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Three-DimensionalTransmission Electron Microscopy Observations of Supported Palladium Particles G. Fuchs,*~fD. Neiman,* and H. Poppa IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Received March 19,1991. In Final Form: July 18, 1991 Transmission electron microscopy (TEM) is an extensively used technique for the characterization of small supported particles. In addition to the usual projectional TEM data such as the size distribution, number density, and crystallographic structure of the aggregates, the three-dimensional shape of these clusters can also be determined when “microsupports”are used. This work presents the first TEM and high-resolution transmission microscopy (HRTEM) results of supported palladium clusters grown by UHV vapor deposition on alumina microspheres in the surface profile imaging mode. For this metal support system, the viability of TEM analysis with and without stabilizing “fixing layers” is assesse Furthermore, in order to extend this technique to other metal/support systems, we will examine whether a sublayer between the particle and the microsupport can be used to easily change the nature of the support surface. The results of this study also emphasizeimportant limitations of the present profile-TEM technique, especially when applied to cluster deposits of chemically active metals.
k
Introduction The first stage of growth of metallic deposits on oxides is the formation of small metal aggregates, which are the result of cluster nucleation and particle growth. In order to control the growth of ultrathin films on such supports, the determination of the details of the growth mechanism is essential. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) are two of the most common and useful techniques for determining the main structural and topographical characteristics of these supported aggregates. One of the most important limitations of these techniques is the difficulty in determining the threedimensional shape of the supported particles. This problem can be solved if profile-TEM observations are performed on microsupports in the shape of microcubes (MgO) or microspheres (TiO2, A1203, Si, Si02, ...), where the aggregates may be seen in cross-sectional profile. This surface profile imaging technique has been fully demonstrated for nonreactive metals, especially gold,’-3 and for some metals such as Rh on Ti02 or Si024-6and Pd on Mg0.’ The main advantage of this profile-TEM technique is that it can characterize the supported particles in three dimensions without image background interference from the support if the microsupports are prepared with the help of holey support *Jlms. The purpose of this paper is to expand existing profileTEM and profile-HRTEM to other application-oriented metal/support systems. In the light of the results obtained with the palladium/alumina system, which is a system ~~
+ On leave of absence from: DBpartement de Physique des Ma-
tbriaux, Universitb Claude Bernard, 43 Boulevard du ll Novembre 1918,69622Viileurbanne Cedex, France.
t Department of Chemistry, Stanford University, Stanford, CA 94305. (1)Iijima, S.J. Electron Microsc. 1986,34(4),249. (2)Marks, L. D.; Smith, D. J. Nature 1983,303,316. (3)Iijima, S.;Ichihashi, T. Phys. Reo. 1986,56(6),616. (4)Chakraborti, S.;Datye, A. K.; Long, N. J. J. Catal. 1987,108,444. (5).Logan, A. D.; Braunschweig, E. J.; Datye, A. K.; Smith, D. J. Ultramicroscopy 1989,31,132. (6)Logan, A. D.; Braunschweig,E. J.; Datye, A. K.; Smith, D. J. Langmuir 1988,4,827. (7) Giorgio, S.; Henry, C. H.; Chapon, J. M.; Penisson, J. M. J. Cryst. Growth 1990,100, 254.
