Carbon Nanocoil-Based Fast-Response and Flexible Humidity Sensor

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Carbon Nanocoil-Based Fast-Response and Flexible Humidity Sensor for Multifunctional Applications Jin Wu,*,‡ Yan-Ming Sun,*,† Zixuan Wu,‡ Xin Li,† Nan Wang,∥ Kai Tao,§ and Guo Ping Wang*,† †

College of Electronic Science and Technology, Shenzhen University, 3688 Nanhai Boulevard, Shenzhen 518060, China State Key Laboratory of Optoelectronic Materials and Technologies and the Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China § The Ministry of Education Key Laboratory of Micro and Nano Systems for Aerospace, Northwestern Polytechnical University, Xi’an 710072, China ∥ Center for Environmental Sensing and Modeling (CENSAM) IRG, Singapore-MIT Alliance for Research and Technology (SMART), Centre 1 CREATE Way, Singapore 138602

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ABSTRACT: Carbon nanocoils (CNCs) are employed to fabricate fast, high-resolution, and reversible humidity sensor based on a flexible liquid crystal polymer (LCP) substrate. The humidity sensor displays fast-response (1.9 s) and recovery time (1.5 s), a broad relative humidity (RH) detection range (4−95%), linearity, repeatability, and stability. The rapid response and recovery are considered to benefit from the hydrophobic effect of the LCP substrate and high purity of the CNCs, which give rise to weak physical adsorption. Meanwhile, the high sensitivity results from both the unique helical structure of CNCs and the microporous structure of the LCP substrate. The distinctive structure-related properties enable the sensor to reliably perceive an extremely small RH variation of 0.8%, which is too small to be detected by most humidity sensors reported previously. These features allow the sensor to monitor a variety of important human activities, such as respiration, speaking, blowing, and noncontact fingertip sensation, accurately. Furthermore, different human physical conditions can be distinguished by recognizing the respiration response patterns. In addition, the long-term operation and mechanical bending do not adversely affect the sensing performance. KEYWORDS: flexible humidity sensor, carbon nanocoils (CNCs), liquid crystal polymer (LCP) substrate, respiration monitoring, human activities monitoring

1. INTRODUCTION Humidity sensing is of great importance for industrial, agricultural, medical, and domestic applications.1−5 Recently, flexible humidity sensors emerge as great candidates for the potential applications in electronic skin (e-skin), wearable electronic systems, soft robotics, and noncontact sensation.1,4,6,7 Particularly, real-time monitoring of the moisture level of human skin or the environmental humidity around the human body has attracted increasing attention in personal healthcare and e-skin applications.1 For example, continuous monitoring of the humidity level of human skin can provide various metabolic, physiological, and health information, which further facilitates the evaluation of human health conditions and the effectiveness of cosmetic products.1 Furthermore, the healing conditions of the wound on the skin can be evaluated by monitoring the moisture level around it.1,8 Note that the wearable humidity sensor can also be employed to monitor respiration, which is one of the most important physiological signals closely related to human health and activities.7,9−11 In © XXXX American Chemical Society

addition, a humidity sensor array can be used for noncontact sensation and noncontact interface localization applications, which reduce the risk of bacterial transmission as occurred in conventional touch screens.4 Up to date, a variety of materials, including polymers, metal oxide, metal nanowires, carbonbased nanomaterials, and porous ceramics, have been explored as humidity transducing elements.6,12−22 Among these materials, carbon-based materials such as graphene and carbon nanotubes (CNTs) have attracted widespread research interest in chemical sensing due to their unique structural and electronic properties, including a large specific surface area, small size, high electronic mobility, and high sensitivity to the electrical disturbances from water or other gaseous molecules.12,23−28 For instance, the resistances of graphene and CNTs change with varied environmental humidity due to the Received: October 24, 2018 Accepted: January 4, 2019

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DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a, b) Schematics illustrating the synthesis of the CNCs via thermal CVD and the fabrication of CNC humidity sensor based on a twoelectrode chemiresistor configuration, respectively. (c) Scanning electron microscopy (SEM) image showing that CNCs bridged the gaps on the Au interdigital electrodes (IEs), which were fabricated on the porous LCP substrate by MEMS technology. The bright and dark strips correspond to the IEs and LCP regions, respectively. (d) I−V curves of the blank IEs and CNC-based chemiresistor. (e) The Raman spectrum of the CNCs. (f) High-quality field emission SEM (FE-SEM) image showing the overall view of the CNCs. (g) The high-resolution transmission electron microscopy (HRTEM) image of a single CNC. (h) The X-ray diffraction (XRD) patterns of CNCs (red curve). The black line in Figure 1h indicates the peak position of carbon in the standard XRD spectrum (JCPDS No. 26-1077). The corresponding crystal planes are marked above the peaks.

