Drawn on Paper: A Reproducible Humidity Sensitive Device by

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Drawn on Paper: A Reproducible Humidity Sensitive Device by Handwriting Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, and Teng Fei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05181 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Drawn on Paper: A Reproducible Humidity Sensitive Device by Handwriting Hongran Zhao, Tong Zhang, Rongrong Qi, Jianxun Dai, Sen Liu, Teng Fei* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P.R. China KEYWORDS: Sensor on paper; humidity sensing; drawing pencil electrodes; drawing sensitive layer; flexible sensor

ABSTRACT

This paper describes the development of a kind of full carbon-based humidity sensor fabricated on the paper substrate by handwriting. The electrodes were written by commercial pencils and the sensitive layer was drawn with an oxidized multi-walled carbon nanotubes (o-MWCNTs) ink marker. The resultant devices exhibit good reproducibility and stability during the dynamic measurement. The response of the optimized paper-based sensor exhibits about five times higher than sensors fabricated on the ceramic substrate, which is owing to the hydrophilic property of the paper substrate. The structure of the sensitive layer formed by dispersing sensitive materials in the porous surface of paper substrates alleviates the inner stress in the process of bending. The response of printing paper-based sensors only shows the 6.7% decay even under an extremely high bending degree.

ACS Paragon Plus Environment

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1. Introduction Following the increasing demand of industrial production, environmental protection and healthcare etc., chemical sensors have become more and more important. In last decades, many efforts were focused on improving sensitive properties of chemical sensors, such as sensitivity, response speed and accuracy.1,2 With the increasingly improved sensing performance of sensors, the cost and practical application have become the new limitation. In recent years, paper-based devices have gradually caused extensive concern due to their advantages of low-cost, portable and environmentally friendly.3-5 Nanostructured forms of carbon materials are favored as an ideal choice for sensors work at room temperature. Carbon nanotubes (CNTs), one of the most ubiquitous materials among them, have been demonstrated as a novel sensing material with high sensitivity to toxic gases and physical variables.6-10 Recently, functionalized pencils or wax pencils generated from mixture of CNTs/graphite and polymers were used for fabricating volatile organic compounds (VOCs) sensors or microfluidic devices by handwriting.11-13 A pencil generated from CNTs has also been reported to fabricate chemiresistive sensor for NH3 detection.14 Pure CNTs are difficult to show significant signal and avoid the interference of air components during the detection of low concentration gases. Hence, the monitoring of high concentration gas molecules, such as water molecules might be a more appropriate application for CNTs. The water adsorption could cause the conductance of p-type CNTs decrease. However, it is hard for hydrophobic CNTs to adsorb water molecules without additional treatments. In order to monitor water molecules, using the functionalized CNTs as the sensitive material is an effective solution.15


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As for electrodes fabrication, noble metals were usually used for fabricating electrodes on the paper substrate via magnetron sputtering, screen printing and inkjet printing techniques.16-19 However, the hazardous chemical solvents and surfactants were often used for dispersing noble metals in the inks applied by conventional printing technique.20,21 In a recent report, the normal pencil trace was utilized as the electrodes on the surface of ceramic.22 This draw-on-paper technique with a carbon-based pencil lead (CPL), could present the advantage of the paper substrate better due to the low price, abundance and facile process. Furthermore, most of carbonbased materials generally exhibit low hardness, which is beneficial for stripping and leaving trace on the substrate with a rough surface.23 Generally, most reports focused on fabricating the CPL to draw sensitive films, but not drawing electrodes with carbon-based materials. Herein, we describe the design of a type of full carbon-based humidity sensor by handwriting electrodes and the sensitive materials on paper substrates. The commercial pencil was used to fabricate electrodes, and the oxidized MWCNTs (o-MWCNTs) were utilized as the sensitive material. A marker was fabricated by injecting the ink (aqueous dispersion of o-MWCNTs) in the refill of a blank maker and drawn on the sensitive region of devices. The o-MWCNTs exhibit excellent dispersibility in the water with good stability, which is beneficial for solution process. Compared with solid-state process, solution process could introduce the sensitive material into the top layer of paper and form a structure of o-MWCNTs loaded porous cellulose layer by a permeating process. Such structure could increase the contact area between the sensitive material and water molecules, meanwhile, the top layer of hydrophilic cellulose paper could participate in the water adsorption process and enhance the response of the humidity sensor. Furthermore, the formation of sensitive layer does not rely on the abrasion between the substrate and bulk made from the sensitive material, which is beneficial for maintaining the completeness of electrodes.


