Contamination of TiO₂ thin films spin coated on rutile and soda–lime–silica substrates

Anatase thin films spin coated on soda–lime–silica (SLS) glass and unpolished (001) rutile were annealed at 200–550 °C for 8 h, followed by GAXRD, Raman, XPS, SIMS, AFM, TEM, UV–Vis, ellipsometry, and MB dye degradation. Films on SLS substrates annealed at 200–350 °C were amorphous but those annealed at 450°–550 °C consisted of anatase; the rutile substrates gave epitaxial rutile films. Annealing caused diffusion of glass ions into the films and counterdiffusion of Ti into the glass substrates. The decrease in glass ion concentrations during aqueous MB testing shows that grain boundary diffusion occurred. The AFM, TEM, and UV–Vis data were affected by the substrate topographies, where SLS was smooth and rutile was rough/uneven. Decreasing E_g in the anatase films with increasing annealing temperature was attributed to increasing crystallinity (heterogeneous nucleation) while the same effect in the rutile films was attributed to the substrate topography (homogeneous nucleation). The anatase films photocatalytically outperformed the rutile films; this was attributed to the potential to form midgap states from oxygen vacancy formation and/or Ti vacancy formation (deriving from the Ti counter-diffusion). However, the deep energy levels of these defects suggest that the performance is dominated by the crystallinity and blockage of the active sites.