Sensors and Actuators B 133 (2008) 638–643

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Properties of humidity sensing ZnO nanorods-base sensor fabricated by screen-printing Qi Qi a , Tong Zhang a,b,∗ , Qingjiang Yu c , Rui Wang a , Yi Zeng a , Li Liu a , Haibin Yang c a

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China Key Laboratory of Low Dimensional Materials & Application Technology, Ministry of Education, Xiangtan 411105, PR China c National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China b

a r t i c l e

i n f o

Article history: Received 29 December 2007 Received in revised form 25 March 2008 Accepted 31 March 2008 Available online 8 April 2008 Keywords: Humidity sensitivity Flower-like ZnO nanorods Complex impedance plots Sensor

a b s t r a c t The humidity sensitive characteristics of a sensor fabricated from flower-like ZnO nanorods by screenprinting on a ceramic substrate with Ag–Pd interdigital electrodes have been investigated. The sensor shows high humidity sensitivity, rapid response and recovery, small hysteresis, and good stability. It is found that the impedance of the sensor decreases by about five orders of magnitude with increasing relative humidity (RH) from 11 to 95%. The response and recovery time of the sensor is about 5 and 10 s, respectively. These results indicate that the flower-like ZnO nanorods can be used in fabricating high-performance humidity sensors. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Fabrication of sensitive chemical sensors has gained special focus driven by their diverse applications in air-quality detection, inflammable-gas inspection, environmental monitoring, healthcare, defense and security, and so on [1–5]. Recently, inspired by the advantages of high surface-to-volume ratios, fabrication of nanomaterial electronic devices and exploration of their properties are of current interest [6–18]. Hitherto, different types of sensors based on three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) architectures have been successfully obtained [10–18]. Among those nanostructures, 1D sensors, based on ceramic structures (SnO2 [19], TiO2 [20], ZnO [14–18], In2 O3 [21], and WO3 [22]), have attracted much focus owing to their high surface area and low dimensionality, which could facilitate fast mass transfer of the analyte molecules to and from the interaction region as well as require charge carriers to transverse the barriers introduced by molecular recognition along the 1D nanostructures [23]. Although many successes have been obtained, most of those papers focus on the gas sensors (e.g. CO, O2 , and C2 H5 OH) [10–18], and few papers on humidity sensors have

∗ Corresponding author at: State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China. Tel.: +86 431 85168385; fax: +86 431 85168270. E-mail address: [email protected] (T. Zhang). 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.03.035

been explored. Additionally, the fabrication of sensitive and stable humidity sensors with rapid response and recovery is still in great demand. In this paper, we report a highly sensitive humidity sensor with rapid response and recovery, which is based on the flower-like ZnO nanorods. ZnO has been chosen in our experiment for its versatile properties in optoelectronic devices, sensors, lasers, transducers, and photovoltaic devices [24,25]. Additionally, ZnO nanostructures are believed to be nontoxic, bio-safe, and possibly biocompatible, and have been used in many applications in our daily life. We believe that our method not only provides a new avenue for fabricating highly effective humidity sensors, but also offers a powerful platform to understand and design desirable humidity sensors. 2. Experimental 2.1. Preparation and characterization of materials Flower-like ZnO nanorods were synthesized by a simple wet chemical method [26]. All chemicals (analytical grade reagents) were purchased from Beijing Chemicals Co. Ltd. and used as received without further purification. Deionized water with a resistivity of 18.0 M cm−1 was used in all experiments. In a typical synthesis process, 100 mL of an aqueous solution of zinc nitrate and 100 mL of a hexamethylenetetramine (HMT) aqueous solution of equal concentration (0.05 M) were mixed together and kept under mild magnetic stirring for 5 min. Then the solution was transferred

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Fig. 1. (a) Scheme of the sensor structure and (b) top-view optical micrograph of the sensor.

