Evidence of oxygen vacancy induced room temperature ferromagnetism in solvothermally synthesized undoped TiO2nanoribbons

Abstract

We report on the oxygen vacancy induced ferromagnetism (FM) at and above room temperature in undoped TiO₂ nanoporous nanoribbons synthesized by a solvothermal route. The origin of FM 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 reveal the TiO₂(B), TiO₂(B)-anatase, and anatase–rutile mixed phases of TiO₂ structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal nanoribbons with uniform pore distribution and nanopits/nanobricks formed on the surface. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which dramatically enhanced after vacuum annealing. Direct evidence of oxygen vacancies and related Ti³⁺ in the as-prepared and vacuum annealed TiO₂ samples are provided through X-ray photoelectron spectroscopy analysis. Micro-Raman, infrared absorption and optical absorption spectroscopic analyses further support our conclusion. The observed room temperature FM in undoped TiO₂ nanoribbons is quantitatively analyzed and explained through a model involving bound magnetic polarons (BMP), 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. Our analysis interestingly shows that the calculated BMP concentration scales linearly with concentration of oxygen vacancies and provides a stronger footing for exploiting defect engineered ferromagnetism in undoped TiO₂ nanostructures. The development of such highly porous TiO₂ nanoribbons constitutes an important step towards realizing improved visible light photocatalytic and photovoltaic applications of this novel material.