Trap-state distributions and carrier transport in pure and mixed-phase TiO2:: Influence of contacting solvent and interphasial electron transfer

We report the room-temperature photoluminescence spectra of nanocrystalline TiO2 in the anatase and rutile phases and in mixed-phase samples obtained commercially (Degussa P25) and by thermal treatment of nanocrystalline anatase. The photoluminescence spectrum of anatase spans a broad range of visible wavelengths, while the much more intense rutile emission is found in the near-infrared. Photoluminescence spectra as a function of contacting fluid provide insight into the microscopic nature of the luminescence, the basis for its breadth, and the influence of solvent on inter- and intraparticle electron transfer. Anatase photoluminescence results from at least two spatially isolated trap-state distributions, one of which is absent or quenched in P25 and in the presence of hole scavengers. TiO2 nanocrystalline films containing a small amount of rutile show solvent-dependent relative intensities of the anatase and rutile photoluminescence that reveal carrier transport between the two phases. Photoluminescence spectroscopy is shown to be a useful approach for determining the energetic distribution of midband gap states.