A study on the kinetics and structure of tungsten oxide nanopowder synthesized by an electrochemical oxidation process

Tungsten oxide possesses unique properties owing to its multiple oxidation states. They are produced by several techniques with each having their advantages and limitations. In this study, the hydrated tungsten oxide nanopowders with varied morphology were synthesized by electrochemical oxidation of WC-6Co scrap at room temperature. This process is efficient and requires low capital investment. The effect of processing parameters, namely voltage, molarity, temperature, and electrolyte stirring on yield, structure, morphology, and energy bandgap is studied. The X-ray diffraction (XRD) analysis showed that at low voltage and low molarity monoclinic WO₃.2H₂O nanoparticles are synthesized. In contrast, at high molarity and high voltage, orthorhombic WO₃.H₂O nanoparticles are synthesized. Further, the size of crystal decreases with the increase in voltage during electrochemical oxidation of WC-6Co pellet. The in-situ XRD analysis showed progressive transformation of as-synthesized nanopowder from orthorhombic to cubic crystal structure. Thermal treatments using microwave radiation and muffle furnace resulted in partial phase transformation of hydrated tungsten oxide to cubic WO₃.H_0.5 phase. The scanning electron microscopy and transmission electron microscopy analyses confirmed the formation of nanoplates, nanorods, and quantum dots depending on the processing parameters. The ultraviolet-visible spectroscopy showed a relatively lower energy bandgap of as-synthesized tungsten oxide nanopowder.