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.