High-Entropy Alloy Nanocrystals with Controlled Compositions and Surface Structures

NON-TECHNICAL SUMMARY

This 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 SUMMARY

This 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.