Instantaneous Reduction of Graphene Oxide Paper for Supercapacitor

Jun 2, 2014 - 3D graphene nanomaterials for binder-free supercapacitors: scientific design for enhanced performance. Shuijian He , Wei Chen. Nanoscale...
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Instantaneous Reduction of Graphene Oxide Paper for Supercapacitor Electrodes with Unimpeded Liquid Permeation Zheng Bo,*,† Weiguang Zhu,† Xin Tu,‡ Yong Yang,§ Shun Mao,∥ Yong He,† Junhong Chen,∥ Jianhua Yan,† and Kefa Cen† †

State Key Laboratory of Clean Energy Utilization, Department of Energy Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China ‡ Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool L69 3GJ, United Kingdom § State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science & Technology, Wuhan, Hubei 430074, China ∥ Department of Mechanical Engineering, University of WisconsinMilwaukee, Milwaukee, Wisconsin 53211, United States S Supporting Information *

ABSTRACT: Scanning positive column of atmospheric-pressure glow discharge (AGD) plasma is proposed as an eco-friendly and cost-effective strategy toward the instantaneous (∼2 s) reduction of graphene oxide (GO) paper, further leading to a reduced graphene oxide (rGO) paper with unimpeded liquid permeation for high performance supercapacitors. Unlike previous finding that AGD plasma has no effect on GO, a well-designed AGD plasma containing scanning positive column region is demonstrated to be capable of converting GO paper to rGO paper. With the synergy of energetic electrons (electron density 1.03 × 1016 cm−3) and surface touch heating (translational temperature ∼800 K), restoration of graphene’s excellent electrical properties is realized, evidenced by a substantially (5 orders of magnitude) improved electrical conductivity. Moreover, the ultrafast reduction of oxygen functionalities results in the creation of exposed open channels and the expansion of graphene layers, leading to a rGO paper supercapacitor electrode with unimpeded liquid permeation. Taking the above advantages, the as-obtained rGO paper exhibits obviously better capacitive and rate performance than the parent GO and chemically reduced rGO counterparts in terms of higher specific capacitance, better charge/discharge rate response, and faster ion diffusion, holding a great promise for energy storage applications.

1. INTRODUCTION The great interest of graphene-based supercapacitors mainly stems from graphene’s high electrical conductivity and huge specific surface area.1−3 Although graphene monolayer with a specific surface area of 2675 m2/g can theoretically store an ultrahigh electric double-layer capacitance (EDLC),4 stacks of reduced graphene oxide (rGO) layers are commonly preferred for practical supercapacitor application.5−8 It thus calls for (i) effective conversion of graphene oxide (GO) to rGO with the restoration of graphene’s excellent electrical properties and (ii) unimpeded electrolyte permeation within the rGO interlayer channels for the formation of EDLC with an optimum utilization of graphene’s huge specific surface area. Conversion of GO to rGO usually relies on either chemical agents or high temperature treatment, mainly including the traditional chemical or thermal reduction,9,10 and their emerging combinations or derivatives such as photochemical reduction,11 thermochemical reduction,12 electrochemical reduction,13 and microwave-induced thermal reduction.14−16 Among the above strategies, chemical reduction shows a great promise for the bulk production of rGO with a low cost. © 2014 American Chemical Society

Unfortunately, most of the widely used chemical reductants are highly toxic and/or explosive (see our previous review17 and the references therein), which could potentially induce environmental issues and safety risks. Moreover, chemical, thermochemical, and electrochemical reduction of GO occur under wet conditions with the presence of reducing agents or electrolytes easily lead to impurity of the products, even for the cases employing eco-friendly reductants.18,19 To this end, thermal reduction operated under a dry condition is considered as an environmentally friendly and product-contaminant-free alternative method.20,21 Thermal reduction usually takes a much shorter processing duration (less than 1 h) than chemically induced counterparts (tens of hours). However, since an elevated temperature is required to initiate the removal or reduction of oxygen functionalities such as epoxide, hydroxyl, carbonyl, and carboxyl groups, considerable energy is consumed or lost during the preheating and cooling of bulk Received: April 17, 2014 Revised: June 2, 2014 Published: June 2, 2014 13493

