Sensors and Actuators B 139 (2009) 520–526
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
First-principles study of O2 adsorption on the LaFeO3 (0 1 0) surface
Xing Liu a , Jifan Hu a,∗ , Bin Cheng a , Hongwei Qin a , Ming Zhao a , Chuanlu Yang b
a
b
State Key Laboratory of Crystal Material, Department of Physics, Shandong University, Hongjialou 5#, Jinan 250100, PR China
Department of Physics and Electronic Engineering, Ludong University, Yantai 264025, PR China
a r t i c l e
i n f o
Article history:
Received 10 December 2008
Received in revised form 9 March 2009
Accepted 21 March 2009
Available online 31 March 2009
Keywords:
Perovskites
Surface structure
Oxygen adsorption
Density functional theory (DFT)
a b s t r a c t
The adsorption of O2 on LaFeO3 (0 1 0) surface is studied with first-principles calculation based on density
functional theory. Calculations for LaFeO3 (0 1 0) surface show that the surface states are near Fermi energy
level and mainly caused by Fe 3d orbital. Calculations for O2 adsorption predict that Fe ion site is most
favorable for molecular oxygen adsorption, and the side-on adsorption mode is more stable than endon one, while the end-on adsorption molecular oxygen is easily excited than side-on one. The bonding
mechanism of O2 adsorption on the Fe ions is also studied.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
The rare earth perovskite-type complex oxides (LnBO3 ) that contain transition metal ions (B) show a range of interesting properties.
They are used as materials for fuel cells [1,2], photocatalysis [3] and
catalysis [4,5]. A number of perovskite oxides have been proposed
earlier as gas-sensor materials because of their stability at high
temperature and in chemical atmospheres [6–11]. As gas sensing
material, LaFeO3 and its derivative have been paid much attention
for their excellent sensitivity [12–15].
According to the classic theory of gas sensing [16], the basis
of gas sensing behaviour in LaFeO3 (p-type semiconductor) is the
observed increase in surface conductivity due to the adsorption
of oxygen. Oxygen ionosorbs on to the surface, extracting valence
electrons. The hole concentration in surface region increases,
resulting in the increase of conductivity. In general, for O2 adsorption on metal clusters or surface, there are three reaction types
with the substrate: a molecular physisorption state, a molecular
chemisorbed state [17,18] and a dissociative chemisorbed state [19].
The widely used model of adsorption process is thought to be:
O2(gas) ↔ O2(physisorbed)
−
O2(physisorbed) + e ↔ O2
−
−
O2 + e ↔ 2O
(1)
−
−
(2)
(3)
Two ionosorbs species are assumed to form. One is a superoxo,
O2 − , which does not react with reducing gases. The other is atomic
∗ Corresponding author. Tel.: +86 531 8856 6143; fax: +86 531 8856 5167.
E-mail address: hu-jf@vip.163.com (J. Hu).
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.03.052
ion O− , which reacts rapidly with reducing gases. Nevertheless this
model may be viewed as a useful but limited guide to the surface
process of oxygen on LaFeO3 , which makes no reference to, such as
the placement or geometry of the oxygen species.
The oxygen adsorption processes play an important role to the
function of any LaFeO3 based sensing device. A deeper understanding of the adsorption, preferably at an atomistic and quantum
mechanical level, would provide a basis for a more detailed characterization of sensing action [20]. At present we have found no
reports on the oxygen adsorption on LaFeO3 used density functional
theory (DFT) calculations. The current work therefore aims to use
DFT calculations to generate more accurate models of the oxygen
adsorbate species that form on LaFeO3 surface.
2. Computational method
Computations for our present work were performed at the
spin unrestricted generalized approximation (GGA) level to density
functional theory (DFT) using the DMol3 suit of programs [21,22].
