- Email: [email protected]
Study of Pd single crystals grown by crucibleless zone melting
Journal of Crystal Growth 192 (1998) 410— 416
Study of Pd single crystals grown by crucibleless zone melting B. Moest!, V.G. Glebovsky",*, H.H. Brongersma!, R.H. Bergmans!, A.W. Denier van der Gon!, V.N. Semenov" ! Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands " Institute of Solid State Physics, 142432 Chernogolovka, Russian Federation Received 29 January 1998; accepted 22 April 1998
Abstract The electron-beam floating zone melting technique has been used to grow oriented single crystals of high-purity Pd. Ultra-high vacuum cleaning procedures of Pd(1 1 0) were developed. Low-energy ion scattering spectrometry showed a crystallographically well-defined monocrystalline structure free of contaminants after cleaning procedures. Growth and segregation of Pt on the Pd(1 1 0) surface as a function of the temperature was studied. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Single crystals of high-purity transition metals, especially Pd and Pt, are widely used in modern material science and technology, catalysis and surface physics. In the past few decades many fundamental studies have reported complex extraordinary gas absorption behavior of different Pd surfaces. Pd is also known to have excellent properties leading to high activity and selectivity on a number of supported Pd catalysts. In order to study many of the catalytic and electronic properties of Pd it is necessary to have well-defined and clean monocrystalline surfaces of different crystallographic orientations.
* Corresponding author. E-mail: [email protected].
Palladium has the fcc (A 1) structure isotypic with Cu, ¹ "1825 K. Few papers about growing . single crystals of high-purity Pd are available. High-purity Pd can be prepared either by condensation in vacuum or gas [1—3] or by float zoning [3—6]. In both cases, samples of very small sizes can be produced. The formation and growth of small monocrystalline Pd particles (40—250 A_ ) produced by gas evaporation in a flowing argon system have been reported [1]. The morphology of small Pd crystallites prepared by vacuum condensation has been studied by electron microscopy so as to determine the surface structure [2]. Chemical precipitation followed by induction float zone melting was used to prepare high-purity Pd single crystals of 8 mm diameter and a random orientation [3]. By float zone melting Pd, without any chemical purifying procedures, single crystals of 1 mm diameter and a random orientation were grown
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 5 0 - 3
B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
[4—6] whereby a water-cooled eddy current concentrator transferred power to the sample [6]. The molten zone in these narrow rods was more stable than that in larger diameter rods. The molten zone was held in place by its own surface tension and by the levitation force arising from repulsion between induced and inducing currents. An additional DCcurrent was passed through the sample to provide extra heating and a better stabilization of the molten zone. It was found that extensive zoning of Pd in air increased the residual resistance ratio substantially (up to 25,000). There is little evidence in the data [6] about the effects of zone refining to suggest that processes other than zoning are more important. This conclusion coincides with our results on vacuum electron-beam floating zone melting of refractory transition metals [7] when the main process governing the purification of the metals is vacuum evaporation of impurities. The presence of oxygen during HF-float zone melting was thought to be essential for the process [3—6]. However, the high purification effect of float zoning in air is very questionable. More likely, impurities with a high affinity to oxygen in liquid Pd were oxidized and formed liquid oxides as slags which appeared on the surface. It is quite possible that the high residual resistance ratios of Pd samples resulted from an internal oxidation of impurities in solid Pd at high temperatures and formation of relatively large inclusions, which did not influence the free path of electrons. We assume that better evidence can be obtained by using such measurements together with both elemental analysis and microscopic structure studies. Without seeding, Pd single crystals of 1 mm diameter and 25 mm in length were obtained with either a (1 1 0) or a (1 0 0) axis nearly perpendicular to the rod axis [6]. However, because of the sample sizes as well as the temperature distribution in the solidification front, using seeds in the HF-concentrator for growing is rather problematic. During the last few decades the electron-beam floating zone melting (EBFZM) technique has been used very extensively to purify refractory transition metals and to grow single crystals of desired crystallographic orientations. Glebovsky et al. [8—10] purified and grew both single crystals and tubular single crystals of many high-purity refractory metals
411
and alloys by EBFZM in vacuo 5]10~5— 1]10~6 mbar. The EBFZM set-up with a specially designed water-cooled EB-gun allows one to have an optimal temperature distribution at the solidification front and to grow perfect single crystals up to 25 mm in diameter and 900 mm in length. The possibility of producing such single crystals of high-purity metals is very important for many applications. The purpose of this paper is to describe the results of our experiments on growing oriented single crystals of Pd as well as to attempt to characterize the Pd(1 1 0) samples by low-energy ion scattering which is a very sensitive surface technique.
