Then, the Fe@CaO-X (1 g) template was put into a quartz boat and placed in the quartz tube furnace of a CVD system. Gaseous pyridine, which was evaporated by bubbling liquid pyridine with an atmospheric argon flow (200 sccm), flowed through the reaction chamber when the temperature reached 700 °C at a heating rate of 35 °C min−1. The pyridine flow was maintained for 20 min. After cooling down naturally, the product was mixed with diluted hydrochloric acid (1 M, 100 mL) and stirred over 12 h to completely remove the Fe@CaO-X template. The suspension was filtered and washed with ultrapure water and ethanol to obtain the final materials.

Then, the Fe@CaO-X (1 g) template was put into a quartz boat and placed in the quartz tube furnace of a CVD system. Gaseous pyridine, which was evaporated by bubbling liquid pyridine with an atmospheric argon flow (200 sccm), flowed through the reaction chamber when the temperature reached 700 °C at a heating rate of 35 °C min−1. The pyridine flow was maintained for 20 min. After cooling down naturally, the product was mixed with diluted hydrochloric acid (1 M, 100 mL) and stirred over 12 h to completely remove the Fe@CaO-X template. The suspension was filtered and washed with ultrapure water and ethanol to obtain the final materials.