Article pubs.acs.org/JPCC
UV-Assisted Photoreduction of Graphene Oxide into Hydrogels: High-Rate Capacitive Performance in Supercapacitor Ling-Bao Xing, Shu-Fen Hou, Jin Zhou, Shijiao Li, Tingting Zhu, Zhaohui Li, Weijiang Si, and Shuping Zhuo* School of Chemical Engineering, Shandong University of Technology, Zibo 255049, P. R. China S Supporting Information *
ABSTRACT: Self-assembly of graphene oxide (GO) into threedimensional (3D) macroscopic architectures is an efficient strategy to exploit their inherent properties fully and restrict the formation of irreversible aggregates or restack into graphitic structure. In the present work, we first report a photoreduction method for producing the 3D photoreduced graphene hydrogels (PRGHs) through self-assembly of GO suspension under photochemical reduction of Hantzsch 1,4dihydropyridine (HEH). The reduction of GO into PRGHs was confirmed by X-ray powder diffraction (XRD), Raman spectroscopy, Xray photoelectron spectroscopy (XPS), elemental analysis, and Fourier transform infrared spectroscopy (FT-IR). It is observed that most of the oxygenated groups could be efficiently removed, and the sp2-like domains are restored as a result of photoreduction process. The 3D hierarchically porous structures of the resulting PRGHs have been intensively investigated by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and N2 sorption measurement. More interestingly, the supercapacitor based on the PRGHs showed a high specific capacitance of 254 F g−1 at 1 A g−1 in KOH electrolyte. The specific capacitance can still be maintained for 64% with an increase of the discharging current density to 4 A g−1. It also showed that the electrode based on PRGHs has good stability and high reversibility in the charge/discharge cycling test. The specific capacitance of the supercapacitor maintained at 224 F g−1 (capacitance retention ≈ 88%) after 4000 cycles.
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sions.23 In this case, the obtained reduced GO performed high specific capacitance and high rate capability. Since these pioneering works, 3D graphene architectures prepared from hydrothermal or chemical reduction method had been widely used for the preparation of 3D graphene architectures and attracted more and more attention for applications in adsorption, sensors, supercapacitors, catalysis, etc.15−25 Although there are extensive reports on the preparation of hydrogels with 3D macroscopic structures, most of which are in a chemical reduction process by using various chemical reductants such as vitamin C, NaHSO3, HI, N2H4, dopamine, and so on.21 In order to further explore the 3D structures and functions of graphene, it is very necessary to find a new method to prepare graphene hydrogels, whose 3D structures both possess the intrinsic features of graphene and display modified functions arising from the particular microstructures. Recently, Kamat, Müllen, and Sun’s groups successively developed lightassisted photoreduction of GO by using TiO2, ZnO, and Ag nanoparticles as photocatalysts.26−29 Especially in 2012, Wu and co-workers prepared mono or few-layer reduced GO sheets in high quality and in large scale through an effective way by
INTRODUCTION As a two-dimensional (2D) sheet of sp2-hybridized carbon, graphene has shown various potential applications such as batteries,1−3 supercapacitors,4,5 fuel cells,6−8 photovoltaic devices,9,10 photocatalysis11,12 and so on due to its remarkable electrical, mechanical, and thermal properties. However, the 2D graphene obtained are easily interacting with each other to form irreversible aggregates or restacking into graphitic structure because of π−π stacking and hydrophobic interactions between the graphene nanosheets.13,14 Consequently, the inherent properties of the graphene sheets cannot be exploited thoroughly due to a dramatic decrease of the specific surface area.15 In order to solve the problem and take full advantage of the high surface area, much efforts have been devoted to the preparation of stable homogeneous graphene suspensions16,17 and construction of graphene-based three-dimensional (3D) architectures,18,19 in which self-assembly of the 2D graphene sheets has been generally recognized as an efficient strategy for constructing 3D graphene architectures and novel features.20,21 In 2010, Shi and co-workers first reported a facile one-step method to construct hydrogels of GO dispersions with porous 3D network,22 which exhibited high specific capacitance (175 F g−1) in an KOH electrolyte. Soon after, using hydrazine and hydroiodic acid as reductants in a hydrothermal process, they also prepared graphene hydrogels by reducing GO suspen© 2014 American Chemical Society
