Gold Nanorods Function as Nanoscale Capacitors in Direct Photocharging Observation

Physicists have achieved a significant advance in photocatalysis by directly observing and modeling the photocharging mechanism within gold nanorods functioning as light-harvesting catalysts. This research resolves the long-standing question of how charge accumulates in illuminated metallic nanoparticles, establishing a concrete physical framework for optimizing light-driven chemical transformations. The findings, which characterize the illuminated nanorods as "photochemical capacitors," are expected to inform the development of sustainable energy technologies, including solar chemical reactors and novel energy storage systems.

The investigation was led by physicist Dr. Wouter Koopman at the University of Potsdam, with Dr. Felix Stete serving as the first author and scientific coordinator. The work is situated within the Collaborative Research Centre SFB 1636, a program funded by the German Research Foundation (DFG) that began in 2024 to explore elementary processes in light-driven reactions at nanoscale metals. The core observation details that upon light exposure, the nanorods—acting as microscopic antennas converting light into collective electron oscillations—generate electron-hole pairs. While the holes are transferred to surrounding molecules, such as ethanol, the electrons are retained on the particle surface, resulting in the observed photocharging effect.

This phenomenon enables the gold nanorods to establish electrical potentials between the particle and its environment solely through light absorption, a critical finding articulated by Dr. Stete. Dr. Koopman further elaborated that these particles operate analogously to nanoscale electrolyzers, capable of facilitating chemical splitting, such as water into hydrogen, but without requiring an external voltage input. The research team tracked this accumulation in situ by monitoring the sensitivity of the nanorods' longitudinal plasmon resonance to charge density, providing spectroscopic evidence that supports the nanoscale capacitor model for the process.

The established physical framework describes the charging process via a capacitor model where the voltage is governed by the chemical potential of holes from the gold's 5d-bands. This offers a pathway for targeted control over light-induced chemistry, moving plasmonic photocatalysis beyond kinetic enhancement to rational optimization of dynamic charge states. Furthermore, the investigation into intensity dependence confirmed that increasing incident light power accelerates the charging rate and increases the maximum achievable charge density, aligning with the capacitor model for applications in areas like CO2 reduction and water splitting.