Thermal annealing of defects in highly defective NiO nanoparticles studied by X-ray and electron spectroscopies

We studied the electronic structure of highly defective NiO nanoparticles using X-ray and electron spectroscopies. The spectra of NiO nanoparticles exhibit splittings which are not present in the spectra of bulk NiO. These extra splittings are attributed to changes in the crystal field caused by both adsorbates and defects. Thermal annealing of the nanoparticles reduces the splittings observed in the spectra. The changes observed in the spectra up to 300°C are attributed mostly to the desorption of various adsorbates. The changes observed above 300°C are attributed to the annealing out of defects. The spectra annealed at higher temperatures approach the spectra of bulk NiO. We suggest that the defects are unsaturated Ni-O bonds at the surface of the nanoparticles.