Orientation Dependence of the Photochemical Reactivity of BaTi 4 O 9
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Orientation Dependence of the Photochemical Reactivity of BaTi4O9 Jennifer L. Giocondi, Shahrzad Samadzadeh, and Gregory S. Rohrer Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, U.S.A. ABSTRACT BaTi4O9 is a photocatalyst with a pentagonal prism tunnel structure. It has been hypothesized that the tunnels promote the separation of photogenerated carriers and, therefore, lead to the spatial separation of oxidation and reduction half reactions. This hypothesis has been tested by observing the distribution of reduced and oxidized reaction products on BaTi4O9 surfaces over a wide range of orientations. The surface orientations were determined by electron backscattered diffraction and atomic force microscopy was used to examine the structure of the surface both before and after the deposition of reaction products. Reduction products (Ag0) are distributed uniformly. The distribution of oxidation products (PbO2) is also not correlated to the surface orientation or to the orientation of the tunnels with respect to the surface. Based on these observations, we conclude that the tunnels in this structure do not separate photogenerated charge carriers and that this mechanism is not responsible for this compound's relatively high photocatalytic activity. INTRODUCTION Heterogeneous photochemical reactions can occur on ceramic surfaces when the adsorption of light with an energy greater than the band gap creates electrons and holes that migrate to the solid surface where they can participate in reactions with adsorbed species. One goal of photochemistry research during the past three decades has been to promote the efficient photolysis of water using particulate oxide catalysts [1]. The efficient photoassisted decomposition of water into hydrogen and oxygen gas is a promising means of converting solar energy into clean burning fuel (H2 gas). The recombination of photogenerated carriers and the back reaction of H2 and O2 to reform water are frequently cited as factors limiting the efficiency of photolysis using particulate materials. For this reason, recent work has been directed toward the development of catalyst structures that separate the charge carriers and the H2 and O2 production sites [2]. Oxides with highly anisotropic crystal structures have attracted attention in the search for efficient water photolysis catalysts. The best (most efficient) materials have been found to have layered [3,4] or tunnel [5-9] structures and previous workers speculated that this was the key to their high efficiencies [2, 4, 9]. They hypothesize that the reduction and oxidation functions of the catalyst occur at different elements of the structure and, by producing the reduced and oxidized products at separate locations, the rate of the back reaction is suppressed and the overall H2 production efficiency increased. Our objective was to test this hypothesis by measuring the orientation dependence of reduction and oxidation reactions on BaTi4O9, a material with a pentagonal prism tunn
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