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STM study of the (111) and (100) surfaces of PdAg
Surface Science 417 (1998) 292–300
STM study of the (111) and (100) surfaces of PdAg P.T. Wouda a,*, M. Schmid b, B.E. Nieuwenhuys a, P. Varga b a Leiden Institute of Chemistry, Department of Heterogeneous Catalysis and Surface Science, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands b Institut fu¨r Allgemeine Physik, Technische Universita¨t Wien, Wiedner Hauptstrasse 8–10, A-1040 Vienna, Austria Received 17 July 1998; accepted for publication 14 August 1998
Abstract The (111) and (100) surfaces of the Pd Ag alloy have been imaged with atomic resolution by scanning tunneling microscopy. 67 33 On the (111) surface, it was found that Pd atoms appear in the images about 25 pm higher than the Ag atoms. The surface concentration of palladium was determined as a function of annealing temperature (720–920 K ) and was found to vary between 5 and 11%. Analysis of the relative positions of the Pd atoms showed a tendency towards formation of isolated palladium sites. On the (100) surface, the palladium concentration in the first layer is extremely low and the system has to be forced into a nonequilibrium state to find palladium atoms in the first monolayer. Chemical contrast here amounts to a 60 pm apparent height difference. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Low-index single crystal surfaces; Palladium; Scanning Tunneling Microscopy; Silver; Surface segregation
1. Introduction Scanning Tunneling Microscopy (STM ) has become one of the most important tools for the study of fundamental processes at surfaces. As is shown in this and other papers, the STM is capable of imaging with chemical contrast; with the assistance of spectroscopic techniques like AES or ISS, the chemical identity of the observed species can be determined as well as the exact location of adsorbates and substrate atoms. Generally, the basis of chemical contrast is either the elementspecific interaction with an adsorbate at the tip, or a sufficiently large variation in the local density of states (LDOS) between substrate atoms of * Corresponding author. Fax: +31 71 5274451; e-mail: [email protected]
different chemical identity, or a combination of those. This has been shown, for instance, with alloys that are interesting from a catalytic point of view, like PtRh [1,2], PtNi [3,4], CuAg [5], PdCu [6 ], or PtCo [7]. The unique property of the STM is that it provides both the chemical composition of the first atomic layer of a crystal, and direct information on the distribution of the constituent component atoms over the surface and, hence, on ordering or demixing phenomena. A good example is our recent study of PtRh(100) [1], which was the first alloy surface to be imaged with atomic resolution and chemical contrast by STM on the basis of the intrinsic electronic properties of the system. Rhodium was imaged with an apparent height 22 pm greater than that of platinum, and a tendency towards formation of small rhodium clusters was observed.
0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 06 7 3 -6
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Alloys have been popular systems in catalysis research for decades because of their effect on reaction selectivity, their ability to catalyse more than one reaction and their resistivity against poisoning. In this paper, a study of PdAg alloy surfaces is described, a system that is well known to influence catalytic reaction paths by the ensemble size effect [8]. Infra-red absorption studies of CO adsorbed on PdAg catalysts indicated that the change in catalytic properties of Pd upon alloying with Ag can be understood in terms of an ensemble size effect [8]. In the case of PdAg, this means that the degree at which the palladium atoms create isolated sites (for on top adsorption only) or clusters forming hollow sites between Pd atoms determines the properties of the system in chemisorption and catalysis. Let us illustrate this effect with an example of thermal desorption of CO. TDS of CO from Pd(111) features a desorption maximum at 500 K, with the development of low T shoulders at increasing exposures [9,10]. On Pd surfaces, CO preferentially occupies the hollow sites, the on-top sites being less favourable. TDS of CO on the PdAg(111) surface shows one single peak at 400 K, shifting to 375 K at higher exposures [9,10]. Clearly, a large part of the palladium sites consists of isolated Pd atoms in a matrix of silver, making only on-top Pd sites available for CO adsorption. Later, more detailed theoretical and experimental results were published, indicating the importance of ligand effects of the site for CO adsorption on PtCu(111) [11–13] and PtFe(111) [14], in addition to ensemble size effects. The term ‘‘ligand effect’’ here refers to the fact that the electronic state of a metal atom is partly determined by the identity and geometry of its surrounding atoms. This invokes ideas from coordination chemistry and homogeneous catalysis where the state and catalytic behaviour of the metal centre are strongly influenced by its ligands. The interesting question is whether there is sufficient difference between the local density of states on palladium and silver atoms at a PdAg surface to induce chemical contrast. Theoretical studies (e.g. Ref. [15] ] of the PdAg system have pointed out that in the total DOS, the Pd contribution has its maximum within 1 eV off the Fermi
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level, whereas the Ag DOS maximum is found at a 5 eV higher binding energy. Concerning the local DOS (‘‘local’’ here means within a muffin tin sphere around a specific atom) on Pd and Ag atoms in the alloy, the modelling results show that the dominating contributions in the near-E region F come from the respective d bands. Both local densities of d states have shifted relative to the d bands of their pure elemental solids, but their centroids are still well separated, and the LDOS on Pd is much larger in the spectral region that is relevant for STM. Looking at the system from the point of view of charge transfer, it has been predicted in these calculations that Pd gains charge density. Charge transfer has been shown to depend on the number of hetero-atoms surrounding the central atom [15]. The results of X-ray absorption studies agree that the Pd d-states gain more charge with increasing Ag concentration [16,17]. The Pd d-states also become more localized [17]. There is agreement that charge transfer occurs (from Ag to Pd ), but the question as to what degree this concerns sp or d-states remains open. The work presented in this paper is a study of the chemical contrast on PdAg(111) and (100), the first layer concentrations of the respective elements as a function of temperature, and the ordering/demixing properties of the surface. To collect this information, scanning tunneling microscopy is used. Other techniques that specifically give data on the first atomic layer (quantitative low-energy electron diffraction and low-energy ion scattering spectroscopy) are not appropriate for the study of PdAg surfaces as the mass as well as the electron scattering properties of both elements are too similar.
