Laser Direct Writing of a High-Performance All-Graphene Humidity

Jun 22, 2018 - ... electronic circuit simulation can be applied in real-time monitoring on a smartphone based on the Internet of things and big data t...
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Laser Direct Writing of a High-Performance All-Graphene Humidity Sensor Working in a Novel Sensing Mode for Portable Electronics Jinguang Cai,*,†,‡ Chao Lv,†,‡ Eiji Aoyagi,§ Sayaka Ogawa,‡ and Akira Watanabe*,‡ †

Institute of Materials, China Academy of Engineering Physics, Jiangyou 621908, Sichuan, P. R. China Institute of Multidisciplinary Research for Advanced Materials and §Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

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S Supporting Information *

ABSTRACT: This paper reports a fast and highly sensitive all-graphene humidity sensor working in a novel alternating current (ac) detection mode for the first time, which is capable of sensing humidity on a smartphone for portable electronics. The humidity sensor is based on an interdigitated reduced graphene oxide/graphene oxide/rGO (rGO/GO/rGO) structure patterned by a facile laser direct writing method. It works in an ac sensing mode with a rectangular input voltage wave and measures the output voltage wave instead of conventional resistance, impedance, or capacitance, exhibiting a dramatically enhanced sensitivity by about 45 times compared to the low and unstable response in dc mode. The humidity sensor shows an obvious response to the relative humidity (RH) ranging from RH 6.3% to RH 100%. The response and recovery toward humidity change are almost instantaneous, and the corresponding costed times including humidity rise and decay times are less than 1.9 and 3.9 s, respectively, which are among the best results in the literature. The sensor also exhibits outstanding cycling stability, flexibility, and long-term stability (>1 year), as well as good reproducibility of device preparation. Besides, it can be easily connected to an iPhone and the humidity sensing can be conducted with an oscilloscope application on iOS. What’s more, an electronic circuit simulation method was employed to fit the output waves, which can not only explain the sensing mechanism, but also determine the resistance and capacitance of the rGO/GO/rGO structure, agreeing well with the results obtained from the electrochemical measurements. It can be reasonably expected that the approach combining humidity sensing and electronic circuit simulation can be applied in real-time monitoring on a smartphone based on the Internet of things and big data technologies. KEYWORDS: humidity sensor, graphene oxide, laser direct writing, ac sensing mode, internet of things, big data

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INTRODUCTION

a smartphone instead of expensive professional laboratory machines, and meet the demand of big data is challenging but urgently required in the development of portable and wearable electronics. Humidity sensor is an important kind of sensors because of their critical roles in daily life such as indoor humidity sensing, weather forecasting, medical care, and agricultural and industrial environmental monitoring, especially in the fast-developing wearable electronics.5−7 For a humidity sensor, not only the sensing performance such as high and fast

Plenty of efforts have been devoted to the study and commercial development of the Internet of things (IoT) and big data because of their increasing relations with the development of science and technology, human daily life, and world economy.1,2 Innumerable sensors, data flows, analytics, and actuators constitute the ecosystem of IoT and big data, in which sensors are one of the most important interfaces with users.3,4 To some extent, it is the falling sensor cost with the new fabrication techniques that has driven the rapid development of IoT and big data,3 but the development of wearable sensors with high stability that can be fabricated low-costly, work in simple, real-time, and portable sensing with © XXXX American Chemical Society

Received: May 5, 2018 Accepted: June 22, 2018 Published: June 22, 2018 A

DOI: 10.1021/acsami.8b07373 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

which can act as a platform for humidity sensing, data analysis, and data storage based on IoT and big data technology. Although some high-performance humidity sensors have been reported, there are rare works involving the development of humidity sensors measured and analyzed on a smartphone. Here, we demonstrated a high-performance flexible allgraphene humidity sensor based on an interdigitated rGO/ GO/rGO structure prepared by laser direct writing with a lowcost compact 405 nm blue-violet laser. An alternating current (ac) mode with rectangular input and output voltage waves was employed for the first time to exert the advantages of the rGO/GO/rGO structure, endowing it with a dramatically enhanced sensitivity by about 45 times, compared to the unstable and low response in the direct current (dc) mode. The humidity sensor shows an obvious response in the relative humidity (RH) range from RH 6.3% to RH 100%, and the detection limit can be as low as RH 6.3%. It exhibits very short response and recovery times, estimated to be less than 1.9 and 3.9 s, respectively, which are among the best performances reported in the literature. Generally, the capacitive-type humidity sensing with GO shows a relatively slow and gradually decreasing response because of the gradual charging of the MSC-like rGO/GO/rGO structure, whereas the ac sensing mode used in this work can eliminate such influence and transform the real-time response to easily measured voltages, endowing the sensor with high, fast, and stable responses. Besides, the humidity sensor showed excellent cycling stability, good flexibility, and long-term stability for more than 1 year, as well as good reproducibility of device preparation. What’s more, it can be easily connected to an iPhone through the earphone jack and the humidity sensing can be conducted with a commercial oscilloscope application on iOS. In addition, the output waves at different frequencies were fitted using an electronic circuit simulation software (LTspice), which could explain the enhancement mechanism for the humidity sensing. The determination of the resistance and capacitance of the rGO/GO/rGO structure at different RH can also be achieved by the simulation, which showed a good agreement with those obtained by electrochemical measurements. It suggests that this approach can not only be used to measure and analyze the sensing data with a smartphone, but also to obtain the electrochemical properties of the rGO/GO/rGO structure at different RH. Therefore, it can be expected that such a strategy can be applied to various kinds of sensors for real-time monitoring with a smartphone based on IoT and big data technologies.

