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Sep 7, 2014 - Herein, we report a graphene-templated carbon (GTC) hybrid via a facile two-step strategy involving a graphene oxide-directed self-assem...
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Ultralong Cycle Life Sodium-Ion Battery Anodes Using a GrapheneTemplated Carbon Hybrid Xiaosi Zhou,*,† Xiaoshu Zhu,‡ Xia Liu,† Yan Xu,† Yunxia Liu,† Zhihui Dai,*,† and Jianchun Bao† †

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡ Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: Hard carbons have been extensively investigated as anode materials for sodium-ion batteries due to their disordered structure and large interlayer distance, which facilitates sodium-ion uptake and release. Herein, we report a graphene-templated carbon (GTC) hybrid via a facile two-step strategy involving a graphene oxide-directed self-assembly process and subsequent pyrolysis treatment. When evaluated as an anode material for sodium-ion batteries, the GTC electrode exhibits ultralong cycling stability and excellent rate capability. A reversible capacity of 205 mA h g−1 and more than 92% capacity retention were achieved after 2000 cycles at a current density of 200 mA g−1. Even at 10 A g−1 a high reversible capacity of 45 mA h g−1 can be obtained. The superior electrochemical performance is due to the strong coupling effect between graphitic nanocrystallites and the graphene template and the large interlayer distance of the graphitic nanocrystallites, both of which can not only effectively relieve the sodiation-induced stress and preserve the electrode integrity during cycling but also promote the electron and sodium-ion transport.

1. INTRODUCTION Sodium-ion batteries (SIBs) have attracted considerable attention as one of the most promising electric energy storage systems because of their various advantages such as high capacity, low cost, natural abundance, and environmental benignity.1−11 However, there are still three challenging issues in SIBs to be overcome to meet the requirements for highperformance power sources in the future commercial application, involving higher capacity and longer cycle life as well as higher rate performance.12−24 Hard carbons have been extensively investigated as anode materials for SIBs due to their disordered structure and large interlayer distance, which facilitates sodium uptake and release.25−27 Various nanostructures of carbon-based anode materials such as carbon nanowires, carbon nanospheres, and carbon nanosheets have been successfully fabricated, and some promising sodium storage properties have been obtained.27−37 For example, Liu and Cao found that hollow carbon nanowires from pyrolyzed hollow polyaniline nanowires had a high reversible capacity of 206 mA h g−1 after 400 cycles.27 Maier’s group reported that hollow carbon nanospheres synthesized by hydrothermal carbonization of glucose showed a reversible capacity of ∼160 mA h g−1 after 100 cycles.33 More recently, Zhang et al. reported nitrogen-doped porous carbon nanosheets that could retain more than 155 mA h g−1 over 260 cycles.34 However, the practical application of hard carbon is hampered by the sodiation-induced stress problem caused by the volume change during Na uptake/release processes, thus leading to the © 2014 American Chemical Society

destruction of the structure of carbon materials and the crack of electrodes and consequently unsatisfactory cycling stability.27 Graphene as an emerging two-dimensional carbon material has been used as a template to synthesize various kinds of functional composites owing to its extraordinary electronic and mechanical properties.38−44 Herein, we report on a graphenetemplated carbon (GTC) hybrid through direct pyrolysis of a graphene oxide (GO)-doped polyelectrolyte precursor. The asprepared carbon hybrid shows ultralong cycling stability as well as excellent rate performance. A reversible capacity of 205 mA h g−1 and more than 92% capacity retention were obtained after 2000 cycles at a current density of 200 mA g−1. Even at 10 A g−1 a high reversible capacity of 45 mA h g−1 can be attained. From X-ray absorption near-edge structure (XANES) and selected area electron diffraction (SAED) analysis, we found that both the strong coupling effect between graphitic nanocrystallites and graphene and the large interlayer distance of the graphitic nanocrystallites are essential to relieve the sodiation-induced stress, preserve the electrode integrity, and promote the electron and sodium-ion transport, thus enabling good sodium storage properties. Received: June 28, 2014 Revised: September 6, 2014 Published: September 7, 2014 22426

