J. Phys. Chem. 1994, 98, 9606-9613
9606
Small Pt Aggregates Adsorbed on Ni(ll1): A Theoretical Study N. J. Castellanit*'and P. LCgard*$t LERCSI, Universitt Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg, France, and PLAPIQUI (UNS-CONICET), 12 de Octubre 1842, 8000 Bahia Blanca, Argentina Received: March 9, 1994; In Final Form: June 22, 1994@
A theoretical study for small Ptp aggregates (p = 1-4) adsorbed on Ni( 111) has been performed by following a model previously implemented for Ni/Ni( 111) and Pt/Pt( 111). Pt on Ni shows a relatively strong interaction with an adsorption energy close to the value for Pt on Pt. Our results suggest a very distorted overlayer for small Pt coverages favoring linear chains and predicting important relaxations particularly for planar configurations. We give also some insight concerning the changes of the local electronic structure.
1. Introduction The application of bimetallic systems to the development of catalysts with specific activity and selectivity properties has been the object of enhanced and increasing attention because the individual catalytic properties of each metal component can be combined or changed in a unique and new way when they form the bimetallic system.' So, while it could be expected that the Pt-Ni alloys would combine the hydrocarbon cracking capacity of Ni with the remarkably isomerization property of Pt, a high selectivity to isomerization is observed on alloys with up to 70% of Ni2 On the other hand, the Pt-Ni system has shown unusual segregation properties as an alloy. For nondilute compositions, a surface-sandwich segregation has been reported3 which is related to a decrease of chemisorption strengths for hydrocarbons and C0.4 More recently, this bimetallic system has been the object of renewed experimental interest in the form of a Pt overlayer over a Ni substrate. Photoemission of adsorbed xenon (PAX) and work function measurements5 carried out on very thin Pt films deposited on Ni( 111) revealed the presence of Pt disordered domains. They seem to reorganize in a more ordered and stressed overlayer structure andor a surface alloy after annealing treatments. Low-energy electron diffraction (LEED) experim e n t ~ show ~ , ~ a highly disordered overlayer which appears to follow, on average, the (111) pattern of Ni. When the temperature increases, a more ordered structure and, at the same time, a high surface mobility of Pt atoms is observed. X-ray photoelectron spectroscopy (XPS) experiments, performed in our laboratory and interpreted with the use of Born-Haber cycles, indicate a relatively high adsorption energy of monatomic Pt on Ni( 111) and the presence of a distorted Pt overlayer.* The Pt/Ni and NiPt systems are interesting examples for making theoretical studies about overlayer growth, taking into account the large mismatch (1 1%)between t'F and Ni. Embedded-atom method (EAM) calculations have been performed for Pt/Ni(100),9 Ni/Pt(lOO),'O and Pt/Ni(lll)" for very small adsorbed aggregates of different geometrical configurations. However, no molecular orbital calculations related to any of these systems have been reported in the literature. The objective of this paper is to attain more detailed information of the geometrical and electronic structure of the
* To whom correspondence must be addressed. Tel. (33) 88 41 62 04; fax (33) 88 41 61 47; e-mail
[email protected]. LERCSI. PLAPIQUI. @Abstractpublished in Advance ACS Abstracts, August 1, 1994.
'
*
Pt/Ni( 111) system by employing molecular orbital calculations within the cluster approximation. For that we follow the methodology previously developed to study the Ni/Ni( 111)and Pt/Pt( 111) homoepitaxial systems,'* which in the present work are taken as comparative references. The paper is organized as follows. First we describe the theoretical method. Afterward, a section is devoted to monatomic adsorption and another one to p-aggregate (p > 1) adsorption. Finally, we give general conclusions.
