# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 4 Ferroelectric Materials for Photoelectrochemical (PEC) Water-Splitting Devices
### Ferroelectric-Based Materials in Photocathodes for Hydrogen
#### 4.2.1 Ferroelectric Oxide Perovskites
The limitation of the performance of PEC cells used BFO as photocathodes could be overcome by an interlayer TiO\({}_{2}\) as protection layer to avoid direct contact with its electrolyte. This interpretation could show a promising pathway to reduce cathodic photocorrosion at the semiconductor/electrolyte interface by employing a variety of protection layers on the photocathodes. To utilize a wide range of the visible light spectrum, tandem PEC systems composed of both photoanodes and photocathodes have been investigated in many recent studies [59; 60]. Among many tandem architectures, the photocathodes based on the crystal Si-pn\({}^{+}\) were constructed with an improvement in PEC activity [61; 62]. However, the efficiency is far from the desired value because of the remaining photoexcited electrons inside the holes of Si. In a recent report, [PERSON] et al. [54] overcame his shortcoming to fabricate a Si-pnn\({}^{+}\)/ITO/Au/BFO hybrid photocathode with enhanced photocurrent and onset potential by utilizing the local surface plasmon resonance (LSPR) effect of Au nanoparticles and the depolarization electric field of BFO. **(2) LaFeO\({}_{3}\)**  
LaFeO\({}_{3}\) (LFO) is an n-type semiconducting perovskite oxide that has emerged as an attractive candidate for photoelectrochemical water splitting because of its many advantages, such as high stability under illumination in an aqueous environment, and abundant quantity of rare-earth element oxides [59; 60]. The bandgap of LFO is estimated to be from 2.0 eV to 2.6 eV, which is small enough to utilize a portion of the visible spectrum light [39]. Although LFO has stood out as a promising perovskite oxide, it still has some drawbacks that limit its use as an effective photocathode for hydrogen production [63; 64; 65; 66; 67]. The LFO photocathode in PEC shows low photoresponse characteristics because of the lack of a sophisticated preparative technique [68]. The p-type LFO film, which is prepared by more sophisticated fabrication techniques such as pulsed laser deposition (PLD) and atomic layer deposition (ALD) as a photocathode coupling with an n-type Fe\({}_{2}\)O\({}_{3}\) photoanode for stable water splitting, has assisted in the suppression of the aforementioned limitation [63; 69]. High-quality LFO films and good back contacts resulted in the generation of a photocurrent density at 0 V versus RHE of 64.5 \(\mu\)A/cm\({}^{2}\) under AM 1.5 G irradiation for  
Figure 6: Photocurrent potential (J–V) curves of different BiFeO\({}_{3}\) (BFO)-based photocathodes before (**a**) and after (**b**) positively poling the BFO film [53]. Copyright 2017, Royal Society of Chemistry. 100 nm of LFO, while the reaction time was maintained for 120 h without any obvious decline of the rate of oxygen and hydrogen production [63]. In addition, under external bias, the Fe\({}_{2}\)O\({}_{3}\)-LaFeO\({}_{3}\) system shows outstanding performance in both the development of the gas evolution rates and stability, even in alkaline conditions, in comparison with an Fe\({}_{2}\)O\({}_{3}\)-Pt system. Furthermore, the modified LFO photocathode with high performance, especially its long-term stability and low cost, could be considered one of the promising candidates to replace the normal Pt photocathode in PEC systems. Although the low photoresponse problem was solved by PLD instead of the sol-gel spin-coating method, the value of the photocurrent onset potential was ~1.0 V versus RHE, which is lower than the standard potential of 1.23 V for water electrolysis. Recent efforts to enhance the photoelectrochemical behavior of LFO has yielded many positive results. [PERSON] et al. [70] synthesized metal-doped LFO thin film electrodes using the sol-gel method, resulting in a significant improvement in efficiency, with an onset potential as high as 1.4 V versus RHE. The enhancement of the efficiency in PCE devices is attributed to an increase of both majority carrier density and mobility by doping with a relatively large amount of Mg\({}^{2+}\) and Zn\({}^{2+}\) (5% with respect to the iron atoms).