What is Double-Walled Carbon Nanotube?
DWCNTs (double-walled carbon nanotubes) are coaxial nanostructures composed of two nested SWCNTs. Both layers of carbon nanotubes can interact with each other without altering the composition of the inner tube, making DWCNTs physically and chemically unique.
DWCNT inner and outer tubes are of different diameters (inner tube 0.8-1.1 nm, outer tube 1.6-1.8 nm, interlayer spacing 0.34-0.39 nm). It is a structure that provides not just mechanical strength and thermal resistance but also electronic, optical and chemical efficiency.
Compared to single-walled carbon nanotubes, DWCNTs are more stable and don't leak energy, so they're more electrically transmissible. But more than that, because inner and outer tubes are fused together, DWCNTs can also be infused with many other interesting electronic and optical properties, such as metal-metal, metal-semiconductor, semiconductor-metal, and semiconductor-semiconductor.
Double-walled carbon nanotubes, in short are two layers of nested single-walled carbon nanotubes, bonded to each other coaxially and very good mechanical, thermal, electrical and optical properties, they are used in a lot of high technology.
| Double-Walled Carbon Nanotubes | Inquiry |
| DWCNTs, >60% | Inquiry |
| Short DWCNTs, >60% | Inquiry |
Properties of DWNTs
Structural Characteristics
Double-walled carbon nanotubes (DWCNTs) consists of two central single-walled carbon nanotubes, and the interlayer spacing will usually be between 0.33 nm and 0.42 nm. This spacing allows for interactions between the inner and outer nanotube layers.
This structure makes DWCNTs mechanically more resilient and thermally stable than single-walled carbon nanotubes (SWCNTs).
Electronic Properties
Electronic features of DWCNTs are dictated by chirality and interlayer spacing between inner and outer nanotubes. In metal-metal, semiconductor-metal, metal-semiconductor or semiconductor-semiconductor, for example, the interaction can produce different electronic characteristics.
Sometimes, if the structure of the inner and outer nanotubes is tuned, the bandgap can be tuned to determine the semiconductor properties.
DWCNTs were found to be more conductive and their electrical characteristics could be controlled by varying the chirality of the inner and outer nanotubes.
Mechanical Properties
DWCNTs are stronger and stiffer than SWCNTs, capable of withstanding extreme conditions.
Their high Young's modulus indicates better elasticity when subjected to stress.
Chemical Stability
DWCNTs exhibit higher resistance to chemical substances, making them more advantageous for chemical functionalization. For example, the outer layer can be modified with grafted chemical functional groups to improve hydrophilicity and chemical sensitivity, and the inner layer doesn't lose its shape and still has excellent mechanical and electrical properties.
Double-walled carbon nanotubes with their characteristic geometry and unique properties are promising both in science and in industry.
Synthesis Methods
Arc Discharge
DWNTs and MWNTs were found in 1991, by Sumio Iijima, in the cathode deposit of an experiment to synthesize C60 via the arc-discharge process. Yet, until 2001 J. L. Hutchison et al. described the selective growth of DWNTs through arc-discharge. They synthesised DWNTs with purity of 10-20 to 50-70 wt% under the atmosphere of Ar and H2 mixture (1 : 1/v : v) at 350 Torr using a graphite anode catalyst made of a mixture of Ni, Co, Fe and S. The synthesised DWNTs have outer tube diameters ranging from 1.9-5 nm and inner tube diameters ranging from 1.1-4.2 nm as measured by high-resolution transmission electron microscopy.
Peapod Growth
The hollow tubular form of CNTs allows them to be used as nano test-tubes or templates for new nanostructures. The first fullerene filled SWNTs (''peapods'') were spotted in 1998 by Smith et al. during purification and annealing of SWNTs fabricated using laser ablation process. Their follow-up found that electron bombardment of the C60 molecules stuck in a SWNT could drive them to diffuse and coalesce. For the first time, DWNTs have been generated from a C60@SWNT peapod. C60, C70 and ferrocene-filled SWNTs have all been prepared from these precursors to prepare DWNTs by annealing in vacuum or inert gas atmosphere since this initial example.
