Carbon Nanotubes

Carbon nanotubes were discovered in 1991 by Sumiyo Iijima, a Japanese scientist working at the NEC Corporation. A carbon nanotube (CNT) is a tubular form of carbon with a diameter as small as 0.4 nm and length from a few nanometers up to a millimeter.  The length-to-diameter ratio of a carbon nanotube can be as large as 28,000,000:1, which is unequalled by any other material.

Carbon exists in several forms; graphite and diamond are the most familiar. To imagine how a carbon nanotube looks like, think about taking a single layer of a graphite sheet, cutting it into a small piece of any size, and rolling it like you would roll a cigar.  The result is a single-wall carbon nanotube (SWCNT). If you take multiple layers of a graphite sheet and roll them like a cigar, then you get a multiwall carbon nanotube (MWCNT).

Of course, no one is sitting out there rolling graphite sheets in order to make nanotubes. These are grown in laboratories, often using a process called chemical vapor deposition.

Though many sophisticated commercial growth reactors are available, carbon nanotubes can be made in a standard chemistry laboratory. A quartz tube about 2.5 cm in diameter serves as the growth reactor and is inserted inside a tube furnace (a tube furnace is a standard heating device for conducting syntheses and purifications). The nanotube is grown on a silicon wafer that is placed at a central location inside the quartz tube. A thin layer of iron or nickel or cobalt is applied to the silicon wafer to serve as a catalyst to grow the nanotubes. A hydrocarbon such as methane (high purity form of natural gas) or ethane or acetylene is sent through the reactor tube which is heated to 750-900ºC by the furnace.

Materials that are comprised of many small structures often have a very large surface area.  The reason is that the surface areas of the many small structures together contribute to a very large surface area of the bulk material. For example, if a piece of material the size of a shoe box were to be split up into 2 nm spheres, these spheres would contribute to a surface area of the shoe box that correspond to the area of 10,000 football fields. It is not surprising that when the surface area is so large, the physical properties are dominated by surface interactions.

Nanomaterials come in many varieties. A few examples of materials that exhibit interesting properties on the nanoscale are carbon nanotubes, inorganic nanowires, dendrimers, nanoparticles, graphene, and quantum dots.