Photochemical Reactivity of Sr 2 Nb 2 O 7 and Sr 2 Ta 2 O 7 as a Function of Surface Orientation

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Photochemical Reactivity of Sr2Nb2O7 and Sr2Ta2O7 as a Function of Surface Orientation Jennifer L. Giocondi, Ariana M. Zimbouski, and Gregory S. Rohrer Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, U.S.A. ABSTRACT Sr2Nb2O7 and Sr2Ta2O7 have a (110) layered perovskite structure and are efficient photolysis catalysts. Aqueous silver and lead cations were photochemically reduced and oxidized, respectively, on the surfaces of Sr2Nb2O7 and Sr2Ta2O7 crystals with a wide range of orientations. Atomic force microscopy has been used to observe the distribution of photochemically reduced and oxidized products and determine the orientation dependence of the reactivity. On surfaces with the same orientation, reaction products frequently had a non-uniform distribution. The reactivity of both compounds proved to be only weakly anisotropic, with the highest relative reactivity for both oxidation and reduction occurring for surfaces oriented between (010), (110), and (011). These low index orientations have structures similar to the ideal {110} and {100} planes in the perovskite structure, respectively. The relationship of the perovskite structure to the reactivity is discussed. INTRODUCTION When illuminated with ultraviolet light, some ceramic oxides can dissociate water to form H2 and O2. In principle, it is therefore possible to directly convert solar energy to a clean burning, replenishable fuel. Systems based on particulate catalysts have attracted attention because of their potential to produce H2 in relatively simple and inexpensive reactors. Such systems typically have low efficiencies because many of the photogenerated carriers recombine before reacting at the surface and much of the photochemically generated H2 and O2 undergoes a reverse reaction to form water before it can be separated. Therefore, significant amounts of water can be dissociated only if carrier recombination and the back reaction can be suppressed. 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 [1]. Recently, a number of ternary transition metal oxide catalysts have been discovered to dissociate water more efficiently than conventional materials such as titania. These new materials all have anisotropic structures made up of layers [2,3] or tunnels [4,5]. It has been hypothesized that these materials have high efficiencies because the layers and tunnels somehow separate the photogenerated charge carriers and, therefore, the locations of the oxidation and reduction half reactions. If such a separation were to occur, it would suppress carrier recombination and the rate of the back reaction. Two such materials are Sr2Ta2O7 (Cmcm, a = 3.937 Å, b = 27.198 Å, c = 5.692 Å) [6] and Sr2Nb2O7 (Cmc21, a = 3.933 Å, b = 26.726 Å, c = 5.683 Å) [7]. Both of the structures are made up of corner sharing MO6 octahedra that are arranged in (110) perovskite type slabs parallel to (010), and separ

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