The observation of photocatalytic water splitting on the surface of anatase TiO2 crystals
has stimulated many investigations of the underlying processes. Nevertheless, a molecular
level understanding of the reaction is not available. Therefore, the work carried
out here is focused on improving the understanding of the water splitting mechanism
by studying anatase TiO2 surfaces with hybrid-exchange density functional theory calculations.
Initially, a simple and systematic methodology for the simulation of constant current
scanning tunnelling microscopy (STM) images is developed in order to study TiO2
surfaces. The methodology has to overcome a significant limitation of local basis set
calculations: the poor description of the charge density in the vacuum region above the
surface. The methodology is tested on various surfaces and the simulated STM images
are found to accurately reproduce experimental data.
The next step was to re-establish the atomistic structures of the (101), (001) and
(100) surfaces. The relative stability of these surfaces is interpreted in terms of the
coordination of the surface ions and the geometry surrounding them. Furthermore, the
electronic properties are studied. Based on relative positions of the conduction and valence
bands of the low-index surfaces studied the (101) and (001) surfaces were identified
as the most likely destinations for photo-generated electrons and holes, respectively.
The atomistic structure of two surfaces vicinal to the (101) surface – the (514)
and (516) surfaces – was established. The surface formation energy of these surfaces
is low and the (516) surface contributes significantly (27%) to the surface area of an
equilibrium crystallite, which could suggest that this surface is exposed in nanoparticles.
The conduction band edge of this surface was computed to be more negative than that
of the (101) surface. Thus, if the (516) surface is exposed in nanostructures, it would
replace the (101) surface as the most likely destination for photo-generated electrons.
Under-coordinated ions at the steps of these vicinal surfaces were identified as preferential
adsorption sites and water adsorption was studied at these sites. In general,
it was found that water preferentially adsorbs in the molecular adsorption mode on all
sites tested, with the exception of the Ti4c on the (514) surface, where the dissociative
mode is preferred. Nevertheless, at the (516) step the binding energy of the dissociative
adsorption mode is comparable to that of the molecular case. Therefore, if it were
possible to preferentially expose the Ti5c-O2c-Ti5c motif from the (516) surface, or even
the Ti4c of the (514) surface the reactivity for water photolysis could be enhanced.