All chemicals were analytical grade and used as received. All aqueous
solutions were prepared with deionized water. In a typical experiment,
2 mmol FeSO4 and 1 mmol CoSO4 were first dissolved in the
mixed solvents composed of 30 mL ethylene glycol (EG) and 10 mL
water. Similarly, 3 mmol H2C2O4 was added to the identical solution
with the above compositions. The two solutions were slowly mixed
together with continuous stirring. Then, the transparent mixture
was transferred into an autoclave, which was then tightly sealed
and hydrothermally treated at 120 °C for 24 h in an oven. The product
was collected by centrifugation and washed with deionized water
and absolute ethanol several times, and dried in air at 80 °C for