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Imaging of zeolite surface structures by atomic force microscopy
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Surface Science North-Holland
Letters
: :
..I...............,,,,,,,,,.,.,,~,,,,,,
:+::::~.::.:.:..... >:....
281 (1993) L331-L334
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.,., ,,_,
“““‘.‘.‘.::.:.~,~:ii:i
surface science letters
L
Surface Science Letters
Imaging of zeolite surface structures by atomic force microscopy L. Scandella, N. Kruse and R. Prim Swiss Federal Institute Received
27 August
ETH, Institute 1992; accepted
of Chemical for publication
Engineering,
8092 Ziirich, Switzerland
17 November
1992
Atomic force microscopy has been employed to study the (010) surface structure of the natural zeolite heulandite. Images on a macroscopic scale exhibit large terraces separated from each other by half of the unit cell length b. Molecular resolution is obtained when measured in O.lM NaOH. The surface ac unit cell is about 26% larger in area than the bulk unit cell.
1. Introduction
2. Experimental
Zeolites are microporous materials with a framework structure in which tetrahedrally coordinated silicon or aluminium atoms are linked to one another through oxygen bridges [ll. The negative charge of the framework is balanced by cations which are mobile along well-defined channels of the crystal and easily exchangeable. Natural and synthetic zeolites are of great interest in chemical industry due to their applications as catalytically active materials, as molecular sieves or detergents [2]. The bulk structure of many zeolites is well characterised [31; however, comparatively little is known yet about the external surface structure. Scanning tunneling microscopy (STM) [4] and atomic force microscopy (AFM) 151are met with increasing interest in various areas of research in surface chemistry where structural information on the atomic scale is desired. For recent reviews on AFM applications see ref. [6]. Earlier STM and AFM experiments with natural and synthetic zeolites have shown that molecular resolution is possible [7,81. Natural zeolites are interesting candidates for AFM imaging since monocrystals of sufficient size are available. This Letter presents AFM data that have been obtained in a study with a natural heulandite.
Heulandite, Ca4(A18Si28072), has a monoclinic crystal symmetry with trait cell latticeoparameters a = 17.7 A, b= 17.94 A, c=7.42 A and p = 116.24 [9]. Fig. la shows the framework of the heulandite viewed perpendicular to the (001) plane [lo]. Open channels of 8- and lo-member tetrahedral rings intersect the crystal parallel to the ac crystallographic plane [ill. This arrangement of channels is associated with a layering of silicate tetrahedra. The layers are linked by only 4 oxygen atoms per unit cell at heights of 0 and l/2 of the crystallographic b axis. Thus, cleaving the crystal along the ac plane is energetically favoured, i.e., the (010) plane is the cleavage plane. After cleavage, there are oxygen atoms or tetrahedrally coordinated silicon or aluminium atoms in the outermost surface layer. In either case, however, ambient air contact immediately leads to reaction with water and, consequently, to the formation of hydroxyl groups which are more or less perpendicularly oriented to the surface. Their positions within the (010) plane clearly indicate the existence of two different distances along the a axis (in fact, caused by the presence of 8- and lo-member rings) and of a uniform spacing along the c axis (see fig. lb).
0039-6028/93/$06.00
0 1993 - El sevier Science
Publishers
B.V. All rights reserved
background
L. Scandella et al. / Imaging of zeolite surface structures by atomic force microscopy
The atomic force microscope used in this study is a commercially available instrument (Nanoscope II, Digital Instrument) which can be operated in air as well as in a liquid cell. Triangular silicon nitride cantilevers with integrated pyramidal tips were used. For the measurements to be reported here, cantilevers with spring constants of 0.032 and 0.064 N/m were chosen. When measured in contact mode within the liquid, the attractive forces were found to be between 1 and 10 nN. The piezos which control vertical distance between probing tip and samples as well as lateral scanning were carefully calibrated. An accuracy of about 5% has been reached on the basis of comparative studies using a number of substrates (e.g. graphite, MoS,, WS,) with different lattice constants. The data presented have not been subject to any filtering. Samples of natural heulandite were obtained from Poona, India. Temperature gravimetric analysis (TGA) reveals the same characteristics as reported in ref. 181.Crystals have been cleaved in air and fixed by epoxy glue on a sample holder. AFM measurements were performed either in air or in O.lM NaOH. Samples used for AFM measurements in NaOH have been subsequently studied with X-ray diffraction. ThisOresulted in lattice parameters (a = 17.26 f 0.03 A, b = 17.81 f 0.03 A, c = 7.39kO.01 A, p = 113.8” f 0.2) close to the literature values.
