Nickel Ultrathin Bimetallic Overlayers on Yttria-Stabilized ZrO2

by X-ray and UV photoelectron spectroscopy (XPS, UPS) at 300 and 570 K and by temperature-programmed desorption of CO (TPD). Photoemission results ...
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J. Phys. Chem. B 2002, 106, 41-48

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Gold/Nickel Ultrathin Bimetallic Overlayers on Yttria-Stabilized ZrO2 (100) S. Zafeiratos and S. Kennou* Department of Chemical Engineering, UniVersity of Patras and ICEHT-FORTH, P.O. Box 1414, 26 500 Rion, Patras, Greece ReceiVed: June 27, 2001; In Final Form: October 3, 2001

Ultrathin bimetallic Au/Ni films vapor-deposited on yttria-stabilized ZrO2 (100) (YSZ) were characterized by X-ray and UV photoelectron spectroscopy (XPS, UPS) at 300 and 570 K and by temperature-programmed desorption of CO (TPD). Photoemission results indicate that, upon co-deposition of the two metals, the Ni signal intensity is significantly lower than that from pure Ni/YSZ, whereas the corresponding Au signal intensity is practically unaffected by the presence of Ni. The binding energies of the Ni XPS peaks were not affected by co-deposited Au, for Au/Ni atomic ratios close to unity, however, the XPS Au 4f levels, were shifted by 0.2 eV toward lower binding energy with respect to the corresponding values of Au/YSZ for very small Au/Ni atomic ratios in the bimetallic film. Carbon monoxide adsorption/desorption was used as a probe of the reactivity of Ni surface atoms, modified by co-deposited Au. Strong attenuation of the overall TPD curves was observed in all cases, accompanied by a shift of the main peak to lower temperature for very small Au/Ni atomic ratios in the bimetallic film. The combination of the spectroscopic and CO desorption results can be understood in terms of a simple picture, in which Au atoms always segregate on the surface, thus leading to complete covering of the Ni clusters for large Au/Ni atomic ratios and to a partial intermixing of Au and Ni atoms in the outermost surface layer for small Au/Ni atomic ratios.

1. Introduction Bimetallic systems are relevant to the design of new materials, with industrial applications in the areas of heterogeneous catalysis, electrochemistry, and microelectronics.1,2 Despite their significance, only few reports have been devoted to bimetallic supported systems deposited under UHV conditions (for a review see ref 3). These studies were concerned primarily with the effect of gas treatment to bimetallic deposit morphology and less with the nucleation and electronic configuration of the deposit, although the latter can in principle affect the physical and chemical properties of bimetallic systems.3,4 In addition, metals on oxides have been shown in a number of cases to have quite different adsorption or reaction properties than the metal itself.4,5 This justifies the systematic investigation of bimetallic overlayers grown by vacuum condensation on well-defined oxidic supports. Among the bimetallic systems of special interest are those whose components do not form alloys or metallic compounds in the bulk but might interact when one is deposited on the surface of the other, like Au and Ni. Recent scanning tunneling microscopy (STM) experiments and total-energy calculations have revealed that both Au/Ni (110)6,7 and Au/Ni (111)8 can form a two-dimensional surface alloy, below a certain Au coverage and at a certain temperature range, despite the immiscibility gap in the bulk phase diagram of Au and Ni. Photoemission spectroscopy evidence for surface alloying has been also reported for submonolayers of Au/Ni (s) [5(001) × (111)].9 In the Ni-rich end of the Au-Ni system, where the immiscibility is less extreme, Ni overlayers have been shown by photoemission to intermix near the surface region of polycrystalline gold.10 * Corresponding author. FAX: +3061 993 255. Tel.: +3061 996 324. E-mail: [email protected].

The properties of the Au/Ni bimetallic system in lowdimensional structures such as composite overlayers and nanoparticles can be investigated when the two metals coexist on an well-defined oxidic surface, such as single-crystal yttria stabilized zirconia. Although the effect of a more inert metal, like Au, on the properties of an active metal, like Ni, is of an obvious interest in heterogeneous catalysis, no model studies on supported bimetallic Au/Ni systems could be found in the literature. Recent photoemission studies of Ni on polycrystalline YSZ at 300 K have shown that the admetal grows forming 3-D clusters at coverages above 0.5 ML, with a 2-D growth at smaller coverages and a weak Ni/ substrate interaction.11 Recent results for the deposition of Au on YSZ (100), reveal a VolmerWeber (3-D) growth mode and a gradual development of the metallic character of the gold overlayer, with increasing cluster size.12 This work reports on the geometric arrangement and the electronic structure of Au-Ni ultrathin films grown on yttriastabilized ZrO2 single crystals, studied by X-ray and UV photoemission spectroscopy (XPS/UPS) and temperatureprogrammed desorption (TPD) with CO as the probe molecule. The main objective was to investigate the possibility of alloying between the two elements at the nanoparticle scale. The interaction between Au and Ni in various proportions was studied from submonolayer to multilayer coverages, using the Au 4f and Ni 2p core-level binding energy shifts, changes of spectral intensities, as well as modifications in the valence band region. The TPD spectra provide a very sensitive tool for investigating both the composition of the outermost surface layer and possible ligand effects in CO chemisorption. 2. Experimental Section The experiments were performed in an ultrahigh-vacuum (UHV) chamber (base pressure < 5 × 10-10 mbar) equipped

