## 2 Structures, Synthesis, and Magnetic Properties of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles
### Fe\({}_{3}\)O\({}_{4}\) Spherical Nanoparticles (SNPs)
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Diameter (nm)** & **Precursor** & **Method** & **Size Control Factor** & **M\({}_{\text{S}}\) (emu/g) a** & **Ref.** \\ \hline
4 (seeds), 8, 12, 16 & Fe(acac)\({}_{3}\) & Thermal decomposition & Seed-mediated growth & 82 (for 16 nm) & [54] \\ \hline
6 (seeds), 8, 10, 16 & Fe(acac)\({}_{3}\) & Thermal decomposition & Temperature, seed-mediated growth & 83 (for 16 nm) & [55] \\ \hline
5, 9, 12, 16, 22 & Fe(oleate)\({}_{3}\) & Thermal decomposition & Different organic solvents or concentration of surfactant & Unknown & [56] \\ \hline
7, 8, 9, 10 & Fe(acac)\({}_{3}\) & Thermal decomposition & Ratio OAm/solvent. (OAm: surfactant and reducing agent) & 76, 77, 79, 80 & [57] \\ \hline
8, 11, 15, 18 & Fe(acac)\({}_{3}\) & Thermal decomposition & Amount of surfactant & \(\approx\)65–75 & [58] \\ \hline
8, 11 & Fe(acac)\({}_{3}\) & Solvothermal & Different surfactants & 73.1, 109.4 (emu/g Fe) & [59] \\ \hline
4, 12 & FeCl\({}_{3}\cdot\)6H\({}_{2}\)O & Solvothermal & Reaction time & 3, 59 **84 (fermi)** & [60] \\ \hline
15.4, 16.7, 22.4, 31.1 & FeCl\({}_{3}\cdot\)4H\({}_{2}\)O & Hydrothermal & Concentration of reactants and solvent composition & 53.3, 65.1, 81.2, 97.4 & [61] \\ \hline
4.2, 7.4, 8.1, 17, 45 & Fe(acac)\({}_{3}\) & Thermal decomposition & Reducing agent, surfactant & 75, 70, 65, 82, 92 & [62] \\ \hline
6.6, 11.6, 17.8 & FeCl\({}_{2}\cdot\)4H\({}_{2}\)O & Solvothermal & Solvent composition & 71, 77, 83 & [63] \\ \hline
11 & FeCl\({}_{2}\cdot\)4H\({}_{2}\)O & & & & \\ FeCl\({}_{3}\cdot\)6H\({}_{2}\)O & Sonochemistry & None & 80 & [64] \\ \hline \hline \end{tabular}  
* Saturation magnetization (M\({}_{\text{S}}\)) value at 300 K. **Bold** indicates that the size of NPs (diameter) exceeds 25 nm.  
\end{table} 
Table 1: Syntheses and magnetic properties of spherical Fe\({}_{3}\)O\({}_{4}\) NPs with a size <25 nm in a superparamagnetic regime at room temperature. prepared by refluxing at a higher temperature of 300 \({}^{\circ}\)C in the higher-boiling point benzyl ether solvent (boiling point: 296 \({}^{\circ}\)C) [55]. In this method, 1,2-hydrocarbon diols served as excellent reducing agents for the synthesis of high-quality Fe\({}_{3}\)O\({}_{4}\) SNPs with good yields for use as seed NPs. Interestingly, the seed-mediated growth processes normally require long-chain mono-alcohols to grow larger particles rather than diol derivatives. Later, [PERSON] et al. [57] reported a more convenient one-pot method to synthesize Fe\({}_{3}\)O\({}_{4}\) nanospheres with sizes of 7, 8, 9, and 10 nm by simply controlling the ratio of OAm and benzyl ether, as shown in Figure 4a. Here, oleylamine served as a multifunctional reagent: a strong reductive agent and an effective capping agent [57; 65]. The obtained NPs exhibited a homogeneous size and uniform spherical morphology, as verified using transmission electron microscopy (TEM). Selected TEM images of Fe\({}_{3}\)O\({}_{4}\) SNPs with sizes of 7 and 10 nm are presented in Figure 4b,c, respectively. The heating procedure is very important in the thermal decomposition of Fe(acac)\({}_{3}\), which usually requires maintenance at 200 \({}^{\circ}\)C for nucleation prior to refluxing or a quick ramping rate of 20 \({}^{\circ}\)C/min. Therefore, precisely controlling the temperature of the reaction and a powerful heating process represent a technical challenge for large-scale production. Later, [PERSON] and coworkers successfully lowered the reflux temperature to 200 \({}^{\circ}\)C using alkaline metal reagents to assist with the reduction of iron precursors [58]. In the presence of Mg(acetate)\({}_{2}\), monodisperse Fe\({}_{3}\)O\({}_{4}\) SNPs with sizes of 8, 11, 15, and 18 nm were prepared. [PERSON] et al. reported an ultralarge-scale synthesis with 40 g of products obtained per single reaction using the inexpensive and environmentally friendly starting material iron(III) chloride [56]. Nanospheres with sizes of 5, 9, 12, 16, and 22 nm were prepared with a uniform size (size variation < 4.1%) and homogeneous morphology.