Carbon nanotube
A carbon nanotube (CNT) is a tube made of carbon with a diameter in the nanometre range (nanoscale). They are one of the allotropes of carbon. Two broad classes of carbon nanotubes are recognized:
Carbon nanotubes can exhibit remarkable properties, such as exceptional tensile strength[3] and thermal conductivity[4][5][6] because of their nanostructure and strength of the bonds between carbon atoms. Some SWCNT structures exhibit high electrical conductivity[7][8] while others are semiconductors.[9][10] In addition, carbon nanotubes can be chemically modified.[11] These properties are expected to be valuable in many areas of technology, such as electronics, optics, composite materials (replacing or complementing carbon fibres), nanotechnology (including nanomedicine[12]), and other applications of materials science.
The predicted properties for SWCNTs were tantalising, but a path to synthesising them was lacking until 1993, when Iijima and Ichihashi at NEC, and Bethune and others at IBM independently discovered that co-vaporising carbon and transition metals such as iron and cobalt could specifically catalyse SWCNT formation.[13][14] These discoveries triggered research that succeeded in greatly increasing the efficiency of the catalytic production technique,[15] and led to an explosion of work to characterise and find applications for SWCNTs.
Functionalization[edit]
CNTs are known to have weak dispersibility in many solvents such as water as a consequence of strong intermolecular p–p interactions. This hinders the processability of CNTs in industrial applications. To tackle the issue, various techniques have been developed to modify the surface of CNTs in order to improve their stability and solubility in water. This enhances the processing and manipulation of insoluble CNTs rendering them useful for synthesizing innovative CNT nanofluids with impressive properties that are tunable for a wide range of applications.
Chemical routes such as covalent functionalization have been studied extensively, which involves the oxidation of CNTs via strong acids (e.g. sulfuric acid, nitric acid, or a mixture of both) in order to set the carboxylic groups onto the surface of the CNTs as the final product or for further modification by esterification or amination. Free radical grafting is a promising technique among covalent functionalization methods, in which alkyl or aryl peroxides, substituted anilines, and diazonium salts are used as the starting agents.
Free radical grafting of macromolecules (as the functional group) onto the surface of CNTs can improve the solubility of CNTs compared to common acid treatments which involve the attachment of small molecules such as hydroxyl onto the surface of CNTs. The solubility of CNTs can be improved significantly by free-radical grafting because the large functional molecules facilitate the dispersion of CNTs in a variety of solvents even at a low degree of functionalization. Recently an innovative environmentally friendly approach has been developed for the covalent functionalization of multi-walled carbon nanotubes (MWCNTs) using clove buds. This approach is innovative and green because it does not use toxic and hazardous acids which are typically used in common carbon nanomaterial functionalization procedures. The MWCNTs are functionalized in one pot using a free radical grafting reaction. The clove-functionalized MWCNTs are then dispersed in water producing a highly stable multi-walled carbon nanotube aqueous suspension (nanofluids).[115]
Metrology[edit]
There are many metrology standards and reference materials available for carbon nanotubes.[117]
For single-wall carbon nanotubes, ISO/TS 10868 describes a measurement method for the diameter, purity, and fraction of metallic nanotubes through optical absorption spectroscopy,[118] while ISO/TS 10797 and ISO/TS 10798 establish methods to characterize the morphology and elemental composition of single-wall carbon nanotubes, using transmission electron microscopy and scanning electron microscopy respectively, coupled with energy dispersive X-ray spectrometry analysis.[119][120]
NIST SRM 2483 is a soot of single-wall carbon nanotubes used as a reference material for elemental analysis, and was characterized using thermogravimetric analysis, prompt gamma activation analysis, induced neutron activation analysis, inductively coupled plasma mass spectroscopy, resonant Raman scattering, UV-visible-near infrared fluorescence spectroscopy and absorption spectroscopy, scanning electron microscopy, and transmission electron microscopy.[121][122] The Canadian National Research Council also offers a certified reference material SWCNT-1 for elemental analysis using neutron activation analysis and inductively coupled plasma mass spectroscopy.[117][123] NIST RM 8281 is a mixture of three lengths of single-wall carbon nanotube.[121][124]
For multiwall carbon nanotubes, ISO/TR 10929 identifies the basic properties and the content of impurities,[125] while ISO/TS 11888 describes morphology using scanning electron microscopy, transmission electron microscopy, viscometry, and light scattering analysis.[126] ISO/TS 10798 is also valid for multiwall carbon nanotubes.[120]