ZnO nanowires were produced using an electrospinning method and used in gas sensors for the detection of ethanol at 220 °C. This electrospinning technique allows the direct placement of ZnO nanowires during their synthesis to bridge the sensor electrodes. An excellent sensitivity of nearly 90% was obtained at a low ethanol concentration of 10 ppm, and the rest obtained at higher ethanol concentrations, up to 600 ppm, all equal to or greater than 90%

D Li

GS Wu

H Liu

H Wu

J Mass

Jyh-Ming Ting

M Yang

NJ Pinto

Po-Jung Huang

Q Wan

R Siddheswaran

S Choopun

S Thandavamoorthy

SJ Chen

T Gao

TJ Hsueh

W Nuansing

Wan-Yu Wu

X Jiaqiang

X Sui

X Yang

XL Yuan

XM Sui

Y Zhang

YW Heo

English

PubMed

Springer - Publisher Connector

NANO EXPRESS
Electrospun ZnO Nanowires as Gas Sensors for Ethanol Detection
Wan-Yu Wu Æ Jyh-Ming Ting Æ Po-Jung Huang
Received: 30 June 2008 / Accepted: 6 February 2009 / Published online: 4 March 2009
 to the authors 2009
Abstract ZnO nanowires were produced using an elec-
trospinning method and used in gas sensors for the
detection of ethanol at 220 C. This electrospinning tech-
nique allows the direct placement of ZnO nanowires during
their synthesis to bridge the sensor electrodes. An excellent
sensitivity of nearly 90% was obtained at a low ethanol
concentration of 10 ppm, and the rest obtained at higher
ethanol concentrations, up to 600 ppm, all equal to or
greater than 90%.
Keywords Zinc oxide  Electrospinning 
Ethanol sensor  Sensitivity
ZnO is an interesting chemically and thermally stable
n-type semiconductor with a large exciton binding energy
(60 meV) and a large band gap (3.37 eV) energy. ZnO
nanomaterial also appears to be the mostly studied one as it
exhibits a wide variety of nanostructures such as nanowires
[1], nanowalls [2–4], nanobelts [5], nanorods [6], nano-
sheets [7], and so on. Among its applications, ZnO
nanowire is receiving greater interests for use in gas sensors
for detecting, for example, ethanol. ZnO nanowires pre-
pared by a reactive thermal deposition method were used
for ethanol sensing [8]. The sensitivity, i.e., (Ra - Rg)/Ra
where Ra and Rg are, respectively, the resistance of the
nanowires exposed to air without and with the detecting gas,
increases from *47% to *98% while the ethanol con-
centration increases from 1 ppm to 200 ppm at a high
temperature of 300 C. However, at lower temperatures of
200 C and 250 C, the sensitivities drop significantly to
*67% and *86%, respectively. The response time and
recovery time are 10 s and 55 s, respectively, at a ethanol
concentration of 200 ppm and a temperature of 300 C.
Ethanol sensors based on ZnO nanowires prepared using a
self-catalyzed vapor–liquid–solid (VLS) method exhibit a
sensitivity that increases from 18% to 61% while the eth-
anol concentration increases from 50 ppm to 1500 ppm at
a high temperature of 300 C [9]. However, the response
time of the ZnO nanowires to the ethanol is not clear. ZnO
nanowires fabricated using a hydrothermal method show an
ethanol sensitivity of 92% at an ethanol concentration of
100 ppm at 330 C [10]. ZnO nanobelts prepared by RF
sputter-deposition method were also used for ethanol
sensing [5]. The sensitivity to ethanol increases from 86%
to 96% while the ethanol concentration increases from
50 ppm to 1000 ppm at a temperature of 220 C. However,
the response–recovery characteristics of the ZnO nanobelt-
based ethanol sensors were not reported. Common to these
reports is that for the fabrication of gas sensors, the as-
synthesized ZnO nanowires or nanorods must be removed
from a substrate and/or be transferred into a solution, and
then dispersed randomly onto the sensor devices. This
‘‘pick-and-place’’ technique raises a concern of incompat-
ibility with the Si processing [11]. However, by using the
electrospuning process for the synthesis of nanowires, the
‘‘pick’’ step is eliminated as it allows a direct placement of
nanowires onto a sensor chip.
In the past few decades, the electrospinning process was
developed for the fabrication of nanowires. Electrospinning
represents a very simple, versatile, and low cost method for
W.-Y. Wu  J.-M. Ting (&)
Department of Materials Science and Engineering,
National Cheng Kung University, Tainan, Taiwan
e-mail: jting@mail.ncku.edu.tw
J.-M. Ting  P.-J. Huang
Institute of Nanotechnology and Microsystems Engineering,
National Cheng Kung University, Tainan, Taiwan
123
Nanoscale Res Lett (2009) 4:513–517
DOI 10.1007/s11671-009-9271-4
large-scale fabrication of nanowires. This technique has
been successfully used to produce various polymeric,
inorganic, and hybrid nanowires or nanowires [12–14].
Considerable efforts have also been made to fabricate
functional nanodevices using the electrospun nanowires
[15, 16]. However, it is only recently that a few reports
have shown the synthesis of ZnO nanowires using the
electrospinning process [1, 17–19]. Moreover, there has
been no report showing the use of electrospun ZnO nano-
wires as gas sensor, except a very recent one in which
electrospun ZnO nanowires were used as a photoelectric
