Structural Changes in Reduced Graphene Oxide upon MnO2

Article pubs.acs.org/JPCC

Structural Changes in Reduced Graphene Oxide upon MnO2 Deposition by the Redox Reaction between Carbon and Permanganate Ions Suk-Woo Lee,†,⊥ Seong-Min Bak,†,‡,⊥ Chang-Wook Lee,† Cherno Jaye,§ Daniel A. Fischer,§ Bae-Kyun Kim,∥ Xiao-Qing Yang,‡ Kyung-Wan Nam,*,‡ and Kwang-Bum Kim*,† †

Department of Materials Science and Engineering, Yonsei University, Shinchon Dong, Seodaemun Gu, Seoul 120-749, Korea Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States § Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ∥ Central R&D Institute, Samsung Electro-Mechanics Co., Ltd., Suwon, Gyunggi Do 443-743, Korea ‡

S Supporting Information *

ABSTRACT: We explore structural changes of the carbon in MnO2/reduced graphene oxide (RGO) hybrid materials prepared by the direct redox reaction between carbon and permanganate ions (MnO4−) to reach better understanding for the effects of carbon corrosion on carbon loss and its bonding nature during the hybrid material synthesis. In particular, we carried out near-edge X-ray absorption fine structure spectroscopy at the C K-edge (284.2 eV) to show the changes in the electronic structure of RGO. Significantly, the redox reaction between carbon and MnO4− causes both quantitative carbon loss and electronic structural changes upon MnO2 deposition. Such disruptions of carbon bonding have a detrimental effect on the initial electrical properties of the RGO and thus lead to a significant decrease in electrical conductivity. Electrochemical measurements of the MnO2/reduced graphene oxide hybrid materials using a cavity microelectrode revealed unfavorable electrochemical properties that were mainly due to the poor electrical conductivity of the hybrid materials. The results of this study should serve as a useful guide to rationally approaching the syntheses of metal/RGO and metal oxide/RGO hybrid materials.

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energy storage,12,13 catalysis,14 and water treatment15 because of its low cost, environmental friendliness, and natural abundance. Because MnO2 suffers from low electrical conductivity,16 MnO2/RGO hybrid materials have been extensively investigated to improve the electrical properties of MnO2 through its combination with RGO.17,18 Among the various synthetic approaches to MnO2/RGO hybrid materials, which include physical mixing,19 thermal decomposition,20 electrodeposition,21 sonochemical synthesis,22 sol−gel methods,23 and redox deposition (or electroless deposition),24−26 redox deposition has been the most widely used because it offers a straightforward processing protocol (impregnating the carbon substrate into an oxidant solution) and facile control of the nanoscale MnO2 deposits on carbon surface by tuning the solution chemistry (e.g., the pH conditions and concentration of the reactant). However, it should be noted that this method has an inherent risk of damaging the carbon bonding structure, which can disturb the electrical properties of RGO.

INTRODUCTION

Graphene has drawn tremendous research attention in many fields because of its unique physical and chemical properties associated with the single-atomic-layer sp2 carbon network.1 One mainstream approach to graphene-related research is the use of graphene or modified graphene such as reduced graphene oxide (RGO) as a conductive template that hosts multifunctional nanomaterials for various applications.2−4 Metal/RGO or metal oxide/RGO hybrid materials have been widely explored for diverse applications including energy storage devices,5 sensors,6 electrocatalysts,7 and solar cells.8 Improvements to the physical/chemical properties of metal/ RGO and metal oxide/RGO hybrid materials arise from the synergistic combination of metal or metal oxide and RGO.9−11 In these materials, the RGO provides electronic conduction channels to the metal or metal oxide in addition to serving as a substrate on which the metal or metal oxide nanoparticles can be distributed uniformly. Therefore, the original structural properties of RGO, which are closely related to electrical conductivity, must be preserved in the hybrid material during its synthesis. However, little has been reported on the structural changes of RGO during the synthesis of hybrid materials. MnO2 has been considered one of the most attractive materials for various applications such as electrochemical © 2014 American Chemical Society

