Catalytic chemical vapor deposition (CCVD) is a widely used method for producing vapor-grown carbon nanofibers (VGCNFs). This process involves the decomposition of hydrocarbon gases on metal catalyst particles at elevated temperatures, typically between 500°C and 1200°C. The method is favored for its scalability, control over fiber morphology, and ability to produce high-purity nanofibers with tailored properties.
The choice of catalyst is critical in CCVD, with transition metals such as iron (Fe), nickel (Ni), and cobalt (Co) being the most effective due to their high carbon solubility and catalytic activity. These metals facilitate the decomposition of carbon-containing gases like methane, ethylene, or acetylene. The catalyst can be used in bulk form, as nanoparticles, or supported on substrates such as alumina or silica to enhance dispersion and control fiber growth. Among these, Fe-based catalysts often yield nanofibers with higher graphitic order, while Ni and Co catalysts may produce fibers with different structural defects or morphologies.
The growth mechanism of VGCNFs follows a vapor-liquid-solid (VLS) model. Hydrocarbon molecules adsorb onto the catalyst surface, where they decompose into carbon atoms. These atoms dissolve into the metal particle until reaching supersaturation, precipitating out as tubular graphitic structures. The orientation of the precipitated carbon layers depends on the crystallographic facets of the catalyst particle, leading to distinct morphologies. Two common structures are stacked-cone (herringbone) and hollow-core (tubular) nanofibers. Stacked-cone fibers exhibit angled graphene layers relative to the fiber axis, while hollow-core fibers have concentric cylindrical layers.
Post-treatment is often necessary to remove residual catalyst particles and amorphous carbon impurities. Acid washing with hydrochloric or nitric acid dissolves metal residues, while oxidative treatments in air or oxygen can eliminate disordered carbon. Thermal annealing at high temperatures further improves crystallinity and electrical conductivity by promoting graphitization.
VGCNFs exhibit unique properties that make them suitable for various applications. Their high aspect ratio, electrical conductivity, and mechanical strength are advantageous in conductive polymer composites, where they enhance electrical percolation at low loadings. In filtration systems, their porous networks and large surface area enable efficient particle capture. As reinforcement in composites, they improve tensile strength and stiffness without significant weight penalties.
Compared to electrospinning, CCVD offers distinct advantages and limitations. Electrospinning produces polymer-derived carbon nanofibers through pyrolysis of spun fibers, resulting in less graphitic and more disordered structures. While electrospun fibers can achieve smaller diameters and higher porosity, they generally exhibit lower electrical and thermal conductivity than CCVD-grown fibers. CCVD, on the other hand, provides better control over crystallinity and alignment but requires higher temperatures and metal catalysts, which may introduce impurities.
In summary, CCVD is a versatile method for synthesizing high-quality carbon nanofibers with tunable morphologies. The selection of catalyst, growth conditions, and post-treatment steps significantly influence the final properties, enabling tailored performance for conductive fillers, filters, and composite reinforcement. While electrospinning offers an alternative route for specific applications, CCVD remains superior for producing highly graphitic and mechanically robust nanofibers.
The continued development of CCVD techniques, including optimization of catalyst supports and gas precursors, will further enhance the scalability and applicability of vapor-grown carbon nanofibers in advanced materials and industrial processes.