extensively studied in catalysisgloand model catalysis,11J2 we will point out the conditions under which this technique can be used to study the interaction between Pd particles and the A1203 support. The accuracy and limitations of this technique will be illustrated by studying the influence of the imaging electron beam on the metal particles and the influence of the support surface preparation. In particular, we will determine if the metal/support system can easily be changed with a thin overlayer of a different oxide is deposited on the initial microsupport and to what extent a fixing layer can be used to protect the metal particles from the irradiation effects of the electron beam. Experimental Procedure The supports used in this work were crystalline All08 and Ti02 microspheres prepared by plasma techniques18J4 and segregated on holey carbon grids. The diameter of these spheres ranged from 10to 150nm. The aluminasphereshave beeneither used without any further preparation or in situ plasma cleaned and coated with SiO,, which was evaporated from a Knudsen cell. In order to study the interaction between amorphous alumina and palladium, Ti02 spheres coated ex situ with an amorphous AlzOa thin f i and in situ plasma clean4 before the Pd deposition have been also used as supports. The amorphous aluminalayer was deposited by reactive sputtering, as described in a previous paper.l6 In this study,the cleaning procedure used before Pd deposition was the following: the supports were annealed to 573 K for 0.5 h at a pressure below 1W Pa, then transferred to a small sample introduction/preparationchamber,and placed in front of an Al discharge electrode. The preparation chamber was isolated from UHV and backfilled with 6 Pa 02,and an rf discharge was applied for 2 min. The chamber was then pumped to below lo-‘Pa and, the sample reintroduced to the UHV system. The sample was (8) Ruckenstein, E.; Chen, J. J. J. Catal. 1981,70,233. (9) Chen, J. J.; Ruckensbein, E. J. Catal. 1981,85,1606. (10)Ruckenstein, E.; Chen, J. J. J. Colloid Interface Sci. 1982,86,1. (11)Marks, F.A.; Lindau, I.; Browning, R. J. Vac. Sci. Technol. 1990, A8, 3437. (12)Rumpf, F.;Poppa, H.; Boudart, M. Langmuir 1988,4,722. (13)Braunschweig,E.J.;Logan,A.D.;Datye,A. K.Moterio&Research Society Symposium Proceedings; Treacy, M. M., Thomas,J. M.,White, J. M., Eds.; (Materials Pittaburgh, PA, 1988;Vol. 111, p 36. (14)Braunschweig, E.J.; Logan, A. D.; Chakrakrti, S.; Datye, A. .K. Proceedings,9thZnternationalCongress on Catalysls;Terman,M.,P U ips, M. J., Eds.; Chemistry Institute of Canada: Ottawa, ON,Canada, 1988;p 1122. (15)Marks, F. A., Ph.D. Thesis, Stanford University, 1989.
0743-7463/91/2407-2853$02.5Q/Q 0 1991 American Chemical Society
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annealed to 573 K for 0.5 h under 5 X 10-8 Pa vacuum. Auger and X-ray photoelectron spectroscopy have indicated that this procedure produces clean oxide surfaces (free of carbon and saturated with oxygen). Palladium was deposited from a wire Pd source. During the Pd deposition, a vacuum in the 10-8-Pa range was maintained. The support temperature during the metal deposition was 573 K, and the deposition rate, monitored directly by a quartz crystal oscillator substituting temporarily a t the deposition position, was fixed at 0.03 nm/min. The metal exposure E = R t ( R deposition rate, t deposition time) ranged from 0.2 to 0.4 nm, resulting in particle number densities (N)and average particle sizes ( D m )of (2-4.5) X 10l2cm-2and 1.8-2.3 nm, respectively, on alumina supports. The TEM results were obtained with an Akashi EM-OOBB and a JEOL-2000FX electron microscope, both operating at 200-kV accelerating voltage. A few observations have been performed at 100kV to increasethe image contrast. During the observation, the background pressure in the microscope was in the 10-5-Pa range. The beam current density was always adjusted to obtain a 2-s exposure time of the photomaterial (Kodak electron image film SO-163). The particle size and particle number densities were determined from the micrographs with a Zeiss particle size analyzer.
