Computational models of (001) faceted anatase TiO₂ nanoparticles

BACKGROUND: Understanding the structures and properties of photocatalysts requires the developing of computational quantum models. The present study is devoted to calculating structures, bands positions and location of the HOMO/LUMO orbitals in anatase titanium dioxide (TiO₂) nanoparticles comprising of exposed (001) and (100) surfaces of various sizes having different extent of hydroxylation at their edges. The (001) surface was left intact or was reconstructed by introducing an additional row of atoms every four unit cells. Two computational approaches were compared: self-consistent charge density-functional tight-binding (SCC-DFTB) and PM6. RESULTS: The SCC-DFTB method was found to be, for most cases, superior to the PM6 method in terms of structure, band positions and electronic orbitals. Based on the SCC-DFTB approach it was concluded that the presence of the (1 × 4) reconstruction was essential for keeping the (001) surface flat. Otherwise the surface undergoes anisotropic shrinking and bending, which contradicts experimental data. The band gap of the de-hydroxylated or partially hydroxylated nanoparticles was always smaller than that of nanoparticles with hydroxylated edges. No clear quantum size effects were found for these nanoparticles. Photogenerated non-thermalized holes were found to localize around (001) facets and at their edges, while electrons tended to concentrate over the central parts of the (100) facets. CONCLUSION: Efficient separation of charge carriers is predicted for anatase nanoparticles having (001) and (100) external surfaces. This conclusion, and moreover, the approach of using SCC-DFTB calculations to study faceting effects, is likely to be relevant to the developing of new, highly active, photocatalysts as well as for fundamental studies of adsorption.