Article pubs.acs.org/JPCC
Carbon Nanotube Based Humidity Sensor on Cellulose Paper Jin-Woo Han,* Beomseok Kim, Jing Li, and M. Meyyappan Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035, United States ABSTRACT: A humidity sensor on cellulose paper is demonstrated using single-walled carbon nanotubes functionalized with carboxylic acid. The conductance shift of the nanotube network entangled on the microfibril cellulose is utilized for the humidity sensing. Compared to the control sensor made on a glass substrate, the cellulose-mediated charge transport on the paper substrate enhances the sensitivity. The sensor response exhibits linear behavior up to a relative humidity of 75% with good repeatability and low hysteresis. A simple circuit model is used to explain the sensor results. This approach is a step toward future paper electronics for low-cost disposable applications.
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INTRODUCTION Electronic devices built on cellulose paper substrates can be cheaper than equivalent class of solid-state devices while providing reasonable performance. Furthermore, the paperbased devices can be used for flexible, foldable, biodegradable, and disposable applications such as biosensors, intelligent packaging, business cards, and advertising banners. Therefore, several studies on paper electronics have appeared recently with implementation examples of lab-on-paper,1 thin-film transistor,2 nonvolatile memory,3 RFID tags,4 electroluminescence devices,5 dye-sensitized solar cell,6 battery,7 supercapacitor,8 and printed circuit board.9 Chemical and biosensors on inexpensive substrates such as paper are also of interest since their utility covers a wide range of applications. Here, a humidity sensor is built on a paper substrate as a fundamental building block of paper electronics, and performance metrics including linearity, sensitivity, hysteresis, and response/recovery times are assessed. We have utilized single-walled carbon nanotubes (SWCNTs) to construct the sensor on paper, in contrast to conventional silicon, polymer or organic conducting materials employed in previous paper electronics efforts mentioned above.1−9 SWCNTs have been successfully utilized for sensing of a wide range of gases and vapors, and reviews on this subject can be found in refs 10−12. The gas sensors are typically classified according to transduction method into capacitor,13 transistor,14 resistor,15 microbalance,16 and fiber optic.17 Each class has its own strengths and weaknesses, but the resistive type sensors are characterized by their simple structure, low fabrication cost, and simple read-out circuitry. Here, a resistor type humidity sensor was fabricated on cellulose papers.
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Figure 1. (a) Schematic illustration of the humidity sensor built on a cellulose paper substrate, (b) networks of CNTs on the paper showing that the device is flexible and custom-cut, (c) SEM image of the cellulose paper (scale bar 250 μm), and (d) magnified image of the cross-linked CNTs (scale bar 100 nm).
performance of some electronic devices such as transistors and diodes; on the contrary, the roughness and porosity are attractive here because they increase the contact area with the ambient air and promote the adhesion to CNTs. The 10 cm filter paper was conformally coated with COOH-functionalized SWCNTs, and the flexible paper can be custom cut as shown in Figure 1b. The substrate was cut to a size of 2 × 10 mm and the scanning electron microscopy (SEM) images in Figure 1, parts
EXPERIMENTAL METHOD
The fabricated sensor shown in Figure 1a has a network of cross-linked SWCNTs with purity over 99%. An ordinary cellulose paper used for filtration was employed as the substrate. The filter paper exhibits medium porosity with a flow rate of 60 mL/min and particle retention of 5−10 μm. The roughness and porosity of the papers tend to hamper the © 2012 American Chemical Society
Received: August 12, 2012 Revised: September 21, 2012 Published: September 25, 2012 22094
dx.doi.org/10.1021/jp3080223 | J. Phys. Chem. C 2012, 116, 22094−22097
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c and d, show that the nanoscale CNTs are firmly entangled with the microscale cellulose fibers. No heat treatment or vacuum environment was involved in the course of device processing. The SWCNTs were functionalized with carboxylic acid (COOH) to render them hydrophilic, thus increasing the adhesion with the substrate. The functionalized SWCNTs were dispersed in dimethylformamide solution. The film composed of networks of cross-linked CNTs was formed using drop-cast coating followed by evaporation of the solvent. Adhesive copper foil tape was used for contact electrodes and the distance between the two electrodes was about 2 mm. The current−voltage (I−V) characteristics were measured by semiconductor parameter analyzer (Agilent 4156C). The sensors were tested under thermostatic test chamber. The relative humidity (RH) of the test chamber was controlled by mixing appropriate amount of dry and wet air. The relative humidity was varied from 10% to 90% and the chamber was maintained at room temperature.
