Semiconductors as sensitisers for water splitting

Due to its small relative molecular mass, the energy-storage capacity of H2 is approximately 120 kjg a figure that is almost three times that of the energy-storage capacity of oil. 

A semiconductor can act as a photosensitiser, which is characterised by its ability to absorb photons, and then, by utilising the photogenerated electrons and holes, cause the simultaneous oxidation and reduction of reactants. 

The photogenerated electrons and holes may recombine in the bulk of the semiconductor or on its surface within a very short time, releasing energy in the form of heat. However, electrons and holes that migrate to the surface of the semiconductor without recombination can respectively reduce and oxidise the reactants adsorbed by the semiconductor. Both surface-adsorption and photochemical-reaction rates are enhanced by use of nano-sized semiconductor particles, as a greatly enhanced surface area is made available. 

At the surface of the semiconductor, photogenerated electrons can reduce an electron acceptor, A  

From thermodynamic considerations, in order to reduce a species the potential of the CB of the semiconductor must be more negative than the reduction potential of the acceptor species. 

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