2. Experimental procedure Materials. The starting material was chemically processed according to a purifying procedure described elsewhere [3] and, in the form of sponge of 99.99 at% purity, was isostatically pressed and melted by vacuum levitation melting. The levitation melting consisted of suspending the solid sample in an electromagnetic field and melting it by the induced electrical current. The resulting 25 g liquid drops had the form of spheres. To solidify a liquid drop, it was enough to switch off the electric power. The ingots were then vacuum melted by floating crucibleless zone melting [8] resulting in Pd single crystals 10—12 mm in diameter and 100 mm in length. Because of evaporation of Pd, the vacuum in the melting vessel was not better than 5]10~5 mbar; however, it seems an evaporation also promoted the protection of the liquid Pd from possible H and O contamination. The EB-gun with an accelerating voltage of 5—25 kV and an anode current of 100—1000 mA produced a circular electron flux which melted a zone on a vertical Pd rod (Fig. 1). During purification and growing, the gun moved along the vertical axis of the Pd rod. The oriented single crystals were grown by means of two liquid zone passes: one at a rate of 4 mm min~1 and another at 2 mm min~1 with a seed. Seeds of the necessary crystallographic orientations were prepared by electric erosion cutting of a Pd single crystal of
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Fig. 2. Experimental curves of an electron current on the growing crystal. (1) Sharp, (2) diffuse.
Fig. 1. Principal diagram of electron-beam floating zone melting and growing single crystals.
a random orientation. The procedure of seed preparation has been described elsewhere [10]. The seeding procedure consisted of melting a narrow liquid zone between the seed and the rod and then moving the liquid zone along the rod to grow a new single crystal. To control the temperature distribution in the liquid zone and also to decrease rod overheating and palladium evaporation, the electric parameters of the EB-gun were varied. For example, the electron current from the EB-gun was varied from a sharp form during seeding to a diffuse one during growth (Fig. 2). Precautions were taken into account because of the possible heavy evaporation of Pd in high vacuum. As mentioned in Ref. [2], the vapor pressure of Pd at its melting point is so high that a considerable amount evaporates after several minutes, when it is melted in vacuum. For this reason all of their melting was carried out under an Ar atmosphere (350—600 mbar). However, our experiments showed
that the evaporation of Pd during melting in vacuum 5]10~5 mbar was no stronger than the evaporation of such refractory metals as W or V during their purification and growth by EBFZM. Available vapor pressure data for Pd, W and V at their melting points are as follows: P " P$ 4.0]10~2 mbar, P "5.0]10~2 mbar and P " W V 2.6]10~2 mbar [11]. This means that the evaporation of palladium at its melting point in vacuum is high but is not catastrophic as was announced before [2]. At higher temperatures (¹ #150°C) . the vapor pressures of the above metals increase to 0.1 mbar. During EBFZM the melt in the zone should be overheated to get the necessary heat distribution at the growing front. From our measurements the overheating can reach about 100—150°C, which means a higher vapor pressure for Pd and higher evaporation rates from the sample. It has been found that during growth of Pd single crystals in relatively high vacuum the losses of Pd by evaporation are not higher than 5%. This means that for growth of Pd single crystals it is preferable to carry out only few zone passes in vacuum in contrast to the growth procedures for refractory transition metals when the number of
B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
zone passes can be higher. Nevertheless, this vacuum growth technique with EB heating seems to be very suitable for growing single crystals of many transition metals, including Pd, of desired sizes, purity and crystallinity. Typical amounts of impurities in the bulk Pd single crystals, as analyzed by spark source massspectrometry and atomic absorption techniques, were (ppm): Pt 5, Ag 2, Ru 0.1, Os 0.1, Ir 0.1, Na 0.2, K 0.6, Ca 9, Mg 4, Zn 1, Al 0.7, B 0.6, Cu 1, Si 7, P 0.3, As 0.2, S 1, Fe 5, Cl 5. With regard to light elements contents (H, O, C, S), in this study a lowenergy ion scattering technique has been used to carry out a chemical characterization of surface layers of monocrystalline samples. Pd single crystals were also studied by SEM and optical microscopy, as well as by X-ray diffractometry (Cu K ) a after careful polishing and etching in an aqueous solution of HNO and HCl. The damaged layer 3 of a few tens of a lm resulted from spark erosion cutting has been completely removed during these chemical procedures, which was checked by optical microscopy. The fine substructure of the Pd single crystals was represented by small subgrains with a misorientation angle of (0.3°. No second phase inclusions were found during microscopical studies. From the Pd single crystal bar a 10]8]3 mm Pd(1 1 0) sample was prepared by spark-cutting and chemical polishing and etching