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1 DOI: 1.138/NMAT3668 Compositional segregation in shaped Pt alloy nanoparticles and their structural behavior during electrocatalysis Chunhua Cui 1, Lin Gan 1, Marc Heggen 2, Stefan Rudi 1 and Peter Strasser 1 1 The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division, Technical University Berlin, Berlin 1623, Germany 2 Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Ju lich GmbH, Ju lich, Germany 1. Experimental detail Chemicals. Platinum(II) acetylacetonate [Pt(acac) 2 ], ckel(ii) acetylacetonate [(acac) 2 ] and anhydrous ethanol were purchased from Alfa Aesar, and Nafion and Dimethylformamid (DMF) was purchased from Sigma-Aldrich. All the chemicals were used as received without further purification. Synthesis of Octahedral Pt x 1-x (, Pt and ) Nanoparticles. The Pt x 1-x octahedral nanoparticles were synthesized by a one-step, facile, surfactant-free solvothermal method in dimethylfomamide (DMF) solvent. DMF solvent acts as complexing agent, solvent, and reducing agent. First, precursor solutions were obtained by completely dissolving 4 mm Pt(acac) 2 /3 mm (acac) 2, 4 mm Pt(acac) 2 /1 mm (acac) 2, and 4 mm Pt(acac) 2 /28 mm (acac) 2 into 1 ml pure DMF, respectively. Then, the precursor solutions were transferred into a glass tube with a Teflon-lined stainless-steel autoclave. The mixed solutions in sealed autoclave were heated from room temperature to 12 o C within 1 min and the heating rate is ~1 o C/min. The monitored temperature is based on real-time solution temperature other than heating mantle. High heating rate results in short induction time and high nucleation rate generates a large number of small seeds. 1 The lower reaction temperature at 12 o C would benefit the nanocrystal faceting during growth of seeds in a colloidal solution. After long time reaction of 42 h,, Pt and octahedral nanoparticles were synthesized, respectively. Before washing the particles, the carbon support (Vulcan XC-72) was added to the mixing solution. Finally, the Pt x 1-x /C was obtained by washing with ethanol/18.2 MΩ de-ionized water (Millipore) several times and a followed freeze-drying. NATURE MATERIALS 1

2 DOI: 1.138/NMAT3668 Morphology, composition and crystalline phase Measurements. Transmission electron microscopy (TEM) and high revolution TEM (HRTEM) images (Figure S1) were obtained by a FEI TECNAI G2 2 S-TWIN transmission electron microscope with LaB6 cathode. The microscope was carried out by an accelerating voltage of 2 kv. Bulk compositions were obtained by energy-dispersive X-ray spectroscopy (EDX) detector and inductively coupled plasma mass spectrometry (ICP). ICP data were obtained by a 715-ES- ICP analysis system (Varian). The resultant phase of the as-synthesized Pt x 1-x octahedra was examined by X-ray diffraction (XRD). XRD patterns were collected using a D8 Advance-Diffractormeter (Bruker) equipped with a Lynx Eye Detector and KFL Cu 2K X-ray tube. Rotating Disk Electrode (RDE) Thin Film Electrode Preparation. The catalyst ink was prepared by mixing the catalyst Pt x 1-x /C powder in 5 ml of an mixed aqueous and isopropanol solution containing 5 wt% Nafion and ultrasonicated for 15 min. A 1 µl aliquot was dispensed onto the RDE resulting in Pt loading of ~8 µg/cm 2. For Pt/C catalyst, the Pt mass loading is ~14 µg/cm 2. The ink was dried for 2 min in air resulting in the formation of uniform thin film at the RDE surface. No any uncovered parts on glassy carbon surface suggested the successful preparation of the thin film electrode. Electrochemical Measurements. A custom-made, three compartment glass cell was used as the electrochemical cell. A commercial glassy carbon RDE (5 mm diameter,.196 cm 2 ) was polished to a mirror finish and thoroughly cleaned. A Pt-gauze (Sigma Aldrich) and a saturated Hg/HgSO 4 were used as counter electrode and reference electrode. All electrode potentials were recorded with respect to the reversible hydrogen electrode (RHE). A commercial dual-channel potentiostat (BioLogic) and a rotating disk experiment (PINE) were used control the electrochemical experiments and rotating rate..1 M HClO 4 diluted from 7 % HClO 4 (Sigma-Aldrich) by 18.2 MΩ de-ionized water (Millipore) used as the electrolyte. Before the measurements performed, the working electrode was immersed into the electrolyte at.6 V under potential control. The initial three potential cycles performed at 1 mv/s between.6 and 1. V to roughly allow the dealloying of the surface atoms and obtain the surface voltammetric response. The third one was recorded to estimate the electrochemical active surface area (ECSA). The obtained surface was defined as initial surface. The catalysts were further electrochemical activated by potential cycling between.6 and 1. V and the 25th potential cycle was recorded to evaluate the ECSA after activation. The obtained surface was defined as activated surface. The stability test was performed 2 NATURE MATERIALS