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2. EXPERIMENTAL SECTION 2.1. Chemicals. Natural graphite (325 mesh, Alfa Aesar), poly(diallyldimethylammonium chloride) (PDDA) (Mw 150 000, Sigma-Aldrich), and poly(sodium 4-styrenesulfonate) (PSS) (Mw 70 000, Sigma-Aldrich) were used without further purification. 2.2. Synthesis of GTC. GO was first synthesized from graphite through a modified Hummer’s method.45 The synthesis of GTC involves a facile two-step process. In a typical synthesis, 5 mL of GO aqueous suspension (10 mg mL−1) was dispersed in 100 mL of PDDA aqueous solution (GO:PDDA = 1:20 by weight) under sonication. Another suspension was prepared by dispersing 5 mL of GO aqueous suspension (10 mg mL−1) in 100 mL of PSS aqueous solution (GO:PSS = 1:25.5 by weight) under sonication. Then, the latter suspension was poured into the former one. After stirring for 6 h, the resulting mixture was filtered off and washed with plenty of deionized water, and then the precipitate GO/ PDDA−PSS was collected and dried at 70 °C under vacuum for 12 h. In order to produce GTC, the GO/PDDA−PSS was heated to 800 °C and held at that temperature for 2 h under argon atmosphere with a 2 °C min−1 heating rate. Highly disordered carbon (HDC) was synthesized under the same conditions but without the presence of GO. Reduced graphene oxide (RGO) was produced by heating freeze-dried GO to 800 °C and held at that temperature for 2 h under argon atmosphere with a 2 °C min−1 heating rate. We measured yields of GTC, HDC, and RGO from GO/PDDA−PSS, PDDA−PSS, and GO, respectively. With these yield values, we calculated the content of RGO in the hybrid GTC to be around 7 wt %. 2.3. Materials Characterization. Scanning electron microscopy (SEM) measurements were performed on a JEOL JSM-7600F scanning electron microscope operated at 15 kV. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) characterizations were carried out on a JEOL JEM-2100F transmission electron microscope operated at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) analysis and scanning transmission electron microscopy (STEM) measurements as well as elemental mapping characterizations were conducted on a Tecnai G2 F20 U-TWIN field emission transmission electron microscope equipped with an EDAX system. The X-ray diffraction (XRD) pattern was measured on a Rigaku D/max 2500/PC diffractometer using Cu Kα radiation. Raman measurements were performed using a Labram HR800 with a laser wavelength of 514.5 nm. Nitrogen adsorption and desorption isotherms at 77.3 K were recorded by an ASAP 2050 surface area-pore size analyzer. C K-edge XANES measurements were determined by using beamline 4B7B at Beijing Synchrotron Radiation Facility (BSRF), respectively, and the resulting data were normalized to the incident photon flux I0 measured by using a fresh gold target. 2.4. Electrochemical Measurements. Electrochemical experiments were done using CR2032 coin cells. To make working electrodes, GTC, Super-P carbon black, and poly(vinylidene fluoride) (PVDF) with a weight ratio of 80:10:10 were mixed into a homogeneous slurry in N-methyl-2pyrrolidone (NMP) with a mortar and pestle. The resulting slurry was pasted onto pure Cu foil (99.9%, Goodfellow). The electrolyte was 1 M NaClO4 in ethylene carbonate/propylene carbonate (1:1 v/v). Glass fibers (GF/D) from Whatman were used as separators, and sodium metal was utilized as the

counter electrode. The coin cells were assembled in an argonfilled glovebox (H2O, O2 < 0.1 ppm, MBraun). The charge and discharge measurements of the batteries were performed on a Land CT2001A multichannel battery testing system in the fixed voltage window between 0.001 and 3 V vs Na+/Na at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were determined on a PARSTAT 4000 electrochemical workstation. CV was carried out at a scan rate of 0.1 mV s−1, while EIS was tested in the frequency range from 100 kHz to 100 mHz.

3. RESULTS AND DISCUSSION Figure 1 schematically illustrates the preparation procedure of the GTC hybrid. Typically, GO was first coordinated with

Figure 1. Schematic illustration for the synthesis of GTC.

PDDA and PSS, respectively, through van der Waals force. Then, self-assembly between positively charged PDDA and negatively charged PSS will spontaneously take place on the surface of GO after admixing the above two suspensions, resulting in a GO-doped polyelectrolyte composite (GO/ PDDA−PSS), as revealed by Raman measurement, with broad G and D peaks (Figure 2a). The strongly coupled hybrid

Figure 2. (a) Raman spectra of GO/PDDA−PSS and GTC. (b) XRD pattern of GTC.

structure of GTC can be easily achieved through a pyrolysis process under the protection of argon atmosphere. We also prepared HDC by pyrolyzing self-assembled polyelectrolyte composite PDDA−PSS and RGO by pyrolyzing freeze-dried GO, respectively. Detailed synthesis procedures are described in the Experimental Section. After heat treatment under argon atmosphere at 800 °C, GO and the polyelectrolyte composite PDDA−PSS in GO/ 22427