response to humidity change, but low cost, easy fabrication, and good flexibility are also important for practical use, most of which are dependent on the sensing materials and fabrication methods. Although lots of effective sensing materials and methods have been developed to prepare humidity sensors;8−44 it is still a challenge, but important to make the fabrication easier and the performance better. Graphene has been considered as a promising candidate in many application areas including various sensors45−48 because of its unique two-dimensional (2D) atom-thin structure, high specific surface area, and excellent electronic properties.49,50 Though graphene prepared from the mechanical exfoliation, epitaxial growth, or chemical vapor deposition methods shows less defects and better electronic performance, graphene oxide (GO) and reduced graphene oxide (rGO) which can be prepared with chemical methods were considered as the most effective materials in practical use because of the relatively low cost and large-scale fabrication.51 The resistance of rGO is sensitive to the humidity, making rGO a promising humidity sensing material, but the complicated modification on rGO is necessary because of the shortages of pristine rGO, such as low sensitivity and slow response, as well as the irreversible aggregation of rGO nanosheets during the chemical reduction process.8−18 GO is an electrical insulator but becomes a good ionic conductor when water molecules are trapped in; hence, it can serve as an effective sensing material in capacitive-type humidity sensors.21,52 Compared to conventional ceramic humidity sensors,6,7 humidity sensors based on GO or rGO materials showed advantages in the preparation, sensitivity, and mechanical stability, especially flexibility, derived from the superior properties of graphene, which are very important for wearable devices. Generally, a pair of metal electrodes such as Au or Ag with different shapes was first prepared by lithographic methods or mask-assisted deposition on the substrates; then, GO-based sensing materials were deposited on the metal electrodes using a drop-casting or spin-coating method, forming the humidity sensors.8−11,22−25 However, this preparation method is complicated and needs expensive equipment and noble metals. Therefore, it is necessary to develop more facile methods to fabricate GO humidity sensors with high sensing performance. Laser direct writing is a noncontact fast single-step fabrication technique with no need for masks, postprocessing, and complex clean environments, and is promising to be integrated into the current electronic industry. In our previous studies, we have demonstrated the effective preparation of high-performance carbon-based microsupercapacitors (MSCs) and integrated devices by the facile laser direct writing method on a flexible polyimide film.53−56 Besides, laser direct writing technique has been employed in the reduction of GO through the photothermal effect to fabricate rGO microelectrodes with high porosity and conductivity in MSCs,52,57−59 as well as in the preparation of laser-induced graphene from various precursors.60−64 However, there are few studies focusing on humidity sensors by using rGO/GO/rGO-structured microelectrodes prepared by laser direct writing, exclusive of noble metal electrodes, where rGO and GO act as humidity-sensitive conductive electrode and solid-state electrolyte, respectively. Besides, it is also required to improve the sensitivity and response of the humidity sensors based on rGO and GO materials. Another important factor for a humidity sensor used in fieldwork is the humidity sensing without expensive professional laboratory machines, but with a convenient smartphone,

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EXPERIMENTAL SECTION

Preparation of an rGO/GO/rGO Humidity Sensor. The singlelayer GO powders prepared by the Hummers method were purchased from Hengqiu Tech. Inc., China, and dispersed in water under ultrasonic treatment for 2 h, resulting in a homogeneous GO dispersion with a concentration of 3 mg/mL. Then, 16 mL of the dispersion was dropped on a clean polyethylene terephthalate (PET, A-ONE 27054) film with a size of 6 cm × 8 cm and allowed to dry in air. The GO loading of the resulting GO film on the PET substrate is about 1 mg/cm2. Laser direct writing on a GO film was conducted in air under ambient conditions using a laser direct writing system, which employed a compact blue-violet continuous-wave semiconductor laser with a rated power of 90 mW, a wavelength of 405 nm (KBL-90C-A, KINMMOM KOHA, Co., Ltd.), and a 50× magnification objective lens (OLYMPUS SLMPlan N, NA 0.35). The interdigitated rGO/GO/rGO pattern was scanned directly using the system following the designed program in a computer at a scan rate of B