as described above for a growth and segregation study using low-energy ion scattering. In further experiments a thin Pt film was deposited on a clean Pd(1 1 0) surface by an evaporation source. The composition of both the first and second atomic layer was measured as a function of temperature. A well-defined surface structure, free from contaminants, is essential for these experiments. Instrumentation. One of the most suitable techniques to study the composition and structure of monocrystalline surfaces and catalysts is low-energy ion scattering (LEIS) [12,13]. The LEIS technique uses a mono-energetic beam of (noble) gas ions which have binary collisions with atoms of the sample. The energy of the scattered ions and the knocked-out surface atoms (recoils) is a function of the scattering angle and the mass-ratio between the projectile ion and the target atom. In the case of
413
time of flight—scattering and recoil spectroscopy (TOF—SARS) these energies are determined by chopping the incident ion-beam and measuring the flight-time from the sample to a detector. Other LEIS techniques use electrostatic deflection of the scattered ions to determine their energy. Since the scattering angle is known, the energy spectrum of the scattered particles represents a mass spectrum. In backscattering experiments the surface composition can be determined by measuring the areas of the various peaks in the energy spectrum. All experiments were performed in the EARISS (energy and angle resolved ion scattering spectrometer) which has been described elsewhere [12,13]. The nominal base pressure in the vessel is in the low 10~10 mbar range and residual gasses are monitored by a quadripole mass-spectrometer. The apparatus is equipped with a 1450 backscattering spectrometer, TOF—SARS and a low-energy electron diffraction (LEED) screen. For cleaning purposes the UHV chamber also has a seperate sputter ion source.
3. Results and discussion The Pd(1 1 0) sample was mounted on a stainless steel holder with a Ta cover to protect the crystal during e-beam heating. The sample was introduced to the vacuum without any pretreatment other than being cleaned with alcohol. The sample was measured with a 4 keV Ar` beam in a TOF—SARS experiment. The results are shown in Fig. 3. The main sharp peaks at 14 and 16 ls are caused by H and O recoils. The broad peak at 21 ls is caused by single scattering from the Pd. This peak has shifted to lower energies compared calculations and broadened as a result of small-angle collisions with the contaminants at the surface. To remove these contaminants it was sputtered for 10 h with a 1 lA, 2 keV Ar` beam. The measurement after this cleaning cycle is shown in Fig. 4. The hydrogen and oxygen peak have decreased with respect to the previous measurement. The Pd-peak has shifted to the correct position and split-up in a less broad single- and a double-scattering peak. Considering the fact that the TOF—SARS technique is extremely sensitive to lighter elements, it is clear that this first
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B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
Fig. 3. TOF—SARS measurement of the Pd(1 1 0) surface before cleaning.
Fig. 5. TOF—SARS measurement of the Pd(1 1 0) surface after flash to 1000°C.
Fig. 4. TOF—SARS measurement of the Pd(1 1 0) surface after sputtering with 2 keV Ar`.
Fig. 6. TOF—SARS measurement of the Pd(1 1 0) surface after sputtering with 2 keV Ar` at 800°C and flashing for 15 s to 1100°C.
cleaning cycle indeed reduced the contamination level of the surface. The peak at 25 ls is caused by Pd-recoils. LEED measurements showed diffuse spots on a high background which indicated that the surface had no well-defined structure yet. In order to get a better surface structure, the sample was flashed to 1000°C for 15 s. The TOF— SARS measurement after this flash shows no hydrogen, but the segregation of carbon and sulfur from the bulk to the surface (Fig. 5) result in the disappearance of the peak splitting and the Pdrecoils. LEED measurements still showed diffuse spots on a high background. The carbon and sulfur were removed by subsequent cycles of sputtering with 2 keV Ar` at 800°C and flashing for 15 s to a temperature of 1100°C. This resulted in a welldefined surface structure, as could be concluded
from the LEED measurements which showed sharp spots on a low background. The TOF—SARS measurement after the flash, as shown in Fig. 6, only contains Pd-related peaks which indicates that the concentration of any contaminant at the surface is much lower than 0.01 ML. Prior to each growth and segregation experiment, the surface was sputter-annealed and flashed as described above. Thereafter, half a monolayer of Pt was evaporated on the Pd(1 1 0). The Pt/ Pd(1 1 0) surface has been measured with the EARISS. This spectrometer measures azimuthally resolved ions at a total scattering angle of 145°. The azimuthal scans are used to discriminate between a first and a second atomic layer contribution to the backscattered yield. Ions that scatter from the second layer are neutralized by atoms in the first
B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
Fig. 7. An EARISS measurement of the Pd(1 1 0) surface with 3 keV Ne`.
