ZnO Nanocrystals with Hexagonal Disk Shape Grown by Thermal Evaporation Method in Air at Atmospheric Pressure

Abstract

Hexagonal-shaped ZnO nanodisks were synthesized at temperatures above 1000oC via thermal evaporation of a mixture of ZnO, SnO, and graphite powders as the source materials. Notably, the ZnO nanodisks were easily formed in ambient air at atmospheric pressure. The ZnO nanodisks could not be obtained without SnO in the source material, which indicated that the SnO powder played a crucial role in the growth of the hexagonal disk shaped ZnO nanocrystals. The ZnO powder in the source material was reduced to Zn vapor due to the graphite and SnO powders. In particular, the SnO is considered to play a role in promoting the reduction of ZnO. As a result, the concentration of Zn vapor increased quickly, leading to the fast nucleation of Zn crystals with a hexagonal close packed crystal structure. In the growth habit of the Zn crystal, the growth rate of <1010> is much faster than that of [0001] because (0001) has the lowest surface energy, resulting in the hexagonal disk shape. Then the hexagonal-shaped ZnO nanodisks were obtained by oxidation of the Zn nanodisks under air atmosphere. The XRD and EDS results showed that the ZnO nanodisks had a hexagonal wurtzite crystal structure and high purity. The ZnO nanodisks formed at 1000oC exhibited a strong ultraviolet emission centered at 380 nm, which was indicative of high crystalline quality.