Influence of annealing process on ferromagnetism of undoped TiO2 nanoparticles prepared by sol–gel method

Undoped TiO2 nanoparticles are synthesized by the sol–gel method. X-ray diffraction (XRD) analysis reveals that the samples have the pure anatase TiO2 structure and the crystallinity becomes better with increase of the annealed temperature. Scanning electron microscopy (SEM) observation shows that the resulted particles are uniformly shaped like spheres. The vibrating sample magnetometer (VSM) measurement shows that the TiO2 nanoparticles exhibit weak ferromagnetism, and the saturation magnetization is increased with increasing the annealed temperature. X-ray photoelectron spectra (XPS) analyses confirm that the binding energy of Ti 2p peaks shifts to higher energy and the oxygen vacancies are formed in the as-prepared samples. Based on the above investigation, the origin of the ferromagnetism in the TiO2 nanoparticles is attributed to the 3d orbits of a small amount of tetravalent titanium cations which are occupied by electrons, then tetravalent titanium cations convert into trivalent or divalent titanium cations, indicating that the ferromagnetism of the samples is due to charge transfer interactions between Ti3+ and Ti2+ cations.