The effect of Nb substitution on synthesis and photo-response of TiO2 thin films prepared by direct current magnetron sputtering

TiO2 has a limited photoefficiency for solar energy harvesting and conversion due to its large bandgap. Nb substitution into the TiO2 lattice was investigated to red-shift its photoresponse by introducing defect energy levels in the host bandgap. Ti1−xNbyO2 thin films (0 ≥ y ≤ 0.52) were prepared by direct current reactive magnetron sputtering. The effect of Nb concentration on crystal phase formation and subsequent change in optical and electronic properties were determined. Films had an increasing rutile proportion as Nb concentration increased, and all films prepared for y > 0.25 were pure rutile. Nb substituted as Nb(V) or a mixture of Nb(IV)/(V) while Ti remained Ti(IV). Therefore, in these materials cation vacancies formed to electronically compensate for group V doping. Nb-doped rutile films showed no red-shift in optical response while Nb-doped anatase/rutile composites were strongly red-shifted. Photocurrent measurements were made to determine if the enhanced visible light absorption generated mobile electron-hole pairs or was due to localized excitations at color centers. At low Nb concentrations (~ 2 at.%), visible light absorption generated mobile charge carriers, while at higher concentrations ( > 9 at.%), no mobile charge carriers were created. At higher Nb concentrations, the formation of mobile charge carriers was hindered by charge recombination at cation vacancies.