Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 61

to the surface. Further growth results in the extrusion of the carbon nanotubes from
the metal droplet. Tubes prepared by this method typically have metal droplets
attached to the end of the tubes and may also contain metal particles trapped within
the tube or intercalated between the layers in multi walled tubes. An unsupported
method based on similar principle has recently been reported (Li et al. , 2004 ) where
a yarn of nanotubes may be spun from the smoke-like output of a high temperature
furnace.
In all the cases there is usually some residual formation of carbon particles
in the fi nal nanotubes. The trace carbon contamination and the metals may be
removed to some extent by treating with strong acids such as nitric acid. This will
dissolve most metal particles (except gold), digest amorphous carbon contaminants
and may also remove the carbon end caps from the nanotubes. Extended refl uxing
in nitric acid will eventually also begin to digest the tubes, resulting initially in the
formation of carboxylate and hydroxide groups on the surface. Despite rigorous
purifi cation steps it is common to fi nd contamination of nanotubes with various
metals.

2.5.10 Graphene

Graphene, perhaps the material of most recent interest in nanotechnology, is essen-
tially a single layer of graphite. Currently it is still being studied and has so far
shown excellent properties with regards to electron transport, semicondutive pro-
perties and quantum effects (Avouris et al. , 2007). It may prove to be the next
material for microchip manufacture. Currently it is manufactured by molecular
beam epitaxy, where a beam of precursor molecules is decomposed on the surface
of a suitable substrate. This results in small areas of graphene fi lm. The fi lm itself
is one atom thick (0.325 nm) and usually a few microns in diameter.

2.5.11 Carbon Black

Carbon black has been prepared on an on-demand basis for thousands of years in
the form of lamp black. However, the fi rst commercial scale production started in
1840 in Pennsylvania, USA. There are four main methods for producing carbon
black: the channel black process, the lamp black process, the thermal black method
and the furnace method. Each method produces material with slightly different
compositions. The channel black process collects carbon nanoparticles as they are
produced during the burning of natural gas. The carbon is collected on cooled mild
steel plates which have cannels cut into them. This method results in carbon par-
ticles with a size of 9-29 nm. The lamp black process uses the burning of shallow
ponds of coal tar or aromatic oils to produce the carbon particles, which have sizes
of 30-200 nm. The furnace process and thermal processes are similar in that a feed
of oil is fed into a hot furnace (

800 °C). The thermal process results in particles
with diameters of 150-500 nm and the furnace method 13- 100 nm. The exact com-
position of these materials can vary, especially regarding the polyaromatic hydro-
carbon (PAH) content. These are often present due to incomplete reduction of the
oil starting materials.

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