Freestanding Laser-Assisted Reduced Graphene Oxide Microribbon

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Freestanding Laser-Assisted Reduced Graphene Oxide Microribbon Textile Electrode Fabricated on a Liquid Surface for Supercapacitors and Breath Sensors HaoTian H. Shi,† Sumyung Jang,† and Hani E. Naguib*,†,‡,§ †

Department of Mechanical Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada Department of Materials Science & Engineering, 27 King’s College Circle, Toronto, Ontario M5S 1A1, Canada § Institute of Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, Ontario M5S 3G9, Canada

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S Supporting Information *

ABSTRACT: Graphene microribbons (rGO-MRs) are highly desired for their high electrical conductivities and specific surface areas, which contribute to multiple applications in thin, flexible, textile supercapacitors, sensors, and actuators. Herein, we demonstrate a facile method for creating reduced graphene oxide microribbons with microscale architecture utilizing a simple blue-violet diode laser under ambient conditions. This method takes advantage of the photochemical reduction mechanism of self-assembled graphene oxide liquid crystals (GO-LC), allowing rGO-MR patterns to be directly printed on the solution surface. The rGO-MR films demonstrated tunable diameters and can be tailored into any geometries. A maximum intrinsic electrical conductivity for rGO-MR reaching 325.8 S/m was observed. The rGO-MR textile electrodes can be assembled into microsupercapacitors with a high areal specific capacitance of 14.4 mF/cm2, a low charge-transfer impedance, and an exceptional cycling performance with a retained 96.8% capacitance after 10 000 cycles. The rGO-MR films also experience changes in resistance in response to the moisture adsorption from human breaths and therefore can also be employed as a breathing sensor for health monitoring. The presented facile method for creating multilayered rGO-MR films directly on liquid surfaces can further expand the potential for three-dimensional printing graphitic materials for various multifunctional applications. KEYWORDS: reduced graphene oxide microribbons, laser reduction, liquid surface, textile supercapacitors, breathing sensors graphene fibers.10 El-Kady et al. were able to use a simple standard LightScribe DVD optical drive to convert graphite oxide (GO) layers into laser-induced graphene (LIG) for supercapacitor electrodes and achieved relatively high areal capacitance of around 4 mF/cm2.11 Recently, Duy et al. published findings on using lasers to grow a well-aligned vertical LIG fiber forest as long as 1 mm and attaining capacitance as much as 12 mF/cm2 when used in a microsupercapacitor (MSC) setting.12 However, the process requires a complex laser setup using a CO2 laser system on polyimide sheet substrates.12 The utilization of extrinsic doping processes and the inclusion of conductive metallic particles are able to improve the electrical performances of the graphene electrodes as well; however, due to the high cost and the degradation of the metallic nanoparticles, it is not practical to apply these on a large scale for smart textile applications.

1. INTRODUCTION With the rapid growth of the wearables and smart textiles industry, there is a growing need for functional fibers and yarns with desired properties such as high electrical conductivity and high specific surface areas for energy storage and sensing applications.1−4 The introduction of graphene fibers (GFs) signified an interesting transition from extrinsic functionalization and coating of existing natural fibers and yarns to the creation of intrinsically conductive fiber systems with small diameters and a better overall performance.5−7 GF, posed as a more conductive and robust alternative to carbon fibers, has received significant attention in recent years.8,9 Cong et al. have used the wet-spinning technique to assemble twodimensional (2D) graphene sheets into macrostructures such as a textile fiber, which have demonstrated stronger mechanical performance and a desired electrical behavior; however, their fiber resistance was too high to employ them as electrochemical capacitor electrodes due to the hydrophilic nature of the graphite oxide layers and that chemical reduction methods were necessary to reduce the graphite oxide fibers into © XXXX American Chemical Society

Received: April 2, 2019 Accepted: July 5, 2019 Published: July 5, 2019 A

DOI: 10.1021/acsami.9b05811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic demonstrating the fabrication method for freestanding rGO-MR textile films. (A) Sonication of the GO solution assisted with the exfoliation process of the GO layers. (B, C) After the GO solution was sonicated, the solution was stabilized under ambient conditions for 3 h to facilitate the formation of the thin layer of GO-LC on the surface. (D) Then, the patterning was carried out utilizing a simple blue-violet 405 nm, 200 mW rated diode laser at 10−25% power levels. (E, F) As-printed rGO-MR film can be transferred onto any substrate for applications with flexible microsupercapacitors (MSCs) and thin-film breathing sensors.

the MSC and the sensitivity of the breathing sensors. The introduction of nanoscale laser modules brings the possibilities to develop rGO-MR with a finer diameter, and by integration with woven textiles with nanometer resolution, it would be possible to construct desired three-dimensional (3D) functional nanoarchitectures with high intrinsic electrical performances.

