Carbon Dot-Mediated Synthesis of Manganese Oxide Decorated

Apr 25, 2016 - School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, No.100, Shih-Chuan first Road, Kaohsiung, 80708, Taiwan. ACS Sus...
10 downloads 18 Views 8MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Carbon Dot-Mediated Synthesis of Manganese Oxide Decorated Graphene Nanosheets for Supercapacitor Application Binesh Unnikrishnan,†,§ Chien-Wei Wu,‡,§ I-Wen Peter Chen,∥ Huan-Tsung Chang,‡ Chia-Hua Lin,Δ and Chih-Ching Huang*,†,○,⊥ †

Department of Bioscience and Biotechnology and ○Center of Excellence for the Oceans, National Taiwan Ocean University, No. 2, Bei-Ning Road, Zhongzheng District, Keelung, 20224, Taiwan ‡ Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei, 10617, Taiwan ∥ Department of Applied Science, National Taitung University, No. 369, Section 2, University Road, Taitung City 95002, Taiwan Δ Department of Biotechnology, National Formosa University, No. 64, Wunhua Road, Huwei Township, Yunlin County, 63208, Taiwan ⊥ School of Pharmacy, College of Pharmacy, Kaohsiung Medical University, No.100, Shih-Chuan first Road, Kaohsiung, 80708, Taiwan S Supporting Information *

ABSTRACT: In this work, we demonstrate that carbon dots (CDs) can be used as a dispersing agent for graphene as well as a reducing agent for KMnO4 for the synthesis of manganese oxide (MnOx)−graphene hybrid nanocomposites for supercapacitor applications. CDs obtained from the pyrolysis of ammonium citrate under dry heating possess excellent solubility in water due to their oxygen- and nitrogen-containing functional groups. In addition, the sp2-carbon-rich CDs exhibited strong interaction with graphene through π−π stacking for self-immobilizing on graphene in the preparation of water-soluble CD/graphene nanocomposites (CDGs). Interestingly, MnOx could be grown in situ on CDGs after reaction with KMnO4 in aqueous solution under a mild reaction temperature (75 °C). Under the mild reaction conditions, CDs undergo sacrificial oxidation for the formation of MnOx nanoparticles on graphene, whereas the graphene’s graphitic carbons are protected. The as-formed nanostructured MnOx on CDGs (MnOx−CDGs) was employed to fabricate flexible solid-state supercapacitor which exhibited good capacitance properties (specific capacitance ∼280 F g−1) with very high charge−discharge cyclic stability (>10 000 cycles) and good capacitance retention at 90° bending angle. Compared to other graphene-based nanocomposites, our one-pot synthesis route for MnOx−CDGs is relatively green, simple, rapid, and cost-effective and has a great potential for the synthesis of different metal oxide-decorated graphene nanocomposites for energy conversion and storage applications KEYWORDS: Manganese oxide, Graphene, Carbon dots, Nanocomposites, Sacrificial reaction, Supercapacitors

■

theoretical specific capacitance of 1370 F g−1.7 The specific capacitance of MnO2-based supercapacitor is highly dependent on the particle size, morphology, and crystal structure.8,9 However, as a supercapacitor material, MnO2 has critical problems associated with poor electrical conductivity and cyclic instability.10 In response to these shortcomings, MnO2 NPs incorporated with 2- or 1-dimensional nanomaterials have been fabricated. For example, nanostructured supporting materials such as graphene and carbon nanotubes, which have high porosity and electrical conducting properties, have been employed to hybridize MnO2 NPs.11−15 The high porosity of

INTRODUCTION Electrochemical capacitors (also called supercapacitors) will dominate the energy storage industry in the near future because of their high power density, fast charge−discharge rate, long life, and stability.1−3 However, one important challenge in using electrochemical capacitors, especially, electrical double layer capacitors (EDLC), is their low energy density. To an extent, the energy density can be improved by incorporating pseudocapacitive materials such as metal oxide (RuO2, MnO2, Fe2O3, V2O5, Co3O4, NiOx, etc.) nanoparticles (NPs).4−6 Among these metal oxide-based materials, manganese oxidebased materials have received great attention for their abundance, low cost, and nontoxicity.7 Manganese oxidebased materials are a great choice and widely studied nanomaterial for energy storage applications due to their © 2016 American Chemical Society

Received: December 14, 2015 Revised: April 15, 2016 Published: April 25, 2016 3008

DOI: 10.1021/acssuschemeng.5b01700 ACS Sustainable Chem. Eng. 2016, 4, 3008−3016

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Representation of One-Pot, Low Temperature Synthesis of MnOx−CDGs Nanocomposite Using CDs as a Sacrificial Reductant