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2. Experimental Synthesis of o-MWCNTs The o-MWCNTs with different oxidization level were synthesized according to a reported process.24 MWCNTs were treated in the mixture of 98% sulphuric acid and 65% nitric acid in ratio of 3:1 at 50 ºC, 70 ºC and 90 ºC, respectively, for 1 h, followed by filtration of the acid mixture. The residue was, then, washed with deionized water until the pH of filtrate reached neutral, and dried at 40 ºC over night. Fabrication of inks and markers The inks were prepared by dispersing 200 mg of o-MWCNTs in 5 mL of deionized water under sonication for 40 min. Then the inks were injected in the refills of blank makers, markers were kept flat for 24 hours before using. Characterizations The bulk conductance of pencil leads was measured by a multimeter, and the square resistances of pencil trace fabricated on the printing paper were measured by four probe method. Raman spectra were obtained on J-YT64000 Raman spectrometer with 514.5 nm wavelength incident laser light. The scanning electron microscopy (SEM) images were taken by a JSM6700F electron microscope (JEOL, Japan). Optical microscopy was done using a DMM-330C microscope (Caikang, China) equipped with a CCD video camera (CK-300), X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray photoelectron spectrometer using Mg as exciting source. The FT-IR spectra of polymers were obtained on a WQF-510AFTIR spectrometer, using KBr pellets as the reference.


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Fabrication and measurement of humidity sensors

Figure 1. Fabrication schematic of paper-based humidity sensors. (a) Structure design by AutoCAD, (b) printing outline on different paper substrates, (c) writing electrodes by a 8B pencil, and (d) drawing sensitive regions using o-MWCNTs-ink markers. Figure 1 shows the fabrication route of electrodes and sensitive layer on the paper substrate. To fabricate electrodes, the outline of interdigitated electrodes were designed in three sizes by AutoCAD (Figure 1a) and printed out on three types of paper substrates (Figure 1b). Then, an 8B pencil was used to fill in the blank spaces of printed outlines (Figure 1c), and each electrode bar was fabricated by five times of orientation writing with an applied force of 5-8 N. To fabricate sensitive region, three kinds of inks were prepared by dispersing 200 mg of o-MWCNTs with different oxidation levels in 5 mL of deionized water under sonication. Each ink was then injected in the refill of a blank maker to fabricate different markers, and the sensitive layer was fabricated by drawing ink trace on the top of electrodes with an applied force of about 1 N (Figure 1d). Sensors were finally obtained, and three sensors were fabricated to investigate their reproducibility for each type. The obtained sensors were named as p/w/c-CNT50/70/90-S/M/L, in which p/w/c represents printing paper, weighing paper and cardboard substrate, respectively;


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50/70/90 stands for the treating temperature of MWCNTs during oxidization process; S/M/L indicates the small, middle and large size of the sensor substrates. The relative humidity (RH) response of sensors was measured by monitoring the current change under different RH, carried on a CHI660E electrochemical analyzer (CH Instruments, Inc., Shanghai). Measurements of current were performed under a constant applied voltage of 1 V. The humidity environments were produced by different saturated salt solutions in their equilibrium states include LiCl for 11% RH, MgCl2 for 33% RH, Mg(NO3)2 for 54% RH, NaCl for 75 % RH, KNO3 for 95% RH.25 The measurement system of the humidity sensors is shown in Figure 2. The response and recovery time in this work is defined as the time taken by the current achieved 90% of the total change during adsorption and desorption process, respectively.26 Humidity hysteresis is defined as the maximum difference of the humidity sensor between the adsorption and desorption processes.27

Figure 2. Measurement system of the humidity sensors and the photograph of sensors fabricated by o-MWCNTs oxidized at 90 ºC on different paper substrates. 3. Results and discussions As for the selection of the material for drawing electrodes, the most common type of commercial pencils, classified from 6H to 12B with the increasing content of graphite, were


