www.sciencemag.org/content/346/6216/1502/suppl/DC1 Supplementary Materials for Element-specific anisotropic growth of shaped platinum alloy nanocrystals Lin Gan, Chunhua Cui, Marc Heggen, Fabio Dionigi, Stefan Rudi, Peter Strasser* *Corresponding author. E-mail: pstrasser@tu-berlin.de Published 19 December 2014, Science 346, 1502 (2014) DOI: 10.1126/science.1261212 This PDF file includes: Materials and Methods Figs. S1 to S8 References Submitted Manuscript: Confidential 5 November 2014 Supplementary Materials Materials and Methods Synthesis of Octahedral PtNi1.5 and Octahedral PtCo1.5 Nanocrystals (NCs). The PtNi1.5 and PtCo1.5 octahedral NCs were synthesized by a one-step, facile, surfactant-free solvothermal method in dimethylfomamide (DMF) solvent (33)), which acts as both solvent and reducing agent. First, precursor solutions were obtained by completely dissolving 4 mM Pt(acac)2 and 28 mM Ni(acac)2 into 100 ml pure DMF. 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 120 °C within 10 min and the heating rate is ~10 °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. The lower reaction temperature at 120 °C would benefit the nanocrystal faceting during kinetic growth of seeds in a colloidal solution. After long time reaction of 42 h, PtNi1.5 octahedral NCs were synthesized. Octahedral PtCo1.5 NCs were synthesized by replacing Ni(acac)2 with Co(acac)2 at the same concentrations while keeping all other conditions constant. To study the growth pathway of the octahedral Pt bimetallic NCs, the synthesis was stopped after different reaction times (4, 8, 16 and finally 42 hours). The NCs were collected by centrifuge at 1000 rpm and then washed by ethanol. Microscopy and Spectroscopy. Transision electron microscope (TEM) images were acquired on a FEI Tecnai 20 microscope operated at 200 kV. High resolution TEM observations were performed on a FEI-Titan 80-300 microscope with a spherical aberration (Cs) corrector for the objective lens. The STEM-EELS elemental mapping was performed in a FEI Titan 50-300 ‘PICO’ electron microscope equipped with a probe corrector (CEOS) and a high-angle annular dark field (HAADF) detector and operated at 80 kV. ‘Z-Contrast conditions were achieved using a probe semi-angle of 25 mrad and an inner collection angle of the detector of 70 mrad. EELS spectra were recorded with a Gatan image filter Quantum ERS system analyzing the Ni L2,3edge. The energy resolution determined from the full width at half-maximum of zero loss peaks was about 1 eV. For the Ni L-edge EELS mapping measurements, spectra were collected across individual NC at a pixel density between ca. 40*40 pixels and 60*60 pixels and the acquisition time of 0.5 sec/pixel. The elemental mapping using energy-dispersive X-ray spectra (EDX) was conducted on a probe-corrected FEI Titan 80-200 ‘ChemiSTEM’ electron microscope equipped with four symmetric SDD detectors. Structural models of the Pt-Ni nanoparticles at different growth stages were generated by using the VESTA (Visualization for Electronics and Structural Analysis) software (34). Electrochemical Measurements. Electrochemical experiments were performed in a threecompartment glass cell with a rotating disk electrode (RDE, 5 mm in diameter of glassy carbon, Pine Instrument) and a potentiostat (Biologic) at room temperature. A Pt-mesh and a Hg/Hg2SO4 electrode (in saturated K2SO4) were used as counter electrode and reference electrode, respectively. To prepare the working electrode, a certain amount of carbon supported PtxNi1-x catalyst was suspended in 3.98 mL of ultrapure water (Millipore, 18 MΩ), 1.00 mL of isopropanol and 20 µL of 5 wt% Nafion solution with sonication for 15 min to form a uniform ink. Ten µL of the ink