Anomalous Reactivity of Gold Thin Films on Iridium

Gold 2003
Shouhei Ogura , Wilson Agerico Di¥~{n}o , Markus Wilde , Katsuyuki Fukutani , Toshio
Abstract One of the ultimate goals of surface science is
to be able to design surfaces with particular
catalytic reactivity [1]. Thus, we need an
atomic-level understanding of the
fundamental principles (elementary
processes) underlying the bond-making and
bond-breaking at surfaces. Although several
proposals (e.g., [2] and references therein)
have been made, examples [3] of catalyst
design on the basis of these fundamental,
surface science-based insights are extremely
few. Necessity dictates that we now focus on
the local surface properties, and clarify how
they influence reactions. Apparently,
combining different substances to form multi-
component surfaces provides us with a
simple means to do just so [1]. Here, we
report results of a study on the interaction of
hydrogen (deuterium) with thin Au{111} films
grown epitaxially on Ir{111}, using
temperature-programmed desorption (TPD)
and nuclear reaction analysis (NRA). We
found that a hydrogen (deuterium) molecule
can dissociatively adsorb on epitaxially grown
Au{111} films [4]. Furthermore, we also found
that H (D) atoms can be confined at the
interface between the bulk-Ir surface and the
Au thin film. These results indicate that, as we
would expect, the particular surface feature
plays an important role.

Low-energy electron diffraction (LEED)
observations, Auger electron spectroscopy
(AES), and TPD measurements were
performed in an ultra-high-vacuum chamber
in Osaka University [5]. NRA experiments were
done using the van de Graaff tandem
accelerator at the Research Center for
Nuclear Science and Technology (RCNST,
University of Tokyo) [6]. This technique allows
us to measure the hydrogen concentration at
the surface of a sample target and, by
scanning the incident energy of the ion beam,
the hydrogen depth profile. The Ir{111} surface
was cleaned by successive cycles of ion
bombardment, heating at 1200 K in oxygen
atmosphere, and final flashing at 1500 K. The
crystal was exposed to high purity hydrogen
(99.999 %) or deuterium (99.9 %) by
backfilling the chamber.

We studied the growth mode of thin Au films
on an Ir{111} surface at 100 K by AES. The
curves for both Ir and Au signals can be
approximated by a series of straight-line
segments expected for layer-by-layer growth.
Thin Au films grown on Ir{111} do not form
alloys with the Ir atoms on the surface.
Compared with a clean Ir{111} surface, the 3
ML Au film grown on the Ir{111} surface at ~
100 K reveals a diffused (1X1) pattern, with
dimmer integral spots and higher
background. Note that the observed surface
lattice constant of the 3 ML Au film on Ir{111} is
~3 % longer than that of a clean Ir{111}
surface. From the TPD measurements, we
observe a linear decrease in D coverage with
increasing Au coverage until ~1 ML when the
Au-covered surface is exposed to deuterium.
Increasing the Au coverage further, from 1 ML
to 4 ML, does not change the adsorbed D
coverage of ~0.5 ML. We also confirmed that
deuterium molecule can still be dissociatively
adsorbed on the surface of an 8 ML Au thin
film, even though the surface is already purely
covered with Au atoms!

The NRA spectrum at an ion incidence angle
of 65 deg., taken after hydrogen-molecule
exposure of the 4 ML Au{111} film grown on
clean Ir{111} at ~100 K. The spectrum fits well
to a single Gaussian peak centered at the
resonance energy of 6.385 MeV. This
indicates that H atoms produced in the
dissociation of hydrogen molecule on a Au
film are on the surface of the Au film. On the
other hand, the NRA spectrum taken for the 4
ML Au{111} film grown on the H-saturated
Ir{111} clearly shows an asymmetric
lineshape. This implies that some H atoms
are present below the surface. We separate
the spectrum into two components. We
attribute the narrower of the two components
to the surface-H, as it is centered at 6.385
MeV. The other, broader component, which is
shifted by 10.6 keV from the surface peak, we
attribute to H atoms located 1.15 nm below
the surface. This location corresponds to the
interface between the Ir surface and the 4 ML
Au{111} film, strongly supporting the existence
of H atoms at the Au-Ir interface as suggested
by the TPD spectra.

From the LEED observations, our Au thin films
contain disordered structures, e.g., defects.
Thus, we expect local surface features (such
as defects, steps, or those originating from
strain effects) to provide possible dissociation
sites. To get some insight into the role of the
local surface features, we performed density
functional theory-based potential energy
surface calculation. To emphasize a localized
feature of the film surface, we considered a
small cluster to represent the dissociative
adsorption at a particular site (represented by
the Au-Au cluster) on the thin Au film surface.
Since the d-orbitals are intrinsically localized
and fully filled compared with the s-orbital, we
suggest that the Au s-electrons of the
localized surface features with the
intermediate size and low dimensionality play
a more important role for the dissociative
adsorption of hydrogen on Au thin films as
well as on a Au-Au cluster. Since the Au s-
band is half-filled, the "s-band center" is
expected to be fixed at Fermi level, even as the
band width changes. Following the same line
of reasoning as the d-band center model [7],
as we decrease the Au coordination, the s-
band gets sharper, suppressing the repulsive
interaction, and enhancing the attraction. This
localization effect becomes more remarkable
with increased filling of the d-orbitals (of the
substances used as impurities/overlayers).
Furthermore, because of the resulting stable
Au-H complexes, we were able to trap the H at
the Au-Ir interface.

[1] J.H. Sinfelt, Surf. Sci. 500, 923 (2002). [2]
W.A. Di¥~{n}o et al., J. Phys. Soc. Jpn. 69, 993
(2000). [3] F. Besenbacher et al., Science 279,
1913 (1998). [4] M. Okada et al., Surf. Sci., in
press. [5] M. Okada et al., Chem. Phys. Lett.
323, 586 (2000); J. Chem. Phys. 115, 9947
(2001). [6] K. Fukutani et al., Phys. Rev. B 59,
13020 (1999); Phys. Rev. Lett. 88, 116101
(2002). [7] B. Hammer and J.K. Norskov,
Nature 376, 238 (1995).

Keywords: hydrogen, Deuterium
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