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single-walled carbon nanotube composite film
Sensors & Actuators: B. Chemical 283 (2019) 786–792
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Chemiresistive humidity sensor based on chitosan/zinc oxide/single-walled carbon nanotube composite film ⁎
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Haipo Dai, Nana Feng, Jiwei Li, Jie Zhang , Wei Li
Department of Material Science, Taiyuan University of Technology, Taiyuan 030024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chitosan Carbon nanotubes Zinc oxide Humidity sensor
A chemiresistive humidity sensor based on chitosan (CS)/zinc oxide/single-walled carbon nanotube (SWCNT) composite was developed. The sensor film was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The humidity sensing properties were investigated in a wide range of relative humidity (RH) (11%–97%) at room temperature. The SWCNT response to humidity was significantly improved with the aid of conjugate material in the composite, especially the CS. The composite also exhibited good reproducibility. The RH-dependent relationship revealed that the sensing response corresponded to the water sorption characteristic of CS. The sensing mechanism was attributed to the swelling effect of CS surrounding the nanotubes through which the hopping conduction path between nanotubes changed.
1. Introduction Chemiresistive humidity sensor, which depends on the change in electrical resistance, is important for application in many fields, such as atmosphere monitoring, industry, manufactory, and agriculture. Carbon nanotubes (CNTs) are regarded as distinct candidate materials for chemiresistive humidity sensors due to their large surface-to-volume ratios and good electron transport property [1]. However, the weak interaction between water molecules and pristine CNTs is a main limitation in obtaining high-performance humidity sensor, although highly porous CNT films can be prepared for free diffusion and adsorption of water molecule [2]. Generally, in addition to structural optimization, sensing material modifications, such as doping [3], composite formation [4], and chemical surface treatment [5], are effective ways to enhance sensing properties. Inorganic nanoparticle/CNT hybrids have been used as humidity sensors in recent years because of their improved electrical and mechanical properties; such improvement is due to the synergetic effect resulting from the combination of components [6]. For example, MnO2-coated CNT shows better response than that of uncoated CNT [7], and iron oxide/gold nanoparticle-decorated CNTs as humidity sensors exhibit high selectivity [8]. Despite the enhanced humidity sensing performance against pure CNTs, inorganic nanoparticle-functionalized CNTs still suffer from low sensitivity. Therefore, the interaction between the sensor and water molecules should be further strengthened. Chitosan (CS), a common type of polysaccharide that contains many
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functional groups, such as hydroxyl (eOH) and amino (eNH2), displays sufficient affinity to small analytes. Furthermore, CS is soluble in acidic media; thus, it can be simply converted to film form. These advantages make CS a highly suitable modification material to improve the performance of chemiresistive gas sensor, such as CNTs [2], graphene [9], and conducting polymers [10]. Particularly, recent theoretical and experimental studies have demonstrated that CS presents not only reversible water adsorption ability but also good water permeability [11,12], thereby indicating that CS can be applied in humidity sensors. Nevertheless, only few studies have focused on CS-modified chemiresistive humidity sensor. Therefore, in this report, ZnO, a representative and typical inorganic gas sensing material [13], was used as the first functional component to enhance the humidity sensing performance of single-walled CNT (SWCNT). Then, CS was utilized as the second functional component to continue enhancing the humidity sensing performance of the main sensing body of ZnO/SWCNT hybrid. 2. Experimental 2.1. Materials and reagents SWCNTs with carbon content of 95% were purchased from Dekedaojin Science and Technology Co., Ltd. (Beijing, China) and chemically treated with concentrated acid prior to use. All chemicals and solvents, such as CS, zinc chloride, ammonia, and acetic acid, were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (W. Li).
https://doi.org/10.1016/j.snb.2018.12.056 Received 20 May 2018; Received in revised form 9 December 2018; Accepted 10 December 2018 Available online 18 December 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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China). These reagents possessed the highest commercially pure grade, and they were used without further purification. 2.2. Fabrication of ZnO/SWCNT hybrids The pretreated SWCNTs were dispersed in 50 ml of zinc chloride solution by ultrasonication at 60 °C for 2 h. Afterward, ammonia was slowly added into this solution until the pH value of approximately 9 was reached. After continued ultrasonication for 2 h, the solution was centrifuged. The sediment was washed several times with deionized water and subsequently dried at 60 °C in vacuum. Finally, the powder was annealed at 250 °C in a quartz tube furnace in air for 2 h to produce ZnO/SWCNT hybrids. The mass ratio of ZnO to SWCNT was approximately 1:2.
2.3. Fabrication of humidity sensor Fig. 1. XRD patterns of SWCNT, ZnO, and ZnO/SWCNT hybrid.
About 1 mg of ZnO/SWCNT hybrid was dispersed in 5 ml of water, and the solution was ultrasonicated for 2 h to disaggregate the agglomerations and obtain uniform suspensions. Afterward, 10 μl of solution was dropped into a silver interdigital electrode-covered alumina substrate (3.5 × 7.0 mm) by using a microliter pipette. The sensing body of ZnO/SWCNT hybrid was fabricated after solvent evaporation. Finally, 20 μl of CS solution was casted onto the above substrate and dried at 60 °C to produce CS/ZnO/SWCNT composite humidity sensor. The pre-prepared CS solution was obtained by dissolving appropriate amount of CS in 1% acetic acid solution. The CS concentrations were controlled at 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1.00 wt% for comparison, and the related sensors were named as CZS1, CZS2, CZS3, and CZS4, respectively. Other two humidity sensors, SWCNT and CS/ SWCNT composite, were also prepared for comparison. The preparation of SWCNT sensor was the same as the above process of ZnO/SWCNT hybrid sensor. CS/SWCNT composite sensor was prepared by casting CS solution (20 μl, 0.75 wt%) onto SWCNT sensor.
3. Results and discussion 3.1. Characterization of materials XRD patterns are shown in Fig. 1. Two typical peaks at 25.8° and 42.1°, which can be commonly indexed on the basis of the hexagonal close-packed graphite or turbostratic graphite [14], are related to the (002) and (100) planes of SWCNT, respectively. Except for the peak at 25.8°, all the marked peaks in the pattern of ZnO/SWCNT hybrid can be indexed to the hexagonal wurtzite structure of ZnO [15]. Specifically, the strong peaks at 31.9°, 34.5°, and 36.4° correspond to the (100), (002), and (101) diffraction planes, respectively. The peaks at 47.7°, 56.7°, 63.1°, and 68.3° can be assigned to (102), (110), (103), and (112) diffraction planes, respectively. The average crystallite size of ZnO estimated by Scherrer equation is approximately 15 nm. Moreover, under the resolution of the current apparatus, no impurity phase occurs in the XRD patterns. SEM images of ZnO/SWCNT hybrid and CS/ZnO/SWCNT composite are presented in Fig. 2. Dense ZnO/SWCNT hybrid clusters can be observed in Fig. 2a. High-magnification image shows that SWCNT bundles are entangled with each other (Fig. 2b). These entanglements can form a conductive network with high porosity, and such porous structure is suitable for molecular adsorption and electron transport [16]. When CS is cast onto the substrate, this ZnO/SWCNT hybrid porous structure is immersed in a flat CS film (Fig. 2c and d). Inset picture of Fig. 2c confirms the formation of CS/ZnO/SWCNT composite instead of layered structure after the drop-casting. The FT-IR spectra are displayed in Fig. 3. The broad peak at 3450 cm−1 is related to the overlapped −OH and –NH groups in CS. The peaks occurring at 2920 and 1633 cm−1 are attributed to CeH vibration mode and CeO stretching mode, respectively. The peak at 1409 cm−1 is a characteristic of the −CH2– group, and the peak at 1045 cm−1 is related to the CHeOH bond in cyclic compounds. The appearance of some of the above characteristic peaks in the spectra of SWCNT and ZnO/SWCNT hybrid is mainly attributed to the concentrated acid pre-treatment process through which many functional groups such as hydroxyl and carboxyl group may graft on the wall of SWCNT. The weak peak at 451 cm−1 corresponds to the stretching mode of Zn–O [15], which indicates the presence of crystalline ZnO. ZnO/SWCNT hybrid film shows a resistance value similar to that of pure SWCNT film (Fig. 4). The resistance of these two films measured between electrodes is about several hundred ohms. The resistance of CS/ZnO/SWCNT composite is about several thousand ohms, depending on CS concentration. High concentration generally produces thick CS coating according to the cross-section view of SEM (not shown). Consequently, in this study, the composite resistance increases with CS
2.4. Material characterization Scanning electron microscopy (SEM) measurements were carried out with a TESCAN LYRA3 XMH instrument at an accelerating voltage of 5 kV. Fourier transform infrared (FT-IR) spectrum was recorded within the scanning range of 500–4500 cm−1 with a Bruker Tensor27. X-ray diffraction (XRD) was performed with a high-resolution X-ray diffractometer (Rigaku D/max-2500) by using Cu Kα radiation. 2.5. Sensing property measurement The sensing property measurements were conducted under various relative humidity (RH) levels, which can be obtained by several saturated aqueous solutions [13]. Specifically, LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, CuCl2, NaCl, KCl, and K2SO4 in a closed chamber were used to yield approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%, 86%, and 97% RH levels, respectively. A commercial humidity sensor was used to determine the RH level in ambient air. The sensor resistance was recorded with a source meter (Keithley Model 2400) at a fixed voltage as a function of time. The sensor response is expressed as follows:
ΔR R − Ro (%) = 100 × 1 Ro Ro
(1)
where R1 is the resistance of a steady state in a given RH level, and Ro is the initial resistance generally measured in 11% RH level unless otherwise noted. All the experiments were carried out at room temperature ranging from 19 °C to 25 °C. 787
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Fig. 2. SEM images of ZnO/SWCNT hybrid (a and b) and CZS1 (c and d). Insets show the cross-section view of ZnO/SWCNT hybrid and CZS1.
