Carbon Nanotubes: Properties and Applications – AZoNano

§ January 24th, 2021 § Filed under Nanotechnology Journal Comments Off on Carbon Nanotubes: Properties and Applications – AZoNano

Sponsored by MerckJan 21 2021

Fortuitously discovered by Japanese physicist Sumio Iijima while he was studying the surface of graphite electrodes in an electric arc discharge, Carbon nanotubes (CNTs), simply known as 'nanotubes,'are cylindrical carbon allotrope nanostructures.1 Since Iijimas revelation, CNTs have retained a key role in the field of nanotechnology due to their particular electronic, mechanical, and structural properties.1-3

CNTs possess great conductivity and high aspect ratio which enables the formation of a network of conductive tubes. Their exceptional mechanical properties derive from an amalgamation of strength, stiffness and tenacity.4 Integrated into a polymer, CNTs shift their mechanical load to the polymer matrix at a weight percentage significantly lower than those of carbon black or carbon fibers, promoting applications with greater efficiency.

CNTs have also been used for thermal protection as thermal interface materials. Their unique electronic and mechanical properties can be utilized across a diverse range of applications, such as nanocomposite materials,5 nanosensors,6 field-emission displays7, and logic elements.8

The utility of CNTs has been studied in great detail ranging from applications in pioneering electronic fabrication extending to pharmaceutical fields for treatment of many different types of disease.9

Single-walled carbon nanotubes (SWNTs) (Product No. 755710) are smooth ordered cylinders consisting of a layer of graphene. They have exceptional electronic properties which can differ considerably with the chiral vector, C = (n, m), the parameter that specifies how the graphene sheet is rolled to produce a carbon nanotube.10

The electrical conductivity of SWNTs relies on the (n, m) values as illustrated in Table 1. Accordingly, how they are rolled directs the SWNTs' bandgap which can vary from 0 to 2 eV while electrical conductivity can demonstrate metallic or semiconducting behavior.

Table 1. The theoretical electronic conductivity of single-walled carbon nanotubes (SWNTs) depending on roll orientation of the graphene sheet (n, m).10Source:Merck Millipore Sigma

Thermal and electrical conductivities of carbon nanotubes are exceptionally high and comparable to other conductive materials as displayed in Table 2.11

Table 2. Transport properties of carbon nanotubes and other conductive materials.11Source:Merck Millipore Sigma

Multi-walled carbon nanotubes (MWNTs) (Product No. 755133) are comprised of numerous layers of graphene that have been rolled-up. Due to their structural complexity and variety in comparison to SWNTs, MWNTs have not yet been well-defined. However, MWNTs demonstrate some advantages over SWNTs, such as scalability due to simplified mass production, improved thermal and chemical stability, and low production cost per unit.

Generally, when functionalized the electrical and mechanical properties of SWNTs can fluctuate, this is due to the structural defects when C=C bond breakages occur during chemical processes. However, the innate properties of carbon nanotubes can be retained by the surface modification of MWNTs: exposure of the outer wall of MWNTs to chemical modifiers.

Surface modification of CNTs is conducted to establish new properties in carbon nanotubes for distinctive applications that necessitate enhancement of functionality, organic solvent or water-solubilization, dispersion, and compatibility or reducing the toxicity of CNTs.12 Figure 1 exhibits several ways to chemically modify the surface of CNTs.

Common functionalized CNTs, such as MWNT-COOH (Product No. 755125), are acquired via oxidation using a variety of acids, ozone, or plasma, which produces other oxygen functional groups (e.g., -OH, -C=O). The existence of oxygen-containing groups encourages the exfoliation of CNT bundles and improves the solubility in polar media and the chemical affinity with compounds containing ester, such as polyesters.

COOH groups on nanotube surfaces are beneficial sites for advanced modification. Different molecules, such as synthetic and natural polymers can be grafted through the production of amide and ester bonds.13

Figure 1. Schematic examples of surface functionalization of CNTs. (Illustration from Zhao et al.12)

Double-walled carbon nanotubes (DWNTs) (Product No. 755168, 755141) are a synthetic combination of both single-walled and multi-walled nanotubes, exhibiting properties intermediate between the two types. DWNTs are made up of precisely two concentric nanotubes separated by 0.35 0.40 nm, with band gaps appropriate for use in field-effect transistors.14 The inner and outer walls of DWNTs have the optical and Raman scattering attributes of each wall.15

Theoretically, if each wall acts like an SWNT and according to (n, m) values of their inner and outer walls, DWNTs can hold four combinations based on the electronic type (metallic or semiconducting) e.g., metallic-metallic (inner-outer), metallic-semiconducting, semiconducting-metallic, and semiconducting-semiconducting.

Several innovative studies discovered that DWNTs may behave as a metal even though both walls are semiconducting.16 This particular snag concerning their overall electrical behavior has restricted the utility of DWNTs to applications such as thin-film electronics.

Yet, DWNTs also demonstrate a range of advantageous properties observed from MWNTs, such as enhanced lifetimes and current densities for field emission and great stability under hostile chemical, mechanical and thermal treatments along with the flexibility observed with SWNTs.17

Selective functionalization of the DWNTs outer wall has promoted their use as core-shell systems made of a clean carbon nanotube core and chemically-functionalized nanotube shells, which are appropriate as imaging and therapeutic agents in biological systems.18

DWNTs can be employed in gas sensors19 as sensitive materials for the detection of gases such as H2, O2, NO2 or NH3, dielectrics,20 and technically challenging applications, such as photovoltaics and field-emission displays.21

Merck offers premium quality SWNTs, MWNTs, and DWNTs, among which are some of the most electrically conductive additives on the market today, ready for your creative and advanced materials research needs. Where indicated, these nanotubes are developed via the catalytic chemical vapor deposition (CCVD) technique, a well-known industrial process with a proven reliability and scalability factor.

Mercks nanotubes are also purified or functionalized to enhance the performance for research applications where particular chemical properties like high field emission characteristics, large surface area, or transparency are required.

Carbon nanotubes can be utilized across a broad range of new and existing applications including those listed below:

Detailed descriptions of available carbon nanotubes are exhibited in Table 3. The presented specification details will help guide you in selecting the appropriate material for your application.

Table 3. Specification details for carbon nanotubes.Source:Merck Millipore Sigma

*TEM images with reprints permission granted by Nanocyl SA.

This information has been sourced, reviewed and adapted from materials provided by Merck.

For more information on this source, please visit Merck.

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