Effect of Annealing Temperature on the Structural, Electronic and Magnetic Properties of Co doped TiO2 Nanoparticles: An Investigation by Synchrotron-Based Experimental Techniques

The present study examines the effect of annealing temperature on the structural, electronic and magnetic properties of Co-doped TiO2 (TCO) nanoparticles (NPs) synthesized by the sol-gel method. X-ray powder diffractograms of TCO NPs annealed at 400, 600 and 800 °C exhibited the monophasic anatase, diphase (anatase and rutile) and monophasic rutile structures respectively. A morphological investigation using high-resolution scanning electron microscopy (HR-SEM) reveals the presence of nanoparticles with diameters ranging from ∼11 to ∼51 nm. The widening and shifting of the most intense Eg peak in Raman studies and SEM images of TCO NPs suggest Co incorporation in the TiO2 matrix and the formation of oxygen vacancies. Magnetic measurements of TCO at room temperature demonstrate ferromagnetic behaviour which is caused by the existence of oxygen vacancies and the doped Co content. Element specific technique, soft X-ray magnetic circular dichroism (XMCD) was used to study the magnetism in Co doped TiO2 samples. The estimated total magnetic moment by XMCD was in the range of ∼1.3 ± 0.1–2.3 ± 0.2 μB/Co. This huge magnetization was first time observed by the surface-sensitive Total Electron Yield (TEY) mode. The estimated saturation magnetic moment is in the range of 1.0–1.5 μB/Co, also consistent with XMCD analysis and cluster model calculation. The results obtained by XMCD recommend an intrinsic behaviour of ferromagnetism. The X-ray absorption spectra (XAS) of Co-doped TiO2 nanoparticles at Ti L2,3 and Co L2,3 -edge in TEY mode concludes that the valence states of Co and Ti ions are in 2 + and 4 + respectively. Cluster model calculation also confirmed that Co ions are in a 2 + valence state with D2 h high spin crystal-field symmetry at the surface while a random crystal field symmetry in bulk has been observed which is well consistent with experimental data.