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Pd particles supported on ZnO (001) epitaxial thin films, evolution during HRTEM observations
MATERIALS SCIENCE & ENGINEERING E LN EVI E R
Materials Science and Engineering A229 (1997) 169-173
Pd particles supported on ZnO (001) epitaxial thin films, evolution during HRTEM observations S. Giorgio *, H. Graoui, C. Chapon, C.R. Henry CRMC2~-CNRS, Campus de Luminy Case 913, 13288 MarseilIe, Cddex 9, France Received 19 March 1996
Abstract Pd particles were condensed under UHV on ZnO (001) thin films epitaxially grown on mica single crystals. The epitaxial relationships between Pd and ZnO lattices were determined by electron diffraction, (111) Pd//(001) ZnO and [112] Pal//[100] ZnO. A tetrahedral shape of the particles larger than 20 ran was observed, using weak beam dark field electron microscopy. After strong irradiation conditions, an evolution of the structure was observed, which could be due to the formation of P d - Z n alloy. © 1997 Elsevier Science S.A. Keywords: Electron diffraction; Pd particles; Thin films
1. Introduction Methanol synthesis is an important catalytic reaction for industrial applications. Catalysts used are composed of Cu dispersed on a ZnO substrate. The mechanism of this reaction is not yet completely known although a large number of fundamental studies have been undertaken [1]. As an example, the state of the active Cu phase is not yet known. The interaction with the substrate seems to play an important role. Pure Pd or Pd associated with Cu are also used as catalysts in methanol synthesis [2]. Pd/ZnO model catalysts have been studied by surface science techniques [3-5]. The growth mode of Pd is controversial. 2D (Frank Van der Merwe) as well as 3D (Volmer Weber) growths are claimed at room temperature. Diffusion of Pd inside the ZnO substrate was also found [5]. However, nothing is known about the morphology of Pd deposited on ZnO. In previous works [6,7], we studied Pd deposits obtained by ultra high vacuum condensation at 450°C on clean surfaces of ZnO micro-crystals. TEM observations clearly showed the formation of 3D Pd particles. On the lateral neutral (100) faces of the ZnO prisms, all the Pd particles were found as f.c.c, single crystals with * Corresponding author. E-mail: [email protected] Laboratoire associ+ aux universit6s d'Aix-Marseille II et III. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. DTT qflOq
l
~Q~LfO~
I ~q"~O
~w
an epitaxial orientation. No alloying with Zn had been observed either on the particle surfaces nor at the particle-substrate interface, as has been observed in conventional catalysts prepared after high temperature reduction under H2 atmosphere [8-10]. In this paper we describe the preparation of Pd particles obtained by UHV condensation on (001) oriented epitaxial thin films of ZnO prepared in-situ, by electron beam evaporation on mica single crystals. The shape and epitaxial orientation of the particles were determined by H R T E M and electron diffraction. A possible formation of P d - Z n alloy was seen during H R T E M observations, under severe electron irradiation.
2. Sample preparation A freshly cleaved single crystal of mica was mounted in a rotating furnace in a U H V chamber. It was brought in front of an electron gun for the evaporation of ZnO. The thickness of the deposit was controlled with a quartz microbalance. ZnO was grown on the substrate at 360°C, as continuous layers of thicknesses varying between 10 and 60 1am. The pressure during the evaporation of ZnO was in the range 10-7-10 .6 Torr. Then, the sample was moved in front of a Knudsen cell for Pd evaporation at
170
S. Giorgio et al./ Materials Science and Engineering A229 (1997) i69-I73
a calibrated flux (1013 a t o m s cm -2 s-l). Pd was deposited at high substrate temperature (between 360 and 460°C) and post deposition annealed at the deposition temperature for 3 h under 2 x 10 .9 Torr. To avoid contamination or oxidation during the transfert to the microscope, the samples were covered by a carbon film [11]. Small pieces of sample were collected after microcleavage of mica on molybdenum grids of electron microscopy. For that, the grid was stuck on the surface of mica on the Pd/ZnO side, then strongly removed. The edges of the samples were observed in electron microscopy in the thinnest parts of the mica.
