Adsorptions Of Some Hazardous Gases On SnO₂ Based Gas Sensing Materials By First-principles Calcuations

SnO2 gas sensor is a typical metal oxide semiconductor(MOS)gas sensor,it has the advantages of high sensitivity,fast response time,low price and so on,and it has been used to detect many kinds of toxic and harmful gases.Graphene is a semiconductor material which has a zero band gap,it has a large specific surface area and enlarge the contact area between the gas sensor and the gases.So graphene can be used as a new type gas sensor material.Typically,the gas-sensing properties of the pure SnO2 have some shortcomings,such as poor selectivity,high working temperature and so on.The gas-sensing property of the pure graphene is not good,but we could prepare a nano SnO2/GO composite gas sensing material if we take advantage of the heterojunction effect,the advantages of the material could include low working temperature,high sensitivity,quick response and recovery and so on.In this paper,we calculate the adsorption capacities when the four harmful gases(dichloromethane,chloroform,tetrachloromethane and trichloroethylene)are adsorbed into SnO2(110)surface by using the first principle,study the adsorption energies,the charge transfer,and the change of electronic structure between the adsorption processes.In addition,we build three heterojunction models,and study the heterojunction interface reaction,the change of electronic structure and the charge transfer during the formation of heterojunction.Finally,we analyse the adsorption capacity when the trichloroethylene molecule is adsorbed into the surface of SnO2/GO heterojunction.The conclusions are as follows:(1)When three methane chloride molecules are adsorbed onto the SnO2(110)surface,the adsorption process is an exothermic process,the adsorption system will release energy to the outside world.The heat release of dichloromethane is the biggest and the heat release of tetrachloromethane is the fewest.The methane chloride molecules will transfer charge to the SnO2(110)surface between the adsorption process,the transfer charge is0.249e of dichloromethane,0.192e of chloroform,0.144e of tetrachloromethane,the electronic conductivity of SnO2 increases and the resistance reduces after the adsorption process.The electronic structure of SnO2 changes after the adsorption,the adsorptions of dichloromethane and trichloromethane make a band gap appeared in the forbidden band of SnO2,the appearance of the band gap will improve the ability of electronic transition and the electronic conductivity of SnO2 increases,the adsorption of tetrachlormethane makes the widths of the valence band and conduction band of SnO2 narrow down.Taking into account of all the factors of the adsorption energy,the transfer charge,and the change of the electronic structure,it can be analyzed that the detection performance of dichloromethane is the best,the detection performance of chloroform comes second,and the detection performance of tetrachloromethane is the worst.(2)O2 would be adsorbed onto the SnO2(110)surface when the gas SnO2 sensor is exposed to the air,O2 becomes O2~-after it gets electrons from the surface and the resistance of SnO2 increases.The O-O bond will break down and O2~-divides into two oxygen ions,an oxygen ion is connected to a tin atom by an ionic bond.The free oxygen ion will react to the methane chloride molecules,a new substance-ClCH2OCl after the methane chloride molecule reacts to the oxygen ion.The chloroform molecule splits into a chlorine atom and CHCl2~+,the chlorine atom is combined with the oxygen ion to form OCl~-.The tetrachlormethane molecule splits into a chlorine atom and CCl3~+,the chlorine atom is combined with the oxygen ion to form OCl~-.The adsorption process is an exothermic process,the adsorption system will release energy to the outside world.The electronic conductivity of SnO2 increases and the resistance reduces because SnO2 get electrons after the adsorption process.(3)When a heterojunction is formed by SnO2 and graphene,the bridging oxygen atoms in the SnO2 which are closed to graphene and the carbon atoms in the graphene bond together by covalent bonds.That enhances the connection force of the heterojunction structure,thus the stability of the heterojunction structure is increased.The electronic conductivity of SnO2 increases because it get electrons as 0.88e from graphene.There is a successive band distribution appeared between the conduction band and the valence band,the ability of electronic transition and the electronic conductivity of SnO2 increases.(4)When the trichloroethylene molecule is adsorbed onto the SnO2(110)surface,the adsorption process is an exothermic process,the adsorption system will release energy to the outside world.The electronic conductivity of SnO2 increases because the trichloroethylene molecule transfers charge of 0.18e to the surface.A band gap appeared in the forbidden band of SnO2,the appearance of the band gap will improve the ability of electronic transition and the electronic conductivity of SnO2 increases.When the trichloroethylene molecule is adsorbed onto the surface of SnO2/GO heterojunction,the adsorption process is an exothermic process,the adsorption system will release energy to the outside world.The single graphene layer has a structural deformation,the middle structure move far away from SnO2 and the covalent bonds between the carbon atoms in the sunken parts and the bridging oxygen atoms are broken,the structure of both sides keep close to SnO2.The trichloroethylene molecule transfers charge of 0.05e to the SnO2/GO heterojunction,the heterojunction have a charge redistribution,the charge is transferred from SnO2 to graphene.