| As the gradually expand of industry scale, the gas type and quantity of raw gases and produced gases are growing. Some of these gases are flammable or explosive, some of them are poisonous. Their leakage not only causes environmental pollution problems such as the greenhouse effect, acid rain, ozone depletion et al. but also harms to people’s personal and property security, prone to explosions, fires or biological poisoning. At the same time, as the rise of people’s living standard, liquefied petroleum gas, natural gas and city fuel gas are also rapidly popularized as domestic fuel. These can also cause the leakage of gas explosion, fire and poisoning accidents. It is the necessary premise to solve the problem of environmental pollution to effectively detection and alarm these toxic, pollution gases. Although the semiconductor metal oxide such as SnO2and ZnO has been widely used as gas sensing materials, the understanding of the gas-sensing mechanism is still relatively vague. SnO2oxide system is the active direction all over the world. It can be seen as a representative of the n type semiconductor oxide. To study the gas-sensing mechanism of SnO2oxide system is worthwhile.Several types of CO2sensors such as infrared, surface acoustic wave, solid electrolyte, capacitive, resistive and MOS have been found. The measurement accuracy of infrared absorption sensor is high, though meanwhile, the device is large, the price is high, and then the popularization is difficult. Electrochemical Severinghaus electrode sensor, which is susceptible to electromagnetic interference, mainly applied to measure the carbon dioxide in the blood. The NASICON solid electrolyte CO2sensor is also difficult to use widespread because of its actual manufacturing difficulty. The conductance or resistance type CO2gas sensing materials are mainly concentrated in the p-n type composite oxide materials, such as CuO-BaTiO3composites or mixed silver CuO-BaTiO3composites. Because of involving two substances (CuO and BaTiO3), the composite materials need complex preparation technics. In addition, BaTiO3needs higher temperature to crystallize, and the best working temperature of CuO-BaTiO3composites CO2gas sensor is400℃or more. It has been shown that some composite or single phase oxides can be used as the resistive CO2sensors, where there is a change in the electrical resistance of semiconductor upon exposure to CO2in air. The composite oxide films (such as CuO-BaTiO3, La2O3-BaTiO3and ZrO2-BaTiO3) and single phase semiconductors (such as LaOCl, Nd2O2CO3, SmCoO3, GdCoO3, and La1-xSrxFeO3) present an increase of resistance to CO2gas. It was suggested that the possible CO2sensing mechanism of CuO-BaTiO3p-n system was connected with the carbonates of CuO. The height of potential barrier of p-n junctions in CuO-BaTiO3could be modified through thin films of carbonation. The formation of carbonates was also suggested to be origin (or one of origins) of CO2sensing mechanisms for LaOCl and Nd2O2CO3and La1-xSrxFeO3. An ab initio calculation showed that for LaOCl, the bridged or polydentate species/carbonates could be formed, through the adsorption of CO2molecule on the surface atom (such as oxygen) of the lattice. However, it gave no information about electron transfer between LaOCl and CO2. Some experimental results also showed that the oxygen adsorbed on the surface of semiconductor may be involved in the sensing process of CO2in dry air for La1-xSrxFeO3. So far, the gas-sensing mechanism of the single and composite oxides to CO2is still not very clear. There are a lot of controversies. The CO2sensing mechanism needs long-term and in-depth exploration.Compared with the single metal oxide, ABO3perovskite oxide as gas-sensing material has some advantages, not only showed better selectivity and sensitivity of gases but also had higher stability. In addition, the sensitivity, selectivity and stability of ABO3perovskite gas sensing material can be adjusted and controled not only by changing the element in A or B site, but also by partly replacing the element in A or B site with other elements. The structure of ABO3perovskite oxide is so stable that doping will not change its original structure. Because of the lack of the gas-sensing mechanism of universal model as a guide, the gas-sensing experiment has certain blindness until now. To provide a theoretical guidance for further development of new gas sensitive material, and a theoretical explanation for the gas sensitive experimental results, this article, we study the atomic and electronic structure informations of gas sensing material in the sensing reaction processes based on the first principles.The studies of this paper mainly include the following results:1. LaFeO3nanocrystalline powders prepared by sol-gel method can exhibit considerable sensing response to CO2gas. The resistance of the LaFeO3sensor increases with increasing concentration of CO2. At300℃, the responses of the sensor to1000,2000and4000ppm CO2are1.74,2.19and2.74. Its response and recovery times to2000ppm CO2at300℃are about4min and8min, respectively. Our first principles calculations results demonstrate that with adeuate oxygen molecules pre-adsorbed on the LaFeO3(010) surface, the CO2molecule