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

Nanometer-sized zinc aluminate (ZnAl2O4) particles were synthesized from he terometal alkoxides, [ZnAl2(OR)(8)], possessing an ideal cation stoichiomet ry 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 s pectroscopy for chemical analysis. Evaluation of the valence-band spectra o f ZnAl2O4 and ZnS suggested that the hybridization of O 2p and Zn 3d orbita ls is responsible for lowering the bandgap in the latter. The average cryst allite size showed an exponential relationship to the calcination temperatu re (X-ray diffractometry and transmission electron microscopy data). The op tical spectra of different spinel powders (average particle sizes, 20-250 r un) showed that the absorption edge exhibits a blue shift as particle size decreases.