into a 500-mL flask and heated at 90 ◦ C for 3 h with refluxing. Subsequently, the resulting white products were centrifuged, washed with deionized water and ethanol and dried at 60 ◦ C in air for further characterization. X-ray diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu K␣ radia˚ Field-emission scanning electron microscopy tion ( = 1.5418 A). (FE-SEM) images were performed on a JEOL JEM-6700F microscope operating at 3 and 5 kV. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2000EX microscope with an accelerating voltage of 200 kV. Raman-scattering spectrum was measured by an HR-800 LabRam confocal Raman microscope with a backscattering configuration made by JY company in France, excited by the 514.5 nm line of an argon-ion laser at room temperature (25 ◦ C). 2.2. Fabrication and measurement of sensors The flower-like ZnO nanorods powders were ground and mixed with deionized water in a weight ratio of 100:25 to form a paste. The paste was screen-printed on a ceramic substrate (6 mm × 3 mm, 0.5-mm thick) with five pairs of Ag–Pd interdigital electrodes (electrodes width and distance: 0.15 mm) to form a film with a thickness of about 10 ␮m, and then the film was dried in air at 60 ◦ C for 5 h. In order to improve the sensor antipollution, we made a 0.1 g of an ethyl cellulose solution in ethyl ester acetate (4 mL), which was coated on the surface of the sensitive film as a protective layer [27]. Finally, the humidity sensor was obtained after aging at 95% relative humidity (RH) with a voltage of 1 V, 100 Hz for 24 h. Fig. 1 shows the structure and optical micrograph of the sensor. The characteristic curves of humidity sensitivity were measured on a ZL-5 model LCR analyzer (Shanghai, China). The voltage applied in our studies was ac 1 V, and the frequency varied from 40 Hz to 100 kHz. The sensor was successively put into several chambers with different RH levels at a temperature of 25 ◦ C. The RH range of 11–95% was obtained using saturated salt solutions as the humidity generation sources [27]. The six different saturated salt solutions were LiCl, MgCl2 , Mg(NO3 )2 , NaCl, KCl, and KNO3 , and their corresponding RH values were 11, 33, 54, 75, 85, and 95% RH, respectively.

resolution image of the sample, indicating the flower structure composed of closely packed nanorods with lengths of 1.5–3 ␮m and diameters of 200–400 nm. The high-resolution image in Fig. 3(b) clearly reveals that the obtained ZnO exhibits well-defined flowerlike morphology and each of the rods has one end outside and another end binds to other rods. Further morphology characterization of the ZnO sample was performed on a transmission electron microscope (TEM) as shown in Fig. 3(c), which agrees with the FE-SEM results. To further determine the accurate structure of the product, the TEM–selected area electron diffraction pattern of the product was recorded as shown in Fig. 3(d). From the SAED, all the detectable dots are perfectly indexed to the same position as those from hexagonal wurtzite ZnO structure, which grows along the [0 0 0 1] direction [28]. Raman spectroscopy is also carried out to study the vibrational properties of the flower-like ZnO nanorods. Fig. 4 shows the roomtemperature Raman spectrum of the ZnO nanorods. All observed spectral peaks can be assigned to a wurtzite ZnO structure according to the literature values [29]. The peak at 437 cm−1 is attributed to the ZnO nonpolar optical phonon E2 (high) mode. The peak at 409 cm−1 corresponds to the E1 (TO) mode, but it is not obvious. As the characteristic peak of hexagonal wurtzite ZnO, the E2 (high) at 437 cm−1 is very intense and has a full width at half-maximum of 12 cm−1 . The asymmetrical and line-broadening characteristics mask E1 (TO) on the left-hand side of E2 (high). The peak at 579 cm−1 is attributed to the E1 (LO) mode, which is caused by the defects such as oxygen vacancy, zinc interstitial, or their complexes [30]. In addition, the peak at 378 cm−1 corresponds to the A1 (TO) mode. Besides these “classical” Raman modes, the Raman spectrum also shows other modes with frequencies of 333, 541, 661, and 1147 cm−1 . These

3. Results and discussion The structure of the flower-like ZnO nanorods has been characterized by XRD as shown in Fig. 2. All the diffraction peaks can be indexed as hexagonal ZnO with lattice constants a = 3.249 A˚ and ˚ which are consistent with the values in the standard c = 5.206 A, card (JCPDS 36-1451). No diffraction peaks from any other impurities are detected. Fig. 3(a) and (b) shows the FE-SEM images of the as-prepared products at different magnifications. Fig. 3(a) shows the low-

Fig. 2. XRD pattern of the flower-like ZnO nanorods.

