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polyelectrolyte nanocomposite film
Sensors and Actuators B 203 (2014) 263–270
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film Dongzhi Zhang a,∗ , Jun Tong a , Bokai Xia a , Qingzhong Xue b,c a b c
College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, PR China College of Science, China University of Petroleum (East China), Qingdao 266580, PR China State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China
a r t i c l e
i n f o
Article history: Received 16 April 2014 Received in revised form 25 June 2014 Accepted 27 June 2014 Available online 7 July 2014 Keywords: Graphene oxide Nanocomposite film Humidity sensor Layer-by-layer self-assembly
a b s t r a c t A ultrahigh performance humidity sensor based on graphene oxide (GO)/poly(diallyldimethylammonium chloride) (PDDA) nanocomposite film was reported in this paper. The multilayered film of GO/PDDA was fabricated on a polyimide substrate using layer-by-layer self-assembly technique. The structures of the self-assembled films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The humidity sensing behaviors of the film sensor were investigated at room temperature over a wide range of 11–97% relative humidity. Unprecedented response of up to 265,640% was demonstrated for the presented sensor when exposed to varying relative humidity levels, which is better than that of the best conventional humidity sensor. Furthermore, the presented sensor exhibited ultrafast response and recovery times capable of monitoring human breath. Moreover, the possible humidity sensing mechanism of the proposed sensor was discussed by using complex impedance spectra and bode diagrams. This measurement results observed highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors have attracted intensive attentions due to their widely application in various fields, such as industry, medicine, agriculture and environmental monitoring [1,2]. So far, many efforts have been taken in investigations of various transduction techniques to develop humidity sensors with good sensing characteristics, including capacitance [3], resistance [4], optical fiber [5], field effect transistor (FET) [6], surface acoustic wave (SAW) [7] and quartz crystal microbalance (QCM) [8]. Furthermore, a variety of materials such as polymers [9], metal oxide [6], carbon nanotubes [3] and composites [4,7] have been employed to fabricate humidity sensors, but they are hardly to meet all the requirements desired for a humidity sensor such as high sensitivity, swift response, fast recovery, good linearity and long-term stability. Therefore, exploring a novel sensing material and method to fabricate high-performance humidity sensor is highly desirable. Recently, graphene has aroused significant interest for ultrasensitive gas detection because its atom-thick two-dimensional structure, large specific surface area (2630 m2 /g) and remarkable
∗ Corresponding author. Tel.: +86 532 86981813x426; fax: +86 532 86981335. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.snb.2014.06.116 0925-4005/© 2014 Elsevier B.V. All rights reserved.
high carrier mobility of up to 1.5 × 105 cm2 /(V s) [10–12]. Up to now, many groups have reported the promising performance of trace gases detection, such as NO2 , NH3 , NO and CO2 [13–17]. Although some investigations have reported water molecular adsorption on graphene, special problems such as long response time (3–5 min) [18], and narrow humidity detection range [19] restrict its applications. Graphene oxide (GO) as an alternatively precursor of graphene, is decorated with many oxygen functional groups on its basal planes and edges, such as hydroxyl, epoxy and carboxylic acid groups, which make GO facilitate to form film by solution-based fabrication process. These functional groups can increase the hydrophilicity of GO and consequently enhance its sensitivity to water molecular. Therefore, graphene oxide opened pathways for potential application in humidity sensors, and some demonstrations can be found in the recent progress. Zhao et al. fabricated a CMOS interdigital capacitive humidity sensor by dropcasting graphene oxide as sensing material. The sensor displayed a higher sensitivity and a faster response than that of polyimide [20]. Zhang et al. demonstrated a resistance-type humidity sensor with chemically reduced graphene oxide-based nanocomposite film, which is ease of implementation in practical electronic systems [21]. Yao et al. presented a stress-type humidity sensor based on graphene oxide-silicon bi-layer flexible structure, while graphene oxide thin films were spin-coated onto silicon microbridge as a humidity-sensing layer [22].
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In this work, we reported an ultrahigh performance humidity sensor based on graphene oxide/polyelectrolyte nanocomposite film. The film with hierarchical nanostructure was fabricated by using layer-by-layer (LbL) self-assembly approach. The sensing properties of the presented sensor were investigated over a wide range of 11–97% relative humidity at room temperature. As a result, the presented sensor exhibited unprecedented sensitivity as well as quite fast response and recovery times. Moreover, the possible humidity sensing mechanism of the humidity sensor was discussed in detail. The measurement results highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors with high performance for various applications. 2. Experiment Fig. 1. Schematic diagram of the humidity sensor.
2.1. Materials The commercially available high-purity graphene oxide (GO) nanosheets (>99%) supplied by Chengdu Organic Chemicals Co. Ltd (Chengdu, China) were employed in our experiment. The GO used was a graphene nanosheet negatively decorated with oxygen functional groups and carboxylic groups located at the sheet surface, facilitating the uniformly dispersion of GO into deionized (DI) water. The GO suspension was 0.25 wt% in concentration
at pH 4.5. Polyelectrolytes used for LbL assembly were 1.5 wt% poly(diallyldimethylammonium chloride) [PDDA (Sigma-Aldrich Inc.), molecular weight (MW) of 200–350 K, polycation] at pH 7.5 and 0.3 wt% poly(sodium 4-styrenesulfonate) [PSS (Sigma-Aldrich Inc.), MW of 70 K, polyanion] at pH 6.5 with 5 M NaCl in both for better surface coverage.
Fig. 2. Schematic diagram of layer-by-layer fabrication of GO/PDDA film along with its hierarchical structure.
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Fig. 3. Schematic diagram of humidity sensing experimental setup.
