Facile one-pot synthesis of tungsten oxide (WO3−x) nanoparticles using sub and supercritical fluids

•Tungsten oxides were synthesized using sub and supercritical water and methanol.•Crystalline WO3 were formed at temperatures as low as 250°C.•Tungsten blue oxides were successfully formed by using either water and methanol.•Methanol was more effective than water in reducing tungsten oxide. Tungsten oxide (WO3) and tungsten blue oxide (WO3−x) were synthesized using sub and supercritical water and methanol. Crystalline tungsten oxides were formed with subcritical water at 250°C, which shows lower production temperature compared to conventional solid-state methods. Glycerol hydrothermal reduction and supercritical methanol were both found to be effective in obtaining crystalline tungsten blue oxides and this is a great advantage since hydrogen is not required in the process. The synthesized particles were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and near edge X-ray absorption fine structure (NEXAFS). NEXAFS results showed that sub and supercritical methanol was more effective in reduction of tungsten oxides compared to glycerol hydrothermal reduction.