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Langmuir 2007, 23, 4724-4727
Pore Hierarchy in Mesoporous Silicas Evidenced by In-Situ SANS during Nitrogen Physisorption Ozlem Sel,† Astrid Brandt,‡ Dirk Wallacher,‡ Matthias Thommes,§ and Bernd Smarsly*,† Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany, Hahn-Meitner Institute, Glienicker Strasse 100, D-14109 Berlin, Germany, and Quantachrome Instruments, 1900 Corporate DriVe, Boynton Beach, Florida 33426 ReceiVed December 22, 2006. In Final Form: March 2, 2007 The pore hierarchy of a hierarchical porous SiO2 with 14 nm spherical mesopores and 3 nm worm-like pores (KLE1C16) is studied by small-angle neutron scattering (SANS) in combination with in-situ nitrogen sorption at 77 K. A novel setup is used developed at Hahn-Meitner Institute, Berlin. It is demonstrated that in these materials indeed all of the large mesopores are connected through the smaller ones, thus providing invaluable insights into the general phenomenon of pore connectivity in mesoporous materials.
In the past years, significant progress has been made both in the preparation of micro- and mesoporous silicas with an ordered pore arrangement and their characterization by sorption methods.1 However, in spite of the advances in sorption theory, additional independent techniques would be helpful to validate physisorption analysis. In the present study, a novel in-situ SANS-nitrogen sorption setup was applied to study the porosity of novel silica with hierarchical pore architecture, thus allowing us to monitor pore filling by simultaneous in-situ SANS. In general, pore size analysis of mesoporous materials is based on the understanding of pore condensation, connectivity, and the associated hysteresis phenomenon. It was demonstrated recently that a reasonable interpretation of physisorption data is provided by using novel nonlocal density function theory (NLDFT) approaches,2 which where verified by suitable silica materials with well-defined pore hierarchy and connectivity.3 These materials (“KLE1C16” silica) consist of uniform spherical mesopores about 14-15 nm in diameter distributed on a distorted FCC lattice with a pore-topore distance of ca. 22 nm (templated by so-called “KLE” block copolymers4a), connected through worm-like IL mesopores pores of 2-3 nm in diameter (generated by a certain ionic liquid “IL” surfactant, C16mimCl) and also a minor fraction of micropores.4,5 The larger 14 nm mesopores are filled and emptied through the smaller pores, thus providing an ideal system to study the influence of pore connectivity on the evaluation of physisorption data and also to shed further light on the phenomenon of hysteresis. The results of high resolution argon (at 87.3 and 77.4 K) and nitrogen experiments (at 77.4 K) on KLE1C16 silica were reported in ref 3, demonstrating that indeed the “cavitation” mechanism occurred in this material (instead of pore blocking/percolation3) owing to the significant difference in the sizes of the larger * Corresponding author. E-mail:
[email protected]. Tel: +49 331 567 9508. Fax: +49 331 567 9502. † Max Planck Institute of Colloids and Interfaces. ‡ Hahn-Meitner Institute. § Quantachrome Instruments. (1) Thommes, M. In Nanoporous Materials: Science and Engineering; Lu, G. Q., Zhao, X. S., Eds.; Imperial College Press: London, 2004; p 317. (2) (a) Neimark, A. V.; Ravikovitch, P. I. Microporous Mesoporous Mater. 2001, 44, 697. (b) Ravikovitch, P. I.; Neimark, A. V. Colloids Surf. A 2001, 187, 11. (3) Thommes, M.; Smarsly, B.; Groenewolt, M.; Ravikovitch, P. I.; Neimark, A. V. Langmuir 2006, 22, 756. (4) (a) Thomas, A.; Schlaad, H.; Smarsly, B.; Antonietti, M. Langmuir 2003, 19, 4455-4459. (b) Kuang, D. B.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534. (5) Sel, O.; Kuang, D. B.; Thommes, M.; Smarsly, B. Langmuir 2006, 22, 2311.
