Studys On Sensing Mechanism Of New Type Materials To CO And CO₂

Since the pre-industrial times, lots of fossil fuels are digged out from the underground and burned off quickly. The product of fossil fuels combustion can not be consumed by ecological environment. It has caused very serious environmental problems, such as sea levels rising, glacier melting, rainwater acidification, and global warming. In the industrial processes, more and more raw materials are used with more and more products or waste products. The existence of flammable or toxic gases in the waste products causes very serious environmental problems. Domestic gas (LPG and natural gas) is accompanied by fire, explosion, and poisoning. This is a threat to people’s health. Effective monitoring is a necessary prerequisite to solve the environmental pollution and avoid personal injury.Preparation of effective gas sensing device is an effective way to solve these problems. Among them, the gas sensors based on oxide semiconductor are the most important gas sensors which are used widely. Gas sensing materials based on semiconductor include metallic oxide and organic gas sensing materials. The gas sensors based on metallic oxide occupy lots of market share. The metallic oxide gas sensing materials have the advantage of high sensitivity, rapid response and easily compound.Perovskite oxide material is a kind of very important function semiconductor material. The perovskite oxide material has been used in many areas. Due to its stable structure, convenient preparation, and easily compound, it has also received widespread concern in the field of gas sensing. The perovskite oxide materials have good selectivity, high sensitivity and high stability when it is used as gas sensing material. The performance of perovskite oxide materials in selectivity, sensitivity, and stability can also be improved by the two ways:(1) change the element of A site or B site; (2) doped in A site or B site. The two approaches would not change the structure of perovskite oxide materials. On the contrary, the two ways can improve its performance. In order to ensure materials’ reliability, security and repeatability, new technologies and devices need to be developed. At present, theories and models are needed for explaining the sensing mechanism of materials. The shortage of theoretical guidance leads to the gas sensing experimental work is blind. In order to provide guidance and interpretation for experimental work on gas sensing material, we build a simulation model based on DFT theory. Following, we study the materials’ gas sensing and materials’ electrical properties.The main research contents and main results of this paper:1. The La1-xCaxFeO3 materials are prepared by sol-gel. We study its electrical transport properties, its gas sensing performance, and its gas sensing mechanism for CO gas. We find that the resistance of La1-xCaxFeO3 materials varying with the temperature is fit to the small polaron hopping conductivity model. With the increasing of Ca2+doping concentration in La1-xCaxFeO3 material, the concentration of Fe4+ increases. The resistance of La1-xCaxFeO3 materials decreases at first, undergoes a minimum at x=0.3, and finally increases. The ratio of Fe4+/Fe3+is approximately equal to 1 when x= 0.3. When Ca doping concentration x> 0.3, the oxygen vacancies in materials provide lots of electrons. The electrons neutralize the hole in materials which leads to the ratio of Fe4+/Fe3+decreases. Then, the resistance of La1-xCaxFeO3 materials increases again. When Ca2+doping concentration x< 0.2, the gas responses of La1-xCaxFeO3 materials increase with the increasing concentrations of Ca2+. This is because the increasing adsorbed oxygen on La1-xCaxFeO3 materials’ surface. The optimized gas sensing response appears at the Ca2+doping concentration x= 0.2. Its gas response to 200ppm CO is 3.45. However, the results of XPS indicate that the proportion of adsorption oxygen on the surface of La1-xCaxFeO3 materials increase with the increasing Ca doping concentration. When Ca2+ doping concentration x> 0.2, its gas response decreases which is inconsistent to the traditional view (more adsorption oxygen leads to a higher gas response). Experimental results show that at the high Ca2+ion doping concentration x> 0.2, the adsorption oxygens on La1-xCaxFeO3 materials’surface do not effectively release their electrons to the surface of La1-xCaxFeO3. The gas sensing response to reducing gas CO for La1-xCaxFeO3 sensors depends not only upon the amount of adsorbed oxygen on the grain surfaces of sensors, but also upon the capabilities of adsorbed oxygen speccies through reacting with CO. Similar case may also occur in nanocrystalline La1-x(Ba, Pb)xFeO3 sensors.2. We employ the DFT theory to study the adsorption propertiesof CO molecules on LaMnO3 (010) surface with MnO2-termination and the influence of pre-adsorbed oxygen. Our calculated results indicate that the orthorhombic LaMnO3 material can be used to detect CO. When the CO molecules adsorbed on LaMnO3 (010) surface, the most stable adsorption structure is Mn-CO. The reason why CO molecule can be adsorbed on LaMnO3 (010) surface is the hybridization interaction between the CO-2p and Mn-3d orbital. For LaMnO3 (010) surface having one pre-adsorbed O2, CO molecule reacts with the pre-adsorbed oxygen. One oxygen atom of pre-adsorbed oxygen is despoiled by CO molecule to form one CO2 molecule. In this chemical reaction, the CO molecule loses 0.122e electrons. The adsorption energies are 0.122Ha. Thus, the existence of adsorbed oxygen on LaMnO3 (010) surface has an important influence on gas sensing performance of LaMnO3 materials.In addition, we also studied the adsorption properties of CO2 molecule on LaMnO3 (010) surface and the impact of pre-adsorbed oxygen with DFT theory. For pure LaMnO3 (010) surface, CO2 molecule acts as a donor or an accepter in the adsorption