## 3 Result and Discussion
Figure 8C shows the photographs of the suspension of Fe\({}_{3}\)O\({}_{4}\) NPs (S6, S8 and S9) with an external magnet. The Fe\({}_{3}\)O\({}_{4}\) NPs can be easily dispersed in ethyl alcohol solution to form a black suspension in 1 min, and it could also be drawn from the solution to the sidewall of the vial by an external magnetic field. The aggregate NPs could further re-dispersed in solution by removing the external field and slightly shaking. Figure 3.3 The hysteresis loops at T = 300 K of Fe\({}_{3}\)O\({}_{4}\) (**A**) and corresponding partly amplification images (**B**) with different molar ratio of NaAc (**a**) 10 mmol, (**b**) 40 mmol, (**c**) 60 mmol; (**C**) photograph showing magnetic isolation. Figure 3.4 XRD patterns of Fe\({}_{3}\)O\({}_{4}\) obtained with different molar ratio of FeCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O, A: 0.4 mmol, B: 5 mmol, C: 10 mmol, D: 20 mmol. Morphology, Crystal Structure, and Magnetic Property of Fe\({}_{3}\)O\({}_{4}\) NPs with Different Reaction Time  
The reaction time during these hydrothermal routes is also considered. Six experimental groups under different reaction time (5, 6, 8, 10, 12 and 20 h) are carried out, and these sample are named as S10-S15 respectively. Other synthetic conditions in each of these experimental groups are kept the same as those in the typical experiment, and the specific parameters are presented in Figure 3.5 The mixture is transferred into a Teflon-lined stainless steel autoclave (100 mL capacity) for hydrothermal treatment at 200 \({}^{\circ}\)C. Firstly, the SEM images of S10-S15 are presented in Figure 9A-F, respectively. As shown in these images, with increasing the reaction time, the particle sizes of the Fe\({}_{3}\)O\({}_{4}\) samples are increased obviously with good dispersity. In addition, the TEM images and the corresponding size distribution of the as-prepared Fe\({}_{3}\)O\({}_{4}\) particles (S10-S15) are presented in Figure 3.6 With the prolonging of reaction time, the particle sizes of these Fe\({}_{3}\)O\({}_{4}\) NPs (S10-S15) are increased gradually. As shown in the corresponding particle size distribution map, the average particle sizes of S10-S15 are 184 nm, 214 nm, 217 nm, 222 nm, 213 nm and 241 nm, respectively. The narrow particle size distribution Fe\({}_{3}\)O\({}_{4}\) NPs could also be observed. Moreover, the hollow structure of all these Fe\({}_{3}\)O\({}_{4}\) NPs could be observed clearly. However, with the prolonging of reaction time, as show in the magnified TEM images (Figure 10G-L), the hollow structure is increasingly apparent. These results indicate that the sizes and the hollow structures of as-obtained Fe\({}_{3}\)O\({}_{4}\) NPs are increased with the extension of reaction time, because the time-dependent Ostwald ripening process could form the Fe\({}_{3}\)O\({}_{4}\) NPs with controllable hollow/mesoporous structures. The XRD patterns of Fe\({}_{3}\)O\({}_{4}\) NPs (S10-S15) with different reaction time are presented in Figure 3.7 The XRD is conducted to confirm the crystal structure Fe\({}_{3}\)O\({}_{4}\) NPs. As shown in Figure 11, these curves show the characteristic peaks of Fe\({}_{3}\)O\({}_{4}\) NPs, and only these peaks of Fe\({}_{3}\)O\({}_{4}\) NPs are observed, it indicates that the Fe\({}_{3}\)O\({}_{4}\) NPs show good crystallinity with inverse spinel phase. Moreover, the intensities of the peaks in all these curves are almost the same. It indicated that the crystal structure could not influence by the reaction time effectively. The magnetic property of these Fe\({}_{3}\)O\({}_{4}\) NPs (S10-S15) are also investigated by VSM technique. The specific results are shown in Figure 3.8 The magnetic saturation \(M\)s values of these as-obtained Fe\({}_{3}\)O\({}_{4}\) NPs in Figure 12A (a: 5 h (S10), b: 10 h (S13), c: 20 h (S15)) are 71.42, 62.5 and 57.78 emu\(\cdot\)g\({}^{-1}\), respectively.