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Adsorption in Periodically Ordered Mesoporous Organosilica Materials Studied by in Situ Small-Angle X-ray Scattering and Small-Angle Neutron Scattering Simone Mascotto,† Dirk Wallacher,‡ Andreas Kuschel,§ Sebastian Polarz,§ Gerald A. Zickler, Andreas Timmann,^ and Bernd M. Smarsly*,†
Physical Chemistry Department, University of Giessen, 35392 Giessen, Germany, ‡Helmholtz Zentrum Berlin, atsstr. 10, D-78457 Glienicker Strasse 100, D-14109 Berlin, Germany, §University of Konstanz, Universit€ Konstanz, Germany, Institute of Mechanics, Montanuniversit€ at Leoben, Franz-Josef-Strasse 18, A- 8700 Leoben, Austria, and ^Hamburger Synchrotronstrahlungslabor (HASYLAB)/Deutsches Elektronen-Synchrotron (DESY), Notkestrasse 85, D-22603 Hamburg, Germany )
†
Received October 16, 2009. Revised Manuscript Received January 11, 2010 Modified periodically ordered mesoporous organosilica materials were prepared starting from a recently introduced type of sol-gel precursor, containing both organic moieties and hydrolyzable Si-OR groups. In order to thoroughly characterize the mesoporosity and its accessibility, different probe gases were used in conventional gas adsorption experiments. Furthermore, in situ small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were applied to study the mesoporosity and the sorption processes, taking advantage of scattering contrast matching conditions. Thereby, the materials were characterized not only by different probe molecules but also at different temperatures (nitrogen at 77 K, dibromomethane at 290 K and perfluoropentane at 276 K). The comparison between the standard and in situ SAXS/SANS adsorption experiments revealed valuable information about the porosity and microstructure of the materials. It is demonstrated that the organic moieties are homogeneously distributed; that is, they do not phase-separate from silica on the nanometer scale.
Introduction In the last years, progress in the development of inorganic mesoporous materials established systems characterized by well-defined pore architecture and large specific surface areas in the order of 700-1000 m2 g-1. Such inorganic materials with a nanoscale structural setup and large specific surfaces may play important roles in sorption, catalysis, photoconversion, or separation processes. Nevertheless, conventional silica materials are limited in their range of functional surface properties. Therefore, the recent discovery of porous organic-inorganic hybrid systems characterized by high accessible surface area has gained a lot of attention in the scientific community.1 These novel periodically ordered mesoporous organosilica (PMO) materials were reported for the first time in 1999.2-4 PMO materials can be prepared in a similar way as ordered mesoporous silica materials such as MCM-415 and SBA-156 using an organic lyotropic phase as template. However, instead of standard sol-gel precursors, special silsesquioxane compounds characterized by an organic entity bridging between two alkoxysilane functions [(R0 O)3Si-R-Si(OR0 )3] are used as building blocks for the porous network. The main idea behind the growing interest in PMO materials is the possibility to incorporate organic moieties for *To whom correspondence should be addressed. E-mail: bernd.smarsly@ phys.chemie.uni-giessen.de Telephone: þ49 641 99 34590. Fax: þ49 641 99 34509. (1) Chujo, Y. Curr. Opin. Solid State Mater. Sci. 1996, 1, 806–811. (2) Asefa, T.; MacLachan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867–871. (3) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611–9614. (4) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304–307. (5) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (6) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552.
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a wide range of potential purposes.7-9 The chemical variance of functional groups which could already be established in the PMO field was rather restricted until very recently.7 A new family of PMO materials was reported by Polarz and Kuschel, in which the modification of the pore surfaces with nearly any desirable functional group becomes possible.10,11 In particular, the attraction toward separation is enhanced by the chance to have a material possessing an enantioselective surface, being able to allocate a chemical compound in a pure enantiomeric form.12-14 In fact, the possibility for obtaining stereochemically pure and specified molecules (e.g., amino acids) is considered to be a challenge of crucial importance. This functionality can be obtained, for example, by anchoring of chiral building blocks into organic moieties of the PMO materials.13 It was previously reported about a novel PMO material (UKON2a) containing a bridging benzoic acid function along the pore walls.10 The benzoic acid groups in the pore walls are accessible for chemical modification. The mesoporous organosilica UKON2a was treated with H2N-AlaOMe (Ala = alanine) to give the material denoted UKON3a or with the “dipeptide” H2N-Ala-Asp(OMe)2 (Asp = asparagine) to give UKON3b (see Scheme 1). (7) Hoffmann, F.; Cornelius, M.; Morell, J.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45, 3216–3251. (8) Fukuoka, A.; Sakamoto, Y.; Guan, S.; Inagaki, S.; Sugimoto, N.; Fukushima, Y.; Hirahara, K.; Iijima, S.; Ichikawa, M. J. Am. Chem. Soc. 2001, 123, 3373– 3374. (9) Zhang, L.; Zhang, W.; Shi, J.; Hua, Z.; Li, Y.; Yan, J. Chem. Commun. 2003, 210–211. (10) Kuschel, A.; Polarz, S. Adv. Funct. Mater. 2008, 18, 1272–1280. (11) Polarz, S.; Kuschel, A. Chem.;Eur. J. 2008, 14, 9816–9829. (12) Kuschel, A.; Sievers, H.; Polarz, S. Angew. Chem., Int. Ed. 2008, 49, 9513–9517. (13) Alvaro, M.; Benitez, D.; Das, D.; Ferrer, B.; Garcı´ a, H. Chem. Mater. 2004, 16, 2222–2228. (14) Polarz, S.; Kuschel, A. Adv. Mater. 2006, 18, 1206–1209.
