Page 63 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 49

of such clusters have been isolated and characterized by X-ray dif-
fraction. They can assemble themselves into nanostructures enabling
the formation of hybrid organic-inorganic materials [49].
■ Addition of water in substoichiometric amounts does not allow the
substitution of all alkoxo ligands that otherwise leads to oxopoly-
mers. Such precursors are well designed for obtaining coatings or
thin films. The residual OR groups can react with surface hydroxyl
groups of the substrate forming covalent bonds. The films are
strongly adhesive and the organic residues can be then eliminated
by thermal treatment.
■ All alkoxo groups are eliminated in the presence of a large excess of
water (h 10), leading to oxide nanoparticles in suspension. Because
of the high dielectric constant of the medium, the surface hydroxylated
groups are mainly ionized allowing formation of sols or gels similar
to those obtained in aqueous solution.

Nonhydrolytic routes to oxide nanoparticles. Nonhydrolytic sol-gel chem-
istry has proved to be a promising route to metal oxides, as demon-
strated by the work of Corriu and Vioux on silica, titania, and alumina
[50]. It has become a widely explored approach to synthesize metal
oxide nanoparticles under various conditions [8].
In nonaqueous media in the absence of surfactant, one possibility is
the use of metal halides and alcohols (Nierderberger). This approach is
based on the general reaction scheme:

≡ M-X ROH → ≡ M-OH RX
≡ M-OH ≡ M-X →≡ M-O-M ≡ HX

It is widely observed that complexation of water to a transition metal
results in an increase in its Brønsted acidity [9]. Similarly, an increased
acidity of water upon complexation to main group compounds has been
inferred from NMR data. The recent isolation of a series of amine sub-
stituted alcohol complexes [51] has allowed for an estimation of the
change in the acidity of alcohols upon coordination to a metal.
Complexation of a protic Lewis base (e.g., ROH, R NH, etc.) results in
2
the increase in Brønsted acidity discerned by a decrease in pK of about
a
7 for the -proton. This activation of the coordinated ligand by increas-
ing the formal positive charge on the -substituent is analogous to the
activation of organic carbonyls toward alkylation and/or reduction by
aluminum alkyls [52–54].
While reaction of primary and secondary alcohols with tetrachlorosi-
lane is the usual method for preparing tetraalkoxysilanes [40], the same