Preparation And Gas Sensing Properties Of Porous LaFeO₃ Microspheres And Their Composites

With the rapid development of industrialization and the continuous improvement of human living standard,the pollution of atmospheric environment is becoming more and more serious.So,the gas sensors playing a more and more important role in our daily life.They can detect toxic gases in living places,industrial waste gas produced in production,etc.,and play an increasingly important role in environmental pollution monitoring,decoration pollution detection,food safety detection and chemical plant waste gas monitoring.Metal oxide semiconductor gas sensor has been widely used because of its high sensitivity,good stability,relatively low working temperature,easy to carry and so on.Among numerous metal oxides,perovskite oxides containing transition metals and rare earth elements ABO₃ have attracted much attention because of their stable crystal structure.La FeO₃ as a typical perovskite structures ABO₃ type oxide,has shown excellent long-term stability in the field of gas sensing and has thus been studied more and more.However,there are still many shortcomings of pure LaFe O₃ materials as gas sensitive materials,such as high working temperature,poor gas selectivity and low sensitivity.Recently,many studies have shown that by adjusting the morphology of pure LaFeO₃ and doping them with some transition metal elements at A or B sites,surface modification with nanoparticles with catalytic properties and the formation of heterojunctions with other metal oxides can help to improve their gas sensing properties.Therefore,this paper mainly prepared LaFeO₃ porous microspheres with abundant pore structure by hydrothermal method,and used transition metal element Ba,Ni for doping,Au,RuO₂ nanoparticles for surface modification of pure LaFeO₃ porous microspheres,composite with other metal oxides and rGO to obtain a series of composites,and characterization and gas sensitivity testing.The main contents of this thesis are as follows:1.LaFeO₃ microspheres with abundant pore structure were prepared by hydrothermal method,and the A and B sites in the LaFeO₃ lattice were doped by two transition metal elements to control the synthetic La_1-xBa_xFeO₃ and LaFe_1-x-x Ni_xO₃ materials,and the obtained gas-sensitive materials were characterized and studied.The results show that the optimal operating temperature of pure LaFeO₃ is 260oC,while the La_0.98Ba_0.02FeO₃ optimal operating temperature is 200oC.Its operating temperature decreased by 60oC,and its sensitivity to ethanol increased by about 7 times compared with pure LaFeO₃.The optimal operating temperature of LaFe_0.98Ni_0.02O₃ also decreases by 40oC,and has strong recognition ability to acetone,which can resist the influence of other interfering gases.2.The prepared porous LaFeO₃ microspheres were used as the matrix to modify them by using Au,RuO₂ nanoparticles to obtain Au/LaFeO₃,RuO₂/LaFeO₃ nanocomposites.A series of characterization and gas sensing properties of the obtained gas sensing materials were studied.The results obtained are as follows:the prepared Au nanoparticles modified porous LaFe O₃ microsphere gas-sensitive materials,Au the nanoparticles were successfully grown on the surface of the microspheres with a diameter of about 17 nm.And the gas sensitivity test of the composite was carried out.The optimum operating temperature of Au/LaFeO₃nanocomposites decreased by 60oC,showing a good response to formaldehyde gas,which can be attributed to the catalytic effect of Au nanoparticles.Another RuO₂ particle with the same catalytic performance was selected to reduce the cost.The test results of the prepared RuO₂/LaFeO₃ nanocomposites were as follows:RuO₂ nanoparticles with a diameter of about80 nm,successfully grown on the surface of microspheres.The sensitivity of triethylamine gas has improved,and the reasons of increasing gas sensitivity were discussed.3.The prepared porous LaFeO₃ microspheres were used as the matrix to synthesize nanocomposites with heterostructure by hydrothermal method with p-type metal oxide semiconductor NiO,n-type metal oxide semiconductor,respectively.The gas sensing properties of the obtained gas sensing materials was studied.The results show that the NiO grows vertically on the surface of the microspheres in nanosheet shape with uniform size.The gas sensitivity test results show that the obtained NiO@LaFeO₃ p-p heterojunction nanocomposites with core-shell structure show a highly sensitive response to ethanol gas,and the interference performance against other gases is also superior.The SnO₂/LaFeO₃ p-n heterojunction nanocomposites prepared SnO₂ the same nanosheet-like growth on the surface of the microspheres not only showed much higher gas-sensitive response to ethanol than pure LaFe O₃,but also decreased the optimal operating temperature by 80oC.The reasons for the enhanced gas sensitivity response of metal oxide semiconductor gas sensors by heterojunction structure are discussed.4.Using porous LaFeO₃ microspheres as matrix,rGO/LaFeO₃ nanocomposites were prepared by electrostatic attraction method and hydrothermal method.Undering the hydrothermal conditions,GO was reduced to rGO,and the LaFeO₃ microspheres were composite by electrostatic attraction to obtain rGO partially encapsulated porous microsphere nanocomposites.Gas sensitivity tests results show that the sensitivity of the composite to triethylamine has been greatly improved at the optimum operating temperature of 240oC,which can not only achieve rapid response,but also improve the gas selection performance.