N-Doped Graphene Quantum Dot-Decorated Three-Dimensional Ordered Macroporous In₂O₃ for NO₂ Sensing at Low Temperatures

Nitrogen dioxide (NO2) detection is of great importance because the emission of NO2 gas profoundly endangers the natural environment and human health. However, a few challenges, including lowering detection limit, improving response/recovery kinetics, and reducing working temperature, should be further addressed before practical applications. Herein, a series of N-doped graphene quantum dot (N-GQD)-modified three-dimensional ordered macroporous (3DOM) In2O3 composites are constructed and their NO2 response properties are studied. The results show that compared to pure 3DOM In2O3, reduced graphene oxide (rGO)/3DOM In2O3, and N-doped graphene sheets (NS)/3DOM In2O3, the N-GQDs/3DOM In2O3 sensing materials exhibit higher NO2 responses with fast response and recovery speed and low working temperature (100 °C). In addition, the detection limit of NO2 response for the optimal N-GQDs/In2O3 sensor is as low as 100 ppb. Upon exposure to CO, CH4, NH3, acetone, ethanol, toluene, and formaldehyde, only very weak responses could be observed, indicating good selectivity for the synthesized material. More attractively, the responses of the optimized N-GQDs/In2O3 sensor exhibit no obviously big fluctuation over 60 days, implying good long-term stability. We suggest that the formation of heterojunctions between 3DOM In2O3 and N-GQDs and the doping N atoms in N-GQDs play crucial roles in improving the NO2 sensing properties.