a novel ethanol gas sensor based on zno-microwire
TRANSCRIPT
A novel ethanol gas sensor based on ZnO-microwire
Fei Li • Heqiu Zhang • Lizhong Hu •
Yingmin Luo • Yu Zhao • Yu Qiu • Jiuyu Ji •
Lunlun Yue
Received: 28 July 2013 / Accepted: 30 August 2013 / Published online: 11 September 2013
� Springer Science+Business Media New York 2013
Abstract One-dimensional (1D) ZnO microwires were
successfully synthesized by chemical vapor deposition and
their structural and morphological properties were ana-
lyzed by X-ray diffraction and scanning electron micros-
copy, demonstrating that the microwires were single
crystalline with perfect hexagonal structure and smooth
surface. Using these 1D microstructures, we fabricated a
novel ZnO-based ethanol gas sensor. Operating at room
temperature, the sensor was found to have good sensing
characteristics. The reliability and stability of the sensor
could be improved by connecting multiple 1-wire devices
(1-WD) in parallel into a multi-wires device. In interior
natural lighting environment and under 3 V bias, the
response and recovery time of the 1-WD to 200 ppm eth-
anol gas were\10 s and about 300 s, respectively, and the
minimum and maximum detection limit were about 2 and
200 ppm, respectively. A sensing model was proposed for
discussing the performance of the sensor. The simplicity in
fabrication, low power consumption and low cost make the
sensor suitable for practical application in many fields,
especially in identifying driving under the influence and
chemical industry monitoring.
1 Introduction
In recent years, a variety of prototype sensors based on
metal oxide semiconductors, such as ZnO, SnO2 and TiO2
[1–3], have been developed. Especially, ZnO-based sensors
have received the most attention due to ZnO’s colorful
micro/nanostructures, such as nanoflakes, nanohelix, hol-
low microspheres, nanofibers, nanoparticles [4–9], and its
fascinating physical and chemical properties, such as low
toxicity, good thermal stability, good biocompatibility and
high electron mobility. By using ZnO micro/nanostruc-
tures, all kinds of sensors, such as gas sensors, chemical
sensors, biosensors, ultra violet (UV) sensors, have now
been fabricated [1].
ZnO-based gas sensors can detect various toxic and
inflammable gases, volatile organic compounds (such as
CO, NO2, H2S, H2, ethanol) [6, 10–12] and humidity [13].
And for the sake of improving the sensitivity, repeatability
and selectivity, researchers have made great efforts in the
following aspects: the first is to dope or coat the sensing
materials with common or rare/noble metal [9, 14–18], the
second is to use nanostructures with high surface/volume
ratio as sensing materials [12, 19–22], the third is to
employ semiconductor hetero-junction [23] or surface
acoustic wave device structure [13], and the forth is to
make use of UV or visible light assisted technology [12,
25, 26]. All these efforts have made inspiring progress in
enhancing the performance of the sensors. However, most
of these sensors require a higher working temperature
usually in the range of 100–450 �C [2, 4–10, 14–25],
leading to the existence of an additional heating element
that will inevitably increase the size, power consumption
and complexity of a sensor. Therefore, it is necessary to
develop a no heating required, low-power and compact gas
sensor for practical application.
In this study, we present a novel ZnO-based ethanol gas
sensor. The sensor utilizes 1D ZnO microwire as its core
part and has the following advantages: (1) ZnO microwires
can be cheaply produced in large quantities by chemical
F. Li � H. Zhang � L. Hu (&) � Y. Luo � Y. Zhao � Y. Qiu �J. Ji � L. Yue
School of Physics and Optoelectronic Technology,
Dalian University of Technology, Dalian 116024,
People’s Republic of China
e-mail: [email protected]
123
J Mater Sci: Mater Electron (2013) 24:4812–4816
DOI 10.1007/s10854-013-1480-z
vapor deposition (CVD); (2) the sensor can sensitively
detect ethanol gas at room temperature (RT) without a
heating element; (3) it is convenient to connect several
1-WDs in parallel into a multi-wires device (m-WD) for
improving the reliability and stability of the device.
2 Experimental
2.1 Chemicals
All chemicals used in this work were of analytical reagent
grade (AR), purchased from Tianjin Kemiou Chemical
Reagent Co., Ltd., China, including of ZnO powder,
graphite powder, anhydrous ethanol, anhydrous methanol,
acetone and methylbenzene.
2.2 Preparation and characterization of ZnO
microwires
ZnO microwires were synthesized by normal pressure
CVD in a horizontal tube furnace. A mixture of ZnO and
graphite powder, with a mole ratio of 1:2, was used as the
growth source material. The source was placed at the
closed end of a semi-closed small quartz tube in the centre
of the horizontal tube furnace, and the opening of the small
quartz tube was along the gas flow direction. The source
temperature was kept at 950 �C for 90 min under a mixed
gas flow of N2 (80 sccm) and O2 (16 sccm) during reac-
tion. After reaction, ZnO microwires were synthesized at
the opening end of the small quartz tube. When the furnace
was cooled to RT under the protection of N2, the as-grown
ZnO microwires were taken out of the furnace.