2. Theoretical Method Here we give only a brief description of the method already described in a report of the homoepitaxy of Ni on Ni( 111) and Pt on Pt( 111).12 The computations have been performed with an extended Huckel (EH) technique in the framework of the cluster approximation to represent the Ni substrate. This semiempirical molecular orbital method is very approximate mainly because it is not self-consistent and spin-dependent. As a consequence, electron charge drifts are usually exaggerated, and atomic and molecular binding energies cannot be compared on an absolute scale. Nevertheless, its transparency and simplicity make it well suited to study and analyze chemical interactions by perturbing reasoning. Moreover, the relative structural stability and qualitative trends in bonding are well described. This is particularly useful in the case of extended systems such as interfaces, where a large amount of computing time is necessary for determining the electronic structure. The last problem is even more critical when d-orbitals are involved. We can mention that the EH method has been successfully used for studying the band structure of linear polymers13 or of very structurally complicated transition metal carbides.l4 More recently, this approach has also been applied to study the metalceramic adhesive proper tie^,'^ the growth of metaumetal overlayers,16 and the chemisorption on transition metals.17 For Ni and Pt, 4s+4p+3d(double) and 6s+6p+5d(double) Slater basis sets have been used, respectively. Ionization potentials were taken from experimental and Hartree-FockSlater atomic data.18 Slater's exponents are based on selfconsistent atomic cal~ulations.~~ The adsorption energy Ead for the M W n system (p = number of atoms of the adsorbed aggregate, n = number of atoms of the substrate) is calculated as Ead
= AEe
+ Erep
(1)
where AE, is the total (valence) electron energy expressed as a difference with respect to the pM M, fragments. The second
+
0022-365419412098-9606$04.50/0 0 1994 American Chemical Society
Small Pt Aggregates Adsorbed on Ni(111)
J, Phys, Chem,, Vol. 98, No. 38, 1994 9607
e,; =*e-'*:-
Figure 1. 13-Atom cluster and monoatomic adsorption sites. First layer, full circles. Second layer, crossed circles. Third layer, empty circles. The 13-atomcluster has 7, 3, and 3 atoms for the first, second, and third layers, respectively. 1-4 refer to 1-fold, 2-fold, 3-fold octahedral (0),and 3-fold tetrahedral (T) adsorption sites, respectively. term EEP,due to Pt-Pt and Pt-Ni repulsions, was calculated by considering a contribution C E R ,from ~ the repulsions between screened cores and a contribution E R ,from ~ the interactions between the inner 5p levels of each Pt atom and the valence levels of other atoms. Both repulsive terms can be computed separately and are painvise additive. C E R ,was ~ calculated by following the procedure described by Calzafem et a1.20 For E R ,we ~ employed a technique originally proposed by L. W. Anders.21 The E R ,energy ~ for a Pt-Pt pair has been adjusted by the expression ,b1 exp(-alR) where = 26 280 eV, a1 = 4.766 A-1, and R is the interatomic distance. For a Pt-Ni pair the exponential expression for E R ,was ~ ,L?2exp(-a2R), where ,L?2 = 1499 eV and a2 = 3.918 A-l. The optimization of parameters was reduced to a minimum. The Slater's exponent of the Pt-5p orbital was varied to obtain a reasonable Pt-Pt distance for spherically symmetrical FCC clusters (not shown) of 43 atoms. The IP of the Ni-3d orbital was reduced by 0.5 eV in order to obtain a bound Ni2 dimer. All the other parameters used in the EH calculations are the same as for Ni/ Ni( 111) and F't/Pt( 11l).l2 As a preliminary test of the calculation method, the relative stabilities of various free Ni3 and Pt3 trimers were analyzed. We concluded that the triangular trimers are more stable than the linear ones, in very good agreement with ab initio calculations.22 The electronic structure has been analyzed by means of the partial local density of states (partial LDOS) and overlap population density of states (OPDOS) concepts based on a Mulliken population analysis of the molecular orbitals.23 The OPDOS at a given energy can be seen as a weighted density of states, whose magnitude depends on the coupling overlap and on the molecular orbital coefficients. This concept is specially useful because it gives a measure of the bonding character between two atoms. The applicability of clusters to model surfaces and interfaces, notwithstanding its wide employment, requires some comments. The most serious disadvantage is the lack of periodicity in two spacial directions. The presence of this defect in the otherwise infinite solid introduces localized states usually known as a "cluster border effect". On the other hand, chemisorption phenomena show chemical bonds of strongly local nature and are (at small coverages) nonsymmetric for translations along the surface. Hence for this case, the cluster model seems to be adequate. Instead of representing the rest of the solid by embedding techniques which could introduce spurious effects, we decided to check the influence of the cluster size in our results, particularly in their degree of convergence. The (1 11) face of Ni substrate has been modeled with clusters of 13, 43, 50, 73, and 91 atoms, that made possible the study of cluster size and border effects. They are pictured in Figures 1 and 2. The monatomic adsorption was studied mainly with the 13- and 43-atom clusters. For p > 1 we used extensively the 43- and 73-atom clusters. The 50- and 91-atom clusters
c
Figure 2. 43-, 50-, 73-, and 91-atom clusters. First layer, full circles. Second layer, crossed circles. Third layer, empty circles. Fourth layer, large empty circles. The 43-atom cluster has 19, 12, and 12 atoms for the first, second, and third layers, respectively (the atoms of the first layer are connected by solid lines). The 50-atom cluster has a fourth layer of 7 atoms. The 73-atom cluster has 31, 21, and 21 atoms for the first, second, and third layers, respectively (the atoms of the first layer are connected by solid and long-dashed lines). The 91-atom cluster has 37, 27, and 27 atoms for the first, second, and third layers, respectively (the atoms of the first layer are connected by solid, longdashed, and long-short-dashed lines). TABLE 1: Monoatomic Adsorption Energies (in eV) and Equilibrium Adsorbate-Substrate Interatomic Distances (in A) for Pt/Ni(lllY site system 1-fold 2-fold 3-fold (0) 3-fold (T) 2.349 2.355 Pt/Nil3 2.209 2.311 (2.505) (2.470) (2.236) (2.457) F"L3 2.375 2.527 2.580 2.707 (2.418) (2.507) (2.578) (2.554) Interatomic distances in parentheses. were also employed for this purpose. The Ni atoms of these five clusters follow the same periodicity of the infinite solid metal. For the present EH calculations, not any reconstruction of the Ni surface has been considered. We want to underscore that the different clusters used in the present work, particularly for p < 50, could be used in themselves as models to study the chemisorptive and catalytic properties of small transition metal clusters. Moreover, the system (Pt aggregate Ni substrate cluster) can be considered as a model of a mixed transition metal cluster where a preferential surface segregation takes place. This cluster would show various morphologies, according to the configurations of the aggregate. The study of such systems is of special interest in the field of catalysis.