2.2. Fabrication The humidity sensor was fabricated on a flexible polyimide (PI) substrate by using microfabrication technology. Polyimide was employed here is mainly due to its more flexibility and lower cost fabrication, compared with silicon substrate. The schematic diagram of the humidity sensor is shown in Fig. 1. Two coil-like electrodes are used as interdigital electrodes (IDE) and the electrode material is Ni/Cu. The IDE pattern window provided an outline dimension of 5 mm × 5 mm, PI substrate is 75 m thick, the electrode thickness is 20 m, and the width and gap both is 75 m. Fig. 2 illustrates the layer-by-layer (LbL) self-assembly process for fabricating sensing film. First, two bi-layers of PDDA/PSS were self-assembled as precursor layers for charge enhancement, and then five bi-layers of GO/PDDA were performed by using LbL selfassembly technique. The immersing time here used was 10 min for polyelectrolytes and 15 min for GO, and intermediate rinsing with DI water and drying with N2 were required after each monolayer assembly to reinforce the interconnection between layers. Finally, the sensor was dried in the oven at 50 ◦ C for 2 h. 2.3. Instrument and analysis A schematic diagram of the experimental setup used for humidity sensing measurement is illustrated in Fig. 3. The humidity sensing properties were investigated by exposing the GO/PDDA film sensor to various relative humidity (RH) levels, which were achieved by several saturated aqueous solutions. Saturated aqueous solutions in closed vessel at a stable temperature can provide stable and controllable RH levels in their equilibrium states [21]. The experiments were performed at room temperature of 25 ◦ C. Saturated solutions of LiCl, CH3 COOK, MgCl2 , K2 CO3 , Mg(NO3 )2 , CuCl2 , NaCl, KCl and K2 SO4 in a closed vessel were used to obtain approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97% RH levels, respectively. The capacitance response of the presented film sensor was measured using TH2828 precision LCR meter, which was connected to a PC through RS-232 interface. The complex impedance spectra (CIS) of the sensor under different RH levels were measured by using HP 4194A impedance
analyzer. The response of the sensor as a function of RH was performed by exposing the sensor inside the closed vessels with different RH levels for the intake/outtake of water molecules. The figures of merit used for the evaluation of sensor performance are the normalized response (R) and sensitivity (S), determined by R = C/C0 = (Cx − C11 )/C11 × 100% and S = (Cx − C11 )/(RHx − RH11 ), where Cx and C11 are the capacitance of the sensor at the x% and 11% RH levels, respectively. 3. Results and discussion 3.1. Structure characterization The surface morphology of PDDA/PSS and GO/PDDA nanocompoiste were inspected with field emission scanning electron microscopy (FESEM, Hitachi S-4800). Fig. 4(a) presents the SEM of bi-layer PDDA/PSS with PDDA as top layer, showing a robust vermiculate pattern for the morphology of polyelectrolyte deposited, and the rough ridge structure facilitated the sequent self-assembly of GO/PDDA. Fig. 4(b) illustrates the observed morphology of GO/PDDA nanocompoiste with GO as top layer, indicating continuous and wrinkled nanostructures of GO/PDDA film consists of randomly crumpled thin sheets. In order to explore the interlayer spacing behavior of the GO film and GO/PDDA film, X-ray diffraction (XRD) analyses are performed by the X-ray diffractometer (Rigaku D/Max 2500PC, Japan) using Cu K␣ radiation with a wavelength of 1.5418 A˚ at temperature 25 ◦ C and a relative humidity of approximately 45%. Fig. 5 plots the XRD spectra for GO and GO/PDDA films. The measured XRD results show that the interlayer distance of the GO film is 7.76 A˚ (2 = 11.4◦ ), as well we find that the value of interlayer distance of the GO/PDDA film is 8.27 A˚ (2 = 10.7◦ ), which is larger than that ˚ due to the surrounding polyelectrolyte chains of GO film (7.76 A) between interlayer of GO. 3.2. Humidity-sensing properties Fig. 6 shows the capacitance-frequency characteristics of the GO/PDDA film sensor in different RH levels changed in the range of
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Fig. 6. Capacitance-frequency characteristics of the GO/PDDA film sensor at different RH levels.
Fig. 4. SEM characterization of (a) graphene oxide and (b) GO/PDDA film.
11–97%. The scanning range of frequency is from 2500 Hz to 1 MHz. For the presented GO/PDDA film sensor, the capacitance decreases with the increasing frequency at all RH levels, and the capacitance of the sensor increases with rising RH at a fixed frequency. It is
Fig. 5. XRD spectrum for GO and GO/PDDA film.
noteworthy that the capacitance-frequency curve is inclined more flatly with the increasing frequencies. This is due to the electrical field direction changes slowly at low frequencies and the spacecharge polarization of adsorbed water occurs in the film, while the polarization is hard to catch up the electrical field direction changes at high frequencies, lead to a weaker dependent of dielectric constant and capacitance on the higher frequencies [23]. Fig. 7 plots the capacitance of the film sensor vs. RH at different frequencies of 10 kHz, 30 kHz, 50 kHz and 100 kHz. Under all frequencies, it is obviously that the capacitance is monotonically increased with the increasing of RH levels. This can be interpreted as that, the absorbed water molecules are beneficial to enhance the polarization effect and increase the dielectric constant, resulting in the increase of film capacitance with rising RH [24]. Obviously, the capacitance of the sensor at 10 kHz is much higher than that at the other three frequencies. The sensor exhibits a large capacitance change from 50 pF to 133546 pF at 10 kHz when the RH rises from 11% to 97% RH, the corresponding normalized response is up to 265,640%, exhibiting an ultrahigh sensitivity of 1552.3 pF/% RH. Therefore, we selected 10 kHz as the operation frequency in our following experiments. Fig. 8 demonstrates the real-time capacitance measurement at 10 kHz for the GO/PDDA film sensor exposed to varying RH levels. The switching RH test was performed through exposure/recovery cycles for different RH environments between 11% and 23%, 33%, 43%, 52%, 67%, 75%, 85%, 97% RH. Each exposure/recovery cycle
Fig. 7. Capacitance of GO/PDDA film vs. RH at 10 kHz, 30 kHz, 50 kHz and 100 kHz
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Fig. 8. Capacitance measurement of the GO/PDDA film sensor under switching RH.
was carried out by an exposure interval of 125 s followed by a recovery interval of 125 s at 11% RH. Each cycle is indicated by the area between two closely adjacent dotted lines marking start and end. A clear increase in the real-time capacitance measurement is observed with the rising of RH in a large range of 11–97%. Fig. 9 plots the normalized response of the film sensor vs. different RH levels at 10 kHz. The response for the sensor exhibited an exponentially increase with the increasing of RH in a range of 11–97%, and the insert is the response for the sensor exposed to RH lower than 43%. The fitting equation for response Y as a function of relative humidity (RH) X can be depicted as Y = 0.00387eX/7.2162 − 0.01357, and the linear regression coefficient, R2 , is 0.9845. The error bars represent standard deviation (SD) from the mean based on five sensors exposed to given RH. Fig. 10 demonstrates the time-dependent response and recovery curves of the sensor to a RH pulse between 11% and 23%, 33%, 43%, 52%, 67%, 75%, 85%, 97% RH, respectively. As can be seen in Fig. 10, the sensor exhibits rapid response and short recovery times in the RH measurements. Motivated by the fast response of the sensor, we explored the application of this sensor in monitoring human breath. Fig. 11 demonstrated the capacitance response of the humidity sensor for human breath monitoring. The breath response characteristic for a normal adult was measured in 45 s, and 11 repetitive cycles for breathing were observed. The capacitance response exhibited sharp rise during exhaling and dropped while inhaling corresponding to the breathing cycles. It is noteworthy that the response time and recovery time for the sensor is within 1 s, allowing the capture of fine features involved moisture modulation during human breath. As is evident, the sensor is
Fig. 9. Normalized sensitivity of GO/PDDA film sensor at 10 kHz as a function of RH.
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Fig. 10. Typical response and recovery curves of the GO/PDDA film sensor to a RH pulse from 11% to other RH levels.
