Cation Distribution in Co-Doped ZnAl2O4 Nanoparticles Studied by X-ray Photoelectron Spectroscopy and 27Al Solid-State NMR Spectroscopy

CoxZn1-xAl2O4 (x = 0.01-0.6) nanoparticles were synthesized by the citrate sol gel method and were characterized by X-ray powder diffraction and transmission electron microscopy to identify the crystalline phase and determine the particle size. X-ray photoelectron spectroscopy and Al-27 solid-state NMR spectroscopy were used to study the distribution of the cations in the tetrahedral and octahedral sites in CoxZn1-xAl2O4 nanoparticles as a function of particle size and composition. The results show that all of the as-synthesized samples exhibit spinel-type single phase; the crystallite size of the samples is about 20-50 nm and increases with increasing annealing temperature and decreases with Co-enrichment. Zn2+ ions are located in large proportions in the tetrahedral sites and in small proportions in the octahedral sites in CoxZn1-xAl2O4 nanoparticles. The fraction of octahedral Zn2+ increases with increasing Co concentration and decreases with increasing particle size. Besides the tetrahedral and octahedral coordinations, the presence of the second octahedrally coordinated Al3+ ions is observed in the nanopartides. The change of the inversion parameter (2 times the fraction of Al3+ ions in tetrahedral sites) with Co concentration and particle size is consistent with that of the Zn fraction inoctahedral sites. Analysis of the absorption properties indicates that Co2+ ions are located in the tetrahedral octahedral sites as well as in the in the nanoparticles. The inversion degree of Co2+ decreases with increasing particle size.