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Detailed investigation of the microporous character of mesoporous silicas as revealed by small-angle scattering techniques
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
295
Detailed investigation of the microporous character of mesoporous silicas as revealed by small-angle scattering techniques B. Smarsly"', K. Yu^ and C. J. Brinker''^ ^University of New Mexico, Center for Microengineered Materials, Advanced Materials Laboratory, 1001 University Blvd., SE, Albuquerque, NM 87106, USA ^Sandia National Laboratories, MS 1349, Albuquerque, NM, 87185, USA 1. INTRODUCTION Mesoporous
silica bulk materials, prepared
by templating with
amphiphilic
block
copolymers, have recently been shown to contain a substantial degree of additional micropores, located in the silica walls. Significant experimental evidence from sorption and SAXS/SANS (Small-Angle X-ray/Neutron Scattering) suggest that these micropores originate from the hydrophilic poly(ethylene oxide) (PEO) chains being tightly embedded in the matrix and creating voids of similar dimension after template removal [1-2]. This microporosity was found in bulk silicas obtained from several polymer templates such as PEO-PEO-PEO (SBA15) and poly(styrene)-/?-poly(ethylene oxides) (PS-PEO). While the degree of microporosity seems to be tunable by a variation of the preparation conditions, the determination of micropores in the presence of mesopores still is an experimental challenge. In particular, standard evaluations of microporosity based on nitrogen sorption measurements have to be regarded as inappropriate for these materials. T-plot and as-plot analyses do not have the required accuracy because of the lack of suitable reference materials. It was recently demonstrated that suitable SAXS/SANS techniques provide a practical and
accurate
determination of microporosity, and even the micropore size could be determined. Thus, this analytical tool allows studying the influence of different preparation methods on the microporosity. In this study, mesoporous silica films were prepared by using PS-PEO polymers as structure directing agents and applying the pathway of evaporation-induced self'correspondence author: [email protected] This work was supported by Sandia National Laboratories, a Lockheed Martin Company, under Department of Energy Contract DE-AC04-94AL85000, the Air Force Office of Scientific Research Award Number F49620-01-1-0168, the DOE Office of Basic Energy Sciences, the DOD MURI Program Contract 318651, and Sandia's Laboratory Directed Research and Development Program.
296
assembly. Starting from a diluted solution of the polymers in THF/H2O, containing TEOS as precursor, this preparation method is characterized by a very low polymer concentration at the beginning of the evaporation, which is different from the procedures for powder materials. The objective of this study is two-fold. Firstly, it was attempted to produce well-defined mesostructured porous silica films, which were characterized by TEM and SAXS in grazing incidence geometry (GISAXS). Secondly, detailed GISAXS and sorption studies were carried out in order to check for microporosity due to PEO chains. The GISAXS data will be compared with corresponding analyses on powder materials obtained also from PS-PEO templates. 2. EXPERIMENTAL In a typical synthesis, PS(22)-Z7-PEO(70) diblock copolymer, which has 22 styrene units and 70 ethylene oxide units, was dissolved in tetrahydrofiiran (THF) at 1 wt.%. Afterwards, a certain amount of tetraethoxysilane (TEOS), hydrogen chloride (HCl), as well as water (Milli Q) were added to the dilute copolymer solution in THF [3-4]. The quantity of the silica precursor, namely MTES added was such as to achieve a weight ratio of ca. 1 copolymer : 7 precursor. The total amount of HCl and water added was such as to achieve molar ratios of 1 TEOS : 0.004 HCl : 5 H2O. After 30 minutes of sonication, one drop of the solution was cast to obtain a silica/diblock thin film on a silicon wafer. Calcination in Argon (with a heating rate of l°C/min to 400 °C for 3 hours) removed the diblock, as confirmed by thermogravimetric analysis, and produced mesostructured porous silica films. The GISAXS measurements were performed directly on cast thin films, using the 5-meter pinhole instrument in the SAXS laboratory at the Center of Micro-Engineered Materials at the University of New Mexico, with an available range of s values between 0.08 - 1.4 nm', where .v = 2sin(0)/A. and 20 is the scattering angle and X the CuKa wavelength (0.154 nm). 3. RESULTS AND DISCUSSION Fig. 1 shows a TEM (Transmission Electron Mikrograph) picture of a mesoporous silica film, obtained by using a PS(22)-Z7-PEO(70) block polymer. Tilting experiments indicate a cubic lattice. The structure corresponds to a bcc or fee structure, but a differentation between them was not possible based on TEM. It is seen that the pore size is about 5nm assuming spherical mesopores, but the determination of mesopore sizes from TEM involves certain inaccuracies. It could be shown by thermogravimetric analysis that by the calcinations step almost all of the polymer is removed at 400 degrees Celsius. Interestingly, these materials showed almost no absorption in nitrogen sorption measurements, indicating that the mesopores are isolated voids.
