4680
J. Phys. Chem. B 2001, 105, 4680-4683
Determination of Extraframework Cation Positions and Their Occupancies on Heulandite (010) by Atomic Force Microscopy Masaharu Komiyama,* Minming Gu,† and Hai-Ming Wu‡ Department of Vacuum UV Photoscience, Institute for Molecular Science, Okazaki National Research Institutes, Okazaki 444-8585, Japan ReceiVed: December 7, 2000; In Final Form: March 5, 2001
Atomic images of a cleaved heulandite (010) surface were obtained under an aqueous condition by atomic force microscopy (AFM). In addition to the framework oxygen atoms on the (010) plane, extraframework cations on the surface were also imaged. An AFM imaging simulation was performed using published X-ray diffraction data to assist the assignment of the AFM-observed framework and extraframework atoms. By comparing it with the observed AFM images, local variations of position and occupancy of individual cations were determined.
1. Introduction Atomic force microscopy1 (AFM) is a powerful tool for the observation of atoms on nonconductive surfaces under various environments including vacuum, ambient, and aqueous, which has not been possible by any other existing techniques. We have been able to observe in situ atomic images of heulandite and stilbite (010) surfaces under aqueous environments,2,3 and molecular images of liquid-phase-adsorbed pyridine bases and other molecules on these surfaces.2-7 The unprecedented resolution of the AFM imaging on these adsorption systems enabled us to determine, for the first time, the array and orientation of the adsorbed molecules on zeolite surfaces. The present paper exploits the same high-resolution inherent to AFM on nonconductive surfaces to observe the atomic images of the extraframework cations exposed on a heulandite (010) surface and to determine their individual positions and occupancies. Zeolite crystal structures have been mainly determined by means of X-ray diffraction. For heulandite, a number of reports have been published on the analysis of atomic coordinates for natural, heat-treated or ion-exchanged crystals.8-18 While the assignment of a few water and cation sites is still being argued (for instance, see refs 17 and 18), its crystallographic structure may be considered to be well established. Following the most recent reports on a partially Rb-exchanged heulandite by Sugiyama and Takeuchi,18 a unit-cell diagram is constructed in Figure 1 that shows the framework and extraframework atom positions on the (010) surface. A (010) surface unit cell is exemplified in the figure by solid lines connecting the topmost six framework oxygen atoms, which are indicated by open circles. There exist four extraframework cation sites, M(1), M(2), M(3), and M(5), in a half (010) unit cell shown by broken lines, and their symmetry sites in the full unit cell are indicated with primes. The notations for cation sites used in this paper throughout follow those given in ref 18, except for M(1)′ and M(3)′ sites which are the symmetry sites of M(1) and M(3), respectively. While the cation site coordinates do * Corresponding author. On leave from Department of Ecosocial System Engineering, Yamanashi University, Takeda, Kofu 400-8511 Japan. † Presently at Centre for the Physics of Materials, Physics Department, McGill University, Montreal, Quebec H3A 2T8, Canada. ‡ Presently at Research Institute of Innovative Technology for the Earth, Kizu-cho, Kyoto 619-0292, Japan.
Figure 1. A diagram showing the atom positions on the (010) surface of heulandite. A surface unit cell is exemplified by solid lines connecting the topmost six framework oxygen atoms, which are indicated by open circles. In a half-unit cell indicated by broken lines, there exist four extraframework cation sites, M(1), M(2), M(3), and M(5). The symmetry sites for M(1), M(2), and M(3) within a full unit cell are indicated with primes.
