Effects of rare earth element doping on the ethanol gas-sensing performance of three-dimensionally ordered macroporous In2O3

Article abstract of DOI:10.1039/c6ra06816b

Rare earth (Tm, Er, La, Yb and Ce)-doped In₂O₃ nanostructures with three-dimensionally ordered macroporous structures (3DOM) were prepared by a colloidal crystal templating method, and their ethanol sensing properties were investigated. Rare earth doping improved the gas response and lowered the optimum operating temperature. Tm-doped 3DOM In₂O₃ had the best gas-sensing performance, and the gas sensitivity for 100 ppm ethanol was seven times higher than that of pure 3DOM In₂O₃. The reason for the different sensitivities generated by the five doping elements and the underlying mechanism of the enhanced gas response for the samples were studied.

Full text of DOI:10.1039/c6ra06816b

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Effects of rare earth elements doping on ethanol gas-
sensing performance of three-dimensionally ordered
macroporous In2O3
Received 00th January 20xx,
Dongmei Han, Junjun Yang, Fubo Gu, Zhihua Wang*
Rare earth (Tm, Er, La, Yb and Ce) doped In2O3 nanostructures with three-dimensionally ordered macroporous structure
(3DOM) were prepared by colloidal crystal templating method, and their ethanol sensing properties were investigated.
Rare earth doping improved the gas response and lowered the optimum operating temperature. Tm doped 3DOM In2O3
had the best gas-sensing performance, and the gas sensitivity to 100 ppm ethanol was 7 times higher than that of the pure
3DOM In2O3. The reason of the different sensitivities by the five doped elements and the underlying mechanism of the
enhanced gas response for the samples were studied.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
performance.21 Anand et al. fabricated Tb doped In2O3
nanoparticles and the results demonstrated that Tb doping
enhanced the response of In2O3 towards ethanol.19 In addition, Er
doped In2O3 nanoribbon,22 La doped In2O3 nanocrystalline,23 and
Eu doped In2O3 Nanobelt,24 have been investigated as gas sensors.
However, the effects of doping with different rare earth elements
on the morphology, crystal structure, crystal defect and gas
sensing properties in the gas sensing process is not well-
understood.
In this investigation, we choose five typical RE elements, Tm, Er,
La, Yb and Ce as dopants to study their effects on morphology,
crystal structure, surface states and gas sensing response and
selectivity. A facile colloidal crystal templating method was used
for the fabrication of RE doped 3DOM In2O3 with polymethyl-
methacrylate (PMMA) microspheres as a template. The colloidal
crystal templating method is a simple and efficiently synthetic
route for preparing the 3DOM materials with well-interconnected
pore and wall structures, which has attracted considerable
attention in recent achievements.25-27 3DOM structures can affect
the gas response by improving the diffusion and adsorption of gas
molecules and accelerating the electron exchange between the
nanostructure and the target gas. Herein, we found that the
sensors based on RE doped 3DOM In2O3 exhibited good
sensitivity and selectivity as well as fast response property at
relatively low temperature for ethanol detection. The gas sensing
mechanism of In2O3 nanomaterials and the reason of the widely
different enhancement of the gas response by the five RE
elements doping were discussed.
1. Introduction
In2O3, as an important n-type semiconductor with a wide band
gap of about 3.5~3.8 eV,1-2 has been used in gas sensors for
detection of diverse gases, such as NH3,3 NO2,4 H2S,5 ethanol6 and
formaldehyde.7 Recently, the attempt of some methods has
focused on further enhancing the sensitivity of In2O3 gas sensors.
The recent application of doping has sparked intense research
interest in this material due to its effective improvement of
electrical conductivity. Metal elements doping, such as Zn,8 Sn,9
Li,10 Cr,11 Cu12 and so on, has been exploited to tailor the
desirable gas response properties of In2O3 by increasing the free
carrier concentration. For example, Li et al.13 reported Zn-doped
In2O3 hollow spheres obviously improving the response to Cl2. Hu
et al.12 synthesized an excellent NO2 sensor based on Cu doped
In2O3 hierarchical flowers.