+
3. Monoatomic Adsorption The simplest case of Pt adsorption on Ni, i.e. the adsorption of an isolated atom, is an instructive comparative example for studying the Pt-Ni interactions. The different possible sites of adsorption are outlined in Figure 1 for the 13-atom cluster and are classified according to their coordination. Notice that there are two possibilities for 3-fold coordination (0 or T). In Table 1 the values of adsorption energy and equilibrium PtNi interatomic distance are given for Pt adsorbed on Nil3 and Ni43. Looking at Table I, we can observe that, in general, the Ed (absolute value) evaluated with the 43-atom cluster is -0.20.3 eV greater than that corresponding to the 13-atom cluster. Nevertheless, the relative order for Ead between the different coordinations remains the same for both clusters. Particularly, we observe that higher coordinations are favored and that the 3-fold (T) site is more stable than the 3-fold (0)one. The last trend is more evident on the 43-atom cluster results (by -0.13 eV).
Castellani and Ugan5
9608 J. Phys. Chem., Vol. 98, No. 38, I994
E 'T c3. 2 . 5
h
m 5
c c
.e
2
0NVNi
1.5
PVPt
1
PVNi
0
0
$
TABLE 2: Monoatomic Adsorption Energies E+ (in.eV) and Equilibrium Adsorbate-Substrate Interatomic Distances "ft (in A) for Pt Adsorbed on a 3-fold (0)Site for Nils, N43, Niso, Ni73, and Nigl Clusters
Ed (eV) dm*(A)
0.5
1
2
3
4
Adsorption Site
Figure 3. Monoatomic adsorption energy (absolute value, in eV) versus adsorption site for m i ( 1 1 l), Ni/Ni( 1 1 l), and F"t(1 1 1). Adsorption site nomenclature: 1, 1-fold site; 2, 2-fold site; 3, 3-fold (0)site; 4, 3-fold (T) site. Data for Ni/Ni(l 1 1) and €"t(11 1) are taken from ref 12. The substrate cluster has 43 atoms.
We show in Figure 3 a comparison between the adsorption energies obtained for Pt/Ni( 11l), Ni/Ni( 11l), and Wpt( 111). The high coordination preference has been observed previously for the two homoepitaxies12where the 3-fold sites are very close to each other. On the contrary, for Pt on Ni43 these two sites differ more widely in their adsorption energies. The 1-fold to 2-fold increase in the adsorption energy (absolute value) is similar (-0.15 eV) to the Ni/Ni(l 11) case (-0.2 eV) and much smaller than for Pt/Pt (-0.5 eV). Regarding the differences between the 13-atom and the 43atom results, we underscore that they are much less pronounced for the case of homoepitaxy. This observation points out that a Pt atom on a Ni surface represents a more important perturbation than, for example, a Ni atom on a Ni surface. Moreover, it can be argued that Pt on Ni would have a behavior noticeably different from that for the homoepitaxy cases. This latter conclusion can be confirmed by examination of Figure 3. We see that the high coordination values for w N i ( l l 1 ) are -1.3-1.4 eV greater than those for Ni/Ni(lll), and even slightly greater than those for w p t ( l l l ) , indicative of a relatively important interaction between the Pt atom and the Ni surface. This observation is in agreement with an experimental evaluation that gives for monatomic Pt on Ni( 111) an adsorption energy close to the cohesive energy of bulk Pt.* Moreover, we note that the Ead values obtained with the EAM formalism for Pt on the (111)" and ( faces of Ni are greater by -1 and 1.5 eV, respectively, than the &d values for Ni on Ni( 111) and Ni on Ni(100). Despite the fact that there are no experimental values of the adsorption energy for a Pt atom on different adsorption sites of a Ni surface, we can compare the activation energy for monatomic diffusion by hopping displacement with the experimental result available from field ion microscopy (FIM) experiment^.^^ Taking the difference of adsorption energies between the 2-fold and the 3-fold sites, we predict for the diffusion activation energy a value of 0.05 eV (3-fold (0)to 2-fold hopping) or 0.18 eV (3-fold (T) to 1-fold hopping). The estimated value deduced from the experimental information is -0.2-0.3 eV.24 The EAM calculations give an activation energy of -0.1 eV.ll We can infer that our prediction of a relatively low activation energy for diffusion is in reasonable agreement with the experiment and EAM theoretical results. In Table 1 we notice an increasing behavior of the adsorbatesubstrate interatomic distance as a function of the adsorption site coordination. Moreover, Pt on the 3-fold (0)site has a distance very close to Pt on the 3-fold (T) site. Both observations are similar to those obtained for the cases of homoepitaxy and are more evident from the Ni43 cluster results. In general, the distances calculated with the 43-atom cluster are greater than those evaluated with the 13-atom cluster. Particularly, the high