can be absorbed on the LaFeO3(010) surface accompanied by releasing electrons to the surface. The C atom of CO2bonds with the lattice O atom of the LaFeO3(010) surface and releases0.331e charge to the surface, while the two O atoms of CO2bond with the nearest lattice Fe atoms with obtaining0.161e and0.156e charge from the surface. In other words, there is a net charge of0.021e transferring from CO2to the lattice of LaFeO3(010) surface. It means that the calculation result is likely responsible for the observed increase in resistance of LaFeO3when exposed to CO2in dry air.2. The structure of the nano-La0.875Ca0.125FeO3powders prepared by sol-gel method has a single orthogonal perovskite phase. The resistance of Lao.875Cao.i25Fe03increases with increasing the moisture, while the gas response S decreases with increasing the moisture. At320℃, the responses to1000ppm CO2in38%RH and70%RH are1.67and1.53respectively. The possible CO2sensing mechanisms in moist air for La0.875Ca0.125Fe03sensor are investigated by first principles calculations. Calculated results demonstrate that there is a small charge transfer from H2O to La0.875Ca0.125FeO3(010) surface both in molecularly and dissociatively adsorption configuration with0.046e and0.025e respectively. CO2could gain electrons from the surface of La0.875Ca0.125FeO3(010) with pre-adsorbed H2O. The gas response S decreases with increasing the moisture can be elplained as follows:In dry air, there are adsorbed oxygen species on the La0.875Ca0.125FeO3surface. The resistance of the material will increase after exposed to CO2gas. With the presence of H2O, a part of adsorption sites on the surface are occupied, so that the site for CO2to adsorbed on the surface reduced. Besides, the reaction of CO2and pre-adsorbed H2O can capture electrons from the La0.875Ca0.12sFeO3(010) surface. Then the rise of the material resistance is reduced, in other words, the response to CO2is reduced.3. The acetone-sensing properties of the pure and Pd doped perovskite-type oxides LaFeO3were investigated from100℃to340℃. X-ray diffraction (XRD) shows that LaFeO3is an orthorhombic structure. The obtained D values of LaFeO3powders doped with0wt%,1wt%,2wt%,3wt%and5wt%Pd were about50.1,58.6,50.9,56.5and59.5nm, respectively. A giant acetone-sensing response of729is observed when the Pd content in LaFeO3powders is about2wt%in500ppm acetone. An obvious response is also observed for1ppm acetone of the2wt%Pd doped LaFeO3sensor. The response and recovery time of the sensor to the1ppm acetone gas are4and2seconds, respectively. At the same time, it performs a good selectivity to acetone gas and may be a new promising material candidate for the acetone-sensor development.4. The Biomorphic La1-xSrxFeO3(x=0～0.3) hollow fibers with porous walls were fabricated using cotton as biotemplates. XRD patterns indicated that all the materials exhibit perovskite phase with orthorhombic structure. The morphologies of the samples were observed by scanning electron microscopy (SEM). The pore diameter of the tube is basically in the range of2-5μm. The walls of La1-xSrxFeO3(x=0～0.3) tubes are made up of lots of nanocrystalline particles. The average particle sizes of La1-xSrxFeO3(x=0～0.3) are about53nm,66nm,63nm and65nm respectively. Appropriate Sr-doping can restrain improve the response of the LaFeO3based sensor to ethanol gas. The best response to ethanol gas was observed with Sr content equal to0.1mole ratio at the operating temperature260℃. The gas response is about52.8to400ppm ethanol gas. Even though the walls are uneven, bumped and porous, the thicker wall is harmful to the interaction between gas and perovskite molecules. The decreasing of wall thickness is beneficial to improve the gas sensitivity.5. Our DFT calculations show that before the introduction of CO, the oxygen species O2-and O-adsorbed on the oxygen-deficient SnO2(110) surface grab electrons mainly from Sn atoms of SnO2. When CO is introduced, For the case of O2, the CO molecule can react with the oxygen atom of pre-adsorbed O2, which is bonded with one Sn atom, to form CO2. There is another possibility that CO molecule may be adsorbed on the oxygen atom of pre-adsorbed O-2, which is inserted into the initial oxygen vacancy and is bonded with two Sn atoms, to form an intermediate state which may finally transform into CO2. At low O2concentration with fewO2-, CO reacts with the lattice oxygen atom to form CO2, when CO is closed to the lattice oxygen atom, since bond strength of Sn-O for this lattice oxygen atom becomes weak, after the oxygen adsorption. There is another possibility that CO is adsorbed on Sn position. For the case of O-, when CO is introduced, the CO molecule can react with pre-adsorbed O-to form CO2. CO may also react with the bridging oxygen atom of lattice neighbored with the pre-adsorbed O-to form CO2. When SnO2(110) surface is exposed to CO reducing gas, the interactions between CO and pre-adsorbed oxygen species (O-2,0-) as well as some lattice atoms at certain sites on SnO2surface leads to the releasing of electrons back to semiconductor SnO2-The DFT calculation can provide a good description for CO sensing processes on SnO2(110) surface. |
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