The arc discharge method is a well-established technique for synthesizing carbon nanotubes, known for producing high-quality structures with good crystallinity. The process involves creating an electric arc between two graphite electrodes in an inert gas environment, leading to the vaporization of carbon atoms and subsequent condensation into tubular nanostructures. This method has been instrumental in the early discovery and large-scale production of both single-walled and multi-walled carbon nanotubes.

The setup for arc discharge synthesis consists of two graphite electrodes placed in a reaction chamber filled with an inert gas, typically helium or argon, at pressures ranging from 100 to 700 Torr. The anode is usually a pure graphite rod, while the cathode may be pure graphite or contain a metal catalyst, such as iron, cobalt, or nickel, to facilitate the formation of single-walled carbon nanotubes. A direct current power supply is used, with voltages typically between 15 and 30 volts and currents ranging from 50 to 150 amperes. The high current causes the anode to erode, generating a plasma arc with temperatures exceeding 3000°C, sufficient to vaporize carbon atoms from the graphite.

The formation mechanism of carbon nanotubes in the arc discharge process involves several steps. As the anode is consumed, carbon atoms are ejected into the plasma and ionized. These carbon species then condense on the cooler cathode or chamber walls, forming nanotubes. The presence of a metal catalyst is critical for SWNT growth, as it helps nucleate the tubes and stabilize their formation. Without a catalyst, MWNTs are more commonly produced due to the spontaneous stacking of graphene layers. The inert gas environment plays a crucial role in controlling the reaction dynamics, as it prevents oxidation and helps maintain the high temperatures required for carbon vaporization.

The arc discharge method yields a mixture of carbon nanotubes, amorphous carbon, and metal catalyst particles. The product typically contains 30-70% nanotubes by weight, with the remainder consisting of impurities. SWNTs produced by this method often exhibit diameters between 1 and 2 nanometers, while MWNTs can range from 5 to 50 nanometers in outer diameter, with lengths varying from several hundred nanometers to micrometers. The crystallinity of arc-discharge CNTs is generally high, with well-defined graphene layers and minimal structural defects.

Purification of the raw material is necessary to remove unwanted byproducts. Common techniques include acid treatment using nitric or hydrochloric acid to dissolve metal particles, followed by oxidation in air or with oxidizing agents to eliminate amorphous carbon. These steps can improve purity to over 90%, though they may introduce functional groups or defects on the nanotube surfaces. Centrifugation and filtration are often employed to separate nanotubes from residual impurities.

Characterization of arc-discharge CNTs involves multiple analytical techniques. Transmission electron microscopy provides direct visualization of nanotube morphology, revealing the number of walls, diameter distribution, and structural integrity. Raman spectroscopy is used to assess the quality and electronic structure of the nanotubes, with the radial breathing mode being particularly diagnostic for SWNTs. Thermogravimetric analysis helps quantify the amount of amorphous carbon and metal residues, while X-ray diffraction can confirm the crystalline nature of the material.

The advantages of the arc discharge method include the production of nanotubes with excellent crystallinity and fewer structural defects compared to other synthesis routes. The process is relatively straightforward and does not require complex equipment or high vacuum conditions. Additionally, the high temperatures involved promote the formation of straight, well-aligned nanotubes with good electrical and mechanical properties.

However, the method has several limitations. The yield of nanotubes is often low compared to the total carbon consumed, and the process generates significant amounts of unwanted byproducts. Metallic impurities from the catalyst can persist even after purification, which may interfere with certain applications. The high energy consumption and difficulty in scaling up the process are also notable drawbacks. Furthermore, controlling the chirality and diameter distribution of SWNTs remains challenging, leading to mixtures of metallic and semiconducting nanotubes.

The arc discharge method continues to be relevant for applications requiring high-quality carbon nanotubes, particularly where superior crystallinity and electronic properties are essential. Ongoing research focuses on optimizing catalyst composition, gas pressure, and current parameters to improve yield and selectivity while reducing impurities. Despite its limitations, this technique remains a cornerstone in the field of carbon nanotube synthesis, providing material for fundamental studies and specialized applications.