Electron Dynamics of Plasmonic Light Harvesting Studied by Ultrafast Time-Resolved Ambient-Pressure X-ray Photoelectron Spectroscopy*

Heterogeneous light harvesting systems consisting of metal nanoparticles interfaced with transition metal semiconductors are among the most studied platforms for solar photon based approaches to sustainable energy supplies and climate change mitigation. Yet, it remains challenging to disentangle the fundamental electronic dynamics and mechanisms that drive the targeted photocatalytic activity, such as solar fuels production or CO₂ reduction. To address this challenge, we translate the atomic-scale sensitivity of X-ray photoemission spectroscopy (XPS) to interfacial electronic and chemical configurations into the ultrafast time-domain. Utilizing picosecond time-resolved ambient pressure XPS (TRAPXPS) at the Advanced Light Source synchrotron and femtosecond TRXPS measurements at the FLASH Free Electron Laser, we study photoinduced charge transfer dynamics in gold nanoparticle sensitized TiO₂ under ultrahigh vacuum conditions as well as under exposure to water. Dramatic differences between dynamics at dry and water-exposed interfaces indicate that conditions for photocatalytic activity of the interface improve substantially with the introduction of the reactant. First steps have been undertaken to complement the experiments by ab initio calculations and reveal the underlying mechanisms.