Fig. 11. Response characteristic of GO/PDDA film sensor to person’s breath.
suitable for various applications such as space suits, anti-choking, breathing adjustment, health care and environmental monitoring. Fig. 12 presents the long-term stability of the GO/PDDA film coated sensor. The stability of the sensor was measured over different days. The capacitance response of the sensor did not show significantly variation under different RH of 11%, 43%, 67% and 85% RH for 60 days. This results demonstrate that the sensor have good stability and is promising for practical application. Table 1 presents the humidity sensing characteristics of the proposed humidity
Fig. 12. Long-term stability of GO/PDDA film sensor exposed to 11%, 43%, 67% and 85% RH.
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Table 1 Performance of the presented sensor in this work compared with previous work. Sensor type
Sensing material
Fabrication method
Measurement range
Sensitivitya
Reference
Stress-type Quartz crystal microbalance (QCM)-type Surface acoustic wave (SAW)-type Capacitive-type Capacitive-type Capacitive-type
Graphene oxide-silicon bi-layer Graphene oxide Graphene oxide Graphene oxide Graphene oxide GO/PDDA
Spin-coating Spin-coating Droplet-by-droplet atomization Solution dripping Drop casting LbL self-assembly
10–98% RH 6.4–93.5% RH 8–18% RH 25–65% RH 15–95% RH 11–97% RH
28.02 V/% RH 22.1 Hz/% RH 1.54 kHz/% RH −9.5 fF/% RH 46.25 pF/% RH 1552.3 pF/% RH
[22] [25] [26] [20] [27] This paper
a It is calculated by (OH − OL )/(RHH − RHL ), OH and OL are the sensor output at higher RH and lower RH limits, and RHH and RHL the higher RH and lower RH in the measuring range, respectively.
sensor in comparison with previous work [20,22,25–27]. The sensing properties for the prepared sensor are comparable to those of the sensor made from graphene oxide by solution dripping, spincoating, droplet-by-droplet atomization and drop casting methods. To our knowledge, this is the humidity sensor with highest response ever reported. The comparison highlights the unique advantages of layer-by-layer self-assembled GO/PDDA nanocomposite film as an ideal candidate material for building humidity sensors. 3.3. Humidity-sensing mechanism The complex impedance spectra (CIS) were adopted to investigate the humidity sensing mechanism for the sensor. Fig. 13 shows the CIS measurement results of the sensor under different RH levels. Some of the real part and imaginary part were magnified on
Fig. 14. Equivalent circuits of GO/PDDA film sensor under different RH levels.
the same plane to compare several complex impedance plots more conveniently. To a first approximation, the CIS of the sensor can be modeled by the equivalent circuit (EC) shown in Fig. 14(a). Here Rct represents charge transfer resistance, and the constant phase elements CPE1 and CPE2 are used to consider the film impedance and electrode/sensing film interface impedance, respectively. The impedance of a constant phase element ZCPE is generally defined by [28,29] ZCPE = A−1 (j2f )
−n
(1)
and log |ZCPE | = −n log f + log
Fig. 13. Complex impedance spectra of GO/PDDA film sensor at different relative humidity. ImZ: imaginary part; ReZ: real part.
1 (2)n A
(2)
Fig. 15. Exponential behavior of the Rct parameter vs. RH at room temperature for the GO/PDDA film.
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to the permeability of water molecules on the GO/PDDA film, which gives rise to protonation and results in an increase in the density of charge carriers [30,31]. Compared with the CIS curves shown in Fig. 13, it is noteworthy that the curve shape is much different under different RH levels, indicating different humidity sensing mechanism for electrical conductivity and polarization that occurs in GO/PDDA film. At low RH (11%), the CIS curve is similar to a straight line over a frequency range of 2500 Hz–1 MHz. The fitted Rct in EC model is very large (more than 100 M) in this case, and the corresponding EC can be simplified by a CPE [28,29] as shown in Fig. 14(b). This could be proved by the corresponding bode diagram shown in Fig. 16(a), the plot of log |ZCPE | vs. log f is a straight line with a negative slope coefficient as described in Eq. (2), and the phase angle is near to but less than −90◦ . In this case, only a few water molecules are adsorbed onto the GO/PDDA film surface, the ion transport is hard to occur since the coverage of water on the film surface is not continuous. At middle RH range (23%, 33%, 43% and 52% RH), the curve of CIS for the sensor is part of semicircle, which is primarily resulted from the intrinsic impedance of the sensing film. It can be modeled by a simplified EC comprising of a resistor and a capacitor connected in parallel [32,33] as shown in Fig. 14(c). Fig. 16(b) plots the bode diagram at 52% RH. It is obvious that the phase angle increases with the increasing frequency and approach to −90◦ at high frequency region. This is a typical behavior for the RC parallel circuit. In this case, more water molecules are absorbed on the surface of GO/PDDA film and form one or several serial water layers. The protons hopping transport via ionic conductivity was generated by Grotthuss chain reaction, H2 O + H3 O+ = H3 O+ + H2 O [34]. Therefore, the transfer of H+ or H3 O+ within the serial water layers results in a swift decrease of impedance in this stage. At high RH (67%, 75%, 85% and 97% RH), the CIS exists a short straight line at the tail of the semicircle at low frequency region. As the RH increases, the straight lines become increasingly longer while part of the semicircle becomes invisible. The similar phenomena have been proved in the previous work [28,29]. The short line represents Warburg impedance, which is caused by the diffusion process of ions or charge carriers at the sensing film/electrode interface [35]. Therefore, EC shown in Fig. 14(a) can account for the sensing mechanism of GO/PDDA in this case. Fig. 16(c) plots the bode diagram at 85% RH. The phase angle increases with the increasing frequency but is far less than −90◦ at high frequency region. At high RH, the serial water layers further accelerate the transfer of protons. The water molecules permeated into the mesopores of GO/PDDA film and electrolytic conduction takes place at this stage, and hence the impedance continues to decrease sharply. 4. Conclusions
Fig. 16. Bode diagrams of GO/PDDA film sensor at different RH: (a) 11% RH, (b) 52% RH and (c) 85% RH.
