Surface structure of nanocrystals revealed

Characterization

April 3, 2008

The atomic structure of nanocrystal surfaces can be extracted from diffraction patterns obtained using a tightly focused coherent electron beam. (Courtesy of Jian-Min Zuo.)

A new method for probing the surface atomic structure of nanocrystals has been developed by researchers at the University of Illinois at Urbana-Champaign (UIUC) [Huang et al., Nat. Mater. (2008) 7, 308].

Surface atoms have fewer bonds than those in the bulk material and will tend to relax towards atoms in the bulk. Surface reconstruction and bond length contraction is expected to be very important in nanocrystals, where it will affect many phenomena such as interfacial stability and heterogenous catalysis.

While the reconstruction process is well known for the surfaces of bulk crystals, understanding what happens in nanoparticles has been hindered by a lack of suitable characterization techniques.

The UIUC researchers used a coherent beam of high-energy electrons to probe the surface structure of 3–5 nm Au nanocrystals. Diffraction patterns from individual nanocrystals could be obtained using the tightly focused 40 nm electron beam.

“In the nanocrystal diffraction patterns, we see the Bragg peaks and speckles of scattered electron beams around the Bragg peak,” explains lead author Jian-Min Zuo. The asymmetry in the speckles around the peak arises from the bonding involving surface atoms. The team used a model to fit the diffraction patterns and understand the surface bond contractions.

“What we found is that surface atoms do contract, but the amount of contraction depends on the crystal facet,” says Zuo. “Atoms on crystal 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.”

The group want to develop their technique to look at metallic nanocrystals supported on oxides so that they can examine a number of important catalyst systems and determine their surface and interface structure, Zuo told Materials Today.

“The work of Huang et al. represents a major breakthrough in structure determination using coherent diffraction of electrons,” says Ian Robinson of University College London, UK.

Jonathan Wood