J. Phys. Chem. 1991,95, 2477-2483
2417
Interaction of Ultrathin Films of Cu with Rh(100) and Ru(0001): An XPS Study Jose A. Rodriguez, Robert A. Campbell, and D. Wayne Goodman* Department of Chemistry, Texas AdiM University, College Station, Texas 77843-3255 (Received: July 1 1 , 1990)
The interaction of ultrathin fdms of Cu with Rh(100) and Ru(0001) has been examined by using X-ray photoelectron spectrapcopy (XPS). The effects of surface annealing temperature, adsorbate coverage (film thickness), and CO chemisorption were investigated. The XPS data show that the atoms in a monolayer of Cu supported on Rh( 100) or Ru(0001) are electronically perturbed with respect to the surface atoms of Cu(100). The magnitude of the electronic perturbations is larger for Cu/Rh( 100). Measurements of the C U ( ~ P , , XPS ~ ) peak position of Cu/Rh(100) and Cu/Ru(0001) as a function of film thickness show that the Cu-Rh and Cu-Ru interactions affect the electronic properties of two or three layers of Cu atoms. The present results show a correlation between the shifts in the XPS surface core-level binding energies and the variations in the desorption temperature of CO from Cu adlayers. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of (1) variations that occur in the Cu-Cu interaction when Cu adopts the lattice parameters of Rh( 100) or Ru(0001) in a pseudomorphic adlayer and (2) modifications in the electronic properties of the Cu adatoms, caused by the Cu-Rh and Cu-Ru interactions. Chemisorption of CO induces a large decrease in the electron density of the Cu adlayers.
I. Introduction During the past 10 years there has been a substantial effort within the surface science community to understand the physical and chemical interactions that occur in bimetallic systems.'-16 The initial motivation for these studies was the identification of those electronic and structural properties of mixed-metal systems that are responsible for the superior activity and selectivity of bimetallic catalyst^.^^-^^ The Cu-Ru and Cu-Rh systems are typical examples of bimetallic cataly~ts.'~J**~' In the present study we examine the electronic interaction of Cu films with Rh(100) and Ru(0001) substrates using X-ray photoelectron spectroscopy (XPS). The effects of adsorbate coverage (film thickness) and surface annealing temperature on these electronic interactions are investigated. In addition, the CO adsorption properties of Cu adlayers are discussed on the basis of our present results. The properties of ultrathin films of Cu on Ru(0001) have been previously investigated by means of low-energy electron diffraction (LEED),IZJ4Auger electron spectroscopy (AES),I2J4 temperature-programmed desorption (TPD),'2I3 ultraviolet photoelectron spectroscopy (UPS),ISand work function measurement^.^^ The results from recent studies14 indicate that for submonolayer depositions, Cu forms two-dimensional (2D) islands pseudomorphic to the Ru(0001) substrate. This growth pattern leads to a Cu monolayer density on Ru(0001) that is -6% smaller than the surface atomic density of Cu( 1 1 1) and -7% larger than the corresponding value of Cu(100). At 100 K, Cu is adsorbed layer-by-layer on Ru(OOO~).'~Cu films with thicknesses of up to two monolayers (ML) remain stable to an annealing temperature of 900 K." Annealing Cu films with coverages larger than 2 ML to 900 K produces three-dimensional (3D) islands (Cu( 111) cluster crystallites), leaving a large portion of the second monolayer exposed." TPD experiments" show desorption temperatures between 1 100 and 1250 K for the first monolayer of Cu on Ru(0001) and between 1000 and 1150 K for the Cu multilayers. AES, LEED, and TPD have been usedl0 to examine the interaction between ultrathin films of Cu and Rh( 100). On this surface, AES data show layer-by-layer growth of Cu at 110 K.Io Layers in excess of the first two monolayers are metastable, forming 3D islands (cluster crystallites) at temperatures above 600 K.l0 TPD of Cu from Rh(100) yields two discrete features,IO which are related to Cu desorption from the monolayer (1200-1 300 K) and multilayer (1050-1200 K) states. The present paper is organized as follows: the next section summarizes the experimental procedures. The third section presents the XPS results obtained for the Cu/Rh(100) and Cu/Ru(0001) systems and for adsorption of CO on copper adlayers. The fourth section contains a discussion of these results, To whom correspondence should be addressed.
