FeCl2 • 4 H2O and FeCl3 at 1:2 molar ratio were dissolved into a round bottom flask which contains 200 ml of deoxygenated distilled water and kept at desired temperature. While vigorously stirring the reaction mixture, the stoichiometry amount of NaOH was added into the solution. Nitrogen gas was kept passing through the solution during the experiment to prevent the oxidation of Fe2+ in the system in order to maintain the stoichiometry ratio between Fe2+ and Fe3+. Afterward, the resulting black precipitate in the flask was separated by placing onto a permanent magnet to accelerate the settling. The clear salt solution was decanted. The precipitate was washed through several cycles with acetone and distilled water and then dried in oven at 35°C for 24 hours. Table 1 summarizes the conditions for the samples preparation. No further calcinations was conducted for all the samples due to the easily oxidation behavior of Fe3O4 at high temperature.