Metal cluster built up like a Russian matryoshka.
A doll in a doll, and then one more, enveloping them from the
outside—this is how Thomas Faessler explains his molecule. He packs one
atom in a cage within an atom framework. With their large surfaces these
structures can serve as highly efficient catalysts. Just like in the
Russian wooden toy, a hull of twelve copper atoms encases a single tin
atom. This hull is, in turn, enveloped by 20 further tin atoms.
Professor Faessler’s work group at the Institute of Inorganic Chemistry
at the Technische Universitaet Muenchen (TUM) was the first to generate
these spatial structures built up in three layers as isolated metal
clusters in bronze alloys.
fascinating are the images the researchers use to explain these
chemical compounds and their properties. In the laboratory the substance
is an unimpressive, fine, grayish-black powder, yet the structure
models are in color and in various nested shapes. These powders, with
their large surfaces, are interesting as an interim step for catalysts
that transfer hydrogen, for instance. Similar structures made of silicon
could be used in solar cells to capture light from the sun more
people view metals as uniform materials with a rather unspectacular
structure. The metal compounds from Faessler’s institute are quite the
opposite. His desk is piled high with various multicolored cage models
with yellow spheres representing copper atoms and blue ones for tin. The
analogy to the carbon spheres that caused a sensation as Buckyballs can
not be overlooked. Here, too, there are geometric structures made up of
triangles, pentagons and hexagons. However, they are not made of
carbon: heavier metals such as tin and lead can also form such isolated
are basically interested in alloy structures that are out of the
ordinary,” says Faessler. Bronze, for example: this mixture of copper
and tin, which was discovered early on and lent its name to an entire
age of humanity, has a crystalline structure; the atoms of the two
components are distributed evenly throughout the entire crystal and are
densely packed together.
new bronzes from the Faessler laboratory are different. The PhD
candidate Saskia Stegmaier melted a particularly pure form of copper
wire and tin granulate under special conditions—protected from air and
moisture in an argon atmosphere. The bronze produced in this manner was
then sealed into an alkali metal such as potassium in an ampoule made
of tantalum. The melting point of tantalum is 3,000 C, making it
particularly well suited as a vessel for binging other metals into
contact with each other.
is how the new metal clusters, nested inside each other just like the
Russian doll, came into existence. When bronze is heated, together with
potassium or sodium, to 600 to 800 C, the alkali metals
act like scissors that cut up the alloy grid and then edge their way
between the pieces, thereby stabilizing the isolated atomic clusters. On
their own, these clusters cannot organize themselves into dense,
uniformly structured layers to form crystals. They are made up of
pentagons with 20 tin atoms in all—a constellation in which repetitive
patterns are not possible under normal conditions. But “cheating” a
little and using potassium atoms as glue can produce a seemingly normal
crystal. Last year the Israeli scientist Dan Shechtman received the
Nobel Prize for chemistry for the discovery of a similar phenomenon—the
so-called quasi-crystals with five-fold symmetry.
clusters are small units. They are, so to speak, piles of atoms that
are not connected to their neighbors.” That makes them ideal for
catalytic applications: “Because they are consistent in size,” explains
Faessler, “they are much better at steering chemical reactions than
reactions in which hydrogen atoms dock to organic molecule chains with
oxygen atoms, e.g. in the synthesis of artificial flavors, are examples
of such processes. Typically, expensive precious metals like rhodium are
used for this. However, novel polar alloys with magnesium, cobalt and
tin can serve the same purpose.
we need for an efficient reaction is a catalyst with very large surface
area.” The classical method of achieving this is to mix solutions of
two metal salts to precipitate extremely small nanoparticles. “This
results in an entire spectrum of particle sizes,” explains Faessler.
With metal clusters we can tailor the catalyst to our needs, as it
Stegmaier’s and Faessler’s reaction vessel contained more surprises.
Aside from the clusters, the scientists noticed a fiber-like
material—like thin needles—whose ends could be bent a little. “We
suspected,” says Stegmaier, “this could turn out to be exiting.” In the
meantime the yield of the fibers has been improved by using sodium as
scissors to cut up the bronze. This time the result was not spheres, but
multilayered rods. In the middle is a string of tin atoms, surrounded
by a layer of copper atoms, and around that yet another tube of tin
atoms. Just as the hollow Matryoshka molecules are reminiscent of
Buckyballs, the new fibers with their tubes are akin to carbon
nanotubes. Analogously, such fibers could one day be used as molecular
wires with various electrical properties.