**5. Photocatalytic material description and design**

As per earlier studies, a suitable photocatalyst should have a band gap of at least 1.23 eV for overall water splitting. High crystallinity and small particle size are major requirements to minimize the recombination of photo-generated electrons and holes. Metal oxide, nitrides, sulfides, phosphates, Groups I and II metals along with their lanthanides can also be used as photocatalytic material for overall water splitting. For the improvement of the efficiency of the photocatalyst, modification of material by doping some transition metal cations can help to increase the visible light response of the photocatalyst. Also, to exclude the energy backward reaction of water splitting and to increase the hydrogen production yields co-catalyst such as RuO2, Au, Pt, and NiO can be used. This section of the chapter has focused on the heterogeneous photocatalyst including TiO2 and metal oxides.

Fujishima and Honda first confirmed that TiO2 was a potential photo-anode for UV light active photocatalytic water splitting. TiO2 has been widely studied in

#### *New Strategy to Improve Photocatalytic Activity and Mechanistic Aspect for Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.109960*

a number of photocatalytic reactions due to its low cost, environmentally friendly nature, chemical stability, and tunable energy band gap [50–54]. A number of alternative methods have been reported to extend the photocatalytic activity of TiO2 into the visible light region, such as by doping with metal ions e.g. Carbon nanotubes [55, 56]. However, altered mechanistic methodologies have been proposed to explain this enhancement of activity. There are three mechanisms that have been reported to describe the synergistic effect of carbon nanotubes on TiO2, the first prospective mechanism is that carbon can act as an electron sink, which can effectively prevent the recombination process [57]. In another mechanism, carbon acts as a photosensitizer, which can pump electrons into the TiO2 conduction band [58]. In addition to proposed mechanisms, carbon can also act as a template for the dispersion of TiO2 nanoparticles to avoid agglomeration [59]. Further nonmetal ion doping and metallic dopants usually add additional energetic levels in the band gap, which will reduce the energy barrier and introduce a new absorption band gap [58, 59]. Doping of TiO2 with other elements can help to change the optical properties of nanomaterial and reduce the charge carrier recombination sufficiently s. Piskunov et al. suggested improvement in water splitting of TiO2 doped with Fe, where Fe2+ and Fe3+ act as centers for electron trapping and Fe3+ & Fe4+ act as a center for hole trapping. Lu0 et al. confirmed the doping of vanadium into the crystal lattice of TiO2 that shifts the absorption band to the visible range and V4+ V5+ efficiently traps the holes and electrons. Further, anionic doping has been extensively reported for TiO2 by different dopant materials such as C, N, F, S, B, and sCl [58, 59].

Further, other than TiO2 a number of metal Oxides such as Cu2O, Al2O3, CoO and ZrO2, Fe2O3, and Ta2O2 have been widely studied due to their low cost and stability. However, metal oxides suffer from limitations due to their large band gaps which limit their absorption of visible light. Ionic bonded materials have a large band gap because, in a typical metal oxide, the valence band and the conduction band have O2p. To overcome this shortcoming transition metal cation has been used with dn configuration for example Fe2O3 with the band gap value (2.0 eV) and Co3O4 91.3 eV [31]. This may lead to an increase in light absorption but a decrease inefficient charge carrier transport due to high resistivity. Using post-transition metals such as PbO (2.1 eV), SnO2 (2.4 eV), and Bi2O3 (2.5 eV) leads to better charge carrier generation however they are indirect semiconductors; so the optical absorption band edge varies with the square root of photon energy and gives less efficient charge carrier extraction process Therefore ternary metal oxides have been suggested to overcome these issues, for example, Bi20TiO32, SnNb2O6 and BiVO4 [31, 35]. Properties of n and p-type semiconductor properties have been found in BiO4 and high photon to current conversion efficiency [59]. In addition, Fe2O3 as photocatalytic material has a band gap of 2.2 eV which allows photon absorption under the irradiation of visible light. Morales-Guio et al. have proposed a photocatalyst of amorphous iron-nickel oxide (FeNiOx) for the oxygen evolution reaction. Similarly, WO3 has been considered a good photo anode material due to its suitable valance band position, which favors a high water oxidation potential.

In addition, Amer et al. have suggested ZrO2 modification with the deposition of ZrN on ZrO2 thin layers for the preparation of core-shell structures which are visible light active. However, Moniz et al. stated that the main drawback of WO3 is its instability towards corrosion. Due to the low e.g. of these materials, these can be modified with doping with metal cations or by combining with other semiconductors to form heterojunctions. Sivula et al. have confirmed a WO3/Fe2O3 heterojunction for better water oxidation due to its suitable band gap and proper alignment between WO3

and Fe2O3 metal oxide which leads to better electron transfer at the host and guest interface. Ta2O3 (Tantalum Oxide) has been considering an attractive semiconductor for overall photocatalytic water splitting because of its wide band gap value (4 eV), further it is required to narrow the band gap by doping with some doping ions. It is also mentioned by Lu et al. that Ta2O5 nanowires as an active photocatalyst with a high rate of hydrogen generation. Recently, Zhu et al. reported Ta2O3 nanowires that were modified by an aluminum reduction for the improvement of electron density and photoelectrochemical overall water splitting of the material.
