Page 45 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 31

because it allows them to be dispersed in either an aqueous or a non-
aqueous medium. Moreover, these nanoparticles can be modified in
liquid suspension by treatment with various chemical species for appli-
cation and use in a diverse range of technical or biological systems.

Oxides
The most widespread route to fabrication of metal oxide nanoparticles
involves the bottom-up approach by the precipitation in aqueous solution
from metal salts. Organometallic species can also be used in hydrolytic
or nonhydrolytic pathways, but due to their cost and the difficulty in
manipulating these compounds, they are used less frequently and
primarily for high-tech applications. An alternative top-down approach
has been demonstrated for aluminum and iron oxide nanoparticles;
however, it is possible that this methodology could be extended to other
oxides.

From molecular species to nanoparticles
One approach to the creation of oxide nanoparticles is to build from the
“bottom-up,” beginning with individual ions or molecular complexes of
metals. Variations on this approach include the hydroxylation of metal
cations in aqueous solutions, the use of metal alkoxides, nonhydrolytic
routes such as those employing metal halides.

Hydroxylation of metal cations in aqueous solution and condensation:Inorganic
polymerization. The metal cations issued for the dissolution of salts in
aqueous solution form true coordination complexes in which water mole-
cules form the coordination sphere. The chemistry of such complexes, and
especially their acid behavior, provides a framework for understanding how
the solid (oxide) forms via inorganic polycondensation [9, 10].
The binding of water molecules to a cation involves an orbital inter-
action allowing an electron transfer from a water molecule to a cation
following Lewis’s acid-base concept of the coordination bond. Such a
transfer drives the electronic density of water molecules toward the
cation and weakens the O-H bond of the coordinated water molecules.
They are consequently stronger Brønsted acids than the water molecules
in the solvent itself, and they tend to be deprotoned spontaneously
according to the hydrolysis equilibrium:

z (z h)
[M(H 2 O) n ] h H 2 O ⇔ [M(OH) h (H 2 O) n h ] h H 3 O
or by neutralization with a base:

[M(H 2 O) n ] z h HO ⇔ [M(OH) (H 2 O) n h ] (z h) h H 2 O
h