# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 3 Photocatalytic Hydrogen Production from Anatase TiO\({}_{2}\)
### Effect of Exposed Surfaces  
The strong dependence of the anatase nanoparticle photocatalytic activity on the typology of exposed surfaces, depend on its different reaction mechanisms occurring on different facets, due to their characteristic physicochemical properties [70]. In 2015 [PERSON] et al. [92] studied the relationship between the different anatase surface and the H\({}_{2}\) generation yield in methanol photo-steam reforming under UV irradiation. They synthesized three different anatase crystals with shape nearly rectangular (RC), rhombic (R) and nanobar (NB) (Figure 7). They exposed {101}, {001} and, only for the RC, the {010} surfaces. They carried out Electronic Paramagnetic Resonance (EPR) experiments, correlating the amount of Ti\({}^{3+}\) (electron traps) with the H\({}_{2}\) evolution rate. They observed that the higher is the Ti\({}^{3+}\) EPR signal, the higher is the H\({}_{2}\) evolution rate. As we will report later, the {101} surface for anatase seems to be a reducing surface. Then, high exposition of this kind of facet should favor the formation of Ti\({}^{3+}\) centers under illumination and, therefore, better H\({}_{2}\) production. From their results, the RC materials were the most active in methanol photoreforming and showed the largest density of Ti\({}^{3+}\) centers, however these materials had also the lowest {101} surface area, exposing preferentially the {010} surface. This points out that the H\({}_{2}\) production cannot be related only to the extension of the reductive [101] surface, but also to presence of the {010} facet seems to  
Figure 6: Dependence of the photonic band gap (**Left**) and H\({}_{2}\) production rate (**Right**) from the nanotubes inner radius. Reprinted with permission from [91]. Copyright © 2016 American Chemical Society. Figure 7: **Left:** Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM) images of shape-controlled TiO\({}_{2}\) nanocrystals produced by solvothermal synthesis. (**a–c**) Rectangular nanoparticles (RC); (**d–f**) rhombic nanocrystals (R); (**g–i**) nanobars (NB). **Right**: Hydrogen evolution during the photocatalytic steam reforming of methanol on shape-controlled anatase nanocrystals. Adapted with permission from [92]. Copyright © 2015 American Chemical Society. play a role. The authors suggested the presence of a \"surface hetero-junction\" between the {101}, {010} and {001} facets which drives the electrons photogenerated on the {001} not just toward the {101}, but also to the {010}, favoring the charge separation. The idea of \"surface hetero-junction\" is one of the most employed concepts for explaining the synergistic effect of the different surfaces of TiO\({}_{2}\) nanocrystals [93; 94]. Figure 8 reports the charge transfer process using energy level diagram of the \"surface hetero-junction\" between facets {101} and {001} of TiO\({}_{2}\) anatase. The importance of the exposed surfaces is highlighted also in the recent paper of [PERSON], [PERSON] and coworkers [95]. They synthesized anatase NPs with different relative amounts of {101} and {001} surfaces. They obtained that the catalyst with an intermediate amount of the {101} and {001} surfaces, normalizing for the SSA, showed the best UV photocatalytic activity in the H\({}_{2}\) production from a methanol solution (Figure 9). They concluded that a synergistic effect may exist between {001} and {101} facets. In 2012 [PERSON] et al. [49] used TiF\({}_{4}\) as precursor in the synthesis of anatase nanocrystals to produce HF in situ for controlling the crystal shape. Through the change of the ratio of TiF\({}_{4}\) precursor in combination with TiCl\({}_{4}\) and the type of surfactant (oleylamine or octadecanol), a tunability of the titania shape based on bipyramidal geometry was demonstrated (Figure 10). The percentage of {101} and {001} facets was therefore well controlled and depended on the amount of HF released during the reaction, resulting in samples where F was still present on the surface.