Fig. 4. Current vs. voltage of SWCNT, ZnO/SWCNT hybrid, and CZS3.
Fig. 3. FTIR spectra of SWCNT, ZnO/SWCNT, CZS3, and pure CS film.
3.2. Humidity sensing behaviors
concentration. The ZnO/SWCNT hybrid undergoes less significant change in resistance than that of pure SWCNT. This observation means that the almost insulating ZnO at room temperature exerts no prevention effect on the good contact among SWCNT bundles. Additionally, such insignificant change in resistance can be explained by the size effect of nanoparticles. When the nanoparticle size is less than 10 nm, the metal oxide depletion can be extended into SWCTNs, which commonly results in a considerable increase in resistance [16]. The estimated particle size in this work is larger than 10 nm, and the depletion and neutral regions both exist. The depletion region will not penetrate into the SWCNTs. Therefore, the increase in resistance is inconspicuous for ZnO/SWCNT hybrid. However, the CS/ZnO/SWCNT resistance largely increases possibly because the CS penetrating into the porous hybrid networks blocks the electron hopping among SWCNT bundles.
The comparisons of sensing performance among different sensors are shown in Fig. 5. With the RH changing from 11% to 75% at room temperature, CS/ZnO/SWCNT composite films obviously exhibit higher response than the other three sensors, and the highest response occurs at a CS concentration of 0.75 wt%. The optimum sensitivity occurs at an intermediate resistance regime, and such phenomenon is highly common for resistive sensors [16], although the reason is still unclear to date. With consideration of the highest response, CZS3 was selected as an optimal sensor for further analysis. In addition, pure SWCNTs and ZnO/SWCNT hybrid can quickly reach plateaus in 75% RH, and this phenomenon is similar to those in other previously reported CNT-based humidity sensors [13]. The response time for these two sensors is approximately a few seconds during calculation by experiential t90 standard, which defines response time as 788
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Fig. 5. Resistance curves of humidity sensors to a RH pulse between 11% and 75% RH at room temperature (22°C).
Fig. 7. Hysteresis of SWCNT, ZnO/SWCNT hybrid, and CZS3 in different RH at room temperature (22–24°C).
the time required for resistance to reach 90% of final or equilibrium value. The estimated response time of composite sensors is several tens of seconds within the exposure interval (approximately 200 s). Response time commonly depends on film structural factors, such as thickness and porosity, which can affect the diffusion or adsorption of gas molecules. The porous structure of SWCNT and ZnO/SWCNT hybrid causes the quick diffusion of water molecules. Therefore, a fast response occurs. If CS covers or penetrates into these pores, then water molecules may need a long time to diffuse through the polymer layer. Consequently, a relatively slow response appears. A steady-state even upon exposure to 97% RH is difficult to obtain with CS/ZnO/SWCNT composite sensor (Fig. 6). Nevertheless, the composite sensor exhibits a good response–recovery behavior and acceptable repeatability. The hysteresis properties during humidification and desiccation processes are shown in Fig. 7. Despite the evidently high response of CS/ZnO/SWCNT composite, this sensor exhibits a larger hysteresis than those of pure SWCNTs and ZnO/SWCNT hybrid. The high affinity of hydrophilic CS to water molecules may decelerate the desorption process, thereby resulting in a larger hysteresis than those of pure SWCNTs and ZnO/SWCNT hybrids [17]. Sensing material modification commonly fails to optimize each aspect of sensing performance [18]. Hence, the requirement for one sensing property is often conflicting with the requirement for another property. For example, after composite modification or formation, the strong interaction between the gas molecule and sensor often not only guarantees high sensitivity but also leads to poor reversibility [19]. Therefore, this negative effect brought by CS is expected in this work.
Fig. 8. Humidity-dependent response of CZS3 sensor measured at room temperature (22–24 °C).
The RH-dependent response is shown in Fig. 8. A polynomial fitting curve, which is rather than a linear fit curve, can precisely describe the relation between response and RH. Error bars denote the standard deviation from the mean through measuring five sensors under a given RH. No strong influence appears on response when RH is lower than 33%. Additionally, the response evidently increases when the RH is higher than 43%. Three other saturated solutions (CaCl2, 31% RH; CO (NH2)2, 73% RH; and (NH4)2SO4, 83% RH) were used to investigate the applicability of this fitting curve. The related responses are in good agreement with the fitting curve. Similar response–RH relationships have been observed in other CNT-based humidity sensors [20,21]. Furthermore, Fei et al. found that the response–RH relationship of MWCNT/polyvinyl alcohol (PVA) composite humidity sensor is similar to the shape of the sorption isotherm of the water–PVA system [21]. Notably, in our work, the shape in Fig. 8 is also analogous to the water vapor sorption isotherm of CS film at room temperature [22]. The similarity indicates that water sorption mostly by CS can dominate the resistance change of CS/ZnO/SWCNT composite [23,24]. When RH is higher than 43%, the water content largely increases. In addition, water may act as plasticizer to make the CS structure soft [22], thereby evidently disarranging the conductive network. Sensitivity can be commonly calculated by using Eq. (2) [13,25]:
S= Fig. 6. Resistance curve of the CZS3 sensor upon repeat exposure to 97% RH. The initial resistance was measured in 33% RH at room temperature (22 °C).
R a − Rb R b × Δ (%RH)
(2)
where Ra is the resistance after exposure to a given RH above 30%, and 789
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Fig. 9. Comparison of humidity-dependent sensitivity among CZS3 and other reported sensors [30].
