Environmental Engineering Reference
In-Depth Information
and silver nanoparticles may be prepared by simply heating an aqueous solution
of gold chloride or silver nitrate with sodium citrate. This rapidly results in the
formation of a red (gold) or yellow (silver) suspension of nanoparticles. This method
relies on the binding of the citrate to the surface of the nanoparticle to impart
stability and the colloid itself is charge stabilised; the citrate also acts as the reduc-
ing agent for the formation of the nanoparticle. The charge stabilisation of the
nanoparticles means that these colloids are generally prepared at very low concen-
trations (
50 ppm of starting salts) and are very unstable in the presence of poly-
valent metal ions. A similar result may also be achieved by reducing the metal salts
in a more dilute solution of sodium citrate or thiopropanoic acid using a common
reducing agent such as sodium borohydride or hydrazine. In the case of thiopropa-
noic acid, the terminal SH group provides the anchor to the particle surface and
the acid group provides a potential point for charge formation.
It is well known that alloy nanoparticles of gold and silver may also be prepared
in a similar fashion due to the similarity of packing in the two lattices. More recently,
silver nanoparticles have also been prepared by the so called polyol route, where
a solution of metal ions in an alcohol is reduced simply by heating the solution
(Sun et al. , 2003) This results in reduction of the metal ions and oxidation of the
alcohol. There are a number of other methods that have also been used including
using surfactants such as cetyltrimethylammonium bromide (CTAB). Copper
nanoparticles have been similarly prepared by the reduction of copper salts in
microemulsions (Qiu et al. , 1999) and a range of other metal nanoparticles can be
prepared in the same manner.
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2.5.9
Carbon
There are three forms of nanocarbon that have gained specifi c interest for applica-
tions in nanotechnology: C 60 , carbon nanotubes (CNT) and more recently graphene
sheets. C 60 is essentially a soccer ball shaped molecule of carbon. It is known to
dissolve in various organic solvents. There have been several reports of dissolving
in water (Lyon et al. , 2006). However, it seems more likely that the result of these
attempts is actually a suspension of a number of C 60 units some of which, mainly
at the surface, have been chemically altered. C 60 or Buckminster fullerene is com-
monly prepared by the arc discharge method, which is one of the original methods
used to prepare C 60 . During the preparation of the material a high voltage discharge
is passed between two graphite rods in a low pressure argon atmosphere. The result
is a soot which is collected on a cold fi nger and comprises up to 15% C 60 . The soot
also contains so-called higher fullerenes such as C 70 .
Carbon nanotubes are essentially hollow tubes, the wall of which is formed from
a single layer of graphite which wraps round onto itself. The tubes may form
Russian doll like structures with several tubes each inside another. This has resulted
in the terms single-walled (SWCNT) and multi-walled (MWCNT) carbon nano-
tubes. The smallest single walled tubes can, in fact, be less than a nanometre in
diameter and several microns long. It is now possible to selectively prepare either
single-wall or multi-wall nanotubes. Depending on the exact method of preparation
and purifi cation the tubes may be closed at both ends by carbon caps, open at both
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