New approach for synthesis of Co2+ doped TiO2 nanoparticles showing room temperature ferromagnetic behavior, through shape transformation of hydrothermally treated scrolled titania nanotubes in the presence of Co2+ ions is described. The XRD and ICP measurements demonstrated successful incorporation of 0.46 at. % Co2+ ions in preserved anatase crystal structure of TiO2 nanoparticles, without presence of Co-oxide clustering, metallic Co, or various Co-Ti oxide species. HRTEM measurements revealed that majority of the nanoparticles have polygonal shapes with average dimension of ∼ 6−10 nm. Obtained value of 3.06 eV for band gap energy of Co2+ doped TiO2 nanoparticles explained altered optical properties of TiO2 matrix and indicates narrowing of the electronic properties in respect to the undoped anatase TiO2 nanomaterials. The Co2+ doped TiO2 nanocrystals enabled synthesis of optically transparent film that shows room temperature ferromagnetic ordering with a saturation magnetic moment of 0.25 μB per Co atom. The proposed explanation for room temperature ferromagnetic behavior is based on the presence of critical amount of oxygen vacancies that mediate interaction between Co2+ spins trapped in the lattice structure of titania nanoparticles with undercoordinated surface defect sites.

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