10.1021/jp0124591 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/06/2001

42 J. Phys. Chem. B, Vol. 106, No. 1, 2002 with a hemispherical electron energy analyzer (Leybold LHS12) for X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS) and a differentially pumped quadrupole mass spectrometer (Balzers QMS 421) for thermal desorption measurements. Unmonochromatized MgKR (1253.6 eV) radiation from a twinanode X-ray source (VSW, TA10) and He II (40.8 eV) radiation from a gas discharge lamp (Specs GmbH, UVS 10/35) were employed. Electron detection was perpendicular to the surface, and for a clean reference gold foil, the Au 4f7/2 binding energy (EB) and full-width at half-maximum (fwhm) were 84.05 and 1.55 eV, respectively, with 100 eV pass energy. Temperatureprogrammed desorption (TPD) was carried out after exposing the sample to saturation exposures of CO (purity 99.999%) at 273 K. The mass spectrometer had a stainless steel snout, placed less than 1 mm from the sample surface to reduce interference from the heating filament and other unwanted hot sources. The sample temperature was monitored with a thermocouple pressed onto the surface of the Ta sample holder plate. The reading of this thermocouple was calibrated to give a better estimate of the true surface temperature of the single-crystal slab in a separate experiment using a second thermocouple attached to the slab surface by conductive silicone paste. All temperatures, including the TPD linear heating rate of 0.7 Ks -1, are calibrated surface temperatures. The substrates were 12 × 12 × 0.5 mm3 slabs of ZrO2 (100) single crystals stabilized with 9% mol Y2O3 (MaTecK, Germany). The slabs were mounted on a flat tantalum plate, in contact with a resistive heater and a liquid nitrogen cooled copper braid. Prior to metal deposition, the sample surface was cleaned in UHV by Ar+ bombardment (2 keV, 1.5 µA) and annealing cycles (700-800 K for 10 min) to produce an impurity-free surface, as judged by XPS and UPS. This procedure has been found for YSZ to give a well-ordered and stoichiometric surface.13,14 Nickel was deposited by electron beam evaporation from a high purity (99.99%) wire, whereas for Au, a source consisting of a resistively heated tungsten filament wetted with spec-pure (99.999%) Au was used. The deposition rate of the two evaporation sources was kept constant in all cases and was on the order of 1 × 1013 atoms cm-2 s-1, as estimated by XPS in separate evaporation experiments.11,12 Metal coverage is expressed in equivalent monolayers (ML), where 1 ML roughly corresponds to 1.5 × 1015 atoms cm-2 of Au and 2 × 1015 atoms cm-2 of Ni. The slabs exhibited electrostatic charging during room temperature (RT) photoemission measurements. This effect declined with increasing temperature, as the ionic conductivity of YSZ rose sharply. The binding energy scale was always referenced to the Zr 3d5/2 core level at 182.6 ( 0.1 eV for clean YSZ,15 thus assuming negligible chemical interaction between the Zr cations and the metal deposits. This assumption has been used previously in the investigation of separate Au and Ni deposition on YSZ and is supported by the invariance of the Zr 3d peak shape during deposition.11,12 3. Results 3.1. XPS Studies. Various coverages of Au and Ni were deposited on YSZ (100). The chemical state and the Au/Ni arrangement in the deposits were investigated by comparing gold and nickel spectra after co-deposition with those obtained following individual metal depositions under identical experimental conditions. A range of different quantities and relative proportions were examined in order to search for coverage dependent interactions between the two metals. Figure 1 A,B shows Au 4f and Ni 2p XP spectra after simultaneous deposition of 5 ML Au and 8 ML Ni on YSZ at RT

Zafeiratos and Kennou

Figure 1. (A) Gold 4f XP spectra for 5ML Au/YSZ at 300 K (a), 5ML Au-8ML Ni/YSZ as deposited at 300 K (b), and same as panel b, annealed at 570 K for 10 min (c). (B) Ni 2p XP spectra for 8ML Ni/YSZ at 300 K (a), 5ML Au-8ML Ni/YSZ as deposited at 300 K (b), and same as panel b, annealed at 570 K for 10 min (c).