2. Experimental In the work presented in this paper, the (100) and (111) surfaces of single crystalline Pd Ag 67 33 have been studied. The choice of this bulk composition was motivated by the expected strong surface segregation of silver. The surface was extensively cleaned in vacuum by sputtering (2-keV Ar+) and annealing (770 K–820 K ). Using
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Auger electron spectroscopy (AES ), the surface was checked for the presence of contaminants; this was complicated by the fact that the S peak 152eV coincides with one of the minor Pd peaks. By using low-energy ion scattering (LEISS), we did not find any indication of sulphur contamination. The LEISS measurements were obtained with a 1-keV He+ beam and a hemispherical analyser. A comparable problem concerns the C peak that is located in the flank of the Pd peak. No 279eV indication of carbon build-up (usually visible as dark patches; see, for example, Ref. [18]) has been observed during the STM measurements. The annealing procedures for the temperature-dependent composition measurements were such that just after the heating had been turned off, the sample was transferred to the STM chamber where it was stored in close contact with a block of copper for fast cooling. The vacuum system consisted of two chambers: a preparation chamber with a base pressure of 1×10−10 mbar in which all annealing and sputtering procedures took place and an STM/AES chamber in which the pressure during measurements was about 8×10−11 mbar. The scanning tunneling microscope used in this study was a commercial Omicron micro-STM, which has been provided with additional vibration damping: the STM head itself was supported by a viton stack, and the entire vacuum system was suspended in a combined spring–elastomer system as described in Ref. [19]. A tungsten tip was used, and the sample bias was always negative. The AES data have all been recorded at an electron energy of 3 kV and with a cylindrical mirror analyser. The peak-to-peak height ratios have been deduced from the differentiated spectra.
3. Results STM images of Pd Ag (111) with chemical 67 33 contrast have been recorded at tunneling resistances between 1 GV and 50 kV (bias voltage range 0.5 mV–1 V ). Fig. 1a shows an STM image of the Pd Ag (111) surface. In this case, the crystal has 67 33 been equilibrated at 820 K for 80 min. This procedure of annealing at 820 K, measuring and
(a)
(b) ˚ ) of the Pd Ag (111) surFig. 1. (a) STM image (100×100 A 67 33 face equilibrated at 820 K. V=0.5 mV, I =13.3 nA. The tunnel surface contains 5.2% palladium. The white line top left indicates where the line plot in (b) is located. (b) Line plot taken from the image in (a) (see white line top left).
analysis was repeated many times, resulting in a surface composition of 5.2±0.3% of palladium. As the line plot in Fig. 1b shows, the apparent height differences between palladium and silver in Fig. 1a are about 25 pm. The AES Pd / 333eV Ag ratio of the system with this preparation 359eV was 0.40±0.02. A set of images, comparable with that in Fig. 1a, has been recorded after equilibration at various
P.T. Wouda et al. / Surface Science 417 (1998) 292–300
Fig. 2. First layer composition versus T for PdAg(111): experimental data ($) are compared with Langmuir–MacLean curves with a 26.6 kJ mol−1 difference in surface energies (see text) for Pd bulk concentrations of 67% (Ω-Ω-Ω), 75% ( · · · ), 80% (– – –) and 85% (———). During the preparation procedures at T>870 K, the top layers of the crystal become enriched in Pd because of the high vapour pressure of silver. Hence, the surface will be in ‘‘equilibrium’’ with a bulk with more than the nominal 67% Pd. An extra data point (&) is added at T=820 K to indicate the surface composition before the high-temperature annealing.
temperatures and subsequent fast cooling. The results of an analysis of those images is shown in Fig. 2, where the palladium concentration in the first layer is plotted versus equilibration temperature (filled circles). At this point, it should be noted that this series of measurements was started with the equilibration at 920 K: at this temperature, silver has a vapour pressure in the low 10−7-mbar range, which means that the preparation results in a certain degree of palladium enrichment in the top layers of the crystal. To illustrate this, after annealing at 920 K, AES measurements indicated a Pd /Ag peak-to-peak height 333eV 359eV ratio of 0.49. Consequently, the experimental data points (solid circles) in Fig. 2 are representative of a surface in equilibrium with a bulk composition of more than 67% Pd. One data point (filled square) has been added in Fig. 2 to indicate this: it represents the surface composition at 820 K before the high-temperature annealing. At first glance, the position of the palladium atoms is such that most of them are isolated, i.e. entirely surrounded by silver atoms, creating an ensemble size distribution that peaks at the single site configuration. A closer statistical analysis
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Fig. 3. Comparison between counts of various configurations at the Pd Ag (111) surface (equilibrated at 820 K ) and the 67 33 expectations for a random distribution. In the indicated configurations, the dark circles are palladium. The preference for isolated palladium sites is clear.
brings out these interesting features more clearly. A set of 12 independent STM images of the PdAg(111) surface equilibrated at 820 K has been analysed, searching for the exact distribution of palladium atoms in the sea of silver atoms. Fig. 3 gives the results, divided in counts for single Pd atoms, double sites (two nearest Pd neighbours), triple sites (three nearest Pd neighbours), nextnearest neighbour couples and next-next-nearest neighbour couples. For comparison, the simulated data of a random distribution are presented. In Fig. 4, an STM image of the Pd Ag (100) 67 33 surface is shown. On this and most other STM images of the equilibrated PdAg(100) surface, no indication of the presence of a second element was found. A small number of palladium atoms (0.4%) was present in the STM images after the subsurface layers of the crystal had been strongly depleted of silver. This depletion procedure involved sputtering and annealing at the same time for about 10 min: sputtering occurred with 2-keV Ar+ ions (current density 2×10−2 A m−2), and the annealing temperature was 700 K. Such a procedure for the depletion of a segregating species has first been reported by Rehn et al. for the CuNi system [20]. The idea is that due to the relatively high temperature, there is a constant segregation of silver atoms to the surface, whereas in the sputtering process, the atoms of the top layer are continuously removed. This results in a selective removal of silver atoms from the top layers.