3. Electron microscopy studies The samples were first observed with a Jeot 2000FX microscope operating at 200 KV. Using X Ray fluorescence, it was verified that the ZnO layers were continuous on the mica surface. Fig. 1 shows the diffraction pattern of a thick clean ZnO film ( ~ 120 rim) on mica. ZnO has the hexagonal structure (a--0.35 nm, c = 0.52 nm, ~ = 120°) an mica (muscovite) is monoclinic (a=0.52 ran, b =0.90 rim, c sin/3=0.1 nm). The two main reflexions, (100) and (110), from ZnO are visible close to the (200) or (330) reflexions of mica. From the relative position of reflexions, a lattice mismatch of about 7% was measured as expected for bulk crystals. This misfit is only seen at high angle reflexions (t10) of ZnO and (060) or (330) of mica, although the intensity of the (110) reflexions of ZnO is quite weak. Fig. 2a shows a collection of Pd particles with a mean size of 18 nm, on ZnO (thickness ~ 6 0 nm), Fig. 2. (a) Overview, at low magnification of a collection of large Pd particles grown at 460°C on ZnO. (b) Overview, at low magnification of a collection of small Pd particles grown at 380°C on ZnO.
Fig. 1. Electron diffraction pattern of a large area (0.2. gm in diameter), of mica covered by a thick ZnO layer ( ~ 120 nm).
grown at high temperature of the substrate (4600C). Most of the Pd particles have triangular outlines with truncations on the corners. At lower substrate temperature (e.g. 380°C), the nucleation frequency is larger and the size smaller with undefined particle shapes, as seen in Fig. 2(b). The diffraction pattern corresponding to such samples (Pd/ZnO/mica) with large or small particles, is shown in Fig. 3. It displays all the spots of mica and in addition, the three sets of (220) spots from Pd (corresponding to distances of 0.137 rim). Here, the (110) spots from ZnO are too weak to be seen. The epitaxial relations are deduced: (111) Pd//(001) ZnO and [1121 Pd//[100] ZnO. Taking as a reference the spots from mica, the lattice parameter of Pd was measured with a precision better than _+2%. No large expansion appears in Pd particles, as observed on other faces of ZnO (5% on ZnO (100)).
S. Giorgio et al./ Materials Science and Engineering A229 (1997) 169-173
I71
Fig. 3. Electron diffraction pattern of a collection of Pd particles in (111) orientation on ZnO (001).
Two additional rings of very weak intensity are visible in Fig. 3. They correspond to (1tl) and (200) lattice distances of Pd. They are due to a small quantity of particles with a random orientation on the substrate. Indeed, the ZnO layer even perfectly epitaxied on the mica substrate, probably contains surface defects which are responsible from the randomly oriented particles. Fig. 4(a) and (b) show the same area of a sample in bright field and dark field illumination using a (220) reflexion of Pd. This image (Fig. 4(b)) was obtained in weak beam dark field illumination with a tilt of the sample of 4 ° from the Bragg orientation in order to attenuate the contrast of the substrate. Among 16 particles seen in bright field, only four of them which have desappeared in dark field are in another orientation.
v
t ~
~;50 n:m
Fig. 4. (a) Bright field image of Pd particles. (b) Weak beam dark field image of the area seen in (a), using (220) Pd reflexion and a tilt of 4 ° .
Fig. 5, (a)-(c) WBDF images of a particle tilted by 3,5 °, 4 °, and 6°, from a (220) relexion.
The same weak beam dark field imaging technique (WBDF) was used to determine the 3D shape of the triangular particles [12]. Fig. 5(a)-(c) show a particle observed at different tilts relatively to the Bragg orientation. From the width variation of t h e thickness fringes it is possible to identify the crystallographic planes limiting the particles. Therefore, particles are tetrahedra (exposing (111) facets) truncated at the top by a (t 1 t) face and on the three corners by (100) faces. The truncations at the corners vary in the different particles. On the other hand, the truncations at the top are quite regular with an average ratio of the height over the side of the triangular basis 0.7. The same samples have been observed with a Jeol FEG 2010 field emission gun microscope operating at 200 kV, equiped on line with an image acquisition system. The magnification was calibrated on gold just before observation. Fig. 6 shows a large particle with a truncated tetrahedron shape observed during strong irradiation with the field emission gun. At the beginning of the observation, the Pd lattice was in the (111) orientation. The three sets of lattice fringes (220) parallel to the electron beam, with an equidistance of 0.137 nm could not be resolved with the 200 kV microscope. After a strong irradiation, two sets of perpendicular fringes with an equidistance of 0.205 nm appeared, which cannot be due-to the Pd lattice, taking into account the particles geometry. In the case of smaller clusters ( < 5-6 nm), (see Fig. 7), these lattice fringes (with an equidistance of 0.205 nm) are visible since the beginning of the observation. A random orientation of the particles was observed.
