## 1 Introduction  
Hydrogen is an environmentally clean chemical fuel with high energy density. Developing a photocatalyst system to generate hydrogen from water gives us a chance to produce hydrogen from an inexhaustible renewable source, i.e., sunlight and water. Much effort has been devoted to studies of the splitting of water into hydrogen and oxygen in the years following the discovery of photocatalytic water splitting using a semiconductor photoelectrode, TiO\({}_{2}\), as was demonstrated by [PERSON] and [PERSON] in 1972 [1]. From the thermodynamic perspective, it requires 237 kJ to transform one mole of water into hydrogen which is an energetically unfavorable uphill reaction. For a semiconductor photocatalyst for photocatalytic water-splitting reaction, the conduction band edge of the photocatalyst should be more negative than the reduction potential of H\({}_{2}\)O to form H\({}_{2}\), and band edge of valence band should be more positive than the oxidation potential of H\({}_{2}\)O is required to form O\({}_{2}\). The band gap of the semiconductor photocatalyst must be larger than the theoretical dissociation energy of the water molecule (1.23 eV). Several studies have reported that transition metal oxides with d\({}^{0}\) and d\({}^{10}\) electron configurations have showed photocatalytic activity of water splitting under UV light such as NaTaO\({}_{3}\)[2], K\({}_{4}\)Nb\({}_{6}\)O\({}_{17}\)[3] and M\({}_{2}\)Sb\({}_{2}\)O\({}_{7}\) (M = Ca, Sr) [4]. Since 1998, there have been several excellent reviews in photocatalysis water splitting [5; 6; 7]. The sun is the most important light source of our world. At sea level, there is only about 8% of solar ultraviolet radiation (200-400 nm), about 50% is visible light, and about 40% is infrared radiation. Hence, numerous studies have attempted to develop highly efficient photocatalysts for water-splitting reaction which can be activated under visible light irradiation [8; 9; 10]. This improved photocatalytic activity can be achieved by introducing impurities such as transition metal [11] and non-metal elements [12] for a smaller band gap by lowering condition band edge or a rising valence band edge for use of solar light. However, a more negative conduction bend potential and a more positive valance band potential are thermodynamically favorable for the photocatalysis reduction and oxidation reactions. It is clear that satisfying these two conflicting conditions is a challenge for the development of visible light-responsive photocatalysts. In recent years, a semiconductor-based Z-scheme system for overall water splitting has drawn much attention [13; 14; 15]. In a Z-scheme system, photocatalytic H\({}_{2}\) evolution and O\({}_{2}\) evolution occurs on two different photocatalysts and a redox couple as electron mediator. As compared to a conventional photocatalytic system with one photocatalyst, a Z-scheme system combines two photocatalysts for water reduction and oxidation where the electronic structure of the photocatalysts only have to satisfy the thermodynamics requirements to the half reactions for reduction and oxidation and thereby expands the variety of potential candidates for overall photocatalytic water splitting under visible light irradiation. In particular, SrTiO\({}_{3}\) photocatalyst has high potential for photocatalytic applications due to its high photocatalytic activity for water-splitting reaction. However, due to the wide band gap of SrTiO\({}_{3}\) (3.2 eV), intensive research has been carried out on doping transition metal ions in SrTiO\({}_{3}\) to reduce band gap and improve photocatalytic activity under visible light irradiation [16; 17].