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J. Phys. Chem. C 2007, 111, 12052-12057
Zeolite Beta Films Prepared via the Langmuir-Blodgett Technique Lubomira Tosheva,† Valentin P. Valtchev,‡ Boriana Mihailova,§ and Aidan M. Doyle*,† DiVision of Chemistry and Materials, Manchester Metropolitan UniVersity, Chester Street, Manchester, M1 5GD, United Kingdom, Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, UMR-7016 CNRS, ENSCMu, UniVersite´ de Haute Alsace, 3, rue Alfred Werner, F-68093 Mulhouse, France, and UniVersita¨t Hamburg, Mineralogisch-Petrographisches Institut, Grindelallee 48, D-20146 Hamburg, Germany ReceiVed: April 2, 2007; In Final Form: May 17, 2007
The Langmuir-Blodgett technique was explored for the preparation of zeolite Beta monolayer films. The films were obtained using two zeolite Beta samples having similar Si/Al ratios but different crystal size, morphology, and hydrophobic properties. Both zeolites were suspended in methanol. The colloidal zeolite Beta sample required modification with a cationic surfactant for films at the air-water interface to be formed. The hydrophobic microcrystal zeolite Beta synthesized in a fluoride medium was used in the LangmuirBlodgett procedure directly. The different behavior of the two samples was explained by their interaction with methanol, which stabilized the zeolites spread at the air-water interface to a different extent. The floating zeolite Beta films were successfully transferred on silicon wafers by the vertical lifting method. The zeolite Beta crystals and films were studied by SEM, XRD, TG, DLS, zeta potential and nitrogen adsorption measurements, and ATR-IR.
Introduction In the past decade, a number of advanced emerging applications of supported zeolite films and coatings has been suggested. Examples include low-k dielectrics,1 sensor coatings,2 zeolitemodified electrodes,2 photoelectronic devices,3 and third-order nonlinear materials.4 These applications require careful design of the zeolite architectures, which conforms to technological processes for which the materials are tailored. The development of methods for the arrangement of preformed zeolite crystals on flat supports is of particular interest. The main reason is that such crystals can serve as seeds influencing the properties of the product films prepared by secondary growth, namely, film thickness and homogeneity, crystal orientation, smoothness, presence of defects, and pinholes. For instance, the orientation of zeolite films has been manipulated using trimer-tetrapropylammonium (TPA) as a structure-directing template for the synthesis of zeolite seeds5 or zeolite films by secondary growth.6 Colloidal zeolites are most often used as seeds for secondary film growth. Different methods have been developed to attach colloidal zeolites to the support.7 Oriented molecular sieve thin films (e.g., AlPO4-5) have also been grown from aligned rodshaped 0.5-1 µm × 40-80 µm crystals.8 Yoon has been extensively studying different approaches for organizing zeolite microcrystals of uniform size and flat crystal faces on different supports by various covalent and ionic linkages.9 For instance, vertically oriented zeolite L monolayers have been prepared from cylindrical zeolite L rods, whereas hexagonal columnar zeolite L crystals formed horizontally oriented monolayers.10 The same authors have also reported the growth of uniformly aligned Silicalite-1 crystals on polyurethane films with the * Corresponding author. Tel.: +441612471420; e-mail: a.m.doyle@ mmu.ac.uk. † Manchester Metropolitan University. ‡ Universite ´ de Haute Alsace. § Universita ¨ t Hamburg.
