Replacement-Free Growth of Au on Ag Nanocrystal Seeds

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Dong Qin from the Georgia Institute of Technology is supported by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry to grow metal nanocrystals on other ("seed") metal nanocrystals with metal combinations that have not previously been possible. A nanocrystal is a crystal whose size is limited to several billionths of a meter, and such small crystals often have unusual properties. Noble-metal nanocrystals have received steadily growing interest in recent years owing to their fascinating properties and widespread use in applications ranging from catalysis to sensing, imaging, and biomedicine. Today, growing them on seed particles has emerged as a prevalent route to the syntheses of nanocrystals from a number of noble metals, such as silver (Ag), gold (Au), palladium (Pd), and platinum (Pt), as well as some of their bimetallic combinations. Despite the remarkable successes, the seed usually has to be the less reactive molecule or it gets eaten away ("galvanic replacement") during the growth process of the second metal. The project is aimed at finding a way to avoid galvanic replacement even when the seed particle is made of the more reactive metal. The new classes of bimetallic nanocrystals being made can be used in a broad range of applications since they may enhance spectroscopy of nearby molecules and catalysis of chemical reactions. The proposed research encompasses disciplines across materials science, chemistry, colloidal science, solid-state physics, optics, and surface chemistry. The project builds an interdisciplinary education program in nanoscale science and engineering for graduate and undergraduate students, with a major focus on the following components: i) learning that exposes students to a multifaceted, integrated approach to understanding the fundamentals of nanomaterials and their unique properties due to nanoscale sizes; ii) training that offers students hands-on experience in the laboratory and the school user facilities in exploring research tools and discoveries and frontiers of nanotechnology; and iii) engaging that connects the students with community and society to impart and promote a conceptual understanding of nanoscale science and technology in high school students and teachers. This project is to develop a scientific basis for achieving seeded growth with two metals that have been plagued by galvanic replacement reactions. Specifically, the team aims to achieve replacement-free growth of Au on Ag nanocubes using a faster, parallel reduction to kinetically compete with and thus inhibit a galvanic reactions. The proposed research includes the following major thrusts: i) synthesis of Ag nanocubes with edge lengths at 30, 60, and 90 nm as uniform samples, together with different degrees of corner truncation; ii) determination of the kinetic parameters (rate law and activation energy) for the reduction of HAuCl4 by a strong reducing agent, such as ascorbic acid, NH2OH, and NaBH4; iii) measurement of the kinetic parameters for the galvanic reaction between HAuCl4 and Ag nanocubes of different sizes and with different degrees of corner truncation; iv) understanding the role of surface diffusion in controlling the final structure (core-frame vs. core-shell) of the bimetallic nanocrystals; and v) evaluation of the optical properties and chemical stability of the bimetallic nanocrystals. Collectively, a solid understanding of the system involving Ag and Au serves as the foundation for achieving replacement-free growth of a less reactive metal on the seeds made of a more reactive metal. These new classes of Ag-Au nanostructures can find widespread use in optical applications with greatly improved performance in terms of chemical stability and activity, together with their potentials for emerging applications such as sensing, imaging, biomedicine, and photonics, as well as in the conversion of solar light into energy through the field enhancement effect. The principle for the galvanic replacement-free growth of Au on Ag can also be extended to other pairs of noble metals, including Ag-Pd, Ag-Pt, Ag-Rh, and Ag-Ir. The resultant bimetallic nanocrystals with a core-frame or core-shell structure may find immediate use in catalysis and environmental protection.