gas sensor for the detection of oxygen under the illumi-
nation of a 500 W Xe lamp [20]. However, the need of a
Xe lamp limits its applications.
In this article, we demonstrate the use of electrospun
ZnO nanowires in silicon-based gas sensors for the detec-
tion of ethanol with very high sensitivities. Gel of zinc
acetate/polyvinyl alcohol (PVA) was used as the precursor.
The precursor was loaded into a syringe, which was con-
nected to a high-voltage power supply and served as the
positive electrode. The negative electrode was an alumi-
num foil where chips having interdigitated electrodes were
placed. The interdigitated electrode area is 1 mm 9 1 mm
and the distance between two adjacent interdigitated elec-
trodes is 70 lm. Fixing chips on the surface of the
aluminum foil allows the direct placement of electrospun
polymeric nanowires onto its surface, leading to the
bridging of two electrodes by the subsequently formed ZnO
nanowires. For the electrospinning, an electric field of
15 kV was applied. The resulting electrospun polymeric
nanowires were subjected to a calcination process without
or with the chips at 600 C to form inorganic ZnO nano-
wires. The calcination time ranged from 1 h to 7 h. The
morphology of the obtained nanowires was examined using
scanning electron microscopy (SEM). X-ray diffraction
analysis was carried out to determine the crystalline
structures of the nanowires. Cathodoluminescence (CL)
spectroscopy analysis was also performed at the liquid
nitrogen temperature. The gas sensing characteristics were
measured in a cylindrical chamber. The chamber has an
inlet port connected to a gas inlet valve and outlet port
connected to an air pump. Sensors were connected to an
outside multimeter to monitor the resistance changes.
Figure 1a shows a general view of the ZnO nanowires
obtained. The nanowires are abundant and have uniform
diameter. The average diameter ranges from 220 ± 15 nm
Fig. 1 a A general view of electrospun ZnO nanowires. ZnO nanowires obtained at calcination times of b 5 h and c 7 h
514 Nanoscale Res Lett (2009) 4:513–517
123
to 90 ± 10 nm and decreases with the calcination time. For
the nanowires obtained at a calcination time B5 h, the
surfaces are smooth as shown in Fig. 1b for ones obtained
at the calcination time of 5 h. The average diameter is
120 nm ± 10 nm. When the calcination time exceeds 5 h,
surface cracks were observed as shown in Fig. 1c for ones
obtained at a calcination time of 7 h. The average diameter
is 90 nm ± 10 nm. The difference in the diameter and the
existence of the cracks have an important consequence on
the sensitivity as will be discussed later. It was also
observed that the existence of such surface cracks does not
affect the crystallinity and the CL emission. XRD analysis
indicates that all the nanowires are polycrystalline and the
crystallinity becomes better, i.e., the long-range order is
improved, with the calcination time. This is illustrated in
Fig. 2. The nanowires calcined for a time \5 h exhibit
broadened diffraction peaks, especially those appearing at
2h [*47. These diffraction peaks are weak or even
insignificant and also the nanowires contain amorphous
structures left from the precursor, which may be seen, as
shown by the lower spectral line in Fig. 2 for a 1-h sample.
The intensity of the diffraction peaks increases and the
peak widths decrease with the calcination time. The
nanowires calcined for a time equal to or greater than 5 h
shows sharp diffraction peaks as shown by the upper
spectral line in Fig. 2 for a 5-h sample. It appears that a
minimum of 5 h is required for complete calcination. In the
mean time, according to the CL spectroscopy analysis,
well-calcined ZnO nanowires exhibit a strong UV emission
at 368 nm and a very insignificant emission or nearly none
at 465 nm, as shown in Fig. 3. The UV emission is related
to the near-band edge emission. The physical origin of the
very weak emission at 486 nm, as shown in the insert, is
not clearly understood at this stage. It can be attributed to
either the presence of impurities or native defects [21],
surface defects [22], and oxygen vacancies [23]. The CL
characteristics indicate that the well-calcined ZnO nano-
wires exhibit a good structure quality.
As mentioned above, chips having interdigitated elec-
trodes were attached onto the aluminum electrode to allow
direct placement electrospun nanowires. The chips were
subjected to 5-h and 7-h periods of calcination. Figure 4
shows ZnO nanowires bridging the sensor electrodes.
Normally, more or less straight nanowires cross sensor
electrodes when the pick-and-place method is used in
conjunction with other nanowire synthesis techniques. As
shown in Fig. 4, curved ZnO nanowires bridge the elec-
trodes, providing a higher surface area for better
sensitivities as shown below. The sensor was found to
exhibit a wide detecting range, from 600 ppm to as low as
10 ppm at a low temperature of 220 C. Figure 5a shows
the sensitivities of 5-h-calcined ZnO nanowire sensor at
20 30 40 50 60 70 80
0
20
40
60
80
100
120
140
500
1000
1500
2000
2500
3000
   
