# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 5 Conclusions  
From the prior discussion, we can summarize the progresses of the research on photocatalytic H\({}_{2}\) production on TiO\({}_{2}\) into these three major areas:  
1. TiO\({}_{2}\) crystal morphology: * _Structuration._ The crystal shape and size can be modulated in order to maximize the light absorption and, therefore, the charge carrier density under irradiation. The same effect can be achieved with photonic crystal structures, which can increase the performance of TiO\({}_{2}\), as discussed in the Section 3.1. * _Exposed Facets._ Besides light absorption, morphology can also decrease the recombination of the charge carriers, accelerating their interfacial transfer. This was highlighted by several works, demonstrating the different behavior of the exposed facets for TiO\({}_{2}\) anatase and rutile. In the case of hydrogen evolution on anatase, the surface [101] has a prominent role due to his reductive nature. In the presence of a cocatalyst as Pt, the higher is the amount of the [101] surface the higher is the rate of H\({}_{2}\) photoproduction. Conversely, on pristine TiO\({}_{2}\) surface, a synergistic effect results between the different surfaces of anatase crystals, indicating the formation of a \"surface heterojunction\" that increases the efficiency of nanoparticles exposing considerable amount of oxidative surface.
2. Cocatalyst. The role of the cocatalyst is crucial for improving the TiO\({}_{2}\) activity in the H\({}_{2}\) photoproduction. The common characteristics of a good cocatalyst in HER are the ability to drain photoelectrons from the conduction band and then decrease the required overpotential for reducing the proton. Noble metals are still in a predominant position, and especially Pt showsthe best activity, followed by Pd, while Au and Ag perform relatively worse. Other metals, such as Rh, Ni, Mn, Ru, Cr and Co, were recently proposed. The emergence of cheaper alternatives, such as Cu and carbonaceous materials (graphene, C\({}_{3}\)N\({}_{4}\), etc.) could substantially decrease the costs of the H\({}_{2}\) production, especially in the prospect of large-scale production.
* Substrates and Sacrificial Agents: usually, the role of the hole scavenger goes into the background compared to the reductive part in photocatalytic reforming. However, the sacrificial agent largely affects the H\({}_{2}\) photoproduction. While a wide range of alcohols (methanol, ethanol, propanol, etc.) can be exploited mainly for research purposes, to better understand the mechanisms involved in the process, the coupling with biomass-derived substrates, such as glycerol, glucose and sugars, or recalcitrant pollutants, could be a useful way for producing H\({}_{2}\) in a greener manner and to fully exploit the positive valence band potential of TiO\({}_{2}\) and other wide band gap semiconductors.  
[PERSON], [PERSON] and [PERSON] contributed equally to the conceptualization, writing and editing of the Review. [PERSON] and [PERSON] contributed as supervisors and reviewers.  
[PERSON], [PERSON] and [PERSON] acknowledge funding from Regione Piemonte, Bando LR34/2004--\"Studio preliminare e sviluppo di soluzioni innovative di materiali d'attrito in ambito Automotive\". [PERSON] and [PERSON] acknowledge the financial support from project \"Ricerca Locale\", University of Torino.  
The authors declare no conflict of interest.