Detection of NO₂ and 2-Nitrotoluene by MoS₂–ZnO Composite

Trace level and timely detection of multiple gases are desirous for any application that intends to facilitate proper air quality monitoring. In many cases, achieving these attributes simultaneously becomes very difficult with a single sensor. In this article, the MoS2-ZnO sensing layer was synthesized by ultrasonicating MoS2 nanoflakes (synthesized using liquid exfoliation) and ZnO nanoflowers (synthesized using hydrothermal method) to detect NO2 and 2-nitrotoluene (2-NT). The bandgaps of MoS2 and ZnO were found to be 1.71 and 3.12 eV, respectively, through ultraviolet-visible (UV-Vis) spectroscopy. While X-ray photoelectron spectroscopy indicated charge sharing between the materials, the electron microscopy images revealed the coexistence of MoS2 nanoflakes and ZnO nanoflowers thereby, confirming the sensing material to be a composite. The intrinsic and the composite nanomaterials were tested for NO2 and 2-NT at different temperatures (25 °C–300 °C). The composite among all samples exhibited the highest response for NO2 and 2-NT but at two different temperatures, 200 °C and 250 °C, respectively. The response of the sensor varied from 31.5% to 544% and 71% to 193% for 5–15 ppm of NO2 and 2-NT, respectively, at the optimum temperatures. The limit of detection of the MoS2-ZnO was found to be 8 and 62 ppb for NO2 and 2-NT, respectively. The response times of the composite were found to be varying from ~1000 to ~1500 s for NO2 and ~250 to 500 s for 2-NT, yet the prediction time of these gases could be reduced to 200 s with the help of transient analysis and cantor pairing function.