was pipetted onto pre-polished and cleaned RDE electrode and dried at 60 °C for 20 min in air, resulting in a uniform catalyst thin film with ca. 8 µg Pt/cm2. For Pt/C catalyst, the Pt mass loading is ~14 µg/cm2. All potentials reported in this paper were normalized with respect to the reversible hydrogen electrode (RHE). The initial three potential cycles performed at 100 mV/s between 0.06 and 1.0 V to roughly allow the dealloying of the surface Ni 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 0.06 and 1.0 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 between 0.6 and 1.0 V at 50 mV/s for 4,000 cycles. Linear sweep voltammetry (LSV) measurements were conducted in oxygen-saturated 0.1 M HClO4 solution by sweeping the potential from 0.06 to 1.05 V at 5 mV/s (1600 rpm). Mass and specific activities were studied at 0.9 V, and were depicted as kinetic current densities normalized to the real active surface area and loading Pt mass. Fig. S1. Morphology of Pt-based NCs prepared by solvothermal reduction of different kinds of metal precursors in DMF at 120 °C for 8h: (a) spherical Pt NCs prepared by using 4mM Pt(acac)2. (b) spherical Pt-Ni NCs prepared using 4 mM Pt(acac)2 + 28 mM Ni (ac)2 . (c) Branched Pt NCs prepared using 4mM Pt(acac)2 + 56 mM K(acac). (d) Branched Pt-Ni hexapods prepared using 4 mM Pt(acac)2 + 28 mM Ni(acac)2. These results suggest that the acetyl acetone ligand plays a critical role in inducing the anisotropic growth into branched NCs. Fig. S2. Fourier transform infra red (FTIR) spectra of pure DMF, pure bulk acetyl acetone, dried PtNi-8h hexapods in the as-prepared state, and PtNi-8h hexapods after annealed in a thermal gravimetrical analysis up to 300 °C in air. The FTIR results provide unambiguous evidence for the adsorption of the precursor acetyl-acetonate ligands on the hexapod surfaces. The adsorbed ligands on the dried hexapod particle surface (after centrifuging any excess DMF and acetyl acetonate ligand in the supernatant liquid phase and after repeated washing of the hexapods) showed characteristic vibrational modes at 1390, 1519 and 1575 cm-1 as well as three smaller ones at 1261, 1018, and 925 cm-1. The adsorbed ligand can be efficiently removed after annealing to 300 °C in air as confirmed by both FTIR and thermo gravimetric analysis. Fig. S3. Thermo gravimetric analysis (TGA) measurement of dried Pt-Ni-8h hexapod particle powder (after repeated washing of the hexapod particles by ethanol and water and complete drying). Adsorbed acetyl acetonate ligands are gradually removed with the two heat flow peaks evidencing exothermic oxidation. Fig. S4. Energy dispersive X-ray elemental mapping and HAADF imaging of Pt-Ni concave octahedra obtained after 16 hours, showing the deposition of Ni-rich phase at the concave surfaces. Fig. S5. TEM (a) and HRTEM (b) images of octahedral PtNi1.5 NCs after a full reaction time of 42 h. Fig. S6. XRD patterns of Pt-Ni NCs at different growth time from 4 h to 42 h. The Bragg reflections exhibit symmetric Gaussian peaks assigned to Pt-rich phase for Pt-Ni NCs prepared by a reaction time within 16h, whereas the 42h product shows asymmetric diffraction peaks composed of a Pt-rich phase and a Ni-rich phase. Fig. S7. TEM images and EDX measurement of octahedral PtCo1.5 NCs after different reaction time during the DMF-based solvothermal synthesis: (a, d, g) 8 h, (b, e, h) 16 h, and (c, f, i) 42 h, showing that the growth of Pt-rich concave octahedra precedes the final formation of Co-rich octahedra. Fig. S8. (a) ORR polarization curves of the PtNi1.5 octahedral NCs supported on high surface area carbon support (Vulcan XC) after electrochemical activation (25 cycles) and long-term potential