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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:693958 |
| Date | January 2013 |
| Creators | Liberti, Emanuela |
| Contributors | McComb, David ; Shaffer, Milo |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/40093 |
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