Because of the need for a substrate and the extrinsic doping for desired electrical conductivity, along with the high cost and lack of facile fabrication strategies, the wide application of such fibers in flexible electronics was severely limited. As the high electrical conductivity and active surface areas of these fibers and textiles can also contribute to the improvements in moisture and breath-sensing applications, these sensors are designed as an essential component in human health monitoring, as serious diseases and other respiratory issues can be readily determined by observing any anomalies in the breathing frequency and patterns.13 For instance, Wu et al. have used carbon nanocoils as moisture-sensing electrodes, taking advantage of the water doping effect to monitor the resistance change and thus the breathing patterns.14 As the emerging wearable electronics market expands, there is a greater need for cost-effective flexible textile solutions that can be used to effectively store energy or sense moisture with a flexible form factor.15,16 Herein, we formulated a facile strategy for directly printing desired electrode patterns on the liquid surface with the utilization of a common blue-violet diode laser engraver module. The formation of the reduced graphene oxide microribbons (rGO-MRs) on the liquid surface can be easily transfer-printed onto any substrate or be extracted as a selfstanding film for a variety of potential applications, including the energy storage as ECs or flexible breathing sensors. This fabrication strategy requires neither complex chemical vapor deposition processes nor a special fabrication environment, which are often used to form graphene microribbons. This method utilizes the laser-induced reduction of the selfassembled graphite oxide liquid crystals (GO-LC) on the surface of GO solutions. The as-printed multipurpose textile thin film with different patterns can be used as flexible microsupercapacitor (MSC) and breathing sensor electrodes. By changing the patterns and tuning the laser power and the number of scans, it is possible to fine-tune the capacitance of

2. EXPERIMENTAL SECTION 2.1. Materials and Instrumentation. Sodium nitrate (NaNO3, Reagent Grade, Sigma-Aldrich), potassium permanganate (KMnO4, Sigma-Aldrich), hydrogen peroxide (30%, H2O2, Sigma-Aldrich), graphite flakes (median size of 7−10 μm, 99% purity, Alfa Aesar), and sulfuric acid (H2SO4, Caledon) were used in experiments without further modification. Qsonica model Q700 at a low amplitude of 10% (70W) was used as a probe sonicator for ultrasonication processing. Scanning electron microscopy (SEM) analysis was performed with JEOL JSM-IT100 InTouchScope and FEI Quanta FEG 250 Environmental SEM systems. Atomic force microscopy (AFM) maps were captured with the Bruker Multi-Mode 8 system in the mechanical mapping contact mode. The electrochemical tests were performed with a CH Instruments 6054E electrochemistry workstation. Fourier transform infrared (FTIR) spectroscopy was conducted with a Bruker α system in the absorption mode. Raman spectroscopy was determined using a HORIBA XploRA Raman spectrometer (green 532 nm, at 4−5 mW laser power). X-ray photoelectron spectroscopy (XPS) experiments were conducted with ThermoFisher Scientific ESCALAB 250Xi with prior Ar cluster cleaning. A Keithley 2400 Series Sourcemeter with a 4-point probe was utilized to measure the surface electrical conductivity of the thin-film rGO-MR textile samples. The same device was also used in a two-electrode cell for measuring the resistance changes during the breath-sensing tests. 2.2. Fabrication Method. 2.2.1. Graphite Oxide Solution Fabrication. The graphite oxide (GO) solution was prepared through a modified Hummer’s method. Briefly, graphite flakes (1 g) and NaNO3 (1 g) were added to a beaker along with concentrated sulfuric acid (40 mL). The beaker was placed on a magnetic stirrer and the solution was slowly stirred at 100 rpm, and KMnO4 powder (6 g) was B


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nematic GO-LC phase.17 The self-assembly process of the GOLCs on the surface indicated a phase separation of the LC phase from the isotropic phase and therefore can be directly observed as shown in Figure 2A,B, with the dark-brownish transparent film formed on the top surface of the GO solution after 3 h or longer of immobilization under ambient conditions.

