On the Origin of Electrocatalytic Oxygen Reduction Reaction on

May 20, 2013 - Cobalt-carbon nanofibers as an efficient support-free catalyst for oxygen reduction reaction with a systematic study of active site for...
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On the Origin of Electrocatalytic Oxygen Reduction Reaction on Electrospun Nitrogen−Carbon Species Dongyoon Shin,†,# Beomgyun Jeong,†,# Bongjin Simon Mun,‡,§ Hongrae Jeon,† Hyun-Joon Shin,∥ Jaiyoon Baik,∥ and Jaeyoung Lee*,†,‡,⊥ †

Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering, ‡Ertl Center for Electrochemistry and Catalysis, RISE, and §Department of Physics and Photon Science, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea ∥ Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, South Korea ⊥ Department of Physical Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany S Supporting Information *

ABSTRACT: Nitrogen−carbon (N−C) species is a potential electrocatalyst for oxygen reduction reaction (ORR) in electrochemical energy conversion cells, but its mechanistic origin of ORR on the N−C surface is still unclear. We show our facile approach to the synthesis of highly active Co-modified N−C catalyst and investigated the origin of ORR activity of electrospun N−C species by removing the metal with hydroxide carbon etching and acid metal leaching. Through the detailed investigation on the origin of ORR electrocatalysis for electrospun N−C nanofibers, we revealed that pyrrolicN and highly graphitized carbon structure are mainly responsible for the enhanced ORR activity of metal-free N−C nanofiber and embedded Co metal got involved in the creation of the pyrrolic N site.

1. INTRODUCTION Activity enhancement of oxygen reduction reaction (ORR) has long been a holy grail for electrochemical technologies and processes, especially polymer electrolyte fuel cells and metal-air batteries, because ORR is the rate-determining reaction.1−8 As is well known, Pt is the best ORR catalyst, but it is too expensive to be utilized widely to meet cost effectiveness. Thus, great efforts have been devoted to discover non-noble catalysts for the replacement of Pt. These alternative non-noble metal catalysts include transition-metal chalcogenides, conductive polymers, metal oxides, carbides, nitrides, oxynitrides, carbonitrides, and nitrogen−carbon (N−C) species.4,5,8−13 Among them, N−C catalysts based on carbon nanofiber, carbon nanotube, and Ndoped graphene are receiving much attention due to their superior activity and stability to 20 wt % Pt/C catalyst.4,10−12 On the basis of the initial studies on transition-metal macrocycles, it has been widely accepted that the origin of reactive site for oxygen reduction is the metal-N4 or metal-N2 center that is bound to the carbon support. R. Jasinski first reported that the metal-Nx moieties could act as ORR catalyst in 1964 using expensive transition-metal macrocyclic compounds such as cobalt phthalocyanine. Later, some researchers found that catalytically active metal-Nx moieties could be formed when high-temperature heat treatment procedures were introduced to the catalyst synthesis process. The only requirement was a proper metal−N coordination bonded to carbon support resulting from heat treatment to synthesize highly active ORR catalyst using © XXXX American Chemical Society

metal, nitrogen, and carbon precursor materials. However, there are still controversial discussions about the origin of ORR active species.14−16 For example, Lefèvre et al. reported that central metal ion in the macrocycle plays a crucial role in the ORR.17 Nallathambi et al. suggested that the transition metals are not the active species for oxygen reduction but rather they serve primarily to facilitate the stable incorporation of nitrogen into the graphitic carbon during high-temperature pyrolysis of metal−nitrogen complexes.18 The argument is supported by observations that metal-free N−C prepared by metal leaching with a strong acid still maintains the electrocatalytic activity and stability for the ORR. Moreover, some of the N−C ORR catalysts can be manufactured without the inclusion of metal as a promoter.19,20 Therefore, it is needed to reveal the origin of electrocatalytic activity in N−C species for a clear understanding the ORR active species. Herein we report on the exploration of the origin of ORR electrocatalysis of the N-doped carbon nanofibers (N-CNFs), which have various candidates for ORR active sites using electrochemical and high resolution XPS analysis with synchrotron radiation. Received: February 1, 2013 Revised: May 20, 2013

