Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (7), 1188 - 1191
Formation of rod-shaped zinc oxide crystals
by oriented attachment of spherical particles
The precise synthesis of small particles is an important prerequisite for nanotechnology. Especially important are control over the size, shape, and crystal structure of the particles. Only a few examples of the controlled synthesis of rod-shaped semiconductor and metal nanoparticles are known to date. However, researchers from the University of Hamburg can now add nearly perfect nanorods of zinc oxide to the list.
Crystals are formed from crystal nuclei, which form in a supersaturated medium. The larger particles then grow at the cost of the smaller ones. Recent research has shown that there exists another mechanism for crystal growth: with oriented attachment, individual little crystals fuse together - in such a way that their crystal lattices integrate almost perfectly. Horst Weller, Andreas Kornowski, and Claudia Pacholski have shown that rod-shaped zinc oxide microcrystals can also be formed by this mechanism.
While generating their zinc oxide particles from a solution of zinc acetate in alcohol, the researchers determined that the shape of the nanoparticles depends on the conditions of the crystallization. At low zinc oxide concentrations, nearly spherical particles form in the solution. Heating this dilute solution leads to an increase in particle size. In contrast, if the solution is made more concentrated and then heated, nanorods are formed. The longer the solution is heated, the longer the rods. After one day of heating, they are up to 100 nm long and about 15 nm wide (one nm = one millionth of a mm).
Examinations of the crystallizing solutions by electron microscopy demonstrate that the rods are formed exclusively from previously formed spherical particles. They line up like pearls on a string and then fuse into a single, rod-shaped crystal. In addition to this preferred addition to the ends of the rods, the researchers noticed that some particles set themselves down parallel to the rods as well. "In a few places we could even see that the individual particles are lined up like a wall in which the stones of the second row are set exactly on top of those of the first," reports Weller. These places could act as branch points for the generation of three-dimensional nanostructures.