then added and was fully dissolved. The beaker containing the solution was then placed into a water bath kept at 35 ± 5 °C, and constant stirring was applied for 1 h. Distilled water (80 mL) was then added dropwise. Afterward, the beaker was placed in an oil bath kept at temperatures between 70 and 90 °C and the solution was stirred for another 30 min. The reaction was then terminated by adding distilled water (200 mL) and 30% H2O2 (6 mL), accompanied by a color change to bright orange, and the stirring continued for another 2 h. The resulting liquid was then vacuum-filtered to obtain the solid agglomerates from the filter. The yellow paste was then dissolved in distilled water and mixed for 2 h before centrifugation to obtain the GO paste. The paste was then collected and dispersed in distilled water to make the GO solution with a distinctive russet-brown color, characteristic of GO formation. 2.2.2. Fabrication of rGO-MR at the Liquid Surface. The fabrication route of the rGO-MR patterns has been described schematically in Figure 1. The GO paste was first made into a GO solution by adding distilled water in various ratios; it was found that a ratio of 2:1 between distilled water and the GO paste was ideal for creating a homogenous solution with the GO flakes fully dissolved. A sonicator was used at an amplitude of 10 and for a time of 1 min to assist with the dispersion and exfoliation process to obtain graphene oxide layers. As shown in Figure 1B,C, the as-prepared graphene oxide solution was stabilized under ambient conditions for 3 h before the graphene oxide flakes were aligned, and a thin layer of graphene oxide liquid crystals (GO-LC) on the solution surface was formed. The graphene oxide solution with a stable LC formation was then carefully placed under the blue-violet diode laser engraver on an x−y axis printing stage so as not to disrupt the GO-LC layer formation. The desired patterns were preloaded into the computer program, which allows the laser engraver to control the laser power and rate at which the patterning takes place. After the pattern has been formed, a thin glass slide or any other substrate can be used to transfer-print the film from the liquid surface. Ethanol and distilled water were used to wash the remaining GO from the rGO-MR film, which allows further structural modifications depending on the application. 2.2.3. Construction of MSCs Utilizing rGO-MR Electrodes. The MSCs were constructed using a two-electrode configuration utilizing the rGO-MR film as the active electrode. The thin-film rGO-MR electrodes directly adhered onto the stainless steel shim current collector (18-8 stainless steel, 12.7 μm thick, McMaster Carr). A piece of filter paper immersed in sulfuric acid (H2SO4, 1 M) was used as both the electrolyte and the separator. The cell was assembled with a typical symmetrical two-electrode setup with the same weight of active electrodes on both sides of the electrolyte. 2.2.4. Assembly of Breathing Sensors Using rGO-MR Films. To fabricate the breathing sensors utilizing the rGO-MR, the freestanding rGO-MR film was first collected from the solution surface and attached to a thin Plexiglas substrate as the wetting of the film provides the adhesion. As shown in Figure 1F, the two ends of the rGO-MR film are connected to copper foil connectors, which would act as electrodes for the resistance measurements. A rectangular rGOMR sample measuring 1 cm in width and 2 cm in length was used as the active material in the breath-sensing apparatus.

Figure 2. Formation of the GO-LC structures on the GO solution liquid surface. (A) Schematic demonstrating the alignment and selfassembly process of the GO-LC on the top surface of the GO solution after mild sonication followed by 3 h or longer stabilization. (B) Digital photograph of the GO-LC formation after gentle sonication cycle. (C) AFM height map showing the roughness and the structures of a 5 μm GO-LC region. (D) Optical microscopy image of the GOLC layers captured on a glass substrate.

Additionally, from the AFM height mapping data shown in Figure 2C, acquired in a dry state, it is possible to observe that the GO-LC structures are arranged in an aligned fashion after the self-assembly process on the surface of the GO colloidal solution. Figure 2D shows the GO-LC wrinkled nature under an optical microscope, confirming the proposed morphology of the extracted GO-LC thin films. After a reduction process with a simple diode laser, the transparent dark-brown thin sheets of GO-LCs were reduced into silver-colored rGO-MR. With a variety of designs and patterns, as shown in Figure S1, it is possible to create any patterns or shapes for the rGO-MR film and obtain the active layer on the surface of the GO solution. The accuracy of the asfabricated rGO-MR fiber depends on the laser and environmental parameters. Laser parameters, such as laser focus, power, and movement speed, can be readily controlled during the fabrication process; however, environmental factors such as the natural convection of air can also affect the movement of the mobile rGO-MR formation on the liquid surface, resulting in discrepancies in the printing resolution. Therefore, care must also be taken in controlling the printing environment during the fabrication process. It was found that laser powers higher than 40 mW or lower than 20 mW will result in discorded and easily collapsible rGO-MR formation, which therefore cannot be readily removed from the surface. At low laser powers smaller than 20 mW, the insufficient reduction of GO-LC led to inconsistencies in the thin rGO-MR formation. At higher laser powers exceeding 40 mW, the induced temperatures burn the sample as the carbon content in the GO-LC combines with the ambient oxygen, resulting in the loss of the structural integrity of the rGO-MR film. Rectangular

