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Surface structure of natural crystals of mordenite as imaged by atomic force microscopy
20 September 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 260 (1996) 208-214
Surface structure of natural crystals of mordenite as imaged by atomic force microscopy Sadaaki Yamamoto a, Osamu Matsuoka a, Shouko Sugiyama a, Tadatoshi Honda a, Yasuyuki Banno b, Hisakazu Nozoye c a Central Research Institute, Mitsui Toatsu Chemicals Inc., 1190 Kasama-eho. Sakae-ku, Yokohama 247, Japan b Geologieal Survey o f Japan. 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan c National Institute o f Materials and Chemical Research, 1-1 Higashi, Tsukuba, lbaraki 305, Japan
Received I November 1995; in final form 12 July 1996
Abstract Atomic force microscopy (AFM) has been applied to examine the surface of natural mordenite crystals in an 0.1 M NaOH aqueous solution. Wide-scan AP-M images revealed that the crystals have a surface partially covered with aggregates of mordenite fine particles. Atomic-scale images showed surface structure that agreed well with the models of the (100) and (010) surface. The hydroxyl groups on the (100) surface could be resolved. The images also revealed that an important factor that determines the resolution of an AFM image of mordenite is the magnitude of the periodic corrugation on the crystal surface.
1. Introduction Zeolites are microporous crystalline aluminosilicates with a framework structure composed o f corner-sharing SiO 4 - and AIO45- tetrahedra [1]. Much interest has been shown recently in natural and synthetic zeolites due to their practical uses as catalysts, molecular sieves, detergents [2,3], etc. To understand the physical and chemical properties that determine a zeolite's catalytic, ion-exchange and adsorption performances, the surface structure as well as the bulk structure are particularly important. Surface structural information is also important for understanding the growth mechanism of a zeolite [4]. The bulk structures of many types of zeolites have already been determined [5]. However, comparatively little is
known yet about the external surface structure due to the lack o f a suitable characterization method. Recent atomic force microscopy ( A F M ) studies on natural zeolite crystals have demonstrated the suitability of A F M for surface structural elucidation of zeolites [6-9]. However, interpretations of the images obtained thus far are controversial due to the resolution o f the images, Hansma et al. [6,7] obtained A F M images of natural crystals of clinoptilotite, scolecite, stilbite and faujasite and interpreted them as a tetrahedral unit structure of SiO 4 - and AIO45- . On the other hand, Scandella et al. [8] imaged the (010) surface of natural heulandite crystals, which has the isostructural framework of clinoptilolite, in an aqueous sodium hydroxide solution, and claimed to have resolved surface hydroxyl
0009-2614/96/$12.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PII 009-261 4(96)00833-0
S. Yamamoto et a l . / Chemical Physics Letters 260 (1996) 208-214
groups. Recently, Komiyama et al. [9] examined the surface structure of natural stilbite and heulandite and claimed to have resolved corrugations at almost an atomic level. However, the periodic structure on the heulandite surface they obtained did not match the surface structure expected from its bulk crystallographic data. Thus, more work is now needed to clarify the factors leading to these discrepancies in the interpreted resolution of AFM images. In this work, we used AFM to image the surface of natural mordenite crystals. We chose natural mordenite as our specimen for the following two reasons. The first is that among the many classes of zeolite, mordenite is of great interest as a catalyst, thus making its surface structure of particular importance. The second is that a natural mordenite crystal is expected to show three distinct surfaces that differ in the magnitude and periodicity of the corrugation as shown in Fig. 1. Therefore, we expected to be able to examine the effect of the magnitude and periodicity of surface corrugations on the atomic resolution in the AFM image of a zeolite surface. We did this by imaging each surface of mordenite and then comparing the images with the corresponding surface-structure model that is based on bulk crystallographic data.
