WDS'05 Proceedings of Contributed Papers, Part III, 584–589, 2005.
ISBN 80-86732-59-2 © MATFYZPRESS
Photoemission Study of the Pb/Ni(111) Surface Phases
J. Libra and V. Matolín
Charles University, Faculty of Mathematics and Physics, Department of Electronics and Vacuum Physics,
Prague, Czech Republic.
Abstract. Pb/Ni(111) surface phases were investigated by synchrotron radiation
photoemission, low-energy electron diffraction (LEED) and angular resolved ultraviolet
photoelectron spectroscopy (ARUPS). For room temperature evaporation, two surface
ordered layers, (3x3) and (4x4), were observed. The Pb 5d and Pb 4f core levels as well
as valence band spectra indicated a weak chemical interaction between Pb and Ni, and the
formation of a close packed overlayer with Pb atoms in two different adsorption sites.
Annealing of the Pb/Ni(111) surface led to the formation of the (√3x√3)R30°
reconstruction characterized by a topmost layer consisting of a substitutional alloy. The
transformation to the (√3x√3)R30° structure was accompanied by the appearance of a
strong photoelectron diffraction effect confirming embedding of Pb atoms in the Ni(111)
first surface layer. CO adsorption results showed that lead atoms simply blocked the CO
adsorption sites for the unannealed Pb/Ni(111) surface whilst the surface alloy exhibited a
chemical effect of weakening of the CO – Ni bond. The Pb 5d core level shift indicated
charge transfer from Pb to the surface, particularly for (4x4) and (√3x√3)R30° structures.
ARUPS results proved electronic structure dependence on surface structure. The ARUPS
plots for the (√3x√3)R30° structure were substantially different compared to two other
structures whilst the CO adsorption affects only intensities of main spots.
Introduction
During the last few years several studies have shown the structural variety of Pb/Ni(111) surface
phases involving a strong modification of the effective radius of the Pb adsorbate atoms. Pb deposited
on Ni(111) at 300 K forms (3x3) and (4x4) structures. The (3x3) surface phase was observed for 1
monolayer (ML) of Pb atoms forming a close packed hexagonal structure with Pb–Pb atom spacing of
3.74 Å corresponding to that of the bulk Pb(111) plane. We observed a transition from (3x3) to (4x4)
structure after additional deposition of 0.2 ML. (4x4) structure was interpreted as a compressed close
packed structure that was characterized by reduced Pb-Pb spacing and therefore a decrease of Pb atom
radius [Umezawa et al., 1996]. Annealing of the Pb-covered surface was found to give rise to a stable
Ni(111)-(√3x√3)R30°-Pb surface phase [Umezawa et al., 1997] with Pb atoms incorporated into the
first Ni(111) layer and with a Pb atom outward displacement of 0.2 Å. The formation of a Ni(111)(√3x√3)R30°-Pb substitutional alloy was confirmed in [Brown et al., 2000], and the Pb atoms were
suggested to be in fcc sites. It was shown that in the alloy phase the Pb effective radius was 0.4 Å
smaller then in bulk crystalline Pb. Tensor LEED analysis of the Ni(111)-(√3x√3)R30°-Pb surface
[Quinn et al., 2002] confirmed earlier findings of a strong reduction in the effective radius, which was
attributed to the influence of surface valence electron charge smoothing and an associated surface
stress effect. The amplitude of the surface alloy rumpling was found to be 0.73 Å.
Despite detailed studies of the Pb/Ni(111) surface phase structure the chemical properties of such
alloys remains unexplored. Alloys of metals with d- and s,p- valence electrons are generally
characterized by an energy shift of the centroid of the d-electron band away from the Fermi level and
is accompanied by a decrease of surface reactivity, which can be manifested as a decrease of CO
adsorption temperature [Matolín et al., 2001, Tsud et al., 2001, Matolín et al., 2004]. Our recent study
of Pb interaction with a Pd(110) crystal suggested that a major aspect of the Pb poisoning effect on CO
oxidation of the Pd containing catalysts was the reduction of the heat of CO adsorption [Tsud et al.,
2004].
