Surface analysis of the PdCu(110) single crystal alloy at different segregation rates

Pergamon

Vacuum/volume 46/number 2lpages 89 to 9611995 Elsevier Science Ltd

Printed in Great Britain

004%207X(93)E0063-0 0042-207x/95 $9.50+.00

Surface analysis of the PdCu(ll0) single crystal alloy at different segregation rates J Loboda-Cackovic, M S Mousa” and J H Block, Fritz-Haber-lnstitut der Max Planck-Gesellschaft, Berlin- Dahlem, Germany

received for publication 7 6 December 1993

The PdCu( 110) plane, with Pd: Cu = 1: 1 in the bulk, was prepared to have various surface compositions by applying cycles of argon ion sputtering (ion energy of d 750 eV with ion current densities d I fl/cm’) and annealing temperatures (T,,) of 420 < T < 820 K. With this preparation method different compositions of the top surface layers (TL) were obtained.

(i) exclusively Pd atoms; (ii) composition of various CulPd ratios, including the bulk ratio; and (iii) exclusively Cu atoms. The surface region (SR) of N 4 layers depth analysed by AES, and the CO TDS used

for TL-characterization, allowed the average ratio of CulPd in the next three subsurface layers (3SSL) to be evaluated. LEE0 enabled the determination of the surface structure after each step.

The high temperature treatment of the surface followed by low temperature sputtering (T,,) and T,, below that of Cu segregation (T,,,) N 550 K, produced a smooth Pd-rich surface. On the other hand, a large number of sputtering and annealing cycles without prior high T,, produced a rough surface, enabling the Cu atoms lying beneath the TL to be accessible for surface reactions.

A diagram describing LEE0 structures can be drawn from the results which present the large variety of surface structures for the clean PdCu( 110) single crystal surface depending on the TSP and TAN. In addition to this diagram the SR CufPd ratio for various TSP and T,, is given for slow and fast annealing modes at long and short pre- treatment periods. Three phase diagrams comparing experimental surface properties for each of the SR, TL and 3SSL with bulk composition are presented.

1. Introduction

It is known that the chemical composition of an alloy surface

generally differs from that of the bulk’.2. The surface composition of a bimetallic catalyst needs to be determined in order to relate the catalytic properties to the surface properties; hence there is

much interest in characterization of alloy surfaces under well- defined conditions3x4. PdCu alloys are interesting due to the activity of both constituents in the synthesis of methanol from CO and H, (refs 5, 6). Recently’.’ Pd(ll0) was used for studying heterogeneous CO oxidation and demonstrated the highest degree of reproducibility in its reactivity. Theoretically, the PdCu

alloy system is interesting since alloys form a continuous series of solid solutions9 and because of its suitability to investigate the influence of atomic strain on the surface segregation behaviour since the atomic radii of Pd and Cu differ by _ 7% 3.

Several theories are capable of qualitatively predicting which element will be enriched in the surface layer’ “, although none of these theories are able to prognosticate the exact surface composition. All theories examined” failed to predict the correct segregation in a considerably large number of cases ; indicating

* Permanent address : Department of Physics, Mu’tah University, PO Box 7, Al-Karak, Jordan.

that the segregation problem is faceted and that a predictive- interpretative model, which is both simple and of general validity, is still far from being achieved.

In treating surface structures of compound-forming alloys, it is frequently assumed that the surface constitutes a termination of the bulk structure of one of the compounds found in the phase diagram’*. The PdCu system is especially interesting since two compounds, CuPd and Cu3Pd occur in the phase diagram9. These compounds undergo an order-disorder transition to the random solid solution above a critical temperature. Whereas the Cu,Pd compound belongs to the fee lattice, as does the solid solution, CuPd forms a bee lattice with the CsCl structure. It is therefore interesting to examine what type of structure the surface region has and whether a transition between fee and bee is found in the

surface phases. Several ordered crystallographic planes of alloys with an A,B

structure have been studied previously. These include Cu,Au(OOl) or (011)9, Cu3Pd13, Ni3Al14, Pt Sn15 and Pt Ti16. Cu,Pt(l 11) has been studied in this laboratoiy’7

3 and a variable

surface composition was observed. After sputtering and annealing at low temperature, a surface which showed a ~(2 x 2) super- structure corresponding to a Cu,Pt layer was observed. Upon prolonged annealing, this structure changes into one which gives a ~(243 x 2J3)R30° LEED pattern, which corresponds to a Cu enriched surface. The NijPt system has been studied by Gauthier

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J Loboda-Cackovic et al : Analysis of the PdCu (110) single crystal alloy

et al”. They found that the surface composition depends on the crystallographic surface plane, where the (100) and (111) planes showed Pt surface enrichment, whereas the (I 10) plane is enriched with Ni. In all cases, strong concentration oscillations are found with an enriched layer followed by a depleted one.

