Resistive gas sensors based on metal oxides, such as SnO 2 [ 4 ], NiO [ 5 ], CuO [ 6 ], WO 3 [ 7 ], and ZnO [ 8 ], are the most attractive gas sensors because of their low cost, easy operation, simple integration, and feasibility for miniaturisation [ 9 ]. Sharp and intense diffraction peaks confirmed the high degree of crystallisation of ZnO NRs. Then, the resistance values dramatically turned to the previous ones when NO 2 was replaced by air in the system. The ZnO NRs were obtained by a one-step hydrothermal process, whereas the NWs were obtained by a two-step hydrothermal process. High-performance gas sensors, particularly for the detection of toxic and explosive gases, are therefore necessary [ 2 , 3 ]. Here, the diameter of the NWs was approximately twice the Debye length, whereas the diameter of the NRs was much larger than the Debye length. No sign of any amorphous layer was observed on the surface of both NRs and NWs, thereby proving the good crystallinity of the ZnO nanostructures.
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