Towards a Quantitative Knob for Controlling the Shape of Noble-Metal Nanocrystals

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Non-Technical AbstractControlling the shape of nanocrystals has implications that go far beyond aesthetic appeal. For nanocrystals made of precious metals such as silver, gold, palladium and platinum, the shape determines not only their physicochemical properties but also their relevance for applications in catalysis, electronics, photonics, display, sensing, medicine, environmental protection, and energy production and storage. Taking silver nanorods as an example, they can be designed with superior electrical and thermal conductivity, while having no optical absorption in the visible region, to meet the requirements for touchscreen displays, energy-efficient transparent solar films, and smart windows. The shape of a nanocrystal is determined by the twin structure and growth pattern of the seed, which are, in turn, correlated with the reduction kinetics involved in a synthesis. With the support of the Solid State and Materials Chemistry program in the Division of Materials Research, the ultimate goal of this research is to establish the role of reduction rate as a quantitative knob for manipulating the shape of nanocrystals. This research has profound impacts on the advancement of a number of disciplines by forging links between different fields, including chemistry, physics, materials science, catalysis, photonics, electronics, and energy technology. The immediate outputs of this research are advanced nanomaterials with substantially improved performance for a broad range of applications, including those related to energy production (e.g., fuel cells), protection of the environment (e.g., catalytic converters), national security, and public healthcare. The ability to produce nanocrystals of precious metals with well-controlled shapes also offers a practical strategy for achieving sustainable and prolific use of these scarce elements that only exist in the Earth's crust at a level of parts per billion. The multidisciplinary and collaborative activity greatly enhances graduate and undergraduate education and also provides a natural vehicle to promote the diversity in higher education and enrich the K-12 education.Technical AbstractThe principal investigator will study the nucleation and growth of nanocrystals by achieving a quantitative understanding of the correlations between the reduction rate of a precursor and the number of twin defects in a seed, as well as its growth pattern. Using palladium as a model system, the research team will develop spectroscopy methods to determine the kinetic parameters (including rate constant and activation energy) of various reduction reactions used for nanocrystal synthesis and then determine the range of reduction rates responsible for the formation of a specific type of seeds characterized by a single-crystal, singly-twinned, multiply-twinned, or stacking-fault-lined structure. The kinetic parameters will also be applied to analyze the growth patterns of cubic and decahedral seeds (with single-crystal and five-fold twinned structures, respectively) in an effort to achieve a deep understanding of new phenomena such as symmetry breaking or reduction. As a powerful demonstration, the quantitative knowledge about the effects of reduction rate on the nucleation and growth of seeds will be further applied to design synthetic protocols for the production of silver nanorods with no optical absorption in the entire visible region by working with palladium decahedral seeds smaller than 20 nm. Taken together, this research will bring major advances to the field of nanotechnology by unraveling the essential knowledge and design rule for the deterministic syntheses of nanocrystals with well-controlled shapes and related properties central to a broad range of fundamental studies and industrially important applications.