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(a) ˚ ) of the Pd Ag (100) surface Fig. 4. STM image (100×100 A 67 33 equilibrated at 770 K. V=0.5 mV, I =16.6 nA. The area tunnel shown contains no palladium: the differences of apparent height of the atoms are much smaller than those observed between Ag and Pd, indicating that no Pd atoms are present in the first monolayer.
We have observed this effect by an increase in the Pd /Ag AES ratio from 0.40 before 333eV 359eV the procedure to 1.40 after sputtering and annealing. The corresponding STM image is shown in Fig. 5a. In Fig. 5b, a line plot over a palladium atom is shown, indicating that the apparent height difference between Ag and Pd atoms is 40–60 pm. In Fig. 5c, the profile over two of the small local depressions is plotted, depressions that are found in large amounts in Figs. 4, 5a and 6a. The apparent height differences in these areas are usually significantly less than the apparent height of the protrusions identified as Pd, and the transition between these areas is always gradual. In Fig. 6a, the local depressions form regions with a c(2×2) pattern. A line profile over such an area is presented in Fig. 6b.
(b)
(C) ˚ ) of the Pd Ag (100) surFig. 5. (a) STM image (100×100 A 67 33 face prepared by simultaneous sputtering and annealing. V= 0.5mV, I =10.6 nA. The surface contains a low concentunnel tration of palladium (about 0.4%); (b) Line plot A in (a) over
a palladium atom: the apparent height is about 60 pm higher than for silver atoms. (c) Line plot B in (a) over an area with less height variations (approximately 15 pm) attributed to the subsurface structure.
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4. Discussion
(a)
The fact that chemical contrast, i.e. discrimination of palladium and silver in PdAg(111), has been found for a wide range of tunneling resistances with STM points to a considerable difference in local densities of state near the Fermi level. This point is consistent with the current knowledge of the PdAg ( local ) valence band structure [15,17,21]. The difference in apparent heights is comparable to that found for a group VIII alloy surface like PtRh(100) [1]. The only other STM studies of the (111) surface of a group VIII/group Ib alloy, showing chemical contrast, concern the PdAu(111) and NiAu(111) surface alloy systems [22,23]. In the PdAu publication, however, no information is given on the apparent height variations [22]. Gold atoms in the first layer of the Ni(111) surface appear as 20-pm-deep depressions [23]. As mentioned Section 3, the palladium concentration in the first layer of PdAg(111) is 5.2±0.3% when equilibration took place at 820 K. With this value, we can calculate the surface energy difference using the Langmuir–MacLean formula for surface segregation:
C D
X Ds bulk exp surf = 1−X 1−X RT surf bulk X
(b) ˚ ) of the Pd Ag (100) surFig. 6. (a) STM image (100×100 A 67 33 face equilibrated at 770 K. V=0.5 mV, I =5.4 nA. The area tunnel shown contains no palladium atoms. Nanometre-scale areas can be discerned with a c(2×2) structure, featuring depressions already shown in Fig. 5c. (b) Line plot of line A in (a). The depressions forming the c(2×2) are apparent, featuring higher apparent height variations than in the line plot in Fig. 5c where the same phenomenon was shown.
with Ds being the difference in surface energies. This formula does not include the effects of bulk mixing enthalpy; however, as the mixing enthalpy is known to be rather low (−3.9 kJ mol−1 according to Ref. [24]) and no ordered bulk structures are known [25], we neglect this effect here. Because of the small size difference between palladium (r= 137 pm) and silver atoms (r=144 pm), no corrections for lattice strain have been included. The result is a surface energy difference of 24.7 kJ mol−1. Surface energies found in literature for Pd(111) and Ag(111) show some scatter as shown in Table 1. The data of Methfessel et al. [26 ] result in a surface energy difference of 12.5 kJ mol−1, which is quite different from the other literature data of 26.6±0.6 kJ mol−1 as well as from our results. De Boer et al. [27] estimated surface energy values for (111)-like surfaces from
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Table 1 Surface energies (kJ mol−1) for the (111) surfaces Reference
Method
Pd(111)
Ag(111)
Ds
Methfessel et al. [26 ] De Boer et al. [27] Zhu and DePristo [28] Skriver et al. [29]
LMTO-DFT Experimental ( liquid) BOS-EMT TB-LMTO-DFT
65.6 81.0 85.9 74.3
53.1 53.8 59.8 48.2
12.5 27.2 26.1 26.1
The second column gives the applied method. BOS: bond order simulator; EMT: effective medium theory; TB: tight binding; LMTO: linear muffin tin orbital; DFT: density functional theory. The last column gives the surface energy difference (Ds).
