This talk will report the properties of TiO₂-based photosensitive oxide semiconductors that may be used as photoelectrodes for the generation of solar-hydrogen by water splitting using solar energy. It will be shown that defect engineering may be applied to convert commercial TiO₂ into well defined TiO₂, which exhibits a wide range of electrical properties, including n- and p-type semiconducting charge transport, metallic conduction, and mixed conduction. These TiO₂-based materials may be used as components of photoelectrodes for the conversion of solar energy into chemical energy required for photoelectrochemical water splitting and other environmentally friendly applications, such as water purification.