Synthesis of functionalized magnetite nanoparticles using only oleic acid and iron (III) acetylacetonate
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The solvothermal method is a way to obtain magnetite nanoparticles with uniformity in shape and size, good dispersibility and functionalized surface. These properties are desirable in many applications in materials science and medical area. Although methods for obtaining nanoparticles with these properties are reported, the most their use several reagents to act as solvent, surfactant and reducing agent, which makes the synthesis process more difficult and expensive. In this paper, we introduced a simple method for the preparation of magnetite nanoparticles using only iron (III) acetylacetonate as metal precursor and oleic acid for all the other functions. This is possible due the constant liberation of the acetylacetone formed in the reaction, which favors the formation of the oleate, the responsible for the conditions required to obtain magnetite nanoparticles smaller than 20 nm. By this method, nanoparticles were obtained with an average size of 17 nm in a narrow size distribution, uniformity of morphology, high crystallinity and functionalized surface. The organic capping layer allows the preparation of stable colloidal solutions in organic solvents and facilitates the posterior use of nanoparticles in films, ferrofluids and nanocomposite preparation.
KeywordsIron oxide Magnetic oxide Functionalized surface Thermal decomposition Colloidal solution Oleic acid-coated nanoparticles
The search for simple and effective methods for the synthesis of the materials with desired and modulating properties is constant. Since the beginning of the studies with nanomaterials, conditions of synthesis with lower cost and enhanced properties are sought. The synthesis via colloidal process is widely studied due to the control of size and shape caused by compounds chemically or physically adsorbed in the surface of particles, besides the possibility of surface functionalization [1, 2].
Magnetite nanoparticles have interesting properties, such as low toxicity, eco-friendliness and superparamagnetism . These properties allow their use as ferrofluids [4, 5], in catalysis [6, 7], as pigments , in nanocomposite preparation [9, 10] and in medical diagnostics  and treatment [12, 13]. Typically, the nanoparticles are synthetized by sol–gel method [14, 15], coprecipitation or precipitation [16, 17], electrodeposition , hydrothermal route , and solvothermal route [20, 21].
In the solvothermal route, the use of an organic solvent is a way to stabilize the nanostructures, because the organic molecules remain bound in crystal surface and cause a steric impediment. In addition, the presence of molecules transfers the solubility to the nanoparticles, resulting in a stable colloidal solution . The use of this synthesis method is interesting for scientific and technological fields, such as in the medical area where the most problems are related with surface coatings [23, 24]. Besides that, most applications of magnetite require particles smaller than 20 nm in a narrow size distribution for the uniform and improved physical and chemical properties .
Many methods are reported for the synthesis of magnetite nanoparticles smaller than 20 nm by solvothermal route. However, these methods using only a metal precursor and another chemical compound for the functions of solvent, reducing agent and stabilizer to obtain nanoparticles smaller than 20 nm, with uniformity of size and shape and functionalized surface are not reported. Most of the related methods use different agents for each function. Sun and Zeng  used phenyl ether, oleic acid, oleylamine and 1-2-hexadecanediol to obtain 4 nm nanoparticles and for obtaining larger particles (12 and 16 nm) they used a seed-mediated growth method. Haddad et al.  and Pereira et al.  used benzyl ether, oleic acid and oleylamine to obtain nanoparticles smaller than 20 nm. Hou et al.  used ethylene glycol, hydrazine and different stabilizers to obtain nanoparticles with average sizes between 8 and 11 nm, depending on the surfactant.
Here we described a simple method to produce magnetite nanoparticles with size smaller than 20 nm, in a narrow size distribution, with acid oleic-coated surface by solvothermal method using Fe(acac)3 as metal precursor and oleic acid for all other functions (solvent, reducing agent and surfactant). A large morphological and chemical characterization by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR) was made to determine the nanoparticles properties.
2 Experimental procedure
2.1 Synthesis of magnetite nanoparticles
A solution 0.3 mol L−1 of iron (III) acetylacetonate [Fe(acac)3] using oleic acid (Synth P.A.) as solvent was prepared in a glass recipient, and transferred to a stainless steel reactor. The system was heated to 300 °C and hold for 24 h under magnetic stirred in this temperature. The vapor formed in the reaction was constantly released and the pressure of the system has not exceeded 1.5 bar. After cooling to room temperature, acetone was added in the same volume as reaction suspension and the precipitate formed was centrifuged twice with acetone to remove the excess oleic acid and other byproducts (3500 rpm, 30 min). The solid obtained was dispersed in organic solvents, such as chloroform and toluene (Synth P.A.), resulting in a magnetic colloidal solution.
