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

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.