Enhanced NH3 gas sensing with longer ZnO nanorods

Please cite this article in press as: V.A. Minh, et al., Enhanced NH3 gas sensing properties of a QCM sensor by increasing the length of vertically
orientated ZnO nanorods, Appl. Surf. Sci. (2012), http://dx.doi.org/10.1016/j.apsusc.2012.11.028
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APSUSC-24611; No.of Pages7
Applied Surface Science xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Enhanced NH3 gas sensing properties of a QCM sensor by increasing the length of
vertically orientated ZnO nanorods
Vu Anh Minha
, Le Anh Tuanb
, Tran Quang Huyc
, Vu Ngoc Hunga
, Nguyen Van Quya,∗
a
International Training Institute for Materials Science, Hanoi University of Technology, No. 1, Dai Co Viet Road, Hai Ba Trung, Hanoi, Viet Nam
b
Advanced Institute of Science and Technology, Hanoi University of Technology, No. 1, Dai Co Viet Road, Hai Ba Trung, Hanoi, Viet Nam
c
Laboratory for Ultrastructure and Bionanotechnology, National Institute of Hygiene and Epidemiology, No. 1, Yersin Street, Hai Ba Trung, Hanoi, Viet Nam
a r t i c l e i n f o
Article history:
Received 1 August 2012
Received in revised form 6 November 2012
Accepted 7 November 2012
Available online xxx
Keywords:
QCM
Gas sensors
ZnO nanorods
Hydrothermal method
a b s t r a c t
Vertically aligned ZnO nanorods were directly synthesised on a gold electrode of quartz crystal microbal-
ance (QCM) by a simple low-temperature hydrothermal method for a NH3 gas sensing application. The
length of vertically aligned ZnO nanorods was increased to purpose enhancement in the gas sensing
response of the sensor. The length of ZnO nanorods increased with an increase in growth time. The
growth time of ZnO nanorods was systematically varied in the range of 1–4 h to examine the effect of
the length of the ZnO nanorods on the gas sensing properties of the fabricated sensors. The gas sensing
properties of sensors with different ZnO nanorods lengths was examined at room temperature for various
concentrations of NH3 (50–800 ppm) in synthetic air. Enhancement in gas sensing response by increasing
the length of ZnO nanorods was observed.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Zinc oxide (ZnO) is an important II–IV group oxide semicon-
ductor with many excellent properties, such as a direct band gap
of 3.37 eV, a large exciton binding energy of 60 meV at ambient
temperature, high thermal and chemical stability, biocompatibil-
ity and a wide electrical conductivity range. It is a key functional
material for a wide range of technological applications in piezo-
electric devices, optoelectronics, sensors, field emission displays,
transducers and biomedical fields [1–5]. ZnO can exist in many dif-
ferent configurations of nanostructures that can be formed from
one material, and it can be grown on any substrate at relatively
low temperatures. Previously, one-dimensional (1D) ZnO has been
prepared by a variety of techniques, including chemical–vapour
deposition (CVD), thermal evaporation, molecular-beam epitaxy
(MBE), pulsed-laser deposition (PLD), electrochemical deposi-
tion and spray pyrolysis [6–11]. However, these techniques still
comprise many challenges, such as high operating temperature,
expensive equipment, complex processes and difficulty in control-
ling the configurations of the ZnO nanostructures. In recent years,
a simple low-temperature hydrothermal method has been devel-
oped to synthesise ZnO nanorod/nanowires [12]. It was clearly seen
that the hydrothermal method exposed many advantages such as
low-cost, high yield, low-operating temperature and easy control
∗ Corresponding author. Tel.: +84 9 2661 4028.
E-mail address: quy@itims.edu.vn (N.V. Quy).
of the configurations of ZnO nanorods/wires. An important advan-
tage of the method was efficiently used to synthesise both vertically
and horizontally well-aligned ZnO nanorods of high quality, and
uniform length and distribution [13,14].
