Preparation And Gas-sensing Properties Of Self-standing ZnO Film With Hierarchical Structures

ZnO is a chemically and thermally stable semiconductor with direct bandgap energy (3.37 eV at room temperature) and a strong exciton binding energy (60 meV). Owing to those excellent properties, it has been widely utilized in piezoelectric transducers, varistors, solar cells, and so forth. Besides the above potential application fields, its competence as gas sensor has undoubtedly been proved for detection of various oxidizing and reducing gases. Especially, the ZnO gas sensing materials with hierarchical structures have attracted wide research interests. In this thesis, the self-standing zinc hydroxide sulfate (ZHS) films are prepared and thermally treated to obtain self-standing ZnO films with porous and layered structures. On the basis of the self-standing ZnO films, a novel"surface mount technology"has been developed to fabricate the gas-sensing device and the gas-sensing properties are investigated in detail. The main conclusions are summarized as follows:Zinc hydroxide sulfate (ZHS) large ?akes with high diameter-thickness ratio were synthesized by hydrothermal treatment of ZnSO4·7H2O solution at 150°C. Then the large ZHS ?akes were used as building blocks to assemble the self-standing ZHS films. Then the hierarchical porous ZnO thick-film was attained by further heat treatment of the ZHS films. According to XRD,SEM,BET and TG-DTA analysis of the samples at different periods, main conclusions are summarized as follows: Basicity has great effect on the morphology of ZHS flakes. By the addition of NaOH to 0.02 M, the hexagonal flexible flakes with lateral size up to 400μm and 2μm thickness can be obtained. If the pH continue to rise, the diameter-thickness ratio of ZHS flakes will decrease; During the simple vacuum filtering, the ZHS flakes can be self-assembled into ZHS thick-films with high (001) orientation; Along with the rise of treatment temperature to the ZHS thick-films, the surface of the ZnO films emerge from upwarped structure at 600°C to the porous structure at 700°C. Finally at 800°C, the particles began to coarsen. The temperature and heating rate all have a great effect on the porous structure, specific surface area and crystallinity.The hierarchical porous ZnO thick-film are made into gas sensors by using a novel"surface mount technology"and investigate gas-sensing properties such as the optimum working temperature of different gases, selectivity, response and recovery times. The results indicate that the ZnO thick-films proved favorable selectivity to H2S. In addition, both the surface porous structure and crystallinity had influence on the gas-sensing property and the influences are not the same for ethanol, acetone and hydrogen. Furthermore, the gas sensors based on the self-standing ZnO films show significant reproducibility and stability, which indicates that these porous and self-standing ZnO films could promisingly be applied in electronic devices for environmental evaluation.