doping effect of the adsorbed water molecules.26,29−31 Nevertheless, due to the intrinsically limited sensitivity of these unmodified materials, it is difficult to perceive minute variation of relative humidity (RH).4 Graphene oxide (GO), reduced graphene oxide (RGO), chemically modified graphene, and CNTs hold promise in highly sensitive humidity detection due to the interaction between the functional groups and water molecules.4,26,32 However, the humidity sensors based on these materials suffer from sluggish recovery due to the formation of strong chemical bonds.4 For metal oxide and ceramic-based humidity sensors, heating elements are required to accelerate the signal recovery in the dehydration process, leading to the high energy consumption and the occurrence of thermal safety problem.6 Thus, it is imperative to investigate the humidity sensing properties of new materials for the fabrication of highly sensitive, fast-response/recovery, lowtemperature, and flexible humidity sensors for the emerging wearable applications.6 As-grown carbon nanocoils (CNCs), the coiled CNTs, are generally a mixture of amorphous, polycrystalline, and helical structure of sp2 grains and sp3 amorphous hybridized carbon atoms.33−35 The physical properties of CNCs may be different from both sp3 structured amorphous carbon nanofibers and sp2 structured CNTs.33,34 Due to the peculiar helical morphologies, incomplete crystalline structures and nanometer sizes, CNCs display great potential in various applications, including near-infrared sensors, flexible strain sensors, wave absorbers, field emitters, and microelectromechanical systems

(MEMS).36−39 Although many research studies have been carried out to understand the electrical, thermal, and mechanical properties of CNCs, the humidity sensing characteristic of CNCs has never been explored until now. In this work, we pioneered the fabrication of a CNC-based, highly sensitive, and ultrafast humidity sensor on a flexible liquid crystal polymer (LCP) substrate. The LCP is a type of aromatic polymer with excellent chemical, fire, and moisture resistances, and more importantly, mechanical flexibility.40 The unique structural and physical properties enable the LCP to be widely utilized as the substrate and packaging materials in MEMS.40 However, the application of the LCP in chemical sensing has not been explored until now. Here, the CNCs and LCP are employed as the humidity transducing material and substrate, respectively, to fabricate a flexible humidity sensor for the first time. Due to the unique helical structure of CNCs, the minimal oxygen content, together with the microporous structure of the LCP substrate, the humidity sensor displays remarkable sensing performances, including a wide RH detection range (4−95%), rapid response (1.9 s) and recovery time (1.5 s), high-resolution, linearity, reversibility, stability, and room-temperature (25 °C) operation, demonstrating the advantages of CNCs in the fabrication of high-quality chemical sensors. Particularly, this humidity sensor can practically perceive subtle RH variation as small as 0.8%, which has rarely been reported by previous humidity sensors until now.41−43 With sufficient sensitivity, this sensor can monitor various human activities, such as respiration, speaking, blow, B

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

(water saturated) was mixed and then injected into a gas chamber, where the samples were placed. There was a 15 cm distance between the outlet of the airflow and the sample surface. Thus, the RH of the overall chamber volume was measured. The wet air was obtained by bubbling dry air through water sealed in a flask. Mass-flow controllers were utilized to precisely control the flow rates of dry and wet air, which also allowed for the precise control of the volume ratio. Various RH levels ranging from 4 to 95% were obtained by mixing the dry and wet air with different volume ratios. The total flow rate was kept as 500 sccm. Because the chamber was not completely airtight, the lowest RH level (4%) was obtained by injecting dry air to the chamber only, and the highest RH level (95%) was obtained by injecting wet air to the chamber only. A standard RH reference sensor (Sensirion EK-H4) was employed to calibrate the humidity levels in the chamber and near the fingertip. The same sensor was used for the humidity sensing test for four times to get the average response values and the error bars.

and noncontact sensation. Furthermore, slow breaths at calm state and fast breaths after exercise can be distinguished by recognizing humidity response patterns. Neither the bending strain nor the long-term operation impairs the sensitivity, demonstrating the impressive stability of the humidity sensor. On the basis of these attributes, the CNC-based humidity sensor holds great promise for real-time and continuous monitoring of various human physiological activities.