2.349 2.505
2.580 2.578
2.536 2.583
2.642 2.588
2.579 2.588
coordination sites are -0.08 A greater. The increasing behavior of the adsorption energy and the Pt-Ni interatomic distance with the coordination is in agreement with previous experimental and theoretical studies for chemisorption on transition metals.25 It is related to the repulsive pairwise additive forces which increase more quickly than the attractive ones as a function of the coordination. Let us compare the Pt-Ni interatomic distance in the P t N (111) system with the adsorbate-substrate distances obtained in the case of Ni/Ni( 111) and Pt/Pt( 111).l2 Besides the above mentioned increasing tendency with the adsorption site coordination for the three systems, we note that the Pt-Ni distance is close to the Pt-Pt distance for similar adsorption sites. The difference between the two bond lengths is always lower than 0.07 A on the Ni43 cluster, the Pt-Ni bond being the larger, except for the top adsorption site. On the contrary, the Ni-Ni bond is always shorter than the Pt-Ni bond by about 0.23 and 0.33 A for the 3-fold and 1-fold sites, respectively. Indeed, for a Pt atom adsorbed on Ni( 11l), due to the relatively shorter distance between the Ni atoms, the repulsive action of the Ni substrate atoms is relatively more important than the repulsive action of the Pt substrate atoms for a Pt atom adsorbed on Pt(1 1l), giving rise to a higher net Pt-Ni distance. This can be confirmed by looking at the distance corresponding to a PtNi diatomic molecule. In that case we calculated an equilibrium distance of 2.148 A (the cohesive energy is - 1.863 eV/atom), i.e., 0.133 A shorter than the Pt-Pt distance calculated for a Pt;!diatomic molecule (Pt-Pt distance = 2.281 A, cohesive energy = -1.217 eV/atom). In order to study the influence of the cluster size on the equilibrium parameters, we show in Table 2 the Ead and Pt-Ni distance &-M' values for a Pt atom adsorbed on a 3-fold (0) site of the 13-, 43-, 50-, 73-, and 91-atom clusters. From the Ead (absolute) values we conclude that, despite some small decreases for Ni50 and Ni91, a general increasing tendency is observed. Taking into account the amount of E d changes (-0.5 eV), we can say that a reasonable convergence has been achieved around 2.6 eV (f0.05 eV) for the greater clusters (n > 13). The small decrease of -0.04 eV when we go from N43 to Ni5o is evidently related to the different structure of the Ni5o cluster, where a fourth layer is added to N43. Note that this fourth layer can be viewed as an adsorbed Ni heptamer. It may perturb markedly the Ni atoms of the third layer and influence the whole system for reasons of symmetry. This is specially valid for the Pt atom as it is aligned directly above the central Ni atoms of the third layer. It is to be underlined that the E d value for Pt on Ni91 is practically the same as that for Pt on N43, indicating the presence of a small oscillation for the intermediate cluster sizes (i.e. Ni73). Furthermore, regarding the results for the Pt-Ni distance, we observe that &-M' increases smoothly, converging to a value of -2.58-2.59 A, i.e., very near to 2.63 A, that corresponds to the sum of covalent radii of Ni and Pt. The electronic structure for an adsorbed Pt atom on the %fold (0)site of N43 is shown in Figure 4 in terms of the s+p and d-components of the LDOS (local density of states) for the adatom and substrate Ni atom immediately nearest to it. For both Pt and Ni atoms the d-component is predominant in the
