In situ investigations of the phase change behaviour of tungsten oxide nanostructures

This study uses two in situ techniques to investigate the geometry and phase change behaviour of bundled ultrathin W(18)O(49) nanowires and WO(3) nanoparticles. The in situ X-ray diffraction (XRD) results have shown that the phase transition of WO(3) nanoparticles occurs in sequence from monoclinic (room temperature) → orthorhombic (350°C) → tetragonal (800°C), akin to bulk WO(3); however, W(18)O(49) nanowires remain stable as the monoclinic phase up to 500°C, after which a complete oxidation to WO(3) and transformation to the orthorhombic β-phase at 550°C is observed. The in situ Raman spectroscopy investigations have revealed the Raman peak downshifts as the temperature increases, and have identified the 187.6 cm(−1) as the fingerprint band for the phase transition from γ- to β-phase of the WO(3) nanoparticle. Furthermore, WO(3) nanoparticles exhibit the γ- to β-phase conversion at 275°C, which is about 75°C lower than the relaxation temperature of 350°C for the monoclinic γ-W(18)O(49) nanowires. These new fundamental understandings on the phase transition behaviour offer important guidance for the design and development of tungsten oxide-based nanodevices by defining their allowed operating conditions.