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

Titanium dioxide thin films prepared with and without lithium and niobium were as follows: uniform, crack-free, and stoichiometric, amorphous as-deposited at 300 degrees C and below; polycrystalline anatase when deposited at 400 degrees C or annealed at 500-800 degrees C; and rutile when annealed at 900 degrees C. Films prepared around 200 degrees 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 de conductivity of undoped films was lower than 10(-10) (Ohm.cm)(-1) Hall effect measurements indicated that effective electron mobility was below 1 cm(2)/(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 10(6) 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.