Published on Web 03/04/2010
DOI: 10.1021/la903934r
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Article Scheme 1. Schematical Representation of the Post-synthetic Modification of the PMO Material UKON2a Constructed from Benzoic Acid Groups with Alanine, Resulting in UKON3a, or with a Alanine-Asparagine Dipeptide Giving UKON3b. The asterisks evidence the chiral carbon atom.
Although PMO materials can be obtained with many different pore structures, their porosity has not been investigated as deeply as the pure SiO2-based mesoporous architectures. While the porous topology can be identical to those of SiO2 materials, several differences are expected regarding the presence of organic moieties. As a matter of fact, the presence of the bridging, flexible, nonhydrolyzable organic group in the silsesquioxane precursor during the condensation in the sol-gel reaction cannot lead to a densely packed SiO2 glassy structure. Thus, the PMO matrix is expected to own a certain amount of microporosity. Attempts to explain the microporosity in the organosilica materials were already done by Bao et al.15 They referred to it as a consequence of the templating reaction of the surfactant, but the organic moieties constituting the hybrid material were not taken into account. Structural investigation of PMO materials especially regarding the displacement of the organic groups has not been carried out in detail so far. In fact, until now it was assumed, without any direct demonstration, that during the sol-gel reaction an ideal mixing of the silsesquioxane precursors occurs, leading to a homogeneous distribution of the organic moieties in the final material (Scheme 2A). However, different factors might be responsible for an inhomogeneous distribution. Since the organic groups and the hydrophilic siliceous groups are mutually incompatible to hydrolysis, domains (size of a few nanometers only) of several organic groups would form (Scheme 2B). In another scenario (Scheme 2C), it would be possible that the organic groups preferentially arrange around the micellar template because of their dislike for silica. Evidently, standard analysis methodologies applied in the case of mesoporous materials such as transmission electron microscopy (TEM), physisorption using nitrogen and small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) are not sufficient for a profound understanding of the organosilicas’ structure. In particular, the investigation of porosity through physisorption using gases such as nitrogen has to be considered as insufficient because of several aspects. First, nitrogen sorption measurements are usually performed at a temperature of 77 K. PMO materials possess a quite flexible structure due to the presence of organic functionalities, and therefore, a certain amount of micropores is expected. The low temperature conditions of the analysis can contract the material structure closing these cracks. In addition, such physisorption experiments suffer from the fact that only accessible pores are detectable. In this way, a post synthetic grafting with further organic functionalities can lead to a partial obstruction of the pore network affecting the consistence of porosity. The determination of the pore size distributions for PMO materials strongly depends on the probe gas. For this reason theoretical (15) Bao, X.; Zhao, X. S.; Li, X.; Li, J. Appl. Surf. Sci. 2004, 237, 380–386. (16) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373–380.
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models such as the theory of Barret, Joyner and Halenda (BJH)16 or the nonlocal density functional theory (NLDFT),17 usually applied for the pore size determination of pure silica materials, cannot be considered to be accurate. From this point of view, it is evident that the use of a novel approach, especially in the study of porous features in organosilica materials, is necessary. Recently, the coupling of a standard sorption experiment with small-angle scattering technique was proven as an elegant tool in the understanding of the pore architecture in silica materials, extending standard sorption methods.18-22 This socalled in situ SAXS/SANS approach is based on the accomplishment of X-ray and neutron scattering curves, respectively, during physisorption experiments. In principle, a particular adsorptive has to be chosen in order to ensure the same electron density (Fe) or scattering length density (Fb) (Table 1), in the case of X-rays or neutrons, respectively, as the silica matrix, thus fulfilling the contrast matching principle. For the sake of clarity, from here the term “density” will be used for both the electron density and scattering length density. In general, for a two-phase system, the SAXS/SANS intensity is proportional to the square of the difference in density of voids and silica/adsorbate. Thus, upon gradual filling of the pores during an in situ SAXS/SANS physisorption experiment, only unfilled pores contribute to the scattering signal.18-25 Therefore, if performed at several pressures during adsorption and desorption, the SAXS/SANS curves provide information on both the sorption mechanisms and the pore system. In the present work, in situ SAXS/SANS were used for the characterization of the porosity and the structure of PMO materials. For these purposes, two different adsorbates, namely, dibromomethane (CH2Br2) and n-perfluoropentane (C5F12), were chosen, which fulfill the principle of the contrast matching for X-rays and neutrons, respectively. The possibility of using organic fluids for performing physisorption is of fundamental importance in the study of PMO materials for several reasons. (A) Since these organosilicas, possessing a worm-like mesopore structure (Figure 1) and enantiomeric functionalities, are particularly suited for separation application (e.g., chiral amino acids resolution), it is worthy to study their sorption properties by using organic adsorbates at ambient temperature, avoiding possible structural changes at 77 K (nitrogen physisorption). (B) Through the in situ scattering experiments, the structure of the material is directly probed at each sorption step. Thus, the presence of inaccessible pores can be detected. Using different organic adsorbates, structural singularities in the PMO pore architecture can be detected. In this sense, the different molecular size of CH2Br2 in comparison with C5F12 (0.36 and 1.08 nm (see Table 1), respectively, magnitudes calculated through the method presented by Lilov26) leads to a better understanding of the pore connectivity toward different fluids. (17) Ravikovitch, P. I.; Haller, G. L.; Neimark, A. V. Adv. Colloid Interface Sci. 1998, 77, 203–226. (18) Smarsly, B.; G€oltner, C.; Antonietti, M.; Ruland, W.; Hoinkis, E. J. Phys. Chem. B 2001, 105, 831–840. (19) Sel, O.; Brandt, A.; Wallacher, D.; Thommes, M.; Smarsly, B. Langmuir 2007, 23, 4724–4727. (20) Mascotto, S.; Wallacher, D.; Brandt, A.; Hauss, T.; Thommes, M.; Zickler, G. A.; Funari, S. S.; Timmann, A.; Smarsly, B. M. Langmuir 2009, 25, 12670– 12681. (21) Zickler, G. A.; J€ahnert, S.; Wagermaier, W.; Funari, S. S.; Findenegg, G. H.; Paris, O. Phys. Rev. B 2006, 73, 184109. (22) Hofmann, T.; Wallacher, D.; Huber, P.; Birringer, R.; Knorr, K.; Schreiber, A.; Findenegg, G. H. Phys. Rev. B 2005, 72, 064122. (23) Hoinkis, E. Part. Part. Syst. Charact. 2004, 21, 80–100. (24) Zickler, G. A.; J€ahnert, S.; Funari, S. S.; Findenegg, G. H.; Paris, O. J. Appl. Crystallogr. 2007, 40, 522–526. (25) Smarsly, B.; Groenewolt, M.; Antonietti, M. Prog. Colloid Polym. Sci. 2005, 130, 105–113. (26) Lilov, S. K. Cryst. Res. Technol. 1986, 21, 1299–1302.