process. Then, we investigated the influence of pre-adsorbed oxygen on the adsorption properties of CO2 molecule adsorbed on LaMnO3 (010) surface. Compared the two adsorption configurations of CO2 adsorbed on LaMnO3 (010) surface with and without pre-adsorbed O2, we find the pre-adsorbed oxygen on the perovskite LaMnO3 surface can improve its sensitivity.3. DFT theory is used to study the adsorption properties of CO molecule adsorbed on CaMnO3 (001) surface with MnO2-termination and the influence of pre-adsorbed oxygen. Our calculated results indicate that the orthorhombic CaMnO3 material can be used to detect CO. When the CO molecule adsorbed on CaMnO3 (001) surface, the most stable adsorption structure is Mn-CO. The reason why CO molecule can be adsorbed on CaMnO3 (001) surface is the hybridization interaction between CO-2p orbital and Mn-3d orbital of surface. For CaMnO3 (001) surface having one pre-adsorbed O2, CO molecule react with the pre-adsorbed oxygen. One oxygen atom of pre-adsorbed oxygen is despoiled by CO molecule to form one CO2 molecule. In this chemical reaction, CO molecule loses 0.216e electrons. In the process, CO molecule acts as a donor. Thus, the existence of adsorbed oxygen on CaMnO3 (001) surface has an important influence on gas sensing performance of CaMnO3 materials.In addition, we also study the adsorption properties of CO2 molecule on CaMnO3 (001) surface and the influence of pre-adsorbed oxygen with DFT theory. For pure CaMnO3 (001) surface, O2 molecule acts as a donor or an acceptor in the adsorption process. Then, we investigated the influence of pre-adsorbed oxygen on the adsorption properties of CO2 molecules adsorbed on CaMnO3 (001) surface. Compared the two adsorption configurations of CO2 adsorbed on CaMnO3 (001) surface with and without pre-adsorbed O2, we find that the existence of pre-adsorbed O2 urges more electrons tranfer between the CaMnO3 (001) surface and CO2 molecule. The results show that the pre-adsorbed oxygen on CaMnO3 materials’ surface can improve the perovskite structure oxide CaMnO3 materials’ sensitivity.4. We use the DFT theory to study the adsorption properties of CO2 molecule adsorbed on LaCoO3 (110) surface and prepict the gas sensing mechanism of p-type perovskite oxide LaCoO3. For CO2 molecule adsorbed on the surface of pure LaCoO3 (110) surface, our DFT theoretical calculated results indicate that CO2 molecule plays as a acceptor and CO2 molecule despoils electrons from the LaCoO3 (110) surface in all adsorption configuration. In addition, pre-adsorbed oxygen has been proved to adsorb on nanomaterials’ surface and involves in the process of gas sensing response. For LaCoO3 (110) surface with one pre-adsorbed oxygen molecule, the structure of CO2 molecule adsorbed on an oxygen atom of pre-adsorbed oxygen molecule is the most stable adsorption state after optimization. However, the CO2 molecule acts as an acceptor in this mode, which is inconsistent with the experimental results that the resistance of LaCoO3 increases after CO2 molecule is introduced into the system. To this end, we designed another adsorption configuration (one more adsorbed O2 molecule is added). For LaCoO3 (110) surface with two pre-adsorption O2 molecule, the configuration of CO2 molecule on one of pre-adsorbed O2 with C-down (CO2) to the adsorbed site plays an important role in the net charge transfer. In this mode, the CO2 molecule acts as a donor which is responsible for the increasing resistance of LaCoO3, when exposed to CO2 gas.5. We use the first-principles DFT theory to study the adsorption properties of CO2 molecules on AIN and SiC nanosheet. Our calculations results show that the graphene-like structures AIN and SiC nanosheets can be used to detect CO2 gas. For pure AIN nanosheet, its most stable adsorption strucctre is the carbon atoms of CO2 molecules adsorbed on the nitrogen atoms, an oxygen atom of CO2 molecules adsorbed on the adjacent aluminum atoms and the other oxygen atom hung dangling. For AIN nanosheet having one pre-adsorbed oxygen, its most stable adsorption strucctre is the carbon atoms of CO2 molecules adsorbed on the nitrogen atoms and the two oxygen atoms of CO2 molecules adsorbed on the two adjacent aluminum atoms. The hybridization interactions between CO2 molecule and clean/O2 pre-adsorption AIN nanosheets mainly root from two points:(1) the hybridization of C-p and N-p; (2) the hybridization of O-p and Al-p. The results of SiC and AIN nanosheets confirm that the pre-adsorbrd oxygen hinders electrons transfer from SiC/AIN nanosheets to CO2 molecule. The charge distribution of AIN nanosheets with CO2 molecular is analyzed using Milliken method. The results show that the CO2 molecule always despoils electrons from the clean/02 pre-adsorbed AIN nanosheets. However, pre-adsorbrd oxygen hinders electrons transfer from AIN nanosheets to CO2 molecule.For clean SiC nanosheet, its most stable adsorption structure is the carbon atom of CO2 molecules adsorbed on a carbon atom (SiC nanosheets), an oxygen atom of CO2 molecules adsorbed on the adjacent silicon atom and the other oxygen atom hung dangling. For SiC nanosheet having one pre-adsorbed O2, SiC nanosheets can retain CO2 molecules with varieties of structural models. However, its most stable adsorption structure also is the carbon atom of CO2 molecules adsorbed on a carbon atom, an oxygen atom of CO2 molecules adsorbed on the adjacent silicon atom and the other oxygen atom is hung dangling. Similarly, the hybridization interactions between CO2 molecule and clean/02 pre-adsorption SiC nanosheets mainly root from two points:(1) the hybridization of C-p (from CO2) and C-p (from SiC); (2) the hybridization of O-p and Si-p. The results of analysis using Milliken are same to the A1N nanosheets. In conclusion, the results of SiC and A1N nanosheets confirm that the pre-adsorbrd oxygen hinders electrons transfer from SiC/AIN nanosheets to CO2 molecule.