Carbon nanofibers (CNFs), vapor grown carbon fibers (VGCFs), or vapor grown carbon nanofibers (VGCNFs) are cylindrical nanostructures with graphene layers arranged as stacked cones, cups or plates. Carbon nanofibers with graphene layers wrapped into perfect cylinders are called carbon nanotubes.
Carbon has a high level of chemical bonding flexibility, which lends itself to the formation of a number of stable Organic and Inorganic Molecules. Elemental carbon has a number of allotropes(variants) including diamond, graphite, and fullerenes. [2] Though they all consist of elemental carbon, their properties vary widely. This underscores the versatility of CNFs, which are notable for their thermal, electrical, electromagnetic shielding, and mechanical property enhancements. [3] As carbon is readily available at low cost, CNFs are popular additives to composite materials. [4] CNFs are very small, existing at the nanometer scale. An atom is between .1-.5 nm, thus specialized microscopic techniques such as Scanning Tunneling Microscopy and Atomic Force Microscopy are required to examine the properties of CNFs.[ citation needed ]
Catalytic chemical vapor deposition (CCVD) or simply 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: [5] 1) fixed-catalyst process (batch), and 2) floating-catalyst process (continuous).
In the batch process developed by Tibbetts, [6] a mixture of hydrocarbon/hydrogen/helium was passed over a mullite (crystalline aluminum silicate) with fine iron catalyst particle deposits maintained at 1000 °C. 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. [5]
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 sub-micrometre diameters and lengths of a few to 100 μm, 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 1100 °C. 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.[ citation needed ]
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. [2] Fe/Ni, Ni, Co, Mn, Cu, V, Cr, Mo, Pd, MgO, and Al2O3 are also used as catalyst. [10] [11] 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.[ citation needed ]
In 2017, a research group in Tsinghua University reported the epytixial growth of aligned, continuous, catalyst-free carbon nanofiber from a carbon nanotube template. The fabrication process includes thickening of continuous carbon nanotube films by gas-phase pyrolytic carbon deposition and further graphitization of the carbon layer by high temperature treatment. Due to the epitaxial growth mechanism, the fiber features superior properties including low density, high mechanical strength, high electrical conductivity, high thermal conductivity. [12]
The Occupational Safety and Health Act (United States) (1970) was a driving force behind many of the changes made regarding safety in the workplace over the last few decades. One small group of the numerous substances to be regulated by this act is carbon nanofibers (CNF). While still an active area of research, there have been studies conducted that indicate health risks associated with carbon nanotubes (CNT) and CNF that pose greater hazards than their bulk counterparts. One of the primary hazards of concern associated with CNT and CNF is respiratory damage such as pulmonary inflammation, granuloma, and fibrosis. It is important to note, however, that these findings were observed in mice, and that it is currently unknown whether the same effects would be observed in humans. Nonetheless these studies have given cause for an attempt to minimize exposure to these nanoparticles. [13]
A separate study conducted prior to the 2013 annual Society of Toxicology meeting aimed to identify potential carcinogenic effects associated with multi-walled carbon nanotubes (MWCNT). The findings indicated that, in the presence of an initiator chemical, the MWCNTs caused a much greater incidence of tumors in mice. There was no indication of increased presence of tumors in the absence of the initiator chemical, however. Further studies are needed for this scenario. [13]
One of the major hurdles in identifying hazards associated with CNF is the diversity of fibers that exist. Some of the contributing factors to this diversity include shape, size, and chemical composition. One exposure standard (2015) states that the acceptable limit for CNT and CNF exposure is 1 μg/m3 of respirable size fraction elemental carbon (8-hour time-weighted average). This standard was based on information gathered from 14 sites whose samples were analyzed by transmission electron microscopy (TEM). [14]
A recent safety data sheet (SDS) for CNF (revised in 2016) lists the nanofibers as an eye irritant, and states that they have single exposure respiratory system organ toxicity. Smaller CNF possess a greater potential for forming dust clouds when handling. As such, great care must be taken when handling CNF. The recommended personal protective equipment (PPE) for handling CNF includes nitrile gloves, particle respirators, and nanomaterial-impervious clothing (dependent on workplace conditions). In addition to exposure controls while working with the CNF, safe storage conditions are also important in minimizing the risk associated with CNF. Safe CNF storage entails storing the fibers away from oxidizing agents and open flames. Under fire conditions, CNF form hazardous decomposition products though the exact nature of these decomposition products is not currently known. Apart from carcinogenicity and organ toxicity, toxicological data for CNF is currently rather limited. [15]