(spectrum b) and following annealing of the sample at 570 K for 10 min (spectrum c). Spectra a in Figure 1 are reference spectra measured after depositing at RT the same amount (5 ML Au and 8 ML Ni) of each metal separately. The Au 4f doublet (Figure 1A) after co-deposition at 300K does not exhibit detectable differences either in binding energy (uncertainty (0.05 eV) or intensity (uncertainty (10%), compared with the reference spectrum. The same is true after annealing, whereby a ∼15% decrease in intensity is expected, when Au is individually deposited at 300 K on the same substrate and then annealed under the same conditions.12 The binding energies of the Ni 2p doublet, as well as the position of the characteristic satellites at higher binding energies, are unaffected by Au (Figure 1B). On the other hand, upon Au co-deposition, a Ni 2p intensity attenuation by about 30% is observed, with respect to the reference spectrum. Upon annealing, an additional 30% attenuation is observed, comparable with an expected decrease of the Ni 2p XPS peak area by about 25%, when Ni is individually deposited at 300 K on polycrystalline YSZ and then annealed under the same conditions.11 The main Au and Ni photoemission spectral features from Figure 1 are summarized in Table 1, where there are also included, for comparison, the values measured on a clean Au polycrystalline foil and a Ni single crystal in the same apparatus. The chemical state of Au and Ni in the multilayer bimetallic deposit is, within experimental uncertainty, identical to the corresponding bulk metals. This is further confirmed by the measured values of the Auger parameter (R), which is defined as the sum of the main photoelectron peak and the strongest X-ray-induced Auger peak and which is independent of substrate charging and a very sensitive probe of the physical and chemical environment of the analyzed atoms.16 The Auger parameter values of 2099.65 eV (from Au 4f 7/2 and Au M5N67N67) and 1698.7 eV (from Ni 2p 3/2 and Ni L3VV), measured upon codeposition and after annealing, were the same as those measured for the bulk metals. The XPS intensities of the stronger substrate peaks, Zr 3d and O 1s (not shown), are still significant following deposition of 5 ML Au and 8 ML Ni, indicating a particulate form of the deposit and increase by about 50% after annealing. This is due to shrinking and coalescence of the deposit, induced by the increased mobility of the metal atoms upon heating and leads to uncovering part of the substrate. The binding energies and widths of the substrate peaks were unaffected by metal deposition and annealing, consistent with the absence of interaction

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J. Phys. Chem. B, Vol. 106, No. 1, 2002 43

TABLE 1: XPS Core Level Positions, Relative Intensities, and FWHM of Au 4f and Ni 2p Peaks for 5ML Au-8ML Ni/YSZa

a

coverage/ML

EBb/eV Au 4f7/2

intensity/au Au 4f

fwhm/eV Au 4f7/2

EBb/eV Ni 2p3/2

intensity/au Ni 2p

fwhm/eV Ni 2p3/2

5 Au/8 Ni 5 Au/8 Ni annealed 570 K 5 Au 8 Ni metal reference sample

84.05 84.05 84.05 84.05

1.0 1.0 1.0 -

1.60 1.55 1.55 1.55

852.6 852.7 852.7 852.7

0.7 0.4 1.0 -

2.3 2.4 2.3 2.2

Data from Au/YSZ, Ni/YSZ, and metallic foils are included as reference. b Experimental uncertainty, (0.05 eV

Figure 2. Gold 4f XP spectra for 0.4ML Au/YSZ at 300 K (a), 0.4ML Au-0.3ML Ni/YSZ as deposited at 300 K (b), 0.2ML Au-8ML Ni /YSZ as deposited at 300 K (c), and same as panel c, annealed at 570 K for 10 min (d).

of the deposit with the YSZ. The invariance of the substrate chemical state was observed in all experiments reported in this work. With the exception of relative peak intensities, the results presented above, for multilayer quantities of co-deposited Au and Ni with an approximate atomic ratio of 0.6, indicate that the two metals exhibit the same XPS features, as if they were deposited separately. A similar picture is obtained for submonolayer quantities of co-deposited Au and Ni with an atomic ratio close to unity, whereas changes are observed in the region of very small Au-to-Ni ratios, which is more interesting for heterogeneous catalysis. Figure 2 shows Au 4f XP spectra after simultaneous deposition of 0.4 ML Au and 0.3 ML Ni on YSZ at RT (spectrum b) and after simultaneous deposition of 0.2 ML Au and 8 ML Ni on YSZ at RT (spectrum c) and following annealing at 570 K for 10 min (spectrum d). The reference spectrum (a) in Figure 2 is measured after depositing 0.4 ML Au alone on YSZ at RT. The Au 4f7/2 peak in spectrum a exhibits an EB and a fwhm of 84.4 and 1.9 eV, respectively, compared with the metallic values of 84.05 and 1.55 eV. This is a general trend for core level binding energies and peak widths of particulate metal deposits with very small particle size on relatively inert substrates and has been attributed to initial and final state effects, as well as cluster size nonuniformity.17 The core-level shifts could also be affected by slight variations in differential electrostatic charging induced by deposition of the metal. The EB for Au 4f7/2 in spectrum b (Figure 2) is also 84.4 eV, and the peak height is smaller; however, the fwhm of the peak is increased by about 0.3 eV, and hence the area under the peaks remains the same, within an experimental error of (10%, as in Figure 1a. However, upon co-deposition of 0.2 ML Au with 8 ML Ni (Au/Ni atomic ratio less than 0.02), the Au 4f peak (spectrum c) is shifted by 0.2 eV toward lower binding energies without any width change with respect to spectrum a. This shift, which persists after annealing at 570 K for 10 min (spectrum d), cannot be attributed to the smaller Au quantity or to changes in the dispersion of Au, since lower Au coverages are expected to give higher binding energies and larger widths,12 which is in

