Preparation of Monodisperse Iron Oxide Nanoparticles via the Synthesis and Decomposition of Iron Fatty Acid Complexes
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Iron fatty acid complexes (IFACs) are prepared via the dissolution of porous hematite powder in hot unsaturated fatty acid. The IFACs are then decomposed in five different organic solvents under reflux conditions in the presence of the respective fatty acid. The XRD analysis results indicate that the resulting NPs comprise a mixture of wustite, magnetite, and maghemite phases. The solvents with a higher boiling point prompt the formation of larger NPs containing wustite as the major component, while those with a lower boiling point produce smaller NPs with maghemite as the major component. In addition, it is shown that unstable NPs with a mixed wustite–magnetite composition can be oxidized to pure maghemite by extending the reaction time or using an oxidizing agent.
KeywordsIron fatty acid complexes Iron oxide Magnetic nanoparticles Monodisperse
Many studies have shown that long-chain carboxylates, or soaps, of polyvalent metal ions are effective precursors for the large-scale synthesis of metal oxide or metal nanoparticles (NP) via a thermal decomposition process utilizing organic solvents [1, 2, 3, 4, 5, 6, 7, 8]. Of these various soaps, those containing iron have attracted particular attention due to the magnetic properties bestowed in the products. For example, iron (ferrous or ferric) fatty acid complexes (IFACs) have been used to synthesize monodisperse iron oxides and iron NPs at different decomposition temperatures using various types of solvent [9, 10, 11, 12, 13, 14, 15] and without the presence of any form of solvent [16, 17, 18], respectively. The NPs produced using these techniques have a uniform size and are widely applied in a broad range of fields such as magnetic storage media, drug carriers, magnetic resonance imaging (MRI) agents, and hyperthermia [19, 20]. Moreover, the size of the NPs can easily be tuned by adjusting the reaction parameters, e.g. the reaction temperature (as determined by the solvent b.p.) or the concentration of the free fatty acid within the reactant. Polyvalent metal soaps are generally prepared via one of three different routes, namely, (1) metathesis of the corresponding soap of Na or K with metal salts (such as chloride or acetate) in aqueous or nonaqueous polar solvents; (2) fusion (or dissolution) of metal oxides, hydroxides, oxy-hydroxides, hydroxycarbonates, or carbonates in hot fatty acids; and (3) direct reaction of metals with hot fatty acids . The metal soap precursors used for the synthesis of iron oxide or iron NPs are generally prepared using the route 1 method [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. However, it has also been shown that the route 2 method is well suited to the preparation of soaps containing elements such as alkaline, alkaline earth metal, Zn, Cd, and so on [22, 23, 24, 25, 26]. Jana et al.  demonstrated the use of iron oxides as precursors in the synthesis of metal oxide NPs. However, they did not provide any experimental details due to the poor reproducibility of the resulting precipitates. Yu et al.  proposed a one-pot process for the preparation of monodisperse magnetite NPs, in which an amorphous iron oxyhydroxide, known as 2-line ferrihydrite, is fully dissolved in a mixture of oleic acid and 1-octadecene solvent at a temperature of 320 °C. In a recent study, the current group demonstrated the feasibility of dissolving hematite in hot long-chain fatty acids such as oleic acid or linoleic acid to produce IFACs for the synthesis of monodisperse magnetic iron oxide NPs via a controlled dissolution–recrystallization process . However, the preparation of IFACs using the route 2 method requires an elevated reaction temperature, which causes the IFACs to decompose virtually as soon as they are formed. Therefore, in synthesizing IFACs using this route, the challenge lies in finding a temperature that is sufficiently high to ensure the complete dissolution of the hematite powder in the hot fatty acid while simultaneously avoiding the decomposition of the resulting IFACs. Assuming that such a temperature can be found, the possibility then exists to synthesize IFACs in a cheap and efficient manner using readily available hematite powder. Furthermore, having prepared the IFACs, iron oxide NPs of a specified size can easily be synthesized via a decomposition process performed using a solvent with an appropriate boiling point.
It is well known that the rate of dissolution of a solid in a liquid is proportional to its surface area. It is also known that the topotactic goethite-to-hematite transformation generates porous hematite . The voids within this porous hematite provide an effective increase in the surface area, and thus hematite produced in this way is expected to have good dissolution properties. This study demonstrates the feasibility of preparing sodium-free IFACs via the dissolution of porous hematite in hot oleic or linoleic acid at a temperature just below the decomposition temperature of the corresponding acid. It is important in view of that the presence trace amount of sodium salts will affect the morphology of iron oxide nanoparticles obtained . Moreover, monodisperse magnetic iron oxide NPs of various dimensions are then synthesized by decomposing the IFACs in a variety of high b.p organic solvents.
