The Gas Sensing Property Of CO₂ And Low Concentation Acetone Sensing Materials

With the increasing awareness of environmental protection and personal health,gas sensor gradually excite the people's attention.The increase of the CO₂ emissions has caused global warming,sea level rise,glacier melting or other serious ecological problems,attracting researchers highly concerned.Therefore,CO₂ gas sensor has become a popular research.In addition,acetone in human exhaled gases is an important potential biomarker for the detection of diabetes.Respiratory analysis of human respiratory tract components by non-invasive diagnosis of clinical disease and monitoring of human metabolic status of the breath analysis technology?breath analysis?for the diagnosis and monitoring of diabetes opened up a new way.In order to detect and monitor CO₂ and acetone gas effectively,this paper is devoted to the preparation of gas sensors with high sensitivity,response,fast recovery,high selectivity and high stability.Metal oxide semiconductor gas sensor is regarded as one of the most important gas sensor for its simple preparation process,excellent performance,low cost.There have been found many kinds of CO₂ gas sensors:infrared,solid electrolyte,surface acoustic wave,resistive,capacitive and MOS.The accuracy of the infrared absorption sensor is relatively high,but because of its device are huge,expensive,so its hard for the popularity;Severinghaus electrode electrochemical sensor is mainly used to measure the CO₂ concentration in blood,such electrochemical sensors highly susceptible to electromagnetic interference;NASICON solid electrolyte type sensor is difficult to use due to the harsh preparation process,manufacturing difficulty.Resistive and p-n type conductivity composite oxide sensing material are commonly used as CO₂ gas-sensing materials.Such as CuO-BaTiO₃ composites and Ag-modified CuO-BaTiO₃ composites,these materials are relatively complex and the phase temperature of BaTiO₃ material and the working temperature of CO₂ gas are very high,so itsMany metal oxide materials?single phase and composite phase?are used as CO₂ sensing materials.?Such as:Nd₂O₂CO₃,CeO₂,LaOCl,GdCoO₃,SmCoO₃,La1-xSrxFeO₃,La1-xBaxFeO₃?and composite oxide films?such as CuO-BaTiO₃,La₂O₃-BaTiO₃ and ZrO₂-BaTiO₃,etc.?the resistive sensors exposed to CO₂ atmosphere are increased.In the p-n system,such as CuO-BaTiO₃,it is often considered that the CO₂ gas-sensing mechanism is related to the carbonate which formed on the surface of CuO.The carbonate formed on the surface of the material changes the p-n barrier height of CuO-BaTiO₃.Similarly,the mechanisms of CO₂ emissions of LaOCl,Nd₂O₂CO₃ and La1-xSrxFeO₃ are also explained by the formation of carbonates and their derivatives.The first-principles calculation show that when exposed in CO₂,the CO₂ atoms can form bridged and coordinated carbonates between the CO₂ molecules and the lattice atoms?such as O?.But there is still doubt that the specific electron transfer information between CO₂ molecules and gas-sensing materials.Further studies have shown that the chemical adsorption of oxygen on the surface of a semiconductor oxide may be involved in the gas-sensing process of CO₂ gas and materials.But the gas-sensing mechanism for CO₂ gas semiconductor oxides remains controversial.Therefore,there is still much work to exploit CO₂ sensing materials and discuss the possible CO₂ gas sensing mechanisms.Acetone is relatively toxic,and it has low boiling point and easily volatile at room temperature.When the content of acetone in the air upto 1000 ppm,the human body will show dizziness,fatigue and other discomfort,also acetone is an important by-product in human exhaled breath,especially diabetics.Usually the content of acetone in healthy people exhaled gas is about 0.3-0.9 ppm,while the diabetes is of>1.8 ppm.Significant differences made the concentration of acetone in exhalation became an important marker for monitoring diabetes.At present,acetone gas detection methods are include:electrochemical method;fluorescence spectroscopy;spectrophotometry;gas chromatography mass spectrometry;mass spectrometry;combustion and sensors.Trace acetone in breath air detection is mainly based on laser spectroscopy,gas chromatography,mass spectrometry and its derivative methods and electrochemical sensors.At present,some binary and composite semiconductor oxides are used as acetone gas sensing materials,such as:SnO₂,WO₃,V₂O₅,ZnO,TiO₂,NiO,Fe₂O,CO₃O₄,CuO-ZnO and ZnO-In₂O₃.Perovskite oxide material usually means compounds with ABO₃ ratio stoichiometric,which is a very important semiconductor functional material.Use it as a gas sensing material have lots of the advantages.Perovskite oxides materials exhibit excellent selectivity and sensitivity for a lot of gases,and the sensor ia also stability.so it difficult to make the baseline drift or other uncertain factors.Because of its ideal space group is Pm3m,in its structure,Its basic structure is BX6 octahedral vertex connected structure.When the cation of A and B is doped,the octahedron will be deformed,the position