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Preparation of rGO Paper with Positive-Column Plasma. A GO paper was first made by vacuum filtration of the resulting GO dispersion through a membrane filter (GVWP09050, 0.22 μm pore size, Millipore), followed by air drying at room temperature. The as-prepared GO paper was then peeled off and placed on a graphite rod housed in a quartz tube. A quartz sleeve was used to fix the GO paper. The graphite rod was grounded, working as the passive electrode (anode) for AGD. A conical profile tungsten pin (taper ∼1:5; tip radius ∼0.01 mm) connected to a negative direct current (dc) high voltage supply was used as the discharge electrode (cathode). The interelectrode gap was adjustable, which was set as 10.8 cm for the production of a scanning positive-column plasma. Before reduction, a helium gas flow was introduced into the reactor for 10 min to remove the background air. Helium was chosen as the discharge gas due to its high excitation and ionization potential. With applying a negative dc high voltage of −10 kV to the cathode, an AGD plasma containing scanning positive-column was immediately created in the interelectrode space, effectively reducing the GO paper through a synergy of high-density energetic electrons and surface touch direct heating. The discharge/reduction process lasted 2 s at atmospheric pressure and room temperature. The as-obtained plasma reduced rGO paper was denoted as “P-rGO”. Discharge Visualization and Electrical Signals. Images of AGD plasmas were taken using a camera with an Af-S Micro NIKKOR VR 105 mm f/2.8G IF-ED versatile long-reach macro lens (Nikon). Dynamic images of the scanning positive-column plasma were captured by a high-speed, rugged HG-100 K digital camera (CMOS sensor, 1504 × 1128 pixels) with setting the sample frame rate as 1000 frames s−1, recorded and analyzed by a MotionCentral Program. A sensitive picoammeter (range 20 fA−21 mA, resolution 51/2 digit; Model 6485, Keithley Instruments, Inc.) was used to measure the discharge current for a variety of interelectrode gaps. Plasma Diagnosis. The light emitted from plasmas was recorded by an Ocean Optics HR2000+ high-resolution spectrometer with a 2048-element charge coupled device (CCD) array detector and an Acton SP2300 spectrometer (grating 1200 grooves mm−1; blaze wavelength 300 nm) equipped with a CCD camera (Princeton Instruments, PIMAX3-1024i). Material Characterization. The material morphology was investigated by a SU-70 scanning electron microscope (SEM, Hitachi). A DXR 532 Raman spectrometer (Thermo Fisher Scientific) was used to obtain the Raman spectra with an excitation wavelength of 532 nm. X-ray diffraction (XRD) patterns were collected with a XRD-6000 diffractometer using Cu Kα radiation (λ = 0.154 25 nm, Shimadzu). X-ray photoelectron spectroscopy (XPS) investigation was carried out in a VG Escalab Mark II system employing a monochromatic Mg Kα X-ray source (1253.6 eV, West Sussex). Ultraviolet−visible (UV−vis) spectra were recorded on a Shimadzu UV-2550 spectrophotometer (Kyoutofu, Japan). Fourier transform infrared (FTIR) spectra were obtained on a Nicolet 5700 FTIR spectrometer. The material electrical conductivity was measured by a four probe station. Measurement of Brunauer−Emmett−Teller (BET) surface area was conducted with a Micromeritics ASAP 2010 BET analyzer using nitrogen as the adsorbent. Measurement of Liquid Permeation. GO, C-rGO, and P-rGO papers were flatwised on glass substrates. A water droplet was placed on the surface of each sample. The contact

