Preparation And Gas Sensing Properties Of Cobalt-based Metal Oxides Composites By MOF As Template

With the continuous development of science and technology,the emission of pollutants such as automobile exhaust and industrial waste gas is increasing,environmental pollution is becoming more serious,and personal safety is threatened.Among the many volatile and harmful gases,ethanol is a colorless and irritating scented gas.Long-term exposure to ethanol gas can cause health problems such as headache,lethargy and eye irritation.As an important branch of the sensor,gas sensor is often used to detect volatile harmful gases,which is convenient for real-time monitoring to avoid personal safety hazards.Co₃O₄ is a typical P-type semiconductor,which is a common semiconductor metal oxide in the field of gas sensors.MOF are widely used due to their regular morphology and large specific surface area.In this thesis,Co₃O₄/Fe₂O₃,Co₃O₄/ZnO,SnO₂/Co₃O₄@Co₃O₄ composites with different morphologies were synthesized using MOF template by coprecipitation combined with high temperature annealing.Finally,the mechanism of gas sensing materials was discussed in detail.Co₃O₄/Fe₂O₃ composites were successfully prepared by a simple two-step method using cobalt sulfate,potassium ferricyanide and trisodium citrate as raw materials.The composition of the sample was determined by XRD.The surface and internal morphologies of the sample were observed by SEM and TEM.The valence of each element in the sample was determined by XPS.Finally,the Co₃O₄/Fe₂O₃ composites were dissolved in anhydrous ethanol and uniformly coated on a ceramic tube to assemble the device,and its gas sensitivity was studied.The gas sensing properties of the materials were tested.The results show that the response of Co₃O₄/Fe₂O₃ composites to 100 ppm ethanol at 140°C is 27.272,which is 2.2 times that of pure Fe₂O₃.The increase in sensitivity is mainly attributed to the hollow structure of the Co₃O₄/Fe₂O₃ composite and the formation of the P-N heterojunction between Fe₂O₃ and Co₃O₄.Both materials have good repeatability and long-term stability。Co₃O₄/ZnO nanoparticles with core-shell structure were prepared from cobalt nitrate,zinc nitrate and 2-methylimidazole by co-precipitation and chemical etching combined with high temperature calcination.The results of XRD test show that the precursors before and after chemical etching are all Co/Zn-ZIF.After calcination at high temperature,the precursors are completely transformed into Co₃O₄/ZnO composites.The gas sensing properties of the composite material were tested.At 200°C,the response of core-shell structure Co3O4/ZnO to100 ppm ethanol was 38.87,which was 2.8 times that of single-shell Co3O4/ZnO.The formation of P-N heterojunctions between ZnO and Co3O4,the larger specific surface area and more gas active sites are the reasons of development of sensitivity.In this experiment,the core-shell structure of Co3O4/ZnO has been proved to be a potential candidate material for high performance gas sensors.SnO2/Co3O4@Co3O4 composites with unique nanocube structure were prepared by coprecipitation and two-step high-temperature calcination using cobalt chloride,tin chloride and sodium hydroxide as raw materials.The structure of the product was observed by SEM and TEM.The product was nanocubes with diameter about 400 nm,showing core-shell structure.The gas sensors were prepared by dissolving it in absolute ethanol.The gas sensing properties of SnO2/Co3O4 composites were compared with those of pure cubic SnO2/Co3O4composites.The results show that SnO2/Co3O4@Co3O4 gas sensor with core-shell structure has a higher response to 100 ppm ethanol at 140°C reaching 27.69,while the sensitivity of SnO2/Co3O4 gas sensor is only 14.88.In addition,long-term stability measurements show that the response value of SnO₂/Co₃O₄@Co₃O₄ decreased only 4.8%after 30 days,which proved nice stability of core-shell SnO₂/Co₃O₄@Co₃O₄ composites.