One-pot wet chemical synthesis of reduced graphene oxide-zinc oxide nanocomposites for fast and selective ammonia sensing at room temperature

Abstract

The present study focuses upon the response of nanocomposites of reduced graphene oxide (rGO) and zinc oxide (ZnO) towards ammonia gas at room temperature. We have synthesized reduced graphene oxide-zinc oxide nanocomposites (rGO-ZnO) by wet chemical route via in-situ one-pot method. The as-synthesized samples were characterized using FT-IR spectroscopy, Raman spectroscopy, X-ray Diffraction, Scanning Electron Microscopy (SEM) and high resolution Transmission Electron Microscopy (HR-TEM) techniques. The average crystallite size of the ZnO sample and rGO-ZnO sample, determined by Williamson-Hall analysis of the XRD results, were calculated to be 20.62 nm and 13.21 nm respectively. The TEM analysis suggests that these nanocomposites comprise of ZnO nanoparticles of average size about 10.8 nm, anchored on rGO sheets. At 300 K, the chemiresistive gas sensing response of pure rGO and rGO-ZnO towards subsequently increasing concentrations (from 20 ppm to 200 ppm) of ammonia gas has been studied. rGO-ZnO samples were prepared by varying the concentration of precursor GO dispersion (0.5, 0.75, 1, 1.25, 1.5 mg/mL). The optimal nanocomposite material (1 mg/mL) is found to exhibit excellent selectivity towards ammonia with a response of 8.92 % towards 20 ppm NH3 and a response time of 8 s. The response is found to increase with increase in ppm level of gas. The selectivity of the synthesized nanocomposite towards ammonia has been confirmed by exposing it to 20 ppm of various other gases such as methanol, ethanol, acetone, propanol and chloroform. The response towards these gases was lower as compared to that towards NH3. To assess the potential of the material in practical applications, the effect of ambient humidity on the sample’s surface resistance was studied. The ammonia sensing response in the presence of humidity was found to vary only by 1.5 % from 10 % RH to 70 % RH, beyond which it saturated. The rGO-ZnO nanocomposite sensing material also displayed good reversibility over subsequent exposure cycles and efficient stability of response with aging. The mechanism of NH3 sensing is understood on the basis of heterojunction interactions between rGO and ZnO in the nanocomposites.