2. EXPERIMENTAL SECTION 2.1. Synthesis of CNCs. Thermal chemical vapor deposition (CVD) was employed to synthesize the CNCs with the catalyst of Fe/ Sn.33,34 Briefly, the mole ratio of Fe to Sn in the Fe2(SO4)3/SnC precursor solution was controlled to be 60:1. The Sn/Fe catalyst was calcined above 700 °C to avoid the vaporization of Sn during the CVD process. The synthesis of CNCs was carried out in a thermal CVD system at 710 °C for 30 min with the introduction of 30 and 230 sccm C2H2 and Ar gases, respectively, in the system (Figure 1a). Purification of the synthetic CNC samples is necessary. We placed the CNC clusters in a beaker containing deionized water for ultrasonic cleaning for 1 min, and then allowed it to stand for 5 s. After the impurity particles such as amorphous carbon was removed, the supernatant was collected to obtain the pure CNCs. 2.2. Fabrication of the CNC Humidity Sensor. The CNCbased humidity sensing device was fabricated on a flexible LCP substrate. Commercially available LCP 3908 thin film has a thickness of 25 μm, with 18 μm thick copper claddings on both sides. When Au interdigital electrodes (IEs) were patterned on the LCP substrate, a 300 μm thick silicon wafer was utilized as the supporting substrate of the flexible LCP thin film. The fabrication process started with bonding a 25 μm thick LCP thin film on the silicon wafer by using a 5 μm thick AZ 9260 photoresist layer as the intermediate adhesion layer between the LCP and silicon wafer. After bonding, the copper on the upper side of the LCP thin film was etched away, exposing the fresh surface of the LCP. After a 6 μm thick AZ 9260 photoresist layer was spin coated on the LCP/Si wafer, a photolithography process was performed, then 10 nm Cr/300 nm Au was sputtered and the photoresist was stripped off to generate the Au IEs on the LCP substrate. Finally, the LCP thin film was released from the supporting silicon wafer by placing the sample in acetone to remove the photoresist with sonication. The gap, width, length, and the effective area of the Au IEs are 20 μm, 30 μm, 1 mm, and 1 mm2, respectively. For the resistive humidity sensing test, CNCs need to bridge the gaps between the two separated Au electrodes. The CNCs were dispersed in deionized water to form 1 mg/mL CNC aqueous dispersion, which was deposited on the Au IEs via drop-casting. The CNCs bridged the gaps on the Au IEs after water evaporation, and thus served as the conducting channel material for humidity sensing (Figure 1b). 2.3. Materials Characterization. The morphologies of CNCs, Au IEs, and the LCP substrate were characterized by FE-SEM 7600. The INCA (INCA 4.15; Oxford Instruments) installed on the FESEM 7600 was employed to carry out energy-dispersive X-ray (EDX) analysis. A Micro-Raman spectrophotometer system with a 488 nm laser (Renishaw Invia, United Kingdom) was utilized to obtain the Raman spectra of CNCs. The water contact angle on the LCP substrate was measured on an Attension Theta optical tensiometer. A JEOL 2200FS microscope operating at 200 kV was used to acquire the high-resolution transmission electron microscopy (HRTEM) images. The X-ray diffraction (XRD) patterns of CNCs were recorded on a Rigaku Ultima IV diffractometer. 2.4. Humidity Sensing Test. The resistance variation of the sensor was monitored after a constant bias voltage of 0.1 V was applied on the two-electrode chemiresistor. The electrical signals from various human activities, including respiration, speaking, blow with mouth, and the fingertip approach, were monitored on an electrochemical analyzer (CHI 760D potentiostat−galvanostat, CH Instruments Inc.). For all other humidity sensing measurements, a Keithley 2602 Source Meter was employed to record the signals based on a home-built humidity characterization system. Dry and wet air