where A is a real parameter, j the imaginary unit, f the operation frequency, and n a real parameter satisfies 0 ≤ n ≤ 1. The CPE behaves as pure resistor or pure capacitor when n = 0 or 1, respectively. The parameters of Rct , A1 , A2 , n1 and n2 in the EC model for different RH can be obtained by best fit of the impedance spectra data using the software Zview 3.3. In particular, we observed the dependence of Rct on RH can be approximated by an exponential relationship shown in Fig. 15, which can be expressed by log(Rct ) = −0.0522RH + 8.38, with the linear regression coefficient, R2 , is 0.9882. The humidity dependence of Rct may be contributed
In this work, an ultrahigh performance humidity sensor based on GO/PDDA nanocomposite film was fabricated by using layerby-layer self-assembly technique. The morphology and structure of the film were inspected by using SEM and XRD. The humidity sensing properties of the sensor were investigated by exposing to the wide relative humidity range of 11–97% at room temperature. As a result, the presented sensor exhibited not only unprecedented response of up to 265,640% which is better than that of the best conventional humidity sensor, but also fast response and recovery times to detect human breath. Moreover, the possible humidity sensing mechanism of the humidity sensor was discussed by using complex impedance spectra and bode diagrams. This humidity sensing results highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors with high performance for various applications.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51205414), the Promotive Research Foundation for the Excellent Middle-Aged and Youth Scientists of Shandong Province of China (Grant No. BS2012DX044), the Science and Technology Development Plan Project of Qingdao (Grant No. 13-1-4-179-jch), and the Fundamental Research Funds for the Central Universities of China (Grant No. 12CX04065A). References [1] J. Chu, X.Y. Peng, P. Feng, Y. Sheng, J.T. Zhang, Study of humidity sensors based on nanostructured carbon films produced by physical vapor deposition, Sens. Actuators B 178 (2013) 508–513. [2] U. Mogera, A.A. Sagade, S.J. George, G.U. Kulkarni, Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow, Sci. Rep. 4 (2014) 4103. [3] H.P. Hong, K.H. Jung, J.H. Kim, K.H. Kwon, C.J. Lee, K.N. Yun, N.K. Min, Percolated pore networks of oxygen plasma-activated multi-walled carbon nanotubes for fast response, high sensitivity capacitive humidity sensors, Nanotechnology 24 (2013) 085501. [4] Q.Y. Tang, Y.C. Chan, K. Zhang, Fast response resistive humidity sensitivity of polyimide/multiwall carbon nanotube composite films, Sens. Actuators B 152 (2011) 99–106. [5] W.C. Wong, C.C. Chan, L.H. Chen, T. Li, K.X. Lee, K.C. Leong, Polyvinyl alcohol coated photonic crystal optical fiber for humidity measurement, Sens. Actuators B 174 (2012) 563–569. [6] F.X. Liang, L.-B. Luo, C.-K. Tsang, L.X. Zheng, H. Cheng, Y.Y. Li, TiO2 nanotubebased field effect transistors and their application as humidity sensors, Mater. Res. Bull. 47 (2012) 54–58. [7] Y. Li, C. Deng, M.J. Yang, A novel surface acoustic wave-impedance humidity sensor based on the composite of polyaniline and poly(vinyl alcohol) with a capability of detecting low humidity, Sens. Actuators B 165 (2012) 7–12. [8] J. Xie, H. Wang, Y.H. Lin, Y. Zhou, Y.P. Wu, Highly sensitive humidity sensor based on quartz crystal microbalance coated with ZnO colloid spheres, Sens. Actuators B 177 (2013) 1083–1088. [9] M. Kulkarni, S. Apte, S. Naik, J. Ambekar, B. Kale, Ink-jet printed conducting polyaniline based flexible humidity sensor, Sens. Actuators B 178 (2013) 140–143. [10] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [11] K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200. [12] R.T. Weitz, A. Yacoby, Nanomaterials: graphene rests easy, Nat. Nanotechnol. 5 (2010) 699–700. [13] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [14] G.H. Lu, L.E. Ocola, J.H. Chen, Reduced graphene oxide for room-temperature gas sensors, Nanotechnology 20 (2009) 445502. [15] R. Pearce, T. Iakimov, M. Andersson, L. Hultman, A.L. Spetz, R. Yakimova, Epitaxially grown graphene based gas sensors for ultra sensitive NO detection, Sens. Actuators B 155 (2011) 451–455. [16] M.G. Chung, D.H. Kim, H.M. Lee, T. Kim, H.H. Choi, D.K. Seo, J.-B. Yoo, S.-H. Hong, T.J. Kang, Y.H. Kim, Highly sensitive NO2 gas sensor based on ozone treated graphene, Sens. Actuators B 166–167 (2012) 172–176. [17] H.J. Yoon, D.H. Jun, J.H. Yang, Z.X. Zhou, S.S. Yang, M.M.C. Cheng, Carbon dioxide gas sensor using a graphene sheet, Sens. Actuators B 157 (2011) 310–313. [18] A. Ghosh, D.J. Late, L.S. Panchakarla, A. Govindaraj, C.N.R. Rao, NO2 and humidity sensing characteristics of few-layer graphene, J. Exp. Nanosci. 4 (2009) 313–322. [19] Q.W. Huang, D.W. Zeng, S.Q. Tian, C.S. Xie, Synthesis of defect graphene and its application for room temperature humidity sensing, Mater. Lett. 83 (2012) 76–79. [20] C.L. Zhao, M. Qin, W.H. Li, Q.A. Huang, Enhanced performance of a CMOS interdigital capacitive humidity sensor by graphene oxide, in: Proceedings of the 16th International Solid-State Sensors, Actuators and Microsystems Conference, 2011, pp. 1954–1957. [21] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano self-assembly, Sens. Actuators B 197 (2014) 66–72. [22] Y. Yao, X.D. Chen, H.H. Guo, Z.Q. Wu, X.Y. Li, Humidity sensing behaviors of graphene oxide-silicon bi-layer flexible structure, Sens. Actuators B 161 (2012) 1053–1058. [23] S. Pokhrel, K.S. Nagaraja, Electrical and humidity sensing properties of chromium (III) oxide-tungsten (VI) oxide composites, Sens. Actuators B 92 (2003) 144–150. [24] B.H. Cheng, B.X. Tian, C.C. Xiao, Y.H. Xiao, S.J. Lei, Highly sensitive humidity sensor based on amorphous Al2 O3 nanotubes, J. Mater. Chem. 21 (2011) 1907–1912.
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Biographies
Dongzhi Zhang received his B.S. degree from Shandong University of Technology in 2004, M.S. degree from China University of Petroleum in 2007, and obtained Ph.D. degree from South China University of Technology in 2011. From 2009 to 2011, he worked as a visiting scholar of Mechanical Engineering at the University of Minnesota, U.S.A. He is currently an assistant professor at China University of Petroleum (East China), Qingdao, China. His fields of interests are gas and humidity sensing materials, nanotechnology, and polymer electronics.
Jun Tong received his B.S. degree in electrical engineering and automation from China University of Mining and Technology in 2011. Currently, he is graduate student at China University of Petroleum (East China), Qingdao, China. His fields of interests include carbon nanomaterials-based gas sensors, precision measurement technology and instruments.
BoKai Xia received his Ph.D. degree in chemical processing from China University of Petroleum in 2001. He is professor at China University of Petroleum (East China), Qingdao, China. His main areas of interest are precision measurement technology and instruments.
