XPS Analysis of the Spacial Distribution of Metals in Au-Ag Bimetallic Particles Considering the Effect of Particle Size Distribution

Abstract

Expressions are obtained for the IMVV/I3d ratio of the AgMVV Auger line intensity to the Ag3d photoemission line intensity in XPS spectra (AlKα radiation, hν = 1486.6 eV) of Au-Ag bimetallic particles located on the flat surface of the support. Bimetallic hemispherical particles with non-uniform distribution of metals (silver core covered with a gold shell) are considered. The particles are characterized by normal size distribution (with respect to the core radius R) with a maximum at Rmax = 3.5 nm and the standard deviation σ = 0.5 nm, 1.0 nm, and 2.0 nm. The bimetallic system is analyzed in the conditions of R-independence: in one case, the shell thickness δ = const(R); in the other case, the atomic ratio of gold to silver γ = const(R). As the Au-shell is formed around the Ag-core and y increases, the IMVV/I3d ratio is shown to decrease significantly due to more effective screening by the shell of AgMVV Auger electrons as compared to that of Ag3d photoelectrons. The IMVV/I3d ratio as a function of γ weakly depends on σ and on the choice of conditions δ = const(R) or γ = const(R). When an alloy with a uniform spatial distribution of gold and silver atoms over the particle volume is formed, the IMVV/I3d ratio also decreases with increasing γ, though to a much smaller extent. The derived expressions can be used in XPS studies to consider the formation of supported bimetallic particles in model planar systems and the changes occurring with the particles as a result of heat-treatment in the reaction medium.