3. Results and discussion In fig. 2 an AFM image is presented which has been obtained by scanning over an area of 1 x 1 pm* of the heulandite cleavage plane. Large regions with atomically flat cleavage terraces can be recognised. The cleavage tips have an in-plane angle of around 20”. The st,ep heights of the cleavage terraces are about 9 A, which is just half of the unit cell length b. Quite generally, the images show that the crystals have cleaved uniformly along the mirror planes at multiples of b/2.
Fig. 3a shows an AFM image obtained by zooming into a region of 12.5 X 12.5 nm’ of the (010) surface plane. The measurements have been performed in O.lM NaOH after keeping the crystal in the liquid cell for 36 h. Periodic structures are clearly discernible in fig. 3a. They can be related to the a axis and the c axis of the (010) plane of the heulandite crystal. While rows of features appear with alternative distances along the a axis, rows with uniform distance are present along the c axis. This is expected, as there are 8membered ring structures along the c axis but alternating 8- and lo-membered structures along the a axis. The periodicities found in the image can be analysed in greater detail by means of two-dimensional fast-Fourier-transform (2DFFT). Indexing in fig. 3b has been performed by
Fig. 1. (a) Model of the heulandite framework structure. Dots represent tetrahedrally coordinated which form 8- or lo-member rings. (b) b axis projection of the tetrahedral framework. The positions of the (010) surface are indicated additionally.
silicon or aluminium of the hydroxyl groups
atoms on top
L. Scandella et al. / Imaging of zeolite surface structures by atomic force microscopy
Fig. 2. Top view AFM image, obtained for the (010) plane of heulandite in O.lM NaOH. The step heights are determined to be around 9 A.
comparison with the real space. The (001) and (200) spots are the two most intense spots in the reciprocal space. Real space distances of 7.4 and
9.4 A, respectively, are calculated from the positions. These spot values are by 11% and 17% larger than the bulk lattice values of 6.6 and 7.8 A measured with X-ray diffraction [lo]. The angle between the (200) and (001) spots is 115”. On the basis of the 2D-FFT spectra the surface unit cell has been determined to be a = 21.0 A and c = 8.4 A. The surface unit cell area acsin/3 is about 26% larger than the bulk unit cell area. To our knowledge, the observation of a surface unit cell with larger dimensions than the bulk unit cell has not been reported before in AFM studies of zeolites. Recently, Weisenhorn et al. [7] have published AFM studies of the zeolite clinoptilolite in diluted tert-butyl ammonium chloride. The interpretation given by these authors is largely based on direct molecule or cluster imaging. A reconstruction of the zeolite unit cell is not considered, though this, as far as can be judged from their data, cannot be excluded. The structures of heulandite and clinoptilolite are closely related. An important difference is the higher amount of Ca2+ in the framework of the heulandite. It is very probable that in NaOH an ionic exchange of Ca ‘+ by Na+ takes place in the
lb) Fig. 3. (a) Image of heulandite (010) on the molecular scale measured in O.lM NaOH. The image size is 125 X 125 A*. Six unit cells are indicated (a = 21.0 A, c = 8.4 A). (b) Two-dimensional fast-Fourier-transform of fig. 2.
L. Scandella et al. / Imaging of zeolite surface structures by atomic force microscopy
near surface region during our experiments. As the charge densities associated with different hydrated cations change, a framework distortion, at least at the surface, becomes likely. Alterations of the bulk lattice parameters have indeed been observed quite frequently, e.g. after dehydration [12,131 or Ca2+ substitution by Kf, Na+ [14] or NH: [13]. The general trend seems to be an increase of the unit cell dimensions with increasing amounts of divalent cations [15]. However, this must not necessarily be true for the surface regions of the crystal and can possibly be reversed if there are overstoichiometric amounts of Naf as in our case. Of course, this remains a speculation which can be checked by replacing Naf through alkali ions of larger or smaller radius. This will be done in the near future.
