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

Abstract

Regarding how photogenerated charge carriers are transferred in TiO2(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 TiO2(B) by comparing EPR-detected amounts of trapped hole O– accumulated on TiO2(B) with X-ray diffraction (XRD)-determined TiO2(B) bulk phase compositions. Under visible-light irradiation which only activates the TiO2(B) phase, we unambiguously show that electron transfer occurs from TiO2(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 TiO2(B) with holes localized on the N atom in N-doped TiO2(B)/anatase. The result again clearly demonstrates that electrons migrate from TiO2(B) to anatase. Time-dependent decay of the N•-hole EPR signal shows that it is difficult for the transferred electron to return to TiO2(B). Both higher conduction band and valence band edge potentials in TiO2(B) than the corresponding ones of anatase are implicated. This study helps to point the way toward future development of TiO2(B) nanowire based material for photovoltaic and photocatalytic applications.