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Abstract
Titanium dioxide (TiO2) thin films were deposited on fluorine-doped tin oxide (FTO) glass, soda-lime-silica glass, and fused quartz substrates by spin coating. The film mineralogies were determined by glancing angle X-ray diffraction (GAXRD) and laser Raman microspectroscopy. Field emission scanning electron microscopy (FESEM) was used to analyse the film morphologies. The chemical compositions of the films were investigated using X-ray photoelectron spectroscopy (XPS). The optical properties (transmittance and optical indirect band gap) were assessed using UV-VIS spectrophotometry.
In the first set of samples, undoped TiO2 films were coated on fused quartz substrates and annealed at 750° to 900°C. These films were fully dense, ~400 nm thick, and consisted of agglomerated grains. The crystallinity changed from anatase → anatase + rutile →rutile with increasing temperatures. The results confirmed that the counter-diffusion of Ti into the SiO2 substrate occurred leading to a stabilisation of anatase and delay in its transformation to rutile. The transmittance and the optical indirect band gap decreased with increasing temperature (3.05 eV at 750°C to 2.30 eV at 900°C).
In the second set, TiO2 thin films were doped with iron (Fe) and manganese (Mn) in concentrations of 1, 3, 5, and 7 wt% (metal basis relative to Ti), and deposited on both FTO and soda-lime-glass substrates, followed by annealing at 500°C. These films were fully dense and ~500 nm thick. In the Fe-doped films, anatase was the major phase and its crystallinity decreased with increasing doping owing to lattice expansion. The dopant was present as Fe3+ and with increasing doping, the optical indirect band gap decreased from 3.36 eV for undoped to 2.95 eV for 7 wt% Fe.
In Mn-doped TiO2 films, the major phase was anatase and its crystallinity decreased with increasing doping owing to lattice expansion. Mn3+ and Mn4+ were the valence states in the films and these can generate shallow trapping sites at the donor and acceptor levels, and thus lower the optical indirect band gap (3.32 eV for undoped and 2.90 eV for 7 wt% Mn). Lowering the band gap can improve the photocatalytic activity, provided the changes in crystallinity are minimal.