Cu co-doping effect on electronic structure and room temperature ferromagnetism of TiO₂:V nanoparticles

We have synthesized co-doped Ti_0.94−yV_0.06Cu_yO₂ (0 ≤ y ≤ 0.02) nanoparticles by sol–gel. Single rutile phase crystal structure has been confirmed from X-ray diffraction patterns. Decreasing trend of bandgap, resistivity and activation energy with increasing Cu co-dopant has been observed in co-doped samples. We found that the ferromagnetism of the V doped TiO₂ in the rutile phase is initially strongly suppressed for very low Cu concentration but with increasing Cu content the moment increases very significantly. The non-monotonic changes in the magnetization effects are attributed to the role of Cu in introducing oxygen vacancies, i.e. excess electrons due to its lower valence compared to Ti and V. The role of oxygen vacancies in the formation of magnetic polarons may be the reason of inducing and mediation of the ferromagnetic exchange between the transition metal ions. This interpretation is supported by the resistivity and optical measurements that both reveal the existence of low lying electronic states in the Cu doped systems that lead to lowering of resistivity and the lowering of the activation energy for electronic conduction. Hence it is apparent that variable valence of the dopants and their effects on the oxygen vacancies have an important role in determining the electronic properties of magnetically doped TiO₂ semiconductors.