Engineering of ZnO/rGO towards NO[sub.2] Gas Detection: Ratio Modulated Sensing Type and Heterojunction Determined Response

Nanoscale heterostructured zinc oxide/reduced graphene oxide (ZnO/rGO) materials with p–n heterojunctions exhibit excellent low temperature NO[sub.2] gas sensing performance, but their doping ratio modulated sensing properties remain poorly understood. Herein, ZnO nanoparticles were loaded with 0.1~4% rGO by a facile hydrothermal method and evaluated as NO[sub.2] gas chemiresistor. We have the following key findings. First, ZnO/rGO manifests doping ratio-dependent sensing type switching. Increasing the rGO concentration changes the type of ZnO/rGO conductivity from n-type (1.4% rGO). Second, interestingly, different sensing regions exhibit different sensing characteristics. In the n-type NO[sub.2] gas sensing region, all the sensors exhibit the maximum gas response at the optimum working temperature. Among them, the sensor that shows the maximum gas response exhibits a minimum optimum working temperature. In the mixed n/p-type region, the material displays abnormal reversal from n- to p-type sensing transitions as a function of the doping ratio, NO[sub.2] concentration and working temperature. In the p-type gas sensing region, the response decreases with increasing rGO ratio and working temperature. Third, we derive a conduction path model that shows how the sensing type switches in ZnO/rGO. We also find that p–n heterojunction ratio (n[sub.p–n] /n[sub.rGO] ) plays a key role in the optimal response condition. The model is supported by UV-vis experimental data. The approach presented in this work can be extended to other p–n heterostructures and the insights will benefit the design of more efficient chemiresistive gas sensors.