Properties and Applications of Single-, Double- and Multi-Walled Carbon Nanotubes

By: Jia Choi, PhD and Yong Zhang, PhD, Aldrich Materials Science, Sigma-Aldrich Co. LLC

Carbon nanotubes (CNTs) were fortuitously discovered by Iijima while studying the surface of graphite electrodes in an electric arc discharge.1 Since their discovery, CNTs have held a fundamental role in the field of nanotechnology due to their unique structural, mechanical and electronic properties.1-3 CNTs have high conductivity and high aspect ratio which help them to form a network of conductive tubes. Outstanding mechanical properties of CNTs are derived from a combination of stiffness, strength, and tenacity.4 Incorporated within a polymer, CNTs transfer their mechanical load to the polymer matrix at a much lower weight percentage than carbon black or carbon fibers, leading to more efficient applications. CNTs have also been utilized for thermal protection as thermal interface materials. Their interesting electronic and mechanical properties can be used in numerous applications, such as field-emission displays,5 nanocomposite materials,6 nanosensors,7 and logic elements.8 CNTs have been extensively studied for utility in leading-edge electronic fabrication and also extended to pharmaceutical fields for treatment of several types of diseases.9

Single-walled carbon nanotubes (SWNTs) (Aldrich Product No. 755710) are seamless cylinders comprised of a layer of graphene. They have unique electronic properties which can change significantly with the chiral vector, C = (n, m), the parameter that indicates how the graphene sheet is rolled to form a carbon nanotube (Figure 1).10

Carbon nanotube configurations

Figure 1. Carbon nanotube configurations with the chiral vector C and unit vectors a and b.

The dependence of SWNTs electrical conductivity on the (n, m) values is shown in Table 1. Depending on how they are rolled, SWNTs' band gap can vary from 0 to 2 eV and electrical conductivity can show metallic or semiconducting behavior.

Table 1. Theoretical electronic conductivity of single-walled carbon nanotubes (SWNTs) depending on roll orientation of the graphene sheet (n, m).10

(n, m) Form of SWNTs
Electrical Conductivity
(n, 0)
Zigzag
Metallic when n is the multiple of 3, otherwise, semiconducting
(n, n)
Armchair
Metallic
(n, m)
when m ≠ 0, and n

Chiral
Metallic when (2n+m)/3 is an integer, otherwise, semiconducting

Thermal and electrical conductivities of carbon nanotubes are very high, and comparable to other conductive materials as shown in Table 2.11

Table 2. Transport properties of carbon nanotubes and other conductive materials.11

Material Thermal conductivity
 [W/(m·K)]
Electrical Conductivity
[S/m]
Carbon Nanotubes > 3,000 106 – 107
Copper 400 6 x 107
Carbon Fiber – Pitch 1,000 2 – 8.5 x 106
Carbon Fiber – PAN 8 - 105 6.5 – 14 x 106

 

Multi-walled carbon nanotubes (MWNTs) (Aldrich Product Nos. 755133, 755117) consist of multiple rolled layers of graphene. MWNTs have not been well-defined due to their structural complexity and variety when compared to SWNTs. Nonetheless, MWNTs exhibit advantages over SWNTs, such as ease of mass production, low product cost per unit, and enhanced thermal and chemical stability. In general, the electrical and mechanical properties of SWNTs can change when functionalized, due to the structural defects occurred by C=C bond breakages during chemical processes. However, intrinsic properties of carbon nanotubes can be preserved by the surface modification of MWNTs, where the outer wall of MWNTs is exposed to chemical modifiers.

Surface modification of CNTs is performed to introduce new properties to carbon nanotubes for highly specific applications which often require organic solvent or water-solubilization, enhancement of functionality, dispersion and compatibility or lowering the toxicity of CNTs.12  Figure 2 illustrates many of the routes to chemically modify the surface of CNTs. Common functionalized CNTs, such as MWNT-COOH (Aldrich Product No. 755125), are obtained via oxidation using various acids, ozone or plasma, which creates other oxygen functional groups (e.g., -OH, -C=O). The presence of oxygen-containing groups promotes the exfoliation of CNT bundles, and enhances the solubility in polar media and the chemical affinity with ester containing compounds, such as polyesters. COOH groups on nanotube surfaces are useful sites for further modification. Various molecules, such as synthetic and natural polymers can be grafted through the creation of amide and ester bonds.13

Surface functionalization of CNTs

Figure 2. Schematic examples of surface functionalization of CNTs. (Illustration adapted from Zhao et al.12)

