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Preparation and humidity sensitive property of mesoporous ZnO–SiO2 composite
Sensors and Actuators B 149 (2010) 413–419
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Preparation and humidity sensitive property of mesoporous ZnO–SiO2 composite Qing Yuan a , Nan Li a , Jinchun Tu a,b , Xiaotian Li a,∗ , Rui Wang c , Tong Zhang c , Changlu Shao d a
School of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, PR China College of Material and Chemical Engineering, Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Hainan University, Haikou 570228, PR China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China d Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, Changchun 130024, PR China b
a r t i c l e
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Article history: Received 29 December 2009 Received in revised form 27 May 2010 Accepted 11 June 2010 Available online 23 June 2010 Keywords: Humidity sensor ZnO Mesoporous material One-pot synthesis
a b s t r a c t Mesoporous ZnO–SiO2 composites with various Si/Zn molar ratios were synthesized through a simple one-pot sol–gel method and their humidity sensing properties were also examined. Compared with pure SBA-15, ZnO–SiO2 composite shows improved humidity sensing properties and the introducing level of ZnO has a great influence on the humid sensitivity of ZnO–SiO2 composites. The results exhibited that the sample with Si/Zn = 1 showed better humidity sensing properties than others within the range of 11–95% relative humidity (RH). Its impedance changed by more than four orders of magnitude over the whole humidity range. The response and recovery time were about 50 s and 100 s, respectively. High sensitivity and low hysteresis were also observed. A possible mechanism was suggested to explain the humidity sensitive properties. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Chemical sensors play an important role in environmental monitoring, healthcare, industrial production, national security, with economic impact on medicine, agriculture, aerospace industries and so on [1–5]. In recent years, many kinds of sensing materials like metal oxides [6,7], polymers [8,9], and polyelectrolytes [10] have been used to serve these applications. Among these materials, zinc oxide (ZnO), as an n-type semiconducting metal oxide, due to its chemical sensitivity to volatile and radical gases, high chemical stability, easy doping, non-toxicity, and low cost has witnessed an explosion of interest as an excellent candidate material for gas sensing [11–13]. Although many successes have been focused on the gas sensitivity of ZnO, little work has been done on its humid sensitivity [14,15]. Ordered mesoporous materials with large surface area and uniform pore structure may be able to enhance the adsorption of water vapor and accelerate the transmission of water molecules so that the humidity sensitivity can be greatly improved. SBA-15, one of the most well-known mesoporous materials with one-dimensional channel, has been reported as a humidity sensor doped with LiCl [16]. However, this experiment employed a two-step processing, and the mesoporous structure of Li-doped SBA-15 was destroyed. In our paper, an in situ coating ZnO on SiO2 in one-pot synthe-
∗ Corresponding author. Tel.: +86 431 85168445; fax: +86 431 85168444. E-mail addresses: [email protected], [email protected] (X. Li).
sis has been adopted. This method not only operates simply but also retains the mesoporous structure of SBA-15. The mesoporous ZnO–SiO2 composite is expected to show good humidity sensitivity to the environment. 2. Experimental 2.1. Preparation of mesoporous materials Mesoporous silica SBA-15 was synthesized according to the method reported by Zhao et al. [17]. Mesoporous ZnO–SiO2 composites were synthesized via a sol–gel process. Briefly, 2 g of triblock copolymer Pluronic P123 (Aldrich) was dissolved in 60 ml HCl solution (2 mol/L) at room temperature, then a calculated amount of Zn(NO3 )2 ·6H2 O with different Si/Zn molar ratio (the Si/Zn molar ratio was defined as R, where R was selected as 0.5, 1, 3, and 5) was added under stirring for 0.5 h. Thereafter, 4.25 g tetraethyl orthosilicate (TEOS) was added dropwise and stirred at 313 K for 24 h. The resultant mixture was aged at 353 K for 24 h without stirring. The solid obtained was dried at 353 K overnight and calcined at 823 K for 6 h to form ZnO–SiO2 composites. 2.2. Fabrication and test method of humidity sensors The device manufacturing process was described in our previous work [16]. Each sample was mixed and ground with deionized water in a weight ratio of 100:25 to form a paste, and then coated on a ceramic plate (0.5 cm × 0.1 cm) in which a pair of interdigitated
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gold electrodes (200 Å thick, 20 m wide and the distance between fingers is 20 m) was printed by photolithographing. The thickness of the sensing materials was about 500 m. The sensor was heated at 70 ◦ C for 5 h before testing. The measurement was carried out by placing a humidity sensor in a glass vessel with a given relative humidity for approximately 3 min until the resistance of the sensing material reached a stable value. 