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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
journa l homepage: www.e lsev ier .com/ locate /snb
Gas sensing properties at room temperature of a quartz crystal microbalancecoated with ZnO nanorods
Nguyen Van Quy, Vu Anh Minh, Nguyen Van Luan, Vu Ngoc Hung ∗, Nguyen Van Hieu ∗
International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No. 1 Dai Co Viet Road, Hanoi, Viet Nam
a r t i c l e i n f o
Article history:Received 19 March 2010Received in revised form 18 October 2010Accepted 21 October 2010Available online 29 October 2010
Keywords:QCMZnO nanorodsGas sensors
a b s t r a c t
Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed fordetection of NH3 at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novelwet chemical route at a low temperature of 90 ◦C, which was used to grow the ZnO nanorods directlyon 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 nmand 3 �m, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performanceto NH3 gas. The frequency shift (�f) to 50 ppm NH3 at room temperature was about 9.1 Hz. It was foundthat the response and recovery times were varied with the ammonia concentration. The fabricated gassensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity toammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N2O), carbon monoxide (CO), nitrogendioxide (NO2), and carbon dioxide (CO2).
In recent years, many semiconductor metal-oxide materials,such as SnO2, TiO2, CuO, and In2O3, have been used for gassensors [1–5]. In these, the ZnO nanomaterial possesses certainunique properties, such as a direct band gap (3.37 eV), large exci-ton binding energy (60 meV), high thermal and chemical stability,transparence, biocompatibility, and wide electrical conductivityrange [6–8]. Moreover, one-dimensional (1D) ZnO nanostructureshave attracted much attention due to their large aspect ratio, whichmakes them a good candidate for gas sensing applications [9,10].Most gas sensors using semiconductor metal-oxide materials arebased on the change in electrical conductivity with the compo-sition of the surrounding gas atmosphere. Major challenges inconductivity-based gas sensors are the high operating tempera-ture, poor gas selectivity, and unstableness. These sensors are basedon the changes in electrical resistance of the materials upon gasadsorption. Thus, a high temperature is required for charge car-riers of the semiconductor materials to overcome the activationenergy barrier. Therefore, almost all conductivity-based gas sensorsoperate at high temperatures [2–4,9,10].
In gas sensor structures, the principle naturally requires a largersurface-area-to-volume ratio for high sensitivity; for this matter,
1D nanostructures have been extensively studied. However, in theconductivity-based sensors, the sensing layer is required to be con-tinuous to ensure that conductivity exists in two electrodes ofthe sensor. Therefore, the sensing layers usually consist of the 1Dnanostructures in parallel orientation with the substrate. Hence,the exposing area of the sensing layer is reduced significantly. Inparticular, many sensors based on semiconductor metal oxide haveextensively been used for detecting toxic gases such as COx, NOx,CH4, and NH3 [4,11–13]. Among these, NH3 gas presents much haz-ard to both humans and the environment. Due to its highly toxiccharacteristics, even low level concentrations (ppm) pose a seriousthreat. Nowadays, there are many fields of technological impor-tance that need to detect NH3 gas in low concentrations, such asfood technology, chemical engineering, medical diagnosis, environ-mental protection, and industrial processes. Thus, the requirementof using ammonia gas sensors is becoming greater. In previousworks, we have developed a room temperature NH3 gas sensorbased on the composites of SnO2/carbon nanotube and polypyr-role/carbon nanotubes [14,15]. The limitations of these sensors aretheir poor selectivity and non-detect of NH3 gas at much lowerlevels (ppb).