2. Experimental
2.1. Materials and sample preparation The natural mordenite crystals that we used were from Arasawa, Kindaichi-mura, Ninohe-gun, Iwate, Japan (registered specimen of the Geological Survey of Japan, GSJ M16609). The powder X-ray pattern, obtained by a JEOL JDX 8030W EDS, and electron diffraction results, which were obtained by using a high resolution analytical electron microscope (JEOL JEM-2010), identified the crystals as being orthorhombic (unit cell lattice parameters a = 18.10, b = 20.42 and c = 7.51 ,~) with chemical composition (Nal.llK0.a4Ca0.88)A14.26Si23.21070.1. The crystals were cleaned in a 2N H2SO 4 aqueous solution at room temperature for 3 h to remove organic residues and metal ion contaminants. Then, the crystals were
209
rinsed three times with ultra-pure water (18 M,O cm, TORAYPURE LV-08 " T O R A Y " ) . After the H 2SO4 treatment, the amount of Na + in the crystals decreased and the crystals were H + form mordenite. The side of the needle-like crystal with an average size of 1 mm long and several tens /zm was glued onto a steel plate by epoxy resin (Araldite Rapid, CIBA-GEIGY), so that the c axis was parallel to the plate. We have investigated about 10 crystals.
2.2. AFM imaging AFM measurements were done for crystals placed in 0.1 M NaOH aqueous solution in a liquid cell unit (Digital Instruments, Santa Barbara, CA). Before imaging, the crystals were kept in this liquid cell filled with 0.1 M NaOH aqueous solution for = 30 min to allow an equilibrium state to be reached. For flexibility in choosing an imaging spot, we used the cell without an O-ring that was normally used for sealing. The imaging was done using a Nanoscope III a (Digital Instruments, Santa Barbara, CA) operated in constant-force mode with scan rates ranging from 3 to 49 Hz. We used triangular Si3N 4 microcantilevers called nanoprobes (NP-S, Digital Instruments, Santa Barbara, CA) that had a spring constant K of 0.58 and were approximately 100/zm long and 0.6 /xm thick with integrated pyramidal tips. The imaging forces were between 5 and 20 nN. Each scan for the same area gave similar images regardless of imaging force, insuring that the applied forces were so weak that the imaging was nondestructive. The periodic distances of the images at the atomic level were determined within the estimated error of ~< 10% by two-dimensional fast Fourier transform. The images presented here were not subjected to any filtering procedure.
3. Results and discussion
3.1. Wide-scan image Fig. 2 shows a representative wide-scan image of a natural crystal of mordenite. Because the crystals were not crushed, the AFM images accurately repre-
S. Yamamoto et al. / Chemical Physics Letters 260 (1996) 208-214
210
A 0 0 V
A A 0 0
c~
S. Yamamow et al. / Chemical Physics Letters" 260 (1996) 208-214
211
Fig. 2. Top-view AFM images of mordenite on a macroscopic scale in 0.1 M NaOH aqueous solution.
sent the f o r m o f the surfaces as they occurred. The image clearly reveals that the m o r p h o l o g y is a sheaf-like structure c o m p o s e d of needle-like crystals. The A F M images that we obtained r e v e a l e d the crystal g r o w t h - i n d u c e d feature that a flat surface is partially c o v e r e d with aggregates o f mordenite fine particles that are rectangular with their long axis
parallel to the c axis o f the crystal (indicated by an arrow). W e ruled out the possibility that these particles w e r e f o r m e d by H 2 S O 4 etching of a flat surface because natural crystals not treated with H 2 S Q aqueous solution had the same surface m o r p h o l o g y . Thus, the surface m o r p h o l o g y that we o b s e r v e d i s intrinsic for m o r d e n i t e crystals.
Fig. 1. Model for the surface structure of mordenite: (a) (100), (b) (010) and (c) (001). The positions of the hydroxyl groups on top of each surface are indicated by a blue circle. The (001) surface is perpendicular to the c axis of the crystal and has 12-ring pores (7.7 × 6.5 ,~) arranged in a plane. The (100) surface is rather flat, and has four surface oxygen rows and a groove elongated along the c axis. Note that there are no pores. The (010) surface has oxygen rows and a groove composed of 8-ring pores (8.4 x 5.7 ,~) that are linked by T - O - T (where T is either Si or A1) bonds and are aligned along the c axis of the crystal. Four 8-ring pores are outlined by dotted white lines.
