Photostability study of nanoporous TiO2 film electrodes in different pH solutions

The nanoporous TiO2 film electrodes have been prepared by a sol–gel deposition process. The photostability of the electrodes at different pH values have been studied. The results show that the photostability of the electrodes decreases with increasing solution pH, especially in strong basic solutions without methanol. The increase of photocurrent and the decrease of onset potential with solution pH can be attributed to the increase of the adsorbed HO− ions. The adsorbed HO− ions act not only as electron donors but as also potential-determining ions on TiO2 film surface. However, in basic solutions the adsorbed O2− anions on TiO2 surface will form as pH value increases. The reaction of holes to the adsorbed O2− anions produces active O− anions. In the solutions without enough methanol the products of active O− anions oxidize Ti3+ to Ti4+ on TiO2 film surface and subsurface to occupy gradually the oxygen vacancies as electron donors. This results in the nanoporous TiO2 film electrodes being unstable with increasing pH.