# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 3 Photocatalytic Hydrogen Production from Anatase TiO\({}_{2}\)
### Effect of the Cocatalyst  
Numerous are the cocatalysts coupled with TiO\({}_{2}\) proposed to maximize the rate of electron transfer to H\({}^{+}\) and consequently increase the overall efficiency of both water photosplitting and photoreforming. The most efficient cocatalysts are noble metals such as Pt, Au and Pd [99; 100; 101; 102; 103; 104; 105; 106; 107; 108], but also materials based on Ag, Rh, Ni, Mn, Ru, Cr, and Co were proposed [109; 110; 111; 112; 113; 114; 115]. Furthermore, different carbon-based materials (e.g., graphene, graphene oxide, single-wall and multi-wall carbon nanotubes, carbon nanocones, graphitic-C\({}_{3}\)N\({}_{4}\)... ) [116; 117; 118; 119; 120; 121] have been recently reported as active materials to (_i_) delocalize the electron in the conduction band thanks to their proper work functions (significantly lower than the CB of the titania), (_ii_) decrease the overpotentials for the proton reduction. An overall agreement on the role of the carbonaceous phase during the photocatalytic process under irradiated hybrid systems has not been reached yet. One reason is that different materials, or the same materials synthesized with different procedures, can have significantly different electronic structures. The partitioning of the photogenerated charges between the semiconductor and the carbon-based material is strongly affected by their electronic properties (especially in terms of energetic positioning of the bands) and, consequently, this point is not easy to be rationalized comparing the photocatalytic behavior of materials only seemingly similar. An agreement on the role of the carbonaceous phase (graphene, graphene oxide, reduced-GO... ) has been reached for the photocatalytic degradation of dyes on hybrid systems. In this case a dye-sensitized degradation mechanism seems to be active: an electron promoted in the LUMO of the dye--as a consequence of the absorption of a visible photon--is rapidly delocalized toward the carbonaceous phase--where the dye is usually strongly adsorbed--and here it reacts with the dissolved oxygen to generate the radical O\({}_{2}\)\({}^{\bullet-}\)/HO\({}_{2}\)\({}^{\bullet}\). Then the oxidized dye can autonomously evolve forming stable or transients byproducts [122; 123]. [PERSON] et al. investigated the photocatalyst degradation of the colorless diphenhydramine and the organic dye methyl orange and demonstrated that under visible irradiation the self-oxidation of the visible-absorbing species is the dominant mechanism [124]. Under UV irradiation the transfer of the charged carriers between the two solid phases and between the hybrid catalyst and the reactants in solutions (e.g., H\({}_{2}\)O to be reduced for the production of H\({}_{2}\) or a sacrificial organic compound in the case of carbon photoreforming) has not been totally clarified. It was suggested by [PERSON] et al. that on UV-irradiated TiO\({}_{2}\)/rGO hybrids, it is possible both the electron transfer from exited states of rGO onto the titania conduction band and the hole transfer from the TiO\({}_{2}\) to the carbonaceous phase where the sacrificial substrate is adsorbed and oxidized, if and only if the HOMO of the substrate has a less positive redox potential (higher energy) than the empty state of excited rGO [123]. Similarly, [PERSON] and [PERSON] suggested that during the photocatalytic transformation of phenol on visible irradiated hybrids composed of rGO and titanate nanotubes (TNTs), the photogenerated electrons in the graphene can be injected into the TNTs conduction bands although the presence of a Schottky barrier. In this case the degradation of phenol under visible irradiation was related to the indirect formation of highly reactive hydroxyl radical mediated by the formation of O2^-/HO2^- as also suggested by [PERSON] et al. [125]. The kinetics of transfer of electrons from high-energy rGO states toward the TiO2 are characterized by kinetics in the 0.1-0.2 ps range. The ab-initio calculations carried out by [PERSON] et al.