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S. Yamamoto et al. / Chemical Physics Letters 260 (1996) 208-214
3.2. Atomic-scale image Figs. 3a and 3b are representative atomic-scale top-view images obtained by zooming in on the flat surface or the surface of aggregated small particles shown in Fig. 2. The periodic structures that we
observed can be divided into two patterns. The first is shown in Fig. 3a. Rows A of bright spots are separated at an equal distance of about 20.5 ,~. Between these rows are an additional three rows (B, C and D) in which the repeat distances between rows AB, BC, CD and DA are 4.6, 3.5, 4.0 and 8.4 ,~,
Fig. 3. Top-view AFM images of mordenite at an atomic level in 0.1 M NaOH aqueous solution. (a) The (100) surface and (b) the (010) surface. The scan rates are 32.60 and 21.36 Hz, respectively. The image for the (100) surface obtained at a scan rate of 48.83 Hz (c) is shown for comparison.
s. Yamamoto et a l . / Chemical Physics Letters 260 (1996) 208-214
respectively. Between rows A and D runs a relatively wide groove (dark part in the figure) parallel to the c axis. Although the resolution of bright spots along the c axis is poor, a uniform spacing of 7.5 ,~ can be seen locally. These repeat distances agree well with the distances between coordinatively unsaturated surface oxygen atoms calculated from crystal structural data for the (100) surface as shown in Fig. la [10]. In an aqueous solution, these coordinatively unsaturated surface oxygen atoms are likely to react immediately with water molecules, yielding surface hydroxyl groups. Therefore, the spots on each row probably correspond to surface hydroxyl groups on the (100) surface. Dark, relatively wide bands neighboring the rows A possibly correspond to the wide grooves in the (100) surface (Fig. la). The rows A neighboring the wide groove were more clearly imaged as compared with other rows (i.e. B, C and D). Even if rows B, C and D were not able to be seen, rows A were imaged. All AFM images are a product of tip-sample interaction. Thus, a possible reason for clearer imaging of rows A is that the bending which a cantilever experiences when it passes from a groove to a flat surface (or vice versa) is large enough to be detected. Even at higher scan rates, a periodicity of 20.5 ,~ could still be observed. However, the three rows B, C and D, existing in this periodical unit decreased to two rows. In addition, the resolution along the c axis deteriorated. Fig. 3c shows an example of an image obtained at the scan rate of 48.83 Hz. When the scan rate is too high, the cantilever may not be able to follow the real surface corrugation at the atomic level. Moreover, the cantilever bending at a groove may couple with that formed at rows B, C and D, thus leading to the formation of artificial rows. These may be the reasons why imaging of the true atomic configuration of the (100) surface was unsuccessful at higher scan rates. Fig. 3b shows the second periodic structure that we observed. Rows with alternate spacings of 8.4 and 9.0 ,~ run parallel to the c axis. These repeat distances agree well with the values calculated from crysta! structural data for the (010) surface (9.69 and 8.40 A in Fig. lb) [10]. Thus, the image shown in Fig. 3b corresponds to the alignment of surface oxygen atoms (or surface hydroxyl groups as mentioned above) on the (010) surface.
