Carbon nanofibers (CNFs), vapor grown carbon fibers (VGCFs), or vapor grown carbon nanofibers (VGCNFs) are cylindric nanostructures with graphene layers arranged as stacked cones, cups or plates. Carbon nanofibers with graphene layers wrapped into perfect cylinders are called carbon nanotubes.
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Carbon is the building block of a myriad of organic and inorganic matter around us. It is a versatile atom capable of joining to other atoms in sp, sp2, and sp3 hybridized structures giving rise to millions of stable molecules. In its single element form, it has a number of allotropes (polymorphs) like diamond, graphite, and fullerenes with different properties ranging from extremely hard to very soft scope[1]. Carbon can be made to form tubular microstructure called filament or fiber. The unique properties of carbon fibers have expanded the science and technology of composite materials in recent decades.
VGCFs and their smaller size variant, VGCNFs are among short carbon fibers that have drawn lots of attention for their potential thermal, electrical, frequency shielding, and mechanical property enhancements [2]. They are being more and more utilized in different material systems like composites [3] thanks to their exceptional properties and low cost.
Catalytic Chemical Vapor Deposition (CCVD) or simply Chemical Vapor Deposition (CVD) with variants like thermal and plasma-assisted is the dominant commercial technique for the fabrication of VGCF and VGCNF. Here, gas-phase molecules are decomposed at high temperatures and carbon is deposited in the presence of a transition metal catalyst on a substrate where subsequent growth of the fiber around the catalyst particles is realized. In general, this process involves separate stages such as gas decomposition, carbon deposition, fiber growth, fiber thickening, graphitization, and purification and results in hollow fibers. The nanofiber diameter depends on the catalyst size. The CVD process for the fabrication of VGCF generally falls into two categories [4]: 1) fixed-catalyst process (batch), and 2) floating-catalyst process (continuous).
In the batch process developed by Tibbetts [5], a mixture of hydrocarbon/hydrogen/helium was passed over a mullite (crystalline aluminum silicate) with fine iron catalyst particle deposits maintained at 1000⁰C. The hydrocarbon used was methane in the concentration of 15% by volume. Fiber growth in several centimeters was achieved in just 10 minutes with a gas residence time of 20 seconds. In general, fiber length can be controlled by the gas residence time in the reactor. Gravity and direction of the gas flow typically affects the direction of the fiber growth [6].
The continuous or floating-catalyst process was patented earlier by Koyama and Endo [7] and was later modified by Hatano and coworkers [8]. This process typically yields VGCF with submicron diameters and lengths of a few to 100 microns, which accords with the definition of carbon nanofibers. They utilized organometallic compounds dissolved in a volatile solvent like benzene that would yield a mixture of ultrafine catalyst particles (5-25 nm in diameter) in hydrocarbon gas as the temperature rose to 1100⁰C. In the furnace, the fiber growth initiates on the surface of the catalyst particles and continues until catalyst poisoning occurs by impurities in the system. In the fiber growth mechanism described by Baker and coworkers [9], only the part of catalyst particle exposed to the gas mixture contributes to the fiber growth and the growth stops as soon as the exposed part is covered, i.e. the catalyst is poisoned. The catalyst particle remains buried in the growth tip of the fiber at a final concentration of about a few parts per million. At this stage, fiber thickening takes place.
The most commonly used catalyst is iron, often treated with sulfur, hydrogen sulfide, etc. to lower the melting point and facilitate its penetration into the pores of carbon and hence, to produce more growth sites [10]. Fe/Ni, Ni, Co, Mn, Cu, V, Cr, Mo and Pd are also used as catalyst [11][12]. Acetylene, ethylene, methane, natural gas, and benzene are the most commonly used carbonaceous gases. Often carbon monoxide (CO) is introduced in the gas flow to increase the carbon yield through reduction of possible iron oxides in the system.
One of the first technical records concerning carbon nanofibers is probably a patent dated 1889 on synthesis of filamentous carbon by Hughes and Chambers[15]. They utilized a methane/hydrogen gaseous mixture and grew carbon filaments through gas pyrolysis and subsequent carbon deposition and filament growth. The true appreciation of these fibers, however, came much later when their structure could be analyzed by electron microscope [16]. The first electron microscopy observations of carbon nanofibers were performed in the early 1950s by the Soviet scientists Radushkevich and Lukyanovich, who published a paper in the Soviet Journal of Physical Chemistry showing hollow graphitic carbon fibers that are 50 nanometers in diameter [17]. Early in the 1970s, Japanese researchers Koyama and Endo [18] succeeded in the manufacturing of VGCF with a diameter of 1 µm and length of above 1 mm. Later, in the early 1980s, Tibbetts [19] in the USA and Benissad [20] in France continued to perfect the VGCF fabrication process. In the USA, the deeper studies focusing on synthesis and properties of these materials for advanced applications were led by R. Terry K. Baker [21] and were motivated by the need to inhibit the growth of carbon nanofibers because of the persistent problems caused by accumulation of the material in a variety of commercial processes especially in the particular field of petroleum processing. The first commercialization of VGCF was attempted by the Japanese company Nikosso in 1991 under the trade name Grasker® [22], the same year Sumio Iijima published his famous paper introducing the discovery of Carbon Nanotubes (CNTs) to the world. [23] VGCNF is produced through essentially the same manufacturing process as VGCF, only the diameter is typically less than 200 nm. Several companies around the globe are actively involved in the commercial scale production of carbon nanofibers and new engineering applications are being developed for these materials intensively, the latest being a carbon nanofiber bearing porous composite for oil spill remediation [24].