Role of interfacial energy and crystallographic orientation on the mechanism of the ZnO + Al 2 O 3 → ZnAl 2 O 4 solid-state reaction

Abstract

Co-doped MgGa 2 O 4 /SiO 2 nano-glass-ceramic composites with the composition of (1 - y) Co x Mg 1-x Ga 2 O 4 -ySiO 2 (0 ≤ x, y ≤ 1) were synthesized by the sol-gel method and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy was used to study the chemical environments of cations and oxygen in the nanocomposites as a function of annealing temperature, cobalt content and SiO 2 content. The results show that Ga 3+ and Mg 2+ ions occupy both the tetrahedral and octahedral sites of spinel structure in the samples. The inversion parameter (two times the fraction of Ga 3+ ions in the tetrahedral sites) of the nanocomposites is relatively larger in comparison with that of ZnGa 2 O 4 , and the value decreases with the increase of annealing temperature and silica concentration, while increases with cobalt-enrichment. All the Co-doped samples exhibit the intense absorption peak at ∼600 nm, which is characteristic of tetrahedral Co 2+ . But only in the composites of Co 0.3 Mg 0.7 Ga 2 O 4 and 0.6Co 0.6 Mg 0.4 Ga 2 O 4 -0.4SiO 2 , the absorption band at 300-500 nm characteristic of octahedral Co 2+ ions is observed. The intensity of the absorption peak corresponding to octahedral Co 2+ ions increases with decreasing annealing temperature for the Co 0.3 Mg 0.7 Ga 2 O 4 nanocrystals. © 2013 Elsevier B.V. All rights reserved.