Fig. 11. Resistance curves of three CZS3 sensors to a RH pulse between 11% and 75% RH in the absence and presence of 500 ppm interference gas stream at room temperature (22°C).
can cause the protonation of amino groups [27], meaning that molecule chain of CS tends to be positively charged [28]. Comparing with water molecule, ammonia molecule has a much stronger proton affinity and hence, is easier to obtain a proton from CS to form a positive ion [29]. The ionic repulsive force arising between molecule chain and ammonia impedes the penetration of ammonia into CS film although ammonia molecule has a high dipole moment of 1.47 D [28]. Consequently, the composite sensor also has nonresponse to ammonia molecules. It must be pointed out that the composite sensor is still an electronic-type humidity sensor instead of ionic-type humidity sensor even though protonation of CS occurs. A proton is 1840 times heavier than an electron. Hence, the mobility of a proton is much slower than that of an electron. Moreover, a proton may frequently collide with other atoms in an electric field while an electron can travel several interatomic distances before being scattered by colliding. Therefore, although CS can be a proton conductor [12], electron transfer absolutely dominates the conduction when CS combines with electronic conductors [10,26].
Fig. 10. Normalized resistance of CZS3 continuously measured in different RH for 116 h at room temperature (21–24 °C).
Rb is the initial resistance at 30% RH. The comparison of sensitivity among CS/ZnO/SWCNT composites and other CNT-based humidity sensors reported in the recent years is shown in Fig. 9. The present sensing material performs fairly well compared with those of others. Given that the drop casting in this work is an average method, the response may be further improved by optimizing sensing structures through different methods, such as hybrid formation or hierarchical structure fabrication. The stability of CS/ZnO/SWCNT composite humidity sensor is shown in Fig. 10. Similar to other CNT/ploymer composite [21], the acceptable change in resistance indicates that CS/ ZnO/SWCNT composite is fairly stable for a long-term application. The selectivity of composite sensor is shown in Fig. 11. The responses of three CZS3 sensors in the absence and presence of interference gas stream (100 sccm, 500 ppm) are very similar, indicating a fairly good selectivity of CS/ZnO/SWCNT composite. Both ZnO and SWCNT have been very common sensing materials to detect these three interference gases in the past decades [1,3]. Therefore, the selectivity of CS/ZnO/SWCNT composite mainly relies on CS. CS has a strong affinity to polar molecules due to the existence of abundant hydrophilic groups such as hydroxyl groups and amino groups on the molecule chains [26]. The dipole moment of water molecule is 1.84 D, and CS can interact with water molecule through hydrogen bonding. Comparing with water molecule, however, NO (0.15D) and CO (0.11D) molecules encounter great difficulty in permeating the CS membrane due to their non-polar characteristics [12,26]. CS seems to be a filtering membrane to block NO and CO molecules and therefore, the composite sensor has no response to NO and CO gas. The dissolving in weak acetic acid solution
3.3. Sensing mechanism The humidity response of drop-casted SWCNT is generally considered to originate from the carrier hopping over the contact barrier at the junctions among SWCNT bundles [31]. The slightly enhanced response, together with the comparable resistance range, indicates that ZnO contributes slightly to promote the sensing performance of ZnO/ SWCNT hybrid but not to facilitate the adsorption of water through hydrogen bonding [32]. However, this surface hydration of ZnO contributes minimally to the conductivity of CS/ZnO/SWCNT composite because most water is adsorbed by polymer [23,24]. The SWCNT conductive network is still the fundamental electron transport path in the composite, as explained before in the voltage-current characteristic result. The direction of electron movement in CS/ZnO/SWCNT composite includes moving along the nanotubes, hopping over the barrier at the junctions, and electron tunneling (Fig. 12). The functional groups of CS can attract water molecules via hydrogen bonding and consequently result in the swelling of CS [2]. This swelling causes the migration of nanotubes and increases the intertube distance. Moreover, electrons encounter difficulty during transfer due to the increased hopping distance of electron, and the composite resistance also remarkably increases. A large amount of adsorbed water suggests the considerable effect of this swelling [23]. Therefore, the sensor resistance increases upon exposure to high RH because more water molecules will permeate. When water molecules diffuse out from CS, a number of functional groups among interchains will directly interact again and 790
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Fig. 12. Diagram of sensing mechanism of CS/ZnO/SWCNT composite.
References
then, a shrinkage effect is caused, leading to a reconstruction of electron conductive path. Consequently, the sensor resistance decreases upon exposure to low RH. On the other hand, the role of insulating ZnO in the composite should not be ignored because CS/ZnO/SWCNT composite has a higher response than CS/SWCNT composite (Fig. 5). Since CS dominates water sorption of composite, any CS microstructural modification may influence the diffusion of water molecules and eventually affect the sensing response. The most possible situation is the change of free volume which is a major factor to the diffusion process of polymers [33]. Generally, the free volume of polymer/inorganic composite is larger than that of pure polymer [34–36]. The free volume of CS will continue changing after introducing ZnO. Specifically, the free volume hole size and concentration increase in the vicinity of ZnO. Compared with CS/ SWCNT composite, CS/ZnO/SWCNT composite possesses a larger extent of swelling due to the more easy diffusion of water molecules [37]. Consequently, the humidity sensing performance of CS/ZnO/SWCNT significantly increases.
[1] D.R. Kauffman, A. Star, Carbon nanotube gas and vapor sensors, Angew. Chem. Int. Ed. 47 (2008) 6550–6570. [2] W. Li, N.D. Hoa, D. Kim, High-performance carbon nanotube hydrogen sensor, Sens. Actuators B Chem. 149 (2010) 184–188. [3] Y. Hou, A.H. Jayatissa, Influence of laser doping on nanocrystalline ZnO thin films gas sensors, Prog. Nat. Sci. 27 (2017) 435–442. [4] L.L. Guo, X.Y. Kou, M.D. Ding, C. Wang, L.L. Dong, H. Zhang, C.H. Feng, Y.F. Sun, Y. Gao, P. Sun, G.Y. Lu, Reduced graphene oxide/-Fe2O3 composite nanofibers for application in gas sensors, Sens. Actuators B Chem. 244 (2017) 233–242. [5] L.B. Ahmed, S. Naama, A. Keffous, A. Hassein-Bey, T. Hadjersi, H2 sensing properties of modified silicon nanowires, Prog. Nat. Sci. 25 (2015) 101–110. [6] E.I. Ionete, S.I. Spiridon, B.F. Monea, D. Ebrasu-Ion, A. Vaseashta, SWCNT-Pt-P2O5based sensor for humidity measurements, IEEE Sens. J. 16 (2016) 7593–7599. [7] D. Jung, J. Kim, G.S. Lee, Enhanced humidity-sensing response of metal oxide coated carbon nanotube, Sens. Actuators A Phys. 223 (2015) 11–17. [8] J. Lee, S. Mulmi, V. Thangadurai, S.S. Park, Magnetically aligned iron oxide/gold nanoparticle-decorated carbon nanotube hybrid structure as a humidity sensor, ACS Appl. Mater. Interfaces 7 (2015) 15506–15513. [9] K.H. Zhang, R.F. Hu, G.K. Fan, G. Li, Graphene oxide/chitosan nanocomposite coated quartz crystal microbalance sensor for detection of amine vapors, Sens. Actuators B Chem. 243 (2017) 721–730. [10] W. Li, D.M. Jang, S.Y. An, D. Kim, S.K. Hong, H. Kim, Polyaniline–chitosan nanocomposite: high performance hydrogen sensor from new principle, Sens. Actuators B Chem. 160 (2011) 1020–1025. [11] R.Y. Aguirre-Loredo, A.I. Rodríguez-Hernández, E. Morales-Sánchez, C.A. GómezAldapa, G. Velazquez, Effect of equilibrium moisture content on barrier, mechanical and thermal properties of chitosan films, Food Chem. 196 (2016) 560–566. [12] J.P. Zou, K. Zhang, Q. Zhang, Giant humidity response using a chitosan-based protonic conductive sensor, IEEE Sens. J. 16 (2016) 8884–8889. [13] D.Z. Zhang, N.L. Yin, B.K. Xia, Y. Sun, Y.F. Liao, Z.L. He, S. Hao, Humidity-sensing properties of hierarchical ZnO/MWCNTs/ZnO nanocomposite film sensor based on electrostatic layer-by-layer self-assembly, J. Mater. Sci: Mater. Electron. 