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

Heterogeneous light harvesting systems consisting of metal nanoparticles interfaced with wide bandgap transition metal semiconductors are among the most intensely studied platforms for creating a sustainable energy supply and developing climate change mitigation strategies. Yet, it remains challenging to disentangle the fundamental electronic dynamics and mechanisms that drive the desired photocatalytic activity. 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. Using femtosecond time-resolved XPS (TRXPS) at the WESPE endstation of the FLASH Free Electron Laser in Hamburg, we study photoinduced charge transfer dynamics in gold nanoparticle sensitized TiO₂ under ultrahigh vacuum conditions as well as under exposure to water. The measurements provide interfacial site-specific real-time information of the charge buildup during the first ~ps and recombination dynamics up to nanosecond timescales. We will discuss an empirical model capturing the multi-scale interfacial charge dynamics and complementary efforts to gain a deeper understanding of the underlying physics through high-level ab initio electron dynamics calculations.