Low Temperature Synthesis And Characterization Of Hollow Micro/Nano Carbon Materials

Ferrocene and several inorganic ammonium salts were used to prepare hollow carbon spheres, hollow six-armed carbon particles, amorphous carbon nanotubes and hollow carbon spheres encapsulating Fe₃O₄ or Fe3N nanoparticles. The microstructures and formation mechanisms of the products were systematically investigated by XRD, SEM, TEM, Raman, FIB and thermal analysis.The experimental results show that hollow carbon spheres of 1～10μm in diameter and smooth surface can be prepared via the reaction of ferrocene and ammonium chloride in a gas pressure furnace or a sealed quartz tube. With increase of initial gas pressure and synthesis temperature, the yield of hollow carbon spheres is enhanced and reaches a maximum value at 600℃with an initial gas pressure of 2MPa. Several iron compounds, including Fe(NH₃)₂Cl₂, Fe(NH₃)₆Cl₂, (NH₄)₃FeCl₅ and NH₄FeCl₃, are formed in different temperature ranges and Fe(NH₃)₂Cl₂ has a low melting point and serves as the core templates for carbon enwrapping to form hollow carbon spheres. The median diameter increases with the total amount of the reactants with the same weight ratio of ferrocene to ammonium chloride. When the excessive ammonium chloride was used, the hollow carbon spheres of a bimodal diameter distribution are produced. However the median diameter is almost the same. It is also found that amorphous hollow carbon spheres can be prepared in a sealed quartz tube at 500℃via the reaction between ferrocene and ammonium bromide or ammonium carbonate. The graphitization of the as-prepared hollow carbon spheres is slightly improved by the heat treatment between 800～1200℃. The mesopores of 4nm are formed after the heat treatment. The discharge capacity of hollow carbon spheres as anode material in Lithium-ion batteries is improved after heat treatment at 800℃compared with the as-prepared hollow carbon spheres and has a maximum value of 357mA·h/g and still retains 303mA·h/g after 40 cycles. However, the discharge capacity decreases and the cycling performance is improved with the increase of heat treatment temperature. In addition, the coulomb efficiency is raised from 42.1 to 65.4%。Hollow carbon spheres encapsulating magnetite nanoparticles were obtained in high pressure argon at 600℃followed by a hydrolysis of Fe(NH₃)₂Cl₂ in the hollow interiors at room temperature and a heat treatment in argon at 450℃. The weight percent of magnetite nanoparticles is about 13.2%. The dimensions of the equiaxed magnetite nanoparticles range from 15 to 90nm while the acicular magnetite nanoparticles have diameters of ca. 20nm and lengths of 120～450nm. The saturation magnetization, remanent magnetization and coercivity of the hollow carbon spheres encapsulating magnetite nanoparticles are 4.29emu/g, 0.74emu/g and 198.4Oe, respectively. Hollow carbon spheres encapsulating Fe3N nanoparticles were obtained in high pressure argon at 500℃followed by a proper acid washing process. The weight percent of Fe3N nanoparticles is about 38.7%. The acicular Fe3N nanoparticles have diameters of ca. 100nm and lengths of 600～800nm. The saturation magnetization, remanent magnetization and coercivity of the hollow carbon spheres encapsulating Fe3N nanoparticles are 10.61emu/g, 0.67emu/g and 180Oe, respectively.Carbon encapsulated six-armed Fe3O4 particles and a few carbon spheres of 1～2.5μm in diameter are synthesized by the reaction of ferrocene and ammonium acid carbonate in a sealed quartz tube at 500℃. The six arms of the Fe3O4, which have a herringbone structure, grow along <100> and they are equal in length of 4～6μm and perpendicular to each other. Hollow six-armed carbon particles can be obtained after acid washing.Amorphous carbon nanotubes with open ends are prepared via the reaction of ferrocene and ammonium chloride in air at 200℃. The amorphous carbon nanotubes have an external diameter of about 100nm, an internal diameter of 50nm and a length of several micrometers, respectively. Some bamboo-like amorphous carbon nanotubes can also be found. The atoms on the surface of carbon nanotubes become ordered after heat treatment at 2200℃.