We report a nickel oxide (NiO_x) thin film, from a cost-effective sol–gel process, coated n-type silicon (n-Si) as a photoanode for efficient photo-oxidation of water under neutral pH condition. The NiO_x thin film has three functions: (i) serves as a protection layer to improve the chemical stability of the Si photoelectrode, (ii) acts as an oxygen evolution catalyst, and (iii) provides junction photovoltage to further reduce overpotential. The oxygen evolution onset potential is reduced to below the thermodynamic water oxidation level and oxygen evolution was observed at low overpotentials. Our results demonstrate the fabrication of robust photoelectrodes from low-cost NiO_x and Si, which enable a practical solar water oxidation with high efficiency.