X-ray photoelectron spectroscopy investigation on chemical statesof oxygen on surfaces of mixed electronic–ionic conducting

La0.6Sr0.4Co1�yFeyO3 ceramics

Qing Xu*, Duan-ping Huang, Wen Chen, Hao Wang, Bi-tao Wang, Run-zhang YuanState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering,

Wuhan University of Technology, 122 Luosi Road, Wuhan 430070, PR China

Received 4 November 2003; received in revised form 30 December 2003; accepted 30 December 2003

Abstract

The chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1�yFeyO3 ceramics was

characterized by X-ray photoelectron spectroscopy (XPS). It was ascertained that there are five different kinds of oxygen on the

ceramic surfaces, including lattice oxygen (OL), chemisorbed oxygen (OC) in the forms of O2�, O�, and O2�, and oxygen in

hydroxyl environment (OH). The concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/

Fe ratio. In order to examine the relation between the chemical states of oxygen on the surfaces and the electrical nature of the

ceramics, the mixed electronic–ionic conducting properties were investigated. At an identical measuring temperature, the

electronic conductivity and ionic conductivity of La0.6Sr0.4Co1�yFeyO3 ceramics tended to rise with the increase of Co/Fe ratio.

It was considered that the mixed electronic–ionic conducting properties are responsible for the complex chemical states of

oxygen on the ceramic surfaces.

# 2004 Elsevier B.V. All rights reserved.

PACS: 72.60; 79.60

Keywords: La0.6Sr0.4Co1�yFeyO3; Perovskite-type ceramics; XPS; Chemical state; Oxygen; Mixed electronic–ionic conducting properties

1. Introduction

In recent years, there is a growing interest of

investigating perovskite-type complex oxides of

La1�xSrxCo1�yFeyO3 composition because of their

superior mixed electronic–ionic conducting proper-

ties. At elevated temperatures (about 800 8C), the

La1�xSrxCo1�yFeyO3 compositions exhibit electronic

conductivities exceeding 102 S cm�1 and oxygen ionic

conductivities on the order of 10�2 to 1.0 S cm�1,

making them promising candidate materials for many

technical applications, including cathodes for inter-

mediate temperature solid oxide fuel cells, oxygen

separation membranes, membrane reactors for syngas

production, and catalysts for oxidation of hydrocarbons

[1–3]. Among these La1�xSrxCo1�yFeyO3 composi-

tions, La0.6Sr0.4Co1�yFeyO3 (x ¼ 0:4) oxides have

attracted considerable attention [4–12].

The mixed electronic–ionic conduction of

La1�xSrxCo1�yFeyO3 oxides is essentially a kind of

Applied Surface Science 228 (2004) 110–114

* Corresponding author. Tel.: þ86-27-87864033;

fax: þ86-27-87642079.

E-mail address: xuqing@mail.whut.edu.cn (Q. Xu).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2003.12.030

bulk electrical transport property. However, it has been

noticed that the performance of La1�xSrxCo1�yFeyO3

during practical applications highly depended on the

behavior of oxygen on the surface of the material [5–

7]. Therefore, it is necessary to investigate the che-

mical state of oxygen on the surface to increase the

understanding about surface kinetic process of the

material during applications. Despite of the extensive

investigations on La1�xSrxCo1�yFeyO3 oxides invol-

ving electronic and ionic conducting properties [8,9],

bulk diffusion and surface exchange of oxygen [10,11]

and oxidation catalytic activity [12], the information

about the chemical state of oxygen on the surface of

the material is relatively limited. X-ray photoelectron

spectroscopy (XPS) is an effective surface analytical

technique for solids, providing qualitative and quan-

titative information about chemical states of consti-

tuents. XPS has been successfully applied to

characterize the chemical states of oxygen on the

surfaces for various perovskite-type ceramics

[13,14]. In present work, the chemical state of oxygen

on the surfaces of mixed electronic–ionic conducting

La0.6Sr0.4Co1�yFeyO3 ceramics was analyzed by XPS.

Moreover, the relation between the chemical state of

oxygen on surfaces and the electrical properties of the

ceramics was also investigated.

