# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 4 Ferroelectric Materials for Photoelectrochemical (PEC) Water-Splitting Devices
### (3) SrTiO\({}_{3}\)  
Titanium dioxide (TiO\({}_{2}\)) was the first oxide used as a photoanode material for PEC water splitting and was discovered by [PERSON] [13]. Until now, it has been one of the most attractive materials for PEC cells. However, TiO\({}_{2}\) absorbs only in the UV part of the spectrum because of its large bandgap [132], which exhibits low efficiency in water-splitting reactions when TiO\({}_{2}\) is used as a single photoanode [133; 134; 135; 136].  
SrTiO\({}_{3}\) (STO) is a perovskite oxide that shows considerable ferroelectricity [137; 138] and possesses a favorable band edge structure that overlaps with the water redox potentials. In addition, the conduction band edge is more negative than TiO\({}_{2}\), as presented in Figure 9[139]. Similar to TiO\({}_{2}\), STO responds only to the UV region and a negligible portion of the visible light. This has prevented it from being a good single photoanode candidate for PEC cells. As a result, STO has been doped with dopants, or coupled with TiO\({}_{2}\), which the aim of shifting the Fermi level of the dual-phase STO/TiO\({}_{2}\) composite [87], for application as a potential candidate for improving photoconversion efficiency.  
Limited reports have focused on TO thin films with dopants such as (Cr, Rh, Ir, and Nb) [140; 141; 142; 143; 144]. However, a few of these studies have reported some drawbacks such as non-uniform doping and the insignificant effect of such dopants, or even the resulting instability because of the formation of IrO\({}_{2}\)[145]. Recently, further investigation into STO/TiO\({}_{2}\) photoanode materials has revealed some desirable outcomes [145; 146; 147; 148; 87; 139]. [PERSON] et al. [136] studied a durable eutectic system made up of TiO\({}_{2}\) and STO as an active photoanode material for PEC cells. Under 600 mW/cm\({}^{2}\) of solar irradiation, the TiO\({}_{2}\)/STO eutectic photoelectrode yielded a photocurrent density of up to 8.5 mA/cm\({}^{2}\) at 1.5 V versus NHE and stabilized after 30 h of testing. This result not only shows an improvement in the photocurrent density, it also showed a better long-term operation compared with Ir-doped STO (24 h) [145]. Thus, eutectic composite-based photoelectrodes are untapped components for PEC water splitting. In addition, the spontaneous electric polarization in STO can simultaneously enhance charge separation and hole transportation in TiO\({}_{2}\)/STO core-shell nanowires (NWs) [146]. Compared to the TiO\({}_{2}\)/BaTiO\({}_{3}\) ferroelectric PEC system [138], STO ferroelectric material shows better charge separation and hole transport.  
mobility, which will facilitate hole migration inside the ferroelectric layer. Interestingly, the core shell with an optimal STO thickness of 10 nm generates the highest photocurrent density of 1.43 mA/cm\({}^{2}\) and has the charge-separation efficiency of 87.7% at 1.23 V versus RHE. This corresponds to an 83% and 79% improvement in comparison with pristine TiO\({}_{2}\) NWs. This study has paved the way for the application of semiconducting ferroelectric materials to further advance the development of ferroelectric PEC systems.