Results 1. P d Particles Deposited on Flat Amorphous Supports. Figure 1presents TEM and HRTEM images of Pd clusters deposited on an amorphous alumina film after the cleaning procedure. From TEM micrographs such as the one presented in Figure la (E = 0.2 nm), the Pd particle number density (N)and the mean cluster size (D,) have been determined (4.5 X 10l2cm-2 and 1.8 nm, respectively). The projected shape of the particles as well as the atomic arrangement inside the particles (Figure 1b,c) can be studied in detail on this planar and amorphous support, but no information can be directly extracted from these micrographs on the three-dimensional shape of the deposited particles. Image simulationscould be performed to fit the HRTEM micrographs (such as the one presented in Figure IC),which could give some idea of the overall thickness of the supported particle. However, in the case of aggregates of the size of the ones presented here (-600 atoms (Figure IC)),the particles are of spherical shape and not flat; such image simulations are extensive and difficultand would seem unjustified when this information can be directly obtained using microsupports and profileTEM techniques. 2. Pd Deposited on Crystalline A1203 Microspheres. Figure 2 shows TEM micrographs of palladium deposits (E = 0.2 nm (a) and E = 0.4 nm (b)) on as-prepared (not plasma-cleaned) Al2O3. The low-magnification TEM micrograph gives an overview of this type of specimen (Figure 2a). Values of N calculated for the spheres are similar to the ones calculated for the Pd particles deposited on amorphous A1203 substrate, but the uncertainty of the orientation of the microsupport surface with respect to the Pd flux and the probable C contamination of the microsupports prevent any unequivocal conclusion on the influenceof the nature and the preparation of the support on the growth mechanism. At higher magnification, the micrograph of Figure 2b illustrates how the profile of the palladium particles can be imaged with this technique. On these samples, numerous Pd crystallites are simultanously imaged in projection either on the surrounding carbon film or on the top of the alumina spheres. The most interesting particles for this study are those which lie at the edge of the spheres because they can be seen in profile. In this case, the generally spherical shape of the particles and the contact angle between the metal particle and the support would indicate, after a transfer through
Figure 1. TEM (a, top) axid HRTEM (b, middle; c, bottom) micrographs of Pd deposited on an in situ cleaned amorphous alumina film. The metal exposure isE = 0.2 nm. The Pd particle number densities (N) and the mean diameter (Dm)of the particles deposited on the alumina spheres are 4.5 X 10l2cm-2and 1.8 nm, respectively.
air from the UHV deposition system to the microscope, a poor wetting of A1203 by Pd during the imaging stage. It must be emphasized at the present time that the
3-0 TEM Observations of Supported Pd Particles
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Figure2. TEM micrographsof Pd deposited on ex situ prepared A1203 spheres. The metal exposure is (a, top) E = 0.2 nm and (b, bottom) E = 0.4 nm. The Pd particle number densities (N) and the mean diameter (D,)of the particles deposited on the alumina spheres are, respectively, (a) 4.4 X 1012 cm4 and 1.9 nm and (b) 2 X 10'2 cm-2 and 2.3 nm.
mobility and structural instability of small particles under the imaging electron beam prevent any accurate, meaningful, and reproducibledirect measurement of the contact angle of these small metal particles with the alumina support. The beam effects are well-known and are not specific to the Pd/A1203 system. Figure 3 illustrates how sensitive small particles are during high beam intensity TEM observations in the case of the system Au/TiOz. The first series of micrographs (Figure 3a-d) indicates clearly the movement of the gold aggregates on the support. During the first seconds of the TEM observation, the
smallest particles move quickly on the support and tend to coalesce with other aggregates (see, for instance, the area labeled A on panels a and b of figure 3). After this first stage (2 min of observationat 780000X magnification), the number of coalescence events significantly decreases and the aggregates are much less mobile on the support surface. Nevertheless, changes of the shape of the particles appear in profile view (see particle B) as well as in normal view (see particle C). This is more clearly indicated by the second series of micrographs (Figure 3e-g). The mobility and the structural instabilities of the gold aggregateson the support are evident looking, for instance, at the aggregate labeled D. During the HRTEM observation, atomic rearrangementsare induced by the imaging electron beam, and the crystallographicorientation of the supported gold particles changes constantly during the observation. A t the same time, it must be noticed on this series of micrographs that the Ti02 surface also changes continuously (mowed area, for instance). In the case of Pd particles, severe contamination occurs simultaneously with the beam-induced mobility of the supported clusters. These contamination effects become visible around the Pd particles as the electron irradiation time increases and/or the electron flux (accompanying high-magnification observations during HRTEM) increases. In these cases, the Pd particles become surrounded by an amorphous shell, as seen in Figure 