Gas-sensing and electrical properties of perovskite structure p-type barium-substituted bismuth ferrite

Article abstract of DOI:10.1039/c5ra01869b

Pure and Ba-substituted bismuth ferrite (BiFeO₃, BFO) powders were successfully synthesized via a sol-gel method. The effects of the Ba-substitution on the morphology, gas-sensing and electrical properties of BFO were studied. The gas-sensing tests show that the sensor based on Bi_0.9Ba_0.1FeO_2.95 (BBFO10) has high sensitivity, quick response, effective selectivity and excellent long-time stability. The conduction mechanism and gas-sensing mechanism of a BBFO10 sample were investigated by the impedance spectroscopy and it was found that the conduction is dominated by p-type hole conduction. The conductivity of the sensor is dependent on the oxygen partial pressures and the type of gas atmosphere. The enhanced gas-sensing performances of the BBFO10 sensor are attributed to the higher oxygen vacancy concentration which was induced by the substitution of Bi³⁺ ion by an aliovalent Ba²⁺ ion at the A-site of the perovskite structure. This journal is

Full text of DOI:10.1039/c5ra01869b

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PAPER
Gas-sensing and electrical properties of perovskite
structure p-type barium-substituted bismuth
ferrite
Cite this: RSC Adv., 2015, 5, 29618
*
Guangzhi Dong, Huiqing Fan, Hailin Tian, Jiawen Fang and Qiang Li
Pure and Ba-substituted bismuth ferrite (BiFeO3, BFO) powders were successfully synthesized via a sol–gel
method. The effects of the Ba-substitution on the morphology, gas-sensing and electrical properties of
BFO were studied. The gas-sensing tests show that the sensor based on Bi0.9Ba0.1FeO2.95 (BBFO10) has
high sensitivity, quick response, effective selectivity and excellent long-time stability. The conduction
mechanism and gas-sensing mechanism of a BBFO10 sample were investigated by the impedance
spectroscopy and it was found that the conduction is dominated by p-type hole conduction. The
conductivity of the sensor is dependent on the oxygen partial pressures and the type of gas atmosphere.
The enhanced gas-sensing performances of the BBFO10 sensor are attributed to the higher oxygen
vacancy concentration which was induced by the substitution of Bi3+ ion by an aliovalent Ba2+ ion at the
A-site of the perovskite structure.
Received 30th January 2015
Accepted 20th March 2015
DOI: 10.1039/c5ra01869b
www.rsc.org/advances
response and gas selectivity of oxide semiconductor sensors by
electronically sensitizing and chemically sensitizing them,
1. Introduction
respectively.
Metallic oxide-based gas sensors have attracted much
research interest due to the growing attention to environ-
mental problems, chemical process control and medical
applications. Among the available semi-conductive materials,
binary metallic oxides such as ZnO, SnO2 and Fe2O3 usually
exhibit excellent sensing properties.1–5 Yet it is a pity that their
long-term reliability is still questionable, especially when
working in a humid environment.6 It was reported that some
ternary metal oxides with a perovskite ABO3 structure have
more stable and reliable gas-sensing properties than binary
metal oxides do, which could be attributed to the ternary
oxides consisting of two differently sized cations and their
ability to accept a variety of dopants.7 Ferrites with perovskite-
structure, such as BiFeO3, LaFeO3, PrFeO3, EuFeO3, GdFeO3,
were oen found to have different degrees of gas-sensing
ability.6,8,9
Bismuth ferrite (BiFeO3, BFO) is well known for its multi-
ferroic properties owning both ferroelectric ordering and
antiferromagnetic ordering at room temperature, and it was
always considered to be a promising candidate for application
in memory, spintronics and magnetoelectric sensor
devices.10,11 Although BFO is also a p-type semiconductor oxide
that exhibits gas-sensing properties to some extent, the
researches on that area are still limited.12–14 As we know,
properly designed aliovalent dopants can enhance the gas
Herein, we prepared pure and bivalent Ba-substituted
BiFeO3 samples utilizing sol–gel method. The inuences of
the Ba-substitution on the morphology, gas-sensing and elec-
trical properties of BFO were studied in detail. The conduction
and gas-sensing mechanism of the samples were also
discussed.
