High-Entropy Alloy Nanocrystals with Controlled Compositions and Surface Structures

非技术性总结这笔赠款支持为合理和确定地合成高熵合金(HEA，一种由五种或五种以上金属组成的复杂固体)纳米晶体创建知识库的研究工作，这些纳米晶体的原子组成和排列都受到表面控制。由于组成的多样性，HEA纳米晶体的表面自然会出现大量处于不同配位环境中的原子，从而为反应的关键中间体产生接近连续分布的吸附能量。因此，HEA纳米晶为快速开发新的催化材料提供了一个通用的平台。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，这使得定量和准确地描述HEA基催化剂的结构-性能关系变得不可能。在这个项目中，开发了一种变革性的技术，以实现具有可控组成和表面结构的HEA纳米晶的确定甚至可预测的合成。得到的纳米晶可以直接作为先进的催化材料，造福于美国的经济和社会。该项目的多学科和协作性质为加强和丰富学生的教育和培训经验提供了一种手段，同时扩大了未被充分代表的群体对前沿研究的参与。特别是，学生们将第一手学习如何以低成本两倍的速度发现和开发先进的催化材料。该项目的成果也被用来加强课堂教学，包括开发演示和实验，以更好地阐明科学和工程的关键概念。技术总结这项研究代表着首次尝试开发具有明确和可控的组成和表面结构的单相HEA纳米晶。传统上，胶体合成金属纳米晶体涉及金属前体的一次注射。当直接应用于双金属或多金属体系时，这种方法会失败，因为不同的前体具有不同的还原动力学，并且它们在反应溶液中的瞬时浓度会随着反应时间的不同而衰减。与传统方法不同，研究人员将不同前体的溶液滴定到反应溶液中，使每个前体的瞬时浓度在整个合成过程中保持在预定的稳定状态。结果是，金属原子以稳定的、预先指定的速率从不同的前驱体产生，以生成组成均匀的HEA纳米晶，其原子比由它们的前驱体在稳定状态下的还原速率确定。当共形沉积在不同形状的预制晶种上，作为几个原子层厚度的覆盖层时，得到了具有明确小面的HEA纳米晶。种子可以被选择性地蚀刻掉，以释放HEA覆盖层，形成超薄的纳米板。在三层原子层的厚度下，纳米板是通过电子显微镜和光谱分析来解析成分和表面结构的完美材料。同时，对HEA纳米晶进行催化反应测试，重点是确定一种对氧还原和键选择加氢具有最佳活性和耐用性的催化剂。合成方法也可以扩展，以加速其他类型的复杂成分纳米材料的合理开发，用于一系列应用，包括与化工生产、石油工业、国家安全和公共卫生相关的应用。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.