surface tensions of liquid surfaces. This leads to the conclusion that our results are in accordance with literature values, with the exception of the findings of Methfessel et al. [26 ]. In comparison, applying zero-order regular solution theory (RST ) [30] gives an equilibrium Pd concentration of 34.7%, taking the sublimation enthalpies of Ag (284.9 kJ mol−1) and Pd (352.4 kJ mol−1), and the mixing enthalpy of Pd Ag (−3.90 kJ mol−1) from Ref. [24] as 67 33 input. The RST treats the alloy bulk as a solution of one element in the other. A coherence energy is defined for each of the couples A–A, B–B and A–B in the AB alloy, calculated on the basis of sublimation and mixing enthalpies. Surface segregation then occurs by minimizing the energy effect of the missing neighbours of the surface atoms. Applying the Bragg–Williams approximation [31] of the regular solution theory, which includes corrections for surface relaxation and second layer composition, results in a 30% Pd concentration in the first layer, which is also too high. Vurens et al. [24] performed Monte Carlo simulations of the Pd Ag (111) with, as input, coherence energies 70 30 that were derived from experimental values for sublimation and mixing enthalpies. For a surface equilibrated at 870 K, they predicted a surface composition of 46% Pd. These very high values for surface Pd concentration result from neglecting or underestimating the influence of the surface energies, or in other words: the influence of the mixing enthalpy is estimated too high. In Fig. 2, the temperature dependence of the first layer composition of the PdAg(111) is compared with simulated data on the basis of surface energies. Although the simulation curves do not
perfectly fit the measured data points, it is clear that the surface is in equilibrium with a bulk that contains 80–85% palladium. This brings up a problem that is inevitable when building a segregation curve of PdAg surfaces with any useful width of temperature range. As mentioned previously, above 870 K, the vapour pressure of silver has such a high value (more than 10−7 mbar) that preferential sublimation starts to occur, enriching the top layers of the crystal with palladium. One of the most interesting aspects of the PdAg surface from the viewpoint of catalysis is the distribution of the two elements over the surface. In Fig. 3, the analysis of this distribution on PdAg(111) is compared with the random distribution. An excess of isolated Pd sites was found, confirming earlier Monte Carlo simulations [24]. In contrast to PdAg(111), almost no indication of the presence of palladium atoms was found on the (100) surface under equilibrium conditions (e.g. Fig. 4). The occasional Pd atom that was found allows us to estimate a maximum surface Pd concentration of 0.05%. According to the Langmuir–MacLean formula, the surface energy difference between Pd(100) and Ag(100) would be about 53 kJ mol−1 at 770 K, which is far from what is known from the literature (see Table 2). However, exchange of surface atoms and adatoms as well as diffusion over step edges at surfaces with fcc(100) orientation is much faster than at surfaces with (111) orientation (see [32]). This leads us to believe that the STM images of the AgPd(100) surface are representative of a much lower equilibration temperature than the preparation temperature of 770 K. The equilibration temperature of 400 K for instance would lead to the more realistic value for the surface energy difference of
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P.T. Wouda et al. / Surface Science 417 (1998) 292–300 Table 2 Surface energies (kJ mol−1) for the (100) surfaces Reference
Method
Pd(100)
Ag(100)
Ds
Methfessel et al. [26 ] Zhu and DePristo [28] Skriver et al. [29] Fiorentini et al. [33]
LMTO-DFT BOS-EM TB-LMTO-DFT LMTO-DFT
85.9 115.8 86.8 87.8
60.8 78.2 59.8 56.9
25.1 36.6 27.0 30.9
The last column gives the surface energy difference (Ds). BOS: bond order simulator; EM: effective medium; TB: tight binding; LMTO: linear muffin tin orbital; DFT: density functional theory.
28 kJ mol−1. For comparison, in Table 2, literature values of surface energy are listed for Pd(100) and Ag(100). Furthermore, the Langmuir–MacLean equation ignores the effect of mixing enthalpy. The negative mixing enthalpy of PdAg [24] indicates a tendency towards formation of ordered structures, and as atom mobility at the (100) surface is much higher than in the bulk, we can expect some ordering there, even in the absence of bulk order. As the ordered structures favoured by nearest-neighbour interactions (the L1 and the L1 structures in fcc 0 2 lattices) have (100) planes with alternating element concentrations (100%/0% and 100%/50% respectively), any order of this kind can lead to a Ag-terminated surface and thereby strongly enhance the Ag segregation. Depletion of the subsurface layers in silver by simultaneous sputtering and annealing results in STM images as shown in Fig. 5a. A very small amount of palladium atoms (about 0.4%) can be observed. However, AES shows a large increase in Pd AES signal as mentioned in the Section 3 (Pd /Ag changes from 0.4 before to 1.4 333eV 359eV after the procedure). Given the almost 100% Ag concentration in the first layer, this can be explained only if almost all Ag is removed from the subsurface region: as mentioned previously, the simultaneous sputtering and annealing procedure strongly depletes the subsurface layers of silver. Nevertheless, the extremely strong surface segregation of silver keeps the surface at almost 100% Ag. The lines A and B in Fig. 5a correspond to the respective line plots in Fig. 5b and c. The curve in Fig. 5b shows that chemical contrast on PdAg(100) is characterized by an apparent height
difference of about 60 pm between palladium and silver. This value is considerably higher than that found on PdAg(111), which may very well be related to tip–sample interactions, possibly based on the lower coordination number of fcc(100) surface atoms. This value is also exceptionally high in comparison with chemical contrast on PdCu(100) [6 ] (10–30 pm). The plot in Fig. 5c shows 15-pm depressions in the (100) surface lattice. These depressions are also found in the image in Fig. 6a, where they form small areas of c(2×2) structures. A line plot ( Fig. 6b) shows that their corrugation is somewhat deeper than in Fig. 5a. The nature of the phenomenon is not clear, although it seems reasonable to say that it has nothing to do with chemical contrast in view of the gradual transition of the p(1×1) areas with low corrugation into c(2×2) areas with high corrugation. It is more likely to be related to the presence of short-range ordering or other phenomena in deeper layers. In the literature, it is generally assumed that PdAg does not have any ordered phases in its phase diagram [25]. However, experimental curves of heat of formation and excess entropy of formation versus alloy bulk composition by Hultgren et al. may point in the direction of short-range ordering in the concentration range near Pd Ag [34]. 60 40 5. Conclusions Chemical contrast can be achieved on PdAg(111) as well as on PdAg(100) using the STM. Apparent height differences between Pd and Ag are larger on the (100) surface than on the (111) surface. Both surface orientations are char-
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acterized by a high enrichment in silver, which can be explained on the basis of currently known surface energies. On PdAg(111), there is an excess of isolated Pd sites in comparison with the random distribution. On PdAg(100), small c(2×2) areas have been observed that seem to be related to ordering or defects in deeper layers.
Acknowledgements This project was supported by the Fonds zur Fo¨rderung der wissenschaftlichen Forschung (Austrian Science Foundation) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek [Netherlands Organisation for Scientific Research (NWO)] under stipendium number SIR 13-4239.
