Carbon nanotube-metal oxide nanocomposite gas sensing mechanism assessed via NO2 adsorption on n-WO3/p-MWCNT nanocomposites

A series of WO3/multiwalled carbon nanotube (MWCNT) nanocomposite sensors was fabricated by bar-coating slurries using different ratios of MWCNTs to WO3 nanoparticles. The morphology, composition, and structure of the fabricated nanocomposites were examined using electron microscopy, X-ray diffraction, ultraviolet and X-ray photoelectron spectroscopy, Raman spectroscopy, and nitrogen adsorption-desorption measurements, with the aim of completely identifying the physical and electronic structures of the nanocomposites. The effects of the different ratios of the nanocomposites on the electrical conductance and NO2 gas sensing properties were examined and compared with the morphological investigation results. The synergetic properties of the nanocomposite sensors were a result of the combined effect of low-doped semiconducting WO3 and metallic MWCNTs. Because nanoscale sensors exhibit a maximal response on the scale of their depletion depth, individual components with conductivities that are either too low or too high cannot meet the condition. Meanwhile, their mixture can establish the required condition for the maximal response which appears as a synergetic effect. Based on this effect, the optimal nanocomposite sensor (0.5 wt% MWCNT) showed a response of ~18 for 5 ppm NO2 at 150 °C with short response/recovery times (~87 s /~300 s). The synergetic effect in nanocomposite sensors cannot be explained by the interfacial Schottky barrier model, which has been used for sensors of agglomerated particles. © 2020