The crystal phase and the morphology of ZnO micro-
wires were analyzed by X-ray diffraction (XRD-6000,
SHIMADZU) and scanning electron microscopy (SEM,
HITACHI S-4800), respectively.
2.3 Preparation and examination of sensor
The length and diameter of ZnO microwires used in our
experiment are about 3 mm and 30 lm, respectively.
Details of the sensor fabrication are as follows: using a
piece of thin plastic board about 3 mm in width as a base
board, on it we drilled an about 1-mm-diameter hole, and a
tiny slot was also carved across the hole for fixing a ZnO
microwire and connecting metal wires. Then we fixed a
ZnO microwire and connected metal wires at its both ends
with silver paste to prepare a probe type sensor (so-called
1-WD). By using the same process, some other 1-WDs
were also fabricated, as shown in Fig. 1a.
A Keithley 4200 semiconductor characterization system
was used to check the connection between ZnO microwire
and metal wire and the sensitivity of the device to ethanol
gas. And all the tests were conducted at RT.
To test the sensing property of the device, we made a
sealed plastic chamber with a volume of 4 l and on it we
made a small hole through which the probe can be easily
inserted into and extracted from the chamber. The sensi-
tivity of an ethanol gas sensor was defined as S = ((Iair -
Igas)/Iair) 9 100 %, where Iair and Igas are, respectively, the
measured current in air and the steady-state measured
current in ethanol gas at the same bias voltage. A certain
amount of liquid alcohol was injected into the sealed
chamber by a syringe with a minimum scale of 0.01 ml and
the corresponding ethanol gas concentration was calculated
by the ratio of the injected ethanol mass (mg) and the
chamber volume (liter). The detecting process is as fol-
lows: (1) injecting a certain amount of ethanol into the
sealed chamber and forming a certain concentration of
ethanol gas in the chamber by naturally evaporating the
injected ethanol; (2) when having a uniform distribution of
ethanol gas, inserting the probe into the chamber to test the
response-time of the sensor; (3) when Igas running up to a
stable value, drawing out the probe from the chamber to
test the recovery-time. The schematic of home-built sealed
chamber and testing setup are shown in Fig. 1b.
3 Results and discussion
3.1 Structure and morphology
Figure 2 shows a typical XRD pattern and two SEM
images of the as-grown ZnO microwires. Because the axes
Fig. 1 Schematic diagrams of the probe (a) and testing setup (b)
J Mater Sci: Mater Electron (2013) 24:4812–4816 4813
123
of the microwires are along different directions, all the 11
diffraction peaks, appearing in the XRD pattern in Fig. 2a,
match with Bragg reflections of the relevant phases of
hexagonal wurtzite structural ZnO. And all the peaks have
a narrower full width at half maximum, indicating that the
ZnO microwires are good in crystal quality. The SEM
image in Fig. 2b clearly shows that numerous different
thickness ZnO microwires were synthesized and their
length ranges from 1–3 mm. Figure 2c shows a high
magnification SEM image of a single ZnO microwire,
displaying that the ZnO microwire has perfect hexagonal
structure, smooth surface and a diameter of *30 lm.
3.2 I–V characteristics
Typical I–V characteristics of 1-WD in dark and interior
natural lighting environment under different bias voltage
are shown in Fig. 3. From the I–V curves, it is easily
estimated that the conductivity of the device increases by
two orders of magnitude in interior natural lighting envi-
ronment in comparison to that in dark when the bias
voltage is 5 V. Obviously, the increase of conductivity
results from the photon-generated carriers and will greatly
improve the sensing performance of the device. Therefore,
all the sensing data in our experiment were obtained in
interior natural lighting environment.
3.3 Ethanol gas sensing performance
In order to observe the effect of bias voltage on the sensing
behavior of the device, the sensitivity of a 1-WD to
100 ppm ethanol gas was measured at different bias volt-
ages of 3, 5 and 7 V, as shown in Fig. 4a. Comparing the
three curves, it is very clear that this device had a similar
sensing performance under different bias. Therefore, all the
data given below were obtained at 3 V bias.