Up to now, ZnO nanostructures have been widely used for gas
sensing applications. However, in the sensing characters of a sen-
sor, sensitivity and efficiency directly depend on the specific surface
area of the material. Thus, 1D ZnO nanostructures are good can-
didates for gas sensing applications due to having a high specific
surface area. Most gas sensors using semiconductor metal–oxide
materials are based on the change of electrical conductivity in the
surrounding atmosphere, and usually use a comb-like electrode
structure. Therefore the sensing layer is required to be contin-
uous to ensure that conductivity exists in two electrodes of the
sensor. Moreover, the sensing layer consists of 1D nanostructures
orientated parallel to the substrate. Hence, the exposed area of
the sensing layer is not expanded when the length of vertically
aligned 1D nanostructures is increased. On the other hand, major
challenges in conductivity-based gas sensors are a high operating
temperature, poor gas selectivity and instability [15–17].
In previous work, we have developed a room temperature NH3
gas sensor based on a quartz crystal microbalance (QCM) coated
with ZnO nanorods [18]. The sensor indicated high sensitivity and
selectivity with NH3. However, the effects of the length of vertically
aligned ZnO nanorods on the gas sensing properties of the sensor,
as well as the improvement in sensitivity, were not examined in
detail. In this study, we enhanced the response of a NH3 gas sen-
sor based on the QCM coated with vertically aligned ZnO nanorods
0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apsusc.2012.11.028

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2 V.A. Minh et al. / Applied Surface Science xxx (2012) xxx–xxx
Fig. 1. Optical image of as-fabricated QCM.
at room temperature by increasing the length of ZnO nanorods.
The QCM technique has been developed as a sensitive tool that
utilises the piezoelectric properties of quartz crystals to measure
the attached mass on an electrode surface. A change in the res-
onant frequency can be related to change in the mass according
to the Sauerbrey equation [19]. As such, the QCM technique has
been applied in biochemistry, analytical science and other fields
[20–22]. The QCM sensor array, which has different resonance
frequencies, can be applied to detect volatile sulfur compounds
(VSCs), the halitosis-causing substances [23]. On the other hand,
QCM coated with a sensing layer was also applied to the sensor
due to its high reproducibility and stability [18,24,25]. As the QCM
is an extremely sensitive mass device that can detect the change
in mass of a molecule, sensors based on the QCM have high sensi-
tivity and accuracy. The operating principle of a QCM coated with
ZnO nanorods is based on the change in mass of the gas adsorbed
on the surface of the ZnO nanorods. Thus, an increase in the length
of ZnO nanorods, which increases the surface area of the sensing
layer, results in enhancing the gas sensing properties of the QCM
sensor.
In the present paper, the fabrication processes of a mass-based
gas sensor using QCM coated with a sensing layer of vertically well-
aligned ZnO nanorods were presented. The enhancement of NH3
gas sensing properties of as-fabricated sensors by increasing the
length of ZnO nanorods was also reported.
2. Experimental
The experimental condition and process are similar to the pre-
vious report [18]. In brief for fabrication of the QCM device, two
circular electrodes with diameters of 12 and 6 mm were deposited
on both sides polished AT-cut quartz plate with dimensions of
25 mm × 20 mm and thickness of 300 ␮m. Vertically aligned ZnO
nanorods were then directly grown on the larger electrode of the
QCM device by a wet chemical route. In this paper, we focused
on the investigation of the effect of ZnO nanorod length on NH3
gas sensing properties. In order to control ZnO nanorod length,
the growth time was varied in the range of 1–4 h. The chemical
reactions for the formation of the ZnO nanorods on ZnO-coated
substrates can be found elsewhere [26]. X-ray diffraction (XRD)
and field emission scanning electron microscopy (FE-SEM, S4800-
Hitachi) were used to analyse the crystal structure and morphology
of the ZnO nanorods.
Gas sensing measurement of the QCM coated with ZnO nanorods
was conducted using a gas sensing system, as shown in Fig. 1. A
flow-through technique with a constant flow rate of 15 sccm was
employed for the gas sensing test via valve number 3. Valve number
3 is a 4-way directional control valve. Valve number 3 was designed
to ensure that only gas for the sensing test or dried air can be flow,
and that there is no agglomeration of gas in the pipe that may form
a high pressure. It means that the MFC3 and MFC4 always flow at
15 sccm and only one of them is injected into a gas sensing chamber,
Fig. 2. Schematic diagram of the gas sensing measurement system.
namely if testing gas is injected into the gas sensing chamber, dried
air is directed out of the exhaust and vice versa.