Catalytic Chemical Vapor Deposition
Catalytic CVD is the most common method for producing DWNTs. It involves releasing gaseous or volatile carbon over metallic nanoparticles, which become catalytic and nucleation sites for CNT initial growth. Catalytic CVD-generated DWNTs, the first catalytic example of them is attributed to J. H. Hafner et al. who discovered that 70 % of DWNTs were produced by catalytic reduction of C2H4 on a 90 : 9 : 1 alumina : Fe : Mo catalyst at 850 ℃. There have been a number of recipes for synthesising DWNTs since.
Dispersion Guide
Dispersion of DWCNTs is a complex and essential operation because carbon nanotubes aggregate strongly and have a negative effect on applications. Here are some recommendations for spreading DWCNTs:
1. Selection of Dispersion Methods
Physical Methods: These are ultrasonic dispersion, grinding dispersion and ball milling. These technologies use mechanical action to break up mounds of carbon nanotubes and thus make dispersion. Ultrasonic dispersion, for instance, uses the cavitation induced by ultrasonic vibrations to disintegrate CNT chains. Another approach is combined "grinding dispersion followed by ultrasonic dispersion".
Chemical Methods: Surfactants or chemical agents may be added for dispersion. A solvent mixture of N,N-dimethylformamide (DMF) and ethylene glycol can, for instance, disperse DWCNTs. Then there is also Na-CMC (sodium carboxymethyl cellulose) as a dispersant that separates the CNTs by physical segregation and electrostatic attraction.
2. Selection and Dosage of Dispersants
Dispersing requires the right dispersant. Popular dispersants are surfactants like Triton X-100, SDS, and polymers like Na-CMC. The amount of dispersant recommended for the multiwalled carbon nanotubes is 0.1-4 times the carbon nanotube weight, based on the nanotubes' outer diameter.
Functioned carbon nanotubes are more facile to disperse, and the dispersant can usually be decreased. For instance, the dispersant dose for carboxylated single-walled carbon nanotubes can be reduced by half.
3. Optimization of the Dispersion Process
In practical operations, the dispersion process may need to be cycled several times to ensure thorough dispersion. For example, during microfiltration (MF), multiple cycles can significantly improve the dispersion efficiency of CNTs.
Following dispersion, the solution should remain at some concentration and viscosity for future use. Screen printing, for instance, needs a viscosity of 1 Pa-s to 10 Pa-s.
4. Evaluation of Dispersion Effectiveness
It is possible to measure dispersion efficiency using transmission electron microscopy (TEM), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and others. These experiments enable us to validate whether the carbon nanotubes are evenly distributed and if they are structurally sound.
5. Challenges in Industrial Applications
For industrial production in scale, traditional methods like ultrasonic and centrifugal dispersion might suffer from poor dispersion, processing time and high costs. Therefore, developing environmentally friendly and scalable dispersion processes is particularly important. For instance, water and cellulose as solvents and dispersants not only do away with harmful elements but are also highly industrially compatible and scalable.
Dispersion of DWCNTs needs to be performed holistically, taking physical and chemical techniques into account, dispersants should be used, and dispersion must be optimized to get the same stable dispersion solution. This will enable the nanotubes to perform optimally in practical applications.
Applications of DWNTs
1. Energy Storage
Batteries and Supercapacitors: Because of the conductivity and surface area of DWCNTs, they make for ideal electrodes for lithium-ion batteries and supercapacitors. They're stable, so they're long-life, and ideal for energy storage devices that need to be rapidly charged and discharged.
Hydrogen Storage: The high surface area and robust structure of DWCNTs enable efficient hydrogen storage, a promising solution for fuel cell technologies in electric vehicles and sustainable energy systems.
2. Biomedical Applications
Drug Delivery: DWCNTs' biocompatibility and functionalization potential make them suitable carriers for targeted drug delivery. Their stability allows drugs to remain protected until they reach the target site, and the double-walled structure offers an added layer of control over release mechanisms.
Bioimaging and Sensing: Functionalized DWCNTs can be used in bioimaging and diagnostic sensors due to their fluorescence and ability to be tailored for specific biological interactions.