layer. In the ‘open’ directions like [0 1 0] and [1 0 0] this neutralization will be lower compared with the ‘closed’ direction like [1 1 0]. The neutralization of ions that scatter from atoms in the first layer will be equal in all azimuthal directions. Therefore, the constant contribution to the azimuth scan is caused by scattering from the outermost layer whereas the variational part is caused by scattering from the second atomic layer, as explained in more detail in Ref. [14]. The result of an EARISS measurement is shown in Fig. 7. The Pt concentration in the first and second atomic layer has been measured as a function of the temperature. After evaporation at room temperature the Pt concentration of the second layer is already larger than the Pt concentration of the first layer, as can be seen from Fig. 8. With increasing temperature the Pt concentration of both layers decrease as a result of the higher surface free energy of Pt with respect to Pd and the increasing mobility at higher temperatures which enables the Pt to diffuse into the bulk. At all temperatures, except room temperature, the ratio between the Pt concentration in the first and the second layer remains constant. In case of an equilibrium between the first three atomic layers one would expect that all Pt segregates to the second atomic layer, based on the negative mixing energy of Pt and Pd and the lower surface free energy of Pt. A local equilibrium is thus not likely. It is, however, possible for an adsorbed Pt atom to exchange with a Pd atom at the surface. The low mobility causes surface roughness.
415
Fig. 8. Pt concentration of the first and second layer as a function of the temperature.
Because of the shielding by the Pd atom, the Pt atom will belong partly to the first layer and partly to the second layer. If the Pt atom diffuses into the bulk, the contribution to the first and the second layer will vanish, which would explain the constant Pt ratio.
4. Conclusions The EBFZM is very suitable for growing oriented Pd single crystals of both high purity and excellent structure. The existence of a crystallographically well-defined structure free of contaminants was confirmed by low-energy ion scattering. The characterization possibilities of lowenergy ion scattering for Pd monocrystalline surfaces were demonstrated. It was shown that there is a constant ratio between the concentration of Pt in the first layer and the second layer as a function of the temperature. This is probably caused by the low mobility at temperatures below 1000°C, which leads to roughness upon Pt deposition, in combination with the exchange of Pd atoms at the surface by adsorbed Pt atoms.
Acknowledgements This work was supported in part by the Netherlands’ Organization for the Advancement of Scientific Research (NWO), NATO Linkage Grant and
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INTAS. The authors wish to acknowledge the moral support and contributions from Prof. W. Gust of the Stuttgart University and Dr. N.S. Sidorov of the Institute of Solid State Physics, Chernogolovka. References [1] A. Renou, M. Gillet, Surf. Sci. 106 (1981) 27. [2] M. Gillet, A. Renoui, Surf. Sci. 90 (1979) 91. [3] N.S. Sidorov, B.G. Karepov, R.K. Nikolaev, Russ. Metall. 2 (1986) 46. [4] S. Hornfeldt, J.B. Ketterson, L.R. Windmiller, J. Crystal Growth 5 (1969) 289. [5] A. Khellah, R.M. Emrick, J.J. Vuillemin, J. Phys. F: Met. Phys. 17 (1987) 2081.
[6] J.J. Vuillemin, A. Khellaf, J.E. Johnson, Solid State Commun. 76 (1990) 229. [7] V.G. Glebovsky, V.N. Semenov, Int. J. Refr. Met. Hard Mater. 12 (1993/1994) 295. [8] V.G. Glebovsky, V.N. Semenov, V.V. Lomeyko, J. Crystal Growth 87 (1988) 142. [9] V.N. Semenov, B.B. Straumal, V.G. Glebovsky, W. Gust, J. Crystal Growth 151 (1995) 180. [10] V. Alonzo, F. Berthier, V.G. Glebovsky, L. Priester, V.N. Semenov, J. Crystal Growth 156 (1995) 480. [11] R. Hultgren, R.L. Orr, D. Anderson, K.K. Kelly, Selected Values of Thermodynamic Properties of Metals and Alloys, Wiley, New York, 1963. [12] R.H. Bergmans, W.J. Huppertz, R.G.V. Welzenis, H.H. Brongersma, J. Vac. Sci. Technol. 14 (1977) 227. [13] R.J.W. Rabalais, Surf. Sci. 299/300 (1994) 219. [14] R.H. Bergmans, M. van de Grift, A.W. Denier van der Gon, H.H. Brongersma, Surf. Sci. 345 (1996) 303.
