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The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously^,,,,,,,, but the use of photocatalysts to split water into stoichiometric amounts of H₂ and O₂ (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation^,,,. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga_1−xZn_x)(N_1−xO_x)) do not exhibit good activity beyond 440 nm (refs ,,) and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%^,. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized …