Adsorption and Reaction at Ferroelectric Surfaces: Chemical Switches and Switchable Chemistry

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In this research supported by the Analytical and Surface Chemistry Program, we will investigate the conditions under which ferroelectric materials in thin film form can be used as chemical sensors as well as more broadly elucidate the fundamental factors that govern chemistry at the surfaces of these materials. Ferroelectrics are materials that develop macroscopic electric fields that can be switched by applying an external field in analogy to ferromagnets. The electric fields create high surface energies that drive surface restructuring and adsorption of polar molecules; in recent work we have shown that the adsorption is sensitive to the direction of the ferroelectric poling. In this project, we seek to exploit this finding to instead switch the polarization of a thin ferroelectric film where the differences in adsorption energies are sufficient to drive a change in the polarization direction. Because the poling direction can be readily detected, it is anticipated that this effect can be exploited to form chemical sensors. In addition, we seek to further understand the roles of electrostatic, chemical, and structural effects in determining the magnitude of the effect of ferroelectric poling on surface chemistry. Based on the results, strategies will be employed to enhance the effect to not only influence adsorption but to also enable control of catalytic activity and selectivity by switching the ferroelectric polarization. These strategies include deposition of co-catalysts to impart complementary functionality to that found on at least one of the polar surfaces; and growth of reactive epitaxial oxide layers where structural differences on oppositely poled surfaces, would yield one side of the crystal unreactive.The project will have a broad impact through its contributions to education and training and emerging areas of science and technology. The students working on this project will get a unique opportunity to develop new strategies for chemical sensing and reversibly controlling surface chemistry. To meet these objectives the students will need to develop expertise in a number of fields including surface science, materials science, chemical kinetics and dynamics, and spectroscopy. Because of the large polarizations that develop in ferroelectrics, the results promise to broadly impact chemical sensing by making new types of chemical devices such as chemical switches where adsorption on the ferroelectric is sufficient to turn on a field effect transistor without requiring any gate voltage or even a gate electrode. Laying the groundwork for creating catalysts whose activity or selectivity can be altered by applying an electric field will impact catalysis by providing a new reversible lever to control reactions, which will particularly impact the emerging area of microreactors.