Highly sensitive and humidity-independent ethanol sensors based on In2O3 nanoflower/SnO2 nanoparticle composites

Article abstract of DOI:10.1039/c5ra07213a

As an ethanol sensing material, the composites of In₂O₃-SnO₂ were composed of In₂O₃ microflowers and SnO₂ nanoparticles. Both In₂O₃ microflowers and SnO₂ nanoparticles were synthesized by hydrothermal method and then mixed in an ultrasonic environment. The morphology and phase composition of the as-synthesized samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results on gas sensing properties showed that when the mass ratio of In₂O₃ and SnO₂ was 2 : 1, the sensor based on the as-prepared In₂O₃-SnO₂ composite exhibited high response and good selectivity to ethanol at 250°C. The response to 100 ppm ethanol gas was 53.2. UV illumination stabilized the responses of the sensors while the relative humidity increased. The gas sensing mechanism proposed was that the addition of SnO₂ to In₂O₃ enhanced the catalytic activity for the ethanol reaction, which changed the electrical resistance of the materials. Besides, the morphology was helpful to the gas reaction on the surface of the sensing materials.

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Highly sensitive and humidity-independent ethanol sensors based
on In O nanoflower/SnO nanoparticle composites
2
3
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
a
Yang Liu , Shiting Yao , Qiuyue Yang , Peng Sun , Yuan Gao* , Xishuang Liang , Fengmin Liu* and
a
Geyu Lu
DOI: 10.1039/x0xx00000x
Abstract
www.rsc.org/
2 3 2 2 3 2
As an ethanol sensing material, the composites In O -SnO were composed of In O microflowers and SnO nanoparticles.
Both In microflowers and SnO nanoparticles were synthesized by hydrothermal method and then mixed under
2
O
3
2
ultrasonic environment. The morphology and phase composition of the as-synthesized samples were characterized by X-
ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results on gas sensing properties showed that
2 3 2 2 3 2
while the mass ratio of In O and SnO was 2:1, the sensors based on as-prepared In O -SnO composites exhibited high
response and good selectivity to ethanol at 250℃.The response to 100 ppm ethanol gas was 53.2. UV illumination
stabilized response of the sensors while the relative humidity increased. The gas sensing mechanism was that proposed
2 2 3
the addition of SnO to In O enhanced the catalytic activity for the ethanol reaction, which changed the electrical
resistance of the materials. Besides, the morphology was helpful to the gas reaction on the surface of the sensing
materials.
their high active surface area and loose microstructure
favourable for gas adsorption and rapid gas diffusion, and have
dominated the research in enhancing the sensitivity of the
metal oxide gas sensors. Another important sensing parameter
1
.Introduction
The ethanol detection on the breath is an effective way to
is selectivity that can be improved through the use of
12,13,14
1
aid police in apprehending drink driving offenders . In
addition, as a kind of volatile organic compounds (VOCs), the
ethanol detection can potentially be utilized as breath markers
composite metal oxides
. The physical interface between
two dissimilar materials is often referred to as
a
15
heterojunction. In Akbar et al review, the authors detailed
the dominant electronic and chemical mechanisms that
influence the performance of the metal oxide heterojunction
as resistive-type gas sensors.
2
for specific diseases .Thus far, several types of ethanol sensors
3
,4,5
have been reported for breath analysis . In recent years, the
semiconductor gas sensors have attracted widespread concern
to science researchers because of their high sensitivity, low
6
.7,8
Up to date, many composites such as In
1
2
O
3
-CeO
, ZnO-
2
cost and simplicity
. It is well known that the performances
6,17,18,19
SnO , TiO -SnO , SnO -Fe O
2 3
have been reported to be
2
2
2
2
of sensors are significantly influenced by the component,
9
crystalline size and morphology of sensing materials
,10,11
promising sensing materials to obtain the sensitive and
selective gas sensors. As two important kinds of fundamental
materials, In O and SnO have been widely studied due to
.
Generally, the smaller crystal size owns bigger surface
area, which provides more active sites for the target gas
reacting with the sensing materials. However, agglomeration
of fine particles greatly reduces the valid surface area.