297
Fig. 1. TEM of a mesoporous silica film obtained from PS(22)-Z7-PEO(70) polymer. The scale bar corresponds to 50nm. A and B correspond to different tilting angles, the difference is 15 degrees.
/^. 1000 J
y
[200]
100 J CO
10
[110]
[110]
14 0.07 0.10
0.33
0.67 1.00 -ii
scattering vector s [nm" ]
Fig. 2. A. GISAXS data of a mesoporous silica film, templated with PS(22)-6-PEO(70) (curve 1, crosses), mesoporous silica film without any template (curve 2, open circles), SAXS data of mesoporous SEIOIO silica powder according to ref [1] (curve 3, crosses), and Porod's law (s"^). B. 2D GISAXS pattern corresponding to A, curve 1. The peak indexing is based on a bcc cubic structure. The two non-indexed peaks are due to the primary reflected beam.
298
Fig. 2 shows SAXS and GISAXS data from silica films and powder materials. Curve 1 corresponds to a mesoporous silica film prepared by using a PS(22)-Z?-PEO(70) block copolymer template described by the procedure above. The curve was obtained by averaging the 2D GISAXS data in Fig. 2B in the region of the [110] reflections, if the GISAXS pattern is interpreted in terms of a cubic bcc structure. Curve 2 represents the GISAXS data of a silica film prepared under identical conditions as the sample belonging to curve 1, without using a template. The non-templated film was studied to study the microstructure of the silica matrix: if micropores were present, this should be apparent at larger scattering vectors. Since the two GISAXS data sets were obtained under comparable conditions (angle of incidence, etc), the relative scattering intensities can be directly compared. It is seen that curves 1 and 2 are almost identical in shape and intensity at scattering vectors beyond ca. s = 0.3 nm'. This identity at larger s already suggests the absence of additional micropores due to PEO. This assumption is further support by comparing the GISAXS data of the thin films with SAXS data obtained on mesoporous silica powders, which were also obtained from PS-PEO templates (Fig. 2A, curve 3). This material was obtained from a PS(9)-b-PS-23) polymer and shows several reflections in SAXS patterns, resulting from ordered mesopores. These materials were shown to contain a significant degree of microporosity. It was concluded that SAXS curves at larger scattering vectors s significantly differ from a theoretical mesostructure without any microporosity. The microporosity gets apparent by a significant deviation from Porod's law (I(s) a s'^) at larger s. A comparison with the GISAXS patterns in Fig. 2A obtained from the silica films therefore furthermore indicates the absence of additional microporores, which were shown to have sizes of about 0.5-1.5nm. Based on the microporosity in mesoporous silica porous, determined in our previous publications [1-2], the upper limit for the micropore volume can be estimated to be about 0.01 ml/g. The reason for the absence for microporosity in the mesoporous silica films may be related to the significantly different preparation conditions, compared to mesoporous bulk silicas: the self-assembly is carried out over a much longer period of time (about 1 day) and in THF, which is a good solvent for PS. Our results suggest that during the solvent evaporation the polymer continuously gets more insoluble, while the silica framework is still fragile enough to rearrange. Hence, the PEO chains may retract from the matrix as a result of the solvent removal. REFERENCES 1. C. G. Goltner, B. Smarsly, B. Berton, M. Antonietti, Chem. Mater., 13 (2001) 1617. 2. B. Smarsly, C. Goltner, M. Antonietti, W. Ruland, E. Hoinkis, J. Phys. Chem. B, 10 (2001)831. 3. K. Yu, A. J. Hurd, C. J. Brinker, A. Eisenberg, Langmuir, 17 (2001) 7961. 4. B. Smarsly, K. Yu, C. J. Brinker, Mat. Res. Soc. Symp. Proc, SI.9 (2002) 728.