DMol3 uses numerical orbital for the basis functions, where each
function corresponds to an atomic orbital. Double numerical basis
sets with polarization functions (DNP) were used for global orbital
cut off of 5.0 Å and a Fermi smearing of 0.005 Ha were used to
improve the computational performance. The sizes of these DNP
basis sets are comparable to the 6-31G** basis sets of Hehre et
al. [23]. However, they are believed to be much more accurate
than a Gaussian basis set of the same size. A hardness conserving
semilocal pseudopotential, density functional semicore pseudopotential (DSSP) was used. The k-point sampling was done using
Monkhorst-Pack scheme [24]. These sets were used in all the calculations.
X. Liu et al. / Sensors and Actuators B 139 (2009) 520–526
521
Fig. 3. Total DOS of the LaFeO3 (0 1 0) surface: (a) bulk; (b) and (c) layered DOS of
the relaxed surface.
Fig. 1. The side view of the ideal LaFeO3 (0 1 0) surface. La atoms are shown in blue,
Fe atoms in Purple, and O atoms in red. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of the article.)
The LaFeO3 crystal with an orthorhombic perovskite-type structure in the Pnma (62) was calculated with 20 atoms in the unit
cell. The crystal structure parameters used in our calculation came
from experiment [15], and the lattice constants were a = 5.535 Å,
b = 7.888 Å, and c = 5.599 Å, the atomic fractional coordinates are Fe
(0.0000, 0.0000, 0.0000), O1 (0.2709, 0.0347, 0.2294), O2 (0.4940,
0.25, 0.5644) and La (0.5184, 0.25, 0.0089). The bulk of LaFeO3 was
geometrically optimized with a 4 × 3 × 4 k-grid sampling in the brillouin zone. The convergence criteria of optimal geometry based on
the energy, force and displacement convergence, were 2 × 10−5 Ha,
4 × 10−3 Ha/Å´ and 5 × 10−3 Å, respectively. The threshold of density
matrix convergence was set to 1 × 10−6 . After the optimization of
bulk LaFeO3 , the (0 1 0) surface was cleaved, followed by the construction of a 10 Å vacuum layer added to the unit cell of the layers
to simulate periodic boundary conditions, as shown in Fig. 1. The
LaFeO3 (0 1 0) surface was modeled as the 1 × 1 slab (a = 5.535 Å,
b = 16.20406 Å, and c = 5.599 Å) with eight atomic layers which was
stacked according to the order of 2O–2Fe–2O–2LaO along normal to
the surface plane containing four LaFeO3 formula units. The internal coordinates of the atoms were fully relaxed with surface unit
cell fixed. Geometries and adsorption energies of both clean LaFeO3
(0 1 0) surface and oxygen adsorption on the LaFeO3 (0 1 0) surface
were calculated with a 4 × 1 × 4 k-grid sampling for the LaFeO3
surface. The convergence criteria of optimal geometry based on
the energy, force and displacement convergence, were the same
as that of LaFeO3 bulk. The surface atoms and oxygen molecular
were relaxed during O2 adsorption.
The adsorption energy can be expressed as the following equation:
Eads = Esubstrate + Eadsorbate − Esubstrate–adsorbate
(4)
where Esubstrate–adsorbate is the total energy of the adsorbate–
substrate system in the equilibrium state, Esubstrate and Eadsorbate
Fig. 2. The top view of the outermost three layers of the LaFeO3 (0 1 0) surface: (a) the ideal surface and (b) the relaxed surface.
522
X. Liu et al. / Sensors and Actuators B 139 (2009) 520–526
are the total energy of substrate and adsorbate, respectively. By this
definition, a positive value, corresponding to an exothermic process,
indicates a stable adsorption.
The calculated bond energy and bond length for the gas-phase
O2 are 654.340 kJ/mol and 1.214 Å, which was obtained by putting
a molecule of oxygen in a 10 Å × 10 Å × 10 Å cubic crystal. The corresponding experimental values are 506.6 kJ/mol and 1.21 Å [25].
