Page 87 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 73

(a) (b)
Figure 3.31 (a) Spherical Co nanocrystals synthesized in the presence of both oleic acid
and TOPO and (b) Co nanodisks synthesized in the presence of amine. The disks are
stacked in columns because of magnetic interactions. Bars are 100 nm [139].

in size from 3–17 nm. The particle size is tuned by the reaction temper-
ature and the composition of surfactant [138]. In the presence of linear
amines, hcp-Co nanodisks are formed, coexisting with -Co nanospheres
(Figure 3.31) [139]. The length and diameter of the disks are controlled
by variation of the reaction time following nucleation as well as by vari-
ation of the precursor to amine surfactant ratio. It has been observed that
the length of the linear amine carbon chain controls the dimensions of
Co disks (lower disks are obtained with shorter chains) whereas tri-
substituted amines R N hinder the formation of disks. This suggests the
3
function is responsible for disk formation by selective adsorp-
R-NH 2
tion, and steric interactions among neighboring adsorbed molecules may
have an impact on the growth rate of the (001) faces.
It is difficult to synthesize iron nanoparticles by the polyol process
because in the conditions of the process, Fe(0) results from dispropor-
tionation of Fe(II) whereas Co(II) and Ni(II) are quantitatively reduced
[140]. Other methods to synthesize iron nanoparticles have been used.
For instance, reduction of FeCl in THF by V(C H ) in the presence of
2
5 2
5
PVP forms PVP stabilized -Fe nanoparticles 18 nm in mean size.
Sonolysis of Fe(CO) in solution in anisol in the presence of poly
5
(dimethylphenylene oxide) (PPO) yields nonagglomerated spherical Fe
nanoparticles with a mean size of around 3 nm. Interestingly, the smaller
particles ( 2.5 nm) have the bcc structure of -Fe and are superpara-
magnetic, whereas the larger ones (
2.5 nm) adopt the fcc structure of
-Fe and are antiferromagnetic or paramagnetic. These -Fe nanopar-
ticles could result from thermal gradients and very high local tempera-
tures during sonication with rapid quenching avoiding recrystallization,