Full Characterization of Colloidal Solutions of Long-Alkyl-Chain-Amine-Stabilized ZnO Nanoparticles by NMR Spectroscopy: Surface State, Equilibria, and Affinity

Full NMR characterization of ZnO nanoparticles (NPs) stabilized by various amines (hexadecylamine, dodecylamine, and octylamine) in C7D8 demonstrated that the surface of this apparently simple system was very complex. Using different NMR spectroscopic techniques (1H, PGSE-NMR, diffusion-filtered 1H NMR, NOESY, ROESY), we observed at least three different modes of interaction of the amines at the surface of the NPs, in thermodynamic equilibrium with the free amines, the relative populations of which varied with their concentration. The first mode corresponded to a strong interaction between a small amount of amine and the ZnO NPs (kdesorp approximate to 13 s-1). The second mode corresponded to a weak interaction between the amines and the surface of the ZnO NPs (koff(2)approximate to 5060 s-1). The third, and weakest, mode of interaction corresponded to the formation of a second ligand shell by the amine around the NPs that was held together through van der Waals interactions (koff(1)approximate to 25x105 s-1). The second and third modes were in fast exchange on the NMR timescales with the free amines. The strongly interacting amines at the NPs surface (first mode) were in slow exchange with the other modes. A complex hydrogen-bonding network at the NPs surface was also observed, which did not only involve the coordinated amine but also THF and water molecules that remained from the synthesis.