Ultrafast Electron Dynamics in a Heterogeneous Plasmonic Photocatalyst Studied by Time-Resolved X-ray Photoelectron Spectroscopy

Heterogeneous light harvesting systems consisting of metal nanoparticles interfaced with transition metal semiconductors are among the most intensely 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 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 femtosecond and picosecond time-resolved XPS (TRXPS) at the FLASH Free Electron Laser and the Advanced Light Source (ALS) synchrotron, respectively, we study photoinduced charge transfer dynamics in gold nanoparticle sensitized TiO₂ under ultrahigh vacuum conditions as well as under exposure to water. Pronounced, systematic differences between dynamics at dry and H₂O-exposed interfaces indicate that conditions for photocatalytic activity of the interface improve substantially with the introduction of the reactant. We will discuss the experimental findings in light of first results of ab initio calculations aimed at revealing the underlying physical mechanisms.