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Energies 2020, 13, 420 28 of 96 High bandgap semiconductors, due to the wide bandgap, absorb only ultraviolet light that corresponds to a tight range of the solar irradiation spectrum. TiO2 is the semiconductor most widely researched for photocatalyst application. The high bandgap reduces the solar-to-hydrogen efficiency that results up to 1.3% for Anatase TiO2 (bandgap of 3.15 eV), 2.2% for the Rutile TiO2 structure (bandgap of 3.0 eV) and 4.8% for WO3 (see Figure 12a). WO3 has a bandgap of 2.6 eV and a low energy level of the conduction band that results in the need for an externally applied bias [320]. Dye sensitization and doping with transition metal ions, rare earth metal ions, non-metals and noble metals are some possible solutions to stretch the light absorption range, minimize the recombination between electron and hole pairs and to reduce the semiconductor over-potential [313]. Intermediate bandgap semiconductors have a lower bandgap that results in a wider absorption range of the solar spectrum, and therefore in higher STH efficiency. BiVO4 has a bandgap of 2.4 eV and achieves a theoretical maximum STH efficiency of 9.1%, whereas the STH efficiency of Fe2O3 is up to 12.9% and the bandgap is ~2.2 eV (see Figure 12a). BiVO4 is a suitable photocatalyst since it has a low production cost, low toxicity, high photostability and good resistance to photo-corrosion. Although, BiVO4 requires an external bias since the energy level of the conduction band is less negative than the water reduction potential and experiences a high charge recombination process and a low water oxidation kinetics [321]. Fe2O3 is chemically stable, abundant in nature and cost-effective. However, Fe2O3 suffers for electron–hole recombination, low diffusion length and valence-band poisoning due to the low diffusion length of holes [322]. Co-catalyst and nanostructures are typically used to enhance the oxidation kinetics and the holes migration [309]. Low bandgap semiconductors are suitable materials for water splitting due to their low bandgap and, therefore, high light absorption range. High interest is addressed to group III–V materials such as InGaP, GaAs and InP. Khaselev and Turner [323], in 1998, reported an STH efficiency of 12.4% established in a crystalline GaInP2/GaAs tandem cell developed at the National Renewable Energy Laboratory. The main drawback is the electrode instability due to photo-corrosion at the interface between the semiconductor material and the electrolyte. A coating treatment of the photoelectrode with a catalytic material reduces the photo-corrosion of electrodes and the electron–hole recombination rate. The coating also improves the oxidation kinetics optimizing the PEC efficiency [324]. 3.2.2. Modification Techniques Low light absorption range, electron–hole recombination, the low diffusion length of carriers, slow oxidation and reduction kinetics and photo-corrosion are all drawbacks of photoelectrochemical devices. Some modification techniques have been proposed to improve the efficiency delivered by PEC. Hierarchical nanostructured photocatalytic materials improve surface to volume ratio, increasing PEC performance through the rise of the light absorption spectrum minimizing the light reflection, the improvement of the charge transfer as a result of the higher surface area in contact with the electrolyte, the increase in the charge carrier collection because of the reduction of the thickness of the semiconductor film and the lowering of the photogenerated charge recombination rate due to the high degree of crystallinity [325]. Nanomaterials are classified as dimensional structures from zero to three [326]. Quantum dots have an ultra-small particle size (<10 nm) and are defined 0 D nanostructures [327]. Quantum dots improve the efficiency of light harvesting and the photocatalytic activity reducing the electron–hole recombination rate [328,329]. Nanorod, nanowire and nanotube form 1 D nanostructures that ensure a rapid diffusion of charge carriers in only one direction lowering the recombination process [330–333]. 2D nanostructures are realized with nanosheets; they enhance the light absorption and improve the charge carriers transport due to the low thickness and high surface area of the catalytic film [329,332,334]. Finally, 3D nanostructures facilitate the separation of the photogenerated charge carrier, reducing the distance between the electrode and electrolyte [335–338]. Doping is one method among the modification techniques to enlarge the light absorption spectrum to the visible region for wide bandgap semiconductors and to improve their photocatalytic activity [339].PDF Image | Green Synthetic Fuels
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