Fortunately, hierarchical nanostructure oxides emerged due to
2
3
2
20,21,22,23
their responses towards different gases
24,25
. In O -SnO
2 3 2
composites
have also been proved to improve the
properties of sensors. However, to the best of our knowledge,
there are few reports on the morphology of In O -SnO
2
3
2
2
composites and no reports have shown that In O -SnO
2
3
composites have an excellent response and selectivity for
ethanol.
a.
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s
Republic of China. E-mail: 18946580425@163.com, liufm@jlu.edu.cn;Fax: +86-
Commonly, the water vapour strongly interacts with the
oxide semiconductor surface, which leads to a significant
4
31-85167808;Tel:0431-85168384.
Electronic Supplementary Information (ESI) available:. Details of preparations
†
and characterizations; the SEM images using a JEOL JSM-7500F microscope with an deterioration of the sensor performance. There is high
accelerating voltage of 15 kV; the XRD measurements with a Rigaku D/max-2500
humidity in breath, so it is very important to reduce the
diffractometer using Cu-Kα1 radiation; the BET measurements with
Micromeritics Gemini VII apparatus (Surface Area and Porosity System);
See DOI: 10.1039/x0xx00000x
a
influence of humidity on the performance of the sensors.
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In this work, In
Journal Name
2 3 2
O microflowers and SnO
nanoparticles sensors were measured by multimeter (fluke 8846A) under
were both synthesized by hydrothermal method. Then the two constant temperature and humidity. The measurement was
kinds of materials were mixed in the ultrasound environment. processed by a static test system: Ra and Rg were the
The testing on gas sensing properties showed that while the resistances of the sensors in air and tested gas, respectively. A
mass ratio of In
In -SnO composite oxides showed the highest response to
ethanol, the response to 100ppm ethanol gas was 53.2 at 250
.UV illumination was adopted to reduce the influence of
relative humidity on the performances of the sensors.
2 3 2
O and SnO was 2:1, the sensors based on certain amount of the tested gas was injected into a closed
2
O
3
2
chamber, and the sensor was put into the chamber for the
measurement of the sensitive performance. When the
response reached a constant value, the sensor was removed to
air to recover. The response of the sensor was defined as
S=R /R for reducing gases or R /R for oxidizing gases. The
℃
a
g
g
a
response and recovery times are defined as the times taken by
the resistance change of sensor achieving 90% of the total in
the case of adsorption and desorption, respectively.
2
. Experimental
2.1 Synthesis and characterization of the samples
UV illumination was processed according our previous
2
8,29
works
. UV-LED (peak wavelength= 380 nm; operating
2
In O was synthesized according to the literatures .In a
6
voltage =3 V) was chosen as the light source, and the distance
between the UV-LED and gas sensors was 2 mm. The
2
3
3 3 2
typical synthesis process,0.381 g of In(NO ) •4.5H O and 0.15
2
measured power of the UV-LED at this distance was 0.7 Cd/m
g urea were dissolved in 36 mL deionized water with stirring
until the solution was clear. Then, the mixture solution was
transferred into a Teflon-lined stainless steel autoclave, heated
(PR650, California, USA).Relative humidity (%RH) was obtained
by using Humidity generator (Shanghai ESPEC environmental
equipment corp.SETH-Z-022L).
at 160℃ for 4 h. After the autoclave was cooled to room
In order to investigate the repeatability of the sensors
temperature naturally, the result product was washed with
deionized water and ethanol for six times alternately by
.
The fabrication and measurement of sensors were repeated 5
times, the results showed the sensors had similar properties.
centrifuge, and then dried at 80
sintered to 500 at a rate of 2
for 2 h. Then In microflowers were synthesized.
SnO was synthesized according to the literatures .In a
typical synthesis process,0.526 g of SnCl •5H O, 0.6 g of
℃
for 3 h. The products were
℃
℃/min and then kept at 500℃
3
. Results and discussion
2 3
O
2
7
2
4
2
3.1 Characterization of the products
cetyltrimethyl ammonium bromide (CTAB) and 0.2 g of
hexamethylenetetramine (HMT) were added to 40 mL mixed
solvent of ethanol and water (1:1, v/v) with stirring until the
solution was clear. Then, the mixture solution was transferred
into a Teflon-lined stainless steel autoclave, heated at 200
for 4 h.The processes after this were same with the process
used to synthesize In
℃
2
3
O .