Qingzhong Xue received his Ph.D. degree in materials science and engineering from Tsinghua University in 2005. He is currently a professor at China University of Petroleum (East China). His research interest includes the fabrication and characterization of film materials, carbon nanotube/graphene-polymer composites, as well as to exploit their potential applications.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film Dongzhi Zhang a,∗ , Jun Tong a , Bokai Xia a , Qingzhong Xue b,c a b c
College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, PR China College of Science, China University of Petroleum (East China), Qingdao 266580, PR China State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China
a r t i c l e
i n f o
Article history: Received 16 April 2014 Received in revised form 25 June 2014 Accepted 27 June 2014 Available online 7 July 2014 Keywords: Graphene oxide Nanocomposite film Humidity sensor Layer-by-layer self-assembly
a b s t r a c t A ultrahigh performance humidity sensor based on graphene oxide (GO)/poly(diallyldimethylammonium chloride) (PDDA) nanocomposite film was reported in this paper. The multilayered film of GO/PDDA was fabricated on a polyimide substrate using layer-by-layer self-assembly technique. The structures of the self-assembled films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The humidity sensing behaviors of the film sensor were investigated at room temperature over a wide range of 11–97% relative humidity. Unprecedented response of up to 265,640% was demonstrated for the presented sensor when exposed to varying relative humidity levels, which is better than that of the best conventional humidity sensor. Furthermore, the presented sensor exhibited ultrafast response and recovery times capable of monitoring human breath. Moreover, the possible humidity sensing mechanism of the proposed sensor was discussed by using complex impedance spectra and bode diagrams. This measurement results observed highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Humidity sensors have attracted intensive attentions due to their widely application in various fields, such as industry, medicine, agriculture and environmental monitoring [1,2]. So far, many efforts have been taken in investigations of various transduction techniques to develop humidity sensors with good sensing characteristics, including capacitance [3], resistance [4], optical fiber [5], field effect transistor (FET) [6], surface acoustic wave (SAW) [7] and quartz crystal microbalance (QCM) [8]. Furthermore, a variety of materials such as polymers [9], metal oxide [6], carbon nanotubes [3] and composites [4,7] have been employed to fabricate humidity sensors, but they are hardly to meet all the requirements desired for a humidity sensor such as high sensitivity, swift response, fast recovery, good linearity and long-term stability. Therefore, exploring a novel sensing material and method to fabricate high-performance humidity sensor is highly desirable. Recently, graphene has aroused significant interest for ultrasensitive gas detection because its atom-thick two-dimensional structure, large specific surface area (2630 m2 /g) and remarkable
∗ Corresponding author. Tel.: +86 532 86981813x426; fax: +86 532 86981335. E-mail address: [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.snb.2014.06.116 0925-4005/© 2014 Elsevier B.V. All rights reserved.
high carrier mobility of up to 1.5 × 105 cm2 /(V s) [10–12]. Up to now, many groups have reported the promising performance of trace gases detection, such as NO2 , NH3 , NO and CO2 [13–17]. Although some investigations have reported water molecular adsorption on graphene, special problems such as long response time (3–5 min) [18], and narrow humidity detection range [19] restrict its applications. Graphene oxide (GO) as an alternatively precursor of graphene, is decorated with many oxygen functional groups on its basal planes and edges, such as hydroxyl, epoxy and carboxylic acid groups, which make GO facilitate to form film by solution-based fabrication process. These functional groups can increase the hydrophilicity of GO and consequently enhance its sensitivity to water molecular. Therefore, graphene oxide opened pathways for potential application in humidity sensors, and some demonstrations can be found in the recent progress. Zhao et al. fabricated a CMOS interdigital capacitive humidity sensor by dropcasting graphene oxide as sensing material. The sensor displayed a higher sensitivity and a faster response than that of polyimide [20]. Zhang et al. demonstrated a resistance-type humidity sensor with chemically reduced graphene oxide-based nanocomposite film, which is ease of implementation in practical electronic systems [21]. Yao et al. presented a stress-type humidity sensor based on graphene oxide-silicon bi-layer flexible structure, while graphene oxide thin films were spin-coated onto silicon microbridge as a humidity-sensing layer [22].
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In this work, we reported an ultrahigh performance humidity sensor based on graphene oxide/polyelectrolyte nanocomposite film. The film with hierarchical nanostructure was fabricated by using layer-by-layer (LbL) self-assembly approach. The sensing properties of the presented sensor were investigated over a wide range of 11–97% relative humidity at room temperature. As a result, the presented sensor exhibited unprecedented sensitivity as well as quite fast response and recovery times. Moreover, the possible humidity sensing mechanism of the humidity sensor was discussed in detail. The measurement results highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors with high performance for various applications. 2. Experiment Fig. 1. Schematic diagram of the humidity sensor.
2.1. Materials The commercially available high-purity graphene oxide (GO) nanosheets (>99%) supplied by Chengdu Organic Chemicals Co. Ltd (Chengdu, China) were employed in our experiment. The GO used was a graphene nanosheet negatively decorated with oxygen functional groups and carboxylic groups located at the sheet surface, facilitating the uniformly dispersion of GO into deionized (DI) water. The GO suspension was 0.25 wt% in concentration
at pH 4.5. Polyelectrolytes used for LbL assembly were 1.5 wt% poly(diallyldimethylammonium chloride) [PDDA (Sigma-Aldrich Inc.), molecular weight (MW) of 200–350 K, polycation] at pH 7.5 and 0.3 wt% poly(sodium 4-styrenesulfonate) [PSS (Sigma-Aldrich Inc.), MW of 70 K, polyanion] at pH 6.5 with 5 M NaCl in both for better surface coverage.
Fig. 2. Schematic diagram of layer-by-layer fabrication of GO/PDDA film along with its hierarchical structure.
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Fig. 3. Schematic diagram of humidity sensing experimental setup.
