Flexible and Highly Sensitive Humidity Sensor ... - ACS Publications

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A flexible and highly sensitive humidity sensor based on cellulose nanofibers and carbon nanotubes composite film Penghui Zhu, Yu Liu, Zhiqiang Fang, Yudi Kuang, Yazeng Zhang, Congxing Peng, and Gang Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04259 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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A flexible and highly sensitive humidity sensor based on cellulose nanofibers and carbon nanotubes composite film Penghui Zhu,‡ Yu Liu,‡ Zhiqiang Fang, Yudi Kuang, Yazeng Zhang, Congxing Peng and Gang Chen* State Key Laboratory of Pulp and Paper Engineering, Guangdong Engineering Technology Research and Development Center of Specialty Paper and Paper-based Functional Materials, South China University of Technology, Guangzhou 510640, China * Email: [email protected] ‡ These authors contributed equally to this work.

Abstract: Flexible and highly sensitive humidity sensors are crucial to humidity detecting. In this study, a flexible cellulose nanofiber/carbon nanotube (NFC/CNT) humidity sensor with high sensitivity performance was developed via a fast vacuum filtration. CNTs were well dispersed in water by using 2,2,6,6-tetramethylpiperidinyl1-oxyl (TEMPO) oxidized NFC as a dispersant. More importantly, NFC also acted as a humidity sensitive material, achieving superior performance of NFC/CNT humidity sensors. The obtained NFC/CNT humidity sensor with 5 wt% CNTs loading exhibits outstanding sensitive performance and its response value reaches a maximum of 69.9% (ΔI/I0) at 95% relative humidity (RH). It also displays good bending resistance and long-term stability. In addition, the NFC/CNT humidity sensor was employed to monitor human breath. Therefore, we believe that the flexible, highly sensitive and

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simple designed NFC/CNT humidity sensor is a promising candidate for various applications in the field of humidity measurement.

Keywords: carbon nanotube; nanofibrillated cellulose; composite film; flexible humidity sensor

Introduction In recent years, humidity measurement is attracting much attention both in scientific and industrial field. The applications are ranging from weather prediction, agriculture and industrial manufacturing to historic preservation and wearable electronics.1-2 In particular, the measurement of human body humidity is significant for evaluating human health. Humidity sensor, an important tool for detecting changes in humidity, is widely used in our daily life. It is also suitable for real-time monitoring of human breath because it can quickly convert human respiratory signals into visual electrical signals, which greatly facilitates people's lives. However, most commercially available humidity sensors are based on porous ceramics or metal oxides.3-5 The application of the sensors is limited owing to a rigid structure or a complicated integration process. Furthermore, the measurement accuracy and sensitivity of traditional sensors are constrained by high temperature or high humidity. Therefore, achieving humidity sensors with flexibility, wide response range and high sensitivity is vital to its practical application. Carbon nanotubes (CNTs) are famous one-dimensional nanomaterials. They have


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been studied and applied in many fields because of their unique electrical, thermal and mechanical properties.6-9 Meanwhile, they are also the ideal materials to fabricate resistive-type humidity sensor owing to its high aspect ratio and nanoscale hollow structure.4,10 For CNT-based humidity sensors, p-type CNTs whose main carrier is hole are commonly used as the humidity sensitive material for water molecules adsorption.2 The electrons of adsorbed water molecules are transferred to CNTs and compensate their holes, leading to an increase in the resistance of CNTs. However, such humidity sensing property cannot be fully employed for two reasons: (1) poor water molecules adsorption due to the intrinsic hydrophobicity of CNTs and (2) the severe agglomeration caused by the strong van der Waals forces among CNTs in aqueous solutions.11-13 Recently, many efforts have been taken to improve the sensitivity of CNT-based humidity sensors. (1) Introducing oxygen-containing groups on the surface of CNTs by modification treatment.10,14 (2) Blending CNTs with hydrophilic polymers (such as polyimide, polyvinyl alcohol and hydroxyethyl cellulose) to prepare CNT/polymer composite materials.6,15-17 However, the improvement is still very limited for the sensors practical applications. Nanofibrillated cellulose (NFC), a biodegradable material which originates from cellulose fibers, is a promising humidity sensitive material because of the massive hydroxyl groups existed in its molecular chains.9,18-21 It can interact with water molecules through hydrogen bonding. Besides, many researchers have also proven that NFC can be used as a surfactant to facilitate the dispersion of CNTs in aqueous medium owing to its amphiphilicity.9,13,22-24 These make NFC an ideal candidate to prepare