of X ion will be small and the Pm3m space group will be transformed into orthorhombic or tetragonal system.Perovskite oxide gas-sensitive materials on the gas selectivity,sensitivity,etc.can be controlled by A,B-bit elements to achieve.At present,the study of perovskite gas sensitive materials needs to be repeated experiments,select the doped elements and change the doping amount,and doping?Pt,Pd,Au?metals to improve its gas sensitivity,gas selectivity and sensor stability.In addition,it is also necessary to explore the development of advanced component manufacturing techniques to ensure reliable,safe,and repeatability during device operation.In this paper,we focus on some perovskite-related gas-sensing work,focusing on the perovskite oxide and cerium oxide materials on CO₂ and trace acetone gas sensitivity research.The mainly research contents and main results are as follows:1,Series of La1-xBaxFeo₃?x=0,0.1,0.2,0.3?nanocrystalline powders were prepared by sol-gel method.And then make it as thick film sensors and packed powder sensors.Both types sensors exhibit good gas-sensing properties to CO₂.With the increase of Ba doping ratio and the CO₂ concentration increase,the resistance of LaFeO₃ gas sensor increases.At 300 ?,the sensitivity of LaFeO3 gas sensor to 1000,2000 and 4000 ppm CO₂ gas was 1.74,2.19 and 2.74,respectively.The results show that the CO₂ molecules can adsorb on LaFeO₃?010?surface lattice,and the C atom is bonded to the surface lattice O atom,and the surface of the LaFeO₃?010?surface is pre-adsorbed with enough oxygen molecules on the LaFeO₃?010?surface.Part of the charge is transferred from the C atom in the CO₂ to the LaFeO₃?010?surface.The two O atoms in the CO₂ are bonded to the adjacent Fe atoms,and the charge is transferred from LaFeO₃?010?to the two O atoms.A total 0.021 charge is transferred from the CO₂ to the LaFeO₃?010?surface,consistent with the change of the resistancwhen the sensing material exposed to CO₂.2.The xwt%Pd-SmFeO₃?x=0?1,3,5?nanocrystalline powders were prepared by sol-gel method and character:ized by make into packed powder sensors.The electrical properties of the nanocrystalline powders were characterized by gas-sensing mechanism.The gas sensitivity characteristics of low concentration acetone were carried out at 100? to 340?.XRD shows that the undoped SmFeO₃ and the Pd-doped SmFeO₃ material are orthorhombic perovskite structures with the average grain size of about 50-55 nM.XPS results show that Pd 3d peaks are deconvoluted as Pd 3d3/2 at 337 eV and elemental Pd0 at 335 eV.The Ols peak is the lattice oxygen peak at 529.27 eV and the adsorption oxygen peak at 531.4 eV,respectively.3wt%Pd-SmFeO3 components at the optimum operating temperature of 240?,the sensitivity of 500 ppb trace acetone gas S=7.21;response-recovery time is 4 seconds and 3 seconds,with the ambient humidity increases,the sensitivity S of the component is reduced.The gas sensitivity mechanism of SmFeO₃ to acetone is based on the change of material resistance,and the change of resistance is controlled by the number of acetone molecules and the number of adsorbed oxygen Oads on the surface of the material.When air is the carrier gas,the chemical adsorption of oxygen to O₂-or O-adsorption on the surface of the material,encountered acetone gas,chemical adsorption of oxygen and acetone gas reaction,the product is CO₂ and H₂O.Therefore,the presence of H₂O inhibits the reaction process.There are two possible mechanisms for the promotion of gas-sensing properties with Pd doping;electron sensitization processes and chemical sensitization.Pd doping makes the gas-sensing response enhancement attributed to the "spillover effect" and "Fermi control".The simultaneous presence of Pd0 and Pdx+ causes the "spillover effecti and the "Fermi control" effect to occur at the same time.Since the number of PdO in the semiconductor material is much higher than Pd0,so in this systyem the,Fermi control" effects dominate the sensing process.3.A single-phase YFeO₃ nanocrystalline powder was prepared by sol-gel method and doped with PdCl₂ to obtain a series of xwt%Pd-YFeO₃?x= 0,1,3,5?nanocrystalline powders,HRTEM,SAED and other characterization,made of thick film sensor to study its electrical properties and trace of acetone gas sensitivity.XRD results show that YFeO₃ powder is an orthorhombic perovskite structure.The average grain size D of the material is about 40-50 nm.And the size of HRTEM obtained by high resolution transmission electron microscopy.XPS results show that the peaks of Pd 3d are deconvolution into two peak positions,Pd 3d3/2 at 337 eV and peak at 342.5 eV,respectively.The peak at 336.7 eV in the Pd3d peak is Pd2+ in PdO.The results of high resolution transmission electron microscopy?TEM?show that 3wt%Pd-YFeO₃ annealed at 800? is spherical particles with a diameter of about 40-60nm.The dispersion is uniform and the particle size is not much different.It can clearly show that the Pd element is supported on the particle surface.The YFeO₃ crystal is a well-formed single crystal material in the electron diffraction pattern.3wt%Pd-YFeO₃ Gas Sensors The S value of the 100 ppb trace acetone gas at 200? was 3.53 and the response recovery time was 42 seconds and 38 seconds.4.The CeO2?111?nanocrystalline powders were prepared by sol-gel method at different annealing temperatures?400?,600?,800?,900??.CeO2?111?