Letter pubs.acs.org/Langmuir
Fabrication of a Highly b‑Oriented MFI-Type Zeolite Film by the Langmuir−Blodgett Method Zheng Wang,*,†,‡ Ting Yu,† Pei Nian,† Qingchun Zhang,‡ Junkang Yao,† Shan Li,† Zuoning Gao,‡ and Xianglong Yue† †
Key Laboratory of Energy Resources and Chemical Engineering and ‡Institute of Chemistry and Chemical Engineering, Ningxia University, 750021 Ningxia, China S Supporting Information *
ABSTRACT: sec-Butanol-modified rounded-coffin-shaped silicalite-1 (SL) microcrystals were assembled into a compact and highly boriented monolayer extending over the centimeter scale via the Langmuir−Blodgett (LB) technique. For comparison, methanol- or ethanol-modified SL microcrystals could not float and were compressed into a dense film in an LB trough. Subsequently, highly b-oriented MFI films with a thickness of ∼1.5 μm were successfully obtained on the solid substrates by secondary growth of the LB monolayer using tetrapropylammonium hydroxide (TPAOH) as the structure-directing agent. The electrochemical experiments confirmed that the prepared films were defect-free. In general, the LB method is a highly controllable and reproducible method of organizing anisotropic zeolite crystals with a preferred orientation over a relatively large surface area. The LB technique could be further applied as an effective platform for the oriented assembly of different types of zeolite particles and the growth of variously oriented zeolite films.
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INTRODUCTION Zeolite films, used as selective membranes and membrane reactors, have drawn considerable research interest for decades owing to their intrinsic properties with respect to molecular recognition on the subnanometer level and high-temperature operation capability.1−3 A number of nontraditional applications, such as selective chemical/electrochemical sensors, low-k materials, and corrosion-resistant films, have also been extensively explored in recent years.4−6 The preparation of an MFI-type zeolite film, an aluminosilicate zeolite belonging to the pentasil family of zeolites, has been most intensively studied because of its important industrial application. It is well recognized that the preferred orientation of MFI films plays an important role in determining overall membrane performance.7,8 In recent years, the trend in MFI films has been shifted to the oriented MFI film, specifically, b-oriented MFI films.9−12 Yan et al. reported the preparation of highly b-oriented MFI monolayer films on smooth stainless steel plates using the in situ crystallization method.13,14 The b-oriented monocrystal films were defect-free with molecular sieving properties proven by the electrochemical experiments. However, attempts to prepare b-oriented MFI films on precious metals such as gold and platinum electrodes failed when the in situ method was used.14 In recent years, the secondary or seeded growth method has been the dominant approach, compared to the in situ method, for the fabrication of b-oriented MFI films. The crucial steps in this strategy are first to achieve a highly b-oriented and densely packed seed layer, and second, to achieve an epitaxial overgrowth-oriented seed layer in a dense and oriented film. © 2014 American Chemical Society
Considerable research effort has been devoted to the assembly of anisotropic MFI crystals on various supports in an oriented manner. Yoon et al. reported a feasible layer-by-layer method based on the chemical-bonding-induced assembly.15−17 Diverse zeolite crystals with different shapes were successfully organized using different chemical linkers on various substrates. On the basis of this, Tsapatsis et al. reported the successful fabrication of a defect-free and highly b-oriented MFI film, which exhibited excellent molecular sieving properties for the separation of p-/oxylene vapors.11They also reported that a high separation ratio could be achieved only from a b-oriented MFI film, whereas relatively low separation was achieved from a c-oriented film.7 Recently, Yang et al. reported a phase-segregation-induced selfassembly method of assembling anisotropic silicalite-1 (SL) microcrystals into a b-oriented monolayer on various substrates.18,19 Significant achievements have been made on the wellorganized and regularly shaped zeolite crystals; however, nearly all of the above-mentioned methods are mainly based on the manual assembly process. The organization procedures are relatively complicated, which restricts the development and exploration of oriented zeolite films as catalytic or selective membranes. Therefore, a highly controllable and reproducible method of organizing anisotropic zeolite crystals with a preferred orientation on a relatively large surface area is in high demand. Received: January 22, 2014 Revised: April 12, 2014 Published: April 14, 2014 4531
dx.doi.org/10.1021/la500115t | Langmuir 2014, 30, 4531−4534
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Letter
ammonia to remove the unreacted materials and calcined at 500 °C for 6 h in air. The electrochemical data were obtained using a CHI660A electrochemistry workstation (Shanghai Chenhua Instrument Corporation, China). All of the experiments were performed in aqueous solutions with 0.1 M KCl as the supporting electrolyte. The CV responses of [Fe(CN)6]3− were obtained at a concentration of 1 mM. The electrochemical measurements were conducted using a conventional three-electrode setup using a Pt wire counter electrode and a saturated sodium calomel electrode (SCE) as the reference electrode. All of the electrochemical potentials were measured versus an SCE and at a scan rate of 50 mV s−1.
