## 2 Structures, Synthesis, and Magnetic Properties of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles
### Fe\({}_{3}\)O\({}_{4}\) Spherical Nanoparticles (SNPs)
Iron(III) oleate complexes were first prepared from iron(III) chlorides and sodium oleate before refluxing with oleic acid at 320 \({}^{\circ}\)C in different high-boiling point organic solvents to obtain SNPs of different sizes. In addition to the advantages of highly uniform size and morphology, NPs prepared using thermal decomposition usually have hydrophobic surfaces that require surface modifications for applications requiring water-soluble nanosubstances, such as biomedical applications or catalysis in aqueous media. [PERSON] and coworkers approached this problem by applying surfactant-free thermal decomposition of the inexpensive precursor FeCl\({}_{3}\) 6H\({}_{2}\)O in a strongly polar 2-pyrrolidone solvent [60]. This recipe allowed obtaining SNPs in a wider range of sizes from 4 to 60 nm by simply controlling the reaction time. Thus, thermal decomposition is a useful synthesis method producing high-quality Fe\({}_{3}\)O\({}_{4}\) SNPs with good uniformity in size and morphology. Figure 4: (a) Schematic synthesis of Fe\({}_{3}\)O\({}_{4}\) NPs with sizes of 7, 8, 9, and 10 nm. Selected TEM images of Fe\({}_{3}\)O\({}_{4}\) NPs with sizes of (**b**) 7 \(\pm\) 0.5 nm and (**c**) 10 \(\pm\) 0.8 nm. Scale bar 20 nm. Reproduced with permission from ref. [57]. Copyright 2009 American Chemical Society. (d) TEM image of spherical Fe\({}_{3}\)O\({}_{4}\) NPs with a size of 200 nm. Reproduced with permission from ref. [48]. Copyright 2005 John Wiley and Sons. In addition to thermal decomposition, solvothermal methods are also used to synthesize spherical superparamagnetic NPs in this size range below 25 nm at lower temperatures than thermal decomposition methods. [PERSON] et al. prepared Fe\({}_{3}\)O\({}_{4}\) SNPs with sizes ranging from 6.6 to 17.8 nm through the solvothermal mixing of FeCl\({}_{2}\) and FeCl\({}_{3}\) precursors in solvent mixtures using different ratios of diethylene glycol and N-methyl diethanolamine [63]. Fe(acac)\({}_{3}\) has also been used as a precursor in the solvothermal synthesis of 8 and 11 nm SNPs by employing different surfactants [59]. In addition, direct preparation of SNPs with hydrophilic surfaces has also been developed [18; 66]. However, the size distribution and uniformity of the spherical geometry are not as good as SNPs prepared using thermal decomposition at high temperatures. For example, [PERSON] and coworkers synthesized 15 to 31 nm NPs by oxidizing FeCl\({}_{2}\)-4H\({}_{2}\)O in a basic aqueous solution [61]. These NPs exhibit the gradual transition from ferromagnetic to superparamagnetic when decreasing in size. [PERSON] and colleagues developed a facile route for the large-scale sonochemical synthesis of 11 nm Fe\({}_{3}\)O\({}_{4}\) SNPs with M\({}_{5}\) = 80 emu/g using inexpensive and nontoxic reactants, such as FeCl\({}_{2}\), FeCl\({}_{3}\), and H\({}_{2}\)O [64]. Thus, among the various synthesis methods, thermal decomposition is the most effective for the synthesis of Fe\({}_{3}\)O\({}_{4}\) nanospheres with a size less than 25 nm. The size and uniformity of NPs can be effectively controlled in this method by modifying the amounts of surfactants, heating protocol, reducing agents, or solvent compositions. The main drawback of this method is the technical requirements for maintaining high temperatures and fast heating rates. Spherical particles in the size range from 25 to even a few hundred nanometers have been synthesized successfully using various recipes and solvothermal methods [42; 43; 48]. In this size range, spherical Fe\({}_{3}\)O\({}_{4}\) NPs exhibit diverse magnetic behaviors, either superparamagnetic or ferrimagnetic (Table 2). In contrast to single-crystalline SNPs with sizes below 25 nm, magnetite particles in the larger size range (diameter > 25 nm) are usually polycrystalline NPs. These NPs are composed of numerous primary nanocrystals aggregated to form secondary structure NPs.