Most of the studies on solar hydrogen are concentrated on the development of photoelectrochemical cells that involve a semiconductor photoanode (titanium oxide) and a metal cathode inmersed in an electrolyte. The process results in oxygen evolution at the photoanode and hydrogen evolution at the cathode. The reaction at the photoanode is associated with substantial energy loss, mainly due to high overpotentials at the oxygen evolving anode. Some advantages of the titanium oxide semiconductor catalyst are that it is cheap, chemically and biologically inert, and that it is very stable under illumination for water photolysis.

For modelling of this process two different mechanisms are considered: a) charged titanium oxide clusters, this case is similar to the experimental investigations in which one proton is transferred from the chromophore to the titanium oxide surface; b) neutral titanium oxide clusters, as in the studies of titanium oxide in form of nanoparticles. The photocatalytic efficiency of titanium oxide for water splitting is limited due to the high recombination rate of photogenerated electron - hole pairs. To avoid this recombination of electron -hole pairs, the production of oxygen is considered to take place on titanium oxide, and the production of hydrogen is considered to take place for example on Pt. Hydrogen production by electrolysis of water is, associated with substantial energy losses. Most of the overpotential giving rise to the energy losses are related to the electrochemical processes at the anode, where oxygen evolution take place. We concentrate here in understanding how the reaction process of water splitting take place and the important chemical reaction steps for producing the hydorgen and oxygen. We show that the difficult step in the water splitting process is the formation of superoxy-type (OOH) species on the surface.

The results for the water splitting on the charged titanium oxide cluster have been compared with the ones on a neutral titanium oxide cluster. A significant improvement in lowering the energy barrier for the hydrogen and oxygen production have been obtained for the charged titanium oxide cluster system compared to the neutral one. A new model for the water splitting on neutral/charged titanium oxide clusters is proposed and its microscopic characteristics are obtained.