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2. EXPERIMENTAL SECTION Electrospinning process was carried out to produce a Co-PAN web using polymer solution that was made up of polyacrylonitrile (PAN) and Co(II) nitrate hexahydrate (Co(NO3)2·6H2O) dissolved in N,N-dimethylformamide (DMF). The amount of each chemical was 4 g PAN and 1 g Co(NO3)2·6H2O, respectively, that could load 20 wt % Co in CNF, and the total amount of the Co-PAN precursor was 40 g. After complete mixing of PAN in DMF solution, we dissolved Co metal precursor into the 10 wt % PAN/DMF polymer solution. A high voltage of 24 kV was applied between a syringe needle and the drum collector, which are located 15 cm apart from each other. Co-PAN precursor was provided at a feed rate of 1 mL h−1 for 30 h by using a syringe pump. All processes were performed at room temperature (∼25 °C) under a relative humidity of 30%. For stabilizing the formation of graphitic carbon structure, the electrospun Co-PAN web was heated at a ramping rate 1 °C min−1 to 280 °C, then kept for 1 h in air. Later, the stabilized CoPAN web was carbonized at a ramping rate 5 °C min−1 up to 1000 °C for 1 h in a N2 atmosphere. Co-containing carbonized nanofiber, that is, Co-modified N-doped carbon nanofiber (CoN-CNF), could be obtained through these heat treatments.4 For complete removal of Co metal, we conducted carbon etching using hydroxide treatment to Co-N-CNF for exposure of Co metal to the surface. The Co-N-CNF was submerged in sodium hydroxide with a concentration of 17.5 M for one day. Wet Co-N-CNF was washed with distilled and deionized water (D.I. water) to remove sodium ions on the sample and then heated at a ramping rate 5 °C min−1 up to 850 °C for 3 h in a N2 atmosphere.21 To investigate N-derived active species, excluding the role of transition metal in CNF, we leached Co metal with aqua regia immersion solution for one day. Cobalt leaching was confirmed through observation on the color change of aqua regia from yellow to green. The metal leached Co-N-CNF (i.e., Co removed N-CNF) and was washed using D.I. water and then filtered. Electrochemical properties and ORR activity of the synthesized CNF, Co-N-CNF, and N-CNF catalysts were evaluated using a three-electrode cell connected to a potentiostat/ galvanostat (Biologic, VSP). We used a Pt wire as the counter electrode and Hg/HgO as the reference electrode. We prepared a catalyst ink by dispersing 10 mg catalyst in a mixture of 25 μL of 10 wt % Nafion solution (Sigma-Aldrich), 4 mL of D.I. water, and 1 mL isopropyl alcohol. After the catalyst ink was sonicated for 5 min, a 20 μL aliquot of the suspension was dropped onto glassy carbon disk electrode (0.2475 cm2) using a micropipet so that the amount of the catalysts on the electrode is 40 μg/cm2. Cyclic voltammograms (CVs) were obtained in an aqueous solution of 0.1 M potassium hydroxide (KOH) as electrolyte that was saturated by ultrapure N2 bubbling at room temperature. The scan rate was 20 mV s−1 and the potential range was from 0.4 to −0.8 V versus Hg/HgO. The linear sweeping voltammograms (LSVs) were measured to obtain ORR activity at rotating speeds of 100, 225, 400, 625, 900, 1225, 1600, 2025, and 2500 rpm, while oxygen is constantly bubbled at a flow rate of 20 mL/min. The potential was swept from 0.4 to −0.8 V versus Hg/HgO at the scan rate of 10 mV s−1. Morphological characteristics of the electrocatalysts were examined with transmission electron microscopy (TEM, JEOL JEM-2100). X-ray diffraction (XRD, Rigaku Miniflex II) measurements were performed to confirm possible structural changes in the bulk crystallinity depending on Co existence. The

surface functionality and composition was identified using X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out at the beamline 8A1 of the Pohang Accelerator Laboratory of which the photon energy was set to 630 eV and the binding energy resolution was better than 200 meV, providing superior resolution of spectra compared to lab-based XPS. The minimum signal-to-noise ratio of each sample was ∼10. The experiment was performed in an ultrahigh vacuum (UHV) chamber with a base pressure ≤5 × 10−10 Torr. The peak positions of the XPS were calibrated with respect to binding energy of the Au 4f7/2 (84 eV) peak, and the standard Shirley background was used for the analysis.