584
LIBRA AND MATOLÍN: PHOTOEMISSION STUDY OF THE Pb/Ni(111) SURFACE PHASES
In this work the formation of the Pb/Ni(111) interface and surface alloy formation were
investigated by means of synchrotron radiation excited photoemission and LEED in the context of a
study of surface structure, reactivity and charge transfer effects. As a reactivity test we selected the
adsorption properties of CO, a commonly used molecule which is important in many reactions.
Electronic structure variations were also investigated by means of angle resolved ultraviolet
photoemission.
Experimental
The experiments with LEED and synchrotron radiation excited photoemission were performed at
the Materials Science Beamline at the Elettra synchrotron light source in Trieste. It is a bending
magnet beamline with a plane grating monochromator, based on the SX-700 concept [Vašina et al.,
2001].
The core-level spectra were recorded at two photon energies: 48.7 eV for the valence band and Pb
5d core levels and 400.6 eV for the Pb 4f, Ni 3p and C 1s core levels. The total resolution (analyser +
photons + natural line width) was determined by measuring the width of the Fermi level at a
temperature of 120 K and the values were: 120 meV (50 eV) and 310 meV (400 eV). At this
temperature the width of the Fermi level is 33 meV. The photoelectron peak intensities were
normalised to the incident photon flux.
ARUPS experiment was performed at the laboratory of Department of Electronics and Vacuum
Physics in Prague using UV lamp, He I line, hν = 21.22 eV. The total resolution was determined by
measuring the Fermi edge width at 300K and it was 90 meV. ARUPS data were collected by
measuring valence band spectra for each angle. Angular resolution was achieved by sample rotation.
The nickel crystal was a disc of 10 mm diameter and 1.5 mm thickness, oriented to within 0.2° of
the (111) plane. Cleaning was performed by cycles of sputtering and heating. The Pb metal was
evaporated from a Knudsen cell onto the clean Ni (111) surface at 300 K.
Results and discussion
Pb was evaporated onto the Ni(111) surface at 300 K. The overlayer structure was monitored by
LEED. The (3x3) reconstruction was formed for the Pb coverage up to 1 monolayer. The subsequent
deposition led to a (4x4) reconstruction. Annealing of the sample with a complete (3x3) structure at
820 K for several seconds and subsequent cooling to 300 K resulted in a sharp Ni(111)-(√3x√3)R30°Pb structure.
The growth of the Pb overlayer on the Ni(111) surface was also investigated by means of
photoelectron spectroscopy. The evolution of the valence band region and Pb 5d core level was
monitored at photon energy of 48.7 eV. Figure 1 shows the spectra as a function of lead coverage; a
sharp Pb 5d doublet appeared progressively. The 5d5/2 core level spectra are plotted in the inset of the
Fig. 1 for the coverage range from 0.15 to 1.2 ML. The binding energy (BE) of the Pb 5d5/2 peak
varied from 17.70 eV for 0.15 ML to 17.80 eV for 0.9 ML and then shifted to 17.86 eV for 1.05 and
1.2 ML, respectively. This can be explained by charge transfer from Pb atoms due to delocalization of
the Pb 6s,p valence electrons over the Ni surface. The Ni 3d valence band exhibits a sharp Fermi edge
for all deposits without a depletion of Ni states near the Fermi edge of the kind we observed for Pb
adsorption on Pd(110) [Tsud et al., 2004]. The continuous decrease of the Ni 3d band intensity is
caused by Ni signal attenuation due to Pb overlayer formation. The absence of an attenuation of the d
states near the Fermi level suggests that Pb is not strongly bonded on the Ni(111) surface.