In this study, we developed a highly sensitive and reproducibleammonia gas sensor with a combination of ZnO nanorods andquartz crystal microbalance (QCM). In the past decades, the QCMtechnique has been developed as a sensitive tool that utilizes thepiezoelectric properties of quartz crystals to measure the attachedmass on an electrode surface. Change in the resonant frequencycan be related to change in the mass according to the Sauerbrey
N. Van Quy et al. / Sensors and Actuators B 153 (2011) 188–193 189
equation [16]. As such, QCM technique has been applied in bio-chemistry, analytical science, and other fields [17–20]. Abe andcolleagues fabricated a QCM sensor array on a single crystal of aquartz plate (one-chip multichannel QCM). The QCM sensor array,which has different resonance frequencies, can be applied to detectvarious kinds of gases [18]. On the other hand, QCM coated with asensing layer was also applied to the sensor due to its high repro-ducibility and stability [21,22]. As the QCM is an extremely sensitivemass device that can detect the change in mass of a molecule, sen-sors based on the QCM have high sensitivity and accuracy. Theoperating principle of the QCM coated with ZnO nanorods sensor isbased on the change in mass of the adsorbed gas in the sensing layer.Thus, mass-based gas sensors measure directly the adsorptionprocess and do not require the charge carriers of the semiconduc-tor materials to be the sensing layer to overcome the activationenergy barrier. The gas sensor based on the QCM coated withZnO can usually be operated at room temperature. Various meth-ods have been reported for the synthesis of ZnO nanorods, suchas reactive magnetron sputtering, pulsed laser ablation, thermalevaporation, vapor phase transport, and chemical vapor deposi-tion [2,23–25]. However, these methods usually require expensiveequipments and high synthesizing temperatures, which are notcompatible with the QCM substrate in the operating frequency ofthe quartz. The wet chemical route was chosen due to its obviousadvantages of low synthesizing temperature, simple equipment,and easy operation. Moreover, this technique can synthesize onany substrate with a large-scale area, precise position control, andeasy control of synthesis conditions. An important advantage ofthe method is its ability to synthesize the vertically well-alignedZnO nanorods with high quality and uniform length and distribu-tion. In the present paper, the fabrication processes of mass-basedgas sensor using QCM coated with a sensing layer of verticallywell-aligned ZnO nanorods were reported. The characteristics,including sensitivity, reproducibility, response, and recovery timesof as-fabricated sensors, were investigated with different ammoniaconcentrations.
2. Experiment
For fabrication of the QCM device, both-side polished AT-cutquartz crystal plates with dimensions of 25 mm × 20 mm and thick-ness of 300 �m were used. Two circular electrodes with diametersof 12 and 6 mm were deposited on both sides of the quartz plateby sputtering method and were patterned by the lithographytechnique. The circular electrodes were composed of a 40 nm Crunder-layer surface and a top 100 nm Au layer.
Vertically aligned ZnO nanorods were directly grown on the Auelectrode of the QCM device by a wet chemical route. First, zincacetate [Zn(COOCH3)2·2H2O] diluted in butanol was coated on theAu electrode by the drop-coating technique and was followed byheat-treatment at 300 ◦C in air for 30 min to form a seed layer ofZnO nanocrystals. Subsequently, the QCM coated with the seedlayer was vertically floated upside down on the aqueous solutionsurface of equal molar zinc nitrate [Zn(NO3)2·6H2O] and hexam-ethylenetetramine (HMTA) (C6H12N4). The hydrothermal processwas conducted at 90 ◦C for 2 h. After reactions, the substrates wereremoved from the solution, rinsed with de-ionized water, and driedwith N2 blow. ZnO nanorods form by the hydrolysis of zinc nitratein water in the presence of HMTA. The chemical reactions for theformation of the ZnO nanorods on ZnO-coated substrates are [26]:
C6H12N4 + 6H2O → 6CH2O + 4NH3 (1)
NH3 + H2O ↔ NH4+ + OH− (2)
Zn2+ + 2OH− → ZnO + H2O (3)
Fig. 1. Diagram of the testing gas system.
X-ray diffraction (XRD) and field emission scanning electronmicroscopy (FE-SEM) were used to analyze the crystal structureand morphology of the as-grown ZnO nanorods.
Gas sensing measurement of the QCM coated with ZnO nanorodswas conducted using a gas sensing system, as shown in Fig. 1. Aflow-through technique with a constant flow rate (15 sccm) wasemployed for the gas sensing test. The gas concentration was con-trolled by changing the mixing ratio of the parent gases on MFC1and synthetic air on MFC2 in a mixing chamber. The testing processwas conducted by two steps as follows. At the beginning, air flowof 15 sccm (MFC4) was flowed through the chamber to obtain thebaseline of the frequency. Balance NH3 gas flow of 15 sccm (MFC3)was then flowed through the chamber to replace the air flow. Thefrequency shifts of the sensors were monitored by a frequencycounter, QCM200, which was connected to a computer system viathe SRSQCM200 software program and stored in a PC. The resonantcharacteristics of the fabricated QCM device were examined usingan R37CG Network Analyzer (30 kHz–3.8 GHz).
