Structure of Gold Characterized

When reduced in size to nanocrystals containing a few thousand atoms, gold is a surprisingly good catalyst. Finding the key to gold’s chemical reactivity (or that of any metal nanocrystal) has been difficult, as few measurement techniques work at the nanoscale.

Because chemical reactions primarily take place on surfaces, scientists need to know how atoms are arranged on those surfaces. While scanning probe microscopy works for flat surfaces of bulk crystals, a different technique is required to study the surfaces of nanocrystals.

In their study, the scientists used a technique they developed called nano-area coherent electron diffraction. The technique works by illuminating a single gold nanocrystal (about 3 nanometers in diameter and containing close to 1,000 atoms) with a coherent electron beam about 40 nanometers in diameter.

The electron beam is scattered by the atoms in the nanocrystal, resulting in a complicated diffraction pattern made of speckles – similar to what is seen when a laser beam is reflected by a surface. When deciphered, the diffraction pattern describes the structural arrangement and behavior of the atoms, and the number and lengths of chemical bonds in the nanocrystal.

What is surprising is that the contraction depends on the crystal facets. Atoms on facets with fewer bonds dominate, and lead to a much smaller contraction on other facets. This behavior is markedly different from bulk crystalline surfaces, and represents a new pattern of structural dynamics for nanocrystalline materials.

Characterizing small nanostructures and their surfaces is essential for understanding the special properties of nanomaterials. Nano-area coherent electron diffraction makes it possible to probe the surfaces of individual nanocrystals and examine their structure and size-dependent catalytic activity.; Source: University of Illinois