Small Pt Aggregates Adsorbed on Ni( 111) 25
J. Phys. Chem., Vol. 98,No. 38, 1994 9609
T
0 , -1 3
A\ I-\
-1 2
-1 1
-1 0
-9
-8
-7
-6
Energy (eV) Figure 4. Partial LDOS corresponding to a Pt atom adsorbed on a 3-fold (0) site (solid line) and the central Ni atom of the first layer of NL3, one of the nearest neighbors of the adatom (long-dashed line). The d-levels contribution has been shifted by 4 units of LDOS. The s+p levels contribution has not been shifted. The arrow indicates the position of the Fermi level energy. 0'7
T
Energy (eV) Figure 5. OPDOS curves corresponding to the adatom-substrate atom bond for F"i(ll1) (solid line), Ni/Ni(l11) (long-dashed line), and Pt/ Pt(ll1) (short-dashed line). Arrows indicate in each case the position of the Fermi level energy. Data for Ni/Ni(111) and PtPt(ll1) are taken
from ref 12. The substrate cluster has 43 atoms. valence band structure. It is also more localized than the s+p component. The LDOS curve for'the adatom is narrower than the curve for the substrate atom, showing a very well defined peak at --9.5 eV. This observation can be explained by its low coordination.26 The LDOS at the Fermi level is lower for the adatom for the same reason. The d-component LDOS curves for Ni adatoms on Ni( 111) or Pt adatoms on Pt( 111) are relatively broader than for the bimetallic system.'* On the other hand, the s-component for Ni/Ni( 111) is as wide as for Pt/Ni(1 11). The decrease of adatom LDOS (EF)(in comparison with the substrate atom LDOS) for Ni/Ni is similar to that for Pt/Ni. We show in Figure 5 the OPDOS (overlap population density of states) corresponding to the Pt-Ni bond. It shows a dominant bonding contribution of the lowest part of the d-band and a less important antibonding contribution of the highest part of this band, giving a net adsorbate-substrate bonding. For comparative purposes, the OPDOS for Ni/Ni(l 11) and Pt/Pt(1 11) are also given in Figure 5. Note the general resemblance of the three curves. However, the greater similarity between Pt-Ni and Ni-Ni OPDOS curves, particularly, the small peak around -12 eV, the pair of peaks around - 11 eV, and the large peak at -10 eV, must be outlined. The Pt-Ni OPDOS curve shows a more developed peak around - 10.5 eV, and the general profile of the antibonding region is of lower magnitude than the corresponding profile for Ni-Ni. The integration of OPDOS
up to the Fermi level gives an overlap population (0.p.) of 0.276 for the Pt-Ni bond, which is somewhat greater than the value for Ni on Ni of 0.265. Care must be taken in making a direct comparison between these values because they have been evaluated for different interatomic distances. Actually, at the equilibrium distance for Ni on Ni(l1 l), the Pt-Ni 0.p. increases up to 0.330 (that is, 20% greater than the Ni-Ni 0.p.).
4. Polyatomic Adsorption 4.1. Unrelaxed Aggregates. The different aggregates considered for polyatomic adsorption are dimer, trimers, and tetramers adsorbed on the 43-, 73-, and 91-atom clusters. Adsorption of trimers has also been considered on Ni50. These aggregates are in epitaxy with the substrate. The effect of nonperfect epitaxy is examined in the next section. For simplicity and for comparative purposes with the cases of homoepitaxy, adsorption on 3-fold (0)sites has been considered. Figure 6 shows the position of atoms corresponding to the different configurations of Pt aggregates adsorbed on Ni43 and Ni73. The values of E d for the different Pt aggregates are shown in Table 3 together with the corresponding equilibrium Pt-Ni interatomic distance. Due to reasons of computational economy the Pt-Ni interatomic distances for Ptfli91 were taken equal
Castellani and LCga C
9610 J. Phys. Chem., Vol. 98, No. 38, 1994
PVPt
1
2
3
1
Mp/M'n System
Figure 7. Adsorption energy (absolute value, in eV) for planar and linear timers adsorbed on 43- and 73-atom clusters, for Pt/Ni( 11l), Ni/Ni( 11l), and Pt/Pt(111). M@I'" system nomenclature: 1, M3(P)/ M'43; 2, M3(L)/M'43; 3, M3(P)/M'73;4, M3(L)/M173. Data for Ni/Ni(111) and Pt/Pt(11 1) are taken from ref 12.