2. Experimental

Reagent grade La(NO3)2�6H2O, Sr(NO3)2,

Fe(NO3)3�9H2O, Co(NO3)2�6H2O, and glycine were

used as starting materials. La0.6Sr0.4Co1�yFeyO3

(y ¼ 0–1.0) powders were synthesized by a glycine-

nitrate process (GNP), which has been reported else-

where [15]. A single-phase perovskite structure with

rhombohedral symmetry was identified for the pow-

ders by X-ray diffraction (XRD). Scanning electron

microscope (SEM) analysis shows that the powders

consist of homogeneous, fine particles with the mean

sizes of 200–400 nm depending on their composition.

The powders were uniaxially pressed into rectangular

bars (30 mm � 4 mm � 4 mm) and disks (13 mm in

diameter and 2 mm in thickness), respectively. Then

the compressed powders were sintered at 1200 8C for

4 h in air.

XPS measurement was performed at room tempera-

ture by a VG Scientific ESCALAB MK II multi-

technique electron spectrometer using Al Ka radia-

tion. The instrument was operated at a power of 125 W

(12:5 kV � 10 mA) with a passing energy of 50 eVand

a scanning step of 0.05 eV. The XPS survey spectrum

and O 1s spectrum were taken from the ground

surfaces of ceramic specimens in an analysis chamber

under a pressure below 10�6 Pa. The binding energy

of C 1s (284.6 eV) was used as an internal standard.

The ceramic specimens were polished to ensure sur-

face flatness. The rectangular specimens were painted

with platinum paste for measuring electronic conduc-

tivity. The electronic conductivity was then measured

at 20–900 8C by a dc four-terminal method in air. The

oxygen ionic conductivity was measured using disk

specimens by the two-terminal electron blocking elec-

trode method described by Chen et al. [9]. Fig. 1 shows

the configuration of electrochemical cells for measur-

ing ionic conductivity. Y2O3 stabilized ZrO2 (YSZ)

disks with a composition of (Zr0.92Y0.08)O2�d were

used as electron blocking electrodes. The YSZ disks

were prepared using a sol–gel method by sintering at

1450 8C for 4 h. As-sintered YSZ disks were polished

to about 0.5 mm in thickness and then painted with

platinum paste on the outer surfaces. The YSZ disks

were mechanically contacted with the both surfaces of

La0.6Sr0.4Co1�yFeyO3 disks. The ac impedance spec-

troscopy of the electrochemical cells was measured by

a TH2816 precision digital bridge (0.05–150 kHz) at

400–800 8C in air. Taking the geometric factors of

La0.6Sr0.4Co1�yFeyO3 disks into consideration, the

ionic conductivities of the specimens were determined

by fitting measured impedance plots using Zview2.1

software.

3. Results and discussion

Fig. 2 shows the XPS survey spectrum of La0.6Sr0.4-

Co0.4Fe0.6O3 (y ¼ 0:6) specimen. Six relatively strong

Fig. 1. Schematic diagram of electrochemical cells.

Q. Xu et al. / Applied Surface Science 228 (2004) 110–114 111

peaks can be observed, attributed to Sr 3d, C 1s, O 1s,

Fe 2p, Co 2p, and La 3d photoelectrons, respectively.

The C 1s peak was assigned to the adventitious carbon

for calibrating binding energy as a reference. The XPS

survey spectra of the specimens with other composi-

tions are very similar to that of La0.6Sr0.4Co0.4Fe0.6O3

(y ¼ 0:6) specimen. The result of XPS survey spec-

trum analysis is in agreement with the elementary

composition of the specimens.

Fig. 3 shows the O 1s spectra of La0.6Sr0.4-

Co1�yFeyO3 specimens with different compositions.