213
The measured distances between surface oxygen atoms (or hydroxyl groups) on both the (100) and (010) surfaces agree well with those calculated from crystal structural data within the estimated error of ~< 10% which is due to drift and other uncertainties in the AFM system. This close agreement indicates that both the (100) and (010) surfaces of natural mordenite crystals seem to be a termination of the bulk structure without any reconstruction. Based on the present results, we propose that the bending of a cantilever caused by the periodic corrugation of the surface determines the resolution quality of AFM images of mordenite surfaces. If the bending of a cantilever due to a surface corrugation is too great, the small bending of the cantilever due to periodic atomic features will be difficult to detect. Thus, atomic resolution is impossible. This may be the case for the (010) surface of mordenite. The resolution along the c axis of the (010) surface seems to be poorer than that of the (100) surface. The individual oxygen atoms (or hydroxyl groups) could not be resolved. The repeat distance of these oxygen atoms calculated from crystal structural data is 3.5 ,~. The resolution limit of our AFM system can be ruled out as the reason for this poor resolution along the c axis because the repeat distance of 3.5 ,~ between rows B and C on the (100) surface could be resolved (Fig. 3a). The (010) surface has 8-ring pores (the dotted white lines in Fig. l b ) t h a t are linked by T - O - T (where T is either Si or Al) bonds and are aligned along the c axis of the crystal. The interaction of the cantilever with these 8-ring pores may be strong and thus significantly contribute to the large bending of the cantilever. Thus, the resolution along the c axis was so poor that individual oxygen atoms (or hydroxyl groups) could not be resolved as can be seen in Fig. 3b. On the contrary, if the surface corrugation is moderate because the surface has no pores and is relatively flat at the atomic level, then atomic resolution can be achieved by setting a proper measurement parameter such as scan rate. This may be the case for the (100) surface of mordenite as mentioned above. If this hypothesis is correct, AFM images of the (001) surface of mordenite will correspond to ordered 12-ring pores. The reason is that the bending of a cantilever due to the ordered corrugation composed of 12-ring pores will be considerably larger than that caused by surface hydroxyl
214
S. Yamamoto et al. / Chemical Physics Letters 260 (1996) 208-214
groups. Our current research involves the imaging of the (001) surface of mordenite. The (010) surface of stilbite has 6-ring pores elongated along the a axis and has periodic surface corrugation similar to these of the (010) surface of mordenite [11]. Its images [9] resemble our image for the (010) surface of mordenite; namely, the rows of oxygen on the (010) stilbite surface are clearly discernible, however, the resolution along the a axis is poor compared with that resolution perpendicular to the a axis. Thus, periodic surface corrugation may be the predominant factor in the resolution of AFM images of the zeolite surface.
zeolite, other zeolite surfaces must be imaged at the same imaging conditions and then these images must be compared with models of the surfaces.
Acknowledgement This work was performed by Mitsui Toatsu Chemicals, Inc., as a part of a research and development joint project on civil industrial technology supported by the New Energy and Industrial Technology Development Organization.
References 4. Conclusion Atomic force microscopy has been applied to reveal the surface structure of natural mordenite crystals. Wide-scan imaging revealed a flat surface partially covered with aggregates of mordenite particles that are rectangular with their long axis parallel to the c axis of the crystal. Atomic-scale imaging revealed an alignment of hydroxyl groups on both the (010) and (100) surfaces. Particularly, the hydroxyl groups on the (100) surface could be resolved. Both surfaces seem to be an ideal termination of the bulk structure without any reconstruction. By comparing these images with surface structure models that are based on bulk crystallographic data, we concluded that the magnitude of the periodical surface corrugation determines the resolution of the AFM images of mordenite. To confirm whether or not this hypothesis is also valid for other types of
