Preparation Of Spinel Oxide Semiconductor And Its Gas Sensing Properties

With the increasing demand for gas sensors in the fields of atmospheric environment monitoring,resource exploration,industrial/people's livelihood security,and medical diagnosis,the research of gas sensors has attracted the attention of academics and industry.Oxide semiconductor gas sensor is the most important chemical quantity sensor.It has the advantages of all solid state,easy integration,high sensitivity and high reliability;therefore,gas sensor has always been a front and hotspot in the research of gas sensors.Compared with single oxide semiconductors,the sensing properties of binary metal oxide semiconductors are easier to control,not only can be designed on the microstructure,but also can improve the sensing characteristics by controlling the chemical composition.Therefore,in recent years,research and development based on binary metal oxide semiconductors?mainly spinel type AB2O4 and perovskite type ABO3?gas sensors have attracted widespread attention.In this paper,hydrothermal and solvothermal methods were used to prepare spinel-type binary metal oxide semiconductor hierarchical nanostructures with different morphologies,and the high-quality spinel composite oxide was used as the sensing material to construct gas sensors.The influence of the composite oxide micro-nano structure and chemical composition on its sensing characteristics were studied.On this basis,the sensing mechanism is described.The specific research contents are as follows:?1?Hollow nanostructured CdFe₂O₄ nanocages and CuFe2O4 porous spheres sensing materials assembled by nanoparticles were synthesized by hydrothermal synthesis and solvothermal synthesis.The CdFe₂O₄ nanocages have a uniform diameter distribution of about 500 nm.Gas sensing characteristics test results show that CdFe₂O₄ nanocages have good response and selectivity to acetone.At the optimal working temperature of 275?,the response of the sensor to 100 ppm acetone is 13.4,and the response and recovery time are 1 s and 40 s,respectively,which is due to that the hollow nanocage structure has better permeability to facilitate gas diffusion.And the gas sensor based on the hollow CdFe₂O₄ nanocages have a lower detection limit of 300 ppb,and the response of the sensor reaches 1.3.In addition,the porous CuFe2O4 nanospheres synthesized by solvothermal method have a hierarchical structure with a diameter of about 200 nm,and the structural unit is a uniform particle size.Sensing characteristics test results show that the sensor based on porous CuFe2O4 nanosphere has excellent gas response to acetone.The optimum operating temperature for acetone is 250?,at which point the sensor responds to 100 ppm acetone is 20.1.The hollow structure of the material and the porous micro-nano structure are more conducive to the diffusion of gas,improve the utilization efficiency of the sensing material,and achieve structural sensitization.On the other hand,the relatively large specific surface area indicates that there are more surface-active sites,increasing the amount of oxygen adsorbed on the surface,allowing more target gas to react with the adsorbed oxygen,thereby achieving an increase in sensitivity.?2?The heterogeneous structure of porous SnO2/Zn₂SnO₄ nanospheres with uniform size and good dispersibility assembled by nanoparticles was successfully synthesized by one-step hydrothermal method.The spatial distribution of the elements shows that the SnO2 nanoparticles and the Zn₂SnO₄nanoparticles are nested together to form porous SnO2/Zn₂SnO₄ nanospheres instead of forming separate nanospheres.The gas sensing properties of the porous SnO2/Zn₂SnO₄ heterostructures show that the sensor has higher response and better selectivity to ethanol than the single SnO2 and Zn₂SnO₄ nanoparticle materials,at an optimum operating temperature of 250?.The response of the sensor to 100 ppm ethanol is 30.5.The excellent gas sensing properties of sensitive materials are attributed to the fact that the porous nanostructure of SnO2/Zn₂SnO₄ composites increases its specific surface area,thus increasing the recognition function of the material and the number of surface-active sites,making more test gases react on the surface of the material to improve the sensitivity of the sensor.On the other hand,due to the heterojunction generated by the heterogeneous contact between the SnO2 and Zn₂SnO₄ nanoparticles,the electrons flow from the conduction band of Zn₂SnO₄ to the SnO2,resulting in the increase in the width of the surface electrons depletion layer of SnO2/Zn₂SnO₄ composite increases and the initial resistance of the sensor also increased,which in turn increases the sensor's response to ethanol.?3?The Zn₂SnO₄ regular octahedral structure assembled on the surface by nanosheets was prepared by one-step hydrothermal synthesis.The PdO nanoparticles with different mass fractions were loaded on the surface of Zn₂SnO₄ by the subsequent wet impregnation method.The results of SEM,TEM and HRTEM show that the synthesized Zn₂SnO₄ material is a regular octahedral structure,and the surface is assembled by nanosheets.After surface PdO loading,the surface of Zn₂SnO₄ regular octahedron becomes rough.The PdO nanoparticles with a size of 10 nm were uniformly supported on the surface of the Zn₂SnO₄ nanosheet,and the introduction of PdO did not change the morphology of the matrix.The gas sensing test results show that compared with the pure Zn₂SnO₄ octahedron,the surface modified Zn₂SnO₄ with PdO nanoparticles exhibits more excellent gas sensitivity characteristics,which is manifested as:the sensitivity and selectivity of ethanol are greatly improved.And the operating temperature is reduced from 275? to 250?.Among them,2 wt%PdO-supported Zn₂SnO₄ octahedron has the best gas sensitivity characteristics.At 250?,the response to 100 ppm ethanol is 82.3,and the lower detection limit is 0.5 ppm.The key to the improvement of gas sensing properties is the electronic/chemical sensitization mechanism of precious metals.