Applying the principle of water decomposition over photoelectrochemical cells to heterogeneous photocatalytic systems using powdered semiconductors is now a field of great interest, encouraging new fundamental investigations into chemical reactions which take place at the electrode surfaces in the electrochemical cells. In the present review, we have focused on systems, which can convert solar energy into chemical energy by using TiO₂ photocatalysts. The photocatalytic decomposition of water under UV light irradiation has been achieved with systems using various nanoparticle photocatalysts such as TiO₂. However, recently, visible light-responsive TiO₂ thin films photocatalysts have been successfully prepared by a radio frequency magnetron sputtering (RF-MS) deposition method. These thin film photocatalysts were found to have enough potential for the separate evolution of H₂ and O₂ from water under solar light. The TiO₂ thin films were prepared on metal substrates by RF-MS deposition and mounted on H-type containers filled with water. This unique system enabled the separate evolution of H₂ and O₂ from water under sunlight irradiation, opening new opportunities for the practical on-site production of pure and clean H₂ from water using abundant and clean sunlight in a safe, environmentally harmonious way.