First principles identification of hole-polaron configurations and optical signatures in titanium oxide photoanodes

For water splitting, a comprehensive understanding of the underlying reaction intermediates and pathways is crucial for optimizing existing materials or discovering novel ones. Among the most well-known active photoanodes for the oxygen evolution half-reaction are TiO2-based materials. A hole polaron has been suggested as a local reactive oxygen configuration for the oxidative reaction. While first-principles calculations have identified new electronic states in the middle of the band gap due to hole polarons and the influence of hole polaron dynamics on transport, an assignment of hole-polaron configurations to a measured spectrum has been challenging due to broad optical transitions in the visible regime. We combine ultrafast pump-probe spectroscopy with first-principles calculations to study two titanium oxide materials. A principal component analysis isolates the ESA generated at ultrafast timescales in the pump-probe spectroscopy. Using DFT and TD-DFT, we identify stable hole-polaron configurations and simulate their spectral signatures. We then study the vibrational broadening and hopping behavior of the polarons at room temperature using ab-initio molecular dynamics. Our computations help explain the observed shift in excited-state absorption between the two materials and highlight how different crystal structures influence polaron behavior.