Clean World With Hydrogen: Electrochemical Photolysis of Water at a Semiconductor Electrode

Photoelectrochemical splitting of water is an environmentally friendly method of hydrogen generation based on renewable and apparently unlimited natural resources such as water and solar energy. Assisted by solar radiation, direct splitting of water into molecular hydrogen and oxygen was demonstrated for the first time in 1972 by Fujishima and Honda in a photoelectrochemical cell PEC with an n-type semiconductor TiO2 photoanode. (A. Fujishima and K. Honda, Nature 238 (1972), pp. 37–38. )
Since that time, many efforts have been undertaken to improve the conversion efficiency of the process but more than 30 years later this method is still far from commercialization. The reasons for this are fundamental and come as a consequence of a considerable mismatch between the spectra of light absorption in TiO2 and that of solar radiation. Many other semiconductors such as GaAs had been tried as a replacement for wide-band-gap TiO2 (3.0 eV rutile, 3.2 eV anatase) before it was realized that severe requirements imposed on the photoanode material could not be met simultaneously by any existing semiconductor. These requirements include:
(i) High stability and resistivity to corrosion and photocorrosion;
(ii) Low cost and availability;
(iii) Conduction band minimum, EC, above the H2O/H2 electrochemical level of water reduction
(iv) Valence band maximum EV below the O2/H2O electrochemical level of water oxidation
(v) Effective absorption of photons of the solar spectrum related to the band gap in the photon energy range of 1.6–1.9 eV.

As titanium dioxide fulfils all but the last one condition, it has been admitted that the best way to the improvement of the performance of the photoelectrochemical devices would be to modify the absorption spectrum of TiO2.
This can be achieved by shifting the fundamental absorption edge to longer wavelengths or by creating additional absorption features within the band gap. The methods tried up till now include:
1. Cation doping,
2. Sensitization with organic dyes
3. Composite materials
4. Anion doping with N, C or S.
However, whereas it is relatively easy to affect the absorption spectra of TiO2 by these methods, this is not in general true for the photocatalytic efficiency. The limiting factor is the recombination rate of the photoexcited electrons and holes.