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

Simple inorganic routes for the synthesis of WO 3 nanostructures with variable crystal phases, WO 3 ·(H 2 O) 0.5 and (NH 4 ) 0.33 WO 3, 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 (H 2 O/ n -propanol) in the solvothermal reactions without any stabilizing agent or surfactant. The adsorption properties of the WO 3 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 WO 3 . The adsorption isotherms, kinetics, mechanism, and reusability of (NH 4 ) 0.33 WO 3 nanospheres were systematically studied. The nanospheres exhibited an exceptionally high adsorption rate K 2 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, (NH 4 ) 0.33 WO 3 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.