Solvent-Dependent Impact of Spectator Anions on the Thermodynamics of Cation Exchange in CdSe Nanocrystals

Cation exchange reactions in nanoparticles are widely used to synthesize nanostructures that cannot be synthesized via direct colloidal synthesis approaches. Cation exchange in chalcogenide nanocrystals has been utilized to produce a massive library of derivative nanostructures. Understanding the thermodynamic feasibility of these reactions is often based on qualitative considerations (i.e., Pearson's acid-base theory). In our previous publication [ Jharimune, S.; et al. Nano Lett. 2018, 18 (11), 6795-6803 ], we demonstrated isothermal titration (ITC) is a robust method to quantify the thermodynamics of cation-exchange reactions in nanocrystals and examined the influence of nanocrystal diameter, identity of capping ligands, and temperature. We extend the use of ITC to study the influence of anions accompanying the foreign cation, solvents, and foreign cation identity on the thermodynamics of cation-exchange reaction in CdSe nanocrystals. Results indicate a strong correlation between the identity of the anion and the apparent enthalpy of the reaction, whereby harder anions favor the cation exchange reaction of CdSe with Ag+ salts in acetonitrile. When an identical exchange is conducted in a harder solvent, water, the strong impact of anions on the thermodynamics of cation exchange disappears. We examined the exchange of CdSe nanocrystals with Cu2+ that has a comparable hardness to the parent cation (Cd2+). This study probes the impact and interplay of solvent and accompanying anion with exchanging cation on energetics in cation-exchange reactions.