Gas Sensing Properties at Room
Temperature of A Quartz Crystal Microbalance Coated with ZnO Nanorods
Tính nhạy khí ở
nhiệt độ phòng của cảm biến vi cân tinh thể thạch anh được phủ thanh nano ZnO (For international
authors, please make Vietnamese title “empty lines”. The secretary board may
help you)
Le Thanh A1,2, Nguyen Van B1,*
1 Hanoi University
of Science and Technology, No. 1, Dai Co Viet, Hai Ba Trung, Hanoi, Viet Nam
2 Viet Nam Atomic Energy Institute, No. 59, Ly Thuong
Kiet, Hoan Kiem, Hanoi, Viet Nam
Abstract
Gas sensors based on a quartz crystal microbalance (QCM)
coated with ZnO nanorods were developed for detection of NH3 at room
temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel
wet chemical route at a low temperature of 90 ºC, which was used to grow the
ZnO nanorods directly on the QCM for the gas sensor application. The morphology
of the ZnO nanorods was examined by field-emission scanning electron microscopy
(FE-SEM). The diameter and length of the nanorods were 100 nm and 3 µm,
respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent
performance to NH3 gas. The frequency shift (Df) to 50 ppm NH3 at room temperature was about
9.1 Hz. It was found that the response and recovery times were varied with the
ammonia concentration. The fabricated gas sensors showed good reproducibility
and high stability. (from 100 to 150 words)
Keywords: QCM, ZnO nanorods, Gas sensing (from 3 to 5 keywords)
Tóm tắt (Abtract in
Vietnamese)
Từ khóa (Keywords in Vietnamese):
(For international
authors, please make Vietnamese abstract and key words “empty lines”. The
secretary board may help you to translate it. Make sure that after adding Vietnamese
title, abstract, and keywords, total page number do not exceed 6)
1. Introduction[*]
|
In recent years, many semiconductor metal-oxide
materials, such as SnO2, TiO2, CuO, and In2O3,
have been used for gas sensors [1-5]. In these, the ZnO nanomaterial possesses
certain unique properties, such as a direct band gap (3.37 eV), large exciton
binding energy (60 meV), high thermal and chemical stability, transparence,
biocompatibility, and wide electrical conductivity range [6-8]. Moreover,
one-dimensional (1D) ZnO nanostructures have attracted much attention due to
their large aspect ratio, which makes them a good candidate for gas sensing
applications [9, 10]. Most gas sensors using semiconductor metal-oxide
materials are based on the change in electrical conductivity with the
composition of the surrounding gas atmosphere. Major challenges in
conductivity-based gas sensors are the high operating temperature, poor gas
selectivity, and unstableness. These sensors are based on the changes in
electrical resistance of the materials upon gas adsorption. Thus, a high
temperature is required for charge carriers of the semiconductor materials to
overcome the activation energy barrier. Therefore, almost all
conductivity-based gas sensors operate at high temperatures [2-4, 9,10].
2. Experimental
For fabrication of
the QCM device, both-side polished AT-cut quartz crystal plates with dimensions
of 25 × 20 mm2 and thickness of 300 μm were used. Two circular
electrodes with diameters of 12 and 6 mm were deposited on both sides of the
quartz plate by sputtering method and were patterned by the lithography
technique. The circular electrodes were composed of a 40 nm Cr under-layer
surface and a top 100 nm Au layer.
Vertically aligned
ZnO nanorods were directly grown on the Au electrode of the QCM device by a wet
chemical route. ZnO nanorods form by the hydrolysis of zinc nitrate in water in
the presence of HMTA. The chemical reactions for the formation of the ZnO
nanorods on ZnO-coated substrates are [26].