4. Conclusions The structure of (010) surfaces of heulandite has been studied in air and in O.lM NaOH. We have shown by AFM that real space imaging with molecular resolution can be obtained. Typical step heights of the terraces have been determined to be multiples of half of the unit cell length b. The surface unit cell is found to be enlarged as compared to the bulk. An ionic exchange of Cazf by Na+ has been advanced as an explanation for this result.
Acknowledgements
We would like to thank V. Gramlich for single crystal X-ray diffraction experiments, T. Stamm for TGA measurements, Ch. Barlocher for the framework model of heulandite and G. Harvey, H. Kouwenhoven, A. Weisenhorn and H. Haefke for helpful discussions. The Swiss National Science Foundation is acknowledged for financial support.
References
111See, e.g., Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis 58, Eds. H. van Bekkum, E.M. Flanigen and J.C. Jansen (Elsevier, Amsterdam, 1991). PI See, e.g., J. Weitkamp, in: Catalysis and Adsorption by Zeolites, Studies in Surface Science and Catalysis, Vol. 65, Eds. G. Qhlmann, H. Pfeifer and R. Fricke (Elsevier Amsterdam, 1991) pp. 21ff; Occurrence, Properties and Utilization of Natural Zeolites, Eds. D. Kallo and H.S. Sherry (Akadtmai Kidao, Budapest, 19881. [31 J.M. Thomas, in: Zeolites: Facts, Figures, Future, Eds. P.A. Jacobs and R.A. van Santen (Elsevier, Amsterdam, 1989) pp. 3ff; L.B. McCusker, Acta Cryst. A 47 (1991) 297. 141 G. Binnig, H. Rohrer, C. Gerber and E. Weibel, Phys. Rev. Lett. 49 (1982) 57. [51 G. Binnig, C.F. Quate and C.H. Gerber, Phys. Rev. Lett. 12 (1986) 930. [61 See, e.g., D. Rugar and P. Hansma, Phys. Today 43 (1990) 23; E. Meyer and H. Heinzelmann, in: Scanning Tunneling Microscopy II, Vol. 28, Springer Series in Surface Sciences, Eds. R. Wiesendanger and H.-J. Giintherodt (Springer, Berlin, 1992); J. Frommer, Angew. Chem. 104 (1992) 1325. [71 A.L. Weisenhorn, J.E. Mac Dougall, S.A.C. Gould, S.D. Cox, W.S. Wise, J. Massie, P. Maivald, V.B. Elings, G.D. Stuckey and P.K. Hansma, Science 247 (1990) 1330. 181J.C. Jansen, J. Schoonman, H. van Bekkum and V. Pinet, Zeolites 11 (1991) 306; J.E. Mac Dougall, S.D. Cox, G.D. Stucky, A.L. Weisenhorn, P.K. Hansma and W.S. Wise, Zeolites 11 (19911 429; P. Rasch, W.M. Heckel, H.W. De&man and W. HIberle, Mater. Res. Sot. Symp. Proc. 233 (1991) 287. [91 G. Gottardi and E. Galli, Natural Zeolites (Springer, Berlin, 19851. 1101W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types (Butterworths, Washington, DC, 1992). [ill A.B. Merkle and M. Slaughter, Am. Miner. 53 (19681 1120. [=I A. Alberti, Tschermaks Miner. Petr. Mitt. 19 (19731 173. [131 W.J. Mortier and J.R. Pearce, Am. Miner. 66 (19811 309. iI41 A.O. Shepard and H.C. Starkey, Miner. Sot. India IMA (1966) 155. [15] J.R. Boles, Am. Miner. 57 (1972) 1463.
..“““.‘...‘.‘...,.i
. . . . ... .... ..... ..... . ..............~.......
