Design Strategy Of High-Performance Semiconductor Gas Sensor And Sensing Characteristics Of P-n Core-Shell Nanofibers

Metal oxide semiconductor(MOS)gas sensors,a kind of solid-state resistive-type gas sensors,have been widely applied in numerous fields including toxic-and explosive-gas alarms,air quality monitoring,indoor chemical pollutant monitoring,medical diagnostics via breath analysis and so on.These sensors' widespread application domains are due to their high sensitivity,low cost,simplicity,flexibility in production and compatibility with modern electronic devices.At present,in order to improve the performance of metal oxide gas sensors,the development and research of high-performance gas-sensitive materials have been of great concern.In this paper,high-performance metal oxide gas-sensing materials are taken as the research object,and the sensitivity mechanism and performance enhancement strategy of them are studied from two aspects of theoretical summary and experimental exploration.The main work is as follows:(1)After a survey of literature,we have summarized recent progress in engineering crystal structures with exposure of high-energy facets of MOS nanomaterials and their improved gas-sensitive performance and discussed the relationship among crystal morphology,surface structure and gas-sensing properties.For the same type of MOS,either the surface morphology or the specific surface area value cannot totally determine their sensing ability to certain gas.It is the crystal planes exposed on the surface that are the fundamental factor in determination.According to the survey of literature and our calculation,we conclude that the appropriate exposed facet is(0001)facet for wurzite-type ZnO,(332)facet for rutile-type SnO2,(001)facet for anatase-type TiO2,(113)facet for hematite(?-Fe2O3),(111)facet for NiO and(110)facet for Cu2O.To sum up,exposing the crystal surface with more unsaturated metal caitons is the most promising approach to realize superior gas sensing performance.During the gas sensing process,the dangling bonds created by metal cations on the exposed surface will provide sufficient active sites for gas molecules adsorption and react,resulting in excellent gas sensing properties of MOS.(2)One-dimensional metal oxide nanofibers with core-shell structure were fabricated by coaxial electrospinning.Among them,n-type and p-type semiconductor material were selected as core and shell material respectively.The material systems chose in this paper were Co3O4-ZnO core-shell nanofibers and Cr2O3-SnO2 core-shell nanofibers,respectively.The effects of p-n heterojunctions on the gas sensing properties of core-shell nanofibers were systematically studied by testing and analyzing the gas sensing properties of core-shell nanofibers.It is found that compared with ZnO-Co3O4 core-shell nanofibers,pristine ZnO nanofibers and Co3O4 nanofibers,Co3O4-ZnO core-shell nanofibers showed superior sensing properties and typical p-type gas sensing properties.This is due to the formation of p-n heterojunctions with discontinuous conduction band at the interface between core and shell materials.Because of the special energy band structure of heterojunctions,the gas sensing reaction of materials is not only determined by the outer material of fibers,but also by the interaction of core layer,shell layer and the heterojunction.For SnO2 and Cr2O3 systems,SnO2-Cr2O3 core-shell nanofibers showed higher response to target gases.At the same time,the response of nanofibers with core-shell structure to target gas were improved compared with composites with similar Cr2O3 content.This emphasized the importance of core-shell structure to enhance the gas ability of materials.In addition,the heterojunction with continuous energy band in Cr2O3-SnO2 core-shell nanofibers makes the gas sensing properties of the materials mainly determined by the change of grain boundary barrier of the particles.The gas sensing properties of SnO2-Cr2O3 core-shell nanofibers were determined by the shell material Cr2O3,showing a p-type gas-sensing characteristics.And because of the formation of heterojunction,there are more lattice mismatches in the material,and more defects and hanging bonds will be formed at the core-shell interface,which will be very helpful to improve the gas sensing properties of the material.Through the above two aspects of work,this paper not only provides an experimental basis for the optimization of metal oxide semiconductor gas sensing materials,but also provides a theoretical basis and reference for the design of high-performance gas sensing materials.