3 DOI: 1.138/NMAT3668 SUPPLEMENTARY INFORMATION between.6 and 1. V at 5 mv/s for 4, cycles, during which the potential region changed to.6-1. V and scan rate changed to 1 mv/s after 25th, 5th, 1,th, 2,th, 3,th and 4,th cycles to evaluate the ECSA change. Considering the strong interaction between CO ad and surface Pt sites may modify the surface morphology/composition, CO stripping was performed in parallel experiments. 2,3 Before CO-stripping, the solution was saturated with CO by bubbling through the solution for 15 min. The Pt-ECSA was confirmed using CO-stripping to avoid the underestimation of the ECSA owing to the possible suppression of H upd adsorption. Linear sweep voltammetry (LSV) measurements were conducted in oxygen-saturated.1 M HClO 4 solution by sweeping the potential from.6 to 1.5 V at 5 mv/s (16 rpm). Mass and specific activities were studied at.9 V, and were depicted as kinetic current densities normalized to the real active surface area and loading Pt mass. Supplementary Figure S1. TEM and HRTEM images of unsupported Pt x 1-x octahedra. TEM and HRTEM images of Pt x 1-x octahedral nanoparticles with different compositions. A and B,, C and D, Pt, and E and F,. The B, D and F are the corresponding schematic sketches of B, D and F, respectively. NATURE MATERIALS 3

4 DOI: 1.138/NMAT3668 intensity / a.u. (111) Pt (2) shoulder (22) (311) /deg. Supplementary Figure S2. Phase segregated Pt x 1-x octahedra. XRD patterns of carbon-supported, Pt and octahedra conducted at 12 o C for 42 h in DMF solvent. After increasing the composition in the bulk, the segregated phases were observed. The Pt and composition distribution was further studied by STEM-EELS. 4 NATURE MATERIALS

5 DOI: 1.138/NMAT3668 SUPPLEMENTARY INFORMATION Supplementary Figure S3. Z-contrast images and STEM-EELS line scans of (A, B and C) and (C, D and E) octahedra. A, Z-contrast image of shows the well facetted structure and the roughly composition distribution. B and C, STEM-EELS of demonstrate the -richer facets and Pt-richer edges. D, E and F, STEM-EELS line scans of show the -richer facet region and Pt-richer corners and edges. NATURE MATERIALS 5

6 DOI: 1.138/NMAT Pt After E-activation (25 cycles) Supplementary Figure S4. CO stripping of Pt x 1-x octahedra after electrochemical activation (25 cycles). CO stripping of Pt x 1-x octahedra recorded after electrochemical activation of 25 potential cycles. CO stripping was carried out in CO-saturated.1 M HClO 4 solution by bubbling 15 min. Once the CO monolayer on Pt surface was obtained, the electrolyte was purged with nitrogen gas. Then, the CO stripping was performed by positively sweeping the potential. The CO stripping charge, Q CO, versus the H upd charge, Q H, Q CO /2Q H =1.1; Q CO /2Q H Pt=1.17; Q CO /2Q H =1.5. Considering the exceptional activity enhancement toward the ORR may be attributed to the formation of a possible Pt-skin structure after a mild surface dealloying, the Qco/Q H rate rules out the presence of the thermally segregated Pt skin structure with its characteristic ratio of NATURE MATERIALS

