UV Photoconductivity of Nanocrystalline TiO2 Films Annealed in Oxygen

Abstract

The effects of annealing in reactive gases (H2, N2, O2) upon the optoelectric properties of nanophased titanium dioxide (TiO2) prepared by chemical vapour deposition (CVD) were investigated. The nanocrystalline structure containing nanosize grains and pores was analyzed by grazing-incidence small-angle scattering of synchrotron radiation (GISAX). The annealing (up to 1073 K) in H2 and N2 generally proved detrimental to photoconductivity due to the overall increase of the electrical conductivity of the samples. In this work, the result of UV (248–404 nm) photoconductivity measurements on TiO2 films annealed in O2 at 773 K and 1073 K are presented. A rather long illumination time (typically 2 h) enabled us to clearly distinguish two types of nonequilibrium photoconductivity variations with time. A fast exponential photoconductivity increase occurred during the initial stage of irradiation, while a slow power–type increase was observed in the later stage. A nonlinear combination of both functions was used in a numerical fitting procedure, which allowed precise determination of the asymptotic value of exponential photoconductivity increase. A relative quantum efficiency for both as-prepared and annealed samples exhibits a nonmonotonic variation with photon energy. Such wavelength-dependence variation might be due to the electronic density function structure at the valence-band edge, or near-valence-band levels in the gap. Generally, the samples annealed at higher temperatures exhibit a higher quantum efficiency and shorter time constants of the excitation processes, in the examined UV range.