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Scheme 2. Illustration of the Different Possible Scenarios for the Potential Distribution of the Organic Groups (Black) in PMO Materials: (A) Homogeneous Distribution Throughout the Matrix; (B) Inhomogeneous Distribution in the Silica Matrix (Light Gray), i.e., Local Enrichment (Black Objects) of the Organic Groups in the Matrix Due to Microphase Separation; (C) Preferential Enrichment or Derichment at the Pore Surface (the Schematic Is Idealized)
Table 1. Principle Features of the Adsorptives Used in This Work adsorptive CH2Br2 C5F12 N2
molecule diameter (nm)
temperature (K)
0.36 1.08 0.34
290 276 77
Fb / Fe (nm-2) -3
1.86 10 3.55 10-4 3.22 10-4
Fb /Fe (nm-2) for silica -3
1.89 10 3.43 10-4 3.43 10-4
source X-ray neutrons neutrons
(siliceous) and organic parts are detectable. In this way, if the material is composed by a phase-separated system, the organic groups will arrange themselves, forming several domains (Scheme 2B), which should be distinguished in the scattering patterns. On the contrary, by a homogeneous distribution (Scheme 2A), the density differences are smeared and will not contribute to distinct scattering changes.
Experimental Section 1. Materials. The synthesis is described in ref 12. 2. Ex Situ Physisorption Experiments. The ex situ gas
Figure 1. TEM micrograph of the sample UKON3a.
(C) The analysis of PMO materials with the in situ SAXS/SANS method is also important from a theoretical point of view. As presented before, standard sorption models are not suitable for a reliable interpretation of the porosity features of the material due to the unknown interaction of the adsorbate with the adsorbent. Quantitative analyses of the scattering curves are expected to provide valuable structural information.18-24 By means of the analytical method of the chord-length distribution, structural parameters such as pore size, pore volume, and surface area can be obtained for materials without very high degrees of mesostructural order. In this way, such analysis is an effective tool to validate sorption theories for non siliceous porous materials.18,20,27 (D) The use of adsorbates which allow contrast matching conditions with the siliceous part of the organosilica matrix is of fundamental importance in the understanding of the PMO organic moiety displacement. By the analysis of the scattering curves at the filled state, in which no voids contribute to the scattering intensity, only the density differences between the inorganic (27) Smarsly, B.; Antonietti, M.; Wolff, T. J. Chem. Phys. 2002, 116, 2618–2627.
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adsorption measurements have been performed in an automated gas adsorption station (Autosorb-1-MP, Quantachrome Corporation, Boynton Beach, FL). The device is dedicated to standard characterization measurements of nanostructured matter by nitrogen sorption isotherms at 77 K. The instrument data reduction sofware supports the standard data reduction algorithms such as Brunauer-Emmett-Teller (BET), BJH, and so on as well as the newest DFT kernels for typical pore geometries. The system is equipped with a vapor sorption option consisting of a heated manifold and vapor generator, which allows volumetric vapor sorption measurements at temperatures up to 50 C. For the gas adsorption measurements, the samples have been filled in standard glass cuvettes and have been stabilized on the measurement temperature in the liquid nitrogen filled standard dewar at 77 K for nitrogen sorption and in a water thermostatted bath at 290 and 276 K for the dibromomethane and perfluoropentane investigations, respectively. The isotherms have been measured up to 0.95 of the equillibrium vapor pressures p of the different adsorptives (N2, 1013 mbar; CH2Br2, 36 mbar; C5F12, 330 mbar). One measurement took about 5 h for nitrogen and perfluoropentane and about 15 h in the case of the dibromomethane. The pore volume, which could be filled with the specific condensate, has been derived from the adsorbed amount at p/p = 0.95 and the corresponding liquid density of the adsorbate at the given temperature; see Table 2. 3. In Situ SANS/SAXS. The in situ SANS experiment was performed at the small angle scattering instrument V4, which is placed in the cold neutron guide (λ = 0.6 nm) of the Helmholtz Zentrum Berlin. By varying the angle of the sample-detector distance, the scattering intensity was collected in the range of 3.34 10-3 nm-1 < s < 0.52 nm-1. The scattering vector s is defined as s = (2/λ)(sin θ) with λ being the wavelength and 2θ the scattering angle. The relative pressure of C5F12 p/p (where p is DOI: 10.1021/la903934r
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Table 2. BET Surface Area SBET, Pore Volume Vp, Volume Fractions u and Average Mesopore Size of the UKON3a and UKON3b PMO Materials, Using Nitrogen Physisorption at T = 77 K, Calculated by Means of the NLDFT Model for Cylindrical Pores (Adsorption Branch)a sample
SBET (m2/g)
Vp N2 (cm3/g)
Vp CH2Br2 (cm3/g)
Vp C5F12 (cm3/g)
j N2
average pore size (nm)
UKON3a 300 0.22 0.15 0.18 0.33 UKON3b 242 0.17 0.14 0.28 a Pore volume values for CH2Br2 and C5F12 are obtained by the adsorbed volume values of the isothermes at p/p = 0.95.
