Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 62

2.5.12 Inorganic Compounds

2.5.12.1 Oxides

Perhaps the most facile and widely used method for the preparation of metal oxides
relies on the fl ame pyrolysis technique. This has been used to prepare a wide range
of materials and is used to prepare the Degussa P45 titania commonly used in many
ecotox studies. Such materials may also be prepared by the base catalysed hydro-
lysis of suitable salts.
Silica and titania may be prepared by simple hydrolysis of an alkoxide (tetra-
ethylorthosilicate or titanium isopropoxide) in an aqueous medium. Careful tuning
of the pH will result in the formation of a stable dispersion of nanoparticles and
this is a common method for the production of some silica nanoparticles. The use
of various capping agents has also been employed in these types of reaction with
some success. One method which has proven useful for a range of nanoparticles is
hydrothermal synthesis. In this method the reaction is conducted in a sealed bomb
so that the temperature may be raised above the boiling point of the liquid under
standard conditions. This increase in temperature and pressure can result in the
formation of different nanoparticle phases and sizes.
Nanoparticles of ceria (CeO 2 ) and zinc oxide (ZnO) have been prepared by
similar methods. In these cases it is important to add a stabiliser or use a micelle
to control the particle size. For example, zinc oxide is readily prepared by the reac-
tion of zinc nitrate with a suitable base (Hartlieb et al. , 2007 ). Ceria nanoparticles
have been prepared in a similar manner (Liu et al. , 2007 ).

2.5.12.2 Narrow Band Gap Semiconductors

Narrow band gap semiconductors represent a large range of nanoparticles. These
are usually compound of metals with p-block elements such as sulfur, selenium,
tellurium or phosphorous. Often these materials contain heavy metals such as
cadmium, mercury, lead, indium, antimony or bismuth. Clearly the composition of
these materials contains elements which, when in their free ionic form, are known
to be exceptionally toxic. Whilst their current use is limited their potential is large
and therefore it is worth considering their preparation.
The sulfi des are perhaps the easiest to prepare and are readily prepared by the
rapid reaction of a solution of the metal ions with sodium sulfi de in the presence
of a capping agent or within a micelle. This method has been used to prepare both
particle and rod shaped forms of cadmium sulfi de (Simmons et al. , 2002 ). However,
the most common method for the preparation of the other chalcogenides and
phosphides was initially described in 1991 (Murry et al. , 1993). More recently there
have been developments of similar methods to prepare metal nitrides such as
gallium and aluminium nitride (Wells and Janik, 1996). In this general approach a
metal precursor is dissolved in a suitable capping agent such as an alkylphoshine
oxide and heated. The second precursor, such as selenium, is dissolved in a phos-
phine, such as trioctylphosphine, and rapidly injected into the reaction. The result-
ing nanoparticles are precipitated by the addition of methanol. This can result in