Page 89 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 75

Bimetallics and alloys
Bimetallic nanoparticles, composed of two different metals, are of inter-
est in the improvement of catalytic properties [143] and in development
of magnetic properties [144]. For instance, the ordered alloys Co-Pt and
Fe-Pt are particularly interesting for magnetic recording because of
their very high magnetocrystalline anisotropy, making these materials
especially useful for practical applications such as magnetic memory
devices as well as in biomedicine. The synthesis of bimetallics is
generally made by coreduction of metal salts. Coreduction is the most
simple preparative method and it is very similar to that used for
monometallic nanoparticles. Successive reduction, carried out to prepare
core-shell structured nanoparticles, is of little importance and will not be
discussed here.
Au-Pt bimetallic nanoparticles have been obtained by citrate reduc-
tion of the mixture of tetrachloroauric acid, HAuCl 4 , and hexachloro-
platinic acid, H 2 PtCl 6 . The UV-Vis absorption spectra of the citrate
stabilized sol of nanoparticles is not the simple sum of those of the two
monometallic nanoparticles, indicating that the bimetallic particles
have an alloy structure, as confirmed by X-ray diffraction and X-ray
absorption spectroscopy [120]. A similar method produces citrate-
stabilized Pd-Pt nanoparticles. Polymer-stabilized Pd-Pt nanoparticles
have been prepared by simultaneous reduction of palladium chloride
PdCl 2 and hexachloroplatinic acid by refluxing the alcohol/water (1:1 v/v)
mixed solution in the presence of PVP at about 90 C for 1 hour [143].
Similarly, PVP-stabilized Pd-Rh nanoparticles were obtained by reduc-
tion in alcohol.
The polyol process [133] appears to be an efficient way to synthesize
bi- or polymetallic nanoparticles of 3D metals, when heterogeneous nucle-
ation controls the size of ferromagnetic nanoparticles. The addition of
small amounts of a platinum or silver salt, which reduces at a lower tem-
perature than 3D elements, forms nuclei for the growth of cobalt, nickel,
or iron. Polymetallic spherical particles of the alloys Co x Ni (100 x) can be
synthesized by precipitation from cobalt and nickel acetate dissolved in
1,2-propanediol with an optimized amount of sodium hydroxide. The
number of nuclei depends on the relative amount of platinum, allowing
the control of the particle size of Co x Ni (100 x) alloys over a very large
range, from micrometric to nanometric [140]. The sodium hydroxide
allows the precipitation of metal as hydroxides or alkoxides before the
reduction. Their slow dissolution takes place at lower temperatures
than the reduction, and this step likely controls the growth of metallic
particles in solution. In controlling the basicity of the medium of the Pt
or Ru seed-mediated polyol process, Co 80 Ni 20 nanoparticles with sur-
prising anisotropic shapes can be obtained [145, 146]. These particles
are hcp crystallized when x 30 (for x 30, the particles very rich in