27 (2016) 2481–2487. [14] O. Zhou, R.M. Fleming, D.W. Murphy, C.H. Chen, R.C. Haddon, A.P. Ramirez, S.H. Glarum, Defects in carbon nanostructures, Science 263 (1994) 1744–1747. [15] V. Srivastava, D. Gusain, Y.C. Sharma, Synthesis, characterization and application of zinc oxide nanoparticles (n-ZnO), Ceram. Int. 39 (2013) 9803–9808. [16] W. Li, H. Jung, N.D. Hoa, D. Kim, S.K. Hong, H. Kim, Nanocomposite of cobalt oxide nanocrystals and single-walled carbon nanotubes for a gas sensor application, Sens. Actuators B Chem. 150 (2010) 160–166. [17] Y. Li, T.T. Wu, M.J. Yang, Humidity sensors based on the composite of multi-walled carbon nanotubes and crosslinked polyelectrolyte with good sensitivity and capability of detecting low humidity, Sens. Actuators B Chem. 203 (2014) 63–70. [18] E.U. Park, B.I. Choi, J.C. Kim, S.B. Woo, Y.G. Kim, Y. Choi, S.W. Lee, Correlation between the sensitivity and the hysteresis of humidity sensors based on graphene oxides, Sens. Actuators B Chem. 258 (2018) 255–262. [19] R. Potyrailo, R.R. Naik, Bionanomaterials and bioinspired nanostructures for selective vapor sensing, Annu. Rev. Mater. Res. 43 (2013) 307–334. [20] J. Lee, D. Cho, Y. Jeong, A resistive-type sensor based on flexible multi-walled
4. Conclusions The dropcasted CS/ZnO/SWCNT composite film shows higher response than those of pure SWCNT, ZnO/SWCNT hybrid, and CS/ SWCNT composite. The composite exhibits a nonlinear response to RH, and such RH-dependent relation may correspond to the water sorption nature of CS. The sensing mechanism is related to the swelling effect of CS, which can enlarge the gap at the tube-to-tube junctions to further decrease the conductivity of composite structure. The enhanced response and a fairly good reversibility highlight the promising potential of CS/ZnO/SWCNT composite as a humidity sensor. The sensing performance of CS/ZnO/SWCNT composite may be further improved by a distinct dispersion technique or composite structure optimization, such as hierarchical structure or organic–inorganic coaxial hybrid.
Acknowledgements The authors would like to acknowledge the fund support from the National Natural Science Foundation of China (Grant no. 61501318). 791
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[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28] [29] [30]
[31] [32]
[33]
[34]
carbon nanotubes and polyacrylic acid composite films, Solid. Electron. 87 (2013) 80–84. T. Fei, K. Jiang, F. Jiang, R. Mu, T. Zhang, Humidity switching properties of sensors based on multiwalled carbon nanotubes/polyvinyl alcohol composite films, J. Appl. Polym. Sci. 43 (2014) 39726. C. Madeleine-Perdrillat, T. Karbowiak, F. Debeaufort, L. Delmotte, C. Vaulot, D. Champion, Effect of hydration on molecular dynamics and structure in chitosan films, Food Hydrocoll. 61 (2016) 57–65. T. Soboleva, K. Malek, Z. Xie, T. Navessin, S. Holdcroft, PEMFC catalyst layers: the role of micropores and mesopores on water sorption and fuel cell activity, ACS Appl, Mater. Interfaces 3 (2011) 1827–1837. D.R.P. Morris, S.P. Liu, D.V. Gonzalez, J.T. Gostick, Effect of water sorption on the electronic conductivity of porous polymer electrolyte membrane fuel cell catalyst layers, ACS Appl. Mater. Interfaces 6 (2014) 18609–18618. K.P. Yoo, L.T. Lim, N.K. Min, M.J. Lee, C.J. Lee, C.W. Park, Novel resistive-type humidity sensor based on multiwall carbon nanotube/polyimide composite films, Sens. Actuators B Chem. 145 (2010) 120–125. A. Bouvree, J. Feller, M. Castro, Y. Grohens, M. Rinaudo, Conductive polymer nanobiocomposites(CPC): chitosan-carbon nanoparticle a good candidate to design polar vapour sensors, Sens. Actuators B Chem. 138 (2009) 138–147. S.A. Fischer, B.I. Dunlap, D. Gunlycke, Proton transport through hydrated chitosanbased polymer membranes under electric fields, J. App. Polym. Sci. B 55 (2017) 1103–1109. Y. Hirabayashi, Pervaporation membrane system for the removal of ammonia from water, Mater. Trans. 43 (2002) 1074–1077. P.C. Singh, G.N. Patwari, Proton affinity correlations between hydrogen and dihydrogen bond acceptors, J. Phys. Chem. A 111 (2007) 3178–3183. F. Sattarzadeh, M.M. Doroodmand, M.H. Sheikhi, A. Zarifkar, Fabrication of a humidity sensor based on chemical vapor deposition-synthesized single-walled carbon nanotubes, Sci. Adv. Mater. 5 (2013) 557–565. K. Zhang, J.P. Zou, Q. Zhang, Roles of inter-SWCNT junctions in resistive humidity response, Nanotechnology 26 (2015) 455501. D. Raymand, A.C.T. van Duin, D. Spångberg, W.A. Goddard III, K. Hermansson, Water adsorption on stepped ZnO surfaces from MD stimulation, Surf. Sci. 604 (2010) 741–752. S.K. Sharma, P.K. Pujari, Role of free volume characteristics of polymer matrix in bulk physical properties of polymer nanocomposites: a review of positron annihilation lifetime studies, Prog. Polym. Sci. 75 (2017) 31–47. C. Tsonos, A. Kanapitsas, G.C. Psarras, T. Speliotis, Effect of ZnO nanoparticles on
the dielectric/electrical and thermal properties of epoxy-based nanocomposites, Sci. Adv. Mater. 7 (2015) 588–597. [35] S.S. Nasab, S. Zahmatkesh, Preparation, structural characterization, and gas separation properties of functionalized zinc oxide particle filled poly(ether-amide) nanocomposite films, J. Plast. Film Sheet. 33 (2017) 92–113. [36] S. Vural, S. Koytepe, T. Seckin, I. Adiguzel, Synthesis, characterization and dielectric properties of rodlike zinc oxide-polyimide nanocomposites, Polym-Plast. Tech. Eng. 51 (2012) 369–376. [37] S.E. Gamal, A.M. El Sayed, E.E. Abdel-Hady, Effect of cobalt oxide nanoparticles on the nano-scale free volume and optical properties of biodegradable CMC/PVA films, J. Polym. Environ. 26 (2018) 2536–2545. Haipo Dai is now in master course in Materials Science and Engineering, Taiyuan University of Technology, China. His current research interests are fabrication of carbon nanotube-based nanocomposite and their applications to gas sensors. Nana Feng is now in master course in Materials Science and Engineering, Taiyuan University of Technology, China. Her current research interests are fabrication of carbon nanotube-based nanocomposite and their applications to gas sensors. Jiwei Li is now in master course in Materials Science and Engineering, Taiyuan University of Technology, China. Her current research interests are fabrication of graphene-based nanocomposite and their applications to gas sensors. Jie Zhang received her Ph.D in Textile Materials at Donghua University, China in 2013. She is working as a lecturer in the Department of Materials Science and Engineering at Taiyuan University of Technology, China. Her current research interests are nanocomposite materials based on inorganic nanoparticles and biopolymers and their applications as functional materials. Wei Li received his PhD in Materials Science and Engineering at Chungnam National University, South Korea in 2011. He is now an associate professor of Department of Materials Science and Engineering at Taiyuan University of Technology, China. His current research interests are fabrication of nanostructured materials including carbon nanotubes and oxides and their applications to electronic devices of solar cells, gas sensors, biosensor, etc.