Figure 3. (A) Variation with Au coverage of the Ni 2p (b) and Au 4f (O) XPS peak intensities (normalized to the corresponding values for 8 ML Ni and 1.9 ML Au individually deposited on YSZ) for up to 1.9 ML Au gradually deposited on YSZ preexposed to 8 ML Ni at 570 K. (B) Variation with Ni coverage of the Ni 2p (b) and Au 4f (O) XPS peak intensities (normalized to the corresponding values for 8 ML Ni and 3 ML Au individually deposited on YSZ) for up to 8 ML Ni gradually deposited on YSZ preexposed to 3 ML Au, at 570 K.

the opposite direction to that observed here. In addition, the decreased dispersion of Au upon annealing (spectrum d) does not affect the shift. The direction and magnitude of this shift are similar to those previously observed for small Au coverages (e0.5 ML) deposited onto a stepped Ni surface and attributed to surface alloying.9 The Ni 2p XP spectra (not shown) corresponding to the Au 4f spectra in Figure 2 do not exhibit any additional features. The behavior upon co-deposition of 0.2 ML Au with 8 ML Ni is similar to that in Figure 1b, with the Ni signal attenuation upon co-deposition at RT with respect to the 8 ML reference sample being about 15% (instead of 30%), due to the lower Au coverage. Upon co-deposition of 0.4 ML Au with 0.3 ML Ni, both position and intensity of the Ni 2p peak do not change, within experimental error, with respect to the 0.3 ML reference peak. The latter is also shifted to higher EB (Ni 2p3/2 at 853.3 eV compared to 852.7 eV for metallic Ni) due to particle size effects.17 The behavior of Ni and Au XPS intensities upon successive depositions of one metal on pre-deposited quantities of the other, shown in Figure 3 A,B, can yield information about the mutual arrangement of the metals on the YSZ substrate. In Figure 3A, up to 1.9 ML Au were gradually deposited at 570 K onto YSZ

44 J. Phys. Chem. B, Vol. 106, No. 1, 2002

Figure 4. (A) Helium II UP valence band spectra at 570 K for 5ML Au/YSZ (a), 5ML Au-8ML Ni/YSZ (b), and 8ML Ni /YSZ (c). (B) Helium II UP valence band spectra at 570 K for 0.4ML Au/YSZ (a), 0.4ML Au-0.3ML Ni /YSZ (b), and 0.3ML Ni /YSZ (c).

with 8 ML pre-deposited Ni. Prior to Au deposition, the sample was annealed at 570 K for 1 h in order to stabilize the Ni deposit dispersion.11 The Ni 2p and Au 4f XPS intensities in Figure 3A were normalized, respectively, against those of 8 ML Ni and 1.9 ML Au, separately deposited on YSZ under the same conditions. The normalized Au 4f peak intensity is gradually increasing toward unity, whereas the normalized Ni 2p intensity decreases gradually to 0.6, that is, to 60% of its initial value. In Figure 3B, the two metals were deposited onto the substrate in reverse order; that is, up to 8 ML Ni were gradually deposited at 570 K onto YSZ with 3 ML pre-deposited Au. The Ni 2p and Au 4f XPS intensities in Figure 3B were normalized, respectively, against those of 8 ML Ni and 3 ML Au, separately deposited on YSZ under the same conditions. The normalized Ni 2p intensity increases gradually; however, its final value is only 0.6, whereas the normalized Au 4f intensity remains almost constant near unity. 3.2. UPS Studies. At 300 K, UPS measurements exhibited strong differential charging problems due to the low conductivity (electronic and ionic) of YSZ. This problem did not arise in XPS, probably because of stray electrons produced mainly by the X-ray source.17 At 570 K, the ionic conductivity of the substrate increases significantly suppressing electrostatic charging problems; thus, the UP spectra presented here were recorded at 570 K. Helium II valence band spectra for Au/Ni co-deposited on YSZ are shown in Figure 4A,B (curves b) for multilayer (5 ML Au/8 ML Ni) and submonolayer (0.4 ML Au/0.3 ML Ni) coverages. The reference spectra for pure Au/ YSZ and pure Ni/YSZ, with the same metal coverage as that for the corresponding curve (b), are shown at the bottom (curve a) and the top (curve c) of each figure, respectively. The Ni-Au/YSZ valence band is characterized by a large density of states (DOS) immediately below the Fermi level primarily due to Ni 3d states.18 The region between 1.5 and 9 eV, is dominated either by the Au 5d electronic states,19 for large metal coverage (Figure 4A), or by the Zr 4d and O 2p orbitals from YSZ,20 for small metal coverage (Figure 4B). Spectrum a in Figure 4B is practically identical, except for a slight broadening in the region indicated by the arrow, to that for clean YSZ. Comparing spectrum b with a and c in Figure 4A shows that the width and the centroid of the Au 5d band, as well as the position of the Ni 3d states, are not modified upon co-deposition. However, there is a ∼50% attenuation of the Ni 3d intensity at the Fermi edge, consistent with the XPS data in Figure 1b. At low metal

Zafeiratos and Kennou

Figure 5. Difference valence band spectra (normalized to the same height) for various Au amounts co-deposited with Ni at 570 K, keeping the Au/Ni ratio near unity ((20%). The curves were obtained by separately subtracting from the overall spectrum properly scaled spectra of clean metallic Ni and clean YSZ. A UP spectrum from a reference Au foil, normalized to the same height, is also shown for comparison.

