Fabrication And Characteristics Of Two-dimensional Transition Metal Carbons And Dichalcogenides Room Temperature NO₂ Sensors

With the development of industry and the increase in the number of vehicles,the massive emission of nitrogen dioxide（NO₂）has seriously polluted the atmospheric environment and posed a threat to human health.Therefore,developing low-power,high-performance,and highly selective NO₂ sensors has important application value in the fields of air-quality monitoring,leakage alarm,medical protection,and industrial production.Considering the energy consumption,miniaturization,and work safety of sensors,the development of room temperature gas sensors is an important direction for the future development of NO₂ sensors.Due to the advantages of large specific surface area,controllable morphology and adjustable electrical properties,two-dimensional（2D）materials can adsorb a large number of gas molecules and undergo physical/chemical reactions with them at room temperature,significantly affecting the electrical properties of the materials.Therefore,2D materials are considered ideal room temperature gas sensing materials.Among them,2D transition metal carbons and transition metal dichalcogenides are particularly competitive in NO₂ detection due to their rich active sites and strong adsorption ability for NO₂.However,these materials have some bottleneck issues in room temperature gas detection,such as low sensitivity,long response/recovery time,high detection limit,poor long-term stability and unclear gas sensing mechanism,restricting their further development.In response to the aforementioned shortcomings,this dissertation conducted material preparation,gas sensing performance research and mechanism analysis based on 2D transition metal carbides and dichalcogenides through methods such as structural optimization,regulation of surface functional groups,construction of heterostructures,and design of hierarchical composites.This lays the foundation for constructing high sensitivity,fast response,low detection limit,high selectivity and long-term stability room temperature NO₂ sensors.The main research content is as follows:1.Highly sensitive NO₂ room temperature sensors were designed and fabricated based on the interlayer swelling of vanadium carbide（V₂CT_x）.An accordion-like V₂CT_xwas prepared by fluoride-containing solution etching and intercalation with dimethyl sulfoxide.Then,an alkalized V₂CT_x room temperature NO₂ sensor was designed and manufactured by alkaline treatment of V₂CT_x with sodium hydroxide（Na OH）solution.Alkaline treatment increased the ratio of oxygen-fluorine functional groups on the surface of V₂CT_x,expanded the interlayer spacing and successfully intercalated sodium ions（Na⁺）,leading to interlayer swelling as the main factor determining the gas sensing performance of the alkalized V₂CT_x sensor.Compared with the V₂CT_x sensor based on gas adsorption/desorption reactions on material surfaces,the alkalized V₂CT_x sensor exhibited an 80-fold increase in response to 50 ppm NO₂ and two orders of magnitude increase in sensitivity to 5-50 ppm NO₂.In addition,the response of the alkalized V₂CT_xsensor to NO₂ was at least three times that of other interfering gases,indicating the excellent selectivity of the alkalized V₂CT_x sensor.Finally,a gas sensing mechanism model based on interlayer swelling was constructed.2.In order to further improve the sensitivity,detection speed and selectivity of V₂CT_x,a strategy based on the synergistic effect of interlayer swelling and charge transfer was proposed to enhance gas sensing performance.Based on this strategy,the accordion-like V₂CT_x and 2D tin disulfide（Sn S₂）nanoplates were combined by mechanical stirring to design and prepare a V₂CT_x/Sn S₂ room temperature NO₂ sensor.The results manifested that the combination of Sn S₂and V₂CT_x increased the interlayer spacing of the V₂CT_xand increased active sites.As a result,the sensitivity of the V₂CT_x/Sn S₂ sensor to NO₂(29.08%ppm^-1)was more than 581 times higher than that of the V₂CT_x sensor.And ultrafast response（4.82 s）and recovery（4.66 s）were achieved.In addition,the sensor also exhibited excellent selectivity and long-term stability.Finally,based on the experimental results,the sensing mechanism of the V₂CT_x/Sn S₂ composite to different gases was further analyzed,and the gas sensing mechanism model based on the synergistic effect of interlayer swelling and charge transfer was established.3.In order to further improve the long-term stability of the sensors,Sn S₂ with good stability was selected as the main sensing material based on the work in the previous chapter.To solve the problem of high resistance of the pure Sn S₂ at room temperature and easy stacking of the 2D Sn S₂,a room temperature NO₂ sensor based on vanadium tetrasulfide（VS₄）/Sn S₂ composites with n-n heterostructure was prepared by combining VS₄ microspheres and 2D Sn S₂ nanoplates using hydrothermal method.The results showed that the VS₄/Sn S₂ composite had good electrical conductivity at room temperature.Compared with the VS₄ sensor,the VS₄/Sn S₂ sensor exhibited a higher response（8.35）and faster recovery speed（2.25 s）to 5 ppm NO₂.The sensitivity of the VS₄/Sn S₂ sensor to 0.1-5 ppm NO₂(1.42 ppm^-1)was 142 times higher than that of the VS₄ sensor.In addition,the VS₄/Sn S₂ sensor also presented reliable repeatability and good selectivity.During the 8-week long-term stability test,the response of the VS₄/Sn S₂sensor to NO₂ remained more than 80%of the initial value.The improvement in NO₂sensing performance was attributed to the formation of n-n heterojunctions and the unique three-dimensional（3D）hierarchical structure.4.To further improve the sensitivity and selectivity of transition metal dichalcogenides while lowering their detection limit,porous nanosheets-assembled 3D Indium oxide（In₂O₃）microflowers modified with edge-enriched 2D molybdenum disulfide（Mo S₂）nanosheets were designed.On the basis of the construction of heterostructures,a large number of strong NO₂ adsorption sites were exposed,significantly improving gas sensing performance.The experimental results showed that the sensitivity of the In₂O₃/Mo S₂ composite sensor to 0.25-5 ppm NO₂ at room temperature was as high as 72.76 ppm^-1,which was 3638 times and 110.24 times higher than that of the pure Mo S₂ and In₂O₃ sensors,respectively.The response of the In₂O₃/Mo S₂ sensor to NO₂ was at least 30 times higher than other gases,demonstrating the ultra-high selectivity of the sensor.In addition,the composite sensor had an ultra-low detection limit（2.21 ppb）and good repeatability.In a 7-week long-term stability test,the response of the In₂O₃/Mo S₂ sensor to NO₂ was reduced by less than 7%.The outstanding sensing performance of the In₂O₃/Mo S₂ sensor was attributed to the synergistic effect of the p-n heterojunction,abundant active sites provided by the 2D/3D hybrid structure,and the extensive exposure of strong adsorption sites on the Mo S₂ edge.