Effect of annealing on the structure of carbon onions and the annealed carbon coated Ni nanoparticles fabricated by chemical vapor deposition

Structural evolutions of carbon onions and carbon coated nanocrystals of nickel during annealing process were investigated. X-ray diffraction, Raman and transmission electron microscope were used to investigate their structural variation. The results showed that annealing (700 °C) of the carbon onions resulted in the shrinkage of hollow core of carbon onions and reduction of the interlayer spacing between carbon onion shells. And the compressed onion structure took a perfect spheroidal shape. However, the structure of the annealed carbon coated Ni nanoparticles presented that the Ni nanoparticles (5–30 nm) escaped from carbon encapsulation and congregated to large Ni particles (30–180 nm), and the carbon coating was disintegrated to small disorder graphite pieces. When the Ni was removed from the carbon coated Ni nanoparticles and then was annealed at 700 °C for 4 h, it was found that these hollow carbon onions presented the similar structural evolution as the above carbon onions.