coverages (Figure 4B), the valence band is dominated by the YSZ features; however, there is some Ni 3d intensity at the Fermi edge and a weak broadening indicated by an arrow, which is produced by the Au 5d states. Despite the low intensity, it is evident that the Ni 3d intensity is suppressed by about 40%, compared with the Ni/YSZ reference (curve c). It should be noted that the YSZ features are present in the valence band region even at high coverages, confirming the particulate nature of the bimetallic deposits. The Au 5d3/2 and Au 5d5/2 doublet structure evident at 3.7 and 6 eV in Figure 4A arises from the combined effects of spinorbit interaction and band formation. The doublet splitting (∆) is sensitive to the average coordination of Au atoms and can yield information about possible structural changes and Au/Ni interactions.19,21 The overlapping of nickel, gold, and zirconia states in the valence band region complicates the measurement of ∆; hence, it is necessary to take difference curves by separately subtracting the Ni (using a reference Ni sample) and the YSZ contribution to the measured valence band spectra. Figure 5 shows a set of such difference curves for various Au quantities co-deposited with roughly equal amounts of Ni on YSZ. The removal of the Ni 3d contribution is more reliable, since the main part of this peak does not interfere with Au or YSZ states. However, the removal of the YSZ intensity is complicated due to its overlap with the Au 5d states. Thus, it was assumed that the high binding energy slope of the Au 5d 3/2 component is always identical in shape with that for a metallic Au foil reference UP spectrum obtained under the same experimental conditions. This method was chosen as the most appropriate, bearing in mind that changes of the Au chemical state does not greatly affect the shape and position of the Au 5d3/2 component.22 The intensity of the curves in Figure 5 has been normalized to that of the reference Au foil in order to depict clearly changes in Au 5d peak shape and position. As the amount of co-deposited Au increases from 0.5 to 5 ML, the splitting ∆ between the two 5d components increases from about 2.3 eV to 2.65 eV, which is close to the value 2.75 eV obtained for the Au foil. The binding energy of Au 5d3/2 remains practically the same, although that of Au 5d5/2 gradually shifts downward, leading to a 5d band broadening and a centroid shift toward the Fermi edge. Recent UPS results for Au/YSZ (100)

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J. Phys. Chem. B, Vol. 106, No. 1, 2002 45

Figure 6. Carbon monoxide TPD spectra, following 4 L CO exposure at 273 K, from 8ML Ni/YSZ annealed at 570 K for 60 min and then postdeposited with increasing Au amounts up to 5 ML.

exhibit similar features,12 which have been attributed to the increasing Au atom coordination as the gold particle size in the deposit increases. This similarity is consistent with the XPS observations, whereby for Au/Ni atomic ratios close to unity, the Au 4f binding energy is not affected by the presence of Ni. Unfortunately, we could not obtain reliable valence band difference spectra for lower Au coverages and small Au/Ni atomic ratios, whereby evidence for Au/Ni interaction was observed by XPS. Possible Au/Ni intermixing is expected to make ∆ lower than 2 eV, since its limiting value for isolated Au atoms is about 1.5 eV.9,10 3.3. CO TPD Studies. The temperature-programmed desorption (TPD) of CO chemisorbed at 273 K is a sensitive probe for investigating both the presence and the chemical state of Ni atoms on the outermost surface layer of the bimetallic deposit, since Au and YSZ do not adsorb CO under our experimental conditions. The adsorption of CO on various single crystal Ni surfaces has been intensively studied in the past by many surface science techniques, including TPD.23-28 The TPD spectra from saturation coverage of CO on the three low-index clean Ni single-crystal surfaces, (100), (111), and (110), exhibit a dominant high-temperature peak (457-414 K) and a broad shoulder at lower temperatures (350-330 K), which is usually related to repulsive lateral interactions between admolecules. Vibrational studies show that at saturation coverages, CO adsorbs primarily on bridge and on-top sites. Admolecules at on-top sites are generally more weakly bound, compared with those on bridge sites and desorb at lower temperatures in TPD;23,26 however, the order of occupancy of these sites upon increasing coverage may depend on adsorption temperature.28 Figure 6 shows CO TPD spectra, following 4 L exposure to CO at 273 K (1 L ) 1.33 × 10-6 mbar.s), from a series of Au/Ni/YSZ samples with increasing Au quantities evaporated on 8 ML pre-deposited Ni. Prior to Au deposition, the Ni/ YSZ sample remained at 570 K for 1 h in order to stabilize the Ni deposit. This procedure aimed at preventing morphological changes of the freshly deposited Ni particles stimulated by the first TPD run, thus allowing the changes observed in the CO TPD spectra to be assigned to Au. However, the annealing of the Ni film led to changes in the CO TPD spectra, compared with the freshly deposited Ni at RT; namely, the TPD peak area was attenuated due to particle coalescence, and there was a shift of the peak maximum to lower temperatures (by up to 30 K). These changes do not depend on the presence of Au on the

