Replacement-Free Growth of Au on Ag Nanocrystal Seeds

来自格鲁吉亚理工学院的董勤得到化学系大分子、超分子和纳米化学项目的支持，在其他（“种子”）金属纳米晶体上生长金属纳米晶体，金属组合以前是不可能的。 水晶是一种晶体，其大小限制在十亿分之几米，这样的小晶体通常具有不寻常的性质。 近年来，贵金属纳米晶体由于其迷人的性质和广泛的应用，从催化到传感，成像和生物医学，受到了越来越多的关注。如今，在晶种颗粒上生长纳米晶体已经成为从多种贵金属（例如银（Ag）、金（Au）、钯（Pd）和铂（Pt）以及它们的某些金属组合）合成纳米晶体的普遍途径。尽管取得了显著的成功，但种子通常必须是反应性较低的分子，否则它会在第二金属的生长过程中被吃掉（“电流替代”）。该项目旨在找到一种方法，即使种子颗粒由更具反应性的金属制成，也可以避免电化学更换。正在制造的新型纳米晶体可以用于广泛的应用，因为它们可以增强附近分子的光谱和化学反应的催化作用。拟议的研究涵盖材料科学，化学，胶体科学，固态物理，光学和表面化学等学科。该项目为研究生和本科生建立了一个纳米科学与工程的跨学科教育计划，主要关注以下内容：i）让学生接触到多方面的综合方法，以了解纳米材料的基本原理及其因纳米尺寸而产生的独特特性; ii）提供培训，让学生在实验室和学校用户设施中探索研究工具和纳米技术的发现和前沿;以及iii）参与将学生与社区和社会联系起来，以传授和促进高中学生和教师对纳米科学和技术的概念性理解。该项目旨在为实现两种金属的种子生长奠定科学基础，这两种金属一直受到电化学置换反应的困扰。具体来说，该团队的目标是实现Au在Ag纳米立方体上的无沉淀生长，使用更快的平行还原来动力学竞争并从而抑制原电池反应。本论文的主要研究内容包括：i）合成边长为30、60和90 nm的Ag纳米立方体，并对其进行不同程度的角截断; ii）确定动力学参数（速率定律和活化能）用于强还原剂如抗坏血酸、NH 2 OH和NaBH 4还原HAuCl 4; iii）测量不同尺寸和具有不同截角程度的HAuCl 4和Ag纳米立方体之间的电偶反应的动力学参数; iv）理解表面扩散在控制最终结构中的作用（核-框架对核-壳）;和v）评价纳米晶体的光学性质和化学稳定性。总的来说，对涉及Ag和Au的系统的扎实理解用作实现在由更具反应性的金属制成的种子上的较低反应性的金属的无沉淀生长的基础。这些新型的Ag-Au纳米结构可以在光学应用中找到广泛的用途，其在化学稳定性和活性方面的性能大大提高，以及它们在新兴应用中的潜力，例如传感，成像，生物医学和光子学，以及通过场增强效应将太阳光转换为能量。这种无电沉积生长Au的原理也可以推广到其他贵金属对，包括Ag-Pd，Ag-Pt，Ag-Rh和Ag-Ir，所得到的具有核-框架或核-壳结构的纳米晶体可能在催化和环境保护方面立即得到应用。

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.