Supporting Information

Transcription

1 Supporting Information Atomically Flat Pt Skin and Striking Enrichment of Co in Underlying Alloy at Pt 3 Co(111) Single Crystal with Unprecedented Activity for the Oxygen Reduction Reaction Shun Kobayashi, Makoto Aoki, Mitsuru Wakisaka, Teppei Kawamoto, Ryo Shirasaka, Kohei Suda, Donald A. Tryk, Junji Inukai, * Toshihiro Kondo, * and Hiroyuki Uchida * Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu , Japan Division of Life, Medical, Natural Sciences and Technology, Organization for Advanced and Integrated Research, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe , Japan Division of Environmental Engineering, Graduate School of Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama , Japan Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu , Japan Division of Chemistry, Graduate School of Humanities and Sciences, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo , Japan S1

2 No signal Pt80Co20 (111) Pt Arb. Unit After annealing j k / ma cm -2 Bulk Pt-Co Figure S1. LEED (1 1) pattern for the Pt 80 Co 20 (111) electrode after H 2 annealing. E / ev Incident electron beam energy was 133 ev. a Figure S2. LEIS spectra for the Pt 80 Co 20 (111) electrode before and after the annealing under H 2-4 atmosphere. Incident Ne + ion energy and scattering angle were 1000 ev and 136, respectively. a at 0.9 V vs. RHE Co Co content, atom % Figure S3. Kinetically controlled area-specific activities for the ORR (j k ) of the Pt-skin/Pt 100 x Co x (111) rotating disk electrodes at 0.9 V vs. RHE as a function of Co content in air-saturated 0.1 M HClO 4 solution. a LEED and LEIS measurements The low energy electron diffraction (LEED, ErLEED100, SPECS, Germany) and low energy ion scattering using Ne + ions (LEIS, Model 1800C, ULVAC-PHI, Japan) measurements were performed under ultrahigh vacuum (UHV, base pressure = Pa) after annealing in 0.5 atm of pure H 2 stream in a preparation chamber followed by transfer to an UHV analysis chamber without exposing the electrode surface to air. Electrochemical measurements The Pt-Co and Pt single crystal disks were mounted in a Teflon sleeve immediately after annealing in H 2. The IR-loss (ohmic drop) was excluded from the electrode potential in the hydrodynamic voltammograms by measuring the ohmic resistance of 0.1 M HClO 4 electrolyte solution saturated with ultrahigh-purity synthetic air with a potentiostat equipped with an AC impedance analyzer (PGSTAT302N, Metrohm Autolab B. V., The Netherlands). a: Data cited from our previous work: Kobayashi, S.; Wakisaka, M.; Tryk, D. A.; Iiyama, A.; Uchida, H. J. Phys. Chem. C 2017,121, S2

3 In-situ SXS measurements Figure S4. Schematic of the spectro-electrochemical cell used for in-situ SXS measurement. Table S1. Structural parameters obtained from the analyses of the (00) CTR profile of the Pt-skin/Pt 75 Co 25 (111) electrode at 0.4 V vs. RHE in the N 2 -purged 0.1 M HClO 4 solution. Atom or ion Content RMS Layer distance / nm Adsorbed layer - ClO ML 0.30 Topmost layer Pt and Co 98 : Second layer Pt and Co 2 : Third layer Pt and Co 79 : Fourth layer Pt and Co 81 : a Bulk Pt and Co 75 : a Bulk parameters were fixed in the fitting S3

4 Density functional theory (DFT) calculations DFT calculations were carried out with periodic boundary conditions to simulate the pure Pt(111) (4 4) and Pt skin/co underlayer (UL)/Pt 3 Co(111) (4 4) surfaces, both with three layers (Figure S5). For pure Pt, the standard lattice parameter of Å was used. For the Pt skin/co UL/Pt 3 Co(111), the Pt 3 Co fcc structure was used as a basis for modification, with the lattice parameter of Å. 1 The top layer was converted to pure Pt, which has been shown to exist in acid solution. The second layer was converted to pure Co to approximately match the experimental SXS results, while the bottom layer was Pt 3 Co, as in the bulk alloy, again to approximately match the SXS results. During the geometry optimization, only the bottom layer of 16 atoms was fixed. The DMol 3 software package (BIOVIA, Materials Studio, Version 7.0) was used. 2 For the geometric optimization, fine tolerance settings were used (convergence, Ha, maximum force Ha Å 1, maximum displacement Å), with the use of density functional semicore pseudopotentials, 3 with a double-numeric quality basis set with polarization functions (dnp). After optimization, the final energies were calculated with all-electron scalar relativistic corrections and the gradient-corrected GGA functional developed by Perdew, Burke and Ernzerhof (pbe). 4 To facilitate scf convergence, thermal smearing was used, Ha for Pt(111) and Ha for Pt skin/co UL/Pt 3 Co(111). In the final energy calculation, the effect of smearing was minimized by extrapolation to zero smearing. Figure S5. Periodic models used in the DFT calculations: (left) Pt(111) (4 4) and (right) Pt skin/co underlayer (UL)/Pt 3 Co(111) (4 4) surfaces, both with three layers. Figure S6. Total density of states (DOS) for the top layer of Pt atoms on the Pt(111) (4 4) ( ) and Pt skin/co underlayer (UL)/Pt 3 Co(111) (4 4) (- - -) surfaces, both with three layers, as shown in Fig. S5. The total DOS for the Co UL is also shown (... ). The d-band centers were 2.55 ev and 3.00 ev, respectively, for the Pt(111) and Pt skin/co UL/Pt 3 Co(111) surfaces. These values are comparable to those reported, by Kitchin et al., of 2.43 ev and 2.73 ev, respectively, for three-layer Pt(111) and Pt/Co/Pt(111) structures. 5 S4

5 Table S2. Atomic charges (Mulliken) obtained from the all-electron relativistic DFT calculations of the Pt skin/co UL/Pt 3 Co(111) three-layer model (layer 3 is the top layer). Atom Layer Charge Ave. Std. dev. Co Co Co Co Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Co Co Co Co Co Co Co Co Co Co Co Co Pt Pt S5

6 Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt References (1) Geisler, A. H.; Martin, D. L. A New Superlattice in Co-Pt Alloys. J. Appl. Phys. 1952, 23, (2) Delley, B. A Scattering Theoretic Approach to Scalar Relativistic Corrections on Bonding. Int. J. Quant. Chem. 1998, 69, (3) Delley, B. Hardness Conserving Semilocal Pseudopotentials. Phys. Rev. B, 2002, 66, (1-9). (4) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett., 1996, 77, (5) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Modification of the Surface Electronic and Chemical Properties of Pt(111) by Subsurface 3d Transition Metals. J. Chem. Phys. 2004, 120, S6