2. Experimental
2.1. Materials and synthesis
BiFeO3 (BFO) and Bi0.9Ba0.1FeO2.95 (BBFO10) powders were
synthesized via sol–gel method using bismuth nitrate pen-
tahydrate (Bi(NO3)3$5H2O), barium nitrate (Ba(NO3)2), iron
nitrate nonahydrate(Fe(NO3)3$9H2O, 32 mmol), acetic acid
(CH3COOH, 25 mmol), ethylene glycol ((CH2OH)2, 1 mol) and
citric acid monohydrate (C6H8O7$H2O, 32 mmol) as starting
ingredients. Firstly, stoichiometric bismuth nitrate and
barium nitrate were dissolved in acetic acid, the iron nitrate
and citric acid monohydrate were separately dissolved in half
of the prescribed amount of ethylene glycol. Aer stirring for
half an hour, nitrates solutions were mixed together, and
then the citric acid monohydrate solution was added. A stable
sol was obtained by aging the nal transparent precursor for
24 h, aer drying the sol at 100 ꢀC for 24 h, and the obtained
gel was calcined at 450 ꢀC for 2 h to remove the organics.
Finally, aer sintering at 850 ꢀC for 2 h, the samples were
obtained.
State Key Laboratory of Solidication Processing, School of Materials Science and
Engineering, Northwestern Polytechnical University, Xi'an 710072, China. E-mail:
hqfan3@163.com; Fax: +86-29-88492642; Tel: +86-29-88494463
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2.2. Materials characterization
Powder X-ray diffraction (XRD) data were collected by using an
automated diffractometer (X'Pert PRO MPD, Philips, Eind-
hoven, The Netherlands) with a nickel lter (Cu-Ka radiation) in
2q range of 10–130ꢀ at room temperature. The surface
morphologies of the samples were observed by using a eld-
emission scanning electron microscopy (SEM; Supra 55, Zeiss,
Oberkochen, German). The X-ray photoelectron spectroscopy
(XPS) measurements were performed using an Omicron energy
analyser (PHI-5400, Perkin Elmer, Waltham, MA, USA). An AC
impedance analyzer (4294A, Agilent, San Diego, CA, USA) was
connected with gas response instrument to collect the imped-
ance data, which performed at different temperatures and gas
atmospheres over the frequency range of 40 Hz to 10 MHz under
a bias ac voltage of 1 V.
2.3. Gas sensor fabrication and measurements
The typical fabrication process of the gas sensors can be
described as follows.15 The as-synthesized powders were mixed
and ground with glycerol as adhesive in an agate mortar to form
a homogeneous paste, respectively. An alumina tube with Au
electrodes and platinum wires was used as a substrate which
was coated with the paste. A Ni–Cr alloy crossed alumina tube
was applied as a heating resistor for both substrate heating and
temperature controlling. Each element was dried under
Fig. 1 XRD patterns and SEM images (insets) for (a) BFO and (b)
BBFO10 samples.
and it is observed that the grain size decreases obviously as the
Bi3+ ions were replaced by Ba2+ ions at A-site.
ꢀ
The XPS spectra for BFO and BBFO10 samples were
measured and shown in Fig. 2. There are XPS photoelectron
peaks of Bi, Fe, O and C elements on the pure BFO surface, and
the C 1s peak located at 284.6 eV is used as the criterion to
rectify the binding energy of XPS spectra. Besides the charac-
teristic photoelectron peaks for Bi 4f7/2, Bi 4f5/2, O 1s, Fe 2p3/2
and Fe 2p1/2, the characteristic peaks of Ba 3d5/2 and Ba 3d3/2
can also be observed in the survey spectra of BBFO10.
To explore the effects of doping on oxygen state, the O 1s
XPS spectra are displayed in Fig. 2(b). The O 1s XPS spectra
show an asymmetric peak very close to 529 eV, while an
additional peak near 531 eV can be obviously observed in
both BFO and BBFO10 samples. Both curves can be Gaussian
tted by two symmetrical peaks, which are normally assigned
as the low binding energy (LBE) peak and the high binding
energy (HBE) peak of O 1s, respectively. The LBE peak is
ascribed to the O 1s binding energy of the BFO phase, while
the HBE peak is related with the loss of oxygen in the
sample.18–20 Comparing the area ratio of the tted HBE peak
with LBE peak, it is noted that the area ratio of HBE peak is
infrared light for several minutes and then calcined at 500 C
for 2 h in air. In order to improve their stability and repeat-
ability, the gas sensors were aged at 300 ꢀC for 10 days in air.16
The gas-sensing measurements were performed on a gas
response instrument (HW-30A, Hanwei Ltd, Zhengzhou,
China). In the measuring electric circuit of a gas sensor, a load
resistor is connected in series with a gas sensor. The circuit
voltage (Vc) is 5 V, and the output voltage (Vout) is the terminal
voltage of the load resistor (RL). The working temperature of the
sensor is adjusted by varying the heating voltage (Vh). When a
given amount of tested gas was injected into the chamber, the
resistance of the sensor changed, and the output voltage
changed accordingly. The gas-response sensitivity (S) is dened
as the ratio of Rgas/Rair, where Rgas and Rair are the resistance
values measured in reducing atmosphere and in air,
respectively.
