Construction Of Metal Oxide Heterojunction Porous Nanomaterials Based On Nanofibers And MOFs And Their Gas Sensing Properties

With the rapid development of the Internet of Things,the applications of gas sensors have been penetrated into various fields,such as environmental monitoring,medical diagnosis,industrial production,resource exploration,family life,consumer electronics and so on.Metal oxide semiconductor（MOS）gas sensors have shown great application potential in above fields because of their unique advantages.However,there are still many problems that need to be improved for MOS gas sensors,such as low response value,slow response time（rres）and recovery time（τrec）or incomplete recovery at low temperature,high resistance,insensitivity to low-concentration gas,high operating temperature,poor selectivity and stability,and unclear sensing mechanism.The sensing materials are the core of gas sensors.Design and controllable construction of high-performance gas sensing materials are the key to improve the sensing performance of gas sensor.、Based on the above background and questions,the purpose of this dissertation is,to develop high-performance sensing materials for gas sensors.A variety of porous MOS heterojunction nanomaterials have been constructed based on the two kinds of porous materials,nanofibers（NFs）and metal-organic frameworks（MOFs）.The intrinsic relationships between the compositions,microstructures,heterojunction types of sensing materials and the gas-sensing performances have been systematically explored.The composition,pore size,surface,interface of sensing materials have been reasonably regulated by creating and optimizing the preparation methods.Several new high-performance MOS heterojunction sensing materials have been successfully constructed,and several new preparation strategies have been created.The main research works of this dissertation includes the following aspects:（1）Porous NiO-WO3 p-n junction NFs with different Ni/W molar ratios have been constructed by electrospinning technique.The composite NFs exhibited effective p-n heterojunctions,high oxygen species absorbing capacity,high surface area,small grain size and special porous structural feature,contributing to the high sensitivity and short τres and τrec to acetone gas.（2）The development of highly sensitive gas sensors under high humidity condition is still a difficult problem.Porous MoO3-WO3 n-n junction NFs with different Mo/W molar ratios were constructed by electrospinning technique.Through the modulation of Mo/W molar ratios,MoO3-WO3 composite NFs exhibited effective n-n heterojunctions,high specific surface area,abundant porous structure and high surface oxygen adsorption capacity,which made the composite nanofibers highly sensitive and selective to acetone gas even under high humidity conditions.（3）The sensor having both good sensing properties and low resistance is beneficial to its practical application.Thus,porous rGO-WO3 composite NFs were constructed by electrospinning technique.The result showed that the ohmic contact was successfully formed between rGO nanosheets and WO3 nanoparticles（NPs）,which improved the sensing properties of WO3 and reduced the resistance of the sensor.Due to the formation of ohmic contact,ultrahigh surface area and strong gas adsorption capacity of rGO nanosheets,as well as special porous structural features of the rGO-WO3 NFs,the rGO-WO3 NFs exhibited high sensitivity,high selectivity and rapid response to acetone gas.（4）There is no effective method to synthesize ultra-small size noble metal nanoclusters（<2 nm）to modify sensing materials.For the first time,ultra-small Ag nanoclusters with precisely regulated atomic numbers（Agx:atomic number x=55,175,600）have been modified on the surface layer of WO3 nanofibers.The sensing properties of Agx—WO3 NFs to NO2 have been significantly improved at low temperature,including high response,short τres and τrec,low detection limit down to 10 ppb,and excellent selectivity and complete recovery at low temperature.The reasons of the excellent sensitivity and selectivity of the Agx-WO3 NFs to NO2 were analyzed.This study showed that the modification of sensing materials by using,Mass Selective Cluster Beam Source Technology" was an effective method to improve the gas sensing properties of MOS,which produces a new strategy for the construction of ultra-high performance MOS gas sensor.（5）Increase of porosity and functionalization with nanoscale catalysts are two significant ways for achieving high-performance MOS gas sensors.Mesoporous ZnO ZnO hollow nanocages（NCs）with uniform distribution of nanoscale Ag catalyst were constructed by MOFs method.The obtained Ag—ZnO NCs possessed porous topography,high surface area,and gas accessibility.Much improved sensing properties of the Ag-ZnO NCs have been obtained,including high response,short Tres and τrec,good ethanol selectivity,and reproducibility.The contributions of the electron sensitization and catalytic effects of Ag nanocatalysts,porous and hollow structures,high surface area and high surface O-species absorbing capability of Ag-ZnO NCs to ethanol sensing were revealed.This study provides a reasonable strategy for the construction of precious metal nanocatalyst to modify the porous MOS sensing materials.（6）Design of appropriate heterojunction MOS with unique nanostructures and abundant porosities is a crucial strategy to develop superior gas sensors.A new kind of MOFs-based porous Co3O4-ZnO p-n junction NFs was constructed by combining MOFs method and electrospinning technique for the first time.The emerging heterojunction NFs integrated the superiorities of the two kinds of porous materials of BMOFs and NFs,thereby showing abundant wormhole-like pores,high surface area and good anti-aggregation property,making that the Co3O4-ZnO p-n junction NFs exhibited high sensitivity,good selectivity and stability to ethanol.This study provides a general strategy for the construction of multiple types of MOFs-based porous MOS heterojunction network nanofibers for high-performance gas sensing application.（7）Finally,the superiorities of three kinds of nanomaterials of MOFs,rGO nanosheets and NFs were integrated.A new kind of the networked porous rGO-ZnO composite NFs was constructed by combining MOFs method and electrospinning technique.The networked NFs exhibited effective p-n heterojunctions,abundant pores,high surface area,which were conducive to the diffusion and adsorption of gas molecules.Thus,the rGO-ZnO NFs exhibited high sensitivity,short τres and τrec to isopropanol.The results showed that MOFs and rGO-based composite NFs can be used as a new porous nanostructure to construct highly sensitive and selective gas sensors.