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Fig. 6. H R T E M image of a truncated tetrahedral particle on ZnO. On the right side, an enlargement of the figure displays two sets of lattice fringes with a spacing of 0.205 nm, extending in the fight part of the particle.
These fringes do not correspond to any orientation in the Pd lattice, they only appear after a strong electron irradiation, first at the edges and then they spread towards the center. So it is reasonnabte to believe that a transformation occured in the particles. The most probable transformation would be the P d - Z n alloying due to the reducing effect of the electron beam. Indeed, this alloy is always obtained instead of pure Pd particles in the case of a chemical preparation, due to the reduction at high temperature in hydrogen. Then, the fringes appearing during the observation exactly correspond to the (200) and (020) lattice distances of the alloy Pd-Zn (tetragonal structure, a =0.41 rim, c = 0.334 nm), observed in the [001] orientation, already got after reduction of the Pd/ZnO catalysts [9].
4. Discussion
Pd condensed at high temperature (near 360°C) on thin epitaxial (001) layers of ZnO exhibits a 3D shape,
Fig. 7. Small Pd particles on ZnO showing the lattice fringes with a spacing of 0.205 n m since the ~beginning of the observation with the field emission gun microscope.
from observations by weak beam dark field electron microscopy. However, by Auger spectroscopy, a 2D growth of Pd was observed at room temperature on ZnO single crystals [5]. Although we observed a 3D growth mode, at high temperature, a Stranski-Krastanov growth mode cannot be ruled out, as far as the presence of a continuous Pd monolayer below the particles is difficult to recognize in electron microscopy. Indeed, a pseudo Sranski-Krastanov growth mode was observed in the case of Cu deposited on ZnO [13]. In this paper, most of the Pd particles were found with the (111) orientation, with [112] Pd//[100] ZnO. The largest ones ( > 20 nm) have the shape of tetrahedra limited by (111) faces, truncated at the edges by (100) facets and at the top by a (111) facet. Particles with a smaller size have the same epitaxial orientation but their shapes are not well defined. It is well known that Pd particles prepared by UHV condensation at high temperature on single crystalline oxide substrates (MgO [14], mica [15], A1203 [16]), presents regular shapes of truncated half octahedra, or truncated tetrahedra. In the case of industrial catalysts, Pd particles were produced by chemical impregnation of commercial ZnO powders and reduction under H2. Many studies, using different techniques, XPS, UPS and XRD, have shown the formation of P d - Z n alloy in the particles, after the reduction stage [8-10]. On the other hand, Pd particles grown in UHV on clean ZnO micro-crystals did not alloy with Zn [6,7]. In addition, it has been shown by HRTEM and image simulation that the interface between bulk Pd and bulk ZnO is sharp, without formation of a P d - Z n alloy [17]. In the present observations, Pd grown under UHV on ZnO thin films starts to transform only under the strong electron beam irradiation, in the microscope. Until an irradiation dose of > 10 A cm -2 during 6 min, no evolution was detected. Due to the knowledge with the system Pd/ZnO prepared by chemical impreg-
S. Giorgio et al./ Materials Science and Engineering A229 (t997) I69-173
nation, the most probable transformation is the alloying with Zn, in agreement with the measurement of lattice fringes appearing after strong irradiation. We conclude that without strong reduction conditions, Pd particles are stable on a ZnO substrate.
Acknowledgements We thank A. Charai, Director of CP2M, who allowed us access to the Jeol FEG2010 microscope.
