Page 85 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 71

(a) (b)

300 nm Mag = 50.00 KK Signal A = SE1 Time :15:14 200 nm Mag = 80.00 KK Signal A = SE1 Date :12 Nov 2002
EHT = 20.00 kV EHT = 20.00 kV
WD = 15 mm Photo no. = 915 WD = 10 mm Photo no. = 913 Time :15:14
Figure 3.30 SEM images of free-standing Ag nanowires at low (a) and high (b) magnifi-
cation [131].

used as a noninteracting template (Figure 3.30). The Ag self-reduction
process is activated by sonication for 2 hours at 45 C. This process does
not require any reducing agent and directly forms crystalline silver
nanowires. They are recovered by dissolving the templating mem-
brane in NaOH solution at RT. The nanowire dimensions are deter-
mined by the pore dimensions and thickness of the membrane.

Nanoparticles of ferromagnetic
3D transition metals (Co, Fe)
The strategy used for the synthesis of 3D transition metals nanoparti-
cles is analogous to that involved for precious metals, but with harder
conditions because the redox potential of 3D transition metal elements
is much lower. The strategy involves either injecting a strong reducing
agent into a hot nonaqueous solution of a metal precursor containing
surfactants, or injecting a thermally unstable zerovalent metal precur-
sor into a hot solution containing stabilizers. Adjusting the temperature
and the metal precursor to surfactant ratio controls the nanoparticle
size. Higher temperatures and larger metal precursor to surfactant
ratios produce bigger nanoparticles. Surfactants not only stabilize the
nanoparticles in dispersion but also prevent or limit their oxidation
[132]. The following illustrative examples show the variety of methods
developed.
Reduction of cobalt salts by polyalcohols (polyols) at temperatures
between 100 C and 300 C in the presence of stabilizers produce Co
nanoparticles with diameters of 2–20 nm [132, 133]. In a typical synthesis,
cobalt acetate and oleic acid are heated at 200 C in diphenylether in the
presence of trioctylphosphine (TOP). Reduction is started by addition of
1,2-dodecanediol solubilized in hot and dry diphenylether and heating at
250 C for 15–20 minutes. With a Co to oleic acid molar ratio of 1 and Co
to TOP molar ratio of 2, 6–8 nm crystalline (hcp) Co particles are obtained.