Then, Fe(OH)2 was generated as the pH of the reaction system increased, which was attributed to the hydrolysis of Fe2+. Finally, Fe3O4 can just be formed as the pH of the solution is further increased. This result shows that the growth of Fe3O4 nucleus occurred when the pH of solution is lower than 11, while the growth of Fe3O4 nucleus is easier to happen when the pH of solution is higher than 11 [24]. Therefore the pH of 11 will be selected for the optimum pH in further experiment.