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Chemiresistive humidity sensor based on chitosan/zinc oxide/single-walled carbon nanotube composite film ⁎
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Haipo Dai, Nana Feng, Jiwei Li, Jie Zhang , Wei Li
Department of Material Science, Taiyuan University of Technology, Taiyuan 030024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Chitosan Carbon nanotubes Zinc oxide Humidity sensor
A chemiresistive humidity sensor based on chitosan (CS)/zinc oxide/single-walled carbon nanotube (SWCNT) composite was developed. The sensor film was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The humidity sensing properties were investigated in a wide range of relative humidity (RH) (11%–97%) at room temperature. The SWCNT response to humidity was significantly improved with the aid of conjugate material in the composite, especially the CS. The composite also exhibited good reproducibility. The RH-dependent relationship revealed that the sensing response corresponded to the water sorption characteristic of CS. The sensing mechanism was attributed to the swelling effect of CS surrounding the nanotubes through which the hopping conduction path between nanotubes changed.
1. Introduction Chemiresistive humidity sensor, which depends on the change in electrical resistance, is important for application in many fields, such as atmosphere monitoring, industry, manufactory, and agriculture. Carbon nanotubes (CNTs) are regarded as distinct candidate materials for chemiresistive humidity sensors due to their large surface-to-volume ratios and good electron transport property [1]. However, the weak interaction between water molecules and pristine CNTs is a main limitation in obtaining high-performance humidity sensor, although highly porous CNT films can be prepared for free diffusion and adsorption of water molecule [2]. Generally, in addition to structural optimization, sensing material modifications, such as doping [3], composite formation [4], and chemical surface treatment [5], are effective ways to enhance sensing properties. Inorganic nanoparticle/CNT hybrids have been used as humidity sensors in recent years because of their improved electrical and mechanical properties; such improvement is due to the synergetic effect resulting from the combination of components [6]. For example, MnO2-coated CNT shows better response than that of uncoated CNT [7], and iron oxide/gold nanoparticle-decorated CNTs as humidity sensors exhibit high selectivity [8]. Despite the enhanced humidity sensing performance against pure CNTs, inorganic nanoparticle-functionalized CNTs still suffer from low sensitivity. Therefore, the interaction between the sensor and water molecules should be further strengthened. Chitosan (CS), a common type of polysaccharide that contains many
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functional groups, such as hydroxyl (eOH) and amino (eNH2), displays sufficient affinity to small analytes. Furthermore, CS is soluble in acidic media; thus, it can be simply converted to film form. These advantages make CS a highly suitable modification material to improve the performance of chemiresistive gas sensor, such as CNTs [2], graphene [9], and conducting polymers [10]. Particularly, recent theoretical and experimental studies have demonstrated that CS presents not only reversible water adsorption ability but also good water permeability [11,12], thereby indicating that CS can be applied in humidity sensors. Nevertheless, only few studies have focused on CS-modified chemiresistive humidity sensor. Therefore, in this report, ZnO, a representative and typical inorganic gas sensing material [13], was used as the first functional component to enhance the humidity sensing performance of single-walled CNT (SWCNT). Then, CS was utilized as the second functional component to continue enhancing the humidity sensing performance of the main sensing body of ZnO/SWCNT hybrid. 2. Experimental 2.1. Materials and reagents SWCNTs with carbon content of 95% were purchased from Dekedaojin Science and Technology Co., Ltd. (Beijing, China) and chemically treated with concentrated acid prior to use. All chemicals and solvents, such as CS, zinc chloride, ammonia, and acetic acid, were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (W. Li).
https://doi.org/10.1016/j.snb.2018.12.056 Received 20 May 2018; Received in revised form 9 December 2018; Accepted 10 December 2018 Available online 18 December 2018 0925-4005/ © 2018 Elsevier B.V. All rights reserved.
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China). These reagents possessed the highest commercially pure grade, and they were used without further purification. 2.2. Fabrication of ZnO/SWCNT hybrids The pretreated SWCNTs were dispersed in 50 ml of zinc chloride solution by ultrasonication at 60 °C for 2 h. Afterward, ammonia was slowly added into this solution until the pH value of approximately 9 was reached. After continued ultrasonication for 2 h, the solution was centrifuged. The sediment was washed several times with deionized water and subsequently dried at 60 °C in vacuum. Finally, the powder was annealed at 250 °C in a quartz tube furnace in air for 2 h to produce ZnO/SWCNT hybrids. The mass ratio of ZnO to SWCNT was approximately 1:2.
2.3. Fabrication of humidity sensor Fig. 1. XRD patterns of SWCNT, ZnO, and ZnO/SWCNT hybrid.
About 1 mg of ZnO/SWCNT hybrid was dispersed in 5 ml of water, and the solution was ultrasonicated for 2 h to disaggregate the agglomerations and obtain uniform suspensions. Afterward, 10 μl of solution was dropped into a silver interdigital electrode-covered alumina substrate (3.5 × 7.0 mm) by using a microliter pipette. The sensing body of ZnO/SWCNT hybrid was fabricated after solvent evaporation. Finally, 20 μl of CS solution was casted onto the above substrate and dried at 60 °C to produce CS/ZnO/SWCNT composite humidity sensor. The pre-prepared CS solution was obtained by dissolving appropriate amount of CS in 1% acetic acid solution. The CS concentrations were controlled at 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1.00 wt% for comparison, and the related sensors were named as CZS1, CZS2, CZS3, and CZS4, respectively. Other two humidity sensors, SWCNT and CS/ SWCNT composite, were also prepared for comparison. The preparation of SWCNT sensor was the same as the above process of ZnO/SWCNT hybrid sensor. CS/SWCNT composite sensor was prepared by casting CS solution (20 μl, 0.75 wt%) onto SWCNT sensor.
3. Results and discussion 3.1. Characterization of materials XRD patterns are shown in Fig. 1. Two typical peaks at 25.8° and 42.1°, which can be commonly indexed on the basis of the hexagonal close-packed graphite or turbostratic graphite [14], are related to the (002) and (100) planes of SWCNT, respectively. Except for the peak at 25.8°, all the marked peaks in the pattern of ZnO/SWCNT hybrid can be indexed to the hexagonal wurtzite structure of ZnO [15]. Specifically, the strong peaks at 31.9°, 34.5°, and 36.4° correspond to the (100), (002), and (101) diffraction planes, respectively. The peaks at 47.7°, 56.7°, 63.1°, and 68.3° can be assigned to (102), (110), (103), and (112) diffraction planes, respectively. The average crystallite size of ZnO estimated by Scherrer equation is approximately 15 nm. Moreover, under the resolution of the current apparatus, no impurity phase occurs in the XRD patterns. SEM images of ZnO/SWCNT hybrid and CS/ZnO/SWCNT composite are presented in Fig. 2. Dense ZnO/SWCNT hybrid clusters can be observed in Fig. 2a. High-magnification image shows that SWCNT bundles are entangled with each other (Fig. 2b). These entanglements can form a conductive network with high porosity, and such porous structure is suitable for molecular adsorption and electron transport [16]. When CS is cast onto the substrate, this ZnO/SWCNT hybrid porous structure is immersed in a flat CS film (Fig. 2c and d). Inset picture of Fig. 2c confirms the formation of CS/ZnO/SWCNT composite instead of layered structure after the drop-casting. The FT-IR spectra are displayed in Fig. 3. The broad peak at 3450 cm−1 is related to the overlapped −OH and –NH groups in CS. The peaks occurring at 2920 and 1633 cm−1 are attributed to CeH vibration mode and CeO stretching mode, respectively. The peak at 1409 cm−1 is a characteristic of the −CH2– group, and the peak at 1045 cm−1 is related to the CHeOH bond in cyclic compounds. The appearance of some of the above characteristic peaks in the spectra of SWCNT and ZnO/SWCNT hybrid is mainly attributed to the concentrated acid pre-treatment process through which many functional groups such as hydroxyl and carboxyl group may graft on the wall of SWCNT. The weak peak at 451 cm−1 corresponds to the stretching mode of Zn–O [15], which indicates the presence of crystalline ZnO. ZnO/SWCNT hybrid film shows a resistance value similar to that of pure SWCNT film (Fig. 4). The resistance of these two films measured between electrodes is about several hundred ohms. The resistance of CS/ZnO/SWCNT composite is about several thousand ohms, depending on CS concentration. High concentration generally produces thick CS coating according to the cross-section view of SEM (not shown). Consequently, in this study, the composite resistance increases with CS
2.4. Material characterization Scanning electron microscopy (SEM) measurements were carried out with a TESCAN LYRA3 XMH instrument at an accelerating voltage of 5 kV. Fourier transform infrared (FT-IR) spectrum was recorded within the scanning range of 500–4500 cm−1 with a Bruker Tensor27. X-ray diffraction (XRD) was performed with a high-resolution X-ray diffractometer (Rigaku D/max-2500) by using Cu Kα radiation. 2.5. Sensing property measurement The sensing property measurements were conducted under various relative humidity (RH) levels, which can be obtained by several saturated aqueous solutions [13]. Specifically, LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, CuCl2, NaCl, KCl, and K2SO4 in a closed chamber were used to yield approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%, 86%, and 97% RH levels, respectively. A commercial humidity sensor was used to determine the RH level in ambient air. The sensor resistance was recorded with a source meter (Keithley Model 2400) at a fixed voltage as a function of time. The sensor response is expressed as follows:
ΔR R − Ro (%) = 100 × 1 Ro Ro
(1)
where R1 is the resistance of a steady state in a given RH level, and Ro is the initial resistance generally measured in 11% RH level unless otherwise noted. All the experiments were carried out at room temperature ranging from 19 °C to 25 °C. 787
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Fig. 2. SEM images of ZnO/SWCNT hybrid (a and b) and CZS1 (c and d). Insets show the cross-section view of ZnO/SWCNT hybrid and CZS1.