0022-3654/9 1/2095-2477$02.50/0
and, in their light, different chemical phenomena that are observed in bimetallic systems are analyzed. 11. Experimental Section
The experiments were performed in an ultrahigh-vacuum surface analytical chamber (base pressure 1 4 X 1O-Io Torr) equipped with capabilities for AES, LEED, XPS, and TPD. The Rh(100) and Ru(0001) crystals were cleaned following procedures reported in the l i t e r a t ~ r e . ' ~ , " - ' ~ ,The ~ ~ cleanliness
( I ) (a) Bauer, E. In The Chemical Physics of Solid Surfaces and HerElsevier: Amerogeneous Caralysis; King, D. A,, Woodruff, D. P., Us.; sterdam, 1984; Vol. 3. (b) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775. (2) Physical and Chemical Properties of Thin Meral c3erlayers and Alloy Surjiaces; Zehner, D . M.,Goodman, D. W., Eds.;Materials Research Society: Pittsburgh, 1987. (3) He, J.-W.; Goodman, D. W. J. Phys. Chem. 1990, 94, 1502. (4) Berlowitz, P. J.; Goodman, D. W. Longmuir 1988, 4, 1091. (5) Neiman, D. L.; Koel, B. E. In ref 3. (6) Ruckman, M.W.; Jiang, L. Q.Phys. Rev. B 1988, 38, 2959. (7) He, J.-W.; Shea, W.-L.; Jiang, X.; Goodman, D. W. J . Vac. Sci. Technol. A 1990, 8, 2435. (8) Schmitz, P. J.; Leung, W. Y.;Graham, G. W.; Thiel, P. A. Phys. Reu. B 1989, 40, 11477. (9) Lilienkamp, G.; Koziol, C.; Bauer, E. Surf. Sci. 1990. 226, 358. (10) Jiang, X.;Goodman, D. W., manuscript in preparation. (1 1) Campbell, C. T.; Goodman. D. W. J. Phys. Chem. 1988,92,2569. (12) Christmann, K.;Ertl, G.;Shimizu, H. J . Coral. 1980, 61, 397. (13) Yates, J. T.; Peden, C. H. F.; Goodman, D. W. J . Carol. 1985, 94, 576. (14) Houston, J. E.; Peden, C. H. F.; Blair, D. S.;Goodman, D. W. Surf. Sci. 1986, 167, 427. (15) Houston, J. E.; Peden, C. H. F.; Feibelman, P. J.; Hamann, D. R. Phys. Rev. Lerr. 1986, 56, 375. (16) Lin, J. C.; Shamir, N.; Gomer, R. Sur/. Sci. 1990, 226, 26. (17) (a) Clarke, J. K. A. Chem. Rev. 1975, 75,291. (b) Ponec, V. A d a Caral. 1983, 32, 149. (c) Sinfelt, J. H.; Via, G.H.; Lytle, F. W. Carol. Rev.-Sci. Eng. 1984, 26, 81. (18) Sinfelt, J. H. Bimetallic Caralysrs;Wiley: New York, 1983. (19) Rodriguez, J. A.; Goodman, D. W. J . Phys. Chem. 1990,94,5342. (20) (a) Sachtler, J. W. A.; Somorjai, G. A. J . Carol. 1983,81, 77. (b) Godbey, D. J.; Garin, F.; Somorjai, G.A. J . Carol. 1989, 117, 144. (21) (a) Peden, C. H. F.; Goodman, D. W. J . Coral. 1987, 104, 347. (b) Peden, C. H. F.; Goodman, D. W. Ind. Eng. Chem. Fundam. 1986,25, 58. (22) Rodriguez, J. A,; Goodman, D. W . In New Trends in CO Activarion; Guczi, L., Ed.; Elsevier: Amsterdam, 1991; Chapter 111. (23) Gurney, B. A.; Richter, L. J.; Villarubia, J. S.;Ho,W. J. Chem. Phys. 1987, 87, 6710.
0 199 1 American Chemical Society
2478 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
Rodriguez et al.
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Figure 2. Cu(2p3 ,)
XPS binding energy of Cu/Rh(100) as a function of Cu coverage (hm thickness).
