Comparison between titania thick films obtained through sol–gel and hydrothermal synthetic processes

Titania films were obtained through two synthetic processes, a traditional sol–gel method and a hydrothermal route. In SG synthesis, thermal decomposition of the precursor in oven at 400 °C for 2 h led to pure anatase TiO2; in HY synthesis, instead, crystalline anatase grains were obtained in autoclave at 200 °C for 1 h. To investigate the microstructural evolution of SG and HY titania with temperature, each powder was annealed at 650, 750, 850 °C for 1 h and subjected to XRD analysis. Surprisingly, HY titania, contrary to SG, maintained the anatase phase, up to 850 °C, without any introduction of foreign elements. The sensing layers, obtained from as grown powders, were fired at 650, 750 or 850 °C and tested vs. methane and carbon monoxide. Both types of films fired at 850 °C yield insignificant responses to both CO and CH4, demonstrating the lack of influence of the crystalline phase on the gas response. Moreover, as regards the films fired at 650 and 750 °C, the gas responses are higher for SG than for HY samples, despite larger particle size.