Study of Pd single crystals grown by crucibleless zone melting B. Moest!, V.G. Glebovsky",*, H.H. Brongersma!, R.H. Bergmans!, A.W. Denier van der Gon!, V.N. Semenov" ! Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands " Institute of Solid State Physics, 142432 Chernogolovka, Russian Federation Received 29 January 1998; accepted 22 April 1998
Abstract The electron-beam floating zone melting technique has been used to grow oriented single crystals of high-purity Pd. Ultra-high vacuum cleaning procedures of Pd(1 1 0) were developed. Low-energy ion scattering spectrometry showed a crystallographically well-defined monocrystalline structure free of contaminants after cleaning procedures. Growth and segregation of Pt on the Pd(1 1 0) surface as a function of the temperature was studied. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Single crystals of high-purity transition metals, especially Pd and Pt, are widely used in modern material science and technology, catalysis and surface physics. In the past few decades many fundamental studies have reported complex extraordinary gas absorption behavior of different Pd surfaces. Pd is also known to have excellent properties leading to high activity and selectivity on a number of supported Pd catalysts. In order to study many of the catalytic and electronic properties of Pd it is necessary to have well-defined and clean monocrystalline surfaces of different crystallographic orientations.
* Corresponding author. E-mail: [email protected].
Palladium has the fcc (A 1) structure isotypic with Cu, ¹ "1825 K. Few papers about growing . single crystals of high-purity Pd are available. High-purity Pd can be prepared either by condensation in vacuum or gas [1—3] or by float zoning [3—6]. In both cases, samples of very small sizes can be produced. The formation and growth of small monocrystalline Pd particles (40—250 A_ ) produced by gas evaporation in a flowing argon system have been reported [1]. The morphology of small Pd crystallites prepared by vacuum condensation has been studied by electron microscopy so as to determine the surface structure [2]. Chemical precipitation followed by induction float zone melting was used to prepare high-purity Pd single crystals of 8 mm diameter and a random orientation [3]. By float zone melting Pd, without any chemical purifying procedures, single crystals of 1 mm diameter and a random orientation were grown
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 5 0 - 3
B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
[4—6] whereby a water-cooled eddy current concentrator transferred power to the sample [6]. The molten zone in these narrow rods was more stable than that in larger diameter rods. The molten zone was held in place by its own surface tension and by the levitation force arising from repulsion between induced and inducing currents. An additional DCcurrent was passed through the sample to provide extra heating and a better stabilization of the molten zone. It was found that extensive zoning of Pd in air increased the residual resistance ratio substantially (up to 25,000). There is little evidence in the data [6] about the effects of zone refining to suggest that processes other than zoning are more important. This conclusion coincides with our results on vacuum electron-beam floating zone melting of refractory transition metals [7] when the main process governing the purification of the metals is vacuum evaporation of impurities. The presence of oxygen during HF-float zone melting was thought to be essential for the process [3—6]. However, the high purification effect of float zoning in air is very questionable. More likely, impurities with a high affinity to oxygen in liquid Pd were oxidized and formed liquid oxides as slags which appeared on the surface. It is quite possible that the high residual resistance ratios of Pd samples resulted from an internal oxidation of impurities in solid Pd at high temperatures and formation of relatively large inclusions, which did not influence the free path of electrons. We assume that better evidence can be obtained by using such measurements together with both elemental analysis and microscopic structure studies. Without seeding, Pd single crystals of 1 mm diameter and 25 mm in length were obtained with either a (1 1 0) or a (1 0 0) axis nearly perpendicular to the rod axis [6]. However, because of the sample sizes as well as the temperature distribution in the solidification front, using seeds in the HF-concentrator for growing is rather problematic. During the last few decades the electron-beam floating zone melting (EBFZM) technique has been used very extensively to purify refractory transition metals and to grow single crystals of desired crystallographic orientations. Glebovsky et al. [8—10] purified and grew both single crystals and tubular single crystals of many high-purity refractory metals
411
and alloys by EBFZM in vacuo 5]10~5— 1]10~6 mbar. The EBFZM set-up with a specially designed water-cooled EB-gun allows one to have an optimal temperature distribution at the solidification front and to grow perfect single crystals up to 25 mm in diameter and 900 mm in length. The possibility of producing such single crystals of high-purity metals is very important for many applications. The purpose of this paper is to describe the results of our experiments on growing oriented single crystals of Pd as well as to attempt to characterize the Pd(1 1 0) samples by low-energy ion scattering which is a very sensitive surface technique.