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considered as the candidates. As widely known, the leads of commercial pencils are the mixture of clay and graphite. The graphite content determines the conductivity of the pencil trace directly. To choose the optimal electrode material, the bulk conductance of pencil leads from 1B to 12B and square resistance of electrodes fabricated on the printing paper were investigated, the results are shown in Table 1. The bulk conductance of pencil leads is progressive with the increasing graphite content, but the difference of bulk conductance between pencil leads turns inconspicuous when the graphite content becomes an extremely high level. From current results, the electrodes fabricated by 8B pencil exhibit the lowest square resistance, so 8B pencil was selected for electrodes writing in the fabrication for all the sensors. The morphology of electrodes on paper substrate was characterized by the Raman spectrum (Figure 3), scanning electron microscopy (SEM) and optical microscope (Figure 4). In Figure 3, there are three characteristic peaks of graphite appear in Raman spectrum. The peaks at 1352 and 1580 cm-1 correspond to the D-band and G-band, which are ascribed to stretching vibrations of ordered sp2 carbon lattice and the existence of defects or disordered layers, respectively.28,29 Using the intensity ratio of D-band and G-band (about 0.19), the average crystallite size of graphitic layers could be calculated as about 100 nm.30 The single symmetric peak appearing at 2721 cm-1 with the width of about 70 cm-1 corresponds to the 2D-band, from the shape, width and position of 2D-band layer number and stacking periodicity along the c-axis of graphic domains could be estimated, in this work, the 2D-band of pencil trace is typical of turbostratic graphite.31 The misorientation of graphite layers along c-axis could cause the substantial electronic decoupling and lead to a 2D graphite behavior.31,32 SEM images in Figure 4a show that pencil trace fabricated by 8B pencil forms a continuous graphite film with good uniformity on weighing paper and printing paper. Moreover, from the high-powered SEM images (Figure 4b), the uniformity of pencil trace


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decreases with the roughness of the substrate surface. The thickness of electrodes was estimated by observing the cross-sections of electrodes on different substrates with an optical microscope (Figure 4c), which is about just several micrometers. Table 1. The bulk conductance of pencil leads from 1B to 12B and square resistances of pencil trace on the printing paper. The preparation of pencil trace is analogous with the fabrication of electrodes.

Pencil

Conductivity (S/m)

Square Resistance (Ω)

B

1.87*103

8717

2B

2.17*103

6776

3B

2.20*103

4163

4B

2.31*103

2168

5B

2.37*103

509

6B

2.43*103

368

8B

2.46*103

332

10B

2.49*103

410

12B

2.52*103

643


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Figure 3. Raman spectrum of the pencil trace prepared by 8B pencil on the printing paper.

Figure 4. (a) SEM images of boundary between region coated by trace of 8B pencil (the upper area of white dash line) and blank region (the area below white dash line) of various paper substrates. (b) High magnification SEM images of the electrodes fabricated by 8B pencil on


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different substrates. (c) Optical micrographs of the cross sections of the pencil trace on different substrates (taken by 45 º tilt view). The regions between yellow lines are the pencil electrodes on various paper substrates. The black upper areas are the top surface of pencil trace, and the white part below electrodes is the cross section of paper. The chemical structures of o-MWCNTs treated with strong acids under different temperatures were examined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. In XPS plot (Figure 5), the strength of the O1s peak increases obviously after oxidation treatment, indicating plenty of oxygenic groups has been modified on MWCNTs successfully. The content of oxygen rises from 7.1% for pure MWCNTs to 31.1% for o-MWCNTs treated under 90 ºC gradually. From the FT-IR spectroscopy (Figure S1), both the curves of CNTs and o-MWCNTs exhibit the peak corresponding to the stretching of C=C bond at 1580 cm-1. After oxidization, two additional peaks appear at 1720 and 1210 cm-1 corresponding to the stretch of C=O and C-O(H), respectively, indicating the modification of carboxyl groups. The introduction of various oxygen functional groups enhances the dispersibility of MWCNTs in deionized water. From images of inks fabricated by pure MWCNTs and three types of o-MWCNTs taken by a week apart (Figure 6), almost all the pure MWCNTs settle in the bottle bottom after a week standing, but inks made of o-MWCNTs remain stable. In addition, the dispersion degree of inks enhances with the oxidation level of MWCNTs.


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Figure 5. XPS spectra of o-MWCNTs treated under different temperature. CNT50, CNT70 and CNT90 refer to carbon nanotubes oxidized under 50 ºC, 70 ºC and 90 ºC, respectively.