In the last few decades, segregation of one component of a solid metallic alloy to its surface has been widely investigated using AES. Varying results were reported, for instance, denying the occurrence’9-Zo or confirming”-24 Cu segregation to the sur-

face of CuNi alloys. Au was also found to segregate to the surface of AuNi alloy2s,26. Burton et al” discussed the problems involved

in determining the equilibrium surface segregation behaviour of an alloy, mainly slow diffusion and fast evaporation. They easily recognized slow diffusion that was manifested in a hysteresis when plotting the composition of Au;Ni (AES) as a function of treatment temperature. They demonstrated how to attain equi- libriumZ5, and determined that the heat of segregation of Au to the (1 I I) surface of Ni is 12 + 2 kcal/mole.

A study of the surface composition of the CuPd alloy appears

particularly interesting, because here the ordered phase is bee and transforms above the critical temperature of order-disorder

(To.,) of approximately 773 K to a fee disordered solid solutionY. It is of interest to examine whether this transition also becomes

apparent at the surface of the alloy. On the other hand, it was found that the formation of the ordered phase is a very slow process, so that after cooling from 773 to 525 K over a period of 4 months, only - 20% of the ordered P-phase (PdCu) was formed, leaving 80% disordered solid27~2”.

In this paper the initial results are reported of a comprehensive study to characterize the surfaces of Pd,,Cu,,( 110) single crystal alloy. Surfaces with Cu/Pd ratios ranging from - 100% Pd to - 100% Cu at the TL were obtained. The role of Cu diffusion and its segregation to the surface, and the Cu depletion by sputtering processes is analysed. The behaviour of CujPd (SR) is shown to be controlled by preparation temperatures ranging between 420 and 824 K (which are divided into four regions), with different

LEED surface structures produced. Three distinguished phase diagrams, for the SR, TL and 3SSL are presented to compare the surface properties with those for the bulk composition.

controller provided linear heating at a controlled rate. The sample temperature was recorded between 115 and 824 K. The single crystal alloy, which has a Pd/Cu (SR) ratio = 1: 1 in the bulk, could be prepared to have a TL with various compositions ranging from 100% Pd to 100% Cu (with the possibility of having

the bulk composition at the surface). Cleaning procedures of the PdCu( 110) surface to remove con-

taminants such as sulphur (the main one), oxygen and carbon were carried out whenever these contaminants were detected

(AES). Ar+ ions sputtering (ion energy EAI- -0.75 keV and current density ‘JAri - 1 PA) followed by annealing processes at temperatures ranging between 700 and 770 K, were carried out

(to obtain a smooth surface). To analyse the surface composition, Auger lines for Cu (105 eV) and Pd (243 eV) were used. For Cu (105 eV) the escape depth of electrons was calculated to be 3.5 monolayers, and 5 monolayers for Pd (243 eV)3’*3’. The energy of these electrons is close to the minimum in the energy free path curve”. Therefore, the analysis using these particular AES lines

is especially SR sensitive and this SR, with an average of -4 layers, will be used in this investigation. Figure 1 (a-c) represents samples of these characteristic Auger spectra obtained for the clean surface, corresponding to Pd-rich, mixed and Cu-rich sur-

faces shown in Figure 1 (a), 1 (b) and I(c) respectively. Well developed preparation procedures of sputtering and

annealing of smooth surfaces at temperatures below that of TSEG (- 550 K) produced Pd-rich surfaces. Figure 2 (a and b) presents the relationship between CujPd (SR) and each of the sputtering and annealing periods. In Figure 2(a), sputtering at Tsp < TSEG is shown to result in the depletion of Cu atoms in the SR. For r,, = 532 K, two curves are presented, with each one plotted starting from a different Cu/Pd ratio. When the sample was annealed at TAN = TSR < TSEGr no change in the Cu/Pd ratio was recorded, but at higher annealing and sputtering temperatures this ratio increases as shown in Figure 2(b). After 20 min of sputtering, the ratio of Cu/Pd becomes constant at a certain value

that is influenced by Tsp. This follows the empirical equation :

Cu/Pd F (Cu/Pd)START exp [ - 3/2 t (h)].

2. Experimental

The Auger electron spectroscopy (AES) and Low electron energy diffraction (LEED) experiments were performed in a Varian uhv chamber with a base pressure of -8 x lo-” mbar. The system is equipped with four-grid LEED optics, that was also used as a retarding field analyser for AES, and a quadrupole mass spec- trometer that is independently pumped by a turbo pump. In this study, single crystal alloy surfaces were examined by means of AES and LEED, while CO thermal desorption spectroscopy (TDS) was used as a method of characterization for the TL composition. Details ~on~~ing the CO TDS technique, as well as further details of the experimental set-up are given elsewhere4%“. The details of the Video LEED technique were also recently described3’.

dtd dE

The PdCu single crystal alloy had a thickness of - 3 mm and was cut from a single crystal rod by spark erosion in the (110) direction and polished using standard techniques. It was then mounted between 0.25 mm diameter tantalum wires through grooves in the sides of the crystal. Temperatures were measured by means of chromel-alumel thermocouple wires spot-welded to the back of the crystal. A special purpose built temperature

* 105 192 213 279 3261330 EleVI cu I Pd -D

Figure 1. A sample representation of AES spectra taken for the clean surface of Cu/Pd (SR) values of < 0. I, 0.3 + 0.05 and 2.3 i 0. I for curves in (a), (b) and (c) respectively.

