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 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 degrees C) -> tetragonal (800 degrees C), akin to bulk WO3; however, W18O49 nanowires remain stable as the monoclinic phase up to 500 degrees C, after which a complete oxidation to WO3 and transformation to the orthorhombic beta-phase at 550 degrees 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 gamma- to beta-phase of the WO3 nanoparticle. Furthermore, WO3 nanoparticles exhibit the gamma- to beta-phase conversion at 275 degrees C, which is about 75 degrees C lower than the relaxation temperature of 350 degrees C for the monoclinic gamma-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.