Synthesis of semiferromagnetic Ni-doped WO 3 nanoparticles by precipitation method: Evaluation of effect of treatment in hydrogen gas

Pure and Ni-doped WO 3 nanoparticles were synthesized by hydraulic acid-assisted precipitation, followed by thermal calcinations. The procedure utilized sodium tungstate (Na 2 WO 4 ) doped with different amounts of Ni ions. The samples were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and magnetic measurements. The XRD studies demonstrate formation of monoclinic (P21/n) crystal structure for pristine WO 3 at room temperature that gradually transformed into orthorhombic (Pbam) structure by increasing of Ni dopant content. The hydrogenation of the samples did not change their crystalline structure. The optical band gap of the samples was red shifted by Ni doping and blue shifted by hydrogenation. The hydrogenation created ferromagnetic (FM) properties even by small concentrations (∼1%) of Ni dopant. This behavior was discussed within the bound magnetic polarons (BMP) theory. © 2019