Received: November 13, 2013 Revised: January 9, 2014 Published: January 9, 2014 2834

dx.doi.org/10.1021/jp411176b | J. Phys. Chem. C 2014, 118, 2834−2843

The Journal of Physical Chemistry C

Article

Synthesis of R-MnO2/RGO Hybrid Material. The method synthesizing R-MnO2/RGO was explained in detail elsewhere.32 In this study, we synthesized R-MnO2/RGO with various contents of MnO2 by controlling the volume of 0.1 M KMnO4 (99+%, Sigma-Aldrich) aqueous solution. As a typical example, the preparation of 75 R-MnO2/RGO is as follow: 15 mL of 0.1 M KMnO4 aqueous solution was added to an RGO suspension that was prepared by sonicating 43.5 mg of RGO in 200 mL of distilled water for 30 min, and the mixture was stirred until the purple color started to disappear. During the reaction, the solution temperature was kept at 73 °C. The product was washed repeatedly with distilled water and then dried at 80 °C for 24 h. Synthesis of S-MnO2/RGO Hybrid Material. For the preparation of the S-MnO2/RGO hybrid material, cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich) was used as a surfactant. First, 43.5 mg of RGO was dipped into 200 mL of 1 wt % CTAB aqueous solution, which was then ultrasonicated to obtain a uniformly dispersed RGO/CTAB suspension. Next, 15 mL of 0.1 M KMnO4 solution was added to the RGO/CTAB suspension and stirred for 1 h. Ethanol then was slowly added to the solution to reduce the MnO4− to MnO2, and the solution was allowed to stand until the purple color disappeared. After the obtained material was washed with ethanol, ammonium nitrate was added to remove the residual CTA+ through the ion exchange reaction between CTA+ and NH4+.33 The final product was washed repeatedly with distilled water and ethanol, and then dried at 80 °C for 24 h. Fourier transform infrared spectroscopy (FT-IR) was performed to confirm the removal of the residual surfactant (Figure S1 in the Supporting Information). Material Characterization. The X-ray diffraction (XRD, Rigaku, Cu Kα, 40 kV, 20 mA) patterns were taken in the 2θ range between 10° and 80° at a scan rate of 4° min−1 at room temperature. EA (2400 Series II, PerkinElmer) was carried out to measure the amount of carbon in the final products. EA measurements were attempted to quantify the oxygen content of RGO in the hybrid materials. However, we were unable to obtain reliable values due to interference from the oxygen in MnO2. To determine the MnO2 content in the hybrid materials, TGA data were collected on a thermal analysis instrument (TGA/DSC 1, Mettler Toledo) with a heating rate of 10 °C min−1 in an air flow rate of 50 mL min−1. Raman spectroscopy (T64000, Jobin-Yvon) was also carried out to evaluate the number of defects and imperfections in the RGO. The morphologies of the as-prepared samples were characterized by transmission electron microscopy (TEM, CM200, Philips). Carbon K-edge NEXAFS experiments were performed at the National Institute of Standards and Technology (NIST) beamline U7A located at the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory. A toroidal spherical grating monochromator with 600 lines mm−1 was used to acquire the C K-edge data, yielding an energy resolution of approximately 0.1 eV with entrance and exit slits of 30 μm each. The NEXAFS spectra were collected in partial electron yield (PEY) mode with a Channeltron electron multiplier detector with the entrance grid bias set to −150 V to enhance the surface sensitivity. The incident beam was set at the magic angle of 54.7° relative to the sample to eliminate the effects of preferential orientations. A carbon mesh was used for energy calibration of the C K-edge spectra using the π* transition of graphite at 285.1 eV. The samples were pelletized and attached on a sample bar with a Cu tape and mounted

Regarding the redox reaction between permanganate ions (MnO4−) and carbon, Chen et al. were the first to use it for generating MnO2 coatings on a planar graphite surface.27 Later, Huang et al. detected the CO2 gas and the CO32− and HCO3− ions from the reaction between MnO4− and acetylene black during the preparation of the MnO2/acetylene black hybrid mateiral.28 Additionally, Jin et al. proposed a MnO2 deposition mechanism based on nanoscale microelectrochemical cells (MECs) for the deposition of MnO2 on multiwalled carbon nanotubes (MWCNTs). According to their study, MnO2 deposition is likely to be initiated at the edge plane or other defect sites on the carbon wall, and then, because of the good electrical conductivity of the MWCNTs, it will propagate to other surface sites in a process that is coupled to the electron transfer through the nanotubes.29 Analysis of the redox reaction between MnO4− and carbon based on the MEC mechanism suggests carbon corrosion or carbon loss arising from edge plane or other defect sites during the MnO2/RGO synthesis. This carbon corrosion may lead to a disruption in the carbon bonding structure, which will be exacerbated as MnO2 deposition progresses (i.e., as the mass loading of MnO2 increases). Carbon loss and changes in the nature of the bonding from the original two-dimensional sp2 bonding in RGO are sure to cause a detrimental effect on its electrical properties. In this respect, it is highly probable that the redox reaction leading to MnO2 deposition onto RGO will produce a significant change in the carbon bonding structure, because RGO consists of a few layers of graphene with edge planes and defect-rich structural features.30 Although this is presumed to occur, the details of the structural damages to RGO that occur during the synthesis of MnO2/RGO hybrid materials by redox deposition have not been reported. Herein, carbon K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was employed to offer direct evidence on the structural changes in RGO that are associated with the redox reaction between carbon and MnO4− during the syntheses of MnO2/RGO hybrid materials. Cyclic voltammetry was also performed using a cavity microelectrode (CME) to reveal how the structural changes in RGO of the hybrid material affect its electrochemical properties. We aim to advance the understanding of the structural features of MnO2/RGO hybrid materials prepared by the redox reaction between carbon and MnO4−, especially with respect to how carbon corrosion causes carbon loss and structural changes and how they affect the chemical or physical properties of the hybrid materials.

