Solvent-dependent structures and photoluminescence of WO3-x nanomaterials grown in nonaqueous solutions

The substoichiometric tungsten oxide (WO3-x) nanostructures are important for the catalytic, energy and optoelectronic applications, yet it remains challenging to control their morphologies and oxygen vacancies. Here we address the challenge of synthesizing the WO3-x nanoflakes and nanorods in ethanol and ethanoic acid by solvothermal method, where tungsten hexachloride (WCl6) is used as the precursor of tungsten. The results characterized by different analytical methods indicate that the WO3-x nanoflakes and nanorods are crystalline structures composed of different phases, respectively. The formation of different WO3-x nanostructures is related to the difference in the formation mechanisms of WO3-x monomers in ethanol and ethanoic acid. Due to the difference in the mobility of WO3-x monomers caused by the properties of ethanol and ethanoic acid, the WO3-x nanoflakes are formed in ethanol depending on the interface between gas and liquid ethanol. In contrast, the WO3-x nanorods are formed in ethanoic acid due to the anisotropic intermediate product formed by the reaction of WCl6 and ethanoic acid. Furthermore, the photoluminescence (PL) of WO3-x nanoflakes and nanorods is studied at room temperature and low temperature. Both the WO3-x nanostructures generate the ultraviolet, green, red and near-infrared light at room temperature, which are related to the transition from the defect resonant state level in the conduction band to the valence band, the transition between defect state levels and relaxation of polarons, respectively. The PL emission at low temperature was enhanced due to the weak lattice vibration and the increase of oxygen vacancy concentration. Our results not only enrich the synthesis and optical properties of WO3-based nanomaterials but also contribute to the development of WO3-based optoelectronic nanodevices.