Received: August 27, 2014 Revised: October 9, 2014 Published: October 21, 2014 25924
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Scheme 1. Schematic Representation of the Formation of PRGHs through the Photochemical Reduction
was dissolved in DMF (5 mL) and was added to GO suspension (5 mL, 3 mg mL−1). The above mixed solution was degassed with nitrogen for 30 min and then photoreduced by using a 500 W high-pressure Hanovia mercury lamp (λ > 320 nm) in a Pyrex reactor without stirring. In order to keep room temperature, the tube with GO suspension was positioned a given distance (typically 2 cm) with a water cooling cover outside the lamp. After irradiation for 6 h, the PRGHs would be obtained. The excess HEH could be removed by washing twice with acetone for 12 h. Then the PRGHs were freeze-dried by a freeze drier for at least 24 h. Characterization. X-ray diffraction (XRD) patterns were conducted using a Bruker D8 Advance diffraction with Cu Kα radiation (λ = 1.5418 Å). The interplanar crystal spacing was calculated by the Bragg equation (2d sin θ = nλ). Raman spectra were obtained from a LabRAM HR800 (JY Horiba) with a 435 nm wavelength laser. The porous structures of PRGHs were determined by scanning electron microscope (SEM, FEI, Holand), transmission electron microscope (TEM, JEOL2010, Japan), and atomic force microscope (AFM, Multimode NS3A, USA). The reduction of GO to PRGHs was characterized by X-ray photoelectron spectroscopy (XPS, Escalab 250, USA) and Fourier transform infrared spectroscopy (FT-IR, Nicolet 5700, USA). Nitrogen sorption measurements were performed at −196 °C using ASAP 2020 system (Micrometitics USA). Elemental analysis (CHO) was performed at FLASH EA1112 Elemental Analyzer. Electrochemical Measurements. Working electrode was prepared by pressing 2 mg of the freeze-dried PRGHs onto nickel foam under 10 MPa. In the electrochemical test, 6 M KOH solution was used as electrolyte. In order to fill in the ions into the porous structures of PRGHs, the working electrode was vacuum-impregnated with the electrolyte for 30 min. The CHI660D electrochemical workstation (Chenhua Instruments Co. Ltd., Shanghai) was applied to determine the electrochemical capacitive performances of PRGHs electrode. In a three-electrode system by using PRGHs as working electrode, a platinum film as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode, CV, galvanostatic charge−discharge measurement, and electrical impedance spectroscopy (EIS) were performed. The potential window from −0.9 to 0 V vs SCE reference electrode was applied to the electrochemical measurements.
using a photochemical approach, in which HEH was used as a moderate reductant to replace traditional reductants such as hydrazine and NaBH4 to reduce GO suspension.30 Although the reduced GO sheets with high quality could be obtained through photochemical process, there is still no report to prepare hydrogels with 3D architectures by using photochemical method. In the present work, we report a simple method for producing the photoreduced graphene hydrogels (PRGHs) through photochemical reduction of GO suspension with HEH (Scheme 1). In a typical procedure as shown in the experimental section, the GO suspension in mixed solvents of N,N-dimethylformamide (DMF) and H2O is readily photoirradiated by HEH under high-pressure mercury lamp (λ > 320 nm) at room temperature for 6 h to form PRGHs. The excess HEH and the generated pyridine product after photoirradiation could be completely extracted with acetone. The PRGHs with well-defined and cross-linked 3D porous networks would be obtained as its freeze-dried samples. As far as we know, this is the first report about 3D graphene porous hydrogels prepared by photoreduction method. Of great significance is that the supercapacitor based on the PRGHs showed a high specific capacitance of 254 F g−1 at 1 A g−1. The specific capacitance can still be maintained for 64% with an increase of the discharging current density to 4 A g−1. Compared to the reported works in hydrothermal, hydrothermal/chemical, chemical, and solvothermal reduction process (Table S1, Supporting Information), it can be seen that the PRGHs obtained through photochemical reduction method have several advantages including low reaction temperature (room temperature), short reaction time (6 h), moderate specific surface area (244 m2 g−1), high performance (254 F g−1), and good electrochemical stability (4000 cycle number, 88%). The high stability and reversibility of this supercapacitor provides a new method to prepare 3D structures based on GO suspension to make promising applications in high rate charge/discharge.
