Thermodynamic Stability of Semiconductors for Photocatalytic Water Splitting

Band structure engineering design of the light-absorbing semiconductors for water splitting has attracted wide attention recently. One of such design is to use the Z-scheme where a photocathode is connected with photoanode to reduce (generate H2) and oxidize (generate O2) water respectively. This requires the conduction band of photocathode and valence band of photoanode to straddle the redox levels of water. However, equally important in this design is the thermodynamic stability of the semiconductors in the aqueous solution upon illumination, i.e., the semiconductors may be oxidized (or reduced) before the water is oxidized (or reduced), causing the corrosion of the photoanode (photocathode). We will present our theoretical study on the thermodynamic stability of a series of photocatalytic semiconductors, including metal oxides, sulfides and nitrides, through the combination of phenomenological models for the semiconductor corrosion and the first-principles total energy and band alignment calculations. We find that almost all sulfides and nitrides are unstable as photoanode, while most of oxides are stable. This limits the choice of the photoanode materials for oxygen evolution. In contrast, for photocathode, most of the considered semiconductors are stable and resistant to reduction, indicating a much wider choice of the photocathode materials for hydrogen evolution.