Sensors and Actuators B 177 (2013) 1083–1088
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Highly sensitive humidity sensor based on quartz crystal microbalance coated with ZnO colloid spheres Juan Xie a,b,∗ , Hu Wang a,∗∗ , Yuanhua Lin b , Ying Zhou b , Yuanpeng Wu b a b
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University, SWPU), Chengdu 610500, China College of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China
a r t i c l e
i n f o
Article history: Received 14 August 2012 Received in revised form 6 December 2012 Accepted 8 December 2012 Available online 20 December 2012 Keywords: Quartz crystal microbalance (QCM) ZnO colloid sphere Humidity sensitivity Adsorption Desorption
a b s t r a c t Different sizes of ZnO colloid spheres were coated on quartz crystal microbalance (QCM) by the selfassembly method. The device exhibits excellent humidity sensing properties in the whole humidity range from 11% to 95%, such as high sensitivity, good linearity, and fast response/recovery time. The results showed that the humidity sensitivity of these sensors increased with increasing relative humidity (RH). Furthermore, the humidity sensitivity enhanced with increasing size of ZnO colloid spheres. We also discussed the adsorption and desorption of water molecules on different sizes of ZnO colloid spheres in different humidity. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Accurate and reliable measurement of humidity has been playing an increasingly important role in various ﬁelds, such as semiconductor industry, automobile industry, environmental control, pharmaceuticals, and food processing industry [1–3]. Compared to existing sensors and technologies, the main development focuses on improving sensitivity, reliability and repeatability . In recent years, several promising technologies are sought to improve upon reliable and highly sensitive humidity sensors. These methods are based on bulk acoustic wave (BAW) , surface acoustic wave (SAW) , quartz crystal microbalance (QCM) [7–11], and ion-sensitive ﬁeld-effect transistor (ISFET) technique [12,13]. The QCM is an extremely stable and powerful device, which can be highly sensitive to mass changes on the nanogram scale. The QCM method monitors the frequency shift of a coated quartz crystal with sensing element due to adsorption of molecules on the surface of the sensing materials. According to Sauerbrey equation, the frequency shift is directly proportional to the adsorbed mass on the QCM with sensitive coating materials. Therefore, use of a QCM with sensitive coating materials has become an alternative approach for detecting low humidity .
At present, different sensing materials have been developed as the coating ﬁlms to detect various kinds of vapors and humidity. These materials include polymer [14–21], metal and carbon absorption materials [22–24]. Nanosized materials were also used for sensing materials owing to their high surface area, such as carbon nanotube/Naﬁon [25,26], modiﬁed nitrated polystyrene , Naﬁon-Ag , bacterial cellulose [2,3], calyxarene [29,30], ZnO nanostructure [31–35], and polopyrrole/Ag/TiO2 nanoparticles . Among them, ZnO is one of the most promising materials for sensor applications, because it can be grown in different morphologies without much effort and it has chemically active defect sites which enable high sensitivity regardless of the chemical environment in which they are employed. Although it is known that some morphologies of ZnO were coated on QCM, we have not reached any report on ZnO colloid spheres on QCM as humidity sensors. In this work, a series of ZnO colloid particles with different scales were fabricated by the self-assembly method on QCM wafers as humidity sensors. The average diameters of ZnO colloid particles can be well controlled by experimental parameters. The humidity sensor was tested at room temperature by varying the relative humidity (RH) from 11% to 95%. The relationship between humidity sensing and size of ZnO particles was also discussed. 2. Experimental
∗ Corresponding author at: State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University, SWPU), Chengdu 610500, China. Tel.: +86 28 83037346; fax: +86 28 83037346. ∗∗ Corresponding author. Tel.: +86 28 83037346; fax: +86 28 83037346. E-mail addresses: [email protected]
(J. Xie), [email protected]
2.1. Fabrication of ZnO colloid particles on QCM ZnO colloid particles were synthesized by the self-assembly method. 0.01 mol zinc acetate dihydrate was added into 100 mL
0925-4005/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.12.033
J. Xie et al. / Sensors and Actuators B 177 (2013) 1083–1088
Fig. 1. Typical FESEM images of ZnO colloid spheres: (a) S1, 0.5 mL supernatant added; (b) S2, 1 mL supernatant added; (c) S3, 5 mL supernatant added; (d) S4, 10 mL supernatant added; (e) S5, 20 mL supernatant added; (f) The plot of average diamenters of ZnO colloid spheres for S1, S2, S3, S4, and S5.
