The Method Of Gas Sensing Research Based On Density Functional Theory Calculations And Its Application

With the rapid development of society,environmental problems are becoming more and more serious,especially the air quality,which poses a threat to people’s healthy life.Metal oxide semiconductor gas sensor has the advantages of simple manufacture,low cost,easy to use,high sensitivity and can be integrated into electronic products,which make it widely used in environmental monitoring,public safety,aerospace and other fields.However,current MOS gas sensors still have problems,such as long response/recovery time,poor selectivity,insufficient stability and high operating temperature.In addition,’trial and error method’ is mainly used to screen specific gas-sensitive materials and sensors,this process not only increases the experimental cost but also prolonging the design cycle,which seriously hinders the industrial application of gas sensors.Therefore,by the means of revealing the sensing mechanism of metal oxide semiconductor gas-sensitive materials,understanding the causes of these problems and proposing corresponding solutions to achieve accurate design of gas sensors,which is the trend of development in the future.Density functional theory(DFT)calculations can theoretically describe the process of adsorption-desorption,electron transfer phenomena,and possible intermediates in redox processes of gas molecules on the surface of metal oxide semiconductor gas-sensitive materials,all of them are essential for a deep understanding of the sensing mechanism.Therefore,it is expected to reveal the sensing mechanism and lay a foundation for the design of future gas sensor by the means of introducing DFT calculation and combining with specific experiments.However,in the field of gas sensing research,there are few methods to study gas sensing mechanism combining DFT calculations with experiments.In this graduation thesis,we selected NO2,H2S,and chlorobenzene as typical target gases and constructed a variety of metal oxide semiconductor gas sensitive materials,developed a series of research method combined DFT calculations with experiments to study the regulation of influence and sensing mechanism of morphology structure,heterostructure,precious metal modification and vacancy defects of sensing materials on the sensing performances.The main research contents are as follows:(1)In view of the sensing enhancement effect of p-n junction of ZnO/Co3O4 composite material on trimethylamine,based on the DFT calculations,we found the electrons in ZnO will transfer from the conduction band to the Co3O4 conduction band during the formation of heterojunction between ZnO and Co3O4,which resulting in the formation of depletion layer and accumulation layer on the surface of the composite material,and ZnO has a stronger adsorption energy value of-3.31 eV for trimethylamine and obtain 0.26 e during the sensing process.Furthermore,in combination with the in situ XPS analysis technique,it was revealed that the gas-sensitive enhancement of the system was caused by the formation of p-n junction,which promoted the adsorption of ZnO for trimethylamine gas molecules and the surface metallization effect of zinc oxide was induced by the trimethylamine.In addition,In view of the gas-sensitive enhancement effect of AuPd double-noble metal alloy modified SnO2 composite material for dimethyl disulfide,the results of DFT calculations exhibited that in the process of AuPd alloy and SnO2 composite,AuPd alloy would transfer electrons to conducton band of SnO2,which promoted the formation of oxygen adsorption on the surface of SnO2.And,as a result of excellent catalytic activity of AuPd alloy,the dimethyl disulfide molecule can easily ionized into CH3S in the surface of AuPd/SnO2 composite material,the transition state energy barrier is 0.15 eV,and CH3S generated on the surface also can easily dissociated into S,the transition state energy barrier is 0.24 eV,and dimethyl disulfide molecules on the surface of SnO2 has larger transition state energy barrier,can not easy to happen the dissociation process.In addition,in combination with ex-situ XPS analysis,it was revealed that the gas-sensitive enhancement of the system was attributed to the promotion of oxygen adsorption on the surface of SnO2 by AuPd alloy and the excellent catalytic activity of AuPd alloy,which reduced the transition state energy barrier of dimethyl dissulfide on the surface of AuPd/SnO2 composite.(2)We successfully synthesized ultra-small ZnFe2O4 nanoparticles via a typical one-step hydrothermal synthetic procedure.We found that our ZnFe2O4 based sensor exhibited an ultrahigh response(Rgas/Rair=247.7)and fast response time(Tres.=6.5 s)and recovery time(Trec.