3. Results and discussion
3.1. Calculation for the LaFeO3 (0 1 0) surface
Fig. 4. The partial density of state of Fe and O ions of outermost three layer.
The clean LaFeO3 (0 1 0) surface was optimized and no reconstruction of the LaFeO3 (0 1 0) surface was found. Table 1 lists the
ions displacement magnitudes of all atomic layers. For the top layers, O1f , Fe2f , O3f and O4f sites move downward, while La4f upward.
For the bottom layers, Fe2f , O3f , O4f and La4f sites move upward
except O1f (downward). It is found that La atom made the greatest upward relaxation. This indicates an enrichment of Lanthanum
Fig. 5. The configuration of molecular oxygen adsorption on LaFeO3 (0 1 0) surface. The left drawings are the configurations of initial adsorption and the rights are that of
final adsorption.
X. Liu et al. / Sensors and Actuators B 139 (2009) 520–526
523
Table 1
The ion displacement for all the atoms of LaFeO3 (0 1 0) surface.
Displacement (Å) along the [0 1 0] direction
Top layers
Bottom layers
O1f
Fe2f
O3f
O4f
La4f
−0.391
−0.154
−0.203
0.026
−0.027
0.121
−0.080
0.052
0.240
0.428
Positive values imply displacements along the Y-axis of Fig. 1 and negative values
along the opposite direction of Y-axis.
on the LaFeO3 (0 1 0) surface, which is also found in experiments
[26,27].
Fig. 2 shows the top view of uppermost three layers of the
LaFeO3 (0 1 0) surface before and after relaxation. After relaxation,
the entire bond length and bond angle are changed. For example,
before relaxation the equilibrium bond length of O1f –Fe2f (2.014 Å
or 1.933 Å) is equal to that of Fe2f –O3f . However, after relaxation
the bond lengths of O1f –Fe2f and Fe2f –O3f are completely different.
Therefore, the relaxation effect is notable for the surface atoms in
LaFeO3 (0 1 0) surface.
Fig. 3 shows the total density of states (DOS) of the LaFeO3 bulk
and layered DOS of the (0 1 0) surface. It indicates that the peak
shape and position of density of states of the bottom layers are
similar to the bulk. However, the density of states of the top layers
is different from the bulk. For the top layers, the conduction band
overlaps with the valance band, leading to a good surface conductivity. The DOS peaks of surface states are near Fermi energy level,
which is associated with the dangling bond on the surface. It is
Fig. 6. DOS of free O2 and adsorbed O2 on the LaFeO3 (0 1 0) surface for M1–M4.
obvious that Fe ions at the surface are not completely coordinated
for having one missing neighboring O2− . Therefore, the surface
states LaFeO3 (0 1 0) surface is mainly caused by Fe 3d orbital, with
some finite O 2p orbital (Fig. 4). Moreover, these surface states
represent the active bonding orbital. So we can conclude that the
surface Fe ions will be active in gas adsorption process. To con-
Fig. 7. Partial density of states and deformation electron density of adsorbed O2 and Fe ion on which the O2 adsorbed for M1 and M2. In the electron density spectra, charge
flows from the yellow into blue regions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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X. Liu et al. / Sensors and Actuators B 139 (2009) 520–526
Table 2
Properties of the adsorption oxygen molecular on the LaFeO3 (0 1 0) surface.
Configurations
d (Å)
do–o (Å)
Eads (eV)
o–o (cm−1 )
q(O2 )
M1
M2
M3
M4
Calculated free O2
O2 −
1.781
1.894
3.274
4.788
1.259
1.297
1.223
1.223
1.215
1.260a
0.70
0.36
0.06
0.06
–
1319
1202
1487
1491
1588
1098b
−0.12
−0.15
−0.05
−0.06
–
a
b
Ref. [28].
Ref. [29].
firm this we calculated the oxygen adsorption on the LaFeO3 (0 1 0)
surface.