References [1] K. Klier, AppL Surf. Sci., I9 (1984) 267. [2] Yu. A. Ryndin, R.F. Hicks, A.T. Bell and Yu. I. Yermakov, Y. Catal., 70 (1981) 287. [3] W. Gaebler, K. Jacobi and W. Ranke, Surf. Sci., 75 (1978) 335. [4] H. Jacobs, W. Mokwa, D. Kohl and G. Heiland, Surf. Sci., t26 (1983) 318.
173
[5] H. Jacobs, W. Mokwa, D. Kohl and G. Heiland, Surf. Sci., I60 (1985) 217. [6] S. Giorgio, C.R. Henry, C. Chapon and G. Nihoul, in B. Jouffrey and C. Colliex (eds.), Proc. ICEM I3, Vol. 2A, Les Editions de Physique, France, 1994, p. 349. [7] S. Giorgio, C.R. Henry and C. Chapon, Microsc. Microanat. Microstruct., 6 (1995) 237. [8] P.S. Wehner, G.C. Tustin and B.L. Gustafson, J. Catal., 88 (1984) 246. [9] A. Sarkany, Z. Zsoldos, B. Furlong, J.W. Hightower and L. Guczi, or. Carat., 141 (1993) 566. [10] Z. Zsoldos, A. Sarkany and L. Guczi, & Catal., t45 (1994) 235. [11] S. Giorgio, C. Chapon, C.R. Henry and G. Nihoul, Philos. Mag. B, 67 (1993) 773. [12] M.J. Yacaman and T. Ocana, Phys. Stat. Sol. (A) 42 (1977) 571. [13] C.T. CampbelI and A. Ludviksson, 3". Vac. Sci. Technol. A, 12 (1994) 1825. [14] C.R. Henry, C. Chapon, C. Duriez and S. Giorgio, Surf. Sci., 253 (1991) 177. [15] E. Gillet, S. Channakhone, V. Matotin and M. Gillet, Surf. Sci., 152/153 (1985) 603. [16] H. Poppa, F. Rumpf, R.D. Moorhead and C.R. Henry, Mater. Res. Soc. Proc., 1I (1988) I. [17] H. Ichinose, H. Ishii, T. Ichimori and Y. Ishida, in B. Jouffrey and C. Colliex (eds.), Proc. ICEM I3, Vol. 2A, Les Editions de Physique, France, 1994, p. 279.
Materials Science and Engineering A229 (1997) 169-173
Pd particles supported on ZnO (001) epitaxial thin films, evolution during HRTEM observations S. Giorgio *, H. Graoui, C. Chapon, C.R. Henry CRMC2~-CNRS, Campus de Luminy Case 913, 13288 MarseilIe, Cddex 9, France Received 19 March 1996
Abstract Pd particles were condensed under UHV on ZnO (001) thin films epitaxially grown on mica single crystals. The epitaxial relationships between Pd and ZnO lattices were determined by electron diffraction, (111) Pd//(001) ZnO and [112] Pal//[100] ZnO. A tetrahedral shape of the particles larger than 20 ran was observed, using weak beam dark field electron microscopy. After strong irradiation conditions, an evolution of the structure was observed, which could be due to the formation of P d - Z n alloy. © 1997 Elsevier Science S.A. Keywords: Electron diffraction; Pd particles; Thin films
1. Introduction Methanol synthesis is an important catalytic reaction for industrial applications. Catalysts used are composed of Cu dispersed on a ZnO substrate. The mechanism of this reaction is not yet completely known although a large number of fundamental studies have been undertaken [1]. As an example, the state of the active Cu phase is not yet known. The interaction with the substrate seems to play an important role. Pure Pd or Pd associated with Cu are also used as catalysts in methanol synthesis [2]. Pd/ZnO model catalysts have been studied by surface science techniques [3-5]. The growth mode of Pd is controversial. 2D (Frank Van der Merwe) as well as 3D (Volmer Weber) growths are claimed at room temperature. Diffusion of Pd inside the ZnO substrate was also found [5]. However, nothing is known about the morphology of Pd deposited on ZnO. In previous works [6,7], we studied Pd deposits obtained by ultra high vacuum condensation at 450°C on clean surfaces of ZnO micro-crystals. TEM observations clearly showed the formation of 3D Pd particles. On the lateral neutral (100) faces of the ZnO prisms, all the Pd particles were found as f.c.c, single crystals with * Corresponding author. E-mail: [email protected] Laboratoire associ+ aux universit6s d'Aix-Marseille II et III. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. DTT qflOq
l
~Q~LfO~
I ~q"~O
~w
an epitaxial orientation. No alloying with Zn had been observed either on the particle surfaces nor at the particle-substrate interface, as has been observed in conventional catalysts prepared after high temperature reduction under H2 atmosphere [8-10]. In this paper we describe the preparation of Pd particles obtained by UHV condensation on (001) oriented epitaxial thin films of ZnO prepared in-situ, by electron beam evaporation on mica single crystals. The shape and epitaxial orientation of the particles were determined by H R T E M and electron diffraction. A possible formation of P d - Z n alloy was seen during H R T E M observations, under severe electron irradiation.