The gas concentration was controlled by changing the mixing
ratio of the parent gases in MFC1 and synthetic air in MFC2 in a
mixing chamber. The testing process was conducted in two steps
as follows. At the beginning, air flow of 15 sccm (MFC4) was flowed
through the chamber to obtain the baseline of the frequency. A bal-
anced NH3 gas flow of 15 sccm (MFC3) was then flowed through the
chamber to replace the air. The frequency shifts of the sensors were
monitored by a frequency counter, QCM200, which was connected
to a computer system via the SRSQCM200 software program and
stored in a PC. The resonant characteristics of the fabricated QCM
device were examined using an R37CG Network Analyzer.
3. Results and discussion
Fig. 2 shows an optical image of the as-fabricated QCM devices
using an AT-cut quartz crystal plate as a precursor substrate. The
standard AT-cut crystal is obtained by cutting quartz crystal wafers
from a bar at an angle in the vicinity of 35◦15 from the Z axis,
and the mode of vibration is thickness-shear. The AT-cut quartz
crystal has very little “frequency-temperature dependence” at or
near room temperature, which causes minimum changes in fre-
quency due to variations in temperature. A fundamental resonant
frequency (f0) of the fabricated sensor was at 5.48 MHz, the detail of
which was presented in previous work [18]. The ZnO nanorods were
then grown on a larger Au electrode of QCM by the hydrothermal
method. The length of the nanorods could be varied by changing
the growth time. Fig. 3 shows FE-SEM images of the top-view and
side-view of the as-grown ZnO nanorods on the Au electrode of the
QCM as a function of growth time from 1 h to 4 h. The morphology
of the ZnO nanorods with a hexagonal structure was vertically well-
aligned and uniformly distributed on the Au electrode of the QCM.
All top-view and side-view FE-SEM images show that the hexago-
nal prism shape of the ZnO nanorods is independent of growth time.
However, the length of the ZnO nanorods greatly increased and the
diameter expanded slightly when the growth time was increased.
Namely, the length and diameter of ZnO nanorods increased from
∼1200 nm to ∼3000 nm and from ∼80 nm to ∼150 nm, respectively,
when the growth time was increased from 1 h to 4 h. As the increase
in diameter of the ZnO nanorods by growth time was too small in
comparison with the increase in their length, the effect on the gas
sensing response was only considered for the increase in the length
of the ZnO nanorods.
Fig. 4 shows the sensitivity and response time of the QCM
sensors coated with various lengths of ZnO nanorods towards

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V.A. Minh et al. / Applied Surface Science xxx (2012) xxx–xxx 3
Fig. 3. Top-view and side-view FE-SEM images of ZnO nanorods synthesised for (a–b) 1 h, (c–d) 2 h, (d–e) 3 h and (f–g) 4 h.

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Fig. 4. Comparison of the frequency shift of QCM sensors based on ZnO nanorods with different lengths when exposed to various concentrations of NH3 gas: (a) 50 ppm, (b)
100 ppm, (c) 200 ppm and (d) 400 ppm.
different concentrations of ammonia gas at room temperature.
Fig. 4(a) shows plots of frequency shift with 50 ppm (parts per
million) concentrations of ammonia for the different lengths
of ZnO nanorods, formed by various growth times, coated on
the QCM electrode. It is obvious that as the length of ZnO
nanorods increases, the magnitude of the frequency shift increases.
Namely, at a NH3 gas concentration of 50 ppm the frequency shift
of fabricated sensors was 5.8, 8.7, 15.3 and 20.2 Hz when the
length of the ZnO nanorods was 1200, 2000, 2500 and 3000 nm,
respectively. When the fabricated sensors were exposed to other
NH3 gas concentrations (from 100 ppm to 400 ppm), the fre-
quency shift of the QCM sensors based on the longer length of
ZnO nanorods still indicated higher resonance, as clearly shown
in Fig. 4(b)–(d).
