Surface Science 584 (2005) 62–69
www.elsevier.com/locate/susc
On-surface and sub-surface oxygen on idealand reconstructed Cu(100)
T. Kangas a,*, K. Laasonen a, A. Puisto b, H. Pitkanen b, M. Alatalo b
a Department of Chemistry, University of Oulu, P.O. Box 3000, FIN-90014, Finlandb Department of Electrical Engineering, Lappeenranta University of Technology, P.O. Box 20, FIN-53851, Finland
Received 7 September 2004; accepted for publication 9 February 2005
Available online 18 April 2005
Abstract
In order to understand the first steps of the Cu(100) oxidation we performed first principles calculations for on-sur-
face and sub-surface oxygen on this surface. According to our calculations, the adsorption energies for all on-surface
site oxygen atoms increase, whereas the energies of the sub-surface atoms decrease with the increasing oxygen coverage.
At coverage 1 ML and higher on the reconstructed surface, structures including both on- and sub-surface atoms are
energetically more favourable than structures consisting only of on-surface adsorbates. On the ideal (100) surface this
change can be perceived at coverage 0.75 ML.
� 2005 Elsevier B.V. All rights reserved.
Keywords: Density functional calculations; Oxidation; Copper; Oxygen; Single crystal surfaces
1. Introduction
Surface oxidation plays a very important role in
many phenomena such as corrosion and catalysis.
Corrosion is usually a harmful process and a lot ofeffort has been done in developing materials more
resistant to corrosion. A good example of this is
0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserv
doi:10.1016/j.susc.2005.02.061
* Corresponding author. Tel.: +358 8 3551687; fax: +358 8
3551603.
E-mail address: [email protected] (T. Kangas).
stainless steel. On the microscopic level the corro-
sion is still not well understood [1]. Participation
of oxygen is crucial in many catalytic processes,
especially on oxidation reactions [2]. Recently it
has been observed that metal surfaces can be oxi-dized during the catalysis, and thus the active cat-
alyst is in fact the metal oxide not the clean metal
[3]. In this work we concentrated on the early
stages of the oxidation of copper.
Oxygen on Cu(100) has been the subject of sev-
eral theoretical and experimental studies [4–12].
However, the sub-surface oxygen in Cu(100) is
not as carefully examined. Several authors have
ed.
T. Kangas et al. / Surface Science 584 (2005) 62–69 63
been modelling sub-surface oxygen in the case
of Ag(100) [13–15]. Gajdos et al. found that the
oxygen below the surface is not stable by itself,
but when the amount of oxygen on the surface in-
creases the sub-surface sites become stable [13].This happens when the oxygen coverage increases
to 0.75 monolayer (ML). Results of experimental-
ists support the formation of oxide also for copper
surface. Zhou and Yang found in their transmis-
sion electron microscope measurements elongated
islands of copper(I)oxide, Cu2O, perpendicular to
each other on copper surface [16]. Similar islands
have been observed in scanning tunnelling micro-scopy measurements by Hirsimaki and Junell
[17]. These islands are a few hundreds of nanome-
ters long and they have the (2p2 ·
p2)R45�
structure.
In many experimental studies [4,10,11] it has
been shown that the Cu(100) surface is fully
reconstructed to (2p2 ·
p2)R45� orientation as
the oxygen coverage exceeds 0.5 ML. Loffredaet al. [14] noticed that for the Ag(100) surface
the energy of the missing row (2p2 ·
p2)R45�
reconstructed surface is smaller than the energy
of the ideal one when the oxygen coverage on
the surface exceeds 0.5 ML. They also found that
for the non-reconstructed case at oxygen coverage
1 ML the diffusion barrier to the sub-surface sites
are only 25 meV. This indicates that the diffusionof oxygen atoms to the surface requires only little
effort. However it must be noticed that the sub-
surface site modelled by Loffreda is not exactly
similar to the sites studied by Gajdos and in the
present work [13,15]. The barrier between the on-
surface and the sub-surface sites studied here
might be slightly higher since our sub-surface sites
are deeper in the bulk. On the other hand, it isknown that the oxidation of copper occurs more
easily than the oxidation of more noble silver.
