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J. Phys. Chem. C 2008, 112, 14274–14280
High-Silica Zeolite-β: From Stable Colloidal Suspensions to Thin Films Johannes Kobler,† Hayim Abrevaya,‡ Svetlana Mintova,*,§ and Thomas Bein*,† Department of Chemistry and Biochemistry, Ludwig-Maximilians UniVersita¨t (LMU), Butenandtstrasse 5-13 (E), 81377 Munich, Germany, UOP-Honeywell, 25 East Algonquin Road, Des Plaines, Illinois 60017-5017, and Laboratoire de Mate´riaux a` Porosite´ Controˆle´e (LMPC), UniVersite´ de Haute Alsace, UMR-7016 CNRS, 3, rue Alfred Werner, 68093 Mulhouse, France ReceiVed: April 4, 2008; ReVised Manuscript ReceiVed: June 4, 2008
A comprehensive study including (i) the synthesis of colloidal zeolite-β nanoparticles with size below 100 nm from precursor suspensions, (ii) kinetics of the crystallization process of zeolite-β, (iii) stabilization of the β nanocrystals in coating suspensions, (iv) preparation of homogeneous thin films by a spin-coating approach, and (v) physicochemical characterizations of the films are the focus of this paper. It has been demonstrated that the crystallization process of β was completed within 7 days, and the final size and shape of the nanocrystals resemble the amorphous particles in the precursor suspensions. Next, the crystalline β particles were stabilized in colloidal suspensions under addition of inorganic silica-based binders and further applied for a spin-on process of films. It has been confirmed that the number of coating steps, speed of deposition, concentration of coating suspensions, and type of binders had a pronounced influence on the thickness, mechanical, and optical properties of the films. The refractive index (n) of the films prepared with binders is increased slightly due to introduction of amorphous silica, thus leading to the formation of denser and more mechanically stable zeolite-β layers. Introduction Zeolites and zeolite-like materials with their uniform pore sizes, unique structural topologies and composition, well-defined acidity, and good thermal stability are viable commercial catalysts, selective ion exchangers, and absorbers. Although many different crystalline molecular sieves are already known, there is a continuing need for zeolites with new properties and a continuing search for new zeolite applications. More recent efforts have explored the possibilities of synthesizing zeolites in the nanosize range by application of highly supersaturated precursor solutions.1,2 By using organic additives during the synthesis procedure, colloidal solutions were formed and used for the preparation of different nanosized molecular sieves.3–9 Several microporous materials including MFI, LTA, FAU, BEA, and LTL in the form of stable colloidal suspensions with narrow particle size distribution (100 nm) are described in the literature. Intriguingly, among the nanosized zeolites the pure silica forms were only reported for the MFI, MEL, and BEA structure types.10–12 BEA is a large-pore microporous material characterized by three sets of mutually perpendicular channels with 12-membered ring apertures, which can be prepared from classical aluminosilicate gels with a wide range of Si/Al ratios.13 The main features of this structure make the material suitable for numerous applications such as heterogeneous catalysis, sensors, membranes, etc.12,14–16 The number of acid sites and the hydrophobicity of the microporous materials can be tuned by controlling the Si/Al ratios in the final crystallites. One potential application of purly siliceous microporous zeolites exhibiting a high degree of hydrophobicity and a high micropore volume is in the field of nanoscale layers with a low * Corresponding authors. † Ludwig-Maximilians Universita ¨ t. ‡ UOP-Honeywell. § Universite ´ de Haute Alsace.