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STM study of the (111) and (100) surfaces of PdAg P.T. Wouda a,*, M. Schmid b, B.E. Nieuwenhuys a, P. Varga b a Leiden Institute of Chemistry, Department of Heterogeneous Catalysis and Surface Science, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands b Institut fu¨r Allgemeine Physik, Technische Universita¨t Wien, Wiedner Hauptstrasse 8–10, A-1040 Vienna, Austria Received 17 July 1998; accepted for publication 14 August 1998
Abstract The (111) and (100) surfaces of the Pd Ag alloy have been imaged with atomic resolution by scanning tunneling microscopy. 67 33 On the (111) surface, it was found that Pd atoms appear in the images about 25 pm higher than the Ag atoms. The surface concentration of palladium was determined as a function of annealing temperature (720–920 K ) and was found to vary between 5 and 11%. Analysis of the relative positions of the Pd atoms showed a tendency towards formation of isolated palladium sites. On the (100) surface, the palladium concentration in the first layer is extremely low and the system has to be forced into a nonequilibrium state to find palladium atoms in the first monolayer. Chemical contrast here amounts to a 60 pm apparent height difference. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Low-index single crystal surfaces; Palladium; Scanning Tunneling Microscopy; Silver; Surface segregation
1. Introduction Scanning Tunneling Microscopy (STM ) has become one of the most important tools for the study of fundamental processes at surfaces. As is shown in this and other papers, the STM is capable of imaging with chemical contrast; with the assistance of spectroscopic techniques like AES or ISS, the chemical identity of the observed species can be determined as well as the exact location of adsorbates and substrate atoms. Generally, the basis of chemical contrast is either the elementspecific interaction with an adsorbate at the tip, or a sufficiently large variation in the local density of states (LDOS) between substrate atoms of * Corresponding author. Fax: +31 71 5274451; e-mail: [email protected]
different chemical identity, or a combination of those. This has been shown, for instance, with alloys that are interesting from a catalytic point of view, like PtRh [1,2], PtNi [3,4], CuAg [5], PdCu [6 ], or PtCo [7]. The unique property of the STM is that it provides both the chemical composition of the first atomic layer of a crystal, and direct information on the distribution of the constituent component atoms over the surface and, hence, on ordering or demixing phenomena. A good example is our recent study of PtRh(100) [1], which was the first alloy surface to be imaged with atomic resolution and chemical contrast by STM on the basis of the intrinsic electronic properties of the system. Rhodium was imaged with an apparent height 22 pm greater than that of platinum, and a tendency towards formation of small rhodium clusters was observed.
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Alloys have been popular systems in catalysis research for decades because of their effect on reaction selectivity, their ability to catalyse more than one reaction and their resistivity against poisoning. In this paper, a study of PdAg alloy surfaces is described, a system that is well known to influence catalytic reaction paths by the ensemble size effect [8]. Infra-red absorption studies of CO adsorbed on PdAg catalysts indicated that the change in catalytic properties of Pd upon alloying with Ag can be understood in terms of an ensemble size effect [8]. In the case of PdAg, this means that the degree at which the palladium atoms create isolated sites (for on top adsorption only) or clusters forming hollow sites between Pd atoms determines the properties of the system in chemisorption and catalysis. Let us illustrate this effect with an example of thermal desorption of CO. TDS of CO from Pd(111) features a desorption maximum at 500 K, with the development of low T shoulders at increasing exposures [9,10]. On Pd surfaces, CO preferentially occupies the hollow sites, the on-top sites being less favourable. TDS of CO on the PdAg(111) surface shows one single peak at 400 K, shifting to 375 K at higher exposures [9,10]. Clearly, a large part of the palladium sites consists of isolated Pd atoms in a matrix of silver, making only on-top Pd sites available for CO adsorption. Later, more detailed theoretical and experimental results were published, indicating the importance of ligand effects of the site for CO adsorption on PtCu(111) [11–13] and PtFe(111) [14], in addition to ensemble size effects. The term ‘‘ligand effect’’ here refers to the fact that the electronic state of a metal atom is partly determined by the identity and geometry of its surrounding atoms. This invokes ideas from coordination chemistry and homogeneous catalysis where the state and catalytic behaviour of the metal centre are strongly influenced by its ligands. The interesting question is whether there is sufficient difference between the local density of states on palladium and silver atoms at a PdAg surface to induce chemical contrast. Theoretical studies (e.g. Ref. [15] ] of the PdAg system have pointed out that in the total DOS, the Pd contribution has its maximum within 1 eV off the Fermi
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level, whereas the Ag DOS maximum is found at a 5 eV higher binding energy. Concerning the local DOS (‘‘local’’ here means within a muffin tin sphere around a specific atom) on Pd and Ag atoms in the alloy, the modelling results show that the dominating contributions in the near-E region F come from the respective d bands. Both local densities of d states have shifted relative to the d bands of their pure elemental solids, but their centroids are still well separated, and the LDOS on Pd is much larger in the spectral region that is relevant for STM. Looking at the system from the point of view of charge transfer, it has been predicted in these calculations that Pd gains charge density. Charge transfer has been shown to depend on the number of hetero-atoms surrounding the central atom [15]. The results of X-ray absorption studies agree that the Pd d-states gain more charge with increasing Ag concentration [16,17]. The Pd d-states also become more localized [17]. There is agreement that charge transfer occurs (from Ag to Pd ), but the question as to what degree this concerns sp or d-states remains open. The work presented in this paper is a study of the chemical contrast on PdAg(111) and (100), the first layer concentrations of the respective elements as a function of temperature, and the ordering/demixing properties of the surface. To collect this information, scanning tunneling microscopy is used. Other techniques that specifically give data on the first atomic layer (quantitative low-energy electron diffraction and low-energy ion scattering spectroscopy) are not appropriate for the study of PdAg surfaces as the mass as well as the electron scattering properties of both elements are too similar.
