Time-resolved monitoring of photoinduced charge transfer dynamics in wet nanoplasmonic light-harvesting systems using ambient pressure x-ray photoelectron spectroscopy

Photoelectrochemical (PEC) water splitting is an attractive method to produce hydrogen fuel because it is clean and renewable; however, materials for efficient PEC systems are not economically viable at industrial scales compared to non-renewable methods. One option to improve solar light harvesting and charge separation in the photocatalyst is to use plasmonic metal nanoparticles (NPs), which are particularly useful 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). The technique allows monitoring charge dynamics on the electron donor and acceptor sites separately and in real-time. Measurements are performed under two conditions: a high vacuum sample environment and a water vapor background pressure of ~9 Pa. 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 Once water vapor is introduced, charges remain separated for up to ~2 orders of magnitude longer timescales and exhibit more complex recombination dynamics compared to the high vacuum case. A simple physical model is used to interpret the effect that the water environment may have on extending charge recombination lifetimes.

1 Borgwardt, M. et al. J. Phys. Chem. Lett. 11, 5476-5481 (2020)