2. Experimental procedure Materials. The starting material was chemically processed according to a purifying procedure described elsewhere [3] and, in the form of sponge of 99.99 at% purity, was isostatically pressed and melted by vacuum levitation melting. The levitation melting consisted of suspending the solid sample in an electromagnetic field and melting it by the induced electrical current. The resulting 25 g liquid drops had the form of spheres. To solidify a liquid drop, it was enough to switch off the electric power. The ingots were then vacuum melted by floating crucibleless zone melting [8] resulting in Pd single crystals 10—12 mm in diameter and 100 mm in length. Because of evaporation of Pd, the vacuum in the melting vessel was not better than 5]10~5 mbar; however, it seems an evaporation also promoted the protection of the liquid Pd from possible H and O contamination. The EB-gun with an accelerating voltage of 5—25 kV and an anode current of 100—1000 mA produced a circular electron flux which melted a zone on a vertical Pd rod (Fig. 1). During purification and growing, the gun moved along the vertical axis of the Pd rod. The oriented single crystals were grown by means of two liquid zone passes: one at a rate of 4 mm min~1 and another at 2 mm min~1 with a seed. Seeds of the necessary crystallographic orientations were prepared by electric erosion cutting of a Pd single crystal of
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Fig. 2. Experimental curves of an electron current on the growing crystal. (1) Sharp, (2) diffuse.
Fig. 1. Principal diagram of electron-beam floating zone melting and growing single crystals.
a random orientation. The procedure of seed preparation has been described elsewhere [10]. The seeding procedure consisted of melting a narrow liquid zone between the seed and the rod and then moving the liquid zone along the rod to grow a new single crystal. To control the temperature distribution in the liquid zone and also to decrease rod overheating and palladium evaporation, the electric parameters of the EB-gun were varied. For example, the electron current from the EB-gun was varied from a sharp form during seeding to a diffuse one during growth (Fig. 2). Precautions were taken into account because of the possible heavy evaporation of Pd in high vacuum. As mentioned in Ref. [2], the vapor pressure of Pd at its melting point is so high that a considerable amount evaporates after several minutes, when it is melted in vacuum. For this reason all of their melting was carried out under an Ar atmosphere (350—600 mbar). However, our experiments showed
that the evaporation of Pd during melting in vacuum 5]10~5 mbar was no stronger than the evaporation of such refractory metals as W or V during their purification and growth by EBFZM. Available vapor pressure data for Pd, W and V at their melting points are as follows: P " P$ 4.0]10~2 mbar, P "5.0]10~2 mbar and P " W V 2.6]10~2 mbar [11]. This means that the evaporation of palladium at its melting point in vacuum is high but is not catastrophic as was announced before [2]. At higher temperatures (¹ #150°C) . the vapor pressures of the above metals increase to 0.1 mbar. During EBFZM the melt in the zone should be overheated to get the necessary heat distribution at the growing front. From our measurements the overheating can reach about 100—150°C, which means a higher vapor pressure for Pd and higher evaporation rates from the sample. It has been found that during growth of Pd single crystals in relatively high vacuum the losses of Pd by evaporation are not higher than 5%. This means that for growth of Pd single crystals it is preferable to carry out only few zone passes in vacuum in contrast to the growth procedures for refractory transition metals when the number of
B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
zone passes can be higher. Nevertheless, this vacuum growth technique with EB heating seems to be very suitable for growing single crystals of many transition metals, including Pd, of desired sizes, purity and crystallinity. Typical amounts of impurities in the bulk Pd single crystals, as analyzed by spark source massspectrometry and atomic absorption techniques, were (ppm): Pt 5, Ag 2, Ru 0.1, Os 0.1, Ir 0.1, Na 0.2, K 0.6, Ca 9, Mg 4, Zn 1, Al 0.7, B 0.6, Cu 1, Si 7, P 0.3, As 0.2, S 1, Fe 5, Cl 5. With regard to light elements contents (H, O, C, S), in this study a lowenergy ion scattering technique has been used to carry out a chemical characterization of surface layers of monocrystalline samples. Pd single crystals were also studied by SEM and optical microscopy, as well as by X-ray diffractometry (Cu K ) a after careful polishing and etching in an aqueous solution of HNO and HCl. The damaged layer 3 of a few tens of a lm resulted from spark erosion cutting has been completely removed during these chemical procedures, which was checked by optical microscopy. The fine substructure of the Pd single crystals was represented by small subgrains with a misorientation angle of (0.3°. No second phase inclusions were found during microscopical studies. From the Pd single crystal bar a 10]8]3 mm Pd(1 1 0) sample was prepared by spark-cutting and chemical polishing and etching