dielectric constant (low-k) for microelectronic applications, as well as for the preparation of hard coatings with unique properties.17,18 Pure silica zeolite BEA was synthesized by Barrett et al., using tetraethylammonium hydroxide (TEAOH) as a template and fluoride ions at near neutral pH.19 In addition, BEA crystals were recently prepared via a solid-solid transformation method,14,20 and the synthesis of colloidal siliceous BEA from highly templated precursor solutions was reported.12,21 One of the focused applications of the pure silica BEA-type molecular sieve is in the preparation of highly porous and hydrophobic layers, which can exhibit variable optical and electronic properties depending on the method of preparation. Several strategies have been applied for the formation of microporous films, and recently a very effective method was established for the deposition of ultrathin pure siliceous MFItype films on two-dimensional supports via spin-coating of stable colloidal suspensions.10 Due to their unique properties, the particles at the nanometer scale are of great interest in a number of advanced applications. Homogenously dispersed in a solvent, these colloidal suspensions are suitable for the preparation of thin homogeneous and transparent films by spin-coating. As a result of their crystallinity, the coatings have high mechanical stability and resistance to aggressive solvents. Additionally, decreasing the crystal size of particles below 100 nm produces substantial changes in the physicochemical properties of the zeolite, particularly in the external surface of the crystals, as well as the ability of such nanocrystals to pack closely in the film during spin-coating.22 The aim of this paper is to describe a complete route for the synthesis of nanoscale zeolite-β from templated precursor solutions with emphasis on the crystallization kinetics, stabilization of the nanosized crystallites in coating suspensions, and the assembly in thin films exhibiting tunable physicochemical properties.
10.1021/jp802922f CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
High-Silica Zeolite-β Experimental Section Synthesis of Nanosized Zeolite-β. The composition of the precursor solution used for synthesis of nanosized zeolite-β crystals was 25SiO2/1Al2O3/15TEAOH/375H2O. The initial compounds were mixed in one pot until a clear solution was obtained: Cab-o-sil M5 (Riedel-DeHa¨en) as silica source, aluminum 2-butoxide (Aldrich) as aluminum source, TEAOH (35 wt %, Aldrich) as organic template, and deionized water were used. The hydrothermal (HT) treatment of the precursor mixtures was performed in an oven at 140 °C under static conditions using steel autoclaves with 18 mL Teflon liners. Every 24 h one autoclave was taken out, and the product was purified by a series of centrifugation steps (1 h at 47.800g RCF) and subsequently redispersed in water under sonication (1 h in ice) until the pH of the solution reaches 10. A part of the final crystalline zeolite-β was dealuminated according to the procedure described in ref 23. The Si/Al ratio of the as-synthesized and dealuminated zeolite-β was 32 and 100, respectively. Stabilization of Nanosized β Crystals in Coating Suspensions. The as-prepared and dealuminated zeolite-β nanoparticles were treated with an aqueous solution of diluted ammonium chloride and then washed with ethanol two times in order to prepare stable zeolite suspensions and prevent fast agglomeration. After washing and redispersion, the colloidal suspensions were centrifuged at 12 000g (RCF) for 5 min, and the settlement was separated by decanting in order to eliminate the larger agglomerates formed during the purification process. The final solid concentration of zeolite-β nanoparticles was adjusted to 4 wt % in ethanolic suspension and further applied for the preparation of thin films. Three types of binders were used to improve the mechanical stability of the porous films: (1) an aqueous colloidal suspension of Nalco 2326 (SiO2, ∼15 wt %, particle size ∼5 nm) was diluted with ethanol to reach a concentration of 2 wt %; (2) prehydrolyzed tetraethylorthosilicate (TEOS) in a mixture of ethanol, water, and HCl in the ratio of 1TEOS/0.06HCl/27H2O/ 60EtOH, and (3) prehydrolyzed TEOS with cetyltrimethylammonium bromide (CTAB) in the following ratio 1TEOS/ 0.1CTAB/0.06HCl/27H2O/60EtOH. The size of the amorphous silica particles formed in TEOS-based binders (solutions 2 and 3) was kept constant, e.g., about 2 nm in diameter determined from dynamic light scattering (DLS) measurements. The silica nanoparticles (about 2 wt %) in the binding solutions 2 and 3 were stable for about 10 days, and then a slight increase in size was measured. Deposition of Thin β Films on Silicon Wafers. The ethanolic zeolite suspension was either used directly for the film deposition or it was mixed with the same volume of one of the three binders prior to spin-coating. No additional functionalization of the zeolite nanoparticles was carried out. In order to ensure the preparation of smooth and homogeneous films, the coating suspensions were filtrated through syringe filters (450 nm) prior to spinning. The nanoporous films were deposited on polished silicon wafers by spin-coating (Laurell WS-400B-6NPP-Lite-AS) with a spinning rate of 3000 rpm (acc: 5000 rpm s-1) for 30 s. All substrates were precleaned with ethanol and acetone for 10 s under spinning at 3000 rpm. In order to prepare thick β films, the spin-coating procedure was repeated up to six times. After each coating step, the films were calcined at 450 °C in air for about 30 min. From the numerous films prepared with variable thickness the following will be discussed in detail: pure β film free of binder (β-pure), β crystals stabilized with prehydrolyzed TEOS
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14275 TABLE 1: Zeolite-β Films Deposited on Silicon Wafers film
binder
β-pure β-TEOS β-SiO2
no TEOS (2) SiO2
deposition steps
thickness (nm)
refractive index
1 and 3 3 and 6 1 and 6
140-470 440-680 180-950
1.10 1.13 1.15
(β-TEOS), and β crystals stabilized with amorphous silica (βSiO2) (see Table 1). No significant differences were observed between the samples prepared with the two types of TEOS binders (solutions 2 and 3), and therefore only binder 2 will be discussed. Characterization. The degree of crystallinity of the zeolite materials was determined based on X-ray diffraction (XRD) patterns collected from purified powders with a STOE Stadi P diffractometer using monochromatic Cu KR radiation in transmission geometry. Additionally, the degree of crystallinity, size, and morphology of the zeolite particles was proven by collecting transmission electron micrographs (TEM) on a JEOL JEM 2011 instrument with a LaB6 cathode at 200 kV. Samples were prepared on Plano holey carbon coated copper grids by evaporating one droplet of the purified zeolite suspensions diluted with water in the ratio of 1:100.
Figure 1. XRD patterns of the β samples synthesized under hydrothermal conditions.
Figure 2. DLS curves of suspensions hydrothermally treated for 1, 3, 5, 7, and 10 days.
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Figure 3. TEM images of nanoparticles extracted from the suspensions hydrothermally treated from 3 to 10 days.
Besides, the evaluation of the particle size in the suspensions was determined by DLS. The DLS data were collected with an ALV-NIBS/HPPS high-performance particle sizer in PMMA cuvettes at 25 °C under continuous scans and analyzed according to the cumulant and distribution function analysis (DFA). The polydispersity index (PDI) calculated for all samples was used as an indication for a multimodal particle size distribution. The powder β samples were subjected to thermogravimetric (TG) analysis after purification and freeze-drying. The powders were heated in corundum
crucibles from 30 to 900 °C (10 °C/min) in a flow of synthetic air (25 mL/min) using a Netzsch STA 449 C Jupiter thermobalance. The amount of organic template and water in the framework-type β zeolite was determined in the samples prepared from 1 to 10 days of HT treatment. The thin films were investigated with scanning electron microscopy (SEM) (Philips 40 XL) and ellipsometry. The ellipsometric measurements were performed on a Woollam M2000D at different angles (65°, 70°, and 75°) in the entire spectral range of 190-1000 nm. The data were fitted in the range
High-Silica Zeolite-β
Figure 4. TG curves of the solid samples recovered from precursor suspensions subjected to hydrothermal treatment (HT) from 1 to 10 days.
