Density functional theory study of O atom

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					Density functional theory study of O atom adsorption on the
Fe (100) surface
Ying Li, Ming Li*, Baohui Zhang, Silei Wang, Wei Shen, Jinsheng Zhang
Department of Chemistry, Southwest University, Chongqing 400715, Republic of China




*
    Corresponding author. liming@swu.edu.cn

                                                                                     1
Abstract

    First-principles plan wave calculations based on density-functional theory (DFT) are used to

study the oxygen atom adsorbed at Fe (100) surface systematically. As shown, the most stable

adsorption sites on Fe (100) surface are 4F and 2F, the most stable adsorption site under the Fe

(100) surface is tetrahedral site. The existence of on-surface O atom strengthens the adsorption

energy of the sub-surface O atom and the stability of the whole adsorption system.


Keywords: Density functional theory; Adsorption; Relaxation; Adsorption energy




                                                                                              2
1. Introduction

     The adsorption of oxygen on iron surface has attracted much research [1-10] in recent years,

for it is of great fundamental and practical interest in oxidation, passivation and corrosion (rusting).

The comprehension of the nature of oxygen adsorption on iron surface is crucially important in

material science. From a fundamental viewpoint oxygen adsorption on iron may provide important

information for the formation of iron oxides and the embrittlement of the metal. However, the

oxidation process passing through many different phases is not well understood. The surface

structure of originating iron-oxide layers is complex and highly dependent on oxidation

conditions.

     Studies of oxygen adsorption on Fe (100) surface by experiments provide different results

[5-8] regarding surface structure. In 1973, the c(2×2) structure was linked with carbon impurities

rather than with O over layer[8]. In 1975, Legg et al. found that a clean (100) surface exposed to

about 8L of dry oxygen at room temperature (300K) exhibits a (1×1) structure whose low-energy

electron diffraction (LEED) spectra differs from those of a clean surface. In the same year,

Simmons et al. and Brucker et al. concluded that at room temperature O atoms chemisorbed in a c

(2×2) surface at ~3L[6,7]. The experimental investigations of O/Fe(100) system in 1984[4], using

several techniques, allowed to formulate a three-stage model of oxidation, consisting of : (i)

dissociative oxygen chemisorbed at fourfold hollow or twofold bridge sites at coverage up to 3L,

(ii) incorporation of oxygen into the selvage between 3 and 20L, and (iii) formation of γ-Fe2O3

above 20L.

     Recent studies agree on the adsorption geometry [1-3]. Spin- and angular- resolves

photo-emission, and LEED-emission by Kim and Vescovo [3] allowed to identify two well



                                                                                                      3
defined, ordered O coverage : A c(2×2) structure at 0.25ML, followed by a c (3×1) structure at

0.33ML. Scanning tunneling microscopy (STM) and LEED images[2] confirmed that at room

temperature oxygen forms a c(2×2) over layer at 0.25ML coverage.

     Theoretical studies of oxygen adsorption at compact simple metal surface have demonstrated

a possibility of coexistence on- and sub-surface oxygen [11-12]. The possibility of oxygen binding

in subsurface sites and formation of bulk-like iron-oxide layers has been demonstrated by various

experimental techniques. P.Blonski et al. concluded that the oxygen atom binds in tetrahedral (tet)

and octahedral (oct) site on Fe (100) sub-surface. There is no systematic study of the character of

the sub-surface oxygen atom and the character of two O atoms adsorption on Fe (100)

simultaneity. In this work, we discuss these problems and all the calculation is performed in p(1×

1) unit surface.


2. Models and methods

     The Fe (100) surface was model by slabs consisting of seven layers separated by a vacuum

layer of 10Å and repeated periodically throughout the space. For the clean Fe (100) surface and the

on-surface adsorption configuration, allowed the three topmost layers relaxed. For the sub-surface

adsorption configuration, three layers under the O atom and the above layers of the adsorbed O

atom allowed to relax. Brillouin zone sampling is performed on Monkhorst-Pack special points.

