As the size of the bulk crystal reduces to the nanometre scale, anatase titania
exhibits enhanced photocatalytic properties. Nanostructuring of TiO2
involves engineering the crystal facets in a way that speci c types of surfaces
dominate the 3D shape. The atomic structure of the surfaces and 3D morphology
of the crystal determine the electronic properties of the material, and
should be characterized with atomic precision. Due to its high spatial resolution
(0.1 nm), aberration-corrected transmission electron microscopy was
used to obtain morphological and structural information on anatase nanoplatelets
and nanorods. TEM morphological analysis showed that the main
3D shape of the platelets is that of a truncated tetragonal bipyramid, where
f001g facets dominate. This 3D shape is accessible via 2D projections of the
crystal structure. In the nanorod specimens, the types of edge morphology
found link to intermediate or nal stages of growth, occurred via oriented
attachment of primary nanocrystals and classical monomer addition. The
structural characterization of the nanocrystals was carried out by examining
the exit plane wave of the specimen, which was reconstructed from a
serial acquisition of aberration-corrected TEM images of di erent defocus.
The phase of the reconstructed wave reproduces the atomic potential of the
specimen, and provides information with the maximum resolution of the microscope.
The optical properties of the platelets and rods were also analysed
using a combination of STEM imaging and EELS. Due to the high surface
to volume ratio of the platelets, the EELS spectrum is dominated by strong
surface features that arise from the polarization of the surface electrons induced
by the electron beam. The in
uence of the surface excitations on the
EELS spectrum is strongly determined by the thickness of the platelets: by
modifying the crystal thickness below 20 nm, the frequency of the surface
excitations changes, enabling the optical properties of titania to be tuned in
the visible and UV range. Finally, preliminary EELS investigations on the
nanorods suggest that, unlike metallic nanoparticles, the surface excitations
are not in
uenced by the morphology of the crystal, but strongly depend on
its thickness.