background gases, leading to a poor energy efficiency. To this end, the effective, eco-friendly, and energy-efficient reduction of GO still remains a huge challenge. On the other hand, aiming at the formation of EDLC with an optimum utilization of graphene’ surface, a priority should be given to the high quality electrode wetting with electrolyte.22 With the only exception that desolvated electrolyte ions enter into subnanometer pores, for most cases of EDLC formation, the pore size of active materials should be substantially larger than the size of the electrolyte ion and its solvation shell.22,23 Typically for rGO paper, which can be considered as a collection of micrometer-sized graphene crystals with an interlocked layered structure,24 the porous structure originates from graphene interlayer channels. As a consequence, an unimpeded electrolyte permeation within the rGO interlayer channels is highly required to facilitate the material wetting and ion diffusion/adsorption. However, rGO-based supercapacitor active materials obtained from the commonly used wet chemistry methods suffer from the easy restacking of graphene layers due to the van der Waals interactions, presenting negative effects on their rate capability and capacitive behavior.6,25,26 To address the issues mentioned above, we herein proposed a novel strategy for GO reduction, i.e., atmospheric-pressure glow discharge (AGD) plasma containing scanning positive column, further leading to a rGO paper with unimpeded liquid permeation for high performance supercapacitors. AGD is capable of producing plasmas with both high translational and vibrational temperature at atmospheric pressure, holding a great potential for a variety of industrial applications.27 Previous practice, unfortunately, reveals that AGD plasma has no effect on the oxygen functionalities of GO,28 where we strongly suspect that negative glow, instead of positive column, of AGD was applied. While the existence of negative glow region is vital for a stable AGD plasma, the formation of positive column region requires a larger interelectrode spacing with adequate energy supply and operation environment.29 An intriguing nature of positive column is that the translational temperature, electron density, ionization degree, and current density in this region could be significantly higher than those in the negative glow region.27,29 This paper started with the study on the physical properties of the as-proposed positive-column plasma. Then the morphology, structure, and composites of the as-obtained rGO paper were characterized in detail, and a possible reduction route was proposed based on plasma diagnosis and calculation. Subsequently, the wettability and liquid permeation of the resulting rGO paper were studied. Finally, supercapacitors employing the as-obtained rGO paper were assembled and tested. Capacitive properties of the resulting rGO paper were compared with those of the parent GO paper and chemically reduced counterpart.

2. EXPERIMENTAL SECTION Preparation of rGO Paper with Chemical Reduction. GO powder was synthesized from graphite powder by a modified Hummer’s method. Preparation of rGO paper with chemical reduction was conducted using hydrazine hydrate as the reductant. A detailed description on the related procedures can be found in the Supporting Information and our previous work.8 The as-obtained chemically reduced GO paper was denoted as “C-rGO”. 13494

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Figure 1. Transition from normal glow discharge, to homogeneous state positive-column plasma, and to scanning inhomogeneous state positivecolumn plasma: (a−c) optical images of discharges with an increasing interelectrode gap from 6.3 to 10.8 cm; (d) dependence of discharge current on interelectrode gap; (e) high-speed camera images of scanning positive column within 4 ms.

and the negative glow region,31 although they are invisible by naked eyes. For this case, there was no space for the formation of positive column, and it is worth noting that AGD at the above two stages is incapable of reducing GO, consistent with the previously reported observation.28 The ignition of a positive column started with an interelectrode gap of ∼7.5 cm. After this critical value, the positive column became longer with an increasing interelectrode gap. As an example, for an interelectrode gap of 10.5 cm, a relatively homogeneous positive column was clearly observed with reddish-purple in color and few millimeters in diameter (Figure 1b). A further increase of the interelectrode gap led to the formation of a scanning positive column at an inhomogeneous state, as shown in Figure 1c where the interelectrode gap was set as 10.8 cm. The transition of positive column from homogeneous to inhomogeneous state could be attributed to the relatively large reactor volume, high operation pressure, intensive Joule heat release, and gas turbulence induced perturbation.29 As a consequence, the “dancing” positive column with a spatially inhomogeneous state moved chaotically in the interelectrode space. Figure 1e shows a series of high-speed camera images taken in 4 ms, presenting the quick, spontaneous scan of positive column on the anode surface. The instantaneous reduction of GO paper was conducted using AGD plasma with a scanning positive column stage. During the above transitions from negative glow, to homogeneous positive column, and to scanning positive column, the discharge current changed accordingly. Figure 1d shows the dependence of discharge current on the interelectrode gap; each data was a time-averaged value obtained in 30 s (see Supporting Information for a typical fluctuation of discharge current). As shown in Figure 1d, the discharge current first increased with the enlarging interelectrode gap, corresponding to the formation and elongation of the positive column. The subsequent increase of interelectrode gap led to

