A schematic diagram shows a gold nanoparticle stabilized with polyvinyl alcohol (PVA) ligands. |
Gold nanoparticles, says Chris Kiely, are fast becoming some of the most effective diplomats of the nanoworld.
They
facilitate a wide range of chemical reactions between molecules that
would not normally interact or would do so only at much higher
temperatures.
And in most cases, they effect a single favorable outcome with few, if any, unwanted side reactions.
In short, says Kiely, a professor of materials science and engineering, the nanoparticles are extremely good catalysts.
Conventional methods of preparing gold nanoparticles, however, alter the morphology and catalytic activity of the particles.
Now,
an international team of researchers has developed a procedure that
enhances the surface exposure of gold nanoparticles and their catalytic
activity over a range of reactions.
A new procedure improves on convention
The
team reported its results in July in Nature Chemistry in an article
titled “Facile removal of stabilizer-ligands from supported gold
nanoparticles.”
Its
members include Kiely and Graham Hutchings, a chemist at Cardiff
University in Wales in the U.K., who have studied nanogold together for
more than a decade.
“In
industry,” says Kiely, “the most common way of preparing gold
nanocatalysts is to first impregnate a nanocrystalline oxide support,
such as titanium oxide (TiO2) with chloroauric acid. A reduction
reaction then converts the acid into metal nanoparticles.
“Unfortunately,
this leads to a variety of gold species being dispersed on the support,
such as isolated gold atoms, mono- and bi-layer clusters, in addition
to nanoparticles of various sizes.”
An
alternative technique that allows more precise control over particle
size and structure, is to pre-form the gold nanoparticles in a colloidal
solution before depositing them onto the support.
The
disadvantage to this method is that during fabrication the
nanoparticles are coated with organic molecules – ligands – that prevent
them from clumping together. Once they are deposited onto a support,
these ligands tend to impair the nanoparticle’s catalytic performance by
blocking the approach of molecules to active sites on the metal
surface.
A milder form of ligand removal
A micrograph taken by Lehigh’s high-angle annular dark field (HAADF) scanning transmission electron microscope (STEM) shows a gold nanoparticle on a TiO2 support after a hot water wash. |
Previous methods for stripping away these ligands have involved heat treatments of up to 400 degrees C.
“At
these temperatures the morphology of the nanoparticles changes and they
begin to coalesce,” says Kiely. “There is also significant decrease in
their catalytic activity.”
The
Kiely-Hutchings team developed a milder alternative for removing the
ligands from polyvinyl alcohol-stabilized gold nanoparticles deposited
on a titanium oxide support – a simple hot water wash.
Graduate
student Ramchandra Tiruvalam used Lehigh’s aberration-corrected JEOL
2200 FS transmission electron microscope to examine the catalysts before
and after washing and to compare them with those that had undergone
heat treatment to remove the ligands.
“Hot
water washing had very little effect on particle size,” says Kiely, who
directs Lehigh’s Nanocharacterization Laboratory, “and while the
particles retain their cub-octahedral morphology, their surfaces appear
to become more distinctly faceted. This is presumably due to some
surface reconstruction occurring after losing a significant fraction of
the protective PVA ligands.”
“Heating
the samples to 400 degrees C was also effective at removing the ligands
but the average particle size increased from 3.7 to 10.4nm,” says
Kiely. “There was also tendency for the particles to restructure and
develop flatter, more extended interfaces with the underlying TiO2
support.”
For
the oxidation of carbon monoxide to carbon dioxide, catalysts prepared
by this colloidal/hot water wash displayed more than double the activity
of conventional gold/TiO2 catalysts. This particular reaction is
crucial for the removal of carbon monoxide from enclosed spaces such as
submarines and space craft, prolonging the life of fuel cells, and
extending the usable lifetime of a firefighter’s mask.
This
work was funded in part by the National Science Foundation. Tiruvalam
is now a research scientist with Haldor Topsoe, a catalyst company in
Copenhagen, Denmark.
Facile removal of stabilizer-ligands from supported gold nanoparticles
