Surface Magnetism of Cobalt-Doped Anatase TiO2 Nanopowders

Cobalt-doped anatase Ti₁–ₓCoₓO₂ (0 < x ≤ 0.04) nanopowders (with a particle size of 30–40 nm) were produced by the hydrothermal synthesis method. Morphology, structure, and thermal stability of the synthesized compounds were examined using transmission electron microscopy, infrared spectroscopy, and X-ray diffraction analysis. Using X-ray photoelectron spectroscopy, cobalt ions are shown to have an oxidation state of 2+, with titanium ions having a tetravalent state of Ti⁴⁺. In the as-prepared state, all investigated compounds of Ti₁–ₓCoₓO₂ are paramagnetic, with the value of paramagnetic susceptibility growing in proportion to cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals that doping TiO₂ with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F⁺ centers. Further growth of the cobalt content results in a relatively wide line (nearly 600 Oe) in the spectrum, with a g-factor of about 2.005, demonstrating exchange-coupled regions being formed, the fraction of which increases with cobalt content, while the intensity of F⁺-center signals is reduced appreciably. Annealing of Ti₀.₉₆Co₀.₀₄O₂ in vacuum at 1000 K is shown to have resulted in the substantial localization of cobalt atoms in the subsurface layers, resulting in an approximately 3-fold increase in the Co atoms content on the surface of nanoparticles as compared with that in the bulk. This is shown to be accompanied by appearance of spontaneous magnetization at room temperature, the value of which depends on the cobalt content in TiO₂ nanopowders. The value of magnetic moment per Co atom decreases monotonically down to a value of ≃1 μB with cobalt content increasing. A core–shell model proposed to be the most adequate for describing the magnetic properties of TiO₂:Co after the reducing annealing. A hypothesis is put forward suggesting that the defect surface enriched with Co atoms and vacancies is described with itinerant type magnetism, allowing for the delocalized nature of electrons near vacancies.