The Preparation Of Three-dimensional Monodisperse Hollow And Porous ZnO??-Fe₂O₃ Microstructures And The Study Of Their Gas-sensing Properties

With the rapid development of industrialization and the sharp increase of population in the modern society, a variety of explosive, toxic gases and the mixtures gases of them are ubiquitous in mining industry, departments of scientific research and families, and the ensuing explosion accident, air pollution and fire accident become a major threat to human life and property safety. So it becomes a problem need to solve urgently for monitoring these gases and alarm timely to prevent the disasters. Currently, there are many ways to detect these gases, such as electrochemical method, optical method and chromatography. However, the disadvantage of these methods is the cost is too high and the measurement device is complicated, so it cannot be widely extended. The gas sensor has been to one of the most important means for detecting and monitoring these explosive and toxic gases in industry and daily life due to simple preparation, high sensitivity, good stability and easy to carry, which are based on metal oxide semiconductor material. Present research shows that the hierarchical, porous and hollow structures are the ideal structures of building high sensitivity and fast response of gas-sensing materials because of its large specific surface area, high porosity and low density. This paper aim at this research direction, the monodisperse hollow ZnO and porous ?-Fe₂O₃ nanostructures are synthesized by a simple hydrothermal method and followed by annealing. The samples were modified by Cu element and surfactant, and the corresponding gas sensors were also prepared using the synthetic samples. The main work is as following:1. Monodisperse ZnO hollow six-sided pyramids were successfully synthesized by a low-cost and simple hydrothermal method at 160? for 20 h and subsequent by annealing process. The sodium citrate?Na3C6H5O7·2H2O? and glycerol?C3H8O3? played a pivotal role in the growth of hollow six-sided pyramid, and the possible growth mechanism was also proposed based on the experimental facts. Moreover, the gas sensor based on this structure displayed excellent gas sensing properties to ethanol at the optimum operating temperature of 240?. The response was about 187 and the response-recovery time was around 11 and 9 s for 200 ppm ethanol, respectively, which indicated that this structure had high sensitivity, fast response-recovery time and good selectivity to ethanol.2. Hamburger-like alpha-iron oxide??-Fe₂O₃? microparticles were fabricated by a simple and low-cost hydrothermal route at 140? for 16 h and subsequent by annealing process. The microparticles showed monodisperse porous structures with the length and the width about 2.7 and 2.2 ?m, respectively. We prepared the corresponding sensors using this unique structure. Several gases of 500 ppm were tested at the optimal working temperature of 270?, and it can be found that the response value of acetone was about 236, which was much higher than other gases. Moreover, the response-recovery time is also around 10 s. It indicated that this structure had high sensitivity, fast response-recovery time and good selectivity to acetone. Thus, the peculiar structure of the sample endows acetone detection with widely application prospects.3. The monodisperse porous cube, cake and spheroid-like ?-Fe₂O₃ microparticles were successfully synthesized via a facile hydrothermal approach. The as-synthesized ?-Fe₂O₃ microparticles were characterized by some technologies, and the results showed that the diameters of all these microparticles were around 2-3 ?m. Meanwhile, the results of gas-sensing properties revealed that both the reaction time of hydrothermal and the amount of Cu doping could remarkably enhance the performances of gas sensors. In these sensors, the sensor of Cu doping amount is 1.0 wt % and hydrothermal time is 15 h with the best gas sensing performance, that is, the cake-like ?-Fe₂O₃ with the best gas sensing performance. And the response of it to 500 ppm acetone was 205.3 at 270?, which was about 11.9 times higher than that of ammonia?about 17.3?. In addition, both of the response and recovery time were within 10 s, demonstrating the sensor based on 1.0 wt% Cu-doped ?-Fe₂O₃ microcakes has a potential application for acetone detection at the reaction time of 15 h. Finally, the possible formation mechanism and gas-sensing mechanism of ?-Fe₂O₃ microstructures were further studied.