Microtopographic and molecular scale observations of zeolite surface structures: Atomic force microscopy on natural heulandite

ELSEVIER

Microtopographic and molecular scale observations of zeolite surface structures: Atomic force microscopy on natural heulandite G. Binder,*? L. Scandella,? A. Schumacher,* N. Kruse,* and R. Prim* *Technisch Chemisches Labor, Ziirich, and the fLaborato9 fiir Mikro- und Manostrukturen, Paul Scherrer Institut, Villigen PSI, Switzerland Atomic force microscopy (AFM) has been employed to study the surface of the natural zeolite heulandite. Images on a macroscopic scale exhibit growth spirals on the outer surface and cleavage tips on the (010) surface, both with terraces separated from each other by half of the unit cell length b. The angle of the cleavage tips depends on the cleaving direction. Molecular resolution is obtained when measured in 0.1 M NaOH, 0.1 and 1 M KOH, and 6.66 M CsOH. The influence of immersion time, the nature of the ion species, and the ion concentration on the unit cell has been investigated. The lattice constants varied around the bulk values. Keywords:

Atomic

force microscopy;

heulandite

INTRODUCTION Atomic force microscopy (AFM) ‘-a made it possible to study the surface topography of nonconducting materials at high resolution and in real space. Although lateral resolution on the molecular scale usually sets rather narrow limits to the size of the monitored surface area, atomic step heights can still be made visible for scan areas of micrometer extension. In fact, the ultimate vertical resolution of the microscope may provide microstructural information on crystal growthinduced phenomena such as defects, spirals, and cleavage planes. In comparison, the application of decoration4 and replica techniques5 in scanning electron microscopy (SEM) allows one to obtain such information in a less direct manner, with the step heights usually remaining undetermined. Topographic studies of the external surface of zeolites present in a challenging task for AFM. Despite growing interest in such local probe research,&” knowledge is still limited. On the one hand, this is due to the rather complicated zeolite crystal structure which consists of a silicon-oxygen framework, with the silicon being partially exchanged by aluminum so that extra cations have to be placed in the channels to guarantee charge neutrality. On the other hand, the complexity of the system increases if experiments are performed in which the sample is immersed in an electrolyte solution. Address reprint requests to Dr. Binder at the Laboratory Mikround Nanostrukturen, Paul Scherrer Institut, CH-5232 ligen PSI, Switzerland. Received 19 April 1995; accepted 3 July 1995 Zeolites 16:2-6, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas,

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This paper reports on growth-induced features that occur on the surface of a natural heulandite. Microtopographic information on spiral formation and cleavage planes will be provided. Moreover, we will discuss the rather unusual repor? on the enlargement of the surface unit cell as observed in former studies with molecular scale resolution. Based on detailed measurements of varying duration, in electrolyte solutions of different concentrations, we will present a careful interpretation of this phenomenon.

EXPERIMENTAL Heulandite, Ca,(Al,Si,,O,,) , has a monoclinic crystal symmetry with unit cell lattice parameters a = 1.77 nm, b = 1.794 nm, c = 0.742 nm, and p = 116.24”. The channel system is two-dimensional, with lo- and Smembered rings parallel to the [OOl] direction, and S-membered rings along [loo]. For a more detailed description see Scandella et al.,” Gottardi and Galli,‘” and Tsitsishvili and Andronikashvili.14 The atomic force microscope used in this study is commercially available (Nanoscope II, Digital Instruments) and can be operated in air or in liquids. One microtopographic image (J@~re 1) was taken on an Autoprobe CP (Park Scientific Instruments). All measurements were performed with pyramidal tips of triangular silicon nitride cantilevers whose spring constant was 0.06 N/m. The attractive forces that act on the cantilever when it is in contact with the zeolite immersed in the liquid phase ranged from 1 to 10 nN. The piezos are supposed to control the distances with an accuracy of about 5%. The data presented were not filtered.

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0144.2449/96/$15.00 0144-2449(95)00085-K

Microtopographic

Samples of natural heulandite are from Poona, India. Crystals were cleaved in air and attached to a sample holder with epoxy glue. AFM measurements revealing microtopographic information were performed in air; investigations providing resolution on the molecular scale were carried out in air, water, and aqueous solutions of KCl, NaOH (0.1 M) , KOH (0.1 and 1 M) , and CsOH (6.66 M) RESULTS

AND

DISCUSSION

Microtopographic images We begin our presentation by providing topographic information on a number of structural phenomena such as crystal spirals and cleavage tips that are found on the (010) plane of the heulandite sample. Figure 1 shows an AFM image obtained by scanning over an area of 15 x 15 pm2 on the outer surface of the crystal. The occurrence of growth spirals is easily recognized in this figure. The pitch of both spirals was determined to be -0.9 nm (sometimes twice as much, i.e., 1.8 nm), which is in good agreement with half of the unit cell length 6. Spirals were randomly oriented; clockwise as well as counterclockwise behavior were observed. Growth spirals were observed incidentally during our measurements. Atomically flat terraces could usually be recognized after cleaving the heulandite parallel to the (010) cleavage plane. Steps or cleavage tips occurred, and these were investigated systemically. Information about the cleavage behavior might be useful to achieve flat planes of synthesized crystals. The measurements showed that most step edges run in the [loll, [201], and [loo] crystallographic direc-

Figure 1 Top view a natural heulandite in air.

