Spray pyrolysis preparation of porous polycrystalline thin films of titanium dioxide containing Li and Nb

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Abstract
Titanium dioxide thin films prepared with and without lithium and niobium were as follows: uniform, crack-free, and stoichiometric, amorphous as-deposited at 300 °C and below; polycrystalline anatase when deposited at 400 °C or annealed at 500−800 °C; and rutile when annealed at 900 °C. Films prepared around 200 °C were very porous, but the porosity decreased as the substrate temperature increased. Optical absorption spectra revealed an indirect bandgap of 3.0 eV for amorphous and anatase films, and a direct bandgap of the same value in rutile. Dark dc conductivity of undoped films was lower than 10−10 (Ω · cm)−1; Hall effect measurements indicated that effective electron mobility was below 1 cm2/(V · s). The presence of Nb and Li increased the conductivity by 2–3 orders of magnitude, similar to the effect of hydrogen annealing. Illumination increased the conductivity by several orders of magnitude, and the decay followed a multiexponential law extending into the 106 second range after irradiation was stopped. The electronic properties of the films were determined by oxygen-related surface states at grain boundaries. Samples containing Li exhibited considerable sensitivity to water vapor.