4. This amorphousenvelopeis clearly seen on the particlein profile (labeled A on Figure 4) as well as on the particles seen in projection through the alumina sphere (for instance, particle B). We have verified that the latticeplane spacing of the Pd particles and the angle between the lattice planes correspond to the well-known values of bulk palladium. Notice also that this carbon contamination affects only the Pd particles. The surfaceof the microspheres remains unchanged,and no comparable continuousamorphousfilm grows on their surfaces. This seems to be typical for Pd and will be discussed later. If irradiation by the electron beam continues at this stage, further drastic changes are noticed, especially for some of the smallest particles (smaller than 2 nm). The growth of an amorphous column under a palladium cluster initially sitting on the edge of the microsphere has been observed for higher electron exposuretimes and for smaller Pd particles, as shown on Figure 5. The black contrast feature on top of this amorphous column corresponds to the Pd aggregate. The growth of this column can be followed until this column becomes bent. A t the same time, the contrast of the particles that are seen through the support vanishes and it becomes very difficult to distinguish them, indicating severe contamination of the deposit. 3. Pd Deposited on Clean Amorphous Al203-Coated Ti02 Microspheres. In order to determine if the nature and the preparation of the support influence the shape of Pd particles, TEM profile images have been obtained from Pd particles supported on an amorphous alumina thin film deposited onto the surface of microsupports (in this case, Ti02 spheres have been used). Before the Pd deposition,the alumina surface was cleaned as mentioned under Experimental Procedure. The Pd particle number densityand the averagePd particlesizeon the thin alumina film supports have been measured respectively as 2.5 X 1012 cm-2 and 2 nm for a 0.4-nm-thick deposit, and no significant difference in N and D, is seen between the "flat" alumina support and the alumina-coated T i 0 2 spheres. This indicates that the alumina deposit has
Fuchs et at.
2056 Langmuir, Vol. 7, No. 11,1991
A .
.
2 nm H
4
2 nm
Figure 3. Two series of (a-d, e-g) of successive HRTEM micrographs of Au aggregates supported on Ti02 spheres. The time between the two successive micrographs is -1 min. The Au exposure is E = 0.2 nm.
uniformly coated the support surfaces (alumina-coated carbon grid and microspheres). Figure 6 presents a typical area of a 0.4-nm Pd deposit on Al2Os-coatedTi02 microspheres. The alumina coating can be seen on the edge of the crystallinespheres, especially when their orientation is favorable for lattice imaging of the Ti02 structure. The thickness of this layer measured from these micrographs is -5 nm. In this case, the acceleratingvoltage has been decreased to 100kV in order to enhance contrast and to better distinguish the amorphous layer from the microsphere support. In this sample
configuration, very high sensitivity of the alumina layer to the imaging electron beam (at 100 kV as well as 200 kV) is observed, and the TEM observations have to be performed with a minimum dose of irradiation. During the observation, changes in the location and shape of Pd particles are noticed, as illustrated in Figure 7. Moreover, the thickness of the amorphous alumina sublayer continually decreases during the TEM observation. After 5 min of irradiation at 400000x magnification, no alumina film can be detected on the irradiated spheres. A t the
3-0 TEM Observations of Supported Pd Particles
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Figure 4. A later stage of contamination observed under HRTEM conditions for Pd particles (E = 0.4 nm) deposited on ex situ prepared alumina spheres. (Early stages of contamination are characterized by the growth of an amorphous shell around the palladium particles.) ,
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Figure 6. TEM micrographs of Pd deposited (E = 0.4 nm) on an in situ cleaned amorphous A1203 thin film coating over Ti02 spheres. The particle number densities (N)and the mean diameter (D,) of the particles deposited on the amorphous alumina film are 2.5 X 1OI2 cm-2 and 2.8 nm, respectively. The accelerating voltage used for this observation was 100 keV.
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Figure 5. Severe contamination effects observed for Pd deposited on ex situ prepared alumina spheres after an extended time of electron beam exposure (5 min a t 78oooOX magnification). The growth of an amorphous column under the Pd particle can be seen.
same time, the location,size, and shape of the Pd particles supported on this film have changed drastically. 4. Sensitivity of the Fixing Layer under the Electron Beam. In order to stabilizethe aggregates under the electron beam, we have deposited C or SiO, fixing layers over the metal deposit. At low magnification (i.e., correspondingly low electroncurrent density),no aggregate mobility has been noticed. At higher magnification, and especially for high-resolution transmission electron microscopy conditions, this fixing layer no longer suffices for particle stabilization. For Pd as well as for Au aggregates,
Figure 7. Change of the shape and locationof Pd particles during HRTEM observation. The support is an amorphous alumina film deposited on Ti02 spheres and in situ cleaned. The time between the successive micrographs a and b is -1 min.