cycling. (b) Comparison of the Pt-mass and specific activities of the activated (20 cycles) and long-term cycled PtNi1.5 catalyst versus commercial Pt/C catalyst. (c-e) Morphology of the as-prepared (c), activated (d) and long-term cycled (4200 cycles between 0.6 and 1.0V/RHE at a scanning rate of 100 mV/s) (e) Pt-Ni catalyst NCs, respectively. References 1. T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein, M. A. El-Sayed, Shape-controlled synthesis of colloidal platinum nanoparticles. Science 272, 1924–1926 (1996). Medline doi:10.1126/science.272.5270.1924 2. N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, Z. L. Wang, Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity. Science 316, 732–735 (2007). Medline doi:10.1126/science.1140484 3. H. A. Gasteiger, N. M. Marković, Just a dream—or future reality? Science 324, 48–49 (2009). Medline doi:10.1126/science.1172083 4. V. R. Stamenkovic, B. Fowler, B. S. Mun, G. Wang, P. N. Ross, C. A. Lucas, N. M. Marković, Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 315, 493–497 (2007). Medline doi:10.1126/science.1135941 5. M. K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells. Nature 486, 43–51 (2012). Medline doi:10.1038/nature11115 6. G. Wu, K. L. More, C. M. Johnston, P. Zelenay, High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 332, 443–447 (2011). Medline doi:10.1126/science.1200832 7. D. Xu, Z. Liu, H. Yang, Q. Liu, J. Zhang, J. Fang, S. Zou, K. Sun, Solution-based evolution and enhanced methanol oxidation activity of monodisperse platinum-copper nanocubes. Angew. Chem. Int. Ed. 48, 4217–4221 (2009). Medline doi:10.1002/anie.200900293 8. J. Zhang, J. Fang, A general strategy for preparation of Pt 3d-transition metal (Co, Fe, Ni) nanocubes. J. Am. Chem. Soc. 131, 18543–18547 (2009). Medline doi:10.1021/ja908245r 9. J. Zhang, H. Yang, J. Fang, S. Zou, Synthesis and oxygen reduction activity of shapecontrolled Pt3Ni nanopolyhedra. Nano Lett. 10, 638–644 (2010). Medline doi:10.1021/nl903717z 10. J. Wu, A. Gross, H. Yang, Shape and composition-controlled platinum alloy nanocrystals using carbon monoxide as reducing agent. Nano Lett. 11, 798–802 (2011). Medline doi:10.1021/nl104094p 11. M. K. Carpenter, T. E. Moylan, R. S. Kukreja, M. H. Atwan, M. M. Tessema, Solvothermal synthesis of platinum alloy nanoparticles for oxygen reduction electrocatalysis. J. Am. Chem. Soc. 134, 8535–8542 (2012). Medline doi:10.1021/ja300756y 12. C. Cui, L. Gan, H. H. Li, S. H. Yu, M. Heggen, P. Strasser, Octahedral PtNi nanoparticle catalysts: Exceptional oxygen reduction activity by tuning the alloy particle surface composition. Nano Lett. 12, 5885–5889 (2012). Medline doi:10.1021/nl3032795 13. Y. Wu, S. Cai, D. Wang, W. He, Y. Li, Syntheses of water-soluble octahedral, truncated octahedral, and cubic Pt-Ni nanocrystals and their structure-activity study in model hydrogenation reactions. J. Am. Chem. Soc. 134, 8975–8981 (2012). Medline doi:10.1021/ja302606d 14. S.-I. Choi, S. Xie, M. Shao, J. H. Odell, N. Lu, H. C. Peng, L. Protsailo, S. Guerrero, J. Park, X. Xia, J. Wang, M. J. Kim, Y. Xia, Synthesis and characterization of 9 nm Pt-Ni octahedra with a record high activity of 3.3 A/mg(Pt) for the oxygen reduction reaction. Nano Lett. 13, 3420–3425 (2013). Medline doi:10.1021/nl401881z 15. J. Wu, L. Qi, H. You, A. Gross, J. Li, H. Yang, Icosahedral platinum alloy nanocrystals with enhanced electrocatalytic activities. J. Am. Chem. Soc. 134, 11880–11883 (2012). Medline doi:10.1021/ja303950v 16. H. Zhang, M. Jin, J. Wang, W. Li, P. H. Camargo, M. J. Kim, D. Yang, Z. Xie, Y. Xia, Synthesis of Pd-Pt bimetallic nanocrystals with a concave structure through a bromideinduced galvanic replacement reaction. J. Am. Chem. Soc. 