Titania-based photocatalysts represent a promising class of materials to  split water in its elementary components due to their high abundancy and stability [1]. Albeit their low solar visible light exploitation, their crystal  structure and electronic properties are well-known and facilitate the  investigation of the several fundamental aspects involved in photocatalytic water splitting. A detailed understanding of these key steps will subsequently allow for the design of an appropriate photocatalyst.
Most recently, the anatase modification of titanium dioxide (a-TiO₂) emerged as a widely applied material since it is the majority phase of TiO₂-nanoparticles and shows enhanced photocatalytic activity [1,2]. The intermediates during the water oxidation pathway on the a-TiO₂(101) surface were already identified on GGA-PBE level of theory [3]. The key step was determined to consist of the first proton removal induced through a photogenerated hole (H₂O + h⁺ → OH^• + H⁺ ). The influence of the photogenerated hole on the dissociation process was addressed controversially by theoretical studies: whereas combined PBE/HSE06 studies indicate that hole-trapping only occurs after dissociation through the OH-anionic species [4], recent work employing the B3LYP functional gives evidence for a concerted proton/hole transfer [5]. In addition, as observed by experimental TPD and TOF methods, OH^• radicals are found to be ejected from a well-defined a-TiO₂(101) surface after irradiation [6].
In order to get a more sophisticated insight into this yet indetermined reaction, we use two different theoretical approaches in this contribution: We firstly identify the active sites of the H₂O/a-TiO₂ system through periodic slab calculations using hybrid DFT functionals (PBE0/HSE06). Afterwards, these data will provide the basis for an embedded cluster approach allowing for accurate post-HF methods. As a result, we will present potential energy surfaces of a single water molecule on a-TiO₂(101).

[1] F. De Angelis, C. Di Valentin, S. Fantacci et al., Chem. Rev. 114 (2014) 9708.
[2] A. Barnard, P. Zapol, L. Curtiss, Surf. Sci. 582 (2005), 173.
[3] Y.-F. Li, Z.-P. Liu, L. Lui, W. Gao, J. Am. Chem. Soc. 132 (2010) 13008.
[4] W.-N. Zhao, Z.-P. Liu, Chem. Sci. 5 (2014) 2256.
[5] C. Di Valentin, J. Phys.: Condens. Matter. 28 (2016) 074002.
[6] Z. Geng, X. Chen, W. Yang et al., J. Phys. Chem. C 120 (2016) 26807.