Facile synthesis of nitrogen-doped reduced graphene oxide/zinc oxide nanocomposites for enhanced room-temperature ammonia gas detection

The present investigations demonstrate for the first time, fast and reversible ammonia (NH3) gas sensing performance of nitrogen-doped reduced graphene oxide/zinc oxide (N-rGO/ZnO) nanocomposites at room temperature. The motivation to use nitrogen as a dopant stems from the fact that it lies next to carbon in the periodic table, therefore is similar in size and covalent nature. The synthesis of N-rGO/ZnO was carried out by a simple two-step in situ method via the wet chemical route. The NH3 sensing potential of reduced graphene oxide (rGO), nitrogen-doped rGO (N-rGO), and rGO-ZnO nanocomposites synthesized by similar routes was also assessed. N-rGO/ZnO was found to exhibit superior NH3 sensing performance as compared to rGO, N-rGO, and rGO-ZnO. Further, the study of the sensing mechanism also affirms that the improved response in N-rGO/ZnO is attributed to the formation of p-n heterojunction sites and the charge activation due to N-dopant. To study the effect of N-doping levels on the NH3 sensing performance of N-rGO/ZnO, different samples were prepared by altering the amount of nitrogen source ammonia solution (0.05, 0.1, 0.2, 0.3 mu L mg-1 of GO) in the reaction. Consequently, ZnO nanoparticles of different morphologies anchored to flexible N-rGO sheets were obtained. The nitrogen doping has been quantified using X-ray photoelectron spectroscopy analysis. An optimal gas sensing performance of 18.35% toward 10-ppm NH3 with response/recovery time 2.5/72 s was obtained for the 0.1-mu L mg-1 sample. The variation in NH3 sensing response in the presence of different %RH levels of humidity was also assessed. The response changed only by 2.6% when %RH is changed from 10 to 80%. The sensor also displayed appreciable stability with ambient aging.