Structural studies of epitaxial Au nanocrystals in rutile

Besides promising applications for electronic nanostructures or nonlinear optical devices nanocrystalline Au may be used as efficient catalysis material (e.g. chemisorption of CO or hydrocarbons). The catalytic reaction is often very sensitive to the structure and size of the metal crystallites and their crystallographic orientation Both conditions may be controlled by growing the nanocrystals in a single crystalline non-metallic matrix.
Here, Au nanocrystals were generated inside rutile TiO2 by implantation of 260 keV Au ions. The growth in a matrix allows to stabilize the precipitates structurally. Furthermore, no impurities were introduced because of mass separation during the process. However, the host crystal is damaged by the ion impact processes. This damage may be reduced by in situ or post-implantation annealing.
Various x-ray scattering techniques were used for the structural characterization of the material: specular reflection for surface and near-surface studies, diffraction for determination of strain and crystal size, pole figure measurements to reveal the orientation relationship.
In the as-implanted state randomly oriented crystals with a size of 5 … 7 nm as well as crystals oriented in a fibre texture with axis parallel to the rutile surface normal have been obtained. Annealing at 1000 K let the nanocrystals grow to 15 nm; simultaneously the amount of epitaxially oriented Au crystals increases. The orientation relationship is controlled by the crystallographic axis of the rutile matrix surface so that the ensemble of the Au nanocrystals fits to the crystal symmetry of the matrix:

• the surface normal is the two-fold rutile [100] direction:

surface normal - Au < 2 1 1 > || TiO2 < 1 0 0 >
in the surface - Au < 1 1 0 > || TiO2 < 0 0 1 > and Au < 1 1 1 > || TiO2 < 0 1 0 >.
In order to fulfil the two-fold matrix symmetry there are two groups of Au crystallites rotated by 180° each to other around the surface normal.

• the surface normal is the four-fold rutile [001] direction:

surface normal - Au <110> || TiO2 <001>
In order to fulfil the four-fold matrix symmetry there are two groups of Au crystallites rotated by 90° each to other around the surface normal. Each of them is split into two sub-groups rotated by +9.7° and –9.7° around the surface normal from the “ideal” orientation: Au <110> + 9.7° and –9.7° || TiO2 <110 > and Au <100 > + 9.7° and –9.7° || TiO2 <110>.