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

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

0072151 Rohrer 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. 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.