Shibata Tomohiro, Bunker Bruce A, Zhang Zhenyuan, Meisel Dan, Vardeman Charles F, Gezelter J Daniel
Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA.
J Am Chem Soc. 2002 Oct 9;124(40):11989-96. doi: 10.1021/ja026764r.
We report on systematic studies of size-dependent alloy formation of silver-coated gold nanoparticles (NPs) in aqueous solution at ambient temperature using X-ray absorption fine structure spectroscopy (XAFS). Various Au-core sizes (2.5-20 nm diameter) and Ag shell thicknesses were synthesized using radiolytic wet techniques. The equilibrium structures (alloy versus core-shell) of these NPs were determined in the suspensions. We observed remarkable size dependence in the room temperature interdiffusion of the two metals. The interdiffusion is limited to the subinterface layers of the bimetallic NPs and depends on both the core size and the total particle size. For the very small particles (< or =4.6 nm initial Au-core size), the two metals are nearly randomly distributed within the particle. However, even for these small Au-core NPs, the interdiffusion occurs primarily in the vicinity of the original interface. Features from the Ag shells do remain. For the larger particles, the boundary is maintained to within one monolayer. These results cannot be explained either by enhanced self-diffusion that results from depression of the melting point with size or by surface melting of the NPs. We propose that defects, such as vacancies, at the bimetallic interface enhance the radial migration (as well as displacement around the interface) of one metal into the other. Molecular dynamics calculations correctly predict the activation energy for diffusion of the metals in the absence of vacancies and show an enormous dependence of the rate of mixing on defect levels. They also suggest that a few percent of the interfacial lattice sites need to be vacant to explain the observed mixing.
我们报告了使用X射线吸收精细结构光谱（XAFS）在室温下对水溶液中银包覆金纳米颗粒（NPs）的尺寸依赖性合金形成进行的系统研究。使用辐射湿式技术合成了各种金核尺寸（直径2.5 - 20 nm）和银壳厚度的颗粒。在悬浮液中确定了这些纳米颗粒的平衡结构（合金与核壳结构）。我们观察到两种金属在室温下的相互扩散存在显著的尺寸依赖性。相互扩散仅限于双金属纳米颗粒的亚界面层，并且取决于核尺寸和总颗粒尺寸。对于非常小的颗粒（初始金核尺寸≤4.6 nm），两种金属在颗粒内几乎随机分布。然而，即使对于这些小金核纳米颗粒，相互扩散主要发生在原始界面附近。银壳的特征仍然存在。对于较大的颗粒，边界保持在一个单层范围内。这些结果既不能用因尺寸导致熔点降低而增强的自扩散来解释，也不能用纳米颗粒的表面熔化来解释。我们提出，双金属界面处的缺陷，如空位，会增强一种金属向另一种金属的径向迁移（以及在界面周围的位移）。分子动力学计算正确地预测了在没有空位时金属扩散的活化能，并表明混合速率对缺陷水平有极大的依赖性。它们还表明，需要有百分之几的界面晶格位置为空位才能解释观察到的混合现象。
