Realizing room-temperature self-powered ethanol sensing of Au/ZnO nanowire arrays by coupling the piezotronics effect of ZnO and the catalysis of noble metal

Abstract

By coupling the piezotronics effect of ZnO and the catalysis of noble metal, room-temperature self-powered active ethanol sensing was obtained from Au/ZnO nanowire arrays. The piezoelectric output generated by Au/ZnO nanowire arrays acts not only as power source, but also as response signal to ethanol at room temperature. Upon exposure to 1200 ppm ethanol, the piezoelectric output of Au/ZnO nanowire arrays decreased from 1.54 V (in air) to 0.43 V. Our research can stimulate a research trend on the development of the next generation of room-temperature gas sensors and will further expand the scope for self-powered nanosystems.

Full text

Realizing room-temperature self-powered ethanol sensing of Au/ZnO nanowire arrays
by coupling the piezotronics effect of ZnO and the catalysis of noble metal
Lili Xing, Yuefeng Hu, Penglei Wang, Yayu Zhao, Yuxin Nie, Ping Deng, and Xinyu Xue
Citation: Applied Physics Letters 104, 013109 (2014); doi: 10.1063/1.4861169
View online: http://dx.doi.org/10.1063/1.4861169
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Realizing room-temperature self-powered ethanol sensing
of Au/ZnO nanowire arrays by coupling the piezotronics effect
of ZnO and the catalysis of noble metal
Lili Xing,
a)
Yuefeng Hu,
a)
Penglei Wang, Yayu Zhao, Yuxin Nie, Ping Deng, and Xinyu Xue
b)
College of Sciences, Northeastern University, Shenyang 110004, China
(Received 28 November 2013; accepted 17 December 2013; published online 8 January 2014)
By coupling the piezotronics effect of ZnO and the catalysis of noble metal, room-temperature self-
powered active ethanol sensing was obtained from Au/ZnO nanowire arrays. The piezoelectric output
generated by Au/ZnO nanowire arrays acts not only as power source, but also as response signal to
ethanol at room temperature. Upon exposure to 1200 ppm ethanol, the piezoelectric output of
Au/ZnO nanowire arrays decreased from 1.54 V (in air) to 0.43 V. Our research can stimulate a
research trend on the development of the next generation of room-temperature gas sensors and will
further expand the scope for self-powered nanosystems. V
C2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4861169]
Recent research on ZnO nanowires (NWs) has con-
firmed that there are a large amount of point defects (oxygen
vacancy) within them, providing n-type carriers (electrons)
for their semiconducting conductivity.
1,2
Also ZnO NWs
have been intensively investigated due to the piezoelectric
effect.
3,4
As the c-axis of ZnO NW is under externally
applied deformation, a piezoelectric field is created along the
surface. By coupling the semiconducting and piezoelectric
properties of ZnO NWs, piezotronic device, that is ZnO-
based electronic device modulated by the piezoelectric field
from the strain of ZnO, has been proposed such as ZnO
piezo-transistor.
5
Free electrons in the conduction band of
ZnO can screen the piezoelectric field (screen effect) by
screening positive piezoelectric charges at one end, while
leaving the negative ionic piezoelectric charges alone.
6
And
the intrinsic potential of the Schottky contact or p–n junction
in piezotronic devices can be influenced by the change in
free-carrier density of ZnO NWs.
7,8
In our previous report,
by coupling the gas sensing and piezotronic properties of
ZnO, a piezo-gas sensing mechanism has been demon-
strated.
9
The free-electron density at the surfaces of ZnO
NWs can be affected by oxidizing or reducing gas adsorbed
on the surface, greatly changing the screening of the piezo-
electric polarization charges at the interface by the free elec-
trons, thus affecting the piezoelectric output of ZnO. In this
paper, the piezotronic properties of ZnO and the catalysis of
noble metal are coupled, and room-temperature self-powered
ethanol sensing is realized from Au/ZnO NW arrays. Au
nanoparticles have catalytic properties and also can intro-
duce Schottky barriers into the piezo-gas sensing process.
Au/ZnO NW arrays in the self-powered active gas sensor
have two functions: one is as an energy source because the
ZnO NWs can produce piezoelectric output power; the other
is a room-temperature ethanol sensor function because the
piezoelectric output of Au/ZnO NWs is a measure of the
ethanol concentration. Our study can stimulate a research
trend on the development of the next generation of room-
temperature gas sensors and will further expand the scope
for self-powered nanosystems.
