Influence of particle size on the electrocatalytic activity and optical properties of NiO nanoparticles

Abstract

The study of nanoparticles becomes increasingly fascinating since their size can give them unusual behaviors, changing their properties. Due to their outstanding properties, nickel oxide nanoparticles have been applied in several research fields, such as catalysis and optical electronics. In this work, we have investigated the influence of particle size on the optical and electrocatalytic properties of NiO obtained through coprecipitation synthesis (Cp). Thermogravimetric analysis (TG/DTG) measurements reveal that the total decomposition of the precursors occurs at ≈ 545 °C. The X-ray diffraction measurements (XRD) show that it is possible for the formation of only one phase of NiO at 350 °C. Rietveld refinement indicates an increase in crystallite size with the rise of temperature and negative strain values associated with shrinkage of the crystalline lattice. Vibrational spectroscopy (infrared and Raman) data showed a symmetry break and imperfections/defects in the synthesized NiO nanoparticles. The optical properties of these samples were studied by spectrophotometry in the ultraviolet–visible and near-infrared region measurements (UV–vis-NIR). Band gap values decreased with increasing nanoparticle size. The Urbach tail energy estimates corroborate the vibrational spectroscopy measurements, indicating the presence of imperfections/defects in the samples. Density Functional Theory (DFT) studies have been applied from the XRD parameters implemented by the Spanish Initiative for Electronic Simulations with Thousands of Atom (SIESTA) to understand the experimental results better. Despite the computational limitations and the difficulty of describing the dynamic effect presented by the 3d orbitals of Ni, it was possible to obtain satisfactory results within the order of magnitude desired. Also, under the effect of the size of the nanoparticles, all samples present excellent electrocatalytic performance (overpotential of 312–335 mV for a current density of 10 mA/cm2) for Oxygen Evolution Reaction (OER) in alkaline medium.