[1] M.S. Whittingham and A.J. Jacobson, Intercalation chemistry (Academic Press, New York, 1982). [2] J. Weitkamp, Studies in surface science and catalysis, Vol. 65. Catalysis and adsorption by zeolites, eds. G. Ohlmann, H. Pfeifer and R. Fricke (Elsevier, Amsterdam, 1991). [3] D.W. Breck, Zeolite molecular sieves (Wiley, New York, 1974). [4] V. Alfredson, T. Ohsuna, O. Terasaki and J.O. Bovin, Angew. Chem. Int. Ed. Engl. 32 (1993) 1210. [5] W.M. Meier and D.H. Olson, Atlas of zeolite structure types, 3rd Ed. (Butterworth-Heinemann, London, 1992). [6] A.L. Weisenhom, J.E. MacDougall, S.A.C. Gould, S.D. Cox, W.S. Wise, J. Massie, P. Maivald, V.B. Elings, G.D. Stucky and P.K. Hansma, Science 247 (1990) 1330. [7] J.E. MacDougall, S.D. Cox, G.D. Stucky, A.L. Weisenhom, P.K. Hansma and W.S. Wise, Zeolites 11 (1990) 429. [8] L. Scandella, N. Kruse and R. Prins, Surf. Sci. 281 (1990) L331. [9] M. Komiyama and T. Yashima, Jpn. J. Appl. Phys. 33 (1994) 3761. [10] A. Alberti, P. Pavoli and G. Vezzalini, Z. Krist. 175 (1986) 249. [11] E. Galli, Acta Cryst. B 27 (1971) 833.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 260 (1996) 208-214
Surface structure of natural crystals of mordenite as imaged by atomic force microscopy Sadaaki Yamamoto a, Osamu Matsuoka a, Shouko Sugiyama a, Tadatoshi Honda a, Yasuyuki Banno b, Hisakazu Nozoye c a Central Research Institute, Mitsui Toatsu Chemicals Inc., 1190 Kasama-eho. Sakae-ku, Yokohama 247, Japan b Geologieal Survey o f Japan. 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan c National Institute o f Materials and Chemical Research, 1-1 Higashi, Tsukuba, lbaraki 305, Japan
Received I November 1995; in final form 12 July 1996
Abstract Atomic force microscopy (AFM) has been applied to examine the surface of natural mordenite crystals in an 0.1 M NaOH aqueous solution. Wide-scan AP-M images revealed that the crystals have a surface partially covered with aggregates of mordenite fine particles. Atomic-scale images showed surface structure that agreed well with the models of the (100) and (010) surface. The hydroxyl groups on the (100) surface could be resolved. The images also revealed that an important factor that determines the resolution of an AFM image of mordenite is the magnitude of the periodic corrugation on the crystal surface.
1. Introduction Zeolites are microporous crystalline aluminosilicates with a framework structure composed o f corner-sharing SiO 4 - and AIO45- tetrahedra [1]. Much interest has been shown recently in natural and synthetic zeolites due to their practical uses as catalysts, molecular sieves, detergents [2,3], etc. To understand the physical and chemical properties that determine a zeolite's catalytic, ion-exchange and adsorption performances, the surface structure as well as the bulk structure are particularly important. Surface structural information is also important for understanding the growth mechanism of a zeolite [4]. The bulk structures of many types of zeolites have already been determined [5]. However, comparatively little is
known yet about the external surface structure due to the lack o f a suitable characterization method. Recent atomic force microscopy ( A F M ) studies on natural zeolite crystals have demonstrated the suitability of A F M for surface structural elucidation of zeolites [6-9]. However, interpretations of the images obtained thus far are controversial due to the resolution o f the images, Hansma et al. [6,7] obtained A F M images of natural crystals of clinoptilotite, scolecite, stilbite and faujasite and interpreted them as a tetrahedral unit structure of SiO 4 - and AIO45- . On the other hand, Scandella et al. [8] imaged the (010) surface of natural heulandite crystals, which has the isostructural framework of clinoptilolite, in an aqueous sodium hydroxide solution, and claimed to have resolved surface hydroxyl
0009-2614/96/$12.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PII 009-261 4(96)00833-0
S. Yamamoto et a l . / Chemical Physics Letters 260 (1996) 208-214
groups. Recently, Komiyama et al. [9] examined the surface structure of natural stilbite and heulandite and claimed to have resolved corrugations at almost an atomic level. However, the periodic structure on the heulandite surface they obtained did not match the surface structure expected from its bulk crystallographic data. Thus, more work is now needed to clarify the factors leading to these discrepancies in the interpreted resolution of AFM images. In this work, we used AFM to image the surface of natural mordenite crystals. We chose natural mordenite as our specimen for the following two reasons. The first is that among the many classes of zeolite, mordenite is of great interest as a catalyst, thus making its surface structure of particular importance. The second is that a natural mordenite crystal is expected to show three distinct surfaces that differ in the magnitude and periodicity of the corrugation as shown in Fig. 1. Therefore, we expected to be able to examine the effect of the magnitude and periodicity of surface corrugations on the atomic resolution in the AFM image of a zeolite surface. We did this by imaging each surface of mordenite and then comparing the images with the corresponding surface-structure model that is based on bulk crystallographic data.
