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

控制纳米晶体的形状具有远远超出美学吸引力的意义。对于由贵金属如银、金、钯和铂制成的纳米晶体，形状不仅决定了它们的物理化学性质，而且决定了它们在催化、电子学、光子学、显示、传感、医学、环境保护以及能源生产和储存方面的应用。以银纳米棒为例，它们可以被设计成具有上级导电性和导热性，同时在可见光区没有光吸收，以满足触摸屏显示器、节能透明太阳能薄膜和智能窗户的要求。晶种的形状取决于晶种的孪晶结构和生长模式，而这又与合成中涉及的还原动力学相关。在材料研究部固态和材料化学计划的支持下，本研究的最终目标是建立还原率作为操纵纳米晶体形状的定量旋钮的作用。这项研究对许多学科的进步产生了深远的影响，通过建立不同领域之间的联系，包括化学，物理学，材料科学，催化，光子学，电子学和能源技术。这项研究的直接成果是先进的纳米材料，其性能大大提高，适用于广泛的应用，包括与能源生产有关的应用（例如，燃料电池），环境保护（例如，催化转化器）、国家安全和公共医疗保健。生产具有良好控制形状的贵金属纳米晶体的能力也为实现这些仅以十亿分之一的水平存在于地壳中的稀有元素的可持续和多产利用提供了一种实用的策略。多学科和协作活动大大提高了研究生和本科生教育，也提供了一个自然的车辆，以促进高等教育的多样性和丰富的K-12教育。技术摘要首席研究员将研究纳米晶体的成核和生长，通过实现一个前体的减少率和种子中的双缺陷数量之间的相关性的定量理解，以及它的增长模式。使用钯作为模型系统，研究小组将开发光谱学方法来确定用于钯合成的各种还原反应的动力学参数（包括速率常数和活化能），然后确定负责形成特定类型种子的还原速率范围，其特征在于单晶，单孪晶，多孪晶或堆垛层错内衬结构。动力学参数也将应用于分析立方和十面体种子（分别具有单晶和五重孪晶结构）的生长模式，以深入了解对称性破缺或还原等新现象。作为一个强有力的证明，还原速率对种子的成核和生长的影响的定量知识将进一步应用于设计合成方案，用于生产银纳米棒，在整个可见光区没有光吸收，通过与钯十面体小于20 nm的种子。两者合计，这项研究将带来重大进展的纳米技术领域的基本知识和设计规则，为确定性合成的纳米晶体具有良好的控制形状和相关性能的中心，以广泛的基础研究和工业上重要的应用。

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.