2.3. Characterization The structure of all samples was characterized by X-ray diffraction (XRD) operated at 40 kV and 40 mA using Cu K␣ ( = 1.5405 Å) radiation. The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). The Brunauer–Emmett–Teller (BET) specific surface area was calculated using adsorption data in relative pressure range from 0.04 to 0.2. The pore size distribution (PSD) curves were calculated from the analysis of the adsorption branch of the isotherm using the BJH (Barrett–Joyner–Halenda) algorithm. Infrared spectra were taken on a Perkin–Elmer series with a resolution of 4 cm−1 . Transmission electron microscopy (TEM) experiments were performed on a JEOL JEM-2010 electron microscope. The variation curve of impedance as a function of relative humidity was measured on a ZL-5 model LCR analyzer at room temperature, 100 Hz, and 1.0 V. The controlled humidity environments were achieved using saturated aqueous solutions of different salts: LiCl, MgCl2 , Mg(NO3 )2 , NaCl, KCl, and KNO3 in a closed glass vessel at ambient temperature, which yielded 11%, 33%, 54%, 75%, 85% and 95% relative humidity, respectively. 3. Results and discussions 3.1. Structure and morphology 3.1.1. X-ray diffraction Fig. 1 illustrates the XRD patterns of all samples we obtained. As can be seen from low-angle XRD spectra (Fig. 1A), all the ZnO–SiO2 composites show three diffraction peaks indexed as (1 0 0), (1 1 0) and (2 0 0) reflections corresponding to p6 mm hexagonal symmetry, identical to that of SBA-15, which indicated the existence of
mesoscopic periodicity in these composites. Moreover the intensity of reflection (1 0 0) in the obtained samples kept almost constant or some lower in comparison with that of SBA-15, showing that this method led to no large damage of the mesostructure of the SBA-15 host. Whereas, the (1 0 0) reflection of R = 0.5 sample shifts to higher angle and becomes weaker compared with that of other samples. This is probably because the more inorganic salt added, the more ZnO could attach to inside wall of pore and block the pore. So the order of mesoporous structure was decreased and thus the XRD intensity of (1 0 0) decreased accordingly. In order to make comparison, we prepared ZnO/SBA-15 nanocomposites by incorporating zinc nitrate precursor into the channels of mesoporous silica SBA-15 and subsequent calcination using two-solvent strategy [18]. The ZnO/SBA-15 nanocomposites with different ZnO loadings are referred as x wt.% ZnO/SBA-15, where x represents the weight percentage of ZnO in the nanocomposite. From the lowangle XRD spectra, the 40 wt.% ZnO/SBA-15 also showed hexagonal ordered structures even after the mixing and calcination process, indicating that the ZnO, which was introduced into the pore of mesoporous SBA-15, does not collapse the mesoscopic order of a two-dimensional hexagonal structure. Fig. 1B shows wide-angle XRD patterns of all samples. For mesoporous ZnO–SiO2 composites, when ZnO loading content was low, no crystalline phase was detected in XRD patterns, indicating the full dispersion of ZnO nanoparticles on SiO2 matrix. As the ZnO loading amount rose up to R = 0.5, crystalline ZnO appeared in the wide-angle scale. Moreover the 40 wt.% ZnO/SBA-15 sample exhibits the broad diffuse peak attributed to the non-crystalline silica and ZnO, which indicated that ZnO in the ZnO/SBA-15 nanocomposites is non-crystalline or may exist as clusters with ultrafine particle size. This result is consistent with previous reports for other metal oxides clusters inside the channels of SBA-15 [19,20]. 3.1.2. N2 adsorption–desorption characterization The N2 adsorption–desorption isotherms of ZnO–SiO2 (R = 1, R = 3) and 40 wt.% ZnO/SBA-15 samples shown in Fig. 2 are all type IV with a clear H1-type hysteresis loop [21]. For ZnO–SiO2 composites, with the increasing ZnO amount, the BET surface area changed from 541 m2 /g (R = 3) to 262 m2 /g (R = 1). From Fig. 3A, the centered pore size for R = 3 is 6.14 nm and for R = 1 reduces to 5.78 nm. For 40 wt.% ZnO/SBA-15, its BET surface area is 281 m2 /g and its cen-
Fig. 1. Low-angle (A) and wide-angle (B) XRD patterns of SBA-15, ZnO–SiO2 . Composites and 40 wt.% ZnO/SBA-15 sample.
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Fig. 2. Nitrogen adsorption–desorption isotherms of ZnO–SiO2 composites (R = 3, R = 1) (A) and 40 wt.% ZnO/SBA-15 sample (B).
Fig. 3. Pore size distribution of (A) ZnO–SiO2 composites (R = 3, R = 1) and (B) 40 wt.% ZnO/SBA-15 sample.
tered pore size is 4.32 nm as shown in Fig. 3B. These results indicate that the three samples are of ordered mesoporous structure.
3.1.3. IR spectra Fig. 4 represents the infrared spectra of pure SBA-15 and ZnO–SiO2 composites. As can be seen, mesoporous silica SBA-15 shows framework bands at 1092 cm−1 and 808 cm−1 attributed to the antisymmetric stretching and symmetric stretching frequency of Si–O–Si, respectively. The bands observed at 1635 cm−1 and 960 cm−1 were stretching modes of surface Si–OH [22,23]. It was found that with increasing ZnO amount, the adsorption band around 1630 cm−1 first gradually sharpened, then weakened, and the strongest band appeared at R = 1. The IR vibration of the NO3 − ion at 1380 cm−1 is not observed [24], confirming the added nitrate has been completely transformed into metal oxides.
3.1.4. Transmission electron microscopy characterization In order to further explore the structural features of these materials, the obtained powders are studied by TEM. As shown in Fig. 5, highly ordered hexagonal mesostructure can be observed for the sample ZnO–SiO2 (R = 1), the average pore size of the periodical hexagonal pores is about 5 nm. These observations are in good agreement with the results obtained from the XRD and N2 adsorption–desorption analysis.
3.1.5. X-ray photoelectron spectroscopy (XPS) To further confirm the interaction of ZnO and SiO2 , the sample ZnO–SiO2 (R = 1) was characterized by X-ray photoelectron spectroscopy (XPS). As seen in Fig. 6A and B the binding ener-
Fig. 4. IR spectra of pure SBA-15 and ZnO–SiO2 composites.