2. Sample preparation A freshly cleaved single crystal of mica was mounted in a rotating furnace in a U H V chamber. It was brought in front of an electron gun for the evaporation of ZnO. The thickness of the deposit was controlled with a quartz microbalance. ZnO was grown on the substrate at 360°C, as continuous layers of thicknesses varying between 10 and 60 1am. The pressure during the evaporation of ZnO was in the range 10-7-10 .6 Torr. Then, the sample was moved in front of a Knudsen cell for Pd evaporation at
170
S. Giorgio et al./ Materials Science and Engineering A229 (1997) i69-I73
a calibrated flux (1013 a t o m s cm -2 s-l). Pd was deposited at high substrate temperature (between 360 and 460°C) and post deposition annealed at the deposition temperature for 3 h under 2 x 10 .9 Torr. To avoid contamination or oxidation during the transfert to the microscope, the samples were covered by a carbon film [11]. Small pieces of sample were collected after microcleavage of mica on molybdenum grids of electron microscopy. For that, the grid was stuck on the surface of mica on the Pd/ZnO side, then strongly removed. The edges of the samples were observed in electron microscopy in the thinnest parts of the mica.
3. Electron microscopy studies The samples were first observed with a Jeot 2000FX microscope operating at 200 KV. Using X Ray fluorescence, it was verified that the ZnO layers were continuous on the mica surface. Fig. 1 shows the diffraction pattern of a thick clean ZnO film ( ~ 120 rim) on mica. ZnO has the hexagonal structure (a--0.35 nm, c = 0.52 nm, ~ = 120°) an mica (muscovite) is monoclinic (a=0.52 ran, b =0.90 rim, c sin/3=0.1 nm). The two main reflexions, (100) and (110), from ZnO are visible close to the (200) or (330) reflexions of mica. From the relative position of reflexions, a lattice mismatch of about 7% was measured as expected for bulk crystals. This misfit is only seen at high angle reflexions (t10) of ZnO and (060) or (330) of mica, although the intensity of the (110) reflexions of ZnO is quite weak. Fig. 2a shows a collection of Pd particles with a mean size of 18 nm, on ZnO (thickness ~ 6 0 nm), Fig. 2. (a) Overview, at low magnification of a collection of large Pd particles grown at 460°C on ZnO. (b) Overview, at low magnification of a collection of small Pd particles grown at 380°C on ZnO.
Fig. 1. Electron diffraction pattern of a large area (0.2. gm in diameter), of mica covered by a thick ZnO layer ( ~ 120 nm).
grown at high temperature of the substrate (4600C). Most of the Pd particles have triangular outlines with truncations on the corners. At lower substrate temperature (e.g. 380°C), the nucleation frequency is larger and the size smaller with undefined particle shapes, as seen in Fig. 2(b). The diffraction pattern corresponding to such samples (Pd/ZnO/mica) with large or small particles, is shown in Fig. 3. It displays all the spots of mica and in addition, the three sets of (220) spots from Pd (corresponding to distances of 0.137 rim). Here, the (110) spots from ZnO are too weak to be seen. The epitaxial relations are deduced: (111) Pd//(001) ZnO and [1121 Pd//[100] ZnO. Taking as a reference the spots from mica, the lattice parameter of Pd was measured with a precision better than _+2%. No large expansion appears in Pd particles, as observed on other faces of ZnO (5% on ZnO (100)).