J. Xie et al. / Sensors and Actuators B 177 (2013) 1083–1088
diethylene glycol. When the solution was heated slowly to about 180 ◦ C, the precipitation of ZnO spheres was formed. The intermediate product was placed in a centrifuge. The polydisperse powder was discarded and the supernatant was kept for the next step. A secondary reaction was then performed to produce the ZnO colloid spheres, which was conducted in the same way as the primary reaction. Prior to reaching the working temperature, however, typically at 170 ◦ C, a certain amount of the primary reaction supernatant was added into the solution. The preparation process ended when the reactant had been kept stirring at 180 ◦ C for 6 h. The suspension was dropped onto the QCM wafers. Then the coatings were dried in a furnace at 80 ◦ C for 1 h. After evaporating the organic solvent, the white thin ﬁlms were found on the surface of the QCM wafers. 2.2. Measurements The morphology of the ZnO ﬁlms on the QCM was observed with the ﬁeld emission scanning electron microscope (FESEM, JSM7500F). The controlled humidity environments were achieved with saturation aqueous solutions of LiCl, MgCl2 , Mg(NO3 )2 , NaCl, KCl, and KNO3 in a closed glass vessel at room temperature, which yielded 11%, 33%, 55%, 75%, 85%, and 95% RH, respectively. Each of the saturated salt solution was kept in the glass chamber for 24 h before the measurement. The sensing properties were examined by measuring the resonance frequency shifts of QCM, which were caused by the additional mass loading. The resonance frequencies were measured by a QCM digital controller (QCM200, Stanford Research Systems). The QCM sensors were suspended above a halfﬁlled closed container without contacting the solution and they were stored in a dry environment after the test ﬁnished. The frequency signals were measured during the adsorption process (RH from 11% to other higher RH%) and desorption process (RH from other higher RH% to 11%). 3. Results and discussion 3.1. Characterization In this work, 0.5 mL, 1 mL, 5 mL, 10 mL, and 20 mL of the primary reaction supernatants were added into the secondary reaction solution respectively. The samples were labeled as S1, S2, S3, S4, and S5. Fig. 1 shows the FESEM images of the ﬁve samples. It can be seen that the size of the colloid spheres varies with the amount of the supernatant added. It can be observed at the same time that the colloid spheres are composed of a series of primary particles. Therefore, these ZnO colloid spheres are secondary particles. As shown in Fig. 1(f), the average diameters of the secondary particles for all ﬁve samples ascend gradually. Furthermore, the sizes of colloid spheres vary with the amount of added primary supernatant.
Fig. 2. Frequency shifts of QCM humidity sensors of S1, S2, S3, S4, and S5.
indicates the adsorption of the water vapor to the ZnO surface is the multi-molecular-layer adsorption and the amount of the adsorbed water vapor in different RHs is not linear. However, the frequency shifts followed a logarithmic increase with good linear log(−f) toward RH in the range of 11–95%, which is shown in Fig. 3. The data in Table 1 derived from Fig. 3 was computed by linear ﬁtting. The linearity was deﬁned as the square of correlation coefﬁcient value (R2 ) of linear ﬁtting curve in the humidity range from 11% to 95%. It can be observed in Table 1 that the ﬁve samples show excellent linearity. R2 are 0.99760, 0.99917, 0.99924, 0.99894, and 0.99507, respectively. It can also be seen that the slopes of linear ﬁtting curves are 0.01884, 0.01617, 0.01637, 0.01650, and 0.01740 respectively, which are extremely close. Therefore, the plots of linear ﬁtting are almost parallel.In this paper, sensitivity of humidity sensor can be deﬁned as the following equation (Eq. (1)): SH =
fH − fL RH
where fH and fL are the steady-state resonance frequency at high RH and low RH, respectively. RH is the change of relative humidity. Fig. 4 shows the dependence of sensitivity on RH for the ﬁve samples. From the results, it can be found that the sensitivity increased with increasing RH. Especially, at 95% RH, the sensitivity of S5 was about 77 Hz/%. Compared with the results obtained in previous
3.2. Humidity sensitive property Fig. 2 shows the relationship between the frequency shifts and humidity ascending process for the ﬁve samples. It indicates that the resonance frequency shift increased with increasing relative humidity for all the samples. It can be also observed from the ﬁgure, with increasing the diameter of the ZnO colloid spheres, the frequency shift tended to increase under the same humidity condition, indicating an increase in humidity sensitivity. At 95% RH, the maximum of frequency shift for S5 was −6425.54 Hz, which was nearly 3 times higher than that of S1. It can be seen in Fig. 2 that the frequency shifts as a function of the RH do not exhibit a linear tendency with increasing RH. The phenomenon was in accordance with humidity sensors based on QCM in reference . The BET model adsorption theory which
Fig. 3. Dependence of the frequency shifts of sensors for ﬁve samples on RH (33–95%).