The O 1s spectra are identical in shape for the speci-

mens with different compositions, showing two

slightly asymmetric peaks. This implies that the spec-

tra originated from the contribution of oxygen in

different chemical environments on the ceramic sur-

faces. After a deconvolution of measured photoelec-

tron signals, a peak fitting was performed for the O 1s

spectra. During the peak fitting, the full width at half

maximum (FWHM) and Gaussian/Lorentzian ratio of

the O 1s peaks corresponding to different kinds of

oxygen were kept as constant values of 1.8 eVand 0.3,

respectively. Fig. 4 shows the fitting pattern of O 1s

spectrum for La0.6Sr0.4Co0.8Fe0.2O3 (y ¼ 0:2) speci-

men. It can be seen that the O 1s spectrum comprises

five independent peaks with very small chemical

shifts, corresponding to five different kinds of oxygen

on the ceramic surfaces. The O 1s peak at 528.70 eV is

attributed to the lattice oxygen (OL) at the normal sites

of the perovskite structure, while the O 1s peaks at

530.50, 531.45, and 532.35 eV are assigned to the

chemisorbed oxygen (OC) in the forms of O2�, O�,

and O2�, respectively. In addition, the O 1s peak at

533.50 eV is ascribed to the oxygen in hydroxyl

environment (OH). Similar peak fitting results were

obtained for the specimens with other compositions.

The atomic percentages of oxygen in different che-

mical states on the surfaces of La0.6Sr0.4Co1�yFeyO3

ceramics are shown in Table 1. It was found that the

concentration of OC þ OH relative to total detected

oxygen enhances with the increase of Co/Fe ratio from

63.20% for La0.6Sr0.4FeO3 (y ¼ 1:0) specimen to

70.02% for La0.6Sr0.4CoO3 (y ¼ 0) specimen.

In order to examine the relation of the chemical

states of oxygen on the ceramic surfaces to the elec-

trical characteristics of La0.6Sr0.4Co1�yFeyO3 cera-

mics, the mixed electronic–ionic conducting

properties were investigated. Fig. 5 shows the electro-

nic conductivity (se) of La0.6Sr0.4Co1�yFeyO3 speci-

mens as a function of measuring temperature. The

electronic conductivity of La0.6Sr0.4CoO3 (y ¼ 0) spe-

Fig. 2. XPS survey spectrum of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6)

specimen.Fig. 3. O 1s spectra of La0.6Sr0.4Co1�yFeyO3 specimens with

different compositions.

Fig. 4. Fitting pattern of O 1s spectrum for La0.6Sr0.4Co0.8Fe0.2O3

(y ¼ 0:2) specimen.

112 Q. Xu et al. / Applied Surface Science 228 (2004) 110–114

cimen decreases with measuring temperature in the

range of 20–900 8C, while those of the specimens with

other compositions present a rather similar variation,

increasing with measuring temperature through a

maximum value near 600 8C and then decreasing.

Comparing the electronic conductivities of the speci-

mens measured at an identical temperature, it was

found that the electronic conductivity enhances with

the increase of Co/Fe ratio. Fig. 6 shows the ionic

conductivity (sion) of La0.6Sr0.4Co1�yFeyO3 speci-

mens as a function of measuring temperature. The

Arrhenius plots over the whole measuring temperature

range yielded straight lines. For a given composition,

the ionic conductivity increases with the elevation of

measuring temperature. In the case of an identical

measuring temperature, the variation of ionic conduc-

tivity with Co/Fe ratio is generally similar to that of

electronic conductivity, tending to rise with the

increase of Co/Fe ratio. The ionic conductivity of

the specimens varied in the ranges of 2:5 � 10�3 to

5:0 � 10�3 S cm�1 and 4:0 � 10�2 to 6:2 � 10�2 S

cm�1 at 600 and 800 8C, respectively. A previous

research indicates that the ionic conductivities of

La0.6Sr0.4Co1�yFeyO3 ceramics measured by a dc

four-terminal method using electron blocking electro-

des were in the range of 10�2 to 10�1 S cm�1 at

800 8C, showing a decrease of ionic conductivity with

Co/Fe ratio by almost one order of magnitude [1]. The

difference in ionic conductivity values for La0.6Sr0.4-

Co1�yFeyO3 ceramics between present work and the

previous research is presumably attributed to different

preparation processes and methods for measuring

ionic conductivity.