AFM image, 15 x 15 vrn’, crystal. The two growth

of the (010) face of spirals are imaged

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tions. Sometimes the steps had “rough” edges 01 changed direction completely, often from one crystallographic direction to another. Step heights rangecl from half a unit cell to tens of a nm. The flat terrace parts right across the cleavage tips had extensions from less than 1 pm to about 10 pm. The characteristics of the cleavage tips depended on the cleavage direction. When cleaved along a certain crystallographic direction, the tips were similarly aligned, mainly [ 1001 and [ 1011. Figure 2 showsan AFhl image of cleavage tips taken in air. The cleavage direr.tion indicated by the arrow is approximately perpendicular to one side of the cleavage tips. Only randomly aligned cleavage tips were found when the cleavage direction deviated from the above mentioned crystallographic directions. Bethgel already pointed out in his investigation of the cleavage faces of alkaline halides with an NaCl-type structure that cleavage structures are not expected to go in distinct crystallographic dire{.tions. In both cases,however, the majority of tips had heights of up to twice that of the unit cell. In-plane angles of the cleavage tips, between 7 and lj”, were generally observed, depending on the cleavage speed. ” Figure 3 presents an AFM image with a lamellar te\;ture which was found on only one crystal. At the lower left, a step of half a unit cell height can be observed. The lamellae with a periodicity of about 400 nm are aligned parallel to the [loll direction. They are also visible on the higher level plane in Figuw 3, lowerleft. The depth ()f the lamella boundaries is determined to be about 0.3 nm. However, the smallest height of structures found in investigations represented by Figures 1-? is half of the unit cell height, 0.9 nm. Therefore, we suppose that the apparent depth does not originate from topographv.

Figure 2 Top view AFM image, 7 x 7 pm’, of the (010) plane of cleaved heulandite in air. The step heights are determined to be 0.9 nm, the in-plane angles are about 7’. The cleaving direction is indicated by an arrow.

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the rhombs is about 125”. According to Gottardi and Galli,‘” the (100) plane may be a twin and contact plane, but twinning is not so frequent. The recently developed polarization contrast scanning near-field optical microscopy (SNOM)“3’” would be an appropriate technique to prove twinning.

Images with molecular resolution

Figure 3 Top view AFM image, cleaving the heulandite crystal. is 0.9 nm. The lamellae, which aligned with the II011 direction

2.5 The run as

x 2.5 pm*, probed in air after step height in the lower left parallel over the image, are indicated by an arrow.

The contrast is probably a result of changes in the amount of the repulsive forces. Weaker repulsive forces lead to weaker forces acting on the cantilever. As measured in the constant force mode, a change in the topography due to the cantilever deflection was recorded, although no difference in height was present on the sample. A chain of rhombs intersects the lamellae perpendicularly and is oriented in the (100) plane. The angle of

Experimenters with molecular resolution were performed in the liquid cell. Measurements in air failed to reveal the lateral dimension of the surface unit cell. We have already reported on the occurrence of lattice parameters, which were enlarged as compared with those in the bulk”; we then carried out a number of experiments aimed at determining whether this phenomenon was influenced by the nature of the ion species, the immersion time, and the ion concentration. For these studies, atomically flat regions on the heulandite (010) cleavage plane were chosen. We present results obtained from measurements in aqueous solutions of NaOH (0.1 M), KOH (0.1 and 1 hi), and CsOH (6.66 M). The results of probing a region of 12 x 12 nm’ while keeping the sample in 0.1 M KOH are shown in Figure 4. The measurement was performed after a total contact time of 40 h. Periodic structures are clearly visible and can be related to the a and c axes of the (010) plane of the heulandite ctystal. The pattern obtained by twodimensional fast Fourier transform (R7) of the real space image (Figurn 4~) reflects the periodicity of the crystallographic directions in reciprocal space (&XW 4b). The corresponding real space values for the (100) and the (001) peaks were determined to be 1.61 and 0.69 nm, respectively. The angle between these two crystallographic axes was 116” (see inset of Figure 46). The lattice parameters were calculated to be a = 1.77

f’~’ \411fj”

(1 Q

t

\,

F

. (100)

--

+ (001)

Figure 4 Molecular inset of pane/ a.

4

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resolved

heulandite

in 0.1 M KOH.

a, real space,

b, two-dimensional

FiTimage.

