On-surface and sub-surface oxygen on ideal and reconstructed Cu(100)

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.

mailto:[email protected]

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.


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).


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].


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


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.


Financial support from the SURFOX project of

National Technology Agency of Finland, TEKES,

is gratefully acknowledged (Grant No. 40246/04).

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On-surface and sub-surface oxygen on ideal and reconstructed Cu(100)

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