Exposure to Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 309

From an occupational health perspective, the likelihood of aerosol generation and
their availability in bulk quantities makes these nanoparticles of particular
interest.

8.2.3 Nanoparticle Production Processes

Nanoparticles, even from the same material, can be synthesised using a variety of
methods. Different methods are used in order to optimise specifi c properties of the
materials. These properties include, but are not limited to, size (diameter, length,
volume), size distribution, symmetry, surface properties, surface coating, purity, ease
of manipulation, yield and suitability for scaling up. Methods used for the com-
mercial or deliberate manufacture of nanoparticles may be divided into four main
groups. These are:

• Gas phase processes including fl ame pyrolysis, high temperature evaporation and
plasma synthesis;
• Vapour deposition synthesis;
• Colloidal or liquid phase methods, in which chemical reactions in solvents lead
to the formation of colloids;
• Attrition methods including grinding, milling and alloying.

For all of these processes the recovery stage may be quite similar and is likely
to comprise mainly powder or slurry handling techniques.

8.2.3.1 Gas Phase Synthesis Methods

Gas phase processes may be used to produce a wide range of materials. Most, but
not all, nanoparticle synthesis methods in the gas phase are based on homogeneous
nucleation of a supersaturated vapour and subsequent particle growth by conden-
sation, coagulation and capture.
Gas phase synthesis methods have been reviewed by several authors, including
Kruis and Fissan (1998) and Swihart (2003). Multiple approaches can be used to
generate the super-saturated vapour, dependent on the materials (precursors) used
and the form of materials to be produced. In general, the formation of the vapour
occurs within an aerosol reactor at elevated temperatures. The most straightforward
method of achieving super-saturation is to heat a solid and evaporate it into a
background gas. This method is well suited for the production of metal nanopar-
ticles in particular. By including a reactive gas such as oxygen, oxides or other
compounds of the evaporated material can be produced. This method has also been
used to prepare composite nanoparticles and to control the morphology of single
component nanoparticle.
The precursor materials are introduced into the reactor as solids, powders, liquids
or gases. In some cases, the precursors are nanoparticles, produced by a separate
process. In the reactor, the precursors are heated and mixed with a carrier gas. The
super-saturated vapour is produced by cooling or by chemical reaction or by some
combination of these. Cooling may be induced by expansion, mixing with a cooler
gas or by heat transfer to the surroundings. Chemical reactions used are usually
decomposition reactions.