Figure 5. XRD pattern of dealuminated zeolite-β coating suspension (Beta-pure) used for preparation of thin films in comparison with the simulated pattern.
between 250-1000 nm using the Cauchy model for layered materials or using the EMA layer (Bruggeman effective medium approximation) of SiO2 and introducing additional voids.24 Results and Discussion Suspensions of Nanosized β Crystals: Kinetic Study and Stabilization. In order to follow the crystallization kinetics of nanosized zeolite-β, the initial solution subjected to HT was interrupted at various times; the solid phases extracted at the different stages of crystallization were subjected to a comprehensive characterization. The change in the crystallinity of the solid products was followed with XRD, DLS, TEM, and TG. The XRD patterns depicted in Figure 1 show that the amorphous precursor solutions transformed into crystalline zeolite-β after 7 days, and the prolonged HT did not change the degree of crystallinity. The first Bragg reflection at 6.2° 2θ occurs with a very low intensity in the sample heated for 5 days. The extremely low intensity of the Bragg reflection is possibly due to the very small crystalline particles ( 0.5). This confirms the presence of unstable amorphous nanoparticles in the suspensions and the continuous dynamics of agglomeration and disintegration in the reacting aluminosilicate species in the crystallizing system, while after 5 days of heating, two generations of particles, one with an average hydrodynamic diameter of about 20-60 nm and another one with ∼80-200 nm, are detected in the precursor mixture (Figure 2). It is also observed that with increasing crystallization time, the width of the particle size distribution curves is diminishing and the mean size changes to smaller diameters, probably due to disaggregation and higher stability of the growing particles in the samples crystallized from 5 to 7 days. This observation can be explained with the higher stability of the growing particles in the samples crystallized from 5 to 7 days. However, the colloidal zeolites tend to agglomerate and at the same time disintegrate under purification of the reacting mixture and redispersion in water or ethanol under sonication. In the suspensions with a pH lower than 8 and a solid concentration above 6 wt %, a significant increase in the particle diameter is measured. On the basis of the DLS data, one can conclude that the aggregates are formed in the crystalline and intermediate suspensions, and they are measured instead of the individual particle grains. This statement is confirmed by the TEM study, where it can be seen that the nanoparticles tend to agglomerate and do not exist as separated crystals (Figure 3). The aggregation process is also accelerated with prolonging the time of HT (10 days), and the mean size of the particles is increased from 10 up to 40 nm according to the TEM data. At an earlier time (5-6 days) of HT treatment, separation of the crystalline from the amorphous particles was not possible (Figure 3). Crystalline fringes representing the β crystalline phase in almost all spherical amorphous particles can be seen in the samples treated for 5 and 6 days. The crystalline particles have the same size and shape as the amorphous ones. In many cases a good alignment between several crystalline domains in the particles is observed. This alignment is enhanced at the end of the HT. According to the TEM micrographs, particles with sizes in the range of 8-15 nm were present during the entire process of crystallization. However, after 5 days some of the particles contain crystalline fringes and some of them appear amorphous. After 6 days the fraction of crystalline particles is increased while the number of amorphous particles is decreased, which is confirmed by both the XRD and DLS data (see Figures 1 and 2). The sample heated for 7 days contains fully crystalline nanoparticles, which appear more disintegrated in comparison to the ones heated for 10 days. These β particles appear as single crystallites and have an almost spherical shape (Figure 3). A slight increase in the size of the individual particles and additional agglomeration are observed in the sample heated for 10 days due to the longer crystallization time. The transformation of the amorphous aluminosilicate species into crystalline zeolite-β was also followed by TG analysis. The amount of organic template and water is changing depending on the degree of the samples’ crystallinity. As can be seen from Figure 4, the weight loss due to desorption of water and combustion of TEAOH is decreased with increasing the degree of crystallinity of the β samples. The samples heated for 7 and 10 days have almost the same amount of template and an almost negligible amount of water, which is expected for highly siliceous molecular sieves. In contrast, in the less crystalline sample (5 days of HT treatment), the amount of water and TEAOH is higher than in the crystalline β. This can be explained
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Figure 6. DLS curves of zeolite coating suspension: β-pure expressed as unweighted and volume-weighted particle size distribution and picture representing the β-pure (right) and β-TEOS (left) coating suspensions.