The unit

cell is shown

in the

Microtopographic

nm and c = 0.75 nm, in good agreement with the bulk unit cell parameters. The effect of the duration of immersion in different electrolytes was investigated next. ZGg~re 5 presents the values for the a axis versus the duration of measurements in 0.1 M NaOH (triungles), in 0.1 M KOH (squares); earlier measurements’ in 0.1 M NaOH are also indicated (circles). Each symbol represents an AFM image that was analyzed as described above. The bulk value for a is indicated in the image as is an accuracy limit that takes into account a measurement error of 5%. The length of the a axis was evaluated because it could always be resolved with high accuracy, whereas the observation of the shorter c axis is more difficult. Many data points are located outside the 5% limit. However, there is no evidence that a continuous ion exchange is responsible for the enlargement of the unit cell starting with the bulk values immediately after immersion. Such a saturation effect is not visible. Experiments with higher concentrations and larger cation radii were performed to clarify the observed effects. Measurements in 1 M KOH and in 6.66 M CsOH guaranteed that ion exchange took place. If the surface unit cell area was influenced by the ion size incorporated in the zeolite framework, the larger radii of Cs (265 pm) and K (227 pm), as compared with Na (186 pm), would have resulted in higher values for the unit cell parameters. Even if ion hydration is taken into account (for example, the radius of the hydrated Na ion is slightly larger than that of the hydrated K ion’“.“‘), systematically different values should have resulted. I;igure 6 shows the two-dimensional R?r spectra obtained by scanning over an area of 12 x 12 mm2 with molecular resolution in 1 14 KOH. The same periodicity as in the time-dependent investigations in 0.1 M solutions is observed. AFM results in CsOH are consistent with those obtained with the other electrolyte solutions. Although an ionic exchange of the framework Ca+ by alkali ions might take place,” a change in surface lattice constants was not detectable by AFM. Quite remarkably, we observed a significant deterio22

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and molecular

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Figure 6 Two-dimensional FFT image of molecular resolved heulandite in 1 M KOH. The pattern reflects the periodicities in a (100) and c (001) directions.

ration of the resolution of higher concentrations

Tip-sample interaction:

in measurements than presented

with solutions here.

Image interpretation

AFM investigations of crystals of the heulandite group (behavior and clinoptilolite) have produccbd striking results. Komiyma and Yashima8 reported that the heulandite image taken in air does not match the surface structure expected from the bulk crystallographic data. Weisenhorn et al.” reported, howevcar, without further comment, that distances betwecbn atomic rows may be observed with unit cell lattice ])arameters larger than the respective bulk values. The present results show that the data should be interpreted with care. A zeolite surface with its complex chemic-al composition, immersed in an alkaline electrolyte, and interacting with the probing tip, presents a rather complex system that is theoretically not yet fully understood. Instead of interpreting the AFM images as copies of the real surface (with, e.g., surface reconstruction, changes of the unit cell size), we favor an explanation that considers an imaging effect superposed onto I he images. A closer look at Ei;gurr 4 reveals that each cell consists of three rows in the c direction, but only two of thc,rn can be related directly to the crystal structure (comp,ire inwt in ~@LWP 4n). This indicates that a tip-sample interaction must be taken into account. Additionall). it illustrates that only the periodicity of the unit cell collld be resolved. For this reason we avoid the expresslon atomir re,rolulion and use molrculur resolution instead. I urthermore, all attempts to achieve molecular scale rc’so-

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lution in nonalkaline solutions have failed. We were not successful in either air or in an aqueous solution, whether it was deionized water or a low molar KC1 solution. It is most likely that an etching process operates at the interface between the zeolite and the alkaline solutions. At higher concentrations of the solution, the etching rate will be higher, and the freed molecules will therefore prevent the tip from coming into contact with the lattice. Shluger et a1.22 already interpreted their molecular scale resolved pictures of alkali halide crystals as the representation of a deeper, more bulk-like layer. It may be that the AFM tip breaks through a soft, disordered outer layer and that the image originates from a bulk-like plane of constant force. This would explain why higher alkali concentrations are more likely to obstruct molecular resolution as illustrated by a comparison of the different qualities of Figures 4 and 6. In addition, when imaging in a electrolyte solution, the effect of an electrostatic forc;s-$;tween tip and According to ST:!% ,sfi~u,~w~k,“~~~,~~a~~~~den et al.” the imaging force at high pH is governed by the repulsive force of the electrical double layer. The pH and concentration dependence of the electrical double layer force could contribute considerably to an interpretation of the concentration-dependent resolution. Apart from their contribution to the electrical double layer, the ion influence on the image process is not yet understood. Vrdoljak and Henderson” claim to have imaged the adsorbed cesium on clinochlore with AFM. A similar process might also have taken place in our studies, but we favor the tip-sample interaction as an explanation. However, an ultimate interpretation of the observed structures is difficult because of the convolution of information in the AFM images resulting from topography, elastic surface deformation, and electrostatic forces.

We demonstrated the usefulness of the AFM in providing information on crystal growth-induced features of nonconducting zeolite surfaces on a micrometer lateral scale while keeping atomic resolution on the vertical scale. Cleavage tips and spirals were observed and evaluated quantitatively according to their topographic characteristics. Resolution on the molecular scale was obtained by immersing the heulandite crystal into various aqueous solutions of alkaline hydroxyls of differing concentrations. The lattice constants varied around the bulk values. It is assumed therefore that the surface is not reconstructed and the unit cell is not enlarged. An etching process was considered in combination with the

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ACKNOWLEDGMENTS This work was supported by the SwissNational Science Foundation. We thank Fergal Callaham for contributions to the investigation of the cleavage tips.

REFERENCES 1

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