Bimetallic Janus Nanocrystals and Their Derivatives

在~10纳米的尺寸范围内具有良好有序结构的金属原子的集合体被称为纳米晶体。控制纳米晶体中不同金属原子的空间分布提供了一种简单而通用的方法来定制它们的结构和性质。这种调整有可能极大地扩大它们作为重要化学品和药品生产中的催化剂以及用于能源转换和环境保护的工艺的效用。在这个项目中，格鲁吉亚理工学院的夏友南博士和威斯康星大学麦迪逊分校的马诺斯·马夫里卡基斯博士正在开发一种双金属系统，在该系统中，两种金属在空间上通过界面相互分离，形成具有Janus结构的纳米晶体。Janus纳米晶体提供了广泛的独特性质和应用，包括光学响应的调谐和特定化学产品的催化活性和/或选择性的增强。通过转换为Janus纳米晶体及其衍生物，可以大幅减少晶体中贵金属的含量，同时可能提高性能，使社会能够实现贵金属的成本效益和可持续利用，这些贵金属是地壳中最稀缺的元素。然而，只有非常有限数量的关于双Janus纳米晶体的报道，主要是由于缺乏对其化学合成的机理理解和实验控制。作为形成双Janus纳米晶体的主要要求，一种金属的原子只能沉积在由另一种金属制成的双金属表面上的单个位点，并且这种特定的沉积模式必须在整个生长过程中保持。PI集成实验研究和计算建模在他们的发展可预测的，确定性的，和强大的合成的Janus纳米晶体。他们还通过研究生和本科生积极参与这种合作，多学科研究以及课程丰富和发展，为科学，技术，工程和数学（STEM）学科的熟练劳动力做好准备。研究人员完全致力于通过让代表性不足的群体参与这项研究项目来促进高等教育的多样性。在化学部大分子，超分子纳米化学（MSN）计划的资助下，夏博士和Mavrikakis正在开发一个新的知识库，用于设计和确定性合成Janus纳米晶体。他们正在推导出从每个单独的晶体上的单个位点实现成核和生长所需的初始还原速率的确切范围，从而生成Janus结构。他们还研究了Janus纳米晶体在原子相互扩散和表面扩散方面的热稳定性。至于应用，Janus纳米晶体被探索为胶体自组装的构建块和作为用于制造金属纳米盒的牺牲模板，该金属纳米盒在表面上具有单个、明确定义的孔，优选地位于其中一个顶点处。这种纳米盒，当由铂制成时，可以作为一种高活性和持久的催化剂，用于氧还原，这是氢基燃料电池运行的关键反应。沿着实验研究，计算模型被用来解释合成中涉及的趋势和机制，以及在一些纳米晶体上发生的催化作用。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

Assemblies of metal atoms with well-ordered structures in the size range of ~10 nanometers are known as nanocrystals. Controlling the spatial distributions of different metal atoms in nanocrystals offers a simple and versatile way to tailor their structures and properties. Such tailoring has the potential to vastly expand their utility as catalysts in the production of important chemicals and pharmaceuticals as well as processes used for conversion of energy and protection of the environment. In this project, Dr. Younan Xia of the Georgia Institute of Technology and Dr. Manos Mavrikakis of the University of Wisconsin-Madison are developing a bimetallic system, in which the two metals are spatially separated from each other by an interface to form nanocrystals with a Janus structure. The bimetallic Janus nanocrystals offer a broad range of unique properties and applications, including the tuning of optical response and the enhancement of catalytic activity and/or selectivity for specific chemical products. By switching to bimetallic Janus nanocrystals and their derivatives, one can substantially reduce the amount of precious metal in a nanocrystal while potentially improving the performance, enabling Society to achieve cost-effective and sustainable use of precious metals, some of the scarcest elements in the Earth's crust. However, there are only a very limited number of reports on bimetallic Janus nanocrystals, primarily due to the lack of mechanistic understanding and experimental control for their chemical synthesis. As a major requirement for the formation of bimetallic Janus nanocrystals, the atoms of one metal can only be deposited at a single site on the surface of a nanocrystal made of another metal and this particular pattern of deposition must be maintained throughout the growth process. The PIs integrate experimental studies and computational modeling in their development of predictable, deterministic, and robust synthesis of bimetallic Janus nanocrystals. They are also contributing to the preparation of a skilled workforce for science, technology, engineering and mathematics (STEM) disciplines through active engagement of graduate and undergraduate students in this collaborative, multidisciplinary research, as well as curriculum enrichment and development. The investigators are fully committed to promoting diversity in higher education by engaging underrepresented groups into this research project. With funding from the Macromolecular, Supramolecular & Nanochemistry (MSN) Program of the Chemistry Division, Drs. Xia and Mavrikakis are developing a new knowledge base for the design and deterministic synthesis of bimetallic Janus nanocrystals. They are deriving the exact range of initial reduction rate needed for achieving nucleation and growth from a single site on each individual nanocrystal and thus generating a Janus structure. They are also investigating the thermal stability of the Janus nanocrystals with respect to atomic inter-diffusion and surface diffusion. As for applications, the Janus nanocrystals are explored as building blocks for colloidal self-assembly and as sacrificial templates for the fabrication of metal nanoboxes with a single, well-defined hole on the surface, preferably located at one of the vertices. Such nanoboxes, when made of platinum, can serve as a highly active and durable catalyst towards oxygen reduction, a reaction key to the operation of hydrogen-based fuel cells. Along with the experimental studies, computational modeling is used to explain the trends and mechanisms involved in the syntheses, as well as the catalysis taking place on some of the bimetallic nanocrystals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.