3. RESULTS AND DISCUSSION To compare morphological changes of N−C nanofibers with and without Co metal particles, we performed TEM analysis. Figure 1a clearly shows the clean N−C nanofiber. In the TEM image of Co-N-CNF (Figure 1b), Co metal particles are almost covered with graphitic carbon with a layer distance of 3.4 Å. The carbon that covers the Co metal particles would come from diffused carbon atoms of the fiber during heat treatment. In contrast, the

Figure 1. TEM images of (a) pristine CNF, (b) Co-N-CNF, and (c) NCNF. Inset: HR-TEM image of Co-N-CNF in panel b that shows Co particle covered by carbon and HR-TEM image of N-CNF in panel c. B

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carbon layer is effectively removed in the hydroxide-treated CoN-CNF. (See the Supporting Information.) Thus, we expect that the exposed cobalt metal particles in hydroxide-treated Co-NCNF would be removed more easily in the leaching process than that embedded in Co-N-CNF (Figure S1 in the Supporting Information). To study the role of Co metal species in N−C nanofibers, it is important to completely remove the Co from the N−C nanofibers. The TEM image of N-CNF (Figure 1c) shows that the Co metal particles disappeared after the acid leaching process. To confirm the possible changes in the bulk crystalline structure depending on Co existence, we performed XRD measurements (Figure 2). All Co-containing catalysts exhibit

Figure 3. Cyclic voltammograms of (a) CNF, (b) Co-N-CNF, and (c) N-CNF in N2-saturated 0.1 M KOH electrolyte. Scan rate: 20 mV s−1.

CNF. These profiles are attributed to the oxidation of Co(II) species to Co(III) species. It is noted that the Co(II)-Co(III) pseudocapacitive feature disappeared in the voltammetric profile of N-CNF. Moreover, the surface composition of Co 2p XPS displays the complete removal of Co metal in Co-N-CNF (Figure S1 in the Supporting Information). Thus, combining the results of Figures 1−3 and Figure S1 in the Supporting Information we strongly confirm that Co metal particles are fully removed by the chemical treatment to form N-CNF. We conduct the electrochemical characterization to identify the relationship between Co and ORR activity. In the RDE-LSV result for CNF, Co-N-CNF, and N-CNF in comparison with commercial 46.7 wt % Pt/C catalyst (Figure 4), oxygen reduction

Figure 2. X-ray diffractograms of (a) CNF, (b) Co-N-CNF, and (c) NCNF. ●: carbon, ■: cobalt.

three peaks at 46, 52, and 76°, which are assigned to the (111), (200), and (220), respectively, of the metallic Co (labeled with quadrangles).22−24 These Co peaks are removed after acid treatment with aqua regia, as shown in Figure 2c. The XRD pattern of CNF shows a broad XRD peak derived from the entangled turbostatic carbon structure of CNFs. In contrast, the development of the peak at 26°, which is attributable to the (002) plane of graphitic carbon, is observed in the XRD result of Co-N-CNF. In addition, the graphitic carbon structure is still retained in N-CNF despite carbon etching and metal leaching process. The small peak of C(101) is observed near the scattering angle 45° in Figure 2c. It demonstrates graphitic carbon characteristics along with the peak of C(002) at 26°, which is consistent with the TEM result (Figure 1c). From the comparison of XRD results of CNF and Co-N-CNF, it is inferred that inclusion of Co may lead to form a more graphitic carbon structure. This understanding is also supported by the XPS C 1s spectrum (Figure S2 in the Supporting Information). Takahagi et al. reported that carbon material with a complete graphite structure presents a narrower full width at half-maximum (fwhm).25 The fwhm of the XPS C 1s peak obtained from pristine CNF, Co-N-CNF, and N-CNF have 1.504, 1.083, and 1.044 eV, respectively. It means that the Comodified N-CNFs have a higher degree of graphitization than pristine CNF. The improved graphitization of carbon might be beneficial in enhancing electrical conductivity26,27 and effectively forming ORR-active N−C species. Figure 3 depicts CVs of CNF, Co-N-CNF, and N-CNF in 0.1 M KOH solution. In this Figure, distinctive voltammetric profiles between 0 and 0.4 V can be observed for Co-N-CNF, and N-

