Characterization of Atomic Diffusion during Ion Exchange Reactions

在这个研究项目中，康奈尔大学的Richard Robinson博士得到了大分子、超分子和纳米化学（MSN）项目的支持，利用先进的实验方法，包括原位x射线吸收光谱（XAS），研究控制纳米颗粒化学转化的基本化学原理。当科学家们研究非常非常小的材料（所谓的“纳米颗粒”）时，他们发现这些材料的性质与体积性质截然不同。纳米粒子令人惊讶的不同之处在于，原子可以比预期更快、更容易地重新排列自己。化学家们利用这种效应，通过化学转化反应来修饰纳米粒子。这些化学转化反应开辟了令人兴奋的新机遇，因为它们提供了独立操纵纳米颗粒的大小、形状、原子结构或化学成分的能力，克服了传统合成技术的局限性。在这个项目中，理查德·罗宾逊博士正在研究一种化学转化，即离子交换反应，通过使用一种称为x射线吸收光谱（XAS）的先进实验方法。XAS能够精确测量原子周围的局部环境，从而有助于发现离子交换反应过程中原子晶格的变化。为了理解为什么纳米材料中的离子交换过程比块状材料中的离子交换过程快几个数量级，罗宾逊博士正在研究一些关键现象，比如纳米颗粒中扩散的大小依赖性以及原子相（晶格）在转化过程中的化学计量学和稳定性。通过学习如何通过化学转化操纵纳米粒子，该项目创建了一个工具包，为电子、催化和光伏等领域的先进应用量身定制纳米粒子，从而造福社会。该项目通过激发具有新成分和结构的纳米颗粒的新合成方法，广泛地影响了化学。罗宾逊博士正在开发一个外联模块，用来教授学生纳米材料的特性。罗宾逊博士正在培训高中教师使用“纳米演示”工具包，并通过一个借阅图书馆提供工具包。罗宾逊博士在当地和地区的拓展工作包括通过斯隆多样性研究员计划指导研究生，并让本科生参与研究活动。离子交换反应是化学转化反应的一个子集，是原子重组第一代（合成）纳米颗粒的组成和相的有力方法。目前尚不清楚的是，离子动力学在纳米尺度上的表现与在体积尺度上的表现有何不同。了解离子交换过程的关键挑战包括表征纳米颗粒中扩散的尺寸依赖性，反应动力学对离子动力学的影响，以及转变过程中原子相（晶格）的化学计量学和稳定性。为了研究这些挑战，罗宾逊博士在阳离子和阴离子交换过程中对纳米粒子进行原位和非原位XAS，并绘制离子和原子结构的运动图。通过结合时间分辨XAS，远程顺序表征和元素分析，罗宾逊博士描述了控制原子动力学，氧化态的动态变化，以及纳米颗粒离子交换过程中发生的配位化学。罗宾逊博士正在利用这些反应来制造先进的纳米异质结构、亚稳相和掺杂纳米颗粒。通过了解如何通过化学转化来操纵纳米颗粒，该项目创建了一个工具包，为电子、催化和光伏等领域的先进应用量身定制纳米颗粒，从而造福社会。

In this research program, Dr. Richard Robinson of Cornell University is supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) program to investigate the fundamental chemical principles governing chemical transformations of nanoparticles using advanced experimental methods including in-situ x-ray absorption spectroscopy (XAS). When scientists study materials that are very, very small (so-called "nanoparticles") they find that the properties are radically different from bulk properties. One of the surprising differences in nanoparticles is that the atoms can rearrange themselves much faster and easier than expected. Chemists have taken advantage of this effect to modify nanoparticles through chemical transformation reactions. These chemical transformation reactions have opened up exciting new opportunities because they offer the ability to independently manipulate the size, shape, atomic structure, or chemical composition in nanoparticles, overcoming limitations of conventional synthesis techniques. In this project, Dr. Richard Robinson is investigating one type of chemical transformation, the ion exchange reaction, by using an advanced experimental method called x-ray absorption spectroscopy (XAS). XAS enables the precise measurement of the local environment around an atom and thereby helps discover how the atomic lattice changes during the ion exchange reaction. To understand why the ion exchange process is orders of magnitude faster in nanomaterials than in bulk materials, Dr. Robinson is investigating key phenomena such as the size dependence of diffusion in nanoparticles and the stoichiometry and stability of the atomic phase (crystal lattice) during transformation. By learning how to manipulate nanoparticles through chemical transformations, this project is benefiting society by creating a toolkit to tailor nanoparticles for advanced applications in areas such as electronics, catalysis, and photovoltaics. The project broadly impacts chemistry by spurring new synthetic approaches to nanoparticles with novel composition and structure. Dr. Robinson is developing an outreach module that teaches students about nanomaterial properties. Dr. Robinson is training high school teachers to use the "nano demo" kit and is making the kit available through a lending library. Dr. Robinson's local and regional outreach efforts include mentoring graduate students through the Sloan Diversity Fellows Program, and involving undergraduates in research activities.Ion exchange reactions, a subset of the chemical transformation reactions, are a powerful method to atomically restructure the composition and phase of first-generation (as-synthesized) nanoparticles. What is currently unknown is how ion dynamics behave differently at the nanoscale than at bulk scales. The key challenges to understanding the ion exchange process include characterizing the size dependence of diffusion in nanoparticles, the influence of reaction kinetics on the ion dynamics, and the stoichiometry and stability of the atomic phase (crystal lattice) during transformation. To investigate these challenges, Dr. Robinson is performing in-situ and ex-situ XAS of nanoparticles during cation and anion exchange, and mapping the movements of the ions and the atomic structure. By combining time-resolved XAS, long-range order characterization, and elemental analysis, Dr. Robinson is characterizing the governing atomic dynamics, the dynamic changes in oxidation states, and the coordination chemistry that occurs during ion exchange in nanoparticles. Dr. Robinson is exploiting these reactions to create advanced nano-heterostructures, metastable phases, and doped nanoparticles. By understanding how to manipulate nanoparticles through chemical transformations, this project benefits society by creating a toolkit to tailor nanoparticles for advanced applications in areas such as electronics, catalysis, and photovoltaics.