Figure 6. Geometrical configurations for the dimer, trimers, and tetramers adsorbed on Ni43 and Ni73. Numbers indicate adatom positions. Aggregates nomenclature: dimer, 5-6; planar trimer, P, 2-56; planar trimer P', 5-6-8; linear trimer L, 1-5-8; linear trimer L', 5-67; planar tetramer P, 2-3-5-6; linear tetramer L, 4-5-6-7; tetrahedral tetramer T, 2-5-6-9. In the last case atom 9 is over one belonging to the first substrate layer. The atoms of the first layer of the N43 cluster are connected by a solid line. Arrows indicate the vertices of the central triangular region considered in section 5.
unfavorable configuration with an E a d that is -0.07 ev/atom lower than that for the planar configuration. In order to check critically the cluster border and cluster size effects, a series of calculations have also been performed on the Ni91 cluster, particularly taking into account that the terminal atoms of the Ph (L) tetramer adsorbed on Ni73 are still close to nonsaturated bonds. Looking at Table 3, we observe that all the results obtained with the Ni73 cluster are verified on Ni91. The Pt3 (L) and P b (L) aggregates are more favored than the Pt3 (P) and pt4 (P) aggregates by 0.19 and 0.22 eV per atom, respectively. Moreover, the P b (T) aggregate is the most unfavored tetramer, with an Ead that is -0.05 eV/atom smaller than that for the planar configuration. A general view of the columns of Table 3 shows that the #.??ad values corresponding to Ptfli43 and Ptfli91 are, with the exception of the P b (T) case, relatively close (by -0.05 eV per atom). On the other hand, they are -0.1 eV/atom (for tetramers) to 0.2 eV/atom (for trimers) greater than the values calculated for Ptfli73. These consequences resemble those deduced for monatomic adsorption (see Table 2), in relation to the oscillation observed for intermediate clusters (i.e., with 73 atoms). A similar conclusion can be obtained for the dimer adsorption (see Table 3). This clear preference for linear structures over planar ones contrasts with previous results for homoepitaxy obtained with the same theoretical method, where we found that both configurations have very close adsorption energies.12 To provide evidence for this observation and to facilitate the comparison among Pt/Ni( 11 l), Ni/Ni( 11 l), and Pt/pt( 11l), we show in Figure 7 the Ead (absolute) values for trimers P and L corresponding to these last three systems. The adsorption is on clusters M43 and M73. From a general look at Figure 7 we note that the Ead values for Pt3/Ni( 11 1) are almost of the same magnitude as those for Pts/Pt( 1 1 1) and are significantly greater (-1 eV per atom greater) than those for Ni3/Ni( 11 1). Particu-
to those determined for Ptfli73. In this section we focus our attention on the study of different geometrical configurations for trimers and tetramers. The dimer is studied in more detail in the next section, where a comparison with trimers and tetramers will be made. Looking at the Ead values in Table 3 concerned with trimers adsorbed on Ni43, we observe that linear trimers L and L' are more favored than triangular ones P and P' by -0.1 eV per atom. Particularly we note that trimers P and L show lower (absolute) values than trimers P and L', respectively. The reason for this difference is that the last aggregates are close to atoms with unsaturated bonds (cluster border effect). The comparison between trimers P and L', which are less affected by the cluster border, gives a difference of -0.15 eV per atom. The results corresponding to adsorption on the greater Ni73 cluster confirm the trend mentioned above. In this case trimer P does not show a cluster border effect. Nevertheless, trimer L' is still noticeably affected, so we should compare P (or P') and L. Here the comparison gives a difference of -0.2 eV per atom, even greater than that for Pt3 on Ni43. This linear over planar preference is also observed if we add a fourth Ni layer to the 43-atom cluster. Indeed, the results for Pt3 (L) and Pt3 (P) on Ni50 show an E a d (absolute value) difference of -0.16 eV per atom. Let us now consider the results for tetramers. Table 3 shows that pt4 (L) adsorbed on Ni43 or Ni73 is favored over % (P) by 0.2 eV per atom. However, the tetrahedral tetramer stability is strongly influenced by the cluster size. The P b (T) aggregate adsorbed on Ni43 has an Ead (absolute) value slightly lower than that for the linear tetramer (-0.01 eV/atom lower). However, when this aggregate is adsorbed on Ni73, it is the most
TABLE 3: Polyatomic Adsorption Energies E,d (in eV) and Pt-Ni Interatomic Equilibrium Distances (in A) for the Pt Dimer, Trimers, and Tetramers Adsorbed on the 43-, 50-, 73-, and 91-Atom Cluster# trimer
tetramer
substrate
dimer
P
P'
L
L'
P
L
T
Ni43
5.017 (2.621)
7.121 (2.644)
6.944 (2.679)
7.858 (2.709)
7.852 (2.690)
9.301 (2.701)
10.114 (2.699)
10.063 (2.644) (2.564")
Ni73
5.095 (2.656)
7.022 (2.674) 7.247 (2.673)
7.241 (2.683)
7.517 (2.709) 7.858 (2.696)
7.758 (2.709)
9.567 (2.720)
10.330 (2.796)
Ni91
5.004
7.096
9.390
10.281
9.273 (2.673) (2.625*) 9.172
Ni50
7.675
Ead was calculated according to eq 1. For the aggregates nomenclature see Figure 6 and the text. Interatomic distances are in parentheses. The values with an asterisk correspond to the distance between the top atom of the tetrahedron and an atom of the trimer underneath. a
J. Phys. Chem., Vol. 98, No. 38, 1994 9611
Small Pt Aggregates Adsorbed on Ni( 111) 14 T
4 J
A -1 3
-1 2
-1 1
-1
0
-9
-8
-7
-6
Energy (eV) Figure 8. Total LDOS corresponding to a Pt atom of the triangular trimer P adsorbed on N& (solid line) and the central Ni atom of the substrate first layer (long-dashed line). The arrow indicates the position of the Fermi level energy.
