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Calcium Gluconate-Derived Carbon Nanosheet Intrinsically Decorated with Nano-Papillae for Multifunctional Printed Flexible Electronics Yiliang Wang, Haomin Wang, Huimin Wang, Mingchao Zhang, Xiaoping Liang, Kailun Xia, and Yingying Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04060 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019
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Calcium Gluconate-Derived Carbon Nanosheet Intrinsically Decorated with Nano-Papillae for Multifunctional Printed Flexible Electronics Yiliang Wang, Haomin Wang, Huimin Wang, Mingchao Zhang, Xiaoping Liang, Kailun Xia, and Yingying Zhang* Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, PR China KEYWORDS:
electronics; flexible sensors
ABSTRACT: With the blooming of wearable technology, developing active materials that can be printed in large-scale attracts great attention. Particularly, there are abundant genius structure design in nature endowing superior performance, inspiring the design of materials towards high performance wearables. Herein, we report the controllable preparation of bionic carbon nanosheets decorated with in-situ formed nanoparticles (NP-CNS) through the pyrolysis of calcium gluconate (CG), which are further used for printing high-performance humidity/pressure/strain sensors. The transformation from CG to NP-CNS had been studied in detail. Interestingly, there are papillae-like CaO NPs formed on the carbon nanosheets, endowing NP-CNS good dispersion in inks and rapid response to external stimuli. Particularly, the printed 2 ACS Paragon Plus Environment
humidity sensor possesses fast response time (1.7s) and broad detecting range (0% to 96% RH), raising from the high hydroscopicity of the CaO NPs and the thus induced expansion of the NP-CNS. Besides, the strain sensor and pressure sensor also show high sensitivity and broad detecting range, which is derived from the unique bionic structure of the NP-CNS. We further showed their excellent performance in monitoring of pulse wave, breath and human motion, indicating the wide potential applications of the bionic NP-CNS in smart wearables.
Flexible and wearable electronics have attracted enormous academic and industrial interests owing to their high potential in health monitoring, motion tracking, human-machine interfaces, soft robotics and augmented reality.1-2 Recently, various flexible sensors have been designed and broadened the scope of flexible electronics.34
However, it is still challenging to balance the performance and the cost of the
sensors, limiting the development and practical applications of flexible sensors. Printing technique, which can produce functional circuit directly on a variety of substrates, holds great promise for the large-scale production of flexible electronics.5 On the other side, the performance of flexible sensors highly depends on the active materials. Particularly, the key parameters, such as the sensitivity and the sensing range, tightly related to the structure of the materials.6-7 Therefore, developing active materials for flexible sensors, which possess highly sensitive structures and can be facily dispersed in printing inks, is highly desired. 3 ACS Paragon Plus Environment
A variety of materials have been explored as printable materials for flexible electronics, such as conductive polymers,8 metallic nanomaterials,9-11 and carbon nanomaterials.12-14 Among them, carbon nanomaterials have great potentiality due to their good electrical conductivity, excellent flexibility, as well as high chemical and thermal stability. Carbon nanotubes (CNTs),15-16 graphene,17-19 and carbon nanofibers (CNFs),20-21 which are the representative carbon nanomaterials, have been used for printing flexible sensors.22 However, the preparation of these materials generally requires harsh condition and time-consuming process. Besides, it is difficult and complicated for the above-mentioned carbon nanomaterials to form hierarchical structures, which usually lead to higher sensitivity and wider sensing range than planar structures. Considering the above factors, seeking easily dispersible carbon nanomaterials with hierarchical structures are urgently needed for printing highperformance wearable sensors. Nature mother has abundant genius structure and material designing to achieve superior performance, such as the well-known super-hydrophobicity of lotus leaves and the super-sensitivity finger-tips derived from the 3D fingerprint structures. Particularly, the existence of papillae on the tongue and fingerprint gives them supersensitivity. Recently, biomaterial-derived carbon material has attracted significant interests.2,