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EXPERIMENTAL SECTION Preparation of RGO Nanosheets. Graphite oxide (GO) was synthesized from purified natural graphite powder (≤45 μm, Sigma-Aldrich) using a previously described, modified Hummers method.31 RGO nanosheets were produced using a solid-state microwave irradiation method. Briefly, 90 wt % GO powder was uniformly mixed with 10 wt % RGO powder using a ball-miller. The resulting GO/RGO mixture was placed into a quartz bottle, and then tightly sealed inside an Ar filled glovebox to prevent oxidation of the carbon. The GO/RGO mixture in the quartz bottle was then placed in a microwave oven (Mars 5, CEM) and exposed to microwaves at 1600 W in pulsed irradiation mode. Combustion elemental analysis (EA) revealed a C/O atomic ratio of 10.17 for the resulting RGO. 2835



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Scheme 1. Schematic Illustration of the Two Synthetic Methods for MnO2/RGO Hybrid Materialsa

a (a) The carbon-destructive method involves a direct redox reaction between carbon and MnO4− to yield R-MnO2/RGO hybrid materials. (b) The carbon-conservative method introduces a surfactant, and alcohol reduces MnO4− to MnO2 that yields S-MnO2/RGO hybrid materials upon deposition.

surfactant is introduced and alcohol is used as a reducing agent for the deposition of MnO2 from MnO4− ions, for which the hybrid materials obtained are denoted as S-MnO2/RGO. In the first approach, carbon reacts with MnO4− ions directly and is gradually consumed at the expense of MnO2 deposition (Figure 1a), as described by the following reaction:38

inside an ultrahigh vacuum chamber. The PEY signals were normalized using the incident beam intensity obtained from the photoemission yield of a clean Au grid to eliminate the effects of beam fluctuations and monochromator absorption features. All spectra were processed using standard pre- and postedge normalization methods, as described in previously published work.34 The pre-edge jump was subtracted to zero, followed by postedge normalization, which consisted of dividing the preedge normalized spectra by the edge jump intensity obtained far above the C K-edge, beyond 320 eV. Thus, the observed changes in relative spectral intensity arise from each sample and are independent of the total carbon content. The pre- and postedge normalization was performed using the Athena program.35 The electrical conductivity was measured using a two-point probe method with the DC voltage sweep method (VMP2, Bio-Logic), as described in detail in our previous study.36 Electrochemical Test. Cyclic voltammetry was carried out between 0 and 0.8 V (vs SCE) using a potentiostat/galvanostat (VMP3, Bio-Logic). A three-electrode system was used with Pt foil and a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. As the working electrode, we used a CME, which allows testing without any additives (conductive agent or binder), and that ensures the acquisition of electrochemical properties for only the active materials.37 The electrolyte was 1 M Na2SO4 dissolved in distilled water.

4MnO4− + 3C + H 2O → 4MnO2 + CO32 − + 2HCO3−

(1)

In this reaction, the carbon substrate serves as a sacrificial reductant that reduces the aqueous MnO4− ions to insoluble MnO2.

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RESULTS AND DISCUSSION In this study, two different synthetic methods were used to prepare MnO2/RGO hybrid materials, as shown in Scheme 1: (1) a carbon-destructive method involving redox deposition through the direct reaction between carbon and MnO4− ions, for which the hybrid materials obtained are denoted as RMnO2/RGO, and (2) a carbon-conservative method in which a

Figure 1. X-ray diffraction patterns of (a) RGO, (b) 75 R-MnO2/ RGO, (c) 75 S-MnO2/RGO, and (d) JCPDS no. 42-1317. 2836



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On the other hand, because alcohol is used as a reducing agent to convert MnO4− ions to MnO2,39 this approach is expected to protect the original conductive bonding structure of the RGO from the redox reaction during the synthesis of the MnO2/RGO hybrid material (Figure 1b). The proposed reduction mechanism for this approach is as follows:

Table 1. Elemental Analysis Results for RGO, R-MnO2/ RGO Hybrid Materials with Various MnO2 Contents, and 75 S-MnO2/RGO result (wt %) RGO 33 R-MnO2/RGO 50 R-MnO2/RGO 65 R-MnO2/RGO 75 R-MnO2/RGO 75 S-MnO2/RGO

