# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 1 Introduction
### Mechanistic Considerations  
Since the earliest reports, it has been outlined that semiconductors such as ZnO or TiO\({}_{2}\) were able to oxidize organics and anions such as I\({}^{-}\)[20] and CN\({}^{-}\), and reducing O\({}_{2}\) to form hydrogen peroxide or water [34]. The addition of occatalysts such as Pt and other noble metals improved the rates and allowed the evolution of hydrogen in anaerobic conditions. Other occatalyst, such as RuO\({}_{2}\), allowed water oxidation to O\({}_{2}\)[35]. From these evidences, a first picture emerged regarding the mechanism of organics photoreforming on noble metal-modified semiconductors (Figure 2b). The photogenerated electron-hole couples are separated; the holes are transferred to the organics [36; 37], while the electrons are captured by the noble metal, where they can promote water reduction and hydrogen evolution thanks to the low activation energy displayed by most noble metals and especially Pt [38; 39]. At this stage is worth clarifying that the mechanisms of reaction in the dark, or under visible irradiation, where plasmonic resonance may be activated, could be radically different, as recently reviewed by [PERSON] and [PERSON] [40]. This pictorial unpolished mechanism is actually in good agreement with the experimental evidences collected during the years. It has been shown that hole trapping is a fast process, requiring less than 50 fs to happen, and that hole transfer to surface hydroxyl requires \(\approx\)250 ns, while in the presence of methanol the hole transfer significantly accelerates, being completed within 1 ns [41; 42]. Conversely electron trapping is considerably slower, requiring \(\approx\)160 fs. Electron lifetime is in the order of the microsecond in vacuum or in the presence of water vapor, but it is extended to more than one second in the presence of methanol vapors. This demonstrates how electron and hole dynamics could be radically different when specific scavengers are introduced, and that their recombination, predominantly non-geminate and showing second order kinetics, strongly depends on these factors [43]. When TiO\({}_{2}\) is decorated with noble metal particles, electron trapping becomes at least 10-15 times faster compared with pristine TiO\({}_{2}\), whereas hole capture seems almost insensitive to noble metals [43]. This evidence is coherent with previous picture of electron capture by the metal where hydrogen evolution takes place (Figure 2b). A more refined treatment was carried out by [PERSON] and coworkers [14]: upon irradiation the Fermi level of the semiconductor is displaced towards more negative potential, because of the faster transfer of holes and to the accumulation of photoelectrons (Figure 2c). The Fermi level shift in the semiconductor causes an identical shift in the noble metal particles. If this shift is such that water reduction is triggered, hydrogen evolution will occur. This picture is coherent with the noble metal particle forming an ohmic contact with the semiconductor, rather than a Schottky junction, which should repel semiconductor electrons, especially in the case of noble metals, which have large work functions. Because hydrogen evolution occurs on the noble metal, and because of the analogy between irradiated semiconductor slurries and photoelectrochemical cells, the study of the mechanism can take advantage from the enormous amount of work done by electrochemists on the hydrogen evolution reaction (HER) on metal electrodes (Figure 4). HER is an archetypical reaction, in which two electrons and two protons are combined to form the simplest molecule H\({}_{2}\). The reaction requires two steps; the first is the Volmer step, the reductive adsorption or electrochemical discharge [44]:  
\[\mathrm{H^{+}+e^{-}+M\mbox{--}\rightarrowM\mbox{--}H}\]  
In which a proton from solution is reduced to become an adsorbed hydrogen atom on the metal surface. Of course, the Volmer step needs empty metal sites M\(\cdot\)\(\cdot\)\(\cdot\)\(\cdot\)\(\cdot\)\  
step.