| In the past few years,metal oxide semiconductor gas sensors have developed rapidly,and have played a huge role in detecting pollutants and toxic and harmful gases.So far a large number of metal oxide semiconductor materials have been used to make gas sensors.These metal semiconductor oxides include binary oxides(such as ZnO,NiO,WO3,etc.)and ternary oxides(such as ZnFe2O4,NiFe2O4,ZnCo2O4,etc.)and four or more complex metal oxides.These oxides are widely welcomed by researchers because of their simple structure,low power consumption,low cost,good selectivity and stability.The spinel structure,especially the spinel ferrite(AFe2O4,A=Zn,Cu,Mg,Ni,etc.)belongs to the ternary metal oxide semiconductor material and is widely used in various fields,such as gas sensors,photocatalysts,Magnetic materials,lithium-ion batteries and solar cells.Nanotechnology has developed rapidly in recent years.Nanomaterials refer to powders with a grain size in the range of 1-100 nm including oxides and metal compounds.Because of the basic characteristics of nanomaterials such as small size effect,quantum size effect,and surface effect,nanomaterials have shown special properties in physics and chemistry.In the paper,we prepared n-type semiconductor CuFe2O4 and MgFe2O4 nanopowders by sol-gel method,and prepared sensor elements,studied their gas-sensitivity studies on reducing gas acetone and CO,and discussed the gas-sensing physical mechanism.In addition,the response characteristics of the sensor element to acetone and CO gas under ultraviolet and visible light irradiation are discussed,and the physical mechanism of light enhancement of gas sensitivity is given.The main research results of this paper are as follows:1.CuFe2O4 nanopowders were prepared by sol-gel method,and annealed in air at 400?,500?,600?,700? and 800? for 3 hours.XRD results show that the annealed CuFe2O4 has a cubic spinel structure.At normal temperature and pressure.we investigated the gas sensitivity of 3 ppm acetone in the operating temperature range of 120??280? for the materials obtained at different annealing temperatures,and found that the resistance of the device decreased when it met acetone,which was due to the transfer of electrons to the surface of the material.The best annealing temperature is 500? and the best working temperature is 220?.Then the CuFe2O4 nanopowders obtained after annealing at 500? were tested.The optimal response values for 0.5 ppm,1 ppm,3 ppm and 5 ppm acetone at the optimal working temperature of 220? were 1.207,1.83,3.147 and 4.316,respectively.We measured the room temperature forbidden band width of CuFe2O4 nano-powder,which is 1.76 eV.We use ultraviolet light(wavelength 365 nm)and visible light(wavelengths 410 nm and 450 nm)to irradiate the sensor element.Under the power of ultraviolet light emitter 50 mW,the best working temperature under ultraviolet light irradiation is 1800? and the material is the response value of acetone has also been improved.The visible light power is 100 mW,and the best working temperature under the light of 410 nm and 450 nm wavelength is 200? and 220?,and the response value of the material to acetone has also been improved.The photon energy of the ultraviolet and visible light is higher than the width of the forbidden band,and photo-generated electron and hole pairs are generated in the surface area of the semiconductor material.Its photogenerated electrons interact with adsorbed oxygen molecules to produce light-induced adsorbed oxygen ions.These lights induce the further reaction of adsorbed oxygen ions with acetone,thereby enhancing the gas-sensing response.Under ultraviolet light and 410 nm visible light,the optimal working temperature is reduced,indicating that these light-induced adsorption of oxygen ion desorption is easier.2.MgFe2O4 nanopowders were prepared by sol-gel method,and annealed in air at 400?,500?,600?,700? and 800? for 3 hours.At normal temperature and pressure,we investigated the gas sensitivity of 3 ppm acetone in the working temperature range of 120??260? for the materials obtained at different annealing temperatures,and found that the resistance of the device decreased when it met acetone,which was due to the transfer of electrons to the surface of the material.The best annealing temperature is 500'C and the best working temperature is 200?.Then the best response values of MgFe2O4 nano-powders obtained by annealing at 500?to 1ppm.3ppm and 5ppm at the optimum working temperature of 200? were 2.045.3.307 and 5.565.respectively.We measured the room temperature forbidden band width of MgFe2O4 nano-powders,which is 1.98 eV.We use ultraviolet light(wavelength 365 nm)and visible light(wavelengths 410 nm and 450 nm)to illuminate the sensor element.The ultraviolet light wavelength is 365 nm,the light emitter power is 50 mW,the visible light wavelength is 410 nm,450 nm,and the light emitter power is 100 mW.The optimal operating temperature under 410 nm and 450 nm wavelength light irradiation is 160?,180? and 200? respectively,and the response value of the material to acetone is also improved.And we found that under the same luminous power,the shorter the wavelength,the higher the optical frequency.and the higher the energy beyond the forbidden band width.which is conducive to the improvement of the gas-sensitive performance of the material.3.CuFe2O4 and MgFe2O4 nanopowders were prepared by sol-gel method,and were annealed in air at 400?,500?,600?.700? and 800? for 3 hours.At normal temperature and pressure,we investigated the gas sensitivity of 500 ppm CO for the CuFe2O4 and MgFe2O4 materials obtained at different annealing temperatures in the operating temperature range of 160??260?,and found that the resistance of the device decreased when it met CO.which was due to the transfer of electrons to the material by CO surface.The best annealing temperature of CuFe2O4 is 600?,the best working temperature is 220?,and the response value is 2.151.Introducing 410 nm and 450 nm of visible light,the optimal working temperature of CuFe2O4 nano-powder obtained at 600? annealing temperature to 500 ppm CO is 200? and 220? respectively,and the corresponding response values are 3.041 and 2.796.respectively.Tests show that the optimal annealing temperature for MgFe2O4 is 500?.the optimal working temperature is 220?.and the response value is 3.735.Introducing visible light with wavelengths of 410nm and 450nm.the optimum working temperature of 500 ppm CO for MgFe2O4 nano-powders annealed at 500?was tested to be 180? and 200?.and the corresponding response values were 4.245 and 3.942,respectively.Therefore,visible light at 410 nm and 450 nm wavelengths can improve the material's response to CO and reduce the optimal operating temperature,and we found that at the same luminous power,the shorter the wavelength,the higher the optical frequency,and the higher the energy beyond the forbidden band width.This is conducive to improving the gas-sensitive performance of the material. |
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