Atomically thin-layered WS₂ based resistive sensors for detection of CO and NO₂ at room temperature

Abstract

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 WS2 nanoflakes 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-WS2 was found to range between 70 – 200 nm, and that of PS-WS2 varied from 0.6 to 80 nm, indicating the presence of single to few layers of WS2 when characterized using atomic force microscope (AFM). All the WS2 samples 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 NO2 at room temperature. Interestingly, BS-WS2 which comprises of a greater number of WS2 layers exhibited –14.2% response as against –3.4% response of PS-WS2, the atomically thin sample. The BS-WS2 sample was found to be highly selective towards NO2 but was slower (with incomplete recovery) as compared to PS-WS2. The PS-WS2 sample was observed to exhibit –11.9 to – 27.4% response to 2–10 ppm of CO and –3.4 to 35.2% response to 2–10 ppm of NO2 at 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-WS2 sample were considered. The atomically thin WS2-based sensor exhibited a limit of detection (LOD) of 131 and 81 ppb for CO and NO2, respectively.