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Microporous and Mesoporous Materials 61 (2003) 79–84

Molecular resolution images of the surfaces of naturalzeolites by atomic force microscopy

M. Voltolini a,*, G. Artioli a,b, M. Moret c

a Dipartimento di Scienze della Terra, Universit�aa di Milano, via Botticelli 23, I-20133 Milano, Italyb Istituto Sperimentale CNR per la Dinamica dei Processi Ambientali, Sezione di Milano, via Mangiagalli 34, I-20133 Milano, Italy

c Dipartimento di Scienza dei Materiali, Universit�aa di Milano-Bicocca, via Cozzi 53, I-20125 Milano, Italy

Received 22 July 2002; received in revised form 4 December 2002; accepted 4 December 2002

Abstract

Atomic force microscopy (AFM) was used to characterize the morphological and cleavage surfaces in a number of

natural zeolites. The investigated zeolites (stilbite, heulandite, thomsonite, yugawaralite, laumontite, and a few others)

show rather interesting and sample-dependent microtopographical features related to the mechanisms involved in the

surface growth processes at the molecular level. The results obtained by AFM on stilbite, heulandite, and yugawaralite

during the preliminary surface characterization are presented, and the images show that molecular resolution can be

achieved and crystallographically interpreted by careful preparation of the sample.

� 2003 Elsevier Inc. All rights reserved.

Keywords: AFM surface imaging; Natural zeolites; Yugawaralite; Stilbite; Heulandite

1. Introduction

Studies on zeolite surfaces are aimed to under-

stand the crystallography, the crystal chemistry,

and the microtopography of the surface layers, andeventually to interpret the underlying mechanisms

of surface growth and transformation. As the

crystal surface is the interface where chemisorp-

tion, transport, diffusion, and chemical reactions

take place, the in situ investigation of crystal sur-

faces has a fundamental importance in under-

standing all physico-chemical processes involving

the crystal–fluid interactions. In the specific caseof zeolites, the understanding and the optimization

* Corresponding author.

1387-1811/03/$ - see front matter � 2003 Elsevier Inc. All rights rese

doi:10.1016/S1387-1811(03)00357-3

of all applications involving absorption, cation

or water exchange, molecular selectivity, crystalli-

zation or dissolution, must necessarily be based

on the appropriate understanding of surface phe-

nomena.Most zeolite studies are concerned with the bulk

structure and properties of the material, because

of its internal controlled microporosity. How-

ever, many processes such as for example crystal

nucleation and growth, are actually governed

by surface properties and chemical absorption.

Atomic force microscopy (AFM) plays a key-role

in the characterization of crystal surfaces, becauseit allows access to the structural and microstruc-

tural features at nanometric resolution. Further-

more many experiments can also be performed

in situ, with the crystal in contact with the solution.

rved.

80 M. Voltolini et al. / Microporous and Mesoporous Materials 61 (2003) 79–84

The evolution of the microtopographical features

can therefore be followed during surface reactions

[1,2].

Typically, AFM investigations on zeolites and

microporous materials pose a number of problems

caused by the strong tip–surface interactions andby the great surface reactivity, which implies an

almost universal attraction of impurity molecules

on the surface, with subsequent degradation of the

image quality. At present only a limited number of

papers have been published containing images of

zeolite surfaces at the nanoscopic scale. The pio-

neering work of Mac Dougall et al. [3] provides a

few images of natural scolecite, stilbite and fauja-site. Other studied natural zeolites are stilbite [4],

heulandite [2,5,6,8], and mordenite [7–9]. Some

attempts of imaging the surfaces of synthetic zeo-

lites such as LTA and FAU failed to achieve good

lateral resolution at the molecular level [8,10–13].

Interestingly, adsorption phenomena in zeolites

have already been tackled by using AFM tech-

niques. The systems studied are clinoptiloliteand heulandite [14–16]. Moreover, very recently a

comparison between zeolites crystals grown in

microgravity conditions and under standard grav-

ity has been undertaken [17].

