Microwave-Assisted Synthesis of Graphene-SnO₂ Nanocomposites and Their Applications in Gas Sensors

We obtained extremely high and selective sensitivity to NO₂ gas by fabricating graphene-SnO₂ nanocomposites using a commercial microwave oven. Structural characterization revealed that the products corresponded to agglomerated structures of graphene and SnO₂ particles, with small secondary SnO_x (x ≤ 2) nanoparticles deposited on the surfaces. The overall oxygen atomic ratio was decreased with the appearance of an SnO_x (x < 2) phase. By the microwave treatment of graphene-SnO₂ nanocomposites, with the graphene promoting efficient transport of the microwave energy, evaporation and redeposition of SnO_x nanoparticles were facilitated. The graphene-SnO₂ nanocomposites exhibited a high sensor response of 24.7 for 1 ppm of NO₂ gas, at an optimized temperature of 150 °C. The graphene-SnO₂ nanocomposites were selectively sensitive to NO₂ gas, in comparison with SO₂, NH₃, and ethanol gases. We suggest that the generation of SnO_x nanoparticles and the SnO_x phase in the matrix results in the formation of SnO₂/SnO₂ homojunctions, SnO₂/SnO_x (x < 2) heterojunctions, and SnO₂/graphene heterojunctions, which are responsible for the excellent sensitivity of the graphene-SnO₂ nanocomposites to NO₂ gas. In addition, the generation of surface Sn interstitial defects is also partly responsible for the excellent NO₂ sensing performance observed in this study.