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

非技术摘要控制纳米晶体的形状具有远远超出美学吸引力的影响。对于由银、金、钯和铂等贵金属制成的纳米晶，其形状不仅决定了它们的物理化学性质，而且还决定了它们在催化、电子、光子学、显示、传感、医药、环境保护和能源生产和储存等方面的应用。以银纳米棒为例，它们可以设计成具有优异的导电性和导热性，同时在可见光区域没有光吸收，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。纳米晶体的形状由晶种的孪晶结构和生长模式决定，而晶种的孪晶结构和生长模式又与合成中涉及的还原动力学有关。在材料研究部固态和材料化学项目的支持下，本研究的最终目标是建立还原速率作为操纵纳米晶体形状的量化旋钮的作用。这项研究通过在化学、物理、材料科学、催化、光子学、电子学和能源技术等不同领域之间建立联系，对许多学科的发展产生了深远的影响。这项研究的直接成果是性能显著提高的先进纳米材料，适用于广泛的应用，包括与能源生产(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相关的应用。生产形状可控的贵金属纳米晶体的能力也为实现这些仅存在于地壳中的十亿分之几的稀有元素的可持续和多产使用提供了一种实用战略。多学科和协作活动极大地促进了研究生和本科生的教育，也为促进高等教育的多样性和丰富K-12教育提供了一个天然的工具。技术摘要首席研究人员将通过定量了解前驱体的还原速率与种子中孪生缺陷数量之间的关系以及它的生长模式来研究纳米晶体的成核和生长。研究小组将以钯为模型系统，开发光谱方法来确定用于纳米晶体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)，然后确定导致特定类型种子形成的还原速率范围，该特定类型的种子具有单晶、单晶、多晶或堆积断层衬里结构。动力学参数还将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，以努力实现对对称破缺或约化等新现象的深入理解。作为有力的例证，关于还原速率对晶种形核和生长影响的定量知识将被进一步应用于设计合成方案，通过使用小于20 nm的钯十面体晶种来制备在整个可见光范围内没有光吸收的银纳米棒。综上所述，这项研究将为纳米技术领域带来重大进展，揭示具有良好控制形状和相关性能的纳米晶体确定性合成的基本知识和设计规则，这些纳米晶体是广泛的基础研究和工业重要应用的核心。

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.