3. Composite Materials
High-Strength Composites: DWCNTs can be used in industries such as aerospace, automotive and construction for reinforcement materials to create lightweight, high-performance composites. Their mechanical strength makes polymers and metals stronger and more durable — making them light and more strong.
Thermal Management: DWCNTs have good thermal conductivity, which is why they are very well-suited to heat dissipation — for instance, in electronics devices, where efficient cooling is crucial for performance and life.
4. Electronics and Sensors
Flexible Electronics: DWCNTs are compatible with flexible substrates, making them suitable for wearable electronics and sensors. Their high conductivity allows for efficient signal transmission even under stress or deformation.
Environmental and Chemical Sensors: Because they are abrasive to environmental influences, DWCNTs are employed in gas, pollution and chemical sensors. They provide very sensitive and fast response times which is necessary for environmental and industrial monitoring.
Double-Walled Carbon Nanotubes: Challenges and Opportunities
As a nanomaterial with a unique structure, DWCNTs have enormous scope in many fields because of their superior performance and unique structure. But there is a cycle of hindrances and opportunities in their evolution.
Challenges
1. Purity and Separation: Because in DWCNTs manufacturing they are mixed with SWNTs and MWNTs, it is difficult to separate high purity DWCNTs. Even though scientists have invented separation technologies with high efficiency, the problem still hinders DWCNT applications at scale.
2. Cost Issues: Despite the many superior properties of DWCNTs, their production cost remains high, which limits their widespread commercial application. Reducing production costs is critical to promoting the broader use of DWCNTs.
3. Technical Challenges: In use, controlling DWCNTs' growth, alignment and insertion in other materials is still a technical problem. It remains, for example, an impossibility to control DWCNT diameter, length and chirality parameters exactly to fit specific applications.
Opportunities
1. Broad Application Prospects: Due to their special optical, mechanical, and electronic attributes, DWCNTs are able to be applied to multiple areas. So, for instance, they have huge advantages in clear conductive films, ultra-thin fibres, batteries and supercapacitors, sensors, displays, etc.
2. Market Growth Potential: As the global market for high-performance materials grows rapidly, particularly in energy storage, electronics and biomedical applications, DWCNTs demand keeps growing. This industry is going to become a huge market in the future.
3. Technological Innovation and Breakthroughs: Researchers in the past few years have had several significant steps in synthesis, purification, and use of DWCNTs. Through improved catalytic CVD, for instance, DWCNTs of high quality could be more easily synthesized, and they could be explored in diverse domains.
Conclusion
Double-walled carbon nanotubes present industries with unprecedented potential due to their strength, stability and conductivity. Yet there is still manufacturing cost, dispersion, functionalization, and safety to overcome before they can be used to their full potential. With continued exploration and development, DWCNTs could become a basic nanomaterial that can transform energy, electronics, medicine and the environment.
References
- Sobamowo, M. G., et al. "Coupled effects of magnetic field, number of walls, geometric imperfection, temperature change, and boundary conditions on nonlocal nonlinear vibration of carbon nanotubes resting on elastic foundations." Forces in Mechanics 3 (2021): 100010.
- Iijima, Sumio. "Helical microtubules of graphitic carbon." nature 354.6348 (1991): 56-58.
- Hutchison, J. L., et al. "Double-walled carbon nanotubes fabricated by a hydrogen arc discharge method." Carbon 39.5 (2001): 761-770.
- Smith, Brian W., Marc Monthioux, and David E. Luzzi. "Encapsulated C60 in carbon nanotubes." Nature 396.6709 (1998): 323-324.
- Smith, B. W., et al. "Carbon nanotube encapsulated fullerenes: a unique class of hybrid materials." Chemical Physics Letters 315.1-2 (1999): 31-36.
- Hafner, J. H., et al. "Catalytic growth of single-wall carbon nanotubes from metal particles." Chemical Physics Letters 296.1-2 (1998): 195-202.
- Gabaudan, V., et al. "Double-walled carbon nanotubes, a performing additive to enhance capacity retention of antimony anode in potassium-ion batteries." Electrochemistry Communications 105 (2019): 106493.
- Bazli, L., et al. "A review of carbon nanotube/TiO2 composite prepared via sol-gel method." Journal of Composites and Compounds 1.1 (2019): 1-9.