3. Results and discussion
3.1. Crystalline structure, morphology and composition
The XRD results of BiFeO3 (BFO) and Bi0.9Ba0.1FeO2.95
(BBFO10), as shown in Fig. 1, exhibit a single-phase perovskite
structure and no secondary impurities phase was observed. The
diffraction peaks are indexed to be the rhombohedral BiFeO3
with space group R3c.
Table 1 Crystallographic data and structure refinement conditions for
BFO and BBFO10 samples
Sample
BFO
BBFO10
XRD Rietveld renements were carried out with an R3c space
group at room temperature by using FullProf program,17 and
the rened lattice parameters are shown in Table 1. The unit cell
volume of BBFO10 sample is larger than BFO, which might be
attributed to the larger radius of Ba2+ ion than Bi3+ ion. The
insets of Fig. 1 are the SEM micrographs of BFO and BBFO10,
˚
a/A
5.5797(1)
13.8719(2)
374.0116
5.50
5.5986(3)
13.7934(9)
374.4245
4.01
˚
c/A
3
˚
V/A
Rp/%
Rwp/%
7.59
5.08
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Fig. 2 XPS spectra of (a) wide-scan, (b) O 1s and Fe 2p (inset) for BFO
and BBFO10 samples.
Fig. 3 (a) Typical response and recovery curve of sensors based on
BBFO10 exposed to 100 ppm ethanol at different working tempera-
tures; (b) sensitivity of the gas sensors based on BFO and BBFO10
exposed to a 100 ppm ethanol testing gas.
higher in BBFO10 than that of pure BFO, indicating the
higher concentration of oxygen vacancies in BBFO10 sample.
The formation of these oxygen vacancy defects may be
attributed to the substitution of aliovalent Ba2+ ion at Bi3+ site
in BiFeO3 to maintain charge neutrality.21 The valence state of
Fe was also checked by XPS, as shown in the inset of Fig. 2(b).
No obvious differences could be observed for the two
samples. The peak at 711.1 eV combined with two satellite
peaks at 719.2 and 724.5 eV demonstrate that the valence
state of Fe in both samples is Fe3+, and there is no evidence
for Fe2+ existence within a few atomic percent resolution.22
which means that the partial substitution of Bi-site by Ba ion
enhanced the gas sensitivity of BiFeO3 effectively.
The gas-concentration dependence is illustrated by the
response recovery characteristics of BBFO10 sensor which
was exposed to ethanol vapor with different concentrations of
5, 10, 30, 50, 100, 300 and 500 ppm at 400 ꢀC, as shown in
Fig. 4(a). The sensor exhibits good sensitivity in various
ethanol concentrations even exposed to as low concentration
as 5 ppm ethanol, and the sensitivity increases dramatically
as the concentration of ethanol vapor increases. Besides, the
sensors exposed in different concentrations show a quick
response and short recovery time. The response and recovery
curve of BBFO10 sensor under 100 ppm ethanol vapor was
chosen and displayed in Fig. 4(b), and the gas response and
recovery time are 3 s and 10 s, respectively.
3.2. Gas sensing properties
Considering the sensitivity of gas sensors is greatly inu-
enced by operating temperature, parallel experiments were
carried out in the range of 300–430 ꢀC to optimize the
appropriate working temperature of the sensors. Typical
response and recovery curve of BBFO10 sensor which was
exposed to 100 ppm ethanol at different working tempera-
tures are shown in Fig. 3(a). The output voltage decreases
Fig. 5(a) displays the stability of the BBFO10 sensor, which
could be continuously operated over many cycles without
obvious loss of sensitivity, and the results could be repeated
aer two months, illustrating good reproducibility and long-
term stability of the sensor. In order to estimate the sensi-
tivity for different gases, the response of BFO and BBFO10
sensor were examined in different gases with equal concen-
along with the operating temperature and reaches
a
ꢀ
minimum value at 400 C, and the output voltage increases
with further increase of the operating temperature to 430 ꢀC.
