Optical properties of TiO2 thin films deposited by DC sputtering and their photocatalytic performance in photoinduced process

TiO2 thin films were deposited by DC Sputtering varying the deposit time. These films were characterized by XRD, AFM, photoluminescence, UV-Vis, ellipsometry and XPS. The optical properties of TiO2 thin films with different thickness, influenced their photocatalytic behavior in two photoinduced process. When TiO2 thin films were irradiated with a UV light, midgap states were generated and the electrons were placed in lower energies than its band gap, favoring the photocatalytic hydrogen production and CO2 photoreduction. From PL technique analyses it was observed that electrons occupied midgap states between the bands, with lower energies than the band gap. With these results it was possible to propose an energy diagram in order to correlate with photoinduced processes results. The presence of Ti3+ species was reconfirmed by means of XPS analyses. These species could be found in the midgap states, generated by the interaction between the UV irradiation and the film surface, which contributed to the photocatalytic activity of the films. The hydrogen production was similar for all the thin films studied (33-35 mu mol) associated to the presence of similar energy midgap states. In the case of CO2 photoreduction, all films produced CH2O (8951 and 6252 mu mol/g) and the films with a thickness of 330 and 420 nm generated CH3OH (970 and 292 mu mol/g). The extinction coefficient confirmed the XRD results for the film with greatest deposited time, which exhibited the highest crystallinity. All photocatalytic results did not show any dependence with the thin film thickness. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.