Thermodynamic Implications of the Ligand Exchange with Alkylamines on the Surface of CdSe Quantum Dots: The Importance of Ligand-Ligand Interactions

Ligand exchange is a critical step for tuning the properties of quantum dots (QDs), as well as functionalization for wide applications. Previous studies have categorized X-, L-, and Z-types of ligands and classified the ligand exchange process accordingly. Many works have investigated the influences of ligand exchange on the optical properties of QDs. To date, however, the thermodynamic implications of ligand exchange are not well understood, and the mechanisms of distinct effects of the ligands of the same type are unclear. To address these issues, ligand exchange reactions of the oleate-capped CdSe QDs with alkylamines of different carbon chain length, namely, n-butylamine (C4Am), n-octylamine (C8Am), n-dodecylamine (C12Am), were investigated using H-1 nuclear magnetic resonance (H-1 NMR) spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), etc. H-1 NMR studies showed that the equilibrium constants (K-eq) for the desorption of the Z-type ligand (cadmium oleate) from the surface of CdSe QDs, as promoted by the L-type ligands (alkylamines), were in the order of C4Am > C8Am > C12Am, presumably highlighting the small steric barrier of ligands with short carbon chain. In addition, the ligand exchange was studied by fluorescence titration at varied temperatures from a perspective of ligand adsorption following the desorption of cadmium oleate. The binding constants were of magnitude 10(3) M-1. A "two-step ligand exchange" (TSLE) model was developed to describe the ligand desorption-adsorption process with thermodynamic perspectives. The increased length of the carbon chain brought not only increasing steric barrier but also increasing van der Waals interactions, exhibiting a double-edged sword effect in ligand exchange. The ligand exchange with C8Am contributed the most negative value of Delta H and Delta S since the van der Waals interactions between C8Am and neighboring ligands were stronger than that for C4Am, whereas its steric barrier was weaker than that of C12Am. These demonstrated the importance of ligand-ligand interactions, relatively less documented in the previous studies, besides the predominance of the nanocrystal-ligand interactions. UV-vis absorption spectroscopic and fluorescence lifetime studies further showed that the ligand exchange with all three alkylamines affected optical properties of CdSe QDs to some extent. Powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that the ligand exchange process basically did not affect the crystal structure and core size of the CdSe QDs. Our work had proposed a conceivable mechanism for ligand exchange, elucidated the thermodynamic implications, and provided a reasonable guidance for selection of suitable ligands.