Silicalite-1 films with preferred orientation

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Abstract

Smooth and homogeneous monolayer Silicalite-1 seeds were deposited on silicon substrate surfaces via the Langmuir–Blodgett (LB) technique. The LB films were prepared by vertical dipping leaving part of the substrate uncoated. Neither substrate modification nor additives were required to produce densely packed monolayer seeds. The degree of coverage and packing of the seed layers were dependent on the target pressure used for film deposition. Dense Silicalite-1 films with controlled thickness in the range 100–650 nm were synthesized after secondary synthesis. The (0 k 0) reflections dominated in all the XRD patterns, suggesting that the prepared polycrystalline films were preferentially b-oriented perpendicular to the substrate surface. The degree of in-plane orientation was preserved with increasing film thickness. All secondary growth films were very smooth with a clear boundary between the seeded and non seeded substrate sections. The Silicalite-1 films were highly stable upon both ultrasonic treatment and calcination.

Introduction

The preparation of zeolite films has attracted considerable attention due to their potential applications as selective membranes, chemical sensors, low-k films and corrosion-resistant coatings [1], [2], [3], [4], [5], [6], [7], [8]. The most commonly used methods for the preparation of zeolite films are in-situ crystallization and seeding/secondary growth techniques. The in-situ crystallization technique uses direct crystallization of the zeolite crystals on the substrate under hydrothermal treatment. The benefits of this method are that it is realized as a single step process without any preparation of seed layers and excellent adhesion to the substrate. Silicalite-1 films with a thickness of 500–700 nm have been prepared on pretreated yttria doped zirconia supports by the in-situ synthesis method at 185–195 °C for 4 h [9]. Wang and Yan have also reported the synthesis of uniformly b-oriented MFI monolayer films on metal substrates [10], [11]. The studies showed that OH/Si and Na+/Si ratios, crystallization temperature, aging time, and surface roughness play a significant role in controlling the crystal orientation. Wang et al. have successfully prepared oriented MFI films with epitaxial overgrowth on polished quartz supports via direct crystallization method [12]. In-situ growth of continuous b-oriented MFI zeolite membranes with a thickness of 2–3 μm has been prepared on porous α-alumina substrates [13]. The results showed that mesoporous silica precoated layers have a crucial role in maintaining the b-orientation of the crystals on the support after long crystallization times (8 h). In-situ growth of ZSM-5 films on other substrates, e.g., zirconia plates, fused-quartz disks, non-porous α-alumina tubes and plates, have also been investigated [14].

The seeding/secondary growth method involves the deposition of preformed zeolite crystals on the substrate, followed by hydrothermal growth of the seeds in a precursor solution to a continuous layer. Various methods have been used for the fabrication of seed layers, e.g., spin-coating, dip-coating, ionic and covalent linkages, electrostatic deposition, and the Langmuir–Blodgett technique [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Mintova and Bein have demonstrated the preparation of b-orientated Silicalite-1 films by spin-coating of colloidal zeolite in ethanol suspension [15]. However, the prepared films were not continuous containing physically attached seeds. Continuous thin Silicalite-1 films with thickness from 110 to 720 nm have been synthesized on nonporous substrates by secondary growth of seed layers [25], [26], [27]. The substrate was modified with a cationic polymer to facilitate the adsorption of the negatively charged Silicalite-1 nanocrystals, followed by hydrothermal treatment to produce highly intergrown films. Sterte et al. have demonstrated the synthesis of continuous Silicalite-1 films with a thickness ranging from 90 to 430 nm grown on silane modified gold substrates [28]. According to XRD results, the precursor seed layers showed a preferred orientation of (0 k 0) parallel to the substrate surface. Secondary growth of these films produced Silicalite-1 crystals grown in the b-orientation perpendicular to the substrate surface. Mintova et al. have also reported the preparation of Silicalite-1 films on polished silicon wafers [29]. Grazing incidence synchrotron XRD analysis showed that the adsorbed seed layers were oriented in the a-axis perpendicular to the substrate surface. Interestingly, the orientation changed from a- to the b-axis perpendicular to the substrate surface as a function of film thickness. Continuous highly c-oriented MFI films were prepared by coating Silicalite-1 crystals using UV glue followed by hydrothermal treatment for 12 h at 180 °C in a synthesis solution with a molar composition of 8TPAOH:40SiO2:800H2O:160EtOH [30]. Yoon et al. have developed the ionic and covalent linkage method for the preparation of highly b-oriented MFI monolayers on different types of supports (e.g., glass, silica, alumina, etc.) [19]. Defect free b-oriented MFI membranes have been prepared by Lai et al. using the seeded growth approach [1]. b-Oriented ZSM-5 seed monolayers were obtained via the covalent linkage method and the seeded substrates were subsequently grown in a precursor solution containing trimer-TPAOH instead of monomer-TPAOH as a structural directing agent. Choi et al. have also reported the preparation of continuous a-oriented MFI films on silica precoated α-alumina disks [31]. a-Elongated leaf-shaped MFI seeds were synthesized from trimer-TPAOH as the structural directing agents. After hydrothermal secondary growth at 90 °C, a-orientation of the seeds was preserved resulting in continuous MFI membranes with a thickness of about 2 μm.

Recently, we have successfully demonstrated the preparation of monolayer Silicalite-1 films with a thickness of ca. 100 nm from methanol modified Silicalite-1 nanocrystals using the LB technique [22]. Silicalite-1 nanocrystals with enhanced hydrophobicity were prepared by ultrasonic treatment in methanol. The formation of film at the air–water interface and subsequent transferring of the film onto the substrate surface using the LB approach were described in detail. The present study deals with the effect of LB monolayer seeds on the orientation and characteristics of Silicalite-1 films obtained by secondary growth.

Section snippets

Experimental

Silicalite-1 nanocrystals were synthesized from a synthesis solution with the molar composition 9TPAOH:25SiO2:480H2O:100EtOH prepared from tetraethyl orthosilicate (TEOS, 98%, Lanchester), tetrapropylammonium hydroxide (1.0 M solution, Aldrich) and distilled water [32]. After pre-hydrolysis of the solution on an orbital shaker at room temperature for 24 h, the synthesis solution was subjected to hydrothermal treatment at 100 °C for 24 h. The nanosized Silicalite-1 crystals were purified by

Results and discussion

Monolayer Silicalite-1 films with a high degree of coverage and density extending over cm square areas were prepared via the LB technique from methanol modified Silicalite-1 seeds. Fig. 1 shows the SEM images of the monolayers deposited on the Si substrates prepared at different target pressure. The Silicalite-1 seeds used for the preparation of the monolayers were ca. 100 nm in size having an oval morphology according to the SEM images (Fig. 1). A monolayer film of poor crystal density was

Conclusions

The Langmuir–Blodgett technique was used to prepare high quality densely packed monolayers of Silicalite-1 seeds, which were then secondary grown into smooth and homogeneous Silicalite-1 thin films. The procedure used did not require any demanding support pre-treatment and no additives were used to modify the Silicalite-1 suspension used for LB film preparation. The approach developed is simple and yet highly reproducible and can be used for the preparation of seeded layers on different flat

Acknowledgments

The authors thank the Leverhulme Trust and EPSRC (Grant EP/D50645X/1) for funding.

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