Atomic Structure of Polypyrrole-Modified Carbon-Supported Cobalt

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Atomic Structure of Polypyrrole-Modified Carbon-Supported Cobalt Catalyst Juan Wang,† Haiying Qin,‡ Jiabin Liu,§ Zhoupeng Li,∥ Hua Wang,† Ke Yang,† Aiguo Li,† Yan He,*,† and Xiaohan Yu*,† †

Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China ‡ College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, P. R. China § Institute of Applied Mechanics, Zhejiang University, Hangzhou 310027, P. R. China ∥ Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, P. R. China

ABSTRACT: Carbon-supported cobalt catalyst (Co-BP) and polypyrrole-modified carbon-supported cobalt catalyst (Co-PPYBP) are prepared by an impregnation−chemical method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption fine structure, to investigate their electronic and atomic structures. It is found that a Co ion bonds with two pyrrolic N atoms to form a Co−N2 bond in the Co-PPY-BP sample. Those two N atoms together with four O atoms at 2.04 Å constitute a distorted octahedral structure. The Co−N2 bond, in which the central Co ion has a high valence electron density, is confirmed to be the main active site of Co-PPY-BP toward oxygen reduction reaction in alkaline electrolyte.

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INTRODUCTION The direct borohydride fuel cell (DBFC) is considered to be an attractive energy source in mobile and portable applications because DBFCs use a high energy density and aqueous solution fuel that could also act as a cooling medium.1,2 However, to become commercially viable, DBFCs have to overcome the barrier of high cost caused by using platinum- and platinumbased cathode catalysts. Highly efficient and low cost cathode catalysts for DBFCs are an important subject of recent research.3−6 Nonplatinum catalyst materials, such as Ni, Pd, Au, Ag, and MnO2, have been studied as cathode catalysts for DBFC.3−6 Although Ag and MnO2 exhibited an initial performance close to that of Pt, these inorganic catalysts were unstable and intolerant to borohydride. Transition metal (for example, Fe or Co) carbon composites with macrocyclic ligands (Fe/N/C or Co/N/C) such as tetramethoxyphenyl porphyrin,7 phthalocyanines,8,9 polypyrrole (PPY),10 etc., exhibited good electrocatalytic activities and excellent BH4− tolerance as a cathode catalyst in DBFCs. Li et al.11 found that PPY-modified carbon-supported cobalt hydroxide (Co-PPY-C) (one of Co/N/C catalysts) demonstrated fairly good cell performance and durability toward the oxygen reduction reaction (ORR) in DBFC. For cobalt-based catalysts, a pyrrole-type nitrogen atom was preferred,12 while © 2012 American Chemical Society

for iron-based catalysts, the nitrogen atoms on the carbon support must be of a pyridine type.13 The Co-PPY-C catalyst exhibited smaller polarization and better performance than that of the carbon-supported Co catalyst without PPY.11 PPY was thought to be an essential componenet for Co/N/C catalysts to form ORR active sites. Two kinds of ORR active site were suggested: one was Co−Nx (x = 2 or 4) and the other was C N−C (pyridinic nitrogen).14,15 The Co−Nx bond was suggested to be responsible for a four-electron ORR which was a key factor for catalyst performance.14−16 However, the atomic structure of Co atom bonded with the N atom and the reaction path of oxygen reduction on Co−Nx still remained unclear. In this work, synchrotron radiation X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) were mainly used to analyze the electronic and atomic structure of the Co−Nx bond. An atomic structure of Co-PPYC was established, and the corresponding oxygen reduction path was suggested. Received: June 27, 2012 Revised: August 24, 2012 Published: August 24, 2012 20225

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EXPERIMENTAL SECTION

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RESULTS AND DISCUSSION The XRD patterns of the prepared Co-BP and Co-PPY-BP samples as well as BP2000 and Co(OH)2 (labeled as Co(OH)2ref) are shown in Figure 1, respectively. All the peaks of Co-BP

