First, 20 mmol of ZnCl2 (Acros Organics) and 40 mmol of Fe
(NO3)3 6H2O (Acros Organics) is dissolved in 200 mL of MQ water.
The pH of the resulting mixture is then adjusted precisely to 11
using a 1 M NaOH solution (Backer analysis) while stirring vigorously.
The mixture is heated at reflux (70 °C) for 2 h to obtain a
characteristic orange-red ZnFe2O4 precipitate. After the formation
of the precipitate, the supernatant solution is removed by centrifugation.
ZnFe2O4 particles are washed 3 times with ultrapure
water in order to remove the dissolved salts (i.e. Cl, NO3
−, Naþ
and OH); the separation between the liquid and the solid phase
is performed by centrifugation (2500 rpm during 20 min). W