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NFC/CNT composites for humidity sensing. In recent years, some work has been conducted to develop the emerging applications of NFC/CNT composite materials, which including printable flexible electronics, high-strength conductive fiber and energy storage.9,11,18,24 Although these studies have been implemented, there is little literature regarding the humidity sensitive property of the NFC/CNT composite so far. In this work, TEMPO-oxidized NFC was utilized to enhance the dispersion of CNTs in water as well as the adsorption of water molecules by CNTs in the composite films. Flexible humidity sensors with excellent sensitive performance were prepared by NFC/CNT dispersion using layer-by-layer self-assembly method. The humidity sensitive behavior of the obtained NFC/CNT sensors was investigated in detail. Meanwhile, humidity sensitive mechanism was also discussed. This work fully analyzed the humidity sensitive performance of NFC/CNT sensors, which demonstrates the practical application of the NFC/CNT humidity sensors for monitoring human breathing.

EXPERIMENTAL SECTION Materials. Bleached softwood pulp was kindly supplied by Lee & Man Paper Manufacturing Co., Ltd (Chongqing, China). 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was purchased from Sigma Aldrich. Sodium bromide (NaBr) was obtained from Kemiou Chemical Reagent Co., Ltd (Tianjin China). Sodium hypochlorite (NaClO) was purchased from Fuyu Fine Chemical Co., Ltd (Tianjin, China). Commercially available multiwalled carbon nanotubes (>95 wt% purity) was obtained


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from Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). The CNTs are 0.5– 2 μm in length and 10–20 nm in diameter. Preparation of NFC/CNT composite films. The NFC dispersion was prepared from the above bleached softwood pulp through TEMPO mediated oxidation pretreatment and homogenization according to our previous publication.25 The obtained aqueous NFC was then diluted with ultrapure water to a concentration of 0.1 wt%. Next, NFC dispersion (0.1 wt%, 400 mL) was blended with CNTs and stirred for 10 min by using a magnetic stirrer. And then, the mixture was sonicated by an ultrasonic homogenizer (JY92-IIN, Scientz, China) at 650 W and 50% amplitude in an ice bath for 20 min. After ultrasonic treatment, the dispersion was centrifuged at 9500 rpm for 1 h in order to remove the CNTs aggregates from the mixture and the filtrate was collected. The actual concentration of CNTs in the resulting product was measured by the method reported in the literature.24 NFC/CNT dispersions with lower CNTs loading were prepared by adding NFC solution (0.1 wt%) under continuous high-speed agitation for 10 min. Subsequently, The NFC/CNT composite films with a given ratio were prepared by vacuum-assisted filtration using polyvinylidene fluoride (PVDF) filter membranes with 0.22 μm pore size. After filtration, the formed NFC/CNT gels were pressed to dry in the ambient and then peeling from the filter membranes. The thickness of the asprepared NFC/CNT films was controlled to 24 μm. Preparation of NFC/CNT humidity sensors. The resulting NFC/CNT composite films were cut into 20×5 mm2 rectangle shapes. Adhesive copper foil tape was used as contact electrodes, and the exposure part has a size of 10×5 mm2.