CO adsorption was studied by measuring the valence band spectra after 8 Langmuir (8 x 10-6
torr.s) of CO exposure, to give saturation coverage on both clean and Pb covered Ni(111) surfaces. In
Fig. 1 we can see that three main features characterize CO adsorption on the Pb/Ni(111) surface: (i)
appearance of 4σ and 5σ/1π CO molecular orbital intensity, (ii) depletion of Ni 3d valence states, (iii)
shift of Pb 5d peak to higher BE. The 4σ and 5σ/1π CO molecular orbital intensity at 11.0 and 8.4 eV
decreased with increasing Pb coverage. Fig. 1 shows the CO molecular orbital spectra and the sum of
both molecular peak areas. It can be seen that the coverage of CO decreases linearly with Pb
adsorption at 300 K, and is negligible for 1 ML of Pb which corresponds to the complete (3x3)
structure formation. This result is rather surprising because lead is generally considered as a strong
585
LIBRA AND MATOLÍN: PHOTOEMISSION STUDY OF THE Pb/Ni(111) SURFACE PHASES
catalyst poison with a potent inhibitor effect for CO adsorption. We can conclude that Pb atoms in the
(3x3) structure are simply blocking the Ni(111) surface without any long range interaction with the
CO molecules. The total disappearance of the CO adsorption for the Pb coverage of 1 ML confirms a
general assumption that CO is not bonded to Pb atoms, but nevertheless a small shift of the Pb 5d
levels to higher BE can be seen in Fig. 1.
35
350
17.86 eV
Pb 5 d5/2
3
Intensity x10
(arb.u.)
Pb/Ni
- - - - - - CO/Pb/Ni
300
1.20 ML
1.05 ML
0.9 ML
0.75 ML
250
15
17.6
10
12
8
6
b)
48.7 eV
30
3
Intensity
x10(arb.u.)
18.0
5σ
4σ
10
0.3 ML
0.15 ML
18.4
0
ML Pb
0.30
0.45
0.60
0.75
0.90
1.05
20
35
0.45 ML
200
8L CO/Pb/Ni(111)
25
0.6 ML
3
Intensity
x10(arb.u.)
a)
48.7 eV
30
17.2
0.75 - 3 x 3
8L CO/Pb/Ni(111) after annealing
0
ML Pb
0.30
0.45
0.60
25
20
15
10
150
6
8
10
12
0.60 - 3 x 3
BE (eV)
3
30x10
0.45 - 1 x 1
100
17.90
8L CO (4σ+5σ)
8L CO (4σ+5σ) annealed
Pb5/2 BE
25
17.85
0.30 - 1 x 1
0.15 - 1 x 1
50
4σ
CO
5σ
3x3
4 x 4
15
17.80
BE (eV)
I CO, I Pd 5/2
20
10
17.75
5
0
20
15
10
BE (eV)
5
0
0
0.0
17.70
0.2
0.4
0.6
0.8
1.0
1.2
Pb deposit (ML)
Figure 1. Left: Valence band and Pb 5d core level for clean and CO covered surfaces, in the inset the
Pb 5d5/2 core level as a function of coverage. Right top: CO molecular orbital spectra for different Pb
coverages; a) surface formed at 300 K, b) surface annealed to 820 K. Right bottom: CO molecular
orbital intensity (left axis) and Pb 5d5/2 BE (right) as a function of the Pb coverage. Arrows indicate
coverage intervals corresponding to (3x3) and (4x4) reconstructions observed by LEED.
The CO molecular orbital intensity obtained after exposing the annealed Pb/Ni(111) surfaces to
8 L of CO as a function of coverage of deposited Pb is shown in the right bottom panel of Fig. 1. We
can see that in this case the CO uptake decreases more rapidly, compared to that of the non-annealed
surface and no CO adsorbs for coverages above 0.6 ML of Pb on the Ni(111) surface. We observed the
5σ - 4σ molecular orbital separation of ∆ = 2.6 eV for 0.3 ML of Pb, and an increasing shift of the
5σ level to lower BE to ∆ = 3.1 eV for 0.6 ML, indicating weakening of the CO – Ni bond [Miranda et
al., 1984, Kandler et al., 1995]. So in this case the poisoning effect is due to the formation of a less
reactive compound, compared with the pure Ni(111) surface, that is in agreement with the observed
formation of the surface substitutional Pb – Ni alloy.
