NH ₄ -doped anodic WO ₃ prepared through anodization and subsequent NH ₄ OH treatment for water splitting

Tungsten trioxide (WO ₃ ) prepared by anodization of a W foil was doped with NH ₄ through NH ₄ OH treatment at 450 °C. Since aqueous NH ₄ OH was used during doping instead of NH ₃ gas, the treatment step does not require complicated annealing facilities. Moreover, the state of doped N is a form of NH ₃ -W instead of W ₂ N, which lowers the bandgap but increases photocorrosion. We found that incorporation of NH ₄ into WO ₃ leads to reduction of the bandgap from 2.9 eV to 2.2 eV, regardless of the amount of NH ₄ OH 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 X-ray photoelectron spectroscopy were employed to investigate the surface morphologies, crystallinities of tungsten oxides and existence of NH ₄ 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.