Oxide-free oxygen incorporation into Ru„0001…
Raoul Blume and Horst Niehus
Institut fu
¨
r Physik der Humboldt-Universita
¨
t, Invalidenstr. 110, 10115 Berlin, Germany
Horst Conrad
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
Artur Bo
¨
ttcher
a)
Inst. fu
¨
r Physikalische Chemie, Univ. Karlsruhe, Kaiserstr. 12, 76131 Karlsruhe, Germany
͑Received 8 January 2003; accepted 1 December 2003͒
A smooth Ru͑0001͒ surface prepared under ultra-high vacuum conditions has been loaded with
oxygen under high-pressure (pϳ1 bar) and low-temperature (T Ͻ600 K) conditions. Oxygen
phases created in this way have been investigated by means of thermal desorption spectroscopy,
low-energy electron diffraction, and ultraviolet photoelectron spectroscopy. The exposure
procedures applied lead to oxygen incorporation into the subsurface region without creation of
RuO
2
domains. For oxygen exposures ranging from 10
11
to 10
14
L oxygen contents up to about 4
monolayer equivalent could be achieved. The oxygen incorporation is thermally activated. The CO
oxidation reaction conducted at mild temperatures (T Ͻ500 K) at a sample loaded with subsurface
oxygen reaches CO→CO
2
conversion probabilities of 10
Ϫ3
.©2004 American Institute of
Physics. ͓DOI: 10.1063/1.1643724͔
I. INTRODUCTION
Oxidation procedures applied to metal surfaces result at
intermediate stages in the formation of extended, thin oxide
domains covering the surface. The physical and chemical
properties of such metal oxide layers are rather well known.
1
The morphology of those oxides acting as terminating layers
of the metallic substrate underneath is element specific and
depends to a large extent on the oxidation conditions applied
͑partial oxygen pressure p, substrate temperature T, surface
roughness
, etc.͒.
2,3
Usually, the crystallographic structure
of those oxide layers is very similar to the structure of the
corresponding solid oxides grown by applying conventional
methods of crystal growth.
4
Various spectroscopic methods
used to monitor the electronic properties of the surface ox-
ides also reveal overall agreement with those of the well-
known bulk oxides.
However, the very initial as well as almost all interme-
diate stages of the oxidation process are much less under-
stood. The importance of these transient states becomes evi-
dent if the incorporation of oxygen into the metal surface is
considered as the initial step for the restructuring of the
original metal lattice into the different crystallographic struc-
ture of the oxide lattice. In the case of Ru͑0001͒, the impor-
tance of the initial and intermediate oxidation stages became
evident when discovering that these phases, which consist
mostly of oxygen incorporated below the topmost layer, ex-
hibit extraordinarily high reactivity towards the CO oxida-
tion reaction.
5–7
A very efficient way to augment the amount
of incorporated oxygen has been found via applying oxida-
tion procedures at elevated substrate temperatures and high
partial pressures (p Ͻ10
Ϫ2
mbar). Such treatments result in
the formation of rather complex lateral structures, dominated
in the later stages by RuO
2
domains.
3
However, up to now,
neither the selective preparation nor the experimental char-
acterization of the particular intermediates preceding the for-
mation of a compact RuO
2
surface layer has been achieved
in detail.
In the following, we report on the oxygen incorporation
into the Ru lattice through the ͑0001͒ surface at experimental
conditions where the oxide formation is essentially inhibited.
Such a particular state was achieved by exposing the clean
crystal ͑cleaned under UHV-conditions͒ at high oxygen pres-
sures (pϳ1 bar) and low sample temperatures, TϽ600K.
Oxygen incorporation achieved with this treatment re-
sembles strongly those oxygen dissolution processes, which
were observed previously for various polycrystalline solids.
8
Sample cleaning and all high-pressure experiments were
performed in an ultra high vacuum chamber ͑UHV͒. Use of
the established UHV procedures ͑cycles of Ar ion sputtering,
heating at base pressure better than 10
Ϫ9
mbar) provides the
high level standards of cleanliness and smoothness of the
single-crystal surface.
9
Oxygen incorporation has been stud-
ied mainly by thermal desorption spectroscopy ͑TDS͒, which
allows to monitor the thermally driven removal of the incor-
porated oxygen and to quantify the oxygen load. In addition,
the electronic properties of the incorporated oxygen have
been characterized by means of UPS ͑21.2 eV͒. The tech-
niques applied reveal physical properties, which allow to
clearly differentiate between the oxygen incorporated and
that bound in RuO
2
domains. Moreover, incorporated oxy-
gen is characterized by its high reactivity towards the CO
oxidation reaction in the low temperature region (T
Ͻ550 K).
a͒
Author to whom correspondence should be addressed. Telephone:
ϩϩ0721-608-3254; Electronic mail: artur.boettcher@chemie.uni-
karlsruhe.de
JOURNAL OF CHEMICAL PHYSICS VOLUME 120, NUMBER 8 22 FEBRUARY 2004
38710021-9606/2004/120(8)/3871/9/$22.00 © 2004 American Institute of Physics