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 |/| E 0 | = 5.1) and hot‐carrier generation at the cup rim, while kinetic analysis identifies dominant substrate‐assisted desorption ( k 2 = 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.