Study On The Improvement Of Gas Sensing Properties Of In₂O₃ Nanofibers By Doping And Constructing Heterojunction

With the progress of industrial production and the improvement of human life quality,environmental problems have been one of the most concerned topics all over the country and even the world.The establishment of timely gas environment supervision mechanism and the construction of environmental Internet of things has become an urgent problem to be solved.The gas sensor is the core of the gas detection system.The gas sensor is a kind of converter that transforms the kind and concentration of a gas into the corresponding electrical signal.At present,gas sensors are widely used in the detection of toxic,combustible,explosive and organic volatile compounds?VOC?,and they are the most core and basic part of the environmental Internet of things.High performance gas sensor can improve the ability of gas information collection and processing,improve the accuracy of real-time accident prediction,and greatly reduce the occurrence of major accidents.At the same time,it can effectively realize the electronization of safety production supervision and management,change passive disaster relief to active disaster prevention,and make safety production scientific.Among them,the metal oxide semiconductor?MOS?material has become the most widely used sensing material because of its many advantages,such as low manufacturing cost,simple manufacturing,high sensitivity,fast response,long life and simple detection circuit structure.When the MOS-based sensor is in contact with the target gas,the contact reaction of the gas molecules to be measured at the surface/interface of the MOS material involves the steps of charge transfer,ion transport,phase generation and transformation,which leads to the recoverable change of the electrical properties of the sensor.This change can be used to effectively identify the type and concentration of the gas to be measured.However,the disadvantage of MOS materials in gas sensor application is that it must work at high temperature and have no different response to many gases.The surface activity of gas sensing materials decreased at low temperature,it is difficult to reduce the surface potential energy by adsorption of the target gas,so they can not meet the requirements of practical application.As a typical n-type MOS material,In₂O₃ has wide band gap,high conductivity,high light transmittance,and has a high response to NO₂,H₂S and VOC.However,there are still some problems such as high temperature and poor selectivity,which are common shortcomings of all MOS.Therefore,by analyzing the sensitive mechanism of MOS gas sensor and various factors affecting its performance,this paper puts forward some methods to improve the gas sensing performance of In₂O₃ based gas sensor,such as common metal doping,noble metal doping,bimetallic doping,rare earth element doping and building core-shell heterojunction structure,Study on adsorption principle and charge transfer law of gas-solid interface.Materials are mainly synthesized by electrospinning method.Because the electrospinning device is simple,and it is easy to synthesize one-dimensional nanofibers with high surface area,high porosity and electronic transmission,which is the most ideal structure of gas sensitive materials.This paper is mainly divided into the following aspects:?1?Common metal doping improves the gas sensitivity of In₂O₃ nanotubes.Firstly,In₂O₃ nanotubes with different Sn doping concentrations?ITO?were prepared by single axis electrospinning.The influence of doping on the morphology and structure of nanotubes was studied,and its gas sensing performance to formaldehyde gas was tested.The results of gas sensing test show that ITO nanotubes with 7 mol%Sn doping have the best response to formaldehyde gas at lower operating temperature.It is found that Sn doping can increase the oxygen vacancy concentration of ITO nanotubes,and when the doping concentration is 7 mol%,the oxygen vacancy concentration of ITO nanotubes is the largest,so it can improve the electrical performance and chemical catalytic activity of ITO nanotubes,thus enhancing the gas sensing response.Then,In₂O₃ nanotubes with different Ni doping concentrations?NIO?were prepared by the same method.By analyzing the gas sensing performance,it can be found that:Ni doping does not reduce the working temperature of the sensor,but improves the response and selectivity of the sensor to ethanol,and when the concentration of Ni doping is 7 mol%,the response to 100 ppm ethanol reaches 49.74at 220?,which is 3.7 times of the response of the sensor to pristine In₂O₃ gas?13.39?.The enhancement mechanism of gas sensing performance is explained as follows:compared with Sn doping,Ni doping not only introduces more oxygen vacancy concentration,but also realizes the adjustment