mesopores (14 nm) and the connecting 3 nm IL pores. Furthermore, good agreement had been obtained for the pore sizes obtained from the NLDFT method and SAXS analysis, supporting the validity of the NDLFT approach for the characterization of porosity.3,4 In spite of these achievements, which possess significant relevance for physisorption in general, it has to be emphasized that the aforementioned conclusions were based on the assumption that all of the larger mesopores were indeed connected through the smaller pores (2-3 nm mesopores and 1 nm micropores). Although structural characterization described in refs 3-5 was in full agreement with such pore hierarchy (see Figure 1), still it remained desirable to apply an independent technique to verify the presence of such peculiar hierarchical pore architecture because these insights would be then fundamental for the correct interpretation of physisorption data of other mesoporous materials. Therefore, in the present study SANS experiments combined with in-situ nitrogen adsorption at 77 K were performed at SANS spectrometer V4, which is placed at the cold neutron guide (λ ) 6 Å) of the Hahn-Meitner-Institute, Berlin. By varying the sample-to-detector distance from 1 to 16 m, the scattering intensity was recorded in the range of 0.02 nm-1 < s < 0.62 nm-1 (s ) q/2π) and q ) 4π/λ sin(θ). The sample used in the present study was not taken from the same batch as the one in ref 3 but possesses the same texture, which is confirmed by the adsorption data and the NLDFT pore size distribution shown in Figure 2, revealing a trimodal pore size distribution with maxima at around 1.3, 3, and 14 nm (see Table 1 and Figure 2). The experiment is based on the principle of “contrast matching”, the scattering lengths of the amorphous KLE-silica matrix (3.43 × 1010 cm-2) and condensed nitrogen (3.22 × 1010 cm-2) being almost identical, leading to negligible scattering contrast in the filled state.6,7 As shown previously, this concept provided indispensable insights into the pore structure and pore filling mechanism in mesoporous hosts.6-14 (6) (a) Hoinkis, E. Part. Part. Syst. Charact. 2004, 2, 80. (b) Hoinkis, E.; Rohl-Kuhn, B. J. Colloid Interface Sci. 2006, 296, 256. (7) (a) Smarsly, B.; Goltner, C.; Antonietti, M.; Ruland, W.; Hoinkis, E. J. Phys. Chem. B 2001, 105, 831. (b) Smarsly, B.; Thommes, M.; Ravikovitch, P. I.; Neimark, A. V. Adsorption 2005, 11, 653. (8) Steriotis, T.; Mitropoulos, A.; Kanellopoulos, N.; Keiderling, U.; Wiedenmann, A. Physica B 1997, 234, 1016. (9) Zickler, G. A.; Jahnert, S.; Wagermaier, W.; Funari, S. S.; Findenegg, G. H.; Paris, O. Phys. ReV. B: Condens. Matter Mater. Phys. 2006, 73, 184109. (10) Kallus, S.; Hahn, A.; Ramsay, J. D. F. Stud. Surf. Sci. Catal. 2002, 144, 67.
10.1021/la063715+ CCC: $37.00 © 2007 American Chemical Society Published on Web 03/30/2007
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Langmuir, Vol. 23, No. 9, 2007 4725
Figure 1. (a) TEM image of bimodal KLE1C16 silica. (b) Higher magnification of panel a. (Large spherical pores ca. 14 nm (a, b) and ca. 3 nm worm-like mesopores (b) templated by KLE block copolymer and IL templates, respectively.) Table 1. Properties of the KLE1C16 Silica Used in This Study property
value
SBET total pore volume (cm3/g) micropore volume (cm3/g) small mesopore volume (cm3/g) large mesopore volume (cm3/g) mesopore size (nm) (NLDFT) mesopore size (nm) (TEM) mesopore size (nm) (SANS)
710 0.51 0.01-0.02 0.29 0.21 (3.1)small meso(13.9)large meso (3.0)small meso (12.7)large meso (13.3)large meso
(m2/g)
In these experiments the relative vapor pressure p/p0 of the pore condensate could be controlled during the scattering experiment (p0 is the saturation vapor pressure of the bulk condensate at the given temperature). Previously, to each pressure measurement a certain fractional filling n/n0 of the sample material had to be assigned by means of an ex-situ measured gas adsorption isotherm, which had to be recorded under exactly the same degassing and temperature conditions as in the scattering experiment. By contrast, our new gas adsorption sample environment (CGA-PT) allows the direct in-situ measurement of a complete pV isotherm. Thereby, the KLE1C16 substrate is located in a flat copper cell with quartz windows, which is connected to a gas manifold by a capillary. The sample cell is mounted on the cold finger of a closed cycle refrigerator and stabilized to 77.3 ( 0.004 Κ by a temperature controller. The gas uptake V of the sample material at a given pressure p is controlled quantitatively by a volumetric method, which is described elsewhere.1 Figure 1a shows a comparison of the insitu measured nitrogen adsorption isotherm at 77 K of the KLE1C16 sample and the isotherm as measured ex-situ by a commercial gas adsorption device (Quantachrome Autosorb). For clearness, only some of these results are shown in Figure 3a, which represent crucial adsorption states of the sorption process (i.e., micropore filling, layer formation, filling of the small mesopores, and filling of the larger mesopores). In this context, the essential advantage of the contrast matching conditions is the fact that filled pores do not contribute to the SANS pattern of the mesopore structure. Therefore, this method is highly sensitive to the pore structure and pore connectivity and perfectly suited to proof the hierarchical order of the KLE1C16 porous silica. Compared to a partially phase-separated arrangement of mesopores (i.e., a significant fraction of the large mesopores not being connected through the smaller ones), a fully hierarchical (11) Ramsay, J. D. F.; Kallus, S.; Hoinkis, E. Stud. Surf. Sci. Catal. 2000, 128, 439. (12) Ramsay, J. D. F.; Hoinkis, E. Physica B 1998, 248, 322. (13) Hoinkis, E. Langmuir 1996, 12, 4299. (14) Ramsay, J. D. F.; Hoinkis, E. J. Non-Cryst. Solids 1998, 225, 200.
