Comparison of Photocatalytic Activity: Impact of Hydrophilic Properties on TiO2 and ZrO2 Thin Films

Thin films (TFs) of TiO2 and ZrO2 were prepared and characterized to evaluate their structural and optical (SO) properties and, later, to test their efficiency for the photocatalytic degradation (PD) of methylene blue (MB) in aqueous solution. The X-ray diffraction patterns showed that the TiO2 TFs had an anatase crystalline structure, unlike the ZrO2 TFs, which showed a tetragonal crystalline structure that was verified by Raman spectroscopy. The band gap (BG) energies, as calculated from UV-Vis spectroscopy and diffuse reflectance spectroscopy, corresponded to 3.2 and 3.7 eV for the TiO2 and ZrO2 TFs, respectively. SEM examination of the obtained materials was also carried out to assess the surface morphology and topography. The comparative study of the FTIR spectra of the TiO2 and ZrO2 TFs successfully confirmed the composition of the two-metal oxide TFs. The electrical properties of the films were studied by conductivity measurements. The two films also showed a similar thickness of about 200 nm and a substantially different photocatalytic performance for the discoloration of MB in aqueous solution. The corresponding rate constants, as obtained from a pseudo-first-order kinetic model, revealed that TiO2 films promote color removal of the model dye solution almost 20 times faster than the rate observed for ZrO2 modified glass substrates. We suggest that this difference may be related to the hydrophilic character of the two films under study, which may affect the charge carrier injection process and, therefore, the overall photocatalytic performance.