Electronic Structures and Photoanodic Properties of Ilmenite-type MTiO3 Epitaxial Films (M = Mn, Fe, Co, Ni)

We performed systematic study on electronic structures and photo electrochemical (PEC) properties of ilmenite-type MTiO3 (M = Mn, Fe, Co, and Ni) thin-film photoanodes for water-oxidation reactions. Single-phase MTiO3 films were grown on alpha-Al2O3 substrates by using pulsed-laser deposition. Formation of the ilmenite-type structures and oxidation states of M2+Ti4+O3 were revealed by X-ray diffraction and X-ray absorption spectroscopy, respectively. The PEC performance in water was investigated by linear sweep voltammetry under Xe-lamp illumination and incident photon to current effciency measurements under monochromatic illumination. We found that NiTiO3 acted as the most effective photoanode among MTiO3. The mechanism of the different PEC performance was discussed from the viewpoint of the electronic structures investigated by photoemission spectroscopy and band calculation based on density functional theory. We revealed that the stronger hybridization between O 2p and M 3d states was responsible for the more efficient photoanodic activity among ilmenitetype MTiO3.