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Fig. 5. TEM image of ZnO–SiO2 composite (R = 1).
gies of O 1s, Zn 2p1/2 and Zn 2p3/2 are 532.6 eV, 1046.1 eV and 1022.8 eV respectively. These results indicate that ZnO have formed on SiO2 matrix [25]. From Fig. 6B, the Zn 2p binding energy of ZnO–SiO2 (R = 1) (1022.8 eV) is significantly greater than that of pure ZnO (1021.1 eV), showing the formation of Zn–O–Si bond. This is because the electrical negative of Si (∼1.9) is higher than that of Zn (∼1.65), which results in the Zn valence electron density of Zn–O–Si bond is smaller than that of Zn–O–Zn bond (pure ZnO). On the other hand, the binding energy of Zn 3d electron in the sample of ZnO–SiO2 (R = 1) is 11.3 eV (Fig. 6C) 1.65 eV higher than that of pure ZnO, which indicates the formation of Si–O–Zn bond [26,27], since the Zn 3d electrons usually participate in the covalent binding of ZnO [28] to give a binding energy value of 9.65 eV. 3.2. Humidity sensitive property 3.2.1. RH dependence of resistance at various frequencies Fig. 7 shows the dependence of impedance of ZnO–SiO2 (R = 1) on RH at different frequencies. At low relative humidity, the higher the operation frequency adopted, the lower the impedance was.
Fig. 7. Variations of impedance of ZnO–SiO2 (R = 1) as a function of relative humidity at different frequencies of 50 Hz, 100 Hz, 1 kHz, 10 kHz, and 100 kHz.
When the RH increased, five lines overlapped together, indicating that at high relative humidity different operating frequency has little effect on the impedance. As it can be seen in all the test line, the best linearity of the impedance versus RH curve appeared at 50 Hz and 100 Hz. However, when 50 Hz was used, impedance changed dramatically, the testing at which is unstable. Therefore, 100 Hz is the optimum operating frequency in our experiment. 3.2.2. Relative humidity–impedance curves The plots of impedance as a function of relative humidity (%RH) of all samples at room temperature are shown in Fig. 8. The measurement voltage was AC 1 V and the frequency was 100 Hz. All the samples exhibited very high impedance at low relative humidity. For pure mesoporous silica SBA-15, the impedance did not decrease drastically until relative humidity increased to 78%, showing its poor humidity sensitivity. When ZnO was mixed on SiO2 matrix, the sensitivity towards humidity was improved. The more ZnO mixed, the lower impedance was. However when the loading of ZnO was up to R = 0.5, the decline in impedance with increasing humidity is much lower than other ZnO–SiO2 samples. It may because that
Fig. 6. XPS spectra with ZnO–SiO2 composite (R = 1) of (A) O 1s, (B) Zn 2p and (C) Zn 3d.
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Fig. 8. Humidity sensitive properties of pure SBA-15, ZnO–SiO2 composites and 40 wt.% ZnO/SBA-15 sample.
the over-doped ZnO cause the blockage of the pore, which was in line with the above-mentioned results of the XRD results. In the whole relative humidity range of our measurement from 11% to 95%, the impedance of R = 1 sample changed by more than four orders of magnitude with increasing RH. This result is better than our previous work [16], in which the impedance of the optimal doping amount Li-SBA-15 (0.1) changed by three orders of magnitude. Moreover the sensor based on ZnO–SiO2 (R = 1) sample had shown a better linearity than the other ratios samples. To display reproducibility information, R = 1 was repeatedly tested and named as R = 1 (2), which also showed better humidity property as seen from curve f. Therefore, our study was focused on this sample. 3.2.3. Response and recovery behavior Since response and recovery behavior is one of the significant features for estimating the performance of the humidity sensors, ZnO–SiO2 (R = 1) composite was chosen (Fig. 9) to evaluate the
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Fig. 10. Hysteresis characteristic of ZnO–SiO2 composite (R = 1).
recovery and response time. The response time from 11% to 95% RH was measured by transferring the humidity sensor in equilibrium at 11% RH to the other chamber in equilibrium at 95% RH, and the recovery time was measured in the opposite direction. According to the definition [29], the time taken by a sensor to achieve 90% of the total resistance change was defined as the response time in the case of adsorption or the recovery time in the case of desorption. It can be seen from Fig. 9 that the response and recovery times were about 50 s and 100 s, respectively, indicating that the sample has a quick response to the humidity change. 3.2.4. Hysteresis The humidity hysteresis characteristic data of the humidity sensor based on ZnO–SiO2 (R = 1) sample was also provided in Fig. 10. The upper line in the figure stands for the course from low to high relative humidity, corresponding to the adsorption process, while the other stands for the desorption process. For this sensor, the maximum humidity hysteresis is around 2% at 75% RH, in line with the characteristics requirement of ideal humidity sensors. 3.3. Discussion
Fig. 9. Response and recovery characteristic of ZnO–SiO2 composite (R = 1).
