In-situ probing of photoinduced charge transfer dynamics in nanoplasmonic light-harvesting systems using time-resolved ambient pressure x-ray photoelectron spectroscopy

Photocatalytic production of hydrogen fuel via photoelectrochemical (PEC) water splitting is a clean and renewable process that is both less efficient and economically viable than non-renewable methods. To improve solar light harvesting and charge separation in the photocatalyst, plasmonic metal nanoparticles (NPs) are used to sensitize wide-bandgap semiconductors due to their chemical stability and strong absorption at visible wavelengths. Here, a model system of 20 nm gold NPs atop a layer of TiO2 is studied with picosecond time-resolved ambient pressure x-ray photoelectron spectroscopy (TRAPXPS), which allows monitoring of charge dynamics on the electron donor and acceptor sites separately. Measurements are performed under high vacuum conditions and with ~9 Pa of water vapor. In vacuum, a charge injection efficiency of ~2 electrons per NP (~0.1% photon-to-charge efficiency) is observed, followed by recombination over two timescales: 60 ps and ~1 ns.1 After introduction of water vapor, charges remain separated for longer, and relaxation is now described by 3 timescales: ~0.9 ns, ~11 ns, and ~150 ns. Possible physical interpretations, including the role of water dipoles in extending recombination, will be discussed.
1 Borgwardt, M. et al. J. Phys. Chem. Lett. 11, 5476–5481 (2020)