Fig. 5 shows the comparison in frequency shift versus NH3
gas concentration of the QCM sensors based on ZnO nanorods of
different lengths. It is obvious that the frequency shift increases
with an increase in length of the ZnO nanorod in the range
of 1200–3000 nm. At higher NH3 gas concentrations, it is also
observed that the frequency shift increases linearly with an increas-
ing length of ZnO nanorod. This indicated that more NH3 molecules
are adsorbed on the ZnO nanorods with the increase in the NH3 gas
concentration. On the other hand, as the length of the ZnO nanorods
increases the surface area of the ZnO nanorods also expands. This
means the surface area of a sensitive material layer is expanding.
Thus more NH3 molecules are adsorbed on the ZnO nanorods with
the increase in surface area. Therefore, it is clearly seen that the
frequency shift of the sensors increases with an increase in the
length of the ZnO nanorods.
The relation between the resonant frequency of a QCM ( f)
and the mass of NH3 gas adsorbed on the ZnO nanorods can be
calculated according to the Sauerbrey equation [19].
f =
−2f 2
0
A q q
m (1)
Fig. 5. Proportion test for the frequency shift of the QCM sensors based on ZnO
nanorods of various lengths at different NH3 gas concentrations.

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Fig. 6. Response–recovery curves of QCM sensors based on ZnO nanorods with lengths of (a) 1200 nm, (b) 2000 nm, (c) 2500 nm and (d) 3000 nm to different NH3 gas
concentrations at room temperature.
where A is the active area of the QCM electrode in cm2, f0 is the
resonant frequency of the QCM in hertz (Hz), m is the change in
the oscillating mass in grams (g), q is the density of quartz, and q
is the shear wave velocity in the quartz.
The frequency shift of the QCM sensors coated with ZnO
nanorods depends on the number of NH3 gas molecules adsorbed
on the surface of the ZnO nanorods. All fabricated sensors were
the same with respect to the resonant frequency, active area and
quality of quartz plate, thus the change in mass ( m) due to the
adsorption of NH3 gas molecules on ZnO nanorods only depends
on the length of the ZnO nanorods. In order to study the effect of
the length of ZnO nanorods on the sensitivity of the sensors, the
gas sensing properties of the QCM sensors based on ZnO nanorods
with different lengths were investigated in detail. From Eq. (1), the
frequency shift of the QCM sensors based on a length of 1200 nm
of ZnO nanorods, which are grown for 1 h, can be expressed as
f1 =
−2f 2
0
A q q
m1 (2)
It is a similar calculation; the QCM sensors coated with 2000 nm
in length of ZnO nanorods, which were grown for 2 h, can be
expressed as
f2 =
−2f 2
0
A q q
m2 (3)
We have compared the frequency shift and the change in mass
of the sensors based on lengths of 1200 and 2000 nm of the ZnO
nanorods. The relation between the sensors can be expressed as
f2
f1
=
m2
m1
(4)
From Fig. 5, the ratio of the frequency shift between the QCM
sensors based on ZnO nanorods of 1200 and 2000 nm in length at a
NH3 concentration of 50 ppm can be calculated as follows
f2
f1
=
8.8
5.5
= 1.6 (5)
In a similar calculation, the ratio of the frequency shift of the
other sensors, based on 2500 and 3000 nm lengths of ZnO nanorods,
and the sensor based on a 1200 nm length of ZnO nanorods, were
determined to be f3/ f1 = 2.78, and f4/ f1 = 3.67. It was
found that the frequency shift of the sensor is directly proportional
to the length of the ZnO nanorods.
NH3 gas molecule adsorption on the surface area of the ZnO
nanorods resulted in an increase in mass. The change in mass ( m)
depends on the density of the NH3 gas molecules absorbed on
the surface of the ZnO nanorods and the morphology of the ZnO
nanorods. NH3 molecular density adsorbed on the surfaces of the
ZnO nanorods of the different sensors is the same. As the ZnO
nanorods grown on QCM sensors were cultivated in the same con-
ditions except for growth time, their quality and density can be
consider the same. Otherwise, although the diameter of the ZnO
nanorods slightly expanded during the growth time in the range of
1–4 h, the change in mass only depended on the length of the ZnO

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Fig. 7. Proportion test for the frequency shift of the QCM sensors based on ZnO
nanorods of various lengths to different NH3 concentrations.
nanorods (L). Hence, the relation of the change in mass between
sensors coated with 1200 and 2000 nm lengths of ZnO nanorods
can be expressed as
m2
m1
=
L2
L1
(6)
A calculation of the frequency shift and change in mass of the
sensors as a function of the ZnO nanorod length using Eqs. (4) and
(6) finds that the sensitivity of QCM coated with ZnO nanorods
increases linearly with the length of the ZnO nanorods. That means
that if the density and diameter of the ZnO nanorods were kept
constant, an increase in length of ZnO nanorods would enhance
the NH3 gas sensing properties of the sensors.
