Study On The Gas Sensing Properties Of Co₃O₄ And Reduced Graphene Oxide Composites And TiO₂ And Reduced Graphene Oxide Composites

With the development of industry and automotive transportation industry, air pollution is getting worse in the city, one of the main ingredient is VOCs. Indoor air pollution also exists because of interior architecture and decoration emitting VOCs. Rising to a certain concentration of VOCs is harmful to health, so it is very important to detect the VOCs. Traditional metal oxide semiconductor gas sensors can detect many kinds of VOCs, but the traditional metal oxide semiconductor gas sensors have some shortcomings. The traditional metal oxide semiconductor gas sensors usually work in high temperature, and the selectivity is not good. To overcome the shortcomings, development of new gas-sensitive materials is the most effective way to improve the sensing properties of gas sensors. Since graphene discovered, graphene as gas-sensitive material has caused extensive research for its special physical and electrochemical characteristics. This work mainly studies on the improvement in the gas sensing properties of metal oxide-reduced graphene oxide composites compared to graphene and metal oxide.The metal oxide and reduced graphene oxide composites, like TiO2-rGO、Co3O4-rGO、 ZnO-rGO, were prepared using a hydrothermal method. The mesoporous TiO2 microspheres and NiO-wrapped mesoporous TiO2 composite microspheres were prepared using sol-gel. Using XRD, SEM, TEM, BET and Raman to show the microstructure. In the SEM graphs of Co3O4-rGO, we saw the intercalation of CO3O4 nanocrystals into the rGO nanosheets. The SEM graphs of TiO2 shows the mesoporous microstructure. We made gas sensors using the materials, and tested the sensing properties of the sensors at room teperature. Pure mesoporous TiO2 microspheres based chemoresistive gas sensors showed response to several organic volatile compounds (VOCs) such as formaldehyde, methanol, ethanol, ammonia and acetone under the UV illumination in the humid air at room temperature. TiO2-rGO sensor showed almost no response to VOCs.With the loading of NiO at the porous surface of TiO2 microspheres, the sensor using NiO-wrapped mesoporous TiO2 composite microspheres showed a good sensitivity to ammonia from 10 ppm to 100 ppm whereas almost no response to other VOCs indicating an excellent selectivity to detect ammonia in the humid air with UV illumination at room temperature. The Co3O4-rGO composite based sensors showed faster response to methanol at room temperature compared to the rGO based sensors. And among the Co3O4-rGO composites, CO3O4-5 wt% rGO showed the highest response to 60 ppm NO2 at room temperature, the Rs is 80%, it’s 20 times to rGO. Two possible reasons have been discussed including the increased surface area of the rGO thick film by the intercalation of C03O4 nanocrystals and the Co3+-carbon coupling effect for the rapid response. The sensors using Co3O4-rGO composite, ZnO-rGO composite and rGO showed a fast response and full recovery to methanol. This has been proposed to be exclusively due to the interaction of methanol with the sp2 bonding of the carbon. But the sensors response to methanol are very low. In addition, Co3O4-rGO composite and ZnO-rGO composite have high response to NO2 and ammonia, but the sensors response to NO2 and ammonia were not fully recovered within the measurement time (-20 minutes), this problem is to be further studied.