Faculty of Natural Sciences

The formation mechanism of spinel ZnAl2O4 has been studied to challenge the conventional approach for its hydrothermal synthesis obtained by trial and error. PDF analysis has provided evidence why particular impurities tend to form as well as disproven the common assumption that a specific metal to hydroxide ratio is required. Through in situ PXRD the influence of synthesis conditions and intermediate phases has been established and compared to various conventional hydrothermal methods synthesized ex situ. For the latter, synthesis conditions were chosen to ensure intermediate impurities and structurally compared to samples synthesized without intermediates. For generality, several synthesis techniques have been applied. Comparing the large portfolio of samples as well as characterization techniques, the structural defects have been established, beyond simple inversion of cations. Based on our knowledge of the prenucleation clusters and impurities, the morphology, crystallite size, band gap and defects of ZnAl2O4 can be tailored.

Figur 1: pH dependent formation mechanism of ZnAl2O4. Al dimers (orange polyhedrons) form independent of pH. Top) high M/OH: the initial octahedral Zn (green) must recoordinate to tetrahedral coordination and attach to the Al dimer in order to form the first ZnAl2O4 nucleus. Bottom) low M/OH, ZnO nanocrystals are initially present in solution. The Zn tetrahedrons dissolving from the ZnO cluster can directly attach to the Al dimerand form the first nucleation site for ZnAl2O4.