DOI: 10.1021/acsami.8b07373 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces 300 mm/min, where the laser power was 9.5 mW to reduce the influence of heat diffusion and to obtain a pattern with a high resolution. Characterizations. The original GO films and rGO structures obtained by laser direct writing were characterized by powder X-ray diffraction (XRD, Rigaku SmartLab 9SW, Cu Kα radiation), optical microscopy (OM) (Olympus BX51), and scanning electron microscopy (SEM, Hitachi S4800, 10 kV). The Raman spectra were measured on a micro-Raman spectrometer. The topographical images of the structures obtained by laser direct writing were observed by a color three-dimensional (3D) laser microscope (KEYENCE VK9700). The sheet resistance was measured using a four-point probe resistance meter. Electrochemical Measurements. The rGO/GO/rGO pattern obtained by laser direct writing was connected to the electrochemical instruments by using copper tapes and silver paste to ensure good conductivity. The device was placed into a sealed box with a glycerol aqueous solution at different glycerol concentrations, which can produce different RH.21 After equilibrium, the cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) measurements of the device were conducted on an electrochemical workstation (HZ5000, Hokuto Denko Ltd.) and an impedance meter (3522 LCR HiTester, Hioki), respectively. The specific capacitances of MSCs were calculated from the CV curves by using the same equation as reported previously.53 Humidity Sensing Measurements. The rGO/GO/rGO pattern was first connected into an electronic circuit in a breadboard and then put in a sealed box with constant RH using a glycerol aqueous solution. A rectangular ac wave with a peak-to-peak (pk−pk) voltage of 1 V was applied to the electric circuit from a function generator (WF1946A, NF Corporation), and the output wave was observed by a digital oscilloscope (TBS1052B, Tektronix). An oscilloscope application in iPhone (Oscilloscope, ONYX Apps) which works as both the oscilloscope and function generator was also employed in the measurements. For the sensing of flow mist, the experiment was conducted with a function generator and an oscilloscope or using an oscilloscope application on iOS by connecting the sensor to the earphone jack of an iPhone. The RH around the rGO/GO/rGO structure was changed by the mist flow from an ultrasonic mist generator. The software, LTspice (Linear Technology Corporation), was employed to fit the sensing data using the same electronic circuit.

Figure 1. Schematic illustration for the preparation of rGO patterns by laser direct writing (a), and top view (b,c) and cross-sectional (d,e) SEM images of the structures obtained by laser direct writing.

dehydrated rGO film.52 The above results were also confirmed by the Raman spectra and X-ray photoelectron spectroscopy (XPS) data of the films before and after laser direct writing. The Raman spectra at the unirradiated part, edge, and center of a laser-irradiated line are shown in Figure 2a, which clearly

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RESULTS AND DISCUSSION Preparation of rGO Structures. The process for the preparation of rGO structures is schematically shown in Figure 1a, which involves two simple steps, casting a GO film on a PET substrate followed by a laser direct writing process. A GO film with an areal loading of 1 mg/cm2 was prepared on a cleaned flexible PET substrate, which showed a rather smooth surface and a layered structure with a thickness of about 3 μm (Figure S1). Figure 1b−e shows the SEM images of the single lines with a gap of 100 μm obtained by laser direct writing at a typical laser power of 28.7 mW, which indicate that a thicker film of about 7 μm with micrometer-sized pores was produced because of the gas evolution of the adsorbed water and decomposed gases caused by laser irradiation. The expansion of the film can also be confirmed by the topographic image in Figure S2 taken by a 3D laser microscope, which indicates an obvious rise and fall at irradiated and unirradiated parts. The cracks on the surface at the edge of the laser-irradiated line shown in Figure 1c are ascribed to the tension induced by the laser thermal effect. The XRD patterns in Figure S3 indicate that the interlayer spacing of about 0.86 nm for the original GO film was reduced to about 0.36 nm after laser direct writing, which suggested the decomposition of −COOH groups of GO and the evaporation of water adsorbed between the GO layers, forming a

Figure 2. Raman spectra at different positions of a laser-irradiated line (a), XPS spectra of the films before and after laser direct writing (b), and C 1s XPS spectra of the films before (c) and after (d) laser direct writing. Inset in (a) is an OM image for labeling the position of the Raman spectra.

indicate that the intensities of the G peak to D peak (IG/ID) and 2D peak are gradually increased from the unirradiated GO to the center of the laser-irradiated line. An increase in the IG/ ID value indicates a transition of the laser-reduced GO from the original GO to a more ordered state with fewer defects, and the remarkable increase of the 2D peak indicates the production of graphitic features, both suggesting a better graphitization C

