Effects of acidity on the formation and adsorption activity of tungsten oxide nanostructures prepared via the acid precipitation method

This study systematically investigated the effects of the acidity of precursor suspension on the formation, physical properties, and adsorption activity of tungsten oxide nanostructures prepared via the acid precipitation method at room temperature. In highly acidic suspension (−0.85 ≤ pH ≤ −0.34), nanoplate orthorhombic tungsten oxide dihydrates (WO3·H2O) with a high crystallinity but with low adsorptive activity were formed. In moderate acidic suspension (0.04 ≤ pH ≤ 0.97), a novel hexagonal tungsten oxide, H-WO3, formed with a mixed morphology and a low crystallinity but with a high adsorptive activity. The effect of acidity was due to the vital role of high H+ ion concentration platform that acted as a scaffold for long-order arrangement and a breaker for confining the nanostructure of tungsten oxide. H-WO3 samples exhibited a high adsorption capacity (~30 mg/g) and a short adsorption–desorption equilibrium time (~15 min) with methylene blue. The adsorption kinetics of the samples followed the pseudo-second-order model, whereas their adsorption isotherm followed the monomolecular adsorptive Langmuir model. This study provides a simple energy-saving method for controlling the crystal structure and morphology of tungsten oxide nanostructures.