Synthesis of carbon nanofibers made from nickel catalysts confined within hollow carbon spheres

Abstract

Hollow carbon spheres (HCSs) are known to exhibit exceptional properties such as having a high surface to volume ratio, encapsulation ability and excellent chemical as well as thermal stability. The hollow cavity of the spheres can act as a nanoreactor to build structures such as carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through an encapsulated metal, metal alloy or metal oxide catalyst. As such, this study reports on the encapsulation of nickel oxide inside hollow carbon spheres (NiO@HCSs). For comparison, NiO was also deposited outside HCSs to give NiO/HCSs catalyst. NiO particles were completely reduced to Ni at 450 °C under the continuous flow of H2gas (100 mL/min) to give Ni@HCSs for the particles deposited inside and outside HCSs, respectively. The synthesis of CNFs was achieved by the decomposition of acetylene gas (100 mL/min) over Ni@HCSs (0.5, 5, and 10 wt% Ni loading) using a chemical vapour decomposition (CVD) technique. Transmission electron microscopy (TEM) studies showed that Ni/HCSs resulted in enormous growth of CNFs and agglomeration of the Ni catalyst particles. Ni@HCSs catalysts resulted in minimal agglomeration of Ni nanoparticles (NPs) at a low loading (0.5 wt%) when compared to a high loading (10 wt%). The average Ni grains were determined to be 12 ± 8 nm, 22 ±5nm and 51 ± 12 nm for 0.5 wt%, 5 wt% and 10 wt% Ni loading, respectively. To selectively prepare CNFs with a controllable size and morphology, the NiO@HCSs at lower loading (0.5 wt%) was used for the synthesis of the CNFs using acetylene and trichloroethylene (TCE)and further studies. The effect of reaction parameters on the synthesis of the CNFs including temperature, growth time and flow rate was investigated. CNFs synthesized using acetylene showed the successful growth of monomodal and bimodal growth with straight and helical morphology at elevated temperatures. TEM analysis revealed that as the temperature increased, nickel fracturing occurred which resulted in the formation of thin CNFs (ca. 11 nm). The growth time had a significant impact on the CNFs yield and CNFs with variable lengths were observed. It was observed that H2 flow rate modified the morphologies of the CNFs synthesized. The CNFs synthesized using TCE as a carbon source resulted in a high yield of carbon. Various fiber morphologies (tripod-like, flower-like, CNFs with a hollow central core and rough outer edges) were observed. This indicated that the Ni particles underwent major reconstruction during CNF growth. It was also demonstrated that different nickel precursors (nickel acetate hexahydrate, nickel chloride hexahydrate and nickel nitrate) only had a slight effect on the morphology of the CNFs. Raman studies revealed that the CNFs synthesized using nickel acetate resulted in the formation of a more graphitic carbon compared to other nickel compounds. The TGA data corroborated this finding