Although the Pb-Ni system is an example of immiscible metals (Pb shows very limited solid
solubility in Ni [Massalski, 1990]), formation of the single-layer substitutional alloy of Ni2Pb
stoichiometry can be activated by surface annealing at 820 K [Quinn et al., 2002]. The spectra of the
Pb 5d core levels show a rather surprising effect of a considerable enhancement of the peak intensity
after 820K annealing of the Pb adlayer of 0.3 and 0.6 ML on the Ni(111) surface. The most likely
explanation of this behavior is an increase of intensity due to photoelectron diffraction, and linked to
the surface phase transformation and embedding of Pb atoms into the first Ni layer. This effect cannot
be caused by any outward diffusion of lead atoms to the surface from the subsurface because we did
586
LIBRA AND MATOLÍN: PHOTOEMISSION STUDY OF THE Pb/Ni(111) SURFACE PHASES
not observe this behavior for Pb 4f core level intensities measured at a photon energy of 400.6 eV. The
high intensity modulation (for normal emission) of the Pb 5d core level could be explained by a
contribution of scattering from neighbouring substrate atoms that is enhanced in the case of Pb atoms
incorporated into the substitutional sites of the Ni surface. The angular dependence of this behavior is
clearly seen in the right panel of Fig. 2, which presents the intensity of the Pb 5d5/2 core level intensity
at normal emission and 60° emission with respect to the crystal normal along the [-1-12] azimuth. This
hypothesis is supported by cluster calculation using the EDAC software [García de Abajo et al., 2001].
____
200
Pb 5d5/2
Pb/Ni(111) at R.T.
- - - Pb/Ni(111) annealed at 820 K
40
1.2 ML
0.6 ML
3
Intensityx10
(arb.u.)
18.4
17.6
18.0
17.2
1.2 ML Pb 4x4
100
3
Intensity
x10(arb.u.)
0.3 ML
150
0.6 ML Pb 3x3
30
Pb/Ni(111), 0.3 ML Pb
300 K , θ 60°
820 K, θ 60°
300 K, θ 0°
820 K, θ 0°
20
10
50
0.3 ML Pb 1x1
20
15
10
BE (eV)
5
0
18.4
0
18.2
18.0
17.8
17.6
17.4
17.2
BE (eV)
Figure 2. Spectra before and after annealing to 820 K. Left: valence band and Pb 5d5/2 peak for normal
photoelectron emission. Right: Pb 5d5/2 peak for normal and 60° photoelectron emission.
For calculations we propose a model shown in Fig. 3. Ni surface is represented by a cluster
composed of 700 Ni atoms in 8 layers. For (3x3) structure Pb atoms occupy two different positions on
the Ni(111) surface; fcc hollow sites (midpoint labeled ●) and morphologically identical positions
with midpoints (labeled +) shifted from the nearby hcp sites by the same distance in [-1-12] directions.
Annealing of the surface to 820 K leads to the (3x3) → (√3x√3)R30° surface phase transition. It
involves the lead atoms occupying substitutional fcc sites to form a Pb-Ni surface alloy. Due to the
large difference between the covalent radii of Ni and Pb atoms this process, which is driven by the
tendency to decrease the surface corrugation, is accompanied by a strong reduction of the Pb atom
effective radius and consequently by surface charge redistribution.
Figure 3. Plan and section views of the (3x3) overlayer and fcc substitutional site models of the
Pb/Ni(111) (3x3) → (√3x√3)R30° transition.