as described above for a growth and segregation study using low-energy ion scattering. In further experiments a thin Pt film was deposited on a clean Pd(1 1 0) surface by an evaporation source. The composition of both the first and second atomic layer was measured as a function of temperature. A well-defined surface structure, free from contaminants, is essential for these experiments. Instrumentation. One of the most suitable techniques to study the composition and structure of monocrystalline surfaces and catalysts is low-energy ion scattering (LEIS) [12,13]. The LEIS technique uses a mono-energetic beam of (noble) gas ions which have binary collisions with atoms of the sample. The energy of the scattered ions and the knocked-out surface atoms (recoils) is a function of the scattering angle and the mass-ratio between the projectile ion and the target atom. In the case of
413
time of flight—scattering and recoil spectroscopy (TOF—SARS) these energies are determined by chopping the incident ion-beam and measuring the flight-time from the sample to a detector. Other LEIS techniques use electrostatic deflection of the scattered ions to determine their energy. Since the scattering angle is known, the energy spectrum of the scattered particles represents a mass spectrum. In backscattering experiments the surface composition can be determined by measuring the areas of the various peaks in the energy spectrum. All experiments were performed in the EARISS (energy and angle resolved ion scattering spectrometer) which has been described elsewhere [12,13]. The nominal base pressure in the vessel is in the low 10~10 mbar range and residual gasses are monitored by a quadripole mass-spectrometer. The apparatus is equipped with a 1450 backscattering spectrometer, TOF—SARS and a low-energy electron diffraction (LEED) screen. For cleaning purposes the UHV chamber also has a seperate sputter ion source.
3. Results and discussion The Pd(1 1 0) sample was mounted on a stainless steel holder with a Ta cover to protect the crystal during e-beam heating. The sample was introduced to the vacuum without any pretreatment other than being cleaned with alcohol. The sample was measured with a 4 keV Ar` beam in a TOF—SARS experiment. The results are shown in Fig. 3. The main sharp peaks at 14 and 16 ls are caused by H and O recoils. The broad peak at 21 ls is caused by single scattering from the Pd. This peak has shifted to lower energies compared calculations and broadened as a result of small-angle collisions with the contaminants at the surface. To remove these contaminants it was sputtered for 10 h with a 1 lA, 2 keV Ar` beam. The measurement after this cleaning cycle is shown in Fig. 4. The hydrogen and oxygen peak have decreased with respect to the previous measurement. The Pd-peak has shifted to the correct position and split-up in a less broad single- and a double-scattering peak. Considering the fact that the TOF—SARS technique is extremely sensitive to lighter elements, it is clear that this first
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B. Moest et al. / Journal of Crystal Growth 192 (1998) 410–416
Fig. 3. TOF—SARS measurement of the Pd(1 1 0) surface before cleaning.
Fig. 5. TOF—SARS measurement of the Pd(1 1 0) surface after flash to 1000°C.
Fig. 4. TOF—SARS measurement of the Pd(1 1 0) surface after sputtering with 2 keV Ar`.
Fig. 6. TOF—SARS measurement of the Pd(1 1 0) surface after sputtering with 2 keV Ar` at 800°C and flashing for 15 s to 1100°C.
cleaning cycle indeed reduced the contamination level of the surface. The peak at 25 ls is caused by Pd-recoils. LEED measurements showed diffuse spots on a high background which indicated that the surface had no well-defined structure yet. In order to get a better surface structure, the sample was flashed to 1000°C for 15 s. The TOF— SARS measurement after this flash shows no hydrogen, but the segregation of carbon and sulfur from the bulk to the surface (Fig. 5) result in the disappearance of the peak splitting and the Pdrecoils. LEED measurements still showed diffuse spots on a high background. The carbon and sulfur were removed by subsequent cycles of sputtering with 2 keV Ar` at 800°C and flashing for 15 s to a temperature of 1100°C. This resulted in a welldefined surface structure, as could be concluded
from the LEED measurements which showed sharp spots on a low background. The TOF—SARS measurement after the flash, as shown in Fig. 6, only contains Pd-related peaks which indicates that the concentration of any contaminant at the surface is much lower than 0.01 ML. Prior to each growth and segregation experiment, the surface was sputter-annealed and flashed as described above. Thereafter, half a monolayer of Pt was evaporated on the Pd(1 1 0). The Pt/ Pd(1 1 0) surface has been measured with the EARISS. This spectrometer measures azimuthally resolved ions at a total scattering angle of 145°. The azimuthal scans are used to discriminate between a first and a second atomic layer contribution to the backscattered yield. Ions that scatter from the second layer are neutralized by atoms in the first
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Fig. 7. An EARISS measurement of the Pd(1 1 0) surface with 3 keV Ne`.