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BINDING ENERGY, eV
Figure 1. Effect of Cu coverage on the Cu(2p3/,) XPS spectra of Cu/
Rh(100). The relative peak intensities can be obtained by multiplying each spectrum by the corresponding scaling factor shown in the left side of the figure. The Cu was deposited at a sample temperature of -350 K. After Cu deposition the sample was heated briefly to 500 K to desorb any accumulated CO. and long-range order of the surfaces were verified by means of A S , XPS, and LEED. Cu was vapor-deposited onto the clean tungsten Rh( 100) and Ru(0001) surfaces by heating a 0.20." filament wrapped with a 0.10-mm high-purity Cu wire housed in a collimating shroud. The Cu doser was outgassed thoroughly prior to Cu deposition. Copper was deposited only onto the front face of the crystals, usually at a sample temperature of -350 K. After dosing Cu, the Rh(100) and Ru(0001) surfaces were annealed to 500 K to desorb any accumulated CO, and no impurities were detected by AES or XPS. The Cu coverages were determined by TPD area a n a l y ~ i s .In~ this ~ ~ work, ~ ~ ~ adsorbate ~~ coverages are reported with respect to the number of substrate surface atoms (Rh( 100) = 1.38 X 1OIs atoms/cm2 and Ru(0001) = 1.64 X 1OIs atoms/cm2). One Cu adatom per substrate surface atom corresponds to Ocu = 1 ML. The O(ls), Cu(2p), Rh(3d), and Ru(3d) spectra of section I11 were recorded with AI Ka radiation. The variations in the binding energies of the O(1s) and Cu( 2p) XPS regions were determined by referencing against a core level of the substrate (Ru(3dsl2) peak for Ru(0001) and Rh(3dS,,) for Rh(100)). Detection was normal to the surface in AES and XPS. 111. Results 1 . Cu on Rh(100). Figure 1 shows representative spectra of the Cu(2p3/,) XPS region for various coverages of Cu on Rh( 100). All spectra were taken after vapor depositing Cu on the clean Rh(100) surface at -350 K and flashing the sample to 500 K. Cu coverages were determined by using TPD area analysis.1° For adsorbate coverages in excess of a monolayer, the XPS spectra represent a product of the combination of electrons emitted from subsurface and surface Cu atoms. The Cu(2p3/,) peak position was monitored by referring to the Rh(3d5,,) peak of Rh(100) and has an experimental error of f0.03 eV. In Figure 1 the Cu(2p3/,) peak shifts from 932.3 eV for the 0.35- and 0.86-ML coverages to a binding energy of 932.9 eV at 9 ML. The effect of Cu coverage (film thickness) on the Cu(2p3/,) peak position is illustrated in Figure 2. Coverages of less than 1 ML show no shift in peak position from 932.3 eV. This phenomenon can be attributed to the formation of 2D islands of Cu on the Rh( 100) surface. A similar behavior has been observed for submonolayer coverages of Cu on Ru(OOO~).~* For coverages larger than 1 ML, there is a monotonic increase in the Cu(2p3/,) peak position up to a value of 932.9 eV at -5 ML. Coverages
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ANNEALINQ TEMPERATURE, K Figure 3. Variation of the C U ( ~ P ~ XPS , ~ peak ) position and Cu(2p)/
Rh(3d) XPS intensity ratio with annealing temperature for four Cu coverages on Rh(lO0): 1.05, 1.70, 5 , and 15 ML. greater than -5 ML show no further change in peak position, a result that indicates the short-range nature of the electronic interactions between copper and rhodium. In Figure 2 the electronic perturbation induced by the Rh( 100) substrate on the second adlayer of Cu atoms is much weaker than that seen for the first monolayer. The Rh(3d) region of the Cu/Rh(100) system was also monitored for various Cu coverages (not shown) with no change in line shape or in the separation between the Rh(3d5/,) peaks. Only a decrease in intensity with increasing Cu coverage was observed. In Figure 3 the binding energy of the Cu(2p3/,) peak position along with the Cu(2p)/Rh(3d) intensity ratio is plotted as a function of annealing temperature for four different Cu films (ecu = 1.05, 1.70, 5, and 15 ML). For these experiments, the Cu films were