3. RESULTS AND DISCUSSION The purified CNC samples were dispersed in an aqueous solution, which was deposited on an LCP substrate (Figure 1a,b). After the evaporation of solvent, the CNCs bridged the gap on the Au IEs, generating the humidity sensor with a twoelectrode chemiresistor configuration. The scanning electron microscopy (SEM) images in Figure 1c depict that more than one gap on the Au IEs was bridged by the CNCs. Importantly, it is found that a great many of micropores appear on the surface of the LCP substrate, indicating the porous and rough surface, which is different from the conventional flat silicon surface (Figures 1c and S1). The microporous LCP substrate is compatible with MEMS fabrication technology for the fabrication of Au IEs on the surface. The three-dimensional porous LCP structures provide an enlarged interaction surface area between CNCs and water molecules, because the interfacial area between CNCs and the underlying substrate is significantly diminished compared with the conventional two-dimensional flat substrate. More importantly, the rough and porous LCP surface coupled with the low surface energy contributes to the hydrophobic properties of the LCP substrate, which was verified by the measured contact angle of the water droplet on the LCP substrate (117°) (Figure S2). A linear current versus voltage (I−V) relationship was observed for the CNC-coated IEs, indicating the occurrence of Ohmic contact between CNCs and IEs (Figure 1d).44 Thus, the Schottky barrier is absent. The contact resistance plays a negligible role in determining the response of the humidity sensor and, the intrinsic humidity sensing properties of CNCs can be assessed.44 The resistance of 726 Ω was extracted from the linear I−V correlation of the CNC sensor. The Raman spectrum of CNCs exhibits a D-band at 1352 cm−1 and a G band at 1585 cm−1, which are two characteristic peaks of graphite-like materials.33 Generally, the D-band results from the structural defects, such as disordered structures of graphitic domains or functional groups attached on the carbon basal plane.44 Hence, the high intensity of the D-band should be attributed to the existence of numerous defects, which are active adsorption sites for water molecules. The G band is originated from the first-order scattering of the E2g mode.44 The EDX analyses indicate that the CNCs are mainly composed of a C element with a very small amount of catalyst (Figure S3). A high-quality FE-SEM image in Figure 1f shows an overall view of the CNCs, which allows for better characterization of the surface morphology of CNCs. HRTEM and XRD analyses are also performed to study the C

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) Dynamic responses of the CNC sensor to the linearly descending RH from 80 to 4%. (b) The plot of the response variation versus RH using the data in (a). (c) Dynamic response of the sensor to 4% RH. (d) Analyses of the response and recovery time t90 in the detection of 16% RH. (e) Comparison of the dynamic responses of the sensor to 80% RH with descending and ascending RH in the reversible sensing processes. (f) Dynamic response of the CNC sensor to linearly ascending RH from 4 to 80% with a small step of 4% RH. (g) The plot of the response variation versus RH using the data in (f). All the humidity sensing tests in this figure were performed at 25 °C.

condensation of H2O molecules on CNCs. The CNC sensor displays distinct capability to distinguish different RH levels. For example, the response increased from 0.5 to 12.2% with increasing RH from 4 to 80%, an indication of 24.4 times increased response. Importantly, a linear correlation between the response and RH was observed, which is advantageous for practical applications (Figure 2b).44 With sufficiently high sensitivity, this sensor can experimentally detect the RH level as low as 4%, which is also the lowest RH level provided by our current setup with the bubbling method (Figure 2c). It demonstrates the low detection limit of our CNC-based humidity sensor. The doping effect of adsorbed water molecules is responsible for the intrinsic resistance change of CNCs.29 Similar to CNTs, the charge transport through CNCs may display p-type semiconducting behavior.46 Thus, the charge carriers are dominated by holes. The adsorbed H2O molecules on CNCs act as the electron donors, and therefore decrease the charge carrier concentration in CNCs, increasing the resistance level.1,29 Since the LCP substrate is hydrophobic, the adsorption of water molecules on the LCP substrate is negligible. However, the microporous structure of the LCP can decrease the contact interface between CNCs and LCP, increasing the interaction surface area between water molecules and CNCs. Compared with the linear structure of

structure and defects of CNCs. As shown in Figure 1g, the structure of a single CNC is confirmed. The rough surface and wrinkle structure of the CNCs can be clearly seen from the TEM image, indicating the existence of defective sites. These defects function as active adsorption sites for gaseous water molecules. Also, our previous work proved that the structure of the CNCs used in our experiment is the polycrystalline− amorphous structure, which shows different sensing characteristics compared to other carbon nanomaterials.34 The XRD patterns agree well with the characteristic peaks of carbon (JCPDS No. 26-1077) (Figure 1h, red curve).45 No XRD peak of impurity was observed, demonstrating the pure composition of carbon. The humidity sensing properties of the CNC sensor were evaluated by monitoring their resistance variation upon exposure to different RH levels after a small bias voltage of 0.1 V was applied on the chemiresistor. The small bias voltage endows low energy consumption. The response is defined as the normalized resistance variation ΔR/R0 (%), in which ΔR is the resistance variation relative to the original value R0. Figure 2a shows the dynamic response of the CNC sensor to a variety of RH levels ranging from 80 to 4%. It is clear that the resistance increased dramatically with increasing RH in the chamber, which was attributed to the physical adsorption and D

