(104b) Ferroelectric Oxide Surfaces: Switchable Chemistry and Chemical Switches

It has long been recognized that the polar and switchable nature of ferroelectric surfaces can potentially lead to polarization direction-dependent surface chemistry that may be exploited to create switchable catalysts and chemical sensors. Therefore, we have been studying the polarization dependence of the structure and chemistry of ferroelectric oxides. Despite the expectation that the bare polar surfaces that result from ferroelectric polarization would reconstruct, we only observed (1x1) surface diffraction patterns for both positively and negatively poled LiNbO₃ (0001) surfaces. Ion scattering and photoelectron spectroscopies indicated that the surfaces were predominantly oxygen-terminated, also independent of the polarization direction. Despite the structural and spectroscopic similarities between the positively and negatively poled LiNbO₃ surfaces, it was found that the polar molecules acetic acid and 1-propanol adsorbed more strongly on the positively poled surface, while non-polar dodecane was insensitive to the polarization direction. Although the differences in adsorption energies were not large, 11 kJ/mol for 2-propanol, they were still comparable to the energy barrier required to switch the polarization of ~10 nm thick films suggesting that chemical switching of ferroelectric thin films is possible. Results for chemical switching of lead zirconate titanate thin films in response to changes in the oxidizing/reducing nature of the environment will be discussed. In an effort to enhance the sensitivity of the surface chemistry to the polarization direction, we explored the impact of ferroelectric polarization on the properties of supported catalytic metals and oxides. For Pd on LiNbO₃ it will be shown that the Pd tends to cluster into particles on the LiNbO₃ surfaces and that CO adsorption on these Pd particles was similar to CO adsorption on Pd on inert supports and was independent of the polarization direction. It was concluded that the Pd clusters were too thick for their surfaces to be influenced by the polarization of the underlying ferroelectric. Alternate approaches to increasing the reactivity of ferroelectric surfaces will be presented including controlling the surface termination of titanate ferroelectrics and epitaxial growth of reactive oxides.