Carbon-supported cobalt catalyst and polypyrrole-modified carbon-supported cobalt catalyst samples (hereafter labeled as Co-BP and Co-PPY-BP) were prepared by an impregnation− chemical method using Black Pearl carbon (BP2000) as a carbon support, which has been described in our previous works.17 A 1.4 g amount of BP2000 and 2.5 mL of glacial acetic acid were first dispersed into 150 mL of deionized water, followed by stirring at room temperature for 30 min to obtain carbon dispersion. Then 2 mL of pyrrole was added into the carbon dispersion and stirred for 5 min, and light was occluded using tin foil to constrain decomposition of pyrrole. Then 10 mL of H2O2 (10 wt %) was added and constantly stirred at room temperature for 3 h to afford the polypyrrole-modified carbon dispersion (PPY-BP). This dispersion was filtered, washed, and then dried at 80 °C for 3 h. To synthesize Co-BP and Co-PPY-BP, appropriate quantities of BP2000 (1.35 g) and PPY-BP (1.35 g) were stirred with 30 mL of deionized water for 30 min at 80 °C, respectively, in a three-necked bottle. Then 0.74 g of Co(NO3)2·6H2O dissolved in 7.5 mL of deionized water was added to the three-necked bottle equipped with a reflux condenser under stirring at 80 °C for 30 min. A solution, which consisted of NaBH4 (1.57 g), NaOH (0.111 g), and deionized water (150 mL), was added to the three-necked bottle slowly and stirred at high speed for 30 min. Finally, the obtained Co-BP and Co-PPY-BP catalysts were filtered, washed, and then dried at 80 °C for 3 h. DBFCs with an active area of 6 cm2 were assembled, using the method described in ref 18. The cathode was prepared by coating Co-BP or Co-PPY-BP slurry onto a piece of carbon cloth with a catalyst loading of 3 mg cm−2. The anode was prepared by coating the Zr−Ni composite catalyst slurry onto a piece of Ni foam with a catalyst loading of 10 mg cm−2. Nafion membrane N117 was used as the electrolyte, and an alkaline NaBH4 solution (5 wt % NaBH4 and 10 wt % NaOH) was used as the fuel. The cell performance was measured at a fuel flow rate of 10 mL min−1 and a dry O2 flow rate of 150 mL min−1 under ambient conditions. The test temperature was about 15 °C. The electronic structural measurements of the Co and N atoms were performed using XPS on an RBD upgraded PHI5000C ESCA system. All spectra were referenced to the C 1S level at 284.6 eV to correct the peak shift that occurred due to charge accumulation on the sample. The data analysis was carried out by using the RBD AugerScan 3.21 software. The crystal structure of the Co-BP and Co-PPY-BP powders was investigated by X-ray diffraction (XRD) at an energy of 10 keV. The XRD experiment was performed on BL14B at Shanghai Synchrotron Radiation Facility (SSRF) in China. The electron beam energy of the storage ring at SSRF was 3.5 GeV, and the maximum stored current was about 210 mA. To determine the neighboring structure and bonding state of the Co atom, XAFS measurements were performed on BL15U1 and BL14W at SSRF. All measurements were performed in transmission mode using two ion chambers (incidence I0 and transmitted I1), in which nitrogen was used. Data were recorded using a Si(111) double crystal monochromator. In the energy range selected for the experiments, a detuning of 30% between silicon crystals was performed to suppress the high harmonic content. Computer program IFEFFIT was used to analyze the XAFS data.

Figure 1. X-ray diffraction patterns for Co(OH)2-ref, BP2000, Co-BP, and Co-PPY-BP.

correspond to the admixture of BP2000 and Co(OH)2-ref (space group P3̅m1), implying that Co exists as a form of Co(OH)2 in Co-BP. Compared with pure Co(OH)2, the lower intensity and broadening of diffraction peaks in the Co-BP sample are attributed to the decrease in crystallinity. No obvious diffraction peaks arising from Co(OH)2 phase are observed in the Co-PPY-BP sample. It can be supposed that the long-chain of PPY may constrain the crystallization of the Co(OH)2 in Co-PPY-BP. The cell performances of DBFCs using Co-BP or Co-PPYBP cathode are illustrated in Figure 2. A maximum power density of 35.7 mW cm−2 is achieved at 0.45 V for the cell using Co-PPY-BP as cathode catalyst, while that of the cell using CoBP as cathode catalyst is only 16.5 mW cm−2 at 0.33 V. These