Rare earth (RE) elements have attracted considerable attention
due to their particular characteristics.14-17 The high surface
basicity, fast oxygen ion mobility and excellent catalytic properties
of rare earth oxides are the important features for chemical
sensing.18 More importantly than all of that, rare earth ions can
be easily introduced into the body of In2O3 and replace the sites
of In3+ due to their similar ion radius. Substitution of REx+ ion in
In2O3 lattice will cause the lattice distortion, which results in the
increase of lattice strain.19-20 Han et al. reported that Ce doped
In2O3 porous nanospheres could enhance methanol gas-sensing
a.State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, P. R. China. E-mail:
zhwang@mail.buct.edu.cn
†Electronic Supplementary Information (ESI) available: The figures of SEM, XRD
，
XPS and gas-sensing measurements are given in electronic supplementary.
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the crystal structures of the samples at a scan rate of 10°/min
with the scope of 5~90°. The crystal phases were identified by
2. Experimental
2.1 Synthesis of 3DOM In2O3 and RE doped 3DOM In2O3
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DOI: 10.1039/C6RA06816B
the JCPDS Database. The chemical character and elemental
analyses of the samples were examined by energy dispersive
spectrometer (EDS, S-4700). X-ray photoelectron spectroscopy
(XPS, VG Scientific ESCALAB 250X) with Al Ka excitation was
carried out to characterize the rare earth, indium species and
surface properties.
All the chemical regents were analytical grade without further
purification. The well-arrayed hard-template PMMA microspheres
were synthesized according to the procedures described
elsewhere.28 3DOM In2O3 and RE (Tm, Er, La, Yb, Ce) doped 3DOM
In2O3 nanostructures were prepared by colloidal crystal templating
method. 10 mmol In(NO)3·4.5H2O and 0.5 mmol RE(NO3)x·yH2O (all
with purity 99%, RE=Tm, Er, La, Yb, Ce) were dispersed into 5 mL
methanol with strongly stirring for 1 h. 2 g citric acid was added into
the mixed solutions with magnetic stirring at room temperature for
1 h. 1.5 g PMMA were quickly added into the clarify solutions and
totally wetted for 4 h. After filtration and drying, the precursors
were transferred into a ceramic boat and thermally treated under
N2 flow in a tubular furnace with a flow rate of 50 mL/min at a ramp
2.3 Gas sensing measurements
The CGS-8TP (Beijing Elite Tech. Co., Ltd, China) gas sensing
intelligent system was used to test the sensing properties of
the sensors. The relative humidity range of the testing
environment was 25±5%. The obtained samples were mixed
with ethanol to form a homogeneous paste, which was coated
on an alumina ceramic tube with a pair of Pt wires covered on
Au electrodes, and aged at 300 oC for 24 h in air in Muffle
furnace. Then the ceramic tube was welded on a substrate and
a Ni–Cr wire coil throughout the tube was employed as a
heater to control the working temperature by varying the
heating current. The sensor sensitivity to gas was defined as
Ra/Rg in which Rg is the resistance of the sensor in the target
gas and Ra is the resistance in air. Response time of the sensor
is defined as the time taken by the sensor to reach 90% of its
maximum value after exposure to test gas and the recovery
time is the time taken for the sensor to reach 10% of its
original resistance value once the desorption begins.
Schematic illustration of the experimental procedure was
shown in Fig .1.
o
o
of 1 C/min from RT to 300 C. The samples were naturally cooled
down to 50 oC and the atmosphere was changed to air with 100
o
o
mL/min at a ramp of 1 C/min from current temperature to 300 C
for 3 h and then to 600 oC for 3 h.
Tm, Er, La, Yb and Ce doped 3DOM In2O3 are denoted as 3DOM
In2O3-Tm, 3DOM In2O3-Er, 3DOM In2O3-La, 3DOM In2O3-Yb and
3DOM In2O3-Ce.
2.2 Characterizations
The morphology, size and microstructure of the samples were
studied using scanning electron microscopy (SEM, S-4700) and
transmission electron microscopy TEM (H-800). The powder X-
ray diffraction (XRD, Bruker/AXS D8 Advance) with Cu Ka
radiation (λ=0.15406 nm) and nickel filter was used to measure
Fig. 1 Schematic Illustration of 3DOM In2O3 materials experimental procedure
layer board is complete, and the channel structure is obvious.