Figure 7. Carbon monoxide TPD spectra, following 4 L CO exposure at 273 K, from the following: (A) 5ML Au-8ML Ni /YSZ (a), 0.2ML Au-8ML Ni /YSZ (b), and 8ML Ni /YSZ (c) as deposited at 300 K; (B) 5ML Au-8ML Ni/YSZ (a), 0.2ML Au-8ML Ni /YSZ (b), and 8ML Ni/YSZ (c), after the first TPD and annealing at 570 K for 10 min.

surface and are not discussed here. No CO2 desorption was ever detected in the TPD experiments. After the Ni stabilization procedure, CO was found to desorb from Ni/YSZ in two overlapping states, a broad shoulder centered around 375 K (state A) and a main peak at 435 K (state B), with an estimated area ratio A/B ≈ 0.4. These features are similar to those reported before for CO desoption from Ni(111) and Ni (100) single crystals.24,26 Increasing the amount of deposited Au has the following effect on the TPD spectra: (1) the total area under the spectrum gradually decreases and practically disappears (negligible CO adsorption) after 5 ML Au. (2) For 0.25 ML Au, the main peak B shifts to lower temperature by about 25 K, whereas the position of shoulder A does not change appreciably. (3) For Au coverages 0.8 ML and higher, state B is much more quickly attenuated than state A and shifts back to its initial position. Further evidence of the influence of Au is derived from a set of CO TPD experiments in Figure 7A,B, whereby Ni and Au were co-deposited at RT just before the CO exposure, thus skipping the Ni film stabilization procedure. Figure 7A shows CO TPD spectra from 8 ML Ni, co-deposited with 0.2 and 5ML of Au (curves b and c, respectively), following 4 L CO exposure at 273 K immediately after metal deposition. A reference spectrum from 8 ML Ni individually deposited on YSZ is also shown (curve a). Figure 7B shows the same TPD spectra, obtained after the first desorption cycle (Figure 7A) and then annealing of the sample at 570 K for 10 min. The general behavior of states A and B in Figure 7 is similar with those in Figure 6. The total spectrum area is attenuated with increasing amount of Au, with state A practically unaffected in position (375 K) and exhibiting less initial attenuation compared with state B, which practically vanishes for 5 ML Au. State B shifts to lower temperature by 13 K in the presence of 0.2 ML Au (curve b in Figure 7A). The shift becomes 30 K after the short preannealing at 570 K (curve b in Figure 7B), which is comparable to that for 0.25 ML Au in Figure 6. The lower

46 J. Phys. Chem. B, Vol. 106, No. 1, 2002

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Figure 8. Carbon monoxide TPD spectra, following 4 L CO exposure at 273 K, from Ni/YSZ (1) and Au-Ni/YSZ with equal Ni coverage (2) from the following: (A) 3 ML Ni and 1 ML Au as deposited at 300 K; (B) 1.5 ML Ni and 0.5 ML Au as deposited at 300 K; (C) 3 ML Ni and 1 ML Au after the first TPD and annealing at 570 K for 10 min; (D) 1.5 ML Ni and 0.5 ML Au after the first TPD and annealing at 570 K for 10 min.

adsorption energy of CO on Ni in the presence of small amounts of Au, which indicates a ligand effect of Au, is correlated with the observed Au 4f7/2 binding energy downshift by 0.2 eV for 0.2 ML Au co-deposited with 8 ML Ni (Figure 2). Carbon monoxide TPD experiments were also performed using smaller Ni amounts with a Au/Ni atomic ratio of about 0.3, initially co-deposited at RT. No binding energy shifts for Au 4f7/2 were detectable in this case. Figure 8A-D shows results for 3 and 1.5 ML of Ni co-deposited with 1 and 0.5 ML of Au, respectively. After the first TPD cycle (Figure 8A,B, curve 2), the sample was annealed for 10 min at 570 K, and the TPD experiment was repeated (Figure 8C,D, curve 2). The reference TPD curves from equal amounts of Ni individually deposited on YSZ and subsequently treated under the same experimental conditions are included in all cases for comparison (curve 1 in Figure 8A-D). The main observation from Figure 8 is that the temperature downshift of state B becomes negligible as the amount of Ni decreases and that there is considerable attenuation of the spectrum area after sample preannealing, especially of state B (Figure 8C,D). After preannealing, the reference TPD curve 1 is also considerably downshifted (by up to 25 K), indicating changes in the Ni deposit upon heating, which are not related to the presence of Au. 4. Discussion The combination of the photoemission (XPS, UPS) and CO TPD results can yield information on two important aspects of the Au/Ni bimetallic films: (1) the mutual spatial arrangement of Au and Ni on YSZ and (2) the concomitant electronic interactions between the two constituents, especially with respect to their effect on the CO/Ni adsorptive behavior. Both of these

