The crystal structure of the WO3 nanoparticles was investigated by X-ray diffractometry (XRD) (Bruker D8 Advance) with a Cu K radiation source (40 kV and 40 mA) and scanning step of 0.02◦. The morphology of the samples was examined by using scanning electron microscopy (SEM) (FEI Sirion 200) and high-resolution field emission transmission electron microscope (HRTEM) (JEM-2100F, JEOL Corporation).