The synthesis and decomposition experiments were performed using the following chemicals: goethite (α-FeO(OH); 99.9%, Strem), oleic acid (OA, 90%, Showa), oleyl alcohol (85%, Alfa Aesar), benzyl ether (99%, Acros), 1-octadecene (ODE, 90%, Acros), tri-n-octylamine (90%, Kanto), 1-eicosane (99%, TCI), and trimethylamine N-oxide dehydrated (98%, Alfa Aesar). Note that all of the chemicals were used in an as-received condition without further purification.
Synthesis of IFACs from Porous Hematite Powder
The porous hematite was prepared via the thermal decomposition of commercially available goethite powder at a temperature of 300 °C for 2 h. In the IFAC synthesis process, porous hematite was mixed with oleic or linoleic acid in a molar ratio of 1:9 and was then loaded into a three-necked round-bottom flask and heated to a temperature of 290 °C at a rate of ~15 °C/min under a flow of Argon. During the reaction process, the reactant bubbled vigorously and spilled out of the flask as a result of the generation of water. Moreover, the original red color of the hematite gradually changed to brown as the reaction process proceeded. The change in color was consistent with the disappearance of the hematite diffraction peaks observed in the sampled aliquot. After 4 h, XRD and IR analyses showed that the hematite was completely dissolved within the fatty acid. The resulting brown grease-like product was allowed to cool to room temperature and was then diluted with a mixture of hexane and acetone (1:20) and centrifuged at a speed of 5,000 rpm. Finally, the precipitate was washed with acetone and dried at a temperature of 60 °C. The products were then characterized by XRD, FTIR, EA, DSC, TGA, XPS, and SQUID, respectively.
Thermal Decomposition of IFACs in Various Solvents
Reaction parameters used in synthesis of monodisperse iron oxide NPs
NP diameter (nm)
4.5 ± 0.4
8.1 ± 0.5
10.1 ± 1.0
13.1 ± 0.8
15.3 ± 0.9
20.4 ± 1.1
Oxidation of NPs from FeO/Fe3O4to γ-Fe2O3
The NP products containing a mixture of wustite (FeO) and inverse spinel (Fe3O4or γ-Fe2O3) were transformed to pure γ-Fe2O3via treatment with an organic oxidizing agent. In the oxidation process, trimethylamine N-oxide dehydrate was added to the reactant in a flask, and the mixture was then heated at a temperature of 140 °C for 2 h under a flow of argon.
The phases of the various products were characterized using an X-ray powder diffractometer (Shimadzu XRD-6000) with Cu Kα radiation. The TEM samples were observed using a transmission electron microscope (JEOL JEM 1200EX) with an accelerating voltage of 80 kV and a high-resolution transmission electron microscope (Philips Tecnai G2 F20 or JEOL JEM 2010) with an accelerating voltage of 200 kV. SEM micrographs of the hematite powder were obtained using an FE-SEM (JEOL JSM 7000). The surface area of the hematite powder was determined in accordance with the BET method at a temperature of 77 K using a Micromeritics ASAP 2010 (Accelerated Surface Area and Porosimetry) system. The IR absorbance was analyzed using a Fourier transform infrared spectrometer (FTIR, Nicolet 5700) with a resolution of 4 cm−1. Finally, the amount of iron oxide in the dried precipitates was measured using a thermogravimetric analyzer (TGA) under a constant flow of nitrogen gas.
Results and Discussion
Dissolution of Hematite in Oleic Acid
Characterization of Iron–Oleate Complexes
Synthesis of Monodisperse Iron Oxide NPs with Tunable Dimensions
Phase Composition of Monodisperse Iron Oxide NPs
On the basis of the evidence presented above, it is concluded that the as-prepared NP samples with larger size such as run 6 has a mixed structure comprising both magnetite and wustite. However, with an increasing oxidation time, the wustite transforms initially to magnetite, and then finally to maghemite. For smaller NPs obtained such as run 1, the smaller size is beneficial to the wustite–magnetite–maghemite conversion process, the as-formed wustite change efficiently to magnetite during the work-up process.
A simple method has been presented for the synthesis of sodium-free iron(III) oleate complexes via the direct dissolution of porous hematite powder in unsaturated fatty acids (oleic or linoleic). It has been shown that these complexes represent suitable precursors for the subsequent preparation of iron oxide NPs using an organic solvent under reflux conditions and the corresponding fatty acid as a surfactant. The results have shown that the size of the NP precipitates increases with an increasing reaction temperature (as governed by the boiling point of the organic solvent) and an increasing concentration of free acid in the reactant. Moreover, it has been shown that the larger NPs are comprised primarily of wustite. However, these NPs undergo a wustite–magnetite–maghemite conversion following oxidation. By contrast, the smaller NPs are comprised principally of maghemite, indicating that a smaller NP dimension is beneficial in prompting the wustite–magnetite–maghemite transformation process. Finally, it has been shown that the as-prepared NP products with a mixed wustite–magnetite composition can be transformed to pure γ-Fe2O3products with a super-paramagnetic behavior via the use of an organic oxidizing agent or following prolonged exposure to air.
The authors gratefully acknowledge the financial support provided to this study by the National Science Council of Taiwan.
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