As a preliminary work towards the character-

ization of natural zeolite surfaces for chemical re-

actions, the present study investigated the crystal

surfaces of three zeolites having similar lamellarmorphology. Stilbite, heulandite, and yugawaralite

all exhibit tabular habit with the {0 1 0} form being

the most developed. The (0 1 0) face is of course a

perfect cleavage plane, fit to be investigated by

AFM.

2. Experimental

2.1. Materials

Natural crystals of zeolites from secondary

crystallization assemblages in basaltic rocks were

used as source materials. Provenance localities are:

heulandite from Osilo, Sardegna, Italy, yugaw-

aralite from Poona, India, and stilbite from Fun-ningsfjordur, Faer Øer Islands, Denmark. All

crystals have well developed tabular habit, and

they were used without any prior chemical treat-

ment. Sample identification and purity was con-

trolled by XRPD on crystals ground from the same

specimens.

2.2. Sample preparation and instrumental parame-

ters

All the crystal samples were mechanically

cleaved with a stainless steel blade just before in-

sertion into the AFM sample cell. AFM mea-

surements were mostly performed in air, although

a few tests were also performed with the sample

submerged in de-ionized water and in dilutedH2SO4 (0.05 M), HCl (0.1 M), NaOH (0.1 M). No

substantial differences were obtained in the two

cases. The best results in terms of lateral resolution

were obtained in air or in sodium hydroxide so-

lutions; the use of acidic solutions produced sev-

eral problems of surface contamination. The

quality of the cleaved surface is crucial for the

imaging experiment: surfaces that allowed highresolution imaging in air also provided high

quality images in water or NaOH solutions.

The instrument used was a Nanoscope III

(Digital Instruments, Santa Barbara, CA) oper-

ated in contact mode. Commercially available

Si3N4 triangular cantilevers with pyramidal tips

were used, with nominal spring constant k varying

from 0.06 to 0.6 N/m. The forces applied on thesurfaces were variable (depending on k and the

environment); a proper set point value was always

chosen to minimize the interaction forces between

tip and crystal surface. A key factor was the choice

of appropriate values of the scan rate: during this

study the scan rate was set at values in the range 3–

40 Hz, using values in the range 12–40 Hz for

optimal results when trying to obtain molecularresolution. Images were also recollected using

different scan angles in order to check for instru-

mental artefacts.

The x and y-directions of the D piezo-scanner

(scan size: 12.5 lm) used for our experiment were

calibrated with the conventional calibration grid

provided by Digital Instruments; the z-axis was

calibrated using the elementary steps on an etchedsynthetic mica foil. The calibration was checked by

imaging the mica sample at molecular resolution.

M. Voltolini et al. / Microporous and Mesoporous Materials 61 (2003) 79–84 81

To avoid artefacts due to external vibrations the

instrument is operating in a quiet place and posi-

tioned on a table supported on a specially designed

pneumatic suspension system.

3. Results

3.1. Heulandite

Heulandite crystals present a well developed

(0 1 0) morphological pinacoid, and after careful

cleavage they provide an excellent surface for

AFM studies. On the cleaved faces there are flatareas of many lm2 showing elementary steps

about 0.91(2) nm high. The measured thickness of

these steps nicely corresponds to b=2 (heulandite

Fig. 1. Surface structure of heulandite. The first image (a) is an unfilte

a unit cell outlined, the third (c) is the calculated surface structure an

cell: a ¼ 17:70, b ¼ 17:94, c ¼ 7:42 �AA, b ¼ 116:4�[18]). Experiments in various environments were

carried out on the Sardinian heulandite crystals,

although the best images were obtained in NaOH

0.1 M. They are shown in Fig. 1.

3.2. Yugawaralite

Yugawaralite is here investigated for the first

time by AFM. Yugawaralite is another zeo-

lite providing very good clavage surfaces with

some b=2 high elementary steps. The measured

step is about 0.72(2) nm (yugawaralite cell: a ¼6:73, b ¼ 13:95, c ¼ 10:03 �AA, b ¼ 111:5� [18]).Growth spirals are also frequently observed. The

images shown in Fig. 2 were taken in de-ionized

water.

red AFM image, the second one (b) is a FTT filtered image with

d the last (d) is the FFT of the surface image.

