Effect of tungsten-doping on the properties and photocatalytic performance of titania thin films on glass substrates

Highly-transparent TiO₂ thin films with varying W-dopant levels (0.010-0.100 mol%) were deposited on soda-lime-silica glass substrates by spin coating, followed by annealing in air at 450°C for 2 h. Characterization was by GAXRD, AFM, SIMS, XPS, UV-vis, and MB degradation. The films were comprised of single-crystal anatase grains. Increasing doping levels increased the crystallite sizes, grain sizes, and film thicknesses; they also changed the grain morphology from anhedral to euhedral. W⁶⁺ ions formed a substitutional solid solution in anatase and probably exhibited charge compensation by Ti vacancy formation, which led to lattice contraction. Contamination from the glass substrates was homogeneously distributed on the grain boundaries. Doping decreased the optical indirect band gap. However, at the highest doping level, it is not certain if this was an artificial effect of precipitation from overdoping or a true effect of lattice stress. The 0.010 mol% W sample showed the best performance probably owing to its formation of a homogeneous solid solution within the range of semiconducting properties, which could have narrowed the band gap or introduced mid-gap states. The 0.100 mol% W sample may have been overdoped and caused WO₃ precipitation. If so, the presence of the TiO₂-WO₃ couple may have enhanced charge separation and hence photocatalytic performance.