the C or SiO, fixing layer is very sensitive to the electron beam, as shown on Figure 8. At the beginning of the observation (Figure 8a), the C-coated aggregates remain stable but the resolution is lowered due to the presence of the amorphous fixing layer. As observation time increases, the thickness of the fixing layer decreases, and after 2 min of electron beam exposure at 98oooOX magnification, the fixing layer has totally disappeared from large areas (Figure 8b) in which the metal particles show strong changes in location and contrast. The behavior of this fixing layer is quite similar to that of the alumina sublayer presented in Figure 7a,b.
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2058 Langmuir, Vol. 7, No. 11,1991
Figure 8. Change of the C-fixing layer during HRTEM observation of Au particles. The thickness of this amorphous layer decreases as the irradiation time increases. The time between the successive micrographs a and b is -2 min. The support is a Ti02 microsphere.
Discussion This surface profile imaging technique has been shown to be a powerful tool for the study of small particles in the case of and our results are in agreement with these previous results. We have verified that gold particles grown on microspheres can be easily imaged in this TEM profile mode. The high contrast and chemical inertness of gold particles constitute definitive advantages for imaging the shape and atomic structure of such particles located on the periphery of microspheres. We have also noticed the well-defined crystallographic shapes of Au particles and the remarkable structural instability of these particles under the imaging electron beam.3916 For gold particles, we have not noticed any carbon contamination induced by the beam, even in the HRTEM mode. In this respect, Pd deposits behave quite differently. The aim of our study was to elucidate the limitations of particle shape determinations by this new profile imaging technique in general and specifically for the system Pd/A1203. We also wanted to study how particle shapes might be influenced by the nature of the support. In the case of the Pd/A1203 system, which is chemically much more reactive than supported Au but also much more interesting from a catalysis point of view, the present TEM observations show that the surface profile imaging technique allows for some characterization of the shape of the Pd particles. A further feature of this technique is the capability of providing an accurate image of the surface of the supported particle without any disturbing backgound contrast coming from the support. Therefore, this technique should be useful to examine, for instance, the formation of a thin film coating on the particle surface. The present results show that in the case of a reactive metal such as Pd the shape of the particle can be determined if electron irradiation effects are minimized by using low-dose TEM techniques. However, electron beam effects in general impose serious limitations upon this technique. As was demonstrated, the main effect of the electron beam is the contamination of the Pd particles by what is assumed to be amorphous carbon. This assumption is consistent with the well-known strong ~~
~~
(16) Mitome, M.; Tanishiro, Y.; Takayanagi, K. 2.Phys. D 1989,12, 45.
interaction between Pd and C.17-20 Also, the penetration of carbon into palladium during catalytic hydrogenation of acetylene2' or during heating in an atmosphere of ethylene, acetylene, or carbon monoxide22and the decomposition of CO by small Pd particles23* has been reported. Another example is the ability of Pd to catalyze the graphitization of amorphous carbon at high temperature (1150 K).25 Since all our TEM profile observations have not been made under UHV conditions, it is difficult to ascribe the formation of the carbon coating of the Pd particles to the reaction of Pd with the residual carboneous contamination on the microsupport or to the reaction with background gases in the electron microscope. However, we have never observed any such contamination effects on Au particles under the existing experimental conditions, and therefore, we assume that the well-known electron beam induced specimen contamination caused by the cracking of hydrocarbons is not the source of contamination in this case. The contamination we observed on Pd seems typical of this metal and can probably be ascribed to the catalytic decomposition of surface carbon compounds, with a mechanism similar to the one previously proposed to explain the decomposition of CO on Pd particles." It seems inescapable at this point to conclude that these severe electron irradiation