It would have been very interesting to study the
shape of the islands found by experimentalists but
in this work we had to limit ourselves to the simpler
investigation of sub-surface oxygen. Sub-surface
oxygen on clean and oxygen precovered Cu(100)
have been studied using first principles calculationsfor ideal and reconstructed surfaces. In addition,
several combinations of the cases with on-surface
and sub-surface oxygen were modelled.
2. Computational methods
All calculations were carried out using Vienna
Ab-initio Simulation Package (VASP) [18] with
ProjectorAugmentedWave (PAW) [19–22]methodwith the generalized gradient approximation
(GGA) [23] for the exchange and correlation func-
tional and cut-off energy of 430 eV for the plane
waves. In surface calculations, we used five atomic
layers with the lowest one kept fixed. In the case
of ideal Cu(100) surface, a p(2 · 2) surface cell
was used. For the k-point sampling we used a mesh
of 6 · 6 · 1 of the Monkhorst–Pack scheme [24]. Inthe case of missing row reconstruction, a (2
p2 ·p
2)R45� cell and Monkhorst–Pack mesh of
8 · 4 · 1were used.We obtained a lattice parameter
of 3.64 A for Cu, the experimental value being
3.61 A [25]. Calculations for oxygen atoms on and
in copper surface were spin averaged, whereas the
calculation for the oxygen molecule was spin polar-
ized. For O2, we calculated the bond length of1.235 A, and a vibrational frequency of 1561
cm�1, whereas the experimental values are 1.21 A
and 1580 cm�1, respectively [26]. For the clean sur-
face, we observed inward relaxation of 3.2% com-
pared to the experimental value of 2.1% [27].
3. Results and discussion
We have studied the adsorption of oxygen both
on ideal and on reconstructed Cu(100). It is
known that this surface reconstructs when the oxy-
gen coverage reaches 0.5 ML and for this reason it
is more valid to compare results for the recon-
structed surface than the results of ideal surface
to experimental results. However we decided to re-port both results since we noticed that for sub-sur-
face adsorption same kind of phenomena could be
found in both cases. Our explanation to this is that
at atomic level Cu–Cu and Cu–O interactions in
the surface are still the same.
3.1. On-surface oxygen on the ideal surface
We found that the hollow sites are the most
favourable sites for the on-surface oxygen inde-
pendent of the oxygen coverage. Additionally, as
Fig. 1. Sub-surface adsorption sites. (a) Top view of the
tetrahedral site. (b) Side view of the tetrahedral site. (c) Top
view of the octahedral site. (d) Side view of the octahedral site.
The arrows in (a) and (c) show the direction of views used in (b)
and (d).
64 T. Kangas et al. / Surface Science 584 (2005) 62–69
the oxygen coverage increases the adsorption ener-
gies for the oxygen atoms at the hollow sites in-
crease. This can be interpreted as existence of a
repulsive effective interaction between oxygen
atoms. At the coverage of 0.75 ML one out ofthree oxygen atoms at the hollow sites moves
slightly below the top layer coppers when relaxing
the structure. However, this new location is near
the topmost layer, distinct from the tetrahedral
and octahedral sub-surface sites modelled in this
study. The adsorption energies for different oxygen
coverages and sites are collected in Table 1. The
adsorption energies were calculated by the formula
Eads ¼EO=Cu � ECu � NO
EO22
NO
; ð1Þ
where EO/Cu is the total energy of the oxygen cov-
ered surface, ECu is the energy of the clean copper
surface, EO2is the energy of an oxygen molecule
and NO is the number of oxygen atoms in the sim-
ulation cell.