The Langmuir−Blodgett (LB) technique is an elegant method for forming highly dense nanocrystal assemblies over unprecedented surface areas from tens of nanometers to micrometers.20,21 Zheng et al. have reported the deposition of a dense monolayer of methanol-modified spherica zeolite nanocrystals on a silicon substrate using the LB technique, whereas the randomly oriented MFI films were prepared by the secondary growth of the seed layer.22 Herein, we report the efficient application of the LB technique to the assembly of rounded-coffin-shaped SL microcrystals into a highly compact b-oriented monolayer extending over the centimeter scale for the first time. As shown in Scheme 1, the assembly process of the SL microcrystals in an
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RESULTS AND DISCUSSION A typical pressure (π) versus area isotherm was obtained during the compression of the sec-butanol-modified SL microcrystals using the LB technique (Figure 1B). In contrast, no isotherm
Scheme 1. LB Technique for Assembling Anisotropic Silicalite-1 Microcrystals into a b-Oriented Monolayer and Secondary Growth for a Highly b-Oriented MFI Film
oriented manner using the LB technique can be mainly divided into three steps: (a) sec-butanol-modified SL microcrystals were spread on the water surface in the LB trough (JML04C2, POWEREACH, China), (b) after the evaporation of secbutanol, LB barriers moved toward and compressed the floating SL crystals into an LB monolayer, and (c) the LB monolayer was transferred to the solid substrates in a vertical manner at a constant compressing pressure. The LB monolayer was further used as a seed layer for the secondary growth of highly boriented and defect-free silicalite-1 films.
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Figure 1. Pressure−area isotherms of (A) aqueous and (B) secbutanol-modified silicalite-1 microcrystals.
was obtained during the compression of the purified aqueous SL microcrystals in the LB trough, indicating the increasing hydrophobicity of the sec-butanol-treated SL crystals. It is reported that alcohol molecules can react directly with the hydroxyl groups on the zeolite surface by the esterification reaction.22Yang et al. reported the sec-butanol modification of the SL crystals; they proposed that the −OCHCH2(CH3)2 group might anchor onto the surface of the MFI microcrystals, which renders the zeolite particles floating at the air−water interface.19 Methanol- or ethanol-modified SL microcrystals could not float and were compressed into a dense film in the LB trough in our experiments. Therefore, we deduced that the alcohol-modified SL crystals could be more hydrophobic upon increasing the number of carbon atoms in the alcohol molecules. The effects of the surface pressure on the zeolite microcrystal arrangement of Langmuir−Blodgett (LB) monolayers deposited onto the stainless steel plate were studied (Figure S1). When the surface pressure was 26 mN/m, the voids between ordered zeolite particles were quite obvious. When the surface pressure was raised to 35 mN/m, most of the particle domains were in contact and formed a solid monolayer. On the other hand, when the surface pressure was higher than 35 mN/m, the content of a-oriented zeolite microcrystals increased, and the ridge caused by the overlapped particles was detected. Therefore, the surface pressure of 35 mN/m is the optimum for a compact and homogeneous zeolite monolayer. Figure 2A shows the typical SEM image of the zeolite microcrystals deposited onto the stainless steel plate transferred at a surface pressure of 35 mN/m. It clearly shows the formation of a compact and homogeneous monolayer of zeolite crystals.