2.2. Fabrication The humidity sensor was fabricated on a flexible polyimide (PI) substrate by using microfabrication technology. Polyimide was employed here is mainly due to its more flexibility and lower cost fabrication, compared with silicon substrate. The schematic diagram of the humidity sensor is shown in Fig. 1. Two coil-like electrodes are used as interdigital electrodes (IDE) and the electrode material is Ni/Cu. The IDE pattern window provided an outline dimension of 5 mm × 5 mm, PI substrate is 75 m thick, the electrode thickness is 20 m, and the width and gap both is 75 m. Fig. 2 illustrates the layer-by-layer (LbL) self-assembly process for fabricating sensing film. First, two bi-layers of PDDA/PSS were self-assembled as precursor layers for charge enhancement, and then five bi-layers of GO/PDDA were performed by using LbL selfassembly technique. The immersing time here used was 10 min for polyelectrolytes and 15 min for GO, and intermediate rinsing with DI water and drying with N2 were required after each monolayer assembly to reinforce the interconnection between layers. Finally, the sensor was dried in the oven at 50 ◦ C for 2 h. 2.3. Instrument and analysis A schematic diagram of the experimental setup used for humidity sensing measurement is illustrated in Fig. 3. The humidity sensing properties were investigated by exposing the GO/PDDA film sensor to various relative humidity (RH) levels, which were achieved by several saturated aqueous solutions. Saturated aqueous solutions in closed vessel at a stable temperature can provide stable and controllable RH levels in their equilibrium states [21]. The experiments were performed at room temperature of 25 ◦ C. Saturated solutions of LiCl, CH3 COOK, MgCl2 , K2 CO3 , Mg(NO3 )2 , CuCl2 , NaCl, KCl and K2 SO4 in a closed vessel were used to obtain approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97% RH levels, respectively. The capacitance response of the presented film sensor was measured using TH2828 precision LCR meter, which was connected to a PC through RS-232 interface. The complex impedance spectra (CIS) of the sensor under different RH levels were measured by using HP 4194A impedance
analyzer. The response of the sensor as a function of RH was performed by exposing the sensor inside the closed vessels with different RH levels for the intake/outtake of water molecules. The figures of merit used for the evaluation of sensor performance are the normalized response (R) and sensitivity (S), determined by R = C/C0 = (Cx − C11 )/C11 × 100% and S = (Cx − C11 )/(RHx − RH11 ), where Cx and C11 are the capacitance of the sensor at the x% and 11% RH levels, respectively. 3. Results and discussion 3.1. Structure characterization The surface morphology of PDDA/PSS and GO/PDDA nanocompoiste were inspected with field emission scanning electron microscopy (FESEM, Hitachi S-4800). Fig. 4(a) presents the SEM of bi-layer PDDA/PSS with PDDA as top layer, showing a robust vermiculate pattern for the morphology of polyelectrolyte deposited, and the rough ridge structure facilitated the sequent self-assembly of GO/PDDA. Fig. 4(b) illustrates the observed morphology of GO/PDDA nanocompoiste with GO as top layer, indicating continuous and wrinkled nanostructures of GO/PDDA film consists of randomly crumpled thin sheets. In order to explore the interlayer spacing behavior of the GO film and GO/PDDA film, X-ray diffraction (XRD) analyses are performed by the X-ray diffractometer (Rigaku D/Max 2500PC, Japan) using Cu K␣ radiation with a wavelength of 1.5418 A˚ at temperature 25 ◦ C and a relative humidity of approximately 45%. Fig. 5 plots the XRD spectra for GO and GO/PDDA films. The measured XRD results show that the interlayer distance of the GO film is 7.76 A˚ (2 = 11.4◦ ), as well we find that the value of interlayer distance of the GO/PDDA film is 8.27 A˚ (2 = 10.7◦ ), which is larger than that ˚ due to the surrounding polyelectrolyte chains of GO film (7.76 A) between interlayer of GO. 3.2. Humidity-sensing properties Fig. 6 shows the capacitance-frequency characteristics of the GO/PDDA film sensor in different RH levels changed in the range of
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Fig. 6. Capacitance-frequency characteristics of the GO/PDDA film sensor at different RH levels.
Fig. 4. SEM characterization of (a) graphene oxide and (b) GO/PDDA film.
11–97%. The scanning range of frequency is from 2500 Hz to 1 MHz. For the presented GO/PDDA film sensor, the capacitance decreases with the increasing frequency at all RH levels, and the capacitance of the sensor increases with rising RH at a fixed frequency. It is
Fig. 5. XRD spectrum for GO and GO/PDDA film.
noteworthy that the capacitance-frequency curve is inclined more flatly with the increasing frequencies. This is due to the electrical field direction changes slowly at low frequencies and the spacecharge polarization of adsorbed water occurs in the film, while the polarization is hard to catch up the electrical field direction changes at high frequencies, lead to a weaker dependent of dielectric constant and capacitance on the higher frequencies [23]. Fig. 7 plots the capacitance of the film sensor vs. RH at different frequencies of 10 kHz, 30 kHz, 50 kHz and 100 kHz. Under all frequencies, it is obviously that the capacitance is monotonically increased with the increasing of RH levels. This can be interpreted as that, the absorbed water molecules are beneficial to enhance the polarization effect and increase the dielectric constant, resulting in the increase of film capacitance with rising RH [24]. Obviously, the capacitance of the sensor at 10 kHz is much higher than that at the other three frequencies. The sensor exhibits a large capacitance change from 50 pF to 133546 pF at 10 kHz when the RH rises from 11% to 97% RH, the corresponding normalized response is up to 265,640%, exhibiting an ultrahigh sensitivity of 1552.3 pF/% RH. Therefore, we selected 10 kHz as the operation frequency in our following experiments. Fig. 8 demonstrates the real-time capacitance measurement at 10 kHz for the GO/PDDA film sensor exposed to varying RH levels. The switching RH test was performed through exposure/recovery cycles for different RH environments between 11% and 23%, 33%, 43%, 52%, 67%, 75%, 85%, 97% RH. Each exposure/recovery cycle
Fig. 7. Capacitance of GO/PDDA film vs. RH at 10 kHz, 30 kHz, 50 kHz and 100 kHz
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Fig. 8. Capacitance measurement of the GO/PDDA film sensor under switching RH.
was carried out by an exposure interval of 125 s followed by a recovery interval of 125 s at 11% RH. Each cycle is indicated by the area between two closely adjacent dotted lines marking start and end. A clear increase in the real-time capacitance measurement is observed with the rising of RH in a large range of 11–97%. Fig. 9 plots the normalized response of the film sensor vs. different RH levels at 10 kHz. The response for the sensor exhibited an exponentially increase with the increasing of RH in a range of 11–97%, and the insert is the response for the sensor exposed to RH lower than 43%. The fitting equation for response Y as a function of relative humidity (RH) X can be depicted as Y = 0.00387eX/7.2162 − 0.01357, and the linear regression coefficient, R2 , is 0.9845. The error bars represent standard deviation (SD) from the mean based on five sensors exposed to given RH. Fig. 10 demonstrates the time-dependent response and recovery curves of the sensor to a RH pulse between 11% and 23%, 33%, 43%, 52%, 67%, 75%, 85%, 97% RH, respectively. As can be seen in Fig. 10, the sensor exhibits rapid response and short recovery times in the RH measurements. Motivated by the fast response of the sensor, we explored the application of this sensor in monitoring human breath. Fig. 11 demonstrated the capacitance response of the humidity sensor for human breath monitoring. The breath response characteristic for a normal adult was measured in 45 s, and 11 repetitive cycles for breathing were observed. The capacitance response exhibited sharp rise during exhaling and dropped while inhaling corresponding to the breathing cycles. It is noteworthy that the response time and recovery time for the sensor is within 1 s, allowing the capture of fine features involved moisture modulation during human breath. As is evident, the sensor is
Fig. 9. Normalized sensitivity of GO/PDDA film sensor at 10 kHz as a function of RH.
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Fig. 10. Typical response and recovery curves of the GO/PDDA film sensor to a RH pulse from 11% to other RH levels.