3. Results and discussion
Fig. 2 shows a photograph and the resonant characteristics ofthe as-fabricated QCM device using AT-cut quartz crystal plate asa precursor substrate. The AT-cut quartz crystal is well known asa piezoelectric material suitable for the QCM due to its high sen-sitivity to mass change on the surface. The resonant frequency (f0)in this work was evaluated from the conductance peak. It has beenobserved that the conductance versus frequency curve shows a fun-damental resonance peak at 5.48 MHz (Fig. 2b). The ZnO nanorodswere then grown on one side of the Au electrode-coated QCM by thewet chemical route. Fig. 3 shows SEM images of the top-view (3a)and side-view (3b) of as-grown ZnO nanorods on the Au electrodeof the QCM. The morphology of the ZnO nanorods with a hexagonalstructure was vertically well-aligned and uniformly distributed onthe Au electrode of the QCM. This shows that the exposed area of thesensing layer was remarkably enhanced compared with the sensinglayer of the ZnO nanowires [27]. The average diameter and lengthof ZnO nanorods were around 100 nm and 3 �m, respectively. Incomparison with the ZnO nanowires that were first synthesizedby evaporating high purity zinc pellets at 900 ◦C and were thendistributed on the QCM [27], the wet chemical route has manyadvantages such as low cost, low temperature operation, high pre-ferred orientation, and environmental friendliness. This methodcan also directly grow ZnO nanorods with high uniform distributionon a large area.
The XRD pattern of the as-grown ZnO nanorods grown on theAu electrode of QCM is shown in Fig. 4. The XRD diffraction peaksat 31.28◦, 34.64◦, 36.32◦, 47.90◦, and 62.90◦ represent the (1 0 0),(0 0 2), (1 0 1), (1 0 2), and (1 0 3) planes, respective, of the hexago-nal ZnO structure with lattice constants of a = 3.24 A and c = 5.20 Ain accordance with the JCDPS card 36-1451. The over-whelmingly
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Fig. 2. (a) Optical image and (b) conductance characterization of the fabricated QCM.
high intensity of the (0 0 2) reflection peak compared to the othercrystallographic planes directly implies that a majority of the ZnOnanorods grew with the c-axis direction perpendicular to the sub-strate. Higher peak ratios of (0 0 2)/(1 0 0) and (0 0 2)/(1 0 1) wereobserved, suggesting the vertical alignment of the nanorods.
Fig. 5a shows the response transients of the ZnO nanorod-coatedQCM sensor to switching-on and off of the NH3 gas-flow with dif-ferent concentrations (50, 100, and 200 ppm) at room temperature(25 ◦C). In the first stage, the sensor flushed a reference air gas flowof 15 sccm to obtain a baseline. The sensor was then exposed to aNH3 gas flow of 15 sccm with a certain concentration, which leadsto frequency response until a steady stage was reached, indicatingmaximum adsorption of NH3 gas onto the QCM sensor. The NH3gas flow was finally replaced by the air gas flow and the sensorreturned back to its baseline. In this experiment, the flow rate ofthe diluted ammonia gas and dry air was fixed at 15 sccm. Hence, inthe gas sensing chamber, the flow and pressure were ensured to beconstant. The change in resonant frequency of a QCM (�f) can berelated to the change in mass (�m) due to the adsorption of NH3gas molecules on ZnO nanorods using the Sauerbrey equation asfollows [16].
�f = −2f 20
A�q�q�m, (4)
where A is the active area of the QCM electrode in cm2, f0 is theresonant frequency of the QCM in hertz (Hz), �m is the change inthe oscillating mass in grams (g), �q is the density of quartz, and �q
is the shear wave velocity in the quartz.Fig. 5a also shows three time-cycling responses of the sensors for
each NH3 gas concentration (50, 100, and 200 ppm). The sensor hadalmost the same response for the three cycles, indicating that thesensor had good reproducibility. Additionally, we found that when
Fig. 3. SEM images of ZnO nanorods grown by wet chemical bath deposition: (a)top-view and (b) side-view.
the NH3 gas flow was replaced by air flow, the resonant frequencyreturned to its original value. This indicates that the absorbed NH3molecules were completely removed. The vertically aligned ZnOnanorods grown on the QCM electrodes might have enhanced thisfact. The temporary change in frequency during the exposure ofNH3 was supposed to be due to the weak physical adsorption of
Fig. 4. X-ray diffraction pattern of ZnO nanorods.
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N. Van Quy et al. / Sensors and Actuators B 153 (2011) 188–193 191
Fig. 5. (a) Reproducibility and (b) proportion test for the frequency shift of the sensorbased on QCM coated with ZnO nanorods with different NH3 concentrations.
