Assigning Surface Hole Polaron Configurations of Titanium Oxide Materials to Excited-State Optical Absorptions

For water splitting, a comprehensive understanding of the underlying reaction intermediates and pathways is crucial for optimizing catalyst design. Among the most well-known active photoanodes for the oxygen evolution half-reaction are TiO₂-based materials. A hole polaron, which consists of a metal-oxide distortion around trapped holes, has been suggested as a local reactive oxygen configuration. While first-principles calculations identify new electronic states in the middle of the band gap and the influence of trapped hole 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. Here, we compare the excited-state absorption (ESA) for two titanium oxide materials with a similar electronic structure but differing crystal structure. The ESA maximum for ultrafast time scales (<1 ps) is isolated by a principal component analysis and shifts from 3.1 eV in rutile TiO₂ (100) to 2.2 eV in perovskite SrTiO₃. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations predict the energies of the midgap states for stable hole polarons and their corresponding spectra. The shift in the ESA is rationalized by the transition optical dipole originating from both edge and deeper states in the valence band being bright for certain configurations of hole polarons in rutile TiO₂ (100) (terminal O) versus STO (lateral Ti₂O^•-). The spectral assignment of a shifting ESA between two titanium oxide materials informs the assignment of hole polaron configurations for oxygen evolution catalysis and, more generally, photodriven processes.