the saturation pressure at 276 K) was controlled during the experiment. Before performing each scattering measurement, the sample was filled with a certain amount (n/n) of adsorptive by using an appropriate gas adsorption sample environment (CGA-PT) which allows a direct in situ measurement of an entire p-V isotherm. The PMO material was situated in a flat copper cell with quartz windows, which was connected to a gas manifold by a capillary. The sample cell was mounted on the coldfinger of a closed cycle refrigerator and stabilized to 276 ( 0.004 K with a temperature controller. For the in situ SAXS measurements, a special custom-made apparatus was designed. For further details of the setup, see refs 21 and 24. The sample powder was carefully pressed to stable pellet of 3 mm diameter and 0.5 mm thickness. Before starting a sorption experiment, the sorption cell was evacuated at a pressure below 10-3 mbar at a temperature of 353 K for 1 h. For sorption experiments, the cell was thermostatted to be the coldest point in the system, while the liquid reservoir stayed at ambient temperature of 298 K. The sorption cell was remotely controlled by a custom-written software program, which allowed continuous adsorption and desorption scans. The in situ SAXS measurements were performed at the beamline BW428 at the Hamburger Synchrotronstrahlungslabor (HASYLAB)/Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. The synchrotron radiation was monochromatized by a double crystal monochromator to an energy E of 8.989 keV, focused by two mirrors (vertical and horizontal), and the cross section of the beam was defined by aperture slits to 0.5 0.5 mm2 at the sample position. For the detection of the scattered photons, a charge-coupled device (CCD) X-ray area detector (marCCD 165, marUSA, Evanston, IL) with a resolution of 2048 2048 pixels (pixel size: 79.1 79.1 μm2) was used. The precise sample-to-detector distance was determined by calibration with a standard sample of silver behenate.29 A total range of the scattering vector s of 0.03 nm-1 < s < 0.45 nm-1 was covered. In order to avoid air scattering, a vacuum flight tube was inserted between the sample and the detector. An exposure time of 60 s yielded a scattering pattern with excellent counting statistics. The scattering patterns were corrected for background scattering, electronic noise, transmission, and polarization by using the data reduction software program FIT2D.30 All specimens showed isotropic scattering patterns, which were azimuthally averaged for equal radial distances from the central beam. The SANS and SAXS patterns were then analyzed by means of the chord length distribution (CLD) method (see theoretical part). This approach can be applied just in the case of ideal two-phase systems. In order to respect this condition by subtracting of a constant background, the Porod law31 was fulfilled (s-4 asymptote at high values of s). The experimental uncertainties in the Porod regime can be due to data noise and three-dimensional fluctuations of scattering length density. After this treatment, the corrected SANS and SAXS data were evaluated by calculating the CLD in a parametrized form.27 (28) Roth, S. V.; D€ohrmann, R.; Dommach, M.; Kuhlmann, M.; Kr€oger, I.; Gehrke, R.; Walter, H.; Schroer, C.; Lengeler, B.; M€uller-Buschbaum, P. Rev. Sci. Instrum. 2006, 77, 085106. (29) Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y. J. Appl. Crystallogr. 1993, 26, 180–184. (30) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; H€ausermann, D. High Pressure Res. 1996, 14, 235–248. (31) Porod, G. In Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982; pp 17-51.
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Figure 2. (a) Nitrogen physisorption isotherms of UKON3a and UKON3a. (b) Pore size distribution of UKON3a and UKON3b calculated from the adsorption branch by means of the NLDFT method on the basis of a cylindrical pore model for the system nitrogen (77.4 K)/silica.
Results and Discussion 1. Ex Situ Physisorption with Nitrogen. The materials analyzed in this study are two PMO materials, namely, UKON3a and UKON3b. In the following, diverse physisorption and in situ SAXS/SANS experiments are presented to address the porosity and also the structure of the matrix regarding the distribution of the organic moieties. The hysteresis loops of the nitrogen physisorption isotherms (Figure 2a) are classified as H2 type according to the IUPAC classification,32 characteristic for mesoporous systems with relatively broad distributions of pore size and shape. This interpretation is confirmed by the pore size distribution analysis (PSD) achieved by means of the NLDFT method, assuming cylindrical mesopores. Note that for the NLDFT model a pure SiO2 surface is assumed, because so far no suitable kernels are available for the PMO studied in the present work. As seen in Figure 2, the PSD plot shows a main maximum centered at about 4 nm for both samples; furthermore, UKON3a possesses a small fraction of bigger mesopores of about 10 nm size. It is important to underline (32) Sing, K. S. W. Pure Appl. Chem. 1982, 54(11), 2201–2218.