EXPERIMENTAL SECTION
The SL microcrystals with an average size of 1.8 × 1.4 × 0.8 μm3 were synthesized using a synthesis solution of SiO2/TPAOH/H2O/EtOH in a molar ratio of 5:1:500:20 at 150 °C for 13 h.7 Typically, 7.6 g of TPAOH (1 M, Aldrich) and 7.5 g of TEOS (98 wt %, Kermel) were mixed in 60 g of distilled water. The reaction mixture was stirred for 24 h at room temperature and then loaded into a Teflon-lined autoclave. After the reaction at 150 °C for 13 h, the prepared silicalite-1 microcrystals were purified by centrifugation (two times) and redispersed into distilled water. After the third centrifugation, the microcrystals were redispersed in sec-butanol (99%, Aladdin), centrifuged, and redispersed twice in fresh sec-butanol. Finally, the purified SL microcrystals were mixed with sec-butanol to obtain a 0.5 wt % suspension and stirred vigorously at room temperature for at least 3 days. The stainless steel plate (2.5 × 3.5 cm2) was treated with piranha solution (H2SO4/H2O = 2:1, volume ratio) for a short period and washed with ethanol. The platinum (Pt) plate electrode was cleaned with ethanol. The suspension was spread using a microsyringe on a water subphase in an LB trough (JML04C2, POWEREACH, Shanghai Zhongchen, China). The pressure−area isotherms were recorded at room temperature at a compression speed of 50 cm2/min. The monolayer LB films were prepared on substrates in the upstroke direction at a target pressure of 35 mN/m with a dip speed of 2 mm/ min. The prepared LB films were calcined at 500 °C for 6 h in air and placed into a synthesis solution for secondary growth. The synthesis solution used for the film growth has the molar composition 3TPAOH/ 25SiO2/1500H2O/100EtOH. After the hydrothermal treatment at 100 °C for 96 h, the sample was rinsed with 0.1 M 4532
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Letter
result in a (10 0 0) reflection. Most recently, Hrabanek et al. prepared preferentially b-oriented silicalite-1 layers on various supports. Crystallographic preferred orientation (CPO) indices and preferred orientation coverage (POC) values were formulated for the quantification of preferred crystal orientation in a film based on XRD data.12 The CPO index of (0 10 0) and the POC (b, 10°) value calculated using the formulas reported in this reference were 0.69 and 85, respectively, for the films prepared in our work. The calculated results further confirm that the prepared SL films are highly b-oriented. It is well recognized that twin crystal growth is unavoidable when tetrapropylammonium hydroxide (TPAOH) is used as the structure-directing agent (SDA). Twin crystals often nucleate close to the center of the (0 1 0) faces of the MFI crystals, and several twin crystals may merge to form an a-oriented intergrowth layer.23 The Tsapatsis group reported noncommercialized trimer-TPAOH as the SDA used to preserve the crystallographic orientation of the b-oriented seed layer.11 Recently, it was reported that the twin growth of a b-oriented MFI seed layer can be suppressed by the prehydrothermal treatment of the synthesis solution24 or by adjusting the concentration of TPAOH.25 Further studies on the growth of twin-free and continuous b-oriented films are currently underway in our laboratory. Besides stainless steel substrates, identical procedures were employed to assemble the zeolite microcrystals and the growthoriented MFI film on silicon, glass, and platinum plates (Figure S2). Optical photographs show that both the LB seed layer and synthesized zeolite films can reflect specific light like a mirror indicating the homogeneous film thickness. The films synthesized on stainless steel are stable enough even after being calcined at 500 °C for 5 h. Virtually identical results were obtained independently of the support type, which proves that the LB method is a highly controllable and reproducible method of organizing anisotropic zeolite