Yu Chengfei, Koh Shirlaine, Leisch Jennifer E, Toney Michael F, Strasser Peter
Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004, USA.
Faraday Discuss. 2008;140:283-96; discussion 297-317. doi: 10.1039/b801586d.
Anomalous small angle X-ray scattering (ASAXS) is shown to be an ideal technique to investigate the particle size and particle composition dynamics of carbon-supported alloy nanoparticle electrocatalysts at the atomic scale. In this technique, SAXS data are obtained at different X-ray energies close to a metal absorption edge, where the metal scattering strength changes, providing element specificity. ASAXS is used to, first, establish relationships between annealing temperature and the resulting particle size distribution for Pt25Cu75 alloy nanoparticle electrocatalyst precursors. The Pt specific ASAXS profiles were fitted with log-normal distributions. High annealing temperatures during alloy synthesis caused a significant shift in the alloy particle size distribution towards larger particle diameters. Second, ASAXS was used to characterize electrochemical Cu dissolution and dealloying processes of a carbon-supported Pt25Cu75 electrocatalyst precursor in acidic electrolytes. By performing ASAXS at both the Pt and Cu absorption edges, the unique power of this technique is demonstrated for probing composition dynamics at the atomic scale. These ASAXS measurements provided detailed information on the changes in the size distribution function of the Pt atoms and Cu atoms. A shift in the Cu scattering profile towards larger scattering vectors indicated the removal of Cu atoms from the alloy particle surface suggesting the formation of a Pt enriched Pt shell surrounding a Pt-Cu core. Together with XRD and TEM, ASAXS is proposed to play an increasingly important role in the mechanistic study of degradation phenomena of alloy nanoparticle electrocatalysts at the atomic scale.
反常小角X射线散射（ASAXS）被证明是一种在原子尺度上研究碳载合金纳米颗粒电催化剂的粒径和颗粒组成动力学的理想技术。在该技术中，SAXS数据是在接近金属吸收边的不同X射线能量下获得的，此时金属散射强度会发生变化，从而提供元素特异性。ASAXS首先用于建立退火温度与Pt25Cu75合金纳米颗粒电催化剂前驱体的最终粒径分布之间的关系。Pt特定的ASAXS谱图采用对数正态分布进行拟合。合金合成过程中的高退火温度导致合金粒径分布显著向更大粒径偏移。其次，ASAXS用于表征碳载Pt25Cu75电催化剂前驱体在酸性电解质中的电化学Cu溶解和脱合金过程。通过在Pt和Cu吸收边处进行ASAXS，展示了该技术在探测原子尺度上的组成动力学方面的独特能力。这些ASAXS测量提供了关于Pt原子和Cu原子尺寸分布函数变化的详细信息。Cu散射谱向更大散射矢量的偏移表明Cu原子从合金颗粒表面被去除，这表明在Pt - Cu核周围形成了富含Pt的Pt壳层。与XRD和TEM一起，ASAXS被认为在合金纳米颗粒电催化剂降解现象的原子尺度机理研究中发挥着越来越重要的作用。
