Particle-Size- and Ru-Core-Induced Surface Electronic States of Ru-Core/Pt-Shell Electrocatalyst Nanoparticles

The unique electrocatalytic properties of core/shell-structured bimetallic nanoparticles (NPs) are generally attributed to core-induced electronic effects on surface atoms. For small core/shell NPs, however, size-dependent electronic modifications can be non-negligible. Here, using synchrotron-based photoelectron spectroscopy, we identified both size- and core-induced modifications in the surface electronic states of core/shell NPs with approximately monolayer-thick Pt shells completely covering Ru cores. The embedded Ru cores were found to induce a downward shift in energy of the d states of the surface Pt atoms. However, with a decrease in core size from 1.9 to 1.3 nm, these Ru-core-induced, low-lying d states increased in energy and decreased in intensity. Extended X-ray absorption fine structure measurements indicated that this size-induced electronic modification can be associated with a decrease in the number of Pt-Ru bonds per surface Pt atom. The observed size- and core-induced electronic modifications were confirmed to affect the electrochemical desorption of CO from the core/shell NPs. Our findings indicate that both size- and core-induced electronic effects must be considered in the rational design of real-world core/shell electrocatalyst NPs.