Polycrystalline silicalite-1 layers:: texture and kinetics of growth

Self-supporting silicalite-1 layers were grown on mercury surface from clear solutions at temperatures 140-180degreesC. Synthesis time was varied. Kinetics of layer growth was measured and the corresponding apparent activation energy was estimated to 52kJ/mol. Layer morphology and thickness were characterized by SEM. Phase purity of silicalite-1 and orientation of crystals were determined by XRD. Density measurements of polycrystalline layers were carried out by helium pycnometry and by hydrostatic method. Layers with the thickness >40mum were found to be highly compact. Relevancy of Brownian motion of colloidal particles besides their sedimentation is shown. HF etching of as-synthesized polycrystalline silicalite-1 layers caused formation of deep sharp slits along grain boundaries and thus it visualized individual crystal blocks. Contrary to this result etching of calcined layers led to a kind of a negative image represented by preserved thin shells at crystal boundaries which were found to be impregnated by carbon residues formed during template removal. Results of etching experiments together with results of recent XPS strongly indicate a presence of a thin layer at the interfaces of neighbouring crystals which is composed of only partially consolidated fine particles of silica material. (C) 2004 Elsevier Inc. All rights reserved.