Structural and vibrational studies of fatty acids-functionalized iron oxide nanoparticles via alkaline co-precipitation route

The preparation and functionalization of iron oxide magnetic nanoparticles (MNPs) are of great importance in a diverse range of applications. However, their structural and magnetic properties can be influenced by the process of coating making these MNPs undesirable for certain applications. For that reason, this work is focused in the understanding of iron oxide nanoparticles functionalized with fatty functional lauric acid (LA) and oleic acid (OA) organic molecules. The synthesis was carried out using the aqueous alkaline co-precipitation route of Fe⁺² and Fe³⁺ salts. The characterization was done by several spectroscopy techniques. X-Ray Diffraction (XRD) measurements reveal uncoated iron oxide nanoparticles with mean diameter of 11.8 ± 2 nm obtained from Debye-Sherrer formula. The spinel crystal structure and lattice parameter was found to be 8.38 ± 0.01 Å which means that the stoichiometric structure is not affected by the functionalization process. This size decreased at around 9.7 ± 2 and 8.6 ± 2 nm for coated with OA and LA which are in agreement with Transmission Electron Microscopy (TEM) measurements. X-ray Photoelectron Spectroscopy (XPS) measurements determined that iron atoms have valences of +3 and +2, with a total ratio of iron atoms Fe⁺³: Fe⁺² of 2:1 which confirmed the stoichiometric relation used for magnetite (Fe₃O₄). Fourier Transform Infrared Spectroscopy (FTIR) measurements show that LA and OA molecules are chemisorbed onto MNPs surface ascribed to bridging bidentate interaction. The thermal stability of the functional groups anchored to magnetite surface was studied by thermogravimetric analysis (TGA) at certain range of temperatures. From TGA measurements we calculate the number of functionalized molecules anchored to the surface of Fe₃O₄ nanoparticles. Micro-Raman spectroscopy was introduced to study the stabilization of organic molecules on MNPs surface.