Ultrafast Charge Transfer Dynamics in a Plasmonic Photocatalyst Studied by Femtosecond Time-Resolved X-ray Photoelectron Spectroscopy*

Heterogeneous interfaces between metal nanoparticles (NPs) and transition metal semiconductors (SCs) attract significant attention due to their potential to provide routes to renewable, carbon-free energy production and storage through processes such as photocatalytic hydrogen generation from water. A particularly intensely studied model system for photochemical water splitting consists of plasmonically active, spherical gold nanoparticles (AuNPs) attached to nanocrystalline TiO₂ substrates. Despite significant efforts, the overall efficiencies of plasmonic light harvesting systems generally remain rather low, requiring a deeper understanding of the fundamental dynamics underlying their function. Currently, charge transfer (CT) between metal NPs and semiconductors (SC) is commonly described by either plasmon-induced hot-electron transfer (PHET) or plasmon-induced metal-to-semiconductor interfacial charge transfer transitions (PICTT). To gain a deeper understanding of these competing processes, we have measured the ultrafast interfacial charge injection and recombination dynamics for AuNP sensitized TiO₂ at the FLASH Free Electron Laser in Hamburg under UHV and water exposed conditions via femtosecond Time-resolved X-ray Photoelectron Spectroscopy (TRXPS). The results will be discussed within a kinetic model taking into account various CT and relaxation channels. The model is complemented by high-level ab-initio calculations based on constrained density functional theory.