S. Giorgio et al./ Materials Science and Engineering A229 (1997) 169-173
I71
Fig. 3. Electron diffraction pattern of a collection of Pd particles in (111) orientation on ZnO (001).
Two additional rings of very weak intensity are visible in Fig. 3. They correspond to (1tl) and (200) lattice distances of Pd. They are due to a small quantity of particles with a random orientation on the substrate. Indeed, the ZnO layer even perfectly epitaxied on the mica substrate, probably contains surface defects which are responsible from the randomly oriented particles. Fig. 4(a) and (b) show the same area of a sample in bright field and dark field illumination using a (220) reflexion of Pd. This image (Fig. 4(b)) was obtained in weak beam dark field illumination with a tilt of the sample of 4 ° from the Bragg orientation in order to attenuate the contrast of the substrate. Among 16 particles seen in bright field, only four of them which have desappeared in dark field are in another orientation.
v
t ~
~;50 n:m
Fig. 4. (a) Bright field image of Pd particles. (b) Weak beam dark field image of the area seen in (a), using (220) Pd reflexion and a tilt of 4 ° .
Fig. 5, (a)-(c) WBDF images of a particle tilted by 3,5 °, 4 °, and 6°, from a (220) relexion.
The same weak beam dark field imaging technique (WBDF) was used to determine the 3D shape of the triangular particles [12]. Fig. 5(a)-(c) show a particle observed at different tilts relatively to the Bragg orientation. From the width variation of t h e thickness fringes it is possible to identify the crystallographic planes limiting the particles. Therefore, particles are tetrahedra (exposing (111) facets) truncated at the top by a (t 1 t) face and on the three corners by (100) faces. The truncations at the corners vary in the different particles. On the other hand, the truncations at the top are quite regular with an average ratio of the height over the side of the triangular basis 0.7. The same samples have been observed with a Jeol FEG 2010 field emission gun microscope operating at 200 kV, equiped on line with an image acquisition system. The magnification was calibrated on gold just before observation. Fig. 6 shows a large particle with a truncated tetrahedron shape observed during strong irradiation with the field emission gun. At the beginning of the observation, the Pd lattice was in the (111) orientation. The three sets of lattice fringes (220) parallel to the electron beam, with an equidistance of 0.137 nm could not be resolved with the 200 kV microscope. After a strong irradiation, two sets of perpendicular fringes with an equidistance of 0.205 nm appeared, which cannot be due-to the Pd lattice, taking into account the particles geometry. In the case of smaller clusters ( < 5-6 nm), (see Fig. 7), these lattice fringes (with an equidistance of 0.205 nm) are visible since the beginning of the observation. A random orientation of the particles was observed.
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Fig. 6. H R T E M image of a truncated tetrahedral particle on ZnO. On the right side, an enlargement of the figure displays two sets of lattice fringes with a spacing of 0.205 nm, extending in the fight part of the particle.
These fringes do not correspond to any orientation in the Pd lattice, they only appear after a strong electron irradiation, first at the edges and then they spread towards the center. So it is reasonnabte to believe that a transformation occured in the particles. The most probable transformation would be the P d - Z n alloying due to the reducing effect of the electron beam. Indeed, this alloy is always obtained instead of pure Pd particles in the case of a chemical preparation, due to the reduction at high temperature in hydrogen. Then, the fringes appearing during the observation exactly correspond to the (200) and (020) lattice distances of the alloy Pd-Zn (tetragonal structure, a =0.41 rim, c = 0.334 nm), observed in the [001] orientation, already got after reduction of the Pd/ZnO catalysts [9].
4. Discussion
Pd condensed at high temperature (near 360°C) on thin epitaxial (001) layers of ZnO exhibits a 3D shape,
Fig. 7. Small Pd particles on ZnO showing the lattice fringes with a spacing of 0.205 n m since the ~beginning of the observation with the field emission gun microscope.