angles and droplet volume above the sample top surface were measured using a digital goniometer fabricated with an automatic dispensing needle at ambient environment. The contact angle was evaluated by the images in different time via Young−Laplace, conic, and circle methods for each specific condition. Electrochemical Measurement. Supercapacitors with different active materials (GO, C-rGO, and P-rGO papers), current collectors (nickel foil and nickel foam), and electrolytes (6 M KOH and 1 M tetraethylammonium tetrafluoroborate in acetonitrile solvent, i.e., TEABF4/AN) were assembled and tested. The test cells using aqueous electrolyte and foil type current collector were assembled in a two-electrode system with a layered structure, and all the components were sandwiched between two pieces of plastic sheet.8 The ones using organic electrolyte and foam type current collector were assembled with the commonly used equipment and procedure for battery coin cells (CR2032), conducted in a glovebox to avoid oxygen and moisture.30 The active materials of all the working electrodes were identical in mass (∼0.5 mg for one piece). Electrochemical performances of all the supercapacitors were tested by cyclic voltammetry (CV), galvanostatic charge/ discharge, and electrochemical impedance spectroscopy (EIS) on an electrochemical workstation (PGSTAT302N, Metrohm Autolab B.V.) at room temperature. The calculation methods of specific capacitance based on CV curves and charge/discharge plots were presented in detail in our previous work.8

3. RESULTS AND DISCUSSION Discharge Characterization. Figures 1a−c exhibit the optical images of AGD plasmas produced with different interelectrode gaps. As shown in Figure 1a for the AGD with an interelectrode gap of 6.3 cm, the bright luminous layer filling up the interelectrode space was mostly attributed to the negative glow emission. In general, Aston dark space, cathode glow, and cathode dark space also exist between the cathode 13495

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Figure 2. Photographs of (a) GO and P-rGO papers, (b) bendable P-rGO paper, and (c) area-selectively reduced paper.

Figure 3. (a) XPS survey spectra of GO, C-rGO, and P-rGO papers. (b) Curve fit of C 1s spectrum of P-rGO paper. (c) FTIR spectra of GO, CrGO, and P-rGO papers. (d) Raman spectra of GO, C-rGO, and P-rGO papers.

According to the XPS survey spectra shown in Figure 3a, the C/O ratio of P-rGO (7.6) was found to be close to that of CrGO (8.5), but significantly higher than that of GO (2.2). Figure 3b shows the Gaussian line fitted C 1s XPS spectrum of P-rGO. The fractions of C atoms in CC/C−C (∼284.5 eV), C−O (∼286.2 eV), CO (∼287.2 eV), and OC−O (∼289.2 eV) were calculated as 59%, 10%, 15%, and 16%, respectively. Figure 3c shows the FTIR spectra of GO, C-rGO, and P-rGO. The peaks at 1738 cm−1 (CO stretching vibration), 3413 and 1383 cm−1 (O−H vibration and deformation), 1228 cm−1 (epoxy C−O stretching vibration), and 1054 cm−1 (alkoxy C−O stretching) of GO FTIR spectrum dramatically decreased or almost disappeared in the C-rGO and P-rGO spectra. Moreover, upon reduction, the C C plasmon peak of UV−vis spectrum (see Supporting Information for the UV−vis spectra of different samples) was red-shifted from ∼230 nm (GO) to 264−268 nm (C-rGO and P-rGO), suggesting the increasing π-electron density. An obvious concern on plasma-assisted GO reduction processes is the possible damages of the graphitic structure induced by energetic plasma. To this respect, Raman

the decrease of discharge current until the vanishing of plasma, where the applied electric energy was no longer sufficient to sustain a stable AGD. The current level confirmed the formation of a typical AGD plasma.27 Material Characterization. Figure 2a shows the photographs of GO and P-rGO papers. The brown, transparent GO paper turned to black, opaque P-rGO paper, indicating the successful reduction.32,33 The mechanical strength of P-rGO paper is worse than that of parent GO; however, it still presented a good structural and mechanical integrity. As shown in Figure 2b, the as-obtained P-rGO paper is freestanding and flexible, which can be readily used as promising active materials for the binder-free fabrication of electrodes of bendable power sources.8,34,35 Furthermore, the reduction can be conducted at selective areas. For example, once parts of the parent GO paper were covered by an insulating mask, the reduction only occurred at the uncovered area, which can be recognized by the color difference of the covered and uncovered regions (see Figure 2c). Characterization of GO, C-rGO, and P-rGO papers were conducted with XPS, FTIR, and UV−vis measurements. 13496

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Figure 4. (a) Optical emission spectra of the positive column and negative glow in the atmospheric pressure glow discharge. (b) Typical Voigt fit of the recorded Hα atomic line (positive column). λL: FWHM of the Lorentz profile; λG: FWHM of the Gaussian profile. (c) Schematic of a possible mechanism on the positive-column plasma assisted GO reduction process.