In this work, we utilized the π electron character for decorating graphene with CDs to facilitate the dispersion of graphene in aqueous media. CDs were adsorbed on the graphene’s surface mainly through π−π interactions and produced a highly dispersed CD−graphene (CDG) composite solution (Scheme 1). Moreover, the CDs itself can be a sacrificial reductant for the reduction of MnO4− ions to MnOx NPs, thus protecting the structure of the graphene. The nanosized MnOx were deposited in situ on CDG to form MnOx−CDG nanocomposite. Thus, graphene nanosheets were employed as a carbon support for MnOx to achieve high electrical conductivity and mass loading. This straightforward synthetic route for preparation of MnOx−CDGs nanocomposites is very simple and cost-effective. Moreover, asprepared MnOx−CDGs nanocomposites exhibit superior performance in supercapacitor applications.

the supporting nanomaterials results in a high surface area for MnO2 deposition which enables high mass loading and rate capability. The porosity also provides easy diffusion of electrolyte reachable to maximum MnO2 NPs, thereby increasing the rate capability16,17 and specific capacitance.18,19 In addition, the high conductivity of the supporting material reduces the equivalent series resistances (ESR) of the capacitor.20,21 The electrolyte diffusion and electron transfer also can be increased by using MnO 2 nanotubes, as demonstrated by Li et al. using carbon supported double walled MnO2 nanotubes which highly enhance the rate capability of the supercapacitor.22 Although graphene, which has scaffolding nanostructures and good electrical conductivity, enables high mass loading of MnO2 and improves the energy density of supercapacitors,23,24 dispersion of graphene in an aqueous medium is still a challenge for graphene−MnO2 nanocomposite synthesis. Therefore, some studies have reported the use of graphene oxide (GO) to synthesize composites with MnO2 NPs.25 However, the capacitance of GO nanosheets as a nanostructured scaffold is limited due to its poor electrical conductivity. Thus, GO needs further treatment with reductants or light irradiation to obtain reduced graphene oxide (RGO) in order to form electronic conduction channels that can improve the electric conductivity of nanocomposites for supercapacitor applications.26,27 However, the RGO’s sp2hybridized carbon structures are prone to destruction during the formation of MnO2 NPs and deposition on RGO. Therefore, complicated modification of RGO for protection by surfactants or polymers and careful use of appropriate reducing agents during composite synthesis are indispensable.28 Carbon dots (CDs) have emerged as an attractive material due to their extremely small size (2−5 nm), ability to host diverse functional groups (e.g., hydroxyl, carboxylate, carbonyl) on their surfaces, and ultrastable photoluminescence properties.29 The carbon core of a CD mainly comprises CC bonds that possess delocalized electrons.30 Recently, different types of CDs and graphene quantum dots have been successfully applied for energy device applications such as Li and Na ion batteries, metal free, and metal oxide based supercapacitors, and fuel cells.31−34 CD networks in hybrid nanomaterials enhance the accessibility of the charged ions and thus facilitate the charge transport and ionic motion during charge−discharge processes in supercapacitors.35 Hybrid nanomaterials synthesized from nitrogen and sulfur codoped graphene quantum dots and reduced GO has excellent capacitance properties as well as catalytic properties for supercapacitor and fuel cell applications, respectively.36

■

EXPERIMENTAL SECTION

Materials. Ammonium citrate was purchased from Showa Chemical Co. Ltd. (Tokyo, Japan). KMnO4, Na2SO4, poly(vinyl alcohol) (PVA; Mw 80 000−100 000), H3PO4 (85%), N-methyl-2pyrrolidone (NMP), and polyvinylidene difluoride (PVDF; Mw ∼534 000 Da) were obtained from Sigma-Aldrich (Milwaukee, WI, USA). Graphene (surface area ∼400 m2 g−1; electrical conductivity 10−20 S cm−1) was obtained from Enerage Inc. (Arcadia, CA, USA). Carbon paper electrodes with a thickness of 0.1 mm were purchased from Beijing Jixing Sheng’an Industry & Trade Co., Ltd. (Beijing, China). Synthesis of CDGs. The CDs were synthesized by heating ammonium citrate (0.1 g) at 180 °C for 2 h in an oven, as demonstrated in our previous work, and dissolved in deionized (DI) water (10 mL).37 The obtained CD solution was centrifuged at a relative centrifugal force (RCF) of 30 000g to remove aggregated particles. Subsequently, the solution was dialyzed using a dialysis membrane (MWCO = 1.0 kDa; Float-A-Lyzer G2, Spectrum Laboratories, Rancho Dominguez, CA, USA) for 6 h in DI water. The DI water was replaced every 1 h. For the preparation of CDGs, graphene (5 mg) was added to purified CD solution (250 μg mL−1; 20 mL) and then sonicated for 2 h using a S4000, 1/8 in. mircotip probe, 20 kHz (Misonix, Newtown, CT, USA). The excess or unadsorbed CDs were removed by filtering through a filter paper with a pore size of 0.22 μm and a vacuum filtration system. The amount of CDs adsorbed on the graphene sheets was determined to be ∼0.3 mg CDs per mg graphene by measuring the fluorescence of the filtrate. Synthesis of MnOx−CDG nanocomposites. The as-obtained CDG solution (0.5 mg/mL; 20 mL) was mixed with different concentrations of KMnO4 (1−10 mM) in a 50 mL beaker and then heated at 75 °C for 2 h under continuous stirring. The appearance of effervescence due to the evolution of CO2 and the color change of the solution to black indicated the formation of MnOx. The as-synthesized MnOx−CDG nanocomposites was centrifuged, washed twice with DI water, and dried. Desired quantities of MnOx−CDG were dispersed in 3009

