Validation of a constrained 2D slab model for water adsorption simulation on 1D periodic TiO₂ nanotubes

Solar light driven hydrogen evolution is one focus of modern materials research. Among the different emerging technologies, particular interest is devoted towards metal oxide photocatalysts in the form of various 1D nanostructures. Presently, the mismatch between regular structures that can be synthesized and the largest structures that are feasible for computer simulation is still very large. For example, an in-depth study of water adsorption on nanotube (NT) surfaces requires, in addition to DFT calculations, molecular dynamics simulations to take into account the disordered nature of the aqueous phase. To completely immerse even a very thin nanotube into an aqueous system requires very large system sizes, the modeling of which is computationally extremely expensive. On the other hand, the typical surface science approach when modeling planar interfaces is computationally much less demanding, but can be adequate only for NTs of very large diameters possessing low strain energies. Here, we present three simplified slab models derived from local NT geometries as approximations for inner and outer interfaces of thin TiO₂ NTs. In order to describe the role of NT strain on the electronic structure of the water/NT interface, these models contain additional constraints on some of the atoms to alleviate some of the shortcomings of the slab approach. We use the energy of water adsorption, equilibrium interatomic distances, calculated densities of states (DOS) and Mulliken charge distribution as criteria for estimating the model quality. Specifically we analyze the suitability of this approach for the very thin TiO₂ (001) slab and the thicker TiO₂ (001) NT.