region can selectively stimulate the required reactions for GO reduction, e.g., in radio-frequency inductively coupled plasmas (500 mTorr, 1−5 min),41 electron beam plasmas (20−100 mTorr, 30−90 s),42 remote microplasmas (atmospheric pressure, 30 min),33 and dielectric barrier discharge plasmas (atmospheric pressure, 2 min).43 In this work, the optical emission spectroscopic (OES) technique was performed to obtain the emission spectra in the regions of positive column and negative glow. As shown in Figure 4a, the spectrum of the positive-column plasma exhibited a significantly higher emission intensity than that of the negative glow counterpart. Atomic lines and molecular bands originating from the working gas (He) and ambient background gases (e.g., N2, O2, and H2O) were identified in the spectrum of the positive column. Besides the He I line at 668 nm (21P → 31D), a strong OH (A2Σ → X2Π, 0−0) band at 309 nm, intensive N2 (C3Πu → B3Πg) second positive system (Δv = 1, 0, −1, −2), N2+ (B2Σu+ → X2Σg+) first negative system at 391 nm, Balmer transition of H atoms at 656 nm, weak atomic oxygen lines at 777 and 846 nm, and NO (A2Σ+ → X2Π) γ system at 200−300 nm were also observed.27,44 However, for the current work at the same spectrometer setting (integration time 1 s; number of pixels in processed spectrum 2048), all the above bands were either not observed or with a much lower intensity in the negative glow region. Different spectral characteristics from two different discharge areas suggest a higher gas temperature and electron density in the positive column region.

measurements on GO, C-rGO, and P-rGO were conducted. As shown in Figure 3d, the intensity ratio of D peak-to-G peak I(D)/I(G) was in the order of P-rGO < GO < C-rGO. The increase of I(D)/I(G) after chemical reduction, from 0.97 for GO to 1.41 for C-rGO, is consistent with the previously reported results,36−38 which can be attributed to the creation of defects and vacancies with the removal of oxygen, as well as the newly formed graphitic domains with a size smaller than those in parent GO.36,39 According to the Tuinstra−Koenig relation,40 the average sp2 cluster size of GO (4.53 nm) is larger than that of C-rGO (3.11 nm). In contrast, P-rGO presented an I(D)/I(G) value of 0.78, obviously lower than those of GO and C-rGO, corresponding to an average size of the in plane sp2 domains of 5.53 nm. The above results indicate that, with the scanning positive column treatment within a short duration of 2 s, the as-applied plasma tends to recover the hexagonal network of carbon atoms with oxygen reduction, rather than damaging the graphitic structure. Reduction Mechanism. Previous work has demonstrated the successful plasma-assisted GO reduction using a variety of plasma sources with different manifestations.15,16,33,41−43 The working styles of plasma-assisted GO reduction can be generally divided into the two categories. First, plasma can be used as an efficient, rapid heating source. For example, GO film can be heated to hundreds of °C through the direct absorption of microwave plasma irradiation, and then effective GO reduction can be achieved in a short duration of 2−60 s.15,16 Second, at a mild or low temperature, the energetic electrons, reactive radicals, active ions, and other species in the plasma 13497