 (
20
0)
1-hr Sample
2
   
 (
20
2)
   
 (
00
4)
  (
20
1)   
 (
11
2)
   
 (
10
3)
   
 (
11
0)
   
 (
10
2)
   
 (
10
1)
  (
00
2)
(1
00
)
In
te
ns
ity 5-hr Sample
Fig. 2 XRD spectrum of ZnO nanowires obtained at 1 and 5 h of
calcination
300 350 400 450 500 550 600
0
5000
10000
15000
20000
co
un
ts
Emission Wavelength (nm)
368nm
440 460 480 500 520 540 560 580 600
300
400
500
600
700
486nm 
Fig. 3 CL spectrum of ZnO nanowires obtained at a calcination
temperature and time of 600 C and 5 h, respectively
Fig. 4 ZnO nanowires on the electrodes (dark area)
Nanoscale Res Lett (2009) 4:513–517 515
123
this ethanol concentration range. The sensitivities were
determined to be 32, 73, 80, 85, and 89% at ethanol
concentrations of 10, 160, 240, 400, and 600 ppm,
respectively. The response time and recovery time are
approximately 16 s and 25 s, respectively. The response
time is defined as the time required for the resistance to
reach 90% of the equilibrium value following an injection
of the test gas, and the recovery time is the time necessary
for the resistance to return to 10% above original resistance
in air after the removal of the test gas [10]. Furthermore,
the sensitivity can be further increased by using 7-h-
calcined ZnO nanowires, as shown in Fig. 5b. Now the
sensitivities are 89, 90, 92, 93, and 94% at ethanol con-
centrations of 10, 160, 240, 400, and 600 ppm,
respectively. Sensitivity was determined from measured
resistances, Ra and Rg, each of which is an average of five
data points. The measured resistance are very consistent
such that the standard deviations are typically less than
0.5%. This makes the sensitivities distinguishable. It is
noteworthy that the sensitivity at the lowest ethanol con-
centration of 10 ppm is nearly 90%, and the rest are all
C90%. In fact, it was found that the sensitivity increases
with reduced nanowire diameter. The enhanced sensitivity
is attributed to the increased specific surface area (area/
volume) and improved crystallinity, both of which increase
with reduced nanowire diameter. In addition to the
enhanced sensitivity, the 7-h-calcined ZnO nanowire sen-
sor also exhibits a similar response time of 19 s. However,
the recovery time was found to be several times higher.
This is also attributed to the existence of the surface cracks
as shown in Fig. 1c. It is believed that the cracks easily
absorb the ethanol, which is then trapped inside the cracks,
and therefore takes time to be desorbed. For comparison, it
was found that depending on the ethanol concentration, the
sensitivities are in general better than or at least compa-
rable to the previously reported values as noted above. At
the same sensing temperature of 220 C, we have obtained
sensitivities of 89 and 93% at 10 and 400 ppm, respec-
tively; while in Ref. [5], the sensitivities are 86 and 95% at