2. Experimental
2.1. Materials and sample preparation The natural mordenite crystals that we used were from Arasawa, Kindaichi-mura, Ninohe-gun, Iwate, Japan (registered specimen of the Geological Survey of Japan, GSJ M16609). The powder X-ray pattern, obtained by a JEOL JDX 8030W EDS, and electron diffraction results, which were obtained by using a high resolution analytical electron microscope (JEOL JEM-2010), identified the crystals as being orthorhombic (unit cell lattice parameters a = 18.10, b = 20.42 and c = 7.51 ,~) with chemical composition (Nal.llK0.a4Ca0.88)A14.26Si23.21070.1. The crystals were cleaned in a 2N H2SO 4 aqueous solution at room temperature for 3 h to remove organic residues and metal ion contaminants. Then, the crystals were
209
rinsed three times with ultra-pure water (18 M,O cm, TORAYPURE LV-08 " T O R A Y " ) . After the H 2SO4 treatment, the amount of Na + in the crystals decreased and the crystals were H + form mordenite. The side of the needle-like crystal with an average size of 1 mm long and several tens /zm was glued onto a steel plate by epoxy resin (Araldite Rapid, CIBA-GEIGY), so that the c axis was parallel to the plate. We have investigated about 10 crystals.
2.2. AFM imaging AFM measurements were done for crystals placed in 0.1 M NaOH aqueous solution in a liquid cell unit (Digital Instruments, Santa Barbara, CA). Before imaging, the crystals were kept in this liquid cell filled with 0.1 M NaOH aqueous solution for = 30 min to allow an equilibrium state to be reached. For flexibility in choosing an imaging spot, we used the cell without an O-ring that was normally used for sealing. The imaging was done using a Nanoscope III a (Digital Instruments, Santa Barbara, CA) operated in constant-force mode with scan rates ranging from 3 to 49 Hz. We used triangular Si3N 4 microcantilevers called nanoprobes (NP-S, Digital Instruments, Santa Barbara, CA) that had a spring constant K of 0.58 and were approximately 100/zm long and 0.6 /xm thick with integrated pyramidal tips. The imaging forces were between 5 and 20 nN. Each scan for the same area gave similar images regardless of imaging force, insuring that the applied forces were so weak that the imaging was nondestructive. The periodic distances of the images at the atomic level were determined within the estimated error of ~< 10% by two-dimensional fast Fourier transform. The images presented here were not subjected to any filtering procedure.
3. Results and discussion
3.1. Wide-scan image Fig. 2 shows a representative wide-scan image of a natural crystal of mordenite. Because the crystals were not crushed, the AFM images accurately repre-
S. Yamamoto et al. / Chemical Physics Letters 260 (1996) 208-214
210
A 0 0 V
A A 0 0
c~
S. Yamamow et al. / Chemical Physics Letters" 260 (1996) 208-214
211
Fig. 2. Top-view AFM images of mordenite on a macroscopic scale in 0.1 M NaOH aqueous solution.
sent the f o r m o f the surfaces as they occurred. The image clearly reveals that the m o r p h o l o g y is a sheaf-like structure c o m p o s e d of needle-like crystals. The A F M images that we obtained r e v e a l e d the crystal g r o w t h - i n d u c e d feature that a flat surface is partially c o v e r e d with aggregates o f mordenite fine particles that are rectangular with their long axis
parallel to the c axis o f the crystal (indicated by an arrow). W e ruled out the possibility that these particles w e r e f o r m e d by H 2 S O 4 etching of a flat surface because natural crystals not treated with H 2 S Q aqueous solution had the same surface m o r p h o l o g y . Thus, the surface m o r p h o l o g y that we o b s e r v e d i s intrinsic for m o r d e n i t e crystals.
