# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 2 Overview of Solar Water Splitting
### Basic Mechanism for Solar Energy to Hydrogen Conversion System  
The concept of photocatalytic water splitting (PWS) was introduced by hydrogen production via photoelectrochemical water splitting with a rutile TiO\({}_{2}\) (anode) as the photocatalyst, and a platinum  
Figure 1: The solutions for solar hydrogen via water splitting. (**a**) Particulate photocatalytic (PC) water splitting system, (**b**) Photoelectrochemical (PEC) water splitting system and (**c**) Photovoltaic-photoelectrochemical hybrid (PV–PEC) system [10]. Copyright 2017, Elsevier.  
(cathode). This was first reported by [PERSON] and [PERSON] in 1972 [13]. Hydrogen could be generated directly from water, and solar light could be generated through the solar energy conversion process.  
In PWS systems, photocatalysts play an important role in the conversion of solar energy to hydrogen in response to visible light in the water-splitting system. The hydrogen gas production process involves a photocatalytic reaction such as the direct conversion of solar energy into hydrogen gas in a semiconductor with an energy bandgap that is positioned between their energy band structures.  
A photocatalytic water-splitting reaction via semiconductor-based photocatalysts is conducted in terms of charge carrier generation, separation, transport, and transfer. This refers to the following three-step process [10; 11; 12; 13; 14].  
Step 1: Light (photons) is absorbed near the surface of the semiconducting materials with energies greater than their bandgap energy (e.g., \(\sim\)1.7 eV). Excited electrons (excitons) and holes are generated inside the semiconducting materials by bandgap excitation.  
Step 2: The electrons and holes henceforth referred to as photogenerated electrons and holes are separated in the bandgap and drift to the surface of the semiconducting materials, causing the release of oxygen from water (catalytic water oxidation reaction).  
Step 3: The photogenerated electrons combine with H\({}^{+}\) via the metal electrode (catalytic water reduction reaction).  
Figure 2 shows the principle of photocatalytic water-splitting reactions with photoelectrons and holes that are generated by the absorption of light. The photoelectrons and holes reduce and oxidize water on the surface of the semiconductors as the two half-reactions of water splitting via the following reactions [10; 11; 12].  
The water-splitting reaction is thermodynamically nonspontaneous, and therefore is an uphill reaction. As such, the net Gibbs free energy (\(\Delta\)G \(\approx\) 237 kJ/mol) needs to be increased. Namely, if a photocatalyst has a bandgap that is larger than the energy required for water splitting (\(\sim\)1.23 eV), then its conduction and valence band edge should contain the oxidation and reduction potential of water [8].  
It is clear that the O\({}_{2}\)/H\({}_{2}\)O redox potential difference is 1.23 V. Therefore, the requirement of a bandgap is larger than the required energy for water splitting (1.23 eV). For more details, the valence band potential must be more positive than the O\({}_{2}\)/H\({}_{2}\)O redox potential of 1.23 V versus Normal hydrogen electrode (NHE, pH = 0) to permit water oxidation, and the conduction band must be more negative than the H\({}^{+}\)/H\({}_{2}\) redox potential of 0 V versus NHE to facilitate water reduction [8; 9; 10].  
Figure 2: Principle of photocatalytic water-splitting (PC) reactions [7]. Copyright 2014, Elsevier.