depend on the chemical composition of the crystal, their variations are less than 6%: for instance the fractional x coordinate of M(1) site is 0.1460 for the Rb-exchanged heulandite18 whereas for a nonexchanged natural heulandite13 it is 0.1526, and for M(2) 0.0425 and 0.0401, respectively. The differences are too small to be apparent in Figure 1. The cation site occupancies measured and reported so far are also summarized in Table 1. In their Rb-exchanged heulandite, Sugiyama and Takeuchi18 found that site M(5) is sparsely populated (occupancy of 0.08). Sites M(1) and M(3) (and also M(1)′ and M(3)′) are a pair of forbidden sites that only one of them is occupied within a unit
10.1021/jp004405h CCC: $20.00 © 2001 American Chemical Society Published on Web 04/26/2001
Extraframework Cation Positions on Heulandite (010)
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4681
TABLE 1: Site Occupancies for Various Heulandites heulandite (origin)
site
Rb-exchanged (Makinokawa, Japan) M(1) and M(1)′ M(2) and M(2)′ M(3) and M(3)′ M(5) natural Ca (Azerbaijan, Iran) M(1) and M(1)′ M(2) M(2)′ M(2) and M(2)′ Ag-exchanged (Azerbaijan, Iran) M(1) and M(1)′ M(2) and M(2)′ M(3) and M(3)′ natural (Faro¨er Islands) M(1) and M(1)′ M(2) M(2)′ M(2) and M(2)′
occupancy ref 0.45 0.41 0.35 0.08 0.50 0.40 0.50 0.45 0.41 0.43 0.13 0.70 0.18 0.74 0.46
18
13
14 9
cell due to their proximity to each other (respective occupancy of 0.45 and 0.35). Site M(2) and its symmetry site M(2)′ also constitute a pair of forbidden sites, with their combined occupancy being 0.41. Rubidium cations were found to exchange almost exclusively the M(3), M(3)′, and M(5) sites. On the other hand, when Ag ion is exchanged on a natural Ca heulandite, monovalent Na ions at M(1) and M(1)′ sites are exchanged first, then divalent cations at M(1), M(1)′, and M(2) (or M(2)′) sites.13,14 In this latter case, the site occupancies in the natural Ca heulandite is 0.50 for M(1) and M(1)′ combined, and 0.45 for M(2) and M(2)′ sites. When it is fully Agexchanged the occupancies become 0.41 and 0.43, respectively. The distribution factor between the pair of forbidden sites M(2) and M(2)′ were 0.45 and 0.55 for the natural Ca heulandite. Yet another reports the M(2) and M(2)′ combined occupancy of 0.46 for a natural heulandite, while its distribution factor being 0.2 and 0.8.9 Of all those crystallographic data, it should be stressed that they are all average values over a very large number of unit cells, and it has not been possible to determine individual atomic positions or their occupancies locally with X-ray diffraction or any other existing techniques. With the advent and the progress of AFM techniques, we are now able to observe and determine the individual atom positions on nonconductive surfaces. The present paper is the first of such attempts applied to zeolite systems. 2. Experimental Section Heulandite has perfect (010) cleavage caused by a layering of the silicate tetrahedra comprising its structural framework. The heulandite from Ross Creek, Nova Scotia, Canada, was cleaved with a razor blade along its (010) (or (020)) plane under ambient conditions and immediately placed in a sealed AFM liquid cell and contacted with deionized and membrane-filtered water (conductivity of ca. 90 µS/cm). The AFM examinations were performed using a Nanoscope II contact-mode AFM (Digital Instruments) in the repulsive force range with a tip load of ca. 5 nN. Type NP cantilevers (Digital Instruments) were used, which have Si3 N4 tips with a force constant equal to 0.38 kN/m. It is known that stick-slip imaging mechanism prevails in the contact-mode AFM operation at high tip loads (a few hundred nN) under ambient conditions.19 The present measurements, however, were done at very low tip load (ca. 5 nN) under aqueous conditions, and characteristics of stick-slip phenomenon are not apparent in the obtained AFM images. A simulation of AFM imaging of the surface was performed following a previously reported method,20 using an algorithm similar to the previously developed AFM simulator ACCESS21-23
Figure 2. An AFM image of a heulandite (010) surface obtained under aqueous conditions, after 2-D FFT with a cutoff period of 0.25 nm. Imaged area is 8 nm × 8 nm, and the gray scale is 0.3 nm full scale. Three unit cells are indicated by solid lines connecting the topmost framework oxygen positions (shown by open circles). Extraframework cations existing within the three unit cells are indicated with their respective numbers.