4MnO4 − + 3C2H5OH → 4MnO2 + 3CH3COOH + 4OH− + H 2O

(2)

In this synthesis, the interaction between RGO and the hydrophobic segment of the surfactant (i.e., CTA+ ion) causes a steric repulsion that improves the dispersion of RGO in the aqueous solution. In addition, the surface charge of RGO becomes positive, because the positively charged headgroup of the CTA+ is oriented facing outward from the RGO, as revealed by the zeta potential measurement (Figure S2 in the Supporting Information). This positive surface charge of the RGO with CTA+ leads to an electrostatic interaction with MnO4− in the solution. Thus, the surfactant not only plays the role of a linker between the carbon and MnO4−, preventing a direct redox reaction between them, but also improves the dispersion of RGO in the solution. The molar feed ratio of KMnO4 to RGO was used to calculate the MnO2 content in the MnO2/RGO hybrid material. The numbers in the sample names used in this study indicate the loading wt % of MnO2 in the hybrid material as calculated from the molar feed ratio of KMnO4 to RGO (i.e., 33 R-MnO2/RGO is expected to contain 33 wt % MnO2). Figure 1 shows the XRD patterns of (a) RGO, (b) 75 RMnO2/RGO, (c) 75 S-MnO2/RGO, and (d) JCPDS card 421317 as a reference. The broad peaks observed at 2θ values of around 23° and 43° in Figure 1a correspond to the (002) and (100) planes of RGO, respectively. After MnO2 deposition, as shown in Figure 1b and c, four characteristic peaks at 2θ values of around 12°, 24°, 37°, and 66° are observed for 75 R-MnO2/ RGO and 75 S-MnO2/RGO, which can be indexed to birnessite-type MnO2.40 The broad peaks are attributed to a poorly crystallized structure originating from the small particles and the pseudoamorphous nature of the hybrid material. The broader and weaker diffraction peaks of 75 S-MnO2/RGO as compared to those of 75 R-MnO2/RGO reflect the more amorphous structure and poorer crystallinity of the 75 SMnO2/RGO, which is likely due to the low-temperature (25 °C) synthesis. Elemental analysis (EA) was performed to evaluate the amount of carbon contained in each sample. The analysis was conducted three times for each sample, and the average values are reported in Table 1. The precursor RGO for the synthesis of MnO2/RGO contained 84.13 wt % carbon. The relative amounts of carbon remaining in the 33 R-MnO2/RGO, 50 RMnO2/RGO, and 65 R-MnO2/RGO samples were estimated to be 50.83, 32.68, and 14.98 wt %, respectively. The amount of carbon contained in each sample decreased with an increasing KMnO4 feed ratio, and the amounts of carbon in the 75 RMnO2/RGO and 75 S-MnO2/RGO samples were 5.96 and 24.39 wt %, respectively. Although the same amounts of RGO and KMnO4 were used for synthesis of the 75 R-MnO2/RGO and 75 S-MnO2/RGO hybrid materials, the relative amounts of carbon that remain in the final products were quite different. Therefore, it is evident that carbon is oxidized and consumed during the synthesis of 75 R-MnO2/RGO. The trace of

a

carbon

hydrogen

nitrogen

sulfur

84.13 50.83 32.68 14.98 5.96 24.39

0.85 1.71 1.63 1.44 0.01 1.77

0.02 0.04 0.04 0.01 0.01 0.87

1.31 0.19 0.18 0.03 NDa 0.08

Not detected.

nitrogen detected in 75 S-MnO2/RGO might be due to residual NH4+ remaining after the ion exchange process. To determine the weight ratio of MnO2 in the R-MnO2/ RGO hybrid materials and 75 S-MnO2/RGO hybrid material, thermogravimetric analysis (TGA) was conducted in air. Figure 2a shows the obtained TGA curves of RGO and R-MnO2/ RGO hybrid materials with various MnO2 contents. The thermal decomposition of RGO in the temperature range from 200 to 500 °C led to the most weight loss for each sample, and the RGO showed particularly rapid weight loss around 500 °C.

