Effects of film topology and contamination as a function of thickness on the photo-induced hydrophilicity of transparent TiO2 thin films deposited on glass substrates by spin coating

Transparent TiO2 films (~50–540 nm thick) were spin-coated in multiple cycles by sol–gel on soda-lime-silica glass and annealed at 450 °C for 2 h. Analyses included mineralogy (XRD), optics (UV–Vis), chemistry (XPS), microstructure and topography (SEM, AFM), and hydrophilicity (sessile drop). The microstructures were discontinuous (1 cycle), continuous (4–11 cycles), and damaged (13–15 cycles), with the highest quality films being those fabricated using 7–11 cycles. The thickest films were damaged as a result of shrinkage during annealing, which may have occurred from a two-stage process of dehydroxylation followed by pyrolysis. The single-crystal grain sizes were ~27 nm for the films and the roughness increased with increasing thickness. The hydrophilicity showed clear correlations with the roughness and surprisingly, not with the Ti3+, –OH, and Na+ contents. The role of the glass also was critical to the absorption/transmission of radiation as well as the wetting. That is, the discontinuous and damaged films showed high wetting angles and the continuous films showed low wetting angles. Correlation was observed between the amounts of different contaminants from glass and the free energies of formation for TiO2 to Ti2O3 under the influence of these contaminants. This relation was attributed to the driving force for oxide bond formation, where the only slightly negative free energy of reaction with Na+ allowed its relatively free diffusion through the microstructure until it achieved saturation solubility, although all of the main glass components effectively achieved saturation solubility under the modest kinetics of equilibration that were used.