However, the annealing temperature is 650 oC (Fig. 2(e, f)), the
channel structure and layer board are slightly damaged. Therefore,
the samples of pure and RE doped 3DOM In2O3 materials were
prepared at 600 oC for the following research.
3DOM In2O3 materials have ordered macroporous hierarchical
structure with an average pore diameter of ~200 nm and wall
thickness of ~25 nm (Fig. 2(a, b)), which correspond to a shrinkage
3 Results and discussion
3.1 Characterizations of pure and RE doped 3DOM In2O3
The SEM images of PMMA microspheres with an average diameter
300 nm were shown in Fig. S1. The influence of calcination
temperature to the morphology of 3DOM In2O3 were recorded in
Fig. S2. PMMA templates are not degraded completely, and the
channel structure is not obvious for 3DOM In2O3 annealed at 550 oC
o
(Fig. S2 (a, b)). For 3DOM In2O3 annealed at 600 C (Fig. S2(c, d)),
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of 30% in comparison with the initial size (ca. ~300 nm) of PMMA crystal In2O3 samples with lattice parameter of a=10.12 Å (JCPDS
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o
o
o
microspheres. The samples annealed at 600 oC present hexagonal PDF# 06-0416) (Fig. S3). The main peaks atD3O0I.:510,.13053.94/C,65R1A.0068a1n6Bd
structure with regular laminated structure.29-30 It can be seen that 60.7o correspond to the (222), (400), (440) and (622) lattice planes,
the structures of all the RE doped 3DOM In2O3 are similar to the respectively, which illustrate the samples with well-defined
structure of 3DOM In2O3, and the RE doping doesn't destroy the crystalline are obtained34-35. For the RE doped 3DOM In2O3, no
regular structure. The average pore diameters of all RE doped diffraction peaks of RE are observed. The (222) diffraction peaks of
3DOM In2O3 are approximately 200 nm and the wall thicknesses are RE doped samples have small angle shift to lower 2θ values
25 nm. In order to further observe the microscopic structure of the compared with pure In2O3 (Fig. 4(b)), suggesting that the structural
3DOM materials, TEM technique was applied to analysis the RE defects of In2O3 caused by rare earth dopants.36-37 The difference of
doped 3DOM In2O3 materials. Fig. 3 shows the complete skeleton ionic radius leads to the deformation of crystal lattice of In2O3.
structure at higher magnification to reveal more details. All 3DOM Because the radius of REx+ are larger than that of In3+, doping with
structures keep ordered macroporous and the skeletons are the REx+ will lead to lattice distortion of In2O3, restrain the crystal
assembled by In2O3 nanoparticles to form a wall thickness of 18~25 grain growth, and eventually result in the large lattice deformation.
nm. There is no remarkable change of surface morphology among The diameters of six kinds of crystal size were determined by
these samples, which may result from two reasons: 1) These rare Debye−Scherrer formula and the lattice constants were calculated
earth elements possess comparable ionic radius, In3+ (0.80 Å)，La3+ from the XRD peaks. The calculated values are listed in Table 1. The
(1.06 Å), Ce3+ (1.03 Å), Er3+ (0.88 Å), Tm3+ (0.87Å), Yb3+ (0.86 Å) and result indicates that the lattice constant will increase when In3+ sites
In3+ is substituted by RE ions without changing crystal structure of are substituted by the rare earth ions. On the basis of the above
In2O3. 31-32 2) The relative low concentration of rare earth dopants33 XRD analysis, it can be concluded that RE3+ ions will uniformly
in this work may not significantly change the morphology of the as replace the In3+ sites. 38-39
fabricated samples.
