Acetate-Induced Disassembly of Spherical Iron Oxide Nanoparticle Clusters into Monodispersed Core–Shell Structures upon Nanoemulsion Fusion

Abstract

It has been long known that the physical encapsulation of oleic acid-capped iron oxide nanoparticles (OA–IONPs) with the cetyltrimethylammonium (CTA+) surfactant induces the formation of spherical iron oxide nanoparticle clusters (IONPCs). However, the behavior and functional properties of IONPCs in chemical reactions have been largely neglected and are still not well-understood. Herein, we report an unconventional ligand-exchange function of IONPCs activated when dispersed in an ethyl acetate/acetate buffer system. The ligand exchange can successfully transform hydrophobic OA–IONP building blocks of IONPCs into highly hydrophilic, acetate-capped iron oxide nanoparticles (Ac–IONPs). More importantly, we demonstrate that the addition of silica precursors (tetraethyl orthosilicate and 3-aminopropyltriethoxysilane) to the acetate/oleate ligand-exchange reaction of the IONPs induces the disassembly of the IONPCs into monodispersed iron oxide–acetate–silica core–shell–shell (IONPs@acetate@SiO2) nanoparticles. Our observations evidence that the formation of IONPs@acetate@SiO2 nanoparticles is initiated by a unique micellar fusion mechanism between the Pickering-type emulsions of IONPCs and nanoemulsions of silica precursors formed under ethyl acetate buffered conditions. A dynamic rearrangement of the CTA+–oleate bilayer on the IONPC surfaces is proposed to be responsible for the templating process of the silica shells around the individual IONPs. In comparison to previously reported methods in the literature, our work provides a much more detailed experimental evidence of the silica-coating mechanism in a nanoemulsion system. Overall, ethyl acetate is proven to be a very efficient agent for an effortless preparation of monodispersed IONPs@acetate@SiO2 and hydrophilic Ac–IONPs from IONPCs.

Photoelectrochemically Active N‐Adsorbing Ultrathin TiO2 Layers for Water‐Splitting Applications Prepared by Pyrolysis of Oleic Acid on Iron Oxide Nanoparticle Surfaces under Nitrogen Environment

“…Additionally, Grazing Incident XRD experiments (Gi‐XRD) were performed using an X'pert3 PANalytical diffractometer working with CuKα radiation (45 kV and 40 mA) and an incident angle of ω = 0.3°. Experiments were performed for Fe 3 O 4 /Mica samples (Figure S10, Supporting Information) showing a structure consistent with that of magnetite (ICDD PDF:04‐005‐4319), in agreement with EELS experiments and previous reports …”

“…To further investigate the effect of thermal annealing on the microstructure of the NPs, a transmission electron microscopy (TEM) grid was prepared and annealed in similar conditions as the Fe 3 O 4 /FTO samples. Figure b shows a high‐resolution transmission electron microscopy (HR‐TEM) micrograph of a typical Fe 3 O 4 particle, with well‐resolved atomic configuration and single crystalline structure; further details can be found elsewhere . Figure d shows the morphology changes on the NPs; it can be observed that for the most part, NPs remain well defined and spherical.…”

“…In our previous reports, we have extensively studied the synthesis mechanism and magnetic properties of Fe 3 O 4 NPs, focusing on their role as initial product for the Fe 3 O 4 /SiO x core–shell structure. We have also conclusively shown magnetite phase of the NPs (Fe 3 O 4 ) by both electron energy loss spectroscopy (EELS) and the presence of the characteristic low‐temperature phase transition (Verwey transition).…”

“…Synthesis and Characterization of Fe 3 O 4 Nanoparticles : The synthesis of magnetite nanoparticles follows a well‐known procedure from literature, which was slightly modified in our previous report . In detail, 3.6 g of readily available iron oleate complex and 0.57 g of oleic acid were dissolved in 200 g of 1‐octadecene.…”

Raman spectroscopy to unravel the magnetic properties of iron oxide nanocrystals for bio-related applications

“…Whereas most metal-oxygen lattice vibrations occur below 750 cm À1 , the main vibrations of organic molecules take place above 1000 cm À1 , and therefore, do not interfere with each other. [43][44][45] This implies an arrangement of magnetic phases that will delineate the magnetic behavior. For instance, the presence of magnetic materials with different magnetic orders can establish exchange coupling between two phases, contributing to the effective magnetic anisotropy.…”