and followed by the addition of about 2 mL 25% NH4OH to precipitate the iron oxides at 40 °C under N2 gas with magnetic stirring, the pH of mixture solution was kept in the range of pH 9–10. Then,
the solution was stirred rapidly at 60 °C for 1 h,
and then cooled to room temperature.
The black products were collected by a magnet and washed with water for several times.