H2O Splitting and the Production of

The reaction mechanism, in terms of the chemistry of the water-splitting and catalyst redox reactions, may alternatively be thought of as  

For obvious reasons, this reaction led to an enormous amount of research into the use of solar radiation in the photosynthesis of hydrogen. However, the process suffers from a number of drawbacks that limit its usefulness, as discussed below. 

The most serious problem however relates to the relatively large bandgap of titanium dioxide. Titanium dioxide (anatase) has a bandgap energy of 

Doping titanium dioxide with metal ions that have been substitutionally positioned in the place of titanium within the lattice is one approach and has been carried out extensively using metals such as iron, molybdenum and copper. With subsurface doping, movement of electron and hole pairs to the surface (where they can react with substrates) is difficult and the metal ions can also act as recombination centres. An optimum doping concentration, above which photocatalytic activity decreases due to promotion of such electron-hole recombination, exists. 

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