It means that the best operating temperature for BBFO10
sensor is around 400 ꢀC which might be attributed to the
competition between adsorption and desorption of the
chemisorbed gases.23
ꢀ
tration of 100 ppm at operating temperature of 400 C, and
the results are shown in Fig. 5(b). The results show that the
response of the BBFO10 sensor is much higher than that of
the pure BFO sensor for all the gas tested. The sensor shows
much stronger responses to ethanol, acetone and gasoline,
while little or no response to toluene, methanol, ammonia
and formaldehyde. It is also noteworthy that the
The sensitivities of the sensors based on BFO and BBFO10
(dened as the ratio of Rgas/Rair) at different working tempera-
tures, are displayed in the Fig. 3(b). It is observed that the
sensitivity of BBFO10 sensor is obviously higher than the
undoped BFO sensor over the full operating temperature range,
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3.3. Gas sensing mechanism
For better understanding of the conduction mechanism of
Bi0.9Ba0.1FeO2.95 sensor, the variable frequency impedance
measurements were performed and shown in the Fig. 6. In
order to determine the inuence of oxygen partial pressure on
the conductivity, the BBFO10 sensor was exposed to O2, air and
N2 atmosphere at 400 ꢀC for 20 h, respectively, and the complex
impedance spectra were displayed in Fig. 6(a). Resistance
values, the reciprocal of conductance, could be obtained from
intercepts on the real Z0 axis. The impedance data, regardless of
the processing conditions or measured temperatures, show a
single semicircle. It means that the electrical properties of
samples are dominated by one electrical component (Grain).
The impedance spectra for the O2- and air-processed
samples are similar, but the conductivities of the O2-annealed
samples are a little higher, indicating a p-type conduction
mechanism,25 and the hole concentration can be derived from
and controlled by the schematic reaction:
1
VcOc þ O2/Oꢁ þ 2h
(1)
c
O
2
Fig.
4 (a) Gas response and recovery curve of BBFO10 sensor
exposed to ethanol at concentrations ranging from 5 to 500 ppm at
400 ꢀC and (b) 100 ppm.
representative indoor pollutant like toluene have minimum
interference to the detection of other gases, which had been a
difficult task using conventional n-type oxide semiconductor
gas sensors.24
Fig.
6 Impedance complex plane plots for BBFO10 sensor (a)
annealed and measured in O2, Air and N2; (b) Z00/M00 spectroscopic
plots; (c) at different temperatures; (d) exposed to different gases of
the same 100 ppm concentration; the resistances of the BBFO10
Fig. 5 (a) Stability of BBFO10 sensor to 100 ppm of ethanol at 400 ꢀC, sensor (e) measured at different temperatures in air; (f) exposed to
(b) sensitivity for different gases of BBFO10 sensor to different test different gases of the same concentration evaluated from the
gases with a concentration of 100 ppm at 400 ꢀC.
impedance spectrum.
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According to the eqn (1), the conductivities of BBFO10
sample are affected dramatically by hole concentration which is
dominated by oxygen partial pressure. As shown in the Fig. 6(a),
the N2-annealed samples are much more insulating due to the
fact that the hole concentration decreases dramatically.
The imaginary parts of impedance (Z00) and modulus (M00) as
a function of frequency are combined and shown in Fig. 6(b).
The data in Z00/M00 spectroscopic plots show a single peak in
both cases with maxima at similar frequencies, indicating that
the sample was electrically homogeneous.25 The temperature
dependence of impedance spectroscopy for BBFO10 sensor is
shown in Fig. 6(c). It could be observed that the diameter of the
semicircle decreases monotonously, which means the resis-
tance decreases as the temperature increases. The correspond-
ing resistances evaluated from the impedance spectrum are
shown in Fig. 6(e). This is due to that the conductivity and
density of carriers in semiconductors greatly increase with
temperature.26
To study the effects of the gases on the resistance of the
BBFO10 sensor, impedance spectroscopy is also measured
when the sensor is exposed to 100 ppm concentration of
different gases at 400 ꢀC, including ethanol, toluene, methanol,
gasoline, acetone, ammonia, formaldehyde, and the results are
summarized in Fig. 6(d) and (f). The resistance values all
increase different degrees when the sensor was exposed to in
the reducing gases, and among them, the resistance increases
most when the sensor were exposed to ethanol gas which
indicating an effective ethanol gas sensitivity.
