Morphology Control And Modification Of Ln₂O₃ And SnO₂ Gas Sensitive Materials And Research On NO₂ Gas-sensing Performance

Nitrogen dioxide（NO₂）is one of the most common toxic gases,mainly from automobile exhaust emissions and fossil fuel combustion.The emission of NO₂ will not only cause the formation of acid rain and photochemical smog,but also cause great harm to human health.Therefore,it is of great significance to develop a sensor that can quickly and accurately detect NO₂ in the environment.In recent years,a class of metal oxide semiconductors（MOS）represented by Sn O₂ and In₂O₃ have been widely used in the field of NO₂ sensing due to their unique physical and chemical properties.However,most MOS including Sn O₂ and In₂O₃ have high working temperature,poor selectivity and other defects,which limit their practical application.Therefore,further improving the NO₂ sensing performance of MOS has become the focus of current research.As we all know,graphene is an ideal low temperature or room temperature NO₂ sensing material due to its advantages such as large specific surface area and high carrier mobility.However,graphene-based NO₂ sensors often have the problems of low sensitivity and slow response/recovery speed.Considering the sensing characteristics of MOS and graphene,the high-performance NO₂ sensing material at low temperature or room temperature can be prepared by constructing graphene/MOS heterojunction.In addition,doping,functional group modification and size adjustment can effectively change the electronic properties of graphene and increase the adsorption active site of NO₂,thereby further enhancing its response to NO₂.Based on the above considerations,this thesis takes In₂O₃ and Sn O₂ as the research objects,and synthesized nitrogen-doped graphene quantum dots（N-GQDs）/In₂O₃,3-aminopropyl-3-ethoxysilane（APTES）functionalized r GO/In₂O₃and N-GQDs/Sn O₂ three low temperature or room temperature NO₂ gas sensing materials.The composition,micromorphology,crystal structure,surface valence state,specific surface area and pore size distribution of the as-prepared sensing materials were analyzed through a series of characterizations,and the NO₂ sensing performance was studied.In addition,the NO₂ sensing mechanism of composite materials has also been discussed.The specific research contents are as follows:（1）3DOM In₂O₃ and N-GQDs were prepared by template method and hydrothermal method,respectively.Then a series of N-GQDs were modified on the surface of 3DOM In₂O₃ by a one-step hydrothermal method to prepare a series of N-GQDs/3DOM In₂O₃ composites.The as-prepared materials were characterized and analyzed by XRD,FT-IR,AFM,SEM,TEM,XPS,BET and UPS,and their sensing performance was investigated.The results show that the optimal working temperature of the composite is 100oC,which is lower than that of pure 3DOM In₂O₃.In addition,the response value of N-GQDs with a mass fraction of 1 wt%of N-GQDs/In₂O₃（NG/In₁）complex to 1 ppm NO₂ was 81.7,which was 5.8 times that of pure 3DOM In₂O₃.More importantly,the actual detection limit of this sensor can reach 100 ppb,and the response value is 4.1.In addition,the sensor also shows good repeatability,selectivity,long-term stability,fast response/recovery time（95 s/36 s）,and good sensing performance under high humidity.（2）First,In-MIL-67 precursor was prepared by a solvothermal method,and then calcined to obtain hierarchical In₂O₃ nanotubes.Next,a series of r GO/In₂O₃composites and APTES-functionalized r GO/In₂O₃ composites were prepared by hydrothermal method.The as-prepared materials were characterized and analyzed by XRD,FT-IR,SEM,TEM and BET,and their sensing performance was investigated.The results show that the modification of r GO and APTES can significantly improve the NO₂ sensing performance of In₂O₃ at room temperature.The optimal APTES-functionalized r GO/In₂O₃（AG/In₁）response to 1 ppm NO₂ is 12,which is4.8 times that of pure In₂O₃ nanotubes.In addition,the actual detection limit of AG/In₁ for NO₂ at room temperature can reach 20 ppb.The sensor also shows good selectivity,repeatability,long-term stability,and good sensing performance under high humidity.（3）Using Zn Sn（OH）₆ hollow cubes as precursors,Sn O₂ hierarchical hollow cubes were prepared by sulfidation-oxidation,and then N-GQDs were modified on the surface by impregnation-calcination method.The formation process,physical and chemical properties of N-GQDs/Sn O₂ were studied by XRD,SEM,TEM,XPS and BET.The NO₂ sensing performance of N-GQDs/Sn O₂ was also studied.The best N-GQDs/Sn O₂(NG/Sn_1.5)response to 1 ppm NO₂ can reach 417,which is 2.2 times that of pure Sn O₂.In addition,NG/Sn_1.5 also has fast response/recovery time（59 s/33s）,very good selectivity,repeatability and long-term stability.