DOI: 10.1021/acssuschemeng.5b01700 ACS Sustainable Chem. Eng. 2016, 4, 3008−3016

Research Article

ACS Sustainable Chemistry & Engineering

with constant current and the specific capacitance (F g−1) was calculated using eq 1.38

DI water to make a film on carbon paper electrodes for electrochemical measurements. The same conditions were used to synthesize the MnOx−CD nanocomposite by the reaction of CDs with KMnO4 for characterization and to understand the reaction mechanism of the reduction of KMnO4 to MnOx. Characterization of the Nanocomposites. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the MnOx−CDG nanocomposites were taken using a Philips/FEI Tecnai 20 G2 S-Twin transmission electron microscope (Hillsboro, Oregon, USA) and a HR-FESEM Hitachi Field-Emission S-4800 (Schaumburg, IL, USA), respectively. The sample films were subjected to gold sputtering before SEM analysis. The fluorescence and UV−visible spectra of the CDs were recorded by a monochromatic microplate spectrophotometer (Synergy 4 MultiMode, Biotek Instruments, Winooski, VT, USA). X-ray photoelectron spectroscopy (XPS) was performed using an ES-CALAB 250 spectrometer (VG Scientific, East Grinstead, UK) with Al Kα X-ray radiation as the X-ray source for excitation. Binding energies were corrected using the C 1s peak at 284.6 eV as a standard. An Agilent Cary 640 FT-IR spectrometer (Santa Clara, CA, USA) was used to record FT-IR spectra of the materials. Thermogravimetric analysis data of the materials was recorded using a TGA instrument (Q500, TA Instrument, New Castle, DE, USA). The electrochemical characterizations of the materials were performed with a CHI 405B electrochemical workstation (Austin, TX, USA). The electrical conductivities of the materials were measured using a four-point probe system using the DC voltage sweep method. A probe diameter of 80 μm and a distance of 1.6 mm between two adjacent probes were used. Measurements were performed using a KeithLink probe station (New Taipei City, Taiwan) in order to provide current from −5 to 5 mA. The electrochemical impedance spectroscopy of the materials was conducted and the data were fitted for a two-electrode configuration using Na2SO4 (1.0 M) as electrolyte, by CHI 760D electrochemical work station and EIS Spectrum Analyzer (http://www.abc.chemistry. bsu.by/vi/analyser/), respectively. Fabrication of MnOx−CDG Supercapacitors. To study the electrochemical properties, diffusion of electrolyte, and cyclic stability of the MnOx−CDG nanocomposites, a two-electrode supercapacitor system using 1.0 M Na2SO4 as electrolyte was prepared. First, a carbon paper electrode (1.2 × 1.0 cm2) was pasted onto a polyethylene terephthalate (PET) film (1.3 × 1.2 cm2) and the MnOx−CDG nanocomposite dispersion (2 mg/mL; 0.5 mL) was applied to an area of 1.0 × 1.0 cm2 of the carbon electrode. This was then vacuum-dried to obtain a uniform thin film. The two electrodes of the supercapacitor were separated by an Advantec 5C quantitative ashless filter paper. The electrolyte (1.0 M Na2SO4, 300 μL) was injected between the electrodes using a microsyringe. Fabrication of Flexible Solid-State Supercapacitor. To improve the capacitive retention of MnOx−CDG composite at higher number of charge−discharge cycles, we fabricated a solid-state supercapacitor using NMP-PVDF as binder and H3PO4/PVA gel as electrolyte. To prepare H3PO4/PVA gel electrolyte, 10 g of PVA was added to 20 mL of DI water in a beaker and the mixture was heated at 60 °C under continuous stirring for 2 h until a clear solution of PVA was obtained. The obtained PVA solution was cooled to room temperature and 10 g of H3PO4 was added drop by drop with continuous stirring for 1 h to obtain a homogeneous H3PO4/PVA gel. To prepare flexible solid-state supercapacitor, MnOx−CDG nanocomposites dispersion (2 mg/mL containing 5% NMP−PVDF; 0.525 mL) was applied (active materials 1 mg/cm2) on carbon paper electrodes having area 2.0 × 1.0 cm2 and vacuum-dried to obtain a uniform thin film. Then, H3PO4/PVA gel was applied on a MnOx− CDGs film modified carbon electrode and another carbon electrode modified with MnOx−CDGs was placed on it and pressed. The electrochemical measurements (cyclic voltammetry and charge− discharge experiments) were conducted using a CHI 405B electrochemical workstation and LANDT Instrument CT2001. The cyclic voltammograms were recorded in the voltage range of 0 to 1 V. The same potential difference was used for charge−discharge experiments

Csp = 4C /m

(1)

Where, C (F) is the capacitance measured for the two-electrode cell and m (g) is the total mass of the capacitive material. The E and P values were calculated according to eqs 2 and 3: E = 0.5C(ΔV )2 /3.6

(2) (3)

P = E /Δt −1

Where, E is the energy density (W h kg ), P is power density (W kg−1), C is the total measured capacitance (F), ΔV is the voltage difference (V), and Δt is the total discharge time (h).