50 and 500 ppm, respectively. At 300 C, the sensitivities
reported in Ref. [8] are 93% at 50 ppm. In Ref. [9], the
sensitivities obtained 300 C, i.e., from 18 to 61% at 50–
1500 ppm, are all lower than that obtained in this study. In
Ref. [10], the sensitivity obtained at an even higher tem-
perature of 330 C is 92% for 100 ppm methanol.
We have fabricated electrospun polycrystalline ZnO
nanowires directly placed onto interdigitated electrodes to
form gas sensors for the detection of ethanol. Sensors
exhibiting very high sensitivities, and fast response time
and recovery time were demonstrated. The average nano-
wire diameter ranges from 220 ± 15 nm to 90 ± 10 nm
and decreases with the calcination time. The ZnO nano-
wires exhibit a strong UV emission at 368 nm and a very
insignificant emission or nearly none at 465 nm. For the
detection of ethanol at 220 C, an excellent sensitivity of
nearly 90% was obtained at a low ethanol concentration of
10 ppm and the rest obtained at higher ethanol concentra-
tions, up to 600 ppm, are all C90%. Fast response time and
recovery times of 16 s and 25 s, were also obtained.
Acknowledgments This work was supported by the National Sci-
ence Council in Taiwan under grant No. 97-2120-M-006-001 and the
Top University Program at the National Cheng Kung University in
Taiwan under R048/D97-3360.
References
1. X. Yang, C. Shao, H. Guan, X. Li, J. Gong, Inorg. Chem.
Commun. 7, 176–178 (2004). doi:10.1016/j.inoche.2003.10.035
2. Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren,
S.J. Pearton, J.R. LaRoche, Mater. Sci. Eng. Rep. 47, 1 (2004).
doi:10.1016/j.mser.2004.09.001
3. G.S. Wu, T. Xie, X.Y. Yuan, Y. Li, L. Yang, Y.H. Xiao, L.D.
Zhang, Solid State Commun. 134, 485 (2005). doi:10.1016/j.ssc.
2005.02.015
4. Y. Zhang, K. Yu, S. Ouyang, Z. Zhu, Mater. Lett. 60, 522 (2006).
doi:10.1016/j.matlet.2005.09.028
5. S. Choopun, N. Hongsith, P. Mangkorntong, N. Mangkorntong,
Physica E 39, 53–56 (2007). doi:10.1016/j.physe.2006.12.053
6. T. Gao, T.H. Wang, Appl. Phys. A 80, 1451–1454 (2005). doi:
10.1007/s00339-004-3075-2
7. S.J. Chen, Y.C. Liu, C.L. Shao, R. Mu, Y.M. Lu, J.Y. Zhang,
D.Z. Shen, X.W. Fan, Adv. Mater. 17(5), 8 (2005)
8. Q. Wan, Q.H. Li, Y.X. Liang, T.H. Wang, Appl. Phys. Lett.
84(18), 3654 (2004). doi:10.1063/1.1738932
0 500 1000 1500 2000 2500
-20
0
20
40
60
80
100 600ppm
S
en
si
tiv
ity
 (%
)
10ppm
160ppm
240ppm
400ppm
0 500 1000 1500 2000 2500 3000 3500
0
20
40
60
80
100 600ppm400ppm240ppm160ppm10ppm
S
en
si
tiv
ity
 (%
)
Time (s)Time (s)
(A) (B) Fig. 5 Sensitivities of the a 5-h
calcined and b 7-h calcined ZnO
nanowires gas sensors measured
at ethanol concentrations
ranging form 10 ppm to
600 ppm
516 Nanoscale Res Lett (2009) 4:513–517
123
9. T.J. Hsueh, C.L. Hsu, S.J. Chang, I.C. Chen, Sens. Actuators B
