Are organic templates responsible for the optical and magnetic response of MgO nanoparticles?

Abstract

Herein, we report the optical and magnetic response of magnesium oxide nanoparticles synthesized using magnesium nitrate in a citric acid host. The precursor obtained during the synthesis process was annealed at 350 °C, 400 °C, 500 °C, 700 °C, 900 °C and 1100 °C to obtain magnesium oxide nanoparticles. The rock salt phase of the synthesized nanoparticles was investigated by Rietveld analysis of their X-ray diffraction (XRD) patterns. The temperature-dependent XRD investigation revealed a stable rock salt phase at the measured temperature of 300 °C for these nanoparticles. The nanoparticles exhibited varying crystallite sizes in the range from 6.9 ± 0.2 nm to 29.7 ± 3.9 nm as the annealing temperature increased from 350 °C to 1100 °C. Furthermore, scanning electron microscopy and transmission electron microscopy revealed the formation of regular particles above 500 °C; however, below this temperature, the particles appeared to be embedded in a carbonaceous-like matrix. A variation in color with annealing temperature was visually observed for the nanoparticles, which was associated with the presence of the carbonaceous-like matrix. Near-edge X-ray absorption spectroscopy (NEXAFS) measurements carried out at the O K-edge also revealed a carbonaceous-like matrix, and these results were supported by C and N K-edge measurements. These measurements indicated the presence of C–O–, CO–, C–N–, and C–H-like bonds attached to MgO crystallites having dominant contribution at low annealing temperatures. This behavior was also reflected by the X-ray photoelectron spectroscopic measurements at O 1s, C 1s and N 1s. The Mg K-edge NEXAFS measurements exhibited slight distortion in the Mg²⁺ ion coordination at lower annealing temperatures, which may be due to the presence of Mg²⁺ ions in MgO- and MgCO₃-like co-ordinations, as investigated from Mg 1s spectra of these nanoparticles. Diffuse reflectance measurements showed almost 100% reflectance for the nanoparticles at an annealing temperature of 900 °C and wavelength of 600 nm; the reflectance of these nanoparticles reduced when the annealing temperature was lowered, and it reduced to almost 8% at an annealing temperature of 350 °C. Additionally, O K-edge X-ray magnetic circular dichroism (XMCD) measurements obtained for these materials showed the onset of magnetism in these C-rich materials.