■

RESULTS AND DISCUSSION CD-Assisted Dispersion of Graphene Nanosheets. The CDs were synthesized by the carbonization of ammonium citrate through a simple dry heating (180 °C, 2 h) method. The as-synthesized CDs with a diameter of 2−5 nm (Figure 1Aa) were readily dissolved in water. The CDs comprised doped nitrogen and oxygen and possessed many functional groups (e.g., carbonyl, carboxyl) on their surfaces (Figure S1A, Supporting Information), as well as C−N and CN bonds.39 Reports suggest that nitrogen doping in carbon

Figure 1. (A) TEM images of (a) CDs and (b) CDG nanocomposite. (inset) HRTEM of (a) CDs and (B) MnOx−CDGs. (B) Photographic images of (a) graphene, (b) CDs, (c) CDGs, and (d) MnOx−CDGs dispersion in water. (C) Fluorescence spectra of CDs (250 μg mL−1) in the (a) absence and (b) presence of graphene (500 μg mL−1) in sodium phosphate buffer (10 mM, pH 6) solution. 3010

DOI: 10.1021/acssuschemeng.5b01700 ACS Sustainable Chem. Eng. 2016, 4, 3008−3016

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. (A) SEM, (B) TEM, and (C) HRTEM images of as-prepared MnOx−CDG nanocomposites.

prepared MnOx−CDG, in which MnOx appears as dense nanoparticles on the graphene nanosheets. The HRTEM image of MnOx−CDG (Figure 2C) reveals the lattice fringes of the MnOx with a lattice spacing of 0.21 and 0.26 nm corresponding to the d-spacing of (202) and (301) crystal planes, respectively.44,45 However, the XRD analysis of MnOx−CDG and MnOx−CD samples did not show any significant peaks (data not shown) for MnOx indicating that the MnOx obtained by the CD mediated reduction of KMnO4 was predominantly amorphous. EDS mapping (Figure S3, Supporting Information) showed that the elements Mn and O homogeneously coexisted on the graphene nanosheets. Moreover, the Mn−O bending and stretching peaks at 490 and 760 cm−1,46 respectively, were observed only in the FT-IR spectra of MnOx−CDG and not in that of the CDG further confirmed the formation of MnOx on the graphene nanosheets (Figure S4, Supporting Information). The thermogravimetric analysis of CDG and MnOx−CDG (Figure S5, Supporting Information) suggests a mass composition of MnOx of approximately 28% in the MnOx− CDG composite. In comparison with the FT-IR spectra of CQDs and CDG, MnOx−CDG exhibited enhanced peaks at 1070 and 1635 cm−1 corresponding to C−O−C and C−O bondings, respectively, (Figure S4D in the Supporting Information), which suggested that the defective sites of CDs on the graphene nanosheets were readily oxidized by KMnO4. CDs exhibited broad D (∼1350 cm−1) and G (∼1600 cm−1) bands in the Raman spectra at the same wavenumber as that of graphene oxide (Figure S6, Supporting Information), suggesting the presence of graphitic carbons (sp2 hybridized) in the core, as well as significant defects in the CDs. The full width at half-maximum (fwhm) of the D and G bands of CDG decreased slightly after reaction with KMnO4, which indicated the KMnO4 indeed oxidized the CDs on the graphene rather than the graphene itself. The Raman peak at 635 cm−1 exhibited by the asprepared MnOx−CDG nanocomposite was due to the Mn−O stretching vibration, which indicated the formation of MnOx.46 The O 1s XPS spectra of MnOx−CDG (Figure 3A) shows Mn−O and Mn−O−C bonding peaks at binding energies of 529.6 and 530 eV, respectively, further confirmed the formation of MnOx on the graphene surfaces. The peaks at 531 eV indicated the presence of some hydrated manganese oxides in addition to CO bonding originated from CQDs. A peak appearing at relatively higher binding energy, 532.4 eV, was originated from C−OH and C−O−C groups of CQDs in the composite. Moreover, the deconvolution of Mn 2p XPS spectrum (Figure 3B) of the MnOx−CDG nanocomposites revealed that the spin−orbit doublet of Mn 2p3/2 centered at

