Study On Gas Sensing Properties And Mechanism Of SnO₂ /TiO₂ Systems

In the field of gas sensor research, the researchers will focus on exploring new type of sensing materials to enhance their sensitivity, selectivity and stability. On the other hand, the development of advanced manufacturing technology can reduce the costs of the sensor and ensure its reliability, safety and repeatability. Owing to lack of the gas sensing and simulation mechanism, the current exploring of new sensing material is mainly based on experimental studies via a trial-and-error design fashion. Thereby, how to select the suitable dopant effectively and understand how the dopant affect the gas sensing properties, are the two crucial issues in the field of sensing material research. The main purpose of our work is to establish the gas sensing mechanism model and provide the instruction to further explore the new type of gas sensing material, and explain the experimental results theoretically from first-principles.What we selected for the study of gas sensing material is SnO₂/TiO₂ system. SnO₂-TiO₂ composite oxide has attracted a wide range attention as the most important semiconductor functional material. So in the study of gas sensing mechanism, we can treat the SnO₂/TiO₂ as the typical n-type semiconductor oxide, and then establish their simulation mechanism. In our work, we firstly prepared TiO₂/Sn, SnO₂/Ti and Ti-Sn-O₂ nano powders and measured their sensing properties in different gas circumstance. We established the sensing mechanism of the anatase TiO₂ system, rutile SnO₂ system and Ti-Sn-O₂ solid solutions system based on the first-principles. The atomic structure of the surface and their gas adsorption properties are investigated by calculation. Then we discussed the essence of the gas sensing effect of SnO₂ and TiO₂. The experimental phenomenon was clarified through analysis of the atomic structure and electronic property of the oxide surface. Then, we demonstrated the feasibility of adsorption model in our work.The main results in this work are as follow:①The morphology and particle size of TiO₂ has no changes after doping Sn with value of 1%⁵%. The Sn-doped TiO₂ samples maintain anatase structure. The result of gas sensing measurement shows that the operating temperature of TiO₂ can decrease after doping Sn. Simultaneity, Sn dopant will also enhance the sensitivity of TiO₂ to detect the reduce gas such as ethanol methanol and improve the linear response of the sensor in different gas concentration. Referenced to the surface adsorption model, we proposal the gas sensing mechanism of anatase TiO₂. In the case of the anatase TiO₂ （101） face, adsorbed oxygen prefer to locate at Ti5C side, the adsorption energy is relativity low. The electronic structure of surface has no significant changes after oxygen adsorption, indicating weak interaction between adsorbed oxygen and surface of TiO₂. The defective （101） face is mainly formed by oxygen vacancy in the inner layer, while the oxygen vacancy in the subsurface affect the adsorption performance slightly. The adsorption site is mainly at Ti5c for case of defective （101） face. That means the anatase （101） face containing oxygen vacancy has the same gas adsorption mechanism as the perfect （101） face. The oxygen adsorption energy increased in the case of Sn doped TiO₂ surface. The electronic structure of Sn doped TiO₂ surface has obviously changed after oxygen adsorbed at Sn5C site. One can see that an acceptor likes locate at the top of VB. The DOS of adsorbed oxygen also changed significantly as compare to that of the free oxygen. The electronic peaks of adsorbed oxygen split and enlarge near the Fermi level, furthermore, charge transfer between the surface and adsorbed oxygen has also increased evidently. All this information can explain that the gas sensing properties of TiO₂ can improve after doping Sn. According to the experimental result, it is demonstrated that the adsorption model we established uses in explaining the anatase TiO₂ gas mechanism should be feasible.②The morphology and particle dispersive of SnO₂ has changed slightly after doping Ti with value of 1%⁵%. The Ti doped SnO₂ samples maintain rutile structure. Although the microstructure of the Ti doped SnO₂ has no significant changes, the sensitivity of SnO₂ will enhance evidently after doping Ti. Moreover, the rutile SnO₂ show better sensing properties than that of undoped and doped Sn in anatase TiO₂. Oxygen adsorption process mainly occurs at the defective （110） face of rutile SnO₂. The composition of defective （110） face is mainly contributed by the surface O_2C vacancy. The high adsorption energy and the more adsorption active spot reveal that the （110） face containing oxygen vacancy have excellent oxygen adsorption properties. Therefore, formation of the surface oxygen vacancy may be a key factor for the rutile SnO₂ show better sensing properties than that of anatase TiO₂. The calculation result of atomic structure and vacancy formation energy show that Ti doped SnO₂ surface can produce the O_2C vacancy easier as compare to that of pure SnO₂ surface. Furthermore, in the case of Ti doped SnO₂ surface, the oxygen adsorption energy will be enhanced and the charge transferred between adsorbed oxygen and the surface will be increased. That means the sensitivity of SnO₂ would be enhanced after doping Ti.③We investigated the gas-sensing and microstructure of SnO₂-TiO₂ composition with the value of Sn/Ti=1/1. The results show that TiO₂ and SnO₂ will mix in different way after different calcined temperature, leading to different sensing properties of the samples. In the case of the sample calcined at 650℃, the particles are mainly composed of Rutile SnO₂ and anatase TiO₂ grain connecting closely. Some of the Ti4+ will substitutes for Sn⁴⁺ or vice versa. The main reason for the gas sensing effect is the sample contained rutile SnO₂ as well as part of Ti ion substituted Sn ion. In the case of the sample calcined at 1050℃, Coupled SnO₂-TiO₂ materials will form a solid solutions. Although the samples keep the rutile structure, crystal lattice of Ti-Sn-O₂ solid solution has changed significantly owing to the same value of Sn and Ti. This effect also influences the sensing performance of the Ti-Sn-O₂ solid solution. The surface atomic structure of the solid solution show the same character as that of rutile SnO₂ （110） face. This character may be the main reason for the Ti-Sn-O₂ solid solution could maintain relativity good sensing properties. The change extent of the bond length of atom at solid solution surface increased as compare to that of at pure rutile SnO₂ surface. Moreover, the bond force connection of surface O_2C and 6 fold coordination atoms at solid solution surface are stronger than that of pure SnO₂ surface. All this character indicated that surface O_2C vacancy is more difficult to form at the solid solution than that at pure SnO₂ surface. In the case of perfect （110） face of solid solution, oxygen adsorption effect is very weak, oxygen adsorption mainly occur at the surface containing oxygen vacancy. All these effects are similar as the perfect （110） face of SnO₂. The surface vacancy will decrease at the surface of solid solution owing to Ti and Sn composed at a high ratio. The reduction of the amount of surface oxygen vacancy at solid solution may lead to the Ti_0.5Sn_0.5O₂ show worse sensing properties.In the current work, we established the gas sensing simulation mechanism of anatase TiO₂ system, rutile SnO₂ system and Ti-Sn-O₂ solid solution system. We also explained the experimental result theoretically during the gas sensing measurement. These models and mechanism proposed in our work may provide the instruction for further exploring the new type of semiconductor oxide sensing material.