In situ Observation of Nanoparticle Photocharging: Gold Nanorods as Photochemical Capacitors

Light can photochemically charge plasmonic nanoparticles and in doing so modulate their Fermi level. Although this effect is increasingly applied to rationalize the kinetic enhancement observed in plasmonic photochemistry, the underlying charging mechanism is challenging to trace experimentally. Here, we introduce a method to observe the charging process of gold nanorods \textit{in situ} by tracing the longitudinal plasmon resonance during the photoinduced charge transfer from a hole scavenger. The obtained data provides spectroscopic evidence that the charging process can be understood in the frame of a nanoscale capacitor model, were the applied voltage is governed by the chemical potential of holes generated in the 5d-bands of gold. We also investigated the influence of particle size, oxygen content in the solvent and ligand on the charging, which further corroborates the proposed capacitor model. This work presents the first step for the rational engineering of Fermi levels in plasmon photochemistry.