A variety of methods have been developed to synthesize nanomaterials of various sizes and composition. Often these methods rely on a hot injection synthesis scheme, but these are generally developed on a case-by-case basis and not appropriate for all compositions of matter. Cation exchange of nanocrystals is a useful method for the modification of well-defined template materials into a number of derivatives with controlled composition and crystallinity that are otherwise unavailable via traditional synthetic methods. While a general thermodynamic understanding of the processes that occur in cation exchange is established, very little quantitative data of the thermodynamics and kinetics of cation exchange at the nanoscale have been published. These measurements are hindered by the fact that cation exchange occurs in solution at a buried solid-liquid interface. Traditional techniques used to characterize cation exchange primarily probe structural and optical properties which provide no information on the thermodynamics of cation exchange. Recently, we have applied isothermal titration calorimetry (ITC) to study the thermodynamics of CdSe cation exchange. These ITC experiments involve the incremental titration of a cation source into a cell containing CdSe nanocrystals and measurement of the heat evolved. With incremental titration, the enthalpy and the equilibrium constant for a reaction are simultaneously measured. We have applied this method to measure how nanocrystal size, cation source (Ag, Cu), and temperature impact the thermodynamics of the reaction. Additionally, the kinetics of cation exchange can be determined from the calorimetry data, and compared directly to a more traditional kinetic measurement technique, stopped-flow spectroscopy. We will demonstrate that the kinetics are influenced by expected variables, such as temperature, but the particle size is a major influence on the observed kinetics of cation exchange. We will summarize the similarities and differences between the thermodynamics and kinetics of cation exchange in nanoscale and bulk materials. The fundamental thermodynamic and kinetic data measured in this work will enable the development of robust synthesis-structure relationships in cation-exchange that will enable the development of nanoscale materials with tailored properties for catalysis, photonics and electronics.

Thermodynamic and kinetic measurements of cation exchange in chalcogenide nanocrystals by isothermal titration calorimetry

249th National Meeting (2015)

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