It is noteworthy that the variation trend of the

concentration of OC þ OH relative to total detected

oxygen with Co/Fe ratio is rather consistent with those

of electronic conductivity and ionic conductivity. It

infers an essential relation between the chemical states

of oxygen on the surfaces and the electrical nature of

the ceramics. This can be qualitatively interpreted

with respect to the formation process of OC and OH

on ceramic surfaces [13,16]. On one hand, oxygen

vacancies as a kind of defect on the ceramic surfaces

provided suitable adsorption sites for oxygen mole-

cules. On the other hand, the trapping of mobile

electrons of La0.6Sr0.4Co1�yFeyO3 ceramics by the

adsorbed oxygen molecules and dissociated oxygen

atoms resulted in different kinds of OC such as O2�,

O�, and O2�. Furthermore, the reaction between OC

and adsorbed gaseous H2O as a surface contaminant

generated hydroxyls. It indicates that the formation of

Table 1

Atomic percentages of oxygen in different chemical states on the

surfaces of La0.6Sr0.4Co1�yFeyO3 specimens

y Atomic percentage (%) (OC þ OH)/P

Oi (%)a

OL OC OH

O2� O� O2�

0 29.98 26.73 27.42 11.03 4.84 70.02

0.2 31.47 20.78 30.88 11.52 5.35 68.53

0.4 33.53 22.21 27.00 11.93 5.33 66.47

0.6 34.19 24.15 26.09 11.40 4.17 65.81

0.8 35.94 19.67 29.84 10.28 4.27 64.06

1.0 36.80 21.14 24.30 13.72 4.04 63.20

a The term ðOC þ OHÞ/P

Oi represents the concentration of

OC þ OH relative to total detected oxygen.

Fig. 5. The electronic conductivity (se) of La0.6Sr0.4Co1�yFeyO3

specimens as a function of measuring temperature.

Fig. 6. The ionic conductivity (sion) of La0.6Sr0.4Co1�yFeyO3

specimens as a function of measuring temperature.

Q. Xu et al. / Applied Surface Science 228 (2004) 110–114 113

OH is closely associated with the appearance of OC on

the ceramic surfaces. The electronic conductivity and

ionic conductivity of La0.6Sr0.4Co1�yFeyO3 ceramics

depend on the concentration and mobility of electrical

carriers. It has been well established that small polaron

hopping and oxygen vacancy diffusion are responsible

for the electronic conduction and ionic conduction of

La1�xSrxCo1�yFeyO3 compositions, respectively [8].

For small polaron hopping, it is well known that the

mobility of small polarons is mainly determined by

temperature. For Sr-substituted lanthanum-transi-

tional metal perovskite-type oxides, the mobility of

oxygen vacancies is also highly dependent on tem-

perature [8] and is relatively insensitive to composi-

tion when temperature is unchanged [17]. Therefore, it

can be deduced that the concentration of electrical

carries appears to be the main contributing factor to

the electrical transport properties of La0.6Sr0.4-

Co1�yFeyO3 ceramics at an identical temperature.

Hence, the rising of electronic conductivity and ionic

conductivity with Co/Fe ratio can be attributed to an

increase in the concentrations of mobile electrons and

oxygen vacancies. It benefits the formation of OC and

OH on the ceramic surfaces. As a result, the concen-

tration of OC þ OH relative to total detected oxygen

enhanced with the increase of Co/Fe ratio.

4. Conclusion

The chemical state of oxygen on the surfaces of

mixed electronic–ionic conducting La0.6Sr0.4-

Co1�yFeyO3 ceramics was characterized by XPS. It

was detected that there are five different kinds of

oxygen on the ceramic surfaces, including lattice

oxygen, chemisorbed oxygen such as O2�, O�, and

O2�, and oxygen in hydroxyl environment. An essen-

tial relation between the chemical states of oxygen on

the surfaces and the electrical nature of the ceramics

was certified. The concentration of OC þ OH relative

to total detected oxygen enhanced with the increase of

Co/Fe ratio.

Acknowledgements

This work was financially supported by the Special

Research Found for the Doctoral Program of High

Education (grant no. 20330497008), the Natural

Science Foundation of Hubei Province of China (grant

no. 2001ABB075), and the Foundation for Excellent

Youths of Wuhan City of China (grant no.

20015005031). The State Key Laboratory of

Advanced Technology for Materials Synthesis and

Processing also provided partial financial support

for this work.

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