126, 473–477 (2007). doi:10.1016/j.snb.2007.03.034
10. X. Jiaqiang, C. Yuping, L. Yadong, S. Jianian, J. Mater. Sci. 40,
2919–2921 (2005). doi:10.1007/s10853-005-2435-4
11. H. Wu, D. Lin, R. Zhang, W. Panw, J. Am. Ceram. Soc. 91(2),
656–659 (2008). doi:10.1111/j.1551-2916.2007.02162.x
12. S. Thandavamoorthy, G.S. Bhat, R.W. Tock, S. Parameswaran,
Appl. Polym. Sci. 96, 557–569 (2005). doi:10.1002/app.21481
13. W. Nuansing, S. Ninmuang, W. Jarernboon, Mater. Sci. Eng. B
131, 147–155 (2006). doi:10.1016/j.mseb.2006.04.030
14. X.M. Sui, C.L. Shao, Y.C. Liu, Appl. Phys. Lett. 87(11), 117115
(2005). doi:10.1063/1.2048808
15. N.J. Pinto, A.T. Johnson, A.G. MacDiarmid, C.H. Mueller,
N. Theofylaktos, D.C. Robinson, F.A. Miranda, Appl. Phys. Lett.
83(20), 4244–4246 (2003). doi:10.1063/1.1627484
16. H. Liu, C.H. Reccius, H.G. Craighead, Appl. Phys. Lett. 87(25),
253106 (2005). doi:10.1063/1.2149980
17. R. Siddheswaran, R. Sankar, M. Ramesh Babu, M. Rathnaku-
mari, R. Jayavel, Cryst. Res. Technol. 41(5), 446–449 (2006).
doi:10.1002/crat.200510603
18. H. Wu, W. Pan, J. Am. Ceram. Soc. 89(2), 699–701 (2006). doi:
10.1111/j.1551-2916.2005.00735.x
19. X. Sui, C. Shao, Y. Liu, Polymer 48, 1459–1463 (2007)
20. M. Yang, T. Xie, L. Peng, Y. Zhao, Appl. Phys. A 89, 427–430
(2007). doi:10.1007/s00339-007-4204-5
21. X.L. Yuan, B.P. Zhang, J. Niitsuma, T. Sekiguchi, Mater. Sci.
Semicond. Process 9, 146–150 (2006). doi:10.1016/j.mssp.2006.
01.037
22. D. Li, Y.H. Leung, A.B. Djurisic, Z.T. Liu, M.H. Xie, S.L. Shi
et al., Appl. Phys. Lett. 85(9), 1601–1603 (2004). doi:10.1063/
1.1786375
23. J. Mass, M. Avella, J. Jimenez, M. Callahan, E. Grant, Super-
lattice. Microst. 38, 223–230 (2005). doi:10.1016/j.spmi.2005.
08.001
Nanoscale Res Lett (2009) 4:513–517 517
123


Electrospun ZnO Nanowires as Gas Sensors for Ethanol Detection

Abstract ZnO nanowires were produced using an electrospinning method and used in gas sensors for the detection of ethanol at 220 &#176;C. This electrospinning technique allows the direct placement of ZnO nanowires during their synthesis to bridge the sensor electrodes. An excellent sensitivity of nearly 90% was obtained at a low ethanol concentration of 10 ppm, and the rest obtained at higher ethanol concentrations, up to 600 ppm, all equal to or greater than 90%.</p

Huang Po-Jung

Wu Wan-Yu

Ting Jyh-Ming

Directory of Open Access Journals

Nanoscale Research Letters

Crossref

2500 -20 100 600ppm S e n s i t i v i t y ( % ) 10ppm 160ppm 240ppm 400ppm 0

file:///data/core-remote/dit/data/Springer-OA/pdf/efa/aHR0cDovL2xpbmsuc3ByaW5nZXIuY29tLzEwLjEwMDcvczExNjcxLTAwOS05MjcxLTQucGRm.pdf

Electrospun ZnO Nanowires as Gas Sensors for Ethanol Detection

Abstract

Similar works

Full text

Available Versions

Springer - Publisher Connector

Springer - Publisher Connector

Directory of Open Access Journals

Crossref