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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

Metal Particles and Cluster Compounds 529

New preparative methods and better techniques for
characterization have allowed considerable progress in
this field of endeavor.
One synthetic approach involves the codesposition
of metal vapors with vapors of hydrocarbon solvents at
liquid nitrogen temperatures (77 K). This approach has
been termed the solvated metal atom dispersion (SMAD)
method for preparation of ultrafine metal powders. In a
typical experiment 1–2g of metal (almost any metal) is va-
porized from a high temperature crucible and codeposited
over 1–2 hr with 100 g of hydrocarbon in a 3-liter vac-
uum chamber cooled to liquid nitrogen. Upon warming
form 77 K to room temperature, the nanoscale metal par-
ticles form by atom aggregation, and the growth process
is controlled by choice of solvent, and warmup rate. As FIGURE 11 Illustration of the synthesis of gold nanocrystals in
the particles grow, atom by atom, they become less mo- an inverse micelle pocket.
bile, and growth stops as weak solvation completes fa-
vorable with slower and slower particle movement. The cylindrical shaped metal particles by use of these tubular
SMAD method has been used to prepare a wide variety of shapes (vesicles).
useful nanomaterials including catalysts, nonaqueous col- There is considerable promise in the inverse micelle
loids, metal particles in polymers, and bimetallic magnetic “nanoreactor” approach since there are many kinds of sur-
and catalytic materials, and synthetic apparatus as been factants available, including cationic, anionic, and neutral
built to allow 100-g quantities to be produced over several molecules (Fig. 12).
hours. Likewise, water content, surfactant concentration, and
Chemical-reducing agents have also been used to ad- polarity of organic solvent can be adjusted to yield differ-
vantage to prepare metal particles through atom-by-atom ent sizes and shapes of the nonreactor pockets. However,
growth. Metal ions in solution (polar organic solvents or particle synthesis and growth is a dynamic process, with
water) can be reduced to metal atoms, which then aggre- rapid exchange of micelle contents occurring, and so only
gate to particles. Some metal ions, such as those of gold a rough control of resultant nanoparticle size and shape
2+
(Au ), silver (Ag ), cobalt (Co ), and nickel (Ni ) can can be expected.
2+
+
3+
be reduced by sodium borohydride (NaBH 4 ) resulting in Numerous other metal particle growth and entrapment
metallic nanoparticles. For metals that are more difficult environments have been reported. These include silica-
to reduce, stronger reagents have been used, such as potas- alumina zeolites, bridge polysilesquioxanes, gels, and
siummetaloralkylborohydrides,sothatevenmagnesium, phosphates.
aluminum, titanium, chromium, and other fine, reactive One of the consequences of being able to prepare
small particles can be produced. metal particles in the nanometer size range, and with very
In recent years a most notable advance has been the narrow size distribution, is that these particles begin to
controlled growth of metal particles in the small pockets behave as “super atoms.” For example, when 6-nm spher-
of inverse micelles. Micelles are formed in a sea of water ical gold particles were ligand-stabilized by adding a long
when dissolved surfactant molecules aggregate with long chained thiol they were found to readily form nanocrystal
hydrocarbon chains together. Inverse micelles are formed superlattices. Figure 13 shows electron microscope pic-
in a sea of hydrocarbon (gasoline, kerosene, octane, etc.) tures of ordered arrays of 6-nm gold particles in two and
where the polar end groups of the surfactant aggregate. three dimensions. This represents a new type of crystal,
Small amounts of water are collected in these regions, and is an ordered assembly of nanocrystals. Such materi-
and these aqueous pockets can serve as nanometer sized als are expected to exhibit a range of new properties, and
reactor zones. Metal salts can be dissolved in these reac- are just now becoming available for detailed studies.
tor pockets and chemically reduced to zero-valent metal
particles. Indeed, by varying the amount of water and sur-
III. METAL CLUSTER COMPOUNDS
factant, the size of these reactor pockets can be controlled
and this can be used to roughly control the size of the
A. Importance and Background
resultant metal nanoparticles (Fig. 11).
Under certain conditions tubular shaped pockets can be Many transition-metal cluster compounds, where the
formed and there have been reports of the synthesis of metal particle (cluster) is surrounded and stabilized by