crystal orientation controlled by the nature of the polymer films.11 Oriented close-packed monolayers of hexagonal ZSM-2 nanoplates have been prepared recently by convective assembly.12 Convective assembly has also been used to prepare ultrathin mesoporous silica films13 as well as colloidal-based antireflective coatings from silica nanoparticles.14 The Langmuir-Blodgett (LB) method is another technique allowing the production of highly ordered films. An important application of LB films is for the immobilization of biomaterials.15 The LB method has also proved to be suitable for the preparation of active catalysts.16 In the past few years, the LB method has been extended to the preparation of clay mineral films17 and colloidal crystals.18 However, the LB method has hardly been explored to prepare zeolite films.19 Besides applications for secondary film growth and supports for supramolecular organization of molecules and particles, LB zeolite films would be of interest for fundamental studies of the properties of the individual zeolite crystals. Another potential area of application of LB zeolite films is catalysis.20,21 In this work, we report on the preparation of zeolite LB films using zeolite Beta crystals of different crystal size, morphology, and hydrophobic properties. Zeolite Beta was chosen for this study because of its technological importance22 and the possibility of varying its hydrophobic properties.23 In addition, reports on supported zeolite Beta films are somewhat limited as compared to other zeolite-type films, suggesting that the synthesis of zeolite Beta films has yet to be optimized. Experimental Procedures Colloidal zeolite Beta crystals were synthesized from a clear solution with the molar composition 0.31Na2O/9TEAOH/ 0.5Al2O3/25SiO2/489H2O prepared from silica sol (Bindzil 30/ 220, a gift from Eka Chemicals AB), aluminum isopropoxide (Alfa Aesar), and tetraethylammonium hydroxide (TEAOH, 20% aqueous solution, Alfa Aesar). Aluminum isopropoxide
10.1021/jp0725679 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
Zeolite Beta Films Prepared via LB Technique was dissolved in a portion of TEAOH with heating, and distilled water was added to compensate for the evaporation weight losses. This solution was added to the silica solution made up from the silica sol mixed with the rest of the TEAOH. The clear synthesis mixture was stirred for 30 min, transferred to a polypropylene reactor, and heated at 90 °C for 19 days. The resultant milky suspension was purified 3 times by centrifugation and redispersion in methanol (HPLC grade, Prolabo) in an ultrasonic bath. The zeolite concentration was determined from the difference in weight between the suspension and the product obtained after drying at 100 °C. Prior to using the suspension for LB film preparation, 0.7 mM cationic surfactant, cetyltrimethylammonium bromide (CTAB, C16H33N(CH3)3Br, Aldrich, critical micelle concentration ) 0.9 mM), was added to the 1 wt % zeolite methanol suspension and subjected to ultrasonic treatment for 1 h. The 1 µm zeolite Beta crystals were synthesized from a gel with the molar composition 1.0SiO2/0.03Al2O3/0.60TEAOH/ 0.60HF/5.0H2O. The reactants used were pyrogenic silica (Aerosil 130, Degussa), aluminum powder (99.99%, Avocado), TEAOH (20% aqueous solution, Fluka), hydrofluoric acid (HF, 40%, Fluka), and distilled water. The aluminum powder was dissolved in TEAOH followed by the addition of Aerosil and HF. The obtained thick gel was freeze-dried and ground, and the required amount of water was added. The synthesis was performed at 150 °C for 21 days in a PTFE-lined stainless steel autoclave. After the synthesis, the zeolite was separated from the mother liquor by filtration and thoroughly washed with distilled water. The sample (1 wt %) was dispersed in methanol and subjected to ultrasonic treatment for 30 min prior to LB film preparation. The zeolite dispersions were spread by a microsyringe on the water subphase in a LB trough (NIMA 1232D1D2, Nima Technology). The volumes used were 250 µL for the CTABcolloidal Beta and 3000 µL for the larger Beta crystals. The water used was obtained from a Synergy water purification system (Millipore) with a resistivity of 18.2 MΩ cm. Surface pressure-area isotherms were recorded upon compression at a barrier speed of 50 cm min-1. The floating films at the airwater interface were transferred onto silicon (100) wafers by a vertical lifting method at a target surface pressure of 10 mN m-1 using a dip speed of 1 mm min-1. Similar experiments were performed with the microcrystal Beta sample using concentrations of 3 and 5 wt % with volumes inserted in trough of 1000 and 750 µL, correspondingly. The Si wafers were precleaned by subsequent treatment with acetone and 2-propanol in an ultrasonic bath for 15 min followed by repeated rinsing with distilled water. The cleaned wafers were stored in distilled water and air-dried prior to use. X-ray diffraction (XRD) patterns were collected using a STOE STADI-P diffractometer with Ge filtered Cu KR radiation. Scanning electron microscopy (SEM) imaging was performed on a JEOL 5600LV scanning electron microscope. Attenuated total reflection infrared (ATR-IR) spectra of the LB films were taken with a Nicolet NEXUS FTIR spectrometer equipped with a Continuµm microscope using a slide-on ATR attachment with a Spectra-Tech Si crystal. Spectra were made up of 1024 scans with the resolution set at 4 cm-1. ATR-IR spectra of the zeolite suspensions in methanol were continuously recorded upon methanol evaporation and subsequent exposure to air with a Bruker Equinox 55 FT-IR spectrometer equipped with a horizontal reflection Pike MIRacle ATR accessory and Ge ATR crystal. The spectra were collected averaging 121 scans with a resolution of 4 cm-1. The particle size of colloidal Beta
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Figure 1. XRD patterns of as synthesized (a) colloidal and (b) microcrystal zeolite Beta.
