Atomically thin-layered WS₂based resistive sensors for detection of CO and NO₂at room temperature

The number of layers present in a two-dimensional (2D) nanomaterial plays a critical role in applications that involve surface interaction, for example, gas sensing. This paper reports the synthesis of 2D WS₂nanoflakes using the facile liquid exfoliation technique. The nanoflakes were exfoliated using bath sonication (BS-WS₂) and probe sonication (PS-WS₂). The thickness of the BS-WS₂was found to range between 70 and 200 nm, and that of PS-WS₂varied from 0.6 to 80 nm, indicating the presence of single to few layers of WS₂when characterized using atomic force microscope. All the WS₂samples were thoroughly characterized using electron microscopes, x-ray diffractometer, Raman spectroscopy, UV-Visible spectroscopy, Fourier transform infrared spectroscope, and thermogravimetric analyser. Both the nanostructured samples were exposed to 2 ppm of NO₂at room temperature. Interestingly, BS-WS₂which comprises of a greater number of WS₂layers exhibited -14.2% response as against -3.4% response of PS-WS₂, the atomically thin sample. The BS-WS₂sample was found to be highly selective towards NO₂but was slower (with incomplete recovery) as compared to PS-WS₂. The PS-WS₂sample was observed to exhibit -11.9% to -27.4% response to 2-10 ppm of CO and -3.4%-35.2% response to 2-10 ppm of NO₂at room temperature, thereby exhibiting the potential to detect two gases simultaneously. These gases could be accurately predicted and quantified if the response times of the PS-WS₂sample were considered. The atomically thin WS₂-based sensor exhibited a limit of detection of 131 and 81 ppb for CO and NO₂, respectively.