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Furthermore, the Hα line and OH (A2Σ → X2Π) bands were measured by using the Acton SP2300 spectrometer of a high resolution. The electron density for each region was determined from a Stark broadening of the Hα line (see Supporting Information for the calculation method of electron density). The spectral line broadening is a complicated function of the environment of the radiating atoms or ions, and the measured shape of a spectral line is the result of the combined effects of several broadening mechanisms. In our experiment, the total width of the spectral line is a convolution of the Lorentzian profile (Stark broadening and van der Waals broadening) and the Gaussian one (Doppler broadening and instrumental broadening). The Stark broadening (full width at half-maximum, FWHM) of the recorded Hα line was obtained by fitting a Voigt function of the spectral line, as shown in Figure 4b. The detailed calculation of spectral line broadenings (Stark, instrumental, Doppler, and van der Waals broadenings) and the convolution procedure are presented in the Supporting Information. The resonance and natural broadenings are usually negligible due to very low FWHM values in comparison to other broadening mechanisms. The Stack broadening was calculated as 0.092 nm for the positive column. The electron density was obtained from the formula proposed by Ashkenazy et al.45 For the current work, the electron density of the positive column was calculated as 1.03 × 1016 cm−3, while that of the negative glow was not available due to a weak intensity of the Hα line in this area. According to Staack et al.’s work for dc glow discharge in air, the electron density in positive column was ∼20 times higher than that in negative glow.27 The rotational temperature of OH in the discharge was determined by comparing the measured OH (A2Σ → X2Π) band to the simulated one using LIFBASE.46 For each rotational temperature in the 300−2000 K range, the absolute error between each experimental peak and its simulated value are calculated and averaged over all the selected peaks in the OH band (306− 311 nm). The rotational temperature can be determined when the best matching is achieved with a minimum error.47 Because of the high collision and fast rotational relaxation rates in the plasma, it can be assumed that the rotational temperature is close to the gas temperature. In this work, the rotational temperature of OH was around 800 K in the positive column, much higher than that in the negative glow (∼300 K). A possible mechanism on the positive-column plasmaassisted GO reduction process is presented in Figure 4c. Based on the above plasma diagnosis results, it is reasonable to conclude that the instantaneous reduction of GO paper could be attributed to the synergy of high-density energetic electrons and direct contact heating induced by the positive-column plasma. On one hand, the collision induced by electrons will lead to the elimination of oxygen functional groups on GO surface.43 Notably, the working duration of positive-column plasma induced GO reduction process lasts an ultrashort time (2 s), and consequently the electron bombardment will not obviously damage the graphitic structure, as evidenced by the Raman results shown in Figure 3d. On the other hand, thermally induced degradation of oxygen functional groups could also happen during the reduction process.48 Compared with the “indirect” heating in the traditional thermal reduction processes that using bulk gases as heating media, the direct contact between GO paper and positive column allows a rapid thermal transport and thus benefits the energy efficiency. Moreover, the synergy of electrons and heating is believed to work on the liquid water remaining on the GO surface and

trapped within the GO stacked layers, leading to the formation of additional OH and H radicals. Finally, gaseous products such as CO2, H2O, and CO are formed through the above reactions, resulting in a rapid buildup of sufficient pressure within the graphene interlayers and the expansion of graphene layers.10,48 Wettability and Liquid Permeation. Figure 5a shows the changes of droplet volume on the top surface of GO, C-rGO, P-

Figure 5. (a) Changes of droplet volume on the top surface of GO, CrGO, and P-rGO papers with a continuous monitoring 30 s. Water contact angels of (b) GO, (c) C-rGO, and (d) P-rGO at 0, 10, and 30 s.

rGO papers within 30 s. The typical apparent water droplet contact angles at 0, 10, and 30 s for different samples are presented in Figure 5b−d. The initial (0 s) apparent contact angles of GO, C-rGO, and P-rGO papers were 30.1°, 92.9°, and 78.5°, respectively. As is well-known, the material wettability is an intrinsic property for the liquid resting on a solid substrate, resulting from the intermolecular interactions between liquid and solid. For graphene monolayer, the van der Waals interactions between graphene and liquid placed on top are negligible, leading to a wettability dominated by short-range chemical bonding and the so-called “wetting transparency” phenomenon.49 However, with an increasing graphene layer number, the transparency became poor and finally reaches a water contact angle of ∼90°.49 As for the current work, the initial contact angles of different samples indicate the change of the materials from hydrophilic GO to hydrophobic C-rGO and P-rGO, corresponding to the reduction of oxygen functionalities with different extents.9 The following changes of apparent water contact angle are related to the liquid permeation of different samples. After 30 s, the droplet volume above the GO and C-rGO papers was almost kept as a constant, while the one above the P-rGO paper decreased by 76%, indicating a substantially improved liquid permeation. Accordingly, the apparent water contact angles of GO and C-rGO were found to be almost steady, while that of 13498

