Reversible Switching from P- to N-Type NO2 Sensing in ZnO Rods/rGO by Changing the NO2 Concentration, Temperature, and Doping Ratio

Abstract

Gas sensors based on hybrid materials of reduced graphene oxide/metal oxide semiconductors are an effective way to improve sensor performance. In this paper, rGO (reduced graphene oxide) was uniformly and compactly anchored on the surface of presynthesized ZnO rods using a facile hydrothermal synthesis. The ZnO rods and reduced graphene oxide combined to form an np-type heterojunction structure for use in a nitrogen dioxide (NO2) sensor. Compared with pure ZnO rods and rGO, the ZnO rod/rGO composites display abnormal sensing behavior in the form of reversible transitions from p- to n-type sensing induced by changes in the NO2 gas concentration (C) and temperature (T). Based on familiar phase diagrams, a binary (T–C) transition diagram was created in terms of the gas-sensing response, which can be directly used to design and control p–n transitions. In addition, we also found that as the doping ratios (c) change, the semiconductor type of the composite also changes. Finally, the mechanisms underlying the NO2-sensing transitions are described based on the dual conduction path model: electron transfer in the internal region and chemical reactions at the surface. This reversible p–n transition of ZnO/rGO materials upon NO2 sensing is interesting and holds great potential for the efficient detection of NO2.