FTIR spectroscopy revealed nonplanar conformers, chain order, and packaging density in diOctadecylamine- and octadecylamine-passivated gold nanoparticles

Abstract

This work reveals that diOctadecylamine (diODA) outperforms its primary amine counterpart ODA in intra- and intermolecular order, forming passivation monolayers on 5 nm small gold nanoparticles. We support this conclusion by meticulously analyzing the differences in the Fourier Transform Infrared (FTIR) spectrum of diODA and ODA molecules in their pure- and coordinated-to-gold-nanoparticle phases. Detailed diODA spectral data are scarce in the literature and are absent for diODA-passivated nanoparticles, whose merits have gone unnoticed. The analysis compares the signatures of defects and all indicators for molecular packing in the ligand. The results align with the High-Resolution Transmission Electron Microscopy (HR-TEM) based analysis. We observe unexpectedly that pure diODA molecules are not prone to form trans-gauche (TG) [CH3-CH2-gauche] end-conformation. At the same time, an intensive absorption structure at the relevant TG position around 760 cm−1 indicates its presence in pure ODA. The spectrum of pure ODA indicates higher energy of the internal kink defects in the vicinity of 1306 cm−1 than pure diODA. In the coordinated phase, both molecules reduce their initial energy. However, the observed better-pronounced red shift of the structures reveals a crystal-like chain arrangement for diODA. Consequently, the HR-TEM images reveal a spontaneous formation of 2D arrangement only for diODA passivated nanoparticles having nearly twice as large gold core separation as the one provided by ODA molecules. The present analysis can be used as a preliminary test in demanding applications where tightly packed coatings of nanoparticles with impenetrable organic passivation are essential.