# Solvothermal Synthesis of Size-Controlled Monodispersed Superparamagnetic Iron Oxide Nanoparticles
## 3 Results and Discussion
### Formation Mechanism of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles with Tunable Sizes  
As a typical solvothermal reaction, the formation process of magnetic nanoparticles includes two main steps: nucleation and ripening growth [29]. In this case, the ethylene glycol and diethylene glycol act as the solvent and reductant, respectively, the iron (m) chloride as the iron source, and the sodium acetate (NaAC) provides the hydroxyl-ion and maintains the acid-base equilibrium of the reaction system. The PVP and Na\({}_{3}\)Cit are structure guide agents.  
As we previously reported [30], during the reaction process, parts of the hydrolyzed Fe\({}^{3+}\) ions reduce to Fe(OH)\({}_{2}\), and combine with the remaining Fe(OH)\({}_{3}\) to finally form Fe\({}_{3}\)O\({}_{4}\) nanoparticles. As is shown in Figure 5a, the small-molecule citrate groups that have adhered on the magnetic nanocrystals act as both resistance and bridging agents, which prevent the further growth of nanocrystals and agglomerate the neighboring nanocrystals into nanoclusters with low remanent magnetization. As for the PVP long-chain molecules shown in Figure 5b, in the initial process, the macromolecule stabilizers mostly act as bridging agents to guide the neighboring nanocrystals to self-assemble into larger nanoparticles with high saturation magnetization. After the larger nanoparticles are fully covered by the PVP long-chain molecules, the PVP long-chain molecules act as space blocks and prevent the further growth of the nanoparticles. In order to obtain superparamagnetic nanoparticles with high saturation magnetization, the binary Na\({}_{3}\)Cit/PVP is added into the reaction system as a structure guide agent.  
In addition, the EG/DEG binary solvent system also plays a key role in controlling the size of the Fe\({}_{3}\)O\({}_{4}\) nanoparticles [31]. From the above-mentioned analysis, the formation of magnetic nanoparticles is closely related to the aggregation process. When pure EG is used, the precursor iron alkoxides transform into crystal nuclei and further agglomerate rapidly into large nanoparticles due to their high surface energy. When DEG is added to form a binary solvent with EG, the size of Fe\({}_{3}\)O\({}_{4}\) nanoparticles decreases as the \(V_{\text{EG}}/V_{\text{DEG}}\) ratio decreases. DEG forms a more stable coordination iron alkoxide with Fe ions than EG, so the growth rate of Fe\({}_{3}\)O\({}_{4}\) grains will slow down, consequently decreasing the size. Furthermore, when EG/DEG coordinates with FeCl\({}_{3}\) to form an iron alko  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Saturation Magnetization (\(M_{\text{s}}\))** & **Remanent Magnetization (\(M_{\text{r}}\))** \\  & **emu/g** & **emu/g** \\ \hline a & 72.14 & 3.34 \\ b & 75.94 & 3.97 \\ c & 80.28 & 3.26 \\ d & 85.41 & 4.28 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Magnetic parameters of the prepared nanoparticles.  
Figure 5: Schematic illustration of the process of obtaining magnetite nanospheres: (**a**) structure guided by sodium citrate (Na\({}_{3}\)Cit) and (**b**) structure guided by polyvinyl pyrrolidone (PVP).  
from the reaction. The accumulation of HCl will inhibit further iron alkoxide formation. When NaAc is introduced, it can react with HCl to reduce the concentration of H\({}^{+}\), and allow the coordination reaction to be completed [32; 33].