DOI: 10.1021/acsami.8b07373 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

current, resistance, impedance, or capacitance is measured in the sensing, but a rather simple way for a portable device is to measure the voltage. Here, the rGO/GO/rGO structure was connected to an electronic circuit including two resistors (Figure 3c), and for the first time a rectangular ac wave with a pk−pk voltage of 1 V was applied on the electronic circuit from a function generator (Figure 3d) and the output wave on the 10 kΩ resistor was observed by an oscilloscope. The output waves responding to an input dc voltage of 0.5 V and a rectangular ac wave with a pk−pk voltage of 1 V under various RH conditions are shown in Figure 4. In the case of dc

degree of the laser-reduced rGO than that of the original GO.65 The XPS data in Figure 2b shows an increase in the carbon content from 68.6% for the GO film to 81% for the film obtained by laser direct writing and a corresponding decrease of oxygen content from 29.5 to 16.4%. Meanwhile, the fitting of C 1s peaks for the original GO in Figure 2c and the laserirradiated film in Figure 2d indicates that the sp3 carbon was reduced significantly after laser direct writing, whereas the sp2 carbon was increased, suggesting the oxygen-containing functional groups in GO were comparably removed after laser irradiation. The laser power has some influence on the laser-irradiated structures. As shown in Figure S4, both the width and thickness of the laser-irradiated lines are increased as the laser power increases, which can be attributed to the increasing heat diffusion with increasing laser power. Correspondingly, the surface resistance was gradually reduced from almost insulating to 100 Ω level (Figure S5). Therefore, GO can be readily reduced to rGO with a highly enhanced conductivity by a facile laser direct writing method without any other treatments. Humidity Sensing. An rGO/GO/rGO interdigitated pattern including 48 electrodes with a length of 2.5 mm, a width of 160 μm, and a gap of 40 μm between two adjacent electrodes was prepared by laser direct writing (Figure S6), where the laser power was set at 9.5 mW to reduce the influence of heat diffusion for a high resolution. The preparation time for one rGO/GO/rGO pattern here is about 15 min, but the productive efficiency can be significantly improved by using laser arrays because of the low cost of semiconductor lasers or a galvanometer scanner with an ultrafast scan rate up to 10 m/s, which can reduce the preparation time to less than 10 s per pattern. Both the ways can be integrated with the roll-to-roll technique to improve the overall productive efficiency. Therefore, the laser direct writing technique is a really low-cost and efficient method for producing such sensors. Figure 3a shows a digital photograph

Figure 4. Output waves of the humidity sensor responding to a dc voltage of 0.5 V at RH 100% (a) and a rectangular ac wave with a pk− pk voltage of 1 V at 0.04 (b), 4 (c), and 400 Hz (d) at different RH, and the change of the sensing peak voltages toward RH at different frequencies (e) and the change of the sensing peak voltages toward frequency at different RH (f).

mode at RH 100% (Figure 4a), in the first cycle, the output voltage showed a peak value of 0.31 V, but it was gradually decreased to 0.04 V within 10 s and then to a constant voltage of 0.016 V in 30 s. In the second cycle, the output voltage showed a largely reduced peak value of 0.04 V, and it was gradually decreased to a constant value of 0.016 V within 5 s. The gradual decrease of the output voltage was attributed to the gradual charging of the rGO/GO/rGO structure which acted as an MSC at high RH. In the second cycle, theoretically, if the rGO/GO/rGO MSC was still in a fully charged status, the output voltage should be a very low constant value, just the same as that in the first cycle. The peak voltage of the output curve in the second cycle is caused by the self-discharge of the rGO/GO/rGO MSC fully charged in the first cycle. The charging and self-discharging behavior of the rGO/GO/rGO structure suggested that the device could not produce a stable and constant response to the RH change in the dc mode. On the contrary, in the ac sensing mode, the peak values of the output waves are much higher and more stable than those in the dc mode, and the peak voltage changes toward RH at different frequencies are also clearly observed in Figure 4b−d. Take the data measured at 0.04 Hz in Figure 4b, for example:

Figure 3. Interdigitated pattern of rGO/GO/rGO prepared on a flexible PET film (a) and corresponding OM image (b); electronic circuit of the rGO/GO/rGO humidity sensor (c) and schematic profile of the applied ac voltage (d).

of an as-prepared rGO/GO/rGO interdigitated pattern on PET with good flexibility. An OM image in Figure 3b clearly indicates the laser-written electrodes and gaps between them, which have very distinct interfaces without a short circuit. This can be confirmed by a 3D topographic image obtained with a 3D laser microscopy (Figure S7), in which the laser-irradiated parts become thicker than the unirradiated parts. Generally, D

