In a typical experiment (S1 sample), 0.01 mol FeCl2∙4H2O and 0.02 mol FeCl3∙6H2O were dissolved in 100 mL degassed deionized water and added into a 250 mL three-neck flask, which was immersed in a room temperature (25 °C) water bath. Point-zero-eight moles of NaOH were dissolved in 100 mL degassed deionized water