Vertically aligned Au/ZnO NW arrays were synthesized
by a two-step method. First, ZnO NW arrays grown on a Ti
foil were synthesized via a hydrothermal route. Prior to the
growth of ZnO NW arrays, a piece of Ti foil as the substrate
was cleaned with deionized water and alcohol, and dried at
60 C, as shown in Fig. 1(a). Then ZnO NW arrays were
grown on the Ti substrate, as shown in Fig. 1(b). Zinc nitrate
(0.4 g) and ammonia (2 ml) were added into 38 ml of deion-
ized water in sequence to prepare a reaction solution. After
dissolved evenly, the mixture was transferred into a 50 ml
Teflon-lined autoclave that was heated to 90 C and kept for
24 h with the pre-cleaned Ti foil immersed in the solution.
After cooling down to room temperature, the substrate
coated with ZnO NW arrays was removed from the solution,
rinsed with deionized water, and dried at 60 C. Second, the
ZnO NW arrays were uniformly coated with Au nanopar-
ticles by a wet chemical method, as shown in Fig. 1(c). ZnO
NW arrays were dipped in H
2
AuCl
6
aqueous solution
(0.5 M, 100 ll) for 30 min and then dried at 60 C. Finally,
the coated ZnO NW arrays were annealed at 400 C for 2 h.
Fig. 1(d) is a schematic diagram of the flexible self-
powered ethanol sensor. There are three parts: Au/ZnO NW
arrays, Ti foil and Al layer as electrodes, Kapton boards as
frames. Ti foil acted not only as the substrate for Au/ZnO
NW arrays, but also as a conductive electrode. A sheet of
flexible Al layer (0.05 mm thick) was positioned on the top
of the Au/ZnO NW arrays as the counter electrode. And two
terminal copper leads were glued with silver paste on the Ti
foil and Al layer for electrical measurements, respectively.
Then the finished device was fixed between two sheets of
Kapton boards firmly to maintain its stability. The device
was connected to the outside circuit that monitored the
change of piezoelectric output voltage upon exposure to
ethanol gas under externally applied deformation. A low-
noise preamplifier (Model SR560, Stanford Research
Systems) was used to measure the piezoelectric output volt-
age. Fig. 1(e) is an optical image of the flexible device,
showing that the device can be easily bent by fingers. The
a)
L. Xing and Y. Hu contributed equally to this work.
b)
E-mail: xuexinyu@mail.neu.edu.cn
0003-6951/2014/104(1)/013109/5/$30.00 V
C2014 AIP Publishing LLC104, 013109-1
APPLIED PHYSICS LETTERS 104, 013109 (2014)
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flexibility of the device is a good feature that makes it effec-
tively convert tiny mechanical energy into electricity. The
device has a length of 2.5 cm and a width of 2.5 cm.
Fig. 2(a) is a top-view scanning electron microscopy
(SEM) image of Au/ZnO NW arrays, showing that Au/ZnO
NWs are vertically aligned on the Ti substrate and the cross-
sectional shape of Au/ZnO NWs is hexagonal. A side-view
SEM image of Au/ZnO NW arrays is shown in Fig. 2(b), fur-
ther confirming that Au/ZnO NWs are uniformly and verti-
cally aligned on the Ti substrate. The length of Au/ZnO NW
arrays is about 1.5 lm, and the average diameter of the NWs
is about 200 nm. The inset is an enlarged view of a single
NW, showing that Au nanoparticles are uniformly distrib-
uted on the whole surface of ZnO NW. Fig. 2(c) is a trans-
mission electron microscopy (TEM) image of Au/ZnO NWs,
showing that ZnO NW is uniformly coated with Au nanopar-
ticles. A high-resolution transmission electron microscopy
(HRTEM) image of the tip region of Au/ZnO NWs is shown
in Fig. 2(d). The interface between Au and ZnO can be
clearly observed: the contrast difference between ZnO and
Au is apparent, and both ZnO and Au are of single-crystal
structures. The interplanar distance of 0.52 nm corresponds
FIG. 2. (a) A top-view SEM image of
the vertically aligned Au/ZnO NW
arrays. (b) A side-view SEM image of
the vertically aligned Au/ZnO NW
arrays. The inset is an enlarged view of
a single Au/ZnO NW, showing that Au
nanoparticles are uniformly loaded on
the whole surface of ZnO. (c) TEM
image of one single Au/ZnO NW. (d)
High-resolution TEM image of the tip
region of Au/ZnO NW. (e) XRD pat-
tern of Au/ZnO NW arrays. (f) EDS
spectrum of Au/ZnO NW arrays.
