## 3 Result and Discussion  
The influences of reaction time, molar ratio of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O and anhydrous NaAc on the sizes, morphologies, crystal structures and magnetic properties of these hollow or mesoporous Fe\({}_{3}\)O\({}_{4}\) NPs are carefully investigated by SEM, TEM, XRD and VSM techniques. Morphology, Crystal Structure, and Magnetic Property of Fe\({}_{3}\)O\({}_{4}\) NPs with Different Amount of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O  
In this section, four experimental groups are carried out for investigating the influence of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O on the morphology of Fe\({}_{3}\)O\({}_{4}\) NPs. By only changing the initial amount of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O (0.4 mmol (S1), 5 mmol (S2), 10 mmol (S3) and 20 mmol (S4)), the Fe\({}_{3}\)O\({}_{4}\) NPs with different performances are obtained (The specific parameters are presented in Table 1). Firstly, the SEM images of as-synthesized Fe\({}_{3}\)O\({}_{4}\) NPs are presented in Figure 1. As shown in the SEM images, the mean particle size of Fe\({}_{3}\)O\({}_{4}\) NPs (S1) are smaller than 100 nm when 0.4 mmol of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O was added (Figure 1A). The mean particle size of Fe\({}_{3}\)O\({}_{4}\) NPs (S2) is increasing to 200 nm when the amount of FeCl\({}_{3}\cdot\)6H\({}_{2}\)O is 5 mmol. (Figure 1B), and the dispersity is better than these NPs in Figure 1A. As shown in Figure 1C,  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Sample No. **} & \multirow{2}{*}{**A mmol**} & \multirow{2}{*}{**B mmol**} & \multirow{2}{*}{**A.B**} & \begin{tabular}{c} **Reaction** \\ **Time** \\ **(hours)** \\ \end{tabular}  & \multirow{2}{*}{ \begin{tabular}{c} **Reaction** \\ **Temperature** \\ **(°C)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Average** \\ **(mm)** \\ \end{tabular}  } & \multirow{2}{*}{**Major Morphology**} \\ \hline \multirow{4}{*}{n(A)} & S1 & 0.4 & 40 & 1:100 & 10 & 200 & 101 & \begin{tabular}{c} Little hollow structure with \\ small solid particles \\ \end{tabular}  \\ \cline{2-7}  & S2 & 5 & 40 & 1:8 & 10 & 200 & 222 & Hollow structure \\ \cline{2-7}  & S3 & 10 & 40 & 1:4 & 10 & 200 & 116 & Mesoporous structure \\ \cline{2-7}  & S4 & 20 & 40 & 1:2 & 10 & 200 & 91 & Flowerike nanospheres \\  & S5 & 5 & 5 & 1:1 & 10 & 200 & 85 & \begin{tabular}{c} Microcrystalline grain, bulk \\ structure \\ \end{tabular}  \\ \cline{2-7}  & S6 & 5 & 10 & 1:2 & 10 & 200 & 122 & Hollow structure with hole in the wall \\ \cline{2-7}  & S7 & 5 & 20 & 1:4 & 10 & 200 & 103 & \begin{tabular}{c} Flowerike nanospheres \\ with Mesoporous structure \\ \end{tabular}  \\ \cline{2-7}  & S8 & 5 & 40 & 1:8 & 10 & 200 & 222 & Hollow structure \\ \cline{2-7}  & S9 & 5 & 60 & 1:12 & 10 & 200 & 256 & Hollow structure \\ \hline \multirow{4}{*}{Reaction} & S10 & 5 & 40 & 1:8 & 5 & 200 & 184 & \ \begin{tabular}{c} large particles without \\ obvious hollow structure \\ \end{tabular}  \\ \cline{2-7}  & S11 & 5 & 40 & 1:8 & 6 & 200 & 212 & \begin{tabular}{c} large particles without \\ obvious hollow structure \\ \end{tabular}  \\ \cline{2-7}  & S12 & 5 & 40 & 1:8 & 8 & 200 & 217 & Hollow structure \\ \cline{2-7}  & S13 & 5 & 40 & 1:8 & 10 & 200 & 222 & Hollow structure \\ \cline{2-7}  & S14 & 5 & 40 & 1:8 & 12 & 200 & 213 & Hollow structure \\ \cline{2-7}  & S15 & 5 & 40 & 1:8 & 20 & 200 & 241 & Hollow structure \\ \hline \hline \endnearly monodispersed Fe\({}_{3}\)O\({}_{4}\) NPs (S3) with mean particle sizes between 100 and 200 nm are obtained when the FeCl\({}_{3}\cdot\)6H\({}_{2}\)O increase to 10 mmol. The SEM images in Figure 1D indicate that the average size of as prepared Fe\({}_{3}\)O\({}_{4}\) NPs (S4) is less than 100 nm. The TEM images and corresponding particle size distribution of the as-prepared Fe\({}_{3}\)O\({}_{4}\) NPs are presented in Figure 2. The S1 showed spherical morphology with average particle size of 101 nm (Figure 2A,E). But the size distribution is wide, and the hollow structure is not obvious.