Thermodynamics and Kinetics of Hybrid Perovskite Amino-Deliquescence and Efflorescence

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With support from the Chemical Structure, Dynamics and Mechanisms-A (CSDM-A) Program of the Division of Chemistry, James F. Cahoon and his research group at the University of North Carolina-Chapel Hill will investigate deliquescence, the phenomenon in which a salt takes up water to form a liquid droplet. An analogous set of phenomena that can occur with a class of materials termed hybrid perovskites will be investigated in this research. When hybrid perovskites are exposed to nitrogen-containing amines, the amines play the same role as water, causing the liquefaction and crystallization of the hybrid perovskite material through processes that have been termed amino-deliquescence and amino-efflorescence, respectively. Although by appearances one may think that the salt has melted, what has occurred at a microscopic scale is the condensation of water from the atmosphere to dissolve the salt crystal. The reverse of this phenomenon, termed efflorescence, can occur when the humidity decreases, causing evaporation of water from the droplet and crystallization of the salt. Hybrid perovskites are an exciting class of materials that are being widely studied because of their tunable properties, including strong light absorption and emission, that make them amenable to a range of potential technologies. This project will examine the fundamental physical chemistry that governs the amino-deliquescence/efflorescence process. The results of this study should provide guidelines that can be used to control composition and morphology of such materials to engender them with specific physical characteristics. This project at the interface of physical chemistry and materials science will form the basis of a well-rounded research experience for the high school, undergraduate and graduate students involved.Hybrid perovskites such as methylammonium lead halide are composed of organic and inorganic constituents that have found unique functionality as thin films, macroscopic single crystals, and colloidal nanocrystals. In this project, the liquefaction/crystallization of hybrid perovskites when exposed to amines (e.g. methylamine, butylamine, etc.) at controlled temperatures and pressures will be quantitatively analyzed using a set of in situ measurements and microscopies in a home-built reactor. Analysis of amino-deliquescence and amino-efflorescence phase transitions should permit a detailed thermodynamic and kinetic analysis of the processes, to extract the relevant enthalpic and entropic parameters for these transitions. Understanding nucleation and growth kinetics should provide a means to also understand and control crystallinity, orientation, morphology, and composition. Both three-dimensional and two-dimensional layered perovskites will be studied using small and large organic ammonium or diammonium cations and their corresponding amines or diamines. The project is expected to provide information on an important, fundamental phenomenon seen with hybrid perovskites, yielding insights that could motivate further theoretical study and assist in future chemical and structural modifications of these systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.