2.2 Characterization of magnetite nanoparticles
2.2.1 X-ray diffraction (XRD)
The crystallinity and the average size of nanocrystals were evaluated by XRD. It was used a diffractometer Rigaku MiniFlex 600 equipped with copper anode emitter CuKα radiation (λ = 0.154 nm) operated at 40 kV and 15 mA using a detector D/teX Ultra, using scan speed 3 (deg/min).
2.2.2 Fourier-transform infrared spectroscopy (FTIR)
For the evaluation of oleic acid bonded at nanoparticles surfaces, the solvent was evaporated, and the magnetite powder were analyzed in a spectrometer Agilent Technologies Cary 360 with attenuated total reflection accessory (ATR).
2.2.3 Transmission electron microscopy (TEM)
The structural, morphological and chemical composition by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) were evaluated by transmission electron microscopy using a FEI Titan Cubed Themis, with double correction and voltage acceleration of 200 kV.
3 Results and discussion
Characterization, structural and morphological, indicates the successful in the synthesis methods using only precursor acetylacetonate and oleic acid. Besides that, literature indicates the use of additional components to control the growth and other components to promotes reduction of iron, then obtain controlled narrow size distribution and well crystalline magnetite nanoparticles. The proposed method in this synthesis protocol can be considered as a simple method to obtain controlled nanoparticles. Details about methods were analyzed considering each component as described in next section.
3.1 Rule of chemical components
3.2 Formation equilibrium of the reaction intermediate
3.2.1 Oleic acid as a solvent
Considering the chemical equilibrium and synthesis conditions, it is possible to synthesize shape controlled crystalline magnetite nanoparticles, under adequate conditions. As mentioned in the introduction, the majority synthesis protocol describes a significant complex system using different reducing agents, surfactant and solvents, if compared a synthesis involving only oleic acid and iron acetylacetonate. Also, a quick and indiscriminate release of acetylacetone from reactional system, promotes the rapid conversion of the oleate, resulting in hematite or requires additional chemical components to obtain magnetite. On the other hand, if the reaction was processed in a closed system, with pressures eluted acetylacetone, would lead to the formation of wustite phase (FeO).
3.2.2 Oleic acid as a reducing agent in the formation of the mixed oxide
In the second step, occurs the partial reduction of the Fe3+ ions. This step can occur simultaneously with the first step. As mentioned, the presence of a reducing agent is necessary for the Fe3+ reduction to obtain Fe2+, and the oleic acid can be used to this function, as described by Kwon and collaborators  and after by Kemp and collaborators . The formation of Fe2+ is initiated at approximately 180 °C and completed at 320 °C by two main mechanisms: the oxidative decarboxylation and the 1-octadecene oxidation. The first is based in an oxidative decarboxylation of a metallic carboxylate via homolytic cleavage of the metal-oxygen bond, which forms the reduced metal and the carboxylic radical. This mechanism was evidenced by the detection of by-products of the redox reaction, such as alkanes C8-C12 and alkenes. The other mechanism is the 1-octadecene oxidation and was studied by these groups concluding that this process is also responsible for the formation of Fe2+. In this work, as only oleic acid was used with potential reducing agent, the first mechanism of reduction was presented.
3.2.3 Oleic acid as a surfactant agent
Hence, the nanoparticles obtained under these conditions have characteristics that allow their application without any additional chemical modification. The hydrophobic nature of the nanoparticle surface allows an improvement in the compatibilization between the nanoparticle and organic matrices. Another possibility to be studied is the exchange of surface ligands by hydrophilic groups, which would facilitate dispersion in aqueous media for biological studies and applications.
In this work we presented a simple method for synthesis of magnetite nanoparticles with size smaller than 20 nm using only two reagents: Fe(acac)3 and oleic acid. This was possible due the constant liberation of the acetylacetone formed in the conversion of Fe(acac)3 into iron (III) oleate, which promotes displacement of the equilibrium to obtain greater quantities of the reaction intermediate. Furthermore, iron oleate acts in the reduction of Fe3+ to Fe2+ and in the control of the nucleation and growing of the nanoparticles. The techniques of chemical and morphological characterization showed the uniformity in size and shape of the nanoparticles, high crystallinity, narrow size distribution and good dispersibility in organic solvents, due the presence of organic capping layer in the nanoparticle surface after the synthesis process. These properties obtained by these conditions make possible the use of the nanoparticles in different applications, such as medical area, nanocomposite preparation, film deposition, among others.
The authors thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa e Inovação do Espírito Santo (Fapes) and Federal University of Espírito Santo (Ufes) for the research funding and the scholarships.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
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