperature and annealing at 1000 K) in UHV condition. (2) The substrate temperature was heated to 573 K and kept the temperature through all the Co and Pt deposition process. (3) The 0.4nm-thick Co (denoted as first layer Co; Figure 1 in the text) was deposited on the Pt(111) surface by using an arc-plasma deposition source of Co. The thickness was fixed for all the model catalysts. (4) The second layer Pt was deposited onto the first layer Co deposited surface by using another arc-plasma deposition source of Pt at given thickness from 0.8 nm to 3.6 S1

2 nm. (5) The third layer Co whose thickness from 3.2 nm to 0.4 nm was then deposited on the second layer Pt deposited surfaces. Sum of the thickness of (4) and (5) was fixed to be 4.0nm. (6) Lastly, the 1.6nm-thick forth layer Pt was deposited on the third layer Co deposited surfaces as the fourth layer Pt. The above-described deposition sequences (3)~(6) generate the model catalysts. We estimated the Co and Pt deposition thickness by using a quartz-crystal microbalance installed in the APD chamber. Arc-plasma deposition parameters for the Co and Pt are listed in Table S1. The 1 st -Co and 2 nd -Pt layers are inserted as buffer layers in the layered structures to mitigate large lattice mismatch between deposited Co and Pt substrate. If the rather thick 3 rd Co-layer is directly deposited on the Pt(111) substrate, the Co lattices would be laterally expanded and the compressive strain on the Pt-shell should become weak. Then, the buffer layers effectively works to weaken lattice mismatch of 3 nd Co-layer against the underlayer (2 nd -layer) Pt and increase the lattice strain effect on the Pt-shell. Table S1 Arc-plasma deposition parameters for Co and Pt Arc-voltage Pulse-frequency Pulse-repetitions Time interval for every (V) (Hz) for the 0.4nm-thick-deposition 0.4nm-thick (s) layers Co Pt S2

3 2. XPS spectra for model catalysts Intensity (a.u.) cf;pt(111) Pt4f 7/2 71.2eV Binding Energy (ev) Pt4f Intensity (a.u.) Co 3.2nm Co 1.6nm Co 0.8nm Co 0.4nm Binding Energy (ev) Co2p Figure S1 XPS spectra for the UHV-prepared Co 0.4nm, Co 0.8nm, Co 1.6nm, and Co 3.2nm model catalysts. All the black line are smoothed spectra of corresponding samples. Table S2 Chemical shifts of Pt4f 7/2 bands for the model catalysts Chemical shift in Pt4f 7/2 vs. Pt(111) (ev) Co 0.4nm 0.10 Co 0.8nm 0.14 Co 1.6nm 0.20 Co 3.2nm 0.29 X-ray photoelectron spectroscopic (XPS) analysis of the catalysts were performed in another UHV chamber that settled semi-spherical electron energy analyzer and X-ray source (SPECS PHOIBOS100 and XR50). XPS spectra of the Co2p (left) and Pt4f (right) bands for the model catalysts are presented in Figure S1. With increasing the underlying Co thickness, the intensities of the Co 2p increase, though the chemical shifts of Co cannot be discussed because of weak intensities. In contrast, the peak positions (binding energies) of the Pt4f 7/2 shift to higher energy side depending upon S3

4 the underlying Co deposition thickness (Table S2), indicating that electronic properties of the Pt(111)-shells are also modified with the DT 2nd-Pt / DT 3rd-Co. XPS sampling depth is ca. 1.6 nm considering our experimental set up and escape depth of Pt and Co 1. S4

5 3. Evaluation of average atomic distances Figure S2 Cross-sectional HAADF-STEM image of the Co 1.6nm. Average atomic distances in the <100> direction are estimated from each 100 atoms in the right hand (Pt(111)-shell) and left hand (Pt(111) substrate) magnified images of the STEM image, respectively. The average atomic distances of the Pt(111)-shell regions for the Co 1.6nm and Co 3.2nm are ca and nm; respective surface stain of the Pt(111)-shells can be estimated to be -2.3 % and -3.1% relative to the value of the Pt(111) substrate region (0.392 nm). S5

6 4. LSVs and estimated ORR activity enhancements for the model catalysts 0 Clean Pt(111) j k / ma cm Co 0.4nm Co 0.8nm Co 1.6nm Co 3.2nm E / V (vs. RHE) Figure S3. Linear-sweep voltammograms recorded in O 2 -saturated 0.1 M HClO 4 for the Pt/Co/Pt(111) model catalysts. The LSV measurement under O 2 -saturation of the 0.1M HClO 4 solution was conducted after the CV measurement; the result is shown in Figure S3. The LSV curves collected at 1600 rpm of disk rotation show that the half-wave potential for the Pt/Co/Pt(111) model catalysts shift in positive direction compared with the clean Pt(111) (gray dashed line), revealing the enhanced ORR activity of the model catalysts. S6

7 Figure S4. Schematic model of (220) planes of fcc-pt structure References [1] Tanuma, S.; Shiratori, T.; Kimura, T.; Goto, K.; Ichimura, S.; Powell, C. Experimental Determination of Electron Inelastic Mean Free Paths in 13 Elemental Solids in the 50 to 5000 ev Energy Range by Elastic-Peak Electron Spectroscopy. J. Surf. Interface Anal. 2005, 37, S7