Ligand exchange of colloidal semiconductor nanocrystals is widely utilized to modify the surface chemistry of quantum dots (QDs) to enhance their solubility and stability and, importantly, manipulate their electrical and optical properties. Recently, quantitative and qualitative measurements have been made in order to obtain a better understanding of ligand exchange reactions on QDs. We present the implementation of isothermal titration calorimetry (ITC) in order to quantitatively determine the thermodynamic properties of the ligand exchange reaction from oleate to n-dodecylphosphonic acid (DDPA) on the surface of 3.1 nm cadmium selenide (CdSe) nanocrystals. ITC directly measures the heat of reaction of the ligand exchange as a dynamic process; this is an improvement upon other investigations that have been conducted to date which infer the thermodynamics of the reaction by post-exchange studies after equilibrium has been established. By monitoring the heat generated as DDPA was titrated into oleate-capped CdSe QDs, we found this ligand exchange reaction to be an exothermic process, and the enthalpy was determined to be -14.86 kJ mol<sup>-1</sup>. We interpret the enthalpy of reaction in the context of bond energies, notably as a comparison of the magnitude of bonds broken with respect to bonds formed; these results are in agreement with previous research that indicates that DDPA binds to CdSe QDs more strongly than oleate. We also calculate the Gibb’s free energy to be -33.81 kJ mol<sup>-1</sup> indicating this is a spontaneous displacement of surface ligands. Interestingly, analysis of the ITC isotherm indicates the number of oleate ligands displaced by DDPA to be about 56 per QD (~33% exchange). Thermogravimetric analysis (TGA), however, was utilized to externally validate the number of ligands displaced and found complete exchange of surface ligands by DDPA. Current investigations are underway to reconcile the difference in ligands exchanged as determined via ITC versus that found by TGA. This study demonstrates the potential of ITC as a method to quantify, in real time, ligand exchange reactions to provide a detailed understanding of the modification of the surface chemistry of colloidal CdSe QDs.

Quantifying the thermodynamics of the surface modification of oleate-capped CdSe quantum dots with dodecylphosphonic acid via isothermal itration calorimetry

255th National Meeting (2018)

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