Adsorption behavior of NO2 molecules in ZnO-mono/multilayer graphene core–shell quantum dots for NO2 gas sensor

•The fabrication of a sensor with ZnO-mono/multilayer graphene core–shell QDs via simple hydrothermal synthesis method for improved NO2 gas sensing performance.•The gas response value of ZnO-multilayer graphene (ZMLG) QDs was about 7.2 and 19 times higher than that with ZnO-monolayer graphene (ZG) and ZnO QDs.•The change value of surface potential of ZMLG QDs increased up to 14.25 times compared to ZnO QDs and 6.33 times increased compared to ZG QDs.•The description of mechanism for the improvement of the detection behavior that is occurred by covering the graphene shells into ZnO QDs.