Then In O3 microflowers and SnO2 nanoparticles were
2
mixed physically in certain mass ratio under ultrasound
environment. Four In
concentration of 0% SnO
SnO were obtained, represented by S0, S1, S2 and S3,
2
O
3
-SnO
2
composites samples with mass
2
, 33.3% SnO
2
, 70% SnO and 100%
2
2
respectively. Until this step, In
obtained.
2 3 2
O -SnO composites were
2.2 Fabrication and measurement of sensors
Gas sensors were fabricated as follows: the prepared
materials (S0, S1, S2 and S3) were mixed with deionized water
to form something like slurry, and then spread on an alumina
tube (4 mm in length, 1.2 mm in external diameter, and 0.8
mm in internal diameter, attached with a pair of gold
electrodes, each electrode was connected with two Pt wire)by
a small brush until forming a film thick enough to cover the
gold electrode entirely. A Ni-Cr heating wire went through the
tube to support heat for the sensors. The resistances of the
Fig.1 XRD of In
2 3 2 2 3
O microflowers, SnO nanoparticles and In O -
SnO composites.
2
2
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The crystal phase of In O3 microflowers, SnO2
2
nanoparticles and In
2
O
3
-SnO
2
composites were investigated by
XRD with a Rigaku D/max-2500 diffractometer using Cu-Kα1
radiation. From the Fig.1, we can see that In O -SnO
2
2
3
composites has all peaks of In
agreement with the JCPDS file of In
and SnO (JCPDS NO.77-451).It proved that In O microflowers
2
O
3
and SnO
2
.They were in good
2
O
3
(JCPDS NO.71-2195)
2
2 3
2
and SnO nanoparticles were mixed entirely by physical route
and they had no chemical reacts. No diffraction peaks from
any other impurities were observed, indicating the high purity
of the products.
3
.2 Structural and morphological characteristics
The morphologies and structures of In
SnO nanoparticles and In -SnO composites were identified
2 3
O microflowers,
2
2
O
3
2
by SEM using a JEOL JSM-7500F microscope with an
accelerating voltage of 15 kV as shown in Fig.2. Fig.2 (a)
presents the SEM image of SnO
2
particles, it can be seen that
SnO particles are composed of relatively uniform SnO
2
2
nanoparticles with a length of 20-30 nm. It can be seen that
the surfaces of nanoparticles are coarse. As is shown in Fig.2
(
b), the In
uniform size of 1-1.5 μm, it can be seen from the image that
the In microflowers are composed of aggregated
nanosheets.Fig.2 (c) shows In -SnO composites, SnO
nanoparticles seems to be surrounded by In O microflowers,
2 3
O microflowers have a good dispersion and
2 3
O
2
O
3
2
2
2
3
SnO
SnO
2
and In
2 3 2 3
O was marked with boxes. Composition of In O -
2
composites had been characterized using EDS as shown
in Fig.2 (d). The EDS spectrum showed that components of the
materials are In, Sn and O. The Si was attributed to the
substrate used in the SEM measurement.
In O
2
3
Fig.3 shows nitrogen adsorption-desorption isotherm and
pore size distribution (inset of Fig. 3) of the In
2 3
O microflowers,
SnO nanoparticles and In -SnO composites. Pore size
2
2
O
3
2
distribution curves were calculated from the desorption
branch of a nitrogen isotherm by the BJH method using the
SnO2
Halsey equation. The BET surface area of the In
2 3
O
microflowers, SnO nanoparticles and In -SnO composites
2
2
O
3
2
2
2
2
were calculated to be 19.5 m /g, 41.0 m /g and 34.4 m /g
respectively by the Brunauer-Emmett-Teller (BET) method with
a Micromeritics Gemini VII apparatus (Surface Area and
Porosity System).
