Defect Engineering Of The Zinc Oxide And Its Gas Sensing Studies

ZnO nanomaterials have been widely used in real-time monitoring of the toxic gases because of their unique optical-electrical characteristics, and they have been the hot topic of gas sensing studies. As a typical surface-controlled sensing material, the crystal facet exposing and the surface defect distribution showed significant influence on its sensing properties. Up till now, there are many researches discussing about the relation between the surface defect and the gas sensing properties. However, the studies focus on improving sensing properties by surface micro-structures engineering are rarely reported. In this work, ZnO nanomaterials with different crystal facet exposing have been prepared to stuy the crystal facet effect on their sensing performances and sensing mechanisms. In addition, various modification methods have been adopted to regulate the surface defect of the ZnO products to get better sensing properties. And the relevance between surface structure and sensing property has been built successfully. The specific contents are summarized as follows:1. ZnO nanopyramids (NPys) with exposed crystal facet of {1011} were synthesized via a one-step solvothermal method using oleylamine as the surfactant. Controllable defect redistribution of the{1011} facet was completed via the calcinating heat treatments, and for the overall goals of morphology maintaining and defect engineering, the effect of aging temperature on the gas sensing properties of the ZnO NPys was studied. Characterizations of PL and XPS were used to analyse the surface structures of the products. Moreover, model of defect redistribution on the{1011} facet was built to illustrate the relation between the surface micro-structures and the sensing properties. The results showed that the ZnO NPys come to a huge shape change and crystallinity decreasing when they were calcinated at high temperatures, which leads to the lower gas response. While at the aging temperature of 300℃, due to the thermal activation, the defect of the{1011} crystal facet redistributed to produce much electron donors such as Zni, which help the material to absorb more oxygens to get a better sensing property. In addition, based on the different interactions between the surface structures and the modified atmospheres, the ZnO NPys were calcinated in different atmospheres to further regulate their surface structures. The results indicated that after calcinated in the atmosphere of oxygen, the gas resonse of the ZnO NPys has been improved three times, which achieved the purpose of sensing performance improving successfully. All of the results above will be noticeable for the improvement of the sensing properties of the ZnO nanomaterials with special crystal facet exposing.2. ZnO nanomaterials with different crystal facet exposing were synthesized via the solvothermal and hydrothermal methods. The dominant exposed crystal facet of the ZnO nanopyramids (NPys), nanoflakes (NFs) and nanorods (NRs) were {1011},{0001} and {1010}, respectively. The sensing tests indicated that all of the three nanomaterials exhibited a prominent selsctivity to ethanol, and the ZnO NPys showed the highest sensitivity, while the response of the ZnO NFs was lower and that of the ZnO NRs was lowest. The results of PL and XPS characterizations showed that the surface defect distribution of the three crystal facet were absolutely different. The order of the content of the surface electron donor and the absorbed oxygen is NPys> NFs>NRs, which is in accordance with the gas sensing results. Moreover, temperature programmed technologies were used to detect the adsorbed-desorbed species of the ethanol molecule on different ZnO crystal facets to get a deeper unstanding of the relation between the surface component and the sensing mechanism. The results indicated that due to the different redox characteristics of the three exposed crystal facets, the ethanol absorbed on the surface generated different intermediates, which leads to some different sensing mechanisms. The results built the real sensing mechanisms of the ZnO nanomaterials at their own working temperature, which provides a well theoretical basis for the performance oriented material structure optimization.