““.’ ‘...l’.‘. ...I.‘.. . . . ..i... .:‘.:‘::“.‘...,“;‘... ::::.:.:.::::::::::.:.!+ ..\..
: : ::t
Surface Science North-Holland
Letters
: :
..I...............,,,,,,,,,.,.,,~,,,,,,
:+::::~.::.:.:..... >:....
281 (1993) L331-L334
....
.,., ,,_,
“““‘.‘.‘.::.:.~,~:ii:i
surface science letters
L
Surface Science Letters
Imaging of zeolite surface structures by atomic force microscopy L. Scandella, N. Kruse and R. Prim Swiss Federal Institute Received
27 August
ETH, Institute 1992; accepted
of Chemical for publication
Engineering,
8092 Ziirich, Switzerland
17 November
1992
Atomic force microscopy has been employed to study the (010) surface structure of the natural zeolite heulandite. Images on a macroscopic scale exhibit large terraces separated from each other by half of the unit cell length b. Molecular resolution is obtained when measured in O.lM NaOH. The surface ac unit cell is about 26% larger in area than the bulk unit cell.
1. Introduction
2. Experimental
Zeolites are microporous materials with a framework structure in which tetrahedrally coordinated silicon or aluminium atoms are linked to one another through oxygen bridges [ll. The negative charge of the framework is balanced by cations which are mobile along well-defined channels of the crystal and easily exchangeable. Natural and synthetic zeolites are of great interest in chemical industry due to their applications as catalytically active materials, as molecular sieves or detergents [2]. The bulk structure of many zeolites is well characterised [31; however, comparatively little is known yet about the external surface structure. Scanning tunneling microscopy (STM) [4] and atomic force microscopy (AFM) 151are met with increasing interest in various areas of research in surface chemistry where structural information on the atomic scale is desired. For recent reviews on AFM applications see ref. [6]. Earlier STM and AFM experiments with natural and synthetic zeolites have shown that molecular resolution is possible [7,81. Natural zeolites are interesting candidates for AFM imaging since monocrystals of sufficient size are available. This Letter presents AFM data that have been obtained in a study with a natural heulandite.
Heulandite, Ca4(A18Si28072), has a monoclinic crystal symmetry with trait cell latticeoparameters a = 17.7 A, b= 17.94 A, c=7.42 A and p = 116.24 [9]. Fig. la shows the framework of the heulandite viewed perpendicular to the (001) plane [lo]. Open channels of 8- and lo-member tetrahedral rings intersect the crystal parallel to the ac crystallographic plane [ill. This arrangement of channels is associated with a layering of silicate tetrahedra. The layers are linked by only 4 oxygen atoms per unit cell at heights of 0 and l/2 of the crystallographic b axis. Thus, cleaving the crystal along the ac plane is energetically favoured, i.e., the (010) plane is the cleavage plane. After cleavage, there are oxygen atoms or tetrahedrally coordinated silicon or aluminium atoms in the outermost surface layer. In either case, however, ambient air contact immediately leads to reaction with water and, consequently, to the formation of hydroxyl groups which are more or less perpendicularly oriented to the surface. Their positions within the (010) plane clearly indicate the existence of two different distances along the a axis (in fact, caused by the presence of 8- and lo-member rings) and of a uniform spacing along the c axis (see fig. lb).
0039-6028/93/$06.00
0 1993 - El sevier Science
Publishers
B.V. All rights reserved
background
L. Scandella et al. / Imaging of zeolite surface structures by atomic force microscopy
The atomic force microscope used in this study is a commercially available instrument (Nanoscope II, Digital Instrument) which can be operated in air as well as in a liquid cell. Triangular silicon nitride cantilevers with integrated pyramidal tips were used. For the measurements to be reported here, cantilevers with spring constants of 0.032 and 0.064 N/m were chosen. When measured in contact mode within the liquid, the attractive forces were found to be between 1 and 10 nN. The piezos which control vertical distance between probing tip and samples as well as lateral scanning were carefully calibrated. An accuracy of about 5% has been reached on the basis of comparative studies using a number of substrates (e.g. graphite, MoS,, WS,) with different lattice constants. The data presented have not been subject to any filtering. Samples of natural heulandite were obtained from Poona, India. Temperature gravimetric analysis (TGA) reveals the same characteristics as reported in ref. 181.Crystals have been cleaved in air and fixed by epoxy glue on a sample holder. AFM measurements were performed either in air or in O.lM NaOH. Samples used for AFM measurements in NaOH have been subsequently studied with X-ray diffraction. ThisOresulted in lattice parameters (a = 17.26 f 0.03 A, b = 17.81 f 0.03 A, c = 7.39kO.01 A, p = 113.8” f 0.2) close to the literature values.