of adsorbed ions on the surface of the material and the adjustment of carrier concentration and lifetime at the interface due to the catalysis of Ni itself and the formation of NiO/In₂O₃heterojunction.It not only improves the gas sensing response of the gas sensor based on In₂O₃,but also greatly improves the selectivity of ethanol.The results of the first principle calculation also show that the adsorption energy of ethanol on the Ni doped In₂O₃ surface is enhanced.?2?Noble metal doping and bimetallic doping enhance the gas sensing properties of In₂O₃ nanotubes.Firstly,the In₂O₃ nanofibers modified by Au were prepared by one-step electrospinning,and the morphology and structure were characterized in detail.Different with common metal doping,Au did not enter the In₂O₃ lattice,but modified on the surface of the In₂O₃ nanotubes.The results of gas sensing show that the Au modification significantly reduces the optimal working temperature of the gas sensor based on In₂O₃,from 220? to 160?.However,the gas sensitive response of the Au-In₂O₃ nanofibers was not significantly improved.Combined with previous research conclusions,the Ni/Au bimetallic modified In₂O₃ nanotubes were further prepared by one-step electrospinning.The combination of Ni doping and Au doping not only improves the response of the sensor,but also reduces the working temperature of the sensor.The results of gas sensing test show that the optimal temperature of the Ni/Au bimetallic modified In₂O₃ nanotubes is reduced from 220? to 180?,and the response to 50 ppm ethanol is 18 times higher than that of the unmodified In₂O₃ nanotubes.In-depth analysis shows that there are two main reasons for improving the gas sensing performance of bimetallic doping,one is the influence of Ni catalysis on the adsorbed ions,and the regulation of interface carriers by NiO/In₂O₃ heterojunction;the other is the chemical sensitization caused by the spillover effect of Au,and the formation of Au/In₂O₃ Schottky barrier to jointly control the properties of surface adsorbed ions and interface carriers.?3?Rare earth element doping improves the performance of In₂O₃ based gas sensor.Five kinds of rare earth elements?Ce,Tm,Eu,Er and Tb?doped In₂O₃nanotubes were prepared by single axis electrospinning.Firstly,the influence of different rare earth elements doping on the morphology and structure of the nanotubes was studied.Secondly,the results of gas sensing test show that the response of Tb-In₂O₃-based sensor is the most obvious improvement compared with other rare earth element doped In₂O₃ gas sensors.The response of Tb-In₂O₃ to 100 ppm ethanol reached 157.27 at its optimum operating temperature of 220?,which is 11 times higher than that of In₂O₃ at the same condition?14.09?.Finally,the enhancement mechanism of rare earth element doping on the gas sensor based on In₂O₃ is analyzed.It is found that the rare earth element doping is similar to the general metal element doping,and the rare earth ion entering into the In₂O₃ lattice results in lattice distortion and grain size reduction.However,different rare earth doping can affect the adsorption type of oxygen ions on the surface of In₂O₃,for example,Ce doping enhances the adsorption of water molecules and hydroxyl groups,which is unfavorable to the gas sensing reaction,while Tb doping enhances the adsorption of oxygen ions,thus enhancing the gas sensing response compared with the commen metal doping.?4?Construction of core-shell heterojunction to enhance the performance of gas sensors.The In₂O₃@ZnO n-n core-shell nanofibers?IZO CSNF?and the NiO@ZnO p-n core-shell nanotubes?CSNT?with controllable shell structure and thickness were prepared by electrospinning and atomic layer deposition?ALD?methods.By adjusting ALD parameters to control the thickness of the shell,the morphology changes of core-shell structure in the formation process were studied.The results of gas sensitivity test show that the thickness of shell ZnO has a great influence on the gas sensitivity of heterojunction materials,whether it is IZO CSNF or NiO@ZnO CSNT.It is found that the formation of heterojunction improves the charge transfer process,enhances the regulation of carriers,and enhances the gas sensing response by enhancing the electrical properties of the sensor material.This paper aims at developing a high response,high selectivity,and low operating temperature gas sensor based on In₂O₃ nanotubes.The relationship between the morphology and crystal structure of one-dimensional In₂O₃ sensitive material and the performance of gas sensor was systematically studied.Through doping and constructing heterojunction,the performance of the In₂O₃-based sensor is improved,and the application of In₂O₃ material in the field of gas sensor is enriched,which provides experimental basis and theoretical support for the development of high-performance sensors.