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Figure 3. SAXS pattern of UKON3b.
the fact that no micropores were suggested by nitrogen sorption analysis. Other relevant porous features such as BET surface area SBET and pore volume Vp are summarized in Table 2 and compared with the results using different gases (see below). 2. Ex Situ SAXS Analysis. Small-angle scattering constitutes a fundamental method for the structural analysis of porous materials. Since both materials under examination possess very similar structural conformation, in Figure 3 only the scattering pattern of UKON3b is presented. The scattering curve indeed is attributable to a mesoporous organization with local order and a scattering maximum centered at s = 0.15 nm-1, corresponding to an average pore-to-pore distance of ca. 6-7 nm. Its width indicates a significant polydispersity of the cavities’ sizes and their mutual distance. The analysis of SAXS data is in good agreement with the nitrogen sorption analysis. 3. Ex situ Physisorption Using CH2Br2 and C5F12. The PMO materials UKON3a and UKON3b were both investigated by in situ SAXS during sorption of CH2Br2 at 290 K, while the in situ SANS experiment with C5F12 at 276 K was performed on UKON3a. In addition to the in situ SAXS/SANS measurements, high-precision ex situ physisorption experiments using these organic probe gases were performed. The CH2Br2 isotherms for UKON3a and UKON3b at 290 K (Figure 4a) look quite similar showing a pronounced hysteresis. As expected, the use of a different adsorptive and higher temperature than in nitrogen physisorption experiments (T = 77 K) affects the shape and the position of the isotherm, especially the hysteresis; that is, the lower closure point of hysteresis is p/p ∼ 0.4 for N2 (Figure 2a) and p/p ∼ 0.17 for CH2Br2 (Figure 4a).33 The sample UKON3a was additionally investigated by C5F12 physisorption at 276 K (Figure 4b). As a main feature, the isotherm shows a very narrow hysteresis loop at this temperature. Already the bare ex situ physisorption data reveal interesting differences between the different probe gases. In particular, for UKON3a, the use of C5F12 shows a similar mesoporosity, expressed in terms of pore volume, as CH2Br2, determined as the adsorbed volume at a relative pressure close to p/p = 1 (Table 2). Hindered adsorption of C5F12 due to its larger molecular size can therefore be excluded, since the mesopores constitute a through-connected network and are not connected through small necks or micropores. (33) (a) Page, K. S.; Monson, P. A. Phys. Rev. E 1996, 54, 6557–6564. (b) Thommes, M. In Nanoporous Materials: Science and Engineering; Lu, G. Q., Zhao, X. S., Eds.; Imperial College Press: London, UK, 2004; pp 317-364.
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Figure 4. (a) CH2Br2 physisorption isotherms for UKON3a and UKON3b at 290 K. (b) C5F12 physisorption isotherm of UKON3a at 276 K.
4. In Situ SAXS during Physisorption of CH2Br2. The in situ SAXS measurements of sample UKON3a are presented in Figure 5. A prominent feature of the in situ SAXS curves is the decrease in intensity of the main scattering maximum (Bragg maximum) upon increase in relative pressure of the probe gas (Figure 5a). By such in situ physisorption SAXS experiments, important features of the pore architecture are revealed. A first significant aspect is the constant shifting of the peak to smaller scattering values (bigger values of pore-to-pore distance) for the initial adsorption steps, that is, until p/p ∼ 0.3. This shift apparently corresponds to an increase in the average pore-topore distance between empty mesopores as a consequence of the fact that at a certain pressure all pores below a certain size are filled. Since only the empty pores contribute to the scattering, the average distance between the remaining empty pores increases, assuming that the size of individual mesopores does not correlate with their position. Parallel to this SAXS feature, the intensity at smaller scattering vectors (0.05 nm-1 < s < 0.1 nm-1) rises up. This finding is a consequence of the broad size distribution of the mesopores, as described and interpreted previously.18 By further increasing the pressure of CH2Br2, the SAXS maximum becomes lower in intensity and very broad (p/p = 0.34). The remaining pores get filled, and the SAXS intensity decreases significantly in the filled state. This variation in SAXS features is analyzed more quantitatively in the subsequent chapters. The scattering intensity almost vanishes when the mesopores are filled. A small hump at the Bragg peak position can still be recognized even at p/p = 1, probably because a small fraction of mesopores is still void and was not accessed by CH2Br2. As demonstrated in previous DOI: 10.1021/la903934r
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Figure 5. Typical series of in situ SAXS pattern for adsorption and desorption of CH2Br2 in UKON3a (a, c) and in UKON3b (b, d). The values given in the figure legend refer to the relative gas pressure p/p.