vapor deposited at a crystal temperature of -300 K, and the spectra were taken after annealing the sample at the specified temperature for 2 min. For OCu 5 1.7 ML an increase in annealing temperature from 300 to 900 K does not produce any significant change in the Cu(2p)/Rh(3d) XPS intensity ratio. At these coverages a small decrease in the Cu(2p3/,) binding energy is observed upon annealing the sample to temperatures above 600 K, with the C U ( ~ P , /peak ~ ) always at a binding energy equal or higher than that seen in Figure 2 for 1 ML of Cu on Rh(100). Annealing films with Cu coverages of 5 and 15 ML from 500 to 900 K produces an appreciable decrease in the Cu(2p3,,) peak position and in the Cu(2p)/Rh(3d) XPS intensity ratio. The Cu(2p3/,) peak positions following the 900 K anneal for 5 and 15 ML of Cu are very close to the corresponding value for two unannealed monolayers of Cu (see Figure 2). The reduction in Cu(2p)/Rh(3d) XPS intensity ratio and the shift in the Cu(2p) binding energy observed in Figure 3 upon annealing 5 and 15 ML of Cu can be attributed to the formation of 3D islands of Cu
Interaction of Cu with Rh(100) and Ru(0001)
I O ( l d XPS
The Journal of Physical Chemistry, Vol. 95, No. 6,I991 2479 I
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Figure 4. O(Is) XPS spectra for CO adsorbed on Cu/Rh( 100) surfaces: (a) clean Rh(100), (b) Rh(100), (c) 0.5 ML of Cu on Rh(100), (d) 1.3 ML of Cu on Rh(100), and (e) 1.9 ML of Cu on Rh(100). The relative peak intensities can be obtained by multiplying each spectrum by the corresponding scaling factor shown in the left side of the figure. The Cu films were vapor-deposited at a crystal temperature of -350 K and annealed to 500 K before dosing CO. The spectra were taken after dosing 30 langmuirs of CO onto the surfaces at 1 IO K.
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Cu(2pm) XPS CO/Cu/Rh(lOO)
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BINDING ENERGY, eV
Figure 6. Cu(2p3/,) XPS spectra of Cu/Ru (0001) as a function of Cu
coverage (film thickness). The relative peak intensitiescan be obtained by multiplying each spectrum by the corresponding scaling factor shown
in the left side of the figure. The Cu was vapor-deposited at a crystal temperature of -350 K. The surfaces were annealed to 500 K before taking the spectra.
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Figure 7. Variation in the CU(~P,,~) XPS peak position of Cu/Ru(0001) surfaces with Cu coverage.
939
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BINDING ENERGY, eV
Figure 5. Effect of CO adsorption on the Cu(2p3,,) spectra of Cu/Rh(100) surfaces. The relative peak intensities can be obtained by multiplying each spectrum by the corresponding scaling factor shown in the right side of the figure. The Cu adlayers were deposited at -350 K and annealed to 500 K before dosing 30 langmuirs of CO at 110 K.
-
(cluster crystallites), which leave a large portion of the second monolayer e ~ p o s e d . ' ~ A series of O(1s) XPS spectra for C O adsorbed on Rh( 100) and on Cu/Rh(100) ( 0 ~ "= 0.5, 1.3, and 1.9 ML) is shown in Figure 4. The spectra were taken after dosing 30 langmuirs of CO onto the surfaces a t 110 K. The Cu adlayers were vapor-deposited on the clean Rh(100) crystal a t -350 K and annealed to 500 K before dosing CO. For Rh( 100) the large CO exposure used in our experiments is enough to saturate the surface (Oca = -0.75), producing CO molecules adsorbed on a-top and bridge sites of the ~ u b s t r a t e . ~ Results ~ J ~ of infrared reflection absorption spectroscopy (IRAS) for adsorption of C O on Cu/ Rh( 100) surfaces with submonolayer coverages of CuZ5show bonding of the CO molecules to Rh and Cu a-top sites and to Rh bridge sites. In Figure 4 the O(1s) features for C O on clean Rh(100) appear at lower binding energy (-2 eV) than those for
-
(24) Leung. L.-W. H.; He, J.-W.; Goodman, D. W.J. Chem. Phys. 1990, 93, 8328. (25) He, J.-W.; Kuhn. W. K.; Leung, L.-W. H.; Goodman, D. W. J . Chem. Phys. 1990,93,7463.
