Photochemical Reduction and Oxidation Reactions on Barium Titanate Surfaces
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Photochemical Reduction and Oxidation Reactions on Barium Titanate Surfaces
Jennifer L. Giocondi and Gregory S. Rohrer Department of Materials Science and Engineering Carnegie Mellon University Pittsburgh, Pennsylvania 15213-3890
ABSTRACT
The influence of the ferroelectric domain structure of BaTiO3 on the photochemical reactions that occur on its surface has been examined using atomic force microscopy. Both the photochemical reduction of aqueous silver cations and the oxidation of steric acid thin films were studied. During reduction, silver selectively deposits on the surface in patterns determined by the ferroelectric domain structure. Based on the analysis of domain polarization in single crystals, we find that the photochemical reduction reaction occurs preferentially on the positive ends of the dipoles. The most likely explanation for this phenomenon is that when the static dipolar field is oriented with the positive end of the dipole on the surface, photogenerated electrons are driven to the solid-liquid interface where they reduce metal cations. The oxidation of steric acid films, on the other hand, is not spatially selective. During oxidation, the films dissipate uniformly as they are converted to CO2 and H2O. In this case, we conclude that the oxidation occurs indirectly. Photogenerated holes create hydroxyl radicals which can migrate on the surface before reacting with the steric acid molecules.
INTRODUCTION
Heterogeneous photochemical reactions can occur on ceramic surfaces when the absorption of light with an energy greater than the band gap creates electrons and holes that, instead of recombining, become trapped on the surface and react with species in the surrounding liquid or vapor. Photochemical reactions can be used to produce ceramic surfaces with superhydrophilic [1] and self-cleaning properties [2]. The photochemically stimulated dissociation of water to produce H2 and O2 is of particular interest as a renewable energy source [3]. In this paper, the influence of the ferroelectric domain structure of BaTiO3 on the photochemical reduction of aqueous silver cations and the oxidation of steric acid thin films is described. Both of these reactions are known to occur on TiO2 surfaces and have already been well-documented [4-9]. Because these reactions lead to readily observable changes in the surface topography, they can be used as probes of the photochemical process. For example, the oxidation of steric acid thin films leads to dissipation of the film as it is converted to CO2 and H2O [7-9]. The reduction of silver, on the other hand, leaves an opaque, insoluble product on the surface. The associated change in optical properties can therefore be used to monitor the progress of the photochemical reaction [6]. The photochemical deposition of silver on TiO2 has AA7.4.1
also been used to evaluate the anisotropy of the reduction reaction. By determining the quantity of Ag deposited on surfaces with different orientations, it was concluded that rutile surfaces with the {101} orientation are more rea
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