The Gas Sensing Performances Of Novel Composite Metal Oxide Materials To Reducing Gases

Since the reform and opening,the total production volume and structure in China have reached a new level,the industry develops rapidly and the status of a manufacturing country has been initially established.However,a large number of organic solvents(ketones,alcohols,aldehydes,etc.)are used in the industrial production process.These organic solvents are toxic.For example,ethanol and acetone can affect the nervous system of human and cause nervous system neuritis;acetone and methanol can stimulate the mucosa and cause skin allergies for human.Meanwhile,these organic solvents have lively chemical properties,which are extremely flammable and volatile.In addition,the incomplete combustion of fuel in industry can lead to the generation of toxic and harmful gases.Therefore,the detection to these harmful vapors or gases is an important foundation for solving environmental problems and maintaining the safety of human.At present,the semiconductor gas sensor has the largest production scale and widest range of applications on the market.It is mainly to convert the relevant information of the measured gas into electrical signals,so as to realize the purpose of detecting for measured gas.The metal oxide gas sensor is a kind of typical semiconductor gas sensor,which has low cost,large production and high sensitivity.But the shortcomings such as high optimal operating temperature,poor selectivity and stability also inhibited the development of these sensors.Therefore,numerous researchers are devoted to the research of novel semiconductor sensors and the improvement of existing semiconductor sensors.The sensing performances of composite metal oxide materials to reducing gases were studied in this paper.The rare earth ferrite materials(such as LaFeO3 and DyFeO3),the Ca-Fe composite oxide materials(such as Ca2Fe2O5,CaFe2O4/Ca2Fe2O5,Ca2Fe2O5/CaFe2O4 and CaFe2O4)and PdO/WO3 materials are included.In this thesis,the factors(such as structure and morphology of material,annealing temperature,operating temperature,humidity and illumination)affecting the gas sensing performances of sensors were studied.Meanwhile,the adsorption models of gas molecules on the surface of sensor were simulated based on DFT calculations,which explains the gas sensing mechanism of sensors to the detected gas.The main research results of this paper:(1)LaFeO3 nanocrystalline powders with different grain sizes were prepared by using the sol-gel method and annealing at different temperatures.Furthermore,the gas sensing performances of LaFeO3 based sensor to ethanol vapor at different operating temperatures and different humidity were studied.We found that the LaFeO3 based sensor annealed at 800℃ has the best sensing response to ethanol vapor at 140℃.Meanwhile,with the increase of humidity,the ethanol sensing response of sensor shows a decreasing trend.In addition,we compared the gas sensing performances of sensors coated with different thicknesses to ethanol vapor,and found that a suitable thickness can greatly improve the gas sensing performances of sensor.We also simulated the adsorption of ethanol molecules on the surface of material based on DFT calculations.First,oxygen molecules are easy to capture electrons from the material,and then pre-adsorbed on the surface of material in the form of ions.When ethanol molecules are introduced,they can react with the pre-adsorbed oxygen and release electrons to the sensor,causing the resistance of the LaFeO3 material(p-type semiconductor material)to increase.The calculation result provides computational model support for the explanation of the experimental phenomenon.(2)The DyFeO3 nanopowders with different grain sizes were prepared by using sol-gel method and annealing at 600℃,700℃,800℃ and 900℃.Then we studied the acetone sensing performances of DyFeO3 nanopowders.We found that the DyFeO3 nanopowders annealed at 800℃ has the best sensing response to acetone vapor due to its excellent micro-morphology.The DyFeO3 based sensor annealed at 800℃ has an optimal response value of 3.81 to 2 ppm acetone vapor at the operating temperature of 190℃.During this process,the response and recovery time of the sensor were 42 s and 44 s,respectively,which indicates that the DyFeO3 material annealed at 800℃ has good recoverability.In addition,DyFeO3 material has excellent selectivity to acetone compared with other gases(ammonia,methanol,formaldehyde,ethanol and acetylacetone).Therefore,DyFeO3 is a promissing material in the application of acetone sensors.(3)The Ca-Fe composite based sensors were prepared by using sol-gel method and annealing at different temperatures.We found that the CaFe2O4/Ca2Fe2O5(CF2/CF)based sensor exhibits the better sensing response to ethanol vapor due to the p-p heterojunction structure and the excellent properties of both Ca2Fe2O5 and CaFe2O4 materials.Meanwhile,the Ca-Fe composite material can promote the ionization process of water and the progress of the hydrolysis reaction.Therefore,the ethanol sensing performances can be enhanced under appropriate relative humidity.In addition,the ethanol sensing response of CF2/CF based sensor can also be greatly improved under UV-visible illumination.Three factors can be considered.First,Ca-Fe composite oxide material is a good catalyst,which can directly promote the decomposition of water and the degradation of ethanol under illumination.Finally,the p-p heterojunction at the interface of CF2/CF based sensor can effectively promote the transport of carriers and inhibit the recombination of electron-hole pairs.Thus,CF2/CF can be regarded as an excellent ethanol sensor material due to its excellent electrochemical and photochemical properties.(4)The CO sensing performances and mechanism of PdO/WO3 based sensors were investigated based experiments and DFT calculations.We found that the decoration of PdO can greatly improve the CO sensing responses of WO3 based sensors,which is mainly due to two factors:the chemical catalytic effect(catalyst)and the electronic effect(p-n heterojunction).On the one hand,PdO is an excellent catalyst that can promote the adsorption of oxygen ions.On the other hand,the p-n heterojunction between PdO and WO3 can promote the migration of carriers and inhibit the recombination of electrons and holes,which can promote the pre-adsorption of more oxygen ions.In addition,the DFT calculation results prove that the decoration of PdO can provide more adsorption sites for oxygen ions.When CO molecules were introduced,more electrons could be transferred from CO molecule to PdO/WO3 structure,resulting in the enhancement of CO sensing performances for sensor.The decoration of PdO can also greatly narrow the band gap energy of the sensor material and expand its absorption range of visible light.Due to the existence of p-n heterojunction in PdO/WO3 based sensor,the photogenerated electron-hole pairs can be effectively separated under light,which can promote the progress of.the electrochemical reaction.In conclusion,PdO/WO3 materials can be widely used in CO gas sensors,and can effectively utilize UV-visible light to solve the CO pollutants.(5)The adsorption mechanism of acetone molecules on PdO decorated hexagonal WO3(0001)(PdO/h-WO3)surface was investigated based on DFT calculations.We found that PdO/h-WO3(0001)surface can provide more adsorption sites for acetone molecules,which can cause more electrons to be transferred from acetone molecules to the surface of material and the resistance of material decreases(n-type semiconductor material).In addition,we found that the concentration of pre-adsorbed oxygen and the operating temperature have an important effect on the adsorption of acetone molecules on PdO/h-WO3(0001)surface.First,O atoms in acetone molecule are easily adsorbed on the W site or Pd site of the substrate material in an environment with low oxygen content.As the operating temperature increases,acetone molecules can desorb from the surface of the material,which verifies that the h-WO3 based sensor has the best acetone sensing performances at certain temperature in experiments.In an environment with high oxygen content,acetone molecules can react with pre-adsorbed oxygen on the surface of the material to form indirect products.It provides a further DFT calculation analysis of acetone sensing mechanism for PdO/h-WO3 based sensors.