Photocatalytic TiO2 sol–gel thin films: Optical and morphological characterization

• TiO2 thin films have been prepared onto glass substrates by dip-coating deposition.
• Optical, morphological and structural changes with temperature have been analyzed.
• A rigorous method was developed for the optical and morphological characterization.
• Samples annealed at 500 °C show small anatase crystals and highly porous network.
• They show the lower electron–hole recombination rate within the annealing series.

TiO2 thin films have been deposited onto glass substrates by sol–gel dip coating technique. The morphological and optical characterization of the as-deposited and annealed films at different temperatures in the range 150–700 °C have been carried out by UV–vis spectroscopy and ellipsometry. An original rigorous optical characterization method based on both the transmission spectrum of a tri-layer optical system and the Tauc-Lorentz dispersion model, has been developed and successfully applied to calculate the thickness, optical constants, absorption edges and optical bandgaps of the dip-coated TiO2 films. Micro-Raman spectroscopy and thermoanalytical measurements have been also performed to complete the characterization of the films and support the morphological and optical results. As-deposited films have been proved to be amorphous and the onset of the formation of anatase phase has been found at annealing temperature of 400 °C. The occurrence of anatase-phase and the increase of crystal size with increasing annealing temperature have been both evidenced by Raman spectroscopy measurements performed in the samples. Such evolution has been accompanied by larger values of the refractive index and a slight decrease in the values of the optical bandgap. Results indicate that samples annealed at 500 °C are organic residues free, and they shows small anatase crystals, highly porous network, as well the lower electron–hole recombination rate within the annealing temperature series.

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