Crystal Phase and Size-Controlled Synthesis of Tungsten Trioxide Hydrate Nanoplates at Room Temperature: Enhanced Cr(VI) Photoreduction and Methylene Blue Adsorption Properties

Abstract

Controlling the crystal phase of a material using solution-based method is a challenging task and has significant consequence to the material’s properties. Herein we report the phase and size-controlled synthesis of tungsten oxide hydrates at room temperature via a simple precipitation method. In the absence and presence of oxalic acid, orthorhombic WO3·H2O and monoclinic WO3·2H2O nanoplates of size in the range of 200–600 (thickness <50 nm) and 40–200 nm (thickness <20 nm) were respectively synthesized. Oxalic acid is found to play the central role in the phase transition due to its chelating nature that facilitates bonding of oxalate ions to tungsten cations leading to formation of WO3·2H2O. Upon annealing at 400 °C for 2 h under air, both WO3·H2O and WO3·2H2O nanoplates were converted to monoclinic WO3 nanoplates. These nanoplates were demonstrated to be highly efficient for the photocatalytic detoxification of toxic Cr(VI) in the acidic pH under the visible light irradiation. The best Cr(VI) reduction performance was obtained with WO3·2H2O nanoplates due to its smaller band gap and larger effective surface area. In addition, a lower pH value is found to facilitates the Cr(VI) reduction. Furthermore, highly concentrated methylene blue was efficiently removed (>95%) by adsorption on the nanoplates within a minute, suggesting the importance and potential of a material that can be synthesized at room temperature.