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J Loboda-Cackovic et al : Analysis of the PdCu(ll0) single crystal alloy

Sputtering Time [Uin]

Figure 2. Cu/Pd (SR) vs : (a) sputtering time, demonstrating the depletion process of Cu atoms in SR for T,, < T,,, (-550 K). For T,, = 532 K, two curves are presented with each one starting at a different Cu/Pd value, (b) annealing time at r,, = r,, below and above TsEo (- 550 K).

To obtain smooth Pd-rich surface in our measurements, the depletion of Cu atoms was done in a careful manner. High temperature (- 700 K) annealing for variable periods of time, that depended on the history of the treatment of the alloy, were carried out. The smoothness of the surface was confirmed by applying the CO-thermal desorption technique, that has been developed as a method of characterization4. Applying this technique enabled detection of the roughness of the Pd-rich surfaces from the TD spectra obtained. Such spectra for rough Pd-rich

surfaces showed the rise of a new low temperature shoulder at - 190 K. The reason behind this shoulder is the presence of Cu

atoms beneath the top layer, in the grooves of the rough surface. The influence of Cu atoms in these grooves on the appearance

of the 190 K shoulder in the CO TD spectra has been extensively dealt with elsewhere4.

In order to have &-rich surfaces sputtering and/or annealing in the range of 700 < T < 824 K was carried out. Fast and slow heating rates, as well as short and long annealing periods, were applied. The influence of each one of these factors, that have extreme importance, on the Cu/Pd ratio (SR, TL and 3SSL) as well as on the surface structure, will be accounted for in detail in the next section.

3. Results and discussion

3.1. Cu segregation and depletion of the surface region. Cu/Pd ratios presented in Figure 3 were measured by AES and are

averaged over a surface region of -4 surface layers. The most important processes, that determine different values of Cu/Pd

1

Figure 3. Cu/Pd (SR) values vs the annealing temperature for the PdCu (110) single crystal surface. The Cu/Pd (SR) was obtained after sputtering (7) and annealing up to 824 K. The behaviour of Cu depletion or segregation defines regions I-IV. Measurements of annealing time for 20 min and 3 1 h are denoted by (0) and (0) respectiveiy. 1 is the 773 K temperature that is the boundary of order-disorder transition for the bulk of Pd : Cu = 1: 19. Heating above the 773 K, that followed sputtering at 710 K, is denoted by (A), and (A), for the slow step by step, and the fast heating rate, respectively.

(SR), are the diffusion of Cu atoms and their segregation to the surface, and the Cu depletion by sputtering processes. In Figure 3 the Cu/Pd (SR) behaviour is dominated by the changes in the

preparation temperatures and thus the plot of Cu/Pd (SR) against TAN is divided into four regions. In these regions, different LEED patterns were obtained (Section 3.2).

In region I, that has a Pd-rich surface composition, with tem-

peratures up to - 550 K, Cu/Pd (SR) of < 0.4 is obtained and this could be divided into two ranges : Ia, with temperatures up to -500 K and Ib, with temperatures between 500 and 5.50 K. In region Ia, a Pd-rich SR was obtained with Cu/Pd (SR) < 0.1, after sputtering the PdCu( 110) single crystal surface at 420 K and annealing at temperatures below 495 K. The length of the annealing period (up to 1 h) did not result in a change in the Cu/Pd (SR) ratio. In such a SR the Cu atoms act as ‘impurities’ in the Pd fee crystal lattice4,34. Sputtering and annealing the

surface at a slightly higher temperature, i.e. - 519 K, produces CujPd (SR) values of -0.2-0.4 shown in region Ib. In the last region both types of atoms, the Cu and Pd, build a bee latice in the SR which is similar to that of the bulk4,34. Region I can also be defined as a temperature range that is characterized by surface depletion of Cu atoms (during sputtering) as the main process, while no important Cu diffusion to the surface takes place.

The beginning of Cu segregation and its behaviour after this point vs the preparation temperature define regions II and III of Figure 3. Region II, ranges between - 550 and 700 K, and region III is characterized by a temperature range up to - 773 K, which corresponds to the boundary between statistically face centred disordered and body centred ordered structures of the bulk, in thecaseofPd:Cu = 1:19~27*25.

The Cu/Pd (SR) ratio increases in region II approximately monotonously up to - 700 K. The results obtained after a short annealing time (< 20 min) deviate from those obtained after

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J Loboda-Cackovic et al : Analysis of the PdCu(l10) single crystal alloy

prolonged annealing (2 1 h). At these temperatures (region II) diffusion of Cu atoms is comparable with annealing time. At temperatures >700 K (region III), the speed of diffusion of Cu atoms is enhanced while the duration of annealing cycles has

no influence on the segregation in the SR any more. The Cu concentration in the SR thus reaches a state of equilibrium almost immediately, as can be seen in Figures 2(b) and 3.

