An investigation of the thermal decomposition of nickel citrate as a precursor for Ni−NiO composite nanoparticles

Highlights: •Preparation of Ni−NiO composite via the thermal decomposition of nickel citrate. •Cauliflower-like Ni−NiO composite is formed at 400–700 °C. •Ni content in the composite decreases on increasing the calcination temperature. •The magnetization of the Ni−NiO composite increases on increasing the Ni content. -- Abstract: Composite Ni−NiO nanoparticles have been prepared via thermal decomposition of nickel citrate. Based on the thermal analysis results, the citrate precursor was pretreated for 1 h, in air, over the temperature range of 150–700 °C. The obtained solids were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was observed that the Ni−NiO composite starts to form as a result of calcining the precursor at 300 °C. Moreover, pretreatment of the precursor at 300–700 °C allows control over the Ni to NiO ratio. FE-SEM investigation of the synthesized powders showed that the obtained Ni−NiO composite consists of a porous network of particles with a cauliflower-like morphology. The TEM study revealed that the particle sizes of the composites obtained at 400 and 700 °C were in the ranges of 4–21 nm and 8–50 nm, respectively. The magnetic coercivity (H_c) of Ni−NiO composite is found to decrease from 139 to 73 Oe upon increasing the pretreatment temperature from 400 to 700 °C.