Previous works have reported several biomaterial-derived carbon
materials, such as carbonized silk,20 carbonized cotton24 and carbonized paper,25 which have been used for fabricating low-cost and scalable electronics. Obviously, the
structure of the obtained carbon material largely depends on the initial structure and the chemical composition of the precursor biomaterial. Particularly, calcium gluconate (CG) is a well-known glucose derivative, which is commonly used as drug and food additives.26-27 Interestingly, CG can transform to low-density and closed-cell carbon aerogel composed of carbon nanosheets when exposed to heat,28 which can serve as active materials for printing flexible sensors. Compared with other reported works,29-30 the fabrication process is more facile and rapid. In addition, the intrinsic hierarchical structures of carbon aerogel can be easily controlled, which brings a new idea for tuning sensor performance.31 Herein, we report the preparation of CG-derived bionic carbon nanosheets decorated with in-situ formed CaO nanoparticles (NPs), and further show their applications in printing high-performance flexible humidity, pressure and strain sensors. We systematically studied the transformation process and the structure evolution of CG in a high temperature treatment process. Interestingly, we found that there were in-situ formed papillae-like CaO NPs on the carbon nanosheet surface when the annealing temperature was around 700−800 °C, and the CaO NPs would invade the carbon nanosheets while the temperature was above 1100 °C. The nanopapillae decorated carbon nanosheet (NP-CNS) could respond to multiple external stimuli (humidity, strain, and pressure) and change the shape, enabling it to be a promising candidate material for electronic sensors. We used the as-obtained NPCNS as precursors and prepared stable printing inks by simple agitation with ethanol,
where the NP-CNS can maintain their hierarchical structure without collapsing after printing. The NP-CNS can be used for printing of flexible humidity/strain/pressure sensors, which all show high performance due to the unique structure of the bionic NP-CNS. The sensors were further used for tracking of breath, arterial pulse, and joint motion. The facile preparation of NP-CNS from CG presents a unique and practical strategy for obtaining high-performance and printable materials towards large-scale production of flexible electronics. 2. RESULTS AND DISCUSSION Preparation Process of NP-CNS from CG. Figure 1a illustrates the transformation process of CG to a closed- cell carbon aerogel in N2. The process includes melt and water loss, pyrolysis and expansion, carbonization and catalytic decomposition, which will be discussed in details in the later part. Figure 1b shows images of a CG tablet and the as-formed materials treated at different temperature. Figure 1c shows an SEM image of the as-formed closed-cell aerogel. The aerogel has a very low density, which is about 4 mg/cm3. The size of the cells ranges from a few micrometers to one hundred micrometers and every cell consists of several carbon nanosheets as the walls. Figure 1d shows the morphology of a cell edge and a nanoparticle decorated carbon nanosheet, which is very similar to the hierarchical micro/nano-structures of lotus leaves (Figure 1e).32
Figure 1. Preparation process and the application of NP-CNS. (a) Schematic illustration of the transformation process of CG to closed-cell carbon aerogel in N2. (b) Photograph of a CG tablet and the residues obtained at different temperature. (c, d) Scanning electron microscopy (SEM) images of carbon aerogel and NP-CNS obtained at 900 °C. The inset in (c) is the photograph of a carbon aerogel on flowers. (e) SEM images showing the structure of the lotus leaf with papillae. Pyrolysis and Carbonization Process of CG. The pyrolysis and carbonization process of CG in N2 was systematically studied. The evolution of volatilized products formed during thermal degradation of CG was monitored using thermogravimetric analysisFourier transform infrared spectroscopy (TG-FTIR).33 Figure 2a shows the thermogravimetric and Grame−Schmidt (TG-GS) curves, which include five weight lose steps. The first step located in 50−170 °C corresponds to the release of the
