Effect of compressive stress inducing a band gap narrowing on the photoinduced activities of sol-gel TiO2 films

TiO₂ thin films grown on different kinds of substrates were obtained by sol-gel process. X-ray diffraction revealed that the TiO₂ lattice parameter c decreased continuously, indicating a continuous variation in the compressive stress, a negligible compressive stress of the film grown onto Soda-Lime Glass (SLG), medium compressive stress of the film grown onto BoroSilicate Glass (BSG) and large compressive stress of the film deposited onto the Quartz Substrate (QS). UV-Vis absorbance spectra exhibited a red-shift of the absorbance edge of the TiO₂ films suggesting a lowering of the band gap, which is a direct consequence of the increase of the compressive stress. X-ray photoelectron spectroscopy revealed that the surface composition of titania films was similar except for sodium-ion concentration. The rate observed during the photo-oxydation of the stearic acid on TiO₂/QS was twice as high as that of TiO₂/BSG and about 1000 times superior to that of TiO₂/SLG. The photoinduced wettability shows an identical dependence of the compressive stress. According to these results, the compressive stress could be used to tune the band gap of the titanium oxide in order to enhance the photoinduced properties.