Figure 2. Cell performance of DBFC obtained at about 15 °C. Cathode: the as-prepared Co-BP (red) or Co-PPY-BP (black) coated on carbon cloth with a catalyst loading of 3 mg cm−2; dry O2 at a flow rate of 150 mL min−1 (1 atm). Anode: Zr−Ni composite coated on a piece of Ni foam with a catalyst loading of 10 mg cm−2; fuel: 5 wt % of NaBH4, 10 wt % of NaOH solution at a fuel rate of 10 mL min−1. Membrane: N117. 20226

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Figure 3. N 1S XPS spectra of PPY-BP(a1) and Co-PPY-BP(a2) and Co 2P XPS spectra of Co-BP(b1), Co-PPY-BP(b2). The black curves were obtained by experiments, while the red curves were fitted. Others are the component peaks.

Table 1. XPS Fit Results of N 1S for PPY-BP and Co-PPY-BP NI

NII

NIII

sample

position (eV)

concentration (%)

position (eV)

concentration (%)

position (eV)

concentration (%)

PPY-BP Co-PPY-BP

399.70 400.20

94.7 85.4

402.09 403.52

5.3 11.4

− 397.18

− 3.2

results indicate that the Co-PPY-BP cathode should exhibit catalytic activity toward ORR higher than that of the Co-BP cathode. To characterize the electronic structure of samples, N 1S core-level XPS for PPY-BP and Co-PPY-BP together with Co 2P core-level XPS for Co-BP and Co-PPY-BP catalysts was performed. Their corresponding curves are indicated by the black line in Figure 3. As shown in Figure 3a1 and 3a2 and Table 1, except for the two nitrogen states that exist in PPY-BP and Co-PPY-BP, a new state (NIII as shown in Figure3a2) can be observed in Co-PPY-BP sample. The main peaks (NI in Figure 3a1 and 3a2) can be attributed to the pyrrolic N bonding with hydrogen (H−N type) in the PPY.19−23 The small peak NII at a higher energy position is closer to the peak expected for the N-oxide function group.24 The new nitrogen state (NIII in Co-PPY-BP) located at a lower energy position is speculated to be a Co−N bond, due to the smallest electronegativity of Co (1.88) compared with that of H (2.20), C (2.55), and O (3.44). If N is bonded with Co in the Co-PPY-BP sample, the valence electron density of N in the Co−N bond should be higher than that in the N−H, N−C, and N−O bonds, finally resulting in a lower binding energy of the Co−N bond. To prove this speculation, the Co 2p core-level XPS analysis for the spectra of Co-BP and Co-PPY-BP was performed. Four Gaussians (Figure 3b1 and 3b2) of Co 2P were used to perform the deconvolution, and the fitting results are summarized in Table 2. The difference between binding energies (ΔBE) of Co 2P1/2 and Co 2P3/2 in Co-BP (16.09) and Co-PPY-BP (15.98), consistent with that in standard Co(OH)2 (16.0),25 indicates that Co ions in both samples exist in the Co2+ form, and the local electronic

Table 2. XPS Fit Results of Co 2p for Co-BP and Co-PPYBP sample

CoI (eV)

SSI (eV)

CoII (eV)

SSII (eV)

ΔBE (eV)