absorbed and crystal H2O. CG began to melt and expand at about 170 °C, which is consistent with the results in Figure 1b. The second step (170−320 °C) shows a dramatic loss in mass which is caused by the pyrolysis of CG molecule. As shown in the 3D diagram (Figure 2b), the volatilized products are mainly CO2 (2360 cm−1) and H2O (3950−3500 cm−1).34 Besides, there are also other products (1500-1750 cm−1), which can be assigned to gaseous pyrolysis compounds (CxHyOz) containing carbonyl group, carbon-carbon double bond, and phenyl, such as ketones, aldehydes, lipid, and aromatic compounds.35 Meanwhile, the molten CG would be blown into a yellow aerogel with these pyrolysis gases. The third step around 400 °C is caused by the carbonization of the aerogel. Most of the oxygen and hydrogen in CG molecule are lost in this process, so the color of aerogel begins to turn to black (Figure S1). The further weight loss above 400 °C is mainly related to the further transformation of the solid pyrolysis products, which will be discussed later. Finally, NP-CNS is obtained after annealing the sample at a temperature above 780 °C. Figure 2c shows the X-ray diffractometer (XRD) patterns of the obtained product with annealing temperature varied from 600-1100 °C. The samples are noted as NPCNS-600/700/800/900/1000/1100 with the terminal number corresponding to the final annealing temperature of the sample. As seen in Figure 2c, there are only diffraction peaks belonging to CaCO3 in the XRD patterns of NP-CNS-600 and NP-CNS-700. While for the other samples, peaks corresponding to CaO, Ca(OH)2 and CaCO3 could be found, indicating the CaCO3 began to decompose above 700 °C.36
samples were taken out from the annealing system and exposed to the ambient condition, the generated CaO NPs could react with H2O and CO2 in the air, so Ca(OH)2 and CaCO3 would be formed. The C 1s spectrum of NP-CNS-1100 (Figure 2d) supplies more evidence, five peaks located at 284.6, 285.6, 286.6, 288.4 and 290.2 eV can be found, which are attributed to C-C, C-OH, C-O-C, C=O, and CaCO3, respectively.37-38 The X-ray photoelectron spectroscopy (XPS) spectra (Figure S2) confirms that NP-CNS contains C, O and Ca, which is in consistence with elemental mapping images (Figure S3).
Figure 2. Pyrolysis and carbonization process of CG. (a) TG-GS curves (50-1100 °C, 5 °C/min, N2). (b) TG-FTIR results of CG pyrolysis. (c) XRD patterns of CG residues obtained at different temperature. (d) The C 1s spectrum of NP-CNS-1100. (e) Raman spectra of NP-CNS-800, NP-CNS-900, NP-CNS-1000 and NP-CNS-1100.
Figure 2e shows the Raman intensity ratio of D to G bands (ID/IG) of NP-CNS800/900/1000/1100, which are 0.94/0.97/1.02/1.04, respectively, indicating the defects in the carbon nanosheets increase with increasing of the annealing temperature.39 However,
the defect density of carbon materials usuallydecreases
with the increase of annealing temperature in the inert atmosphere,40 which implies a different trend in this study. This interesting observation can be ascribed to the elevated catalytic decomposition of carbon nanosheets at high temperature induced by the CaO NPs, which will be further discussed later. Besides, the thermal degradation and expansion of CG in air can supply more information (Supporting Information, Video 1). Morphology of NP-CNS. The morphology of the obtained NP-CNS was investigated in detail (Figure 3). As illustrated in Figure 3a, the CaO NPs on the carbon nanosheet are very similar to the papillae on lotus leaves. As shown in Figure 3b, neighboring NP-CNS form closed-cell aerogel and the maximum diameter of the cell is about 100 μm. Figure 3b, c show the products obtained at 800 °C and 1000 °C, respectively, indicating closed-cell aerogels can be formed at different temperatures. However, higher annealing temperature results in bigger CaO NPs, which can be concluded from the high magnification SEM images of NP-CNS-800 and NP-CNS-1000 (Figure S3a, b). Besides, there are some partly expanded carbon cells which are composed of thick NP-CNS. As shown in Figure 3d, the CaO NPs at the cell edges and thick NP-CNS are bigger than that on the thin NP-CNS, which can be ascribed to the fact that there
is more calcium in these areas. Combined with the XRD results, we surmise that the calcium in CG can migrate to the carbon nanosheet surface and form CaCO3, which will decompose into CaO at high temperature.