The plane-wave energy cutoff is set to 340eV for all the model and the k-points mesh is set to 9×

9×1. The smearing width is set to 0.1eV and the pseudo-potential approximation is set to

ultra-soft (USPP), which includes non-linear core corrections to account for possible valence-core

charge interaction due to the 3d valence states overlapping with the 3p semi-core states in iron

[25]. In this work, we use 1F, 2F and 4F to figure the adsorption configuration of O atom adsorbed



                                                                                                 4
on top, bridge and hollow site, use Fe (1), Fe (2) etc to figure the Fe atom on the first and second

layers and use O (1), O (2) to figure the O atom adsorbed on on-surface and sub-surface

respectively.

     All the calculation were performed within the frame-work of density-functional theory (DFT)

using a basic set consisting of plane waves, as implemented in CASTEP Package program [14] of

the materials studio 3.1 of the Accelrys Inc. We use the GGA-PW91 for all geometry

optimizations and report the energy from GGA-PBE [15-20]. Table 1 shows the results of the

clean Fe (100) surface with three topmost and four topmost layers relaxed respectively. Table 2

shows the results of the different adsorption sites of O atom on Fe (100) surface. Table 3 shows

the results of the different adsorption sites of O atom on sub-surface. Table 4 shows the results of

two O atoms adsorb on on-surface and sub-surface simultaneously. Figure 1 shows the geometry

of all adsorbed configuration. Figure 2 shows the density of states of the clean Fe (100) surface

and the 2F+tet (1-2) adsorption configuration.


3. Results and discussion

3.1. Clean Fe (100) surface

     The result of the Fe (100) with three topmost layers and four topmost layers relaxed is shown

in table 1. It indicates that the distance of the first two layers is compressed and the distance of the

second two layers is extended. The result of the first two layers is similar to the other reference

[14,22,23]. The distance of the second two layers approaches to lattice constant.


3.2. On-surface O atom

     The adsorption energy of on-surface O atom was calculated using the formula:

   E ad  ( E slabFe  E slab  E O ) . E slabFe is the total energy of the O/Fe system. E slab is the total
              O/         Fe               O/                                               Fe




                                                                                                          5
energy of the clean Fe(100) surface. E O is the half energy of an isolated O molecule.

     The adsorption energy of O atom and the O-Fe bond population analysis indicate that the

stability sequence of the different O atom adsorption sites on Fe (100) surface is 4F>2F>1F. The

adsorption energy of 4F is similar to 2F, indicating that 2F and 4F adsorption sites are all stable

on-surface adsorption sites. The O atom on the 4F site is the closest to the Fe (100) surface. The

upright distance from 4F O atom to the surface is 0.401Å and the O-Fe (1) bond length is 2.047 Å,

which agrees with the experiment [5] (0.53±0.6Å and 2.08 Å respectively). The O-Fe (1) bond

length is shorter than the O-Fe bond length in the FeO about 4%. The 1F O atom is the most

further to the surface; 2F O atom is in the middle. The interaction between the 4F O atom and the

surface is strong and the interaction between the 1F O atom and the surface is feeble. The

existence of 1F O atom compresses the first two layers while 2F O atom and 4F O atom extend the

first two layers. From the analysis, we can see that the charge of d type orbital transfers from the

surface Fe atom to the O empty p type orbital. It induces O atom chemisorbed on Fe surface. The

more negative charge the O atom is, the more stable adsorption.


3.3. Sub-surface O atom

     P.Blonski et al modeled the adsorption of O atom on sub-surface [13] and concluded that the

O atom can adsorb in tetrahedral (tet) and octahedral (oct) site on sub-surface. The tet adsorption

site is more stable. But there is no systematically analysis about the O sub-surface adsorption site.

     Research indicates that tet (1-2) is the only one stable adsorption site between the first and

the second layer. The adsorption energy is -2.51eV. There are three adsorption sites between the

second and the third layer. They are oct1 (2-3), tet (2-3) and oct2 (2-3) with the adsorption energy

of -1.75eV, -2.52eV and -2.04eV, respectively. The upright distance from the oct1 (2-3) site O



