Impact of various dopants (X = Zn, Mg, and Bi) on the structural, optical, and adsorption properties of (CoNiX)O nanostructures.

This study aimed to enhance the adsorption efficiency of spinel CoO against methylene blue dye removal, a significant environmental treatment. Hence, metal elements (X = Zn, Mg, and Bi) doped (CoNi X )O nanoparticles (NPs) were synthesized by the co-precipitation method, with a crystallite size range between 17 and 23 nm. The X-ray diffraction (XRD) analysis with the Rietveld refinement confirmed the spinel single-phase for Zn and Mg-doped (CoNi)O NPs without any secondary phases. However, the Bi-doped (CoNi)O NPs exhibited a secondary BiOCl phase, indicating the lack of Bi ions incorporation into the (CoNi)O lattice. Accordingly, the Fourier transform infrared spectroscopy (FTIR) confirmed the BiOCl secondary phase, and X-ray photoelectron spectroscopy (XPS) analysis verified the formation of the spinel structure in all samples. Morphologically, the scanning electron microscope (SEM), energy-dispersive X-ray (EDX), and transmission electron microscope (TEM) of doped samples revealed the presence of agglomerated particles with spherical and hexagonal nanoparticles. Subsequent investigations with high-resolution resolution-(HRTEM) and selected-area electron diffraction (SAED) demonstrated that high crystalline spinel structures. The Raman spectra exhibited vibrational modes related to the (CoNi)O cubic structure. The optical band gap increased with Mg-doping, and decreased with Bi-doping as compared to the Zn-doped sample. The PL intensity of Zn-doped (CoNi)O was lower than Mg and Bi samples, indicating the slower recombination rate of photogenerated charge carriers in the Zn-doped sample. Eventually, the highest adsorption capacity of 94.4 mg.g, was reached by the Zn-doped (CoNi)O NPs. Afterward, the adsorption behavior was studied by changing the contact time, initial dye concentration, and pH. The adsorption of methylene blue onto the synthesized adsorbents was best described by the Freundlich isotherm model. These findings highlight the promising performance of the prepared NPs, supporting their potential application as effective adsorbents for water treatment.