Defect-related photoluminescence properties of as-synthesized and annealed NiO nanostructures via hydrothermal method

•Amorphous and nano-grained NiO nanowires were synthesized.•The structures have changed from amorphous to crystalline after annealing.•The different PL properties may have resulted from annealing effects.

A hydrothermal-based synthesis technique was used to produce both amorphous and nano-grained NiO nanowires. The scanning electron microscopy images showed the morphologies before and after annealing treatment at 350 and 400 °C for 2 h. The microstructural features of the mixed amorphous and local crystalline in as-synthesized NiO and polycrystalline in annealed NiO were characterized by X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy, respectively. The corresponding elemental compositions were determined via energy-dispersive X-ray. In addition, annealing-dependent effects on the photoluminescence spectra were observed in at least two different energy band regions, e.g., at ~ 395, 660, and 680 nm. The photoluminescence properties, including intensity and energy, of the individual NiO nanowires varied significantly with microstructural changes arising from nickel and oxygen defects from annealing the as-synthesized amorphous NiO. The microstructural and optical origin of as-synthesized amorphous and annealed nano-grained NiO nanowires is discussed on the basis of the aforementioned results.