Decoupling the Roles of Ligand-Field and Charge-Transfer Excitations on the Photocarrier Yield in Transition-Metal Oxides: Insights from MTiO3 (M = Ni, Co, Mn).

Open d-shell transition-metal oxides are widely studied as photoactive materials for solar-energy conversion but typically exhibit low yields of mobile photocarriers. This inefficiency has been linked to ligand-field (LF) states that induce ultrafast relaxation, although their impact across open d-shell oxides remains poorly understood. Here, epitaxial ilmenite-type MTiO3 (M = Ni, Co, Mn) thin films were stabilized by pulsed laser deposition. In this isostructural series, the M2+ cation tunes the d-electron configuration and the energetic separation and spectral overlap of LF and charge-transfer excitations. Combined optical and time-resolved microwave conductivity measurements, benchmarked against rutile TiO2 as a d0 reference, together with photoelectrochemical measurements of the ilmenites, reveal reduced yield-mobility products compared to TiO2. NiTiO3 and CoTiO3 show measurable photoconductivity only under UV charge-transfer excitation, whereas MnTiO3, dominated by a dense LF manifold, exhibits a negligible response. These results indicate that mobile charge-carrier generation contributing to photoelectrochemical activity originates from UV charge-transfer excitations, likely dominated by O2- → Ti4+ ligand-to-metal charge-transfer (LMCT) excitation into Ti 3d-derived states, while LF states primarily act as relaxation pathways that do not yield mobile carriers on nanosecond time scales. These findings decouple the roles of distinct optical excitations in governing photocarrier localization in open d-shell oxides.