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Fig. 3. (a) Low-resolution FE-SEM image, (b) high-resolution FE-SEM image, (c) TEM image of flower-like ZnO nanorods, and (d) SAED pattern of a single nanorod.

additional peaks cannot be explained within the framework of the bulk phonon modes, which are attributed to multiphonon scattering processes [29]. The dependence of the impedance on RH is measured for the flower-like ZnO nanorod sensor as shown in Fig. 5. From the curve, it can be clearly seen that the impedance of the film decreases remarkably with increasing the frequency at low RH, and the impedance difference between adjacent two working frequencies becomes progressively smaller with increasing RH. The reason can be explained to be that at higher frequencies, the adsorbed water cannot be polarized and the dielectric phenomenon does not appear [31]. In order to gain high RH sensitivity and good linearity over the entire RH range, low working frequency should be

applied. At a frequency of 100 Hz, the impedance change is found to be about five orders of magnitude, which is more than those of many humidity sensors reported in the literature. It is well known that response and recovery behavior is an important characteristic for evaluating the performance of humidity sensors. Fig. 6 shows response and recovery characteristic (corresponding to water molecules adsorption and desorption process) curves for one cycle based on the ZnO sensor. The response time (as the humidity changes from 11 to 95% RH) is about 5 s and the recovery time (as the humidity changes from 95 to 11% RH) is about 10 s for our sample, which is super-rapid than all the results reported before (it is important to note that the time taken by a sensor to achieve 90% of the total impedance change is defined as the response time in the case of adsorption or the recovery time in the case of desorption).

Fig. 4. Room-temperature Raman spectrum of the flower-like ZnO nanorods.

Fig. 5. Impedance vs. RH of flower-like ZnO nanorod sensor.

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Fig. 6. Response of the flower-like ZnO nanorod sensor measured at 100 Hz.

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Fig. 8. Hysteresis of the flower-like ZnO nanorod sensor measured at 100 Hz.

Fig. 7 shows the properties of capacitance versus frequency at different RH. It can be seen that the change of capacitance is inconspicuous at high frequency (10 and 100 kHz), and the large change of capacitance can be obtained at lower frequency (40 and 100 Hz). This is because the electrical field direction changes slowly at low frequency and there obviously appears the space–charge polarization of adsorbed water. The higher the RH is and the more the water molecules are adsorbed, the stronger the polarization is, and then the larger the dielectric constant is. When the frequency is high, the electrical field direction changes fast, the polarization of the water cannot catch up with it, and hence the dielectric constant is small and independent of RH [31,32]. Fig. 8 shows the humidity hysteresis characteristic of the humidity sensor based on our products. The solid line in the figure is measured from low RH to high RH, i.e. for the adsorption process, and the dotted line is for desorption process (measured in the opposite direction). A hysteresis of about 2% is observed under 80% RH. This indicates a good reliability of the sensor. Moreover, the linear dependence of the impedance on RH is observed in the range of 54–95% RH. To test the stability, the sensor was exposed in air for 30 days, followed by measuring impedances at various RH. As shown in Fig. 9, there was almost no change in the impedances, which directly confirms the good stability of the sensor. From the criteria as discussed above, the sensor has prominent stability and is quite promising for a practical humidity sensor. The analysis of complex impedance plots is useful for studying the sensing behavior of humidity sensors [33–35]. The complex impedance plots of the flower-like ZnO nanorod sensor at differ-