The beginning of Cu diffusion and its segregation at > 550 K agree with the temperature of - 550 K that we measured for this alloy. 4, 3 4 At this temperature, deviation from the typical Debye- Waller behaviour has been obtained. By plotting the heating temperature T, of the alloy versus Ln I (intensity of various

LEED spots) a deviation from the typical linear behaviour was obtained at temperatures >500 K; this deviation will be dealt with as part of another study to determine the changes in important physical factors with surface composition at the (110) face

of the PdCu single crysta134. It is also striking to note that both temperature limits, at - 500 K (region Ib) and that of enhanced Cu segregation at - 700 K that is followed by a Cu( 1 x 2) structure, are in good agreement with the surface roughening temperatures for the (110) planes of pure Pd (- 503 K) and pure Cu (- 704 K) 35. For Pd-rich surfaces, the mobility of the Pd atoms in the TL, that starts at the roughening temperature, enables the diffusion of Cu atoms from the bulk to the surface; this Cu diffusion is enhanced by the mechanism of roughening of the Cu enriched TL.

In region IV, limited by temperatures >773 K, two different annealing experiments were done. Before carrying out any of

these experiments the surfaces in both cases were prepared under identical conditions, i.e. at 710 K (region III) with Cu/Pd (SR) = 2.3, and the TL consisted of Cu atoms only, as determined by CO adsorption-desorption analysis“. LEED pattern showed a Cu(1 x 2) surface structure.

The first experiment was carried out by slowly increasing the temperature of the sample, in steps of 20 K, from 710 to 824 K; while keeping the temperature constant for 1 h at each step. Despite exceeding the 773 K temperature (limit of order-disorder for the bulk”) by - 50 K, at the end of the experiment, a Cu/Pd (SR) value of - 2 was normally attained and the Cu-rich surface with Cu (1 x 2) LEED structure remain unchanged.

During the second experiment, the heating temperature was increased in large steps (firstly 70 and secondly 20 K) from 710 to 800 K while holding the temperature constant for only a few minutes at each step. After that the temperature was kept cori-

stant for about 1 h. In this case the Cu/Pd (SR) ratio decreased strikingly from 2.3 down to 0.7, while the surface structure changed from a Cu-rich (1 x 2) to mixed Cu and Pd (1 x 1) surface.

When the temperature was increased too fast in the second experiment, the Cu atoms were depleted’6 in at least the four layers measured for the SR by AES. By slowly heating the alloy in the first experiment, no Cu depletion occurred, even at temperatures as high as those of the fast heating experiment explained above.

3.2. The structures of differently treated surfaces. Figure 4 illus- trates the various structures obtained by LEED after certain surface preparation treatment procedures, including well defined sputtering and/or annealing temperatures and the length of each treatment. The regions I-IV belong to the same distribution of surface treatment temperatures as that in Figure 3. The processes of Cu depletion (in region I) and Cu segregation to the surface

92

T,,,IKI PdCu (1101 Clean Surface

Ttinx ------_-------__-_---- __-----___________(

1

Cu(lx21 I 800

4 t 11x1)

! ! I

I ! I ! I

I I I I I ! I ! I I ! I

!

f

- TSp[KI

Figure 4. LEED surface structures of PdCu (110) clean surface obtained after sputtering and annealing at different temperatures (Tsp and TAN). Dashed lines denote the experimentally reached levels TM,, (TSp = 734 K and TAN = 825 K), while the (- - - -) lines identify the four regions of different structures. The arrows denote the Cu segregation at TsEC and the surface structure transitions after sputtering and/or annealing. TE-DIF is the temperature of enhanced Cu diffusion (h 700 K). 7’b.n (773 K) is the temperature limit that defines the boundary between statistically face centred disordered and body centred ordered structures’,“.

(for T 2 550 K) are decisive factors for obtaining the large variety of surface structures.

In region Ia with temperatures up to T,, -420 K and T,,

-495 K, only a (1 x 1) LEED pattern was obtained, where the reciprocal lattice cell dimensions in [Ol] and [lo] directions have

a ratio of 1.4. Relatively high background intensity and broad

spots are dominating characteristics of the LEED pattern from the PdCu(ll0) single crystal alloy3”. The domains on the surface contain only a few lattice cells and the TL is the (110) plane of the fee lattice that is built up by Pd atoms4.34. In region Ib with Tsp TV TAN E 540 K, the TL consists of Pd atoms as well. The type of crystal lattice changes into bee and the (1 x 2) super- structure arises with the spots at l/2 of the large lattice cell edge.