/?110?nanospheres was prepared by one-step hydrothermal method;CeO₂ nanorods?11O?were prepared by hydrothermal method.XRD,HRTEM,SAED and so on were used to characterize the gas-sensing properties of CO₂ gas.XRD results show that the CeO2 nanocrystalline material annealed at 400?,600?,700?,800? and 900? is fluorite structure?space group Fm3m?.XPS results show that CeO₂ is mainly in the form of Ce₃₊.The results show that the CeO₂ nanocrystalline particles are spherical particles with a diameter of about 40-60nm.The results show that the CeO2 nanocrystalline particles are the spherical particles with diameter of about 40-60 nm.It can be seen that the diffraction rings are bright and arranged in order to show that the CeO₂ crystal is a polycrystalline material with good appearance.The Snanocrystalline ceO₂?111?>S spherical CeO₂?111?/?110?>SCeO₂?110?is shown to exhibit the gas sensing properties of CO₂.The adsorption structure of CO₂ molecules on cerium oxide nanocrystalline powders was studied by means of reflection absorption infrared spectroscopy?RAIRS?and electron structure of XPS energy spectrum.It was found that CO₂ adsorption on the surface of cerium oxide nanocrystalline powder(CO_32-)and bridged carbonate and a small amount of carboxylate?CO₂-?,and the carboxylate?CO₂-?species and their instability are weak.5.The SmFeO₃ prepared by sol-gel method,and then was doped with PdCl₂,HAuCl₄ and H₂PtCl₆ respectively.The Pt,Pd,Au-SmFeO₃ doped nanopowders were made into thick film sensors,and then measured its sensing sensitivity and their electrical properties,gas-sensing properties,gas-sensing mechanism and gas-sensitive changes under O₂ background and humidity were studied.The HRTEM of 3wt%Au-SmFeO₃ shows that 3 wt%Au-SmFeO₃ annealed at 800? is spherical particles having a diameter of about 60-100 nm and Au particles are spheres with a diameter of about 8 nm.Au0?111?crystal face when the crystal plane spacing is 0.2355 nm.The interplanar spacing of 0.2041 nm shows the Au0?200?plane.The XPS diffraction spectra show that the peaks of Au 4f in 3%wt Au-SmFeO₃ are deconvolution into two peak positions,Au0 4f7/2 at 83.7 eV and Au³⁺ 4f 5/2 at 87.5 eV,respectively.Fe 2p2/3 is located at 710.75 eV and the Fe 2p1/2 peak is located at 725 eV.Gas sensing results show the sensitivity of each sensors:SAu-SsmFeO₃?SPt-SmFeO₃>SPd-SmFeO₃?SSmFeO₃.The sensitivity S for 200 ppb acetone was 1.58,1.75,2.48 and 4.67.This indicates that the amount of Au doped with SmFeO₃ gas sensor on the performance of acetone gas is greatly improved.The improved gas-sensing performance of Au particles to acetone is attributed to aspects as follows:First,the catalytic oxidation of acetone by Au nanoparticles can accelerate the reaction rate of acetone and chemical adsorption of oxygen ions on the surface of the material;Second,the Au nanoparticles on the semiconductor oxide band modulation:due to the large work function of Au,when the semiconductor surface and acetone molecules in contact with the occurrence of gas-sensitive reaction,the chemical adsorption of oxygen ions back to the semiconductor oxide,So that the width of the depletion layer caused by Au in the semiconductor oxide becomes narrower,resulting in a greater resistance change.Third,Au nanoparticles increase the chemical adsorption of the surface of the semiconductor by the "spill over effect".6,We discuss the gas sensing property of 3wt%Pd-SmFeO₃ to acetone under UV enhanced.Also we discuss the possible gas-sensing mechanism.The sensitivity of the 3 wt%Pd-SmFeO₃ based gas sensor under UV light to 200 ppb,500 ppb,1 ppm and 5 ppm acetone gas at 200? is 3.6,7.9,17.1,20.5,respectively,compared to the sensitivity in the dark.Increased by 89.5%,113.5%,143.7%,98.1%,74.3%and 53.1%respectively.After the gas-sensing test system is excited by ultraviolet light,the surface of Pd-SmFeO₃ material produces a large number of photogenerated holes.Hole can capture free hydroxyl groups?hydroxyl groups present in water and acetone?to form hydroxyl radicals.Hydroxyl radicals are strongly oxidizable and accelerate the rate of oxidation of the reducing gas adsorbed on the surface of Pd-SmFeO₃,while accelerating Gas desorption.In addition,acetone molecules contain hydroxyl groups,under the photocatalytic action of Pd-SmFeO₃ easily with the surface of the adsorbed oxygen Oads redox reaction.The photo-excited gas-sensing process has the following possibilities:The coupling of SmFeO₃ with Pd under the excitation of ultraviolet light enables Pd to improve the efficiency of charge separation and change the charge transfer direction in the material;it can provide a special path for photogenerated transport and can store semiconductors The photocatalytic activity of SmFeO₃ was enhanced by the photocatalytic activity of SmFeO₃,and the gas photoactivity was improved.