3.2. Molecular oxygen adsorption on the LaFeO3 (0 1 0) surface
Some adsorption modes were chosen to simulate the O2 adsorption on the LaFeO3 (0 1 0) surface. The configurations of initial and
final adsorption were depicted in Fig. 5. It is obvious that oxygen
molecular on the top sites of La and O ions become far away from the
La and O ions sites but close to the Fe ions sites. This may indicate
that Fe ions play an important role in the oxygen adsorption. Further calculations were done, the equilibrium distance (d) between
adsorbed oxygen and the adsorption site, the optimized O–O bond
lengths (do–o ), adsorption energies (Eads ), Mulliken charges (q(O2 ))
and the O–O vibrational frequencies (o–o ) for equilibrium modes
are listed in Table 2. The equilibrium distance Oad –O (3.274 Å) and
Oad –La (4.788 Å) for M3 and M4 are much longer than that of La–O
and O–O in the bulk. The adsorption energies of O2 adsorption on O
(M3) and La (M4) sites are only 0.06 eV and the O–O bond lengths
are both found to be 1.223 Å, slightly elongated by 0.008 Å. All these
results show that the O2 adsorption on O (M3) and La (M4) sites are
weak and unstable. So the sites of surface La and O ions are not the
active sites for oxygen adsorption.
For the modes (side-on Fe ion) M1 and (end-on) M2, the equilibrium distances are 1.781 Å and 1.894 Å which are shorter than
that of Fe–O (about 2.00 Å) in bulk. It shows that the adsorbed oxygen forms bonds with Fe ion. The O–O bonds of the adsorption O2
are elongated to be 1.26 and 1.30 Å, and vibrational frequencies
are 1319 and 1202 cm−1 , respectively. Referring the bond length
(1.26 Å) [28] and vibrational frequency of O2 − (1098 cm−1 ) [29], it
suggests the existence of a superoxo O2 − . Meanwhile, it indicates
that the Fe ions dominate the adsorption of oxygen gas, and this is
consistent with the experiment result that the catalytic activity of
perovskite compounds is essentially controlled by the B-site metal
[30–33].
Further insight into the bonding mechanism for O2 on the
LaFeO3 (0 1 0) surface can be obtained by analyzing the density of
the state of the adsorbed oxygen. As shown in Fig. 6, the DOS of O2
adsorbed at M3 and M4 sites are both similar to the DOS of free
O2 only with the bands shift downward. It indicates that characters
of O–O bond in O2 adsorption on La and O ions are not changed. It
should be a physical adsorption. However, the DOS of O2 adsorbed
at M1 and M2 sites changes greatly. The splitting and broadening of DOS peaks of adsorbed O2 at M1 and M2 indicate a strong
Fig. 8. Frontier orbitals HOMO-1, HOMO and LUMO of M1and M2 together with their corresponding energies. The molecular orbitals correspond to isovalues of 0.03 a.u.
X. Liu et al. / Sensors and Actuators B 139 (2009) 520–526
interaction between 2p orbital of adsorbed O2 and 3d orbital of
the Fe ion (see Fig. 7(a and c)). As a result, the ␲g states shift to
lower binding energies, thereby increasing their electron population. This shift is the largest for M2, suggesting that in this mode
the oxygen molecule has the strongest interaction with the Fe ions.
This assessment is further supported by data on the electron charge
distribution for M1 and M2 (Table 2). The oxygen molecule takes
electrons from Fe ions, thus filling its ␲g orbital, grabbing −0.15e
in M2 and −0.12e in M1. This extra charge is distributed equally
between the oxygen atoms in M2; in M1, however, most of which
is allocated on the far O atom.
