We report a systematic study of Si|ZnO and Si|ZnO| metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n(+)-ZnO) and Si|ZnO(N(2)) (thin films prepared in N(2)/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.