School of Engineering and Science - Jacobs University

The advantage of use of organometallic route in preparation of nanoclusters is obvious
when it is needed to stabilize (modify) nanoparticles by hydrophobic surfactant or in
other words, when required stabilizer is water insoluble.
An interesting method of preparation of metal nanoparticles is through electrochemical
approach established by Reetz [152]. In this method, an anode is made up of the
specific metal (required for nanoparticles) and a cathode is of any other metal. Under
the suitable applied current density the anode sacrificially dissolves in the electrolyte,
the metal ions migrate towards the cathode and reduction occurs. The nucleation and
growth of reduced metal atoms occurs at the electrode surface. The stabilizing agent in
the reaction vessel arrests the growth of the nanoparticles and facilitates the formation
of stable nanostructures. The final step is dissolution of nanoparticles from the
electrode surface into the bulk of the solution. The advantages of the electrochemical
pathway are that the contamination with by-products resulting from chemical reducing
agents is avoided and that the products are easily isolated from the precipitate. Further,
the electrochemical preparation allows for size-selective particle formation. The
particle size obtained by the electrochemical route depends on many factors, the
distance between the electrodes, reaction time, temperature, and polarity of the solvent
contribute to the particle size. Experiments have also shown that the applied current
density also has a major influence on the particle size.
Vapor phase synthesis
As was mentioned above metal atoms can be obtained not only through reduction of
metal salts, but also through solid to gas phase transformation of metal. The Solvated
metal atom dispersion (SMAD) techniques is based on this method. Typically, the metal
of interest is heated in a crucible at elevated temperature evaporated under vacuum and
then co-condensed with a specified amount of liquid ligand substrate on the liquid
nitrogen cooled walls of the reactor vessel. The reactor is removed from the liquid
nitrogen and the reactor is allowed to warm slowly. After the warming stage, particles
can be stabilized either sterically (by solvation) or electrostatically (by incorporation of
negative charge). The SMAD techniques has been successfully applied e.g. in
preparation of thiol stabilized gold colloid [153]. The main advantage of this “clean”
method of preparing nanoclusters in gram scale is in fact, that it does not involve
starting materials containing counter ions like nitrate, chlorides, sulfates which are
common in reduction synthetic methods. However, the use of SMAD method is limited
because of difficulties in the operation of the apparatus and it is difficult to obtain
narrow particle size distributions.
Another interesting method is through the laser ablation of bulk metal target crystal
being in liquid phase. The recent example is in preparation of Pt nanoparticles by laser
ablation of Pt crystal in water [154-156]. Due to the laser induced evaporation of Pt
atoms into the liquid phase with formation of 1-15 nm nanoparticles, it was found that
the yield of nanoparticles depends on the wavelength of the used laser beam whereas it
has no significant influence on the average size of produced nanoparticles. The laser
ablation method has the same important advantages as SMAD method demonstrates,
moreover there is not need to work with very high temperatures (for metal evaporation)
and keep system under high vacuum. However with respect to the chemical methods
the laser ablation has the following disadvantages. There is more insight about the
reaction at plasma and liquid interface and it is relatively more expensive to obtain the
large amount of products, in fact the rate of nanoparticles production was about 4.4
mg/h.
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