Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 56

There is still great debate in the literature over various other mechanisms for the
formation of nanoparticles and there are several other processes that may contrib-
ute to the growth stage which are not discussed in detail here. The actual kinetic
of the growth process may override any thermodynamic sink for the structure of
the fi nal material and this can result in particles which contain more than one
crystal phase (Christian and O' Brien, 2005, 2008 )
This simple method for preparing nanoparticles can take many forms and the
medium in which supersaturation occurs can be as diverse as a plasma or a con-
ventional solvent.

2.5.7.1 High Temperature Methods

There are several varieties of high temperature methods for the production of
nanoparticles. Generally they are well suited to the formation of either metal oxide
or metallic nanoparticles and have been used to prepare a range of commercial
materials. One excellent method for the preparation of nanoparticles on a large
scale is aerosol fl ame synthesis or spray pyrolysis. The process is relatively simple.
A solution is prepared containing the ions required for the synthesis. The solution
is then injected into a high temperature fl ame of up to 3000 K and the particles
essentially form in the fl ame and are collected as they settle out of the atmosphere.
Materials prepared by this method tend to be crystalline and well formed. The
method has been used for some time for the preparation of nanoparticles of materi-
als such as carbon, titania, silica and alumina at rates of tens of tonnes per hour
(Ulrich, 1984). The result of this type of preparation methods is a nanomaterial
with an uncoated surface. It is, therefore, not unusual for the particles to be aggre-
gated and diffi cult to redisperse. There are some excellent reviews on the applica-
tion of this method to the preparation of metal and metal oxide nanoparticleS with
application in catalysis (Wooldridge, 1998; Pratsinis, 1998).

2.5.7.2 Wet Methods

There are several variations on wet methods for the preparation of nanoparticles.
The term refers to the use of a solvent in which the reaction is performed. There
are two subsets of this method: micelle encapsulation and arrested precipitation.
Many of these methods can be used with out supplying energy to the system.
However, in cases where energy is required to initiate the reaction there is a lot of
work investigating the use of light, cavitation, ionising radiation and microwaves
instead of thermal heating.

Micelle e ncapsulation
Micelle encapsulation relies on the use of a micelle to control the particle size. As
already discussed, it is well known that certain surfactants will form micelles where
an oil may be dispersed in water. In fact, it is also possible to form inverse micelles,
where water is dispersed in a hydrophobic medium. These water-in-oil microemul-
sions are used as reactors which control the particle growth, size and, in some cases,
shape. Two microemulsions are prepared containing the two reagents (Figure 2.16);
for example, if cadmium sulfi de was to be prepared then one emulsion might
contain cadmium chloride and the other sodium sulfi de. Upon mixing the micelles