Presence of Room Temperature Ferromagnetism in Co²⁺ Doped TiO₂ Nanoparticles Synthesized through Shape Transformation

Abstract

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.

Synthesis of visible light-responsive cobalt-doped TiO2 nanoparticles with tunable optical band gap

“…In the literature, such band narrowing was also observed by several other groups [28,[41][42][43]. In most of the cases, when dopant concentration is too high, the band gap either remains unaltered or increases with dopant concentration [44][45][46]. This is either due to the formation of …”

Room-temperature ferromagnetism in Ni2+ doped TiO2 nanocrystals synthesized from nanotubular precursors

“…As we showed in our previous work the existence of undercoordinated surface defect sites that terminate layers of titania nanotubes enables initial adsorption of dopant ions [20]. Also, it should be mentioned that the bigger portion of these defect sites are located in the interior/interface between the layers that constituted the scrolled nanotubes [21].…”

“…Also, it is well known that ferromagnetism in 3d-metal doped metal oxide nanocrystals can be activated by the fusion of defects from the interior of nanocrystals to the interface and by the dopant ions randomization [20,44]. On the other hand, the appearance of positively charged oxygen vacancies in doped TiO 2 nanocrystals, as was mentioned earlier, also induces more charge carriers which have the role of mediator of exchange interaction between magnetic dopant atoms inducing overall enhancement of magnetic ordering [45].…”

“…Also, it should be mentioned that the bigger portion of these defect sites are located in the interior/interface between the layers that constituted the scrolled nanotubes [21]. It allows synthesis of Ni 2+ doped TiO 2 faceted nanocrystals through encapsulation of nanotube's surface-bound Ni 2+ ions with extra layers of TiO 2 during shape transformation [20,21].…”

“…The magnetization process in diluted magnetic semiconductor nanocrystals greatly depends on the 3d-metal concentration, the particle size, their aggregation density and of interfacial or grainboundary defects [20,42,43]. Also, it is well known that ferromagnetism in 3d-metal doped metal oxide nanocrystals can be activated by the fusion of defects from the interior of nanocrystals to the interface and by the dopant ions randomization [20,44].…”