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shown in Figure 7b,c, the CV curves of C-rGO became evidently distorted with an increasing scan rate from 20 to 500 mV s−1, while those of P-rGO were kept as a quasirectangular shape. For a certain scan rate, the specific capacitance of P-rGO was significantly higher (1.51−1.92 times) than that of C-rGO. The influence of the pseudocontributions (by functional surface groups and electrolyte redox couples) can be considered as a minimal due to the similar C/O ratios of P-rGO (7.6) and CrGO (8.5) papers. Consequently, the above observation could be mainly attributed to the less stacking and open graphene interlayer channels, where a higher electrolyte accessible surface area was achieved. Galvanostatic charge/discharge tests were conducted on CGO and P-rGO based supercapacitors at different current densities. The galvanostatic charge/discharge plots of both supercapacitors at a current density of 1 A g−1 can be found in the Supporting Information. Figure 7d shows the galvanostatic charge/discharge plots of both working electrodes at a relatively high current density of 50 A g−1 (1 M TEABF4/AN, nickel foam). The P-rGO-based supercapacitor presented an obviously lower voltage (IR) drop than the C-rGO counterpart. Meanwhile, with an increasing current density from 1 to 50 A g−1, the retention of the specific capacitance of C-rGO based supercapacitor was only 18.3% (from 93.2 to 17.1 F g−1) while that of the P-rGO counterpart was 77% (from 161.6 to 124.5 F g−1), proving the better charge/discharge rate response of PrGO-based supercapacitor to the applied potential. A further comparison between the capacitive properties of CrGO and P-rGO working electrodes was conducted with EIS test. Figure 7e shows the Nyquist plot of both working electrodes. As is well-known, the slope of the 45° portion of the Nyquist curve is related to the Warburg finite-length diffusion stage, and the projected length of which on the real axis characterizes the ion diffusion process from solution into the graphene interlayers.6,52 As shown in Figure 7e, the P-rGO working electrode presented a much shorter Warburg-type line than that of C-rGO counterpart, indicating the faster ion diffusion. The above results were corroborated by the Bode plots of the frequency response of the capacitance. The operating frequency at half-maximum capacitance of each working electrode was calculated from the Bode plots (calculation method can be found in our previous work8). The corresponding characteristic relaxation time constant (CRTC) of the P-rGO-based supercapacitor was calculated as 420 ms, obviously lower than that of the C-rGO counterpart (935 ms), consistent with the better charge/discharge rate performance. Similar observation was also obtained with the foil type current collector and aqueous electrolyte, as shown in Figure 7f for the Nyquist plots of C-rGO- and P-rGO-based supercapacitors employing nickel foil as the current collector and 6 M KOH as the electrolyte. For this case, the CRTC of PrGO and C-rGO working electrodes were calculated as 464 and 772 ms, respectively. The frequency response of the capacitance of both supercapacitors with different current collectors and electrolytes can be found in the Supporting Information.

P-rGO decreased to 22.1°, close to that of the bare glass substrate (15.8°; see Supporting Information for the image of water droplet on a glass). The liquid permeation performance is strongly related to the material morphology. Figure 6a−d

Figure 6. SEM images of (a) C-rGO and (b) P-rGO. (c, d) Magnified SEM images on the typical open channels of P-rGO.

shows the SEM images of C-rGO paper and P-rGO paper. As shown in Figure 6a, C-rGO presented a surface-wrinkled, corrugation morphology as the typical characteristics of a chemically rGO paper.8,25 In addition to the ripples and wrinkles kept well on the surface of P-rGO, a considerable number of newly formed exposed open channels were observed, as shown in Figure 6b−d. According to the XRD patterns (see Supporting Information for the XRD patterns of different samples), the interlayer spacing (d-spacing) of P-rGO was 0.39 nm, larger than that of C-rGO (0.37 nm). The BET surface area of P-rGO paper was measured as 371 m2 g−1, significantly larger than that of the compact graphene paper obtained from chemical route.50 For GO or rGO papers, the liquid permeation of the submicrometer paper can be described as the liquid transport in numerous nanocapillaries. 24 According to Hagen−Poiseuille’s law, the pressure loss along a capillary decreases with an increasing capillary cross-section diameter. Taking advantage of exposed open channels with an expansion of graphene interlayer spacing, improved liquid permeation of P-rGO than C-rGO was achieved. Supercapacitor Application. Figure 7a shows the CV curves of GO, C-rGO, and P-rGO working electrodes at a scan rate of 10 mV s−1 (1 M TEABF4/AN, nickel foam). The areas of CV curves were in the order of GO < C-rGO < P-rGO, and the corresponding specific capacitances of GO, C-rGO, and PrGO papers were calculated as 22.9, 120.0, and 181.4 F g−1, respectively. Both the CV curves of C-rGO and P-rGO papers presented a quasirectangular shape (a predominant EDL capacitive behavior) while that of GO paper was quite distorted (see Supporting Information for the enlarged CV curve of GO paper at 10 mV s−1). The above results are consistent with the substantially improved active material electrical conductivity. GO paper presented a poor electrical conductivity (∼5 × 10−2 S m−1 for the current work and 8.5 × 10−2 S m−1 for ref 51), mainly attributed to the extensive presence of saturated sp3 bonds induced by oxygen. In contrast, the as-obtained P-rGO paper presented a high electrical conductivity of 5.9 × 103 S m−1, a level similar to that of C-rGO (6.5 × 103 S m−1) but around 5 orders of magnitude higher than that of GO paper. As