studies,19 this effect could also be interpreted as an imperfect contrast matching condition between the average electron density of CH2Br2 and PMO material, which was a priori assumed to be as the silica one. However, the substantial decrease of the SAXS intensity for p/p = 1, reaching almost a constant background scattering, indicates almost perfect contrast matching, which is reasonable taking into account that the SAXS intensity is dominated by the heaviest elements of a sample (Si in the present case.). Thus, these in situ SAXS measurements can be indeed used for further insights into the mesopore structure, based on a Porod analysis. Another important aspect deduced from the in situ the SAXS curves is the absence of micropores in the PMO matrix. The scattering intensity is very weak at s>0.35 nm-1 and does not change during adsorption. If micropores were present, a significant change in the SAXS signal would occur between the evacuated state and higher pressures. The scattering intensity observed at larger values of s thus is attributable only to density fluctuations of the constituting matter, creating a constant SAXS background. Since the micropores can be expected to generate SAXS intensity for s > 0.4 nm-1,20 it can be excluded that the materials contain a significant amount of micropores. More importantly, as pointed out in previous studies,19,20 if micropores existed one should observe an increase of the peak’s intensity for the first adsorption steps (p/p = 0 - 0.1) due to the enhanced contrast between the voids and the matrix, which is not the case for the present materials. The adsorption behavior of UKON3b (Figure 5b) presents the same characteristics as those of UKON3a, especially that one can conclude that all mesopores are totally accessible as seen by the almost total contrast matching. Furthermore, also desorption of CH2Br2 from the PMO structures was investigated by in situ SAXS. Figure 5c, d shows desorption in situ SAXS patterns of the samples UKON3a and UKON3b. As expected, a constant increase in SAXS scattering intensity is observed. In both samples, decreasing the pressure of 6588 DOI: 10.1021/la903934r
CH2Br2 shifts the main scattering peak maximum to larger scattering vector values. These experiments prove that the adsorption and desorption proceed reversibly. 5. In Situ SANS during Physisorption of C5F12. Physisorption in combination with in situ SANS was also performed using C5F12 on the sample UKON3a at 276 K. The SANS curves for adsorption and desorption plots are presented in Figure 6. The possibility to perform this kind of experiment with an adsorptive such as C5F12, possessing a molecular size three times bigger than that of CH2Br2, was expected to provide additional insights into the PMO structure, since hindered sorption would occur, if the mesopores were connected through small micropores. Figure 6a clearly shows that the adsorption behavior looks rather different from CH2Br2. According to the ex situ physisorption measurements, beyond p/p ∼ 0.35 no significant uptake was observed, indicating that all of the mesopores were filled. However, the in situ SANS patterns reveal a pronounced scattering maximum even beyond p/p = 0.37. Since the ex situ physisorption experiment showed an almost identical overall mesopore volume, this remaining signal has to be attributed to mismatch of the scattering length density for C5F12. In fact, assuming that the organic part of the material has the same mass density of graphite (F = 2.2 g/cm3), the corresponding scattering length density will be FbC = 7.33 10-4 nm-2 which is significantly different from the C5F12 and silica one (FbSiO2/C5F12 ≈ 3.5 10-4 nm-2 (Table 1). On the contrary, this difference is not as relevant in the case of X-ray scattering, where the electron densities of carbon and silica are not so different (FeC = 1.87 10-3 nm-2 FeSiO2 = 1.89 10-3 nm-2), thus explaining the almost perfect contrast matching for CH2Br2. 6. Chord Length Distribution Analysis. In order to analyze the sorption process in PMO materials, a quantitative analysis of the scattering curves is required. Since the systems presented here possess a mesoporous structure with only local order, the SAXS curves can be suitably analyzed by means of the methodology of the chord length distribution (CLD),18,20,27 which was previously Langmuir 2010, 26(9), 6583–6592
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Figure 6. In situ SANS patterns for adsorption (a) and desorption (b) of C5F12 in UKON3a.
Figure 7. CLD plots of CH2Br2 adsorption in UKON3a (a, c) and in UKON3b (b, d).
introduced by Mering and Tchoubar.34 This evaluation method for small-angle scattering data provides reliable information on the structural and topological details of general two-phase systems with sharp or diffuse phase boundaries. Since only for CH2Br2 contrast matching was observed, the calculation of the CLD is only meaningful for these in situ SAXS data, because the CLD has direct physical meaning only for ideal two-phase systems (i.e., the material is composed of two species of different, average density, in this case the silica/organic hybrid and the voids). The autocorrelation function of the scattering intensity is related to the CLD by the relation: gðrÞ ¼ lp γ00 ðrÞ, r > 0 where r is the distance, lp is the averaged chord length (Porod length) of the system, and lp is also the first momentum of g(r). Figure 7 presents the CLD plot for the samples UKON3a and UKON3b during adsorption of CH2Br2. Interestingly, the value of g(0) in the CLDs of the two samples (Figure 7a and c) falls to zero for p/p = 0.2. Generally, a positive value of g(0) is caused by small micropores and angularity of the pore system.27,35 The very small value of g(0) indicates in this case (34) Mering, J.; Tchoubar, C. J. Appl. Crystallogr. 1968, 1, 153–165. (35) Gille, W. Eur. Phys J. B 2000, 17, 371–383.
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the presence of a small degree of a certain angularity, most properly, from the irregular pore surface due to the grafting of the organic functionalities. The presence of micropores can indeed be excluded, since it is observed neither in N2 sorption analyses nor in the scattering patterns. The analysis of the morphological changes in the UKON3a and UKON3b materials during the sorption experiment can be visualized by r*g(r) plots in Figure 7b and d, respectively. The r*g(r) plot reveals the most prominent pore size of the system, which is in good agreement with nitrogen sorption analysis (about 4 nm). The following minimum and maximum at 8 and 12 nm correspond to the pore-to-pore distance and the chord penetrating two interfaces, thus corresponding to “two pores and pore wall”, respectively.20 For sample UKON3a, the CLD illustrates nicely that the pore-to-pore distance (e.g., the distance between remaining unfilled pores) apparently increases with increasing pressure. Parallel to this, the maximum at ca. 4 nm (containing the contributions of the pore size and the pore wall to the CLD) remains unchanged. This effect is given by the mutual elimination of the changes of the pore walls and pore size signals during the physisorption process. In fact, due to multilayer formation in the pores, the maximum of the pore walls shifts to larger r values, while the maximum of the void pores shifts to lower r values,18 with the result that the superposed maximum, sum of both, does not changes its position. DOI: 10.1021/la903934r
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Figure 8. CLD plots of CH2Br2 desorption in UKON3a (a, b) and in UKON3b (c, d).