3. RESULTS AND DISCUSSION 3.1. GO-LC and rGO-MR Morphologies. After stabilizing the GO-LC solution under ambient conditions for more than 3 h, it was found that the GO-LC alignment starts to take place on the surface of the GO solution, forming a large sheet of yellow transparent GO-LC thin films. The mild ultrasonication process assisted in the exfoliation and formation of GO-LC from the GO solution produced by the as-described modified Hummer’s method. The formation of the GO-LC films has been studied extensively in the past since their discovery by Kim et al. and Xu et al.17,18 The observed appearance of the GO solution included inhomogeneous dark-brown agglomeration features, which are indicative of the formation of the C


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Figure 3. (A) GO-LC formation on top of the stabilized GO solution can be observed, whereas the gray portion of the solution surface is the laserscanned patterns resulting from photochemical reduction. (B) SEM image showing the clear patterns created via the laser scans. (C) AFM height map of a 10 μm corrugated region on the rGO-MR fibers. (D) Crisscross-patterned rGO-MR can be printed by setting the appropriate patterning parameters to create a freestanding film. (E, F) Optical images demonstrating the patterns formed by two separate laser scans perpendicular to each other to create the crisscross patterns with the intersecting fibers fused together.

patterns printed with a variety of laser powers are compared in Figure S2. The optimal laser power for forming a film that is easily removable and coherent is at around 30 mW power or 15% of the rated power. The as-formed thin film was studied with a variety of microscopy methods. As demonstrated in Figure 3A, the laser patterns can be directly created on the liquid surface by photochemically reducing the GO-LC layers. Depending on the thickness of the GO-LC layers, in conjunction with the laser penetration depth through the GO-LC layers, it is possible to create rGO-MR with a variety of ribbon thicknesses. The patterns can also be controlled to obtain the desired geometries. Figure S3 shows that with a single scan, if the blue-violet laser is in focus and is at a lower laser power level, each rGO-MR line could have diameters as low as 10 μm. The diameters of the laser-scanned lines are consistent throughout the patterned film. A very corrugated characteristic rGO feature with large folded randomly oriented rGO sheets was observed in the SEM image shown in Figure 3B. Through the AFM height map shown in Figure 3C, it is possible to observe the wrinkled rGO sheets apparent on the microribbon surface. However, if only one laser scan was performed, the rGO-MR fibers were not connected in any way and therefore the printed pattern tends to come apart during the transfer process due to the stress from the applied surface tension. Additionally, the mechanical properties of each individual rGO-MR are fairly weak and thus they cannot sustain their own weight when removed from the GO solution. This can be attributed to the fact that the laser movement of the printing system is not designed to move at a precise and consistent pace and therefore may contribute to defect formation through the length of the fiber, contributing to weak connection points. Several parameters can be tuned to obtain the ideal rGO-MR with appropriate thicknesses and diameters that contribute to an enhanced mechanical performance. To further enhance the mechanical and electrical properties of the rGO-MR, it is also possible to increase the number of scans with different directionalities. To create a crisscross pattern similar to that of a woven textile, a specially designed