Fig. 2. Yugawaralite surface. The first image (a) is an unfiltered AFM image, a FFT filtered image is shown on the right (b), and the

calculated surface structure is presented below (c).

82 M. Voltolini et al. / Microporous and Mesoporous Materials 61 (2003) 79–84

3.3. Stilbite

Stilbite has non-perfect cleavage, but nonethe-

less it provides reasonably flat surfaces when mech-

anically cleaved. As in heulandite and yugawaralite,

the elementary steps observed on the surface are

about 0.92(2) nm, that is b=2 high (stilbite cell:

a ¼ 13:61, b ¼ 18:24, c ¼ 11:27 �AA b ¼ 127:85�[18]). The images shown in Fig. 3 were taken in air.

Overview of the surface with several growth spiralsis shown in Fig. 4.

4. Discussion and conclusion

AFMhas proven to be an appropriate and useful

technique to investigate zeolite surfaces in terms of

microtopography, although to reach a spatial res-

olution of sufficient quality to interpret the surface

structure at near-atomic level very flat and cleansurfaces have to be selected [7]. Imaging of other

natural and synthetic zeolites (i.e. chabazite, Na-

LTA, laumontite) did not produce nanometric

resolution images at the lattice scale, probably be-

cause of degree of roughness of the surfaces at the

atomic level. The best resolution achieved when

imaging the (0 1 0) surface of yugawaralite (Fig. 2)

is limited for the same reason. In the yugawaraliteimage the single tetrahedral vertices, probably

surface hydroxyl groups, can not be resolved and

only groups of two tetrahedra are visible. In the

stilbite images (Fig. 3) the single tetrahedra can be

identified, and the resemblance of the calculated

and the FFT filtered image is evident.

The environmental fluid in contact with the

surface does not seem to greatly improve the final

Fig. 3. Stilbite surface. The first image (a) is a (0 1 0) area at molecular resolution, the second image (b) was FFT filtered. The cal-

culated structure (c) and its FFT (d) are shown below.

Fig. 4. Stilbite (0 1 0) cleavage surface. The microtopographic image (a) at the left exhibits elementary steps having b=2 height, which

are generated by several dislocation sources. The image at the right (b) represents one of such steps at molecular resolution.

M. Voltolini et al. / Microporous and Mesoporous Materials 61 (2003) 79–84 83

quality of the images with respect to those imaged

in air, provided that the selected surface is flat and

clean. In general, an acceptable spatial resolution

can be obtained in air, although working with a

fluid cell can provide more stability and limit the

forces acting on the surface. The use of acidic

84 M. Voltolini et al. / Microporous and Mesoporous Materials 61 (2003) 79–84

fluids, even if very diluted and operating on

chemically resistant zeolites, always produces a

little surface etching. In spite of keeping the sur-

face clean, some reaction products are formed at

the surface, quickly degrading the quality of the

images. Basic solutions like NaOH 0.1 M seem toproduce better results, as previously observed by

other authors [19].

AFM studies of the (0 1 0) surfaces of three

natural zeolites at molecular resolution were com-

pleted, as a preliminary step into interpreting the

behaviour of zeolite surfaces in contact with

aqueous solutions. The scarcity of high resolution

images of zeolites in the literature is to be related tothe difficulty of obtaining appropriate surfaces for

AFM imaging. The study will progress by follow-

ing in situ chemical reactions such as adsorption

and dissolution on zeolite surfaces.

Acknowledgements

The work has been carried out in the frame of

the project ‘‘Mineral surface chemical reactions:

intercalation and sorption processes’’ (Coordina-

tor Prof. G. Artioli), and financed by MURST

COFIN 2000.

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