J. Xie et al. / Sensors and Actuators B 177 (2013) 1083–1088
Table 1 Data computed by linear ﬁtting derived from Fig. 3. Sample
Average diameter (nm)
Fitting linear equation
Related coefﬁcient (R2 )
S1 S2 S3 S4 S5
100 370 500 560 680
Y = 1.5949 + 0.01884X Y = 1.90294 + 0.01617X Y = 1.95791 + 0.01637X Y = 2.03375 + 0.01650X Y = 2.12118 + 0.01740X
0.99760 0.99917 0.99924 0.99894 0.99507
Fig. 5. The response time curves of the ﬁve samples at different RH. Fig. 4. Sensitivity curves of the ﬁve samples in the RH range from 33% to 95%.
work [11,32], our results show that the sensors have extremely highly humidity sensitive property. As shown in Fig. 4, with increasing the diameter of ZnO colloid spheres, the sensitivity tended to increase at the same RH. A reasonable explanation is that under the same humidity environment, the water molecules could be adsorbed easily on the big colloid spheres due to steric effect of the secondary particles. 3.3. Response and recovery property The response and the recovery time are important merits for a humidity sensor. Response time in absorption process and recovery time in desorption process are both deﬁned as the time of achieving 90% variable quantity. In a humidity sensor, the response and recovery time correspond to the adsorption and desorption of water molecules on the surface of sensing materials. The two parameters are determined by alternatively exposing the sensing ﬁlm to solutions of the lowest and the highest RH values. Figs. 5 and 6 show the response time and the recovery time for S1, S2, S3, S4, and S5. As can be seen in Fig. 5, the response time increased with increasing RH for all the samples. It can also be observed that the response time increased with the increase of the ZnO particle diameters. The water adsorption model based on physical and chemical actions can be used to explain the response to humidity of ZnO colloid spheres. Firstly, water molecule is chemically adsorbed onto the active sites by means of hydrogen bond, forming an adsorption complex. It is a chemisorption process. Secondly, more water molecules are adsorbed and condensed on the surface to form water layers. It is a physisorption process . The ZnO colloid spheres can establish an adsorption–desorption equilibrium under the actions of van der Waals force and hydrogen bond in a certain humidity environment. Changing the humidity environment will cause equilibrium shift, indicating a change of resonance frequency of the QCM caused by the change of mass . As mentioned above, there is enough space on the big-sized colloid spheres for adsorption of water molecules. Hence, the resonance frequency shift will ascend
and in this situation it needs relatively long time to establish the new equilibrium. According to Figs. 5 and 6, at low RH (33% and 55%) the recovery time (from 62.2 s to 19.6 s) is longer than response time (from 1.1 s to 3.5 s) for all the samples. However, at high RH (75%, 85%, and 95%), it exhibits shorter recovery time (from 18.1 s to 8 s) than response time (from 38 s to 167.7 s). As shown in Fig. 7, chemisorption occurs at relatively low RH value to form two surface hydroxyls per water molecule. In this adsorption process, the equilibrium time is short, namely, the response time is short. However, when humidity rises, physisorption of water molecules takes place. In this adsorption process, the equilibrium time is relatively long. Therefore, with increment of humidity values, physisorption and frequency shift will increase and the response time will enhance as well. On the other hand, the chemisorptions–desorption equilibrium time is longer than the physisorption–desorption equilibrium time.
Fig. 6. The recovery time curves of the ﬁve samples at different RH.
J. Xie et al. / Sensors and Actuators B 177 (2013) 1083–1088
Fig. 7. Adsorption models for ZnO colloid spheres with different size.