Figure 7. Pictures of a bare silicon wafer and β-TEOS films prepared by one-, two-, and six-step depositions (from left to right).
Figure 8. SEM pictures of films deposited on silicon wafers from coating suspensions of β-pure (A) and β-SiO2 (B).
with extra amount of template adsorbed on the surface and in the mesopores of the aluminosilicate amorphous grains. The weight loss of the washed and dried products is decreased in the crystalline samples, and also the combustion of the template is shifted to higher temperatures. This is due to the different location of the template in the amorphous or crystalline matrix, i.e., the TEA cations are encapsulated in the zeolite channels instead of the surface or in the mesopores of amorphous nanoparticles. In conclusion, the amount of water and template encapsulated in the material is decreased, whereas the temperature of decomposition is increased in highly crystalline zeolite-β samples. β Nanocrystals Stabilized in Coating Suspensions and Films. The interfacial chemistry between the binders and the zeolite particles was studied. Dynamic light scattering measure-
ments were performed in pure zeolite suspensions as well as in binder-containing coating suspensions in order to study the stability prior to film deposition. The β crystals used for preparation of thin films were obtained according to the procedure described above (140 °C, 7 days); some amount of the purified sample was subjected to dealumination. As a high silica content of β crystals is required for low-k materials, the aluminum was partially removed from the BEA-type framework structure through leaching to increase the hydrophobicity. In order to weaken the acidity of the resultant coating suspensions, the particles were subsequently washed with ammonium chloride and then dispersed in ethanol to obtain homogeneous colloidal suspensions. The XRD pattern from the solid dealuminated sample synthesized in a large amount (500 mL, solid concentration of 6 wt %) confirms the presence of pure BEA-type zeolite
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J. Phys. Chem. C, Vol. 112, No. 37, 2008 14279
Figure 9. Refractive index for films prepared from coating suspensions of β-pure, β-TEOS, and β-SiO2 measured with ellipsometry (left) and experimental data for a thick β-TEOS film (680 nm) measured at different angles with the corresponding fitting (right).
(Figure 5). The DLS results of the same β-pure suspension reveal particles with a hydrodynamic diameter in the range of 20-80 nm (Figure 6), which are attributed to the zeolite-β as single crystals and probably as agglomerates in a comparable amount. This conclusion is based on the different presentation of the scattering intensity. The unweighted distribution is expressed as scattering intensity, which is strongly sizedependent, whereas the volume-weighted size distribution is pointing out that a considerable amount of the small-sized fraction exists in the samples. The volume-weighted scattering term represents the real size of the particles in the suspension containing particles with the same density, crystallinity, and chemical composition. The particle size distribution of this coating suspension does not change with time (at least 1 week), and this confirms the colloidal stability of β nanoparticles at ambient condition (see Figure 6, right images). The coating suspension was mixed with the three binders and stayed stable with no sedimentation during 1 week (Figure 6, right). The suspensions with all binders appear transparent with minor scattering, similar to the ones shown in Figure 6. The thin films prepared with and without binders appear homogeneous and transparent to the eye. In general, the thickness of the films is enlarged by increasing the number of deposition steps of the coating suspensions, resulting also in a change of the defined interference color of the films (Figure 7). By variation of the conditions of spin-coating, number of deposition steps, concentration of coating suspensions, and types of binders numerous films were deposited on silicon wafers with variable optical properties. The thickness of the films varied from 140 to 900 nm, and the refractive index from 1.1 to 1.2 depending on the porosity and degree of binder loading. In general, the films prepared with and without binders exhibit a smooth surface (Figure 8) and do not show big differences. Homogeneous and continuous films at a centimeter scale are obtained by one-to-six step spin-coatings, and the homogeneity of the deposited layer is preserved by the close packing of the β nanocrystals surrounded by the binding agents. The individual crystallites in the films prepared from β-SiO2 and β-TEOS suspensions are more closely packed in comparison with pure β suspensions. Besides, the β-pure film has lower adhesion in comparison to the films with added binder. An increased surface roughness of the multilayered films is observed as well. This can be explained by the higher surface roughness of the intermediate porous layers in comparison to the bare Si wafer which is propagated and amplified with each coating step. One can consider that the smoothness of the films could be improved by the application of smaller particles (Figure 8A). However, small particles tend to agglomerate faster in the coating suspensions than the bigger ones, and therefore the stabilization