materials enhances their capacitance properties and increases the electrical conductivity.40,41 The HRTEM image of CDs (the inset of Figure 1Aa) reveals the graphitic interlayer spacing of 0.32 nm, which is in good agreement with the X-ray diffraction (XRD) pattern (2θ = 22.8°; (002) plane) as displayed in Figure S1B (Supporting Information). The CDs showed an absorption band at 240 nm ascribed to the π−π* transition of CC bonds, whereas a shoulder band at 340 nm was attributed to the n−π* transition of CO bonds (Figure S2A, Supporting Information).39 The excitation dependent emission of the CDs (Figure. S2B, Supporting Information) and the broad emission peak are probably due to the different size distribution and/or the heterogeneity of their chemical compositions.42 In this work, the CDs were used without any surface passivation, which retained several surface functional groups and their surface emissive traps result in the low fluorescence quantum yield, ca. 10% (at excitation/emission maxima of 365 and 440 nm; in comparison with quinine reference).39 However, the low QY obtained for the CDs in this work is immaterial, because our primary interests are the π-electron character of the carbon core for interaction with graphene and the polar functional groups for enhancing dispersibilty in aqueous solution. Modification of the synthetic method and post-treatment of the CDs could improve the QY for fluorescence applications. The dispersion of graphene (1−2 μm; Figure 1Ba) in water is quite low (10 nm) are formed (Figure S10B−D, Supporting Information), which resulted in a decrease in the specific capacitance of MnOx−CDGs. The measured conductance for the composite synthesized from 1.0, 5.0, and 10 mM KMnO4 were 8.51, 6.90, and 3.20 S cm−1, respectively. Therefore, the decreasing performance of the composite at higher mass loading of MnOx could be also due to the agglomeration of the nanocomposites that resulted in poor conductivity. Figure S11A (Supporting Information) shows that the specific capacitance almost remains stable upon increasing the mass loading of MnOx−CDG from 0.48 to 1.5 mg cm−2 on the electrode surface. We noted that the areal capacitance of the system was increased with the increase in mass loading (Figure S11B), implying good electrolyte diffusion and rapid electron transfer. The lateral view of the MnOx−CDG film is displayed in Figure S11C. The folded graphene structures provide plenty of macroporous channels and contact among adjacent graphene sheets, which are highly favorable for electrolyte diffusion and electrical conductivity, respectively. The MnOx−CDGs exhibited high charge−discharge cyclic stability, retaining 94% specific capacitance of the initial value after 5000 cycles. Solid-State Supercapacitor Application of MnOx−CDG Nanocomposites. For practical applications of capacitive nanomaterials, charge−discharge cyclic stability is important. 3014

DOI: 10.1021/acssuschemeng.5b01700 ACS Sustainable Chem. Eng. 2016, 4, 3008−3016

ACS Sustainable Chemistry & Engineering current discharge curves and the Ragone plot was constructed (Figure S13, Supporting Information). It can be seen that the energy density of the supercapacitor decreased gradually with the increase in power density. At a power density of 15.63 kW kg−1 the supercapacitor exhibited an energy density of 19.53 W h kg−1. The low rate capability of the MnOx−CDGs could be due the poor diffusion rate of electrolyte at higher current densities. Further modification of the synthesis procedure such as heat treatment or modification of CDs and gel electrolyte could improve the porosity, charge transport and ionic motion during charge−discharge process, which ultimately improve rate capability. The stable working potential range of the solidstate supercapacitor was 1.0 V which was not sufficient to demonstrate its practical application. Therefore, we connected two devices in series which showed good capacitive properties in the potential range from 0 to 2.0 V, as can be seen from Figure 5E. Figure 5F shows the working of an LED powered by two devices connected in series.