In gas sensing application, sensitivity and efficiency directly
depend on the ratio of exposed surface area to volume. The ratio
of exposed surface area to volume can be increased by increas-
ing the length or reducing the diameter of 1D nanostructures. In
conductivity-based sensor, the sensing layers usually consist of the
1D nanostructures in parallel orientation with the substrate [27].
Moreover, Kim et al. investigated a series of open space nanowire
structures of WO3 with diameters varying from 35 to 82 nm, and
the sensor response was maximum with nanowires of ∼40 nm [27].
This phenomenal was also found in a single SnO2 nanowire based
sensor [28]. It is clearly proven that the gas sensing properties of the
conductivity-based sensor are not completely affected by reducing
diameter or increasing length of the 1D nanostructures. However,
gas sensing properties of mass-typed sensor exposed much depen-
dence on the length of ZnO nanorods.
Fig. 6 shows the response–recovery curves of the QCM sensors
based on ZnO nanorods of various lengths to the NH3 gas-flow with
different concentrations (50, 100, 200, 400 and 800 ppm) at room
temperature. In the first stage, the sensors were flushed with a ref-
erence air gas flow of 15 sccm to obtain a baseline. The sensors were
then exposed to a NH3 gas flow of 15 sccm of a certain concentra-
tion, which lead to a frequency response until a steady stage was
reached, indicating maximum adsorption of NH3 gas onto the QCM
sensors. The NH3 gas flow was finally replaced by an air gas flow and
the resonant frequency of the sensors returned to its baseline. The
sensors show a good response to NH3 gas. The response increased
as the concentration of NH3 gas was increased from 50 to 800 ppm.
Fig. 6 also shows a good recovery of the sensors. The frequency of
the sensors completely recovered the initial value when NH3 gas
was replaced by the air. The recovery time of each sensor increased
with an increase in the NH3 gas concentration. The recovery time
of the sensor based on a 1200 nm in length of ZnO nanorods to 50,
100, 200, 400, and 800 ppm NH3 was about 300, 650, 770, 1400,
and 1800 s, respectively. The effect of varying length of ZnO on the
recovery time of the sensor was also exposed in Fig. 6. At 50 ppm
NH3 the recovery time of the sensors based on 1200, 2000, 2500,
and 3000 nm in length of ZnO nanorods was 330, 460, 890, and
950 s, respectively. At other NH3 concentrations the recovery time
of the sensor also increased slightly with increasing the length of
ZnO nanorods.
Fig. 7 shows the frequency shift of the QCM sensors coated with
various lengths of ZnO nanorods as a function of NH3 gas concen-
tration. Fig. 7 clearly indicates that when the sensors have a longer
length of ZnO nanorod they also have a greater frequency shift. The
frequency shift of each sensor linearly increases with an increase
in NH3 gas concentration in the range of 50–800 ppm.
4. Conclusion
Vertically well-aligned ZnO nanorods were uniformly synthe-
sised on a substrate using the hydrothermal method. The results
indicated that it is possible to adjust the average diameter and
length of ZnO nanorods by changing the growth time. The ZnO
nanorods grown on an Au electrode of QCM were used to fabri-
cate a gas sensor to detect NH3 gas at room temperature. The effect
of the length of ZnO nanorods on gas sensing properties was sys-
tematically investigated. Enhancement in the gas sensing response
by increasing the length of the ZnO nanorods was clearly observed.
The increase in the length of ZnO nanorods increases the sensitive
surface area of ZnO and improves the sensing characteristics.
Acknowledgment
This work was supported by the National Foundation for Science
and Technology (NAFOSTED) research program (Code: 103.02-
2010.37).
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