# A Deeper Insight on the Stability of Water-Induced Reconstruction of Anatase (001) Surface
## 1 Introduction  
Bulk TiO\({}_{2}\) and TiO\({}_{2}\) surfaces of the various crystal phases (anatase, rutile, and brookite) are being considered since many years for a variety of applications with special mention to environment-and energy-related applications, such as the photo-decomposition of organic pollutants, solar cells, and solar-hydrogen production [1; 2; 3]. Titanium oxides are also important in surgical implants where the implant surface is oxidized and in contact with the bio-moieties in the human body [4]. All the mentioned systems and problems require the knowledge of the surface-water interaction processes because they imply the usage of either nano-sized TiO\({}_{2}\) or TiO\({}_{2}\) surfaces in water. For this reason, the atomistic scale characterization and modeling of the TiO\({}_{2}\)-water interface is being studied intensively [5; 6; 7; 8]. Among the crystal phases, the metastable anatase form is known to be photo-catalytically more active than the ground state rutile phase, and becomes the most stable phase at the nano-scale. This is why the anatase is increasingly considered. While the (101) is the most stable anatase surfaces, the minoritary (001) has been claimed to be more reactive in water [7; 9; 10; 11; 12; 13; 14] and this is the reason why many articles have focussed on the (001) anatase surface in water. Moreover, recent results have reported on the successful synthesis of TiO\({}_{2}\) particles with minority facets [15; 16; 17]. The reactivity of the (001) surface with respect to the water adsorption and dissociation has been debated for long time with no conclusive statements yet. While the bare (unreconstructed) TiO\({}_{2}\) (001) surface has been predicted as a dissociative reactive surface for water absorption and, therefore, a good candidate to obtain photo-reactive water splitting for energetic purposes, the experimental results are rather controversial and do not confirm such predictions [18; 19; 20]. The (001) unreconstructed (dry) surface is made of rows of under-coordinated atoms along the [100] direction formed by sequences of five-fold titanium atoms (Ti\({}_{5c}\)) and two-fold oxygen atoms (O\({}_{2c}\)) with alternating long and shortTi\({}_{5C}\)-O\({}_{2c}\) bonds and an inward relaxation of the topmost layer with respect to the unrelaxed slab (Figure 1). There is a general consensus that such a surface should be highly reactive for water dissociation [7; 9; 10; 11; 12; 13; 14] but this is in marked contrast with the experiments that show, quite surprisingly, that this is not the case [18; 19; 20; 21]. Moreover, in-situ experimental findings have shown that stable (001) anatase surface shows a (1 \(\times\) 4) reconstruction character in vacuum. Atomistic models, based on popular first principles techniques such as the Density Functional Theory (DFT), have been constructed starting from the well known ADM model [22] that is able to reproduce the (1 \(\times\) 4) surface reconstruction observed in vacuum. However there have been increasing difficulties to explain the experimentally observed surface intertress for water dissociation using the ADM model [21; 23]. The reactivity loss has been related to the decreasing of the surface stress with respect to the unreconstructed surface [11; 12; 13] but, while the ADM model predicts a reduced surface stress due to the addition of TiO\({}_{2}\) molecules on the surface, this appears to be not sufficient because some residual surface reactivity, that is not confirmed by experiments, has been found [23]. Moreover it has been shown also that point defects, such as vacancies, oxygen ad-atoms or interstitials, might even enhance the ADM reactivity [23]. For this reason the ADM model has been further modified proposing that the observed surface reactivity might be related to the oxidized or reduced state of the surface [21].