A review on optical bandgap engineering in TiO2nanostructures via doping and intrinsic vacancy modulation towards visible light applications

Abstract The prospect of engineering the bandgap in semiconductor nanostructures all the way from ultraviolet to visible is highly significant in various applications such as photocatalysis, sensing, optoelectronics and biomedical applications. Since many semiconductors have their bandgaps in the UV region, various techniques are used to tune their bandgaps to the visible region. Doping and co-doping with metals and non-metals have been found to be highly effective in bandgap narrowing as doping creates a continuum of mid-bandgap states which effectively reduces the bandgap. Other than these techniques, the modulation of intrinsic vacancies is an effective way to control the bandgap. Among all semiconductors, titanium dioxide (TiO 2 ) is a well-studied material for UV photocatalytic applications. TiO 2 has oxygen and titanium vacancies as intrinsic defects which influence the bandgap based on its phase of existence. The oxygen vacancies generate unpaired electrons associated with Ti 3+ species, resulting in the creation of donor levels within the bandgap. Trivacancies give a p-type nature to TiO 2 due to excess holes and generate acceptor levels in the bandgap. The existence of a continuum of such intrinsic defect states within the bandgap appears to narrow the bandgap and enhances the visible light absorption in TiO 2 , although the effect is an apparent narrowing. Doping and co-doping of TiO 2 with metals such as Au, Ag, Fe, Co, Ni, Pt and Pd and non-metals such as B, C, N, Br and Cl, doping with Ti 3+ ions and hydrogenation have all been found to narrow the bandgap of TiO 2 . In this review, we focus on such intrinsic vacancy-modulated bandgap narrowing in TiO 2 . This review covers significant recent advancements in bandgap engineering of TiO 2 .