Page 79 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 65

A similar “strings of pearls” morphology has been observed for the solu-
tion decomposition of Co (CO) in the presence of soluble organic poly-
2
8
mers [115].
The small sizes and pseudospherical geometry of the InSe and GaSe
particles suggests a vapor phase component is operative during the
deposition of these metal selenide films. The fragmentation of the pre-
cursors in the vapor phase may combine to form particles that will remain
in the vapor phase until their size is such as to “precipitate” out of the gas
stream. The deposition of particles from the vapor phase due to gravita-
tional or thermophoretic forces has been studied for SiC [116]. Factors that
may control the particle size include: precursor concentration, vacuum
versus atmospheric growth, growth temperature, and the thermal sta-
bility of the precursor.

Metallics, Bimetallics, and Alloys
The most currently synthesized metallic nanoparticles include two cat-
egories of metals: noble or precious metals (Ag, Au, Pd, Pt), less exten-
sively Ru and Cu, and ferromagnetic metals (Co, Fe, Ni). Silver, gold,
and copper are essentially used for their color, yellow to red, due to
their plasmon resonance located within the visible domain of the elec-
tromagnetic spectrum. Palladium, platinum, and ruthenium are largely
used for heterogeneous catalysis. Cobalt, iron, and nickel are interest-
ing as magnetic nanomaterials for various applications such as infor-
mation storage in recording devices, ferrofluids, and microwave composite
materials.
The two main routes for the synthesis of metallic nanoparticles are the
reduction of metallic salts in solution, involving a large variety of salts
and reducing agents, and the decomposition of zerovalent metal com-
pounds. Whatever the reaction involved, the formation of monosized
nanoparticles is achieved by a combination of a low concentration of solute
and a protective layer (polymer, surfactant, or functional groups) adsorbed
or grafted onto the surfaces. Low concentrations are needed to control the
nucleation/growth steps, and polymeric layers reduce diffusion causing
diffusion to be the rate-limiting step of nuclei growth, resulting in uni-
formly sized nanoparticles. The protective layer also limits or avoids irre-
versible aggregation of nanoparticles.

Reduction mechanism
The basic reaction involved in the production of metallic nanoparticles
is the reduction of metal cations in solution:

z 0
M Red → M Ox