Tungsten trioxide (WO3) prepared by anodization of a W foil was doped with NH4 through NH4OH treatment at 450 degrees C. Since aqueous NH4OH was used during doping instead of NH3 gas, the treatment step does not require complicated annealing facilities. Moreover, the state of doped N is a form of NH3-W instead of W2N, which lowers the bandgap but increases photocorrosion. We found that incorporation of NH4 into WO3 leads to reduction of the bandgap from 2.9 eV to 2.2 eV, regardless of the amount of NH4OH treatment, lowering the onset potential and increasing the current density at fixed potential for oxygen evolution reaction under illumination. Scanning electron microscopy, X-ray diffraction and Xray photoelectron spectroscopy were employed to investigate the surface morphologies, crystallinities of tungsten oxides and existence of NH4 doping, respectively. The bandgap energy was determined by UV-Vis spectroscopy to measure the transmittance and refraction. The water splitting performance of each sample was measured by electrochemical linear sweep voltammetry in a 3-electrode configuration under illumination. (C) 2014 Elsevier B.V. All rights reserved.