3.2. Sub-surface oxygen on the ideal surface
For the sub-surface oxygen we have considered
the octahedral and tetrahedral sub-surface sites
(Fig. 1). In contrast to the on-surface adsorption
the sub-surface sites become more favourable as
the oxygen coverage increases. That means that
there are effective attractive interactions betweenthe sub-surface atoms. We also found that when
only a small amount of oxygen is on the sub-sur-
face sites the adsorption energies for the tetrahe-
dral and octahedral sites are similar. As the
oxygen coverage on the sub-surface sites increases,
Table 1
Adsorption energies per one oxygen atom with different coverages
Adsorption energies (eV)/atom 0.25 ML 0.
p(2 · 2)O c(
Top �0.27 �Bridge �1.40 �Hollow �2.16 �Octahedral +0.14 �Tetrahedral +0.023 �a At coverage 0.75 ML all the oxygen atoms in the surface cell are
the tetrahedral sites become slightly more favour-
able than the octahedral sites. This seems to bedue to the fact that when more oxygen atoms are
added in the surface, the bonding between the
5 ML 0.75 ML 1.0 ML
2 · 2)O p(2 · 2)3Oa p(1 · 1)O
0.19 +0.20 +0.47
1.20 �0.62 �0.331.95 �1.44 �0.760.42 �0.59 �0.610.42 �0.65 �0.66on same sites.
T. Kangas et al. / Surface Science 584 (2005) 62–69 65
copper atoms in the first and second layer are
weakened and the surface layer relaxes upwards.
Therefore there exists more space for the oxygen
to occupy the tetrahedral sites. Minimum distance
of first and second copper layer for systems wherethere are sub-surface oxygen atoms at different
coverages are shown in Table 2. Since the tetrahe-
dral sites are between these layers, the topmost
layer moves up when increasing the coverage.
The octahedral sites are within the second copper
layer and therefore the distance between the two
uppermost copper layers does not depend on cov-
erage. The same phenomenon can be seen whencomparing the minimum bond lengths of copper
and oxygen atoms (Table 2). As we can see, nor-
mal Cu–O bond lengths at on-surface sites are
1.74–2.03 A. In the unrelaxed structure the Cu–O
bond lengths are 1.82 A for the octahedral sites
and very short, 1.54 A, for the tetrahedral sites.
After relaxing these structures minimum distances
have risen to 1.84 A and 1.91 A for tetra- and octa-hedral sites, respectively. Gajdos et al. found a
similar trend for the sub-surface oxygen sites in
Ag(100) [13]. Geometric changes are similar in
both cases, but effects on adsorption energies are
clearer in the case of silver.
3.3. Sub-surface and on-surface hybrid structures
on the ideal surface
We performed also several calculations for hy-
brid systems which had sub-surface as well as
on-surface oxygen atoms. Adsorption energies
for these structures are listed in Table 3 (see Fig.
2 for the numbering of oxygen sites of Table 3).
We found that the octahedral sub-surface sites
Table 2
Distances of first and second copper layer and minimum oxygen–cop
Distances (A) Adsorption sites 0.25 ML
DCu–Cu Tetrahedral 2.53
Octahedral 2.52
DCu–O Tetrahedral 1.84
Octahedral 1.95
Hollow 2.00
Bridge 1.83
Top 1.74
The distance of copper atoms in the bulk is 2.54 A.
are more favourable when the on-surface oxygen
atoms are on the hollow sites. For example, the
adsorption energy for the octahedral sites with
hollow site oxygen atoms at 0.75 ML coverage is
almost 1 eV lower than the energy of octahedraloxygen atoms with bridge site atoms. Comparing
the sub-surface sites, the octahedral sites with
adsorbates at the hollow sites are more favourable
than the tetrahedral sites with atoms at the hollow
sites at all coverages except 0.75 ML.
When comparing the values of pure on-surface
and sub-surface adsorption energies to hybrid ones
we noticed that the energy is non-additive. Thatmeans that there is a strong effective attractive
interaction between the on-surface and sub-surface
atoms. As an example, if we calculate the energies
of 0.5 ML octahedral oxygen atoms and 0.5 ML
hollow site oxygen atoms from Table 1, we get
�1.19 eV ((�0.42 eV Æ 2–1.95 eV Æ 2)/4), which is
over 0.5 eV higher than the adsorption energy of
structure 2octahedral + 2hollow (�1.61, Table 3).We calculated the same values for some other
structures also and noticed that the difference be-
tween these values and the adsorption energies of
combined structures increase with increasing
coverage.