Figure 3. Morphology of NP-CNS. (a) Schematic diagram of the lotus leaves-liked NPCNS. (b, c) SEM images of NP-CNS obtained at 800 °C and 1000 °C, respectively. (d) SEM image of a partly expanded carbon cell in NP-CNS-900. (e, f) Transmission electron microscopy (TEM) images show three nanosheets of NP-CNS-1000 and NPCNS-1100, respectively. (g), (h, i) TEM images of NP-CNS-1000 and NP-CNS-1100 after treating with 0.3M HCl. In Figure 3i, several residual CaO NPs could be found. The CaO NPs formed in the pyrolysis process can in turn catalyze the reactions and decomposition of the pyrolysis products during the transformation process of CG. 11 ACS Paragon Plus Environment
As a common solid base catalyst, CaO can catalyze the aldol addition and transesterification reactions between different oxygen-containing groups.41-42 And CaO can also facilitate the decomposition of hydrocarbon fragments during the chemical vapor deposition (CVD) growth of graphene.43 In our process, as the annealing temperature rose, the CaO NPs grew up, and the catalytic decomposition became faster. Compared with NP-CNS-1000 (Figure 3 e), there are many bigger CaO NPs in NP-CNS-1100 (Figure 3 f), which brings the NP-CNS-1100 higher strength. The CaO NPs can be removed by acid treatment. Figure 3g-i show TEM images of the acid-treated NP-CNS. The acid treated NP-CNS-1100 has many holes (Figure 3h, i), which cannot be found in the acid treated NP-CNS-1000 (Figure 3g), indicating that the CaO NPs can invade the carbon nanosheet at 1100 °C and lead to faster catalytic decomposition of the carbon nanosheets. Noted that the acid treated holey NP-CNS1100 may have a high surface area and serve for applications in adsorption, catalysis and energy field.
Figure 4. The preparation and properties of the NP-CNS inks. (a) Photograph of the NP-CNS ink. (b) Optical image of the NP-CNS ink coated on a glass slide. (c) Atomic force microscopy (AFM) image of the NP-CNS ink coated on a silicon substrate. (d) Photograph of the printed strain sensors with different NP-CNS inks. (e, f) SEM images showing the cross-section of the NP-CNS layer in a strain sensor. The Preparation and Properties of NP-CNS Inks. The bionic CG-derived NP-CNS can be used to prepare printing inks for flexible electronics, where the carbon nanosheets can maintain their pristine hierarchical structure without collapsing. The printing inks of pressure and strain sensors were easily prepared by mixing NP-CNS and ethanol (0.1g:10 ml) (Figure 4a). Figure 4b is an optical image of the NP-CNS deposited on a substrate from a diluted ink, which shows that the pristine morphology of the NP-CNS is well preserved. The AFM image shows the thickness of NP-CNS is about 50 nm (Figure 4c). We further demonstrated the screening printing of the ink on flexible substrates in Figure 4d. The printed film could withstand cycling tensile strain up to 80% (Supporting Information, Video 2). Figure 4e, f show the cross section of the printed layer, further evidencing the well reserved hierarchical structure of the NP-CNS which lead to high sensitivity and large sensing range for strain sensors. NP-CNS Humidity Sensor. We had chosen NP-CNS-1000 to fabricate humidity sensors which can guarantee the wide sensing range and high sensitivity. Because the NP-CNS-1000 owns a relatively high strength and electrical conductivity (Figure 4d). Meanwhile, the CaO NPs are still located on the surface of NP-CNS-1000. Figure 13 ACS Paragon Plus Environment
5a illustrates the structure of the NP-CNS-1000 humidity sensor. A polyethylene terephthalate (PET) film was used as the substrate and the NP-CNS-1000 ink was mixed with silver paint which served as an adhesive for the printing of humidity sensors. Figure 5b shows the relative change in current ((I0-I)/I0) versus humidity applied to the NP-CNS humidity sensor, which can be used to evaluate the sensitivity of sensors. The curve obeys to an exponential function, which means the sensor has enhanced sensitivity for high humidity. For example, the sensitivity for 50% RH and 90% RH are 4.39% and 8.25%, respectively, which is comparable with other reported high-sensitivity humidity sensors.44 Besides, the sensor shows a higher sensitivity at a lower temperature (Figure S4). And the humidity sensor had a wide detecting range from 0% to 96% RH, which is obviously larger than most of the reported humidity sensors (Table S1). Cycling moisture on/off had been applied on the NP-CNS humidity sensor to investigate its reliability. As shown in Figure 5c, while the humidity changes between RH of 9.5 % and 46.3%, the sensor showed a rapid and reliable response, proving the excellent performance of the humidity sensor.
