The difference between single-walled carbon nanotubes and multi-walled carbon nanotubes

 Carbon nanotubes are simply classified according to the number of layers of graphene sheets: single-walled carbon nanotubes and multi-walled carbon nanotubes. The difference between the two is mainly reflected in:

1. Discover the time difference
In 1991, when Li Ji-ma, an electron microscope expert in the Basic Research Laboratory of NEC Corporation, Japan, observed the C60 structure under a high-resolution transmission electron microscope, he accidentally discovered a diameter of 4-30nm, a length of μm, and a graphite structure on the tube wall. Multilayer carbon molecules, this is the carbon nanotubes that are widely concerned today. However, what Li Ji-ma discovered at the time was not single-walled carbon nanotubes. The minimum number of layers is 2, called double-walled carbon nanotubes. In 1993, the research teams of NEC and IBM successfully synthesized single-layer carbon nanotubes (SWCNTs) at the same time. Figure 1

2. Different structure
Multilayer carbon nanotubes are hollow small tubes constructed from a coaxial cylindrical surface sleeve of multiple hexagonal lattices of carbon atoms, while single-walled carbon nanotubes are composed of only one carbon cylindrical surface. The wall of this tubular structure is composed of hexagonal grids similar to graphite sheets. The intersection of the hexagonal grids is where the carbon atoms are located, and each carbon atom is adjacent to the surrounding carbon atoms. At the same time, both ends of the tube (end caps) are closed by a polygonal structure composed of pentagonal carbon rings.

3. Different preparation methods
Single-walled carbon nanotubes have huge potential application prospects, and the research on single-walled carbon nanotubes has made great progress, but the large-scale production and controllable growth of single-walled carbon nanotubes have not yet been realized. Hongwu Nano is moving towards large-scale technology. With the continuous deepening and expansion of basic research and application research, the demand for single-walled carbon nanotubes will become greater and higher, and higher and higher requirements will be placed on the structure. How to realize the large-scale production of single-walled carbon nanotubes How to control the diameter, length and helicity of single-walled carbon nanotubes has become the focus of research in Hongwu’s nanotechnology R&D department. The current methods for preparing single-walled carbon nanotubes mainly include arc discharge, chemical vapor deposition (CVD), and laser evaporation.
The main synthesis method of multi-wall carbon nanotubes is chemical vapor deposition. The basic principle is that the gas containing carbon source (C6H6, C2H2, C2H4) decomposes when flowing over the surface of the catalyst to produce carbon nanotubes.

4. Different purification
The properties of single-walled carbon nanotubes and multi-walled carbon nanotubes are different, so their purification methods are also different, and because different preparation methods and test conditions introduce different impurities, the purification methods also differ depending on the specific preparation methods. It can be roughly divided into physical methods and chemical methods. The most feasible physical method at present is the filtration method. Chemical purification methods mainly include gas-phase oxidation, liquid-phase oxidation and so on.

Purification of single-walled carbon nanotubes
Carbon nanotubes are generally prepared by arc discharge, laser evaporation, and catalytic pyrolysis, and the prepared products contain impurities such as amorphous carbon and catalysts, which need to be purified. The purification methods generally include physical and chemical methods. For different preparation methods, purification methods are also different. For example, the purity of the original sample of single-walled carbon nanotubes prepared by the arc discharge method is about 50%, and the impurities mainly include: amorphous carbon, metal catalysts and graphite particles . For the impurities contained, there are the following purification methods:
(1) Soak the single-walled carbon nanotube sample in alcohol for 24 hours to remove organic impurities in the sample;
(2) The immersed single-walled carbon nanotube sample was refluxed and boiled with 3MHNO3 solution for 30h, aiming to strip and oxidize the carbon particles and amorphous carbon attached to the single-walled carbon nanotube, and at the same time remove the metal catalyst particles in the sample ；
(3) Boiling the sample in 30% H202: for 15 minutes to further oxidize the residual carbon impurities in the sample; (4) Place the sample in a NaOH solution with a pH of 10 to 11, oscillate ultrasonically for 2 hours, and use hydrogen and oxygen The combination of root ions and carbonaceous fragments strips off the carbonaceous fragments produced in the two steps (2) and (3) above and attached to the surface of the carbon tube bundle, thereby obtaining a single-walled carbon nanotube sample with a clean surface; (5) ) Heat the sample at 900°C for 1h under the protection of Ar atmosphere. After each of the above steps is completed, rinse the sample with deionized water to neutrality and filter.

The above purification method. It can effectively remove amorphous carbon and metal catalyst particles in the original sample of single-walled carbon nanotubes. The purity of the purified sample can exceed 95%, the yield is about 35%, and the process is simple and less time-consuming.

5. The parameters are different
Nano-single-walled carbon tube, outer diameter of 2 nanometers, long tube 5-20UM, short tube 1-2UM. Purity 90,95%.
Nano double-walled carbon tube, the outer diameter is 2 nanometers, the long tube is 5-20UM, and the short tube is 1-2UM. The purity is 90,95%.
Nano multi-walled carbon tube, outer diameter 10-30 nanometer, 30-60 nanometer, 60-100 nanometer, long tube 5-15UM, short tube 1-2UM. Purity 97-99%.

6. Different applications
At present, SWCNT single-walled carbon nanotubes have potential applications in field-effect transistors, single-molecule devices, and various chemical, physical, and biological sensors, and many results have been achieved.
MWCNT multi-walled carbon nanotubes are mainly used in catalyst carriers, nano electronic components, field effect emission materials, polymer enhancers, etc. At present, SAT NANO can be mass-produced.