pore architecture should theoretically show a measurable difference in the SANS patterns (i.e., a system with completely phase separated domains of pores of different sizes would show a steady decrease in the SANS intensity as the relative pressure increases). In the present study, we focus on a semiquantitative analysis of the SANS data acquired during the adsorption branch to address the pore hierarchy, while a more detailed evaluation will be presented in a separate publication. The points were chosen to represent the crucial adsorption processes (i.e., micropore filling, layer formation, filling of the small mesopores, and also filling of the larger mesopores). The SANS curves showed various interesting features regarding the porosity and the filling process. First, the pressure-dependent experiments proved that the SANS data themselves could be attributed to a superposition of three different types of pores: at p/p0 ) 0, the shape of the curve s < 0.25 nm-1 was attributable to the 14 nm KLE mesopores. The broad maxima at s ) 0.55 nm-1 and s ) 0.3 nm-1 were assigned to the micropores and small IL mesopores, respectively, based on the relative pressures at which these scattering contributions disappeared (see the isotherm). Interestingly, at p/p0 > 0.35 further oscillations, corresponding to the form factor of the large sperical KLE mesopores, became visible. Second, and more importantly, the relative overall intensity of the SANS curves showed interesting changes as a function of p/p0. Starting from p/p0 ) 0, the SANS curve (below s ) 0.15 nm-1) underwent a significant, continuous overall increase up to p/p0 ) 0.3-0.35, which coincides with the filling of the micropores and small mesopores. In particular, this increase in I(s) is the most pronounced during the filling of the 2-3 nm mesopores. These findings and a basic evaluation provide direct evidence of the hierarchical nature according to the following considerations based on contrast matching: In general, the SANS contribution at small scattering vectors s < 0.15 nm-1 is governed by the difference in scattering length density between the 14 nm spherical mesopores and the average scattering length density of the surrounding matrix, i.e., I(s) ∝ (F1 - F2)2 ) F12 ) FjKLE1C16,evacuated2, if we set F2 ) Fvacuum ) 0. Hence, for the evacuated sample the scattering length density F1 corresponds to the average density FjKLE1C16,evacuated of the silica matrix surrounding the 14 nm KLE mesopores, being lower than the density of nonporous silica due to the presence of the small IL mesopores. Thus, filling the IL mesopores changes the average scattering length density of the porous matrix surrounding the spherical KLE mesopores to FjKLE1C16,filledIL ≈ FSiO2. In conclusion, assuming that all of the IL mesopores are located between the KLE mesopores, at sufficiently small s the ratio of SANS intensities (intensity maximum for s < 0.15 nm-1) of the evacuated sample IKLE1C16,evacuated(s) (Scheme 1a) to that of the stage where the IL mesopores are filled, but the KLE pores not yet (IKLE1C16,filledIL(s), Scheme 1b), is given by
IKLE1C16,evacuated(s) IKLE1C16,filled IL(s)
)
2 F KLE1C16,evacuated 2 F KLE1C16,filled IL
)k
(1)
Thus, we obtain
FKLE1C16,evacuated FKLE1C16,filled IL
) xk
(2)
Also, in general in the evacuated state (p/p0 ) 0), the average scattering length density of the silica matrix is lowered as
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Figure 2. (a) Nitrogen sorption isotherm (red circles show the p/p0 values of available SANS patterns, while empty circles correspond to the p/p0 values for which SANS patterns are presented in Figure 3a. (The steep step in the adorption isotherm at p/p0 ca. 1 is due to bulk condensation.) (b) NLDFT pore size distribution calculated from the adsorption branch from high-precision ex-situ sorption experiments using a commercial instrument. The hybrid kernel used here is based on a cylindrical pore model for the micropore and small mesopore (smaller than 5 nm) range and a spherical pore model for the mesopore range where hystersis occurred.