Figure 5. Performance and working mechanism of NP-CNS-1000 humidity sensor. (a) The structure of NP-CNS-1000 humidity sensor. (b) Relative change in current of the sensor versus the humidity at 10 °C. (c) Reliability of the sensor against moisture on/off cycles. (d) The current curve when the humidity sensor was exposed to exhaling and inhaling of human breath. The inset shows magnified curve of three breath cycles. (e) Respond time and recovery time of humidity sensor (39% to 10% RH). (f) Photograph showing a NP-CNS-1000 humidity sensor assembled on a respiration mask. (g) Working mechanism of the humidity sensor based on the hygroscopicity of CaO NPs and thus expanded space between neighboring carbon nanosheets. The humidity sensor showed fast response and excellent reliability in monitoring the exhaling and inhaling of human breath. As shown in Figure 5d, the current
decreased dramatically as exhaling, and increased rapidly during inhaling of breath. The results can be understood considering the higher moisture in the exhaled air than the ambient humidity. Figure 5e shows that the humidity sensor owns a fast response (1.7s, 10% to 39% RH) and recovery (100.1s, 39% to 10% RH) ability. Based on the good flexibility and the excellent performance of the printed humidity sensor, it can be facilely assembled on respiration mask for breath monitoring (Figure 5f). Figure 5g illustrates the working mechanism of the humidity sensor, which is based on the hygroscopicity of CaO NPs.45 When the sensor is put in high humidity, the CaO NPs will absorb water rapidly and grow into large sizes. It can lead to the change in the shape of NP-CNS as well as the expansion of the space between neighboring nanosheets, thus the resistance increases accordingly. In an environment with low humidity, the shape can recover, resulting in a decrease of resistance. In the NP-CNS1000 humidity sensor, the CaO NPs are located on the surface, which could forbid the adhesion of neighboring NP-CNS and supply enough space for the expansion and shape change. The abundant CaO NPs on the surface and the hierarchical structures of the NP-CNS enables the fast response, high sensitivity and broad sensing range of the sensor. NP-CNS Strain Sensor and Pressure Sensor. The NP-CNS ink was also used for printing highly sensitive strain and pressure sensor. Figure 6a illustrates the structure and the working mechanism of the strain sensor. Strain sensors had been fabricated using inks with NP-CNS samples prepared at different temperature. All strain sensors
can tolerate strains up to 500% (Figure 6b). Furthermore, the printed strain sensors showed high sensitivity as evaluated by gauge factor (GF) corresponding to the slope of the ∆R/R0~strain curves.39 For example, the GF of NP-CNS-800 sensor is 21.9 for strain range of 0−350% and 99.9 for strain range of 350–500%. Besides, the respond time of the printed strain sensors is about 70 ms (Figure S5a, b). The performances of the NP-CNS strain sensors are comparable or even better than typical reported flexible strain sensors (Table S2). Figure 6b also shows that the sensitivity of the strain sensors decreases with the annealing temperature of the NP-CNS. This phenomenon may be ascribed to the evolution of CaO NPs, which gradually invade carbon nanosheets at high temperature, leading to reduced sensitivity of the printed NP-CNS layer. In addition, the NP-CNS strain sensors have excellent reliability and stability, especially the NP-CNS-1100 sensor (Figure 6c).
Figure 6. Performance and working mechanism of flexible NP-CNS pressure sensor and strain sensor. (a) The structure of NP-CNS strain sensor. (b) Relative change in resistance of the strain sensor versus the applied tensile strain (strain rate 7.5 mm min –1).
(c) Relative resistance changes of NP-CNS-1100 sensor against cyclic loading and
unloading of strain at a frequency of 0.1 Hz. (d) The structure of NP-CNS pressure sensor. (e) Current–voltage (I–V) curves of NP-CNS pressure sensor under different applied pressure. (f) Multiple cycles of pressure response under pressure of low pressures (50 Pa) to high pressures (600 Pa) at a frequency of 0.125 Hz.