=11s)toward 10 ppm NO2 at a low operating temperature of 125℃,which are superior to the vast majority of NO2 semiconducting sensors,and more importantly,a sensor based on spinel structure materials presenting such excellent sensing performances for NO2 has barely been reported before.Based on DFT calculations,we found that ZnFe2O4 has a strong adsorption energy with a value of-1.32 for NO2 and loses 0.35 e during the adsorption process.Moreover,the high sensing response of ZnFe2O4 nanoparticles to NO2 is realized based on charge transfer through the characterization of ex-situ photoluminescence spectra.In addition,it has been confirmed that the presence of oxygen vacancy can enhance the adsorption energy of ZnFe2O4 for NO2 molecule(△Eads=-1.48 eV)and promote the charge transfer between them.(3)The pristine ZnFe2O4 yolk-shell spheres composed of ultrathin nanosheets and ultrasmall nanoparticles decorated with nano-sized Au particles with diameter of 1-2 nm are fabricated using the method of solution-phase deposition-precipitation.As a result,the Au@ZnFe2O4 composite based sensor exhibits significantly sensing performances for chlorobenzene.In comparison with pristine ZnFe2O4,the response(Rair/Rgas=90.9)of Au@ZnFe2O4 composite based sensor with a low detection limit of 100 ppb increases four-fold exposed to 10 ppm chlorobezene at 150℃.Excitingly,the sensing response for chlorobenzene is the highest among metal oxides based sensors and the Au@ZnFe2O4 composite based sensor also exhibits outstanding selectivity.In additional,the relationship between the sensing performances of pristine ZnFe2O4 and Au@ZnFe2O4 composite for chlorobenzene and the factors of Au loading amount,operating temperature and humidity was also fully investigated in this paper.The method of matrix change is proposed to identify gas species and improve the selectivity.In addition,based on the DFT calculations,we found that some electrons in ZnFe2O4 will transfer to Au surface during the process of Au nanoparticles compounding with ZnFe2O4,which resulting in the formation of depletion layer on the surface of the composite.Moreover,Au/ZnFe2O4 composite has a stronger adsorption energy for chlorobenzene(△Eads=-3.19 eV),and can obtain 0.164e in the adsorption process.Therefore,it was revealed that the gas sensitivity enhancement mechanism of Au/ZnFe2O4 composite system for chlorobenzene was originated from the formation process of Au/ZnFe2O4 composite,which promoted the increase of oxygen content adsorbed on the surface of Au nanoparticles and the adsorption of Au/ZnFe2O4 to chlorobenzene molecule.(4)By means of in-situ growth and high temperature annealing with different heating rates,the porous NiO nano wires possessed oxygen vacancies were prepared.The materials have nice sensing response to H2S and excellent selectivity.At the same time,we used DFT calculations to study the influence of oxygen vacancy concentration on the electronic structure of NiO.It was found that with the increase in the number of oxygen vacancies,the more the oxygen vacancy occupied the 3d orbitals of Ni,the more obvious the degree of electron localization,and the more obvious the electron interaction of each atom in NiO.It was further found that the optimal adsorption sites of H2S,HS,S and H on the surface of NiO(NiO)without oxygen vacancy were all located at the T-Ni site except S at the B or H sites,and the corresponding adsorption energies of H2S,HS,S and H were-0.73 eV,-2.47 eV,-2.49 eV and-2.09 eV,respectively.The optimal adsorption sites of H2S,HS,S and H on the surface of NiO(Vo-NiO)with oxygen vacancy were all located at T-O site(oxygen vacancy)except H2S at the T-Ni site,and the corresponding adsorption energies were-0.97 eV,-3.59 eV,-5.43 eV and-2.57 eV,respectively,which were significantly higher than the adsorption energies on the surface of NiO.In addition,we also discovered the dissociative path and transition state energy barrier of H2S molecule on the surface of the NiO and Vo-NiO.The correspondings energy barrier of H2S molecule dissociated into HS and H was 0.70 eV on the surface of the NiO.The energy barrier of HS dissociated into S and H was 1.53 eV.However,the energy barriers of the above two dissociation process in the surface of Vo-NiO were 0.37 eV and 0.17 eV,respectively,which showed H2S molecule on the surface of Vo-NiO is easily dissociated into S and H.In addition,in combination with the ex-situ XPS analysis technology,the gas-sensing enhancement mechanism of oxygen vacancy in NiO for H2S was revealed.Specifically,the presence of oxygen vacancy can increase the charge density on the surface of NiO,which promoted the adsorption of H2S,HS,H and S on the surface of NiO,and reduced the energy barrier which H2S molecule need to overcome when they dissociated to form S and H on NiO surface.