under two-dimensional periodic boundary conditions parallel to the sample surface. With a model AFM tip consisting of a Si(OH)4 cluster the topmost cleaved plane of heulandite(010) was scanned. Each OH- group in the Si(OH)4 cluster tip was taken as one atomic entity, following the previous method.20 The forces acting between the atoms in the tip and those in the sample were calculated by using the previously employed interatomic potential function that include Coulomb and exchange repulsion.20 The parameter values also follow our previous report, including the potential parameters for OHgroups in Si(OH)4 cluster tip that are represented with those of O2- ions with a charge Zi of -1. The same was done for the topmost oxygen atoms on heulandite (010), which reflects the most likely situation under the present AFM examination conditions. For simplicity, the potential parameters for extraframework cations, predominantly Ca, were replaced by those for Si, except for the charge of +2. The z-position of the tip was adjusted by z-position feedback so that the total force on the tip comes within 0.2% of a set value of 10 nN (the present AFM simulator works in the repulsive mode), and then the z-position of the tip is recorded. This was repeated for each of the tip positions as it scans the surface with steps of 0.5 Å both in x and y directions, and the contour map of z-position of the tip was obtained. 3. Results Figure 2 shows an atom-resolved AFM image of a heulandite (010) surface obtained under an aqueous environment, after filtering by two-dimensional fast Fourier transform with a cutoff period of 0.25 nm. White color in the figure indicates protrusions, and dark color depressions (the gray scale is 0.3 nm full scale). Thus bright spots are where the atoms are exposed on the (010) surface. With close examination of the AFM image, one might notice that there exist brighter spots arranged in a very periodic manner. Since the framework oxygen is the only framework atoms that
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Komiyama et al.
TABLE 2: Local Variations of Cation Site Positions and Their Occupancies Determined from the AFM Image Shown in Figures 2. Deviations Are Calculated as Scalar Values from the X-ray-Determined Cation Positions cation sites
occupancy
positional deviation (standard, Å)
M(1) M(1)′ M(1) and M(1)′ combined M(2) M(2)′ M(2) and M(2)′ combined M(3) M(3)′ M(3) and M(3)′ combined M(5)
0.77 0.13 0.46 0.38 0.37 0.38 0.09 0.57 0.33 0.26
0.76 1.12 0.82 1.24 1.37 1.31 1.32 1.32 1.32 2.27
are expected to be exposed on this cleaved surface (it is probably in the form of hydroxyls considering the aqueous environment the AFM observation was made), these highly periodic brighter spots are assigned to the framework oxygen. Three unit cells are overlaid in Figure 2, with the outermost framework oxygen atom positions indicated by open circles. It is noted that the overlaid unit cell dimension in Figure 2 is derived from bulk crystallographic data. The almost perfect match of the bulk-terminated unit cell with the observed AFM atomic image means that the atomic arrangements on this surface are the same with that of bulk, and no surface relaxation or surface reconstruction was taking place under the present conditions as far as the framework oxygen atoms are concerned. With the brighter and more periodic spots attributed to the framework oxygen atoms, it is then noted in Figure 2 that there exist less bright and less periodic spots among those framework oxygen. Comparison of the AFM image with the X-ray derived atomic coordinates shown in Figure 1, it is found that those spots closely correspond to the extraframework cation positions. Following the notation by Sugiyama and Takeuchi,18 those extra spots were identified and labeled as found in the unit cells indicated in Figure 2. Looking at the entire image, it is apparent that on this particular surface, AFM-imaged cations are located at all the M(1), M(2), M(3), M(5) and their respective symmetry sites. Moreover, it is also apparent that those sites are not always occupied, reflecting the nonunity occupancies for those extraframework sites. Since AFM provides local atomic information in contrast to the space-averaged one by XRD, we can determine from this AFM image the local variations of those cation site positions and their occupancies. The results obtained from Figure 2 are listed in Table 2. Figure 3 shows the result of AFM image simulation using the X-ray derived crystallographic structure. Here brighter spots are due to the framework oxygen, and less-bright spots the extraframework cations. Again a unit cell is indicated by solid lines connecting the topmost oxygen atoms shown by open circles. Numbers inset the unit cell indicate cation sites following the above notation. 4. Discussion First of all, we would like to discuss the appropriateness of the atom assignments for the bright spots found in Figure 2. Here we assigned the brighter and more periodic spots to the framework oxygen atoms, and less bright and less periodic ones to extraframework cations. For the framework oxygen assignments, the periodicity found in Figure 2, which is in almost perfect agreement with the bulk-terminated oxygen positions,
Figure 3. A simulated AFM image of a heulandite(010) based on X-ray crystallographic data. All the possible cations are shown (i.e., all the cation occupancies are taken as unity). A unit cell is indicated by connecting the topmost framework oxygen atoms which are shown by open circles. In the unit cell extraframework cation positions are indicated by their respective numbers.
