pH-controlled assembly of three-dimensional tungsten oxide hierarchical nanostructures for catalytic oxidation of cyclohexene to adipic acid
Tungsten oxide has a wide range of applications in chromic devices and catalysis. Effective control over the morphology of tungsten oxide remains a major challenge. Here, we present a facile pH-controlled strategy for synthesizing three-dimensional tungsten oxide architectures with different morphologies without using any templates or surfactants. These hierarchical architectures with rod-like, disk-like and sphere-like morphologies are assembled from one-dimensional tungsten oxide nanowires/nanorods. The influence of the single parameter of the pH value on the morphologies and crystal structures of tungsten oxide is systematically studied. The results indicate that different polytungstate anions obtained at various pH values give rise to different crystal nuclei by a dehydration process. These crystal nuclei grow into similar phases (crystal structures) but distinct morphologies via hydrothermal treatment. Here, the similarity of the crystal structures of h-WO3 and o-WO3·0.33H2O plays a key role in the structural transformation. Further experiments show that these three-dimensional tungsten oxide architectures display promising applications as catalysts in green chemistry for the oxidation of cyclohexene to adipic acid using an environmentally friendly oxidant (hydrogen peroxide).