Crystal Phase and Morphology-Controlled Synthesis of Tungsten Oxide Nanostructures for Remarkably Ultrafast Adsorption and Separation of Organic Dyes.

Abstract

Simple inorganic routes for the synthesis of WO3 nanostructures with variable crystal phases, WO3·(H2O)0.5 and (NH4)0.33WO3, and their self-assembled structures as nanoplates and nanospheres, respectively, were reported. The morphologies and formation of nanoplates and nanospheres were controlled by changing the solvent amount (H2O/n-propanol) in the solvothermal reactions without any stabilizing agent or surfactant. The adsorption properties of the WO3 nanostructures were studied, and it was found that nanospheres show remarkably higher and ultrafast adsorption of methylene blue (MB) in comparison to nanoplates and commercial WO3. The adsorption isotherms, kinetics, mechanism, and reusability of (NH4)0.33WO3 nanospheres were systematically studied. The nanospheres exhibited an exceptionally high adsorption rate K2 of 17.24 g·mg-1·min-1 and the maximum adsorption capacity of 116 mg·g-1 for MB. The adsorption cycle of nanospheres was examined, and the removal efficiency of MB remained at ∼98-99% even after three regeneration cycles. In addition, (NH4)0.33WO3 nanospheres exhibited excellent selective adsorption performance toward several cationic dyes, including MB, malachite green (MG), safranin O (SO), crystal violet (CV), and separate MO, a negatively charged dye, with a separation efficiency of 99.93 and 77.31% from binary and pentanary dye mixture solutions, respectively, at neutral pH.