relative humidity is too high, water dissociation occurs on the moist cellulose fiber under the applied bias, creating H+ and OH− ions. Thus, the current can flow due to ionic conduction. Figure 2b exhibits the threshold humidity defined as the RH that initiates the current conduction. The bare paper substrate acts as insulator under low RH but current begins to flow at high RH above 85%. The intrinsic paper may be therefore limitedly used as threshold humidity detector, but it is impractical for ordinary sensor applications. In contrast, the CNT resistor sensor responds over the entire range of RH as shown in Figure 2c. The conductance decreases linearly with increasing humidity up to RH = 75%, and then increases slightly with humidity. The sensitivity is defined as the ratio of the relative conductance difference over the relative humidity difference: ((Sx − S0)/S0)/(RHx − RH0) where the subscripts x and 0 correspond to specific RH under investigation and RH = 10%, respectively. The sensitivity of CNT resistor sensor is 6% in the linear regime as shown in Figure 2c. The humidity effects on electrical conductivity of CNT have been well established through theoretical calculations and experimental demonstrations.11,18,19 The CNT network shows global p-type semiconducting behavior, where the electrical conduction is dominated by holes.20 The H2O molecules adsorbed on the surface are known to behave like electron donors. In other words, an increase in humidity results in a reduction of hole density of the p-type nanotubes. Besides the electron donation model, other mechanisms for the humidity effect have been proposed as well: hydrogen bonding on the oxygen defect sites present on the nanotubes; possible introduction of charge traps on the nanotubes arising from direct adsorption of water molecules on the substrate.11 In any case, the conductivity decrease for RH > 75% here seems to be canceled out by some factors. If the conduction via cellulose fibers is attributed to this offset, the transition should occur around RH = 85%. However, the observed transition point is smaller than the threshold humidity of the bare cellulose, which implies the possibility of some other mechanism. This inference is reasonable due to the fact that the conductance increase of bare paper is of the order of nS whereas the conductance decrease of the CNT resistor is of the order of μS. The possible cause of compensation can be the counter doping effect.21 When the nanotubes are exposed to moisture, the p-type CNTs can contain excessive electrons as the H2O molecules act as electron donors. In other words, the intrinsic p-type semiconductor can behave like an n-type semiconductor in a humid environment since the generated excess electrons completely compensate the intrinsic majority holes. In the present case, the compensation doping effect seems to occur at RH = 75%, which is apparently smaller than the conduction threshold humidity of bare paper (RH=85%). Beyond this compensation humidity, the CNT is no longer p-type but n-type characteristics are seen and the humidity increases the conductivity of ntype semiconductor. Therefore, the trend offset at RH > 75% can be explained by compensation doping effect caused by high humidity. To compare the sensing performance between devices built on different substrates, some control sensors were fabricated on glass using an identical process. The paper device exhibits superior sensitivity to its glass counterpart shown in Figure 2d. At RH = 10%, the conductivity of the sensor on glass is roughly 1 order of magnitude lower than that on paper. The porous and rough surface of the paper is favorable to accommodate individual CNTs tightly compared to the relatively uniform and
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RESULTS AND DISCUSSION Figure 2a shows the I−V characteristics of bare paper and CNT resistor for RH = 10% and RH = 50%. The current did not flow
Figure 2. (a) Current−voltage (I−V) characteristics of intrinsic paper and CNT resistor for RH = 10% and 50%. Conductance responses of (b) bare cellulose paper substrate, (c) CNT sensor on cellulose paper substrate, and (d) CNT sensor on glass substrate.
on the unprocessed paper implying that the cellulose network is an insulator. The CNT resistors show ohmic characteristics, and the conductance decreases with increasing moisture content. Since the differential resistance is constant over the supply voltages, the sensitivity is identical regardless of the operation bias. In other words, a low voltage operation does not hinder the sensitivity thus allowing low power operation. The low power feature is important in self-powered applications such as ubiquitous sensor networks. The operation voltage in this work is fixed at 1 V. To clarify the impact of the substrate on humidity, bare cellulose paper was tested first. When the 22095
dx.doi.org/10.1021/jp3080223 | J. Phys. Chem. C 2012, 116, 22094−22097
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Figure 3. Schematic illustrations of humidity sensing mechanism for (a) CNT sensor on glass substrate and (b) CNT sensor on cellulose paper substrate. (c) Schematic illustration of carrier conduction between unconnected nanotubes assisted by cellulose fiber.
