Preparation And Gas-sensing Preformance Of Cobalt(copper) Oxide/Mxene Nanocomposites

With the development of modern industry and economic society,environmental pollution is becoming increasingly serious.Especially air pollution,which attracts more and more people's attentions.Nitrogen oxides(NO_x)and ammonia(NH₃),as the main pollutants of air pollution,have a serious impact on environmental protection,human and animal health.Once the NO_x concentration in the air is higher than 25 ppm,it will lead to acid rain,respiratory,heart disease,and even life-threatening.Therefore,it is very important to prepare a highly sensitive,safe and reliable gas sensor at room temperature to monitor its content in the atmosphere.Nowadays,traditional gas-sensing materials are mainly metal oxide semiconductor materials.The shortcomings of metal oxide semiconductor gas-sensing materials such as small specific surface area,low adsorption capacity for gas,low reactivity,low conductivity,high operating temperature and high energy consumption have limited their applications.MXenes,a class of transition metal carbon,nitrogen and carbon/nitrogen,has excellent physical,chemical and electrical properties,so that it has a wide range of applications in the field of gas sensing materials.To solve the problems existing in the room temperature NO_x gas sensor,Co and Cu oxides/Ti₃C₂T_x MXene composites with different morphologies were prepared controllable with Ti₃C₂T_x MXene as matrix and Co and Cu oxides as active components in this thesis.We have adopted different materials and methods to control the specific surface areas,morphologies of the composites and constructed heterogeneous structures between Co(Cu)oxides and Ti₃C₂T_x MXene nanosheets.The structure,morphology and composition of composites were analyzed by SEM,TEM,XRD and BET.The gas-sensing performances(sensitivity,response/recovery time,long-term stability,repeatability and selectivity,etc)of the composites to NO_x and NH₃ were systematically studied.Main researches of this thesis were as follow:Firstly,the thesis had chosen a simple hydrothermal method,used cobaltosic oxide nanoparticles as gas-sensitive active sites,branched polyethyleneimine(PEI)as the assembler,and Ti₃C₂T_x MXene nanosheets as the substrate,preparing Co₃O₄/PEI/Ti₃C₂T_xMXene(Co PM)nanocomposite with wonderful gas-sensing performance at RT.The results showed,Co PM-24 composite material had a high gas response to NO_x(R₀/R_g=27.9,100 ppm),short response time and low detection limit(30 ppb).The wonderful gas-sensing performance of Co PM-24 was mainly attributed to the heterogeneous structure between Co₃O₄ nanoparticles and Ti₃C₂T_x MXene nanosheets,and there were a large number of oxygen adsorption species on the surface of Co PM-24 which contribute to the surface adsorption capacity of the material.Secondly,the thesis had studied the effects of porous structure,microstructure and heterogeneous structure on the gas-sensing performance of composite.We had prepared the Co Al-LDH/Ti₃C₂T_x MXene(CAM)template by a hydrothermal method and synthesized three-dimensional porous Co₃O₄/Al₂O₃/Ti₃C₂T_x MXene(CAM-AC)composite by a high-temperature calcination method.The CAM-AC composite showed an excellent gas-sensing performance with a high sensitivity of 40.3 to 100 ppm NO_x,short response/recovery time(1.3/13.3 s)and low detection limit(10 ppb).The CAM-AC composite also had an excellent repeatability,long-term stability and selectivity.The excellent gas-sensing performance was attributed to the three-dimensional structure,abundant mesoporous structure and heterostructure of the composite.Finally,the thesis had studied the effects of microstructure and heterogeneous structure on the gas-sensing performance of composite.Cu_xO/Ti₃C₂T_x MXene(CM)composites were synthesized by anchoring copper oxide(Cu_xO)with variable copper element on the surface of Ti₃C₂T_x MXene nanosheets by refluxing method.Due to the heterogeneous structure between Cu_xO and Ti₃C₂T_x MXene nanosheets,the gas diffusion rate and the electrons transfer rate within the CM composites were increased.The CM-2composite had a high gas response to NO_x(R₀/R_g=37.8,100 ppm),shorter response/recovery time(2.0 s/21.3 s)and detection limit of 10 ppb.In addition,owning to copper oxide with variable copper element was chosen as the gas sensitive active substance,the CM-2 composite also showed wonderful gas-sensing performance to NH₃.The CM-2 material showd a good gas response to 1000 ppm NH₃ at RT(S=(R_g-R₀)/R₀=51.4%),which was much higher than that of pure Cu_xO(21.1%).