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Humidity Sensing Properties of the Sensors Made of Different Materialsa sensing materials

detection range (% RH)

response/recovery time (s)

fabrication methods

GO RGO/PDDA RGO/PU CNT/PI WS2 PANI LiCl/TiO2 MoS2 CNCs

10−95 11−97 10−70 20−90 20−90 11−95 11−95 0−35 4−95

50/3 108/94 3.5/7 5/∼600 5/6 760/170 3/7 10/60 1.9/1.5

laser interference layer-by-layer self-assembly thermal annealing of GO in situ polymerization tungsten sulfurization pre-stretching and self-assembly electrospun CVD CVD

ΔR/R0 (%)/(% RH) 25/75 4.7/38 6.7/70 21 000/45

40/10 12.2/80

refs 14 52 1 29 53 54 55 4 this work

a

In this table, PDDA, PU, PI, and PANI are the abbreviations of poly(diallyldimethylammonium chloride), polyurethane, polyimide, and polyaniline, respectively.

Figure 3. Dynamic responses of the CNC sensor to 24% (a), 48% (b), and 95% RH (c), respectively, in 11 repeated sensing cycles at 25 °C. (d) One of the selected sensing cycles in the detection of 24% RH. (e) The plot of response variations versus experimental cycle for the repeated detection of 95, 48, 24, and 8% RH. (f) Recovery percentage of the sensor versus experimental cycle in the repeated detection of 48% RH.

GO and graphene synthesized by plasma-enhanced chemical vapor deposition,49 the CNC sensor displays smaller response, but much faster response and recovery speeds (Figure S5 and Table S1). For example, the response and recovery time of the GO-based humidity sensor were calculated to be as long as 152 and 83 s, respectively. The response and recovery time of the graphene-based humidity sensor were calculated as 118 and 149 s, respectively. The faster response and recovery speeds of the CNC sensor compared with corresponding sensors made of GO and graphene are attributed to the absence of oxygenated functional groups in CNCs. Note that the short response and recovery time were obtained when the humidity sensing test was performed in the large chamber (1.8 L), where there was a delay in making the RH in the chamber reach the preset values using the dynamic mode of RH supply (Figure S6). Thus, the real response and recovery time may be shorter. Notably, the signal can recover completely and rapidly in the relaxation process with a small baseline drift, indicating good reversibility. The rapid response/recovery speeds and the reversibility should be attributed to both the hydrophobic effect of the LCP substrate and the minimal oxygen content in CNCs. Previously, first-principles studies of water adsorption on carbon materials reveal that the doping effect of H2O molecules strongly depends on the substrate.50 Since the LCP

CNTs, the helical structure of CNCs contributes to an enlarged surface area, which facilitates the moisture adsorption by providing increased number of adsorption sites.45 Nitrogen adsorption and desorption tests were also performed to investigate the porosity of CNCs, as shown in Figure S4. The Brunauer−Emmett−Teller surface area of the CNCs was measured to be 55.22 m2 g−1, indicative of the porous structure of the spiral CNCs. The well-defined pore system is mainly composed of mesopores and inner hollow carbon tubes.47 The mesoporous structure facilitates gas adsorption during gas sensing.45 Furthermore, the large amount of defective sites on CNCs also serves as active sites to facilitate the adsorption of water molecules. In addition, the electron hopping in the helical CNCs promotes the transport of the charge carrier, increasing the signal level.33 The typical response and recovery time t90 of the sensor are defined as the time required for 90% signal change in the full magnitude of response factor.1,25,48 As such, the response and recovery time of the CNC sensor are calculated as short as 1.9 and 1.5 s, respectively (Figure 2d), which are shorter than or comparable to those of humidity sensors based on other materials, including GO, RGO, CNT, WS2, polyaniline (PANI), LiCl-doped TiO2, MoS2, etc. (Table 1). Compared with other carbon-based humidity sensing materials, such as E