Therefore, under the low water vapor condition, the recovery time is longer than response time. On the contrary, under the high water vapor condition, the recovery time is shorter than response time. 4. Conclusion In summary, the scales of ZnO colloid spheres could be well controlled by experimental parameters. The sensors represent highly sensitive performance. The inﬂuences of the size of ZnO colloid spheres and humidity values on humidity sensors have been investigated. The results show that with the increase of humidity values and particle diameters, the adsorption of water molecules will increase remarkably. In adsorption–desorption process, the response and recovery property varies with the relative humidity values due to different behavior of establishing equilibrium. The investigation of the humidity sensitive characteristics of the QCM sensors indicates that this material has good prospects in application of humidity sensor. Acknowledgments This work was ﬁnancially supported by the Fundamental Research Funds of Southwest Petroleum University (No. 2012XJZ017), Sichuan College Laboratory of Materials of Oil and Gas Field (12YQT016), and Innovative Research Team of Southwest Petroleum University (2012XJZT002). References  H. Jamil, S.S. Batool, Z. Imran, M. Usman, M.A. Raﬁq, M. Willander, M.M. Hassan, Electrospun titanium dioxide nanoﬁber humidity sensors with high sensitivity, Ceramics International 38 (2012) 2437–2441.  M.Y. Su, J. Wang, Y.W. Hao, Development of Y3+ and Mg2+ -doped zirconia thick ﬁlm humidity sensors, Materials Chemistry and Physics 126 (2011) 31–35.  Y.H. Zhu, H. Yuan, J.Q. Xu, P.C. Xu, Q.Y. Pan, Highly stable and sensitive humidity sensors based on quartz crystal microbalance coated with hexagonal lamelliform monodisperse mesoporous silica SBA-15 thin ﬁlm, Sensors and Actuators B 144 (2010) 164–169.  K. Ocakoglu, S. Okur, Humidity sensing properties of novel ruthenium polypyridyl complex, Sensors and Actuators B 151 (2010) 223–228.  H. Seh, T. Hyodo, H.L. Tuller, Bulk acoustic wave resonator as a sensing platform for NOx at high temperatures, Sensors and Actuators B 108 (2005) 547–552.  M. Penza, G. Cassano, Relative humidity sensing by PVA-coated dual resonator SAW oscillator, Sensors and Actuators B 68 (2000) 300–306.  Y.L. Sun, Y.Z. Chen, R.J. Wu, M. Chavali, Y.C. Huang, P.G. Su, C.C. Lin, Poly(llactide) stabilized gold nanoparticles based QCM sensor for low humidity detection, Sensors and Actuators B 126 (2007) 441–446.  Y.L. Sun, R.J. Wu, Y.C. Huang, P.G. Su, M. Chavali, Y.Z. Chen, C.C. Lin, In situ prepared polypyrrole for low humidity QCM sensor and related theoretical calculation, Talanta 73 (2007) 857–861.  P.G. Su, Y.L. Sun, C.C. Lin, Novel low humidity sensor made of TiO2 nanowires/poly(2-acrylamido-2-methylpropane sulfonate) composite material ﬁlm combined with quartz crystal microbalance, Talanta 69 (2006) 946–951.  Y.S. Zhang, K. Yu, R.L. Xu, D.S. Jiang, L.Q. Luo, Z.Q. Zhu, Quartz crystal microbalance coated with carbon nanotube ﬁlms used as humidity sensor, Sensors and Actuators A 120 (2005) 142–146.
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Biographies Juan Xie received her B.Sc. degree in applied chemistry from University of Electronic Science and Technology of China (UESTC) in 2003, and received her M.Sc. degree in materials science from UESTC in 2006. Currently, she is lecturer in Southwest Petroleum University, Chengdu, China. Her research interests include surface and interface science, humidity and gas sensors, thin ﬁlm solar cells.
J. Xie et al. / Sensors and Actuators B 177 (2013) 1083–1088
Hu Wang received his B.Sc. and M.Sc. degree in materials science from Beijing University of Chemical Technology in 2001 and 2004. He received his Ph.D. degree in applied chemistry from Sichuan University in 2010. Currently, he is associate professor at Southwest Petroleum University, Chengdu, China. His research interests are in the areas of surface and interface science, nanomaterials.
Chinese Academy of Science in 2007. He received his doctor degree from University of Zurich, Switzerland in 2010. Currently, he is professor at Southwest Petroleum University, Chengdu, China. His research interests are in the areas of hydrothermal synthesis, in situ studies and environmental applications of oxide nanomaterials.
Yuan-Hua Lin received his Ph.D. degree from Southwest Petroleum University, in 1999. Currently, he is professor at Southwest Petroleum University, Chengdu, China. His research interests are applications of advanced materials in oil ﬁeld.
Yuan-Peng Wu received his Ph.D. degree in polymeric chemistry and physics from Chengdu Institute of Organic Chemistry of Chinese Academy of Sciences in 2010. Currently, he is lecturer at Southwest Petroleum University, Chengdu, China. His research interests are functional nanomaterials, smart polymer materials.
Ying Zhou received his B.Sc. and M.Sc. degree in materials science from central South University in 2004 and Shanghai Institute of Optics and Fine Mechanics,