One of the first technical records concerning carbon nanofibers is probably a patent dated 1889 on synthesis of filamentous carbon by Hughes and Chambers. [24] 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 microscopy. [2] 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. [25] Early in the 1970s, Japanese researchers Morinobu Endo, now the director of the Institute of Carbon Science and Technology at Shinshu University, reported the discovery of carbon nanofibers, including that some were shaped as hollow tubes. [26] He also succeeded in the manufacturing of VGCF with a diameter of 1 μm and length of above 1 mm. [27] Later, in the early 1980s, Tibbetts [6] in the USA and Benissad [28] 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.[ citation needed ] They 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. In 1991, Japanese researchers Sumio Iijima, while working at NEC, synthesized hollow carbon molecules and determined their crystal structure. The following year, these molecules were called "carbon nanotubes" for the first time. [29] 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. [30]
A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized:
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
Laser ablation or photoablation is the process of removing material from a solid surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. While relatively long laser pulses can heat and thermally alter or damage the processed material, ultrashort laser pulses cause only minimal material damage during processing due to the ultrashort light-matter interaction and are therefore also suitable for micromaterial processing. Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approximately 200 nm.
Tungsten disulfide is an inorganic chemical compound composed of tungsten and sulfur with the chemical formula WS2. This compound is part of the group of materials called the transition metal dichalcogenides. It occurs naturally as the rare mineral tungstenite. This material is a component of certain catalysts used for hydrodesulfurization and hydrodenitrification.
As the world's energy demand continues to grow, the development of more efficient and sustainable technologies for generating and storing energy is becoming increasingly important. According to Dr. Wade Adams from Rice University, energy will be the most pressing problem facing humanity in the next 50 years and nanotechnology has potential to solve this issue. Nanotechnology, a relatively new field of science and engineering, has shown promise to have a significant impact on the energy industry. Nanotechnology is defined as any technology that contains particles with one dimension under 100 nanometers in length. For scale, a single virus particle is about 100 nanometers wide.
Morinobu Endo is a Japanese physicist and chemist, often cited as one of the pioneers of carbon nanofibers and carbon nanotubes synthesis at the beginning of the 1970s. He demonstrated carbon fibers can be grown by gas pyrolysis and traveled to Orléans, France in 1974 working with Madame Agnès Oberlin at CNRS in her laboratory. He discovered carbon nanotubes in 1976 as part of his studies at University of Orleans in France. He has been awarded the Charles E. Pettinos Award from the American Carbon Society in 2001, "For his pioneering work and applications of carbon nanotubes", Medal of Achievement in Carbon Science and Technology from the American Carbon Society in 2004, "for the discovery of, and early synthesis work on, carbon nanotubes".
Silicon nanotubes are nanoparticles which create a tube-like structure from silicon atoms. As with silicon nanowires, they are technologically important due to their unusual physical properties, which differ fundamentally to those of bulk silicon. The first reports on silicon nanotubes appeared around the year 2000.
Single-walled carbon nanohorn is the name given by Sumio Iijima and colleagues in 1999 to horn-shaped sheath aggregate of graphene sheets. Very similar structures had been observed in 1994 by Peter J.F. Harris, Edman Tsang, John Claridge and Malcolm Green. Ever since the discovery of the fullerene, the family of carbon nanostructures has been steadily expanded. Included in this family are single-walled and multi-walled carbon nanotubes, carbon onions and cones and, most recently, SWNHs. These SWNHs with about 40–50 nm in tubule length and about 2–3 nm in diameter are derived from SWNTs and ended by a five-pentagon conical cap with a cone opening angle of ~20o. Moreover, thousands of SWNHs associate with each other to form the ‘dahlia-like' and ‘bud-like’ structured aggregates which have an average diameter of about 80–100 nm. The former consists of tubules and graphene sheets protruding from its surface like petals of a dahlia, while the latter is composed of tubules developing inside the particle itself. Their unique structures with high surface area and microporosity make SWNHs become a promising material for gas adsorption, biosensing, drug delivery, gas storage and catalyst support for fuel cell. Single-walled carbon nanohorns are an example of the family of carbon nanocones.
