Nano sculptures in gold

New method clarifies the structure of uncharged gold nanoparticles

Giza in the Nanoworld: The gold particles were placed on virtual desert sand. A cluster of seven atoms forms an equilateral triangle, to which an atom hangs as an additional corner. 20 atoms pile up into a pyramid with a triangular base. With one atom less, the pyramid loses its point. © Fritz Haber Institute of the Max Planck Society
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If you are loaded, you may change the color of your face, but you do not want to tear off one arm to assemble it as the third leg. However, this is different with clusters of seven gold atoms. These arrange themselves in the loaded condition differently than in the unladen, as scientists report now in "Science".

Chemically sluggish and expensive: these properties dampened the enthusiasm that chemists felt for gold - at least since the days of alchemy have passed. But interest has been revived for some years now. For nanoparticles of the precious metal may be suitable as catalysts for important reactions in the chemical industry. The tiny gold particles are very picky in the reactions they support.

Whether the gold nanoparticles can favor certain reactions depends strongly on their structure. Researchers at the Fritz-Haber-Institut of the Max Planck Society, the Steacie Institute for Molecular Sciences in Canada and the Dutch free-electron laser FELIX at the FOM Institute Rijnhuizen have now developed a method that takes the form of neutral gold To determine clusters. Chemists have known for some time how some of these ensembles look like when loaded with up to a few dozen atoms. As catalysts but especially uncharged particles would be interesting. And sometimes they take on a very different shape than charged clusters with the same number of atoms.

From pyramid to hexagon

The Berlin researchers headed by André Fielicke have studied clusters of 7, 19 and 20 atoms. For the uncharged particles of 19 and 20 atoms, they observed the same structures that are known from their negatively charged counterparts: 20 gold atoms stack to a tetrahedron, a pyramid with a triangular base. One atom less costs the pyramid the top. "Seven gold atoms form a triangle with an additional corner in the uncharged state, " explains Fielicke.

In a simply positively charged cluster, however, seven atoms form a hexagon with an atom at its center. In the uncharged form, there are three gold atoms on each edge of a triangle. At one edge, two atoms are bridged by another, creating the additional corner. "This structure probably prefers the gold atoms in the uncharged form, because the electrons can better get out of it, " says Fielicke. display

In order to structurally scan the uncharged nanoparticles, the Berlin scientists had to tackle a number of problems: "The clusters are rather unstable, you can not just buy them as powders, " explains Philipp Gruene, who has a large Part of the experimental work was done. The scientists therefore have to produce the gold clusters in the same apparatus in which they also determine the shape of the particles. For this purpose, they vaporize small amounts of the precious metal with a laser beam from a gold rod. In this way, gold clusters of different sizes and shapes are formed.

Method combination brings the success

Usually, chemists separate such a particle mix in a mass spectrometer. This device initially ionizes the particles, so it dissolves them electrically. Then it separates them in an electric field according to their mass, since the field accelerates light particles faster than heavy - if both carry the same charge. If a larger quantity of a particle type is available, its structure can also be determined in an infrared spectrometer.

Since the Berlin scientists want to look at the structures of uncharged particles and can only produce very few particles, this procedure is ruled out. Nevertheless, the Berlin scientists use the two methods, they only combine them in a sophisticated way. Before they separate the particle clutter, they fire with a very intense infrared laser of certain wavelength on the mix and separate the clusters in a rather rabid way. The particles vibrate so much that they burst.

After selection with the infrared laser, the researchers send the mixture of the remaining particles through a mass spectrometer. The particles, which were excited and destroyed by a certain wavelength of infrared light, hardly leave any traces in the mass spectrum. The fact that there is something missing at a certain point in the mass spectrum, the researchers conclude with a comparison experiment: they also separate a mixture of gold clusters in the mass spectrometer, which they have not previously subjected to the special treatment with the intensive infrared laser.

Laser bombardment over many wavelengths

Making such an experiment with a laser beam of a single wavelength does not do much. Only a complete oscillation spectrum reveals the shape of a particle. "So we have to repeat the experiment at about 200 different wavelengths of the infrared laser, " explains Fielicke. This creates the next problem for the scientists: laser light, which is intense enough over a large part of the spectrum to cause the clusters to burst, is only supplied by a free-electron laser.

Therefore, the Berlin researchers have clarified the structure of the gold cluster on the Free Electron Laser for Infrared eXperiments, Felix in short, in the Dutch Nieuwegein. From the mass spectrometric measurements at different wavelengths of the infrared laser, they then reconstruct the oscillation spectrum for certain clusters and can then read their structure.

Among other things, the Berlin scientists are using their investigations to help them find a catalyst for epoxidation - a technically important reaction in which chemists attach hydrocarbon molecules to an oxygen atom. This is often the first step to more complicated molecules. Whether in the end the desired product comes out depends on where the oxygen atom is attached to the hydrocarbon. And that's exactly where gold clusters could serve as pilots.

(MPG, 04.08.2008 - NPO)