may be taken as an evidence that supports the appropriateness of their assignments. In addition, the AFM simulation result shown in Figure 3 also gives a support to the present assignments. In this simulated AFM image it is found that framework oxygen is brighter and larger than extraframework cations. This is due to the fact that in the present simulation the interaction between the tip and the surface includes two terms, as described in our previous work.20 The first one is a long-range electrostatic interaction which is proportional to q1‚q2/r2 (q: charge, r: distance) and the second a short-range hard-core interaction among atoms, which is always repulsive. In the present simulation the apex of the tip consists of a pseudo O atom with a charge of -1, and on the surface atomic charges are O(-1) and Ca(+2). The AFM simulator was operated in the repulsive mode. Thus, over an pseude O atom z-position of the tip is mostly determined by the first term, since (-1) × (-1) ) +1. Over Ca atom, however, the first term acts as an attractive one and the tip will go closer to Ca until the hard-core interaction dominates, thus making Ca appear smaller in size and lower in height. While there still exists a room for parameter optimization in the current AFM simulation (such as modeling the tip and the sample OH with a true O and H rather than the pseudo O-, or optimizing the parameters in the second term that could change the way in which the tip feels the hard-core repulsion), it will have a limited effect for the appearance of framework and extraframework atoms on this simulated heulandite (010) image, certainly not to the extent to reverse the order of the atomic brightness that is found in Figure 3. Thus assigning the brighter, more periodic spots in Figure 2 to framework oxygen atoms and other extra spots to extraframework cations, we go on to discuss the local occupancy and positional deviations of the latter. Referring to Table 2, in which the data are collected over the entire AFM image shown in Figure 2, a few conclusions may be drawn. First, on this particular sample surface examined here, occupation of M(1) site dominates that of M(1)′. The site occupancy factor is 0.86 and 0.14, respectively, within the area found in Figure 2. While
Extraframework Cation Positions on Heulandite (010) the reason for this dominant M(1) site occupation is not clear, this large difference seems rather curious. Since M(1) makes a pair of forbidden site with M(3), and also its symmetry site M(1)′ with M(3)′, the above observation necessarily be reflected to M(3) and M(3)′ occupation factor, which comes out to be 0.14 and 0.86, respectively. Second, it is found in Table 2 that for the pair of forbidden sites M(2) and M(2)′ their occupancy factor is 0.51 and 0.49. This number is similar to those obtained for a natural heulandite from N. E. Azerbaijan, Iran (0.45 and 0.55),13 and very different from those measured on a natural heulandite from Faro¨er Islands (0.2 and 0.8).9 It is also noted that the occupancy of the M(5) site is relatively high (0.26), in comparison to the heulandite from Makinoshima, Japan (which shows the occupancy of 0.08).18 A part of the above-discussed differences in the occupancies and their ratios with other heulandite samples may be attributed to their origin. The chemical compositions of natural samples vary depending on their deposits, and thus the values listed in Table 2 as well as those found in the literature may not simply be compared to each other. Another possible cause for the above differences may be introduced by the fact that the number of unit cells counted in the present AFM (total of ca. 50) is very small compared to those included in X-ray analysis. Small sample numbers necessarily increase margin of errors. There is another point to note in relation to the cation occupancies. We should keep in mind that the AFM observation is made on a cleaved surface. Because the extraframework cations exist as charge-compensating entities for the zeolite framework, cleavage at the (010) (or (020)) plane