Figure 2. Thermogravimetric analysis of (a) 33, 50, 65, and 75 RMnO2/RGO and RGO and of (b) 75 R-MnO2/RGO and 75 SMnO2/RGO obtained in air at a heating rate of 10 °C min−1. 2837



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The weight loss of 7−8% below 200 °C is attributed to the evaporation of the absorbed water in R-MnO2/RGO samples.41 On the basis of the weight loss of water in the samples, the weight ratios of MnO2 in 33 R-MnO2/RGO, 50 R-MnO2/ RGO, 65 R-MnO2/RGO, and 75 R-MnO2/RGO were determined to be 45, 59, 76, and 95 wt %, respectively. Because RGO is oxidized and consumed during the synthesis of R-MnO2/RGO, the measured weight ratios of MnO2 in the RMnO2/RGO hybrid materials are higher than those calculated from the molar feed ratio of KMnO4 to RGO. In fact, the difference between the calculated MnO2 content and measured MnO2 content tends to increase as more MnO2 is deposited onto RGO, because the carbon loss is accompanied by the redox deposition of MnO2. Figure 2b shows the TGA curves of 75 R-MnO2/RGO and 75 S-MnO2/RGO. On the basis of the 12 wt % of absorbed water, the actual weight ratio of MnO2 in 75 S-MnO2/RGO was estimated to be 72 wt %, which is close to the calculated value of 75 wt %. Because the 75 R-MnO2/ RGO and 75 S-MnO2/RGO hybrid materials were synthesized using the same amounts of RGO and KMnO4, the fact that the actual MnO2 weight ratio of 75 R-MnO2/RGO is larger than that in 75 S-MnO2/RGO confirms that carbon loss does occur during the direct redox reaction between MnO4− and RGO. Figure 3a shows the Raman spectra of RGO, 75 R-MnO2/ RGO, and 75 S-MnO2/RGO. In the 75 R-MnO2/RGO and 75 S-MnO2/RGO spectra, there are three major peaks at around 510, 565, and 630 cm−1.42 The Raman band at 630 cm−1 can be assigned to the symmetric stretching vibration (Mn−O) of the MnO6 groups. The bands located at around 510 and 565 cm−1 are usually attributed to the (Mn−O) stretching vibration in the basal plane of the MnO6 sheets.43 Raman spectroscopy also provides information on the structural properties of the carbonaceous materials, including the amounts of disorder, defects, and imperfections in the structures. All of the Raman spectra are dominated by the two well-understood G and D peaks, which are found in all graphene-like systems. It is known that the G band corresponds to the first-order scattering of the E2g phonon of the sp2 carbon domains in a two-dimensional hexagonal lattice, whereas the D band comes from the disorderinduced mode associated with structural defects and imperfections.44 Therefore, the intensity ratio I(D)/I(G) has been generally used as a standard to evaluate the quality of graphitic structures. To improve the accuracy in the determination of the intensity and the peak position, the D and G peaks were both fitted (Figure 3b−d). The D peak was fitted to a Lorentzian, and the G peak was fitted to a Breit− Wigner−Fano (BWF) equation because of its asymmetric line shape.45 The calculated I(D)/I(G) ratios of RGO, 75 R-MnO2/ RGO, and 75 S-MnO2/RGO were 0.80, 0.93, and 0.85, respectively. The fact that the I(D)/I(G) ratio of 75 R-MnO2/ RGO is larger than that of RGO indicates that a number of defects and imperfections are introduced and/or there is a decrease in the average size of the sp2 domains in the RGO during the synthesis of 75 R-MnO2/RGO. For 75 S-MnO2/ RGO, however, the I(D)/I(G) ratio was similar to that of RGO, which suggests that the original carbon structure is well preserved during the surfactant-mediated synthesis of the 75 SMnO2/RGO hybrid material. The different I(D)/I(G) ratios of the as-prepared samples indicate that the direct redox reaction between carbon and MnO4− generates defects and induces a decrease in the sp2 domain size through carbon loss. Additionally, the G peak position was shifted by 11 cm−1 (from 1596 to 1607 cm−1) after the redox deposition of MnO2

Figure 3. (a) Raman spectra of RGO (blue line), 75 R-MnO2/RGO (red line), and 75 S-MnO2/RGO (black line) and the D and G peak fitting for (b) RGO, (c) 75 R-MnO2/RGO, and (d) 75 S-MnO2/ RGO. The positions of the D and G peaks were determined by fitting to Lorentzian and BWF functions, respectively. The R2 values indicate the goodness of fit.

(compare Figure 3b and c), which is similar to the behavior observed for heavily oxidized graphite.46 This shift of the G peak is attributed to the presence of isolated double bonds separated by functional groups or defects in the carbon network of RGO in the 75 R-MnO2/RGO hybrid material.47 Figure 4 compares the TEM images of (a) 75 R-MnO2/RGO hybrid materials and (b) 75 S-MnO2/RGO hybrid materials. Interestingly, the difference between the 75 R-MnO2/RGO and 75 S-MnO2/RGO hybrid materials is easily discerned, which corresponds to the different thicknesses of the MnO2 layer 2838



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Figure 4. Transmission electron microscopy images of (a) 75 RMnO2/RGO and (b) 75 SMnO2/RGO (insets show magnified views).

