Using thermal annealing process, graphene oxide (GO) films were synthesized into reduced graphene oxide (rGO) composite films. Compared with the NO2 gas sensors using ZnO and rGO sensing membranes, the performances of the NO2 gas sensors using ZnO-rGO sensing membrane were improved. The improvement mechanisms were attributed to the removal of oxygen-containing functional groups, the supply of electrons from the oxygen vacancies of ZnO material, and the formation of C–O–Zn bonds. To study the dependence of sensing performances on the ZnO/GO ratio, various ZnO/GO ratios in the ZnO-rGO sensing membrane were used in the NO2 gas sensors. For the best ratio of 0.08, the sensing responsivity, response time, and recovery time of the ZnO-rGO NO2 gas sensors were respectively improved to 47.4%, 6.2 min, and 15.5 min compared with 19.0%, 10.3 min, and 75.9 min of the rGO NO2 gas sensors operated under 100 ppm NO2 environment at room temperature. Furthermore, the minimum detective NO2 concentration of 5 ppm and linear sensing responsivity from 10 ppm to 100 ppm were achieved using the ZnO-rGO gas sensors operated at room temperature.
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