Fabrication Of MOFs-Derived Cobalt Oxides And Zinc Oxides And Their CO-Sensing Performances

Carbon monoxide gas sensors have extensive application requirements in aerospace,nuclear industry,and military fields.This puts forward higher requirements on the sensor’s response,selectivity and detection concentration range.Currently,the response values of the metal oxide-based CO sensors are generally low,and the detection concentration range is relatively narrow,which is difficult to meet the application requirements.Metal-oxides derived from metal-organic frameworks（MOFs）can inherit the characteristic of high specific surface area and porosity,which is beneficial to increase the response and broaden the detection range.In this thesis,MOFs were used as templates to synthesis Co₃O₄ and Pt@ZnO polyhedrons with good CO-sensing performances by the co-precipitation method and the heat treatment process.On the basis,in order to further improve the CO-sensing performances,we synthesized cobalt-based bimetallic oxide nanosheets（Co-M-O;M=Cu,Mn,Ni,and Zn）and cobalt-based multi-element metal oxide nanosheets（Sn/Mn-Co₃O₄）by adjusting the composition and optimizing the structure of ZIF-67 precursors.The composition,microstructures,and CO-sensing performances of these materials were studied systematically.Meanwhile,the relationship between the composition and structures and CO-sensing performances was preliminarily established.The CO-sensing mechanism was also clarified.This provides a new idea for fabricating high-performance CO-sensing materials.First,the ZIF-67 polyhedrons templates were synthesized using dimethylimidazole as the ligand and cobalt nitrate as cobalt source by the co-precipitation method.Then Co₃O₄ was synthesized by heating ZIF-67 templates.The obtained Co₃O₄ exhibits polyhedron structure and has high specific surface area(83.7 m²·g^-1).The relative content of Co³⁺in Co₃O₄ can be adjusted by adjusting the heat treatment temperatures and heating rates of ZIF-67.The CO-sensing test results show that the response of Co₃O₄ polyhedrons increases with the increase of Co³⁺content.Compared with the Co₃O₄ nanoparticles prepared by the non-template method,the response of Co₃O₄ polyhedrons derived from ZIF-67 has increase by 20 times,and the limit of detection has been reduced to 500 ppb.The response of Co₃O₄ polyhedrons to 100 ppm CO at 150 ^oC is 210%,and the response-recovery time is 10 s and 2 s,respectively.The good CO-sensing performances are mainly due to the relatively high content of active sites of Co³⁺in Co₃O₄ derived from ZIF-67and the high specific surface area and porosity.In order to enhance the response to CO at low-temperature condition,we synthesized ZIF-8 templates by co-precipitation method using dimethylimidazole as the ligand and zinc nitrate as zinc source.Then Pt@ZnO nanocomposite was obtained through the dipping strategy and the subsequent heat treatment process.The obtained Pt@ZnO presents polyhedral structure,and the average particle size of Pt nanoparticles is 3.26 nm.There is no obvious agglomeration of Pt nanoparticles on the surface of ZnO,which illustrates that the Pt nanoparticles have good dispersibility.This is mainly due to the fact that the isolated chamber structure of ZIF-8 can anchor Pt nanoparticles well,avoiding further agglomeration during high-temperature heat treatment.The response of Pt@ZnO to 50 ppm CO reaches 90%at low temperature（100 ^oC）.Meanwhile,the response to 500ppb CO at 300 ^oC is 9%,and the limit of detection is reduced to 100 ppb.These excellent CO-sensing properties are mainly because the MOFs conversion strategy improves the dispersibility of Pt nanoparticles on the surface of ZnO,and enhances their catalytic efficiency for CO.On the other hand,Pt@ZIF-8-derived Pt@ZnO possesses high specific surface area,which is conducive to the adsorption and reaction of CO.In order to improve the CO-sensing of ZIF-67-derived Co₃O₄,ZIF-67 was doped with heterogeneous elements by co-precipitation method to prepare ZIF-Co M（M=Cu,Mn,Ni,and Zn）polyhedrons,and then ZIF-Co M polyhedrons were converted to Co M-MOF nanosheets by ion-assistant solvothermal method.The cobalt-based bimetallic oxides（Co-M-O,M=Cu,Mn,Ni,and Zn）were obtained by low-temperature annealing of Co M-MOF nanosheets.The results show that Co-M-O materials exhibit porous nanosheets structure and have high specific surface areas(146.4-220.7 m²·g^-1).Moreover,Cu、Mn、Ni and Zn are doped into Co₃O₄ in the form of+2 valence.Among these four samples,compared with ZIF-67-derived Co₃O₄ polyhedrons,the response of Co-Mn-O nanosheets to CO have increased by 25.7%.The response to 100 ppm CO is 264%at 175^oC.In addition,it still shows a clear response signal to CO.The improved CO-sensing properties is mainly because that the oxygen vacancies is increased through replacing of part of Co²⁺with the cations(M²⁺).On the other hand,the active sites can be fully exposed because of the nanosheets structure,leading to sufficient reaction for CO.Finally,in order to further enhance the response to CO,ZIF-67 was co-doped with Sn and Mn,and then converted into Sn/Mn-Co-MOF nanosheets using similar strategies.The cobalt-based multi-element metal oxide（Sn/Mn-Co₃O₄）was obtained by annealing the Sn/Mn-Co-MOF templates.The results show that the co-doping with Sn and Mn does not change the phase of Co₃O₄,and they are doped into the crystal lattice of Co₃O₄ in the form of Sn⁴⁺and Mn²⁺,respectively.The obtained Sn/Mn-Co₃O₄ inherits the nanosheets structure of Sn/Mn-Co-MOF precursors and has a large number of mesopores and high specific surface area(175.5 m²·g^-1).Compared with pure Co₃O₄,single-doped Sn-Co₃O₄and Mn-Co₃O₄,the response of Sn/Mn-Co₃O₄ to CO increases by 5.0 times,3.3 times and1.8 times,respectively.The response of Sn/Mn-Co₃O₄ sensor to 100 ppm CO at 150 ^oC is 600%,and the response-recovery time is 30 s and 38 s,respectively.Moreover,it can detect high concentration of CO（100000 ppm）.The improved CO-sensing performances is because the replacement of part of Co²⁺by Mn²⁺can increase the oxygen vacancies content of Co₃O₄.Besides,doping with Sn⁴⁺introduces more electrons into Co₃O₄,leading to the decrease of hole concentration of Co₃O₄ in air.The synergy of these two factors together enhance the CO-sensing performances.The thesis focused on the MOF-derived cobalt oxides and zinc oxides materials system.The response value of the metal oxide-based CO sensor and the upper limit of the detection concentration have been elevated by means of morphology design,composition regulation,and defect construction.The relationship between the composition and morphology of the materials and the gas-sensing performances was revealed,and the CO-sensing mechanism was discussed in detail.This thesis provides theoretical and experimental support for the preparation of high-performance CO-sensing materials.