laser path is used to create crisscross patterns that lay on top of one another as shown in Figure 3E,F. It is possible to perform multiple laser scans when the film is still situated on the GO solution surface. Without removing the thin rGO-MR film, the mechanical properties of the rGO-MR patterned film can be improved through multiple scans in perpendicular directions to become freestanding. Thus, it is possible to create any desired patterns that can form electrically conductive networks with different rGO-MR fiber diameters, which would contribute to various flexible electronics applications. It is also interesting to note that Figure 3F shows that the intersections between the perpendicular rGO-MR fibers are fused together, resulting in the textile film acting as a single layer. This effect is because that even after the first laser scan, the rGO-MR is still highly hydrophilic, contributing to the possibility of absorbing the GO-LC onto its surface and therefore further laser scans can subsequently reduce the GO layer on top of the already formed rGO-MR, resulting in improved adhesion between the layers. Figure S4A demonstrates that the rGO-MR fibers can fold upon themselves to create conductive pathways with desired directionalities, where an arrow pattern was created by folding the sheets in a straight line. The rGO sheets were distributed evenly along each laser-scanned line, creating a microribbon structure with thin layers of graphene sheets randomly oriented on the surface of each fiber. Figure S4B,C shows zoomed-in microscopy images showing the rGO-MR-resembling typical graphitic materials. The rGO-MR structures and dimensions were further verified via a cross-sectional scan of a single rGOMR fiber as shown in Figure S5, where the sample was positioned so that a single rGO-MR was protruding from the top of the substrate and therefore demonstrated a flat microribbon morphology. The thickness of the rGO-MR sample was measured using electron microscopy with the sample oriented vertically so that the sample cross-section is exposed to the electron beam. By making consecutive measurements of the sample thickness from the SEM images, it was possible to find that the average thickness of each rGOMR is around 15 μm. D


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Figure 4. Scanning electron microscopy (SEM) images and a digital photograph of the crisscross rGO-MR textiles washed with different liquids. (A−C) Crisscross-patterned rGO-MR films washed with distilled water. (D−F) Crisscross-patterned rGO-MR films washed with ethanol. Inset: Freestanding crisscross rGO-MR film extracted from the solution surface.

Figure 5. (A) Raman spectroscopy performed with a laser wavelength of 532 nm, at 4−5 mW for pristine GO, GO-LC, and rGO-MR samples with an overall comparison for all samples with D, G, 2D, and D + G peaks observed at 1346, 1592, 2685, and 2930 cm−1 respectively. The ID/I2D ratio is measured to be 1.63 for the rGO-MR samples. (B) More detailed examination of the ID/IG ratio for the rGO-MR vs GO-LC. (C) XPS spectrum of the pristine GO film. (D) XPS spectrum of the rGO-MR film showing a significant increase in C−C intensities.

would lead to increased specific surface areas and therefore contribute to higher capacitance if used in an MSC as a thinfilm electrode. It is also noted that the observed porous formation indicates that the hydrophilic nature of the rGO-MR film helped the absorption of the liquid during the washing process and that due to the fast evaporation of ethanol as compared to water, the process induced consistent microsized porous architectures throughout the rGO-MR fiber textiles. As shown in the inset of Figure 4D, the rGO-MR film extracted after repeated washing was freestanding and demonstrated a gray-silvery color. It is also shown in Figure S6 that the extracted freestanding film can withstand bending and flexing without suffering permanent deformations. The bending and flexing of the rGO-MR film, even in the cases with only one laser scan, offered a significant improvement in the mechanical properties over the GO-LC films. The GO-LC films are not

After the rGO-MR textile sample was retrieved from the solution surface, ethanol and distilled water were used to wash the sample to remove any excess GO from the fabrication process. It is also interesting to observe from the scanning electron microscopy (SEM) analysis shown in Figure 4 that there is a different effect depending on which liquid was used in washing the as-printed crisscross rGO-MR thin film. The average thickness of the freestanding rGO-MR film was measured using a micrometer to be around 21 ± 3 μm. Figure 4A−C shows the effect of washing with distilled water, which resulted in the retention of most of the large-sized rGO flakes and formation of only a small amount of porous structures. However, if ethanol was used in washing the film extracted from the GO solution, then a large amount of consistent porous structure formation was observed. It is believed that the formation of such a large number of pores E


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Figure 6. Water angle characterization test demonstrated the hydrophilicity change for the rGO-MR thin-film cloth. (A) Glass slide used as the substrate for the hydrophilicity test demonstrated an angle of 53.6°, which shows a slight hydrophilicity as expected. (B) Glass slide coated with a thin layer of GO-LC showed a reduced contact angle of 41.8°. (C) Glass substrate when coated with the crisscross rGO-MR made with a laser at 30 mW showed the lowest contact angle of 20.5°.

Figure 7. Electrical conductivity of rGO-MR changes with (A) different laser powers during fabrication and (B) different number of scans applied during fabrication.

