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

Abstract

This study uses two in-situ techniques to investigate the geometry and phase change behaviour of bundled ultrathin W18O49 nanowires and WO3 nanoparticles. The in-situ X-ray diffraction (XRD) results have shown that the phase transition of WO3 nanoparticles occurs in sequence from monoclinic (room temperature) → orthorhombic (350 ºC) → tetragonal (800 °C), akin to bulk WO3; however, W18O49 nanowires remain stable as the monoclinic phase up to 500 °C, after which a complete oxidation to WO3 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 identified the 187.6 cm-1 as the fingerprint band for the phase transition from γ- to β-phase of the WO3 nanoparticle. Furthermore, WO3 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-W18O49 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.