Despite that, the average pore size deduced from the CLD at the void state (ca. 4 nm) corresponds well to the physisorption analysis using nitrogen as probe gas. The behavior of the CLD plots for the desorption branch (Figure 8) represents the reverse of the adsorption. Looking at the g(r) representation, the g(0) values increase by decreasing the pressure and reach almost the same value as in the void state. The minima in the r * g(r) plots, referred to the pore-to-pore distance, decrease to smaller values according to the scattering results. For C5F12, the CLD can formally be calculated, but it must not be interpreted in terms of a two-phase system (see the Supporting Information) because of incomplete contrast matching. The changes in the scattering intensity patterns allow one to calculate the relative interfacial surface area as function of the relative pressure, by means of the calculation of the Porod invariant Q.18 As one can see (Figure 9), for both samples, the surface area decreases until p/p = 0.4, in agreement with the ex situ sorption analyses. Interestingly, a further evidence of the absence of microporosity in the systems is given by the smooth drop of the surface area for p/p = 0-0.1. If micropores were present, a more drastic decrease in relative interface area should be noted at small relative pressure, as shown for ink-bottle pore systems.20 7. Analysis by the Rosenfeld Approach. As shown recently, the small-angle scattering of mesopore systems with only local order can be analyzed by an approach based on the Percus-Yevick (PY) structure factor.18,36,37 A similar model which uses the same assumptions of the Percus-Yevick, but refers to cylindrical systems, instead of spherical ones, is the Rosenfeld (RF) approach.38 The scattering data were thus fitted using an algorithm already discussed in ref 38, based on a system of polydispersed hard disks to model the cylindrical mesoporosity, while the distorted mutual arrangement of the pores is described by the structure factor introduced by Rosenfeld (see the Appendix). The good matching between the experimental data and (36) Siemann, U.; Ruland, W. Colloid Polym. Sci. 1982, 260, 999–1010. (37) Smarsly, B.; Thommes, M.; Ravikovitch, P. I.; Neimark, A. V. Adsorption 2005, 11, 653–655. (38) Rosenfeld, Y. Phys. Rev. A 1990, 42, 5978–5989.
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Figure 9. Relative interface area of UKON3a and UKON3b during CH2Br2 adsorption.
the RF fitting is shown in Figure 10. As a main advantage, this procedure provides a small number of meaningful physical parameters, namely, the volume fraction ηRF, the RF radius RRF, which represents half of the pore-to-pore distance, the average pore radius Rav, and its variance σ, which represents the polydispersity of the pore size (Table 3). By the RF analysis of the adsorption curves, it can be seen that, during the sorption process, RRF increases, while the average pore size Rav slightly decreases, due to multilayer formation of the adsorbate on the surface of the void pores. As explained above, this trend could not be observed in the CLD since the signal of the pore size is superposed with the one of the pore walls. Nevertheless, RRF changes and the respective calculated morphological parameters are in good agreement with the results of the CLD, providing the reliability of both methods in the interpretation of the SAXS data. Furthermore, the polydispersity σ of the systems is enhanced at higher pressure, thus precisely explaining the rising of the intensity at small scattering vectors in Figure 5a, b. In addition, the volume fraction of Langmuir 2010, 26(9), 6583–6592
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Figure 10. Experimental curve (circles) and Rosenfeld fitting (bold line) of UKON3a at p/p = 0.17. Table 3. Analysis of SAXS Data during CH2Br2 Adsorption by the Modified Percus-Yevick Analysis (the Filled State Was Obviously Not Fitted)36 p/p
ηRF
RRF
σ
Rav
0.62 0.65 0.7 0.72
2 1.9 1.85 1.8
0.6 0.6 0.62 0.65 0.68
2 1.9 1.8 1.78 1.75
UKON3a 0 0.17 0.3 0.34
0.36 0.32 0.25 0.22
0 0.11 0.19 0.26 0.33
0.34 0.34 0.32 0.28 0.22
3.65 3.7 3.85 3.85 UKON3b 3.6 3.7 3.7 3.75 3.8
the mesopores, simulated by ηRF, decreases. In particular, the ηRF values for the void state are in good agreement with the volume fraction obtained by ex situ sorption analysis (Table 2).
Conclusions In the present study, the pore structure of two recently introduced PMO materials with a mesopore structure with local order was studied by physisorption using N2, CH2Br2, and C5F12, using both ex situ methods and in situ SAXS/SANS during physisorption. As main result, total contrast matching was observed for physisorption of CH2Br2 in both samples. Aside from the precise determination of porosity parameters, the current study brought further insights into the distribution of the organic moieties within the pore walls (see Scheme 2). The fact that contrast matching is observed in SAXS with a certain fluid (CH2Br2) proves that the pore walls constitute a homogeneous matrix in terms of the electron density and thus the mass density. Therefore, it is concluded that the organic moieties are randomly distributed and do not “phase-separate”, that is, not even on the scale of nanometers (Scheme 2A). In this context, the terminology “homogeneous” deserves a more detailed discussion and has to be defined properly, in particular with respect the differentiation between scenarios A-C (Scheme 2). Our conclusion that the SAXS experiments did not present any indication for scenarios B or C is limited by the experimental resolution, that is, the range of scattering vectors being accessible in our experimental setups. The maximum Langmuir 2010, 26(9), 6583–6592