In the relaxed structure ‘‘4octahedral(4),(5),
(7),(8) + 4hollow(10),(11),(12),(13)’’ at coverage of
2 ML, the on-surface oxygen atoms at the hollowsites have moved 0.66 A below the first copper layer
of surface when the sub-surface oxygen atoms were
at octahedral sites. However they did not reach the
depths of the octahedral or tetrahedral sub-surface
sites. With structures where the sub-surface oxygen
atoms were on tetrahedral sites oxygen atoms re-
mained on on-surface sites.
per bonds at different coverages
0.5 ML 0.75 ML 1.0 ML
3.28 3.34 3.43
2.55 2.52 2.61
1.93 1.90 1.93
1.95 1.92 1.91
1.97 1.91 2.03
1.82 1.81 1.82
1.74 1.77 1.77
-2.5
-2
-1.5
-1
-0.5
0
0.5
0 0.5 1 1.5 2 2.5Coverage [ML]
Ad
sorp
tio
n e
ner
gy/
per
ato
m [
eV]
On-surface sites
On-surface+sub-surface sites
Fig. 3. Comparison of adsorption energies per oxygen on on-
surface and sub-surface sites on ideal Cu(100) surface versus
coverage.
Table 3
Adsorption energies per oxygen atom for the hybrid systems
Adsorption sites Coverage h (ML) Adsorption energy (eV)/atom
Sub-surface site + on-surface site
Octahedral(5) + top(5) 0.5 �0.28Octahedral(5) + top(7) 0.5 �0.11Octahedral(5) + hollow(12) 0.5 �1.15Octahedral(5) + 2bridge(5–8),(2–5) 0.75 �0.36Octahedral(5) + 2hollow(11),(12) 0.75 �1.212Octahedral(5),(7) + 2hollow(11),(12) 1.0 �1.612Octahedral(5),(7) + 2top(4),(8) 1.0 �0.37Octahedral(5) + 4hollow(10),(11),(12),(13) 1.25 �1.172Octahedral(5),(7) + 4hollow(10),(11),(12),(13) 1.5 �1.282Octahedral(5),(7) + 4top(4),(5),(7),(8) 1.5 �0.204Octahedral(4),(5),(7),(8) + 4hollow(10),(11),(12),(13) 2.0 �0.874Octahedral(4),(5),(7),(8) + 4top(4),(5),(7),(8) 2.0 �0.29Tetrahedral(5–8) + bridge(2–5) 0.5 �1.06Tetrahedral(5–8) + 2hollow(10),(11) 0.75 �1.562Tetrahedral(2–5),(4–7) + 2bridge(5–8),(1–4) 1.0 �1.322Tetrahedral(2–5),(4–7) + 2hollow(11),(12) 1.0 �1.424Tetrahedral(7–8),(8–9),(4–5),(5–6) + 4bridge(7–8),(8–9),(4–5),(5–6) 2.0 �0.744Tetrahedral(7–8),(8–9),(4–5),(5–6) + 4hollow(10),(11),(12),(13) 2.0 �0.814Tetrahedral(7–8),(8–9),(4–5),(5–6) + 4top(4),(5),(7),(8) 2.0 �0.28See Fig. 2 for the numbering.
Fig. 2. The 2 · 2 simulation cell on ideal copper surface. The
atoms are numbered for identifying the different oxygen sites
(see Table 3).
66 T. Kangas et al. / Surface Science 584 (2005) 62–69
The adsorption energies for the most favour-
able situations for the ideal surface are compared
in Fig. 3. As we can see, when the coverage is in-
creased, the hybrid sites become most favourable.At the coverage of 0.75 ML the structure with oxy-
gen atoms at the hollow sites and also below the
surface at the tetrahedral sites actually becomes
more favourable than the structure with the hol-
low sites. This seems to be an important step
towards the formation of oxide. When the cover-
age exceeds 1 ML, the calculated energies per
one oxygen adsorbate begin to increase but the
combined total adsorption energies calculated to
all atoms are still lowering. Increasing trend is very
likely due to the repulsion of the on-surface oxy-gen atoms.
