Green synthesis by carambola fruit juice coprecipitation route, effect of nickel doping, and optical properties of spherical copper oxide nanoparticles

Abstract

Doping CuO with Ni^2 + has been preferred because of the closeness of the ionic radii of Cu²⁺ (0.72 Å) and Ni²⁺ (0.69 Å) but the trends in crystallite sizes as a consequence of Ni^2+_ doping has been contradicted. Therefore, an independent study on the effect of doping CuO with Ni²⁺ was necessary. In this study, spherical nanostructured pure and nickel-doped cupric oxide (CuO) are synthesised by co-precipitation using Carambola fruit juice. The single molecular precursors are decomposed at a predetermined decomposition temperature of 350°C for 3 h. The Nickel doping concentrations used are 2.0 %, 4.0 %, 6.0 %, and 10.0 %. The single molecular precursors obtained were characterised by IR and TGA, while the decomposed samples were characterised by IR, SEM/EDS, PXRD, and UV–vis. The IR results revealed that the single molecular precursors were metal oxalates, whereas the decomposed samples were metal oxides (CuO and Ni-doped CuO). The SEM results show discrete spherical nanoparticles with a predominant spherical shape but with the porous morphology and particle size varying as Ni²⁺ are introduced in the CuO matrix. The EDS results indicate the presence of Cu and the progressive incorporation of Ni²⁺ ions in the CuO matrix., suggesting that the decomposed samples were CuO and Ni-doped CuO. X-ray diffraction (XRD) shows the formation of pure polycrystalline CuO with tenorite phases, which belong to the monoclinic structure. Two competing predominant adsorption peaks were observed corresponding to the (111) and (11͞1) crystal planes. Furthermore, the determination of crystal sizes from XRD data showed that the particle sizes ranged between 9 nm and 24 nm and, were greatly modified by the doping and dopant concentrations of Ni²⁺. The optical band gap of the decomposed samples fluctuated between 2.84 and 3.01 eV as the precursor concentration of nickel ions increased from 0.00 % to 10.0 %. Other unit cell parameters were calculated using particle size absorbance/transmittance data extracted from PXRD, and the results demonstrated an irregular trend. These results attribute the contradiction reported in literature to redistribution of particle sizes of the crystallites which correlate positively to the prevalent diffraction Braggs angles. Furthermore, the fundamental understanding of the effect of Ni²⁺ doping and increase in dopant concentration in the CuO matrix is governed by many factors including the size of the dopant ion, concentration of the dopant ions in the host matrix and morphology of the crystallites. These factors are all closely linked to the method of synthesis or crystallite growth.