Figure 3. (a) In-situ SANS curves of KLE1C16 recorded at different relative pressures p/p0 during adsorption of nitrogen. (b) Sketch showing the features discussed in this section and the nomenclature used in the calculations (IKLE1C16,evacuated(s): evacuated sample; IKLE1C16,filledIL(s): only IL mesopores are filled.) Scheme 1a
a (a) Evacuated bimodal porous silica. (b) Filling of the IL mesopores (wormlike micelles). (c) Layer formation of the adsorbate on the KLE mesopores (connected spheres through IL mesopores). (d) After pore condensation (grey: silica matrix, white: pores, light grey: pores filled with nitrogen, p refers to the increasing pressure).
compared to bulk amorphous SiO2, due to the presence of the IL mesopores to
By combining eqs 1-3 we obtain
FKLE1C16,evacuated ) FSiO2 × (1 - φsmall mesopores)
(1 - φsmall mesopores) ) xk )
(3)
where φsmall mesopores is the volume fraction of the IL within the SiO2 matrix around the KLE mesopores with φsmall mesopores: ) Vsmall mesopores/(Vsmall mesopores + Vsilica).
x
IKLE1C16,evacuated(s) IKLE1C16,filled IL(s)
(4)
Equation 4 allows therefore the estimation of porosity φsmall mesopores in the KLE pore walls based on the analysis of the relative
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SANS intensity at s < 0.15 nm-1 of the evacuated sample as compared to the maximum of I(s) at the pressure corresponding to a complete filling of the IL mesopores (i.e., at p/p0 about 0.35). It should be emphasized again that these considerations only hold for I(s) at those scattering values (s < 0.15 nm-1), which corresponds to the packing of the KLE mesopores and therefore can be regarded as almost unaffected by the direct SANS contribution of the IL mesopores. Also, the influence of layer formation within the 14 nm mesopores can be neglected. Considering the above equations and using these SANS intensities (s < 0.15 nm-1) at p/p0 ) 0 and p/p0 ) 0.38 from Figure 3a, the volume fraction of pores below 4 nm in size was calculated as φsmall mesopores ) 0.42. Interestingly, only a moderate increase in the SANS intensity of the first maximum (i.e., maximum value of I(s) of the interference maximum) was observed at small p/p0, where micropore filling takes place. This finding is in conformity with the low microporosity found by high-precision physisorption.3 It should be mentioned that it was difficult to separate the volume fractions of the micro- and IL mesopores referred to the SiO2 matrix, the latter thus being ca. 0.4, having an uncertainty of (0.02 at maximum. Nitrogen physisorption provided an IL mesopore volume fraction of φsmall mesopores ) 0.36 (Figure 1, also (0.02) and a micropore volume of ca. 0.010.02 mL/g. The theoretical value, calculated using the amount of IL used as porogen, is φIL,theoretical ) 0.41, but this value has to be taken with care because it is based on a guess of the density of the pure IL surfactant distributed inside a silica matrix, which can only roughly be estimated. Still, the experimentally calculated values of the IL mesoporosity derived from two independent techniques, namely, physisorption and in-situ SANS, were in excellent agreement, implying that at least 90% of all IL
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mesopores in the specimen were located between KLE mesopores, thus implying almost ideal pore hierarchy. Note that in a completely phase-separated material (i.e., KLE and IL pores forming separate mesostructured domains), the SANS contribution of the KLE mesopores would continuously decrease from p/p0 ) 0 to larger p/p0 due to layer formation and condensation of the KLE mesopores at higher p/p0 and also the filling of the IL mesopores. The influence of these different sorption processes on the SANS curves of such complex mesopore architectures will be described in detail in a further publication. In conclusion, our results further supports our recent detailed physisorption studies,3,5 which had already indicated that a vast majority of the 14 nm spherical mesopores were connected through the smaller pores. However, the present in-situ SANS experiments provided the first direct evidence for the homogeneity of the material in terms of the hierarchical arrangement of the 14 nm KLE mesopores and the IL mesopores between them. Further analysis will be dedicated to a quantitative evaluation of the SANS data by suitable modeling procedures. In particular, we will address the dependence of the layer thickness on the pressure and the KLE mesopore filling process. Furthermore, additional experiments will be devoted to the investigation of the nature of the pore emptying mechanism. Acknowledgment. Hahn-Meitner Institute is gratefully acknowledged for financial support and the possibility to perform the in-situ experiments. In particular, U. Keiderling and A. Wiedenmann are thanked for their help with the setup at the V4 beamline. LA063715+