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Real-time response of the CNC sensor to linearly increased RH from 64.8 to 69.6% with a small step of 0.8% at 25 °C. (b) Plot of response variation versus RH using the data in (a). (c) Real-time response of the sensor to linearly increased RH from 51.2 to 62.4% with a small step of 1.6% at 25 °C. (d) Plot of response variation versus RH using the data in (c).

different cycles are highly similar, which features the goodcycling behavior. The CNC sensor displayed nearly constant responses with small variations in the repeated detection of the same RH. For instance, the standard deviation values of the sensor in the repeated detection of 8, 24, 48, and 95% RH were as small as 0.09, 0.14, 0.12, and 0.24%, respectively, indicating the remarkable stability and repeatability (Figures 3e and S8). Furthermore, the response of the sensor increased monotonically with RH, which was consistent with the results in Figure 2a. The good repeatability in response was related to the impressive signal recovery percentage (close to 100%) (Figure 3f). Since the adsorbed water molecules readily leave the CNC surface in the relaxation process, the active adsorption sites become available for the adsorption of new water molecules in the next sensing cycle, leading to a nearly unchanged response signal in the repeated detection process. Note that this CNC sensor displays a wide RH detection range (4−95%), which is wider than those of the previously reported humidity sensors based on other materials (Table 1). The capability to detect tiny humidity variation is vital for the precise monitoring of environmental humidity in practical applications. Nevertheless, this capability has rarely been reported by previous humidity sensors.41−43 Here, the CNC sensor displays the distinct capability to detect the humidity variation as minute as 0.8% (Figure 4). Specifically, the response increased from 9.9 to 10.3% when the RH increased from 64.8 to 65.6% (Figure 4a,b). Furthermore, the response increased monotonically from 9.9 to 11.3% with linearly increased RH from 64.8 to 69.6% with a small step of 0.8%. To the best of our knowledge, the ability to differentiate such small response variation has rarely been reported for previous humidity sensors based on other materials,41−43 demonstrating the unique advantage of CNCs in high-resolution humidity detection. The response versus RH correlation was linearly fitted in the small linear region from 66.4 to 69.6% RH (Figure S9). The extracted sensitivity (0.16/% RH) from this linear fitting is close to that derived from the linear fitting within a large RH range (Figure 2b).

substrate is hydrophobic, the interaction between the substrate and H2O molecules is very weak. The weak adsorption facilitates the release of H2O molecules in the relaxation process, speeding up the signal recovery. Furthermore, the CNCs do not contain impurities like oxygenated functional groups. Thus, the interaction between H2O molecules and CNCs is dominated by a weak physical force, such as van der Waals’ force, rather than strong chemical bonds. The weak physical adsorption leads to a fully reversible sensing process with rapid response and recovery speeds. Notably, good linearity was also observed when ascending RH levels were introduced in the chamber in the reversible sensing process (Figure 2f,g). Furthermore, the dynamic responses of the sensor to the same RH are nearly constant, whether descending or ascending RH was introduced in the chamber in the reversible sensing process, demonstrating the good reproducibility of the humidity sensor (Figure 2e). To calibrate the humidity sensor, the relationship between the response and RH was linearly fitted, producing a linear function as: ΔR/R0 (%) = 0.15RH − 0.39 (Figure 2b). The sensitivity of a chemical sensor can be defined as the slope of the linearly fitted response versus RH line.29,51 With this approach, the sensitivity calculated in the reversible sensing with descending RH is coincident with that extrapolated from the reversible sensing with ascending RH, which verifies the correct identification of the sensitivity (0.15/% RH) for the CNC humidity sensor (Figure 2b,g). The hysteresis characteristics of the CNC humidity sensor were studied with varied RH from 0 to 60% (Figure S7). The responses in humidification and desiccation processes were very close. A small hysteresis loop with the maximal hysteresis of 7% was observed at 40% RH. The repeatability and stability are significant properties of humidity sensors in the practical sensing applications.1 To evaluate the repeatability, the time-dependent resistance variations of the sensor to different RH levels, such as 8, 24, 48, and 95% RH, were monitored in consecutive 11 sensing cycles (Figure 3a−d). The response curves to the same RH in F

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. (a) Dynamic response of the CNC sensor to slow breaths (0−24 s) at calm state and fast breaths after running for 10 min (30−54 s). (b, c) Dynamic responses of the sensor to the speaking and the blow with mouth, respectively. (d) Plot of RH versus the distance from the fingertip to the sensor. The RH was calibrated by a reference RH sensor. (e) Dynamic response of the sensor to the approach of a fingertip with the gaps of 3, 2, and 1 mm, respectively, between the fingertip and this sensor. (f) Response variation versus the distance between the fingertip and the sensor. All the humidity sensing tests in this figure were performed at 25 °C.