3.2.2. X-ray Photoelectron Spectroscopy (XPS). The C 1s XPS spectra of the pristine GO and the rGO-MR samples are shown in Figure 5C,D, respectively. The binding energy of the C−C bonding is assigned to around the 284 eV region, whereas the CO and C−O bondings have binding energies of 288 and 286 eV, respectively. The rGO-MR film demonstrated a significant increase in the C−C bonding intensities compared with the pristine GO films and therefore validating that the laser reduction process led to formation of rGO-MR structures. 3.2.3. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectroscopy was also used to further validate the reduction process. The FTIR spectra in Figure S7 show the variations in the observed absorption peaks between the GOLC and the extracted rGO-MR thin-film samples. In Figure S7, it is possible to observe that the pristine GO-LC samples show the characteristic peaks of graphite oxide. The observed broad absorption peak at around 3200 cm−1 can be assigned to the isolated −OH groups and intercalated H2O molecules.21 The water content within the GO-LC can be further verified with the corresponding peak at 1617 cm−1 for H2O.22 The peak at 1044 cm−1 is consistent with the C−O vibrations.23 It is also possible to assign the absorption peaks at 1220 and 1720 cm−1 to phenolic C−O and ketonic CO, respectively, which demonstrates the presence of phenol and carboxylic acid groups.21 However, after the laser reduction process, it is clearly shown that most GO peaks disappear in the resulting FTIR spectrum of rGO-MR and only the C−O stretching, phenolic C−O stretching, and CO stretching are retained, which is similar to what was observed in the literature for rGO, which validates that the GO-LC has been reduced to rGO-MR on the liquid surface.21,23,24 3.3. Hydrophilic Nature of the rGO-MR Films. For determining the hydrophobicity of the rGO-MR films, a standard water droplet contact angle test was used. A

strong enough to hold their own weight, whereas the rGO-MR films are able to be extracted from the liquid surface and were freestanding at ambient conditions. 3.2. Compositional Characterization. 3.2.1. Raman Spectroscopy. Raman spectroscopy was conducted with a low laser power of 4−5 mW so as to prevent any unwanted damages induced by the laser to the sample surface, which can affect the interpretation of the results. From the Raman spectra shown in Figure 5A, it is clear that the two main bands appear for the pristine GO and GO-LC films centered at 1346 and 1592 cm−1, representing the D and G bands, respectively. There is a noticeable increase in the ID/IG ratio to around 0.96 in the GO-LC samples from the pristine GO samples, indicating a more disordered nature of the GO flakes after the sonication process. However, after the laser treatment was conducted and the rGO-MR patterns were formed, as shown in Figure 5A,B, a decrease in the ID/IG ratio to 0.76 and a significant increase in the 2D peak centered at 2685 cm−1 were observed. The G band arises from the C−C bond stretching in graphitic materials and indicates the presence of carbon sp2 networks. The increase in the G band intensity is indicative of that the laser reduction process created more ordered rGO flakes with fewer defects compared to the original GO-LC films. The 2D band is due to the second-order two-phonon process, characteristic of few-layer graphitic edges. The appearance of the 2D peak further demonstrated the increase in graphene-like features.19 There is also a noticeable slight shift in the G peak between the GO-LC and the rGO-MR samples, along with an increase in the G peak intensity, as indicated in Figure 5B. The G band is shifted from 1590 cm−1 in GO-LC to 1587 cm−1in rGO-MR, which can demonstrate a slight increase in the number of graphitic layers in the rGOMR samples.20 F


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Figure 8. Electrochemical analysis of the electrode samples used in a two-electrode device: (A) cyclic voltammetry (CV) diagram for rGO-MR samples with varying scan rates from 50 to 1000 mV/s. (B) CV diagram for pristine GO samples with varying scan rates from 50 to 1000 mV/s. (C) Comparison of the Nyquist plots of the electrochemical impedance spectroscopy between the pristine GO and rGO-MR samples. (D) Galvanostatic charge/discharge (GCD) test on the rGO-MR two-electrode MSCs. (E) Rate capability of rGO-MR electrode capacitance with increasing scan rates. (F) Capacitance variations with cycling from 1st to 10 000th cycles, with the inset showing the CV at 100 mV/s scan rates between the 1st and 10 000th cycle.