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Characterization. The morphology of the NFC/CNT film was determined by a field scanning electron microscope (Merlin, Zeiss, Germany) in 5 kV. The XRD patterns of the samples were performed by an X-Ray diffractometer with Cu-Ka radiation (D8 Advance, Bruker, Germany). The Raman spectra of the samples were recorded by a Raman spectrometer (LabRAM Aramis, Jobin Yvon, France) equipped with a laser with an excitation wavelength of 532 nm. Fourier transform infrared spectra (FTIR, VERTEX 70, Bruker, Germany) of the samples were measured in the 3600-600 cm−1 range. The morphology of NFC and NFC-dispersed CNTs was characterized by an atomic force microscope (Multimode 8, Bruker, Germany) in tapping mode under ambient conditions. Transmission electron microscopy images were obtained using a transmission electron microscopy (JEM-1200EX, JEOL, Japan) at an acceleration voltage of 120 kV. The zeta potential of NFC solution and NFC/CNT dispersions was measured by using a laser scattering particle size distribution analyzer (LA-960S, HORIBA, Japan). Humidity sensing measurements. The electrical property of the as-prepared humidity sensors was characterized by an electrochemical workstation (CHI660E, CHI, China). The current-voltage (I–V) curves of the humidity sensors were measured under dry environment. And the dry condition was prepared by P2O5 powder. The electrical performance of the samples under different humidity conditions was recorded by placing them in many closed bottles with saturated solutions of different salts at 23 ℃ under an applied voltage of 1 V, respectively. The various humidity environment produced by saturated solutions of different salts were listed as follows: LiCl (11%),


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CaCl2·6H2O (29%), NaBr·2H2O (57%), NaCl (75%) and KNO3 (95%).

RESULTS AND DISCUSSION Fabrication of NFC/CNT humidity sensors. Figure 1a illustrates a schematic for the fabrication of homogeneous NFC/CNT solution. NFC was isolated from the wood fibers by TEMPO/NaClO/NaBr oxidation system pretreatment and a mechanical homogenization. Strong negatively charged COO- groups were introduced to the surface of NFC in this process. Meanwhile, it is known that NFC is an amphiphilic material owing to the presence of hydrophobic C-H moieties and hydrophilic O-H groups on cellulose molecular chain.26-27 NFC can interact with CNTs through the hydrophobic sites of CNTs and specific crystalline faces (hydrophobic (200) planes) of NFC. As a result, the electrostatic repulsive forces provided by COO- groups and the hydrophobic interaction between NFC and CNTs facilitate the dispersion of CNTs in NFC solution. The obtained NFC/CNT solution can keep well-dispersed even after one month. In contrast, CNTs undergo severe agglomeration in water without NFC while using the same sonication process (Figure S1). Morphological characterization and zeta potential measurement of the NFC/CNT solution further confirm that the uniform dispersion of CNTs by NFC (Figure S2, S3). The obtained solution can be assembled into NFC/CNT composite films with hierarchical structure by using a facile vacuum filtration (Figure 1b). Here, NFC plays an important role for improving the humidity sensitive performance of CNTs network since water molecules can be firmly absorbed by the massive hydroxyl groups in


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cellulose molecular chains. Without NFC, the hydrophobicity of CNTs makes it hard for the water molecules adsorption, which greatly restricts its humidity sensitive behavior. Compared to previous publications, our work for the fabrication of NFC/CNT humidity sensor presents many advantages: (1) CNTs were dispersed in water by NFC. Compare with the commonly used surfactants, such as cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and polyoxyethylene (10) stearyl ether (Brij 76).12,28-33 NFC originates from wood fibers, which is green and biodegradable. It is also considered that this dispersion method is scalable and environmental friendly. (2) Conventional humidity sensors are mainly based on porous ceramics or metal oxides whose rigid structure hinders their applications in large part. In contrast, the novel humidity sensor prepared by NFC/CNT film is highly flexible, which can meet the requirement of wearable electronic device.

Figure 1. Schematic illustrating the dispersion mechanism of CNTs by NFC and the hierarchical structure of the obtained NFC/CNT film. (a) The dispersion of CNTs by the hydrophobic-hydrophobic interaction between NFC and CNTs. (b) The hierarchical structure of the prepared NFC/CNT film. The adsorption of water molecules by the composite film was enhanced by NFC.

