ZnS and ZnO Nanosheets from ZnS(en)0.5 Precursor: Nanoscale Structure and Photocatalytic Properties

Wurtzite ZnS and ZnO porous nanostructures have been obtained by annealing ZnS(en)(0.5) nanosheets in air at different times and temperatures. The evolution of the morphological and structural transformation has been investigated at the nanoscale by transmission and scanning electron microscopy (TEM and SEM) analysis. At the annealing temperature of 400 degrees C, the ZnS(en)(0.5) hybrid decomposes by a topotactic transformation, giving ZnS nanosheets. By increasing the annealing time, the gradual transformation ZnS -> ZnO is observed to take place at the nanoscale. The transformation completes with the formation of highly porous ZnO crystals when the annealing temperature is increased to 600 degrees C. Remarkably, all the structural and morphological transformation steps during the annealing preserve the nanosheet crystalline character. These processes have been related to the materials photocatalytic activity in methylene blue degradation under UV-visible irradiation. The ZnS(en)(0.5) precursor is the weakest photocatalyst, whereas porous ZnO is the strongest. Samples treated at 400 degrees C show degradation efficiencies that are intermediate between ZnS(en)(0.5) and ZnO platelets and decrease with increasing annealing time due to the formation of ZnS/ZnO heterojunctions.