Growth of oriented molecular sieve crystals on thin organic layers

Sue M Feng, Purdue University

Abstract

Oriented zeolite molecular sieve crystal films show great promise for size-selective chemical sensors and separation membranes. Crystal growth techniques inspired by biological processes have attracted a growing amount of interest in view of the potential for enhanced synthetic control over complex assemblies. We have developed strategies for the deposition of oriented single layer zeolite crystals on organic layer modified gold surfaces. In order to control the crystal growth, three systems were studied and analyzed with ellipsometry, reflection absorption infrared spectroscopy (RAIR), wettability measurements, atomic farce microscopy, scanning electron microscopy, X-ray diffraction, and quartz crystal microbalances. In the first system, single layer pure organo-phosphate layers were prepared by adsorption of 11-mercapto-1-undecanol (MUD) followed by phosphorylation of the hydroxyl group. Single-layer mixed phosphate films were prepared by adsorption of mixed solutions of 1-dodecanethiol and MUD. The single-layer pure phosphate layers were found to adopt the ($\surd 3\ \times x \surd 3)$ R30$\sp\circ$ adlayer structure on Au(111)/mica. The single-layer mixed phosphate films were formed as a densely packed monolayer and had slightly preferred adsorption of the methyl terminated species over the hydroxyl terminated species. These above mixed films did not formed large domains. It was observed that single layer pure organo-phosphate films can promote the growth of oriented zinco-phosphate molecular sieve crystals while the single layer mixed phosphate films had strong effects on crystal density. In the second system, tri-layer pure organo-phosphonate films were prepared by adsorption of the phosphorylated MUD layer sequentially into zirconyl chloride and 1,10-decanediylbis (phosphonic acid). RAIR results indicated that the tri-layer phosphonate films were more thermally stable than the single-layer phosphate layers. We have discovered strategies to grow both zinco-phosphate and aluminophosphate molecular sieve crystals on gold substrates pre-modified with organo-phosphonate multilayer films. We learned that several factors are important for the surface nucleation process: the presence of a phosphonate surface, a specific templating agent, substrate orientation, gel concentration, and synthesis temperature. Scanning electron microscope images indicated that most crystals are attached to the surface with one of their triangular faces in the case of zinco-phosphate and with oriented vertical channels in the case of aluminophosphate molecular sieves. Grazing angle X-ray diffraction experiments reveal the high orientation in both cases. Nitrogen sorption in these structures on a piezoelectric device confirms the presence of zeolite microporosity.

Degree

Ph.D.

Advisors

Bein, Purdue University.

Subject Area

Chemistry|Chemistry|Materials science

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