Fig. 4 XRD patterns of (a) the pure and RE doped 3DOM In2O3 samples, (b) high
magnification of the (222) peaks
Fig. 2 SEM images of 3DOM In2O3 (a, b); 3DOM In2O3-Tm (c, d); 3DOM In2O3-Er (e,
f); 3DOM In2O3-La (g, h); 3DOM In2O3-Yb (I, j); 3DOM In2O3-Ce (k, l)
Fig. 5 EDS patterns of the samples of (a) 3DOM In2O3; (b) 3DOM In2O3-Tm; (c)
3DOM In2O3-Er; (d) 3DOM In2O3- La; (e) 3DOM In2O3-Yb; (f) 3DOM In2O3–Ce
Fig. 3 TEM images of (a) 3DOM In2O3; (b) 3DOM In2O3-Tm; (c) 3DOM In2O3-Er; (d)
3DOM In2O3-La; (e) 3DOM In2O3-Yb; (f) 3DOM In2O3-Ce
The elemental component can be achieved precisely by applying
the EDS technique. It can be found that RE doped samples consist of
Fig. 4(a) shows the XRD patterns of the pure and RE doped 3DOM
elemental In, O and doped RE element (Tm or Er, La, Yb, Ce) (shown
In2O3 samples. The samples are identical to those of the cubic
in Fig. 5(a~f)). According to the EDS analysis, the actual ratio of RE
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to In is about 0.041~0.055, which is close to the proposed doping The gas sensing properties for 3DOM In2O3 with different doping
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concentration. The max value of In peak is 11.31 K at 3.3 KeV. In ratio of rare earth elements were measuredDinOIo:u10r.e10x3p9e/rCim6ReAn0t6s8a1n6Bd
peak values of Tm, Er, La, Yb, Ce doped 3DOM In2O3 are 6.74 K, 7.43 the similar rules had been obtained among these elements. Taking
K, 7.60 K and 6.92 K and 6.26 K, respectively, which are distinctly Tm as its example, the gas sensing properties for 3DOM In2O3 with
lower than that of the pure 3DOM In2O3.
2%, 5%, 8% doping ratio are shown in Fig. S5. The experiment
The XRD and EDS results indicate that REX+ ions are successfully results show that the optimum performance can be achieved with
doped into the crystal lattice of In2O3 matrix without forming RE 5% doping ratio of Tm.
oxides or any other impurities.
The gas response usually depends on the working temperature of
sensors. Fig. 7(a) shows the gas responses to 100 ppm ethanol
Table 1 crystal size, lattice parameters and doping ratio of pure and RE doped measured at different operating temperatures for the pure and RE
3DOM In2O3
doped 3DOM In2O3. It can be seen that the optimum working
temperature of the 3DOM In2O3 decreased from 230 oC to 175 oC by
doping RE elements. The responses of all the RE doped 3DOM In2O3
are dramatically higher than the response of the 3DOM In2O3 at 175
oC. The 3DOM In2O3-Tm has the best gas sensing property, following
by the the 3DOM In2O3-Er, 3DOM In2O3-La, 3DOM In2O3-Yb, 3DOM
In2O3-Ce and the pure 3DOM In2O3. The result indicated that RE
doping is an efficient way to improve the gas response of
semiconductor-based gas sensor.It is worth noting that the gas
sensitivity of the 3DOM In2O3-Tm is 7 times higher than that of the
pure In2O3.
Crystal sizea Lattice parameters RE/In
Samples
(nm)
a=b=c (Å)
10.1200
10.1326
10.1294
10.1373
10.1210
10.1685
(mol%)b
--
3DOM In2O3
22.2448
21.9386
22.1375
18.3579
21.6361
18.8294
3DOM.In2O3-Tm
3DOM In2O3-Er
3DOM In2O3-La
3DOM In2O3-Yb
3DOM In2O3-Ce
4.8
4.5
4.1
5.4
5.4
aDetermined by XRD, bDetermined by EDS
XPS analysis is conducted to confirm the change of the amount of
defect at the surface of In2O3 caused by RE doping. The survey XPS
spectra of the samples are shown in Fig. S4 (a~f). C, In and O
elements are presented in the all spectra. For the pure 3DOM In2O3,
the peaks centered at 703.0, 665.7, 450.96 and 443.91 eV are
assigned to In 3p1/2, In 2p3/2, 3d3/2 and In 3d5/2, respectively, which
suggests that In only exists in a trivalent oxidation state in the
samples.40 No significant changes of the full-range survey XPS
spectra for the RE doped 3DOM In2O3 can be observed because of
the low content of RE doping.
