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

Expressions are obtained for the I MVV / I 3 d ratio of the Ag MVV Auger line intensity to the Ag3 d photoemission line intensity in XPS spectra (Al K α radiation, h ν = 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 R max = 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 I MVV / I 3 d ratio is shown to decrease significantly due to more effective screening by the shell of Ag MVV Auger electrons as compared to that of Ag3 d photoelectrons. The I MVV / I 3 d 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 I MVV / I 3 d 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.