Carbon nanotube supported catalyst is a novel supported catalyst, using carbon nanotubes as the support instead of the conventional alumina or silicon support. The exceptional physical properties of carbon nanotubes (CNTs) such as large specific surface areas, excellent electron conductivity incorporated with the good chemical inertness, and relatively high oxidation stability makes it a promising support material for heterogeneous catalysis.
Filamentous carbon is a carbon-containing deposit structure that refers to several allotropes of carbon, including carbon nanotubes, carbon nanofibers, and microcoils. It forms from gaseous carbon compounds. Filamentous carbon structures all contain metal particles. These are either iron, cobalt, or nickel or their alloys. Deposits of it also significantly disrupt synthesis gas methanation. Acetylene is involved in a number of method of the production of filamentous carbon. The structures of filamentous carbon are mesoporous and on the micrometer scale in dimension. Most reactions that form the structures take place at or above 280 °C (536 °F).
A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.
Gallium nitride nanotubes (GaNNTs) are nanotubes of gallium nitride. They can be grown by chemical vapour deposition.
In nanotechnology, carbon nanotube interconnects refer to the proposed use of carbon nanotubes in the interconnects between the elements of an integrated circuit. Carbon nanotubes (CNTs) can be thought of as single atomic layer graphite sheets rolled up to form seamless cylinders. Depending on the direction on which they are rolled, CNTs can be semiconducting or metallic. Metallic carbon nanotubes have been identified as a possible interconnect material for the future technology generations and to replace copper interconnects. Electron transport can go over long nanotube lengths, 1 μm, enabling CNTs to carry very high currents (i.e. up to a current density of 109 A∙cm−2) with essentially no heating due to nearly one dimensional electronic structure. Despite the current saturation in CNTs at high fields, the mitigation of such effects is possible due to encapsulated nanowires.
Techniques have been developed to produce carbon nanotubes (CNTs) in sizable quantities, including arc discharge, laser ablation, high-pressure carbon monoxide disproportionation, and chemical vapor deposition (CVD). Most of these processes take place in a vacuum or with process gases. CVD growth of CNTs can occur in a vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth are making CNTs more commercially viable.
In materials science, vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.
Graphenated carbon nanotubes (G-CNTs) are a relatively new hybrid that combines graphitic foliates grown along the sidewalls of multiwalled or bamboo style carbon nanotubes (CNTs). Yu et al. reported on "chemically bonded graphene leaves" growing along the sidewalls of CNTs. Stoner et al. described these structures as "graphenated CNTs" and reported in their use for enhanced supercapacitor performance. Hsu et al. further reported on similar structures formed on carbon fiber paper, also for use in supercapacitor applications. Pham et al. also reported a similar structure, namely "graphene-carbon nanotube hybrids", grown directly onto carbon fiber paper to form an integrated, binder free, high surface area conductive catalyst support for Proton Exchange Membrane Fuel Cells electrode applications with enhanced performance and durability. The foliate density can vary as a function of deposition conditions with their structure ranging from few layers of graphene to thicker, more graphite-like.
A graphene morphology is any of the structures related to, and formed from, single sheets of graphene. 'Graphene' is typically used to refer to the crystalline monolayer of the naturally occurring material graphite. Due to quantum confinement of electrons within the material at these low dimensions, small differences in graphene morphology can greatly impact the physical and chemical properties of these materials. Commonly studied graphene morphologies include the monolayer sheets, bilayer sheets, graphene nanoribbons and other 3D structures formed from stacking of the monolayer sheets.
A probe tip is an instrument used in scanning probe microscopes (SPMs) to scan the surface of a sample and make nano-scale images of surfaces and structures. The probe tip is mounted on the end of a cantilever and can be as sharp as a single atom. In microscopy, probe tip geometry and the composition of both the tip and the surface being probed directly affect resolution and imaging quality. Tip size and shape are extremely important in monitoring and detecting interactions between surfaces. SPMs can precisely measure electrostatic forces, magnetic forces, chemical bonding, Van der Waals forces, and capillary forces. SPMs can also reveal the morphology and topography of a surface.
Gilbert Daniel Nessim is a chemistry professor at Bar-Ilan University specializing in the synthesis of 1D and 2D nanomaterials for electronic, mechanic, and energy applications.