Fig. 11. Response characteristic of GO/PDDA film sensor to person’s breath.
suitable for various applications such as space suits, anti-choking, breathing adjustment, health care and environmental monitoring. Fig. 12 presents the long-term stability of the GO/PDDA film coated sensor. The stability of the sensor was measured over different days. The capacitance response of the sensor did not show significantly variation under different RH of 11%, 43%, 67% and 85% RH for 60 days. This results demonstrate that the sensor have good stability and is promising for practical application. Table 1 presents the humidity sensing characteristics of the proposed humidity
Fig. 12. Long-term stability of GO/PDDA film sensor exposed to 11%, 43%, 67% and 85% RH.
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Table 1 Performance of the presented sensor in this work compared with previous work. Sensor type
Sensing material
Fabrication method
Measurement range
Sensitivitya
Reference
Stress-type Quartz crystal microbalance (QCM)-type Surface acoustic wave (SAW)-type Capacitive-type Capacitive-type Capacitive-type
Graphene oxide-silicon bi-layer Graphene oxide Graphene oxide Graphene oxide Graphene oxide GO/PDDA
Spin-coating Spin-coating Droplet-by-droplet atomization Solution dripping Drop casting LbL self-assembly
10–98% RH 6.4–93.5% RH 8–18% RH 25–65% RH 15–95% RH 11–97% RH
28.02 V/% RH 22.1 Hz/% RH 1.54 kHz/% RH −9.5 fF/% RH 46.25 pF/% RH 1552.3 pF/% RH
[22] [25] [26] [20] [27] This paper
a It is calculated by (OH − OL )/(RHH − RHL ), OH and OL are the sensor output at higher RH and lower RH limits, and RHH and RHL the higher RH and lower RH in the measuring range, respectively.
sensor in comparison with previous work [20,22,25–27]. The sensing properties for the prepared sensor are comparable to those of the sensor made from graphene oxide by solution dripping, spincoating, droplet-by-droplet atomization and drop casting methods. To our knowledge, this is the humidity sensor with highest response ever reported. The comparison highlights the unique advantages of layer-by-layer self-assembled GO/PDDA nanocomposite film as an ideal candidate material for building humidity sensors. 3.3. Humidity-sensing mechanism The complex impedance spectra (CIS) were adopted to investigate the humidity sensing mechanism for the sensor. Fig. 13 shows the CIS measurement results of the sensor under different RH levels. Some of the real part and imaginary part were magnified on
Fig. 14. Equivalent circuits of GO/PDDA film sensor under different RH levels.
the same plane to compare several complex impedance plots more conveniently. To a first approximation, the CIS of the sensor can be modeled by the equivalent circuit (EC) shown in Fig. 14(a). Here Rct represents charge transfer resistance, and the constant phase elements CPE1 and CPE2 are used to consider the film impedance and electrode/sensing film interface impedance, respectively. The impedance of a constant phase element ZCPE is generally defined by [28,29] ZCPE = A−1 (j2f )
−n
(1)
and log |ZCPE | = −n log f + log
Fig. 13. Complex impedance spectra of GO/PDDA film sensor at different relative humidity. ImZ: imaginary part; ReZ: real part.
1 (2)n A
(2)
Fig. 15. Exponential behavior of the Rct parameter vs. RH at room temperature for the GO/PDDA film.
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to the permeability of water molecules on the GO/PDDA film, which gives rise to protonation and results in an increase in the density of charge carriers [30,31]. Compared with the CIS curves shown in Fig. 13, it is noteworthy that the curve shape is much different under different RH levels, indicating different humidity sensing mechanism for electrical conductivity and polarization that occurs in GO/PDDA film. At low RH (11%), the CIS curve is similar to a straight line over a frequency range of 2500 Hz–1 MHz. The fitted Rct in EC model is very large (more than 100 M) in this case, and the corresponding EC can be simplified by a CPE [28,29] as shown in Fig. 14(b). This could be proved by the corresponding bode diagram shown in Fig. 16(a), the plot of log |ZCPE | vs. log f is a straight line with a negative slope coefficient as described in Eq. (2), and the phase angle is near to but less than −90◦ . In this case, only a few water molecules are adsorbed onto the GO/PDDA film surface, the ion transport is hard to occur since the coverage of water on the film surface is not continuous. At middle RH range (23%, 33%, 43% and 52% RH), the curve of CIS for the sensor is part of semicircle, which is primarily resulted from the intrinsic impedance of the sensing film. It can be modeled by a simplified EC comprising of a resistor and a capacitor connected in parallel [32,33] as shown in Fig. 14(c). Fig. 16(b) plots the bode diagram at 52% RH. It is obvious that the phase angle increases with the increasing frequency and approach to −90◦ at high frequency region. This is a typical behavior for the RC parallel circuit. In this case, more water molecules are absorbed on the surface of GO/PDDA film and form one or several serial water layers. The protons hopping transport via ionic conductivity was generated by Grotthuss chain reaction, H2 O + H3 O+ = H3 O+ + H2 O [34]. Therefore, the transfer of H+ or H3 O+ within the serial water layers results in a swift decrease of impedance in this stage. At high RH (67%, 75%, 85% and 97% RH), the CIS exists a short straight line at the tail of the semicircle at low frequency region. As the RH increases, the straight lines become increasingly longer while part of the semicircle becomes invisible. The similar phenomena have been proved in the previous work [28,29]. The short line represents Warburg impedance, which is caused by the diffusion process of ions or charge carriers at the sensing film/electrode interface [35]. Therefore, EC shown in Fig. 14(a) can account for the sensing mechanism of GO/PDDA in this case. Fig. 16(c) plots the bode diagram at 85% RH. The phase angle increases with the increasing frequency but is far less than −90◦ at high frequency region. At high RH, the serial water layers further accelerate the transfer of protons. The water molecules permeated into the mesopores of GO/PDDA film and electrolytic conduction takes place at this stage, and hence the impedance continues to decrease sharply. 4. Conclusions
Fig. 16. Bode diagrams of GO/PDDA film sensor at different RH: (a) 11% RH, (b) 52% RH and (c) 85% RH.