was determined by dynamic light scattering (DLS) with a Malvern Zetasizer Nano ZS instrument. Zeta potential measurements of the colloidal and CTAB-modified colloidal Beta samples were performed using the same instrument with a folded capillary cell (DTS1060). For these measurements, the powder obtained upon drying the colloidal Beta was ground and dispersed in methanol or 0.7 mM CTAB-containing methanol to obtain a 1 wt % zeolite concentration and subjected to ultrasonic treatment for 1 h. Thermogravimetric (TG) curves were recorded using a NETZSCH TG 209 thermal analyzer at a heating rate of 10 °C min-1 in air. Nitrogen adsorption measurements were performed with a Micromeritics ASAP 2020 surface area analyzer. As-synthesized zeolite Beta samples were degassed at 140 °C overnight prior to analysis. Specific surface areas were calculated with the BET equation. The Si/Al ratio of the zeolite powder samples was determined by wavelength dispersive X-ray fluorescence spectroscopy using a Philips MagiX instrument. Results and Discussion Zeolite Beta Samples. The size and morphology of the zeolite Beta samples were studied by SEM. The size of the colloidal Beta was about 180 nm, and the particles were of spherical morphology. The microzeolite Beta crystals were truncated square bipyramids with a size of ca. 1 µm (c/f). For simplicity, the two Beta samples will be designated in the text as CB (colloidal Beta) and MB (micrometer-sized Beta). According to the chemical analysis, the Si/Al ratio was 14.5 and 16.1 for CB and MB, respectively. The MB sample also contained 1.11 wt % F. The high crystallinity and phase purity of the two samples were confirmed by XRD (Figure 1). The peak broadening of the XRD peaks in spectrum a was due to the nanosized crystal nature of that sample. The XRD pattern of MB was better resolved, and additional polymorph A features (marked with arrows) appeared that can be related to the limited number of defects in this sample associated with synthesis in a fluoride medium.24 The BET surface areas of the as-synthesized samples (containing the organic structure-directing template) were 221 and 28 m2 g-1 for CB and MB, respectively. Nanozeolites exhibit large external surface areas attributed to their small crystallite size, which explains the difference observed.25 On the other hand, the external zeolite surface area is directly proportional to the number of terminal silanol groups.26 Thus, from the BET surface area values, one can expect a high concentration of silanol sites for the CB sample. As mentioned, the MB sample contains limited framework defects and hence should be more hydrophobic. The TG analysis confirmed this conclusion. The weight loss in the region up to about 150 °C was ca. 2.5 wt % for CB and 0.5 wt % for MB (see Supporting Information).
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Figure 2. Pressure-area isotherms obtained by spreading 1 wt % methanol suspensions of (a) CTAB-modified CB and (b) MB.
