Preparation And Gas Sensing Properties Of Heterostructure Based On Co₃O₄

With the development of economic, more and more people are troubled by environmental pollution, especially air pollution, therefore, the need of gas sensors is rising in response to the proper time and conditions, and they hope that gas sensors can monitor and even control the poisonous and harmful gases, including NO2, CO, H2S, VOCs and so on. So far, from the reported literatures, we can see that the most of gas sensor elements based on semiconductor metal oxides are n-type semiconductor, while the percentage of p-type semiconductors acting as gas sensing materials to be studied is relatively small. The aim of this paper is to study the preparation and gas sensing behaviors of sensors based on Co3O4. Co3O4 acting as a representative p-type semiconductor, is different from n-type semiconductor in gas sensing mechanism, and its major charge carriers are holes. Therefore, the surface of Co3O4 exposed to air forms hole-accumulation layer (HAL), not electron-depletion layer (EDL).To improve the gas sensing behavior, we designed p-n heterojunction at interface between SnO2 and Co3O4 by utilizing electrospinning method to prepare both metal oxides. We selected dimethyl formamide (DMF) as solvent, polyvinylpyrrolidone (PVP) as sacrificial template and under the appropriate conditions (The feeding rate is 0.0006 mm·s-1, the distance between syringe needle and collector is 20 cm, and potential is 20 kV), the SnO2@Co3O4 p-n nanotubes were obtained. The morphology and structure were also investigated by a series of characterizations. In addition, we carried on the gas sensing tests of composites couples with different SnO2 content, and found that the gas sensing behaviors of composites were apparently more excellent than pure Co3O4, especially in the enhancement of sensitivity, decreasing of operating temperature and shortening of response time. The improvement of gas sensing properties is ascribed to the information of p-n junction between two type semiconductor oxides and the amount of forward bias p-n junction.This paper also successfully prepared urchin-like hollow Co(CO3)0.5(OH)·0.11H20 microsphere via a facile hydrothermal strategy with no template, and after heating treatment of 400℃ for 2 h, we obtained the expected Co3O4. By a series of characterization including scanning electron microscopy (SEM), X-ray diffraction (XRD) measurements, energy dispersive spectrometer (EDS), thermogravimetric (TG) analysis, we found that the obtained product had well-defined morphology, porosity and larger surface area, and can be acted as a basis for the further modification. To improve the gas sensing properties of materials, noble metal Ag was loaded on the surface of Co3O4 microsphere by a facile in-situ reducing of wet chemical method. the gas sensing tests of composites indicate the loaded Ag cannot only enhance the sensitivity to formaldehyde, but also decrease the optimum operating temperature. Furthermore, the gas sensing properties depend on the loaded Ag amounts. Besides, we also discussed the gas sensing mechanism from the formation of Schottky barrier at the interface of metal-semiconductor and the catalysis of Ag nanoparticles.