High-Entropy Alloy Nanocrystals with Controlled Compositions and Surface Structures

非技术概述：该基金支持为合理和确定合成高熵合金（HEA，一种由五种或更多金属组成的复杂固体）纳米晶体的研究工作，这些纳米晶体在原子组成和排列方面都受到控制。由于组成的多样性，HEA纳米晶体的表面自然呈现出大量的原子在不同的配位环境中，为反应的关键中间体产生近乎连续的吸附能分布。因此，HEA纳米晶体为新催化材料的快速发展提供了一个多功能平台。尽管近年来取得了一些进展，但控制HEA纳米晶的组成和表面结构仍然是一个巨大的挑战，这使得无法定量准确地描述HEA基催化剂的结构-性能关系。在这个项目中，一种革命性的技术被开发出来，以实现具有控制成分和表面结构的HEA纳米晶体的确定性甚至可预测的合成。所获得的纳米晶体可以直接作为先进的催化材料，造福美国经济和社会。这个项目的多学科和合作性质提供了一个工具，以加强和丰富学生的教育和培训经验，同时扩大代表性不足的群体参与前沿研究。特别是，学生们可以直接学习如何以较低的成本以两倍的速度发现和开发先进的催化材料。这个项目的成果也被用于加强课堂教学，包括发展演示和实验，以更好地说明科学和工程的关键概念。技术综述：本研究首次尝试开发具有良好定义和可控成分和表面结构的单相HEA纳米晶体。传统上，金属纳米晶体的胶体合成需要一次注射金属前驱体。当直接应用于双金属或多金属体系时，这种方案容易失败，因为不同的前驱体具有不同的还原动力学，它们在反应溶液中的瞬时浓度随着反应时间的推移而衰减不同。与传统方法不同，研究人员将不同前驱体的溶液滴入反应溶液中，使每种前驱体的瞬时浓度在整个合成过程中保持在预定的稳定状态。结果，金属原子以稳定的、预先指定的速率从不同的前驱体中产生，以生成具有均匀组成的HEA纳米晶体，原子比由其前驱体在稳定状态下的还原速率决定。当在不同形状的预成型种子上共形沉积时，可以得到具有明确晶面的HEA纳米晶体。种子可以被选择性地蚀刻掉，以释放HEA覆盖层作为超薄纳米板。在三层原子层的厚度上，纳米板非常适合通过电子显微镜和光谱学来分析成分和表面结构。同时，HEA纳米晶体在催化反应中进行了测试，重点是在氧还原和键选择性氢化方面具有最佳活性和耐久性的催化剂的鉴定。该合成方法还可以扩展，以加速其他类型复杂成分纳米材料的合理开发，这些材料可用于化学生产、石油工业、国家安全和公共卫生等一系列应用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARYThis grant supports research efforts to create a knowledge base for the rational and deterministic synthesis of high-entropy alloy (HEA, a complex solid comprised of five or more metals) nanocrystals with controlled surfaces in terms of both atomic composition and arrangement. Because of the diversity in composition, the surface of a HEA nanocrystal naturally presents an enormous number of atoms in distinct coordination environments to produce a near-continuum distribution of adsorption energies for the key intermediate of a reaction. As such, HEA nanocrystals offer a versatile platform for the rapid development of new catalytic materials. Despite some progress in recent years, it remains a grand challenge to control the composition and surface structure of HEA nanocrystals, making it impossible to quantitatively and accurately describe the structure-property relationship of HEA-based catalysts. During this project, a transformative technique is developed to achieve deterministic and even predictable synthesis of HEA nanocrystals with controlled compositions and surface structures. The as-obtained nanocrystals can directly serve as advanced catalytic materials to benefit U.S. economy and society. The multi-disciplinary and collaborative nature of this project offers a vehicle to enhance and enrich the education and training experiences of students while broadening the participation of underrepresented groups in cutting-edge research. In particular, the students learn firsthand how to discover and develop advanced catalytic materials twice as fast at a fraction of the cost. The results from this project are also adapted to enhance classroom teaching, including the development of demonstrations and experiments to better illustrate the key concepts of science and engineering. TECHNICAL SUMMARYThis research represents the first attempt to develop single-phase HEA nanocrystals with well-defined and controllable compositions and surface structures. Traditionally, colloidal synthesis of metal nanocrystals involves one-shot injection of the metal precursor. When directly applied to a bi- or multi-metallic system, such a protocol is subject to failures because different precursors have distinct reduction kinetics and their instantaneous concentrations in the reaction solution would decay differently with reaction time. Parting from the traditional method, the investigators co-titrate the solutions of different precursors dropwise into a reaction solution so that the instantaneous concentration of each precursor is kept in a predefined steady state throughout the synthesis. As a result, metal atoms are produced from the different precursors at stable, pre-specified rates for the generation of HEA nanocrystals with a uniform composition, with the atomic ratios determined by the reduction rates of their precursors in the steady states. When conformally deposited on preformed seeds with different shapes as overlayers of a few atomic layers in thickness, HEA nanocrystals with well-defined facets are obtained. The seeds can be selectively etched away to release the HEA overlayers as ultrathin nanoplates. At a thickness of three atomic layers, the nanoplates are perfect for resolving the composition and surface structure by electron microscopy and spectroscopy. In parallel, the HEA nanocrystals are tested for catalytic reactions, with a focus on the identification of a catalyst optimal in activity and durability toward oxygen reduction and bond-selective hydrogenation. The synthetic method can also be extended to accelerate the rational development of other types of nanomaterials with complex compositions for a range of applications, including those related to chemical production, petroleum industry, national security, and public health.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.