necessarily separates the cations on the plane into two parts, a part of which is left on the plane for the AFM observation while the other is taken away with the other (010) plane. Thus one may expect that statistically only a half of the cations are present on the AFM observed (010) plane shown in Figure 2. This expectation is met with the fact that none of the combined cation occupancies (M(1) and M(1)′, M(2) and M(2)′, and M(3) and M(3)′) exceeds 0.5 in Table 2. Thus for the discussion of local cation occupancies, and their habits such as M(1) site dominance over M(1)′, we have to know whether the cation separation into two parts is random or it has any particular tendency. At this moment it is an open question to be answered. As for the positional deviation, it is found in Table 2 that the deviation is relatively large for all the cation sites. This may be an inherent character specific to the present heulandite, or it may have something to do with the cleaving of the plane (and
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4683 the absence of the other plane in contrast to bulk). Site M(5) shows particularly large deviation. This again may be attributed to a very small M(5) occupancy, and hence a very small sample number counted (ca. 10) in the present measurement. Nevertheless, the authors would like to point out that almost all the observed M(5) sites are shifted toward +c direction (cf. Figure 2, inset), and toward this direction at 2.6 Å apart from the M(5) site there exists a site attributed by Sugiyama and Takeuchi to a water (W(5) according to their notation).18 There is another point to note in the analysis of Figure 2 that is not apparent in Table 2: in no occasion the pairs of forbidden sites, M(1) and M(3), M(1)′ and M(3)′, or M(2) and M(2)′, were occupied simultaneously. We would like to note that the confirmation of this fact comes from the very local nature inherent to the AFM technique, and should be stressed as one of the prominent characteristics of the technique. References and Notes (1) Binning, G.; Quate, C. F.; Gerber, C. Phys. Lett. 1986, 56, 930. (2) Komiyama, M.; Gu, M. Jpn. J. Appl. Phys. 1996, 35, 3775. (3) Komiyama, M.; Koyama, T.; Shimaguchi, T.; Gu, M. J. Phys. Chem. 1996, 100, 15198. (4) Komiyama, M.; Gu, M. J. Vac. Sci. Technol. B 1997, 15, 1325. (5) Komiyama, M.; Gu, M.; Shimaguchi, T.; Wu, H.-M.; Okada, T. Appl. Phys. A 1998, 66, S635. (6) Komiyama, M.; Shimaguchi, T.; Kobayashi, M.; Wu, H.-M.; Okada, T. Surf. Interface Anal. 1999, 27, 332. (7) Komiyama, M.; Kobayashi, M. J. Phys. Chem. B 1999, 103, 10651. (8) Merkle, A. B.; Slaughter, M. Amer. Mineral. 1968, 53, 1120. (9) Alberti, A. Tschermaks Mineral. Petrogr. Mitt. 1972, 18, 129. (10) Alberti, A. Tschermaks Mineral. Petrogr. Mitt. 1973, 19, 173. (11) Bartl, H. Z. Krist. 1973, 137, 440. (12) Alietti, A.; Gottardi, G.; Poppi, L. Tschermaks Mineral. Petrogr. Mitt. 1974, 21, 291. (13) Bresciani-Pahor, N.; Calligaris, M.; Nardin, G.; Randaccio, L.; Russo, E.; Comin-Chiaramonti, P. J. Chem. Soc., Dalton Trans. 1980, 1511. (14) Bresciani-Pahor, N.; Calligaris, M.; Nardin, G.; Randaccio, L. J. Chem. Soc., Dalton Trans. 1981, 2288. (15) Mortier, W. J.; Pears, J. R. Amer. Mineral. 1981, 66, 309. (16) Alberti, A.; Vezzalini, G. Tschermaks Mineral. Petrogr. Mitt. 1983, 31, 259. (17) Galli, E.; Gottardi, G.; Mayer, H.; Preisinger, A.; Passagalia, E. Acta Cryst. B 1983, 39, 189. (18) Sugiyama, K.; Takeuchi, Y. Stud. Surf. Sci. Catal. 1986, 28, 449. (19) Fujisawa, S.; Kishi, E.; Sugawara, Y.; Morita, S. Phys. ReV. B 1995, 51, 7849. (20) Tsujimichi, K.; Tamura, H.; Hirotani, A.; Kubo, M.; Komiyama, M.; Miyamoto, A. J. Phys. Chem. B 1997, 101, 4260. (21) Komiyama, M.; Tsujimichi, K.; Tazawa, K.; Hirotani, A.; Yamano, H.; Kubo, M.; Broclawik, E.; Miyamoto, A. Surf. Sci. 1996, 357-358, 222. (22) Komiyama, M.; Tsujimichi, K.; Ohkubo, S.; Tazawa, K.; Kubo, M.; Miyamoto, A. Jpn. J. Appl. Phys. 1995, 34, L789. (23) Komiyama, M.; Ohkubo, S.; Tazawa, K.; Tsujimichi, K.; Hirotani, A.; Kubo, M.; Miyamoto, A. Jpn. J. Appl. Phys. 1996, 35, 2318.