Characterization of NFC/CNT humidity sensors. Typically, NFC/CNT


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dispersions with lower CNTs content were prepared by diluting with 0.1 wt% NFC solution (as shown in Figure S4). Based on this, a series of NFC/CNT composite films with CNTs weight ratios of 1%, 2%, 3%, 5% and 10% (abbreviated as CNT01, CNT02, CNT03, CNT05 and CNT10, respectively) were obtained by vacuum filtration method. As shown in Figure 2a, a CNT10 humidity sensor with two electrodes was assembled. The sensor exhibits high flexibility and its bending radius is less than 0.15 cm (Figure 2b). Microstructure of the CNT10 film was characterized by scanning electron microscopy, it can be observed that the film surface is relatively smooth, there are no obvious micropores or cavities within the film (Figure 2c). At high magnification, NFC and CNTs can be easily distinguished by their diameters (Figure S5). They tightly intertwine with each other and form a fibrous conductive network (Figure 2d). Figure 2e shows the cross-sectional morphology of CNT10 film, in which NFC/CNT building blocks stack together and a regularly ordered layered structure is created. The special lamellar structure is significant to introduce more water molecules acceptors for humidity sensing.34 From Figure 2f, it can be seen that CNTs distribute uniformly in the layers of the film, revealing the excellent dispersion performance of NFC. The interaction between CNTs and NFC in the composite film was studied by X-ray diffraction. As shown in Figure 2g, three peaks at 2θ = 14.6°, 16.5° and 22.4° correspond to the (1-10), (110) and (200) lattice planes, respectively, which suggest that the prepared NFC exhibits a typical cellulose I crystalline form.35 With regards to CNTs, a peak at 2θ = 26° assigns to (002) lattice plane, but the intensity of the peak becomes so weak in CNT10 composite film. Furthermore, the effect of CNTs on the


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index of crystallinity (CrI) of NFC in the composite film was studied. Compare with NFC film, the CrI of CNT10 film decreases from 59.1% to 49.9%, indicating the crystalline regions of cellulose were partly destroyed by the interaction between CNTs and NFC. To clarify the interaction between CNTs and NFC, the composite film was performed by Raman spectra (Figure 2h), the peak at about 1092 cm-1 stands for the stretching vibration of C-O-C band of NFC, whereas it disappears for the composite film with 10% CNTs loading. Two characteristic peaks at 1332 cm-1 and 1590 cm-1 represent the D-band and G-band, respectively. D-band is induced by defects and curvature in the nanotube lattice. G-band is attributed to the in-plane vibration of the C–C bonds.32 The intensity ratio of D-band and G-band (ID/IG) is commonly used to evaluate the degree of defects in carbon materials. Compare with CNTs, the ID/IG of CNT10 film decreases from 1.45 to 1.32. The slight change indicates that the structure of CNTs retained integrated in the ultrasonic process. Meanwhile, it also implies that the noncovalent interaction was formed between NFC and CNTs without introducing covalent defects.36


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Figure 2. (a) Photograph of the obtained CNT10 humidity sensor. Adhesive copper foil tape was used as contact electrodes, and the exposure part has a size of 10×5 mm2. (b) Photograph shows the high flexibility of the prepared sensor. R represents bending radius of the sensor. (c) Low-magnification and (d) high-magnification SEM images show the NFC/CNT network structure of the CNT10 film. (e) Lowmagnification and (f) high-magnification SEM images show the layer-by-layer structure of the CNT10 film. (g) The XRD patterns, (h) Raman spectra and (i) FTIR of CNT, CNT10 film and NFC film.