where A is a real parameter, j the imaginary unit, f the operation frequency, and n a real parameter satisfies 0 ≤ n ≤ 1. The CPE behaves as pure resistor or pure capacitor when n = 0 or 1, respectively. The parameters of Rct , A1 , A2 , n1 and n2 in the EC model for different RH can be obtained by best fit of the impedance spectra data using the software Zview 3.3. In particular, we observed the dependence of Rct on RH can be approximated by an exponential relationship shown in Fig. 15, which can be expressed by log(Rct ) = −0.0522RH + 8.38, with the linear regression coefficient, R2 , is 0.9882. The humidity dependence of Rct may be contributed
In this work, an ultrahigh performance humidity sensor based on GO/PDDA nanocomposite film was fabricated by using layerby-layer self-assembly technique. The morphology and structure of the film were inspected by using SEM and XRD. The humidity sensing properties of the sensor were investigated by exposing to the wide relative humidity range of 11–97% at room temperature. As a result, the presented sensor exhibited not only unprecedented response of up to 265,640% which is better than that of the best conventional humidity sensor, but also fast response and recovery times to detect human breath. Moreover, the possible humidity sensing mechanism of the humidity sensor was discussed by using complex impedance spectra and bode diagrams. This humidity sensing results highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors with high performance for various applications.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51205414), the Promotive Research Foundation for the Excellent Middle-Aged and Youth Scientists of Shandong Province of China (Grant No. BS2012DX044), the Science and Technology Development Plan Project of Qingdao (Grant No. 13-1-4-179-jch), and the Fundamental Research Funds for the Central Universities of China (Grant No. 12CX04065A). References [1] J. Chu, X.Y. Peng, P. Feng, Y. Sheng, J.T. Zhang, Study of humidity sensors based on nanostructured carbon films produced by physical vapor deposition, Sens. Actuators B 178 (2013) 508–513. [2] U. Mogera, A.A. Sagade, S.J. George, G.U. Kulkarni, Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow, Sci. Rep. 4 (2014) 4103. [3] H.P. Hong, K.H. Jung, J.H. Kim, K.H. Kwon, C.J. Lee, K.N. Yun, N.K. Min, Percolated pore networks of oxygen plasma-activated multi-walled carbon nanotubes for fast response, high sensitivity capacitive humidity sensors, Nanotechnology 24 (2013) 085501. [4] Q.Y. Tang, Y.C. Chan, K. Zhang, Fast response resistive humidity sensitivity of polyimide/multiwall carbon nanotube composite films, Sens. Actuators B 152 (2011) 99–106. [5] W.C. Wong, C.C. Chan, L.H. Chen, T. Li, K.X. Lee, K.C. Leong, Polyvinyl alcohol coated photonic crystal optical fiber for humidity measurement, Sens. Actuators B 174 (2012) 563–569. [6] F.X. Liang, L.-B. Luo, C.-K. Tsang, L.X. Zheng, H. Cheng, Y.Y. Li, TiO2 nanotubebased field effect transistors and their application as humidity sensors, Mater. Res. Bull. 47 (2012) 54–58. [7] Y. Li, C. Deng, M.J. Yang, A novel surface acoustic wave-impedance humidity sensor based on the composite of polyaniline and poly(vinyl alcohol) with a capability of detecting low humidity, Sens. Actuators B 165 (2012) 7–12. [8] J. Xie, H. Wang, Y.H. Lin, Y. Zhou, Y.P. Wu, Highly sensitive humidity sensor based on quartz crystal microbalance coated with ZnO colloid spheres, Sens. Actuators B 177 (2013) 1083–1088. [9] M. Kulkarni, S. Apte, S. Naik, J. Ambekar, B. Kale, Ink-jet printed conducting polyaniline based flexible humidity sensor, Sens. Actuators B 178 (2013) 140–143. [10] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [11] K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200. [12] R.T. Weitz, A. Yacoby, Nanomaterials: graphene rests easy, Nat. Nanotechnol. 5 (2010) 699–700. [13] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [14] G.H. Lu, L.E. Ocola, J.H. Chen, Reduced graphene oxide for room-temperature gas sensors, Nanotechnology 20 (2009) 445502. [15] R. Pearce, T. Iakimov, M. Andersson, L. Hultman, A.L. Spetz, R. Yakimova, Epitaxially grown graphene based gas sensors for ultra sensitive NO detection, Sens. Actuators B 155 (2011) 451–455. [16] M.G. Chung, D.H. Kim, H.M. Lee, T. Kim, H.H. Choi, D.K. Seo, J.-B. Yoo, S.-H. Hong, T.J. Kang, Y.H. Kim, Highly sensitive NO2 gas sensor based on ozone treated graphene, Sens. Actuators B 166–167 (2012) 172–176. [17] H.J. Yoon, D.H. Jun, J.H. Yang, Z.X. Zhou, S.S. Yang, M.M.C. Cheng, Carbon dioxide gas sensor using a graphene sheet, Sens. Actuators B 157 (2011) 310–313. [18] A. Ghosh, D.J. Late, L.S. Panchakarla, A. Govindaraj, C.N.R. Rao, NO2 and humidity sensing characteristics of few-layer graphene, J. Exp. Nanosci. 4 (2009) 313–322. [19] Q.W. Huang, D.W. Zeng, S.Q. Tian, C.S. Xie, Synthesis of defect graphene and its application for room temperature humidity sensing, Mater. Lett. 83 (2012) 76–79. [20] C.L. Zhao, M. Qin, W.H. Li, Q.A. Huang, Enhanced performance of a CMOS interdigital capacitive humidity sensor by graphene oxide, in: Proceedings of the 16th International Solid-State Sensors, Actuators and Microsystems Conference, 2011, pp. 1954–1957. [21] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano self-assembly, Sens. Actuators B 197 (2014) 66–72. [22] Y. Yao, X.D. Chen, H.H. Guo, Z.Q. Wu, X.Y. Li, Humidity sensing behaviors of graphene oxide-silicon bi-layer flexible structure, Sens. Actuators B 161 (2012) 1053–1058. [23] S. Pokhrel, K.S. Nagaraja, Electrical and humidity sensing properties of chromium (III) oxide-tungsten (VI) oxide composites, Sens. Actuators B 92 (2003) 144–150. [24] B.H. Cheng, B.X. Tian, C.C. Xiao, Y.H. Xiao, S.J. Lei, Highly sensitive humidity sensor based on amorphous Al2 O3 nanotubes, J. Mater. Chem. 21 (2011) 1907–1912.
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Biographies
Dongzhi Zhang received his B.S. degree from Shandong University of Technology in 2004, M.S. degree from China University of Petroleum in 2007, and obtained Ph.D. degree from South China University of Technology in 2011. From 2009 to 2011, he worked as a visiting scholar of Mechanical Engineering at the University of Minnesota, U.S.A. He is currently an assistant professor at China University of Petroleum (East China), Qingdao, China. His fields of interests are gas and humidity sensing materials, nanotechnology, and polymer electronics.
Jun Tong received his B.S. degree in electrical engineering and automation from China University of Mining and Technology in 2011. Currently, he is graduate student at China University of Petroleum (East China), Qingdao, China. His fields of interests include carbon nanomaterials-based gas sensors, precision measurement technology and instruments.
BoKai Xia received his Ph.D. degree in chemical processing from China University of Petroleum in 2001. He is professor at China University of Petroleum (East China), Qingdao, China. His main areas of interest are precision measurement technology and instruments.
Qingzhong Xue received his Ph.D. degree in materials science and engineering from Tsinghua University in 2005. He is currently a professor at China University of Petroleum (East China). His research interest includes the fabrication and characterization of film materials, carbon nanotube/graphene-polymer composites, as well as to exploit their potential applications.