Preparation And Formaldehyde Sensitivity Of Hierarchical Nanosheet/Nanofibers Gas-Sensitive Composites

Formaldehyde is a colorless but irritating toxic gas.Long-term exposure can cause coughing,nausea and sneezing,and even death from poisoning.However,the formaldehyde sensors in the market still have the disadvantages of high cost,low sensitivity and poor limit of detection,which can not meet the actual application demand.Therefore,the design and development of low-cost and high-efficiency formaldehyde gas sensors are of great significance for environmental protection and safety protection.This paper is mainly aimed at the problems of low sensitivity,poor selectivity,long response/recovery time and poor stability of formaldehyde gas sensors prepared by traditional nanomaterials.Taking the design and construction of new nanosheets/nanofibers hierarchical structure materials as the starting point,it fully combines the structural characteristics and advantages of nanofibers and nanosheets to realize the design and development of new formaldehyde sensitive materials.By studying the effect of the hierarchical structure on the gas sensing performance of the material,the gas sensing mechanism of the hierarchical core-shell nanomaterials was clarified,and the application value of the nanosheets/nanofibers hierarchical structural materials in the field of formaldehyde sensors was explored.The research work mainly includes the following three parts:(1)Preparation of SnO₂ nanofiber/nanosheets hierarchical structure materials and study on gas sensing performance:SnO₂ nanofibers prepared by electrospinning method are used as main materials,and SnO₂ nanofibers/nanosheets with hierarchical structure are prepared by hydrothermal method.The morphology of SnO₂ NF/NSs is that the nanosheets(NSs)arrays are evenly distributed on the surface of nanofibers.Furthermore,the gas sensing performance is investigated.The results show that at an optimal operating temperature of 120°C,the sensitivity of SnO₂ NF/NSs to 100 ppm formaldehyde reaches 57,the limit of detection is 500 ppb,and the response/recovery time is 4.7 s/11.6 s,respectively.In addition,in order to study the gas sensing mechanism of hierarchical structure materials,the single structure SnO₂ NFs,SnO₂ NSs and SnO₂mixture(NFs-NSs-Mix)gas sensors are prepared and tested.The results show that compared with SnO₂ NFs,SnO₂ NSs and SnO₂ NFs-NSs-Mix gas sensors,the sensitive performance of hierarchical structure materials can be improved by 2?3 times under the same test conditions.The improvement of sensitive performance can be attributed to the synergism of nanosheets and nanofibers,the construction of hierarchical structure and the large specific surface area.(2)Graphene oxide(GO)sensitized SnO₂ nanofiber/nanosheets:In order to further improve the sensitivity of the material to formaldehyde,using GO as a sensitizer,a three-dimensional hierarchical GO@SnO₂ NF/NSs nanocomposite sensitive material was prepared.The gas sensing test results show that the optimal working temperature of GO@SnO₂ NF/NSs nanocomposites is 60?,and the optimal GO compounding ratio is1 wt%.At the optimal working temperature,the sensitivity of 1 wt%GO@SnO₂ NF/NSs nanocomposite to 100 ppm formaldehyde gas reaches 280.Furthermore,the compounding ratio of GO in the composite was studied.The results show that the potential barrier at the contact between the shell SnO₂ NSs and GO can effectively adjust the initial resistance,promote the electron transfer between the materials,achieve the improvement of the sensitivity of sensitive materials,improve its sensitivity while reducing the working temperature and optimizing the selectivity.Through the research work in this chapter,it is proved that the sensitivity of three-dimensional structural materials can be improved by using GO sensitization.However,the low temperature process will cause problems such as slower response/recovery speed and increased interference from humidity.(3)Preparation and sensitivity study of In₂O₃@SnO₂ hierarchical core-shell materials:In order to further improve the comprehensive sensitivity of materials,heterogeneous core-shell composite materials were designed and constructed.Using In₂O₃ NFs as the core material,the In₂O₃@SnO₂gas sensitive composite with hierarchical core-shell structure was prepared by hydrothermal growth method.The test results showed that the sensitivity of In₂O₃@SnO₂ to 100 ppm formaldehyde gas was about 180.1at the optimal working temperature of 120?,and it had excellent selectivity,fast Response/recovery speed(3 s/3.6 s),good stability,moisture resistance and lower limit of detection(10 ppb).Compared with the single structure In₂O₃ NFs and SnO₂ NSs,it is found that the construction of hierarchical core-shell structure can effectively promote the electron transfer between the inner and outer layer materials,form the Schottky barrier,and effectively regulate the resistance,which is conducive to the improvement of formaldehyde sensitivity.It is proved that the design of nanosheets/nanofibers hierarchical core-shell structure is one of the effective methods to improve its sensing performance.