3. Results and discussion In fig. 2 an AFM image is presented which has been obtained by scanning over an area of 1 x 1 pm* of the heulandite cleavage plane. Large regions with atomically flat cleavage terraces can be recognised. The cleavage tips have an in-plane angle of around 20”. The st,ep heights of the cleavage terraces are about 9 A, which is just half of the unit cell length b. Quite generally, the images show that the crystals have cleaved uniformly along the mirror planes at multiples of b/2.
Fig. 3a shows an AFM image obtained by zooming into a region of 12.5 X 12.5 nm’ of the (010) surface plane. The measurements have been performed in O.lM NaOH after keeping the crystal in the liquid cell for 36 h. Periodic structures are clearly discernible in fig. 3a. They can be related to the a axis and the c axis of the (010) plane of the heulandite crystal. While rows of features appear with alternative distances along the a axis, rows with uniform distance are present along the c axis. This is expected, as there are 8membered ring structures along the c axis but alternating 8- and lo-membered structures along the a axis. The periodicities found in the image can be analysed in greater detail by means of two-dimensional fast-Fourier-transform (2DFFT). Indexing in fig. 3b has been performed by
Fig. 1. (a) Model of the heulandite framework structure. Dots represent tetrahedrally coordinated which form 8- or lo-member rings. (b) b axis projection of the tetrahedral framework. The positions of the (010) surface are indicated additionally.
silicon or aluminium of the hydroxyl groups
atoms on top
L. Scandella et al. / Imaging of zeolite surface structures by atomic force microscopy
Fig. 2. Top view AFM image, obtained for the (010) plane of heulandite in O.lM NaOH. The step heights are determined to be around 9 A.
comparison with the real space. The (001) and (200) spots are the two most intense spots in the reciprocal space. Real space distances of 7.4 and
9.4 A, respectively, are calculated from the positions. These spot values are by 11% and 17% larger than the bulk lattice values of 6.6 and 7.8 A measured with X-ray diffraction [lo]. The angle between the (200) and (001) spots is 115”. On the basis of the 2D-FFT spectra the surface unit cell has been determined to be a = 21.0 A and c = 8.4 A. The surface unit cell area acsin/3 is about 26% larger than the bulk unit cell area. To our knowledge, the observation of a surface unit cell with larger dimensions than the bulk unit cell has not been reported before in AFM studies of zeolites. Recently, Weisenhorn et al. [7] have published AFM studies of the zeolite clinoptilolite in diluted tert-butyl ammonium chloride. The interpretation given by these authors is largely based on direct molecule or cluster imaging. A reconstruction of the zeolite unit cell is not considered, though this, as far as can be judged from their data, cannot be excluded. The structures of heulandite and clinoptilolite are closely related. An important difference is the higher amount of Ca2+ in the framework of the heulandite. It is very probable that in NaOH an ionic exchange of Ca ‘+ by Na+ takes place in the
lb) Fig. 3. (a) Image of heulandite (010) on the molecular scale measured in O.lM NaOH. The image size is 125 X 125 A*. Six unit cells are indicated (a = 21.0 A, c = 8.4 A). (b) Two-dimensional fast-Fourier-transform of fig. 2.
