자기집합막을 이용하여 수열조건하에서 TS-1 제올라이트 박막 제조

조규진, 서차연, 문일식, 설용건*

순천대학교 화학공학과, 연세대학교 화학공학과*

Fabrication of Ultra-thin TS-1 Zeolite Films under Hydrothermal Condition Through Self-Assembled Arrays

Gyoujin Cho, Chayeon Seo, Il-Shik Moon, Yong-Gun Shul*

Department of Chemical Engineering Sunchon National University

Department of Chemical Engineering Yonsei University*

Introduction

After Barrer and coworkers successfully synthesized zeolites at 1940, zeolites have been used as a major class of porous inorganic oxides that have wide applications in industry for separation and catalysis.1 In addition to these typical applications of zeolites, there has been a tremendous surge in the preparation of thin films or membranes of zeolites in the past decade because they are very promising as highly selective separation membranes, sensors, conductors, and optoelectronic devices. Many of these applications are more effective when thinner films of zeolite are used.

There are a variety of techniques to fabricate zeolite films, including dip-coating, spin coating, sputtering, and laser ablation.2 Besides those typical techniques, "wet" colloid chemical methods have been intensively investigated recently. Since colloidal materials are usually in the range of micrometer-to-submicrometer in size and are well dispersed in solution, the spontaneous adsorption of colloidal particles onto a substrate could lead to  micrometer- or submicrometer-structured materials.

By mimicking the natural principle of self-assembly, the spontaneous adsorption of molecules or nanoparticles onto a substrate can lead into the formation of self-assembled multilayer films. At least conceptually, self-assembly studies can be traced to the study of surfactant mono-, bi-, and multilayers to Langmuir-Blodgett (LB) films for more than a century. However, the recognition of the potential application of self-assembled layers as advanced materials truly started in the 1980s with Sagiv's self-assembled monolayer formation.3

For the application of self-assembled monolayers to zeolites, Yan and Bein successfully used self-assembled thiol-silane monolayers to fabricate zeolite Y on gold surfaces for application as a chemical sensor.  After first being introduced by Decher, self-assembly techniques using electrostatic interaction between colloidal nanoparticles and adsorbed polyelectrolytes in the fabrication of ceramic thin films have been intensively investigated by J. Sterte, T. E. Mallouk, J. H. Fendler, T. Kunitake, etc. As a completement to these successful methods were successful in preparing ceramic films, the investigation for a new and simple method using a van der Waals interaction for the fabrication of ceramic thin films has been reported by Cho and coworkers.4

The van der Waals interaction through self-assembled monolayers was successful in fabricating crack free TS-1 zeolite thin films from nano-sized colloidal dispersions of TS-1 solution.4 However, the adhesion of resulting TS-1 films often problematic for preparing stable films for industrial application. This problem can be avoided if TS-1 films directly nucleated and growth on the surface of substrate. Since TS-1 is a bicomponent ceramic zeolite of TiO2 and SiO2, the crack free and homogeneous thin film formation of TS-1 is cumbersome by using alkoxy precursors of TiO2 and SiO2 which have extremely different hydrolysis ratios in sol-gel process.

In this paper, we are developing a novel method for the direct fabrication of TS-1 ultra-thin films through self-assembled monolayers of polyethyleneglycol (PEG) under hydrothermal method. Self-assembled monolayers of PEG will effectively chelate the precursors of TiO2 under aqueous phase and retard the hydrolysis rate of the TiO2 precursors. Therefore, we can expect reasonable ratio of Ti and Si on TS-1 zeolite ultra-thin films.

Experimental Procedure

Surface modification of glass slides using 3-(triethoxysilyl)propyl isocyanate was carried out based on a previously reported method.5 Self-assembled monolayer of polyethylene glycol (PEG) were simply prepared by adding PEG onto the surface modified glass slides which was immersed methylene chloride solution containing catalytic amount of stannous 2-ethylhexanoate, under N2 atmosphere. The resulting PEG self-assembled monolayer on glass were characterized using contact angle meter, and surface reflection IR.

The zeolite system used in our work is TS-1 zeolite, which can be prepared under hydrothermal condition with 95% crystallinity. The fabrication for TS-1 zeolite films was carried out by adding both cleaned and surface modified slide glasses (1 x 2 cm2) into the reaction vessel containing various amounts of tetraethyl orthosilicate (TEOS), titanium (IV) butoxide (TBD),  tetrapropylammonium hydroxide (TPAOH), isopropyl alcohol, and water. The resulting vessel was kept in a thermostated bath at 80 ℃ overa period of 7 days. The resulting glass slides were rinsed with stirring water for 5 min and sintered at 100 ℃ for 5 h and then further heated at 550 ℃ for 3 h. The resulting ultra-thin TS-1 films deposited on glass slides were characterized FT-IR, SEM, XRD, and AFM.

Results and discussion

The contact angles of solid surfaces with water provide a useful insight into the wettability, roughness, and molecular motions at interfaces. The results for the contact angles of bare glass, TPC modified glass, and PEG self-assembled glass are 0□□ 5, 100□□ 10, and 30□□ 10 respectively. All reported values are the average of at least five measurements taken at different locations on the glass surfaces. If we consider that the contact angle of the completely hydrophobic surface of polyethylene is 120□and the end group of TPC on glass is the cyanate group (-N=C=O), it is reasonable to conclude that almost perfect packing of TPC on glass was achieved. In addition, after PEG self-assembles on TPC modified glass, the advancing contact angle is lower than that of TPC modified glass. Considering the hydrophilicity of PEG, the observed contact angle for the PEG self-assembled glass is reasonable one. In addition, IR spectra for the TPC and PEG modified glass were taken. For the TPC modified glass, the characteristic frequency of the cyanate group at 2156 cm-1 was observed with a weak intensity. For PEG modified glass, the characteristic C-O stretching peak at 1130 cm-1 was observed with a strong intensity. All these results provide a reasonable basis to postulate that the self-assembled monolayers (SAM) of PEG actually exists and can be used as a two dimensional liquid on glass surface.

As shown at Fig. 1, TS-1 zeolite thin films were successfully fabricated on SAM of PEG on glass. This image (Fig. 1) also shows that the film is continuous with relatively defect free. The morphology of the zeolite films was strongly affected by the concentration of TEOS and TBD. As lowering the concentrations of both TEOS and TBD, the particle size of the films was decreased upto 10 to 5 nm.

The bonding of the zeolite film to the glass substrate is strong as substantiated by the fact that the film withstands by removal of Scotch tape. Bonds between the TS-1 film and substrate are most likely created when the SAM of PEG on substrate is preferentially adsorbed seed and interwined with the films with the growth of film.

Acknowledgment

This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Research and Development Center for Automobile's Parts and Materials (RCAPAM) at Sunchon National University.

Fig. 1. SEM image for TS-1 zeolite films

Reference

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[2] T. Bein, Chem. Mater., 1996, 8, 1636.

[3] J. Sagiv, J. Am. Chem. Soc. 1980, 102, 92.

[4] G. Cho, I. S. Moon, Y. G. Shul, K. T. Jung, J. S. Lee, B. M. Fung,

Chem. Lett, 1998, 355..

[5] T. E. Browne, Y. Cohen, Ind. Eng. Chem. Res. 1993, 32, 716.