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EXPERIMENTAL SECTION Materials. Natural flake graphite powder (325 mesh) was purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. All other chemicals were purchased from Sinopharm Chemical Reagents Co. Ltd. and used directly without further purification. Preparation of GO and PRGHs. GO was prepared on the basis of modified Hummers method as in previous work.4,30 In a typical procedure for preparation of PRGHs, HEH (120 mg) 25925
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Figure 1. XRD (a) and Raman (b) spectra of GO (black) and PRGHs (red).
Figure 2. XPS survey (a); C 1s spectra (b); O 1s spectra (c); and FT-IR spectra of GO and PRGHs (d).
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RESULTS AND DISCUSSION The formation of PRGHs was carried out by a photochemical reduction of GO aqueous dispersion (1.5 mg mL−1) in mixed solvents of DMF and H2O (V/V = 1:1) at room temperature. As confirmed by AFM image of GO suspension (Figure S1, Supporting Information), the obtained GO sheet is monolayered (∼1 nm) with the size from several hundred nanometers to a micron. Typically, the DMF solution of HEH was added to GO aqueous suspension. Soon after, the mixed solution in a test tube was degassed with nitrogen for 30 min. Then the above solution was irradiated by a high-pressure mercury lamp in a Pyrex reactor without stirring. Along with the photoirradiation, the color of the solution turned from brown to dark after 30 min. After irradiated for 2 h, a welldefined black hydrogel with a cylinder shape was formed. Meanwhile, the apparent size of hydrogel decreased obviously within the initial 4 h and subsequently changed little (Figure S2, Supporting Information). However, although the color of solution turned from brown to dark when the GO solution was irradiated under the same condition in the absence of HEH, no gelation phenomena were formed (Figure S3, Supporting Information). Even if the above solution was irradiated for 12 h, there is still no graphene hydrogels formed. The excess HEH
and the generated pyridine product after photoreduction could be simply extracted with acetone. Then the PRGHs were freeze-dried for at least 24 h for characterizations (Figure S4, Supporting Information). The XRD and Raman spectra demonstrate the structural change from GO to PRGHs in detail as shown in Figure 1. It can be seen that the peak intensity of GO at 9.74° almost disappeared with photoreduction. Meanwhile a broad bump near 24.78° appeared in the freeze-dried PRGHs. According to the literature report,22 the new peak corresponding to an interplanar spacing of 3.68 Å can be attributed to the existence of π−π stacking interactions between graphene sheets in the PRGHs, which is between GO precursor (6.94 Å) and graphite (3.36 Å). In addition, the graphene sheets self-assemble into few-layer stacked graphene with random order through π−π stacking interactions, which can be reflected by the broad XRD peak in the freeze-dried PRGHs sample. In the Raman spectra (Figure 1b), the specific vibrations of D bands (the A1g symmetry mode) in the vicinity of 1354 cm−1 and G bands (the E2g mode of the sp2 carbon atoms) in the vicinity of 1598 cm−1 were obviously observed both in the GO and PRGHs (Figure 1b). The intensity ratio of ID/IG is 0.82 in GO and increases to 1.36 in PRGHs, which agree well with other 25926
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Figure 3. SEM images with different magnifications of the PRGHs interior microstructures (a,b), N2 sorption isotherms (c), and pore size distribution (d).