L. Scandella et al. / Imaging of zeolite surface structures by atomic force microscopy
near surface region during our experiments. As the charge densities associated with different hydrated cations change, a framework distortion, at least at the surface, becomes likely. Alterations of the bulk lattice parameters have indeed been observed quite frequently, e.g. after dehydration [12,131 or Ca2+ substitution by Kf, Na+ [14] or NH: [13]. The general trend seems to be an increase of the unit cell dimensions with increasing amounts of divalent cations [15]. However, this must not necessarily be true for the surface regions of the crystal and can possibly be reversed if there are overstoichiometric amounts of Naf as in our case. Of course, this remains a speculation which can be checked by replacing Naf through alkali ions of larger or smaller radius. This will be done in the near future.
4. Conclusions The structure of (010) surfaces of heulandite has been studied in air and in O.lM NaOH. We have shown by AFM that real space imaging with molecular resolution can be obtained. Typical step heights of the terraces have been determined to be multiples of half of the unit cell length b. The surface unit cell is found to be enlarged as compared to the bulk. An ionic exchange of Cazf by Na+ has been advanced as an explanation for this result.
Acknowledgements
We would like to thank V. Gramlich for single crystal X-ray diffraction experiments, T. Stamm for TGA measurements, Ch. Barlocher for the framework model of heulandite and G. Harvey, H. Kouwenhoven, A. Weisenhorn and H. Haefke for helpful discussions. The Swiss National Science Foundation is acknowledged for financial support.
References
111See, e.g., Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis 58, Eds. H. van Bekkum, E.M. Flanigen and J.C. Jansen (Elsevier, Amsterdam, 1991). PI See, e.g., J. Weitkamp, in: Catalysis and Adsorption by Zeolites, Studies in Surface Science and Catalysis, Vol. 65, Eds. G. Qhlmann, H. Pfeifer and R. Fricke (Elsevier Amsterdam, 1991) pp. 21ff; Occurrence, Properties and Utilization of Natural Zeolites, Eds. D. Kallo and H.S. Sherry (Akadtmai Kidao, Budapest, 19881. [31 J.M. Thomas, in: Zeolites: Facts, Figures, Future, Eds. P.A. Jacobs and R.A. van Santen (Elsevier, Amsterdam, 1989) pp. 3ff; L.B. McCusker, Acta Cryst. A 47 (1991) 297. 141 G. Binnig, H. Rohrer, C. Gerber and E. Weibel, Phys. Rev. Lett. 49 (1982) 57. [51 G. Binnig, C.F. Quate and C.H. Gerber, Phys. Rev. Lett. 12 (1986) 930. [61 See, e.g., D. Rugar and P. Hansma, Phys. Today 43 (1990) 23; E. Meyer and H. Heinzelmann, in: Scanning Tunneling Microscopy II, Vol. 28, Springer Series in Surface Sciences, Eds. R. Wiesendanger and H.-J. Giintherodt (Springer, Berlin, 1992); J. Frommer, Angew. Chem. 104 (1992) 1325. [71 A.L. Weisenhorn, J.E. Mac Dougall, S.A.C. Gould, S.D. Cox, W.S. Wise, J. Massie, P. Maivald, V.B. Elings, G.D. Stuckey and P.K. Hansma, Science 247 (1990) 1330. 181J.C. Jansen, J. Schoonman, H. van Bekkum and V. Pinet, Zeolites 11 (1991) 306; J.E. Mac Dougall, S.D. Cox, G.D. Stucky, A.L. Weisenhorn, P.K. Hansma and W.S. Wise, Zeolites 11 (19911 429; P. Rasch, W.M. Heckel, H.W. De&man and W. HIberle, Mater. Res. Sot. Symp. Proc. 233 (1991) 287. [91 G. Gottardi and E. Galli, Natural Zeolites (Springer, Berlin, 19851. 1101W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types (Butterworths, Washington, DC, 1992). [ill A.B. Merkle and M. Slaughter, Am. Miner. 53 (19681 1120. [=I A. Alberti, Tschermaks Miner. Petr. Mitt. 19 (19731 173. [131 W.J. Mortier and J.R. Pearce, Am. Miner. 66 (19811 309. iI41 A.O. Shepard and H.C. Starkey, Miner. Sot. India IMA (1966) 155. [15] J.R. Boles, Am. Miner. 57 (1972) 1463.