3.3 Gas-sensing properties
2 3
As we know, the mass ratio of In O microflowers and
SnO2 nanoparticles and the working temperature both
influenced greatly gas responses in this research, so the gas
sensing properties of S0 to S3 to 100 ppm ethanol under
different temperatures were investigated, the results were
shown in Fig.4.The VOCs were got from liquid evaporation,
concentration calculation method is according to ideal gas
Fig.2 SEM images of (a) SnO
2 2 3
nanoparticles, (b) In O
microflowers and (c) In
2
O
3
-SnO
2
composites (d) EDS spectra
of In -SnO composites.
2
O
3
2
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Fig.4 Responses of sensors based on S0-S3 to 100 ppm
ethanol.
Fig.5 Relationship between responses of sensors based on S1
and ethanol concentration at 250
dynamic response.
℃, the inset displaying
the response is nearly saturated. Dynamic response towards
00-1000 ppm ethanol at 250 was shown in the inset of
Fig.5.
1
℃
The response and recovery characteristics were
investigated which were shown in the Fig.6 (a).The results
indicate that the In
has fast response-recovery kinetics. The response and
recovery times of the In -SnO composites sensors are
about 15 s and 60 s, respectively. Five reversible cycles of the
response curve indicate stable and repeatable
characteristic, as shown in Fig.6 (b).
Selectivity is also an important performance of the sensor
based on the as-prepared In -SnO composites to various
gases, such as NO, H S, NO , C CH , C O, CH OH, HCHO
and CH CH OH.All of the gases were tested at an operating
temperature of 250 as shown in Fig.7. The response of the
sensors based on In -SnO composites (S1) to toluene,
acetone and methanol is much lower than ethanol, and the
response to other gases or vapours such as NO, H S, NO and
formaldehyde is negligible. The results indicate that the as-
prepared In -SnO composites display superior
2 3 2
O -SnO composites sensor based on S1
Fig.3 Typical N
2
adsorption-desorption isotherms and pore
microflowers, SnO nanoparticles
composites.
2
O
3
2
size distribution of In
and In -SnO
2
O
3
2
2
O
3
2
a
2 3 2
equation of state (PV=nRT).The sensors based the In O -SnO
composites (S1) showed the best response while the mass
ratio of In O and SnO was 2:1.The best response to 100
2
O
3
2
H
5
3
3
H
6
3
2
3
2
2
2
6
ppm ethanol was 53.2 at 250
℃
. A comparison between the
3
2
sensing performances of the sensor and literature
℃
2
6,27,30,31,32
reports
Fig.5 shows the responses of the sensors based on S1 to
different concentrations ethanol at 250 .Before 500 ppm,
were summarized in Table 1.
2
O
3
2
℃
2
2
the responses increase with the ethanol concentration like a
line, when the ethanol concentration is more than 500 ppm,
2
O
3
2
4
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Table 1
Material
Fabrication
approach
Ethanol
concentration(ppm)
Temperatu Sensor
Reference
re (℃)
response
In
SnO
SnO
In
In
In
2
O
2
3
microflowers
nanostructures
Hydrothermal route
100ppm
RT
275℃
RT
275℃
RT
250℃
~1.5
~9.6
~3.0
~21.0
~18.7
53.2
[26]
[27]
[30]
[31]
[32]
Hydrothermal route
Sol–gel and electrospinning
Hydrothermal route
100ppm
100ppm
100ppm
100ppm
100ppm
2
–In
2
O
3
2
2
2
O
O
O
3
3
3
nanospheres
-SnO
-SnO
2
nanocomposites Mechanochemical reaction
composites Hydrothermal route
2
This work
photochemical water splitting (Water can be decomposed
3
into hydrogen and oxygen under UV illumination ).
3
3
.4 Gas mechanism
The formation of metal oxide composites is known to
increase the response and selectivity to gases. For this work,
Fig.7 Comparison of responses of In
sensors to various gases at 250
2 3 2
O -SnO composites (S1)
℃.