DOI: 10.1021/acsami.8b07373 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

that all the peak voltages obtained at different frequencies in ac mode are significantly higher than the constant voltages obtained in dc mode. As the RH increases, the peak voltage obtained in ac mode at each frequency is monotonically increased, suggesting the rGO/GO/rGO structure can act as an effective humidity sensor in ac sensing mode. From Figure 4f, it can be found that, as the input wave frequency increases, the peak voltages of the output waves are gradually decreased and become constant when the frequency is higher than 40 Hz. Besides sensitivity, response is another very important factor for a humidity sensor. To reduce the time consumed in the environmental humidity change around the humidity sensor, a mist-flow switch setup was employed to evaluate the response of the humidity sensor. Mist was generated by an ultrasonic mist generator and introduced to the surface of the rGO/GO/ rGO humidity sensor connected to a function generator and an oscilloscope or an iPhone (Figure S13). The switch of the mist flow was controlled by a three-way cock connected to a diaphragm pump and an ultrasonic mist generator. The flow rate was set at 5 L/min controlled by a flow meter. The mist flow condition corresponds to RH 100%. Figure 5a,c,e show

the peak value of the output wave at RH 11% is 0.014 V, and such a low value is attributed to the very high impedance of the rGO/GO/rGO structure caused by a very low ionic conductivity of GO at the low humidity. When the RH was increased to 50 and 81%, the peak values of the output waves were increased to 0.124 and 0.44 V, respectively. As the RH was further increased to 100%, the peak value of the output wave was increased to 0.72 V, which was significantly improved by about 45 times compared to the constant voltage of 0.016 V at RH 100% in dc mode. One main reason for the improvement of the peak voltage with increasing RH is attributed to the increase of the ionic conductivity of GO and subsequent decrease of the impedance of the rGO/GO/rGO structure. It can be clearly found that the output waves obtained at 0.04 Hz show transient decay behaviors. The increase of the output voltage up to 0.72 V accompanying a transient decay behavior can be explained by considering the charging and discharging processes of the rGO/GO/rGO structure. Once the polarity of the input rectangular ac wave is changed from negative to positive, the charges kept in the rGO/GO/rGO structure are discharged instantaneously, leading to a dramatic increase of the peak voltage and the transient decay behavior. Compared to the humidity sensing in dc mode, the input rectangle wave in ac mode can provide a freshly charging and discharging process to the rGO/GO/rGO structure, thus endowing the humidity sensor with a high, fast, and repeatable response. It should be noted that the output waves with decay at a low frequency show very good reproducibility because of the stability of the rGO/GO/rGO structure. To exert the capability of the detection limit, the output waves of the humidity sensor under a lower RH of 6.3% provided by a saturated LiBr solution were measured at different frequencies and shown in Figures S8 and S9, which indicate an obviously higher peak voltage of 0.0064 V than the 0.00496 V in vacuum. Therefore, the detection limit of this humidity sensor can be as low as RH 6.3%. The obvious response difference between RH 6.3% and vacuum suggests the potential for sensing a further lower humidity based on the rGO/GO/rGO structure. Notably, not only the sensing mode, but also the input waveform in ac mode influences the sensing performance. The humidity sensing data obtained with sine input waves are provided in Figures S10−S12 in the Supporting Information. The humidity sensing behaviors with sine input waves are quite different from those with rectangular input waves, which are discussed detailedly in the Supporting Information. Another remarkable feature in ac mode is the influence of the input wave frequency. The peak value of the output wave was decreased with increasing frequency. For example, at RH 100%, the peak values of the output waves at the frequencies of 4 and 400 Hz were decreased to 0.424 and 0.392 V, respectively (Figure 4c,d). However, the waves at high frequencies showed a much less decay (Figure 4c,d), which can be attributed to the short charging and discharging time at high frequencies. RH- and frequency-dependent peak values of the output waves for the rGO/GO/rGO humidity sensor are shown in Figure 4e,f. It should be noted that the peak voltage taken from the output wave measured at each frequency and each RH for one same sensor was perfectly the same without any deviation, and no baseline drift was observed during the measurements because the ac mode sensing can provide a freshly charging and discharging process to the rGO/GO/rGO structure in each period. It can be clearly seen from Figure 4e

Figure 5. Mist flow on/off measurement of the humidity sensor at different frequencies: 40 (a), 4 (c), and 0.4 Hz (e), response and recovery of the humidity sensor at different frequencies: 40 (b) and 4 Hz (d), and mist flow on/off cycling test of the humidity sensor at the frequency of 4 Hz (f).

the on/off response of the humidity sensor at the input frequencies of 40, 4, and 0.4 Hz, respectively, which exhibit very fast response and recovery. It can be determined from Figure 5b,d that the response times (defined as the rise time required to reach 90% of the peak voltage) from room humidity to RH 100% after turning on the three-way cock and the recovery times (defined as the decay time required to reach 90% change of the peak voltage) after switching off the mist flow are 1.9 and 3.9 s, respectively. It should be noted that they are not the real response times because there is a time delay for the humidity change of the surrounding RH after switching the mist flow on or off. It can be clearly seen from a sensing video (Video S1) that the response and recovery are almost instantaneous. The response and recovery of the rGO/GO/ rGO humidity sensor are among the best results compared to E