and O in GO and PRGHs also confirmed the decrease of oxygen content from 50.9 to 12.1 (Table S3, Supporting Information). Figure 2d shows the FT-IR spectra of GO and the freeze-dried PRGHs. The broad and intense band at 3000− 3500 cm−1 and peaks at 1720, 1610, 1410, 1174, and 1040 cm−1 can be attributed to the stretching vibrations of O−H groups, CO stretching vibrations, CC flexible vibrations from unoxidized graphitic domains, O−H bending vibrations, and C−O stretching vibrations, respectively. For PRGHs, the peak at 1720 cm−1 related to CO or OC−O and the band related to O−H (3000−3500 cm−1) decreased. Moreover, the peaks at 2915 and 2843 cm−1 attributed to the stretching and bending vibrations from C−H were increased. Meanwhile, the new peaks at 1554 and 1428 cm−1 attributed to stretching vibrations of aromatic CC appeared. The results of FT-IR further demonstrate removal of oxygenated groups, and the restoration of sp2 domains resulted from the HEH photochemical reduction process. The porous architectures of PRGHs was characterized by field emission scanning electron microscopy (FESEM). As shown by SEMs of freeze-dried PRGHs in Figures 3a,b and S5a, Supporting Information, a typical three-dimensional network with porous structures of PRGHs formed by crosslinking of graphene sheets is particularly uniform in the large scale, and the hierarchical pore with the wide size distribution ranges from submicrometer to several micrometers. According to the previous literature report,16,17 the cross-linked 3D graphene porous network was formed by partial overlapping or aggregating of the graphene nanosheets through π−π stacking and hydrophobic interactions during photochemical reduction. The high resolution transmission electron microscopy (HRTEM) image (Figure S5b, Supporting Information) also showed wrinkled and folded paper-like textures of graphene sheets in line with the SEM images. The BET specific surface area of PRGHs (Figure 3c) was measured to be 244 m2 g−1, indicating the overlap of graphene sheets within PRGHs. From the pore size distribution plot obtained from N2 sorption analysis as shown in Figure 3d, we can see that the prepared
chemical reduction methods and confirms the reduction of GO.21 The increased ratio of ID/IG in the reduction of GO into PRGHs may be attributed to the removal of oxygenated groups, which changed the structure of GO and introduced structural defects in the gelation process. XPS experiments were carried out to further illuminate the variation of surface chemical properties with photoreduced gelation process. As shown in Figure 2a, the two different peaks at 286 and 533 eV are related to C and O, respectively. The atomic concentration calculated by using XPS spectra showed that the carbon and oxygen concentrations of GO were relatively increased and decreased, respectively, by the photoreduction. The content of oxygen (atm.%) dramatically decreased from 29.1 to 10.1, implying that most of the Ospecies have been efficiently removed by light-assisted photoreduction treatment. The atom binding states of PRGHs were also investigated by high-resolution XPS measurements (Figure 2b,c and Table S2, Supporting Information). Figure 2b exhibits the C1s deconvolution spectra of GO and PRGHs. The CC/C−C, C−O (epoxy and hydroxyl), CO (carbonyl), and OC−O (carboxyl) groups give four different peaks centered at 284.6, 286.4, 287.0, and 288.8 eV, respectively. The evolutions of functional groups could be reflected by the relative changes of different peaks. C1s XPS spectra showed that the sp2 carbon peak (284.6 eV) increases due to the restoration of the sp2 lattice as a result of the photoreduction (Figure 2b). The dramatic decrease of oxygenated carbon components at 286− 289 eV confirms the removal of oxygenated groups. Similar conclusions could be derived from the O1s deconvolution spectra of GO and PRGHs as shown in Figure 2c and Table S2, Supporting Information. After irradiation for 6 h, three different peaks centered at 531.8, 532.7, and 533.6 eV, corresponding to CO (carbonyl), C−O (epoxy and hydroxy), and OC−O (carboxyl) groups, have significant changes in line with C1s spectra, in which the obvious decrease of CO (carbonyl) also indicates the removal of oxygen-containing groups. In line with the XPS analysis, the relative elemental analysis (mass %) of C 25927
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Figure 4. Electrochemical capacitive performance of PRGHs: (a) cyclic voltammograms of the supercapacitor based on PRGHs at scan rates of 10, 20, 50, and 100 mV s−1; (b) Galvanostatic charge/discharge curves of PRGHs at a charging/discharging current density of 1, 2, 3, and 4 A g−1; (c) electrochemical impedance spectra; and (d) long-term cycle test (inset shows the retention ratio of the capacitance).
generating reversible capacitive behavior even at high power output. It should be noted that the specific capacitance per surface area is very high, up to 1.04 F m−2 at 1 A g−1, which seems not to be simply interpreted by double layer capacitance as a result of the 3D hierarchical porosity. As shown in Figure 2, the PRGHs still have lots of oxygenated groups. The reversible redox reactions of hydroquinone/quinone groups could introduce the extra pseudocapacitance improving the specific capacitance. Furthermore, the O-rich pore surface also increases the hydrophilicity and polarity of carbon materials and thus facilitates the wettability between the electrolyte and pore surface. Figure 4c shows the electrochemical impedance spectra (EIS) of the PRGHs in a frequency range from 0.01 to 105 Hz. The obtained Nyquist plot (Figure S7, Supporting Information) is simulated by ZView software, and the equivalent circuit is also illustrated as an inset. As shown in Figure 4c, a typical Nyqusit plot of carbon-based supercapacitor is obtained, which is composed of an uncompleted semicircle, a 45° slope and a straight line upward. At very high frequencies, the imaginary impedance (Z″) is near to zero and the corresponding real resistance (Z′) is the sum of ohimic resistance including the electrolyte and the contact between the work electrode and the current collector (Rs). The low value of Rs demonstrates that the conductivity of the supercapacitor is good. The uncompleted semicircle loop observed in the range of medium-high frequencies is ascribed to faradic charge transfer process (Rct) at the interface of electrode/electrolyte. The 45° slope region in the middle frequencies, which is called to be Warburg impedance (W), reflects the frequency dependence of ion migration from electrolyte into the internal porosity of PRGHs material. The short length of this slope indicates that the electrolyte ions diffuse fast due to the 3D hierarchical porosity and O-riched polar pore surface. At low frequencies, an almost vertical line was obtained in EIS, demonstrating the dominance of ideal double-layer capacitive behaviors.
