Study Of Carbon-Metal Oxide Composites For Ammonia Gas Sensor

Metal oxide semiconductor gas sensors have the advantages of simple structure,high sensitivity and fast response.New carbon materials like graphene have advantages such as high conductivity.Exerting the synergy of them is a new way to design advanced gas sensor and to solve the problems of metal oxide semiconductor gas sensor,such as the poor stability,elevated working temperature and poor selectivity.On the other side,it also overcomes the problem that the carbon materials are not such good gas sensors alone.There are three main purposes of this paper.Firstly,based on the percolation theory model,the critical concentration range of n-p phase transition was predicted after the mixing of p-type rGO and n-type SnO2.rGO/SnO2 composite based is prepared using physical mixing method.The critical concentration range was verified by experiments based on the rule that the resistance of the sensing material will change as it has response to reducing gas;Secondly,a new composite structure of amorphous carbon/reduced graphene oxide(rGO)-wrapped Co3O4 nanofibers is conducted and characterised,and the microstructure and conductance of the composite is finely tuned to optimize the sensing performance by using in situ oxidation etching;Thirdly,gas sensing mechanism of amorphous carbon/reduced graphene oxide(rGO)-wrapped Co3O4 nanofibers is analysed by the density functional theory calculation.Based on the percolation theory model with consideration of 2D structure of rGO,the critical concentration range of n-p phase transition was predicted after the mixing of p-type rGO and n-type SnO2.RGO-SnO2 composite based gas sensors with different rGO mass fractions in this range were prepared.The critical concentration range of n-p phase transition sensing behavior of rGO-SnO2 composite based gas sensors was verified by experiments based on the rule that the resistance of the sensing material will change as it has response to reducing gas.It was found that the composite tended to be stable and the response speed increased after the transformation from n-type to p-type.The switch from n-type response to p-type response may be due to the doping of ammonia to graphene matrix.The n-type response to ammonia is due to the n-type SnO2 in the composite and the auxiliary role of rGO.The enhanced p-type response derived from p-type rGO in the composite may be attributed to the formation of p-n junction between n-type SnO2 and p-type rGO,and SnO2 nanoparticles that prevented the agglomeration of rGO layers.The p-type response behavior showed better gas sensing performance to ammonia gas,which has a good guiding significance for the further design of high-performance sensitive materials based on rGO-metal oxide composite.Amorphous carbon/reduced graphene oxide(rGO)-wrapped Co3O4 nanofibers was preprared by electrospinning process.The microstructure and conductance of the nano fiber composite material formed is further improved systematically by finely tuning the amount of amorphous carbon using in situ oxidation etching.The prepared sensor was optimized and indicated a good sensitivity and the fast response(?20 s:50 ppm)to different concentrations of ammonia from 1-100 ppm and good selective to methanol,ethanol,formaldehyde,toluene,benzene,acetone and water at room temperature,and a brilliant long-term stability for about four-years.The behind chemical sensing process is examined using the in-situ Fourier transform infrared spectroscopy indicating a water-mediated catalytically oxidation transducing mechanism.DFT theoretical calculation was conducted.The model of rGO/Co3O4(110B)interface to absorb O2,H2O and NH3 was established.Through the optimised calculation results including energy,DOS,charge transfer,etc,it is found that O2 can promote the adsorption of NH3.Different coverage level of carbon on Co3O4(110B)would affect the adsorption of NH3.When the coverage level is low,the coupling of rGO/Co3O4(110B)interface enhances the adsorption of NH3 by the Co-C bond.The C atom on the Co-C bond has a stronger adsorption for NH3 because of the coupling effect which explained theoretically why samples with rGO have higher response to ammonia.It was found that by H2O had both promoting and hindering effects to the adsorption of NH3 by simulation and analysis of effects of H2O on ammonia adsorption.When adsorption of NH3 occurs at the good NH3 adsorption site of Co-C,the H2O and NH3 promote dissociated adsorption to each other and bring about a larger charge transfer which proved that H2O play an important role in catalytic oxidation conversion mechanism of carbon-wrapped Co3O4 nanofibers composite on ammonia.