DOI: 10.1021/acsami.8b07373 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

frequencies at RH 100% (Figure S17), and their peak voltages were similar to each other with a deviation less than 10% (Figure S18), suggesting that this method has a good reproducibility for device preparation. In addition, the rGO/ GO/rGO structure was also employed to conduct the sensing to other gases, including saturated ethanol, methanol, hexane, and 5% H2 in Ar. From Figures S19 and S20, it can be clearly seen that the sensor showed no response toward the saturated hexane gas and 5% H2 in Ar, and it showed a slight response toward the saturated ethanol gas with a peak voltage of 0.0096 V, whereas the response toward the saturated methanol gas becomes stronger with a peak voltage of above 0.07 V, even though it is still much lower than 0.712 V for the sensor responding to RH 100%. The response behavior of the sensor toward different gases can be easily understood because the response is mainly dependent on the proton diffusion of the GO electrolyte. The sensor showed no response to nonpolar gases, such as hexane and H2, because they have no influences on the proton diffusion of GO. The slight response to ethanol and relatively high response to methanol can be attributed to their different contributions to the proton diffusion of the GO electrolyte. Therefore, in principle, the rGO/GO/rGO sensor will respond to the gases which can provide protons when interacting with the GO electrolyte. The humidity sensing on an iPhone was demonstrated with this humidity sensor for portable electronics. The electronic circuit involving the rGO/GO/rGO structure was connected to an iPhone through the earphone jack, and the humidity sensing was conducted using an oscilloscope application on iOS, which worked as a signal generator and an oscilloscope (Figure S13). The humidity sensing measurement was carried out by using the same mist flow switch setup, and the frequency of the input rectangle wave was set at 400 Hz. Figure 7a shows the iPhone 5S connected with the humidity sensor

the previously reported humidity sensors based on graphene, TiO2, SnO2, CuO, black phosphorus, W2S, SnS2, and MoS2/ SnO2 materials (Table S1). Such a fast response can be attributed to the narrow gap between the rGO electrodes and the relatively thin thickness of the GO layer. Another rGO/ GO/rGO humidity sensor with a wider gap of 200 μm was prepared and the response toward mist flow was evaluated to elucidate the influence of the gap. As shown in Figure S14, the humidity sensor with a gap of 200 μm showed the response and recovery times of 3.1 and 6.0 s, respectively, which are longer than the 1.9 and 3.9 s for the humidity sensor with a gap of 40 μm. Therefore, the narrow gap between the rGO electrodes accounts for the fast response of the humidity sensor because of the short proton diffusion distance. In addition, the stability and reproducibility of the sensor are also very important for practical use. The mist flow sensing test for some cycles in Figure 5f suggested that the rGO/GO/rGO humidity sensor exhibited a good cycling performance. Besides, the rGO/GO/rGO structure on the substrate shows a very good flexibility. As shown in Figures 6a,b, and S15, when the

Figure 6. Photographs of the rGO/GO/rGO structure before and after bending the substrate from 20 to 15 mm (a), peak voltage retention of the output waves at different bending cycles at RH 81% (b), and peak voltages of the output waves obtained at RH 100% at different frequencies before and after keeping the sensor in air for 1 year (c).

substrate with the rGO/GO/rGO structure was bended from 20 to 15 mm at RH 81% (Figure 6a), the output waves for the first three bending cycles from the original (20 mm) to the bending status (15 mm) were almost overlapped (Figure S15), and the response intensity kept a constant value after 1200 bending cycles (Figure 6b), indicating a high strain resistance because of the good mechanical stability of the GO and rGO layers on the PET substrate. What’s more, even after being kept in air for more than 1 year, the rGO/GO/rGO humidity sensor showed reproducible output waves with similar peak voltages without apparent degradation (Figures S16 and 6c), suggesting an extremely long-term stability, which can be attributed to the stable physical and chemical properties of GO and rGO materials in air. Besides, it is also important to reduce the productive variation of sensors in the performance for practical use. Here, four rGO/GO/rGO structures were prepared with the same method, and the sensing performances were measured to evaluate the reproducibility. Surprisingly, they showed coincident output waves at all the measured

Figure 7. Photographs of the smartphone and humidity sensor (a,c) and measuring data on an iPhone screen (b,d) when the mist flow was turned on and off toward the humidity sensor at a frequency of 400 Hz: mist flow off (a,b) and mist flow on (c,d).

