We assumed that the increase of Ti 3+ state could affect the electronic structure of the a-TiO2 state. Therefore, valence state XPS measurement was carried out to investigate the distinct structure of hybrid PEI/TiO2 compared to normal a-TiO2. BiVO4/TiO2 exhibited a valence band composed of O 2p orbitals and a defect state was observed below 1.5 eV from the Fermi level, with a width of 0.75 eV (Supplementary Fig. ). These values coincide with the valence band and "leaky" state reported in previous studies. However, BiVO4/PEI/TiO2 showed a widened defect which was twice as broad as that in normal a-TiO2 (Supplementary Fig. ). We conclude that the wide defect state is due to a reduction of Ti 4+ by incorporated nitrogen. The long absorption tail observed in UV-Vis spectroscopy also supports the presence of the reduced Ti 4+ state, as suggested in previous reports (Supplementary Fig. ). Based on these results, we propose that the partial reduction of Ti 4+ state in hybrid PEI/TiO2 widens the defect band, thereby enhancing conductivity through increased density of states (DOS) in TiO2.