Effect of annealing temperature on the structure of carbon encapsulated Fe3O4 nanospheres

Abstract

Carbon-encapsulated Fe₃O₄ nanospheres were synthesized by a one-step solvothermal method. The effect of annealing temperature on the structure of carbon-encapsulated Fe₃O₄ nanospheres was investigated. The morphologies and microstructures of all the products were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, N₂ adsorption–desorption isotherms, thermogravimetry and vibrating sample magnetometry. The results show that the as-obtained carbon-encapsulated Fe₃O₄ nanospheres possess a core–shell structure with a size of about 120 nm. The thickness of the carbon layer, specific surface area and pore volume are 20 nm, 33 m² g⁻¹ and 0.06 cm³ g⁻¹, respectively. With the increase of annealing temperature from 300 to 500 °C for 1 h, the carbon layer of the carbon-encapsulated Fe₃O₄ nanospheres becomes thinner, and the specific surface area and pore volume become larger. After annealing at 500 °C, the carbon layer thickness, specific surface area and pore volume of the carbon-encapsulated Fe₃O₄ nanospheres are 13 nm, 191 m² g⁻¹ and 0.14 cm³ g⁻¹, respectively. After annealing at 600 °C, the carbon-encapsulated Fe₃O₄ nanospheres change to a yolk–shell structure. When annealed at 700 °C, the carbon layer is destroyed and some Fe nanoparticles are exposed. A plausible formation mechanism for yolk–shell Fe₃O₄@C nanospheres is proposed.