even surface of the glass. Furthermore, the paper can soak up CNTs from the suspension solution due to its high wettability, leading to firm adhesion. As a result, the connectivity of SWCNTs is promoted easily on paper. The sensitivity of the sensor-on-glass is 1% in the linear regime and thus, the paper sensor displays higher sensitivity than the glass sensor. To explain the observed results, a simplified circuit model is presented and a schematic of the conduction mechanism is shown in Figure 3. The CNTs are randomly networked on glass whereas they are entwined with the cellulose fibers as backbone on the paper. On glass, the current pathway is formed only across CNTs. The overall resistance on glass (RGlass) is composed of n resistances of kth CNT (rCNTk) in series:
microfibril cellulose (RCFm) in series, but its contribution to RCF can be neglected because the global cellulose fiber network tends to be insulating. However, the localized cellulose fibers segmented by CNTs can significantly impact RPaper. The RCNT,CF is the total sum of the localized resistance of nth cellulose fibers (rCNT,CFn) segmented by CNTs. The cellulose is a straight chain polymer with rod-like conformation. The cellulose molecules have multiple hydroxyl groups that form hydrogen bonds within and between cellulose molecules. In addition, these hydroxyl groups can also connect to the surface of CNTs by hydrogen bonding. Therefore, although the microfibril fiber is an insulator, these segments electrically bridge the adjacent disconnected CNTs. Then, carrier hopping and tunneling boost intertube conduction. As a result, the increase in percolation path and the reduction of the effective conduction distance result in an increase in sensitivity. Figure 4a shows the response and recovery curves of the present sensor. At t = 0 s, the sensor in an ambient of RH = 10% is exposed to RH = 60% air and switched back to RH = 10%. The sensor response time is about 6 s and the sensor
R Glass = rCNT1 + rCNT 2 + ··· + rCNTk − 1 + rCNTk
On cellulose, the current pathways are formed by both nanoscale CNTs and microscale cellulose fibers. Therefore, the overall resistance on paper (RPaper) is considered to be a parallel combination of the resistance via CNTs (RCNT), the resistance of cellulose fibers (RCF), and the resistance through CNTs and cellulose fibers (RCNT,CF). 1 RPaper
=
1 R CNT
+
1 1 + R CF R CNT , CF
R CNT = rCNT1 + rCNT 2 + ··· + rCNTl − 1 + rCNTl R CF = rCF1 + rCF 2 + ··· + rCFm − 1 + rCFm
R CNT , CF = rCNT , CF1 + rCNT , CF 2 + ··· + rCNT , CFn − 1 + rCNT , CFn
RCNT is composed of l resistances of lth CNT (rCNTl) in series, which is similar in form to RGlass, but RCNT is smaller than RGlass since CNTs are closely and densely packed as previously deduced. The RCF is composed of m resistances of mth
Figure 4. (a) Response and recovery curves for a change between 10 and 60% RH and (b) repeatability curves of the CNT sensor on paper. 22096
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recovery time to reach RH = 95% of the final state is about 120 s. Figure 4b shows the good repeatability of the sensor when alternatively exposed to different humidity levels. The dynamic response shows that the conductance at a given humidity is identical, implying a negligible hysteresis effect. Finally, the performance of our sensor on cellulose paper is comparable to other published CNT-based humidity sensors on various substrates. 19,22 For example, MWCNTs modified with MnWO4 show a sensitivity of about 8% but very high response and recovery times; in contrast, LiClO4/ MWCNTs show a higher sensitivity and a fast recovery time of 1 min.19 Response and recovery times of 30 and 25 s respectively have been obtained for switching between 8−93% RH for PMMA/ MWCNT thin films doped with KOH.22 Compared to resistive sensors, field effect transistor sensors with CNTs appear to provide msec response times.23 Our sensors here also perform better than oxide nanowire based humidity sensors24−26 in terms of sensitivity and response/recovery times, besides the attractive room temperature operation.
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CONCLUSIONS Resistor-type humidity sensors were demonstrated on cellulose paper, employing carboxylic acid functionalized single walled carbon nanotubes as sensing material. The sensors were solution-processed without heat and vacuum treatment. The conductivity linearly decreased with respect to the relative humidity up to 75%. The sensor on paper showed better sensitivity than the control sensor on glass. The cellulosebridged charge transport between intertubes boosts the moisture contact area and thus, the sensitivity. The present paper-based sensor features low-cost, flexible, foldable, biodegradable, and disposable characteristics. The sensor application can be extended to other gases and vapors and construction of an electronic nose with an array of sensors.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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