Here, we try to give a possible mechanism qualitatively to explain the humidity sensitive properties based on ZnO–SiO2 (R = 1) sample. In order to make comparison, we synthesis the 40 wt.% ZnO/SBA-15 sample and test its humidity property. From Fig. 8, we see the impedance of this sample is still high with the relative humidity increased, showing its poor humidity sensitive property. This result proved that the encapsulated ZnO in mesoporous SBA-15 was not sensitive to humidity. However with the ZnO amount mixed on SiO2 matrix increased, the impedance of ZnO–SiO2 samples was obviously dropped, showing better sensitivity than 40 wt.% ZnO/SBA-15. Moreover the above-mentioned XPS analysis results indicate that Si–O–Zn exists in ZnO–SiO2 composites. From the view of material structure, the visible difference between mesoporous ZnO–SiO2 composites and ZnO/SBA-15 is that the proportion of Si–O–Zn in ZnO–SiO2 composites is much higher than that in ZnO/SBA-15. So we believe that the reason why the humidity sensitive property of ZnO–SiO2 composites is bet-
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ter than that of ZnO/SBA-15 is probably the presence of Si–O–Zn bond. Among all the mesoporous ZnO–SiO2 composites, with a Si/Zn molar ratio 1:1, the proportion of Si–O–Zn is higher than in other samples with different Si/Zn molar ratio expect for R = 0.5. The higher Si–O–Zn proportion, the more water molecules can be adsorbed by Si–O–Zn. The Si4+ , Zn2+ of Si–O–Zn adsorb the OH− of H2 O and release H+ to participate in conduction. Moreover for ZnO–SiO2 (R = 1) composite, not only the Si–OH groups played a major role in increasing the conductivity of the senor material, but also the large surface areas and ordered porous structure [30–32] created by mesoporous ZnO–SiO2 (R = 1) could be in favor of humidity device on the absorption of water molecules. In existing results reported recently on humidity sensitivity performance, LiCl was always used as an electrolytic material for humidity sensors [33]. From the experimental results, the humid sensitivity property of ZnO–SiO2 (R = 1) is as well as LiCl/SBA-15 [16], LiCl/SBA-16 [34]. However, LiCl is easily soluble even in room environment. Although LiCl is added into SBA-15, SBA-16 as the guest material to prevent the loss of LiCl, a long time can cause the devices to loss part of LiCl. For our mesoporous ZnO–SiO2 composites, because ZnO is not soluble in water, the humidity device based on ZnO can be more stable than that based on LiCl. So the sample of ZnO–SiO2 (R = 1) composite is selected as a good candidate of humidity sensitive materials. 4. Conclusions Mesoporous ZnO–SiO2 composites with various Si/Zn ratios were synthesized through a simple one-pot sol–gel method and their humidity sensing properties were also examined. Comparing with pure mesoporous silica SBA-15, ZnO–SiO2 composites showed improved humidity sensing properties within the range of 11–95% RH. The optimal mixing molar ratio was obtained for sample with Si/Zn = 1:1, which exhibited an excellent linear correlation between impedance and humidity in the whole range. The response time is 50 s and the maximum humidity hysteresis is about 2%. These results display the great potential of mesoporous materials as a sensor support to fabricating humidity sensors. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20743005 and 50572014), and the Science Foundation for Young Teachers of Northeast Normal University (NCET-05-0322). References [1] W.Y. Weng, S.J. Chang, T.J. Hsueh, C.L. Hsu, M.J. Li, W.C. Lai, AlInN resistive ammonia gas sensors, Sens. Actuators B 140 (2009) 139–142. [2] A. Barbaro, C. Colapicchioni, E. Davini, G. Mazzamurro, A. Piotto, F. Porcelli, CHEMFET devices for biomedical and environmental applications, Adv. Mater. 4 (1992) 402–408. [3] L. Wadsö, F.G. Galindo, Isothermal calorimetry for biological applications in food science and technology, Food Control 20 (2009) 956–961. [4] F. Osterloh, H. Hiramatsu, R. Porter, T. Guo, Alkanethiol-induced structural rearrangements in silica-gold core–shell-type nanoparticle clusters: an opportunity for chemical sensor engineering, Langmuir 20 (2004) 5553–5558. [5] S. Zampolli, I. Elmi, F. Ahmed, M. Passini, G.C. Cardinali, S. Nicoletti, L. Dori, An electronic nose based on solid state sensor arrays for low-cost indoor air quality monitoring applications, Sens. Actuators B: Chem. 101 (2004) 39–46. [6] R.R. Salunkhe, D.S. Dhawale, D.P. Dubal, C.D. Lokhande, Sprayed CdO thin films for liquefied petroleum gas (LPG) detection, Sens. Actuators B 140 (2009) 86–91. [7] M. Sorescu, L. Diamandescu, A. Tomescu, D. Tarabasanu-Mihaila, V. Teodorescu, Structure and sensing properties of 0.1SnO2 –0.9␣-Fe2 O3 system, Mater. Chem. Phys. 107 (2008) 127–131. [8] H. Mishra, V. Misra, M.S. Mehata, T.C. Pant, H.B. Tripathi, Fluorescence studies of salicylic acid doped poly(vinyl alcohol) film as a water/humidity sensor, J. Phys. Chem. 108 (2004) 2346–2352. [9] C.-Y. Lin, V.S. Vasantha, K.-C. Ho, Detection of nitrite using poly(3,4ethylenedioxythiophene) modified SPCEs, Sens. Actuators B 140 (2009) 51–57.
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Biographies Qing Yuan received her BS degree from the Department of Material Science in Jilin University, China in 2007. Presently she is a graduate student, majored in mesoporous materials. Nan Li received her BS degree in Department of Chemistry, Jilin University, in 1999. Then she obtained her PhD from Jilin University in 2004. Her main research interests are inorganic chemistry and chemical sensors. Jinchun Tu received a BS degree in Department of Material Science, Jilin University in 2003. Now, he is a PhD student in Jilin University after two year’s MS course. His major interests are mesoporous silicon dioxide and chemical sensors technology.
Q. Yuan et al. / Sensors and Actuators B 149 (2010) 413–419 Xiaotian Li is a professor of Department of Material Science in Jilin University. He received a BS, MS degree in Department of Electronic Science in Jilin University in 1989 and 1992, respectively. He obtained his PhD degree in the Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, in 2000. His main research interests are inorganic chemistry. Rui Wang received her BS, MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2005 and 2007, respectively. She entered the PhD course in 2007, majored in microelectronics and solid state electronics, and engaged in novel sensing materials and humidity sensors. Tong Zhang received her MS degree in major of Semiconductor Materials in 1992 and PhD degree in the field of Microelectronics and Solid State Electronics in 2001
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from Jilin University. She was appointed as full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, she is interested in the field of sensing functional materials, gas sensors and humidity sensors. Changlu Shao is a professor in College of Physics in Northeast Normal University. He received his BS, MS degree in Department of Chemistry in Northeast Normal University in 1987 and 1993, respectively. He obtained his PhD degree in the Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, in 2000. His main research interests are inorganic chemistry.