deposited on the RGO surfaces. As shown in Figure 4a, the 75 R-MnO2/RGO hybrid material has a thicker MnO2 layer than that of the 75 S-MnO2/RGO hybrid material. According to the previous report by Jin et al., the nucleation of MnO2 is initiated at an edge plane or other defect site on the carbon wall of a MWCNT, which then propagates to other surface sites as the MnO2 layer grows. In their study, MnO2 deposits became thicker as the deposition progressed, and a thick layer of MnO2 could be obtained on the surface of the MWCNT. In the same manner, as shown in the TEM image of 75 R-MnO2/RGO, which had a quite high MnO4− feed ratio, the deposited MnO2 layer is thicker at the edge plane or other defect sites because of the strong redox reaction between carbon and MnO4−. However, for 75 S-MnO2/RGO, the introduction of the CTAB surfactant on the surface of RGO led to a thin deposited MnO2 layer by enhancing the dispersion of RGO in the aqueous solution and providing electrostatic interactions between CTA+ and MnO4−. Furthermore, the use of ethanol as a reducing agent to convert MnO4− to MnO2 minimized the carbon consumption, as confirmed by the EA in Table 1. To gain more insight into the structural changes that occur during the redox deposition of MnO2 onto the RGO surface, we performed C K-edge NEXAFS to obtain complementary information on the electronic and bonding structure of RGO. NEXAFS, which is based on the excitation of core electrons to empty or partially filled states, is a powerful technique for probing the electronic band structure of carbon-related materials.48 Figure 5a shows the acquired C K-edge NEXAFS spectra for R-MnO2/RGO hybrid materials with various MnO2 contents, the 75 S-MnO2/RGO hybrid material, and RGO. The two main peaks at around 285 and 292 eV for all of the samples are attributed to the transitions from C 1s to the unoccupied states of CC π* and C−C σ* character, respectively. The broadening of the 285 eV peak regions may be a consequence of the irregular, small-sized graphitic sp2 domain of RGO, which would be expected to permit more resonances near 285 eV.49 The spectra of the MnO2/RGO hybrid materials exhibited a weaker π* resonance and new peaks at 288 eV. In accordance with the literature, this absorption peak is assigned to unoccupied CO π* and C−O π* states that correspond to chemically functionalized carbon atoms and/or defect states.50,51 Because the birnessite-type MnO2 has a layered structure with potassium ions located between the MnO6 octahedral sheets, the absorption peaks for the potassium (K) L3 and L2 edges also appeared in the spectra of the MnO2/RGO hybrid materials at about 298 and 300 eV, respectively.

Figure 5. (a) C K-edge NEXAFS spectra acquired for R-MnO2/RGO hybrid materials with various MnO2 contents, 75 S-MnO2/RGO hybrid material, and RGO. (b) Normalized spectra for R-MnO2/RGO hybrid materials with various MnO2 contents and the RGO precursor. (c) Normalized spectra for 75 R-MnO2/RGO, 75 S-MnO2/RGO hybrid materials, and RGO precursor.

The pre- and postedge normalized NEXAFS spectra (Figure 5b) provide direct evidence of the structural changes in the RGO associated with the redox reaction between carbon and MnO4− during the synthesis of the R-MnO2/RGO hybrid materials. Upon deposition of MnO2 by the redox reaction, the intensities of the CC π* and C−C σ* resonances are substantially decreased, which indicates significant disruption of the carbon bonding structure in the RGO sheets. Meanwhile, the intensities of the CO π* and C−O π* resonances at 288 eV are increased, which may suggest that there is an oxidized carbon environment at the interface within the hybrid materials that withdraws a significant amount of charge from those carbons, and results in a localized electronic structure of RGO in the hybrid material after disruption of the carbon bonding during the redox deposition of MnO2. These features can be interpreted as indication of a strong Mn−O−C covalent bonding interaction between MnO2 and RGO in the R-MnO2/ RGO hybrid materials.52 Such covalent bonding probably causes the observed significant changes to RGO’s electrical properties.53 To clearly show the different structural changes in RGO that occur, for different synthesis methods, the normalized spectra of 75 R-MnO2/RGO and 75 S-MnO2/RGO are presented along with the RGO spectrum in Figure 5c. There is an obvious difference between the spectra of 75 S-MnO2/RGO and 75 RMnO2/RGO. The changes in the peak intensities of the CC π* and C−C σ* resonances of 75 S-MnO2/RGO are less than 2839