Figure 4b shows the typical sensing responses of a
1-WD to different ppm ethanol gas. It is obvious that the
device had different sensitivity to different ppm ethanol gas
and the minimum and maximum detection limit were about
2 and 200 ppm, respectively. To lower concentration
(B50 ppm) ethanol gas, the response and recovery time
were about 60 and 300 s, respectively. To higher concen-
tration ([50 ppm) ethanol gas, the response time would
reduce to 20 s or less, while the recovery time was still
Fig. 2 a Typical XRD pattern of the ZnO microwires. b Low-
magnification SEM image of the ZnO microwires. c High-magnifi-
cation SEM image of a single ZnO microwire
Fig. 3 I–V characteristic curves of a 1-WD in dark and interior
natural lighting environment, the inset is the amplified I–V charac-
teristic corresponding to the red rectangle (Color figure online)
4814 J Mater Sci: Mater Electron (2013) 24:4812–4816
123
about 300 s. Comparing to the sensors working at RT
reported by Yu et al. [23] and Xu et al. [26], our device is
superior to theirs in the response, but inferior to theirs in
the recovery. For example, to 200 ppm ethanol gas, the
response and recovery time of our 1-WD were \10 and
about 300 s, respectively, while their corresponding data
were about 62 and 83 s, respectively. In addition, our
device has a prominent advantage of simplicity in
fabrication.
Figure 4c, d show the sensing responses of a 2-WD and
a 4-WD exposed to different ppm ethanol gas. It can be
clearly seen that 1-WD and m-WD can all be used as
ethanol gas sensor and have the same sensing ability to
different ppm ethanol gas. But the reliability and stability
of m-WD should be better than 1-WD.
3.4 Ethanol gas sensing mechanism
It is common sense that the surface layer resistance of ZnO
nanomaterial will increase after chemisorbing O2 mole-
cules on its surface. This characteristic has become a basic
premise for most of ZnO-based sensors. The corresponding
detection mechanism has been described in many litera-
tures [1–4, 7, 8, 14, 15, 18, 21–24], in which C2H5OH gas
to be detected was found to be a reductive gas able to
release electrons in reduction reaction process and thus
increase the carrier concentration and decrease the resis-
tance of the nanomaterial. However, in our research, it was
found that C2H5OH gas did not act as a reductive gas, but
played as an oxidative gas to decrease the carrier concen-
tration and thus increase the resistance of ZnO microwire.
Similar result was got by Peng et al. [12] and Xu et al. [26].
So in our experiment, the probable detection mechanism
can be described by following Eqs. (1–4).
MOHðgasÞ $ MOHðadsorbedÞ ð1Þ
MOHðadsorbedÞ $ MO� þ Hþ ð2Þ
MO� þ hþ $ MOðadsorbedÞ ð3Þ
Hþ þ e� $ HðadsorbedÞ ð4Þ
where MO- is the group such as ethoxy or methoxy ion,
H? is hydrogen ion, e- is electron and h? is positive hole.
Alcohols can be written as MOH in chemical structure, and
their ability of ionizing into MO- and H? is sequentially as
follows [27]: H2O [ ethanol [ methanol. According to the
research result of Chuasiripattana et al. [28], the chemi-
sorption ability of OH, MO and H on the surface of ZnO is
in turn as follows: OH [ MO [ H. Therefore, during the
response or recovery, the chemisorption or desorption of
Fig. 4 a Sensing response of a 1-WD to 100 ppm ethanol gas under different bias and sensing responses of a 1-WD (b), 2-WD (c) and 4-WD
(d) to different ppm ethanol gas at 3 V bias
J Mater Sci: Mater Electron (2013) 24:4812–4816 4815
123
MOH molecules on the surface of ZnO microwire will
capture or release a large number of photon-generated
carriers to cause a decreasing or increasing of the detected
current. If the change of the detected current is large and
fast, indicating the sensing behavior of the sensor is
excellent. Figure 5 shows the sensing responses of a 1-WD
to 100 ppm H2O gas, methanol gas and ethanol gas. From
it we can see that the three corresponding sensitivities are
sequentially as follows: H2O [ ethanol [ methanol, being
consistent with their degree of ionization. This result is a
convincing evidence to support our detecting mechanism.
4 Conclusions
We have successfully synthesized ZnO microwires by
CVD and characterized them by XRD and SEM. With
these 1D ZnO microwires we fabricated a novel ethanol
gas sensor. At RT the sensor was found to have good
sensing characteristic and the reliability and stability could
be improved by parallel-connection of multiple 1-WDs. To
200 ppm ethanol gas, the response time of 1-WD was
within 10 s and the recovery time was about 300 s. The
mechanism of the ethanol gas sensor was also discussed. It
was found that C2H5OH molecule in our sensors plays as
an oxidative agent to increase the resistance of ZnO
microwire.
Acknowledgments The research was supported by the National
Natural Science Foundation of China, No. 60777009 and the Fun-
damental Research Funds for the Central Universities, No.
DUT11LK46.
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Fig. 5 Sensing responses of a 1-WD to 100 ppm H2O gas, methanol
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