Fig. 6 In 3d XPS spectra of pure and RE doped 3DOM In2O3
Furthermore, compared with the pure 3DOM In2O3, the binding
energies of the In3d XPS peaks of the RE doped 3DOM In2O3 shift to
higher values, as shown in Fig. 6. This phenomenon indicates that
the electronic interaction existed between the In2O3 and the
dopants. In Fig. 3 (b-f), the peaks located at 176.44, 168.74, 838.52,
184.77 and 884.85 eV belong to Tm 4d peak, Er 4d, La 3d5/2, Yb 4d
and Ce 3d5/2 , respectively, which are higher than that of the pure
rare earth oxide. It indicates the successful RE-doping into the In2O3
lattice20 as a further proof.
Fig. 7 (a) Gas response of pureand RE doped 3DOM In2O3 to 100 ppm ethanol at
different operating temperatures; (b) Dynamic response curve and (c) Linear
relationship response of pure and RE doped 3DOM In2O3 to different
concentration of ethanol.
3.2 Gas sensing properties and sensing mechanism
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The gas responses and the response/recovery time of the pure
and RE doped 3DOM In2O3 are shown in Fig. S6 and S7. RE doped
View Article Online
DOI: 10.1039/C6RA06816B
Good selectivity is required for practical applications of gas
3DOM In2O3 exhibit fast response time (~9 s), recovery time (~13 s) sensors. Therefore, the responses of the pure and RE doped 3DOM
and high gas response for 100 ppm ethanol. Dynamic response In2O3 to 100 ppm methanol (CH3OH), acetone (C3H6O), ammonia
curve with various concentration of ethanol for all samples are (NH3) and cyclohexane (C6H12) are also measured at 175 oC, as
shown in Fig. 7(b). The gas responses are enhanced gradually with shown in Fig. 8(a). It is obviously found that the responses of all
the increase of the ethanol concentration. The linear relationship samples for other gases are very low, which indicates that the RE
between gas response and the concentration of ethanol is good at doped 3DOM In2O3 in our work have very good selectivity for
the range of 20~100 ppm for all samples (Fig. 7(c)). With the further ethanol.
increase of the concentration of ethanol, the increasing amplitude
of gas response decrease, as shown in Fig. S8.
In practice, the stability is also a critical factor for sensors. The
stability tests were carried out for the sensors stayed in air for 10
days, 30 days and 60 days, as shown in Fig. 8 (b). Even after 60 days,
the samples still have good response, and the changes of the gas
sensing properties are not obvious. A comparison of different In2O3-
based sensors for ethanol detection is illustrated in Table 2. It can
be clearly seen that Tm, Er and La doped 3DOM In2O3 exhibit higher
gas response and lower optimum working temperature than those
reported in the literatures.
Fig. 9 O 1s XPS spectra of pure and RE doped 3DOM In2O3
Fig. 8 (a) Selectivity of pure and RE doped 3DOM In2O3 to 100 ppm different gases
under the optimum working temperature. (b) Sensing stability to 100 ppm
ethanol for nearly two month.
Table 2 Comparison of the responses of In2O3 nanostructures to ethanol
T
Concentration
Sample structure
3DOM In2O3-Tm
3DOM In2O3-Er
3DOM In2O3-La
Ga2O3–In2O3 nanocomposite
Cr2O3 decorated In2O3
CuO/In2O3 core–shell nanorod
Bi2O3 decorated In2O3
Tb doped In2O3 nanoparticles
Response
Reference
/oC /ppm
175
175
175
300
200
300
200
300
100
122
112
Present work
Present work
Present work
100
100
300
500
200
200
50
78.9
69.1
15.03
7.26
17.74
40
41
42
43
44
19
-
attributed to adsorbed oxygen (O2 , O− and O2− ions) species, and
The surface structure is an important factor to influence the gas OІІI species is assigned to hydroxyl oxygen.