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Preparation and humidity sensitive property of mesoporous ZnO–SiO2 composite Qing Yuan a , Nan Li a , Jinchun Tu a,b , Xiaotian Li a,∗ , Rui Wang c , Tong Zhang c , Changlu Shao d a
School of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, PR China College of Material and Chemical Engineering, Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Hainan University, Haikou 570228, PR China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China d Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, Changchun 130024, PR China b
a r t i c l e
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Article history: Received 29 December 2009 Received in revised form 27 May 2010 Accepted 11 June 2010 Available online 23 June 2010 Keywords: Humidity sensor ZnO Mesoporous material One-pot synthesis
a b s t r a c t Mesoporous ZnO–SiO2 composites with various Si/Zn molar ratios were synthesized through a simple one-pot sol–gel method and their humidity sensing properties were also examined. Compared with pure SBA-15, ZnO–SiO2 composite shows improved humidity sensing properties and the introducing level of ZnO has a great influence on the humid sensitivity of ZnO–SiO2 composites. The results exhibited that the sample with Si/Zn = 1 showed better humidity sensing properties than others within the range of 11–95% relative humidity (RH). Its impedance changed by more than four orders of magnitude over the whole humidity range. The response and recovery time were about 50 s and 100 s, respectively. High sensitivity and low hysteresis were also observed. A possible mechanism was suggested to explain the humidity sensitive properties. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Chemical sensors play an important role in environmental monitoring, healthcare, industrial production, national security, with economic impact on medicine, agriculture, aerospace industries and so on [1–5]. In recent years, many kinds of sensing materials like metal oxides [6,7], polymers [8,9], and polyelectrolytes [10] have been used to serve these applications. Among these materials, zinc oxide (ZnO), as an n-type semiconducting metal oxide, due to its chemical sensitivity to volatile and radical gases, high chemical stability, easy doping, non-toxicity, and low cost has witnessed an explosion of interest as an excellent candidate material for gas sensing [11–13]. Although many successes have been focused on the gas sensitivity of ZnO, little work has been done on its humid sensitivity [14,15]. Ordered mesoporous materials with large surface area and uniform pore structure may be able to enhance the adsorption of water vapor and accelerate the transmission of water molecules so that the humidity sensitivity can be greatly improved. SBA-15, one of the most well-known mesoporous materials with one-dimensional channel, has been reported as a humidity sensor doped with LiCl [16]. However, this experiment employed a two-step processing, and the mesoporous structure of Li-doped SBA-15 was destroyed. In our paper, an in situ coating ZnO on SiO2 in one-pot synthe-
∗ Corresponding author. Tel.: +86 431 85168445; fax: +86 431 85168444. E-mail addresses: [email protected], [email protected] (X. Li).
sis has been adopted. This method not only operates simply but also retains the mesoporous structure of SBA-15. The mesoporous ZnO–SiO2 composite is expected to show good humidity sensitivity to the environment. 2. Experimental 2.1. Preparation of mesoporous materials Mesoporous silica SBA-15 was synthesized according to the method reported by Zhao et al. [17]. Mesoporous ZnO–SiO2 composites were synthesized via a sol–gel process. Briefly, 2 g of triblock copolymer Pluronic P123 (Aldrich) was dissolved in 60 ml HCl solution (2 mol/L) at room temperature, then a calculated amount of Zn(NO3 )2 ·6H2 O with different Si/Zn molar ratio (the Si/Zn molar ratio was defined as R, where R was selected as 0.5, 1, 3, and 5) was added under stirring for 0.5 h. Thereafter, 4.25 g tetraethyl orthosilicate (TEOS) was added dropwise and stirred at 313 K for 24 h. The resultant mixture was aged at 353 K for 24 h without stirring. The solid obtained was dried at 353 K overnight and calcined at 823 K for 6 h to form ZnO–SiO2 composites. 2.2. Fabrication and test method of humidity sensors The device manufacturing process was described in our previous work [16]. Each sample was mixed and ground with deionized water in a weight ratio of 100:25 to form a paste, and then coated on a ceramic plate (0.5 cm × 0.1 cm) in which a pair of interdigitated
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gold electrodes (200 Å thick, 20 m wide and the distance between fingers is 20 m) was printed by photolithographing. The thickness of the sensing materials was about 500 m. The sensor was heated at 70 ◦ C for 5 h before testing. The measurement was carried out by placing a humidity sensor in a glass vessel with a given relative humidity for approximately 3 min until the resistance of the sensing material reached a stable value. 