Co-BP Co-PPY-BP

783.18 782.64

787.60 787.66

799.27 798.62

804.78 804.67

16.09 15.98

structures around Co are similar to that of Co(OH)2. Interestingly, the binding energies (Table 2) of the Co 2p electron in the Co-PPY-BP shift toward low energy as compared with those in the Co-BP, indicating the increase in valence electron density of the Co ions. This chemical shift might be a result of the low electronegativity of N (∼3.04) compared to that of O (∼3.44). Supposing that the bonding between Co ions and pyrrolic N is possible, the valence electron density of Co in the Co−N bond in Co-PPY-BP should be higher than that in the Co−O bond in Co-BP, because nitrogen has a lower ability to bind electrons than oxygen. Combined with the presence of the new nitrogen state in Co-PPY-BP compared to that of Co-BP, it can be confirmed that the Co−N interaction in Co-PPY-BP is reasonable. Thus, the peak NIII located at 397.18 eV can be attributed to a Co−Nrelated bond. Furthermore, we also find that the decrease in the ratio of peak intensity of NI to peak intensity of NII is accompanied by the formation of peak NIII for the Co-PPY-BP sample, indicating that the component N of the Co−Nx bond should come from the pyrrolic nitrogen (NI). In a series of investigations,15,19,23,26 the enhanced catalytic activity of Co-PPY-C can be attributed to this Co−Nx bond, which acts as an ORR active site in the Co-PPY-C catalyst. At 20227

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present, two types of active site structure for this Co−Nx bond are postulated in the literature: one involving Co coordinated by four N ions,19 and the other with two N ions.15 To analyze the atomic structure of the active site in the Co-PPY-BP, the coordination of N to the Co ions was resolved by XAFS spectroscopy. The normalized Co K-edge X-ray absorption near-edge structure (XANES) for the Co(OH)2-ref, Co-BP, and Co-PPY-BP are exhibited in Figure 4. Apparently, the

Figure 5. Fourier transformed k3-weighted EXAFS functions at the Co K-edge EXAFS (solid curves) for Co-BP (black) and Co-PPY-BP (red) and their simulated functions (dash curves, Co-BP green, CoPPY-BP blue).

Figure 4. XANES spectra of Co(OH)2-ref (black), Co-BP (red), and Co-PPY-BP (green).

intensity of pre-edge peak (marked by A in Figure 4) in Co-BP and Co-PPY-BP samples is higher one by one, compared with that of the Co(OH)2-ref. This increase can be attributed to the distortion of its microstructure, which induces more electrons transition from the 1s to 3d orbital of the Co ions. Thus, the higher intensity of the pre-edge peak means a greater structural distortion in the Co-PPY-BP sample, which is due to the introduction of PPY. The abrupt peak B, corresponding to the transition of the 1s to 4p orbital, is mainly determined by the type and coordination of the first shell neighbors around the Co ion, because the amplitude of the photoelectron scattering of Co is much smaller than O.27 Herein, the obvious differences in the peaks B of Co-BP and Co-PPY-BP, compared with that of Co(OH)2-ref, implies the position or type change of the first coordinate neighbor atoms of the Co ion. To further estimate the atomic structure that the condition of first shell neighbors around Co, EXAFS measurement was carried out. Figure 5 shows the Fourier transforms of the Co K-edge XAFS spectra for Co-BP and Co-PPY-BP samples together with FEFF fits. Splitting of the first neighbor shell is observed for Co-BP and Co-PPY-BP samples, attributed to their distorted structure as shown by XANES spectra. It has been confirmed that the Co atoms in Co-BP and Co-PPY-BP samples exist as a form of Co(OH)2 as shown by XPS and XRD analysis, so Co(OH)2 (ICSD no. 53994) with octahedral structure was used as the initial fitting model. The optimized structure of Co-BP via the fitting result is illustrated in Figure 6a. The fitting result displayed a splitting coordination sphere with two O atoms at 1.88 Å and four O atoms at 2.07 Å, and an outer coordination sphere with about six Co atoms at 3.15 Å. The six O atoms bonded by the central Co ion forms a distorted six-coordination octahedral structure in Co-BP. This distorted structure produces more electron transfer from the 1s to 3d orbital of the Co ions while they are induced by X-ray. For the Co-PPYBP sample, the Co−N bond that has been confirmed by XPS is

Figure 6. Schematic representations of Co-BP (a) and Co-PPY-BP (b).