Pressure-Area Isotherms. The different properties of CB and MB resulted in different behavior of the zeolites at the airwater interface when spread from methanol suspensions. Methanol was chosen as a dispersion medium for the following reasons. First, methanol wets both hydrophilic and hydrophobic surfaces and thus can be used as a solvent for both zeolite Beta samples.27 Second, methoxy groups can be grafted on the zeolite surface and increase the particle hydrophobicity. Such an effect has been reported for methanol-washed Sto¨ber silica particles.28 LB films were prepared from the modified silica particles dispersed in methanol without any addition of surfactants. The authors concluded that methanol reacts not only with the surface hydroxyl groups but also with defect sites. Zeolite surface esterification via alcohol treatment resulting in products of high hydrophobicity has also been reported.29 The CB suspension in methanol was spread in the LB trough. However, no changes in the surface pressure were detected upon compression, indicating that zeolite Beta films were not formed at the air-water interface. Thus, the surface of CB was modified by physical adsorption of CTAB.30,31 The modified CB suspension was spread in the LB trough, and a typical pressure-area isotherm obtained upon compression is shown in Figure 2a. The CTAB treatment did not substantially alter the size or particles size distribution of CB according to DLS analysis (see Supporting Information). In addition, both CB and CTAB-modified CB methanol suspensions were stable for more than 6 months without any detection of particle sedimentation. The interaction between CTAB and CB was also confirmed by the Z-value measurements: -24.7 for CB and -10.7 for CTAB-CB. These values cannot be used to quantify the interaction because the measurements were performed after drying the zeolite, during which particle aggregation occurred. Films at the air-water interface were formed using the MB methanol suspension without any surfactant modification (Figure 2b). The higher collapse pressure for MB was indicative of the higher hydrophobicity of MB and the higher rigidity of the MB film as compared to CTAB-CB. The large volume needed to obtain this isotherm, 3000 µL, indicates that part of the sample sediments upon compression. However, sedimentation was minimized upon subsequent compressions. Experiments were also performed using MB suspensions of 3 and 5 wt % concentrations. The collapse pressure and compressibility decreased with an increase in the sample concentration (see Supporting Information). Role of Methanol. In situ ATR-IR measurements of the zeolite suspensions in methanol upon methanol evaporation and exposure to air were performed to study the role of methanol for the stabilization of the zeolite particles at the air-water interface. These measurements can be used for qualitative
Figure 3. ATR-IR spectra recorded upon in situ methanol evaporation and exposure to air of (a) CB, (b) CTAB-CB, and (c) MB suspensions in methanol. The spectrum of methanol (1) is included in all graphs for clarity, and the time of exposure is increasing in the order 2, 3, etc. The inset magnifies the spectral range of 950-1150 cm-1. The arrows in panel a show the additional absorption peaks due to H2O. The lines in the insets trace the position of the C-O stretching mode of methanol.
comparison only as the results are dependent on the suspension volume and concentration as well as the external conditions (e.g., temperature). Results for the three suspensions used, CB, CTABCB, and MB, are shown in Figure 3. For clarity, the ATR spectrum of methanol is included in each graph. The spectral range below 700 cm-1 could not be measured because of the limitation of the Ge ATR crystal. The spectra collected after the solvent evaporation contain features characteristic of a highly crystalline zeolite Beta: bands at about 782, 1060, 1170, and
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Figure 5. SEM micrographs of LB films prepared from MB. Figure 4. SEM micrographs of LB films prepared from CB.
1217 cm-1.32,33 First, the methanol-CB system was studied. The methanol bands together with weak zeolite Beta bands were detected in spectrum 2. The intensity of the former decreased in spectrum 3. At the same time, the shape of the O-H stretching band (spectral range of 3000-3500 cm-1) was changed due to additional IR absorption near 3400 cm-1, and an additional band near 1650 cm-1 that is typical for H-O-H bond bending vibrations appeared. Both spectral features indicate the formation of water.34 In spectrum 4, a small peak at 1033 cm-1 was still present; however, it is slightly shifted from the C-O methanol band at 1025 cm-1 (spectrum 1).35 We attribute the peak at 1033 cm-1 to -OCH3 groups attached to the zeolite Beta crystal surface via reaction with the terminal silanol groups: CH3OH + -OH f -OCH3 + H2O. The appearance of water is in agreement with the proposed reaction. The trans-
formation from the stage corresponding to spectrum 2 to stage 5 was very fast, 15 min), suggesting that the hydrophobic coating of the -OCH3 groups was stabilized. Thus, films were formed at the air-water interface using CTAB-CB. The zeolite crystals spread in the LB trough are not only exposed to air but also in contact with the water subphase. Methanol and water are fully miscible, which might also provide a reservoir of solvent in the neighborhood of the surface, additionally stabilizing the floating particles.36 In the case of MB, the peak at 1033 cm-1 was detected in the spectra recorded after >15 min air exposure, explaining the successful preparation of MB floating films at the air-water interface (Figure 3c). This may be related
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Figure 6. ATR-IR spectra of LB zeolite Beta films prepared from (a) MB and (b) CB.