NH3 onto the ZnO, as the chemical adsorption of NH3 on the ZnOnanorods could not be rinsed with the air flow [27,28]. However, thetheory calculation of the adsorption of NH3 gas on 1D ZnO nanos-tructures was chemical adsorption as previously reported [29]. Thismeans neither the experimental data nor the theory calculationdata as previously reported can be used to explain our observation.Thus, we propose the NH3 sensing mechanism of our sensor as fol-lows. The H2O molecules from air (moisture) could be expected toadsorb physically on the surface of the ZnO nanorods at room tem-perature. Upon exposure to ammonia, the surface reaction of NH3with the physical adsorption H2O could occur:
NH3(g) + H2O(surface) → NH4OH(g) (5)
Ammonium hydroxide, NH4OH, produced during the surface reac-tion is volatile in nature. The high volatility of NH4OH explains thecomplete recovery of the sensor by rinsing the air flow.
Fig. 5b shows the frequency shift as a function of NH3 gas con-centration. It is obvious that the frequency shift linearly increaseswith an increase in the NH3 gas concentration in the range of50–200 ppm. This indicates that more NH3 molecules are adsorbedon the ZnO nanorods with the increase in the NH3 gas concentra-tion. Although a higher NH3 gas concentration was not tested in thiswork due to a limitation in our testing system, we recognize that theincrease in the shift frequency tends to saturate with the increase inconcentration, as previously reported [27]. It can be also observedthat the shift frequency upon exposure to 50 ppm is about 9.1 Hz.This suggests that ZnO nanorod-coated QCM sensor can detect NH3
Fig. 6. High magnification of the response and recovery time of the fabricated sensorto (a) 50, (b) 100, and (c) 200 ppm of NH3 concentrations.
gas at lower concentration and even at the ppb level. Our group iscurrently working on this.
In order to study the response and recovery times of the sen-sor, the high magnification of the frequency shift versus time ateach concentration of NH3 is plotted in Fig. 6. The times to reach90% variation in the frequency shift upon exposure to gas and airwere defined as the 90% response time [t90%(air-to-gas)] and the 90%recovery time [t90%(air-to-gas)], respectively. The 90% response timefor gas exposure [t90%(air-to-gas)] and that for recovery [t90%(gas-to-air)]were calculated from the frequency shift–time data from Fig. 6.The t90%(air-to-gas) values in the sensing of 50, 100, and 200 ppm NH3were 226, 231, and 239 s, respectively, while the t90%(gas-to-air) valuein the sensing of that were 368, 394, and 398 s, respectively. Theresponse and recovery times slight increased with an increase in theNH3 gas concentration. It can be seen that the response times werealso relatively shorter than the recovery time. This means that theadsorption process of NH3 was faster than the desorption process.This observation agrees well with the previous reports [27,28].
One of the most important characteristics of the gas sensor isits selectivity. To examine the selectivity of the QCM coated withZnO nanorods gas sensor, we tested the sensitivity of the sensorwith several gases including of carbon monoxide, carbon dioxide,nitrogen dioxide, nitrous oxide and LPG (which consists of hydro-
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Table 1Brief summary of results reported on the response of NH3 gas sensors.
No. Gas sensor types Testing gases Response, S Reference
carbons like CH4, C3H8, C4H10). The conductivity-based gas sensorusing ZnO thin film indicates significant sensitivity to LPG signifi-cantly [28]. For the gas sensing properties, the fabricated sensorswere not responsive at a concentration of about 1% LPG. The changein frequency of the sensors was realized when the concentration ofLPG was about 2%. Fig. 7 shows the comparison of the resonantfrequency of the device exposed to N2O, CO, NO2, CO2, and LPGwith that exposed to ammonia. Although the concentration of LPGwas 2%, the frequency shift of the sensor only changed by 0.8 Hz.In comparison with 50, 100, or 200 ppm NH3, the response of thesensor with LPG was insignificant. Specially, our NH3 sensors arealso good selective over the compound of nitrogen (N2O and NO2).We have tested sensor with 0.5% N2O and 0.1% NO2 concentration
7006005004003002001000Time (s)
0.5% N2O 150 ppm CO 0.1% NO2
2% CO2
2% LPG 50 ppm NH
3
Gas in
Air in
-5
0
Δ f (
Hz)
Fig. 7. Comparison of the frequency change of the fabricated sensor when exposedto LPG, CO2, N2O and NH3.
the frequency shift are about 0.4 Hz and 0.6 Hz, respectively. It canbe recognized that the compound of nitrogen gas as concentra-tion of few hundred ppm cannot interfere the sensor to NH3 gas. Agood selectivity of the sensors to NH3 gas was also observed whenexposed to carbon monoxide (CO) and carbon dioxide (CO2). Thefrequency shift of the sensors with both of carbon monoxide andcarbon dioxide was even less than that with the LPG. Fig. 7 showsclearly the frequency shift of the sensor when exposed to them. At150 ppm carbon monoxide and 2% carbon dioxide concentrations,the change in frequency was only 0.5 and 0.6 Hz, respectively.