In Figure 4, as in Figure 3, for regions II (540 < T < 700 K) and III (700 < T < 773 K) the beginning of Cu segregation (- 550 K) and the enhanced Cu diffusion (-700 K) are the processes that are responsible for the different structures obtained. The importance of the SR composition through a large number of layers (even more than four, as determined by AES and explained in Section 2) is demonstrated by the preparation conditions that produced either (1 x 1) or Cu (1 x 2) structures in region III. The type of surface structure at the beginning of the experiment is a crucial factor in determining which one of the two structures mentioned above will be obtained. On one hand a Pd (1 x 2) LEED structure prepared in region Ib shows continuous changes, as a result of annealing to higher temperatures up to region III (illustrated in Figure 4). These changes


involve Pd (1 x 2) pattern changes to a (1 x 1) structure by decreasing the intensity of the (h, 1/2k) spots. This (1 x 1) struc-

ture obtained from Pd (1 x 2) could only have a surface with

Cu/Pd (SR) ratio N 1.

On the other hand, it is possible to obtain much higher Cu concentrations in the SR. Cu-rich surfaces with Cu/Pd (SR) > 2, with the TL consisting only of Cu atoms and corresponding Cu (1 x 2) structure, were produced by sputtering at temperatures in region II and annealing at temperatures within region III (illustrated in Figure 4) or sputtering and annealing at temperatures within region III.

This difference in the surface structure obtained in the above mentioned two cases, where annealing of the alloy was carried out up to the same temperature in region III, could be explained

by variation of surface preparation prior to annealing in region

III. When the sample is prepared in region I, the surface layers (SR) are Cu depleted. Cu atoms can segregate to the surface by annealing at high enough temperatures. The degree of this segregation is obviously not sufficiently high to fill the large number of existing Cu depleted layers (that exist for Pd-rich surfaces), which is required before establishing a Cu-rich surface. Only the removal of some depleted surface layers at high tem-

perature (sputtering in regions II or III), accompanied by sim- ultaneous Cu diffusion, could sufficiently change the composition through a large number of layers and result in Cu-rich compositions.

The thermal treatment of the alloy in the two experiments with slow and fast heating rates from region III into region IV has already been discussed in Section 3.1 and its influence is presented in Figure 3. The Cu-rich (1 x 2) structure in Figure 4, remains unchanged when increasing the temperature up to 50 K higher than the 773 K (when slow heating rate is applied) or with drastic changes to (1 x 1) accompanied by the loss of Cu atoms (for fast heating rate). These experiments will be further discussed in the

following sections.

3.3. The dependence of the SR composition of the alloy on the temperature of preparation. The composition of the SR changes

with preparation temperatures, i.e. both Tsp and TANI and with the number of cycles of sputtering and annealing. All the points (0) shown in Figure 5 are measurements that belong to the prolonged annealing time. The measurements reported by Hansen9 that were obtained for various compositions and only after treating the Pd-Cu alloys at high temperatures (in the range between 700 and 900 K) are also presented in Figure 5 and demonstrate two regions of ordered structures for the bulk.

The measurements obtained in this study demonstrate the most important temperature boundaries for the SR compositions :

(9 TSEGl the temperature at which the Cu atoms start to segregate (- 550 K), similar to what has been shown in Figure 3

and

(ii) To.,, the temperature of ordered-disordered boundary (- 773), where, below and above, this temperature the body centered-ordered and the face centred-disordered structures for Pd : Cu = 1: 1 in the bulk have been respectively obtained 9.

Figure 5 clearly demonstrates a larger variety of SR compositions with ordered structures, than has been originally proposed for the bulk composition’. The first region is extended between 30 and 75% of Pd concentration in the SR of the PdCu single crystal alloy, while the second one is for concentrations between 90 and 100% Pd. This variation arises from the differ-

t cu Seg

500-

400- I I

d I 1

I I

0 i d ll5,,,,,,,,,,,

0 10 20 30 40 50 60 70 80 90 100 cu ATOMIC PER CENT Pd Pd

Figure 5. The influence of the preparation temperature after prolonged annealing of > 1 h, on the atomic percentage of Pd atoms in the SR of the PdCu(ll0) alloy. The measured points (0 and 0) are compared with those reported by Hansen for the bulk’, presented as (- .-). Two regions of ordered structures, that we measured, are limited by thick dashed lines. The line (--) represents the boundary for Cu segregation (Tsro) at 550 K. (B) represents the sample that had SR, TL and 3SSL compositions similar to that of the bulk. The surfaces presented with ( x ) were prolonged sputtered at 519 K and 420 K, followed by 60 min annealing at -705 K (in region III)4. In spite of high annealing temperature TL containing - 100% Pd atoms and the 3SSL with - 67% Cu were obtained. Full circles belong to the sufaces heated at T > 773 K. The results for various groups of SR measurements (l-7) are summarized in the figure.

ence in the temperatures of sputtering and annealing applied during preparation procedures, whether they were carried out above or below the TSEG.

The surfaces prepared at temperatures higher than TSEG, such as in the groups 1,2 and 3 in Figure 5, have SR compositions all of which belong to the ordered region proposed for the bulk’.

Such SR have a bee crystal lattice4. LEED shows (1 x 2) patterns for groups 1 and 2 while for group 3 (1 x 1) structure was obtained (Figure 4). For surfaces that have a (1 x 2) structure, the TL consists only of Cu atoms, as has been demonstrated by the CO thermal-desorption measurements4. Further groups of

ordered structures, groups 5-7, where Tsp < TSEG, lie at higher Pd percentages than was proposed for the bulk composition’.

