# Solar Concentration for Wastewaters Remediation: A Review of Materials and Technologies
## 3 Advanced Oxidation Processes and Photocatalysis
### Principles of Solar Photocatalysis
#### 3.1.1 Heterogeneous Photocatalysis  
In heterogeneous photocatalysis processes, the catalyst is generally a semiconductor material activated by absorption of UV or UV-Visible photons. Several semiconductors have been investigated as catalysts, such as TiO\({}_{2}\), ZnO, Fe\({}_{2}\)O\({}_{3}\), CdS, GaP, Co\({}_{3}\)O\({}_{4}\), ZnS, etc. Among these, TiO\({}_{2}\) has received the most investigation efforts since its good photocatalytic properties under UV irradiation were already known in the early 1970s. However, novel types of catalysts focused to solar applications are rapidly emerging, especially those which are able to efficiently absorb visible light. Heterogeneous photocatalysis by semiconductors can be explained in the framework of the band theory for the electronic structure of solids. In Figure 9 the first step is shown, where an electron (\(e^{-}\)) in the valence band (VB) is excited to the vacant conduction band (CB) by absorption of a photon of energy _hv_ (see Figure 9) equal to or greater than its optical band gap, leaving a positive hole (\(h^{+}\)) in the VB. The excited electrons can reduce available acceptor species, such as oxygen in proximity of the surface, while holes are often powerful oxidants. The photo-generated electrons and holes can thus drive redox reactions with species for which the involved redox potentials are an appropriate match. However, the electrons and holes can also recombine without participating in any redox process, in a competing deactivation event. This is a major issue in the design of materials, and is the reason why the time scale of the recombination process must be kept as large as possible. In the case of TiO\({}_{2}\) photocatalysis in water with UV photons of energy \(\geq\)3.2 eV for anatase and 3.0 eV for rutile (corresponding to about 387 and 413 nm wavelength respectively), photo-generated electron-hole pairs at the solid-liquid interface can promote redox reactions with adsorbed water and dissolved oxygen. This process results in the formation of OH\({}^{\bullet}\) and O\({}_{2}\)\({}^{-}\) radicals respectively, which are powerful oxidant species capable of reacting with most organic substances. The two most common configurations for heterogeneous photocatalytic reactors are: (1) reactors where the photocatalyst is in powder form suspended in water, and (2) reactors where the photocatalyst is immobilized on a surface. The first configuration is more explored, due to the simplicity of synthesis methods for powders; however, it needs an additional separation step, generally sedimentation and/or filtration, to recover the catalyst material. In the case of TiO\({}_{2}\), commercial products such as P25 and P100  
Figure 8: Publications on photocatalysis compared to those on solar photocatalysis. Data from Scopus comparing “photocatalysis” with “solar AND photocatalysis” as search terms within the “article title, abstract, keywords” search field. from Degussa are the most commonly-employed in the literature. Catalyst separation and recovery is a major drawback for large-scale applications [1]; thus, it is possible to identify here an open space on the research field aimed at the immobilization of the photocatalyst. This is highlighted by emerging manufacturing technologies, as exemplified by the use of pulsed laser deposition (PLD) for the production of nanocatalysts with controlled properties [85; 86]. Discussions highlighting the use of TiO\({}_{2}\) catalysts with sunlight on PTR and IPC reactors can be found in the literature [87; 88]. Also, advances in photocatalyst immobilization for TiO\({}_{2}\), to be used with concentrated sunlight, present alternatives as mesoporous clays, nanofibers, nanowires, nanorods, membranes and surface doping modifications [1]. The possibility to produce glasses or polypropylene tubes with supported TiO\({}_{2}\) has also been reported [45]. In practice, however, novel applications of TiO\({}_{2}\) have been hindered due to their wide optical band gaps allowing only little or no absorption of visible-light [89; 90].