Double-walled carbon nanotubes (DWNTs) (Aldrich Product Nos. 755168, 755141) are a synthetic blend of both single-walled and multi-walled nanotubes, showing properties intermediate between the two types. DWNTs are comprised of exactly two concentric nanotubes separated by 0.35 – 0.40 nm, with sufficient band gaps for use in field-effect transistors.14 The inner and outer walls of DWNTs have optical and Raman scattering characteristics of each wall.15 Theoretically, if each wall behaves like a SWNT, DWNTs can consist of four combinations based on the electronic type (metallic or semiconducting) according to (n, m) values of their inner and outer walls, e.g., metallic-metallic (inner-outer), metallic-semiconducting, semiconducting-metallic, and semiconducting-semiconducting. Some experimental studies found that even though both walls are semiconducting, DWNTs may behave as a metal.16 This complication of their overall electrical behavior has limited the utility of DWNTs to applications such as thin film electronics. However, DWNTs also exhibit several beneficial properties observed from MWNTs, such as improved lifetimes and current densities for field emission and high stability under aggressive chemical, mechanical, and thermal treatments along with the flexibility observed with SWNTs.17 Selective functionalization of the outer wall has led to the use of DWNTs as core-shell systems made of a pristine carbon nanotube core and chemically-functionalized nanotube shells, which are applicable as imaging and therapeutic agents in biological systems.18 DWNTs can be utilized in gas sensors19 as sensitive materials for the detection of gases such as such as H2, NH3, NO2 or O2, dielectrics,20 and technically demanding applications, such as field-emission displays and photovoltaics.21 An example of DWNT solar cell fabrication process is shown in Figure 3.

Schematic of the fabrication process employed to develop a DWNT/n-type Si solar cell

Figure 3. Schematic of the fabrication process employed to develop a DWNT/n-type Si solar cell.21

Aldrich® Materials Science offers high quality SWNTs, MWNTs and DWNTs, some of which are the most electrically conductive additives available today, for your innovative and advanced materials research needs. When indicated, these nanotubes are produced via the catalytic chemical vapor deposition (CCVD) technique, a proven industrial process well-known for its reliability and scalability, and are purified or functionalized to increase the performance for research applications where special chemical properties like high surface area, transparency, or high field emission characteristics are required.  

Carbon nanotubes offered by Aldrich Materials Science can be used for a wide range of new and existing applications:

  • Conductive plastics
  • Structural composite materials
  • Flat-panel displays
  • Gas storage
  • Antifouling paint
  • Micro- and nano-electronics
  • Radar-absorbing coating
  • High functional textiles
  • Ultra-capacitors
  • Atomic Force Microscope (AFM) tips
  • Batteries with improved lifetime
  • Biosensors for harmful gases
  • Extra strong and conductive fibers
  • Targeting Drug Delivery
  • Bioengineering applications such as energy storage and conversion devices, radiation sources, and hydrogen storage media

Detail descriptions of carbon nanotubes available from Aldrich Materials Science are shown in Table 3. Specification details provided will help you select the right material for your application.

Table 3. Specification details for carbon nanotubes available from Aldrich Materials Science

 

Aldrich Product No. TEM Image Description
755710
  • Single-walled isolated and bundled carbon Nanotubes powder
  • 2 nm (diameter, measured by HRTEM) x several µm (length, measured by TEM / SEM)
  • Carbon purity : > 70 % (by TGA)
  • Metal oxide impurity: < 30 % (by TGA)
  • High specific surface area (> 1000 m2/g by BET)
755168
  • Double-walled isolated and bundled carbon nanotubes powder
  • 3.5 nm (diameter, by HRTEM) x 1 - 10 µm (length, by TEM / SEM)
  • Carbon purity : > 90 % (by TGA)
  • Metal oxide impurity: < 10 % (by TGA)
  • Specific surface area:  >500  m2/g (by BET)
  • High filed emission characteristics
  • Transparency
755141
  • Short double-walled isolated and bundled carbon nanotubes powder
  • 3.5 nm (diameter, by HRTEM) x 3 µm (length, by TEM / SEM)
  • Carbon purity : > 90 % (by TGA)
  • Metal oxide impurity: < 10 % (by TGA)
  • Surface chemistry characteristics
  • Ease of dispersability
755133
  • Thin multi-walled (avg. 7~9 walls) carbon nanotubes powder
  • 9.5 nm (diameter, by TEM) x 1.5 µm (length, by TEM)
  • Carbon purity : > 95 % (by TGA)
  • Metal oxide impurity: < 5 % (by TGA)
  • High level of purity
755125
  • Thin multi-walled (avg. 7~9 walls) -COOH functionalized carbon nanotubes powders
  • COOH functionalization:  > 8%
  • 9.5 nm (diameter, by TEM) x 1.5 µm (length, by TEM)
  • Carbon purity : > 95 % (by TGA)
  • Metal oxide impurity: < 5 % (by TGA)
  • High level of purity 
755117
  • Short thin multi-walled (avg. 7~9 walls) carbon nanotubes powder
  • 9.5 nm (diameter, by TEM) x < 1 µm (length, by TEM)
  • Carbon purity : > 95 % (by TGA)
  • Metal oxide impurity: < 5 % (by TGA)
  • Surface chemistry characteristics
  • Ease of dispersability

*TEM images with reprints permission granted by Nanocyl SA.

 

 References

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  19. I. Sayagoa, H. Santosa, M.C. Horrillo a, M. Aleixandrea, M. J. Fernándeza, E. Terradob, I. Tacchini, R. Arozc,W.K. Maserb, A.M. Benitob, M.T. Martínezb, J. Gutiérreza, E. Munoz, Talanta, 2008, 77, 758-764.
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