adsorption of the molecule on surfaces covered with 1.3 and 1.9 ML of Cu. In the case of C O on Cua5/Rh(100), the O(1s) spectrum is characterized by the presence of broad features in a large range of binding energies. This spectrum is the product of a combination of electrons emitted from CO bonded to free regions of Rh( 100) and from CO adsorbed on 2D islands of Cu. Figure 5 illustrates the effect of CO adsorption on the Cu(2p3/,) binding energy of Cu layers supported on Rh( 100). The formation of Cu-CO bonds induces a shift of 0.7 eV toward higher binding energy in the Cu(2p3/,) peak position of 0.5 ML of Cu on Rh(100). For Cu coverages of less than 1 M L the CO-induced shift in C U ( ~ P ~peak / ~ )position was always the same: 0.7 eV. This phenomenon is a consequence of the formation of 2D islands of Cu on the Rh( 100) surface. For Cu coverages larger than 1 ML, the magnitude of the CO-induced shift decreased with increasing Cu coverage: 0.5 eV for 1.3 ML, 0.3 eV for 1.9 ML, and 0.1 eV for 20 ML. The effects of C O adsorption upon the Cu(2p3/,) binding energy of the Cu films were reversible. Annealing the CO/Cusl/Rh(lOO) systems to 500 K produced desorfition of CO and a Cu(2p3 *) binding energy of 932.3 eV. 2. Cu on du(OO01). Figures 6 and 7 show the effect of Cu coverage (determined by TPD area analysisI3J4) on the Cu(2p3 2) XPS spectra of ultrathin films of Cu on Ru(0001). The 6 u adlayers were prepared following the same procedure as for Cu/Rh(100) (see above). The binding energy of the Cu(2p) region was determined by referencing against the Ru(3dSIz) peak for Ru(0001). The formation of 2D islands of Cu leads to a constant Cu(2pJI2)binding energy for Cu coverages below 1 ML. In Figure 7 the Cu(2p3 ,) peak position varies from 932.6 eV a t OCu 5 1 ML to 932.9 e\/ for Cu coverages in excess of 5 ML. The total shift in CU(ZP~,~) binding energy is 0.3 eV. This shift is smaller than the shift of 0.6 eV seen for Cu/Rh(100). The electronic perturbation induced by the Ru(0001) substrate on the second layer of Cu atoms (see Figure 7) is much weaker than that
2480 The Journal of Physical Chemistry, Vol. 95, No. 6,1991
Rodriguez et al. CU(2PW2) XPS COlCulRu(0001)/p',
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BINDING ENERGY, eV CO/CU/FtU(0001)
Figure 10. Effect of CO adsorption on the Cu(2p3,,) spectra of Cu/ Ru(0001) surfaces. The relative peak intensities can be obtained by multiplying each spectrum by the corresponding scaling factor shown in the right side of the figure. The Cu films were deposited at -350 K and annealed to 500 K before dosing 30 langmuirs of CO at 110 K.
-
are very close to the value of 932.75 eV reported in Figure 7 for two unannealed monolayers of Cu. Figure 9 displays O(1s) spectra taken after dosing 30 langmuirs of C O onto clean Ru(0001) and Cu/Ru(0001) surfaces at 110 K. This large gas exposure produces a CO saturation coverage of -0.68 admolecule/Ru surface atom on clean Ru(0001).26 Results of IRAS for CO/Cu( 111),27 CO/Ru(0001),t6j28 and CO/Cu/Ru(0001)** show adsorption of the CO molecules on Ru and Cu a-top sites. The O(1s) spectrum for CO on Ru(0001) appears at 1 eV lower binding energy than that for adsorption of the molecule on a surface covered by 1 ML of Cu. A similar trend was observed for CO adsorbed on Cu/Rh( 100) surfaces (see above). Under the experimental conditions of Figure 9 (Cu deposition at a sample temperature of -350 K, annealing to 500 K before dosing CO), Cu films with submonolayer coverages form pseudomorphic 2D islands on the Ru(0001) surface.I4 The spectrum observed for CO/Cuo,6/Ru(0001)is a combination of CO adsorbed on free domains of Ru(0001) and CO bonded to 2D islands of C U . ~For~ adsorption of C O on Cu,,o/Ru(OOO1) and Cu1.6/Ru(0001), the O(1s) spectra are very broad, showing unresolved satellites that extend to higher binding energy than the main peaks. A very similar broadening has been seen in the O(1s) XPS spectra of C O adsorbed on polycrystalline Cu29and Cu( 1 In those cases the features toward high binding energy in the spectra have been attributed to strong shakeup transition~.~~.~~ Figure 10 shows Cu(2pyZ) spectra taken before and after dosing C O onto Cu/Ru(0001) surfaces (Ocu = 0.6, 1.0, and 1.6 ML). Adsorption of CO on Cu films with dCu I1 ML produced a shift of 0.5 eV toward higher binding energy in the Cu(Zp,,,) peak position. The CO-induced shift in binding energy was somewhat smaller than that seen for CO/Cu
