Environmental Engineering Reference
In-Depth Information
polymers, such as polythiophenes in conjunction with a nanoparticle such as cadmium
selenide. Some of the current challenges lie in being able to prepare such solar cells
with high effi ciency and using more benign materials.
2.5
Nanoparticle Preparation
There is an ever increasing number of methods and techniques being developed to
prepare and manipulate nanoparticles. Whilst some of these methods have been
commercialised on a large scale many have yet to be scaled up to mass production.
As already seen, nanomaterials are actually complex mixtures. Therefore, having
an understanding of how they are prepared can be key to understanding their
behaviour. There are several challenges currently facing the community which need
to be overcome before mass production of many materials can become a reality
and, as will be seen later, the exact quality, properties and amounts of materials
required can vary widely from application to application. Therefore, some time is
spent at the end of this chapter considering the applications in which nanoparticle
have been, are being and may be used.
2.5.1
The Challenges of Nanoparticle Synthesis: Scale Up
The so-called bottom-up approach to nanoparticle preparation currently offers the
best route to mass produced nanoparticles. This type of preparation method essen-
tially builds nanoparticles up from molecules or atoms. It generally requires low
concentrations of nanoparticle to be prepared in order to maintain a narrow size
distribution of nanoparticle diameters. Simply increasing the concentration of these
reactions generally results in the formation of larger nanoparticles with a wider
distribution of sizes. For some applications this is not a signifi cant issue; for others,
however, it is critical.
2.5.2
Reactivity
As already discussed, nanoparticles have very large surface areas. This can make
them prone to reactions at the surface which modify and degrade the performance
of the particles. Many nanomaterials will undergo oxidation or hydrolysis at the
surface, resulting in the alteration of the surface chemistry of the nanoparticles.
It is well known that the surface chemistry can be key to the properties of the
nanoparticle. In fact, in some cases it has been speculated that this oxidation process
is the reason why some nanoparticles exhibit the properties which are desired.
Perhaps the only exception to this problem is oxide-based nanoparticles, those of
noble metals and some polymer nanoparticles. Attempt to circumvent this problem
currently focus on developing new capping agents and surfactants and in producing
layered nanoparticles with chemically inert coatings.
2.5.3
Dispersability
The ability to form a stable colloidal dispersion of many nanoparticles is key to
their processing and incorporation into the fi nal product. In some cases, enhance-
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