Fig. 4. Current vs. voltage of SWCNT, ZnO/SWCNT hybrid, and CZS3.
Fig. 3. FTIR spectra of SWCNT, ZnO/SWCNT, CZS3, and pure CS film.
3.2. Humidity sensing behaviors
concentration. The ZnO/SWCNT hybrid undergoes less significant change in resistance than that of pure SWCNT. This observation means that the almost insulating ZnO at room temperature exerts no prevention effect on the good contact among SWCNT bundles. Additionally, such insignificant change in resistance can be explained by the size effect of nanoparticles. When the nanoparticle size is less than 10 nm, the metal oxide depletion can be extended into SWCTNs, which commonly results in a considerable increase in resistance [16]. The estimated particle size in this work is larger than 10 nm, and the depletion and neutral regions both exist. The depletion region will not penetrate into the SWCNTs. Therefore, the increase in resistance is inconspicuous for ZnO/SWCNT hybrid. However, the CS/ZnO/SWCNT resistance largely increases possibly because the CS penetrating into the porous hybrid networks blocks the electron hopping among SWCNT bundles.
The comparisons of sensing performance among different sensors are shown in Fig. 5. With the RH changing from 11% to 75% at room temperature, CS/ZnO/SWCNT composite films obviously exhibit higher response than the other three sensors, and the highest response occurs at a CS concentration of 0.75 wt%. The optimum sensitivity occurs at an intermediate resistance regime, and such phenomenon is highly common for resistive sensors [16], although the reason is still unclear to date. With consideration of the highest response, CZS3 was selected as an optimal sensor for further analysis. In addition, pure SWCNTs and ZnO/SWCNT hybrid can quickly reach plateaus in 75% RH, and this phenomenon is similar to those in other previously reported CNT-based humidity sensors [13]. The response time for these two sensors is approximately a few seconds during calculation by experiential t90 standard, which defines response time as 788
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Fig. 5. Resistance curves of humidity sensors to a RH pulse between 11% and 75% RH at room temperature (22°C).
Fig. 7. Hysteresis of SWCNT, ZnO/SWCNT hybrid, and CZS3 in different RH at room temperature (22–24°C).
the time required for resistance to reach 90% of final or equilibrium value. The estimated response time of composite sensors is several tens of seconds within the exposure interval (approximately 200 s). Response time commonly depends on film structural factors, such as thickness and porosity, which can affect the diffusion or adsorption of gas molecules. The porous structure of SWCNT and ZnO/SWCNT hybrid causes the quick diffusion of water molecules. Therefore, a fast response occurs. If CS covers or penetrates into these pores, then water molecules may need a long time to diffuse through the polymer layer. Consequently, a relatively slow response appears. A steady-state even upon exposure to 97% RH is difficult to obtain with CS/ZnO/SWCNT composite sensor (Fig. 6). Nevertheless, the composite sensor exhibits a good response–recovery behavior and acceptable repeatability. The hysteresis properties during humidification and desiccation processes are shown in Fig. 7. Despite the evidently high response of CS/ZnO/SWCNT composite, this sensor exhibits a larger hysteresis than those of pure SWCNTs and ZnO/SWCNT hybrid. The high affinity of hydrophilic CS to water molecules may decelerate the desorption process, thereby resulting in a larger hysteresis than those of pure SWCNTs and ZnO/SWCNT hybrids [17]. Sensing material modification commonly fails to optimize each aspect of sensing performance [18]. Hence, the requirement for one sensing property is often conflicting with the requirement for another property. For example, after composite modification or formation, the strong interaction between the gas molecule and sensor often not only guarantees high sensitivity but also leads to poor reversibility [19]. Therefore, this negative effect brought by CS is expected in this work.
Fig. 8. Humidity-dependent response of CZS3 sensor measured at room temperature (22–24 °C).
The RH-dependent response is shown in Fig. 8. A polynomial fitting curve, which is rather than a linear fit curve, can precisely describe the relation between response and RH. Error bars denote the standard deviation from the mean through measuring five sensors under a given RH. No strong influence appears on response when RH is lower than 33%. Additionally, the response evidently increases when the RH is higher than 43%. Three other saturated solutions (CaCl2, 31% RH; CO (NH2)2, 73% RH; and (NH4)2SO4, 83% RH) were used to investigate the applicability of this fitting curve. The related responses are in good agreement with the fitting curve. Similar response–RH relationships have been observed in other CNT-based humidity sensors [20,21]. Furthermore, Fei et al. found that the response–RH relationship of MWCNT/polyvinyl alcohol (PVA) composite humidity sensor is similar to the shape of the sorption isotherm of the water–PVA system [21]. Notably, in our work, the shape in Fig. 8 is also analogous to the water vapor sorption isotherm of CS film at room temperature [22]. The similarity indicates that water sorption mostly by CS can dominate the resistance change of CS/ZnO/SWCNT composite [23,24]. When RH is higher than 43%, the water content largely increases. In addition, water may act as plasticizer to make the CS structure soft [22], thereby evidently disarranging the conductive network. Sensitivity can be commonly calculated by using Eq. (2) [13,25]:
S= Fig. 6. Resistance curve of the CZS3 sensor upon repeat exposure to 97% RH. The initial resistance was measured in 33% RH at room temperature (22 °C).
R a − Rb R b × Δ (%RH)
(2)
where Ra is the resistance after exposure to a given RH above 30%, and 789
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Fig. 9. Comparison of humidity-dependent sensitivity among CZS3 and other reported sensors [30].