Figure 6. Photographs of inks fabricated by o-MWCNTs with different oxidation levels with a concentration of 50 mg/mL before and after a week of standing. The state of sensitive material on the substrate directly influences the sensing performance of devices. Hence, the morphology of o-MWCNTs on the surface of printing paper was observed under the SEM and optical microscope (Figure 7). To contrast the change of paper substrates


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before and after drawing sensitive region, the sensitive material was only drawn on the half side of the printing paper substrate with electrodes, while the other side is blank. From the SEM image of the boundary between sensitive region and blank region (Figure 7a), the drawn side and blank size exhibit obvious brightness difference, and the morphologies of two sides are analogous. At high magnification, the blank cellulose behaves smooth surface (Figure 7b), by contrast, the structure of cellular fibers coated by o-MWCNTs could be observed at sensitive region (Figure 7c). From the optical micrographs of cross sections of sensitive region taken by 45º tilt view (Figure 7d), it can be seen the sensitive material does not form a sensitive film on the surface of substrate, but being introduced into the top layer of paper and forming a sensitive layer with the structure of o-MWCNTs loaded porous cellulose. The thickness of the sensitive layer was measured to be about 262 µm from cross sectional SEM image (Figure 7e).

Figure 7. The morphology of sensitive region fabricated by o-MWCNTs oxidized under 90 ºC on printing paper. (a) Top view SEM image of boundary between sensitive region (right side) and blank region (left side). (b) The high magnification SEM images of the blank region and (c) sensitive region of the device surface. (d) The optical micrographs of cross sections of the paper substrate with sensitive region (right side) and blank region (left side) taken by 45 º tilt view.


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Region between green dot lines is the cross section of paper, and the area above the upper dot line is the top surface of blank region. The region between yellow dash lines is the sensitive layer constructed of o-MWCNTs loaded porous cellulose layer. The area above the upper dash line is the top surface of sensitive region, and the white part between the lower dash line and dot line is the blank layer of sensitive region. (e) SEM image of cross sections of sensitive region, the view is analogous to (d). After preparing the electrodes and sensitive layer, the humidity sensing properties of the obtained devices were appraised by a dynamic measurement under different relative humidity (RH). In order to contract the humidity sensitivity between different sensors, the current signal was treated as the normalized response, which is defined as - ∆I/I0 [%] (- ∆I/I0 [%] = (II0)/I0*100%). In this equation, I0 is the current at 11% RH and I stands for the current at targeted RH environment. The effects of o-MWCNTs’ oxidation level, size of the sensor substrate and various types of paper substrates on the response of sensors under different RH are summarized in Figure 8 and Figure S2. For each RH environment, five cycles of response-recovery process were tested, and each of a single adsorption or desorption process was measured for 300 s, meanwhile, the sensing curves were tested by three devices for each type of sensor fabricated under the same condition. The measurement method for different devices is analogous in this work. The humidity response of the obtained sensors is mainly attributed to the p-type characteristic of oMWCNTs, the water adsorption could cause the conductance decrease of o-MWCNTs through charge transfer between o-MWCNTs and water molecules, finally, leading to the decrease of the current. From the resultant plots, the response curves of the devices with same parameters almost coincide, and the baseline and end value of 300 s response have not exhibited obvious drift


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during five circles dynamic measurement under certain RH, indicating good stability and reproducibility of the devices by handwriting. The dynamic sensing response at 33%, 54%, 75% and 95% RH (from 11% RH) of all the printing paper-based sensors are shown in Figure S2. All the curves exhibit apparent plateaus under low RH, and the plateaus turn less obvious with the increase of RH, due to the high water vapor level make the response process need a longer time to reach equilibrium. At 95% RH, the recovery process could not reach the initial value in 300 s measurement, but this does not affect the end value of response during the following circles. From three plots of Figure S2, the response of sensors decreases when the size turns lager, the analogous phenomenon occurs on devices fabricated by o-MWCNTs with three oxidation levels. In order to investigate the influence of o-MWCNTs’ oxidation level on the response of devices, the dynamic sensing response and the linear fitting curves (the detail parameters are shown in Table S1) of response vs. RH relationship of sensors with the same size but different oxidation levels are shown in Figure 8a and Figure 8b, respectively. Obviously, sensors fabricated by oMWCNTs with higher oxidation level exhibit the faster rate of change and better linearity in response vs. RH relationship. The response vs. RH cure of p-CNT50-S turns to saturation at high humidity region, by contrast, the extra oxygenic groups make the response of p-CNT90-S continue to increase and the response vs. RH curve exhibits higher slop (0.41) and better linearity (R2 = 0.9976) among 33% to 95% RH range. Above phenomenon could be explained as follows: Firstly, the electron transfer between adsorbed water molecules and o-MWCNTs would become easier when the electrode distance turns closer. Secondly, sensors based on o-MWCNTs with higher oxidation level exhibit higher response because the oxygenic groups could enhance the water adsorption capacity.


