Surface Structure-property Relationships for Ceramics with Unusually High Photochemical Activities

0072151 rohrer当用紫外光照射时，一些陶瓷氧化物能将水分解成氢和氧。因此，原则上，直接将太阳能转化为清洁燃烧、可补充的燃料是可能的。这一目标尚未实现，因为最好的光催化材料将光和水转化为氢和氧的效率太低，无法实现实际的大规模能量转换。因此，材料研究人员在这一领域面临的主要挑战是确定能够更有效地催化水解离的化合物。由于缺乏可用于识别潜在有用化合物的表面结构-性质关系，搜索工作受到阻碍。然而，在1999年6月，韩国一个研究小组发表的一篇论文表明，某些三元铌酸盐和钛酸盐光催化解离水的效率比传统材料高出一到两个数量级。这些观察为我们提供了一个独特的机会来发展表面结构-性质关系，可用于寻找具有较高光化学活性的氧化物。所有具有高光化学活性的材料（包括Sr2 Nb2 O7和La2 ti2o7与Ni促进）都可以可视化地由沿（110）平面切片的钙钛矿结构层构建而成。基于这一观察，我们假设存在这些相共有的特定结构成分，它们与高光化学活性有机械联系。此外，这些组分在某些表面上的存在将导致各向异性反应性。本研究项目的目的是检验这一假设。将测量Sr2 Nb2 O7、La2 Ti2 O7和Ba Ti4 O9光化学反应性的取向依赖关系，以确定最活跃的表面。镍促进剂的位置也将通过使用透射电子和原子力显微镜的直接显微检查来确定。为了证实这些表征实验得出的结构-性能关系适用于所研究环境中的颗粒材料，我们将测量具有不同习性和宽高比的Sr2 Nb2 O7颗粒的H2和O2的析出速率。这些样品将暴露特定切面的不同分数区域。如果这个假设是正确的，这些实验将使我们有可能识别出导致高光化学活性的表面和结构成分。有了功能结构成分的知识，将有可能选择候选材料，可以测试高光化学活性。此外，基于对大多数反应表面的了解，光化学活性陶瓷可以被织构成具有高反应性的微观结构。作为提议项目的一部分，将为材料缺陷课程开发一个新的本科实验室单元。该实验室的目标是利用原子力显微镜和反向散射电子衍射图，通过测量表面表面取向和几何形状来估计相对表面能。当用紫外光照射时，一些陶瓷氧化物可以将水分解成氢和氧。因此，原则上，直接将太阳能转化为清洁燃烧、可补充的燃料是可能的。这一目标尚未实现，因为最好的光催化材料将光和水转化为氢和氧的效率太低，无法实现实际的大规模能量转换。如果该项目成功，它将提供设计和生产材料所需的信息，这些材料将使清洁燃烧燃料的商业生产成为可能，从而对社会和环境产生巨大影响。

0072151RohrerWhen illuminated with ultraviolet light, some ceramic oxides can dissociate water to form hydrogen and oxygen. In principle, it is therefore possible to directly convert solar energy to a clean burning, replenishable fuel. This goal has not been realized because the efficiency with which the best photocatalytic materials convert light and water to hydrogen and oxygen is too low for practical, large-scale energy conversion. Therefore, the central challenge for a materials researcher in this area is to identify compounds that will catalyze the dissociation of water more efficiently. The search has been hindered by the absence of a surface structure-property relationship that could be used to identify potentially useful compounds. However, in June of 1999, a publication from a group in Korea demonstrated that certain ternary niobates and titanates photocatalytically dissociate water with an efficiency that is one to two orders of magnitude greater than conventional materials. These observations provide us with a unique opportunity to develop a surface structure-property relationship that can be used in the search of oxides with higher photochemical activities. All of the materials with high photochemical activity (including Sr2 Nb2 O7 and La2 Ti2 O7 promoted with Ni) can be visualized as being built from layers of the perovskite structure sliced along (110) planes. Based on this observation, it is hypothesized that there are specific structural components, common to these phases, that are mechanistically linked to the high photochemical activity. Furthermore, the presence of these components on certain surface planes will lead to anisotropic reactivity. It is the objective of this research project to test this hypothesis. The orientation dependence of the photochemical reactivity of Sr2 Nb2 O7, La2 Ti2 O7, and Ba Ti4 O9 will be measured to determine the most reactive surfaces. The location of the Ni promoter will also be determined by direct microscopic inspection using transmission electron and atomic force microscopy. To confirm that the structure-property relationship resulting from these characterization experiments applies to particulate materials in the environment of interest, H2 and O2 evolution rates will be measured from Sr2 Nb2 O7 particles with different habits and aspect ratios. These samples will expose different fractional areas of specific facets. If the hypothesis is correct, these experiments will make it possible to identify the surfaces and structural components responsible for the high photochemical activity. With knowledge of the functional structural components, it will be possible to select candidate materials that can be tested for high photochemical activity. Further, based on knowledge of the most reactive surface planes, photochemically active ceramics can be textured to have high reactivity microstructures. As part of the proposed project, a new undergraduate lab unit will be developed for a course on defects in materials. The lab's goal will be to estimate relative surface energies from measurements of surface facet orientation and geometry using atomic force microscopy and back scattered electron diffraction patterns. When illuminated with ultraviolet light, some ceramic oxides can dissociate water to form hydrogen and oxygen. In principle, it is therefore possible to directly convert solar energy to a clean burning, replenishable fuel. This goal has not been realized because the efficiency with which the best photocatalytic materials convert light and water to hydrogen and oxygen is too low for practical, large-scale energy conversion. If the project is successful, it will provide the information necessary to design and produce materials that will enable the commercial production of a clean burning fuel, thus having an immense impact on society and the environment.