Arterial pulse waves monitored by the pressure sensor. The inset shows a magnified waves with“P” (percussion), “T” (tidal), and “D” (diastolic) peaks. (i) Monitoring bending of a finger with the strain sensor. 18 ACS Paragon Plus Environment
The structure and the working mechanism of the pressure sensor are illustrated in Figure 6d. The NP-CNS-1100 ink was printed on two PDMS films, which were then placed face-to-face for the fabrication of the pressure sensor. Figure 6e shows the current–voltage (I–V) curves of the pressure sensor under different pressure, which displays good linear Ohmic characteristics and indicates the sensor has a stable response to pressure. And the pressure sensor has good performance for detecting low pressure. The low detection limit of the pressure sensor is 5 Pa and the respond time is about 32 ms (Table S3, Figure S5b, Figure S6). Figure 6f shows the curves while different pressure at the same frequency is applied to the sensor, further confirming the good stability of the pressure sensor. The lightweight and highly flexible sensors could be conformally attached to skin, which made them be a portable and wearable sensing device in daily life. For example, the pressure sensor can be attached to the wrist surface for detecting arterial pulse waves (Figure 6g). It demonstrated a good real-time dynamic response curve, as shown in Figure 6h. Besides, the high-performance strain sensors can be used to monitor and recognize full-range human activities, including subtle physiological signals and large joint motions. For demonstration, Figure 6i shows its application for tracking the bending of the finger. In addition, we fabricated a real-time motion tracking system using the strain sensors with the assistance of a motion capture module, which showed excellent performance (Supporting Information, Figure S7 and Video 3).
The superior performance of the pressure sensor and the strain sensor can be ascribed to the unique structure of the bionic NP-CNS. The papillae-like CaO NPs on the carbon nanosheets, which can hinder the tight adhesion between overlapping nanosheets, contribute to the formation of the hierarchical structures in the printed layer
disconnection/reconnection of the neighboring conductive layer induced by the loading/releasing of pressure or strain will lead to the resistance change of the sensors (Figure 6a, d). Therefore, by monitoring the current variation of the sensor, the pressure and strain can be monitored in real-time. In short, the unique bionic structure of the NP-CNS endows the printed sensors high sensitivity, broad sensing range and good durability (Figure S9). 3. CONCLUSIONS In summary, we reported the preparation of bionic CG-derived carbon nanosheets decorated with papillae-liked CaO NPs, which can be easily dispersed into printing inks for large-scale production of high-performance flexible humidity/pressure/strain sensors. The transformation process and the structural evolution of the CG in the hightemperature treatment process were systematically studied. We found that papillaelike CaO NPs could in-situ form on the surface of the carbon nanosheets, which can hinder the collapse of the nanosheets and the tight interaction between overlapping nanosheets in the printing inks. The NP-CNS was further used for printed of multifunctional high-performance sensors, including humidity sensors, strain sensors, 20 ACS Paragon Plus Environment
and pressure sensors. Remarkably, the printed NP-CNS humidity sensor showed high sensitivity (4.39% for 50% RH), the fast response time (1.7 s) and a broad sensing range (0% to 96% RH), which can be ascribed to excellent hydroscopicity of the CaO nano papillae. Besides, the printed strain and pressure sensors also showed high sensitivity, wide sensing range, fast response and excellent reliability, which is also derived from the unique hierarchical structure of the NP-CNS. Particularly, the NPCNS strain sensor showed high sensitivity (GF up to 99) and wide workable strain range (0−500%). Based on the superior performance, the sensors were further demonstrated for applications in the tracking of breath, arterial pulse, and joint motion. The facile preparation of bionic NP-CNS with unique structure enables the low-cost fabrication of high-performance wearable electronics through printing technique.