We have made comparison the selectivity of our QCM-coatedZnO nanorods sensors to various kind of the sensors recentlyreported as presented in Table 1 [30–33]. It can be seen that Ru-doped ZnO-based sensor has quite good selectivity to NH3 gascompared with the other sensors (see Table 1). For instance, theratio of response of NH3 to LPG and NO2 (SNH3 /SLPG and SNH3 /SNO2 )at 1000 ppm is around 10.7 and 43, respectively. These ratios in oursensor are comparable with reported results, which are round 26and 28.3. It should be noted that we have tested our sensor withlower NH3 concentration and higher concentration of LPG and NO2.
The good selectivity to NH3 gas of the QCM coated with ZnOnanorods is an important factor for developing NH3 gas sensor. Thischaracteristic is difficult to achieve with resistive-sensors based onthe semiconductor metal oxides. For further application of the sen-sors, the mechanism of this issue needs to be elucidated in detail.Although present study cannot fully understand the mechanism, itcan be somehow explained based on the viewpoint of Eq. (4).
4. Conclusion
In this paper, we presented the first study on the use of ZnOnanorods-coated quartz crystal microbalance as a NH3 sensor.Vertically well-aligned ZnO nanorods were successfully grownon the Au electrode of QCM by the wet chemical method. The
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ZnO nanorods were uniformly distributed on the substrate witha diameter and height of around 100 nm and 3 �m, respectively.As-developed sensor showed a good response to NH3 gas at roomtemperature and it could be used detect at low level concentrationof NH3 gas. Additionally, the sensor has a good selectivity to NH3gas over various gases such as LPG, N2O, NO2, CO, and CO2. This plat-form provides a promising NH3 gas sensor with high response, highselectivity, rapid response, and operating at room temperatures.
Acknowledgment
This work was supported by the application-oriented basicresearch program (2009-2012, Code: 05/09/HÐ-ÐTÐL).
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Biographies
Nguyen Van Quy received his M.S and Ph.D degrees from Materials Science andEngineering at Chungnam National University, South Korea in 2006 and 2009,respectively. He is currently a research lecturer at International Training Institutefor Materials Science (ITIMS), Hanoi University of Science and Technology (HUST),Vietnam. His research interests include synthesis of carbon nanotubes and applica-tions to nano-electronic devices of field emission, solar cell and sensors, MEMS, andsensitive materials.
Vu Anh Minh received his B.S and M.S degrees from Faculty of Physics at Hanoi Uni-versity of Science, Vietnam National University, in 1992 and 2001, respectively. Heis currently a Ph.D student at International Training Institute for Materials Science(ITIMS), Hanoi University of Science and Technology (HUST), Vietnam. His currentinterests include nanomaterials synthesis, characterizations, and applications toelectronic devices, gas sensors and biosensors.
Nguyen Van Luan received his B.S from Institute of Engineering Physics (IEP), HanoiUniversity of Science and Technology (HUST), Vietnam. He is currently a mastercourse student Department of Energy Science, Sungkyunkwan Advanced Instituteof Nanotechnology, Sungkyunkwan University, South Korea.
Vu Ngoc Hung received the B.S. degree in physics from Kishinev University (USSR), in1979 and the Ph.D. degree from Hanoi University of Science and Technology (HUST),Vietnam in 1991. He is currently an Associate Professor at the International TrainingInstitute for Materials Science (ITIMS), HUST. His current research interests are inthe area of MEMS inertial and QCM sensors.
Nguyen Van Hieu joined the International Training Institute for Material Science(ITIMS) at Hanoi University of Science and Technology (HUST) in 2004, where he iscurrently associate professor. He received his PhD degree from the Faculty of Electri-cal Engineering at University of Twente, The Netherlands in 2004. In 2007, he workedas a post-doctoral fellow at the Korea University. Currently, he is the vice directorof ITIMS and chairs the research group of gas sensors. His current research inter-ests include nanomaterials, nanofabrications, characterizations and applications toelectronic devices, gas sensors and biosensors.
