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Abstract
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 various 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 for the oxidative reaction. While first principal calculations identify 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. Here, we compare the excited state absorption (ESA) for two titanium oxide materials with a similar electronic structure, but different titanium-oxide crystal structure and find that the maximum in the broad spec-trum shifts from 3.1 eV in rutile TiO2 (100) to 2.2 eV in perovskite SrTiO3. A principal component analysis isolates the ESA generated at ultra-fast timescales (< 1 ps) in the pump-probe spectroscopy. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations predict the energies of the mid-gap states for stable hole-polarons and the corresponding spectra from the valence band, respectively. When compared to the experimental spectra, we can rationalize the shift in the ESA by the transition optical dipole, considering both the edge and deeper states in the valence band, being bright for certain configurations of hole polarons in rutile TiO2 (100) (terminal O•-) and STO (lateral Ti2O•-). The spectral assignment of the ESA in two transition metal oxides materials via their distinctive crystal structures paves the way for the assignment of hole-polaron configurations and their dynamics in other transition metal oxides for oxygen evolution catalysis and more-generally, photo-driven processes.
Supplementary materials
Title
Supplementary Information
Description
Parameterization of the hybrid functional; Additional spectroscopic details including ellipsometry data, OC vs. CC dataset comparison, and influence of probe polarization on TiO2 TR spectra; Detailed de-scription of SVD approach to data analysis; Large doping dependent TiO2 and STO datasets with cor-responding constrained SVD analysis; Constrained SVD analysis on large, pH dependent STO datasets in OC and CC conditions; Determination of spectral maximum in TiO2 and STO; Polaron binding ener-gy and vertical transition level from DFT; Optical dipole of other surface polarons; Valence band states contributing to vertical transition; Experimental spectra compared to less probable polaron transitions predicted by theory.
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