A comparative study of magnetic, photocatalytic and dielectric properties of NiO nanoparticles synthesized by sol-gel and composite hydroxide mediated method

In this study, Nickel oxide (NiO) nanoparticles were synthesized by conventional sol-gel (SG) route using citric acid as an end-capping agent and composite-hydroxide-mediated (CHM) approach. XRD and FTIR confirmed the formation of NiO nanoparticles and the average crystallite size was found to be 68 and 34 nm for SG and CHM method, respectively. Raman spectroscopy revealed higher intensity of one-phonon longitudinal optical (1LO) mode in CHM nanoparticles which is attributed to the presence of defects such as Ni or oxygen vacancies. The two-magnon (2 M) band at 1472 cm(-1 )confirmed antiferromagnetic (AFM) nature of SG nanoparticles whereas its suppression in CHM nanoparticles indicate superparamagnetic (SPM) nature of the nanoparticles due to decrease in average crystallite size. The magnetic measurements also confirmed the AFM and SPM state of SG and CHM nanoparticles, respectively in accordance with Raman spectroscopy. A sharp and low blocking temperature peak in ZFC/FC also indicates smaller and uniform sized NiO nanoparticles synthesized by CHM method. Optical studies revealed a large value of energy band gap for CHM nanoparticles due to their small average crystallite size and is attributed to quantum confinement effect. A higher photocatalytic performance was observed for CHM nanoparticles due to their large surface area and higher concentration of oxygen vacancies in accordance with Raman spectra. A higher dielectric constant was also observed for CHM nanoparticles due to increased number of grains per unit volume in smaller crystallites. In summary, CHM is one of the most simple synthesis approach that provides fine single-phase NiO nanoparticles at low calcined temperature with improved magnetic, photocatalytic and dielectric properties as compared to SG method.