Fig. 1. Model for the surface structure of mordenite: (a) (100), (b) (010) and (c) (001). The positions of the hydroxyl groups on top of each surface are indicated by a blue circle. The (001) surface is perpendicular to the c axis of the crystal and has 12-ring pores (7.7 × 6.5 ,~) arranged in a plane. The (100) surface is rather flat, and has four surface oxygen rows and a groove elongated along the c axis. Note that there are no pores. The (010) surface has oxygen rows and a groove composed of 8-ring pores (8.4 x 5.7 ,~) that are linked by T - O - T (where T is either Si or A1) bonds and are aligned along the c axis of the crystal. Four 8-ring pores are outlined by dotted white lines.
212
S. Yamamoto et al. / Chemical Physics Letters 260 (1996) 208-214
3.2. Atomic-scale image Figs. 3a and 3b are representative atomic-scale top-view images obtained by zooming in on the flat surface or the surface of aggregated small particles shown in Fig. 2. The periodic structures that we
observed can be divided into two patterns. The first is shown in Fig. 3a. Rows A of bright spots are separated at an equal distance of about 20.5 ,~. Between these rows are an additional three rows (B, C and D) in which the repeat distances between rows AB, BC, CD and DA are 4.6, 3.5, 4.0 and 8.4 ,~,
Fig. 3. Top-view AFM images of mordenite at an atomic level in 0.1 M NaOH aqueous solution. (a) The (100) surface and (b) the (010) surface. The scan rates are 32.60 and 21.36 Hz, respectively. The image for the (100) surface obtained at a scan rate of 48.83 Hz (c) is shown for comparison.
s. Yamamoto et a l . / Chemical Physics Letters 260 (1996) 208-214
respectively. Between rows A and D runs a relatively wide groove (dark part in the figure) parallel to the c axis. Although the resolution of bright spots along the c axis is poor, a uniform spacing of 7.5 ,~ can be seen locally. These repeat distances agree well with the distances between coordinatively unsaturated surface oxygen atoms calculated from crystal structural data for the (100) surface as shown in Fig. la [10]. In an aqueous solution, these coordinatively unsaturated surface oxygen atoms are likely to react immediately with water molecules, yielding surface hydroxyl groups. Therefore, the spots on each row probably correspond to surface hydroxyl groups on the (100) surface. Dark, relatively wide bands neighboring the rows A possibly correspond to the wide grooves in the (100) surface (Fig. la). The rows A neighboring the wide groove were more clearly imaged as compared with other rows (i.e. B, C and D). Even if rows B, C and D were not able to be seen, rows A were imaged. All AFM images are a product of tip-sample interaction. Thus, a possible reason for clearer imaging of rows A is that the bending which a cantilever experiences when it passes from a groove to a flat surface (or vice versa) is large enough to be detected. Even at higher scan rates, a periodicity of 20.5 ,~ could still be observed. However, the three rows B, C and D, existing in this periodical unit decreased to two rows. In addition, the resolution along the c axis deteriorated. Fig. 3c shows an example of an image obtained at the scan rate of 48.83 Hz. When the scan rate is too high, the cantilever may not be able to follow the real surface corrugation at the atomic level. Moreover, the cantilever bending at a groove may couple with that formed at rows B, C and D, thus leading to the formation of artificial rows. These may be the reasons why imaging of the true atomic configuration of the (100) surface was unsuccessful at higher scan rates. Fig. 3b shows the second periodic structure that we observed. Rows with alternate spacings of 8.4 and 9.0 ,~ run parallel to the c axis. These repeat distances agree well with the values calculated from crysta! structural data for the (010) surface (9.69 and 8.40 A in Fig. lb) [10]. Thus, the image shown in Fig. 3b corresponds to the alignment of surface oxygen atoms (or surface hydroxyl groups as mentioned above) on the (010) surface.
