Size Matters: d-Band Holes Drive Plasmonic Chemistry in Gold.

Light absorption in plasmonic gold nanoparticles (AuNPs) generates short-lived nonthermalized electrons and holes that can drive redox reactions, making them attractive for photocatalytic applications. However, characterizing these processes and disentangling competing effects due to photothermal heating or plasmonic field enhancement remain challenging. Here, we exploit the light-driven synthesis of silver shells around AuNPs as a model reaction to study the role of nonthermalized carriers in plasmonic gold photochemistry. By coupling in situ extinction spectroscopy with Mie theory calculations, we extract internal quantum efficiencies (IQEs) for AuNPs with different diameters. The size dependence of the IQE scales with the probability of light absorption within 2-3 nm of the Au nanoparticle surface, highlighting the critical role of short-lived d-band holes, which recombine on ultrafast time scales, in driving our chemistry. More broadly, our approach provides a simple yet powerful framework to characterize activation mechanisms and quantify carrier dynamics in plasmonic photocatalysis.