to the fewer silanol groups present in MB due to its smaller external surface area and limited number of defects. Thus, the use of methanol as a solvent was sufficient to stabilize the MB particles at the air-water interface and allowed film formation. LB Zeolite Films. SEM micrographs of the LB zeolite Beta films prepared from CTAB-CB and MB methanol suspensions are shown in Figures 4 and 5, respectively. In both cases, the films were visually homogeneous and smooth, covering the whole wafer area available for film deposition (Figures 4a and 5a). The films prepared from CB were transparent, whereas the films from MB were whitish. The results were highly reproducible: films of similar quality were obtained by spreading fresh suspensions. It was also possible to prepare several consecutive samples of similar appearance by repeated expansion and compression of the floating films to the selected target pressure. The SEM images in Figures 4c and 5a show the edge of the corresponding LB films. It seems from these images that the films were generally monolayers. The films prepared from CB were more densely packed (Figure 4b) as compared to the ones from MB (Figure 5b), which can be attributed to the different morphology of the crystals. The smaller particle size and spherical morphology allowed a more compact arrangement of the CB crystals. In addition, the majority of the MB crystals constituting the LB film seemed attached to the support with their pinacoidal face (Figure 5b,c). It should be mentioned here that for the successful preparation of microcrystal zeolite LB films, a narrow crystal size distribution was essential. Thus, although it was possible to obtain floating films at the airwater interface from a zeolite Beta sample containing microcrystals of varying particle size (see Supporting Information), only isolated crystals were attached to the Si wafer upon dipping (not shown). The zeolite Beta structure of the MB LB films was confirmed by XRD (see Supporting Information). However, no XRD zeolite peaks could be detected by conventional XRD for the CB LB films, possibly because of the limited thickness of these films. For this reason, ATR-IR spectra for the films prepared from both samples were recorded and shown in Figure 6. The characteristic peaks of zeolite Beta discussed previously can be seen in both spectra. Finally, the LB technique can be used to prepare multilayered coatings. An example is provided in Figure 7, in which a layer of MB was deposited onto a layer of CB. The data reported indicate that the LB technique can successfully be applied to prepare zeolite films. This result is of particular importance for the preparation of LB films of micrometer-sized crystals. Whereas colloidal zeolites can be arranged on the substrates by less complicated methods such as spin-coating or layer-by-layer (LBL) deposition,1,7,20 the
Figure 7. SEM micrograph of a bilayered sample consisting of CB (bottom) and MB (top) LB films.
methods for the deposition of microcrystals are somewhat limited. To prove the superiority of the LB technique for preparing MB films, the LB films were compared to MB films prepared by spin-coating and the LBL method (see Supporting Information). The latter films were characterized by poor crystal density and random crystal distribution. Conclusion Zeolite Beta films were deposited on Si wafers by the LB method. Two zeolite Beta samples suspended in methanol were used. The 180 nm sample was characterized by a large external surface area and required additional surfactant modification prior to spreading in the LB trough. The 1 µm sample was synthesized in a fluoride medium and contained a limited number of framework defects. LB films were prepared from this sample without any modification. The zeolite suspensions in methanol were studied by ATR-IR. It was suggested that methanol stabilizes the zeolite particles at the air-water interface to a different extent. LB zeolite Beta monolayer films of high quality were prepared using both samples. The results from this work suggest that the LB technique can be used to prepare films of various zeolite-type structures provided that monosized crystals are available. Multilayer zeolite films of the same or different zeolite-type structures can also be prepared by the method. The prepared zeolite films can be used as seeded layers for the preparation of zeolite-supported films by secondary growth. The films are also of interest as hosts for supramolecular structures, for antireflective coatings, and for the immobilization of biomaterials. The high degree of ordering of the micrometersized crystals on the support allows for fundamental studies of the properties of the individual zeolite crystals. Acknowledgment. This work was supported by EPSRC (Grant EP/D50645X/1) and the Leverhulme Trust. Supporting Information Available: TG curves for CB and MB, DLS analysis of CB and CTAB-modified CB, pressurearea isotherms for MB suspensions of different concentrations, SEM micrographs of zeolite Beta sample unsuitable for LB film preparation as well as films prepared by spin-coating and LBL method, and XRD pattern of LB MB film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, Z.; Johnson, M. C.; Sun, M.; Ryan, E. T.; Earl, D. J.; Maichen, W.; Martin, J. I.; Li, S.; Lew, C. M.; Wang, J.; Deem, M. W.; Davis, M. E.; Yan, Y. Angew. Chem., Int. Ed. 2006, 45, 6329-6332.
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