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It was accidentally discovered by Japanese physicist Sumio Iijima while studying the surface of graphite electrodes in arc discharge that carbon nanotubes (CNT), abbreviated as "nanotubes", are cylindrical carbon allotrope nanostructures. 1 Since Iijima's enlightenment, CNT has been playing a key role in the field of nanotechnology due to its special electronic, mechanical and structural characteristics. 1-3
Carbon nanotubes have good conductivity and high aspect ratio, and can form a conductive tube network. Its excellent mechanical properties are derived from the fusion of strength, stiffness and toughness. 4 Integrated into the polymer, the carbon nanotube transfers its mechanical load to the polymer matrix at a weight percentage significantly lower than that of carbon black or carbon fiber, thereby promoting the application with higher efficiency.
Carbon nanotubes are also used as thermal interface materials for thermal protection. Their unique electronic and mechanical properties can be used in a variety of applications, such as nanocomposites, 5 nanometer sensors, 6 field emission displays 7 and logic elements. 8
The utility of carbon nanotubes has been studied in great detail, extending from the pioneering applications of electronics manufacturing to the field of medicines for the treatment of many different types of diseases. 9
Single-walled carbon nanotubes (SWNT) (Cat. No. 755710) are smooth and ordered cylinders composed of a layer of graphene. They have special electronic properties, which may be very different from the chiral vector C = (n, m). This parameter specifies how to curl graphene sheets to produce carbon nanotubes. 10
The conductivity of SWNT depends on the value of (n, m), as shown in Table 1. Therefore, their rolling method will guide the band gap of SWNT, which can vary from 0 to 2 eV, and the conductivity can exhibit metallic or semiconductor behavior.
Table 1. The theoretical conductivity of single-walled carbon nanotubes (SWNT) depends on the rolling direction (n, m) of the graphene sheet. 10 Source: Merck Millipore Sigma
As shown in Table 2.11, the thermal and electrical conductivity of carbon nanotubes is very high, comparable to other conductive materials
Table 2. Transmission characteristics of carbon nanotubes and other conductive materials. 11 Source: Merck Millipore Sigma
Multi-walled carbon nanotubes (MWNT) (Cat. No. 755133) are composed of many rolled graphene layers. Compared with SWNT, MWNT has not yet been clearly defined due to their structural complexity and diversity. However, compared with single-walled carbon nanotubes, multi-walled carbon nanotubes show some advantages, such as scalability due to simplified mass production, improved thermal and chemical stability, and low unit production costs.
Generally, when functionalized, the electrical and mechanical properties of SWNT fluctuate due to structural defects when the C=C bond breaks during the chemical process. However, the inherent properties of carbon nanotubes can be preserved by surface modification of MWNT: exposing the outer wall of MWNT to a chemical modifier.
Surface modification of carbon nanotubes is to establish new properties in carbon nanotubes for unique applications that require enhanced functionality, organic solvents or water solubility, dispersion and compatibility, or reduction of the toxicity of carbon nanotubes. 12 Figure 1 shows several methods of chemically modifying the surface of carbon nanotubes.
Common functionalized carbon nanotubes, such as MWNT-COOH (Cat. No. 755125), are obtained by oxidation using various acids, ozone or plasma, which generate other oxygen functional groups (for example, -OH, -C=O) . The presence of oxygen-containing groups promotes the peeling of CNT bundles, and improves the solubility in polar media and the chemical affinity with ester-containing compounds (such as polyester).
The COOH group on the surface of the nanotube is a favorable position for advanced modification. By generating amide bonds and ester bonds, different molecules can be grafted, such as synthetic and natural polymers. 13
Figure 1. Schematic example of surface functionalization of carbon nanotubes. (Illustration by Zhao et al. 12)
Double-walled carbon nanotubes (DWNT) (Cat. No. 755168, 755141) are a synthetic combination of single-walled and multi-walled nanotubes, exhibiting characteristics between these two types. DWNT is composed of two concentric nanotubes precisely spaced by 0.35 – 0.40 nm, with a band gap suitable for field effect transistors. 14 The inner and outer walls of DWNT have the optical and Raman scattering properties of each wall. 15
In theory, if each wall is like a SWNT and according to the (n, m) values of their inner and outer walls, DWNT can maintain four combinations according to the type of electron (metal or semiconductor), for example, metal-metal (inner-outer ), metal-semiconductor, semiconductor-metal and semiconductor-semiconductor.
Several innovative studies have found that although both walls of DWNT are semiconductors, it may behave like metal. 16 This particular obstacle to its overall electrical behavior limits the uses of DWNT, such as applications such as thin-film electronics.
However, DWNTs also exhibit a series of favorable properties observed from MWNTs, such as enhanced lifetime and field emission current density and excellent stability under harmful chemical, mechanical, and heat treatments, as well as the flexibility observed with SWNTs. 17
The selective functionalization of the outer walls of DWNTs promotes their use as core-shell systems made of clean carbon nanotube cores and chemically functionalized nanotube shells, which are suitable for imaging and therapeutic agents in biological systems. 18
DWNT can be used for gas sensor 19 as a sensitive material for detecting gases (such as H2, O2, NO2, or NH3), dielectric 20, and technically challenging applications (such as photovoltaics and field emission displays). twenty one
Merck offers high-quality SWNT, MWNT and DWNT, including some of the most conductive additives on the market today, to meet your creative and advanced material research needs. As shown, these nanotubes were developed through catalytic chemical vapor deposition (CCVD) technology, a well-known industrial process with proven reliability and scalability factors.
Merck's nanotubes are also purified or functionalized to improve the performance of research applications where special chemical properties such as high field emission characteristics, large surface area or transparency are required.
Carbon nanotubes can be used in a wide range of new and existing applications, including the following applications:
A detailed description of the available carbon nanotubes is shown in Table 3. The specification details provided will help guide you in choosing the right material for your application.
Table 3. Specification details of carbon nanotubes. Source: Merck Millipore Sigma
*TEM image with reprint permission granted by Nanocyl SA.
This information is derived from, reviewed and adapted from materials provided by Merck.
For more information on this source, please visit Merck.
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