ACKNOWLEDGMENTS

■

REFERENCES

(1) Miller, J. R.; Simon, P. Electrochemical Capacitors for Energy Management. Science 2008, 321, 651−652. (2) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326−1330. (3) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities. Adv. Energy Mater. 2014, 4, 1300816−1300859. (4) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797− 828. (5) Li, F.; Ran, J.; Jaroniec, M.; Qiao, S. Z. Solution Combustion Synthesis of Metal Oxide Nanomaterials for Energy Storage and Conversion. Nanoscale 2015, 7, 17590−17610. (6) Augustyn, V.; Simon, P.; Dunn, B. Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energy Environ. Sci. 2014, 7, 1597−1614. (7) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Manganese Oxide-Based Materials as Electrochemical Supercapacitor Electrodes. Chem. Soc. Rev. 2011, 40, 1697−1721. (8) Chen, W.; Rakhi, R. B.; Wang, Q.; Hedhili, M. N.; Alshareef, H. N. Morphological and Electrochemical Cycling Effects in MnO2 Nanostructures by 3D Electron Tomography. Adv. Funct. Mater. 2014, 24, 3130−3143. (9) Ghodbane, O.; Pascal, J. L.; Favier, F. Microstructural Effects on Charge-Storage Properties in MnO2-Based Electrochemical Supercapacitors. ACS Appl. Mater. Interfaces 2009, 1, 1130−1139. (10) Hou, Y.; Cheng, Y.; Hobson, T.; Liu, J. Design and Synthesis of Hierarchical MnO2 Nanospheres/Carbon Nanotubes/Conducting Polymer Ternary Composite for High Performance Electrochemical Electrodes. Nano Lett. 2010, 10, 2727−2733. (11) Peng, L.; Peng, X.; Liu, B.; Wu, C.; Xie, Y.; Yu, G. Ultrathin Two-Dimensional MnO2/Graphene Hybrid Nanostructures for HighPerformance, Flexible Planar Supercapacitors. Nano Lett. 2013, 13, 2151−2157. (12) Fan, Z.; Yan, J.; Wei, T.; Zhi, L.; Ning, G.; Li, T.; Wei, F. Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density. Adv. Funct. Mater. 2011, 21, 2366−2375. (13) Huang, Y.; Li, Y.; Hu, Z.; Wei, G.; Guo, J.; Liu, J. A Carbon Modified MnO2 Nanosheet Array as a Stable High-Capacitance Supercapacitor Electrode. J. Mater. Chem. A 2013, 1, 9809−9813. (14) Li, L.; Hu, Z. A.; An, N.; Yang, Y. Y.; Li, Z. M.; Wu, H. Y. Facile Synthesis of MnO2/CNTs Composite for Supercapacitor Electrodes with Long Cycle Stability. J. Phys. Chem. C 2014, 118, 22865−22872. (15) Zhu, J.; He, J. Facile Synthesis of Graphene-Wrapped Honeycomb MnO2 Nanospheres and Their Application in Supercapacitors. ACS Appl. Mater. Interfaces 2012, 4, 1770−1776. (16) Gao, H.; Xiao, F.; Ching, C.-B.; Duan, H. High-Performance Asymmetric Supercapacitor Based on Graphene Hydrogel and Nanostructured MnO2. ACS Appl. Mater. Interfaces 2012, 4, 2801− 2810. (17) Jin, Y.; Chen, H.; Chen, M.; Liu, N.; Li, Q. Graphene-patched CNT/MnO2 Nanocomposite Papers for The Electrode of HighPerformance Flexible Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2013, 5, 3408−3416.

CONCLUSIONS We demonstrated a green approach to disperse graphene in aqueous medium using CDs, and its application for MnO2− CDGs hybrid nanocomposites synthesis. The CDs used in this study effectively reduced MnO4− ions to nanostructured MnOx on the graphene surface at a low temperature of 75 °C. The materials used in this work are green and cheap. KMnO4 readily oxidized the carbon bonds in the CDs at low temperature, thereby protecting the graphitic carbons in the graphene. This protection preserved the characteristic properties of the graphene, such as its good electrical conductivity, making it highly useful in many applications. The controlled loading of MnOx on the graphene surface improved electrical contacts between the adjacent graphene sheets and provided an efficient path for electron flow from MnOx. The NMP−PVDF binder and H3PO4/PVA gel electrolyte improved the stability of MnOx−CDG (high retention capacity of 94.7% after 10 000 cycles) as well as flexibility of the solid-state supercapacitor device. The power density and energy density can be improved by higher mass loading of MnOx−CDG by using porous electrodes. We believe that our work demonstrated a simple method to deposit MnOx on graphene. This method may be highly feasible for in situ deposition of nanostructured MnOx on three-dimensional carbon materials for various applications. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01700. XRD, XPS, UV−visible, fluorescence, FT-IR, Raman, and EIS spectroscopy results; elemental scanning TEM image, TGA, SEM, cyclic voltammograms, Ragone plot, and Table S1 (PDF)

■

■

This study was supported by the Ministry of Science and Technology of Taiwan under the contracts 104-2628-M-019001-MY3, 104-2622-M-019-001-CC2, and 102-2113-M-019001-MY3. We are grateful for the assistance of Ms. Ya-Yun Yang and Ms. Ching-Yen Lin from the Instrument Center of National Taiwan University (NTU) for TEM and SEM measurements.

■

■

Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: 011-886-2-2462-2192 ext 5517. Fax: 011-886-2-24622320. E-mail: [email protected]. Author Contributions §

B.U. and C.-W.W. contributed equally to this work.