This noncovalent interaction was further confirmed by the Fourier transform infrared spectra in Figure 2i. The peak in the range of 3600 cm-1 to 3000 cm-1 represents the stretching vibration of formed O-H band. It is worth noting that this peak can also be observed for the sample of CNTs, indicating that there exist oxygen-containing groups on the surface of CNTs. Therefore, it can be concluded that the noncovalent interaction between CNTs and NFC is hydrogen bonds, and the formed hydrogen bonds are crucial to achieve the outstanding humidity sensitive performance of NFC/CNT sensors. Humidity sensing properties of NFC/CNT humidity sensors. Figure 3a shows the current-voltage (I–V) curves of CNT/NFC films under dry environment. As we can see there is linear relationship between voltage and current, which demonstrates that the composite films display typical ohmic characteristic.37 To explore the effect of CNTs


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content on the humidity performance of prepared sensors, the relationship between the electric property of sensors and relative humidity (RH) was investigated. As shown in Figure 3b, CNT01, CNT02 and CNT03 films act as insulators under low RH conditions, but the current begins to flow through the films when RH is higher than 57%. It is because the water molecules were dissociated into H+ and OH- ions on the films surface, and the ionic conduction results in the generation of current.37 Meanwhile, the films were also connected to a circuit with two batteries as power source (3 V) to further verify their electrical conductivity in the ambient, respectively (Figure S6). Not surprisingly, the LEDs were not lit, which confirm that the resistance of the films with low CNTs content is too high for practical application even though they are able to respond to high RH. In contrast, the current-RH curves of CNT05 and CNT10 films were shown in Figure 3c-d. It can be found that the current decreases almost linearly with the increase of RH, demonstrating typical p-type CNTs humidity response behavior. Similarly, the CNT05 film and CNT10 film were also connected to the circuit. As a result, both LEDs were lit, and the LED in CNT10 circuit was obviously brighter than that in CNT05 circuit (Figure S6). It is evident that the increase of CNTs loading leads to a decrease in resistance of NFC/CNT composite films. Notably, compare to CNT03 film, the resistance of CNT05 film decreases by nearly 3 orders of magnitude. It can be deduced that the conductive paths are formed inside the composite film when CNTs loading reaches the critical state of 5 wt%. Therefore, the percolation threshold of the composite films is about 5 wt% CNTs.


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Figure 3. Electrochemical characterization of the assembled NFC/CNT humidity sensor. (a) Currentvoltage (I–V) curves of CNT/NFC films with different CNTs loading under dry environment. (b) Current-RH curves of CNT01, CNT02 and CNT03 films. Current-RH curves of (c) CNT05 and (d) CNT10 films.

Based on the above results, the sensing performance of CNT05 and CNT10 sensors was further studied. The tests were conducted with an assembled device which was illustrated in Figure S7. The dynamic response and recovery curves were recorded between 11% and 29%, 57%, 75% and 95% RH, each humidification or dehumidification process lasted 5 min. The response is used to evaluate the sensors performance, which is defined by the following equation: Response = -ΔI/I0, where ΔI = IRH － I0, I0 is the initial current measured at 11% RH. IRH represents the current flowing through the film when exposed to different RH conditions. The real-time response of CNT05 sensor is plotted as function of RH in Figure 4a. For each RH condition, three repeated response-recovery processes were performed. It can be seen that the CNT05 sensor can make a fast response to the change of RH. And the highest response is achieved at 95%RH. It is because more water vapors can be captured by the film at high RH and lead to the great change in current. However, the response curve


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shows a slight difference between three repeated dynamic measurement. This is attributed to the long-time adsorption of water molecules that make the sensor unable to reach equilibrium within 5 min. Figure 4b shows the response as a function of RH for CNT05 sensor, there is a linear correlation (R2 = 0.9822) between response and RH, which suggests that the sensor we prepared have excellent linearity. The dynamic response curve of CNT10 sensor was displayed in Figure 4c, in which a curve tendency similar to that of CNT05 sensor was observed. It implies that the CNT10 sensor also need more than 5 min for the water molecules to complete adsorption or desorption. Furthermore, the relationship between the response of CNT10 sensor and RH was plotted in Figure 4d. As we can see the response increases linearly (R2 = 0.9841) with RH. It also shows good linearity. However, compare to CNT05 sensor, response of CNT10 sensor is obviously weaker over the entire range of RH. This can be explained by the effect of percolation threshold. From the mentioned above, we have known that the conductive CNTs paths are just formed in the composite film when CNTs loading reaches 5%. These conductive paths are susceptible to NFC due to its swelling behavior under high RH. But the swelling behavior of NFC has less impact on CNT10 sensor because there are more conductive paths in CNT10 sensor. Therefore, the change in RH is more likely to result in great response for CNT05 sensor.


