# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 3 Photocatalytic Hydrogen Production from Anatase TiO\({}_{2}\)
### Effect of Crystal Shape and Structure  
The interaction of the TiO\({}_{2}\) crystals with the light can be changed modifying their size and shape in order to maximize the light absorption and the amounts of photogenerated charge carriers, in accordance with the Mie theory [80; 81]. In 2011, [PERSON] and coworkers [82] investigated the role of the TiO\({}_{2}\) nanoparticles shape in the photocatalytic hydrogen evolution. They compared nanospheres and nanorods using Pt as cocatalyst (1 wt %) and ethanol as sacrificial agent. They found that TiO\({}_{2}\) nanorods have higher UV photocatalytic activity due to a decreased probability of \(\rm{e_{cb}}^{-}/h_{vb}{}^{+}\) recombination. The authors, however, did not investigate the role of the different surfaces in their materials, probably responsible for the different electron hole recombination. In 2011, [PERSON] et al. [83], synthesized hierarchically branched TiO\({}_{2}\) nanorods for more efficient PEC devices, due to their large contact area, excellent light absorption and the conductive path for the charge carrier collection. They obtained three different nanostructured films, made of TiO\({}_{2}\) nanoparticles (NPs), nanorods (NRs) and branched nanorods (B-NRs). Under Xe lamp irradiation (880 W m\({}^{-2}\) total output, UV matched to AM 1.5G) NRs and B-NRs had higher performances when used in PEC devices, due to their enhanced light absorption and charge carriers transport. In particular, the higher surface area and the more effective charges separation in branched TiO\({}_{2}\) nanorods (B-NRs), resulted in an increased activity in comparison to simple nanorods (NRs). Figure 5: Equilibrium shape using Wulff construction for macroscopic crystals of rutile (**A**), anatase (**B**) and brookite (**C**). (**A** and **B**): adapted with permission from [60]. Copyright Elsevier, 2003. (**C**): adapted with permission from [78]. Copyright American Physical Society, 2007. Nanorods were studied also by [PERSON] et al. in 2013 [PERSON] et al. (2013). They synthesized different immobilized anatase nanostructures from titanate nanowires by means of hydrothermal treatment. Changing the conditions of synthesis, two different structures were obtained: in neutral or acidic aqueous solution a chemical dehydration of titanate to anatase porous framework (TiO\({}_{2}\)-PF) and nanowires (TiO\({}_{2}\)-NW) occured, on the other hand mild alkaline conditions led to a dissolution-recrystallization mechanism, in this case nanorods were the final product with well-defined {010} and {101} surfaces (TiO\({}_{2}\)-NR). Under solar simulated spectrum (41 W m\({}^{-2}\) in the UV), the authors found that the presence of the reactive {010} and {101} facets promoted the photocatalytic hydrogen evolution, overcoming the benefits of the intimate inter-particle contact of the hierarchical structures synthesized in neutral and acidic media. Among different TiO\({}_{2}\) architectures, photonic crystal can also be exploited for studying the correlation between shape and photocatalytic activity. Photonic crystals are periodic optical nanostructure in which the refractive index varies periodically in the space. In this way they can affect the light propagation in the material [85]. If a given wavelength matches the lattice periodicity, a constructive interference of the reflected waves inside the material will be possible. At that wavelength, a complete reflection occurs and the light cannot propagate because of the photonic band gap. The group velocity of the light is strongly reduced at the edges of the photonic band gap (where it can propagate inside the crystal); therefore, the interaction with the material increases. This stronger interaction is called \"slow light\" or \"slow photons\" and can be exploited in several fields, including photocatalysis [86; 87; 88; 89].