Effect of the morphology and phases of WO3 nanocrystals on their photocatalytic efficiency

Abstract

Understanding of the effect of the morphology and crystal phase on photocatalytic efficiency and their precise control are still a great challenge in photocatalysis. In this paper, we use WO₃ as an example to study how to control the morphology through understanding the effect of solution pH, EtOH and polymeric surface modulator on the morphology, crystalline phase, band gap and the photocatalytic efficiency of WO₃ nanostructures. The photocatalytic efficiency of nano WO₃ is a compromise of the band gap, crystal phase, morphology, and the oxidation state. Cuboid-shaped m-WO₃ with E_g = 2.7 eV and nanoneedle h-WO₃ with E_g = 3.1 eV give high photocatalytic efficiency, while nanowire h-WO₃ with E_g = 2.9 eV gives the lowest photocatalytic efficiency. Detailed control of the morphology and crystal phase of nanoscale WO₃ were also presented in this paper. pH 1.05 was found to be a transition point for the crystalline phase, crystal size and band gap. pH values lower than 1.05 preferred monoclinic, whereas pH values higher than 1.05 favoured hexagonal WO₃ formation. At pH 1.05, the crystal shape changed from cuboid to a fine nanoneedle shape, which was followed by a sudden size decline and an exponential increase in the aspect ratio. At the transition pH, the band gap reached a peak.