of zeolite grains with a size less than 10 nm could be a great challenge. Another possibility is to use an additional top layer of binder to fill out all gaps between the crystalline particles deposited in β-pure films. However, the deposition of the top layer of amorphous silica will influence the surface chemistry and more specifically the hydrophilicity of the entire film, which will increase, whereas the porosity will decrease. Introducing porosity into thin films strongly influences the optical properties compared to dense ones prepared from the same material. The refractive index decreases with increasing the voids or porosity in the nanoporous material. For the ellipsometric measurements, a theoretic model was developed based on the assumption that the film consists of nanoparticles and that a certain fraction of voids are present. Thus, the Bruggeman effective medium approximation (EMA) of the films was assumed using an average of two or more sets of optical functions. The model used to describe the experimental ellipsometric data was designed as follows: base layer (0) is a silicon substrate with a thickness of 1 mm, layer (1) is a thermal oxide (SiO2) with a thickness of 2 nm, and layer (2) is a porous film described as a Cauchy layer. A very good correlation between the experimental and the modeling data for the films is shown in Figure 9. The thickness of the films determined by ellipsometry is in very good correspondence with the values determined with SEM (the thickness was measured in mechanical scratches made in the films). The refractive index (n) of the β films with and without binders was also determined (Figure 9, left), and the n-values at a wavelength of 589 nm were used for comparison of the optical properties of the films (Table 1). All films showed a very low refractive index, which is expected for porous high-silica materials. Although the films appear dense to the eye, the refractive index is in the range of 1.10-1.15. Similar results for the thickness and refractive index of the films were calculated by replacing the Cauchy layer with the EMA layer consisting of a SiO2 interlayer and additional voids. The mechanical properties of the films were investigated by tape tests according to ASTM D3359 standard. In all films except β-pure, the mechanical stability was high and the films were not removed from the silicon wafer and not even partially destroyed. However, the strongest adhesion was diagnosed in films prepared with amorphous SiO2 as binder. One can conclude that the efficiency of the binders mainly depends on the size of the individual particles. Small particles of SiO2 with a high external surface are able to form many bonds (Si-O-Si) between the zeolite crystals and the SiO2 from the silicon wafer, thus resulting in the formation of mechanically stable coatings.
14280 J. Phys. Chem. C, Vol. 112, No. 37, 2008 Conclusions Colloidal suspensions of zeolite-β nanocrystals with low aluminum content were prepared, and the crystallization kinetics were studied. The crystallization process was completed within 7 days, and the size and the shape of the first crystalline particles detected in the crystallizing suspensions resemble the ones in the amorphous suspensions. Under prolonged synthesis time, an agglomeration in the crystallizing suspensions is observed; however, the size of the crystalline zeolite-β particles did not exceed 80 nm. The crystalline β nanoparticles were stabilized in colloidal suspensions under addition of inorganic silica-based binders and applied for the preparation of films by a spin-on process. The thickness of the films was varied from 140 to 950 nm, controlled by repeating the number of coating steps, speed of deposition, and β concentration of the coating suspensions. The β films exhibit good mechanical properties, smooth surfaces, and show a low refractive index, which is typical for highly porous silicabased materials. The refractive index of the films prepared with binders is increased slightly due to additional amorphous silica introduced in the film which is leading to the formation of denser layers. The presented spin-coating approach allows the preparation of films with variable hydrophilicity, thickness, and mechanical stability on supports with different shapes and sizes. Acknowledgment. The authors gratefully acknowledge funding from UOP-Honeywell. References and Notes (1) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494–2513. (2) Mintova, S. Collect. Czech. Chem. Commun. 2003, 68, 2032–2054. (3) Holmberg, B. A.; Wang, H.; Yan, Y. Microporous Mesoporous Mater. 2004, 74, 189–198.