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and 24.39 wt % carbon, respectively. Therefore, it can easily be predicted that 75 R-MnO2/RGO will have unfavorable electrochemical properties due to the low electrical conductivity of MnO2 and its low conductive carbon content (i.e., low amount of RGO in the MnO2/RGO hybrid material). To distinguish the effects of the changes in the carbon bonding structure of the RGO from the effect of the carbon content in the MnO2/RGO electrodes on the electrochemical performance, we prepared one more CME from 75 R-MnO2/RGO with additional RGO as a conductive agent. We added a sufficient amount of the additional RGO conductive agent such that the amount of RGO in the sample was theoretically equivalent to that in the 75 S-MnO2/RGO hybrid material, based on the EA result in Table 1. The CME made from 75 R-MnO2/RGO (Figure 6a) exhibited highly distorted CVs at all scan rates, which indicates resistive behavior that may be due to the diminished amount of conductive carbon (i.e., ca. 5.96 wt % in the electrode material) and the resulting low electrical conductivity. When the amount of conductive carbon in the electrode was increased by the addition of RGO (ca. 24.39 wt % carbon in the electrode material), more rectangular CVs were obtained (Figure 6b), which indicates a decrease in resistivity and a resulting improvement in the capacitive performance. However, because of the relative decrease in active material content (i.e., MnO2 content in the MnO2/RGO hybrid material), the current intensity was still smaller than that obtained from a single 75 RMnO2/RGO hybrid material. This result may reveal why 5−20 wt % of conductive agents was required to obtain reasonable electrochemical performance of the MnO2/RGO electrode in a previous study on the redox reaction between carbon and MnO4−.57 For the 75 S-MnO2/RGO hybrid material (ca. 24.39 wt % carbon in the electrode material), rectangular CVs were obtained even at the high scan rate of 500 mV s−1 (Figure 6c). Although it contained the same amount of carbon, the 75 SMnO2/RGO electrode exhibited better capacitive behavior than the 75 R-MnO2/RGO electrode with additional conductive RGO. As was discussed with the NEXAFS results accompanying Figure 5, the difference in performance might be due to disruption of the carbon bonding structure that was induced by the direct redox reaction between carbon and MnO4− in the 75 R-MnO2/RGO sample. The charge retention ratios of each CME electrode obtained at various scan rates are shown in Figure 6d. Because the mass of the active material in the CME is very small, the charge is used to compare the retention ratio instead of the specific capacitance. At a 100 mV s−1 scan rate, the charge retention ratios of the 75 R-MnO2/RGO, 75 R-MnO2/RGO with additional RGO, and 75 S-MnO2/RGO electrodes were 61.2%, 82.8%, and 84.9%, respectively. For the 75 R-MnO2/ RGO electrode, the charge degenerated very seriously at high scan rates, whereas for the 75 S-MnO2/RGO electrode the charge retention ratio was still 80.2% at 200 mV s−1 and even 78.9% at 500 mV s−1. Additionally, these superior charge retention characteristics of 75 S-MnO2/RGO are better than those of the 75 R-MnO2/RGO with additional RGO (e.g., a charge retention ratio of 62.1% at 500 mV s−1). In the MnO2/ RGO hybrid material, RGO acts as the electronic conductive channel as well as the template for the deposition of MnO2, which reduces the internal resistance of the electrode and improves the electrochemical performance of MnO2. However, in the 75 R-MnO2/RGO sample, the RGO could not reduce the internal resistance of the electrode because of the carbon

those of 75 R-MnO2/RGO, because the direct redox reaction between MnO4− and carbon is effectively prevented by the surfactant and the use of alcohol as the reducing agent. Although we found these differences to be sufficiently clarified for this investigation of the structural degradation of RGO during the redox deposition of MnO2, the spectra of 75 SMnO2/RGO still have smaller intensities for the CC π* and C−C σ* peaks but larger intensities for the CO π* and C−O π* resonances at 288 eV. From the aforementioned results, we conclude that the redox reaction between MnO4− and RGO gives rise to not only quantitative carbon loss, as revealed by the EA and TGA results, but also to changes in the electronic structure of the remaining carbon after the redox deposition of MnO2 onto the RGO surface. Both of these changes in RGO induced by the direct redox reaction with MnO4− have a detrimental effect on the electrical properties of the hybrid materials. Therefore, efforts must be made to suppress such a strong redox reaction, or a different synthetic approach must be chosen. Table 2 shows the electrical conductivity for RGO, 75 RMnO2/RGO, 75 S-MnO2/RGO, and MnO2 powder. The Table 2. Electrical Conductivity for RGO, 75 R-MnO2/RGO, 75 S-MnO2/RGO, and MnO2 electrical conductivity (S m−1) RGO 75 R-MnO2/RGO 75 S-MnO2/RGO MnO2

7.41 × 102 54,a 2.65 × 10−3 2.97 × 10−1 10−3−10−455,a

a

Conductivity of RGO and MnO2 reported in refs 54 and 55, respectively.