Fig. 11. Resistance curves of three CZS3 sensors to a RH pulse between 11% and 75% RH in the absence and presence of 500 ppm interference gas stream at room temperature (22°C).
can cause the protonation of amino groups [27], meaning that molecule chain of CS tends to be positively charged [28]. Comparing with water molecule, ammonia molecule has a much stronger proton affinity and hence, is easier to obtain a proton from CS to form a positive ion [29]. The ionic repulsive force arising between molecule chain and ammonia impedes the penetration of ammonia into CS film although ammonia molecule has a high dipole moment of 1.47 D [28]. Consequently, the composite sensor also has nonresponse to ammonia molecules. It must be pointed out that the composite sensor is still an electronic-type humidity sensor instead of ionic-type humidity sensor even though protonation of CS occurs. A proton is 1840 times heavier than an electron. Hence, the mobility of a proton is much slower than that of an electron. Moreover, a proton may frequently collide with other atoms in an electric field while an electron can travel several interatomic distances before being scattered by colliding. Therefore, although CS can be a proton conductor [12], electron transfer absolutely dominates the conduction when CS combines with electronic conductors [10,26].
Fig. 10. Normalized resistance of CZS3 continuously measured in different RH for 116 h at room temperature (21–24 °C).
Rb is the initial resistance at 30% RH. The comparison of sensitivity among CS/ZnO/SWCNT composites and other CNT-based humidity sensors reported in the recent years is shown in Fig. 9. The present sensing material performs fairly well compared with those of others. Given that the drop casting in this work is an average method, the response may be further improved by optimizing sensing structures through different methods, such as hybrid formation or hierarchical structure fabrication. The stability of CS/ZnO/SWCNT composite humidity sensor is shown in Fig. 10. Similar to other CNT/ploymer composite [21], the acceptable change in resistance indicates that CS/ ZnO/SWCNT composite is fairly stable for a long-term application. The selectivity of composite sensor is shown in Fig. 11. The responses of three CZS3 sensors in the absence and presence of interference gas stream (100 sccm, 500 ppm) are very similar, indicating a fairly good selectivity of CS/ZnO/SWCNT composite. Both ZnO and SWCNT have been very common sensing materials to detect these three interference gases in the past decades [1,3]. Therefore, the selectivity of CS/ZnO/SWCNT composite mainly relies on CS. CS has a strong affinity to polar molecules due to the existence of abundant hydrophilic groups such as hydroxyl groups and amino groups on the molecule chains [26]. The dipole moment of water molecule is 1.84 D, and CS can interact with water molecule through hydrogen bonding. Comparing with water molecule, however, NO (0.15D) and CO (0.11D) molecules encounter great difficulty in permeating the CS membrane due to their non-polar characteristics [12,26]. CS seems to be a filtering membrane to block NO and CO molecules and therefore, the composite sensor has no response to NO and CO gas. The dissolving in weak acetic acid solution
3.3. Sensing mechanism The humidity response of drop-casted SWCNT is generally considered to originate from the carrier hopping over the contact barrier at the junctions among SWCNT bundles [31]. The slightly enhanced response, together with the comparable resistance range, indicates that ZnO contributes slightly to promote the sensing performance of ZnO/ SWCNT hybrid but not to facilitate the adsorption of water through hydrogen bonding [32]. However, this surface hydration of ZnO contributes minimally to the conductivity of CS/ZnO/SWCNT composite because most water is adsorbed by polymer [23,24]. The SWCNT conductive network is still the fundamental electron transport path in the composite, as explained before in the voltage-current characteristic result. The direction of electron movement in CS/ZnO/SWCNT composite includes moving along the nanotubes, hopping over the barrier at the junctions, and electron tunneling (Fig. 12). The functional groups of CS can attract water molecules via hydrogen bonding and consequently result in the swelling of CS [2]. This swelling causes the migration of nanotubes and increases the intertube distance. Moreover, electrons encounter difficulty during transfer due to the increased hopping distance of electron, and the composite resistance also remarkably increases. A large amount of adsorbed water suggests the considerable effect of this swelling [23]. Therefore, the sensor resistance increases upon exposure to high RH because more water molecules will permeate. When water molecules diffuse out from CS, a number of functional groups among interchains will directly interact again and 790
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Fig. 12. Diagram of sensing mechanism of CS/ZnO/SWCNT composite.
References
then, a shrinkage effect is caused, leading to a reconstruction of electron conductive path. Consequently, the sensor resistance decreases upon exposure to low RH. On the other hand, the role of insulating ZnO in the composite should not be ignored because CS/ZnO/SWCNT composite has a higher response than CS/SWCNT composite (Fig. 5). Since CS dominates water sorption of composite, any CS microstructural modification may influence the diffusion of water molecules and eventually affect the sensing response. The most possible situation is the change of free volume which is a major factor to the diffusion process of polymers [33]. Generally, the free volume of polymer/inorganic composite is larger than that of pure polymer [34–36]. The free volume of CS will continue changing after introducing ZnO. Specifically, the free volume hole size and concentration increase in the vicinity of ZnO. Compared with CS/ SWCNT composite, CS/ZnO/SWCNT composite possesses a larger extent of swelling due to the more easy diffusion of water molecules [37]. Consequently, the humidity sensing performance of CS/ZnO/SWCNT significantly increases.
[1] D.R. Kauffman, A. Star, Carbon nanotube gas and vapor sensors, Angew. Chem. Int. Ed. 47 (2008) 6550–6570. [2] W. Li, N.D. Hoa, D. Kim, High-performance carbon nanotube hydrogen sensor, Sens. Actuators B Chem. 149 (2010) 184–188. [3] Y. Hou, A.H. Jayatissa, Influence of laser doping on nanocrystalline ZnO thin films gas sensors, Prog. Nat. Sci. 27 (2017) 435–442. [4] L.L. Guo, X.Y. Kou, M.D. Ding, C. Wang, L.L. Dong, H. Zhang, C.H. Feng, Y.F. Sun, Y. Gao, P. Sun, G.Y. Lu, Reduced graphene oxide/-Fe2O3 composite nanofibers for application in gas sensors, Sens. Actuators B Chem. 244 (2017) 233–242. [5] L.B. Ahmed, S. Naama, A. Keffous, A. Hassein-Bey, T. Hadjersi, H2 sensing properties of modified silicon nanowires, Prog. Nat. Sci. 25 (2015) 101–110. [6] E.I. Ionete, S.I. Spiridon, B.F. Monea, D. Ebrasu-Ion, A. Vaseashta, SWCNT-Pt-P2O5based sensor for humidity measurements, IEEE Sens. J. 16 (2016) 7593–7599. [7] D. Jung, J. Kim, G.S. Lee, Enhanced humidity-sensing response of metal oxide coated carbon nanotube, Sens. Actuators A Phys. 223 (2015) 11–17. [8] J. Lee, S. Mulmi, V. Thangadurai, S.S. Park, Magnetically aligned iron oxide/gold nanoparticle-decorated carbon nanotube hybrid structure as a humidity sensor, ACS Appl. Mater. Interfaces 7 (2015) 15506–15513. [9] K.H. Zhang, R.F. Hu, G.K. Fan, G. Li, Graphene oxide/chitosan nanocomposite coated quartz crystal microbalance sensor for detection of amine vapors, Sens. Actuators B Chem. 243 (2017) 721–730. [10] W. Li, D.M. Jang, S.Y. An, D. Kim, S.K. Hong, H. Kim, Polyaniline–chitosan nanocomposite: high performance hydrogen sensor from new principle, Sens. Actuators B Chem. 160 (2011) 1020–1025. [11] R.Y. Aguirre-Loredo, A.I. Rodríguez-Hernández, E. Morales-Sánchez, C.A. GómezAldapa, G. Velazquez, Effect of equilibrium moisture content on barrier, mechanical and thermal properties of chitosan films, Food Chem. 196 (2016) 560–566. [12] J.P. Zou, K. Zhang, Q. Zhang, Giant humidity response using a chitosan-based protonic conductive sensor, IEEE Sens. J. 16 (2016) 8884–8889. [13] D.Z. Zhang, N.L. Yin, B.K. Xia, Y. Sun, Y.F. Liao, Z.L. He, S. Hao, Humidity-sensing properties of hierarchical ZnO/MWCNTs/ZnO nanocomposite film sensor based on electrostatic layer-by-layer self-assembly, J. Mater. Sci: Mater. Electron. 