587
LIBRA AND MATOLÍN: PHOTOEMISSION STUDY OF THE Pb/Ni(111) SURFACE PHASES
<-1-12> <1-10>
fcc subst.
atop
fcc hollow
-90
-60
-30
0
Polar angle [°]
30
60
90
Figure 4. Intensity plots of diffracted photoelectrons for directions [-1-12] and [1-10]. Bottom axis is
angle to sample normal in degrees.
Calculated polar intensity plots along the [1-10] and [-1-12] azimuthal directions for different Pb
adsorption sites are shown in Fig. 4. It can be seen that the calculated signal enhancement in the
normal emission due to diffraction is in good agreement with experimental values presented in Fig. 2.
Also the experimental and calculated intensity change for a polar angle of 60° is in good agreement.
The electronic structure of the Pb/Ni(111) surface was studied by angle resolved ultraviolet
photoemission. Measurements were done in two modes. In the first one, the region of binding energy
from 0.6 eV to -0.1 eV was acquired for various azimuthal and polar angle with step of 2° in both axis.
Ni(111)
CO/Ni(111)
Pb/Ni(111)
(√3 x √3) R30°
a)
b)
c)
d)
Figure 5. a) b) c) ARUPS constant initial states maps shows stereographic projection of valence band
spectra intensities at binding energies of a) -0.01 eV, b) 0.07 eV, c) 0.19 eV. Polar angle at the edge of
those maps is 70°, d) maps of polar-angle scanned energy distribution curves (EDCs) taken along the
azimuth of direction [1-10]. Horizontal axis is binding energy going from 1.3 eV to -0.15 eV
(left→right), vertical axis is surface-parallel wave vector k|| going from 0 to 2.4 A-1 (bottom→top).
High intensities are shown in white in all maps.
588
LIBRA AND MATOLÍN: PHOTOEMISSION STUDY OF THE Pb/Ni(111) SURFACE PHASES
Figure 5abc shows the angular distribution of the intensity for three chosen binding energies, i.e. –
0.01, 0.07, 0.19 eV. In the second mode, the azimuthal angle was fixed and the region of binding
energies from 4 eV to -0.1 eV was acquired for various polar angle with step of 1°. Fig. 5d shows a
part of the intensity plots , from 1.3 to –0.15 eV, for [1-10] azimuthal direction in k|| versus EB axes.
Three surface structures were investigated by ARUPS: clean Ni(111) surface, Ni(111) surface
with adsorbed CO at the saturation coverage and (√3x√3)R30° Pb/Ni(111) surface reconstruction.
Figure 5 shows variations of ARUPS data for these surface structures. It can be seen, that difference
between the clean Ni(111) surface and one with adsorbed CO is mainly in intensities of the spots on
the ARUPS plots. On the contrary Ni-Pb surface alloy causes small shifts of the spots, that confirms
the formation of the Ni-Pb intermetallic phase on the Ni(111) surface with the electronic structure
characteristic of the alloy.
Conclusion
Surface structure and CO adsorption properties of the Pb/Ni(111) system were investigated by
means of SRPES, LEED and ARUPS. At 300 K Pb atoms form (3x3) and (4x4) superstructures on the
Ni(111) surface. Annealing at 820 K induces the (3x3) → (√3x√3)R30° transition indicated by
intensity changes of the Pb 5d core level caused by diffraction effects. Structural models were
developed in order to compare measured effects with theoretical calculations and were confirmed by
agreement between the experiment and diffraction calculations. The CO adsorption investigation
showed that the Pb-poisoning effect on CO catalysis is strongly dependent on the surface structure of
the Pb/Ni(111) system.
Acknowledgments. This work was supported by the Czech Grant Agency under the Grant No.
202/05/H003. Author wishes to thank I. Matolínová, N. Tsud, S. Fabík, V. Dudr, V. Cháb and K.C. Prince for
colaboration on the experiment at the Materials Science Beamline at the Elettra synchrotron light source in
Trieste.
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