Single-source sol-gel synthesis of nanocrystalline ZnAl2O4:: Structural and optical properties

Nanometer-sized zinc aluminate (ZnAl2O4) particles were synthesized from heterometal alkoxides, [ZnAl2(OR)(8)], possessing an ideal cation stoichiometry for the ZnAl2O4 spinel. ZnAl2O4 is formed at 400 degreesC, which is the lowest temperature reported for the formation of monophasic ZnAl2O4. Al-27 magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl2O4 possesses an inverse structure at < 900 degreesC, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:AI stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl2O4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20-250 run) showed that the absorption edge exhibits a blue shift as particle size decreases.