# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 4 Ferroelectric Materials for Photoelectrochemical (PEC) Water-Splitting Devices
### ZnS  
Zinc sulfide (ZnS) is regarded as one of the most important II-VI semiconductors, and is an appealing candidate for water splitting because of its own outstanding properties, which include an ability to generate mobile photoexcited charge carriers [174], fast electron transfer because of high CB potential, and high catalytic activity for H\({}_{2}\) generation under conditions without a co-catalyst [175; 176]. However, the performance of pure ZnS-based photoelectrode in water-splitting devices is limited, because absorption is only in the UV region due to the wide bandgap value of 3.66 eV of this material [177]. Furthermore, overcoming the high charge recombination is also a serious challenge [178; 179]. As a result, many modifications have been investigated for ZnS-based electrodes to increase visible light absorption. To enhance photocatalytic activity, transition metal-doped ZnS (Cu, Ni, Mo) or doping with GaN were investigated as potentially efficient photocatalysts to generate H\({}_{2}\) under visible light irradiation [180; 181; 182]. However, due to the remaining challenges of dopant introduction [181; 183; 184; 185; 186] an alternative method by controlling defects in nanostructured ZnS has been exploited through PLD to enhance the overall PEC properties of ZnS [187]. The fabrication of ZnS-based heterojunctions with other semiconductors for more advanced designs has been considered to enhance PEC performance for water splitting. Typically, the ZnO/ZnS heterostructure has drawn tremendous attention because of the extended visible absorption that it affords and its appropriate alignment [188; 189; 190; 191; 192]. Among ZnO/ZnS heterostructures with different morphologies such as nanowires [193] and nanorings [194] synthesized by means of chemical processes, the PEC behavior is scarcely introduced by anodization, which facilitates electron-hole separation, and consequently, the enhancement of photoelectrochemical activity for water splitting [195]. Moreover, the design of sandwich-structured ZnO/ZnS photoanodes with a third component, such as noble metals Au or graphite-like carbon nitride (C\({}_{3}\)N\({}_{4}\)), have been demonstrated with significantly superior PEC activity compared to those of a pristine ZnO photoanode [196; 197]. Nevertheless, the remaining challenge is the wide bandgap of the ZnO/ZnS heterojunction, which still limits large-range visible light absorption. The systems that result from coupling ZnS with narrow bandgap chalcogenide CdS exhibit much higher performance than those of every single material-based system [198; 199; 200; 201]. 3.3 Hybrid Halide Ferroelectric CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) Perovskite Tandem System Approach  
Tandem cell configuration is a facile and novel approach to address the limitation of single or heterojunction PEC devices for water splitting. Organic-inorganic hybrid perovskite CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) has attracted tremendous attention in perovskite solar cells (PSCs) because of their superb light-harvesting characteristics, large electron/hole diffusion lengths, and high crystallinity [203; 204; 205; 206; 207]. Based on the latest research, the certified highest efficiency of PSCs is 22.1% fabricated on limited to very small areas (\(\sim\)1 cm\({}^{2}\)) [208]. Ferroelectric domains have been observed in CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\), despite the centrosymmetric structure of this perovskite material, which is ascribed to the reduced symmetry of molecular CH\({}_{3}\)NH\({}_{3}\)\({}^{+}\) dipoles [209; 210]. Recently, [PERSON] et al. [211] demonstrated two perovskite solar cells connected in series as a tandem cell that can serve as a separated and external power supply for water splitting, as presented in Figure 11. Although this device can achieve a high photocurrent density of \(\sim\)10 mA/cm\({}^{2}\), the electrodes were not directly settled in the photoreactions.