Electron Paramagnetic Resonance Investigation of Charge Transfer in TiO2(B)/Anatase and N–TiO2(B)/Anatase Mixed-Phase Nanowires: The Relative Valence and Conduction Band Edges in the Two Phases

Regarding how photogenerated charge carriers are transferred in TiO 2 (B)/anatase mixed-phase nanowires, no unified conclusion has been reached. Electron paramagnetic resonance (EPR) spectroscopy is employed to investigate the vectorial charge transfer in this material. When the material is subjected to UV irradiation, we show that holes stimulated in anatase are transferred to TiO 2 (B) by comparing EPR-detected amounts of trapped hole O – accumulated on TiO 2 (B) with X-ray diffraction (XRD)-determined TiO 2 (B) bulk phase compositions. Under visible-light irradiation which only activates the TiO 2 (B) phase, we unambiguously show that electron transfer occurs from TiO 2 (B) to anatase. Without intervention of other charge carriers generated by bandgap excitation, we monitor exclusively the fate of conducting electrons generated by specific excitation of N – midgap states of TiO 2 (B) with holes localized on the N atom in N-doped TiO 2 (B)/anatase. The result again clearly demonstrates that electrons migrate from TiO 2 (B) to anatase. Time-dependent decay of the N • -hole EPR signal shows that it is difficult for the transferred electron to return to TiO 2 (B). Both higher conduction band and valence band edge potentials in TiO 2 (B) than the corresponding ones of anatase are implicated. This study helps to point the way toward future development of TiO 2 (B) nanowire based material for photovoltaic and photocatalytic applications.