Characterization of Atomic Diffusion during Ion Exchange Reactions

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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.