7 DOI: 1.138/NMAT3668 SUPPLEMENTARY INFORMATION 1 Relative at. % Pt As-prepared Activated Stability Supplementary Figure S5. Composition changes in the bulk. The relative atomic fraction of in the bulk of as-prepared samples and electrochemical treated samples was measured by EDX. The composition information was obtained by measuring all particles of a selected region other than a single particle. After electrochemical activation of 25 potential cycles, the bulk composition of the three catalysts was changed to near Pt 3. It means that more in the starting materials, more has been leached out after 25 potential cycles. After 4, stability test, the composition of all samples was changed to around Pt 8 2. The results indicate that more Pt in the starting materials prevent the leaching of in acidic electrochemical conditions. NATURE MATERIALS 7

8 DOI: 1.138/NMAT3668 Supplementary Figure S6. Overview of morphology changes in the Pt x 1-x octahedra after 25 potential cycles. TEM images of Pt x 1-x octahedra after electrochemical activation of 25 potential cycles. (A, B), (C, D )Pt, and (E, F). 8 NATURE MATERIALS

9 DOI: 1.138/NMAT3668 SUPPLEMENTARY INFORMATION A D.5. after after 5 after 1, after 2, after 3, Pt -1. after 4, after 25 after 5 after1, after 2, after 3, after 4, Pt B ECSA E ECSA Potential cycle Pt Potential cycle C F intial LSV After 4, cycles initial LSV After 4, cyces Pt G.3 H 6 5 I -1 initial LSV After 4, cyces. -.3 after 25 after 5 after 1, -.6 after 2, after 3, after 4, ECSA Potential cycle j 15 k 15 Normalized specific activity Pt Initial after 4, Stability test Normalized mass activity Pt Initial after 4, Stability test Supplementary Figure S7. Specific and mass activities during the stability test. CV and ECSA changes of Pt x 1-x octahedra were recorded during stability test and the polarization curves were obtained after stability test. (A-C) CVs, ECSAs and LSVs for, respectively. (D-F) CVs, ECSAs and LSVs for Pt, respectively. (G-I) CVs, ECSAs and LSVs for, respectively. (J-K) normalized specific and mass activities of Pt x 1-x octahedra. NATURE MATERIALS 9

10 DOI: 1.138/NMAT3668 Supplementary Figure S8. Overview of morphology changes in Pt x 1-x octahedra after stability tests. TEM and HRTEM images of Pt x 1-x octahedra obtained after stability test of 4, potential cycles. (A) TEM image of after stability test shows octahedral morphology, (B) TEM image of Pt losses the facet morphology and (C) TEM image of demonstrates the concave structure. Some particles only had six corners as shown in the inset. (E) HRTEM image of octahedra. (F) HRTEM image of a dendritic nanoparticle after selective leaching in the facets. References 1 Snyder, J., McCue, I., Livi, K. & Erlebacher, J. Structure/Processing/Properties Relationships in Nanoporous Nanoparticles as Applied to Catalysis of the Cathodic Oxygen Reduction Reaction. J. Am. Chem. Soc. 134, , (212). 2 Mayrhofer, K. J. J., Juhart, V., Hartl, K., Hanzlik, M. & Arenz, M. Adsorbate-Induced Surface Segregation for Core-Shell Nanocatalysts. Angew. Chem. Int. Ed. 48, , (29). 3 van der Vliet, D. F., Wang, C., Li, D., Paulikas, A. P., Greeley, J., Rankin, R. B., Strmcnik, D., Tripkovic, D., Markovic, N. M. & Stamenkovic, V. R. Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces. Angew. Chem. Int. Ed. 51, , (212). 1 NATURE MATERIALS