295
Detailed investigation of the microporous character of mesoporous silicas as revealed by small-angle scattering techniques B. Smarsly"', K. Yu^ and C. J. Brinker''^ ^University of New Mexico, Center for Microengineered Materials, Advanced Materials Laboratory, 1001 University Blvd., SE, Albuquerque, NM 87106, USA ^Sandia National Laboratories, MS 1349, Albuquerque, NM, 87185, USA 1. INTRODUCTION Mesoporous
silica bulk materials, prepared
by templating with
amphiphilic
block
copolymers, have recently been shown to contain a substantial degree of additional micropores, located in the silica walls. Significant experimental evidence from sorption and SAXS/SANS (Small-Angle X-ray/Neutron Scattering) suggest that these micropores originate from the hydrophilic poly(ethylene oxide) (PEO) chains being tightly embedded in the matrix and creating voids of similar dimension after template removal [1-2]. This microporosity was found in bulk silicas obtained from several polymer templates such as PEO-PEO-PEO (SBA15) and poly(styrene)-/?-poly(ethylene oxides) (PS-PEO). While the degree of microporosity seems to be tunable by a variation of the preparation conditions, the determination of micropores in the presence of mesopores still is an experimental challenge. In particular, standard evaluations of microporosity based on nitrogen sorption measurements have to be regarded as inappropriate for these materials. T-plot and as-plot analyses do not have the required accuracy because of the lack of suitable reference materials. It was recently demonstrated that suitable SAXS/SANS techniques provide a practical and
accurate
determination of microporosity, and even the micropore size could be determined. Thus, this analytical tool allows studying the influence of different preparation methods on the microporosity. In this study, mesoporous silica films were prepared by using PS-PEO polymers as structure directing agents and applying the pathway of evaporation-induced self'correspondence author: [email protected] This work was supported by Sandia National Laboratories, a Lockheed Martin Company, under Department of Energy Contract DE-AC04-94AL85000, the Air Force Office of Scientific Research Award Number F49620-01-1-0168, the DOE Office of Basic Energy Sciences, the DOD MURI Program Contract 318651, and Sandia's Laboratory Directed Research and Development Program.
296
assembly. Starting from a diluted solution of the polymers in THF/H2O, containing TEOS as precursor, this preparation method is characterized by a very low polymer concentration at the beginning of the evaporation, which is different from the procedures for powder materials. The objective of this study is two-fold. Firstly, it was attempted to produce well-defined mesostructured porous silica films, which were characterized by TEM and SAXS in grazing incidence geometry (GISAXS). Secondly, detailed GISAXS and sorption studies were carried out in order to check for microporosity due to PEO chains. The GISAXS data will be compared with corresponding analyses on powder materials obtained also from PS-PEO templates. 2. EXPERIMENTAL In a typical synthesis, PS(22)-Z7-PEO(70) diblock copolymer, which has 22 styrene units and 70 ethylene oxide units, was dissolved in tetrahydrofiiran (THF) at 1 wt.%. Afterwards, a certain amount of tetraethoxysilane (TEOS), hydrogen chloride (HCl), as well as water (Milli Q) were added to the dilute copolymer solution in THF [3-4]. The quantity of the silica precursor, namely MTES added was such as to achieve a weight ratio of ca. 1 copolymer : 7 precursor. The total amount of HCl and water added was such as to achieve molar ratios of 1 TEOS : 0.004 HCl : 5 H2O. After 30 minutes of sonication, one drop of the solution was cast to obtain a silica/diblock thin film on a silicon wafer. Calcination in Argon (with a heating rate of l°C/min to 400 °C for 3 hours) removed the diblock, as confirmed by thermogravimetric analysis, and produced mesostructured porous silica films. The GISAXS measurements were performed directly on cast thin films, using the 5-meter pinhole instrument in the SAXS laboratory at the Center of Micro-Engineered Materials at the University of New Mexico, with an available range of s values between 0.08 - 1.4 nm', where .v = 2sin(0)/A. and 20 is the scattering angle and X the CuKa wavelength (0.154 nm). 3. RESULTS AND DISCUSSION Fig. 1 shows a TEM (Transmission Electron Mikrograph) picture of a mesoporous silica film, obtained by using a PS(22)-Z7-PEO(70) block polymer. Tilting experiments indicate a cubic lattice. The structure corresponds to a bcc or fee structure, but a differentation between them was not possible based on TEM. It is seen that the pore size is about 5nm assuming spherical mesopores, but the determination of mesopore sizes from TEM involves certain inaccuracies. It could be shown by thermogravimetric analysis that by the calcinations step almost all of the polymer is removed at 400 degrees Celsius. Interestingly, these materials showed almost no absorption in nitrogen sorption measurements, indicating that the mesopores are isolated voids.
