Adsorption of oleic acid on magnetite facets

M Creutzburg and M Konuk and S Tober and S Chung and B Arndt and H Noei and RH Meissner and A Stierle, COMMUNICATIONS CHEMISTRY, 5, 134 (2022).

DOI: 10.1038/s42004-022-00741-0

The microscopic understanding of the atomic structure and interaction at carboxylic acid/oxide interfaces is an important step towards tailoring the mechanical properties of nanocomposite materials assembled from metal oxide nanoparticles functionalized by organic molecules. We have studied the adsorption of oleic acid (C17H33COOH) on the most prominent magnetite (001) and (111) crystal facets at room temperature using low energy electron diffraction, surface X-ray diffraction and infrared vibrational spectroscopy complemented with molecular dynamics simulations used to infer specific hydrogen bonding motifs between oleic acid and oleate. Our experimental and theoretical results give evidence that oleic acid adsorbs dissociatively on both facets at lower coverages. At higher coverages, the more pronounced molecular adsorption causes hydrogen bond formation between the carboxylic groups, leading to a more upright orientation of the molecules on the (111) facet in conjunction with the formation of a denser layer, as compared to the (001) facet. This is evidenced by the C=O double bond infrared line shape, in depth molecular dynamics bond angle orientation and hydrogen bond analysis, as well as X-ray reflectivity layer electron density profile determination. Such a higher density can explain the higher mechanical strength of nanocomposite materials based on magnetite nanoparticles with larger (111) facets. Metal oxide nanoparticles functionalized by organic molecules are important building blocks for nanocomposites. Here, the authors study the adsorption of oleic acid on magnetite (001) and (111) surfaces experimentally by SXRD, LEED, FT- IRRAS and XRR, and employ a molecular dynamics force field specifically developed for magnetite/organic interfaces to determine molecular adsorption geometries, coverages, layer thicknesses and adsorption- induced structural changes in the substrate.

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