effects have to be controlled and limited to extend the profile-TEM technique from gold particles to more reactive metals. When the microscopy is restricted to low-magnification observations to minimize the electron irraidation effects, it is still possible to determine the overall shape of Pd particles; in the diameter range 1.8-3 nm, the Pd particles are approximately spherical when deposited on asprepared crystalline A1203 spheres. However, HRTEM images are much more difficult to obtain with Pd than with Au. Just as in the case of gold deposited on Si023or on Ti02 (this study), the location and the shape of the Pd particles change when the current density is too high. Nevertheless we have noticed some important and reproducible differences between Pd and Au particles observed in profile. First, when particles on the same support (for instance, TiO2) are compared, the Pd particles are rarely found as well-defined microcrystallites as it is the case for Au (the Pd/TiO2 results are unpublished to date). Profile images of Au particles usually show welldefined crystallographic facets while the shape of the Pd particles is generally smooth and rounded, and the profile lattice images of Pd aggregates are more difficult to obtain. This may be due to the lower atomic number and the much higher tendency for contamination under the imaging beam since the lattice spacing of these two fcc structures (gold (dl11 = 0.2355 nm) and palladium (dl11 = 0.2246 nm)) are similar. The SiO, fixing layer used in this study was found helpful in controlling the contamination of the Pd particles and reducing the changes in shape and location of the Pd (17) Baetzold, R. C. Surf. Sci. 1972,36, 123. (18) Hamilton, J. F.; Logel, P. C. Thin Solid Films 1973, 16,49. (19) Egelhoff, W. G., Jr.; Tibbets, G. G. Solid State Commun. 1979, 29, 79. (20) Lamber, R,Jaeger, N.; Schulz-Ekloff, G. Surf. Sci. 1990,227,15. (21) Starchurski,J.; Frackiewia,A. J. Less-CommonMet. 19%, 108, 249. (22) Ziemecki, S. B.; Jones, G. A. J. Catal. 1985,95,621. (23) Ichikawa, S.; Poppa, H.; Boudart, M. In Catalytic Materiuls: Relationships between Structure and Reactivity;Whyte, T. E., Jr., Dalla Betta, R. A., Derouane, E. G., Baker, R. T. K., Eds.; ACS Sympmium Series 248; American Chemical Society: Washington, DC, 1984. (24) Doering, D. L.; Dickinson, J. T.; Poppa, H. J . Catal. 1982,73,104. (25) Holstein, W. L.; Moorhead, R. D.; Poppa, H.; Boudart, M. In Chemistryand Physics of Carbon;Walker, P. L., Ed.;Dekker: New York, 1982; Vol. 18, p 139.
3-0 TEM Observations of Supported Pd Particles particles induced by the microscope electron beam. With this fixing layer, the aggregates remain stable at low magnification (low current density) but contrast and resolution decrease somewhat compared to unfixed samples. However, under the influence of a high current density electron beam, this layer disappears and particle changes induced by the beam become intolerable. To our knowledge no other HRTEM profile study on Pd/A1203 has been reported. Only C-coated Pd supported on microcubes of MgO has been studied by profile imaging.7 This report also states that Pd particles must be protected with a (carbon) fixing layer to avoid major beam effects and to stabilize the Pd aggregates during HRTEM observations, but the stability of the fixing layer has not been addressed. The surface profile imaging technique has to date been used for very few metal/support systems. However, this technique could be very helpful, especially in studying the role of general metal/support interactions, which are essential for the understanding of oxidation/reduction treatments routinely used in catalysis. For instance, the interaction mechanisms invoked by Wang and Schmidt26 for the system Ir/SiOz, by Derouane et ala2'for the system Cu/MgO, by Nakayama et a1.28 for the system Ni/A1203, and by Ruckenstein and Chu29for the system Pt/alumina could be studied in more detail by employing profile imaging. Among the very few reactive metal/support systems studied with the TEM profile technique, the system Rh/SiOz can be menti~ned.~*S 'In this study, the electron beam effects are also mentioned in spite of the use of more stable Rh aggregates of larger mean size (- 5 nm). The Rh aggregates were deposited by impregnation of microsupports followed by thermal treatments at high temperature (773 K). A possible explanation for the limited use of this method could be the difficulty of finding suitable microsupport materials. In the literature some examples of oxide microsupports are given. Various syntheses of such microsupports have been reported. Support materials such as 7-AI203,l amorphous Si02130 and a - F e ~ O 3have ~ ~ been utilized as have been semiconductor microsupports such as Si (oxidized on the surface)' and even metal microsupports such as Co, Cu, Ag, Pt, and Ni.32 It must be emphasized that special care has to be exercised in the synthesis of these microsupports. Even during the apparently easy preparation of MgO microcubes,33 special attention has to be paid to preparation details in order to limit the roughness of the cube ~ u r f a c e . 