Photolysis by Oxides with Internal Dipolar Fields

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Certain transition metal oxide ceramics can catalyze water photolysis and produce hydrogen from water and sunlight. Particulate systems that catalyze this process remain impractical because of their low energy conversion efficiency, which is limited by carrier recombination and the back reaction of reduced and oxidized intermediates before they can form more stable molecular species. The basic idea of this research is use the bulk photovoltaic effect to build an internal field into every crystallite in a particulate catalyst, so that the photogenerated charge carriers will be separated and the reduced and oxidized products formed at different locations. An internal field can be incorporated within the individual crystallites by using a ferroelectric material. Thus, experiments will be conducted to test the hypothesis that internal dipolar fields can be used to increase the quantum efficiency of water photolysis in particulate systems. Specifically, the photolysis efficiency of catalysts in the ferroelectric and paraelectric state will be compared. The influence of dipolar fields on the reactivity of thin catalytic films on ferroelectrics, such as TiO2 on BaTiO3, will also be explored. A composite will allow the charge separating and catalytic functions to be optimized separately. The efficiency will probably be affected by the size of the ferroelectric particle and by the thickness of the film. These length scale phenomena will be explored independently by comparing the efficiencies of catalysts with different particle sizes and by testing the reactivity of films of different thicknesses in a planar geometry. As a fuel for the future, hydrogen is attractive because it has three times the energy density of oil and its combustion does not create dangerous emissions, greenhouse gases, or radioactive byproducts. It is currently possible to produce this sustainable, clean burning fuel from sunlight and water. However, the potential societal benefits of this energy source will not be realized until the cost become competitive with established fuel sources (fossil and nuclear). Therefore, the overarching goal of this research is to develop new materials that will make photolytic hydrogen economically feasible.