Figure 6. (a) Photograph depicting the tiny size of the CNC sensor in comparison with that of 1 S$ coin. (b) Optical image showing the CNC sensor fabricated on the LCP substrate can be conveniently bent with a bending radius of 1 cm. (c) Dynamic responses of the CNC sensor to 80% RH under flat and bent states at 25 °C. (d) Dynamic responses of this sensor to 80% RH after utilization for 0, 3, and 6 months at 25 °C. (e) Comparison of the responses of the same sensor to 80% RH under flat and bent states, and after utilization for 6 months. (f) Dynamic responses of this sensor to respiration under flat (0−24 s) and bent states (31.5−55.5 s) at 25 °C. (g) Dynamic responses of this CNC sensor fabricated on the SiO2/Si substrate to 50% RH at 22, 34, and 54 °C, respectively. (h, i) Plots of the response values and recovery time of the CNC sensor versus substrate temperature in the detection of 50% RH, respectively.

G

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

respectively (Figure 6f). No noticeable response degradation was observed after the bending strain was applied. The good mechanical flexibility of the humidity sensor is attributed to the flexible nature of both the CNCs and the LCP substrate.36 After the CNC humidity sensor was fabricated on a SiO2/Si substrate, the temperature dependence of the sensor was investigated (Figure 6g−i). The response of the CNCs to 50% RH decreased from 6.8 to 0.4% with an elevated substrate temperature from 22 to 54 °C, indicating a strong negative correlation between the response and temperature. A competition exists between the thermal energy and adsorption energy for the adsorption of water molecules on the CNC surface.56,57 The adverse effect of temperature on the response is attributed to the decreased adsorption energy and increased thermal energy at elevated temperatures.56 In contrast, the signal recovery time was reduced from 365 to 99 s with elevated temperatures from 22 to 54 °C, suggesting the accelerated recovery process by heating. This is because the heating can lower the energy barrier of moisture desorption, and thus impels the adsorbed water molecules on the CNC surface to vibrate, facilitating their detachment from the CNC surface.56 Compared with the CNC sensor fabricated on the conventional SiO2/Si substrate, the CNC sensor fabricated on the LCP substrate displays higher sensitivity, faster response, and recovery speeds, demonstrating the distinct advantages of the LCP platform in improving the performance of humidity sensors.

The practical applicability of the CNC humidity sensor was further validated by employing it to monitor various human activities, including respiration, speaking, blowing with mouth, and the noncontact fingertip approach, in real-time (Figure 5). Generally, the humidity of exhaled gas by human is slightly higher than that in ambient air.1 When this sensor was brought close to the nose of a volunteer for the real-time monitoring of his breath, the resistance of the sensor increased and decreased sharply in the exhaling and inhaling processes, respectively. Notably, the respiration periodicities of the volunteer at the calm state and after running for 10 min were measured to be 3.5 and 2.5 s, respectively, demonstrating the capability of this sensor to distinguish different physical states of human (Figure 5a). In addition to the periodicity of the response patterns, the intensity of the breath peak may also contain some important physiological health information, such as dehydration.1 Due to the rapid response/recovery speed and high sensitivity, this sensor can also be exploited to detect speaking with the frequency of 1.85 Hz (Figure 5b). Thus, the speed of speaking can be quantitatively measured by this humidity sensor. In addition, the sensor also showed well-defined response kinetics to the blow with the mouth (Figure 5c). Importantly, the CNC humidity sensor can also be used for noncontact sensation, such as the noncontact detection of the approach of a fingertip (Figure 5d−f). The fingertip served as a humidity source in this study. When the wet fingertip moved close to the sensor, the resistance of the sensor increased sharply. Furthermore, the resistance recovered to the initial level when the fingertip was removed. Furthermore, the response increased rapidly with the decreased distance between the fingertip and the sensor. For example, the response increased dramatically from 0.82 to 5.1% with the decreased distance from 3 to 1 mm, which was attributed to the increased RH level with the shortened distance. This noncontact humidity sensation capability of the CNC sensor holds promise for the human−machine interface and noncontact localization applications.4 It is worth noting that the CNC sensor fabricated on the flexible LCP substrate provides the distinct advantages of small size and good flexibility. The comparison in the size between the two CNC sensors and one Singapore dollar coin in a photograph indicates the relatively small size of the humidity sensor (Figure 6a). Notably, the CNC sensor displayed similar response curves to 80% RH under flat and bent states, suggesting the immunity of this flexible sensor to the large bending strain with the bending radius of 1 cm (Figure 6b,c). The long-term stability was evaluated by employing the same CNC sensor for humidity sensing within 6 months. No appreciable response degradation was found after utilization of this sensor for as long as 6 months, demonstrating the impressive stability and relatively long life span of the sensor. The long-term stability of the sensor is attributed to the unique material properties of both the sensing material (CNCs) and the LCP substrate. As the CNCs do not contain purities such as hydrophilic oxygenated groups, the gaseous chemicals in ambient air hardly form chemical bonds with the CNCs, enabling the CNCs to retain their intrinsic humidity sensing properties. Furthermore, the hydrophobic LCP substrate has very low permeability for moisture, nitrogen, carbon dioxide, oxygen, hydrogen, etc.40 The moisture absorption is also extremely low (∼0.02%).40 Thus, the interplay between the moisture and the LCP in the long-term is also negligible. The flexibility of the CNC sensor was further evaluated by employing it to detect respirations at flat and bent states,

