Page 90 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

76 Principles and Methods

c

100 nm
250 nm
(a) (b)
100 nm 100 nm

(c) (d)
Figure 3.33 Particles Co 80 Ni 20 obtained by polyol process with molar ratio HO/(Co Ni)
1.25 (a, b), 1.8 (c) and 2.5 (d) [146].

nickel are fcc structured and always isotropic in shape). At low hydrox-

ide concentrations (molar ratio [HO ]/[Co Ni] 2), growth is favored
along the c-axis, forming nanowires 8 nm in diameter and 100–500 nm
in length with cone-shaped ends (Figure 3.33). Theses wires appear
linked to a core, forming a sea-urchin–like shape (Figure 3.33b). With

molar ratios [HO ]/[Co Ni] 2, the growth occurs preferentially per-
pendicular to the c-axis resulting in rods that are 20–25 nm in diame-
ter and 75–100 nm in length. The particular shape of the ends of
nanowires seen in Figure 3.33b probably results from a lower growth
rate at the end of the reaction, when the Co(II) and Ni(II) concentration

fall (as the result of their reduction) and when the molar ratio [HO ]/
[Co Ni] is increased. If it is assumed that the shape of wires is
controlled by the growth rate, the head, formed at the end of the reac-
tion, grows under conditions of low supersaturation inducing growth
perpendicular to the c-axis. The polyol process has also be used to form
Fe-Pt nanoparticles from the acetylacetonates Fe(acac) and Pt(acac) in
3
2
ethylene glycol [146, 147].
Astrong reducing agent such as hydrazine N H has been used to reduce
2
4
metal salts and to form Fe-Pt nanoparticles in water at low temperature.