## 3 Result and Discussion
### Formation Mechanism of Fe\({}_{3}\)O\({}_{4}\) NPs  
These hollow/mesoporous Fe\({}_{3}\)O\({}_{4}\) nanostructures are synthesized by solvothermal method. In the synthetic process, it is usually proposed that the EG acts both as a high-boiling-point solvent and a reducing agent, and sodium acetate provides as a structure-directive agent. Specifically, EG as a reducing agent is oxidized under heating to generate glycolaldehyde, as shown in formula (1). Then, glycolaldehyde can reduce Fe\({}^{3+}\) to Fe\({}^{2+}\), and the glyoxal is generated, as shown in formula (2). Moreover, with the increasing of temperature gradually, the Fe[Ac]\({}_{3}\) and Fe[Ac]\({}_{2}\) are formed by the reaction of Ac\({}^{-}\) and Fe\({}^{3+}\) (Fe\({}^{2+}\)), such as the formulas (3) and (4). As the temperature rising, the Fe(OH)\({}_{3}\) and Fe(OH)\({}_{2}\) are formed by the hydrolytic and alcoholysis process of Fe[Ac]\({}_{3}\) and Fe[Ac]\({}_{2}\). These processes are shown in formulas (5)-(8). Finally, the followed Fe\({}_{3}\)O\({}_{4}\) primary NPs are generated via dehydration of these hydrolytic products. \[2 HOCH_{2}CH_{2}OH+O_{2}\xrightarrow{\Delta}2 HOCH_{2}CHO+2H_{2}O \tag{1}\]  
\[HOCH_{2}CHO+2F^{3+}\to OHC-CHO+2H^{+}+2F\mathrm{e}^{2+} \tag{2}\]  
\[Fe^{3+}+3 Ac^{-}\to Fe[Ac]_{3} \tag{3}\]  
\[Fe^{2+}+2 Ac^{-}\to Fe[Ac]_{2} \tag{4}\]  
\[Fe[Ac]_{3}+3H_{2}O\xrightarrow{\Delta}Fe(OH)_{3}+3 HAc \tag{5}\]  
\[Fe[Ac]_{2}+2H_{2}O\xrightarrow{\Delta}Fe(OH)_{2}+2 HAc \tag{6}\]  
\[Fe[Ac]_{3}+3 ROH\xrightarrow{\Delta}Fe(OH)_{3}+3 RAc \tag{7}\]  
Figure 12: The hysteresis loops at T = 300 K of Fe\({}_{3}\)O\({}_{4}\) (**A**) and corresponding partly amplification images (**B**) with different reaction time (**a**) 5 h, (**b**) 10 h, (**c**) 20 h; (**C**) photograph showing magnetic isolation. \[Fe{[Ac]}_{2}+2 ROH\xrightarrow{\Delta}Fe{(OH)}_{2}+2 RAc \tag{8}\]  
\[2 Fe{(OH)}_{3}+Fe{(OH)}_{2}\to Fe_{3}O_{4}+4H_{2}O \tag{9}\]  
Afterwards, in order to reducing the surface energy, these Fe\({}_{3}\)O\({}_{4}\) primary particles could aggregate with magnetic dipole alignment by oriented attachment mechanis [26]. The schematic diagram is presented in \[HOCH_{2}CHO+2F^{3+}\to OHC-CHO+2H^{+}+2F\mathrm{e}^{2+} \tag{2}\]0 However, the formation of the hollow structures without using any templates in this formation process need be further understood. As the reaction goes on, the formation mechanism of these hollow Fe\({}_{3}\)O\({}_{4}\) NPs can further explained by Ostwald ripening mechanism. The inner primary particles could be dissolved and transformed into outer shell. In other words, reducing the overall surface energy provides the driving force for inner particles ripening. As a result, the large-scale synthesis of Fe\({}_{3}\)O\({}_{4}\) nanospheres with hollow structures are formed finally. Moreover, during the formation mechanism of these Fe\({}_{3}\)O\({}_{4}\) NPs, the Ostwald ripening mechanism play an important role in formation of hollow structure. However, the influence of anhydrous NaAc to the performance of Fe\({}_{3}\)O\({}_{4}\) NPs could also be taken into account. As shown in Table 1, the final Fe\({}_{3}\)O\({}_{4}\) NPs with mesoporous structure or amorphous structure are preferred to form, when the ratio of FeCl\({}_{3}\)-6H\({}_{2}\)O and anhydrous NaAc is greater than 1:8. It means that the less anhydrous NaAc is facilitated to form the mesoporous structure. With the increasing of anhydrous NaAc, the Fe\({}_{3}\)O\({}_{4}\) NPs with hollow structure are easily formed. In these reactions, we infer that all these Fe\({}^{3+}\) are participated into the reaction, about 1/3 of Fe\({}^{3+}\) could be reduced into Fe\({}^{2+}\). Theoretically, the three times amount of Ac\({}^{-}\) could coordinate with Fe\({}^{3+}\), and two times amount of Ac\({}^{-}\) could coordinate with Fe\({}^{2+}\).