Research On Surface Properties Regulation And Sensing Properties Of Low-dimensional Tin Oxide

With the increasing demand for energy and manufacturing,air pollution has become inevitable.Previous studies have shown that these pollutants are harmful for human health.Therefore,it is imperative to develop high-performance sensing materials to detect toxic gas.Semiconductor metal oxide based（SMO）chemical resistive gas sensors have been widely used in semiconductor manufacturing,diagnostic medicine,biomedicine,and environmental monitoring due to their low cost,easy integration,and real-time detection.As an important n-type semiconductor,tin oxide（SnO₂）has received great attention in the field of gas sensors.Various toxic and flammable gases can be detected by SnO₂-based gas sensors,includ ing CO,H₂S,acetone,formaldehyde,3-hydroxy-2-butanone,etc.At present,great efforts have been devoted to improving the sensing performance by finely tuning the SnO ₂ surface structure.Among the numerous types of structures,two-dimensional（2D）nanosheet structures have attracted extensive attention due to their structural advantages such as high exposure of surface atoms,large specific surface area,and easy modification.The current methods for synthesis of 2D nanosheets suffer from cumbersome steps and low yields,so it is important to develop a new method to synthesize 2D nanosheets and apply it to the field of gas sensing.This study found that the construction of heterojunctions is an effective strategy to improve the gas sensing performance of S n O₂-based gas sensors.In this dissertation,we investigate the preparation of low-dimensional SnO₂ gas sensing materials and their enhanced sensing mechanisms.The work can be divided into three sections（corresponding to the synthesis and synthesis mechanism of porous metal oxide nanosheets,the construction of heterojunctions to generate asymmetric oxygen sites,and the construction of in-plane heterojunctions）:（1）Crystalline metal oxide nanosheets show exceptional catalytic performance owing to the large surface-to-volume ratio and quantum confinement effect.However,it is still a challenge to develop a facile and general method to synthesize metal oxide nanosheets.Herein,we report a cocrystallization induced spatial self-confinement approach to synthesize metal oxide nanosheets.Taking the synthesis of SnO₂ as an example,the solvent evaporation from KC l and Sn C l₂ solution induces the cocrystallization of KCl and K₂Sn C l₆,and the obtained composite with encapsulated K₂Sn C l₆ can be in situ converted into SnO₂ nanosheets confined in KC l matrix,after water washing to remove KCl,porous SnO₂ nanosheets can be obtained.Notably,a series of metal o xide nanosheets can be obtained through this general and efficient green route.In particular,porous Ce O₂/SnO₂ nanosheets with improved surface O^-species and abundant oxygen vacancies exhibit superior gas sensing performance to3-hydroxy-2-butanone.（2）The local environment of active sites and the number of oxygen vacancies can greatly affect the reactivity of semiconductor metal oxides（SMOs）.However,rare work has been reported to investigate the relationship between the state of oxygen vacancies and sensing performance of SMOs.Herein,a series of porous Ce O ₂-SnO₂ hetero-structure nanosheets with fine modulation of the local environment active sites and oxygen defect activity have been successfully constructed.We have investigated their sensing perfo rmance towards 3-hydroxy-2-butanone,which is a biomarker of food pathogenic microbe Listeria monocytogenes.We found that the amount of asymmetric Ce-O-Sn sites rather than total oxygen vacancies amount of Ce O₂/SnO₂ materials can greatly affect their sensing performance.Impressively,the porous Ce O₂/SnO₂-400 nanosheets,which possessed abundant active O^-（ad）species originated from the asymmetric Ce-O-Sn sites,exhibited high sensitivity（Ra/Rg=637.94 to 50 ppm）,fast response（29 s）and recovery（172 s）,excellent selectivity and high stability toward 3-hydroxy-2-butanone at a work ing temperature of160℃.Both the surface O^-（ad）species associated with the asymmetric oxygen sites and porous nanosheet hetero-structure contribute to the enhanced gas adsorption and diffusion process,further boosting their sensing performance.Our work provides new sights for identification of active sites in sensing materials as well as paves the way for detection of pathogenic microbes in food.（3）The huge challenge for detection of formaldehyde（HCHO）,a toxic volatile organic compound for human health,with ppb level at low working temperature lies in the activation of surface oxygen species of sensing materials.Herein,we have designed in-plane SnO₂-Sn₃O₄ heterojunctions with hierarchical nanoflower morphology for efficient HCHO sensing,and the lattice strains,surface properties as well as the electronic structure around the SnO₂-Sn₃O₄ phase boundaries can be finely optimized.Notably,lattice strain and Schottky junction dual regulation ensures the in-plane SnO₂-Sn₃O₄ heterojunctions with excellent HCHO sensing performance at low working temperature of 120°C.Particularly,Sn₃O₄-400 displays high sensing response（Ra/Rg=637.94,50 ppm）,high repeatability and excellent selectivity to HCHO.Furthermore,geometric phase analysis,multiple structural analysis and DFT calculations demonstrate the lattice strain and Schottky junction dual regulation tunes the surface oxygen properties and electron structure,which facilitate the formation of rich surface O^-（ad）species and enhance the interaction between materials and HCHO around the abundant SnO₂-Sn₃O₄ phase boundaries,thus enhancing the HCHO sensing performance.This research provides nove l ideas for selecting high-tech sensing materials for the detection of dangerous volatile organic chemicals.