Measurements of groups 2 and 4 were obtained after surface preparation at the same temperature of 710 K and subsequent annealing to higher temperatures (> 773 K). Group 2 is obtained after careful annealing, while the annealing with large temperature steps produces group 4; thus demonstrating a Cu depletion process in the SR. Details concerning those two experiments have already been given in the previous two sections.

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J Loboda-Cackovic et al : Analysis of the PdCu (110) single crystal alloy

For Pd percentages of 60-75% (Figure 5), two groups, 5 and 6, appear in this region. Group 5 was obtained after surface preparation at temperatures Tsp g TAN g 519 K which is below TsEG and which is followed by annealing at -620 K. Group 6 was obtained after surface preparation at temperatures Tsp -495 K and TAN - 519 K. In this region (60-75% Pd) the bee crystal lattice exists4, and LEED shows a (1 x 1) pattern. For higher Pd percentages, of - 70%, a (1 x 2) structure of low intensity could be detected after careful analysis of the shape of LEED spots34.

Concerning the region for 9&100% Pd, group 7 was obtained from surfaces prepared at low temperatures where T,, -420 K

and TAN - 495 K. These surfaces show ordered (1 x 1) structures, but the crystal lattice is now of the fee type4 and the TL consists of a Pd( 110) plane.

The circles shown at 115 K are the measurements that define the constant trend in both the surface composition and structure for every recorded measurement down to the lowest registered temperatures4.

3.4. Dependence of the TL composition on the temperature of surface preparation. The varying compositions of the TL were determined by CO thermal-desorption from PdCu(ll0) surfaces, that was developed as a method of characterizing surface com-

position4. In Figure 6 the Cu/Pd values for the TL versus the TAN are presented in a similar manner to Figure 3, for SR, and Figure 8, for the 3SSL. In region Ia and, almost, in Ib, the TL consists only of Pd atoms. In regions II and the beginning of region III, the top layer has Cu/Pd values of - 1. In the last part of region III, the Cu/Pd ratios increase sharply to high values that correspond to a (1 x 2) LEED pattern for &-rich surfaces, as shown in Figure 4.

The measurements for the TL composition are also presented in the phase diagram of Figure 7, similar to that of Figure 5, for SR, and of Figure 9 for 3SSL, and comparable with those for bulk ‘.

The ordered structures, obtained in our measurements, are localized within four regions :

(i) For Pd percentages in the TL of < 10% (where Tsp > T,,,)

groups 1 and 2 are obtained ;

Figure 6. Cu/Pd (TL) vs TAN, for annealing time of > 1 h, arc presented. The temperature is divided into four ranges, similar to those in Figures 3.4 and 8.

0 10 20 30 40 50 60 70 80 90 100 cu ATOMIC PER CENT Pd Pd

Figure 7. Plot similar to Figure 5 but the measurements belong to the TL. The results for the 4 regions (groups I-7) of ordered structures are summarized in the figure.

(ii) 43% < Pd% < 70% (where Tsp is larger or smaller than

7’sEG) groups 3,4 and 5 are obtained, depending on the amount of sputtering and annealing cycles carried out;

(iii) 90% < Pd% < 100% (where Tsp = TAN < TSEG) group 6 is obtained ; and

(iv) for TL of - 100% Pd (where Tsp and T,, << TSEG) group 7 is obtained.

In this phase diagram for the TL, only one part of region (ii) overlaps with one of the regions proposed for the bulk9, while other measurements of the TL lay out of these regions. This overlapping region contains our measurements for the PdCu( 110) alloy with the composition of the TL similar to that of the bulk.

3.5. Dependence of the 3SSL composition on the temperature of surface preparation. When the Cu/Pd compositions in the SR and the TL are known, it is possible to determine the compositions in the three layers beneath the TL, which make it possible to plot

Figures 8 and 9 for the same surfaces shown in Figures 5 and 7. The results concerning the 3SSL, which are relevant to the previously presented Figures 5 and 7, are of interest and will be discussed here.

The ratios of Cu/Pd (3SSL) versus TAN are presented in Figure 8 with T,, defined in each region. By comparing this figure with Figure 3, it can be seen that in region Ib the values for the 3SSL have higher ratios, while in regions II and III these values are lower than that obtained for the SR. Such behaviour is a conse- quence of the enrichment of the TL with Cu atoms when

TAN > TSEG, especially after enhanced Cu diffusion for T > 700 K (Figure 6). For Tsp (-500 K) < TsEG the 3SSL will contain

94


Figure 8. Plot similar to Figure 3, presenting the ratio of Cu/Pd (3SSL) vs TAN after prolonged annealing treatment of 2 1 h. Various sputtering temperatures, Tsp, during preparation are identified for the 4 regions.

more Cu than the TL that is suffering from Cu depletion. For

Tsp ( -420 K) << TsEG the depletion of Cu atoms spreads through the layers where Cu/Pd ratios for the SR and the 3SSL are comparable. In this case, the Cu atoms act as impurities in the Pd lattice.