Fig.6 (a) Dynamic response resistance of the sensor (S1) and
b) five periods of response curve of the sensor to 100 ppm
ethanol at 250 (20%RH).
(
℃
selectivity to ethanol against the other interference gases and
was very suitable for sensing ethanol at 250
℃.
UV illumination was used to enhance the performance of
the sensors (S1). Although the sensitivity and the response
and recovery times were almost not affected by UV
illumination, UV illumination greatly reduced the influence of
humidity on the sensitivity of the sensors, the results were
shown in Fig.8. It can be seen that the sensitivity is almost
stable below 80% RH under UV illumination. When the
relative humidity is even up to 95%RH, the sensitivity still
maintains 75%, which is higher than that without UV
illumination. UV illumination can decompose the water
vapour on the surface according to the mechanism of
Fig. 8 Responses of the sensors (S1) in different relative
humidity, the inset is the resistances in air vs relative
humidity.
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Besides, In O and SnO were both active site, but each
2
3
2
promoted different breakdown profiles of the ethanol
vapours being sensed. Because of their complementary
catalytic activity (catalytic activity reaches an optimum level
2 3 2
when the mass ratio of In O and SnO is 2:1), the mixed
oxides permitted a more complete breakdown of the ethanol
vapours, leading to the observed enhancement in sensitivity.
In addition, the porous structure is helpful for gas diffusion
and its reaction on the surface of the sensing materials.
(b)
4. Conclusions
In summary, In O -SnO composites had been successfully
2
3
2
2 3 2
obtained through a mixing of In O microflowers and SnO
nanoparticles. The nanostructure is in favour of permeation
of the tested gas. The In O -SnO sensors exhibit high
2
3
2
response and good selectivity to ethanol at 250
℃. It can
detect 100ppm ethanol with a response value of about 53.2
and with little interference from other gases. The
Fig.9 (a) Schematic diagram of the proposed mechanism of
In -SnO composites and (b) Energy band structure of In
SnO and In –SnO composites.
2 3 2
enhancement in gas-sensing property on In O and SnO
2
O
3
2
2 3
O ,
composites may be ascribed to heterostructure. Because of
the depletion layer manipulation and complementary
catalytic activity, the composite oxides permitted a more
complete breakdown of the ethanol vapours, leading to the
observed enhancement in sensitivity and selectivity. The
sensitivity is stable while the relative humidity increasing
under UV illumination due to photochemical water splitting.
2
2
O
3
2
both In
2
O
3
and SnO
2
increase the response and selectivity to
and SnO are n-type
gases. For this work, both In
2
O
3
2
semiconductor oxides. The surface of the sensor material in
− 2−
air is covered with chemisorbed oxygen ions such as O , O
2
−
and O
2
, the electrons on chemisorbed oxygen ions come
from the material, which decrease the electron density in n-
type material and in turn increase the resistance of the oxide.
If reducing gas is introduced, it reacts with the surface
adsorbed oxygen, the electron will be donated back into the
semiconductor that causes a decrease in the resistance.
Adsorbed ethanol undergoes two possible mechanisms of
dehydrogenation to an aldehyde and dehydration to an
Acknowledgments
This work is supported by the National Nature Science
Foundation of China (Nos. 61074172, 61134010, and
61327804) and Program for Chang Jiang Scholars and
3
4
alkene, shown in reactions (1) and (2) .
C H OH (g)
Innovative Research Team in University (No. IRT1017).
National High-Tech Research and Development Program of
China (863 Program, No. 2013AA030902).
→
CH CHO (g) + H (g)
2
(1)
(2)
2
5
3
C
2
H
5
OH (g)
→
C
2
H
4
(g) + H
2
O (g)
Following this process consecutive reactions happen
which consume ionic oxygen species and release electrons, so
the resistance of the oxide is reduced. The electrons only
removed to a certain depth from the surface known as the
depletion region. The depletion region width may change as
the test gas or oxygen is adsorbed on the surface, which in
turn caused a measurable change in the resistance. This
process was shown in Fig.9 (a).
Notes and references
1
2
8
3
2
4
2
5
2
6
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SnO composites may be mainly ascribed to heterostructure
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