sensing properties. XPS analysis was carried out to explore the
The adsorbed oxygen contents are important to gas sensing
difference of gas sensing properties of all samples. The Gauss fitting property. OII species are advantageously utilized to the ethanol
curves of O1s XPS spectra of the pure and the RE doped 3DOM decomposition reaction on the sensor surface, which is beneficial to
In2O3 are shown in Fig. 9. Three peaks centred at 529.50 eV (OІ), the improvement of gas response. The data of the relative amount
531.04 eV (OІІ) and 532.29 eV (OІІI) are assigned to OI, OII and OIII of oxygen species calculated from the O1s XPS spectra are listed in
species.45-47 The OІ species belongs to lattice oxygen, OІІ species is
Table 3. It is clearly observed that the 3DOM In2O3-Tm has the
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highest relative amount of OII specie, following by the 3DOM In2O3- the same time, a thick space-charge layer is formed on the surface
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DOI: 10.1039/C6RA06816B
Er, 3DOM In2O3-La, 3DOM In2O3-Yb, 3DOM In2O3-Ce and the pure of the In2O3 to increase their potential barrier with a higher
3DOM In2O3. From the gas sensing performances and O1s XPS resistance. As previously mentioned, Tm doped 3DOM In2O3 with
results, it can be concluded that OII specie is the main factor to the highest amount of OII specie means that it can adsorb the most
affect the gas response, and the sensor performances can be oxygen molecules among all RE doped samples. This leads to the
improved with the increase of the amount of adsorbed oxygen. The most electrons being extracted on the surface and the widest
3DOM In2O3-Tm with the largest relative amount of adsorbed space-charge layers being formed (shown in Fig. 10). Consequently,
oxygen exhibits the highest gas sensitivity, and the RE doped 3DOM the initial resistance (Ra) will increase. Along with the relative
In2O3 is larger than that of the pure 3DOM In2O3. Therefore, the amount of OII specie decreasing from Er to Ce gradually, the width
analysis of XPS has provided a satisfactory explanation on the of space-charge layers lessens in sequence.
difference of the gas-sensing properties resulted from the different
doping of 3DOM In2O3.
O2 (gas) → O2 (ads)
(1)
(2)
(3)
(4)
(5)
(6)
O2 (ads) + e- → O2- (ads)
Table 3 The date of O1s Gauss fitting of pure and RE doped 3DOM In2O3
O2- (ads) + e- → 2O- (ads)
O- (ads) + e- → O2- (ads)
CH3CH2OH (ads) + 6O- (ads) → 2CO2 (g) + 3H2O (l) + 6e-
CH3CH2OH (ads) + 6O2- (ads) → 2CO2 (g) + 3H2O (l) + 12e-
Samples
OІ (%)
OІІ (%)
OІІI (%)
3DOM In2O3
3DOM In2O3-Tm
3DOM In2O3-Er
58.97
51.50
51.66
23.81
42.34
39.61
17.22
6.16
8.73
As shown in Equations 5~6, when the In2O3 sensors are exposed
to ethanol, the oxygen species will react with ethanol to release the
trapped electrons back to conduction band simultaneously, and
finally the thickness of the electron depletion layer is reduced.
Accordingly, the resistance of In2O3 semiconductor materials will
decrease. 3DOM In2O3-Tm with the most adsorbed oxygen has the
strongest reaction with ethanol and release the more electrons
than other samples. 3DOM In2O3-Tm has the best gas performance
with obvious resistance change signal followed by the 3DOM In2O3-
Er, 3DOM In2O3-La, 3DOM In2O3-Yb, 3DOM In2O3-Ce in order.
3DOM In2O3-La
48.15
38.10
13.75
3DOM In2O3-Yb
3DOM In2O3-Ce
57.37
58.61
30.34
24.54
12.29
16.85
The gas-sensing mechanism of the In2O3 sensors can be explained
by surface-depletion model. When the sensors are exposed in air,
the oxygen molecules will be adsorbed on the surface of the In2O3
to capture electron from conduction band to form adsorbed oxygen
−
species (O2 , O2− and O−)48-50 as represented in Equations 1~4. At
Fig. 10 Schematic illustration of gas sensing mechanism (EV–valence band edge; EC–conduction band edge; EF–Fermi energy)
6 | J. Name., 2012, 00, 1-3
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Acknowledgments
The authors acknowledge the National Natural Science Foundation
of China (No.21275016，21575011)
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50. A. Shanmugasundaram, B. Ramireddy, P. Basak, S. V. Manorama
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Graphical abstracts:
Rare earth 3DOM In2O3 were prepared by colloidal crystal templating method,
and their ethanol sensing properties were investigated.