SOL-GEL-DERIVED TIO2 FILM SEMICONDUCTOR ELECTRODE FOR PHOTOCLEAVAGE OF WATER - PREPARATION AND EFFECTS OF POSTHEATING TREATMENT ON THE PHOTOELECTROCHEMICAL BEHAVIOR

The effects of postheating on the photoelectrochemical behavior of TiO2 film electrodes prepared by the sol-gel method have been investigated over a heating temperature range of 600-900-degrees-C. TiO2 in the sol-gel-derived films were anatase on heating below 800-degrees-C and transformed into rutile. It was found that the photocurrent shows a maximum at a certain heating time for each of the heating temperatures at 900-degrees-C. At the same time, other properties such as the flatband potential shift and donor density also showed a maximum at the corresponding heating time. These results are explained as follows. An increase in the photocurrent of the as-prepared TiO2 film electrode with increasing heating time results from the improvement of crystallinity and the increasing Ti3+ concentration due to the reduction of Ti4+ by the undecomposed organic residue. Further heating, however, causes a decrease in the specific surface area and Ti3+ concentration due to the reoxidation of Ti3+ to Ti4+, resulting in the reduction of the photocurrent. TiO2 (anatase) has only an indirect bandgap at 3.03 eV, while TiO2 (rutile) has an indirect bandgap at 2.92 eV and a direct bandgap at 3.13 eV.