Orientation Dependence of Photochemical Reduction Reactions on SrTiO 3 Surfaces
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Orientation Dependence of Photochemical Reduction Reactions on SrTiO3 Surfaces Jennifer L. Giocondi and Gregory S. Rohrer Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, U.S.A. ABSTRACT Polished and annealed surfaces of randomly oriented crystallites were used to study the orientation dependence of the photochemical activity of SrTiO3 surfaces. Silver cations reduced from an aqueous solution produce solid silver metal at the reaction site. The amounts of silver produced by a fixed exposure were used as a relative measure of each grain’s activity. The surface structure of the grains was observed using atomic force microscopy and the surface orientation of each grain was determined by electron backscattered diffraction. Surfaces annealed in air for 6h at 1200° C were bound by some combination of the following three planes: {110}, {111}, and a complex facet inclined approximately 24° from {100}. By correlating the orientations of individual grains to the amount of deposited silver, we conclude that surfaces with the complex {100} facet are the most active. 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. The photochemical properties of transition metal oxides are important because of their relationship to water photolysis [1], super hydrophilicity [2], and contaminant degradation in air and water purification systems [3]. By identifying the most reactive surfaces of transition metal oxide photocatalysts, we hope to develop structure-property relationships that can be used to identify improved materials for these applications. In this paper, the influence of surface orientation on the photochemical reduction of aqueous silver cations on individual SrTiO3 crystallites is described. This "probe" reaction leaves behind a solid silver product at the site of reduction that can be observed with standard microscopic techniques [4,5]. The amounts of silver observed on surfaces of different orientations are taken as a measure of relative reactivity. This method was recently used to study the anisotropy of the photochemical reduction of silver on rutile surfaces and showed that surfaces with a {101} orientation were more reactive than surfaces with either {100} or {110} orientations [6,7]. The photochemical deposition of silver has also been used to describe the photochemical reactivity of BaTiO3 [8-10]. In this case, any crystalline anisotropy is overwhelmed by the effect of dipolar fields associated with ferroelectric domains. EXPERIMENTAL DETAILS Polycrystalline SrTiO3 ceramics were made by first uniaxially compacting commercially available powders (Alderich, 99.5%) under 150 MPa to form disk-shaped pellets with a thickness of 3 mm and an approximate diameter of 11 mm. The pellets were then placed in an alumina crucible with an excess of the
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