Supporting Information

Nanocrystal and Surface Alloy Properties of Bimetallic Gold-Platinum Nanoparticles

Derrick Mott, Jin Luo, Andrew Smith, Peter N. Njoki, Lingyan Wang, and Chuan-Jian Zhong*

Department of Chemistry, State University of New York at Binghamton,

Binghamton, New York 13902

Direct current plasma - atomic emission spectroscopy (DCP-AES): The nanoparticle composition was determined by DCP-AES on an ARL Fisons SS-7 Spectrometer. Measurements were made on emission peaks at 267.59 nm and 265.95 nm, for Au and Pt, respectively. The nanoparticle samples were dissolved in concentrated aqua regia, and then diluted for the analysis. Details are given in J. Luo, et al, Langmuir, 2006, 22, 2892; and J. Luo, et al, Chem. Mater. 2005, 17, 3086.

X-ray photoelectron spectroscopy (XPS): XPS measurements were made on a Physical Electronics Quantum 2000 Scanning ESCA Microprobe with a multichannel detection system. The percentages of individual element detected were determined from the relative composition analysis of the peak areas of the bands. XPS data in the spectral regions for Au(4f), Pt(4f), C(1s), and S(2p) before and after the thermal treatment are shown in Table S1. The S(2p) bands characteristic of any thiolate or sulfur species from the capping /linking molecules were not detected after the thermal treatment at 300 oC, demonstrating the effective removal of sulfur-containing species to a level at least below the detection limit. In addition, the C(1s) band is dramatically decreased to a level at which only adventitiously adsorbed hydrocarbons are present. The relative ratios of the two metal components are largely consistent with those determined by DCP analysis. The results show that the surface capping molecules and sulfur species are effectively removed by the thermal treatment.

Table S1. XPS analysis of Binding Energy and Elemental Composition for Alloyed Nanoparticles.

Catalyst/SiO2
treatment at 400 C / Binding Energy (eV) / Elemental Composition (%)
C(1s) / Au(4f) / Pt(4f) / C / S a) / Au / Pt / Au:Pt
Au72Pt28 / 284.6 / 83.4 / 71.2 / 1.7 / - / 0.11 / 0.03 / 79:21
Au82Pt18 / 284.2 / 83.8 / 70.7 / 2.2 / - / 0.39 / 0.10 / 80:20

a) “–“ means “below detection limit of XPS”.

Cyclic Voltammetry (CV). CV is a powerful technique to detect the surface composition of the nanoparticles if the surface elements are electroactive. In comparison with the redox waves for the monometallic Au and Pt catalysts (Fig. 1A), the detection of redox waves corresponding to Au and Pt in the bimetallic Au72Pt28/C and Au35Pt65/C catalysts (Gig. 1B) demonstrate the presence of a bimetallic surface composition consistent with the bimetallic nanoparticle core composition.

(A) (B)

Figure 1. CV curves of the carbon-supported nanoparticles on a glassy carbon electrode in 0.5 M KOH, 50 mV/s. (A) Au/C and Pt/C catalysts. (B) Au72Pt28/C and Au35Pt65/C catalysts.

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