Observation of Optical Band-Gap Narrowing and Enhanced Magnetic Moment in Co-Doped Sol–Gel-Derived Anatase TiO₂ Nanocrystals

The magnetic behavior of TiO₂ and doped TiO₂ nanocrystals has been a challenge due to the unambiguous nature of defects present in oxide semiconductors. Here, a simple, low-temperature sol–gel method is developed for the synthesis of low-dimensional and highly efficient stable anatase TiO₂ nanocrystals. The X-ray powder diffraction pattern and Raman spectra confirm the formation of a single-phase anatase structure of TiO₂. High-resolution transmission electron microscopy studies reveal the crystalline nature of the sol–gel-derived nanocrystals. The increase in lattice parameters together with the shifting and broadening of the most intense E_g(1) mode in micro-Raman spectra of Co-doped TiO₂ nanocrystals indicate the incorporation of Co in TiO₂. Shifting of the absorption edge to the visible region in UV–visible spectra indicates narrowing of the band gap due to Co incorporation in TiO₂. X-ray photoelectron spectra confirm the presence of Co²⁺ and Co³⁺ in Co-doped TiO₂ samples. Oxygen vacancy defects lead to the formation of bound magnetic polarons which induces a weak ferromagnetic behavior in air-annealed 3% Co-doped TiO₂ at room temperature. It is observed that irrespective of the dopant ion, whether magnetic or nonmagnetic, the overlapping of bound magnetic polarons alone can induce ferromagnetism, while the magnetic impurities give rise to an enhanced paramagnetic moment for higher Co concentrations. A detailed understanding on the variation of these magnetic properties by estimating the concentration of bound magnetic polarons is presented, which is in corroboration with the photoluminescence studies. The observed band-gap narrowing in Co-doped TiO₂ nanostructures and the mechanism underlying the magnetic interactions associated with the magnetic impurity concentration are advantageous from an applied perspective, especially in the field of spintronic and magneto-optic devices.