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.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Suk-Wah Tam-Chang
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Georgia Tech Research Corporation
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