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Figure 8. Humidity sensing response of sensors based on o-MWCNTs with different oxidation levels and various types of paper substrates. (a) Five circles dynamic sensing response of the sensors to 33%, 54%, 75% and 95% RH (from 11% RH), respectively. The duration was 300 s for each adsorption or desorption process, and the overlay curves were tested by three sensors fabricated under the same condition. (b) Response vs. RH relationship of sensors fabricated by oMWCNTs with various oxidation levels. Each error bar was generated by five circles response values of three devices fabricated under the same condition. (c) Dynamic sensing response and (d) response vs. RH relationship of sensors based on weighing paper, printing paper and cardboard with analogous measurement methods. The effect of different substrates is significant for our sensors. Figure S3 and Figure 8d show the response of sensors based on ceramic substrate (the detailed fabrication process could be found in the Supporting Information) and different paper substrates under different RH. From current results, the substrate material shows a strong effect on the response of sensors. Response of sensors based on ceramic substrate is clearly weaker than paper-based sensors, among the


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paper-based sensors, weighing paper-based sensors exhibit the highest response (~33%) and the best linearity (R2 = 0.9978) from 33% to 95% RH, while the response vs. RH relationship of sensors based on cardboard shows the slowest rate of change (slop = 0.27) and the worst linearity (R2 = 0.9368). The response difference between the sensors based on three kinds of substrates is mainly owing to the different hydrophilic properties of weighing paper, printing paper and cardboard. The precision of w-CNT90-S sensor, p-CNT90-S sensor and c-CNT90-S sensor were calculated as ± 5.8% RH, ± 6.0% RH and ± 10.2% RH, respectively. Meanwhile, the deviation between three sensors of w-CNT90-S is only 6.8%, which is in the similar level to the reproducibility of sensors fabricated by other solution casting method.6 In order to investigate the effect of blank paper substrates on the response of sensors, the humidity sensitive properties of blank papers are shown in Figure S4. The resistance of blank paper substrates decreases with increasing RH, but the current variation is only a few tens of nA, demonstrating there is almost no electron transfer between adsorbed water molecules and paper substrates.33 However, after loading o-MWCNTs, the water molecules adsorbed by top layer of paper could donate electrons to o-MWCNTs, resulting in further increase of resistance, thus enhancing the response of the devices. In addition, the dependence of temperature on sensors response were also measured, including the dynamic sensing response curves of the sensors between 11% RH and 95% RH (Figure S5a) and the response vs. RH curves of sensors (Figure S5c-e) under different temperatures (20-50 ºC). The response of sensors based on different substrates increased with the rising temperature. Meanwhile, the response drift turns more obvious when the temperature increases, the maximum response drift of w-CNT90-S sensor, p-CNT90-S sensor and c-CNT90S sensor is 0.45/ºC, 0.25/ºC and 0.43/ºC around 95% RH, respectively (Figure S5b).


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To further investigate the sensing performance of our sensor, the hysteresis and the response and recovery time of w-CNT-90-S sensor, p-CNT90-S sensor and c-CNT90-S sensor were measured by conventional method, recording the current signal at variation RH environments after it reach equilibrium, and shown in Figure S6 and Figure S7. From the adsorption and desorption curves shown in Figure S6, it can be seen the hysteresis of w-CNT-90-S sensor, pCNT90-S sensor and c-CNT90-S sensor was 7.6% RH, 9.8% RH and 5.9% RH, respectively. Meanwhile, the response time and recovery time of sensors based on different paper substrate are calculated from the response and recovery curves (shown in Figure S7). The p-CNT90-S sensor exhibits the shortest recovery time (412 s) and the relatively fast response speed (reach equilibrium in 328 s). The c-CNT90-S sensor shows the fastest response speed (reach equilibrium in 311 s) but the longest recovery time (643 s). Both of the response and recovery time of w-CNT90-S sensor are relatively long (470 s and 500 s, respectively) among three sensors. In order to evaluate the properties of our sensor, some present reports of various sensors were collected and the detail data are presented in Table 2. By comparing with reported sensors, our sensors exhibit good sensitivity and fast response and recovery speed among paper-based sensors. The paper-based devices generally exhibit slow response and recovery speed, the phenomenon of reaching a constant response in a fix time which observed in our measurement could reduce the test time. In order to evaluate the life time of paper-based sensors we fabricated, the dynamic sensing response curves and the response vs. RH relationship fitting curves of the sensors based on different substrates to 33%, 54%, 75% and 95% RH (from 11% RH) measured with an interval of nine months were measured and provided in Figure S9. Among three types of sensors, the p-