                                                                                                    6
atom to the second Fe layer, and from the oct2 (2-3) site O atom to the third Fe layer is 0.146 Å,

and 0.156 Å respectively. Tet (3-4) is the only stable adsorption site between the third and the

fourth layer with the adsorption energy of -2.50eV. We have tried to optimize the structure with

the O atom adsorbed on oct (1-2) and oct (3-4) site, and found that O atom on oct (1-2) move to

the site between the second and the third layer formed oct (2-3) stable adsorption site, and the O

atom on oct (3-4) move to the site between the fourth and the fifth layer formed oct (4-5) stable

adsorption site. The adsorption energy of O atom on oct (4-5) is -2.03eV and the O atom is almost

on the same plane with the fourth Fe layer. In tet (1-2), O-Fe (1), O-Fe (2) bond length is 1.761 Å

and 1.886 Å respectively. In tet (2-3), O-Fe (2), O-Fe (3) bond length is 1.960 Å and 1.884 Å

respectively. In tet (3-4), O-Fe (3), O-Fe (4) bond length is 1.901 Å and 1.902 Å respectively. The

site of O atom tends to regular tetrahedral and regular octahedral along with the O atom moving to

the interior of Fe bulk. It indicates that O atom has entered into the Fe bulk when it is adsorbed

under the third Fe layer. The most stable site of the O atom adsorbed on sub-surface of Fe (100) is

tet site. The adsorption energy of O atom adsorbed between the different Fe layers is similar.

There are two octahedral stable adsorption sites between the second and the third layer. Tet is only

one stable adsorption site between the first and the second layer, and between the third and the

fourth layer respectively. Only two stable adsorption structures, regular tetrahedral and octahedral,

exist under the fourth Fe layer. The existence of sub-surface O atom induces the more relaxation

of Fe (100) in z direction. This makes the surface structure of Fe (100) more relaxed.

     The charge of all the tet site O atom is similar, so does all the oct site O atom. The negative

charge of O atom comes mainly from the closest Fe atom’s d type orbital in the z direction. The

difference of O-Fe bond population at the same absorption site is few: the range is 0.50-0.69 for



                                                                                                   7
all the tet sites and 0.82-0.88 for all the oct sites severally. It indicates that oct site O-Fe bond is

more stable than tet site O-Fe bond. The total energy of oct adsorption system is lower about

0.16-1eV than tet adsorption system. The bond population of O-Fe3 and O-Fe4 in tet (3-4) site are

all 0.62. It further indicates that O atom is in the regular tet absorption site under the third Fe layer.


3.4. Two O atoms adsorbed on on-surface and sub-surface

simultaneously

     For the further study of the adsorption of O atom on Fe (100) surface, we study the structure

with two O atoms adsorbed on on-surface and sub-surface simultaneously. From calculation we

found three absorption structures: (1) 1F+oct1(2-3), means one O atom absorbed on 1F site, and

the other on oct (1) (2-3) site; (2) 2F+tet(1-2), means one O atom absorbed on 2F site, and the

other on tet (1-2); (3) 1F+tet(1-2), means one O atom absorbed on 1F site, and the other on tet (1-2)

(see fig1). The result is listed on table 4.

     In two O atoms adsorption configuration, the absorption energy of single O atom is defined

as: E ad  E slab  E slab          E O , E ad(1) means the adsorption energy of O(1) atom,
      O (1)  2 O / Fe O ( 2 ) / Fe           O



                                                                    O(
Eslab/ Fe means the total energy of two O atom adsorption system, E slab2 ) / Fe means the total
 2O



                                       O
energy of the O(2)/Fe system and E         means half energy of isolated oxygen molecule.

     For configuration 1F+oct1(2-3), the adsorption energy of on-surface O atom (2.62eV) being

compared with 1F adsorption energy (2.89eV) is slightly reduced and the distance from on-surface

O to the surface is the same as the 1F adsorption structure. The adsorption energy of sub-surface O

atom (2.62eV) apparently increased compared with oct1(2-3) adsorption configuration (1.75eV).

Compared with oct1(2-3) adsorption configuration, the O atom is closer to the third Fe layer. For

configuration 2F+tet(1-2), the adsorption energy of on-surface O atom (3.94eV) is slightly



                                                                                                        8
increased compare with 2F adsorption structure (3.82eV). The on-surface O atom is slightly away

from the surface, compared with 2F configuration (the distance of the on-surface O atom to the Fe

atom on the first layer change from 1.722Å to 1.74Å). The adsorption energy of sub-surface O

atom (3.62eV) is apparently increased compare with tet (1-2)(2.51eV). The sub-surface O atom is

closer to the second Fe layer. It is obvious that the existence of on-surface O atom strengthen the

adsorption energy of sub-surface O atom and the stability of the whole system. The analysis of

Density of state (see fig.2) indicates that there is strong interaction between two O atoms and

induces the system is more stable. For 1F+tet (1-2), the adsorption energy of on-surface O atom is

2.28eV, lower than 1F adsorption configuration (2.89eV). The upright distance is the same as 1F

adsorption structure from on-surface O atom to the surface. The adsorption energy of sub-surface

O atom (2.02eV) is lower than in tet(1-2) configuration (2.51eV). The sub-surface O atom is much

closer to the second Fe layer. We also tried to geometry the 4F+tet(1-2) configuration and the

result is the on-surface 4F O atom move to on-surface 2F site and then formed 2F+tet(1-2)

configuration.

