Page 86 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

72 Principles and Methods

Increasing the concentration of oleic acid and TOP by a factor of 2 yields
smaller, 3–6 nm nanoparticles [132].
Reduction by superhydride LiBEt H of anhydrous cobalt chloride at
3
200 C in solution in dioctylether in the presence of oleic acid and
alkylphosphine yields Co nanoparticles with a complex crystalline
structure ( -Mn). The average particle size is coarsely controlled by
the type of phosphine. Bulky P(C H ) limits the growth and produces
17 3
8
2–6 nm particles, while less bulky P(C H ) leads to larger (7–11 nm) par-
9 3
4
ticles [134]. The combination of oleic acid and trialkylphosphine produce
a tight ligand shell, which allows the particles to grow steadily while pro-
tecting them from aggregation and oxidation. In effect, trialkylphosphine
reversibly coordinates the metal surface, slowing but not stopping the
growth. Oleic acid used alone is an excellent stabilizing agent, but it
binds so tightly to the surface during synthesis that it impedes particle
growth.
Reduction by dihydrogen of the organometallic compound,
3 4
Co( -C H )( -C H ) in THF in the presence of polyvinyl pyrolidone
12
8
8
13
(PVP) forms very small Co nanoparticles with fcc crystalline structure,
around 1 nm and 1.5 nm depending on the temperature, 0 and 20 C,
respectively [135]. The decomposition at 150 C under H of the same
2
organometallic precursor, but in anisole and in the presence of oleic
acid and oleylamine (1:1 Co/oleic acid and Co/amine molar ratios)
produces initially (3 hours of reaction) spherical 3 nm Co nanoparticles.
After 48 hours, nanorods 9 40 nm are obtained [136]. The role of sur-
factants is very important in controlling the shape of particles.
Increasing the concentration by a factor of 2 of oleic acid produces very
long (micron range) nanowires of 4 nm in diameter, whereas the nature
of the amine changes drastically the dimensions of nanorods having an
aspect ratio varying between 1.7 and 22. The combination of lauric acid
and hexadecylamine produces monodisperse 5 85 nm nanorods form-
ing spontaneously 2D crystalline superlattices [137].
Cobalt nanoparticles have also been obtained from micellar media.
Reverse micelles are water droplets dispersed in oil and stabilized by a
surfactant monolayer, typically sodium bis-(2-ethylhexyl)sulfosuccinate
[Na(AOT)]. The size of the micelles depends on the amount of solubilized
water and varies from 0.5 to 1.8 nm. In the liquid, collisions between
droplets induce exchange between water pools [138]. By mixing two
and the other one NaBH ,
micellar solutions, one containing Co(OAT) 2 4
well-crystallized fcc Co nanoparticles are formed with a mean size around
6.4 nm. Extraction of nanoparticles with TOP forms a protective layer
against aggregation and oxidation.
Pyrolysis around 180 C of cobalt carbonyl Co (CO) dissolved in anhy-
2
8
drous o-dichlorobenzene containing surfactant (mixtures of oleic acid,
lauric acid and TOP) produces monodisperse -Co nanoparticles ranging