In the phase diagram shown in Figure 8, the values of Cu/Pd (3SSL) behave in a similar manner to that of the SR in Figure 5. Groups 1, 2 and 3, obtained from measurements carried out on

600

8 500

b A b B b A I I I I t 1 I II 10 20 30 40 50 60 70 80 90 100

ATOMIC PER CENT Pd Pd

Figure 9. Plot similar to Figure 5, but the measurements belong to the 3SSL. The results for the 3 regions (groups l-7) of ordered structures are summarized in the figure.

surfaces prepared at high temperatures, agree with the region of

ordered structures proposed for Pd-Cu bulk composition9 ; but other groups of ordered structures, obtained from measurements

made for surfaces prepared at T,, < TSEC, lay out of this region.

4. Conclusions

A comprehensive study of the PdCu(ll0) single crystal alloy, using AES, LEED and CO-TDS, produced a detailed analysis of

the behaviour and characteristics of this bimetallic alloy. Infor- mation about the composition of the SR, TL and 3SSL as well as different LEED structures was obtained. The alloy preparation temperatures determining the alloy characteristics were defined. Cu/Pd (SR) < 0.1 up to 2.4 were obtained, thus providing surfaces ranging between a Pd-rich to Cu-rich TL.

For PD: Cu = 1: 1 in the bulk, three temperatures play an important role in Cu diffusion and ordering-disordering of the surface. These temperatures are :

(i) the temperature at which Cu atoms begin to segregate (TSEG

-550 K); (ii) the temperature at which the enhanced diffusion of Cu

occurs ( TE_DIF - 700 K) ; and (iii) the temperature above which the face centred disordered

and below which the body centred ordered structures exist in the

bulk (To., - 773 K). The first two temperatures approximately agree with the roughening temperatures of pure Pd(l10) and pure Cu( 110) 3 5. This fact explains the beginning of Cu diffusion at such temperatures where the Pd atoms are mobile and start to form a rough surface. For this reason, it is not surprising that a Pd (1 x 2) structure that yields some roughness can be obtained by preparation temperatures near -550 K. The enhanced Cu diffusion occurring at -700 K and the production of a Cu (1 x 2) surface structure agree with the roughening process of the

Cu(ll0).

Both the PdCu( 110) surface composition and structure can be altered by the following processes :

(i) Cu segregation, that results from increasing the heating temperature to a value higher than 550 K, and

(ii) Cu depletion, that results from sputtering of the TL.

Moreover, the last process could proceed through a large number of subsurface layers until a state could be reached, where the Cu atoms act as minority compounds in the crystal lattices. These

lattices are built up by Pd atoms, and the type of lattice changes from bee in the bulk to fee for the SR. During experiments described in this work, SR characteristics do not change at temperatures lower than that of preparation“.

* The phase diagrams for SR, TL and 3SSL are compared with the phase diagram for the bulk composition’. Our measurements cover a much wider range of regions, where ordered surface structures exist for the PdCu(ll0) alloy. SR and 3SSL compositions with Pd percentages between 30% and 50% (i.e. for surfaces prepared at temperatures higher than TSEG) are comparable with values obtained for different bulk compositions. Surfaces with Pd percentages < 30%, which should show an fee region proposed for the bulk, have not been obtained from our PdCu(llO) alloy. This result could be explained in terms of the fact that the SR of the investigated PdCu(l10) alloy was developed on the bee lattice of the bulk, and so has no chance of getting Cu,Au type structure with fee lattice. The fee type of lattice measured in this study belonged to the Pd-rich SR only.

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J L oboda-Cackovic et al : Analysis of the PdCu (110) single crystal alloy

Table 1. Temperature regions I-IV (Figure 3) defining preparation temperatures and the Pd percentages at both TL (Figure 7) and 3SSL (Figure 9) for each region

Region T (Preparation)/K TL 3SSL .~._~

(i) Ia <495 E 100% Pd 85% < Pd < 100% (ii) lb 520 < T< 550 90% < Pd < 100% 63% < Pd < 71% (iii) II 550 < T < 700 63%<Pd<71% 63%<Pd<71% (iv) III 700 < T< 773 0% < Pd < 10% 32% < Pd < 50%

42% < Pd < 71% (v) IV 773 < T 0% < Pd < 10% 32% < Pd < 70%

z 65% Pd

In this case, the SR was built up only by one component, i.e. Pd, while the other one, i.e. Cu, was the minority component in the fee lattice of the first component. Here, the SR contained a Cu composition of < lo%, and the TL consisted of Pd(ll0) plane.

The changes that were obtained for TL and for the 3SSL

compositions in terms of temperature of preparation are classi- fied into four regions in Table 1. For low temperatures of preparation (T < TsEG), in region I, Pd-rich TL and 3SSL were measured. Ia is the region of the Pd(1 IO) SR developed on the bee bulk PdCu(ll0) alloy and in Ib the crystal lattice for the SR changed to bee type. For T > TseG TL and 3SSL could have approximately the same compositions (region II). The com-

position of the TL strongly depends on the preparation temperature in the narrow temperature region III ; from - 70% Pd to - 100% Cu. Heating at temperatures - 50 K above the order- disorder boundary of -773 K (region IV) resulted in ordered surface structures. Various heating rates in this temperature region to the same temperature resulted in producing completely

different TL compositions, consisting of - 100% Cu or -65% Pd atoms.