crystals extending over the centimeter scale. The platinum electrode was modified with the prepared zeolite film and further used as the working electrode in the flowing electrochemical experiments to prove that the prepared zeolite films were defect-free.14 The cyclic voltammetry (CV) experiments were carried out on the basis of the literature reports.24 Negatively charged ion [Fe(CN)6]3− (diameter 7.2 Å) was selected as the redox probe, considering that the defects and grain boundaries generated in the zeolite film are in general >1 nm. If the oriented film is not continuous or has tiny defects, then [Fe(CN)6]3− may directly pass through these defects to generate a redox response at the electrode. A solution of [Fe(CN)6]3− generates well-defined redox peaks on the bare platinum electrode. However, only a straight line was observed when the electrode coated with the b-oriented MFI film was used. It is generally believe that amorphous materials plugged into the intercrystalline spaces produce defects in zeolite films. The CV experimental results clearly proved that the prepared zeolite films in our works were free of nonzeolitic pores. This can be associated with the relatively large seeds and longer synthesis time employed for film preparation in our work. It should be clarified that most of the zeolite films on platinum disappeared with calcination at 500 °C owing to the inert property of platinum metal and high temperature.
Figure 2. (A) SEM image and (B) XRD pattern of a silicalite-1 monolayer transferred onto a stainless steel plate. Inset: enlarged image of peak (0100).The reflection labeled with * emanates from the stainless steel substrate.
Nearly all of the microcrystals were firmly anchored to the substrate with their b axes perpendicular to the support surface. However, a few twin seeds were a-oriented. The X-ray diffraction (XRD) results further confirmed that the [0k0] reflections dominated in the diffraction pattern, confirming the high b orientation of the deposited LB monolayer. Similar results were obtained from the different areas of the samples and from the samples prepared from the different batches at the same target pressure. The highly b-oriented SL monolayer obtained using the LB technique was further employed as an oriented seed layer for the secondary growth of the oriented MFI films. Figure 3A,B
Figure 3. (A) Top view of the SEM image. (B) Side view of the SEM image. (C) XRD pattern of the calcined silicalite-1 film prepared by the secondary growth of the LB monolayer. Inset: enlarged image of peak (0 10 0). (D) Cyclic voltammograms of [Fe(CN)6]3− on (a) the bare Pt electrode and (b) the b-oriented MFI-film-coated Pt electrode.
shows the top and side views of the SEM images of the SL film prepared on the stainless steel substrates. The SEM results proved that a well-developed continuous zeolite film with a thickness of ∼1.5 μm was formed after the secondary growth of the seed layer at 100 °C for 96 h. It is quite obvious that the intercrystal gaps were completely filled by the competitive lateral and vertical growth of the rounded-coffin-shaped seeds. Notably, the twin crystals were present on the surface of the film after the secondary growth step. The XRD results support the SEM observations. The inset of the XRD pattern shows the evolution of (10 0 0) and (0 10 0) reflections for the film. The intensity of the (0 10 0) reflection is stronger, whereas the a-oriented twin crystals
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CONCLUSIONS sec-Butanol-modified rounded-coffin-shaped SL microcrystals were assembled into a compact and highly b-oriented 4533