aspects depend on the total loading and relative amounts of Au and Ni and, to some extent, on the metal deposition and subsequent treatment conditions. The total loading varied between 0.7 ML (0.4 ML Au and 0.3 ML Ni) and 13 ML (5 ML Au and 8 ML Ni), whereas emphasis was placed on roughly equiatomic and Ni-rich deposits, down to a Au/Ni atomic ratio of 0.02 (0.2 ML Au and 8 ML Ni). All the experimental evidence suggests a particulate nature for the deposits; however, in the absence of microscopy studies, there is no direct evidence for the size of the deposited particles. On the basis of indirect model calculations, assuming reasonable particle number densities and hemispherical particle shape, it can be estimated that the size ranges from 1 or 2 nm (lowest metal loading) to several nanometers for the highest loading. 4.1. Mutual Spatial Arrangement of Au and Ni on YSZ. The variation of the Au 4f and Ni 2p XPS signal intensities upon co-deposition (Figure 1) and upon successive deposition of one metal on a pre-deposited amount of the other (Figure 3) clearly show that, at least for total loadings above 1 ML, the two metals are not separated on YSZ but are in intimate contact with each other in the form of bimetallic islands or clusters. If this were not the case, the signal intensity of each metal would not be affected by the presence of the other. Even for submonolayer co-deposits, there exists an interaction between the two elements, as evidenced by the increased fwhm of the Au 4f7/2 peak when 0.4 ML Au is co-deposited with 0.3 ML Ni (Figure 2, spectrum b). The above picture is also supported by both the UPS and CO TPD results. The Ni 3d intensity at the Fermi level drops off by 40% upon co-deposition with Au (Figure 4). In addition, the presence of Au changes the CO TPD spectra of Ni/YSZ for all metal coverages studied (figs. 6,7,8).

Overlayers on Yttria-Stabilized ZrO2 (100) The next question is whether Au and Ni forms an extended solid solution (bulk like alloy) on YSZ regardless of the metal coverage. The answer to this is negative on the basis of the following observations: (i) Although the Ni XPS intensity is strongly affected by co-deposited Au, this does not happen to a comparable extent for the Au XPS intensity (figs.1 and 3). (ii) Each of the Au 4f7/2 and Ni 2p3/2 binding energies, for roughly equiatomic bimetallic deposits, is not affected by the presence of the other metal (Figure 1). It has been reported that for a AuNi metastable bulk alloy the Ni 2p3/2 peak is shifted by 0.6 eV to lower binding energy.29 (iii) The valence band Au 5d splitting, ∆, for roughly equiatomic bimetallic deposits, is not affected by the presence of Ni (Figure 5). If an alloy were formed, the dilution of Au by Ni atoms would lead to ∆ values less than 2.10 The absence of extended alloy formation in the case of bimetallic Au/Ni nanoclusters is thus in accordance with the bulk phase diagram where Au and Ni are essentially immiscible near RT, especially in the Ni-rich side. Since Au and Ni do not intermix extensively, the only possible arrangement for comparable amounts of the two metals in a bimetallic aggregate is in the form of phase-separated nanoclusters with the outermost surface layer of the cluster occupied predominantly by Au or Ni. The results in Figure 3 immediately show that the bimetallic clusters must be covered by Au, and this is also supported by the fact that the CO adsorption capacity of the bimetallic deposits practically vanishes for roughly equal amounts of Au and Ni (Figures 6 and 7). No evidence for adsorbed-CO-induced Ni migration toward the surface of the bimetallic particles was observed. Thus, the thermodynamically expected wetting of bulk Ni by Au, due to the lower surface energy of the latter, is also observed in the case of bimetallic nanoclusters. The only remaining question is what happens for small Au/Ni atomic ratios when the amount of Au is not sufficient to cover the Ni cluster completely. In this case, Au could form either an adsorbed overlayer or intermix with the outermost layer of the Ni cluster forming a surface alloy, as already reported for submonolayer amounts of Au on extended Ni single-crystal surfaces.6-9 These two situations are discussed below in terms of the experimental evidence for electronic interaction between Au and Ni and for ligand effects in the CO adsorption on Ni. 4.2. Gold-Nickel Interaction at the Surface of the Bimetallic Nanoclusters. The only detectable spectroscopic evidence for a Au/Ni interaction is shown in Figure 2 for 0.2 ML Au co-deposited with 8 ML Ni, whereby a small but distinct downshift of the Au 4f7/2 binding energy by 0.2 eV is observed. Whether this is due to surface intermixing or it can also originate from Au atoms forming submonolayer islands over Ni is not immediately obvious. This shift is similar to that previously reported for the Au 4f7/2 peak, when less than ∼0.5 ML Au were deposited on a Ni stepped surface.9 In that case, the shift was attributed to a Au-Ni surface alloy, mainly based on the UPS results for the Au 5d band splitting, which, in the present work, could not be measured reliably at RT and for small Au/ Ni ratios. The 0.2 eV downshift in Au 4f7/2 binding energy, when going from 0.4 ML Au on YSZ to 0.2 ML Au on 8 ML Ni/YSZ (Figure 2a,c), could be also the result of differences in final state effects.30 The increased electron density around Au atoms due to the presence of Ni enhances the extra atomic screening of the photoelectron hole and may lead to a decrease in Au 4f binding energy by an unspecified amount. This effect could be in principle observable both for a Au overlayer and an intermixed layer, although one would expect it to be more pronounced in the latter case, due to the closer vicinity of Au