297
Fig. 1. TEM of a mesoporous silica film obtained from PS(22)-Z7-PEO(70) polymer. The scale bar corresponds to 50nm. A and B correspond to different tilting angles, the difference is 15 degrees.
/^. 1000 J
y
[200]
100 J CO
10
[110]
[110]
14 0.07 0.10
0.33
0.67 1.00 -ii
scattering vector s [nm" ]
Fig. 2. A. GISAXS data of a mesoporous silica film, templated with PS(22)-6-PEO(70) (curve 1, crosses), mesoporous silica film without any template (curve 2, open circles), SAXS data of mesoporous SEIOIO silica powder according to ref [1] (curve 3, crosses), and Porod's law (s"^). B. 2D GISAXS pattern corresponding to A, curve 1. The peak indexing is based on a bcc cubic structure. The two non-indexed peaks are due to the primary reflected beam.
298
Fig. 2 shows SAXS and GISAXS data from silica films and powder materials. Curve 1 corresponds to a mesoporous silica film prepared by using a PS(22)-Z?-PEO(70) block copolymer template described by the procedure above. The curve was obtained by averaging the 2D GISAXS data in Fig. 2B in the region of the [110] reflections, if the GISAXS pattern is interpreted in terms of a cubic bcc structure. Curve 2 represents the GISAXS data of a silica film prepared under identical conditions as the sample belonging to curve 1, without using a template. The non-templated film was studied to study the microstructure of the silica matrix: if micropores were present, this should be apparent at larger scattering vectors. Since the two GISAXS data sets were obtained under comparable conditions (angle of incidence, etc), the relative scattering intensities can be directly compared. It is seen that curves 1 and 2 are almost identical in shape and intensity at scattering vectors beyond ca. s = 0.3 nm'. This identity at larger s already suggests the absence of additional micropores due to PEO. This assumption is further support by comparing the GISAXS data of the thin films with SAXS data obtained on mesoporous silica powders, which were also obtained from PS-PEO templates (Fig. 2A, curve 3). This material was obtained from a PS(9)-b-PS-23) polymer and shows several reflections in SAXS patterns, resulting from ordered mesopores. These materials were shown to contain a significant degree of microporosity. It was concluded that SAXS curves at larger scattering vectors s significantly differ from a theoretical mesostructure without any microporosity. The microporosity gets apparent by a significant deviation from Porod's law (I(s) a s'^) at larger s. A comparison with the GISAXS patterns in Fig. 2A obtained from the silica films therefore furthermore indicates the absence of additional microporores, which were shown to have sizes of about 0.5-1.5nm. Based on the microporosity in mesoporous silica porous, determined in our previous publications [1-2], the upper limit for the micropore volume can be estimated to be about 0.01 ml/g. The reason for the absence for microporosity in the mesoporous silica films may be related to the significantly different preparation conditions, compared to mesoporous bulk silicas: the self-assembly is carried out over a much longer period of time (about 1 day) and in THF, which is a good solvent for PS. Our results suggest that during the solvent evaporation the polymer continuously gets more insoluble, while the silica framework is still fragile enough to rearrange. Hence, the PEO chains may retract from the matrix as a result of the solvent removal. REFERENCES 1. C. G. Goltner, B. Smarsly, B. Berton, M. Antonietti, Chem. Mater., 13 (2001) 1617. 2. B. Smarsly, C. Goltner, M. Antonietti, W. Ruland, E. Hoinkis, J. Phys. Chem. B, 10 (2001)831. 3. K. Yu, A. J. Hurd, C. J. Brinker, A. Eisenberg, Langmuir, 17 (2001) 7961. 4. B. Smarsly, K. Yu, C. J. Brinker, Mat. Res. Soc. Symp. Proc, SI.9 (2002) 728.