NANOPOROUS AGGREGATES OF ZNS NANOCRYSTALLITES

During the synthesis of ZnS powders by wet chemical precipitation, the formation of nanoporous spheres is observed. The powders have been in vestigated using thermogravimetric analysis, X-ray diffraction and opt ical spectroscopies. Nanopore formation can be explained by several st ages of growth. The formation of nanoparticles as primary particles is followed by their agglomeration Forming secondary particles. These se condary particles are monodispersed spheres with a considerable porosi ty, because the agglomeration of the nanoparticles is unlikely to be v olume-filling. The voids or nanopores formed by this agglomeration pro cess in the secondary particles is estimated to comprise around 35% of the sphere volume. They are mainly filled with water and the residues of the chemical reagents. Water in the pores partially reacts with Zn S and forms hydrated sulphates, The chemical reagents used for the pre cipitation reactions are also found to be bound to the nanocrystallite 's surfaces as ligands in some cases, Depending on the reaction condit ions and reagents, the agglomeration of the nanoparticles can also be modified or hindered by the use of complexing agents acting as a steri cally stabilizing surface layer on the nanocrystallites. The agglomera tion of nanoparticles to larger units being a general phenomenon, this use of complexing agents to control pore formation and agglomerate si ze should be applicable to other nanocrystalline systems. (C) 1998 Joh n Wiley & Sons, Ltd.