Notes

The authors declare no competing financial interest. 3015

DOI: 10.1021/acssuschemeng.5b01700 ACS Sustainable Chem. Eng. 2016, 4, 3008−3016

Research Article

ACS Sustainable Chemistry & Engineering

Biofunctional Carbon Quantum Dots for Bacterial Labeling. Biosens. Bioelectron. 2015, 68, 1−6. (38) Stoller, M. D.; Ruoff, R. S. Best Practice Methods for Determining an Electrode Material’s Performance for Ultracapacitors. Energy Environ. Sci. 2010, 3, 1294−1301. (39) Yang, Z.; Xu, M.; Liu, Y.; He, F.; Gao, F.; Su, Y.; Wei, H.; Zhang, Y. Nitrogen-Doped, Carbon-Rich, Highly Photoluminescent Carbon Dots from Ammonium Citrate. Nanoscale 2014, 6, 1890− 1895. (40) Hulicova-Jurcakova, D.; Kodama, M.; Shiraishi, S.; Hatori, H.; Zhu, Z. H.; Lu, G. Q. Nitrogen-Enriched Nonporous Carbon Electrodes with Extraordinary Supercapacitance. Adv. Funct. Mater. 2009, 19, 1800−1809. (41) Zhou, J.; Lian, J.; Hou, L.; Zhang, J.; Gou, H.; Xia, M.; Zhao, Y.; Strobel, T. A.; Tao, L.; Gao, F. Ultrahigh Volumetric Capacitance and Cyclic Stability of Fluorine and Nitrogen Co-Doped Carbon Microspheres. Nat. Commun. 2015, 6, 8503. (42) Sun, Y.-P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; Luo, P. G.; Yang, H.; Kose, M. E.; Chen, B.; Veca, L. M.; Xie, S.-Y. QuantumSized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756−7757. (43) He, P.; Sun, J.; Tian, S.; Yang, S.; Ding, S.; Ding, G.; Xie, X.; Jiang, M. Processable Aqueous Dispersions of Graphene Stabilized by Graphene Quantum Dots. Chem. Mater. 2015, 27, 218−226. (44) Zhai, T.; Wang, F.; Yu, M.; Xie, S.; Liang, C.; Li, C.; Xiao, F.; Tang, R.; Wu, Q.; Lu, X.; Tong, Y. 3D MnO2−graphene Composites with Large Areal Capacitance for High-Performance Asymmetric Supercapacitors. Nanoscale 2013, 5, 6790−6796. (45) Dubal, D. P.; Dhawale, D. S.; Salunkhe, R. R.; Lokhande, C. D. Conversion of Chemically Prepared Interlocked Cubelike Mn3O4 to Birnessite MnO2 Using Electrochemical Cycling. J. Electrochem. Soc. 2010, 157, A812−A817. (46) Hsu, Y.-K.; Chen, Y.-C.; Lin, Y.-G.; Chen, L.-C.; Chen, K.-H. Reversible Phase Transformation of MnO2 Nanosheets in an Electrochemical Capacitor Investigated by in situ Raman Spectroscopy. Chem. Commun. 2011, 47, 1252−1254. (47) Yang, S.; Song, X.; Zhang, P.; Gao, L. Crumpled Nitrogendoped Graphene−Ultrafine Mn3O4 Nanohybrids and their Application in Supercapacitors. J. Mater. Chem. A 2013, 1, 14162−14169. (48) Zhu, S.; Zhang, H.; Chen, P.; Nie, L.-H.; Li, C.-H.; Li, S.-K. Selfassembled Three-Dimensional Hierarchical Graphene Hybrid Hydrogels with Ultrathin β-MnO2 Nanobelts for High Performance Supercapacitors. J. Mater. Chem. A 2015, 3, 1540−1548. (49) Dash, S.; Patel, S.; Mishra, B. K. Oxidation by Permanganate: Synthetic and Mechanistic Aspects. Tetrahedron 2009, 65, 707−739. (50) Jin, X.; Zhou, W.; Zhang, S.; Chen, G. Z. Nanoscale Microelectrochemical Cells on Carbon Nanotubes. Small 2007, 3, 1513−1517. (51) Bakardjieva, S.; Bezdicka, P.; Grygar, T.; Vorm, P. Reductive Dissolution of Microparticulate Manganese Oxides. J. Solid State Electrochem. 2000, 4, 306−313. (52) Han, Z. H.; Seo, D. H.; Yick, S.; Chen, J. H.; Ostrikov, K. K. MnOx/Carbon Nanotube/Reduced Graphene Oxide Nanohybrids as High-Performance Supercapacitor Electrodes. NPG Asia Mater. 2014, 6, e140. (53) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426−430.

