Page 81 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 67

A very large majority of redox systems in aqueous media is also
strongly dependent on pH because the nature of the metal complex in
solution is highly dependent on the acidity of the medium. Consequently,
the control of metal complex chemistry in solution by complexation
and/or by adjusting pH allows control of the reactivity of the species in
a given metal-reducing agent system. It is also important to note that
reduction can be performed in aqueous or nonaqueous media. However,
synthesis in a nonaqueous medium presents serious difficulties because
the metallic salts are generally very weakly soluble. Organometallic
compounds can be used with suitable reducing agents, which are sol-
uble in a specific medium. In some cases, the solvent can play a mul-
tiple role—for instance, primary alcohols (methanol, ethanol), which act
as both solvent and reducing agent in the synthesis of Au, Pd,
Pt nanoparticles, and polyols (e.g., glycerol, diethyleneglycol) can play
a triple role as solvent, reducing agent, and stabilizer in the prepara-
tion of a lot of metallic nanoparticles, including noble metals and tran-
sition metals.
Whatever the reaction involved, the formation of particles occurs, as
for other systems (oxides, chalcogenides), in two steps: nucleation and
growth. Since the metal atoms are highly insoluble, as soon as they are
generated they aggregate by a stepwise addition, forming clusters and
then nuclei when they reach a critical size. This step is favored by a
high supersaturation of metal atoms, and consequently by a high rate
of reduction or decomposition of precursor species in solution. The
growth of nuclei proceeds at first by stepwise addition of new metal
atoms formed in solution, leading to primary particles. Particle growth
may continue by further addition of metal atoms controlled by diffusion
toward the surface, by incorporation onto the surface, or by coalescence
of primary particles and formation of larger secondary ones. In order to
avoid coalescence and limit the growth and also to form stable disper-
sions of nanoparticles, protective agents are introduced into the reac-
tion medium, allowing, as already indicated before, formation of a
surface layer limiting the diffusion and growth, and also the nanopar-
ticle aggregation.

Nanoparticles of noble or precious metals
Gold and silver nanoparticles have been synthesized and studied
extensively for a long time. Colloidal gold, as described in 1857 by
M. Faraday [118], is probably the first monodispersed system ever
reported in the literature (apart from carbon black used for Egyptian
ink, already known in the time of the Pharaohs!). Reduction of chloroau-
ric acid, HAuCl , in aqueous solution by citrate at 100 C forms spher-
4
ical nanoparticles. Their mean diameter increases from around 10 to