# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 4 Ferroelectric Materials for Photoelectrochemical (PEC) Water-Splitting Devices  
### General Requirements for Photoelectrode Materials  
The choice of appropriate materials for photoelectrodes plays a decisive role in the PEC activity for water splitting. Conventional materials, including semiconductors, can be categorized into wide-bandgap and narrow band-gap materials. Traditionally, wide-bandgap materials (TiO\({}_{2}\), SrTiO\({}_{3}\), BaTiO\({}_{3}\), and WO\({}_{3}\)) are considerably stable in an electrolyte solution, but they can have the limited absorbance range in the ultraviolet region. Even though narrow bandgap materials (Cu\({}_{2}\)O, InAs, and CuO) can absorb much more visible light irradiation in the solar spectrum, they have a high possibility of being damaged from strong electrolytic environments. However, we do not have many options for selecting suitable materials for photoelectrodes (photocathodes and photoanodes) of PEC. For photocatalytic water splitting, the selected materials must have bandgaps of approximately 2 eV (at least 1.23 eV) as a result of thermodynamic energy and kinetic loss. Figure 5 shows the bandgap of various materials. Therefore, it is imperative that the following general requirements are met prior to the fabrication of high-performance PECs [12; 29; 30]. Figure 4: Conventional models for the polycrystalline anomalous photovoltaic effect [27]. Copyright 2015, Royal Society of Chemistry. (i) Suitable bandgap energy and band edge positions. The electrolysis of water is composed of two reactions occurring at different electrodes. There is a reduction reaction generating hydrogen with H\({}^{+}\)/H\({}_{2}\) redox potential of 0 V vs. NHE at the photocathode, whilst the O\({}_{2}\)/H\({}_{2}\)O redox potential of 1.23 V vs. NHE (pH = 0) in an oxidation reaction at the photoanode; thus, the bandgap of the materials needs to be larger than 1.23 V in order to be able to occur water split. In addition, the thermodynamic energy losses (0.3-0.4 V) and an overpotential (0.4-0.6 V) should also be considered. Consequently, a minimum bandgap of \(\sim\)1.8 eV is required. Furthermore, visible light is very limited below 390 nm, and this limits the upper value of the bandgap energy to 3.2 eV. Therefore, materials with a bandgap energy in the range between 1.9-3.2 eV are promising candidates for the generation of large photovoltages. When a photoelectrode material that meets these requirements with respect to the bandgap value absorbs incoming photons, electrons are excited to the conduction band (CB) and leave holes in the valence band (VB). The CB edge must be more negative than the H\({}^{+}\)/H\({}_{2}\) redox potential of 0 V versus normal hydrogen electrodes (NHEs) (pH = 0) to induce H\({}_{2}\) production, while the VB edge value must be more positive than the O\({}_{2}\)/H\({}_{2}\)O redox potential of 1.23 V versus NHE to generate water oxidation. (ii) Efficient charge separation and transfer. Low efficiency in PEC devices is mostly attributed to charge recombination and inefficient carrier transportation. As a result, many studies have been performed with the objective of addressing these challenges to achieve enhanced performance on PEC devices. Both the intrinsic properties (hole and electron mobility) and the extrinsic properties (crystallinity, nanostructure) affect the separation and transfer of the photogenerated carriers. (iii) Robust catalytic activity and stability. Generally, there are energy barriers for the transfer of electrons or holes, and these barriers exhibit energy losses in the reaction. Appropriately rapid surface reaction kinetics can overcome these barriers and suppress electron-hole recombination. In PEC systems, the hydrogen evolution reaction (HER) generates H\({}_{2}\) from H\({}^{+}\) (in acid) or H\({}_{2}\)O (in base), whereas oxygen evolution reaction produces O\({}_{2}\) from H\({}_{2}\)O (in acidic) and OH\({}^{-}\) (in base) [31; 32].