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Figure 4. Humidity sensing response of CNT05 and CNT10 sensors. (a) Dynamic response and recovery curves of CNT05 sensor between 11% and 29%, 57%, 75% and 95% RH. The sampling time for each adsorption or desorption process was 5 min. (b) Response of CNT05 sensor as a function of RH. (c) Dynamic response and recovery curves of CNT10 sensor between 11% and 29%, 57%, 75% and 95% RH. (d) Response of CNT10 sensor as a function of RH.

In order to explore the response and recovery time of the humidity sensors, the sampling time was extended to 15 min for the sensors to reach equilibrium state. The response and recovery time in this work is calculated by the time taken by a sensor to achieve 90% of the total current change during humidification-dehumidification process, respectively. Figure 5a shows the response and recovery curves of CNT05 sensor, the response and recovery time of CNT05 sensor is 330 s and 377 s, respectively. In contrast, the CNT10 sensor exhibits a longer response (401 s) and recovery time (418 s) (Figure 5b). It also can be calculated from the curves that the maximum response of CNT05 is 69.9%, which is higher than that of CNT10 sensor (66.0%). The hysteresis of the NFC/CNT sensors was also investigated by measuring the current during RH increasing from 11% to 95% and then conversely decreasing from 95% to 11%. As


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shown in Figure S8, the maximum hysteresis of CNT05 and CNT10 sensor is about 3.2 % and 3.6 % at 75% RH, respectively. Therefore, it can be concluded that the CNTs content has a significant impact on the performance of NFC/CNT sensors. And the sensor with 5% CNTs loading shows the best humidity sensitive property in this work.

Figure 5. Response and recovery curves of (a) CNT05 and (b) CNT10 sensor between 11% and 95% RH.

The obtained NFC/CNT films are highly flexible. In order to investigate the effect of flexibility on the humidity sensitive performance of NFC/CNT sensors, the currentRH curve and dynamic response curves of CNT05 sensor under bending state were measured. As shown in Figure 6a, since the bending angle of the film cannot be accurately measured, we used the distance between the two ends of the film to characterize the bending angle. And the bending angle of the sensor in the test was 1.4 cm. Figure 6b exhibits the relationship between the electric property of CNT05 sensor and RH, it can be seen that the resistance of the sensor increases under bending state. This is because the conductive CNTs path is partially destroyed by the inner stress. Meanwhile, the real-time response and recovery curves of the sensor under bending state from 11% to 57% and 95% RH were also tested (Figure 6c and 6d). Compare to the unbent CNT05 sensor, the response value is somewhat reduced, but the sensitivity


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is still as high as 64.4% at 95% RH. This indicates that the NFC/CNT sensor exhibits good bending resistance.

Figure 6. Humidity sensing property of CNT05 sensor under bending state. (a) A schematic illustration for the bending treatment of CNT05 sensor. (b) Current-RH curve of CNT05 sensor under both unbending and bending states. Dynamic response and recovery curves of CNT05 sensor between 11% and (c) 57% and (d) 95% RH.

The long-time stability is vital to the practical application of the humidity sensors. To evaluate the stability of the prepared NFC/CNT sensors, the dynamic response and recovery curves were re-measured after five months. As shown in Figure S9, the CNT05 sensor can still make a fast response to the change of RH, and the response value does not show obvious changes. Furthermore, the response of the sensor also increases linearly (R2 = 0.994) with RH. These results demonstrate that the NFC/CNT humidity sensor has good long-term stability.


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Table 1. The comparison of the humidity sensors performance between this work and previous publications. Output No.

