Structural, optical and ferromagnetic properties of Cr doped TiO2 nanoparticles

• Incorporation of Cr3+ increases the concentration of oxygen vacancies in TiO2 nanoparticles.
• Doped TiO2 nanoparticles contain absorption peaks corresponding to d–d transition of Cr3+ into TiO2.
• Pure and doped TiO2 nanoparticles contain emission peaks related to oxygen vacancies.
• Pure TiO2 shows diamagnetism while Cr doped TiO2 shows ferromagnetism.
• The ferromagnetism is due to the interaction of Cr3+ ions via oxygen vacancies.

Cr doped TiO2 nanoparticles are prepared with three different concentrations of chromium, 1.5%, 3.0% and 4.5 mol% respectively. Doping decreases the crystallinity and increases the width of the X-ray diffraction peak. The Raman active Eg peak of TiO2 nanoparticles become asymmetric and shifted to higher energy on doping of 4.5% chromium. Electron paramagnetic resonance spectra reveal the presence of Cr3+ in the host TiO2 matrix. The absorption spectra of Cr doped TiO2 nanoparticles contain absorption peaks corresponding to d–d transition of Cr3+ in octahedral coordination. Most of the visible emission peaks are due to the electrons trapped in the oxygen vacancy centers. Undoped TiO2 nanoparticles show diamagnetism at room temperature while all chromium doped samples show ferromagnetism. The magnetization of the doped samples increases at 1.5% and 3.0% and decreases at 4.5%. The ferromagnetism arises owing to the interaction of the neighboring Cr3+ ions via oxygen vacancies. The decrease of magnetization at the highest doping is possibly due to the antiferromagnetic interactions of Cr3+ pairs or due to Cr3+O2−Cr3+ superexchange interaction in the lattice.

Doping of Cr3+ distorts the lattice of TiO2, generate oxygen vacancies and create d-band states in the mid band gap of TiO2. Incorporation of Cr3+ also imparts magnetism in non-magnetic TiO2 by undergoing coupling with the neighboring oxygen vacancies.Figure optionsDownload as PowerPoint slide