Figure 4. Linear sweep voltammograms of (a) CNF, (b) Co-N-CNF, (c) N-CNF, and (d) 46.7 wt % Pt/C in O2-saturated 0.1 M KOH electrolyte at a rotation rate of 1600 rpm. Scan rate: 10 mV s−1.

activity of Co-N-CNF is markedly increased compared with that of pristine CNF. Moreover, N-CNF has just slightly smaller activity than Co-N-CNF, even though Co particles were fully removed by acid treatment. To check the effect of the chemical treatments to the ORR activity, we conducted the same chemical treatments to the pristine CNF. However, there is no essential difference in ORR activity between the acid-treated CNF and the pristine CNF, and it ensures that the metal leaching treatment does not affect the ORR activity derived from nonmetallic C

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species (Figure S3 in the Supporting Information). This observation can be reasonably explained if we hypothesize that the origin of ORR activity of Co-N-CNF is N−C-activated by Co rather than Co-related ORR active species such as metallic Co, Co oxide, and Co-N2 or Co-N4 species. As an ORR electrocatalyst for fuel cell, it is highly desirable for ORR catalyst to reduce O2 to H2O selectively via the fourelectron transfer pathway rather than the two-electron transfer pathway, producing H2O2 as intermediate because H 2O2 damages to the membrane in the fuel cell. Hence, it is necessary to check the selectivity for the complete reduction pathway that was investigated using the Koutecky−Levich plot. (See the Supporting Information.)13,28−30 Figure 5 is drawn to compare each sample’s slope of Koutecky−Levich plot at −0.5 V, where all samples indicate

Figure 6. XPS N1s spectrum of (a) CNF, (b) Co-N-CNF, and (c) NCNF. Each dashed line denotes the various binding energies of doped nitrogen: pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic-N-oxide. Inset image presents a schematic diagram of pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic-N-oxide species incorporated into the carbon.

Figure 5. Koutecky−Levich plot for ORR on (a) CNF, (b) Co-N-CNF, (c) N-CNF, and (d) 46.7 wt % Pt/C at −0.5 V versus Hg/HgO in O2 saturated 0.1 M KOH. The inset shows the dependence of the number of electrons transferred on potential.

eV), graphitic-N (∼401 eV), and pyridinic-N-oxide (402 to 403 eV).31−33 Pyridinic-N refers to N atoms that are located at the edge of the graphene planes where each N atom is bonded to two carbon atoms and donates one p electron to the aromatic π system. Pyrrolic-N is incorporated into five-membered heterocyclic rings that contribute two p-electrons to the π system. Graphitic-N is integrated into the graphene layer and substitutes carbon atoms within the graphene plane. Pyridinic-N-oxides are formed with N atoms bonded to two carbon and one oxygen atom. In pristine CNF (Figure 6a), there are four distinct nitrogen species of pyridinic-N, pyrrolic-N, graphitic-N, and pyridinic-N-oxide. Even though the pyridinic-N and graphitic-N are present as dominant composition in the CNF, the ORR activity of CNF is far less than that of Co-N-CNF and N-CNF. In contrast, the spectrum of the most ORR-active Co-N-CNF exhibits the increase in pyrrolic-N species, shown in Figure 6b and Table 2, while pyridinic-N and graphitic-N components remain on the surface. Moreover, the pyrrolic-N still remained at the same ratio with Co-N-CNF in N-CNF despite the acid metal leaching process (Table 2). This suggests that the pyrrolic-N