taken into account during the simulation. It is found that two N atoms substitute the two nearest O atoms bonded to the Co ion at a distance of 1.89 Å. This result is consistent with the deduction from the theoretical calculation: Co can only accommodate two polypyrrole chains.26 The most possible optimized structure for Co-PPY-BP via the fitting result is illustrated in Figure 6b. The second-shell coordination is four O atoms at 2.04 Å (corresponding to peak B in Figure 5), and the third-shell coordination is four Co atoms at 3.13 Å. Compared with Co-BP, the distorted six-coordination octahedral structure of Co-PPY-BP, which is the coordination structure for cobalt polypyrrole, is constituted by the two pyrrolic N atoms and the four O atoms around the central Co ion. This structure is similar to the structure proposed by R. Bashyam et al.15 and Shi et al.26 Because PPY was inactive toward the ORR,23 it was believed that the enhanced catalytic activity of Co-PPY-BP would be induced by this Co−N2 bond. The ORR on Co-PPY-BP mainly proceeds via a four-electron pathway in alkaline electrolyte, and the Co2+/Co3+ redox couple is responsible for the mechanism of ORR as discussed in our previous work.17,28 The oxygen adsorbed on a Co-PPY catalyst surface is likely to be bound to form a side-on configuration, which indicates a four-electron ORR pathway.26 It is suggested that the Co−N2 of Co-PPY-BP should participate in the formation of stable oxygen adducts in alkaline electrolyte with a side-on configuration. The rate-determining 20228

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step of Co-PPY-BP toward ORR in alkaline electrolyte should be the primary charge-transfer reaction according to the reaction suggested by Deng and Dignam in the following equation:29

REFERENCES

(1) Liu, B. H.; Li, Z. P. J. Power Sources 2009, 187, 291−297. (2) Rostamikia, G.; Janik, M. J. Electrochim. Acta 2010, 55, 1175− 1183. (3) Cheng, H.; Scott, K.; Lovell, K. Fuel Cells 2006, 6, 367−375. (4) Liu, B. H.; Suda, S. J. Power Sources 2007, 164, 100−104. (5) Wang, Y.; Xia, X. Electrochem. Commun. 2006, 8, 1775−1778. (6) Chatenet, M.; Micoud, F.; Roche, I. Electrochim. Acta 2006, 51, 5452−5458. (7) Cheng, H.; Scott, K. J. Electroanal. Chem. 2006, 596, 117−123. (8) Ma, J.; Wang, J.; Liu, Y. J. Power Sources 2007, 172, 220−224. (9) Ma, J.; Liu, Y.; Zhang, P.; Wang, J. Electrochem. Commun. 2008, 10, 100−102. (10) Qin, H. Y.; Liu, Z. X.; Yin, W. X.; Zhu, J. K.; Li, Z. P. J. Power Sources 2008, 185, 909−912. (11) Qin, H. Y.; Liu, Z. X.; Ye, L. Q.; Zhu, J. K.; Li, Z. P. J. Power Sources 2009, 192, 385−390. (12) Bouwkamp-Wijnoltz, A. L.; Visscher, W.; Van Veen, J. A. R.; Tang, S. C. Electrochim. Acta 1999, 45, 379−386. (13) Lefèvre, M.; Dodele,t, J. P. Electrochim. Acta 2003, 48, 2749− 2760. (14) Bezerra, C. W. B.; Zhang, L.; Lee, K.; Liu, H.; Marques, A. L. B.; Marques, E. P.; Wang, H.; Zhang, J. Electrochim. Acta 2008, 53, 4937− 4951. (15) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63−66. (16) Lima, F. H. B.; De Castro, J. F. R.; Ticianelli, E. A. J. Power Sources 2006, 161, 806−812. (17) Qin, H.; Liu, Z.; Lao, S.; Zhu, J.; Li, Z. J. Power Sources 2010, 195, 3124−3129. (18) Qin, H.; Lao, S.; Liu, Z.; Zhu, J.; Li, Z. Int. J. Hydrogen Energy 2010, 35, 1872−1878. (19) Yuasa, M.; Yamaguchi, A.; Itsuki, H.; Tanaka, K.; Yamamoto, M.; Oyaizu, K. Chem. Mater. 2005, 17, 4278−4281. (20) Okada, T.; Gokita, M.; Yuasa, M.; Sekine, I. J. Electrochem. Soc. 1998, 145, 815−822. (21) Faubert, G.; Cote, R.; Dodelet, J. P.; Lefevre, M.; Bertrand, P. Electrochim. Acta 1999, 44, 2589−2603. (22) Wu., G.; Chen, Z.; Artyushkova, K.; Garzon, F. H.; Zelenay, P. ECS Trans. 2008, 16, 159−170. (23) Lee, K.; Zhang, L.; Lui, H.; Hui, R.; Shi, Z.; Zhang, J. Electrochim. Acta 2009, 54, 4704−4711. (24) Casanovas, J.; Ricart, J. M; Rubio, J.; Illas, F.; Jiménez-Mateos, J. M. J. Am. Chem. Soc. 1996, 118, 8071−8076. (25) Brundle, C. R.; Chuang, T. J.; Rice, D. W. Surf. Sci. 1976, 60, 286−300. (26) Shi, Z.; Liu, H.; Lee, K.; Dy, E.; Chlistunoff, J.; Blair, M.; Zelenay, P.; Zhang, J.; Liu, Z. S. J. Phys. Chem. C 2011, 115, 16672− 16680. (27) Ławniczak-Jabłonska, K.; Iwanowski, R. J.; Gołacki, Z. J. Phys. Rev. B 1996, 53, 1119−1128. (28) Liu, Z. X.; Li, Z. P.; Qin, H. Y.; Liu, B. H. J. Power Sources 2011, 196, 4972−4979. (29) Deng, C. Z.; Dignam, M. J. J. Electrochem. Soc. 1998, 145, 3513− 3520.