layer. In the ‘open’ directions like [0 1 0] and [1 0 0] this neutralization will be lower compared with the ‘closed’ direction like [1 1 0]. The neutralization of ions that scatter from atoms in the first layer will be equal in all azimuthal directions. Therefore, the constant contribution to the azimuth scan is caused by scattering from the outermost layer whereas the variational part is caused by scattering from the second atomic layer, as explained in more detail in Ref. [14]. The result of an EARISS measurement is shown in Fig. 7. The Pt concentration in the first and second atomic layer has been measured as a function of the temperature. After evaporation at room temperature the Pt concentration of the second layer is already larger than the Pt concentration of the first layer, as can be seen from Fig. 8. With increasing temperature the Pt concentration of both layers decrease as a result of the higher surface free energy of Pt with respect to Pd and the increasing mobility at higher temperatures which enables the Pt to diffuse into the bulk. At all temperatures, except room temperature, the ratio between the Pt concentration in the first and the second layer remains constant. In case of an equilibrium between the first three atomic layers one would expect that all Pt segregates to the second atomic layer, based on the negative mixing energy of Pt and Pd and the lower surface free energy of Pt. A local equilibrium is thus not likely. It is, however, possible for an adsorbed Pt atom to exchange with a Pd atom at the surface. The low mobility causes surface roughness.
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Fig. 8. Pt concentration of the first and second layer as a function of the temperature.
Because of the shielding by the Pd atom, the Pt atom will belong partly to the first layer and partly to the second layer. If the Pt atom diffuses into the bulk, the contribution to the first and the second layer will vanish, which would explain the constant Pt ratio.
4. Conclusions The EBFZM is very suitable for growing oriented Pd single crystals of both high purity and excellent structure. The existence of a crystallographically well-defined structure free of contaminants was confirmed by low-energy ion scattering. The characterization possibilities of lowenergy ion scattering for Pd monocrystalline surfaces were demonstrated. It was shown that there is a constant ratio between the concentration of Pt in the first layer and the second layer as a function of the temperature. This is probably caused by the low mobility at temperatures below 1000°C, which leads to roughness upon Pt deposition, in combination with the exchange of Pd atoms at the surface by adsorbed Pt atoms.
Acknowledgements This work was supported in part by the Netherlands’ Organization for the Advancement of Scientific Research (NWO), NATO Linkage Grant and
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INTAS. The authors wish to acknowledge the moral support and contributions from Prof. W. Gust of the Stuttgart University and Dr. N.S. Sidorov of the Institute of Solid State Physics, Chernogolovka. References [1] A. Renou, M. Gillet, Surf. Sci. 106 (1981) 27. [2] M. Gillet, A. Renoui, Surf. Sci. 90 (1979) 91. [3] N.S. Sidorov, B.G. Karepov, R.K. Nikolaev, Russ. Metall. 2 (1986) 46. [4] S. Hornfeldt, J.B. Ketterson, L.R. Windmiller, J. Crystal Growth 5 (1969) 289. [5] A. Khellah, R.M. Emrick, J.J. Vuillemin, J. Phys. F: Met. Phys. 17 (1987) 2081.
[6] J.J. Vuillemin, A. Khellaf, J.E. Johnson, Solid State Commun. 76 (1990) 229. [7] V.G. Glebovsky, V.N. Semenov, Int. J. Refr. Met. Hard Mater. 12 (1993/1994) 295. [8] V.G. Glebovsky, V.N. Semenov, V.V. Lomeyko, J. Crystal Growth 87 (1988) 142. [9] V.N. Semenov, B.B. Straumal, V.G. Glebovsky, W. Gust, J. Crystal Growth 151 (1995) 180. [10] V. Alonzo, F. Berthier, V.G. Glebovsky, L. Priester, V.N. Semenov, J. Crystal Growth 156 (1995) 480. [11] R. Hultgren, R.L. Orr, D. Anderson, K.K. Kelly, Selected Values of Thermodynamic Properties of Metals and Alloys, Wiley, New York, 1963. [12] R.H. Bergmans, W.J. Huppertz, R.G.V. Welzenis, H.H. Brongersma, J. Vac. Sci. Technol. 14 (1977) 227. [13] R.J.W. Rabalais, Surf. Sci. 299/300 (1994) 219. [14] R.H. Bergmans, M. van de Grift, A.W. Denier van der Gon, H.H. Brongersma, Surf. Sci. 345 (1996) 303.