213
The measured distances between surface oxygen atoms (or hydroxyl groups) on both the (100) and (010) surfaces agree well with those calculated from crystal structural data within the estimated error of ~< 10% which is due to drift and other uncertainties in the AFM system. This close agreement indicates that both the (100) and (010) surfaces of natural mordenite crystals seem to be a termination of the bulk structure without any reconstruction. Based on the present results, we propose that the bending of a cantilever caused by the periodic corrugation of the surface determines the resolution quality of AFM images of mordenite surfaces. If the bending of a cantilever due to a surface corrugation is too great, the small bending of the cantilever due to periodic atomic features will be difficult to detect. Thus, atomic resolution is impossible. This may be the case for the (010) surface of mordenite. The resolution along the c axis of the (010) surface seems to be poorer than that of the (100) surface. The individual oxygen atoms (or hydroxyl groups) could not be resolved. The repeat distance of these oxygen atoms calculated from crystal structural data is 3.5 ,~. The resolution limit of our AFM system can be ruled out as the reason for this poor resolution along the c axis because the repeat distance of 3.5 ,~ between rows B and C on the (100) surface could be resolved (Fig. 3a). The (010) surface has 8-ring pores (the dotted white lines in Fig. l b ) t h a t are linked by T - O - T (where T is either Si or Al) bonds and are aligned along the c axis of the crystal. The interaction of the cantilever with these 8-ring pores may be strong and thus significantly contribute to the large bending of the cantilever. Thus, the resolution along the c axis was so poor that individual oxygen atoms (or hydroxyl groups) could not be resolved as can be seen in Fig. 3b. On the contrary, if the surface corrugation is moderate because the surface has no pores and is relatively flat at the atomic level, then atomic resolution can be achieved by setting a proper measurement parameter such as scan rate. This may be the case for the (100) surface of mordenite as mentioned above. If this hypothesis is correct, AFM images of the (001) surface of mordenite will correspond to ordered 12-ring pores. The reason is that the bending of a cantilever due to the ordered corrugation composed of 12-ring pores will be considerably larger than that caused by surface hydroxyl
214
S. Yamamoto et al. / Chemical Physics Letters 260 (1996) 208-214
groups. Our current research involves the imaging of the (001) surface of mordenite. The (010) surface of stilbite has 6-ring pores elongated along the a axis and has periodic surface corrugation similar to these of the (010) surface of mordenite [11]. Its images [9] resemble our image for the (010) surface of mordenite; namely, the rows of oxygen on the (010) stilbite surface are clearly discernible, however, the resolution along the a axis is poor compared with that resolution perpendicular to the a axis. Thus, periodic surface corrugation may be the predominant factor in the resolution of AFM images of the zeolite surface.
zeolite, other zeolite surfaces must be imaged at the same imaging conditions and then these images must be compared with models of the surfaces.
Acknowledgement This work was performed by Mitsui Toatsu Chemicals, Inc., as a part of a research and development joint project on civil industrial technology supported by the New Energy and Industrial Technology Development Organization.
References 4. Conclusion Atomic force microscopy has been applied to reveal the surface structure of natural mordenite crystals. Wide-scan imaging revealed a flat surface partially covered with aggregates of mordenite particles that are rectangular with their long axis parallel to the c axis of the crystal. Atomic-scale imaging revealed an alignment of hydroxyl groups on both the (010) and (100) surfaces. Particularly, the hydroxyl groups on the (100) surface could be resolved. Both surfaces seem to be an ideal termination of the bulk structure without any reconstruction. By comparing these images with surface structure models that are based on bulk crystallographic data, we concluded that the magnitude of the periodical surface corrugation determines the resolution of the AFM images of mordenite. To confirm whether or not this hypothesis is also valid for other types of
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