Ultralow-detection limit ammonia gas sensors at room temperature based on MWCNT/WO3 nanocomposite and effect of humidity

Nanocomposite-based NH3 gas sensors working at room temperature were systematically fabricated by compositing multi-walled carbon nanotubes (MWCNTs) and tungsten oxide (WO3) nanobricks at different mass ratios. The response of the nanocomposite-based sensors increases with increasing WO3 content from 0 to 95 wt% then decreases as the WO3 content continuously increases to 100 wt%. The sensor with 95 wt% of WO3 shows a significant enhancement in NH3 gas sensing characteristics, namely, high response (~350% with 60 ppm NH3), ultralow limit of detection (6 ppb), and selectivity to NH3. The enhancement of gas sensing characteristics was attributed to the synergistic effect of the p-p junction between MWCNT and the inversion layer on the WO3 surface caused by the adsorbed oxygen gas and water vapor in ambient air. The response increases while the sensor's resistance decreases with increasing relative humidity from 25% to 65%. Hence, MWCNT/WO3 nanocomposite is a promising material for selective sub-ppm NH3 gas sensor at room temperature.