Therefore, the high catalytic activity of Co-PPY-BP can be attributed to the formation of Co−N2, producing a high valence electron density of Co, which is beneficial to improve the reaction rate of the primary charge-transfer reaction as shown in eq 1. As a result, the reaction kinetics of ORR in alkaline electrolyte can be improved by the improvement of the ratedetermining step, which can contribute to the high electrocatalytic activity of catalyst toward ORR. Therefore, Co−N2, in which the central Co ion has a high valence electron density, is confirmed to be the main active site of Co-PPY-BP toward ORR in alkaline electrolyte.

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CONCLUSIONS In this work, the electrochemical property and structures of CoBP and Co-PPY-BP samples have been investigated. The results are summarized as follows: (1) The existence of a Co−Nx bond is verified and speculated to be Co−N2, which confirms the deduction that Co can only accommodate two polypyrrole chains. It is presumed that the N atoms combined with the Co ion originate from the pyrrolic N atoms in PPY. The two pyrrolic N atoms bonded with the central Co ion with other four O atoms at 2.04 Å constitute a distorted octahedral structure. (2) Co−N2 is confirmed to be the main active site in CoPPY-BP, which is the reason that Co-PPY-BP exhibits better electrochemical performance than that of Co-BP. With the existence of Co−N2, oxygen molecules easily absorb on the catalyst, forming a side-on configuration. In this configuration, oxygen obtains electrons from Co to realize ORR because of the higher valence electron density of Co ions.

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Article

AUTHOR INFORMATION

Corresponding Author

*(Y.H.) Tel.: +86-21-33932078. Fax: +86-21-33933227. Email: [email protected]. (X.Y.) Tel.: +86-21-33933188. Fax: +86-21-33933227. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the use of BL15U1, BL14B, and BL14W in Shanghai Synchrotron Radiation Facilities (SSRF). This work is financially supported by the National Natural Science Foundation of China (grant nos. 21006090, 20976156, and 2010CB934501), the Zhejiang Provincial Natural Science Foundation of China (grant no. Z4110126), the Doctoral Fund from the Education Ministry of China (20100101110042), the Fundamental Research Funds for the Central Universities, and the Qianjiang Talents Project of Science Technology Department of Zhejiang Province (2012R10058). 20229

dx.doi.org/10.1021/jp306355h | J. Phys. Chem. C 2012, 116, 20225−20229