     All of the above discussion indicates that the most stable adsorption configuration of two O

atoms adsorbed on Fe (100) surface is 2F+tet (1-2). The coexistence of on-surface O atom and

sub-surface O atom can strengthen the interaction between the O atom and the Fe(100) surface.

Compared with one O atom adsorption configuration, the charge of O atom on the same

adsorption site is similar. The population analysis indicates O–Fe bond is stable and the O atom is

chemisorbed on Fe (100) surface. The geometry change of on-surface O atom is slight, compared

with one O atom adsorption configuration.


4. Conclusions


                                                                                                 9
     First-principles calculations of oxygen adsorption have been performed for Fe (100) surface.

The results indicate that the most stable adsorption site for one O atom on Fe (100) surface is 4F

and 2F site with a preference of 4Fsite. The most stable adsorption site of O atom adsorbed on

sub-surface is tet site. There are two metastable adsorption oct sites between the second and the

third layer. The O atom locates in regular tet or oct adsorption site when the O atom is adsorbed

under the third Fe layer. The oct adsorption site can be considered as a intermediate state of the

process of O atom shift from the Fe surface to bulk inside.

     The most stable adsorption configuration of two O atoms is 2F+tet (1-2). The existence of

sub-surface O atom strengthens the adsorption energy of sub-surface O atom and the stability of

the adsorption system. The change of the adsorption energy and the geometry of on-surface O

atom is little and can be ignored.



Acknowledge

     This work is supported by the Municipal Science Foundation of Chongqing City of China

(No.CSTC-2004BA4024) and calculations are supported by professional Chen Hong in physics

department of South-West University, all authors here express their deep thanks.

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                                                                                                           11
    1F                       2F                         4F                       tet(1-2)




 oct1(2-3)                  tet(2-3)                 oct2(2-3)                   tet(3-4)




oct(4-5)                 1F+tet(1-2)               1F+oct1(2-3)                   2F+tet(1-2)
Fig.1. Side view of all the adsorption configuration: three on-surface adsorption configuration,

1F, 2F, 4F; seven sub-surface adsorption configuration, tet (1-2), oct1(2-3), tet (2-3), oct2(2-3),

tet (3-4), oct (4-5); three two O atom adsorption configuration, 1F+tet(1-2), 1F+oct1(2-3),

2F+tet(1-2)




   Fig.2. The electronic density of states (DOS) of the clean Fe (100) surface and 2F+tet

     (1-2) adsorption configuration. The vertical lines mark the fermi level.



                                                                                                12
Table1:Relaxation of the clean Fe(100) surface

                                                                                                      reference
                  Three topmost layers relaxed (%)   Four topmost layers relaxed (%)
                                                                                                  a
                                                                                            exp                   theob

     Δd12/d                     -3.58                             -3.78                -5    2%             -3.8% and

                                                                                                                  -3.5%
     Δd23/d                     -0.78                             -0.99                5     2%                    —


     Δd34/d                      -3.6                             -2.1                      —                      —


     Δd45/d                       —                               -3.4                      —                      —


a.     see [24]

b.     see[15],[24]




                                                                                                                          13
Table 2:Results of the O atom adsorbed at on Fe(100) surface

Adsorptio                                               Ead    R(O-Fe)   D(O-sur)   d(O)    d(Fe)   Popu;ation
             Δd1,2 (%)     Δd2,3(%)     Δd3,4(%)
  n site                                               (eV)      (Å)      (Å)        (e)     (e)     (O-Fe)