2.3. Characterization The structure of all samples was characterized by X-ray diffraction (XRD) operated at 40 kV and 40 mA using Cu K␣ ( = 1.5405 Å) radiation. The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). The Brunauer–Emmett–Teller (BET) specific surface area was calculated using adsorption data in relative pressure range from 0.04 to 0.2. The pore size distribution (PSD) curves were calculated from the analysis of the adsorption branch of the isotherm using the BJH (Barrett–Joyner–Halenda) algorithm. Infrared spectra were taken on a Perkin–Elmer series with a resolution of 4 cm−1 . Transmission electron microscopy (TEM) experiments were performed on a JEOL JEM-2010 electron microscope. The variation curve of impedance as a function of relative humidity was measured on a ZL-5 model LCR analyzer at room temperature, 100 Hz, and 1.0 V. The controlled humidity environments were achieved using saturated aqueous solutions of different salts: LiCl, MgCl2 , Mg(NO3 )2 , NaCl, KCl, and KNO3 in a closed glass vessel at ambient temperature, which yielded 11%, 33%, 54%, 75%, 85% and 95% relative humidity, respectively. 3. Results and discussions 3.1. Structure and morphology 3.1.1. X-ray diffraction Fig. 1 illustrates the XRD patterns of all samples we obtained. As can be seen from low-angle XRD spectra (Fig. 1A), all the ZnO–SiO2 composites show three diffraction peaks indexed as (1 0 0), (1 1 0) and (2 0 0) reflections corresponding to p6 mm hexagonal symmetry, identical to that of SBA-15, which indicated the existence of
mesoscopic periodicity in these composites. Moreover the intensity of reflection (1 0 0) in the obtained samples kept almost constant or some lower in comparison with that of SBA-15, showing that this method led to no large damage of the mesostructure of the SBA-15 host. Whereas, the (1 0 0) reflection of R = 0.5 sample shifts to higher angle and becomes weaker compared with that of other samples. This is probably because the more inorganic salt added, the more ZnO could attach to inside wall of pore and block the pore. So the order of mesoporous structure was decreased and thus the XRD intensity of (1 0 0) decreased accordingly. In order to make comparison, we prepared ZnO/SBA-15 nanocomposites by incorporating zinc nitrate precursor into the channels of mesoporous silica SBA-15 and subsequent calcination using two-solvent strategy [18]. The ZnO/SBA-15 nanocomposites with different ZnO loadings are referred as x wt.% ZnO/SBA-15, where x represents the weight percentage of ZnO in the nanocomposite. From the lowangle XRD spectra, the 40 wt.% ZnO/SBA-15 also showed hexagonal ordered structures even after the mixing and calcination process, indicating that the ZnO, which was introduced into the pore of mesoporous SBA-15, does not collapse the mesoscopic order of a two-dimensional hexagonal structure. Fig. 1B shows wide-angle XRD patterns of all samples. For mesoporous ZnO–SiO2 composites, when ZnO loading content was low, no crystalline phase was detected in XRD patterns, indicating the full dispersion of ZnO nanoparticles on SiO2 matrix. As the ZnO loading amount rose up to R = 0.5, crystalline ZnO appeared in the wide-angle scale. Moreover the 40 wt.% ZnO/SBA-15 sample exhibits the broad diffuse peak attributed to the non-crystalline silica and ZnO, which indicated that ZnO in the ZnO/SBA-15 nanocomposites is non-crystalline or may exist as clusters with ultrafine particle size. This result is consistent with previous reports for other metal oxides clusters inside the channels of SBA-15 [19,20]. 3.1.2. N2 adsorption–desorption characterization The N2 adsorption–desorption isotherms of ZnO–SiO2 (R = 1, R = 3) and 40 wt.% ZnO/SBA-15 samples shown in Fig. 2 are all type IV with a clear H1-type hysteresis loop [21]. For ZnO–SiO2 composites, with the increasing ZnO amount, the BET surface area changed from 541 m2 /g (R = 3) to 262 m2 /g (R = 1). From Fig. 3A, the centered pore size for R = 3 is 6.14 nm and for R = 1 reduces to 5.78 nm. For 40 wt.% ZnO/SBA-15, its BET surface area is 281 m2 /g and its cen-
Fig. 1. Low-angle (A) and wide-angle (B) XRD patterns of SBA-15, ZnO–SiO2 . Composites and 40 wt.% ZnO/SBA-15 sample.
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Fig. 2. Nitrogen adsorption–desorption isotherms of ZnO–SiO2 composites (R = 3, R = 1) (A) and 40 wt.% ZnO/SBA-15 sample (B).
Fig. 3. Pore size distribution of (A) ZnO–SiO2 composites (R = 3, R = 1) and (B) 40 wt.% ZnO/SBA-15 sample.
tered pore size is 4.32 nm as shown in Fig. 3B. These results indicate that the three samples are of ordered mesoporous structure.
3.1.3. IR spectra Fig. 4 represents the infrared spectra of pure SBA-15 and ZnO–SiO2 composites. As can be seen, mesoporous silica SBA-15 shows framework bands at 1092 cm−1 and 808 cm−1 attributed to the antisymmetric stretching and symmetric stretching frequency of Si–O–Si, respectively. The bands observed at 1635 cm−1 and 960 cm−1 were stretching modes of surface Si–OH [22,23]. It was found that with increasing ZnO amount, the adsorption band around 1630 cm−1 first gradually sharpened, then weakened, and the strongest band appeared at R = 1. The IR vibration of the NO3 − ion at 1380 cm−1 is not observed [24], confirming the added nitrate has been completely transformed into metal oxides.
3.1.4. Transmission electron microscopy characterization In order to further explore the structural features of these materials, the obtained powders are studied by TEM. As shown in Fig. 5, highly ordered hexagonal mesostructure can be observed for the sample ZnO–SiO2 (R = 1), the average pore size of the periodical hexagonal pores is about 5 nm. These observations are in good agreement with the results obtained from the XRD and N2 adsorption–desorption analysis.
3.1.5. X-ray photoelectron spectroscopy (XPS) To further confirm the interaction of ZnO and SiO2 , the sample ZnO–SiO2 (R = 1) was characterized by X-ray photoelectron spectroscopy (XPS). As seen in Fig. 6A and B the binding ener-
Fig. 4. IR spectra of pure SBA-15 and ZnO–SiO2 composites.