Table 1. Number of Transferred Electrons for the Catalysts at −0.5 V vs Hg/HgO sample

number of transferred electrons

CNF Co-N-CNF N-CNF 46.7 wt % Pt/C

2.60 3.70 3.25 3.85

similar region (Table 1). (Moreover, we also consider the electron-transfer number dependence on potential. However, in the case of pristine CNF, it is difficult to calculate the electron transfer number at −0.2 V due to low kinetics of pristine CNF; therefore, we estimate that at −0.2 V as a dotted line. As seen in this Figure, the electron transfer number for ORR at the N-CNF is always higher than that on the pristine CNF over the entire potential range, even though it shows a smaller number of electron transfer than Co-N-CNF. This result indicates that NCNF is a more efficient ORR electrocatalyst than pristine CNF. To address the difference in the number of electrons and ORR activity adequately, we carried out synchrotron-based highresolution XPS measurements for CNF, Co-N-CNF, and NCNF. In the top of Figure 6, several N-containing functional structures are shown; pyridinic-N (∼398 eV), pyrrolic-N (∼400

Table 2. Relative Ratio of N Species Obtained from the Deconvoluted N 1s Peaks by XPS (%)

D

sample

pyridinic-N

pyrrolic-N

graphitic-N

pyridinic-N-oxide

CNF Co-N-CNF N-CNF

32.84 28.85 27.46

19.97 26.99 27.30

41.85 40.54 39.27

5.35 3.62 5.97

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might be enhanced by Co metals.34 On the basis of these XPS results combined with RDE voltammograms (Figure 4 and 6), we could conclude that pyrrolic-N is contributing as a highly active nitrogen species to enhance ORR kinetics in N-CNF. This understanding is clearly supported by a previous study by Unni et al.33 that pyrrolic-N significantly affects the charge density of the near carbon atoms and it assists adsorption and reduction of oxygen molecule through an energetically favored association. Xia et al. also reported that pyrrolic-N contributes to ORR activity along with pyridinic-N.35