   1F           -1.4         -0.5         -4.7         -2.89    1.591     1.591     -0.26   0.29      0.89

   2F           0.74         -1.44        -4.05        -3.82    1.722     0.975     -0.46   0.58      0.97

   4F           0.59         -0.9         -4.0         -3.83    2.047     0.401     -0.61   0.70      1.03




                                                                                                             14
Table 3:Results of the O atom adsorbed on Fe(100) sub-surface

Adsorption                                                  D(O-Fe)             Δd12             Δd23    Δd34     d(O)              d(Fe)               Population
                  (eV)
                Ead                 R(O-Fe)(Å)
    site                                                     (Å)               (%)              (%)      (%)      (e)              (e)                       (O-Fe)

                                     O-Fe(1): 1.761                                                                            Fe(1): :0.21           O-Fe(1): 0.50;
  tet(1-2)           -2.51                                     —                    9.9          0.22     -2.0    -0.54
                                     O-Fe(2): 1.886                                                                            Fe(2):        0.14      O-Fe(2): 0.59

                                                                                                                               Fe(1): -0.01;
                                                            D(O-layer
oct 1 (2-3)          -1.75                    —                                     2.54         6.64    -1.87    -0.59        Fe(2):0.50;             O-Fe(2): 0.82
                                                            2):0.146
                                                                                                                               Fe(3):-0.07

                                   O-Fe(2): 1.960,                                                                             Fe(2):0.13;            O-Fe(2): 0.57;
  tet(2-3)           -2.52                                     —                -2.79            13.14   -0.86    -0.54
                                   O-Fe(3): 1.884                                                                               Fe(3):0.18             O-Fe(3): 0.69

                                                                                                                               Fe(2):-0.12;
                                                            D(O-layer
oct 2 (2-3)          -2.04                   —                                  -3.22            7.08     3.43    -0.60        Fe(3):0.47;             O-Fe(3): 0.88
                                                            3):0.156
                                                                                                                               Fe(4):-0.02

                                     O-Fe(3): 1.901                                                                            Fe(3):0.23;            O-Fe(3): 0.62;
  tet(3-4)           -2.50                                     —                -4.11            -0.48   12.80    -0.53
                                     O-Fe(4): 1.902                                                                            Fe(4):        0.17      O-Fe(4): 0.62

                                                                                                                               Fe(3):        0.02;
                                                            D(O-layer
 oct(4-5)            -2.03                   —                                      —             —        —      -0.59        Fe(4):0.44;             O-Fe(4): 0.83
                                                            4):0.001
                                                                                                                               Fe(5):0.02




Table 4:Results of O atoms adsorbed on Fe(100) surface and sub-surface simultaneity.

               E             D
                                                                                                                                  D(O-Fe2                             Population
Adsorpt       (O1            (O1        R           d(O1)          d(Fe)                   Population    E(O2)    R(O-Fe2)                           d(O2)
                                                                                                                                         )                            (O2—Fe)
ion site       )             -Fe     (O1-Fe)         (e)              (e)                    (O1-Fe)     (eV)        (Å)                              (e)
                                                                                                                                        (Å)
              (eV)           1)

                                                              Fe(1):        0.30;                                O2-Fe1:1.91                                     O2-Fe1:-0.10;
1F+oct                                                                                                                            O2-Fe2:
              -2.62      1.59          —            -0.28     Fe(2):        0.46;          O1-Fe1:0.85   -2.62   O2-Fe2:2.03                         -0.62       O2-Fe2: 1.93;
1(2-3)                                                                                                                              0.30
                                                              Fe(3):        -0.06                                O2-Fe3:1.87                                     O2-Fe3: 0.17

                                                               Fe1:         0.72;
  2F+                                O1-Fe:                                                                      O2-Fe1:1.89                                     O2-Fe1: 0.41;
              -3.94          —                      -0.46      Fe2:         0.15;          O1-Fe1:0.87   -3.62                          —            -0.59
tet(1-2)                              1.74                                                                       O2-Fe2:1.74                                     O2-Fe2: 0.76
                                                               Fe3:         0.01

                                                               Fe1:         0.52;
1F+tet                                                                                                           O2-Fe1:1.90                                     O2-Fe1: 0.40;
              -2.28      1.59          —            -0.29      Fe2:         0.16;          O1-Fe1:0.80   -2.02                          —            -0.58
 (1-2)                                                                                                           O2-Fe2:1.90                                     O2-Fe2: 0.71
                                                               Fe3:         0.02




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