Microemulsion-mediated synthesis, characterization and optical properties of spherical nickel tungstate nanocrystals

Nickel tungstate (NiWO₄) nanocrystals with wolframite structure were prepared in the presence of microemulsion systems including a cationic cetyltrimethylammonium bromide (CTAB) or nonionic polyethyleneglycol (PEG) surfactant. The water pools formed in the microemulsion system acted as nanoscale reactors for the reaction of nickel and tungstate ions to produce nickel tungstate nanocrystals. X-ray diffraction analysis revealed the formation of monoclinic phase after the heat treatment of the as-prepared materials. Elemental compositions and morphological structures were studied by energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM). SEM analysis showed the spherical morphology of NiWO₄. The average crystallite sizes of NiWO₄ were about 38 and 26 nm in the presence of PEG and CTAB surfactants, respectively. The study of UV–Vis absorption spectrum showed the presence of direct band gap in the NiWO₄ nanomaterial, found to be 2.7 and 2.6 eV in the presence of PEG and CTAB surfactants, respectively. A reasonable crystal growth mechanism has also been proposed.