Catalytic chemical vapor deposition (CVD) is a widely used method for synthesizing carbon nanofibers (CNFs), offering precise control over morphology, diameter, and structural alignment. This technique relies on the decomposition of hydrocarbon precursors over transition metal catalysts, which serve as nucleation sites for fiber growth. The resulting nanofibers exhibit distinct microstructures—such as platelet, fishbone, or tubular—depending on processing conditions and catalyst selection. Unlike carbon nanotubes (CNTs), CNFs typically possess a less ordered, amorphous or partially graphitic structure, with stacked graphene layers at varying angles relative to the fiber axis.

Transition metal catalysts, including nickel (Ni), copper (Cu), iron (Fe), and cobalt (Co), play a critical role in CNF synthesis. These metals facilitate hydrocarbon dissociation and carbon diffusion, enabling fiber growth through a vapor-liquid-solid (VLS) or vapor-solid-solid (VSS) mechanism. Nickel, for instance, exhibits high carbon solubility and diffusion rates, promoting rapid fiber formation. In contrast, copper, with lower carbon solubility, often yields thinner fibers due to slower carbon precipitation. The catalyst particle size directly influences the nanofiber diameter, with smaller particles producing narrower fibers.

Hydrocarbon precursors such as methane, ethylene, acetylene, and carbon monoxide are commonly used in catalytic CVD. The choice of precursor affects the carbon supply rate and the degree of graphitization. For example, acetylene decomposes at lower temperatures than methane, leading to faster deposition but potentially more disordered structures. The carbon atoms dissolve into the catalyst nanoparticles, supersaturate, and then precipitate to form nanofibers. The growth direction and stacking arrangement of graphene layers are determined by the crystallographic orientation of the catalyst surface and the reaction kinetics.

CNF microstructures are classified into three primary types: platelet, fishbone, and tubular. Platelet nanofibers feature graphene layers stacked perpendicular to the fiber axis, resembling stacked plates. This structure arises when the catalyst particle’s exposed facet favors carbon precipitation in a direction normal to the surface. Fishbone nanofibers, also known as herringbone, exhibit graphene layers angled between 15° and 45° relative to the axis, resulting from oblique precipitation. Tubular nanofibers, though less common, resemble CNTs but with a less defined graphitic alignment and occasional branching.

The distinction between CNFs and CNTs lies in their structural order. CNTs consist of seamless cylindrical graphene sheets with sp² hybridization, offering high crystallinity and electrical conductivity. In contrast, CNFs contain stacked graphene segments with interlayer defects, sp³ hybridized carbon, and amorphous regions, leading to varied mechanical and electronic properties. The presence of edge planes in CNFs enhances their chemical reactivity, making them suitable for functionalization in catalytic or adsorption applications.

Process parameters such as temperature, pressure, gas flow rates, and catalyst support materials significantly influence CNF growth. Temperatures between 500°C and 1000°C are typical, with higher temperatures favoring graphitization but risking excessive catalyst sintering. The use of supports like alumina, silica, or magnesium oxide stabilizes catalyst nanoparticles and prevents aggregation. Additionally, introducing hydrogen or ammonia as co-gases can etch amorphous carbon, improving fiber purity.

Challenges in catalytic CVD include achieving uniform fiber diameter distribution, minimizing catalyst contamination, and scaling up production. Post-synthesis treatments such as acid washing or thermal annealing may be employed to remove residual metal particles or enhance graphitic ordering. Despite these challenges, catalytic CVD remains a versatile and scalable method for CNF synthesis, with applications ranging from electrode materials to reinforced polymers—though composite-related uses fall outside this discussion.

In summary, catalytic CVD enables tailored CNF synthesis through careful selection of catalysts, precursors, and growth conditions. Transition metals dictate fiber morphology and growth kinetics, while hydrocarbon decomposition governs carbon supply and microstructure. The amorphous or partially graphitic nature of CNFs differentiates them from CNTs, influencing their suitability for specific applications. Continued optimization of CVD parameters will further advance the controlled production of carbon nanofibers for diverse technological uses.