larly for the triangular trimers, the &d values for Pt3/Ni(111) are somewhat lower than those for Pt3/Pt(111). On the other hand, for the linear trimers they are slightly greater. With respect to the linear versus planar stability, we notice the different behavior of Pt trimers from the Ni to the Pt surface. When they are adsorbed on Ni (specially on Ni73), the linear Pt3 (L) trimer always shows a greater Ed value than the Pt3 (P) aggregate. On the other hand, when adsorbed on Pt, the planar trimer can be slightly favored (on Pt43) or very slightly unfavored (on Pt73), in comparison with the linear chain. These results suggest that in the early stages of Pt growth on Ni(1 11), linear aggregates will perturb the bidimensional agglomeration. Indeed, experimental reports show that it is difficult to obtain an ordered overstructure. From the theoretical point of view we can mention the EAM calculations performed on Pt/Ni(lOOj? Ni/Pt(lOO),10and M i (1 l In the first work, Pt planar trimers adsorbed on Ni(100) are favored by -0.05 eV per atom over the linear ones9 On the other hand, Ni3 linear trimers on Pt( 100) are favored by -0.06 eV per atom, with an important influence due to a relaxation of the Pt surface.1° Furthermore, EAM simulations carried out on deformed Pt3 triangles adsorbed on Ni( 111) favor the symmetrical C3" configuration." The reasons that could explain such different behaviors are not sufficiently clear, and they could be related to the approximations involved in each case. Particularly, we comment that EAM calculations are parametrized to represent 3D systems and do not take into account the minute details of the electronic structure of adsorbates, which could be very important for the correct description of very small adsorbed clusters. It can be observed in Table 3 that the Pt-Ni interatomic distance is slightly bigger (-0.04 %.)for linear trimers than for triangular ones, on Ni43 as well as on Ni73. The same observation is valid for Pt4 on Ni73, while for pt4 on Ni43 the Pt-Ni distances are practically the same for both configurations. A comparison with the homoepitaxy of Ni and Pt trimers on Ni(ll1) and Pt( 111) is interesting. It is remarkable that the calculated Pt-Ni distance is always greater than the Pt-Pt distance for a given geometrical configuration,12the difference ranging from 0.1 A to less than 0.2 A. The Ni-Ni distance is always shorter than the Pt-Ni distance and the Pt-Pt distance as well. This observation has also been pointed out for monatomic adsorption (see section 3) and can be explained by a combination of geometrical and Pt-Ni repulsive force factors. 637
In Figures 8 and 9 are shown the LDOS corresponding to the adatoms and one of the neighbor atoms of the substrate, for Pt3 (P) and P t 3 (L),respectively. The shape of the LDOS curves for adatoms in trimers has the same gross general features as for monatomic adsorption. However, they are broader because of the interadsorbate hybridization. Specially, LDOS curves for terminal atoms as well as for the central atom of the linear chain are dominated by only one prominent peak (see Figure 9), whose shape has a noticeable resemblance to that obtained for an isolated Pt adatom. On the other hand, the LDOS for the atoms of the triangular aggregate show three well-developed peaks (see Figure 8). In this case the valence band is broader than for the atoms of the linear trimer, which can be related to a greater interatomic hybridization. Furthermore, as for the monatomic case, the LDOS curves for the adatoms in trimers are narrower and have a lower LDOS (EF)than the substrate atoms because of their lower coordination. The general shape of the present LDOS curves is in qualitative agreement with LDOS curves obtained with tight-binding recursive method calculations performed on this system.27 Moreover, similar qualitative trends for LDOS curves have been found in our work devoted to Ni on Ni(ll1) and Pt on Pt(ll1) systems.'* The shapes of the adatoms LDOS for P t 3 / N 4 3 are much more similar to those of Ni3/N43 than those corresponding to Pt3Pt43, in particular, the decrease of the LDOS (EF)with respect to the substrate atom value. Finally, as for Ni3 (L)/Ni43 and Pt3 (L)/ Pt43, the integration of the LDOS up to EFfor the terminal atoms of the linear chain gives an electron charge difference of -0.15 of an electron with respect to the central atom; Le., there is a small charge transfer to the ends of the chain. 