hydrophilic flexible thin film offers better adhesion to various substrates and can lead to better facilitation of electrolytic ion diffusion into and out of the electrode 3D architecture during the charging and discharging processes in the case of an electrochemical capacitor.25 As shown in Figure 6, even though the GO-LC wrinkled surfaces led to a more hydrophilic surface compared to the bare glass substrate, the hydrophobicity of the rGO-MR sample decreased even more dramatically in comparison with that of the GO-LC coating, which indicates that the crisscrossed pattern was better suited for the ion diffusion processes and therefore should lead to a lower charge-transfer impedance in the electrochemical characterization. It was also a result of the porous structural formation from the washing process, leading to increased accessible surface areas where the electrolytic ions can access the adsorption sites during the charging process. The hydrophilicity of the sample also allows the rGO-MC to be better attached to various substrates. 3.4. Electrical Conductivity Characteristics. To employ the rGO-MR thin-film electrodes within MSCs, it is important to understand their electrical conductivity performances to ensure that the internal resistance is not too high to hinder the electron transfer to the current collectors. It was found that the electrical conductivity of the rGO-MR freestanding film changes drastically with different levels of laser power applied during the fabrication process. The effect of the laser power ranging from 0 to 60% of the maximum laser power of 200 mW is shown in Figure 7A. It is possible to see that the 15% power laser, which translates to 30 mW of the laser power

applied, would lead to a dramatic increase in the electrical conductivity up to 70.6 S/m. The further increase in the laser power led to a decrease in the electrical conductivity measured, which could be due to the ineffective absorption of the laser energy and burning of the GO-LC during the reduction process, resulting in inconsistent reduction on the formed thin film. As shown in Figure S8, any additional increases in the laser power levels beyond 60 mW resulted in disbanded and inconsistent printed patterns with the formation of cracks, which contributed to the disruption of the conducting network and thus a reduction in the electrical conductivity. It is also observed that with the increasing number of laser scans the rGO-MR layers are stacked and fused on top of one another, ensuring that the surface conductivity is not jeopardized and the electrically conducting network is further improved, resulting in a linear relationship in the number of laser scans and the electrical conductivity of the rGO-MR samples. It is possible to see that with five laser scans a high conductivity of 325.8 S/m can be obtained. However, at a higher number of scans, undesirable burning of the rGO-MR samples can be induced, which would lead to disruption of the conductive networks, resulting in a decrease in the electrical conductivity, as observed. 3.5. Electrochemical Performance of MSCs. The electrochemical performance of the two-electrode symmetrical MSC systems fabricated with the thin-film electrodes made using pristine GO and rGO-MR is shown in Figure 8. As shown in the cyclic voltammetry (CV) graphs in Figure 8A,B, the rGO-MR electrodes performed substantially better than G


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clear in Figure 9 that the rGO-MR breath sensor experiences highly reversible behaviors where within 5−8 s after the

the pristine GO electrodes and demonstrated a more rectangular cyclic voltammogram and higher current densities, indicating less internal resistance and higher capacitance, respectively. The larger capacitance of 14.4 mF/cm2 was observed with 5 mV/s scan rate and can be attributed to the improved porous surface areas on the symmetrical rGO-MR electrodes. The improved electrical conductivity of the rGOMR with laser reduction was the reason behind the decrease in internal resistance and therefore a more symmetrical CV diagram, as shown in Figure 8A. Electrochemical impedance spectroscopy results, shown in Figure 8C, also indicated a low charge-transfer impedance value, which led to the conclusion that the porous structure formation in the rGO-MR electrodes effectively facilitated the transfer of the electrolytic ions into and out of the electrode during charging and discharging processes. The hydrophilic nature of the rGO-MR system also contributed to both the low series resistance and low chargetransfer impedance. The interesting morphology of the ethanol-washed rGO-MR samples included significant porous structures, which increase the specific surface areas for electrode/electrolyte interactions, but this porous structure formation also led to an increase in the charge-transfer resistance, which was observed in the Nyquist plot as shown in Figure S9. Additional electrochemical tests with galvanostatic charge and discharge (GCD) tests mimicking the utilization of a supercapacitor under real-life scenarios with currents ranging from 1 to 0.1 mA are shown in Figure 8D. The rGO-MR GCD tests show symmetric charge and discharge curves with a very minimal IR voltage drop of less than 0.03 V when the discharge process begins, indicative of low internal resistance with the conductive network. Figure 8E shows the rate capability of rGO-MR electrode capacitance with increasing scan rates; even at very high scan rates of 1000 mV/s, the rGO-MR is still able to demonstrate a somewhat rectangular-shaped CV diagram with the capacitance of 5.9 mF/cm2 being retained. The low internal resistance and the uniform network allow consistent capacitance to be observed even at very high scan rates as the MSC is more responsive to ion movements within the electrolyte, without having undesirable structures that hinder the transfer. Cycling tests were conducted to study how the performance of the rGO-MR electrodes changes after 10 000 charge and discharge cycles. As shown in Figure 8F, 96.8% of the capacitance of the rGO-MR electrode was retained even after 10 000 cycles, similar to that of electrochemical double-layer (EDL) materials, demonstrating the expected performance of typical EDL materials. In the inset of Figure 8F, it is also interesting to note that the CV graph taken at 100 mV/s scan rate of the rGO-MR sample after 10 000 cycles kept its initial rectangular shape, indicative of similar impedance response as the 1st cycle, which verifies the formation of graphitic materials that contribute to the superb electrochemical performances. 3.6. Breath-Sensing Performance. Breathing sensors are typically designed for respiratory health monitoring and clinical diagnosis in the biomedical fields. The human breathing patterns can be detected by observing the resistance changes resulting from the moisture adsorption on the microribbon three-dimensional structure of the rGO-MR samples. The adsorbed water molecules on the surface of the rGO-MR structures lead to observable changes in the electrically conductive networks, resulting in a change in the resistance of the sample each time a breathing action is done. It is also

