Water Splitting Semiconductor Photoanodes - a Comparative Study

The long term stability is the critical issue that conditions application of semiconductor materials in photo-electrochemical water splitting devices. This is the reason why the choice of the semiconductors intended to operate as water oxidation photo-anodes is actually restricted to a few metal oxides. However, none among those materials, able to absorb a significant portion of solar light, exhibits band-edge energy levels that match those of hydrogen and oxygen evolution reactions. Neither tungsten trioxide, nor bismuth vanadate and still less hematite ferric oxide have the positions of the conduction band edges more negative than the H₂ evolution potential, a condition necessary to perform unassisted water splitting. Consequently, continuing efforts are devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water. These efforts include, among others, development of new electrocatalysts and incorporation into photo-anodes of plasmonic metal nanostructures that allow incoupling of light into the semiconductor films, by scattering from plasmonic nanostructures and/or resonant coupling of the plasmonic electromagnetic near field to the semiconductor. Recent advances regarding the most investigated photoanode materials: WO₃ (E_g = 2.5 eV); BiVO₄ (E_g = 2.4 eV) and hematite Fe₂O₃ (E_g = 2.2 eV) will be discussed.