The photoelectrochemical behavior of a sintered Nb dopednhyphen;NiTiO3electrode in relation to the photoassisted oxidation of water has been studied. Three bands mediate the different photoprocesses observed: A conduction band (Ti4+:3d) several tenths of one electron volt above the H+/H2redox level; the O2minus;:2thinsp;p6band about 3 eV below the conduction band; and the Ni2+:3d8lsquo;lsquo;bandrsquo;rsquo; above the oxygen band that seems just to overlap the O2/H2O level. Electronic transitions induced by UV illumination are mainly O2minus;rarr;Ti4+transitions; holes photogenerated in the oxygen band move through this band, by the effect of the internal electric field, to the electrode surface where they react with water molecules with an efficiency similar to that observed innhyphen;TiO2or SrTiO3. Visible light produces Ni2+rarr;Ti4+electronic transitions. Hole mobilities in the Ni2+:3d8lsquo;lsquo;bandrsquo;rsquo; appear to be adequate for the photogenerated holes to reach the s.c. surface, where they become trapped at Ni3+surface centers at an energy level too high to oxidize water efficiently. However, the geometry of the NiTiO3ilmenite structure makes the oscillator strength for the Ni2+rarr;Ti4+charge transfer too weak to have an acceptable photoresponse to visible light. Therefore NiTiO3is not a good candidate for water photoelectrolysis in a photoelectrochemical cell. This conclusion is probably valid for other mixedhyphen;metal oxides with ilmenite structure.
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