FIG. 1. Fabrication process of the self-
powered active gas sensor based on
Au/ZnO NW arrays. (a) A pre-cleaned
Ti foil is used as the substrate. (b)
Vertically aligned ZnO NW arrays are
grown on Ti foil. (c) The decoration of
Au nanoparticles on ZnO NW arrays.
(d) Schematic diagram showing the
structural design of the self-powered
active ethanol sensor based on Au/ZnO
NW arrays. (e) The optical image of
the flexible device showing that it can
be easily bent by fingers.
013109-2 Xing et al. Appl. Phys. Lett. 104, 013109 (2014)
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to (001) planes of ZnO, confirming the growth of ZnO NWs
is along [001] direction. The interplanar distance of 0.24 nm
in the supported nanoparticles corresponds to Au (111)
planes. It also can be seen that Au nanoparticles have diame-
ters of 5–8 nm. Fig. 2(e) is an X-ray diffraction (XRD) pat-
tern of Au/ZnO NW arrays. The peaks at 40.12, 52.93, and
70.61can be indexed to Ti (JCPDS file No. 44-1294) aris-
ing from the Ti foil; the peak at 38.22can be indexed to Au
(JCPDS file No. 04-0784) with the plane (111). All other
peaks belong to ZnO NWs (JCPDS file No. 41-1049). No
sharp peaks can be indexed to other impurities, indicating a
high crystal quality of Au/ZnO. The component of Au/ZnO
NWs is examined by using an energy dispersive X-ray spec-
trometer (EDS) attached to SEM. The EDS spectrum of
Au/ZnO NWs on Ti foil is shown in Fig. 2(f). It can be seen
that the four elements (Au, Zn, O, and Ti) exist at this region.
Similar EDS results have been obtained at 10 other different
areas, which further confirm that the Au nanoparticles are
uniformly distributed in the whole system.
Fig. 3shows the piezoelectric output response of a de-
vice in dry air and ethanol (400, 800, and 1200 ppm) at room
temperature (the exposure time is long enough for ethanol
adsorption). The compressive strain applied on the device is
0.01%. In dry air, the piezoelectric output voltage of the de-
vice is about 1.54 V (shown in Fig. 3(a)). Upon exposure to
400, 800, and 1200 ppm ethanol gas, as shown in Figs.
3(b)–3(d), the piezoelectric output voltage of the device is
about 1.50, 1.10, and 0.43 V, respectively. The piezoelectric
output generated by Au/ZnO NWs acts not only as a power
source, but also as a gas sensing signal. The piezoelectric output
of the device is dependent on the outside atmosphere. As the
concentration of ethanol increases, the piezoelectric output of
the device decreases. Similar to the traditional definition of the
sensitivity of gas sensors (S%¼jRaRgj
Ra100%, where R
a
and
R
g
represent the resistance of the sensor in dry air and in the
test gas, respectively),
10,11
the sensitivity S of the self-powered
active ethanol sensor can be simply defined as follows:
9
S%¼jVaVgj
Va
100%;(1)
where V
a
and V
g
are the piezoelectric output voltage of the
device in dry air and test gas, respectively. The sensitivity S
of Au/ZnO NW arrays against 400, 800, and 1200 ppm etha-
nol are 2.5, 28.5, and 72.1, respectively. As the ethanol con-
centration increases, the output voltage decreases and the
sensitivity increases, as shown in Fig. 3(e). The piezoelectric
output current of the device in dry air and ethanol gas under
the same conditions at room temperature is also measured.
As shown in Fig. 3(f), when the device is in dry air, the
FIG. 3. The piezoelectric output volt-
age of the device at room temperature
upon exposure to dry air (a), 400 ppm
(b), 800 ppm (c), and 1200 ppm ethanol
(d), respectively. The compressive
strain applied to the device is 0.01%.
The insets are the enlarge views of the
piezoelectric output. (e) The depend-
ence of the piezoelectric output voltage
and the sensitivity on the concentration
of ethanol gas. (f) The piezoelectric
output current of the device at room
temperature upon exposure to dry air,
400, 800, and 1200 ppm ethanol,
respectively.