This investigation demonstrated that various SR compositions and structures for the CuPd(l10) single crystal alloy could be prepared. Such variety is important for many applications, such as catalysis, materials science and adsorption of gases that depend on both the TL and the 3SSL compositions4.37.

Acknowledgements

The fellowship by the Alexander von Humboldt Stiftung is grate- fully acknowledged by M S Mousa. The authors thank Prof J C Bertolini of the Institut de Recherches sur la Catalyse, CNRS,

France, for discussions ; Mrs B Beran for preparing the crystal and W Feige for valuable assistance in performing the exper-

iments.

References

’ W M. Sachtler and R A van Santen, Appl SurfSci, 3, 121 (1979). ‘M J Kelley and V Ponec, Prog Surface Sci, 11, 139 (1981). ’ A D van Langeveld, H A Hendrickx and B E Nieuwenhuys, Thin Solid Films, 109, 179 (1983). ‘M S Mousa, J Loboda-Cackovic and J H Bloc, Vacuum, in press. ‘R G Herman, K Klier, G W Simmions, B P Finn and J B Bulko, J Catal, 56,407 (1979). ‘M L Poutsma, L F Elek, P A Ibarbia, A P Risch and J A Rabo, J Cutal, 52, 157 (1978). ‘M Ehsasi, M Berdau, A Karpowicz, K Christmann and J H Block, Proc fOth ICC, Budapest, Hungary, 1993 (in press). ‘M Berdau, M Ehsasi, A Karpowicz, W Engel, K Christmann and J H Block, Vacuum, in press. 9M. Hansen and K Anderko, In Constitution of Binury Alloys, p. 612. McGraw-Hill, New York (1958). “P A Dowben and A Miller. Surface Seareaation Phenomena. CRC Press, Boston, USA (1990). d I ” ‘I P M Ossi, Surface Sci, 201, L519 (1988). ’ 2 B Gans P A Knipp, D D Koleske and S J Sibner, Surface Sci, 264,81 (1992). ’ ’ 3 M A Newton, S M Francis, Y Li, D Law and M Bowker, Surfuce Sci, 259,45 (1991). 14D Sondericker, F Jona, V L Moruzzi and P M Marcus, Solid St Commun, 53, 175 (1985). 15A N Haner, P N Ross and U Bardi, Cutal Lett, 8, 1 (1991). “A Atrei. L Pedocchi, U Bardi. G Rovida. M Torrini. E Zanazzi. M A van Hove and P N Ross, Surface Sci, 261,64 (1992). “A Holzwarth, J Loboda-Cackovic, J H Block and K Christmann, Surface Sci, in press. ” Y Gauther, R Baudoing and J Jupille, Phys. Rez: B40, 1500 (1989). 19G Ertl and J Kuppers, Surface Sci, 24, 104 (1971). 20D Quinto, V S Sundaram and W D Robertson, Surface Sci, 28, 504 (1971). 2’ W M Sachtler and G J Dorgelo, J Catal, 4,654 (1965). ‘*Y Takasu and H Shim& J Catal, 29,479 (1973). 23C R Helms and K Y Yu, J Vat Sci Technol, 12,276 (1975). z4C R Helms. J Cutnl. 36, 114 (1975). 25 J J Burton, C R Helms’and R S Polizzotti, / Vuc Sci Technol, 13, 204 (1976). *6 F L Williams and M Boudart, J Catal, 30,438 (1973). “F W Jones and C A Cykes, J Inst Metals, 65,419 (1939). 28 A J Bradley, W L Bragg and S Sykes, J Iron Steel Inst, 13, 121 (1940). “J Goschnick, M Wolf, M Grunze. W N Unertl, J H Block and J Loboda-Cackovic, Su@ce Sci, 178, 83 1 (1986). “J Weimer, J Loboda-Cackovic and J H Block, Surfuce Sci, in press. 3’ D R Penn, J Electron Spectrosc Rei Phenom, 9,29 (1976). j2 Handbook of Auger Electron Spectroscopy, p. 13. JEOL Ltd, Tokyo 1982. 33G Ertl and J Kuppers, Low Energy Electrons and Surface Chemistry, p. 7. VCH Verlagsgesell. GmbH, Weinheim (1985). 34 J Loboda-Cackovic, M S Mousa and J H Block, to be published. “A Trayanon, A C. Levi and E Tosatti, Surface Sci, 233, 184 (1990). ‘6 J Ferrdnte, Scr Metall, 5, 1129 (1971). r’M S Mousa, J Loboda-Cackovic, S Jaenicke and J H Block, Surfuce Sci, in press.

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Surface analysis of the PdCu(110) single crystal alloy at different segregation rates