Synthesis And Modification Of Metal-Organic Framework ZIF-8-based Derivatives For Their Gas Sensing Properties

With the rapid development of industry and the increasing quality of daily life,it is higher desirable to detect the toxic and harmful gases in working or living environments.In recent years,great efforts have been focused on the gas sensors based on the metal-organic framework derivatives as sensing materials because of the good gas-sensing performances.As one of the typical metal-organic frameworks,Zeolitic imidazolate Framework-8?ZIF-8?with zinc ions as metal nodes is widely applied to derive zinc oxide?ZnO?sensing materials for gas sensor application due to its advantages of good thermal stability,adjustable pore channels and easy functionalization.It is helpfully for optimization the morphology,doping or decoration elements and formation heterojunctions.Therefore,it is promising to develop highly gas-sensing performances of sensors based on ZIF-8 derivatives.In this thesis,ZIF-8-based derivatives of metal-organic framework were studied different methods including doping V element,Co-ion phase transition and loading noble element of Pd.The morphologies,structures,compositions and properties of the samples were comprehensive investigated by SEM,XRD,EDS,TEM,XPS,TGA and BET.The gas-sensing properties of as-fabricated gas sensors based on the samples were studies toward hydrogen sulfide?H₂S?and acetone detection.The enhanced gas sensing mechanism of sensors was also discussed.The detailed research results are showed as follows:?1?Pure and V-doped ZnO hollow microcages were synthesized with V-doped ZIF-8 as self-sacrificing template by solvothermal method.After characterization and testing,it was found that V-doped ZnO hollow microcages had better gas sensing properties than pure ZnO.The results demonstrate that the measured response of V-doped ZnO hollow microcages could reach to 90.2toward 5 ppm H₂S at 100°C,and it has a good linear dynamic range of 0.2ppm¹0 ppm and excellent selectivity,and it has faster recovery properties at300°C.?2?The different concentration of V-doped?0,0.5,1,2 mol%?ZnO hollow microcages were synthesized by self-sacrificing template method at room temperature.The characterization results showed that the presence of V can significant influence the morophology size,particle density,oxygen vacancy and adsorbed oxygen ratio,and specific surface area of ZnO microcages.The gas sensing experiments shows that the 1 mol%V-doped V₁ZnO sensor exhibits the enhanced gas-sensing performance.The measured response of V₁ZnO sensor could reach 13.4 toward 100 ppm acetone at 350°C with fast response/recovery time?6 s/3 s?,a wide linear range?0.05¹000ppm?,good selectivity,repeatability and long-term stability.?3?The Pd@Co/Zn-ZIF hollow structures and were synthesized utilizing solvothermal method after introducing Co-ion and Pd into ZIF-8.After that,the CO₃O₄,ZnO,Co₃O₄/ZnO and Pd@Co₃O₄/ZnO hollow microcages were obtained by anneling the Pd@Co/Zn-ZIF templated at high temperature.The morphologies,distribution of elements and existence formation of metal oxides of all samples were analyzed by characterization.The gas sensing properties of sensor show that Pd@Co₃O₄/ZnO sensor exhibits lowest optimum operating temperature?200°C?,high response value?22.5 to 100 ppm acetone?,fast response/recovery time?14 s/15 s?,wide linear range?0.05¹000 ppm?,good selectivity,repeatability and long-term stability.?4?The gas sensing mechanism of V-doped ZnO and Pd@Co₃O₄/ZnO hollow microcages was analyzed in our works.Their enhanced properties can be attributed as follows:1)Hollow structure and more pore between nanoparticles contribute to the adsorption and diffusion of gas molecules;2)The n-n heterojunction between V₂O₅ and ZnO is formed after V doping,which widens the depletion layer thickness of ZnO;3)The p-n heterojunction formation between Co₃O₄ and ZnO after introduction of Co ion,which improves the intergranular grain boundary barrier;4)Pd loading has synergistic and catalytic effects,which reduces the activation energy of the composites and promotes the gas sensing reaction on the surface of the materials.