Synthesis of NiO-In2O3 heterojunction nanospheres for highly selective and sensitive detection of ppb-level NO2

NiO-In2O3 composite nanospheres with p-n heterojunction structure were fabricated by a one-step hydrothermal method. A series of XRD, FTIR, SEM, TEM, and XPS measurements were utilized to characterize the morphology and structure of NiO-In2O3 composite nanospheres, accompanying with pure In2O3 nanospheres for comparison. The results indicated that both the diameters of pure In2O3 and NiO-In2O3 nanospheres were in the range of 400–500 nm, indicating that the introduction of Ni did not change the product morphology. The gas sensing performance showed that Ni functionalization with proper content was conducive to enhancing NO2 sensing performance compared with pure In2O3 nanospheres. The obtained NiO-In2O3 composite nanospheres showed excellent response-recovery properties to NO2 gas and high selectivity, stability, and repeatability. The highest response of 1771 was achieved for 10% NiO-In2O3 composite nanospheres to 10 ppm NO2 gas at 100 °C, and the corresponding response to 10 ppb NO2 gas was 1.2. The gas sensing mechanism of NiO-In2O3 composite nanospheres to NO2 gas was discussed based on the electron depletion theory and the formation of p-n heterojunction structure. The results show that NiO-In2O3 composite nanospheres have favorable potential application in ppb-level NO2 detection.