Self-Assembly of ZnO Nanoparticles - An NMR Spectroscopic Study

The role of ligands (i.e. hexadecylamine, dodecylamine, or octylamine associated with oleic acid) on the formation of ZnO nanoparticle superlattice structures (NSSs) was investigated by NMR spectroscopy in C7D8. This full study demonstrates that ion-paired ammonium carboxylates play a crucial role in NSS formation. Using different NMR spectroscopic experiments, such as 1H NMR, pulsed field gradient spin-echo (PGSE) NMR, and NOESY, we evidenced that the introduction of long-alkyl-chain carboxylic acid molecules into a colloidal solution of ZnO nanoparticles (NPs) stabilized by amine ligands leads to rearrangement of the amines on the surface of the NPs with the formation of ammonium carboxylate. This NMR spectroscopic study evidences the dependence of the nature of the ligands adsorbed on the NP surface on the colloid concentration. At high concentration, several ligand shells with a lot of ion-paired ammonium carboxylate and inclusion of amine are observed. At low concentration, the ion-paired ammonium carboxylate shells are replaced by amine shells. This NMR spectroscopic study suggests that the driving force of NSS formation is the presence of ion-paired ammonium carboxylate shells around the nanoparticle.