Nanostructured WO3/graphene composites for sensing NOx at room temperature

Abstract

WO₃ has emerged as an outstanding nanomaterial composite for gas sensing applications. In this paper, we report the synthesis of WO₃ using two different capping agents, namely, oxalic acid and citric acid, along with cetyltrimethyl ammonium bromide (CTAB). The effect of capping agent on the morphology of WO₃ material was investigated and presented. The WO₃ materials were characterized using X-ray diffraction analysis (XRD), field emission transmission electron microscopy (FETEM), field emission scanning electron microscopy (FESEM), particle size distribution (PSD) analysis, and UV-visible spectroscopic analysis. WO₃ synthesized using oxalic acid exhibited orthorhombic phase with crystallite size of 10 nm, while WO₃ obtained using citric acid shows monoclinic phase with crystallite size of 20 nm. WO₃ obtained using both capping agents were used to study their gas sensing characteristics, particularly for NO_x gas. The cross sensitivity towards interfering gases and organic vapors such as acetone, ethanol, methanol and triethylamine (TEA) was monitored and explained. Furthermore, the composites of WO₃ were prepared with graphene by physical mixing to improve the sensitivity, response and recovery time. The composites were tested for gas sensing at room temperature as well as at 50 °C and 100 °C. The results indicated that the citric acid-assisted WO₃ material exhibits better response towards NO_x sensing when compared with oxalic acid-assisted WO₃. Moreover, the sensitivity of the WO₃/graphene nanocomposite was better than that of the pristine WO₃ material towards NO_x gas. The WO₃ composite prepared using citric acid as capping agent and graphene exhibits sensing response and recovery time of 29 and 24 s, respectively.