modulus of the scattering vector was smax = 0.5 nm-1, corresponding to a dimension of 2 nm in real space, which thus also defines the maximum resolution. In our experiments, the Porod law was almost ideally observable for s < smax, proving the presence of an ideal two-phase system (= “homogeneous” distribution of the organic moieties) with two components (matrix and voids) of constant average electron density beyond a dimension of 2 nm. Assuming a significant inhomogeneity in the distribution of the organic moieties would imply a local abundance (e.g., minimum 3-4 molecules) of these moieties and, therefore, certainly an average distance between them being larger than 2 nm on the average. Thus, any realistic scenario for the distribution of the organic species should result in a significant deviation from Porod’s law, since the electron density of the organic parts is substantially different from silica and the voids. Similarly, also scenario C should result in a visible deviation from Porod’s law. This interpretation is further supported by a previous study on micellar templating using methyltrimethoxysilane (MTMS) as precursor.39 The random distribution of the functional organic moieties was also proven in an independent study on PMO materials of the UKON type containing two different functional groups.40 EPR spectroscopy was utilized to determine the average distance of those two groups, and to predict its dependence on composition of the materials. Contrary to the present study, the material could be heated up to 400-450 C, keeping the mesostructural order intact. The SAXS data revealed a scattering maximum which was interpreted as the consequence of the statistical distribution of the methyl groups in the siliceous matrix. This result is attributable to the high degree of condensation, resulting in a relatively localized distribution of the methyl group and thus in a scattering maximum. However, in the present case, the degree of condensation is much lower owing to the low treatment temperature.10-12 Thus, there is a significant flexibility of the single units, allowing reorganization and, eventually, microphase separation (see Figure S4 in the Supporting Information). Thus, the distribution of the organic moieties is hard to predict a priori and requires suitable SAXS experiments. As a further main conclusion, our in situ SAXS/SANS and ex situ sorption experiments revealed that the templating of the present PMO materials can be regarded as a true liquid-crystaltemplating in the following sense: the SAXS and physisorption evaluation prove the absence of microporosity; that is, the porosity is exclusively constituted by the mesopores originating from the templating action. In essence, the study also shows as a main result that the materials do not contain inaccessible voids. Furthermore, the application of the concept of the chord length distribution allowed for a valid determination of the mesopore size. It is interesting to note that the results obtained from the CLD correspond reasonably with the evaluation of the sorption data. Our study indicates that the usage of the NLDFT approach for the pore size analysis provides pore sizes, which can be regarded as a good approximation. In conclusion, in situ SAXS physisorption measurements using appropriate gases/fluids represent an invaluable tool for the structural characterization of PMO materials. In future experiments, the PMO material without functionalization will be studied to further understand the influence of these organic moieties on the sol-gel templating. Also, it is needed to extend current DFT methodology to gases other than nitrogen (39) Smarsly, B.; Xomeritakis, G.; Yu, K.; Liu, N. G.; Fan, H. Y.; Assink, R. A.; Drewien, C. A.; Ruland, W.; Brinker, C. J. Langmuir 2003, 19, 7295–7301. (40) Kuschel, A.; Drescher, M.; Kuschel, T.; Polarz, S. Chem. Mater. 2010, 22, 1472–1482.
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and to take into account surface roughness, as recently developed for argon.41 Acknowledgment. Mr. Laemthong Chuenchom is gratefully acknowledged for the help in performing the in situ SAXS measurements at HASYLAB/DESY, Hamburg. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 226716. This work was supported by the German Research Foundation (DFG, SM 199/7-1 and SP 780/6-1). Helmholtz-Zentrum Berlin (HZB) is gratefully acknowledged for financial support and the possibility to perform the in situ SANS experiments.
Appendix The analytical calculation of the scattering intensity assuming a polydispersed hard disks model in the Rosenfeld approximation and considering that the position of the cylindrical mesopores is independent from their size, is given by IðsÞ ¼ k½ÆjFðsÞj2 æ þ jÆFðsÞæj2 ðÆjSRF ðsÞj2 æ -1Þ
ðA1Þ
where k is a scaling factor, F(s) is the form factor of the cylinder, SRF(s) is the structure factor, and the brackets Ææ represent the number-average of cylinders.36 The form factor of a cylinder of radius R is given by FðR, sÞ ¼
R J1 ð2πsRÞ s
where the J1(R,s) is a Bessel function of the first kind of order 1. The two contributions of the form factor in eq A1 can be expressed by R2 þ σ 2 exp -2π2 s2 σ 2 FðR, sÞ ðA2Þ ÆFðR, σ, sÞæ ¼ 2 R (41) Muroyama, N.; Yoshimura, A.; Kubota, Y.; Miyasaka, K.; Ohsuna, T.; Ryoo, R.; Ravikovitch, P. I.; Neimark, A. V.; Takata, M.; Terasaki, O. J. Phys. Chem. C 2008, 112, 10803–10813.
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ÆFðR, σ, sÞ2 æ 3 0 1 2π2 s3 R4 þ 6R2 σ 2 þ 3σ 4 FðR, sÞ R 6 7 B C -1A þ 15 ¼ 2 3 4exp -8π2 σ2 s2 @ 2π s R5 2
ðA3Þ The structure factor SRF(s) is obtained through the Rosenfeld approach with the parameters ηRF and RRF "
J1 ðRRF sÞ 2 RRF s ! # -1 BðηRF ÞJ0 ðRRF sÞJ1 ðRRF sÞ GðηRF ÞJ1 ð2RRF sÞ þ þ þ1 RRF s RRF s SRF ðηRF , RRF , sÞ ¼ 4ηRF AðηRF Þ
ðA4Þ
AðηRF Þ ¼
1 þ ð2ηRF -1ÞχðηRF Þ þ 2ηRF GðηRF Þ ηRF
BðηRF Þ ¼
ð1 -ηRF ÞχðηRF Þ -1 -3ηRF GðηRF Þ ηRF
GðηRF Þ ¼
1 ð1 -ηRF Þ3
; χðηRF Þ ¼
1 þ ηRF ð1 þ ηRF Þ3
where J0 and J1 are the Bessel functions having the property that J0(x) and 2J1(x)/x are both equal to 1 for x = 0.38 Supporting Information Available: Porod plot and Porod law, and CLD analysis of SANS data during sorption of perfluoropentane. This material is available free of charge via the Internet at http://pubs.acs.org.
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