Editing Carbon nanotube (section)

== Synthesis ==
{{Main|Synthesis of carbon nanotubes}}

Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, [[chemical vapor deposition]] (CVD) and high-pressure carbon monoxide disproportionation (HiPCO). Among these arc discharge, laser ablation are batch by batch process, Chemical Vapor Deposition can be used both for batch by batch or continuous processes,<ref name="Endo – 2004">{{cite journal | vauthors = Endo M | title = Applications of carbon nanotubes in the twenty-first century | journal = Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences | volume = 362 | issue = 1823 | pages = 2223–2238 | date = October 2004 | doi = 10.1098/rsta.2004.1437 | pmid = 15370479 | s2cid = 20752554 }}</ref><ref name="Zhou – 2003">{{cite journal | vauthors = Zhou Z | title = Producing cleaner double-walled carbon nanotubes in a floating catalyst system | journal = Carbon | volume = 41 | issue = 13 | pages = 2607–2611 | date = January 2003 | doi = 10.1016/S0008-6223(03)00336-1 | bibcode = 2003Carbo..41.2607Z }}</ref> and HiPCO is gas phase continuous process.<ref name="Nikolaev-2004">{{cite journal | vauthors = Nikolaev P | title = Gas-phase production of single-walled carbon nanotubes from carbon monoxide: a review of the hipco process | journal = Journal of Nanoscience and Nanotechnology | volume = 4 | issue = 4 | pages = 307–316 | date = April 2004 | pmid = 15296221 | doi = 10.1166/jnn.2004.066 }}</ref> Most of these processes take place in a vacuum or with process gases. The CVD growth method is popular, as it yields high quantity and has a degree of control over diameter, length and morphology. Using particulate catalysts, large quantities of nanotubes can be synthesized by these methods, and industrialisation is well on its way, with several CNT and CNT fibers factory around the world. One problem of CVD processes is the high variability in the nanotube's characteristics <ref>{{cite book|url=https://books.google.com/books?id=L_zVa647-80C&pg=PA476 |title= Nanomedicine Design of Particles, Sensors, Motors, Implants, Robots, and Devices| vauthors = Schulz MJ, Shanov VN, Yun Y |date=2009 |publisher=Artech House|isbn=978-1-59693-280-7}}</ref> The HiPCO process advances in catalysis and continuous growth are making CNTs more commercially viable.<ref>{{cite journal | vauthors = Takeuchi K, Hayashi T, Kim YA, Fujisawa K, Endo M | title = The state-of-the-art science and applications of carbon nanotubes. | journal = Nanosystems: Physics, Chemistry, Mathematics | date = February 2014 | volume = 5 | issue = 1 | pages = 15–24 | url = http://nanojournal.ifmo.ru/en/articles-2/volume5/5-1/paper01/ }}</ref> The HiPCO process helps in producing high purity single-walled carbon nanotubes in higher quantity. The HiPCO reactor operates at high [[temperature]] 900–1100&nbsp;°C and high [[pressure]] ~30–50 bar.<ref>{{cite journal | vauthors = Bronikowski MJ, Willis PA, Colbert DT, Smith KA, Smalley RE |title=Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study |journal=Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films |date=July 2001 |volume=19 |issue=4 |pages=1800–1805 |doi=10.1116/1.1380721 |bibcode=2001JVSTA..19.1800B |s2cid=3846517 }}</ref> It uses [[carbon monoxide]] as the carbon source and [[iron pentacarbonyl]] or [[nickel tetracarbonyl]] as a catalyst. These catalysts provide a [[nucleation]] site for the nanotubes to grow,<ref name="Nikolaev-2004" /> while cheaper iron-based catalysts like Ferrocene can be used for CVD process.

[[Vertically aligned carbon nanotube arrays]] are also grown by thermal chemical vapor deposition. A substrate (quartz, silicon, stainless steel, carbon fibers, etc.) is coated with a catalytic metal (Fe, Co, Ni) layer. Typically that layer is iron and is deposited via sputtering to a thickness of 1–5&nbsp;nm. A 10–50&nbsp;nm underlayer of alumina is often also put down on the substrate first. This imparts controllable wetting and good interfacial properties.
When the substrate is heated to the growth temperature (~600 to 850&nbsp;°C), the continuous iron film breaks up into small islands with each island then nucleating a carbon nanotube. The sputtered thickness controls the island size and this in turn determines the nanotube diameter. Thinner iron layers drive down the diameter of the islands and drive down the diameter of the nanotubes grown. The amount of time the metal island can sit at the growth temperature is limited as they are mobile and can merge into larger (but fewer) islands. Annealing at the growth temperature reduces the site density (number of CNT/mm<sup>2</sup>) while increasing the catalyst diameter.

The as-prepared carbon nanotubes always have impurities such as other forms of carbon (amorphous carbon, fullerene, etc.) and non-carbonaceous impurities (metal used for catalyst).<ref>{{cite journal| vauthors = Itkis ME, Perea DE, Niyogi S, Rickard SM, Hamon MA, Hu H, Zhao B, Haddon RC |date=2003-03-01|title=Purity Evaluation of As-Prepared Single-Walled Carbon Nanotube Soot by Use of Solution-Phase Near-IR Spectroscopy|journal=Nano Letters|volume=3|issue=3|pages=309–314|doi=10.1021/nl025926e |bibcode = 2003NanoL...3..309I}}</ref><ref>{{cite journal | vauthors = Wang L, Pumera M | title = Residual metallic impurities within carbon nanotubes play a dominant role in supposedly "metal-free" oxygen reduction reactions | journal = Chemical Communications | volume = 50 | issue = 84 | pages = 12662–12664 | date = October 2014 | pmid = 25204561 | doi = 10.1039/C4CC03271C }}</ref> These impurities need to be removed to make use of the carbon nanotubes in applications.<ref>{{cite journal | vauthors = Eatemadi A, Daraee H, Karimkhanloo H, Kouhi M, Zarghami N, Akbarzadeh A, Abasi M, Hanifehpour Y, Joo SW | title = Carbon nanotubes: properties, synthesis, purification, and medical applications | journal = Nanoscale Research Letters | volume = 9 | issue = 1 | page = 393 | date = 2014-08-13 | pmid = 25170330 | pmc = 4141964 | doi = 10.1186/1556-276X-9-393 | bibcode = 2014NRL.....9..393E | doi-access = free }}</ref>