4.2. Relaxed Aggregates. The large lattice mismatch between Pt and Ni is of fundamental importance to examine the influence of nonperfect epitaxy on our calculations. For that, the interadsorbate interatomic distance has been relaxed in order to obtain a minimum in adsorption energy. The optimization for the dimer and the trimers P and L was performed on the Ni43 and Ni73 clusters. On the other hand, because the cluster border effects could be critical for P k (P) and P k (L), the PtPt distance for tetramers was optimized only on the Ni73 cluster. In Table 4 we show the optimized Pt-Pt distance and the corresponding Ead value. The nonrelaxed values are also shown for comparison. We notice that the planar configurations undergo a more appreciable change due to relaxation (-0.1 eV/ atom) than the linear ones (-0.01 eV/atom), for P t 3 and for Pt4
Castellmi and LCgar6
9612 J, Phys, Chem,, Vole 98,No, 38, 1994
l146 12
T
A
+
-1 3
-1 2
-1 1
-1 0
-9
-8
-7
-6
Energy (eV) Figure 9. Total LDOS corresponding to a terminal Pt atom of the linear chain L adsorbed on N43 (solid line), the central Pt atom of this chain (long-dashed line), and the nearest neighbor Ni substrate atom in between them (short-dashed line). The arrow indicates the position of the Fermi level energy. TABLE 4: Optimized Interadsorbate Distances d m (in A) and Adsorption Energies E,d (in eV) for the Pt Dimer, Trimers (P and L Configurations), and Tetramers (P, L, and T CodlgurationsY system PtZm43 Pt3(P)/N43 Pt3(L)/Ni43 Ptfli73 Pt3(P)/Ni73 Pt3(L)/Ni73 Pk(P)/Ni73 Pk(L)/Ni73 Ph(T)/Ni73
d m [AI 2.702 2.842 2.564 2.707 2.839 2.588 2.745 2.546 2.839 2.601**
Ead
[e~]
5.062 7.405 7.478 5.145 7.532 7.885 10.050 10.356 9.886
E&, nonrelax. rev] 5.017* 7.121* 7.449* 5.095* 7.247* 7.858* 9.567* 10.330* 9.273*
Nonoptimized ,??ad values are labeled with an asterisk (for d m =
2.489 A). The values with two asterisks correspond to the distance between the top atom of the tetrahedron and an atom of the trimer
undemeath. aggregates. This provided evidence for the importance of PtPt repulsions on the Ni( 111) surface, as pointed out in a previous work devoted to this system.27 Nevertheless, this relaxation does not compensate the adsorption energy difference between unrelaxed configurations, and hence, linear structures remain more favored than the planar ones. It is noteworthy that the effect of relaxations on Ni/Ni( 111) and F"t(111) can be ignored and for these systems the perfect epitaxy is a good approximation.I2 For dimers, the E,d change due to relaxation is of the same magnitude as for linear chains, but for tetrahedral tetramers it is even more important than for the planar structures (-0.15 eV per atom). However, Ph (T) remains the most unstable of Pt tetramers. Therefore the general conclusions deduced in the last subsection are still valid. A more careful analysis of relaxed E a d values for trimers and tetramers on Ni73 shows that the Pt3 (L)-Pt3 (P) difference in E a d values reaches up to 0.12 eV/atom, while the Pt4 (L) - Ph (P) difference is 0.08 eV/atom. That is, as the aggregate size increases, the planar configuration gains in stability over the linear one. The Pt-Pt distance changes follow the E a d modifications. Indeed, it undergoes a very signifiFant relaxation in the case of planar configurations (-0.2-0.3 A), while for linear chains the relaxation is lower but still remarkable (-0.1 A). Moreover, the changes are relatively greater for trimers than for tetramers, and greater on N43 than on Ni73. Looking at the results obtained
on N43, we observe that for P t 3 (P) the Pt-Pt distance is greater than the value for bulk Pt (= 2.772 A). For Pt3 (L) this distance is near the mean value between bulk Pt-Pt and Ni-Ni values (= 2.630 A). On the other hand, for pt4 (P) it is slightly lower than the bulk Pt value, and for Pb (L) it is somewhat greater than the Ni-Ni distance for bulk Ni (= 2.489 A). For pt4 (T) the distance between the top atom and one of the atoms of the triangle undemeath is lower than the Pt-Pt bulk value by -0.17
A. The experimental results coming from PAX,5 LEED,6s7and XPS8 techniques show that the Pt growth on Ni(ll1) is complex, exhibiting a variety of structures depending on the thickness of the overlayer and the annealing temperature. In the range of very thin layers at low temperatures (