2. Experimental In the work presented in this paper, the (100) and (111) surfaces of single crystalline Pd Ag 67 33 have been studied. The choice of this bulk composition was motivated by the expected strong surface segregation of silver. The surface was extensively cleaned in vacuum by sputtering (2-keV Ar+) and annealing (770 K–820 K ). Using
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Auger electron spectroscopy (AES ), the surface was checked for the presence of contaminants; this was complicated by the fact that the S peak 152eV coincides with one of the minor Pd peaks. By using low-energy ion scattering (LEISS), we did not find any indication of sulphur contamination. The LEISS measurements were obtained with a 1-keV He+ beam and a hemispherical analyser. A comparable problem concerns the C peak that is located in the flank of the Pd peak. No 279eV indication of carbon build-up (usually visible as dark patches; see, for example, Ref. [18]) has been observed during the STM measurements. The annealing procedures for the temperature-dependent composition measurements were such that just after the heating had been turned off, the sample was transferred to the STM chamber where it was stored in close contact with a block of copper for fast cooling. The vacuum system consisted of two chambers: a preparation chamber with a base pressure of 1×10−10 mbar in which all annealing and sputtering procedures took place and an STM/AES chamber in which the pressure during measurements was about 8×10−11 mbar. The scanning tunneling microscope used in this study was a commercial Omicron micro-STM, which has been provided with additional vibration damping: the STM head itself was supported by a viton stack, and the entire vacuum system was suspended in a combined spring–elastomer system as described in Ref. [19]. A tungsten tip was used, and the sample bias was always negative. The AES data have all been recorded at an electron energy of 3 kV and with a cylindrical mirror analyser. The peak-to-peak height ratios have been deduced from the differentiated spectra.
3. Results STM images of Pd Ag (111) with chemical 67 33 contrast have been recorded at tunneling resistances between 1 GV and 50 kV (bias voltage range 0.5 mV–1 V ). Fig. 1a shows an STM image of the Pd Ag (111) surface. In this case, the crystal has 67 33 been equilibrated at 820 K for 80 min. This procedure of annealing at 820 K, measuring and
(a)
(b) ˚ ) of the Pd Ag (111) surFig. 1. (a) STM image (100×100 A 67 33 face equilibrated at 820 K. V=0.5 mV, I =13.3 nA. The tunnel surface contains 5.2% palladium. The white line top left indicates where the line plot in (b) is located. (b) Line plot taken from the image in (a) (see white line top left).
analysis was repeated many times, resulting in a surface composition of 5.2±0.3% of palladium. As the line plot in Fig. 1b shows, the apparent height differences between palladium and silver in Fig. 1a are about 25 pm. The AES Pd / 333eV Ag ratio of the system with this preparation 359eV was 0.40±0.02. A set of images, comparable with that in Fig. 1a, has been recorded after equilibration at various
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Fig. 2. First layer composition versus T for PdAg(111): experimental data ($) are compared with Langmuir–MacLean curves with a 26.6 kJ mol−1 difference in surface energies (see text) for Pd bulk concentrations of 67% (Ω-Ω-Ω), 75% ( · · · ), 80% (– – –) and 85% (———). During the preparation procedures at T>870 K, the top layers of the crystal become enriched in Pd because of the high vapour pressure of silver. Hence, the surface will be in ‘‘equilibrium’’ with a bulk with more than the nominal 67% Pd. An extra data point (&) is added at T=820 K to indicate the surface composition before the high-temperature annealing.
temperatures and subsequent fast cooling. The results of an analysis of those images is shown in Fig. 2, where the palladium concentration in the first layer is plotted versus equilibration temperature (filled circles). At this point, it should be noted that this series of measurements was started with the equilibration at 920 K: at this temperature, silver has a vapour pressure in the low 10−7-mbar range, which means that the preparation results in a certain degree of palladium enrichment in the top layers of the crystal. To illustrate this, after annealing at 920 K, AES measurements indicated a Pd /Ag peak-to-peak height 333eV 359eV ratio of 0.49. Consequently, the experimental data points (solid circles) in Fig. 2 are representative of a surface in equilibrium with a bulk composition of more than 67% Pd. One data point (filled square) has been added in Fig. 2 to indicate this: it represents the surface composition at 820 K before the high-temperature annealing. At first glance, the position of the palladium atoms is such that most of them are isolated, i.e. entirely surrounded by silver atoms, creating an ensemble size distribution that peaks at the single site configuration. A closer statistical analysis
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Fig. 3. Comparison between counts of various configurations at the Pd Ag (111) surface (equilibrated at 820 K ) and the 67 33 expectations for a random distribution. In the indicated configurations, the dark circles are palladium. The preference for isolated palladium sites is clear.
brings out these interesting features more clearly. A set of 12 independent STM images of the PdAg(111) surface equilibrated at 820 K has been analysed, searching for the exact distribution of palladium atoms in the sea of silver atoms. Fig. 3 gives the results, divided in counts for single Pd atoms, double sites (two nearest Pd neighbours), triple sites (three nearest Pd neighbours), nextnearest neighbour couples and next-next-nearest neighbour couples. For comparison, the simulated data of a random distribution are presented. In Fig. 4, an STM image of the Pd Ag (100) 67 33 surface is shown. On this and most other STM images of the equilibrated PdAg(100) surface, no indication of the presence of a second element was found. A small number of palladium atoms (0.4%) was present in the STM images after the subsurface layers of the crystal had been strongly depleted of silver. This depletion procedure involved sputtering and annealing at the same time for about 10 min: sputtering occurred with 2-keV Ar+ ions (current density 2×10−2 A m−2), and the annealing temperature was 700 K. Such a procedure for the depletion of a segregating species has first been reported by Rehn et al. for the CuNi system [20]. The idea is that due to the relatively high temperature, there is a constant segregation of silver atoms to the surface, whereas in the sputtering process, the atoms of the top layer are continuously removed. This results in a selective removal of silver atoms from the top layers.
