Role of Ni2+ substituent on the structural,optical and magnetic properties of chromium oxide (Cr2-xNixO3) nanoparticles

Pristine chromium oxide (Cr₂O₃) and nickel ions (Ni²⁺) substituted Cr₂O₃ nanoparticles were synthesized using a simple co-precipitation technique. The main objective of this work is to investigate Ni²⁺ substituent's role at different concentrations on the structural, morphological, optical, and magnetic properties of Cr₂O₃ nanoparticles. Structural analyses based on X-ray diffraction (XRD), Raman and Fourier transform infra-red (FTIR) data confirmed the successful incorporation of Ni²⁺ into Cr₂O₃ nanoparticles up to x = 0.05 of Ni²⁺ content, without affecting the rhombohedral crystal structure of Cr₂O₃ nanoparticles. Rietveld refinement results showed the variation in lattice parameters and cell volumes alongwith the substitution of Ni²⁺ into Cr₂O₃ nanoparticles. Raman and FTIR spectra also depicted a considerable shift in the characteristic vibration modes of Cr₂O₃ nanoparticles due to strain-induced by Ni²⁺ substitution. Beyond x = 0.05, the structural transformation took place from rhombohedral to cubic crystal structure. Subsequently, new peaks (apart from Cr₂O₃ phase modes) have been observed at x = 0.1 of Ni²⁺ content due to the formation of secondary phase i.e., nickel chromate (NiCr₂O₄). Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) illustrated the changes in the morphology of Cr₂O₃ nanoparticles with Ni²⁺ substitution. UV–Vis analysis revealed a narrowing of optical band energy (E_g) of Ni²⁺ substituted Cr₂O₃ nanoparticles from 3 to 1.85 eV as Ni²⁺ content varies from x = 0 to 0.2, respectively. Afterward, there is an increase in optical band gap energy (E_g) when Ni²⁺ content increased from x = 0.3 to 0.5, as NiCr₂O₄ started dominating the Cr₂O₃ phase. Single-phase Ni²⁺ substituted Cr₂O₃ nanoparticles exhibited a superparamagnetic behavior, whereas the multi-phase compound ascribed to both superparamagnetic and paramagnetic. These changes in optical and magnetic properties can lead to novel strategies to render applications in the field of optoelectronics and optomagnetic devices.