The structural information of synthetized nanoparticles indicates that was obtained magnetite in a pure phase, with high crystallinity, according XRD pattern (Fig. 1). The crystallite size was 17 nm, calculated by the Scherrer equation .
TEM images (Fig. 2a) shows the uniformity of size and shape of synthesized nanoparticles and the dispersibility. The graph of size distribution (Fig. 2b) shows that the average size is 16.84 nm, the same value estimated from XRD by Scherrer equations, as mentioned. Moreover, from the image and standard deviation it is observed a narrow size distribution. Therefore, the nanoparticles dispersed in solution has potential to the magnetite applications where narrow size distribution are required.
Energy dispersive X-ray spectroscopy in scanning transmission electron microscope (EDX-STEM) was used to analyze sample composition and dispersion of iron and oxygen in the sample. The nanoparticles with high crystallinity have iron and oxygen uniformly distributed, as shows in Fig. 3.
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.
Rule of chemical components
Uniformity in size and shape of nanoparticles are associated with solvothermal synthesis method and the ligand agent used to control the growth. These effects can be considered separated or together . From approach used in this work, during reaction the nanoparticles are functionalized with organic molecules of solvent, that act as ligand over nanoparticle surface, besides the presence of the oleic acid being essential for the process of partial reduction of the Fe3+ ion to Fe2+, to compose particles of magnetite with a correct relation between these ions. So, the conversion of the Fe(acac)3 into Fe3O4 nanoparticles can be divided in five steps, as will be described in the following sections and schematically presented in Fig. 4.
Formation equilibrium of the reaction intermediate
Oleic acid as a solvent
In the first step, the precursor Fe(acac)3 is solubilized in oleic acid and reacts with it. During this process, an exchange occurs between the acetylacetonate ion and the oleate ion formed by the deprotonation of the oleic acid. The Fe(acac)3 is converted to iron oleate, soluble in the reaction media, and the acetylacetonate ion is then converted to acetylacetone (Hacac). In fact, the system establishes a chemical equilibrium between oleate and acetylacetonate as described by Eq. (1). In a closed system, under pressure, the equilibrium will be dislocated to left, once that acetylacetonate is a bidentate ligand resulting in six members chelate ring while oleate is a monodentate ligand. So, it is important to release the gas formed in the reaction to promote the formation of the iron oleate in bigger quantity. For this, in reactional conditions, the pressure must be kept lower than 2 bar to eliminate acetylacetone. Besides that, the oleic acid is present in large quantities because it is the solvent, which does not limit the reaction.
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).
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.
Oleic acid as a surfactant agent
The third step is the formation of the Fe3O4 clusters by the thermal decomposition of the iron oleate. The clusters are aggregated until they have enough ΔG, what makes the process irreversible. The clusters that do not reach this level are solubilized again. The formation and the growing of the nanoparticles are the fourth and the fifth steps, respectively. In the fourth step, the oleic acid acts as a surfactant, once that the compound prevents the approximation of the other particles, which would lead to the formation of aggregates. Figure 5 shows the FTIR spectra of the nanoparticles synthetized. As observed, similar vibrational modes are present in oleic acid and in the nanoparticles sample, indicating that the oleate group keeps bounded on the nanoparticle surface after synthesis. Ligands over nanoparticles are bonded from carboxylate group as evidenced: in oleic acid the presence of carboxylic bonds (1722 cm−1 region) is intense and it is reduced when compared with spectra from nanoparticle. Furthermore, a dislocation of vibration region in the surface (1710 cm−1) indicating the coordination of carboxylic group on magnetite surface. The vibration of 580 cm−1 region refers to the Fe-O bond in the crystal interior . Therefore, the oleate presence is the factor responsible for stabilization of nanoparticles in their synthesis to obtain uniformity and good dispersibility, as observed in Fig. 2. In addition, the presence of the oleic acid transfers the solubility to the nanoparticles, which results in colloidal solutions after washed and dispersion in organic solvents such as toluene and chloroform.
The action of the oleic acid as surfactant is also evidenced in the analysis of crystallinity and morphology. These details were analyzed by scanning transmission electron microscope (STEM). Figure 6 presents a high magnification of a nanoparticle with typical facets in the crystal surface, as that the energy in facets promotes the crystal shape. The oleate ligand promotes the control in the growth process of the size and shape. Different ligands can be used to control the growth to conduct different shapes. When the same ligand and solvent are present, the ligand desorption and adsorption rate will be low, due the excess of oleic acid, once that it is the solvent. During the reaction, an exchange occurs between the acetylacetonate ion and the oleate ion formed by the deprotonation of the oleic acid. The acetylacetonate ion is then converted to acetylacetone and leaves the system in gaseous form. Thus, the exchange of the acetylacetonate by the oleate is favored and the impediment to the action of the oleate ion is reduced, facilitating its action in stabilizing the shape and size of the magnetite. Therefore, the ligand present on the surface controls the crystal growth in low rate resulting in high crystalline nanocrystals and to oleic acid with facets almost regular to resulting in a faceted tetra dodecahedron .
Indexation of crystal lattice were realized from a HRTEM images, as presented in Fig. 7. In the images, a nanocrystal oriented in zone axis <5, − 1, 2>, determined from HRTEM Fast Fourier Transform (FFT) and index using CIF of magnetite, with spots associated with crystalline plane, as indicated in Fig. 7b associated CIF (crystallographic Information File to Fe3O4 – PDF #19-629). HRTEM and representation of lattice from CIF, are presented in Fig. 7c and d, match HRTEM image with atomic positions in represented crystalline lattice in zone axis, as indicated in the Figure.
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.