Facile chemical synthesis of nanosheets self-assembled hierarchical H₂WO₄ microspheres and their morphology-controlled thermal decomposition into WO₃ microspheres

A controlled acidic precipitation is reported for the direct synthesis of nanosheets self-assembled hierarchical tungstic acid microspheres, and their annealing-dependent thermal conversion into WO₃ microstructures is studied for the first time. The synthesis was carried out without the use of any structure directing agents and hydrothermal conditions. The microspheres were found to be consisting of many single-crystalline and highly oriented nanosheets that were hierarchically self-aggregated in spherical form for the minimization of surface energy. An experimentally validated time-dependent formation and growth mechanism was proposed for the development of microspheres. The characterization results indicated that the morphology-preserved thermal decomposition of H₂WO₄ into WO₃ microspheres occurred at 400 °C, whereas a morphology distortion was noticed for the sample synthesized at 600 °C. The increase in annealing temperature further increased the crystallinity of WO₃ without changing its monoclinic crystal structure. However, it has a diminishing effect in the specific surface area, pore size and pore volume of the annealed samples. Moreover, the adsorption of methylene blue in aqueous medium was carried out to evaluate its potential environmental application. The WO₃ microspheres synthesized at 400 °C showed high adsorption capacity (63 mg/g) compared to the same at 600 °C (56 mg/g). The high activity could be ascribed to the synergistic effects of high surface area, low particle size, porosity, specific morphology and more negative zeta potential. The adsorption further followed pseudo-second-order kinetics with a best fit of the experimental and theoretical values.