4. CONCLUSIONS CNCs are employed for the first time to fabricate a highresolution, fast-response/recovery, highly sensitive, stable, and linear humidity sensor by integrating on a flexible, hydrophobic, and microporous LCP substrate. The doping effect of water molecules is responsible for the positive resistance shift of the CNCs upon moisture adsorption. Due to the high purity of the CNCs and the hydrophobic effect of the LCP substrate, this humidity sensor exhibits a relatively short response (1.9 s) and recovery time (1.5 s), high reversibility, as well as remarkable stability and repeatability. In addition, the unique helical structure of the CNCs coupled with the microporous structures of the LCP substrate contributes to the high sensitivity, low limit of detection down to 4% RH, and a broad detection range (4−95% RH). It is worth noting that this humidity sensor can experimentally detect the humidity variation as small as 0.8%, which is too small to be detected by previous flexible humidity sensors. On the basis of these attributes, the flexible sensor can be employed to monitor various human activities, such as breath, speaking, blow, and noncontact sensation. Furthermore, human respirations at the calm state and after exercise can be conveniently distinguished. Notably, the response of the sensor is immune to the longterm repeated operation and large mechanical bending, demonstrating good stability. These features provide tremendous opportunity for a variety of practical applications, including personal health and activity monitoring, e-skin, and intelligent wearable systems.1,2,7 In addition, this study provides new insights into the material-structure−property correlation of carbon materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18599. H

DOI: 10.1021/acsami.8b18599 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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SEM image showing that a CNC bridged the gap on the Au IEs; photograph showing that the measured contact angle of a water droplet on the porous LCP substrate; EDX spectra of the CNCs deposited on a Si substrate; nitrogen adsorption and desorption isotherms and Barrett−Joyner−Halenda pore distribution of the CNCs, dynamic responses of the GO- and graphenebased humidity sensors to 70% RH; comparison of the response and recovery time for different carbon-based humidity sensing materials; simultaneous measured responses of the CNC sensor and the commercial sensor to descending and ascending humidity in the chamber; investigation of the hysteresis characteristics of the CNC sensor; plot of the standard deviation of the sensor versus relative humidity in the repeated detection of the same RH for 11 cycles; plot of the response of the CNC sensor versus relative humidity within the small RH range from 66.4 to 69.6% (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.W.). *E-mail: [email protected] (Y.-M.S.). *E-mail: [email protected] (G.P.W.). ORCID

Jin Wu: 0000-0002-3065-6858 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (Grants Nos. 61801525, 11574218, and 11734012), the Guangdong Natural Science Funds Grant (2018A030313400, 2018A030310603), the Open Fund of the Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications (Jinan University) (Grant No. 201700), the Science and Technology Innovation Commission of Shenzhen (Grant No. JCYJ20170818101314276), the Natural Science Foundation of Shenzhen University (Grant No. 2017010), and the Guangzhou Science and Technology Project. The authors wish to acknowledge the assistance on HRTEM observation received from the Electron Microscope Center of the Shenzhen University.

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