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(a) ˚ ) of the Pd Ag (100) surface Fig. 4. STM image (100×100 A 67 33 equilibrated at 770 K. V=0.5 mV, I =16.6 nA. The area tunnel shown contains no palladium: the differences of apparent height of the atoms are much smaller than those observed between Ag and Pd, indicating that no Pd atoms are present in the first monolayer.
We have observed this effect by an increase in the Pd /Ag AES ratio from 0.40 before 333eV 359eV the procedure to 1.40 after sputtering and annealing. The corresponding STM image is shown in Fig. 5a. In Fig. 5b, a line plot over a palladium atom is shown, indicating that the apparent height difference between Ag and Pd atoms is 40–60 pm. In Fig. 5c, the profile over two of the small local depressions is plotted, depressions that are found in large amounts in Figs. 4, 5a and 6a. The apparent height differences in these areas are usually significantly less than the apparent height of the protrusions identified as Pd, and the transition between these areas is always gradual. In Fig. 6a, the local depressions form regions with a c(2×2) pattern. A line profile over such an area is presented in Fig. 6b.
(b)
(C) ˚ ) of the Pd Ag (100) surFig. 5. (a) STM image (100×100 A 67 33 face prepared by simultaneous sputtering and annealing. V= 0.5mV, I =10.6 nA. The surface contains a low concentunnel tration of palladium (about 0.4%); (b) Line plot A in (a) over
a palladium atom: the apparent height is about 60 pm higher than for silver atoms. (c) Line plot B in (a) over an area with less height variations (approximately 15 pm) attributed to the subsurface structure.
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4. Discussion
(a)
The fact that chemical contrast, i.e. discrimination of palladium and silver in PdAg(111), has been found for a wide range of tunneling resistances with STM points to a considerable difference in local densities of state near the Fermi level. This point is consistent with the current knowledge of the PdAg ( local ) valence band structure [15,17,21]. The difference in apparent heights is comparable to that found for a group VIII alloy surface like PtRh(100) [1]. The only other STM studies of the (111) surface of a group VIII/group Ib alloy, showing chemical contrast, concern the PdAu(111) and NiAu(111) surface alloy systems [22,23]. In the PdAu publication, however, no information is given on the apparent height variations [22]. Gold atoms in the first layer of the Ni(111) surface appear as 20-pm-deep depressions [23]. As mentioned Section 3, the palladium concentration in the first layer of PdAg(111) is 5.2±0.3% when equilibration took place at 820 K. With this value, we can calculate the surface energy difference using the Langmuir–MacLean formula for surface segregation:
C D
X Ds bulk exp surf = 1−X 1−X RT surf bulk X
(b) ˚ ) of the Pd Ag (100) surFig. 6. (a) STM image (100×100 A 67 33 face equilibrated at 770 K. V=0.5 mV, I =5.4 nA. The area tunnel shown contains no palladium atoms. Nanometre-scale areas can be discerned with a c(2×2) structure, featuring depressions already shown in Fig. 5c. (b) Line plot of line A in (a). The depressions forming the c(2×2) are apparent, featuring higher apparent height variations than in the line plot in Fig. 5c where the same phenomenon was shown.
with Ds being the difference in surface energies. This formula does not include the effects of bulk mixing enthalpy; however, as the mixing enthalpy is known to be rather low (−3.9 kJ mol−1 according to Ref. [24]) and no ordered bulk structures are known [25], we neglect this effect here. Because of the small size difference between palladium (r= 137 pm) and silver atoms (r=144 pm), no corrections for lattice strain have been included. The result is a surface energy difference of 24.7 kJ mol−1. Surface energies found in literature for Pd(111) and Ag(111) show some scatter as shown in Table 1. The data of Methfessel et al. [26 ] result in a surface energy difference of 12.5 kJ mol−1, which is quite different from the other literature data of 26.6±0.6 kJ mol−1 as well as from our results. De Boer et al. [27] estimated surface energy values for (111)-like surfaces from
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Table 1 Surface energies (kJ mol−1) for the (111) surfaces Reference
Method
Pd(111)
Ag(111)
Ds
Methfessel et al. [26 ] De Boer et al. [27] Zhu and DePristo [28] Skriver et al. [29]
LMTO-DFT Experimental ( liquid) BOS-EMT TB-LMTO-DFT
65.6 81.0 85.9 74.3
53.1 53.8 59.8 48.2
12.5 27.2 26.1 26.1
The second column gives the applied method. BOS: bond order simulator; EMT: effective medium theory; TB: tight binding; LMTO: linear muffin tin orbital; DFT: density functional theory. The last column gives the surface energy difference (Ds).
