UV-Sensitized Generation of Phasepure Cobalt-Doped Anatase: CoxTi1−xO2−δNanocrystals with Ferromagnetic Behavior Using Nano-TiO2/cis-[CoIII(en)2(MeNH2)Cl]2+

Abstract

Anatase nanoparticles, CoxTi1−xO2−δ (x = dopant density in at % and δ = concentration of oxygen vacancy in the lattice) were synthesized using UV (λ = 254 nm)-induced photoreduction of surface preadsorbed cis-[CoIII(en)2(MeNH2)Cl]2+ complex ion on nanoscale TiO2 particle surfaces in an aqueous solution. Photogenerated dopant, CoII, has successfully implanted in the semiconductor host lattice, preserving anatase crystal phase without the formation of impurity phase like rutile. Such materials termed as diluted magnetic semiconductors (DMSs), if realized without undesirable phases, are projected to have significant implications for the evolving fields of spintronics and advanced magneto-optics. The light dosage was systematically varied to prepare nanostructured semiconductor particles with varying dopant density in the anatase matrix leading to: x%,Co/nano-TiO2; 0.91 ≤ x ≤ 1.26 at % nonstoichiometric compound. The compound has been found to show altered optical, compositional, and room-temperature ferromagnetic properties with respect to the undoped TiO2 nanoparticles. X-ray diffraction measurements showed characteristic anatase reflections and supplied information about the absence of coexisting rutile impurity phase. The nanoparticles are ferromagnetic, having their hysteresis loops in the range −4000 Oe < H < +4000 Oe with the specific magnetizations of (14.40 to 0.196) × 10−3 μB/CoII at 300 K. However, the magnetic property depends critically on oxygen vacancy (V0O, V+O) and the Co content. A combined study of PL, XRF, SEM-EDX, HRTEM, XPS, and Raman techniques was used to investigate the size, dopant-distribution, composition, and surface structure. Cobalt dopant is predominantly substitutional, CoIIsub in anatase lattice and in +2 formal oxidation state. SEM and HRTEM images revealed obvious variations in the surface morphology of raw and Co implanted anatase. This strategy provides an alternative route to synthesize nanosized phase pure anatase x%,Co/nano-TiO2 showing room temperature ferromagnetism, which is expected to open up a general method for the synthesis of other transition-metal-doped nano-TiO2.