Supporting Information Synthesis of Composition-tunable Octahedral Pt-Cu Alloy Nanocrystals by Controlling Reduction Kinetics of Metal Precursors Yanyan Jia, Zhenming Cao, Qiaoli Chen, Yaqi Jiang*, Zhaoxiong Xie*, and Lansun Zheng State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, China, 361005 E-mail: [email protected], [email protected] Figure S1. The XRD patterns of the samples prepared by using different amount of n-butylamine. Figure S2. The SEM images of the samples prepared by using different amount of n-butylamine: (a) 0.0 mmol, (b) 0.1 mmol, (c) 0.2 mmol, (d) 0.5 mmol. The SEM images show that the octahedral shape was maintained very well. Figure S3. The pictures showing the yield of Pt nanocrystals at different reaction times with (No.1, 3, 5 samples)/without (No. 2, 4, 6 samples) n-butylamine. It can be seen clearly that the as-prepared Pt NCs with n-butylamine (the former) is less than that formed in the absence of n-butylamine (the latter) at each reaction time. We thus deduced that the reducing rate of Pt in the presence of n-butylamine is much slower than that without n-butylamine. Figure S4. The CV curves measured for three Pt-Cu alloy catalysts with different composition and commercial Pt black in an N2-purged solution of 0.5 M H2SO4. Scan rate: 50 mV s-1. Typical features for the hydrogen desorption description are clearly observed in CV curves of the catalysts in an N2-purged solution of 0.5 M H2SO4. From the representative CV curves in H2SO4 solution, we can see that the voltammogram features in hydrogen underpotential deposition (HUPD) region are similar to those reported for the commercial Pt black. For the three Pt-Cu alloy, the HUPD region shows a nearly features. The ECSA was calculated by measuring the charges collected in the hydrogen desorption region from -0.2 to 0.1 V. The ECSA are 1.14, 1.46, 1.92 and 2.23 cm2 for PtCu, Pt38Cu62, PtCu3 and Pt black. There is an oxidation peak at about 0.48 V for three Pt-Cu alloy compared with Pt black, indicating Cu can be electrochemically dissolved, and the current density increases with Cu content increase displaying Cu dissolution more apparent.