electrical conductivities of 75 R-MnO2/RGO and 75 S-MnO2/ RGO were measured to be 2.65 × 10−3 and 2.97 × 10−1 S m−1, respectively. As MnO2 is deposited on the RGO, the electrical conductivity of MnO2/RGO composites is reduced regardless of the synthetic method. However, the electrical conductivity of 75 R-MnO2/RGO is much smaller than that of 75 S-MnO2/ RGO, because the conduction channels in 75 R-MnO2/RGO are damaged by the carbon consumption and disruption of the sp2 bonding structure. To demonstrate how the disruption of the carbon bonding structure in the MnO2/RGO hybrid material affects its electrochemical properties for supercapacitor applications, cyclic voltammograms (CVs) were obtained using a CME. Because the aim of the electrochemical study was to investigate how the carbon loss and changes in the carbon bonding structure affect the electrochemical properties of the hybrid material, the electrochemical test method was chosen to evaluate the intrinsic properties of MnO2/RGO itself. In this regard, testing with the CME technique in a three-electrode configuration is more relevant than testing in a two-electrode configuration, because it excludes conductive carbon additives or binders in the electrode preparation such that only the intrinsic electrochemical properties of an electrode material are exhibited.56 Figure 6 shows the CVs of (a) 75 R-MnO2/RGO, (b) 75 RMnO2/RGO with additional RGO as a conductive agent, and (c) 75 S-MnO2/RGO, as well as (d) the charge retention ratio of each CME obtained at various scan rates in 1 M Na2SO4 aqueous electrolyte. According to the EA results in Table 1, the 75 R-MnO2/RGO and 75 S-MnO2/RGO samples contain 5.96 2840



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Figure 6. Cyclic voltammograms of (a) 75 R-MnO2/RGO, (b) 75 R-MnO2/RGO with additional RGO as a conductive agent, and (c) 75 S-MnO2/ RGO, as well as (d) charge retention ratios of each CME obtained at various scan rates in 1 M Na2SO4 aqueous electrolyte.

electrode because the carbon was consumed and the πconjugated carbon network was disrupted during the synthesis. In contrast, EA, Raman spectroscopy, and C K-edge NEXAFS confirmed that the 75 S-MnO2/RGO samples synthesized using a carbon-conservative approach better maintained the sp2-bonded carbon network of the RGO, and this sample exhibited better electrochemical performance. This study provides new insight into the structural changes of the carbon in MnO2/RGO hybrid materials, especially with respect to the disruption of the carbon bonding structure as newly elucidated by the C K-edge NEXAFS analysis. This study will also serve as a guide for rational approaches to the syntheses of metal/RGO or metal oxide/RGO hybrid materials that are optimized in accordance with their physical or chemical properties.

consumption and destruction of the π-conjugated carbon network that occurred during synthesis. On the other hand, in 75 S-MnO2/RGO, the sp2-bonded carbon network of RGO is more effectively maintained than in 75 R-MnO2/RGO. Consequently, we conclude that, in addition to the quantity of conductive carbon, a well-preserved electronic structure (i.e., the quality) for the RGO is very important in the MnO2/RGO hybrid material.

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CONCLUSIONS We have demonstrated the changes in the RGO structure that occur during the synthesis of MnO2/RGO hybrid materials by the direct redox deposition of MnO2 onto RGO. Our results demonstrate that the redox reaction between MnO4− and RGO gives rise not only to quantitative carbon loss but also to changes in the electronic structure of the carbon remaining after the redox deposition of MnO2. The direct redox deposition of MnO2 onto RGO, which is a carbon-destructive approach, leads to a substantial carbon loss from the initial RGO structure, as evidenced in our EA results. Additionally, C K-edge NEXAFS results suggest that there is an oxidized carbon environment at the interface within the hybrid materials that results in a localized electronic structure of the remaining RGO in the R-MnO2/RGO hybrid material after the carbon loss during redox deposition of MnO2. Therefore, disruption of the sp2 carbon bonding in RGO and strong Mn−O−C covalent bonding interactions between MnO2 and RGO in the RMnO2/RGO hybrid materials may have detrimental effects on the electrical properties of the hybrid materials. Indeed, the CVs of a CME using 75 R-MnO2/RGO show that the damaged RGO could not effectively reduce the internal resistance of the

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ASSOCIATED CONTENT

S Supporting Information *

Zeta potential measurement and FT-IR spectra; Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-2-365-7745. E-mail: [email protected]. *Tel.: (631) 344-3202. E-mail: [email protected]. Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest. 2841



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ACKNOWLEDGMENTS This work was supported by the Energy Efficiency & Resources portion of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government from the Ministry of Knowledge Economy, Korea (no. 20122010100140). The work done at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE), under contract no. DE-AC02-98CH10886. Certain commercial names are presented in this Article for the purposes of illustration and do not constitute an endorsement by the National Institute of Standards and Technology.

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Structural Changes in Reduced Graphene Oxide upon MnO2

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