3.4. Missing row reconstruction and additional
oxygen on the reconstructed surface
In many experimental studies [28,29] it has been
suggested that after the oxygen coverage on
1
2 3
4
5 6
1
Oxygen
Cu in first layer
Cu in second layer
Cu in third layer
8 7
Fig. 4. The missing row reconstruction in the (2p2 ·
p2)R45�
simulation cell. Atoms are numbered for identifying the
different oxygen sites.
Table 4
Adsorption energies for O in the missing row reconstruction of
Cu(100)
Reconstructed surface,
adsorption sites
h (ML) Eads (eV)
On Hollow(7) 0.75 �1.24On Hollow(8) 0.75 �0.35On Hollow(7),(8) 1.0 �0.80Sub Octahedral(1) 0.75 �0.88 (�0.69)Sub Octahedral(4) 0.75 �0.83Sub Tetrahedral(2–4) 0.75 �0.95 (�0.32)Sub Octahedral(1),(2) 1.0 �1.16 (�0.50)Sub Octahedral(2),(3) 1.0 �1.33Sub Octahedral(1),(2),(3) 1.25 �1.17 (�0.34)Sub Octahedral(1),(2),(3),(4) 1.5 �1.12 (�0.14)The numbering refers to Fig. 3. In structure octahedral(4)
oxygen atom lies above the atom 4, under the missing copper
inside the surface. Adsorption energies calculated by Gajdos
et al. for surface Ag(100) is given in parenthesis [13].
T. Kangas et al. / Surface Science 584 (2005) 62–69 67
Cu(100) reaches 0.5 ML the surface is fully recon-
structed to the (2p2 ·
p2)R45� missing row struc-
ture. This reconstruction is shown in Fig. 4. The
oxygen atoms are in the hollow sites and onerow of Cu atoms is missing from the structure.
We wanted to know what would happen on the
reconstructed surface when 0.5 ML coverage is ex-
ceeded and therefore compared adsorption ener-
gies of pure on-surface sites to the energies of
mixed hybrid ones. Our purpose was to find out
whether the mixed structures would be more opti-
mal than on-surface structures and what would bethe coverage when this occurs. Since some studies
have shown that there are elongated islands of
Cu2O on the surface we wanted to get some insight
to the intermediate phases between reconstruction
and the formed oxide.
We calculated the adsorption energies for on-
surface and sub-surface sites on the reconstructed
surface by formula
Eads ¼EO=Curec. � ECurec. � ðNO � 2Þ EO2
2
NO � 2; ð2Þ
where EO=Curec. is the total energy of the oxygen cov-
ered surface and the reference energy ECurec. thetotal energy of the reconstructed surface with oxy-
gen coverage 0.5 ML (two oxygen atoms). EO2is
the energy of an oxygen molecule in the gas phase
and NO is the number of oxygen atoms on the sur-
face cell. One must notice that ECurec. include ener-
gies of two oxygen atoms of the reconstructed
surface and that is why those have been subtracted
from the overall amount of oxygen NO. Since theamount of copper atoms is not the same in the
cases of ideal and reconstructed surface, the com-
parison of adsorption energies is difficult.
The results for the reconstructed surface are
compiled in Table 4. For the reconstructed surface
we could not calculate adsorption energies of pure
sub-surface sites since the reconstructed surface al-
ways includes at least 0.5 ML coverage of oxygenon on-surface sites. The values in parentheses are
adsorption energies calculated by Gajdos et al.
[13] for same structures on Ag(100) surface. From
the result we noticed that also with the recon-
structed surface the mixed on-surface and sub-sur-
face sites became favourable when the oxygen
coverage increased to 1.0 ML.