133, 6078–6089 (2011). Medline doi:10.1021/ja201156s 17. Y. Wu, D. Wang, Z. Niu, P. Chen, G. Zhou, Y. Li, A strategy for designing a concave Pt-Ni alloy through controllable chemical etching. Angew. Chem. Int. Ed. 51, 12524–12528 (2012). Medline doi:10.1002/anie.201207491 18. X. Liu, W. Wang, H. Li, L. Li, G. Zhou, R. Yu, D. Wang, Y. Li, One-pot protocol for bimetallic Pt/Cu hexapod concave nanocrystals with enhanced electrocatalytic activity. Sci. Rep. 3, 1404 (2013). doi:10.1038/srep01404 19. E. Christoffersen, P. Liu, A. Ruban, H. L. Skriver, J. K. Norskov, Anode materials for lowtemperature fuel cells: A density functional theory study. J. Catal. 199, 123–131 (2001). doi:10.1006/jcat.2000.3136 20. G. Wang, M. A. Van Hove, P. N. Ross, M. I. Baskes, Monte Carlo simulations of segregation in Pt-Ni catalyst nanoparticles. J. Chem. Phys. 122, 024706 (2005). Medline doi:10.1063/1.1828033 21. I. A. Abrikosov, A. V. Ruban, H. L. Skriver, B. Johansson, Calculated orientation dependence of surface segregations in Pt50Ni50. Phys. Rev. B 50, 2039–2042 (1994). Medline doi:10.1103/PhysRevB.50.2039 22. C. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis. Nat. Mater. 12, 765–771 (2013). Medline doi:10.1038/nmat3668 23. C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H. L. Xin, J. D. Snyder, D. Li, J. A. Herron, M. Mavrikakis, M. Chi, K. L. More, Y. Li, N. M. Markovic, G. A. Somorjai, P. Yang, V. R. Stamenkovic, Highly crystalline multimetallic nanoframes with three-dimensional electrocatalytic surfaces. Science 343, 1339–1343 (2014). Medline doi:10.1126/science.1249061 24. J. M. Petroski, Z. L. Wang, T. C. Green, M. A. El-Sayed, Kinetically controlled growth and shape formation mechanism of platinum nanoparticles. J. Phys. Chem. B 102, 3316–3320 (1998). doi:10.1021/jp981030f 25. T. Yu, Y. Kim, H. Zhang, Y. Xia, Platinum concave nanocubes with high-index facets and their enhanced activity for oxygen reduction reaction. Angew. Chem. Int. Ed. 50, 2773– 2777 (2011). Medline doi:10.1002/anie.201007859 26. H. Zheng, R. K. Smith, Y. W. Jun, C. Kisielowski, U. Dahmen, A. P. Alivisatos, Observation of single colloidal platinum nanocrystal growth trajectories. Science 324, 1309–1312 (2009). Medline doi:10.1126/science.1172104 27. J. M. Yuk, J. Park, P. Ercius, K. Kim, D. J. Hellebusch, M. F. Crommie, J. Y. Lee, A. Zettl, A. P. Alivisatos, High-resolution EM of colloidal nanocrystal growth using graphene liquid cells. Science 336, 61–64 (2012). Medline doi:10.1126/science.1217654 28. H.-G. Liao, H. Zheng, Liquid cell transmission electron microscopy study of platinum iron nanocrystal growth and shape evolution. J. Am. Chem. Soc. 135, 5038–5043 (2013). Medline doi:10.1021/ja310612p 29. H.-G. Liao, L. Cui, S. Whitelam, H. Zheng, Real-time imaging of Pt3Fe nanorod growth in solution. Science 336, 1011–1014 (2012). Medline doi:10.1126/science.1219185 30. Y. Xia, Y. Xiong, B. Lim, S. E. Skrabalak, Shape-controlled synthesis of metal nanocrystals: Simple chemistry meets complex physics? Angew. Chem. Int. Ed. 48, 60–103 (2009). Medline doi:10.1002/anie.200802248 31. Y. Wu, D. Wang, X. Chen, G. Zhou, R. Yu, Y. Li, Defect-dominated shape recovery of nanocrystals: A new strategy for trimetallic catalysts. J. Am. Chem. Soc. 135, 12220– 12223 (2013). Medline doi:10.1021/ja4068063 32. Y. Ding, F. Fan, Z. Tian, Z. L. Wang, Atomic structure of Au-Pd bimetallic alloyed nanoparticles. J. Am. Chem. Soc. 132, 12480–12486 (2010). Medline doi:10.1021/ja105614q 33. C. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis. Nat. Mater. 12, 765–771 (2013). Medline doi:10.1038/nmat3668 34. K. Momma, F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 44, 1272–1276 (2011). doi:10.1107/S0021889811038970