Photocatalytic activity of SnWO4 and SnW3O9 nanostructures prepared by a surfactant-assisted hydrothermal process

Flower-like α-SnWO4 and rod-like SnW3O9 nanostructures in the form of a single and a mixed phase were prepared by hydrothermal process at 200 °C for 36 h in a wide pH range, with the assistance of dodecyltrimethylammonium bromide (DTAB). The effects of the pH of the synthesis solution and the Sn2+/W6+ molar ratio on phase compositions, structures and morphologies of the as-prepared powders were investigated. As a single phase, α-SnWO4 and SnW3O9 could be hydrothermally prepared at pHs 7–8 and 1, respectively. In the form of a mixed phase, they could be hydrothermally obtained in the pH range of 2–6. The electron microscopy observations revealed that α-SnWO4 powders presented flower-like structures with the diameter of 2–5 μm and SnW3O9 powders possessed rod-like structures with the width of 250 nm and the length of 2 μm. The X-ray photoelectron spectroscopy (XPS) results confirmed the reduction of tungsten from W6+ to W4+ during the hydrothermal process in accordance with the pH value. The UV–vis diffuse reflectance absorption spectra proved a strong visible-light absorbance of α-SnWO4 powders in the range of 400–650 nm. Compared to phase-pure SnW3O9 and the mixture of α-SnWO4 and SnW3O9 phases, phase-pure α-SnWO4 exhibited a good photocatalytic activity for the degradation of methyl orange under visible-light irradiation. After several cycles, the α-SnWO4 sample retains high degradation efficiency.