Component

Response RH range

Response

signal H2SO4 and HNO3 (3:1) treated 1

MWCNTs/silicone rubber composite

Hysteresis Linearity

Ref.

/recovery time

1.10 pF/% RH Capacitance 11–98% RH

30 s/27 s

4.73%

Linear

39

-

Nonlinear

40

3.99%

Linear

41

-

Linear

10

-

Linear

16

-

Linear

42

4 s/34 s

2.02%

Linear

43

470 s/500 s

7.6%

Linear

38

330 s/377 s

3.2%

Linear

(ΔC/ΔRH) film

2

GO/MWCNTs film

Kappa-carrageenan/MWCNTs 3

H2SO4 and HNO3 (3:1) treated

(43~97% RH)

-/(ΔZ/Z0) 120%

50 s/40 s

(ΔR/R0)

(33~98% RH)

Resistance 11-98% RH

Plasma-treated MWCNTs/polyimide 5

0.00466/% RH Resistance 10-93% RH

composite film

-/(ΔR/R0/ΔRH)

MWCNTs/poly (acrylic acid) 6

0.069/% RH

670 s/380 s

(ΔR/R0/ΔRH)

(50~90% RH)

Resistance 30-90% RH composite film ZnO/H2SO4 and HNO3 (3:1) treated

24.8 Ω/% RH Resistance 0-97% RH

MWCNTs/ZnO nanocomposite film

8

(ΔC/ΔRH) 90%

MWCNTs thin film

7

5 s/2.5 s

Impedance 10–90% RH composite

4

7980 pF/% RH Capacitance 11-97% RH

H2SO4 and HNO3 (3:1) treated

(ΔR/ΔRH) 33% Current

11−95% RH

MWCNTs on paper

(ΔI/I0)

Nanofibrillated cellulose/MWCNTs 9

69.9% Current

composite film

11−95% RH

Our

(ΔI/I0)

work

To further assess the humidity sensitive performance of our prepared sensors, some representative works were listed in Table 1. It can be found that the NFC/CNT sensor we prepared exhibits an outstanding performance with a high sensitivity and a wide humidity response range, but it takes a relatively long response and recovery time. This is mainly due to the fact that the massive hydroxyl groups in the cellulose chains take a long time to adsorb water molecules to reach saturation. To overcome this drawback, we can significantly reduce the test time by reaching a certain response in a fixed time;38 For instance, the CNT05 sensor can achieve 10% response in 1 min.


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Humidity sensing mechanism of NFC/CNT sensors. The humidity sensing mechanism of CNTs have been established through previous studies.7,14,37,44-46 Based on the humidity sensing mechanism of CNTs network, the sensing mechanism of NFC/CNT sensors was proposed. Figure 7 illustrates the schematic of humidity sensing mechanism for NFC/CNT sensors, it is mainly explained in terms of two aspects. (1) Humidity sensitive behavior of p-type CNTs: It is known that CNTs are well suited for adsorbing water vapors because of the porous network formed by its own hollow structure and the entanglement of nanotubes.7 As shown in Figure 7a, when water molecules are adsorbed to CNTs, the electrons donate to the carbon atoms on the surface of CNTs and compensate hole carriers of the p-type CNTs, leading to the increase of resistance. Besides, the adsorbed water molecules also fill the space between nanotubes and create some non-conductive layers, which hinder the electrons transfer and ultimately result in the increase of resistance. (2) The hydrophilicity and swelling behavior of NFC: On one hand, NFC is considered as a hydrophilic substance due to the existence of abundant hydroxyl groups on its surface. Water vapors tend to be adsorbed on the surface of NFC through hydrogen bonding. This interaction greatly promotes the electrons transfer between CNTs and water molecules, which further increase the resistance of CNTs network. On the other hand, NFC is prone to swell when the composite films are exposed to humidity conditions. The swelling NFC disrupts the CNTs conductive network (as illustrated in Figure 7b), causing a sharp decrease in electrical conductance.