(5) Olson, T. S.; Pylypenko, S.; Atanassov, P.; Asazawa, K.; Yamada, K.; Tanaka, H. Anion-Exchange Membrane Fuel Cells: Dual-Site Mechanism of Oxygen Reduction Reaction in Alkaline Media on Cobalt-Polypyrrole Electrocatalysts. J. Phys. Chem. C 2010, 114, 5049− 5059. (6) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241−247. (7) Zhang, J.; Yang, H.; Fang, J.; Zou, S. Synthesis and Oxygen Reduction Activity of Shape-Controlled Pt3Ni Nanopolyhedra. Nano Lett. 2010, 10, 638−644. (8) Lee, J.; Jeong, B.; Ocon, J. D. Oxygen Electrocatalysis in Chemical Energy Conversion and Storage Technologies. Curr. Appl. Phys. 2013, 13, 309−321. (9) Morozan, A.; Jégou, P.; Jousselme, B.; Palacin, S. Electrochemical Performance of Annealed Cobalt−Benzotriazole/CNTs Catalysts towards the Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2011, 13, 21600−21607. (10) Choi, C. H.; Lee, S. Y.; Park, S. H.; Woo, S. I. Highly Active NDoped-CNTs Grafted on Fe/C Prepared by Pyrolysis of Dicyandiamide on Fe2O3/C for Electrochemical Oxygen Reduction Reaction. Appl. Catal., B 2011, 103, 362−368. (11) Geng, D.; Chen, Y.; Chen, Y.; Li, Y.; Li, R.; Sun, X.; Ye, S.; Knights, S. High Oxygen-Reduction Activity and Durability of Nitrogen-Doped Graphene. Energy Environ. Sci. 2011, 4, 760−764. (12) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Exploration of the Active Center Structure of Nitrogen-Doped Graphene-Based Catalysts for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 7936−7942. (13) Wang, B. Recent Development of Non-Platinum Catalysts for Oxygen Reduction Reaction. J. Power Sources 2005, 152, 1−15. (14) Chen, Z.; Higgins, D.; Yu, A.; Zheng, L.; Zhang, J. A Review on Non-Precious Metal Electrocatalysts for PEM Fuel Cells. Energy Environ. Sci. 2011, 4, 3167−3192. (15) Gupta, S.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E. Heat-Treated Polyacrylonitrile-Based Catalysts for Oxygen Electroreduction. J. Appl. Electrochem. 1989, 19, 19−27. (16) Bezerra, C. W. B.; Zhang, L.; Lee, K.; Liu, H.; Marques, A. L. B.; Marques, E. P.; Wang, H.; Zhang, J. A Review of Fe−N/C and Co−N/C Catalysts for the Oxygen Reduction Reaction. Electrochim. Acta 2008, 53, 4937−4951. (17) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 2009, 324, 71−74. (18) Nallathambi, V.; Lee, J.-W.; Kumaraguru, S. P.; Wu, G.; Popov, B. N. Development of High Performance Carbon Composite Catalyst for Oxygen Reduction Reaction in PEM Proton Exchange Membrane Fuel Cells. J. Power Sources 2008, 183, 34−42. (19) Wang, S.; Yu, D.; Dai, L.; Chang, D. W.; Baek, J. B. Polyelectrolyte-Functionalized Graphene as Metal-Free Electrocatalysts for Oxygen Reduction. ACS Nano 2011, 5, 6202−6209. (20) Wang, S.; Yu, D.; Dai, L. Polyelectrolyte Functionalized Carbon Nanotubes as Efficient Metal-Free Electrocatalysts for Oxygen Reduction. J. Am. Chem. Soc. 2011, 133, 5182−5185. (21) Yoon, S.-H.; Lim, S.; Song, Y.; Ota, Y.; Qiao, W.; Tanaka, A.; Mochida, I. KOH Activation of Carbon Nanofibers. Carbon 2004, 42, 1723−1729. (22) Smirnova, A.; Wender, T.; Goberman, D.; Hu, Y.-L.; Aindow, M.; Rhine, W.; Sammes, N. M. Modification of Carbon Aerogel Supports for PEMFC Catalysts. Int. J. Hydrogen Energy 2009, 34, 8992−8997. (23) Zhang, H.-J.; Yuan, X.; Sun, L.; Zeng, X.; Jiang, Q.-Z.; Shao, Z.; Ma, Z.-F. Pyrolyzed CoN4-Chelate as an Electrocatalyst for Oxygen Reduction Reaction in Acid Media. Int. J. Hydrogen Energy 2010, 35, 2900−2903. (24) Nguyen-Thanh, D.; Frenkel, A. I.; Wang, J.; O’Brien, S.; Akins, D. L. Cobalt−Polypyrrole−Carbon Black (Co−PPY−CB) Electrocatalysts for the Oxygen Reduction Reaction (ORR) in Fuel Cells: Composition and Kinetic Activity. Appl. Catal., B 2011, 105, 50−60.

4. CONCLUSIONS We synthesized N-doped carbon nanofibers via electrospinning and subsequent pyrolysis of nitrogen-containing polymer combined with Co metal precursor. We found that the Comodified N-CNFs contain more pyrrolic-N functional structure and a higher degree of graphitization of carbon than pristine CNF. They showed superior activity in comparison with pristine CNF regardless of the existence of Co metal particles. Therefore, it would be possible to conclude that the electrocatalytic activity of the N-CNFs is highly dependent on the amount of pyrrolic-N along with highly graphitized carbon structure, which is promoted by the Co precursor. We expect that effective incorporation of pyrrolic-N to the graphitic carbon structure would result in further enhancement of ORR activity, and ultimately it will lead to successful application of N-CNFs to the cathodes in various electrochemical technologies and processes.

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ASSOCIATED CONTENT

S Supporting Information *

Calculation of electron transfer numbers in ORR, hydroxide treatment for carbon etching, the enhanced graphitization of carbon, and the effect of acid treatment on the metal-free CNF. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-62-715-2440. Fax: +82-62-715-2434. E-mail: [email protected]. Author Contributions #

Dongyoon Shin and Beomgyun Jeong contributed equally to this research and should be considered cofirst authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MEST) (20100022453).

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REFERENCES

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