Rethinking the Reactivity of Nanoscale Metal Oxides

在这个由化学系大分子、超分子和纳米化学项目资助的项目中，耶鲁大学的James Mayer教授正在对各种金属氧化物纳米晶体的化学反应进行新的基本理解。纳米晶体是包含数千个原子的微小颗粒。虽然它们比化学分子大得多，化学分子只包含几个原子，但它们仍然比固体材料小得多，固体材料大到足以用肉眼看到，含有数万亿个原子。纳米材料是独特的，因为它们的大小介于分子和固体之间，但有时它们的特性不容易通过平均两个极端尺寸来预测。纳米晶体的不寻常之处还在于，很大一部分原子位于表面或非常接近表面，因此，纳米晶体中的原子与大气或周围液体的重排、交换和添加要比与相同气体或液体接触的大块固体容易得多，后者的原子通常位于固体内部，无法接近。梅尔教授正在研究特定的金属氧化物纳米晶体，包括二氧化钛（白色油漆中的颜料，染料敏化太阳能电池的主要成分）、氧化锌（某些防晒霜中的紫外线保护剂）和氧化铈（汽车催化转换器的关键部件）。PI和他的团队正在进行实验，阐明了一种思考这些材料表面化学过程的新方法。由于金属氧化物是非常常见的材料，这些研究的结果将在许多领域产生广泛的影响。广泛而基本的方法为研究生提供了良好的培养环境。梅耶尔实验室还通过开放日和暑期实践项目，与初高中学生分享开发一种思考重要科学领域的新方式的兴奋之感。从催化到环境，半导体金属氧化物/溶液界面的氧化和还原反应是普遍存在的。在美国国家科学基金会（NSF）大分子、超分子和纳米化学（MSN）项目的支持下，耶鲁大学Mayer实验室正在开发一种新的氧化物表面化学过程范例。Mayer和同事们正在研究悬浮在有机溶剂中的电子、质子和氢原子向ZnO、TiO2、CeO2和其他材料的胶体纳米晶体的化学计量转移的动力学和热力学。他们的研究表明，目前对电子转移的关注需要扩大到包括平衡电子电荷的阳离子。因此，在许多情况下，反应化学最好被描述为内球过程，在这个过程中，电子转移与化学键的形成和断裂相结合。诸如氢原子转移和多电子过程等反应正在被研究。这种新方法的一部分是对界面反应热力学的重新思考，根据氧化物材料对氢的亲和力建立新的热化学尺度，而不是使用电子能带能。无处不在的界面氧化还原反应使得这项工作具有从能量转换到腐蚀的巨大潜在影响。广泛的方法是为多样化的研究生群体的科学成长提供一个具有挑战性和刺激性的跨学科环境。开发一种新的思维方式的兴奋是梅尔实验室向初高中学生推广的基础，包括开放参观和实践暑期课程。

In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professor James Mayer of Yale University is developing new fundamental understanding of chemical reactions of a variety of metal oxide nanocrystals. Nanocrystals are tiny particles that contain thousands of atoms. While they are much larger than chemical molecules, which contain only a few atoms, they are still much smaller than solid materials that are large enough to see with the naked eye, containing trillions of atoms. Nanoscale materials are unique because they have sizes intermediate between molecules and solids, but sometimes have properties not easily predicted by averaging the two size extremes. Nanocrystals are also unusual in that a large fraction of the atoms are at or very near the surface, so rearrangement, exchange and addition of atoms in the particle with the atmosphere or a surrounding liquid is much easier for a nanocrystal than for a bulk solid in contact with the same gas or liquid, where the atoms are typically inside the solid and not accessible. Professor Mayer is studying particular metal oxide nanocrystals, including titanium dioxide (the pigment in white paint and a primary component of dye-sensitized solar cells), zinc oxide (a UV protectant in some sunscreens), and cerium oxide (a key part of automobile catalytic converters). The PI and his team are performing experiments that elucidate a new way of thinking about chemical processes at the surfaces of these materials. The results from these studies should have broad impact in many fields, since metal oxides are extremely common materials. The broad and fundamental approach provides an excellent training environment for graduate students. The Mayer lab is also sharing with middle and high school students the excitement of developing a new way of thinking about an important area of science, through open houses and hands-on summer programs. Oxidation and reduction reactions at semiconducting metal oxide/solution interfaces are pervasive, from catalysis to the environment. Work in the Mayer laboratory at Yale University, supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program at NSF, is developing a new paradigm for such chemical processes at oxide surfaces. Mayer and co-workers are studying the kinetics and thermodynamics of stoichiometric transfers of electrons, protons, and hydrogen atoms to colloidal nanocrystals of ZnO, TiO2, CeO2 and other materials, suspended in organic solvents. Their studies show that current focus on transferring electrons needs to be broadened to include the cations that balance the electron charge. Thus the reaction chemistry is in many cases better described as inner-sphere processes, in which electron transfer is coupled to the making and breaking of chemical bonds. Reactions such as hydrogen atom transfers and multi-electron processes are being examined. Part of this new approach is a rethinking of the thermodynamics of interfacial reactions, building a new thermochemical scale for oxide materials based on their affinities for hydrogen, rather than using electronic band energies. The ubiquity of interfacial redox reactions makes this work of substantial potential broader impact, from energy conversions to corrosion. The broad approach is providing a challenging and stimulating interdisciplinary environment for the scientific growth of a diverse group of graduate students. The excitement of developing a new way of thinking is the basis of the Mayer lab's outreach to middle and high school students, both via open houses and hands-on summer programs.