3In~this ~ ~ sense, ~ coating microsupports (before the metal deposition) with another support material of interest in order to have a well-defined surface or to extend the use of this method to other metal/support systems seems to be attractive. But, as we show in this paper, such overcoating layers may (26) Wang, T.; Schmidt, L. D. J. Catal. 1980,66,301. (27) Derouane, E. G.;Chludzinski, J. J.;Baker, R. T. K. J.Catal. 1984, 85, 187. (28) Nakayama, T.;Arai, M.; Nishiyama, Y. J. J.Catal. 1983,79,497. (29) Ruckenstein, E.; Chu, Y. F. J. Catal. 1979,59,109. (30) Huizenga, D. G.; Smith, D. M. AIChE J. 1986, 32, 1. (31) Sugimoto, T. MRS Bull. 1989, 14(12) 23. (32) Fievet, F. MRS BuZl. 1989, 14(12), 29. (33) Uchida, Y.; Lehmpfuhl, G.; Weiss, K.; Zemlin, F. Ultramicroscopy 1982,9,325. (34) Duriez, C., Thesis, Marseille University, 1989. (35) Duriez, C.; Chapon, C.; Henry, C. R.; Richard, J. M. Surf. Sci. 1990,230, 123.
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not be stable and may interact with the metal particles during HRTEM. This behavior has been more systematically investigated for A1203 and SiO, coatings on Ti02 and alumina spheres. In our work, the SiO, coating of the microsupports was performed under UHV conditions in order to guarantee the cleanliness of the new support. Even in this case, we have noticed that the irradiation effects are too strong to be inhibited by the interaction between the SiO, film and the clean surface of the support. It should also be recalled that the profile of metal particles can sometimes be imaged on the edge of a hole or on a bent part of the film support. When a particle is observed on the edge of a hole, the support surface profile is not well-defined and care must be taken to compare these edges with flat support surfaces. Nevertheless, lattice images and shape information of the metallic particles may be obtained. However, the shape of small metal particles is very sensitive to the interaction with the support and a well-defined surface is an important criterion for preferring microsupports to holey films for profileTEM studies.
Conclusion Information on the three-dimensional shape of small supported particles has been obtained by TEM not only for nonreactive materials such as Au, but also for a reactive and more application oriented metal such as Pd. In light of the results obtained with the Pd/A1203 system, it can be concluded that the profile-TEM technique using microsupports is capable of imaging the shape of deposited Pd aggregates. Because Pd clusters lying at the edge of the microspheres can be observed without any background contrast coming from the support, this configuration is optimal to also image the surface of the clusters. This technique can, therefore, detect contamination or reaction layers at the surface of the aggregates. The present results have especially highlighted a drastic carbon contamination of Pd clusters during TEM experiments which is not observed for Au and is particularly severe for very small Pd particles. As previously reported for other metals, Pd has been found to be extremely mobile under the imaging electron beam, particularly under HRTEM conditions. This presently prevents any accurate assessment of the contact angle or general wetting properties of the deposited clusters on the support surface. Because of the demonstrated high reactivity to carbon, particle mobility, and particle structural rearrangements under the beam, profiling of Pd particles requires special care. Electron beam effects must be minimized by working with low-dosage TEM techniques and by using a suitable fixing layer to obtain reproducbible and meaningful results. However, the effect of the fixing layer and sample air exposure for routine HRTEM upon the as-deposited particle/support configuration must eventually be elucidated by systematic UHV in situ HRTEM studies. Acknowledgment. We are grateful to Professor A. K. Datye (University of New Mexico) for providing the Ti02 and A1203microspheres used in this work. The UHV deposition experiments were performed at the Department of Chemical Engineering, Stanford University. Registry No. Pd,7440-05-3; A1203, 1344-28-1.
