Photoefficiency and optical, microstructural, and structural properties of TiO2 thin films used as photoanodes

The present paper tries to determine, for TiO2 in the form of thin films, the influence of sample characteristics such as crystallinity and microstructure on photoefficiency. TiO2 thin films have been prepared by ion beam induced and plasma enhanced chemical vapor deposition at room temperature and 523 K. The thin films have also been annealed at increasing higher temperatures to modify their structural and microstructural characteristics. Depending on the preparation protocol and annealing treatments, thin films have been prepared with different crystallographic structure, texture, and microstructure, as determined by X-ray diffraction and scanning electron microscopy. Nonstoichiometric thin films with a high concentration of Ti3+ centers have been also prepared by using an oxygen-poor plasma gas during plasma enhanced chemical vapor deposition. The rutilization behavior of the anatase thin films after annealing at a high temperature has also been investigated. This study has shown that rutilization only starts at 1023 K for the most compact films, while it requires a slightly higher temperature for the more porous thin films. The optical properties of the films have been analyzed by UV-visible absorption spectroscopy and ellipsometry. The photoefficiency of the TiO2 thin films used as photoanodes has been measured in a photoelectrochemical cell, and the results have been correlated with the different sample characteristics. From this comparison, a clear picture arises, showing that the anatase structure, a high porosity, and a columnar-type growth of the TiO2 thin films are beneficial factors favoring the photoefficiency of the system.