Water splitting reaction can be carried out need to meet:
1. The band gap must be appropriate, not only generate enough voltage to decompose water, but also not too much which will cause excite electrons difficult to across the band gap, and thus greatly reduces the absorption of solar energy, the ideal band gap is 1.6~ 2.2eV;
2. With good oxygen evolution catalytic property, must be stable in the electrolyte, and cost low.
The material which can meet all the above requirements is not yet found, but some of potential material system has found, such as WO 3 , Fe 2 O 3 , WS 2 , etc., while WO 3 is one of the most potential. The property of tungsten trioxide can basically meet the main requirements of PEC photocatalytic absorbing layer, with excellent oxygen catalytic property, stability in the electrolyte, and relatively inexpensive, the band gap of 2.4 ~ 2.8eV which is slightly larger than the ideal band gap. For this reason, people have done a lot of study to narrower the band gap, in order to improve sunlight absorption and photocurrent density to meet the requirements of photoelectrochemical water splitting. Studies have shown that doping can adjust the semiconductor band gap.
Photolysis of water hydrogen and oxygen evolution is the co-exist half-reaction, with the rapid development of semiconductor photocatalysis material, tungsten trioxide as water photolysis catalytic material attracted a lot focus. Gratian and their team preliminary studied the oxygen evolution situation of WO3 induced in visible light. Gao and their team research how the sintering atmosphere affect on WO 3 water photolysis catalytic activity and found that the stability of WO 3 photocatalytic activity can be significantly improved by chemical-bit control. Sayama ect. reported the redox system of Fe 3 + / Fe 2 + ions and WO 3 , in which under the existing of Fe 3 + , WO 3 can decompose water to produce oxygen in the visible light, while at the same time Fe 3 + continue to be consumed, Fe 2+ ions are easily oxidized to Fe 3 + ions under UV light to produce hydrogen, and achieve circulatory system.
In addition, China prepared WO 3 by sol - gel method and to get a series of different photocatalytic oxygen vacancies and a crystalline state by at different temperatures to WO 5 + precursor is heat-treated, and then analyze the photolysis of water oxygen evolution activity. They concluded that tungsten trioxide catalyst has the highest photocatalytic activity of oxygen evolution under 350°C for 4 hours treatment.
Experimental study of photocatalytic decomposition of water of Ce / WO 3 photocatalyst showed that photocatalytic water splitting under visible light irradiation oxygen at 600°C, the 0.05% of Ce doped of WO 3 catalyst has the highest catalytic activity, which the oxygen evolution rate increase by 1.5 to 1.7 times compared to the undoped WO 3.