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(10) Pham, T. C. T.; Nguyen, T. H.; Yoon, K. B. Gel-free secondary growth of uniformly oriented silica MFI zeolite films and application for xylene separation. Angew. Chem., Int. Ed. 2013, 52, 8693−8698. (11) Lai, Z.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Microstructural optimization of a zeolite membrane for organic vapor separation. Science 2003, 300, 456−460. (12) Hrabanek, P.; Zikanova, A.; Drahokoupil, J.; Prokopova, O.; Brabec, L.; Jirka, I.; Matejkova, M.; Fila, V.; de la Iglesia, O.; Kocirik, M. Combined silica sources to prepare preferentially oriented Silicalite-1 layers on various supports. Microporous Mesoporous Mater. 2013, 174, 154−162. (13) Li, S.; Demmelmaier, C.; Itkis, M.; Liu, Z.; Haddon, R.; Yan, Y. Micropatterned oriented zeolite monolayer films by direct in situ crystallization. Chem. Mater. 2003, 15, 2687−2689. (14) Li, S.; Wang, X.; Beving, D.; Chen, Z.; Yan, Y. Molecular sieving in a nanoporous b-oriented pure-silica-zeolite MFI monocrystal film. J. Am. Chem. Soc. 2004, 126, 4122−4123. (15) Lee, J. S.; Lee, Y.; Tae, E. L.; Park, Y. S.; Yoon, K. B. Synthesis of zeolite as ordered multicrystal arrays. Science 2003, 301, 818−821. (16) Lee, J. S.; Kim, J. H.; Lee, Y. J.; Jeong, N. C.; Yoon, K. B. Manual assembly of microcrystal monolayers on substrates. Angew. Chem., Int. Ed. 2007, 46, 3087−3090. (17) Yoon, K. B. Organization of zeolite microcrystals for production of functional materials. Acc. Chem. Res. 2007, 40, 29−40. (18) Liu, Y.; Li, Y.; Yang, W. Phase-segregation-induced self-assembly of anisotropic MFI microbuilding blocks into compact and highly boriented monolayers. Langmuir 2011, 27, 2327−2333. (19) Liu, Y.; Li, Y.; Yang, W. Fabrication of highly b-Oriented MFI monolayers on various substrates. Chem. Commun. 2009, 1520−1522. (20) Talham, D. R. Conducting and magnetic Langmuir−Blodgett films. Chem. Rev. 2004, 104, 5479−5502. (21) Tao, A. R.; Huang, J.; Yang, P. Langmuir-Blodgettry of nanocrystals and nanowires. Acc. Chem. Res. 2008, 41, 1662−1673. (22) Wang, Z.; Wee, L. H.; Mihailova, B.; Edler, K. J.; Doyle, A. M. Langmuir−Blodgett deposited monolayers of silicalite1 seeds for secondary growth of continuous zeolite films. Chem. Mater. 2007, 19, 5806−5808. (23) Iwasaki, A.; Hirata, M.; Kudo, I.; Sano, T. Behavior of the (010) face of silicalite crystal. Zeolites 1996, 16, 35−41. (24) Liu, Y.; Li, Y.; Yang, W. Fabrication of highly b-oriented MFI film with molecular sieving properties by controlled in-plane secondary growth. J. Am. Chem. Soc. 2010, 132, 1768−1769. (25) Li, X.; Peng, Y.; Wang, Z.; Yan, Y. Synthesis of highly b-oriented zeolite MFI films by suppressing twin crystal growth during the secondary growth. CrystEngComm 2011, 13, 3657−3660.
monolayer extending over the centimeter scale on various solid substrates by the LB technique. Methanol- or ethanol-modified SL microcrystals could not float and were compressed into a dense film in the LB trough. Subsequently, highly b-oriented MFI films were successfully obtained on solid substrates by the secondary growth LB monolayer with TPAOH as the SDA. The electrochemical experiments confirmed that the prepared films were defect-free. In general, the LB technique could be further applied as an effective platform for the oriented assembly of different types of zeolite particles and the growth of variously oriented zeolite films. Thus, the LB technique is a very promising technique for the development and exploration of oriented zeolite films as sensors, zeolite-modified electrodes, and catalytic or selective membranes.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of the ordered zeolite microcrystals transferred onto stainless steel plates at different surface pressures (S1). Optical images of the zeolite microcrystals deposited onto silicon and stainless steel plates (S2). This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (grant nos. 21066011 and 21366026), the Ningxia Natural Science Foundation (NZ12132), and SRF for ROCS, State Education Ministry.
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REFERENCES
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