(O2)gas 4 (O2)ads
(2)
(3)
(4)
(O2)ads + eꢂ 4 (O2
)
ads
ꢂ
(O)ads + eꢂ 4 (Oꢂ)ads
For the p-type semiconductor, this reduction of the electrons
number will change the electric surface's charge status of the
sensors. It means that an underlying positive accumulation zone
formed by holes, inducing a lower resistance layer which covers
the whole surface of the gas sensor. Thus the surface resistance
decreases by the presence of atmospheric oxygen. This view has
been conrmed by the variation in conductivity of sensors under
different oxygen partial pressure, as discussed earlier.
This leads to the formation of an electronic core–shell cong-
uration (Fig. 7(a and b)), and the conduction in p-type oxide
semiconductors can be explained by the parallel competition
between the resistive core (Rbulk) and the conductive hole accu-
mulation layer (RHAL) shell in addition to the serially connected
neck component. As suggested by Pokhrel et al.,28 the response to
reducing gas in p-type sensors is not affected signicantly by the
particle size of sample. Because if the neck conguration of
particles remains similar, the coarsening of particles will not
decrease gas response to a great extent.29
Correspondingly, accompanied with the electrons attracted by
the adsorbed neutral oxygen molecules, the formation of a thick
space-charge layer will increase the potential barrier, as displayed
in the schematic model of the depletion layer. Furthermore, a
larger quantity of oxygen vacancy in BBFO10 sample induces
higher adsorptions of oxygen without lowering the expansion level
of the depletion layer for pure BFO sample, which results in a
further decrease of the resistance of the BBFO10 sensor.
According to the Wolkenstein's model,27 the adsorbed
neutral oxygen molecules (O2)ads at the semiconductor surface
are partially ionized into O2ꢂ, Oꢂ or O2ꢂ ions by attracting
electron from the valence band of semiconductor at different
temperatures. Since the sensors are operated at 400 ꢀC, the Oꢂ
species is more dominated than any other oxygen adsorbate.
When the sensors are exposed to an atmosphere containing
reducing gas, the oxygen adsorbates are removed by reduction
reaction. This process returns the trapped electrons in the
Fig. 7 The schematic diagram of the proposed gas-sensing mechanism for p-type semiconducting sensor: (a) and (c) in air, (b) and (d) in
reducing gas, (e) simplified equivalent circuit.
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valence band and results in a recombination of electrons and
holes which increases the layer resistance. It is described in the
specic cases of ethanol and acetone by the reaction as follows.
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CH3CH2OH + 6Oads / 2CO2gas + 3H2Ogas + 6eꢂ
(5)
(6)
ꢂ
ꢂ
CH3COCH3 + 8Oads / 3CO2gas + 3H2Ogas + 8eꢂ
For BBFO10, the substitution of Bi3+ by Ba2+ increases the
oxygen vacancy concentration. The oxygen vacancies react with
the atmospheric oxygen to generate more holes and then make
the sample more conductive.30
0:5ðO2Þ þ Vcc 4Oꢁ þ 2h
(7)
c
gas
O
O
where VcOc, OꢁO, hc represent oxygen vacancy, lattice oxygen and
hole, respectively. In this case, more oxygen gas can combine
with these surface defects aer debonding the molecules, and
the conductance change in the Ba-substituted BFO is more
signicant compared to that of the pure BFO. Thus, the Ba-
substituted BFO sensor that has a signicantly better sensing
performance than pure BFO sensor is mainly due to the larger
quantity of oxygen vacancies.
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4. Conclusions
In summary, the gas-sensing properties, conduction and gas-
sensing mechanism of Ba-substituted BFO sample are investi-
gated and compared with pure BFO. It is found that the
conduction of BBFO10 is dominated by p-type hole conduction.
The gas-sensing test revealed that the sensor based on the
BBFO10 sample exhibited a high sensitivity, quick response
time, effective sensitivity for different gases and excellent long-
time stability. It is proposed that the enhanced gas-sensing
properties of Ba-substituted BFO sample could be attributed
to the higher concentration of oxygen vacancy than pure BFO,
and the oxygen vacancy is induced by the substitution of Bi3+
ion by aliovalent Ba2+ ion at A-site of perovskite structure.
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Acknowledgements
This work was supported by the National Natural Science
Foundation (51172187), the SPDRF (20116102130002,
20116102120016) and 111 Program (B08040) of MOE, the Xi'an
Science and Technology Foundation (CX12174, XBCL-1-08), the
Shaanxi Province Science Foundation (2013KW12-02), Aero-
nautical Science Foundation of China (2013ZF53072), the SKLP
Foundation (KP201421), and the Fundamental Research Funds
for the Central Universities (3102014JGY01004) of China.
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