J. Phys. Chem. B, Vol. 106, No. 1, 2002 47 and Ni. Additional information to clarify this question can be obtained from the analysis of the CO TPD results. The presence of Au atoms on the Ni/YSZ surface can alter the CO desorption properties in two basic ways, by modifying the electronic structure of exposed nickel (ligand effect) and by directly reducing the number of Ni surface sites of a given type (ensemble effect). The latter could occur through direct site-blocking, for overlayer adsorption, or through Ni atom replacement, for surface intermixing. Ligand effects were found to induce changes in the CO TPD peak temperature, ranging from -50 to +50 K on Ni monolayers deposited on W, Mo, and Ru due to the formation of bimetallic bonds between Ni and the underlying metal.2 Bond formation was accompanied by upshifts in the Ni 2p3/2 binding energy ranging from +0.35 to +0.05 eV2. On the other hand, submonolayers of Bi (which is inert for CO adsorption and does not intermix with nickel) evaporated on Ni(100) caused an attenuation of the CO TPD spectrum without shifts, which could be explained entirely on the basis of an ensemble effect due to site-blocking.28 In this case, TPD state A exhibited a slightly preferential attenuation compared to state B.28 The TPD results from Figures 6-8 are consistent with a Au/ Ni surface intermixing when small amounts of gold (around 0.2 ML) and several monolayers of Ni coexist, with Au residing on the surface of the bimetallic nanoclusters. The simultaneous existence, in this case, of a ligand effect in CO adsorption (Figure 6 for 0.25 ML Au and Figure 7A,B, curve b) and a Au4f7/2 binding energy downshift (Figure 2, curves c and d), as well as the extinction of these effects for larger Au/Ni ratios, when Au covers a larger fraction of the Ni cluster surface (Figure 6 for g 0.8 ML Au, Figure 7A,B, curve c and Figure 8) suggests that at some critical submonolayer coverage, there is a transition from an intermixed state to an overlayer state. This behavior has been recently observed by STM for Au adsorption on Ni single crystals and the critical Au coverage has been assigned at 0.4 ML.6-8 The same behavior has been deduced by combined XPS/UPS measurements for Au adsorption on a stepped Ni surface.9 The CO adsorption behavior on the Au/Ni bimetallic nanoclusters can thus be described in terms of a combined ligand/ensemble effect for subcritical Au coverage on the Ni cluster surface and a pure ensemble effect for larger Au coverages. The absence of a detectable Ni 2p3/2 binding energy upshift when the ligand effect is present, as previously reported for Ni monolayers on W, Mo and Ru2, could be attributed in this case to the small value of the upshift, which is difficult to resolve for the Ni surface atoms in the presence of multilayers of Ni under our experimental conditions. 5. Summary Ultrathin films of gold and nickel co-deposited on YSZ (100) have been investigated in situ by photoemission spectroscopy and CO Temperature-programmed desorption. Comparison of the XPS intensities from the co-deposited metals with those measured for the corresponding individual deposits showed that the relative intensity of the Ni signal was reduced, whereas that for Au was unaffected upon co-deposition. These results demonstrate a strong Au tendency to segregate on the surface of bimetallic nanoclusters and cover the Ni deposit. For very small Au/Ni atomic ratios, the Au 4f peaks shift to a slightly lower binding energy relative to the same amount of individually deposited Au, consistent with the shifts observed upon Au-Ni surface alloying on extended metal surfaces. At larger Au/Ni ratios, there are no significant differences in the EB and fwhm of the XPS peaks between the co-deposited and individual

48 J. Phys. Chem. B, Vol. 106, No. 1, 2002 deposited systems, and the spin-orbit splitting between the Au 5d valence-band in UPS suggests that the Au atoms mean coordination number is not affected by the presence of Ni on the surface. Carbon monoxide desorption data from the bimetallic deposits confirm the segregation of Au on the surface of the Ni nanoclusters and demonstrate a significant ligand effect of Au on CO adsorption for very small Au/Ni ratios, in accordance with the surface intermixing suggested by the photoemission results. References and Notes (1) Larsen, J. H.; Chorkendorff, I. Surf. Sci. Rep. 1999, 35, 163. (2) Rodriguez, J. A.; Goodman, D. W. Science 1992, 257, 897. (3) Henry, C. R. Surf. Sci. Rep. 1998, 31, 235. (4) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (5) Rodriguez, J. A.; Chaturvedi, S.; Kuhn, M.; van Ek, J.; Diebold, U.; Robert, P. S.; Geisler, H.; Ventrice, C. A. J. Chem. Phys. 1997, 107, 9146. (6) Nielsen, L. P.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E.; Engdahl, C.; Stolze, P.; Jacobsen, K. W.; Norskov, J. K. Phys. ReV. Lett. 1993, 71, 754. (7) Nielsen, L. P.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E.; Engdahl, C.; Stolze, P.; Noskov, J. K. Phys. ReV. Lett. 1995, 74, 1159. (8) Jacobsen, J.; Nielsen, L. P.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E.; Ramussen, T.; Jacobsen, K. W.; Noskov, J. K. Phys. ReV. Lett. 1995, 75, 489. (9) Zafeiratos, S.; Kennou, S. Appl. Surf. Sci. 2001, 173, 69. (10) Santra, A. K.; Rao, C. N. Appl. Surf. Sci. 1995, 84, 397.

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