It is well known that Mulliken populations are not expectation
values. They certainly have no real physical significance. However, an analysis of the difference of electron density maps of the
adsorbed oxygen and Fe ions (Fig. 7(b and d)) confirm this trend; the
density isosurface clearly shows charge transfer from the Fe ion into
the adsorbed oxygen ␲g orbital with charge transfer in M1 having
a lesser extent into the oxygen than that in M2. This charge transfer
weakens the O–O bond with lengthening the bond and reducing its
stretching frequency. It can be seen that the more the charge transfer, the more the O–O bond lengthens (Table 2). Such structural
changes suggest that these molecular sates are likely precursors for
O2 dissociation.
On the other hand, Frontier orbital HOMO-1, HOMO, LUMO of
M1 and M2 together with their corresponding energies are shown
in Fig. 8. For side-on site (M1), the dz2 and dxy orbital of Fe ion compose the HOMO-1 orbital; dyz and dxz compose the LUMO orbital.
For end-on site (M2), dyz and dxz compose the HOMO orbital; dx2 −y2
composes the LUMO orbital. It indicates that different geometries of adsorbed oxygen cause different orbital energy changes
of surface Fe ions. Meanwhile, the HOMO–LUMO gap of the M2
(0.32 eV) is less than that of M1 (0.38 eV). From Table 2 it was
found that the bond length of adsorbed oxygen of M2 (1.297 Å)
was longer than that of M1 (1.259 Å) and adsorbed oxygen of M2
(−0.15) grabbed more electrons than that of M1 (−0.12). This shows
that the adsorbed oxygen with smaller band gap is more easily
excited.
From results and discussions above, the direct dissociation
adsorption of O2 is not found. It indicates that the O2 dissociation
on LaFeO3 surface belong to chemisorbed-precursor mechanism. In
this mechanism, the O2 molecule initially chemisorbs intact. Subsequent thermally driven kinetics determine the selectivity between
desorption and dissociation [34]. This is just the reason that in
experiments LaFeO3 and its derivative need to be heated to an
optimal temperature to reach their best gas response and catalysis.
4. Conclusions
The clean LaFeO3 (0 1 0) surface and O2 adsorption on it have
been investigated at the level of density functional theory. The
surface states of LaFeO3 (0 1 0) surface appear near Fermi energy
level mainly caused by Fe 3d orbital. The surface Fe ions dominate the oxygen adsorption process. The adsorbed O2 on Fe ion is
much stable than that on La and O ions, and the bonding mechanism of adsorbed O2 on surface Fe ions is the strong interaction
between O 2p and Fe 3d orbital. In addition, our calculations show
no direct dissociation adsorption of O2 on the LaFeO3 (0 1 0) surface. It indicates that O2 dissociation on LaFeO3 surface belongs to
chemisorbed-precursor mechanism.
Acknowledgements
This work is supported by the National Natural Science Foundation of China under Grant Nos. 50872069 and 50872074.
525
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Biographies
Xing Liu received her BSc degree from the Department of Physics, Shangqiu Normal
College, Henan, China in 2004. She is currently working toward the PhD degree at
the Department of Physics, Shandong University. Her research interests focus on
solid-state gas sensors.
Jifan Hu received his PhD from the Institute of Physics of Chinese Academy of Science in 1993. He is currently a professor at the Department of Physics, Shandong
University. The current fields of interest are magnetism materials, functional alloy
and rare earth oxide materials.
Bin Cheng received his MS from the Department of Physics and Microelectronics,
Shandong University, China in 2008. The current fields of interest are nano and
magnetism materials.
Hongwei Qin She is currently a professor at the Department of Physics, Shandong
University. The current fields of interest are magnetism materials, functional alloy
and rare earth oxide materials.
Ming Zhao is currently working toward the MS degree at the Department of Physics,
Shandong University, China in 2008. His research interests focus on solid-state gas
sensors.
Chuanlu Yang received his PhD from Sichuan University in 2000. He is currently a
professor at the Department of Physics and Electronic Engineering, Ludong University. His reach field is atomic and molecular physics.