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Fig. 5. TEM image of ZnO–SiO2 composite (R = 1).
gies of O 1s, Zn 2p1/2 and Zn 2p3/2 are 532.6 eV, 1046.1 eV and 1022.8 eV respectively. These results indicate that ZnO have formed on SiO2 matrix [25]. From Fig. 6B, the Zn 2p binding energy of ZnO–SiO2 (R = 1) (1022.8 eV) is significantly greater than that of pure ZnO (1021.1 eV), showing the formation of Zn–O–Si bond. This is because the electrical negative of Si (∼1.9) is higher than that of Zn (∼1.65), which results in the Zn valence electron density of Zn–O–Si bond is smaller than that of Zn–O–Zn bond (pure ZnO). On the other hand, the binding energy of Zn 3d electron in the sample of ZnO–SiO2 (R = 1) is 11.3 eV (Fig. 6C) 1.65 eV higher than that of pure ZnO, which indicates the formation of Si–O–Zn bond [26,27], since the Zn 3d electrons usually participate in the covalent binding of ZnO [28] to give a binding energy value of 9.65 eV. 3.2. Humidity sensitive property 3.2.1. RH dependence of resistance at various frequencies Fig. 7 shows the dependence of impedance of ZnO–SiO2 (R = 1) on RH at different frequencies. At low relative humidity, the higher the operation frequency adopted, the lower the impedance was.
Fig. 7. Variations of impedance of ZnO–SiO2 (R = 1) as a function of relative humidity at different frequencies of 50 Hz, 100 Hz, 1 kHz, 10 kHz, and 100 kHz.
When the RH increased, five lines overlapped together, indicating that at high relative humidity different operating frequency has little effect on the impedance. As it can be seen in all the test line, the best linearity of the impedance versus RH curve appeared at 50 Hz and 100 Hz. However, when 50 Hz was used, impedance changed dramatically, the testing at which is unstable. Therefore, 100 Hz is the optimum operating frequency in our experiment. 3.2.2. Relative humidity–impedance curves The plots of impedance as a function of relative humidity (%RH) of all samples at room temperature are shown in Fig. 8. The measurement voltage was AC 1 V and the frequency was 100 Hz. All the samples exhibited very high impedance at low relative humidity. For pure mesoporous silica SBA-15, the impedance did not decrease drastically until relative humidity increased to 78%, showing its poor humidity sensitivity. When ZnO was mixed on SiO2 matrix, the sensitivity towards humidity was improved. The more ZnO mixed, the lower impedance was. However when the loading of ZnO was up to R = 0.5, the decline in impedance with increasing humidity is much lower than other ZnO–SiO2 samples. It may because that
Fig. 6. XPS spectra with ZnO–SiO2 composite (R = 1) of (A) O 1s, (B) Zn 2p and (C) Zn 3d.
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Fig. 8. Humidity sensitive properties of pure SBA-15, ZnO–SiO2 composites and 40 wt.% ZnO/SBA-15 sample.
the over-doped ZnO cause the blockage of the pore, which was in line with the above-mentioned results of the XRD results. In the whole relative humidity range of our measurement from 11% to 95%, the impedance of R = 1 sample changed by more than four orders of magnitude with increasing RH. This result is better than our previous work [16], in which the impedance of the optimal doping amount Li-SBA-15 (0.1) changed by three orders of magnitude. Moreover the sensor based on ZnO–SiO2 (R = 1) sample had shown a better linearity than the other ratios samples. To display reproducibility information, R = 1 was repeatedly tested and named as R = 1 (2), which also showed better humidity property as seen from curve f. Therefore, our study was focused on this sample. 3.2.3. Response and recovery behavior Since response and recovery behavior is one of the significant features for estimating the performance of the humidity sensors, ZnO–SiO2 (R = 1) composite was chosen (Fig. 9) to evaluate the
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Fig. 10. Hysteresis characteristic of ZnO–SiO2 composite (R = 1).
recovery and response time. The response time from 11% to 95% RH was measured by transferring the humidity sensor in equilibrium at 11% RH to the other chamber in equilibrium at 95% RH, and the recovery time was measured in the opposite direction. According to the definition [29], the time taken by a sensor to achieve 90% of the total resistance change was defined as the response time in the case of adsorption or the recovery time in the case of desorption. It can be seen from Fig. 9 that the response and recovery times were about 50 s and 100 s, respectively, indicating that the sample has a quick response to the humidity change. 3.2.4. Hysteresis The humidity hysteresis characteristic data of the humidity sensor based on ZnO–SiO2 (R = 1) sample was also provided in Fig. 10. The upper line in the figure stands for the course from low to high relative humidity, corresponding to the adsorption process, while the other stands for the desorption process. For this sensor, the maximum humidity hysteresis is around 2% at 75% RH, in line with the characteristics requirement of ideal humidity sensors. 3.3. Discussion
Fig. 9. Response and recovery characteristic of ZnO–SiO2 composite (R = 1).
