Silicalite-1 polycrystalline layers and crystal twins: Morphology and grain boundaries

Silicalite-1 layers as well as individual crystal twins were synthesized from clear solutions using TPABr as template. Their morphology was characterized by scanning electron microscope (SEM). Large crystal twins coloured by iodine were also observed by light microscope. The compactness of a thin layer consisting of spherical crystals (twins) appeared much higher than that of prolonged ones. HF etching of as-synthesized polycrystalline silicalite-1 layers led to the formation of sharp slits along grain boundaries. So the grain boundaries became observable. Etching of calcined layers produced a sort of a negative image represented by preserved thin shells around dissolved crystals. Those shells – films of less organized silica remaining between neighboring crystals – were resistant against HF acid due to the presence of organic species formed during layer calcination. For large isolated silicalite-1 twins the etching patterns differed from those in layers. Non-calcined large twins, after a contact with HF acid, exhibited sharp slits along contact faces of both individual single crystals. These slits were exclusively visible in mildly etched crystal cuts. After calcination large crystal twins dissolved without any order. In contrast to them, small calcined silicalite-1 twins exhibited a high resistance against etching except for boundaries along individuals of the crystal twins.