Single‐Source Sol‐Gel Synthesis of Nanocrystalline ZnAl 2 O 4 : Structural and Optical Properties

Nanometer‐sized zinc aluminate (ZnAl 2 O 4 ) particles were synthesized from heterometal alkoxides, [ZnAl 2 (OR) 8 ], possessing an ideal cation stoichiometry for the ZnAl 2 O 4 spinel. ZnAl 2 O 4 is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl 2 O 4 . 27 Al magic‐angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl 2 O 4 possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in‐depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence‐band spectra of ZnAl 2 O 4 and ZnS suggested that the hybridization of O 2 p and Zn 3 d 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 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.