Here, we try to give a possible mechanism qualitatively to explain the humidity sensitive properties based on ZnO–SiO2 (R = 1) sample. In order to make comparison, we synthesis the 40 wt.% ZnO/SBA-15 sample and test its humidity property. From Fig. 8, we see the impedance of this sample is still high with the relative humidity increased, showing its poor humidity sensitive property. This result proved that the encapsulated ZnO in mesoporous SBA-15 was not sensitive to humidity. However with the ZnO amount mixed on SiO2 matrix increased, the impedance of ZnO–SiO2 samples was obviously dropped, showing better sensitivity than 40 wt.% ZnO/SBA-15. Moreover the above-mentioned XPS analysis results indicate that Si–O–Zn exists in ZnO–SiO2 composites. From the view of material structure, the visible difference between mesoporous ZnO–SiO2 composites and ZnO/SBA-15 is that the proportion of Si–O–Zn in ZnO–SiO2 composites is much higher than that in ZnO/SBA-15. So we believe that the reason why the humidity sensitive property of ZnO–SiO2 composites is bet-
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ter than that of ZnO/SBA-15 is probably the presence of Si–O–Zn bond. Among all the mesoporous ZnO–SiO2 composites, with a Si/Zn molar ratio 1:1, the proportion of Si–O–Zn is higher than in other samples with different Si/Zn molar ratio expect for R = 0.5. The higher Si–O–Zn proportion, the more water molecules can be adsorbed by Si–O–Zn. The Si4+ , Zn2+ of Si–O–Zn adsorb the OH− of H2 O and release H+ to participate in conduction. Moreover for ZnO–SiO2 (R = 1) composite, not only the Si–OH groups played a major role in increasing the conductivity of the senor material, but also the large surface areas and ordered porous structure [30–32] created by mesoporous ZnO–SiO2 (R = 1) could be in favor of humidity device on the absorption of water molecules. In existing results reported recently on humidity sensitivity performance, LiCl was always used as an electrolytic material for humidity sensors [33]. From the experimental results, the humid sensitivity property of ZnO–SiO2 (R = 1) is as well as LiCl/SBA-15 [16], LiCl/SBA-16 [34]. However, LiCl is easily soluble even in room environment. Although LiCl is added into SBA-15, SBA-16 as the guest material to prevent the loss of LiCl, a long time can cause the devices to loss part of LiCl. For our mesoporous ZnO–SiO2 composites, because ZnO is not soluble in water, the humidity device based on ZnO can be more stable than that based on LiCl. So the sample of ZnO–SiO2 (R = 1) composite is selected as a good candidate of humidity sensitive materials. 4. Conclusions Mesoporous ZnO–SiO2 composites with various Si/Zn ratios were synthesized through a simple one-pot sol–gel method and their humidity sensing properties were also examined. Comparing with pure mesoporous silica SBA-15, ZnO–SiO2 composites showed improved humidity sensing properties within the range of 11–95% RH. The optimal mixing molar ratio was obtained for sample with Si/Zn = 1:1, which exhibited an excellent linear correlation between impedance and humidity in the whole range. The response time is 50 s and the maximum humidity hysteresis is about 2%. These results display the great potential of mesoporous materials as a sensor support to fabricating humidity sensors. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 20743005 and 50572014), and the Science Foundation for Young Teachers of Northeast Normal University (NCET-05-0322). References [1] W.Y. Weng, S.J. Chang, T.J. Hsueh, C.L. Hsu, M.J. Li, W.C. Lai, AlInN resistive ammonia gas sensors, Sens. Actuators B 140 (2009) 139–142. [2] A. Barbaro, C. Colapicchioni, E. Davini, G. Mazzamurro, A. Piotto, F. Porcelli, CHEMFET devices for biomedical and environmental applications, Adv. Mater. 4 (1992) 402–408. [3] L. Wadsö, F.G. Galindo, Isothermal calorimetry for biological applications in food science and technology, Food Control 20 (2009) 956–961. [4] F. Osterloh, H. Hiramatsu, R. Porter, T. Guo, Alkanethiol-induced structural rearrangements in silica-gold core–shell-type nanoparticle clusters: an opportunity for chemical sensor engineering, Langmuir 20 (2004) 5553–5558. [5] S. Zampolli, I. Elmi, F. Ahmed, M. Passini, G.C. Cardinali, S. Nicoletti, L. Dori, An electronic nose based on solid state sensor arrays for low-cost indoor air quality monitoring applications, Sens. Actuators B: Chem. 101 (2004) 39–46. [6] R.R. Salunkhe, D.S. Dhawale, D.P. Dubal, C.D. Lokhande, Sprayed CdO thin films for liquefied petroleum gas (LPG) detection, Sens. Actuators B 140 (2009) 86–91. [7] M. Sorescu, L. Diamandescu, A. Tomescu, D. Tarabasanu-Mihaila, V. Teodorescu, Structure and sensing properties of 0.1SnO2 –0.9␣-Fe2 O3 system, Mater. Chem. Phys. 107 (2008) 127–131. [8] H. Mishra, V. Misra, M.S. Mehata, T.C. Pant, H.B. Tripathi, Fluorescence studies of salicylic acid doped poly(vinyl alcohol) film as a water/humidity sensor, J. Phys. Chem. 108 (2004) 2346–2352. [9] C.-Y. Lin, V.S. Vasantha, K.-C. Ho, Detection of nitrite using poly(3,4ethylenedioxythiophene) modified SPCEs, Sens. Actuators B 140 (2009) 51–57.
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Biographies Qing Yuan received her BS degree from the Department of Material Science in Jilin University, China in 2007. Presently she is a graduate student, majored in mesoporous materials. Nan Li received her BS degree in Department of Chemistry, Jilin University, in 1999. Then she obtained her PhD from Jilin University in 2004. Her main research interests are inorganic chemistry and chemical sensors. Jinchun Tu received a BS degree in Department of Material Science, Jilin University in 2003. Now, he is a PhD student in Jilin University after two year’s MS course. His major interests are mesoporous silicon dioxide and chemical sensors technology.
Q. Yuan et al. / Sensors and Actuators B 149 (2010) 413–419 Xiaotian Li is a professor of Department of Material Science in Jilin University. He received a BS, MS degree in Department of Electronic Science in Jilin University in 1989 and 1992, respectively. He obtained his PhD degree in the Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, in 2000. His main research interests are inorganic chemistry. Rui Wang received her BS, MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2005 and 2007, respectively. She entered the PhD course in 2007, majored in microelectronics and solid state electronics, and engaged in novel sensing materials and humidity sensors. Tong Zhang received her MS degree in major of Semiconductor Materials in 1992 and PhD degree in the field of Microelectronics and Solid State Electronics in 2001
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from Jilin University. She was appointed as full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, she is interested in the field of sensing functional materials, gas sensors and humidity sensors. Changlu Shao is a professor in College of Physics in Northeast Normal University. He received his BS, MS degree in Department of Chemistry in Northeast Normal University in 1987 and 1993, respectively. He obtained his PhD degree in the Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, in 2000. His main research interests are inorganic chemistry.