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

Light can potentially charge plasmonic nanoparticles by photochemical processes, significantly modifying their optical and chemical properties. However, the charging process is challenging to track experimentally, which severely hinders its thorough evaluation. Here, we introduce a method to observe the charging of gold nanorods \textit{in situ}. Employing the sensitivity of the rods' longitudinal plasmon resonance to the charge density we were able to track the particles' charge during a light induced reaction. The obtained data provide spectroscopic evidence that the charging process can be understood in the framework of a nanoscale capacitor model, where the applied voltage is governed by the chemical potential of holes generated in the 5d-bands of gold. Experimental observations of the dependence on particle size, oxygen content in the solvent and ligand type further corroborate the proposed capacitor model. The results of this work present the foundation for the rational engineering of dynamic charge states during plasmon driven photoreactions.