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.
In analogy to ferromagnetic materials, ferroelectric materials develop macroscopic electric fields that can be switched by applying an external field. The field, or spontaneous polarization, arises from alignment of electric dipoles in the solid which also dictates that oppositely poled surfaces must be structurally, electronically, or chemically distinct. This suggests oppositely poled surfaces should behave chemically different, opening the door to switchable surface chemistry, including catalysts that may be turned on and off by applying an electric field. We previously demonstrated that the binding strength of polar molecules to ferroelectric surfaces in fact depends on the polarization direction. Unfortunately, we also found that the common ferroelectric materials are fairly inert. Therefore, we have investigated the possibility of inducing switchable chemistry in materials that are in intimate contact with the ferroelectric. This included a study of the influence of ferroelectric polarization on the catalytic metal Pd. While we found that small Pd clusters on the ferroelectric surface behave differently than bulk Pd, these differences did not depend on the polarization direction and thus the Pd reactivity was not switchable. We found that the reason for this lack of switchability was due to a very weak interaction between Pd and the ferroelectric which caused the Pd to cluster into 3D particles whose surfaces were too far from the ferroelectric substrate to be affected by the polarization. We therefore turned our attention to materials that could be grown atomic-layer by atomic-layer onto ferroelectric supports. We showed that we could grow the catalytic oxide Cr2O3 on top of ferroelectric LiNbO3 in such a manner. Further, we found that the surface charges required to stabilize the ferroelectric material migrated to the Cr2O3 surface, giving promise that the materials chemical properties would respond to changes in the ferroelectric polarization of the substrate. Finally, we developed a new method to study the stabilizing surface charges on ferroelectric surfaces. Using this method we showed that the surface charge state of these materials is sensitive to the environment and its history and that the charges seldom reach thermodynamic equilibrium. As chemical interactions of these materials with the environment are largely driven by electrostatics, this finding indicates that the reactivity of ferroelectric surfaces depends sensitively on the sample's history and how it was prepared.