(18) Qian, L.; Lu, L. Fabrication of Three-Dimensional Porous Graphene−Manganese Dioxide Composites as Electrode Materials for Supercapacitors. Colloids Surf., A 2015, 465, 32−38. (19) Wang, G.; Xu, H.; Lu, L.; Zhao, H. One-Step Synthesis of Mesoporous MnO2/Carbon Sphere Composites for Asymmetric Electrochemical Capacitors. J. Mater. Chem. A 2015, 3, 1127−1132. (20) Yu, G.; Hu, L.; Liu, N.; Wang, H.; Vosgueritchian, M.; Yang, Y.; Cui, Y.; Bao, Z. Enhancing the Supercapacitor Performance of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping. Nano Lett. 2011, 11, 4438−4442. (21) Ma, S.-B.; Ahn, K.-Y.; Lee, E.-S.; Oh, K.-H.; Kim, K.-B. Synthesis and Characterization of Manganese Dioxide Spontaneously Coated on Carbon Nanotubes. Carbon 2007, 45, 375−382. (22) Li, Q.; Lu, X. F.; Xu, H.; Tong, Y. X.; Li, G. R. Carbon/MnO2 Double-Walled Nanotube Arrays with Fast Ion and Electron Transmission for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2014, 6, 2726−2733. (23) Huang, Y.; Liang, J.; Chen, Y. An Overview of the Applications of Graphene-Based Materials in Supercapacitors. Small 2012, 8, 1805− 1834. (24) Yang, S.; Song, X.; Zhang, P.; Gao, L. A MnOOH/Nitrogendoped Graphene Hybrid Nanowires Sandwich Film for Flexible AllSolid-State Supercapacitors. J. Mater. Chem. A 2015, 3, 6136−6145. (25) Park, K.-W. Carboxylated Graphene Oxide−Mn2O3 Nanorod Composites for Their Electrochemical Characteristics. J. Mater. Chem. A 2014, 2, 4292−4298. (26) Wei, B.; Wang, L.; Miao, Q.; Yuan, Y.; Dong, P.; Vajtai, R.; Fei, W. Fabrication of Manganese Oxide/Three-Dimensional Reduced Graphene Oxide Composites as the Supercapacitors by a Reverse Microemulsion Method. Carbon 2015, 85, 249−260. (27) Li, Y.; Wang, G.; Ye, K.; Cheng, K.; Pan, Y.; Yan, P.; Yin, J.; Cao, D. Facile Preparation of Three-Dimensional Multilayer Porous MnO2/ Reduced Graphene Oxide Composite and its Supercapacitive Performance. J. Power Sources 2014, 271, 582−588. (28) Lee, S.-W.; Bak, S.-M.; Lee, C.-W.; Jaye, C.; Fischer, D. A.; Kim, B.-K.; Yang, X.-Q.; Nam, K.-W.; Kim, K.-B. Structural Changes in Reduced Graphene Oxide upon MnO2 Deposition by the Redox Reaction between Carbon and Permanganate Ions. J. Phys. Chem. C 2014, 118, 2834−2843. (29) Lim, S. Y.; Shen, W.; Gao, Z. Carbon Quantum Dots and their Applications. Chem. Soc. Rev. 2015, 44, 362−381. (30) Dong, Y.; Shao, J.; Chen, C.; Li, H.; Wang, R.; Chi, Y.; Lin, X.; Chen, G. Blue Luminescent Graphene Quantum Dots and Graphene Oxide Prepared by Tuning the Carbonization Degree of Citric Acid. Carbon 2012, 50, 4738−4743. (31) Chao, D.; Zhu, C.; Xia, X.; Liu, J.; Zhang, X.; Wang, J.; Liang, P.; Lin, J.; Zhang, H.; Shen, Z. X.; Fan, H. J. Graphene Quantum Dots Coated VO2 Arrays for Highly Durable Electrodes for Li and Na Ion Batteries. Nano Lett. 2015, 15, 565−573. (32) Mondal, S.; Rana, U.; Malik, S. Graphene Quantum Dot-Doped Polyaniline Nanofiber as High Performance Supercapacitor Electrode Materials. Chem. Commun. 2015, 51, 12365−12368. (33) Zhu, Y.; Wu, Z.; Jing, M.; Hou, H.; Yang, Y.; Zhang, Y.; Yang, X.; Song, W.; Jia, X.; Ji, X. Porous NiCo2O4 Spheres Tuned through Carbon Quantum Dots Utilised as Advanced Materials for an Asymmetric Supercapacitor. J. Mater. Chem. A 2015, 3, 866−877. (34) Liu, W.-W.; Feng, Y.-Q; Yan, X.-B.; Chen, J.-T.; Xue, Q.-X. Superior Micro-Supercapacitors Based on Graphene Quantum Dots. Adv. Funct. Mater. 2013, 23, 4111−4122. (35) Zhu, Y.; Ji, X.; Pan, C.; Sun, Q.; Song, W.; Fang, W.; Chen, Q.; Banks, C. E. A Carbon Quantum Dot Decorated RuO2 Network: Outstanding Supercapacitances under Ultrafast Charge and Discharge. Energy Environ. Sci. 2013, 6, 3665−3675. (36) Samantara, A. K.; Sahu, S. C.; Ghosh, A.; Jena, B. K. Sandwiched Graphene with Nitrogen, Sulphur Co-doped CQDs: An Efficient Metal-Free Material for Energy Storage and Conversion Applications. J. Mater. Chem. A 2015, 3, 16961−16970. (37) Weng, C.-I.; Chang, H.-T.; Lin, C.-H.; Shen, Y.-W.; Unnikrishnan, B.; Li, Y.-J.; Huang, C.-C. One-Step Synthesis of 3016

DOI: 10.1021/acssuschemeng.5b01700 ACS Sustainable Chem. Eng. 2016, 4, 3008−3016