graphene gel possesses aggregates of plate-like particles giving rise to slit-like pores or voids. The total pore volume of PRGHs is 0.18 cm3 g−1. The special 3D architectures can effectively hold back the restacking of graphene sheets, and the abundant pores can be used as ion storage to realize the fast charge and discharge in supercapacitors. The electrochemical performance of PRGHs was evaluated as supercapacitor electrode in 6 M KOH electrolyte by using a conventional three-electrode system, in which SCE and platinum plate are employed as the reference electrode and counter electrode, respectively. Figure 4a shows the CV of the PRGHs-based supercapacitor at different scan rates of 10, 20, 50, and 100 mV s−1, and all the curves present a typical rectangular-like shape, implying the characteristic of double layer capacitance for the electrode material. This is in accordance with the behavior of the conventional reduced GO hydrogels. What’s more, the PRGHs also show superior rate performance as shown in Figure S6, Supporting Information. Even when the scan rate increases to 150 and 200 mV s−1, the CV of the PRGHs still remains a rectangularlike shape with a slight deviation at lower potentials. This indicates the double layer capacitance based on PRGHs has a quick charge propagation capability. The capacitive property of PRGHs was further tested at different current densities by a galvanostatic charge/discharge experiment (Figure 4b). The specific capacitance could be calculated by the equation of C = It/mV, where C (F g−1) is the specific capacitance of PRGHs, I (A) is the discharge current, t (s) is the discharge time, V (V) is the potential window of the cell (0.9 V in this study), and m (g) is the mass of active materials loading on the work electrode. From the discharge curve, the specific capacitance of the PRGHs electrode was evaluated to be about 254, 191, 170, and 164 F g−1 at constant current density 1, 2, 3, and 4 A g−1 from the discharge curves, respectively. These results indicate that the PRGHs possess good rate performance, which may be attributed to its hierarchical 3D porous structure that allows for highly effective ion diffusion into the active sites, thereby 25928
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To investigate the long-term cyclic stability, a new cell is assembled and measured by using galvanostatic charge/ discharge test up to 4000 cycles at 1 A g−1 (Figure 4d). With the increase of the cycle number, the capacitance of PRGHs decreases gradually. During charge/discharge cycles, some unstable functional groups such as carboxyl groups or pyridinic could be decomposed, which may lead to some loss of pseudocapacitance.31 A high capacitance of 224 F g−1 is obtained even after 4000 charge/discharge cycles, this value is about 88% of the discharge capacitance of the first cycle. The good electrochemical stability and a high degree of reversibility reflect that the prepared PRGHs electrode has promising application in the repetitive charge/discharge cycling test.
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CONCLUSIONS In summary, we report a facile photochemical reduction method for producing the photoreduced graphene hydrogels (PRGHs) with 3D porous structures from GO suspension. More interestingly, the supercapacitor based on the porous PRGHs performed a high specific capacitance of 254 F g−1 at 1 A g−1 in KOH electrolyte. With an increase of the discharging current density to 4 A g−1, the capacitance can still be maintained for 64%. It also showed that the long cycle life can be maintained at ∼88% capacitance retention after 4000 cycle tests at 1 A g−1. It is anticipated that this research line would lead to development of a promising new method to fabricate graphene-based monolith materials with 3D porous architectures for application in advanced materials.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis of graphene oxide (GO), AFM image of GO suspension, contrast experiment, elemental analysis, FESEM and HRTEM images of PRGHs, and cyclic voltammograms (CV). 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/Fax: +86 533 2781664. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (51302156 and 21402108) and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS (PCOM201402).
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