ent RH are shown in Fig. 10 in the frequency range from 40 Hz to 100 kHz. At low RH (11, 33, and 54%), a semicircle due to the film impedance is observed. The semicircle indicates a “non-Debye” behavior, and many investigations have explained that it is due to a kind of polarization and can be modeled by an equivalent circuit of parallel resistor and capacitor [35–37]. With increasing the RH (75, 85, and 95%), a line appears in the low-frequency range and the semicircle becomes small. The higher the RH is, the longer the line and smaller the semicircle is. The line represents Warburg impedance, and is due to the diffusion of the electroactive species at the electrodes [38–41]. The equivalent circuits of such complex impedance plots are shown in Fig. 10(b). Here Rf represents the resistance of the flower-like ZnO nanorod film, which decreases as RH increases. Cf is the capacitance of the film and Zi the impedance at the electrode/sensing film interface. According to Fig. 10(a), Rf  Zi at low RH, and the impedance change of the sensor is mostly determined by Rf . At high RH (Fig. 10(b)), the magnitudes of Rf and Zi are the same and the impedance change of the sensor is determined by both Rf and Zi . A possible qualitative mechanism to explain the humidity sensing properties of the flower-like ZnO nanorods is proposed hereafter. According to Kulwicki [41], water-related conduction in ceramic and porous materials mainly occurs as a surface mechanism. In our case, the large increase in conductivity with increasing RH of the flower-like ZnO nanorods may also relate to the adsorption of water molecules on the surface of the ZnO film. At low humidity, only a few water molecules are adsorbed. Since the cov-

Fig. 7. Capacitance vs. RH of the flower-like ZnO nanorod sensor.

Fig. 9. Stability of the flower-like ZnO nanorod sensor measured at 100 Hz.

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Fig. 10. (a) The complex impedance plots and (b) equivalent circuits of the flower-like ZnO nanorod sensor at different RH.

erage of water on the surface is not continuous, the electrolytic conduction is difficult. Based on the mechanism of Schaub et al. [42], the tips and defects of the ZnO nanorods present a high local charge density and a strong electrostatic field, which promote water dissociation. The dissociation provides protons as charge carriers of the hopping transport. At high humidity, one or several serial water layers are formed among ZnO nanorods, and electrolytic conduction between nanorods takes place along with protonic transport, and becomes dominating in the transport-process. 4. Conclusion A flower-like ZnO nanorod sensor is successfully fabricated. The sensing characteristics to RH are carefully studied. High sensitivity, rapid response and recovery are found in the investigation. These results demonstrate that flower-like ZnO nanorods can be used as the humidity sensing material for fabricating highly sensitive sensors. Acknowledgements This research was financially supported by Science and Technology Office, Jilin Province, China (Grant No. 2006528) and the Open Project of Key Laboratory of Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (Grant No. KF0706). References [1] V.V. Kovalenko, A.A. Zhukova, M.N. Rumyantseva, A.M. Gaskov, V.V. Yushchenko, I.I. Ivanova, T. Pagnier, Surface chemistry of nanocrystalline SnO2 : effect of thermal treatment and additives, Sens. Actuators B: Chem. 126 (2007) 52–55.

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Biographies Qi Qi received his BS degree from the College of Electronics Science and Engineering, Jilin University, China in 2003. He entered the PhD course in 2006, majored in microelectronics and solid state electronics. 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 from Jilin University. She was appointed as a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, she is interested in the field of sensing functional materials and gas sensors and humidity sensors. Qingjiang Yu received his MS degree from National Laboratory of Superhard Materials, Jilin University, China in 2005. He entered the PhD course in 2005, majored in condensed matter physics. Rui Wang received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2007. She entered the PhD course in 2007, majored in microelectronics and solid state electronics. Yi Zeng received his MS degree from National Laboratory of Superhard Materials, Jilin University, China in 2007. He entered the PhD course in 2007, majored in microelectronics and solid state electronics. Li Liu received her MS degree from the College of Electronics Science and Engineering, Jilin University, China in 2000. She entered the PhD course in 2002, majored in microelectronics and solid state electronics. Haibin Yang received his MS in major of materials in 1986, and PhD degree from National Laboratory of Superhard Materials, Jilin University, China in 1990. Now, he is interested in the field of functional nanomaterials.