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

在这项由分析和表面化学计划支持的研究中，我们将调查薄膜形式的铁电材料用作化学传感器的条件，并更广泛地阐明控制这些材料表面化学的基本因素。铁电体是一种产生宏观电场的材料，可以通过施加外部电场来切换，类似于铁磁体。电场产生了高表面能，驱动了极性分子的表面重构和吸附；在最近的工作中，我们证明了吸附对铁电极化的方向很敏感。在这个项目中，我们试图利用这一发现来切换铁电薄膜的极化，其中吸附能的差异足以驱动极化方向的改变。由于极化方向可以很容易地检测到，预计这种效应可以被利用来形成化学传感器。此外，我们试图进一步了解静电、化学和结构效应在决定铁电极化对表面化学影响的大小中所起的作用。在此基础上，我们将采用一些策略来增强这一效应，不仅可以影响吸附，还可以通过切换铁电极化来控制催化剂的活性和选择性。这些策略包括沉积共催化剂，以赋予至少一个极性表面上的功能互补；以及生长反应性外延氧化层，在相反极化表面上的结构差异将产生晶体的一面不反应。该项目将通过其对教育和培训以及新兴科学技术领域的贡献产生广泛的影响。从事这个项目的学生将获得一个独特的机会来开发新的化学传感和可逆控制表面化学的策略。为了达到这些目标，学生需要在许多领域发展专业知识，包括表面科学、材料科学、化学动力学和动力学以及光谱学。由于在铁电材料中发展的大极化，这一结果有望通过制造新型化学设备(如化学开关)来广泛影响化学传感，其中铁电材料上的吸附足以打开场效应管，而不需要任何栅极电压，甚至不需要栅极。为创造其活性或选择性可以通过施加电场来改变的催化剂奠定基础，将通过提供一种新的可逆杠杆来控制反应，从而影响催化，这将特别影响新兴的微反应器领域。

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.