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Figure 7. Schematic of humidity sensing mechanism for NFC/CNT humidity sensor. (a) When water molecules are adsorbed to CNT surface, the electrons donate to the carbon atoms on the surface of CNT and compensate hole carriers of the p-type CNT, leading to the increase of resistance. (b) A number of water molecules are adsorbed to NFC surface and promote the electrons transfer between CNTs and water molecules; The adsorbed water molecules make NFC swells. The swelling NFC disrupts the CNTs conductive network, resulting in a sharp increase in resistance.

Based on previous researches, we have known that the sensitivity of CNT-based or functional CNT-based humidity sensors is generally lower than that of CNT/polymer composite humidity sensors. It can be attributed to the intrinsic hydrophobic property of CNTs, which makes it difficult for water vapors adsorption. In our case, the NFC/CNT sensors exhibit an outstanding humidity sensitive performance compare to the CNT-based humidity sensors reported before. Therefore, it can be confirmed that the hydrophilic NFC dominates the humidity sensitive performance of NFC/CNT sensors. Application in breath monitoring of NFC/CNT sensors. The NFC/CNT composite film sensors are highly desirable for various applications involving humidity measurement. As shown in Figure 8a, the flexible NFC/CNT humidity sensors with high sensitivity performance were demonstrated in the application of breath monitoring. The sensor was placed under the nose and the current was recorded during breathing. Figure 8b-d shows the response of the sensors for detecting human breath at different rates. The exhaled and inhaled breath can change the humidity around the sensor and


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lead to the change in current signal. The breathing rate can be clearly distinguished from the number of peaks in the curves. In addition, the slow, normal and rapid breathing curves were also tested by the NFC/CNT sensor under bending state (Figure S10). It can be seen that there is no difference between the number of respiratory peaks measured by the bending sensor and unbending sensor. These results imply that the bending state of the NFC/CNT sensor has little effect on the monitoring of human respiratory rate. This work demonstrates a flexible, sensitive and readable NFC/CNT sensor for the application in breath monitoring.

Figure 8. (a) Schematic illustrating the application of NFC/CNT humidity sensor for human breath monitoring. The response-time curves of human breathing at a (b) slow, (c) normal and (d) rapid frequency.

CONCLUSION In summary, a flexible humidity sensor with high sensitivity was prepared by the mixture of NFC and CNTs using a facile vacuum-assisted technique. NFC served as a dispersant to enable CNTs disperse in water. Meanwhile, it was also used as a sensitive material to improve the humidity performance of CNT-based sensor. The NFC/CNT humidity sensors exhibit outstanding sensitivity to humidity conditions. Especially, the


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NFC/CNT-5 wt% humidity sensor shows high response of 69.9 % (ΔI/I0) at 95% RH. Compared with the CNT-based humidity sensors reported in previous literatures, the obtained NFC/CNT humidity sensor displays high response over the range of 11%-95% RH, which is critical for the practical humidity measurement. In addition, the NFC/CNT humidity sensor exhibits good bending resistance and long-term stability. And it was also demonstrated to monitor human breath in this work. The favorable combination of easy process ability, flexibility as well as high sensitivity performance may pave the way for the practical application of NFC/CNT humidity sensor.

ACKNOWLEDGMENTS This work was supported by the 2018 Spring's Innovation Fund for Excellent PhD Theses at South China University of Technology, State Key Laboratory of Pulp and Paper Engineering at South China University of Technology (Grant No. 2017ZD01) and National Program on Key Basic Research Project of China (Grant No. 2010CB732206).

Supporting Information The optical images of CNTs dispersion with and without NFC after standing for one month. AFM image and TEM image of NFC/CNT aqueous solution. Zeta potential of NFC dispersion and NFC/CNT-14 wt% aqueous solution. Optical image of NFC dispersion and NFC/CNT aqueous solutions. AFM images and the corresponding height plots of NFC dispersion and NFC/CNT aqueous solution. The photos of LEDs


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connected to the NFC/CNT composite films circuit. The schematic illustration of the humidity measurement device.

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