from observations by weak beam dark field electron microscopy. However, by Auger spectroscopy, a 2D growth of Pd was observed at room temperature on ZnO single crystals [5]. Although we observed a 3D growth mode, at high temperature, a Stranski-Krastanov growth mode cannot be ruled out, as far as the presence of a continuous Pd monolayer below the particles is difficult to recognize in electron microscopy. Indeed, a pseudo Sranski-Krastanov growth mode was observed in the case of Cu deposited on ZnO [13]. In this paper, most of the Pd particles were found with the (111) orientation, with [112] Pd//[100] ZnO. The largest ones ( > 20 nm) have the shape of tetrahedra limited by (111) faces, truncated at the edges by (100) facets and at the top by a (111) facet. Particles with a smaller size have the same epitaxial orientation but their shapes are not well defined. It is well known that Pd particles prepared by UHV condensation at high temperature on single crystalline oxide substrates (MgO [14], mica [15], A1203 [16]), presents regular shapes of truncated half octahedra, or truncated tetrahedra. In the case of industrial catalysts, Pd particles were produced by chemical impregnation of commercial ZnO powders and reduction under H2. Many studies, using different techniques, XPS, UPS and XRD, have shown the formation of P d - Z n alloy in the particles, after the reduction stage [8-10]. On the other hand, Pd particles grown in UHV on clean ZnO micro-crystals did not alloy with Zn [6,7]. In addition, it has been shown by HRTEM and image simulation that the interface between bulk Pd and bulk ZnO is sharp, without formation of a P d - Z n alloy [17]. In the present observations, Pd grown under UHV on ZnO thin films starts to transform only under the strong electron beam irradiation, in the microscope. Until an irradiation dose of > 10 A cm -2 during 6 min, no evolution was detected. Due to the knowledge with the system Pd/ZnO prepared by chemical impreg-
S. Giorgio et al./ Materials Science and Engineering A229 (t997) I69-173
nation, the most probable transformation is the alloying with Zn, in agreement with the measurement of lattice fringes appearing after strong irradiation. We conclude that without strong reduction conditions, Pd particles are stable on a ZnO substrate.
Acknowledgements We thank A. Charai, Director of CP2M, who allowed us access to the Jeol FEG2010 microscope.
References [1] K. Klier, AppL Surf. Sci., I9 (1984) 267. [2] Yu. A. Ryndin, R.F. Hicks, A.T. Bell and Yu. I. Yermakov, Y. Catal., 70 (1981) 287. [3] W. Gaebler, K. Jacobi and W. Ranke, Surf. Sci., 75 (1978) 335. [4] H. Jacobs, W. Mokwa, D. Kohl and G. Heiland, Surf. Sci., t26 (1983) 318.
173
[5] H. Jacobs, W. Mokwa, D. Kohl and G. Heiland, Surf. Sci., I60 (1985) 217. [6] S. Giorgio, C.R. Henry, C. Chapon and G. Nihoul, in B. Jouffrey and C. Colliex (eds.), Proc. ICEM I3, Vol. 2A, Les Editions de Physique, France, 1994, p. 349. [7] S. Giorgio, C.R. Henry and C. Chapon, Microsc. Microanat. Microstruct., 6 (1995) 237. [8] P.S. Wehner, G.C. Tustin and B.L. Gustafson, J. Catal., 88 (1984) 246. [9] A. Sarkany, Z. Zsoldos, B. Furlong, J.W. Hightower and L. Guczi, or. Carat., 141 (1993) 566. [10] Z. Zsoldos, A. Sarkany and L. Guczi, & Catal., t45 (1994) 235. [11] S. Giorgio, C. Chapon, C.R. Henry and G. Nihoul, Philos. Mag. B, 67 (1993) 773. [12] M.J. Yacaman and T. Ocana, Phys. Stat. Sol. (A) 42 (1977) 571. [13] C.T. CampbelI and A. Ludviksson, 3". Vac. Sci. Technol. A, 12 (1994) 1825. [14] C.R. Henry, C. Chapon, C. Duriez and S. Giorgio, Surf. Sci., 253 (1991) 177. [15] E. Gillet, S. Channakhone, V. Matotin and M. Gillet, Surf. Sci., 152/153 (1985) 603. [16] H. Poppa, F. Rumpf, R.D. Moorhead and C.R. Henry, Mater. Res. Soc. Proc., 1I (1988) I. [17] H. Ichinose, H. Ishii, T. Ichimori and Y. Ishida, in B. Jouffrey and C. Colliex (eds.), Proc. ICEM I3, Vol. 2A, Les Editions de Physique, France, 1994, p. 279.