surface tensions of liquid surfaces. This leads to the conclusion that our results are in accordance with literature values, with the exception of the findings of Methfessel et al. [26 ]. In comparison, applying zero-order regular solution theory (RST ) [30] gives an equilibrium Pd concentration of 34.7%, taking the sublimation enthalpies of Ag (284.9 kJ mol−1) and Pd (352.4 kJ mol−1), and the mixing enthalpy of Pd Ag (−3.90 kJ mol−1) from Ref. [24] as 67 33 input. The RST treats the alloy bulk as a solution of one element in the other. A coherence energy is defined for each of the couples A–A, B–B and A–B in the AB alloy, calculated on the basis of sublimation and mixing enthalpies. Surface segregation then occurs by minimizing the energy effect of the missing neighbours of the surface atoms. Applying the Bragg–Williams approximation [31] of the regular solution theory, which includes corrections for surface relaxation and second layer composition, results in a 30% Pd concentration in the first layer, which is also too high. Vurens et al. [24] performed Monte Carlo simulations of the Pd Ag (111) with, as input, coherence energies 70 30 that were derived from experimental values for sublimation and mixing enthalpies. For a surface equilibrated at 870 K, they predicted a surface composition of 46% Pd. These very high values for surface Pd concentration result from neglecting or underestimating the influence of the surface energies, or in other words: the influence of the mixing enthalpy is estimated too high. In Fig. 2, the temperature dependence of the first layer composition of the PdAg(111) is compared with simulated data on the basis of surface energies. Although the simulation curves do not
perfectly fit the measured data points, it is clear that the surface is in equilibrium with a bulk that contains 80–85% palladium. This brings up a problem that is inevitable when building a segregation curve of PdAg surfaces with any useful width of temperature range. As mentioned previously, above 870 K, the vapour pressure of silver has such a high value (more than 10−7 mbar) that preferential sublimation starts to occur, enriching the top layers of the crystal with palladium. One of the most interesting aspects of the PdAg surface from the viewpoint of catalysis is the distribution of the two elements over the surface. In Fig. 3, the analysis of this distribution on PdAg(111) is compared with the random distribution. An excess of isolated Pd sites was found, confirming earlier Monte Carlo simulations [24]. In contrast to PdAg(111), almost no indication of the presence of palladium atoms was found on the (100) surface under equilibrium conditions (e.g. Fig. 4). The occasional Pd atom that was found allows us to estimate a maximum surface Pd concentration of 0.05%. According to the Langmuir–MacLean formula, the surface energy difference between Pd(100) and Ag(100) would be about 53 kJ mol−1 at 770 K, which is far from what is known from the literature (see Table 2). However, exchange of surface atoms and adatoms as well as diffusion over step edges at surfaces with fcc(100) orientation is much faster than at surfaces with (111) orientation (see [32]). This leads us to believe that the STM images of the AgPd(100) surface are representative of a much lower equilibration temperature than the preparation temperature of 770 K. The equilibration temperature of 400 K for instance would lead to the more realistic value for the surface energy difference of
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P.T. Wouda et al. / Surface Science 417 (1998) 292–300 Table 2 Surface energies (kJ mol−1) for the (100) surfaces Reference
Method
Pd(100)
Ag(100)
Ds
Methfessel et al. [26 ] Zhu and DePristo [28] Skriver et al. [29] Fiorentini et al. [33]
LMTO-DFT BOS-EM TB-LMTO-DFT LMTO-DFT
85.9 115.8 86.8 87.8
60.8 78.2 59.8 56.9
25.1 36.6 27.0 30.9
The last column gives the surface energy difference (Ds). BOS: bond order simulator; EM: effective medium; TB: tight binding; LMTO: linear muffin tin orbital; DFT: density functional theory.
28 kJ mol−1. For comparison, in Table 2, literature values of surface energy are listed for Pd(100) and Ag(100). Furthermore, the Langmuir–MacLean equation ignores the effect of mixing enthalpy. The negative mixing enthalpy of PdAg [24] indicates a tendency towards formation of ordered structures, and as atom mobility at the (100) surface is much higher than in the bulk, we can expect some ordering there, even in the absence of bulk order. As the ordered structures favoured by nearest-neighbour interactions (the L1 and the L1 structures in fcc 0 2 lattices) have (100) planes with alternating element concentrations (100%/0% and 100%/50% respectively), any order of this kind can lead to a Ag-terminated surface and thereby strongly enhance the Ag segregation. Depletion of the subsurface layers in silver by simultaneous sputtering and annealing results in STM images as shown in Fig. 5a. A very small amount of palladium atoms (about 0.4%) can be observed. However, AES shows a large increase in Pd AES signal as mentioned in the Section 3 (Pd /Ag changes from 0.4 before to 1.4 333eV 359eV after the procedure). Given the almost 100% Ag concentration in the first layer, this can be explained only if almost all Ag is removed from the subsurface region: as mentioned previously, the simultaneous sputtering and annealing procedure strongly depletes the subsurface layers of silver. Nevertheless, the extremely strong surface segregation of silver keeps the surface at almost 100% Ag. The lines A and B in Fig. 5a correspond to the respective line plots in Fig. 5b and c. The curve in Fig. 5b shows that chemical contrast on PdAg(100) is characterized by an apparent height
difference of about 60 pm between palladium and silver. This value is considerably higher than that found on PdAg(111), which may very well be related to tip–sample interactions, possibly based on the lower coordination number of fcc(100) surface atoms. This value is also exceptionally high in comparison with chemical contrast on PdCu(100) [6 ] (10–30 pm). The plot in Fig. 5c shows 15-pm depressions in the (100) surface lattice. These depressions are also found in the image in Fig. 6a, where they form small areas of c(2×2) structures. A line plot ( Fig. 6b) shows that their corrugation is somewhat deeper than in Fig. 5a. The nature of the phenomenon is not clear, although it seems reasonable to say that it has nothing to do with chemical contrast in view of the gradual transition of the p(1×1) areas with low corrugation into c(2×2) areas with high corrugation. It is more likely to be related to the presence of short-range ordering or other phenomena in deeper layers. In the literature, it is generally assumed that PdAg does not have any ordered phases in its phase diagram [25]. However, experimental curves of heat of formation and excess entropy of formation versus alloy bulk composition by Hultgren et al. may point in the direction of short-range ordering in the concentration range near Pd Ag [34]. 60 40 5. Conclusions Chemical contrast can be achieved on PdAg(111) as well as on PdAg(100) using the STM. Apparent height differences between Pd and Ag are larger on the (100) surface than on the (111) surface. Both surface orientations are char-
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acterized by a high enrichment in silver, which can be explained on the basis of currently known surface energies. On PdAg(111), there is an excess of isolated Pd sites in comparison with the random distribution. On PdAg(100), small c(2×2) areas have been observed that seem to be related to ordering or defects in deeper layers.
Acknowledgements This project was supported by the Fonds zur Fo¨rderung der wissenschaftlichen Forschung (Austrian Science Foundation) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek [Netherlands Organisation for Scientific Research (NWO)] under stipendium number SIR 13-4239.
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