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CNT90-S sensor exhibits the minimum precision drift as 0.2% RH per month. The precision drift c-CNT90-S sensor and w-CNT90-S sensor are 0.4% RH and 0.3% RH per month, respectively. Table 2. Performances of humidity sensors based on different substrates. Materials

Output signal

Substrate

RH range

Response

Response/recovery time

Reference

paper

frequency

paper

20-90%

14.5% (∆f/f0*)

4 min/6 min

34

graphite

voltage

paper

20-70%

215% (∆V/V0*)

~1.5 min/--

35

f-CNTs

resistance

PPE film

70-90%

~100%(∆R/R0)

--/~2 h

36

polycarbonate

resistance

--

39-100%

1.3-2.5 GΩ/%RH

--/4-8 min

37

grapheme

current

glass

35-98%

18.1% (∆I/I0)

--/--

38

PEDPT:PVMA

resistance

paper

11-98%

71-98% (∆R/R0)

--/--

39

o-MWCNTs

current

paper

33-95%

18-33% (∆I/I0)

5-8 min/7-11 min

this work

Considering that flexible papers are employed as the substrate, the influence of bending degree on the resistance and humidity response of sensors based on printing paper and weighing paper was further examined. The dynamic response of p-CNT90-S sensor and w-CNT90-S sensor to 54% and 95% RH (from 11% RH) is measured under different bending degree. The results are shown in Figure S8 and Figure 9. Generally, the paper substrates do not form perfect arc after normal bending, and it is difficult to calculate or measure the bending angle accurately. Hence, we use the distance (d) between the upper end and lower end of sensors to show the bending degree. As shown in Figure 9, both p-CNT90-S sensor and w-CNT90-S sensor remain good stability during the dynamic measurement and show slightly reduced response under


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bending state. When d reaches 0.5 cm, the response of p-CNT90-S sensor and w-CNT90-S sensor shows a decay of 6.7% and 8.9%, respectively, under 95% RH. These results indicate good flexibility robustness of paper-based sensors. The good mechanical stability could also be seen from the dynamic currents under different bending degrees (Figure S8), the baseline does not exhibit obvious drift even when d reaches 0.5 cm. Normally, the signal drift of sensors under inflecting is mainly caused by inner stress of functional film which could result in functional film cracking or lead to the separation of functional film and the substrate. In our work, o-MWCNTs could permeate into the top layer of paper, forming a structure of o-MWCNTs loaded porous cellulose layer, not an o-MWCNTs film on the exterior surface of paper. Such structure could alleviate the inner stress of the sensitive layer, finally, leading to stable response under inflecting.

Figure 9. Dynamic response of sensors based on weighing paper and printing paper to 54% and 95% RH (from 11% RH) under various bending degree. The bending degree was evaluated by distance (d) between the upper end and lower end of sensors. The solid, dash and dot line represents the response curve of sensors as d = 1.5 cm, 1 cm and 0.5 cm, respectively. 4. Conclusion


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In conclusion, a method to fabricate full carbon-based humidity sensor by handwriting oMWCNTs inks on paper substrates is provided. The structure of the obtained paper-based humidity sensor mainly exhibits such advantages: i) o-MWCNTs could permeate into the top layer of paper substrates, such structure could increase the contacting area between o-MWCNTs and gaseous water molecules, meanwhile, the top layer of hydrophilic cellulose paper could participate in the water adsorption process and further enhance the response of humidity sensors; ii) the dispersion of sensitive materials alleviates the inner stress of sensitive layer, leading to good mechanical stability; iii) the paper substrates, electrodes and the sensitive material are all carbon-based, demonstrating the environment friendly and recyclable superiority; iv) drawing on paper technique with the ubiquitous materials, paper, pencil and marker, provides a facile method to fabricate flexible sensitive device on-sit and gives users great freedom to design sensors on-demand. The resultant devices are flexible, disposable and potentially wearable, meanwhile, the solution-based process is feasible for ink-jet printing, which is possible for batch production.

ASSOCIATED CONTENT Supporting Information. The FT-IR spectra of CNT and CNT90; dynamic sensing response measurement results of sensors in different sizes; the response vs. RH relationship of sensors based on ceramic and blank paper substrates; dynamic current-time curves of sensors. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Corresponding Author *E-mail address: [email protected] (T. Fei) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation Committee (NSFC, No. 51103053), Projects of Science and Technology Development Plan of Jilin Province (No. 20160520093JH). REFERENCES (1)

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Drawn on Paper: A Reproducible Humidity Sensitive Device by

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