A hot bath for gold nanoparticles

Aug 02, 2011 By Carol Kiely
A hot bath for gold nanoparticles
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 , the nanoparticles are extremely good catalysts.

Conventional methods of preparing , 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

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.”

A hot bath for gold nanoparticles
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.

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 /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.

Explore further: Mirror-image forms of corannulene molecules could lead to exciting new possibilities in nanotechnology

More information: Nature Chemistry 3, 551–556 (2011) doi:10.1038/nchem.1066

Related Stories

A greener path for the production of a vital chemical

Jan 14, 2011

(PhysOrg.com) -- Nanoparticles of gold and palladium (Au-Pd) could lead to a more efficient and environmentally friendly way of producing benzyl benzoate, a chemical compound used widely in the food, pharmaceutical ...

Improving catalysis

Jun 14, 2011

(PhysOrg.com) -- Cardiff University research may help to improve the way that metal nanoparticles are used in catalysis – the process of making chemical reactions go faster.

Road to greener chemistry paved with nano-gold

Oct 24, 2005

The selective oxidation processes that are used to make compounds contained in agrochemicals, pharmaceuticals and other chemical products can be accomplished more cleanly and more efficiently with gold nanoparticle catalysts, ...

The sweet smell of nano-success

Jan 27, 2006

Materials scientists at Lehigh University and catalyst chemists at Cardiff University have uncovered secrets of the "nanoworld" that promise to lead to cleaner methods of producing, among other things, spices and perfumes.

Recommended for you

Tiny graphene drum could form future quantum memory

Aug 28, 2014

Scientists from TU Delft's Kavli Institute of Nanoscience have demonstrated that they can detect extremely small changes in position and forces on very small drums of graphene. Graphene drums have great potential ...

Graphene reinvents the future

Aug 27, 2014

For many scientists, the discovery of one-atom-thick sheets of graphene is hugely significant, something with the potential to affect just about every aspect of human activity and endeavour.

User comments : 0