Kobler et al. (4) Zhu, G.; Qiu, S.; Yu, J.; Sakamoto, Y.; Xiao, F.; Xu, R.; Terasaki, O. Chem. Mater. 1998, 10, 1483–1486. (5) Kecht, J.; Mintova, S.; Bein, T. Chem. Mater. 2007, 19, 1203– 1205. (6) Larsen, S. C. J. Phys. Chem. C 2007, 111, 18464–18474. (7) Larlus, O.; Mintova, S.; Bein, T. Microporous Mesoporous Mater. 2006, 96, 405–412. (8) Hsu, C.-Y.; Chiang, A. S. T.; Selvin, R.; Thompson, R. W. J. Phys. Chem. B 2005, 109, 18804–18814. (9) Lew, C. M.; Li, Z.; Zones, S. I.; Sun, M.; Yan, Y. Microporous Mesoporous Mater. 2007, 105, 10–14. (10) (a) Bein, T; Mintova, S. PCT/EP01/12512, DE 100 52 075 8. (b) Li, Z. J.; Li, S.; Luo, H. M.; Yan, Y. S. AdV. Funct. Mater. 2004, 14, 1019– 1024. (11) Li, Z.; Lew, C. M.; Li, S.; Medina, D. I.; Yan, Y. J. Phys. Chem. B 2005, 109, 8652–8658. (12) Mintova, S.; Reinelt, M.; Metzger, T. H.; Senker, J.; Bein, T. Chem. Commun. 2003, 326–327. (13) Camblor, M. A.; Corma, A.; Valencia, S. Chem. Commun. 1996, 2365–2366. (14) Serrano, D. P.; Van Grieken, R.; Sanchez, P.; Sanz, R.; Rodriguez, L. Microporous Mesoporous Mater. 2001, 46, 35–46. (15) Wang, Z.; Mitra, A.; Wang, H.; Huang, L.; Yan, Y. AdV. Mater. 2001, 13, 1463–1466. (16) Mintova, S.; Bein, T. AdV. Mater. 2001, 13, 1880–1883. (17) Johnson, M.; Li, Z.; Wang, J.; Yan, Y. Thin Solid Films 2007, 515, 3164–3170. (18) Tatlier, M.; Demir, M.; Tokay, B.; Erdem-Senatalar, A.; KiwiMinsker, L. Microporous Mesoporous Mater. 2007, 101, 374–380. (19) Barrett, P. A.; Camblor, M. A.; Corma, A.; Jones, R. H.; Villaescusa, L. A. Chem. Mater. 1997, 9, 1713–1715. (20) Selvam, T.; Aresipathi, C.; Mabande, G. T. P.; Toufar, H.; Schwieger, W. J. Mater. Chem. 2005, 15, 2013–2019. (21) Schoeman, B. J.; Babouchkina, E.; Mintova, S.; Valtchev, V. P.; Sterte, J. J. Porous Mater. 2001, 8, 13–22. (22) Nery, J. G.; Hwang, S.-J.; Davis, M. E. Microporous Mesoporous Mater. 2002, 52, 19–28. (23) Krijnen, S.; Sanchez, P.; Jakobs, B. T. F.; van Hooff, J. H. C. Microporous Mesoporous Mater. 1999, 31, 163–173. (24) Bruggeman, D. A. G. Ann. Phys. (Berlin) 1935, 24, 665–679.
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