4. CONCLUSIONS The as-proposed positive-column plasma method is capable of converting GO paper to high quality rGO paper (high C/O ratio, excellent graphitic structure maintenance, and good electrical conductivity) within an ultrashort duration, showing the advantages of effective, environmentally friendly, energyefficient, and facile. It presented a series of advantages over the 13499

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Figure 7. (a) CV curves of GO, C-rGO, and P-rGO working electrodes at a scan rate of 10 mV s−1 (nickel foam and TEABF4/AN). CV curves of (b) C-rGO and (c) P-rGO working electrodes at different scan rates between 20 and 500 mV s−1 (nickel foam and TEABF4/AN). (d) Galvanostatic charge/discharge plots of C-rGO and P-rGO working electrodes at a current density of 50 A g−1 (nickel foam and TEABF4/AN). Inset: capacitance retention of C-rGO and P-rGO with an increasing current density from 1 to 50 A g−1. (e) Nyquist plots of C-rGO and P-rGO working electrodes (nickel foam and 1 M TEABF4/AN). (f) Nyquist plots of C-rGO and P-rGO working electrodes (nickel foil and 6 M KOH).

feasible, potentially benefiting applications beyond energy storage.53,54

existing counterparts (see Supporting Information Table S1 for the comparison of positive-column plasma reduction with some typical chemical-, thermal-, and plasma-assisted reduction processes), such as ultrafast operational procedure (∼2 s), eco-friendly (absence of chemical reagents), cost-effective (negligible power consumption of few watts), and mild processing condition (without heating). The instantaneous reduction can be mainly attributed to the synergy of highdensity energetic electrons and surface touch direct heating induced by the positive-column plasma. Moreover, the ultrafast reduction of oxygen functionalities results in a rapid buildup of sufficient pressure within graphene interlayers. It leads to the creation of exposed open channels and the expansion of graphene layers, presenting a unimpeded liquid permeation. The as-obtained rGO paper with high electrical conductivity and unimpeded liquid permeation has been demonstrated as a promising active material candidate for supercapacitors, exhibiting better capacitive and rate performance than the parent GO and chemically reduced rGO counterparts. Finally, production of the rGO patterns using the proposed method is

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of GO powder and C-rGO paper; fluctuation of discharge current for scanning positive-column plasma; UV−vis spectra of GO, C-rGO, and P-rGO papers; water contact angles of glass substrate; XRD patterns of parent GO, C-rGO, and PrGO papers; calculation of electron density; enlarged CV curve of GO paper (10 mV s−1, 1 M TEABF4/AN, nickel foam); galvanostatic charge/discharge plots of C-rGO and P-rGO papers at 1 A g−1 (1 M TEABF4/AN, nickel foam); frequency response of the capacitance; comparison with existing GO reduction methods. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel 86 571 87953055 (Z.B.). 13500

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Author Contributions

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Z.B., J.C., and K.C. designed this research. W.Z. and Y.Y. constructed the glow discharge system. W.Z. synthesized the GO, C-rGO, and P-rGO materials. W.Z. and S.M. conducted the liquid permeation experiments and analysis. W.Z. and J.Y. conducted experimental and analysis work on the physical properties of glow discharge. X.T. and Z.B. conducted the calculation and analysis on plasma diagnosis results. W.Z. conducted the material characterization. Y.H. and J.Y. conducted the plasma diagnosis. W.Z., Z.B., and J.Y. conducted the electrochemical measurement and analysis. Z.B., J.C., and K.C. drafted the manuscript. All authors commented on the final manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51306159), the Zhejiang Provincial Natural Science Foundation of China (No. LY13E020004), the Foundation of National Excellent Doctoral Dissertation of China (No. 201238), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120101120140), and the Fundamental Research Funds for the Central Universities (No. 2014FZA4011).

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