Constructing Of Metal Oxide And Crystalline Nanocarbon Heterostructure And Their Gas Sensing Performance

During the global environmental pollution, air pollution is regarded as one of themost important research subjects. At present, there are more than100hundred kinds ofair pollutants, as we known. SO2, CO and NO2are known as three great environmentalpollutions, especially for NO2, their detection and control has the important practicalvalue and business prospects in gas sensors. Among various kinds of new gas sensormaterials, crystalline carbon materials could be applied as room temperature gas sensors,which was due to the superior electrical conductivity, exceeding electronic transmissionefficiency, and the combining crystalline carbon with the traditional metal oxidesemiconductor to form composite materials can be achieved the fast response to the gasat room temperature. In this thesis, based on the understanding of relation betweenmaterial structure and performance of gas sensors, a series of metal oxide/crystallinecarbon gas sensing composites were designed and already prepared. All the preparedgas sensing materials possess unique structure with high performance, high stability andgreat potential of room temperature NOxgas sensing materials. The main researchcontents are as following.(1) Ferroferric oxide/reduced graphite oxide composite (Fe/rGO-400) has beensynthesized with a two-step synthetic method. Firstly, FeOOH/graphite oxide composite(FeOOH/GO) precursors have been prepared by hydrothermal method, and thenFe/rGO-400were obtained after calcinated at400℃under N2atmosphere. The TEManalysis indicate that Fe3O4nanoparticles with a diameter of20~50nm uniformlydispersed on the surface of the rGO. With the increment of Fe3O4load, the size of Fe3O4nanoparticles in composite gradually became bigger. XPS analysis shows that there is astrong interaction between the metal oxide and rGO substrate. The synthesis ofFe/rGO2-400materials has good gas response to NOxgases at room temperature, in which the gas sensitivity to97.0ppm NOxis35.6%and the response time is29.3s.Fe/rGO-400have special layered structures, which is favor for the diffusion, adsorptionand desorption of the target gas. In addition, the introduction of rGO as carbon substrateincrease the conductivity of composite, restrict the growth of FeOOH nanoparticles, it isnecessary for the preparation of the small size, high dispersion of Fe3O4particles.In2O3/reduced graphite oxide composites (In2O3/rGO) have been successfullysynthesized. In the synthesis, GO was used as carbon resource, Indium nitrate as themetal salts, SDBS as surfactant and urea as precipitant. In addition, differentmorphological In2O3have been prepared in absence of GO. When being as the gassensing materials to fabricate into gas sensors, the room temperature NOxgas test havebeen carried out. Studies have shown that the In2O3/rGO have the N type semiconductorcharacteristic, which shows NOxgas sensing performance with high gas response of1.45and the response time of25.0s to97.0ppm NOxat room temperature. In addition,the sensor based on pure In2O3have better response to NOxthan that of In2O3/rGO, thesensor has higher gas response of17.0and the response time of17.3s to97.0ppm NOxat room temperature. The improvement of room temperature NOxgas sensingperformance for the synthesized products is due to the special morphology and porousstructure.(3) The CeO2/graphene-like nanosheet composites (CeGNCs) have beensynthesized via a facile solvothermal reaction by using expanded graphite as carbonsource and cerium nitrate as metal salts. In the synthesis, Ce(NO3)3precursor waspromoted full infusion into the interlayers of expanded graphite (EG) undervacuum-assisted conditions, and then CeO2nanoparticles grow in situ in the interlayersof inexpensive EG under solvothermal condition to form CeGNCs. The results showthat the CeO2particles (about3nm) are highly dispersed on graphene-like nanosheet(>10layers). Especially, the CeGNCs with46.7wt%of CeO2shows higher NOxgassensing performance with low detection limit of5.0ppm, high sensitivity (10.39%), short response time (7.33s) towards100ppm NOx. The enhancement of roomtemperature gas response to NOxfor the composites is due mainly to tiny CeO2nanoparticles and improved conductivity of the composites. CeO2nanoparticles andgraphene like nanosheet can formed a schottky contact, electronics can be quicklyobtained from conduction band and migrated.(4)3D nanoflower-like CuxO/multilayer graphene composites (CuMGCs) havebeen successfully synthesized as a new type of room temperature NOxgas sensor.Firstly, the expanded graphite was used as carbon resource and activated by KOH andmany moderate functional groups were generated; and secondly, the Cu(CH3COO)2andCTAB were promoted full infusion into the interlayers of activated EG (aEG) by meanof a vacuum-assisted technique and then react with functional groups of aEGaccompanied by the exfoliation of aEG via a reflux treatment. Eventually, the3Dnanoflower consisting of5~9nm CuxO nanoparticles homogeneously in situ grow onaEG. The KOH activation EG plays a crucial role in uniformly formation for CuMGCs.When being used as gas sensors for detection of NOx, the CuMGCs achieved a higherresponse at room temperature than that of the corresponding CuxO. In detail, theCuMGCs shows higher NOxgas sensing performance with low detection limit of97ppb, high gas response of95.1%and short response time of9.6s to97.0ppm NOxatroom temperature. Meantime, the CuMGCs sensor presents a favorable linearity, goodselectivity and stability. The enhancement of the sensing response is mainly attributed tothe improved conductivity of the CuMGCs. A series of Mott-Schottky and EISmeasurements demonstrated that the CuMGCs have much higher donor densities thanthe CuxO and can easily capture and migrate electrons from the conduction band,resulting in the enhancement of electrical conductivity.