The oxygen vacancy induced ferromagnetism at and above room temperature in undoped TiO2 nanoporous nanoribbons synthesized by a solvothermal route was studied. The origin of ferromagnetism in as-synthesized and vacuum annealed undoped nanoribbons grown for different reaction durations followed by calcinations was investigated by several experimental tools. X-ray diffraction pattern and micro-Raman studies revealed the TiO2(B), TiO2(B)-anatase, and anatase-rutile mixed phases of TiO2 structure. Field emission scanning electron microscopy and transmission electron microscopy observations revealed nanoribbons with uniform pore distribution and nanopits/nanobricks formed on the surface. These samples exhibited strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which were dramatically enhanced after vacuum annealing. Direct evidence of oxygen vacancies and related Ti3+ in the as-prepared and vacuum annealed TiO2 samples were provided through X-ray photo-electron spectroscopy analysis. Micro-Raman, infrared absorption and optical absorption spectroscopic analyses further support these conclusion. The observed room-temperature ferromagnetism in undoped TiO2 nanoribbons was quantitatively analyzed and explained through a model involving bound magnetic polarons, which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d1) of Ti3+ ion. The analysis showed that the calculated bound magnetic polaron concentration scaled linearly with concentration of oxygen vacancies and provided a stronger footing for exploiting defect engineered ferromagnetism in undoped TiO2 nanostructures. The development of such highly porous TiO2 nanoribbons constituted an important step towards realizing improved visible light photocatalytic and photovoltaic applications of this novel material.

Evidence of Oxygen Vacancy Induced Room Temperature Ferromagnetism in Solvothermally Synthesized Undoped TiO2 Nanoribbons. Santara, B., Giri, P.K., Imakita, K., Fujii, M.: Nanoscale, 2013, 5[12], 5476-88