Figure 9. Breathing sensor with resistance changes corresponding to the detection of human breath. With an elevated level of moisture and humidity adsorbed on the sample 3D-structured surfaces, there is an observable change in resistance with each human breath.

breathing was detected with an elevated resistance, the resistance returned to an acceptable reference point. The moisture is quickly removed by convection from the ambient air movement and the electrical networks returned to their initial conditions. The observed rate of breathing of humans is an important indicator of potential respiratory diseases, such as sinusitis, asthma, and allergies. By implementing solutions such as breathing sensors, the health conditions of a patient can be continuously monitored and any anomalies can be quickly detected to provide early warnings of potentially life-threatening conditions.

4. CONCLUSIONS In conclusion, a facile method for directly printing rGO-MR patterns on a liquid surface was proposed. The as-created rGOMR demonstrated diameters ranging from 20 to 80 μm and a thickness of 15 μm. Multiple laser scans led microribbons to fuse on top of one another, leading to the possibilities of creating three-dimensional intrinsically conductive functional microstructures. The intrinsic electrical conductivity can reach up to 325.8 S/m with multiple laser scans, and the patterned microribbon electrodes made from the flexible mesh graphene fibers can be assembled into a microsupercapacitor, which showed an areal specific capacitance of 14.4 mF/cm2. A low charge-transfer impedance was attributed to the formation of porous networks facilitating the electrolytic ion transfer inside and out of the electrode structure. An exceptional cycling performance with a retained 96.8% capacitance after 10 000 cycles was also reported. Additionally, the rGO-MR thin film was also employed as a breathing sensor, responsive to the moisture changes from human breaths, and can be used to track patient respiratory conditions. The facile method presented for creating thin freestanding rGO-MR 3D structures can be further used for applications requiring conductive networks and can further lead to the creation of three-dimensional nanostructures if nanoscale lasers were utilized.

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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.9b05811. Patterns printed by different laser parameters and predetermined patterns (Figure S1); comparison with a range of laser powers in creating the rGO-MR samples H


Research Article


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(Figure S2); diameters of the rGO-MR tunable by changing the laser focus, laser power, and scan speeds (Figure S3); further SEM studies of printed rGO-MR patterns (Figure S4); thickness determination of the rGO-MR samples (Figure S5); images of rGO-MR electrodes demonstrating good bendability (Figure S6); Fourier transform infrared (FTIR) spectra of the rGOMR samples in comparison with those of the pristine GO-LC films (Figure S7); SEM images of rGO-MR films created with a range of laser powers (Figure S8); effect of washing rGO-MR in distilled water vs ethanol on the charge-transfer resistance (Figure S9) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hani E. Naguib: 0000-0003-4822-9990 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) (Grant no. 459389 and NSERC PGS-D scholarship) and Canada Foundation for Innovation (CFI) (Grant no. 481796) for the financial support they have provided for this research work.

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Freestanding Laser-Assisted Reduced Graphene Oxide Microribbon

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