A thin film anatase titania photocatalyst having 60% quantum efficiency measured without applied bias potential can produce hydrogen from water at rates of 26 with bias by a mercury lamp as a light source and 0.075 to 0.42 without bias by the sunlight both in units of 1/h/m(2)-irradiation cross section on photocatalyst surface and a single unit photoelectrocatalysis system can be formed with the photocatalyst and an electrocatalyst consisting of zinc oxide and copper which when combined together can stably convert 82% of carbon dioxide to hydrocarbons in terms of current efficiency including 44% for methane and 24% for ethylene without deactivation of the reactions by operating the system under a pulsed bias between the photocatalyst and the electrocatalyst.