# 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
[126] suggested that the electron transfer process can occur from photoexcited states of graphene sheets without defects onto the titania surface with a non-adiabatic mechanism. On the contrary, the EIS evidences reported by [PERSON] et al. investigating the TiO2-rGO/electrolyte interface suggested that electrons in the TiO2 conduction bands can be transferred onto rGO sheets, but also that the holes in the valence bands can move toward the rGO phase promoting an effective degradation of adsorbed molecules [127]. The main conclusion from the analysis of the scientific literature on this topic (see as an example the recent reviews [128; 129]) is that it is quite difficult to generalize because the numerous modifications of the synthesis and the intrinsic differences between materials only seemingly similar can affect abruptly their photocatalytic performance, the electronic structures of the hybrids and--ultimately--the dynamics of the photogenerated charge carries. Recently, numerous Cu-containing species (Cu [105; 130; 131], Cu2O [132], CuOx [133], CuO [134], Cu(OH)2 [135] and Cu2(OH)2 CO3 [136; 137]) showed relatively high photocatalytic rate, even if a complete agreement on the operative electron transfer mechanism has not been reached yet. The complex redox speciation of the copper-based materials seems to force the e^-/TiO2 to be delocalized on the cocatalyst and here react to produce hydrogen. The role of Pt and Au in the photoreforming of methanol under irradiated Pt- and Au-supported titania was investigated by [PERSON] and coworkers [100] by in-situ electron spin resonance (ESR) spectroscopy. Both the nature of the cocatalyst and its loading strongly affected the H2 evolution rate and the nature and relative concentration of the byproducts of the methanol oxidation. The best performance was reported for Pt-TiO2 and this was attributed to the better ability of Pt to act as electron sink with compared with Au. This was directly demonstrated evaluating the concentration of the Ti^-3 paramagnetic sites under irradiation that followed the order: pristine TiO2 > Au-TiO2 > Pt-TiO2. The electrons photopromoted were better delocalized on the metal phase--especially with platinum--giving a lower concentration of Ti^-3 species. Interestingly, in the adopted experimental conditions the oxidation of methanol was only partial and significant amounts of formaldehyde were reported (see Figure 11). In water photosplitting the back reaction giving water from the evolved products (H2 and O2) has to be limited to maximize the quantum yield of the process. As an example, Au, NiO and RuO2 are suitable cocatalysts also because the back reaction hardly proceeds because the reductive and oxidative sites on these catalytic systems are spatially separated. Interestingly, a suitable cocatalyst for  
Figure 11: Hydrogen, formaldehyde, carbon monoxide and carbon dioxide production rates on 4 different TiO2-based photocatalysts. TiO2-A and TiO2-Rf are pristine nanometric TiO2, while Au/TiO2-A and Pt/TiO2-A are as TiO2-A but decorated with nanometric particles of Au and Pt, respectively. Reprinted with permission from [100]. Copyright Elsevier, 2013.  
the H\({}_{2}\) evolution from water on irradiated semiconductor--not only TiO\({}_{2}\)--is NiO, which is usually formed by H\({}_{2}\) reduction of islands of nickel oxide on the semiconductor photocatalyst surface and subsequent partially oxidation with O\({}_{2}\) to form biphasic Ni/NiO layer structure. This structure is usually denoted as NiO\({}_{\rm x}\) and is able to favor the charge carrier separation promoting the evolution of H\({}_{2}\) on the surface of the cocatalyst (in aqueous suspension mainly formed by Ni(OH)\({}_{2}\)) and avoiding the back reaction between H\({}_{2}\) and O\({}_{2}\) being the oxygen evolution sites spatially separated from the cathodic sites. For further information on the electrochemical behavior of NiO, please refer to [138].