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TiOz (rutile) has been reduced in well-defined conditions to give n-type TiOzhx semiconductors in which both the nature and concentration of the defects attributed to reduction are known. The influence of the value of x in TiOz-x on the efficiency of such a material when used as a photoanode is very marked and, in particular, there exists a value of x for which this efficiency is a maximum. The influence of x on the electrochemical behaviour of Ti02-, is only noticeable for electrodes whose active surface has been previously mechanically polished. Finally the broadening of the absorption spectrum towards the visible as x increases can be attributed to the existence of deep acceptor levels which are excited by the light, giving free holes that can be involved in the OH-oxidation process.

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