Single‐Particle Kinetics Unravel Aperture‐Size Dominance in Hot‐Carrier Transfer and Product Desorption for Photocatalytic Reactions in Plasmonic Nanocavities

Photocatalysis with plasmonic nanoparticles has emerged as a promising strategy for solar energy conversion, yet the fundamental relationship between nanocavity geometry and catalytic efficiency remains elusive. In this work, we systematically investigate aperture-size-controlled hot-carrier transfer and mass transport coupling in gold nanocups (AuNCs) with precisely tuned cavity diameters (35-67 nm). Single-particle catalytic imaging reveals that AuNCs with a medium aperture size (≈58 nm) exhibit optimal photocatalytic activity, achieving a maximum turnover rate of 0.59 ± 0.007 s^-1, 2.8-8.4-fold higher than extreme-aperture variants. Theoretic simulations correlate this enhancement with intensified electric field localization (|E_max|/|E₀| = 5.1) and hot-carrier generation at the cup rim, while kinetic analysis identifies dominant substrate-assisted desorption (k₂ = 6.12 ± 0.14 s^-1) and strong spatiotemporal memory effect. The work deciphers the synergy between electromagnetic confinement and molecular transport at the single-particle level, demonstrating that cavity geometry governs reactant adsorption, product desorption pathways, and active-site stability. These findings establish a geometric design principle for plasmonic photocatalysts, advancing the rational engineering of high-efficiency solar-to-chemical conversion systems for sustainable energy applications.