Determining Quasi-Equilibrium Electron and Hole Distributions of Plasmonic Photocatalysts using Photomodulated X-ray Absorption Spectroscopy*

Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices are usually measured using ultrafast pulsed lasers, and the quasi-equilibrium properties are then estimated from the carrier rates and lifetimes. In this work, we prove that steady-state, photomodulated x-ray spectra from a non-time-resolved synchrotron beamline can be used to directly measure electron and hole distributions. A set of plasmonic metal core-shell nanoparticles is designed to systematically isolate photothermal, hot electron, and thermalized electron-hole pairs in a TiO₂ shell. Steady-state changes in the Ti L_2,3 edge are measured with and without continuous-wave illumination of the nanoparticle’s localized surface plasmon resonance. Ab initio excited-state x-ray theory is then applied to model the experimental spectra and extract the steady-state carrier distributions and lattice temperature. For example, we measure that the quasi-equilibrium hot electron distribution exists up to 0.3 eV above the TiO₂ conduction band minimum. The ability to separate heating and quasi-equilibrium carrier distributions from mixed excited-state phenomena opens new avenues for in-situ and operando measurements from non-time-resolved beamlines.