under room condition without mist flow, and the corresponding peak voltage shown on the iPhone screen was about 0.12 V (Figure 7b). When the mist flow was switched on (Figure 7c), the response peak voltage shown on the iPhone screen was increased to about 0.42 V (Figure 7d). The response and recovery of the sensor toward the mist flow on/off observed on the iPhone were same as those on a laboratory-used oscilloscope (Video S1). Therefore, a smartphone can be employed to conduct humidity sensing instead of the professional oscilloscope and function generator. It can be F

DOI: 10.1021/acsami.8b07373 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

expressed by an equivalent circuit consisting of C1 and R1 in parallel (C1R1) and C2 and R2 in series (C2−R2) circuits. The electronic circuit simulation was repeated at different frequencies until the simulations using common C and R showed a good agreement with the output waves at every frequency. Figure 9 shows the typical electronic circuit

reasonably expected that the sensing data using smartphones can be uploaded and analyzed on the web cloud based on the big data technology. Besides, not only humidity sensors, but other various kinds of sensors also can work in this mode on a smartphone, which acts as a platform for data collecting, recording, and analyzing. Electronic Circuit Simulation. The rGO/GO/rGO structure is not only a humidity sensor, but also an MSC, even without adding any electrolytes, because GO itself can work as a solid electrolyte at a high humidity condition because of its good ionic conductivity and electrical insulation when water molecules are trapped in. The sensitivity of the rGO/ GO/rGO humidity sensor toward RH change mainly resulted from the change of its capacitance and resistance. It is helpful for understanding the sensing mechanism by measuring the electrochemical properties. The CV curves and EIS spectra of the rGO/GO/rGO structure were measured at different RH. Figure 8a shows the CV curves of the rGO/GO/rGO structure

Figure 9. Output waves of the humidity sensor responding to a rectangular ac wave with a pk−pk voltage of 1 V at RH 100% (solid lines) and the corresponding electronic circuit simulation curves (broken lines) at different frequencies: 0.04 (a), 0.4 (b), 4 (c), and 40 Hz (d).

simulation curves at RH 100% at different frequencies, which exhibit a good agreement with the experimental output waves. C1, R1, C2, and R2 of the rGO/GO/rGO structure at RH 100% were determined to be 340 μF, 6 kΩ, 170 μF, and 4 kΩ, respectively (Figure S22). The values are in the same order as the C and R obtained from CV and EIS measurements, suggesting the physical image of the electronic circuit model in Figure S21 is reasonable. With decreasing RH, R increased, whereas C decreased. Fox example, C1, R1, C2, and R2 at RH 50% were determined to be 70 μF, 130 kΩ, 40 μF, and 85 kΩ, respectively (Figure S23). Such simulation procedures were also conducted for the output waves at other RH conditions (Figures S24−S29). At the lowest RH of 11%, R1 increased to 360 kΩ, whereas C1 decreased to 0.002 μF (Figure S29), where the transient decay of the voltage was almost negligible because of the low capacitive component. The electronic circuit simulation by the LTspice software reproduced the transient phenomena of the output waves obtained from the rGO/GO/rGO humidity sensor in ac mode by considering the capacitive component. These results confirm that the transient decay of the sensing voltage and the signal enhancement are caused by the charging and discharging behaviors that mainly originated from the capacitive property of the rGO/GO/rGO structure. It can be clearly seen from the plots of the simulation and experimental data that all the simulation curves were wellfitted with the experimental output waves. The capacitance and resistance obtained by the electrochemical measurements and the SPICE simulation on output waves are summarized in Figure 8b,d, in which most of the data points showed a good agreement with each other, although several points have a little deviation because of the difference in the time domain between the electrochemical measurements and the simulation on the output waves. Therefore, the electronic circuit simulation method can be employed to determine the impedance parameters, even to predict the results to evaluate the design of sensors.

Figure 8. CV curves of the rGO/GO/rGO structure at different RH (a), capacitances obtained from CV curves and by simulation at different RH (b), Nyquist plots of the rGO/GO/rGO structure at different RH (c), and resistances obtained from the Nyquist plots and by simulation at different RH (d).

at different RH at a scan rate of 5 mV/s, which exhibit quasirectangle shapes at higher RH, suggesting the capacitive properties. The calculated areal specific capacitances from the CV curves are shown in Figure 8b. It can be clearly found that the capacitance was increased dramatically from 1.32 μF/cm2 to 1.01 mF/cm2 as the RH increased from 11 to 100%. Figure 8c shows the EIS spectra of the rGO/GO/rGO structure at different RH, which indicate that the rGO/GO/rGO structure was changed gradually from the resistive type to capacitive type as the RH increased. The resistance obtained from the xintercept of the Nyquist plots is shown in Figure 8d, which clearly indicated that the resistance was decreased gradually as RH increased. The changes of capacitance and resistance toward RH account for the sensitivity of the humidity sensor. An electronic circuit simulation method with the Simulation Program with Integrated Circuit Emphasis (SPICE) was employed to fit the output waves of the humidity sensor in ac mode. The detailed description and analysis of the simulation process are provided in the Supporting Information. The electronic circuit model is illustrated in Figure S21, in which the electronic circuit of the rGO/GO/rGO structure is G

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Figure 10. Comparison between ac mode sensing, together with the electronic circuit simulation method, and the LCR meter method in the operating and data processing procedures.

can not only make the sensing portable, facile, and real time without professional and expensive lab machines, but can also be applied in other sensors to construct a sensing network in the future.