In Table 4, it can be seen that for on-surfacesites the most favourable site at the coverage
68 T. Kangas et al. / Surface Science 584 (2005) 62–69
0.75 ML is the hollow site (7) further from the
missing copper. The adsorption energy of the hol-
low site (8), which is next to the missing copper
atom, is almost 1 eV higher. In corresponding
mixed structures (octahedral(1), octahedral(4)and tetrahedral(2–4)) oxygen atoms have higher
energies. As in the case of the ideal surface at this
coverage, tetrahedral sub-surface sites are more
favourable than the octahedral ones.
At coverage 1.0 ML it is energetically favour-
able that some of the oxygen atoms lie on the
sub-surface sites. The adsorption energy of the
structure ‘‘octahedral(2),(3)’’ which have two oxy-gen atoms on on-surface and two atoms on sub-
surface sites is clearly lower than the energy of
the on-surface structure ‘‘hollow(7),(8)’’. Also en-
ergy of mixed structure ‘‘octahedral(1),(2)’’ is al-
most as low as ‘‘octahedral(2),(3)’’. Difference
between the energies of these mixed structures is
not significant. On both of these structures one
of the on-surface oxygen atoms has to move belowthe surface. Additionally, the adsorption energy
per oxygen atom decrease with increasing coverage
(�1.24, �1.33).When the oxygen coverage is higher than
1.0 ML, adsorption energies of structures which
include atoms on octahedral sites are still notably
low whereas in the case of silver the adsorption
energies to these structures are substantially lower.While relaxing these structures the oxygen atoms
moved from their initial locations substantially.
For example in the structure ‘‘octahedral(1),
(2),(3),(4)’’ eventually the oxygen atoms have coor-
dination number three or four depending on their
location relative to missing copper row. According
to the coordination number of oxygen atoms the
formed structures resemble more copper(I)oxide,Cu2O, than copper(II)oxide, CuO, in which the
oxygen atoms are coordinated to six copper atoms.
This result is in agreement with experimental re-
sults of Zhou et al. The islands they found consist
of Cu2O. At the coverage higher than 1 ML, large
structures including tetrahedral sub-surface oxy-
gen atoms were difficult to relax. In this study we
could not obtain reasonable structures for thesesystems. This was due to very strained initial
geometries which result very large and disordered
relaxations.
On the whole, the sub-surface sites in both the
ideal and reconstructed surface become more
favourable as the oxygen concentration on the
sub-surface sites increases. This is caused by the
outwards relaxation of the uppermost layer mak-ing more space between the first and second layer.
Comparing our adsorption energies to the values
calculated by Gajdos et al. it is clear that copper
becomes oxidized easier than silver and difference
between these two increases with increasing cover-
age [13].
4. Conclusion
The oxidation process starts when oxygen atoms
approach the surface and adsorption occurs.
According to adsorption energies, oxygen atoms
exist on hollow sites at (and below) coverage
0.25 ML. On the ideal surface there is a repulsive
interaction between on-surface oxygen adsorbates,whereas sub-surface atoms interact attractively.
The full reconstruction of the Cu(100) surface is
attained when the oxygen coverage reaches 0.5
ML. After that the adsorption of available oxygen
atoms continues to on-surface sites until at cover-
age 1.0 ML sub-surface sites are more optimal for
added oxygen atoms than on-surface sites. At high-
er coverages the energies of oxygen atoms are stilllow and the observed structures locally resembles
Cu2O. The conclusion that the energies of the
mixed oxygen structures are lower in copper than
in silver is in agreement with the fact that the oxida-
tion of copper is easier than the oxidation of a cor-
responding silver surface. The existence of the
discovered attractive interaction of sub- and on-
surface oxygen atoms suggests that the sub-surfaceoxygen atoms have an important role in the oxida-
tion, but in this work we have not investigated the
oxidation paths from on-surface to sub-surface
sites. The work is in progress to model the next
phase of oxidation: transformation of the observed
high coverage structures to copper oxide Cu2O.
Acknowledgments
The authors would like to thank the Finnish IT
center for science, CSC, for computer resources.
T. Kangas et al. / Surface Science 584 (2005) 62–69 69
Financial support from the SURFOX project of
National Technology Agency of Finland, TEKES,
is gratefully acknowledged (Grant No. 40246/04).
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