4. EXPERIMENTAL SECTION
Preparation of NP-CNS and NP-CNS inks. NP-CNS was prepared in a tube furnace with N2 atmosphere (100 sccm of 99.9992% N2). CG was heating to the target temperature (600 °C, 700 °C, 800 °C, 900 °C, 1000 °C and 1100 °C) and keeping 180 min, different NP-CNS were prepared after cooling to room temperature. The ramping rate was 5 °C/min. A simultaneous TG-FTIR system (Netzsch 209 F1/Bruker Vertex 70 Hyperion 1000)
was used to analyzed the preparation of NP-CNS. The NP-CNS
ink of humidity sensor was prepared by mixing NP-CNS-1000 and silver paint (mass ratio, 1:5) with a glass rod for 10 min. As for the NP-CNS inks of pressure and strain
sensors, NP-CNS and ethanol (0.1g:10ml) were mixed by supersonic dispersion for 10 min. Characterization of NP-CNS and NP-CNS inks. SEM (Quanta 450 FEG and JSM IT300) and TEM (JEM-2010) were used to characterize the morphology of NP-CNS. An AFM (Asylum Research MFP-3D, Oxford) was used to characterize the thickness of NP-CNS. A Raman spectroscope (Horiba HR800, 633 nm) was applied to obtain the Raman spectra of NP-CNS. XPS (PHI QUANTERA-II SXM) and XRD (Japan Rigaku D/Max-Ra) were used to analyze the surface elements and crystal structures of the NP-CNS, respectively. The optical images of NP-CNS in the ink was obtained with an optical microscope (Leica DM2500M). Fabrication of the NP-CNS Humidity Sensor. Firstly, a thin PET film (100 µm) was treated with plasma for 3 min. A paper mask prepared using CO2 laser writing was put on the targeted substrate, followed by screen printing using the NP-CNS/sliver paint ink. Then, the mask was removed and the film was heated at 80 °C for 10 min. Finally, two copper wires were connected to the printed layer at two ends with silver paint and heated at 80 °C for 30 min. Fabrication of the NP-CNS Pressure Sensor. A micro-structured PDMS (Sylgard 184) film (300 ± 20 µm) was obtained by pouring the precursor on sandpaper (80 mu) and cured at 80 °C for 2 h. Then PDMS/hexane (10 wt% PDMS) solution was spin coated on the film, following by a heating process at 80 °C for 3 min. After that, the NP-CNS1100/ethanol ink was screen printed on the film. Washing the film with ethanol for two 22 ACS Paragon Plus Environment
times and then heated at 80 °C for 30 min. Finally, NP-CNS-1100/PDMS film was obtained. Two pieces of NP-CNS-1100/PDMS films were assembled face-to-face, and two copper wires were glued by silver paint on each of the films. The copper electrode on each NP-CNS-1100 should be isolated from the NP-CNS-1100 layer on the other side. Fabrication of the NP-CNS Strain Sensor and Patterned Circuit. Flexible Ecoflex film (100 ± 20 µm, Supersoft 0050, Smooth-On, Inc.) was used as substrate. The paper mask was prepared using CO2 laser writing. After screen printing with the NPCNS/ethanol inks, the printed layer would be heated at 80 °C for 30 min and then encapsulated with Ecoflex. The thickness of the NP-CNS strain sensor is about 0.6 mm, as shown in the Supporting Information (Figure S10). Characterization of the NP-CNS Sensors. The cross sections of the sensors were characterized using SEM and optical microscope. The electromechanical performance of the sensors was measured with a universal testing machine (Shimadzu AGS-X) and a digital source meter (Keithley 2400) at a constant bias voltage of 5 V. Before the tests, several precycle trainings of each device were needed to achieve a stable operational state. The humidity measurement of NP-CNS humidity sensor was carried out with an airtight chamber, a humidifier, nitrogen gas, a commercialized reference humidity sensor, and a digital source meter. The relative humidity in the chamber was controlled by changing the proportion of water vapors and N2, which was calibrated by the commercialized humidity sensor. To record human body motions, two NP-CNS 23 ACS Paragon Plus Environment
strain sensors were attached to elbows using commercial medical adhesive tapes. The self-designed system for the motion reconstruction includes two main parts, the motion capture module, and the motion reconstruction module. ASSOCIATED CONTENT Supporting Information Photograph of calcium gluconate residues obtained at 400 °C (5 °C/min, N2); XPS spectra of NP-CNS-800, NP-CNS-900, NP-CNS-1000 and NP-CNS-1100; High magnification SEM images and elemental mapping analysis; Sensitivity of the humidity sensor at different temperature; Response times of the strain and pressure sensors; Sensitivity of the pressure sensor; Comparation between our sensors and other reported works; Human motion tracking using NP-CNS-800 sensors; SEM images of the NP-CNS layer in pressure sensor; The durability study for humidity, strain, and pressure sensors; Optical microscope images of the cross-section of strain sensor and A4 paper. (PDF) The expansion of CG; The printed film under different strains; The motion tracking system (avi) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 24 ACS Paragon Plus Environment
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