Energy-efficient NO2 sensors based on two-dimensional layered 2H-WS2 nanosheets

Layered transition metal dichalcogenides (TMDCs) are considered among the next-generation materials for gas sensing. Here, we report exfoliated 2H-WS2 nanosheets for the fabrication of highly performing NO2 sensors. Thermal annealing at several temperatures was performed to investigate the oxidation of WS2. The long-term stability of 2H-WS2 bulk was verified. Using droplet variation method, three batches of conductometric sensors from 2H-WS2 dispersions were fabricated on electrical transducers, namely two layers (2 L), five layers (5 L) and ten layers (10 L) WS2 nanosheets. These sensors were tested towards low NO2 concentrations at different temperatures (Room Temperature (20 ℃), 50 ℃ and 100 ℃) and relative humidity (RH) levels (20%, 40%, 60%, 80% and 90% RH). 2L-WS2 based sensor showed the highest response at room temperature (RT). Excellent repeatability (4 cycles) towards 1 ppm NO2 and long-term stability (more than two months) were achieved. Full selectivity towards NO2 (1 ppm) at RT was observed over NH3 (15 ppm), H2S (15 ppm), ethanol (30 ppm) and acetone (30 ppm). Our results confirm that low-power consumption devices with high sensitivity (even at high RH), long-term stability and excellent selectivity towards NO2 were fabricated using 2H-WS2 nanosheets.