Morphology-function relationship of ZnO: Polar planes, oxygen vacancies, and activity

The relationship between morphology and function of ZnO is demonstrated by investigating its polar planes, oxygen vacancies, and catalytic activity for N-formylation. ZnO with various morphologies is controllably synthesized via simple hydrothermal reactions. Scanning electron microcopy images exhibit a variety of the as-prepared hexagonal zinc oxides: rods, disks, rings, and screw caps as a new member of ZnO morphology family. Each of the morphologies is remarkably different from the others in the proportion of the (0001) and (0001) polar planes in the outside surfaces of ZnO crystals. The analysis of photoluminescence spectra shows that there exist more oxygen vacancies in the samples with large polar planes. The synthesized samples are used as a catalyst for the N-formylation of aniline and show a morphology-dependent activity: ZnO with large polar planes is more catalytically active for the N-formylation reaction. This is attributed to the fact that the polar planes generate easily oxygen vacancies, which are considered as the favored sites for catalyzing the N-formylation reaction. The results suggest a positive relationship among polar planes, oxygen vacancies, and catalytic activity for N-formylation.