Although the LCR meter methods are essential for the studies of sensing devices in laboratory experiments, it is difficult to apply the laboratory machines to a huge number of sensors increasing exclusively in IoT technology because of the cost and nonportability. Therefore, the strategy and approach of the ac mode sensing together with the electronic circuit simulation developed in this work can make the sensing much easier. The comparison between the approach in this work and the conventional LCR meter methods in the sensing and data processing procedures is illustrated in Figure 10. In the LCR meter method, a sine ac voltage is applied to the rGO/GO/ rGO sensor, and the amplitude and phase difference between the input and output sine waves are observed to get a Nyquist plot. A wide frequency range measurement from an ultralow frequency region far below 1 Hz to that higher than several hundred kilohertz is necessary to obtain absolute C and R parameters by the simulation of the Nyquist plot, which takes a long time. Although the rapid detection of C and R parameters is possible by using an LCR meter assuming a simple parallel or series circuit at a constant frequency, they are relative parameters depending on the observed frequencies and assumed circuit, which cause the difficulty in the comparison of the collected data from different sensors and measurement systems (detailed in the Supporting Information and Figure S30). For the ac sensing with the electronic circuit simulation method, a rectangular ac voltage is applied to an electronic circuit containing an rGO/GO/rGO sensor, and the output wave is simulated by an electronic circuit simulator (e.g., LTspice). The simulations are repeatedly conducted for several output waves observed at different frequencies until the simulations using common C and R show a good agreement with the experimental data. What’s more important, the sensor can be connected to a portable device such as a smartphone, which can communicate with the web cloud and use the big data technology to store and analyze the sensing data efficiently. The ac sensing mode and big data compatibility ensure the sensing and data analysis to be fast, exact, and convenient. It can be reasonably expected that such a strategy

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CONCLUSIONS In summary, we reported a new approach to preparing a highperformance humidity sensor based on an interdigitated rGO/ GO/rGO structure prepared by laser direct writing, which worked in a novel ac sensing mode with high stability and enhancement compared to the dc mode. Such a humidity sensor exhibited a high sensitivity, fast response, outstanding flexibility, good cycling stability, and long-term stability because of the very small gap between the rGO electrodes and the stable physiochemical properties of GO and rGO materials. Such sensors with similar performances can be produced with a very good reproducibility. Moreover, the humidity sensor can be connected to an iPhone through the earphone jack to monitor the sensing data using an oscilloscope application on iOS, making the humidity sensing portable and convenient. In addition, an electronic circuit simulation technique was employed to elucidate the sensing mechanism and determine the impedance parameters by fitting the output waves, which showed a good agreement with the results obtained by the electrochemical measurements. It can be reasonably expected that the sensing process, together with the electronic circuit simulation and analysis, can be proceeded on a smartphone connecting to the web cloud and big data in fieldwork in the future. This strategy can be used not only in the humidity sensors but also in various other sensors to make the sensing smart and networked.

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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.8b07373. H

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SEM images of the GO film; XRD patterns of the GO and rGO films; OM and 3D laser microscopy images of lines prepared by laser direct writing; sheet resistance of the rGO films; OM and 3D laser microscopy images of an rGO/GO/rGO interdigitated structure; output waves of the rGO/GO/rGO structure at RH 6.3%; output waves and results of the rGO/GO/rGO structure under a sine input wave; experimental setup for measuring mist flow; mist flow on/off measurement; measurements for mechanical stablity, long-term stability, and preparation reproducibility of the humidity sensor; output waves of the sensor under different gas atmosphere ; electronic circuit simulation for the output waves of the sensor obtained at different RH; simulation on Nyquist plots of the rGO/GO/rGO humidity sensor; comparison of the sensing performance; and electronic circuit simulation method (PDF) Instantaneous response and recovery of the rGO/GO/ rGO humidity sensor (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.C.). *E-mail: [email protected] (A.W.). ORCID

Jinguang Cai: 0000-0003-3641-5083 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (no. 21603201), a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element Blocks (no. 2401)” (JSPS KAKENHI grant number JP24102004) and JSPS KAKENHI grant number JP15H04132, and Institute of Materials, China Academy of Engineering Physics (item no. TP02201303).

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J

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