Thin films of aligned hexagonal WO3 nanotrees were fabricated on metal tungsten substrates by hydrothermal treatment. In the presence of a sacrificial reduction agent, the fabricated WO3 nanotree films oxidized water to evolve oxygen under visible-light irradiation. The photocatalytic water oxidation activity of the WO3 nanotree was higher than that of a dense monoclinic WO3 electrode, and had a quantum efficiency of 3.2% under visible-light irradiation. With the application of an anodic bias potential, the WO3 nanotree electrode produced both hydrogen and oxygen in the absence of a sacrificial agent under visible-light irradiation. (C) 2015 Elsevier B.V. All rights reserved.