27 (2016) 2481–2487. [14] O. Zhou, R.M. Fleming, D.W. Murphy, C.H. Chen, R.C. Haddon, A.P. Ramirez, S.H. Glarum, Defects in carbon nanostructures, Science 263 (1994) 1744–1747. [15] V. Srivastava, D. Gusain, Y.C. Sharma, Synthesis, characterization and application of zinc oxide nanoparticles (n-ZnO), Ceram. Int. 39 (2013) 9803–9808. [16] W. Li, H. Jung, N.D. Hoa, D. Kim, S.K. Hong, H. Kim, Nanocomposite of cobalt oxide nanocrystals and single-walled carbon nanotubes for a gas sensor application, Sens. Actuators B Chem. 150 (2010) 160–166. [17] Y. Li, T.T. Wu, M.J. Yang, Humidity sensors based on the composite of multi-walled carbon nanotubes and crosslinked polyelectrolyte with good sensitivity and capability of detecting low humidity, Sens. Actuators B Chem. 203 (2014) 63–70. [18] E.U. Park, B.I. Choi, J.C. Kim, S.B. Woo, Y.G. Kim, Y. Choi, S.W. Lee, Correlation between the sensitivity and the hysteresis of humidity sensors based on graphene oxides, Sens. Actuators B Chem. 258 (2018) 255–262. [19] R. Potyrailo, R.R. Naik, Bionanomaterials and bioinspired nanostructures for selective vapor sensing, Annu. Rev. Mater. Res. 43 (2013) 307–334. [20] J. Lee, D. Cho, Y. Jeong, A resistive-type sensor based on flexible multi-walled
4. Conclusions The dropcasted CS/ZnO/SWCNT composite film shows higher response than those of pure SWCNT, ZnO/SWCNT hybrid, and CS/ SWCNT composite. The composite exhibits a nonlinear response to RH, and such RH-dependent relation may correspond to the water sorption nature of CS. The sensing mechanism is related to the swelling effect of CS, which can enlarge the gap at the tube-to-tube junctions to further decrease the conductivity of composite structure. The enhanced response and a fairly good reversibility highlight the promising potential of CS/ZnO/SWCNT composite as a humidity sensor. The sensing performance of CS/ZnO/SWCNT composite may be further improved by a distinct dispersion technique or composite structure optimization, such as hierarchical structure or organic–inorganic coaxial hybrid.
Acknowledgements The authors would like to acknowledge the fund support from the National Natural Science Foundation of China (Grant no. 61501318). 791
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[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28] [29] [30]
[31] [32]
[33]
[34]
carbon nanotubes and polyacrylic acid composite films, Solid. Electron. 87 (2013) 80–84. T. Fei, K. Jiang, F. Jiang, R. Mu, T. Zhang, Humidity switching properties of sensors based on multiwalled carbon nanotubes/polyvinyl alcohol composite films, J. Appl. Polym. Sci. 43 (2014) 39726. C. Madeleine-Perdrillat, T. Karbowiak, F. Debeaufort, L. Delmotte, C. Vaulot, D. Champion, Effect of hydration on molecular dynamics and structure in chitosan films, Food Hydrocoll. 61 (2016) 57–65. T. Soboleva, K. Malek, Z. Xie, T. Navessin, S. Holdcroft, PEMFC catalyst layers: the role of micropores and mesopores on water sorption and fuel cell activity, ACS Appl, Mater. Interfaces 3 (2011) 1827–1837. D.R.P. Morris, S.P. Liu, D.V. Gonzalez, J.T. Gostick, Effect of water sorption on the electronic conductivity of porous polymer electrolyte membrane fuel cell catalyst layers, ACS Appl. Mater. Interfaces 6 (2014) 18609–18618. K.P. Yoo, L.T. Lim, N.K. Min, M.J. Lee, C.J. Lee, C.W. Park, Novel resistive-type humidity sensor based on multiwall carbon nanotube/polyimide composite films, Sens. Actuators B Chem. 145 (2010) 120–125. A. Bouvree, J. Feller, M. Castro, Y. Grohens, M. Rinaudo, Conductive polymer nanobiocomposites(CPC): chitosan-carbon nanoparticle a good candidate to design polar vapour sensors, Sens. Actuators B Chem. 138 (2009) 138–147. S.A. Fischer, B.I. Dunlap, D. Gunlycke, Proton transport through hydrated chitosanbased polymer membranes under electric fields, J. App. Polym. Sci. B 55 (2017) 1103–1109. Y. Hirabayashi, Pervaporation membrane system for the removal of ammonia from water, Mater. Trans. 43 (2002) 1074–1077. P.C. Singh, G.N. Patwari, Proton affinity correlations between hydrogen and dihydrogen bond acceptors, J. Phys. Chem. A 111 (2007) 3178–3183. F. Sattarzadeh, M.M. Doroodmand, M.H. Sheikhi, A. Zarifkar, Fabrication of a humidity sensor based on chemical vapor deposition-synthesized single-walled carbon nanotubes, Sci. Adv. Mater. 5 (2013) 557–565. K. Zhang, J.P. Zou, Q. Zhang, Roles of inter-SWCNT junctions in resistive humidity response, Nanotechnology 26 (2015) 455501. D. Raymand, A.C.T. van Duin, D. Spångberg, W.A. Goddard III, K. Hermansson, Water adsorption on stepped ZnO surfaces from MD stimulation, Surf. Sci. 604 (2010) 741–752. S.K. Sharma, P.K. Pujari, Role of free volume characteristics of polymer matrix in bulk physical properties of polymer nanocomposites: a review of positron annihilation lifetime studies, Prog. Polym. Sci. 75 (2017) 31–47. C. Tsonos, A. Kanapitsas, G.C. Psarras, T. Speliotis, Effect of ZnO nanoparticles on
the dielectric/electrical and thermal properties of epoxy-based nanocomposites, Sci. Adv. Mater. 7 (2015) 588–597. [35] S.S. Nasab, S. Zahmatkesh, Preparation, structural characterization, and gas separation properties of functionalized zinc oxide particle filled poly(ether-amide) nanocomposite films, J. Plast. Film Sheet. 33 (2017) 92–113. [36] S. Vural, S. Koytepe, T. Seckin, I. Adiguzel, Synthesis, characterization and dielectric properties of rodlike zinc oxide-polyimide nanocomposites, Polym-Plast. Tech. Eng. 51 (2012) 369–376. [37] S.E. Gamal, A.M. El Sayed, E.E. Abdel-Hady, Effect of cobalt oxide nanoparticles on the nano-scale free volume and optical properties of biodegradable CMC/PVA films, J. Polym. Environ. 26 (2018) 2536–2545. Haipo Dai is now in master course in Materials Science and Engineering, Taiyuan University of Technology, China. His current research interests are fabrication of carbon nanotube-based nanocomposite and their applications to gas sensors. Nana Feng is now in master course in Materials Science and Engineering, Taiyuan University of Technology, China. Her current research interests are fabrication of carbon nanotube-based nanocomposite and their applications to gas sensors. Jiwei Li is now in master course in Materials Science and Engineering, Taiyuan University of Technology, China. Her current research interests are fabrication of graphene-based nanocomposite and their applications to gas sensors. Jie Zhang received her Ph.D in Textile Materials at Donghua University, China in 2013. She is working as a lecturer in the Department of Materials Science and Engineering at Taiyuan University of Technology, China. Her current research interests are nanocomposite materials based on inorganic nanoparticles and biopolymers and their applications as functional materials. Wei Li received his PhD in Materials Science and Engineering at Chungnam National University, South Korea in 2011. He is now an associate professor of Department of Materials Science and Engineering at Taiyuan University of Technology, China. His current research interests are fabrication of nanostructured materials including carbon nanotubes and oxides and their applications to electronic devices of solar cells, gas sensors, biosensor, etc.
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