| The development of industry has brought much more treasures to people. However, more and more poisonous gases produced in the development process are not only non-friendly to the environment but also dangerous to people. Therefore, the real-time detection of harmful gases in the environment has attracted more and more concern in the scientific community. Gas sensor (commonly known as the "electronic nose") is an important branch of the sensing field, which can change the gas composition into a kind of measurement information, such as resistance, potentiometer, capacitor, or frequency changes by physical or chemical effects, in order to realize the detection, monitoring, and analysis functions. They have been applied to a wide range of environmental monitoring, civil, and industrial control, which has become a research focus at home and abroad.In this dissertation, we selected ZnO as the research subject and used electrospinning technique to fabricate a series of ZnO nanofibers with different morphologies. We made the breakthrough in preparing the hollow fibers by single capillary electrospinning, first prepared the SnO2-ZnO nanofibers and explored the formation mechanism. On the other hand, additives wre applied to improve the sensitivity, selectivity and reduce the operating temperature of the gas sensors effectively. A series of sensors with practical value were performed. The results of the research are to provide technical support and effective implementation of the program for the further industrialization. The specific findings are as following.First, we fabricated solid and hollow nanofibers through single capillary electrospinning technique.(1) Solid nanofibers:Two favorable morphologies of Zn(AC)2/PVP composite fibers were synthesized through electrospinning under the optimization conditions, using PVP, Zn(AC)2 as raw materials. The influence of the spinning parameters on fiber morphology were discussed in detail. After sintering, the composite fibers changed into zinc oxide long fibers and rod-like short fibers respectively. From PVP, Zn(AC)2 and PdCl2, Pd-ZnO nanofibers were also fabricated through electrospinning.(2) Hollow nanofibers:ZnO hollow nanofibers were fabricated through a facile single capillary electrospinning and following sintering. DMF and distilled water were used as solvents to dissolve PVP and Zn(AC)2, while ethanol was served as both solvent and the promoter of phase separation. On the basis of the above experiments, SnO2-ZnO hollow nanofibers were synthesized by single capillary electrospinning for the first time, employing ethyl acetate as the promoter of phase separation. This method is easy to operate and can avoid many shortcomings of the co-axial electrosipinning, such as complexity of the device, selection difficulties of inner solvent and tedious process of the post treatment. This approach is conducive to improve the yield of the hollow fibers, and lay the foundation for the further industrialization.Second, we studied the mechanism and affecting factors of the single capillary electrospinning preparation for hollow fiber:The samples have been observed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The results show that the composite fibers are solid before sintering and the hollow structure is formed in the post-sintering process. Based on the summary of the experimental phenomena and the induction of experimental data, we deduce that the choice of solvent and the polymer concentration have a direct impact on the formation of hollow fibers. The solvent should be able to effectively dissolved inorganic salts and polymers to form uniform and stability spinning solution firstly. On the other hand, there should be existed the promote factor to migrate the metal ions to the fiber surface, laying the foundation for the hollow structure. In addition, the concentration of the polymer also need to control strictly. If the concentration is too large, the network structure of the polymer will impede the migration of metal ions, if the concentration is too small, the polymer can not bound the metal ion effectively and the hollow structure will be easy to collapse in the post-sintering process. The relative researches reveal the theoretical basis of the single capillary electrospinning preparation for hollow fibers.Third, we have prepared four kinds of gas sensors with prominent performance.(1) Zinc oxide long nanofibers possess the outstanding ethanol sensing properties at 310℃ with good selectivity and stability. When the ethanol concentration is 10,50,100,150 and 200ppm, the sensitivity is 9,17,32,39 and 48 respectively. The Response and recovery times are 8-11s and 12-15s. This kind of material is a good candidate for ethanol detection in practice.(2) ZnO hollow nanofibers show improved response to acetone at 220’C with good selectivity and stability. When the acetone concentration is lppm, lOppm,20ppm,30ppm,50ppm, 70ppm, 100ppm,150ppm and 200ppm, the sensitivity is 7.1,19.2,25.6,32.3,41.3,49.9,67.7, 79.2 and 87.9. The response and recovery times are 11-17s and 7-15s, respectively. The sensing properties of ZnO hollow nanofibers are better than the solid fibers and those reported in the literature. The sensor is suitable to detect low concentration acetone in the lower operating temperature.(3) SnO2-ZnO hollow nanofibers are all more sensitive to toluene than the pure one because of the more active sites and the n-n heterojunction on the fibers surface. In all the samples, 50at.%SnO2-ZnO nanofibers shows the best sensing properties to toluene at 190℃with good selectivity and stability. When the toluene concentration is 1,5,10,50,100 and 300ppm, the sensitivity is 2.1,4.1,7.3,15.6,26.5 and 50.2, respectively. The response and recovery times are 6-11s and 12-23s, respectively.(4) Compared with pure ZnO nanofiber sensor, the Pd-doped ZnO nanofiber sensor exhibits improved sensing properties to CO at 220℃ through the electronic sensitization of Pd dopping. When CO concentration is 1,5,10,50,100 and 300ppm, the sensitivity is 2.1,4.1,7.3,15.6,26.5 and 50.2, respectively. The response and recovery times are in the range of 25-29 s and 12-17 s, respectively. Pd-doped ZnO nanofibers have opened up the possibility to detect low concentration CO in the lower operating temperature.Fourth, we have analyzed the influence of material microstructure on gas-sensing properties.(1) One dimension (1D) nanomaterials can maintain the shape and size even after high temperature calcinations. The formation of more gas channel and the increasing of effective surface area of 1D nanomaterials are the main reasons for the improved sensing properties superior to the zero-dimensional powder. While the prominent sensing properties of the nanofibers are attribute to the larger length-diameter ratio.(2) Hollow nanofibers have more surface area (inner as well as outer), which will increase the number of active adsorption centers and make the molecules adsorb onto the surfaces of nanofibers easily. Additionally, hollow nanofibers have open channels and rough porous surface, which make the inner surface accessible and provide more gas channels. The electronic transport along the fibers is also improved. In addition, hollow fibers also have many virtues of 1D nanostructure, such as the stable structure and the perfect crystal surface. All these properties lead to the higher response at lower operating temperature and the good stability.In summary, carrying out researches on the preparation methods and surface modification of 1D nano-materials, especially the hollow 1D nanofibers, is effective method to improve the sensing properties of the materials for particular applications. Electrospinning technique can provide a strong platform for designing the ideal materials, which can effectively control the structure of nanofibers and be easily doped. The relationships between the structure and properties of nanofibers have enhanced the nanofibers application value in the sensor field. On the other hand, the relationships also provide experimental and theoretical basis for improving the sensing properties on designing the materials. |
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