Microwave-assisted synthesis and characterization of WO^sub x^ nanostructures for gas sensor application

Tungsten oxide (WOx) nanoparticles were synthesized by a facile and eco-friendly microwave-assisted hydrothermal method without need for using any surfactant. A thorough investigation was performed in order to elucidate the effects of microwave irradiation time (10, 20 and 30 min) on the structural, morphological and optical properties of the as-prepared WOx. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) of the samples revealed the presence of irregular nanosized particles containing some well-structured rod shape particles. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) indicated that these nanoparticles are crystallines with mean crystalline size of about 16 nm exhibiting both monoclinic and orthorhombic WOx crystal structures. The optical properties were investigated by using ultraviolet visible spectroscopy (UV-VIS) and photoluminescence (PL). A blue-shifted optical absorption spectrum with an enhanced defects emission was observed when it was compared to bulk spectrum of WO3. Thermal-aged WOx nanoparticles at mild temperature (350 °C) were also used to fabricate conductometric gas sensors Gas sensing tests showed excellent performance towards ethanol monitoring for each synthesized material. In particular, the highest sensitivity was obtained for the sensor based on WOx synthesized by 10 min irradiation time (S10). At the optimal operating temperature of 300 °C, the S10 sensor showed a response Ra/Rg = 8.5 towards 100 ppm ethanol with fast response time of 10 s, in addition to an excellent selectivity against common interfering gases.