Tungsten trioxide (WO3) is a typical n-type inorganic semiconductor material have great application prospect in gas sensing, electrochromic device, photocatalysis, water splitting, and biosensing, because of its facile preparation, low cost, strong thermal stability, and good chemical stability.

 

WO3 has a band gap between 2.4 and 2.8 eV, making it highly attractive for photocatalysis research. promising semiconductor material for photoelectrochemical (PEC) water splitting. Doping with transition metal is an effective route to improve the electrochemical properties of WO3. It is reported that using gallium (Ga) as dopant to WO3 greatly enhances the WO3. Gallium-doped tungsten trioxide (Ga-doped WO3) has been synthesized as photoelectrodes for photoelectrochemical water splitting, the band gap decreased as the concentration of gallium increased. The synthesis process of Ga-doped WO3 is as below:

 

 

First, the tungstic acid was added into aqueous ammonia (28–30%) to form an ammonium tungstate solution. Excess ammonia can be eliminated by heating. Appropriate amounts of gallium(III) nitrate and PEG 300 were added into the ammonium tungstate solution to obtain the desired atomic ratio, expressed by W:Ga = x:1 − x, where x = 1, 0.95, 0.9, 0.85 or 0.8. Before coating, FTO glass was cleaned with acetone, ethanol and iso-propanol for 5 min in each solvent using an ultrasonic cleaner. Then, the gallium(III) nitrate–ammonium tungstate solution was drop-casted on the FTO glass. The resulting samples were calcined and sintered at 500 °C for 30 min at a constant heating rate of 5 °C s−1. The drop-casting and calcination steps were repeated two times to create three layered thin films. After annealing at 500 °C, the Ga-doped WO3 photoelectrochemical material is obtained.

 

 

In conclusion, gallium-doped tungsten trioxide has been synthesized as photoelectrodes for photoelectrochemical water splitting. The Ga-doped WO3 samples exhibited a greater grain than undoped WO3, and a monoclinic structure was observed for all WO3 samples. Furthermore, the 20% Ga-doped WO3 sample exhibited the best photoelectrochemical performance under the test conditions. Compare to the undoped WO3 (2.74 eV), 20% Ga-doped WO3 has a lower band gap as 2.60 eV. Compared to the undoped sample, all the Ga-doped WO3 samples showed a red-shift in light absorption. This indicates that doping of gallium into WO3 enhances the utilisation of the visible light in solar radiation. The result reveals that the doping of gallium contributes to the band gap reduction of WO3. The band gap decreased as the concentration of gallium increased. Gallium (III) nitrate and tungstic acid has been used as precursor materials.