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The observation of switchable dual conductive channels and related nitric oxide gas sensing properties in N-rGO/ZnO heterogeneous structure.

Gas sensors based on hybrid materials of graphene oxide/metal oxide semiconductors is an effective way to improve sensor performance. In this paper, we demonstrate a high-performance nitric oxide (NO) gas sensor based on nitrogen-doped reduced graphene oxide/ZnO nanocrystals (N-rGO/ZnO) operating at a low work temperature. ZnO nanocrystals, with an average size of approximately 5 nm, can be uniformly and compactly anchored on the surface of N-rGO by using a facile two-step hydrothermal synthesis with the appropriate amount of ammonia as the nitrogen source. The sensor based on the N-rGO/ZnO composite with 0.3 mL of ammonia (N-rGO/ZnO-0.3), in comparison with N-rGO/ZnO with different amounts of ammonia, N-rGO, and rGO/ZnO, exhibited a significantly higher sensitivity (S=Rg/Ra) at the parts per billion (ppb) level for NO gas at 90 °C. The maximum sensitivity at 800 ppb NO was approximately 22, with much fast response and recovery times. In addition, the N-rGO/ZnO-0.3 sensor revealed a great stability, a low detection limit of 100 ppb, and an excellent selectivity toward NO vs other gases (NO2, H2, CO, NH3, and CH4), especially at the ppb level. More interestingly, when exposed to oxidizing and reducing gases, respectively, unlike conventional semiconductor sensitive materials, with the behavior of that the resistances normally change in the opposite direction, only the increase of the resistance is surprisingly and incomprehensibly observed for the N-rGO/ZnO-0.3 sensor. The peculiar sensing behaviors cannot be explained by conventional theory of the adsorption process, redox reactions on the surfaces, and the well-defined p-n junction between N-rGO and ZnO, originating from the chemical bonding of Zn-C. We are here for the first time proposing that switchable contribution from dual-conduction paths including the corresponding ZnO channel with p-n junction and the corresponding N-rGO channel to the sensitivity may exist in the interaction between gases and N-rGO/ZnO-0.3 material. When an oxidizing gas (such as NO) exposures to the N-rGO/ZnO-0.3 sensor, the contribution from conductive channel of ZnO nanoparticles and p-n junction to the sensitivity is dominant. On the contrary, as for a reducing gas (such as H2), the contribution alters to N-rGO channel as dominating mode for sensitivity. For gas sensing behaviour of the NGZ-0.1 and NGZ-0.5 sensors, there is only one conduction path from the N-rGO channel for the sensitivity. The model of switchable dual-conduction paths has addressed the mysterious response observed for different gases, which may be utilized to enlighten understanding other application problems in nanoscale hybrid materials with heterogeneous structure.