Structural, optical and magnetic properties of pure and rare earth-doped NiO nanoparticles

Pure and a series of rare earth element (RE) (Er, Sm, Gd, Pr and Y)-doped NiO nanoparticles (NPs) were fabricated by co-precipitation method in the presence of polyvinyl alcohol as a capping agent. X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques were used to investigate the crystal structure and morphology of the prepared samples, respectively. The XRD patterns indicated that all the samples exhibited a single phase of face-centered cubic crystalline structure. TEM images displayed a spherical morphology of weakly agglomerated NPs. Furthermore, the chemical compositions were identified by proton-induced X-ray emission and Rutherford backscattering analysis. The metal oxide (Ni–O) functional group was confirmed by Fourier transform infrared spectroscopy with its observed band varying from 419.3 to 433.3 cm−1, depending on the RE doping element. The optical properties of the RE-doped NiO NPs were studied using UV–Vis absorption and photoluminescence (PL) spectroscopy. A small blueshift was obtained in the optical spectra of the RE-doped samples as compared to the undoped samples, implying an increase in the optical band gap. The study of the room temperature magnetic properties was done using the vibrating sample magnetometer, which revealed the coexistence of antiferromagnetic and weak ferromagnetic ordering in pure and RE3+-doped NiO NPs. The magnetization increased depending on the magnetic moments of RE dopant ions. Also, the origin of the anomalous ferromagnetism in the prepared samples may be mainly related to Ni vacancy defects, which were evinced from the results of PL.