3. Results and
disscution
Fig. 2 shows a
photograph and the resonant characteristics of the as-fabricated QCM device
using AT-cut quartz crystal plate as a precursor substrate. The AT-cut quartz
crystal is well known as a piezoelectric material suitable for the QCM due to
its high sensitivity to mass change on the surface. The resonant frequency (fo) in this work was
evaluated from the conductance peak. It has been observed that the conductance
versus frequency curve shows a fundamental resonance peak at 5.48 MHz (Fig.
2b). The ZnO nanorods were then grown on one side of the Au electrode-coated
QCM by the wet chemical route. Fig. 3 shows SEM images of the top-view (3a) and
side-view (3b) of as-grown ZnO nanorods on the Au electrode of the QCM. The
morphology of the ZnO nanorods with a hexagonal structure was vertically
well-aligned and uniformly distributed on the Au electrode of the QCM. This
shows that the exposed area of the sensing layer was remarkably enhanced
compared with the sensing layer of the ZnO nanowires [1]. The average diameter
and length of ZnO nanorods were around 100 nm and 3 mm, respectively. In comparison with the ZnO nanowires that were
first synthesized by evaporating high purity zinc pellets at 900 ºC and were
then distributed on the QCM [2], the wet chemical route has many advantages
such as low cost, low temperature operation, high preferred orientation, and
environmental friendliness. This method can also directly grow ZnO nanorods
with high uniform distribution on a large area.
Table 1. The main components of fresh cassava
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Main components
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Composition base on dry weight (%, w/w)
|
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Water concentration
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58.6 - 59.9
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Starch
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28 - 31
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Fig. 5a shows the
response transients of the ZnO nanorod-coated QCM sensor to switching-on and
off of the NH3 gas-flow with different concentrations (50, 100, and
200 ppm) at room temperature (25 ºC). In the first stage, the sensor flushed a
reference air gas flow of 15 sccm to obtain a baseline. The sensor was then
exposed to a NH3 gas flow of 15 sccm with a certain concentration,
which leads to frequency response until a steady stage was reached, indicating
maximum adsorption of NH3 gas onto the QCM sensor. The NH3
gas flow was finally replaced by the air gas flow and the sensor returned back
to its baseline. In this experiment, the flow rate of the diluted ammonia gas
and dry air was fixed at 15 sccm. Hence, in the gas sensing chamber, the flow
and pressure were ensured to be constant. The change in resonant frequency of a
QCM (∆f) can be related to the change
in mass (∆m) due to the adsorption of
NH3 gas.
Fig. 1. SEM images of ZnO nanorods grown by wet chemical bath
deposition: (a) top-view and (b) side-view. (Figure’s
colors are black and white only)
4. Conclusion
Vertically
well-aligned ZnO nanorods were successfully grown on the Au electrode of QCM by
the wet chemical bath deposition method. The ZnO nanorods were uniformly
distributed on the substrate with a diameter and height of 100 nm and 3 mm, respectively. The sensor based on QCM coated with ZnO nanorods
showed good interaction with ammonia and could detect low concentration of
several tens of parts per million. The magnitude of the response of the
fabricated sensor was directly proportional to the concentration of the
ammonia. The result indicated the reproducible and reversible performance of
the sensor.
Acknowledgments
This work was supported by
the application-oriented basic research program.
References
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Thickness dependence of sensor response for CO gas sensing by tin oxide films
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Chu, Structural and optical properties of ZnO thin films on (111) CaF2
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P.A. Lieberzeit, A.
Rehman, B. Najafi, F.L. Dickert, Real-life application of a QCM-based e-nose:
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X. Du, S.M. George,
Thickness dependence of sensor response for CO gas sensing by tin oxide films
grown using atomic layer deposition, Sens. Actuators B 135 (2008) 152-160.
[5]
Y.Z. Wang, B.L.
Chu, Structural and optical properties of ZnO thin films on (111) CaF2
substrates grown by magnetron sputtering, Superlattices Microstruct. 44 (2008)
54–61.
[6]
P.A. Lieberzeit, A.
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Bioanal. Chem. 391 (2008) 2897–2903.
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