013109-3 Xing et al. Appl. Phys. Lett. 104, 013109 (2014)
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output current of the device induced by the compressive
strain is 0.19 lA. In ethanol gas, the output current is
0.17 (400 ppm), 0.12 (800 ppm), and 0.07 lA
(1200 ppm), respectively. Such a room-temperature ethanol
sensing can be attributed to the coupling of the piezotronics
effect of ZnO NW arrays and the catalysis of Au nanopar-
ticles. The detailed piezo-ethanol sensing mechanism of
Au/ZnO NW arrays is discussed as follows:
Upon exposure to dry air and without any compressive
force, as shown in Fig. 4(a), Au/ZnO NW arrays are in the
natural state and no piezoelectric field is created along the
Au/ZnO NWs. The oxygen molecules in air adsorb on the
surface of ZnO NWs trapping free electrons from the con-
duction band of ZnO NWs, and ionosorbed oxygen ions
(O
2
) are formed.
12,13
By the assistance of catalytic Au
nanoparticles, oxygen molecules can be more easily
adsorbed and dispersed on the surface of Au/ZnO NWs by
spillover effect.
14
Since Au is a far better oxygen diffusion
catalyst than ZnO, the coverage of chemisorbed oxygen on
Au/ZnO NWs surface is very large. Also, adsorbed oxygen
can diffuse fast to surface defects of ZnO NWs. Thus, a large
amount of adsorbed oxygen can capture free electrons from
the conduction band to become oxygen ions, which results in
very strong electron depletion within the ZnO.
15,16
At the
same time, the Schottky contacts between the ZnO NWs and
the Au nanoparticles lead to an additional depletion layer at
the interface by electron abstraction from surface defects of
ZnO.
17
Thus, the electron density of Au/ZnO is very low in
dry air. When the device is under a compression strain (Fig.
4(b)), a piezoelectric field is created along the ZnO NWs. As
free electron density in ZnO NWs is very small in air, the
screen effect of free electrons on the piezoelectric output is
very weak.
6
Thus, the piezoelectric output is relatively high
in air. Upon exposure to ethanol, as shown in Fig. 4(c), a se-
ries of reactions between ethanol and oxygen ions can take
place with the assistance of catalytic Au nanoparticles as
follows:
18,19
CH3CH2OHðgasÞ$
Au CH3CH2OHðads:Þ;(2)
CH3CH2OHðads:Þ$
Au CH3CH2Oðads:ÞþHðads:Þ;(3)
CH3CH2Oðads:Þ!
Au CH3CHOðads:ÞþHðads:Þ;(4)
CH3CH2Oðads:Þ!
Au CH3CHOðgasÞþHðads:Þ;(5)
4H ads:
ðÞ
þO
2ads:
ðÞ
$
Au 2H2O gas
ðÞ
þe:(6)
The reaction rate is greatly accelerated due to the catalytic
activity of Au, and the work temperature can be lowered
down to room temperature. As expressed by Eq. (6), elec-
trons are released back to the conduction band of ZnO,
which can increase the electron density. Under the compres-
sion strain (Fig. 4(d)), the high electron density results in a
strong screening effect of free electrons on the piezoelectric
output, thus the piezoelectric output is lower than that in air.
In summary, by coupling the piezotronics effect of ZnO
and the catalysis of Au, room-temperature self-powered active
ethanol sensor was obtained from Au/ZnO NW arrays. The
piezoelectric signal generated by Au/ZnO NW arrays acted
not only as a power source, but also as a response signal to
ethanol at room temperature. The catalysis of Au nanopar-
ticles improves the self-powered active gas sensing properties
of ZnO NWs. Such a development of Au/ZnO self-powered
active ethanol sensor is an important step for the practical
applications in actively detecting gases at room temperature.
This work was supported by the National Natural
Science Foundation of China (51102041 and 11104025), the
Fundamental Research Funds for the Central Universities
(N120205001 and N120405010), and Program for New
Century Excellent Talents in University (NCET-13-0112).
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FIG. 4. The working mechanism of Au/ZnO self-power active ethanol sen-
sor driven by compressive strain. (a) Schematic illustration showing the
electron density and depletion layer in Au/ZnO NWs in air without applied
force. (b) Schematic illustration showing the piezoelectric output of Au/ZnO
NWs in air under mechanical deformation. (c) Schematic illustration show-
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out compression. (d) Schematic illustration showing the piezoelectric output
of Au/ZnO NWs in ethanol under mechanical deformation.
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