# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## Shape Controlled TiO\({}_{2}\) Structures  
While H\({}_{2}\) evolution occurs on the metal, charge separation and the oxidation semireaction efficiency are strongly related to TiO\({}_{2}\) morphology and surface texture.  
Engineering the shapes of some materials to desirable morphologies was actively pursued. Many applications, such as heterogeneous catalysis, molecule adsorption, gas sensing and energy conversion are strongly dependent from surface atomic structures, which can be finely tailored by morphology control. Meaningful advancements in this area have been achieved [49; 50; 51; 52; 53; 54; 55; 56; 57; 58].  
Albeit a large amount of experimental and theoretical studies has been carried out on the characteristics of different surfaces of metal oxides [59; 60; 61; 62; 63; 64; 65; 66; 67], less attention has been paid on the dependence of the functional properties (reactivity, toxicity, osseointegration, etc.) on crystal surfaces exposed by metal oxides [68; 69]. By tuning the ratio of different facets, the functional properties correspondingly change [70]. It is generally accepted that a surface with a high percentage of under-coordinated atoms has a greater reactivity [71; 72; 73; 74]. A cooperative mechanism involving both favorable surface atomic structures and surface electronic structures has been recently proposed in order to explain the differences in reactivity [75]. Moreover, surfaces with higher reactivity usually have higher surface energy, so the possibility to obtain a pristine facet without the use of adsorbates was questioned [73], and upon removing the adsorbates the surface can reconstruct. However, reactive facets are highly desirable for improving and tuning functional properties. The shape evolution during the crystal growth is driven by the continue decrease of the total surface energy until the minimum surface energy point. Such process is conditioned by the presence of shape-controllers in the reaction environment.  
The equilibrium shape of crystal structures can be predicted by using the Wulff construction [76], taking into account the surface energies of the crystal surfaces in the growth medium, if they are available. In the Wulff construction, surface energy minimization is used to optimize the composition of the crystal surface. The disadvantage is that only for clean crystal planes in vacuum are the surface energies available, and only in these conditions is the obtained equilibrium shape valid. The presence of liquid phases, electrolytes and adsorbates can change the equilibrium shape. The main TiO\({}_{2}\) polymorphs are anatase, rutile and brookite. All these three phases are constituted by TiO\({}_{6}\) octahedra, but in the case of anatase and rutile the crystal system is tetragonal, while for brookite is orthorhombic. The TiO\({}_{6}\) octahedra have different arrangements in the three polymorphs. Therefore, the surface terminations in various orientations are different, as well as the equilibrium morphologies of the crystals.  
According to the Wulff construction, the equilibrium shape of the anatase is a lightly truncated bipyramid enclosed with eight trapezoidal [101] surfaces (>94%) and two top squares {001} surfaces, as shown in Figure 5A. Regarding rutile, the predicted equilibrium shape of a macroscopic crystal contains {110}, {100}, {001} and {011} faces (Figure 5B) [77]All the above predicted equilibrium shapes must be considered only in vacuum at 0 K [60]. By means of theoretical approaches, [PERSON] et al. found that the presence of water could influence the equilibrium shapes by changing the size and the aspect ratio of anatase and rutile [79]. Despite the high differences among vacuum conditions used in the calculations and hydrothermal/solvothermal conditions for crystal growth, the most stable facets predicted usually represent the largest fraction also in the real crystal surfaces.