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From the full width at half-maximum (fwhm) of the (002) peak, the c-axis length (Lc) of the graphite can be calculated to be 1.24 nm.27 Therefore, the XRD measurement suggests that the hybrid GTC consists of graphite nanocrystallites with 3−4 (1.24/0.37 = 3.4) layers of graphene sheets. SEM images (Figure 3a) show that the obtained hybrid presents as a micrometer-sized structure with obvious wrinkles exposed on the outer surface. TEM and HRTEM images (Figure 3b−d) show that the graphene surface is covered with a large amount of graphite nanocrystallites, and some graphene sheets protrude over the edge of the carbon hybrid. The ring-like mode in the SAED pattern (inset of Figure 3b) further confirms the presence of graphitic nanocrystallites, which is in good agreement with XRD analysis (Figure 2b). The EDS measurement of the hybrid reveals the carbon hybrid is composed of carbon and oxygen (Supporting Information, Figure S1). Moreover, nitrogen absorption measurement determines a Brunauer−Emmett−Teller (BET) surface area of 7.9 m2 g−1 for GTC (Supporting Information, Figure S2), which is more than 3 times higher than that of HDC (2.5 m2 g−1).37 These results indicate that the graphene sheets might function as a template to guide the orderly formation of graphitic nanocrystallites, thus producing the larger specific surface area to allow more transport channels for the electron and sodium ion. Figure 4a displays the first three cyclic voltammetry (CV) curves of the GTC electrode from 0.001 to 3 V vs Na+/Na at a scanning rate of 0.1 mV s−1. The large irreversible reduction peaks occurred at 0.46 V in the first cathodic scan, which can be ascribed to the formation of the solid electrolyte interphase (SEI) film.27,29−34 The small peak at 0.4 V in the third anodic scan is probably caused by a room-temperature change. Additionally, the pair of broad peaks between 0.2 and 1.2 V corresponds to the charge transfer on the surface of the graphitic nanocrystallites. The pair of sharp peaks in the potential range from 0 to 0.2 V can be assigned to sodium insertion/extraction in the interlayer of the graphite nanocrystallites.27 Remarkably, the CV curves almost overlap after the first cycle, suggesting that both the SEI formation and local

PDDA−PSS were transformed into RGO and HDC, respectively, as proved by Raman measurements with both sharper G peak and broader D peak in comparison with those of the precursor GO/PDDA−PSS (Figure 2a). The XRD pattern of GTC displays two broad peaks around 2θ = 24.3° and 43.2°, corresponding to the (002) plane and the (101) plane of graphite, respectively (Figure 2b). The interlayer distance of the (002) plane (d(002)) can be calculated to be approximately 0.37 nm on the basis of its 2θ degree, which is consistent with the HRTEM observation shown in Figure 3d.

Figure 3. (a) SEM image, (b) TEM image and SAED pattern (inset), and (c, d) HRTEM images of GTC.

Figure 4. (a) CV curves of the first three cycles of GTC at 0.1 mV s−1 scanning rate. (b) Galvanostatic charge−discharge profiles for different cycles of GTC at a current density of 0.2 A g−1. (c) Cycling performance and Coulombic efficiency of GTC under 0.2 A g−1. (d) Rate capability of GTC. 22428

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structure rearrangement induced by sodiation/desodiation mainly take place in the initial cycles. The electrochemical stability of the GTC electrode was investigated under galvanostatic conditions. As shown in Figure 4b, the voltage profiles present sloping lines between 1.2 and 0.2 V and plateaus below 0.2 V, in good agreement with the broad peaks and sharp peaks observed during CV scans. The charge and discharge capacity for the first cycle are 205 and 476 mA h g−1, respectively, with a Coulombic efficiency (CE) of 43.1%. This value is comparable to the reported results,27,29−34 and the large capacity loss is commonly attributed to the decomposition of electrolyte on the surface of the hybrid to form the SEI layer. Note that the specific capacity values are calculated based on the mass of GTC, which contains approximately 7 wt % RGO (see Experimental Section). After the initial cycles, a stable charge capacity of 192 mA h g−1 is obtained for the GTC electrode. Figure 4c shows the cycling performance of the GTC electrode at a current density of 0.2 A g−1 between 0.001 and 3 V vs Na+/Na. The CE of the second cycle exceeds 87% and maintains thereafter above 99% after the 15th cycle. After 2000 deep charge/discharge cycles, the GTC electrode still exhibits a reversible capacity of 190 mA h g−1. To the best of our knowledge, the presented result shows the longest cycle life and stability ever reported for a SIB anode material (Supporting Information, Table S1). In addition, an increase of the capacity of the GTC electrode can be observed during the initial cycles. This activation process might result from the delayed wetting of the electrolyte into the hybrid. Figure 4d shows the rate capability of the GTC electrode. As the current densities increase stepwise from 0.2 to 0.5, 1, 2, 5, and 10 A g−1, reversible capacities remain at 192, 150, 118, 97, 68, and 45 mA h g−1, respectively. When the current density is finally returned to 0.2 A g−1, the charge capacity is still 190 mA h g−1. Control experiments were performed using HDC and RGO as anode materials for SIBs, respectively (Supporting Information, Figures S3 and S4). However, both of them exhibit poorer electrochemical performance than that of GTC (Supporting Information, Figures S5 and S6). After 100 cycles, the charge capacities of HDC and RGO are 169 and 144 mA h g−1, respectively, while GTC delivers a higher capacity of 192 mA h g−1, indicating that the graphene template is indeed beneficial for enhancing the sodium storage properties of GTC. In order to prove this point, the rate capability of HDC was studied under the same testing conditions (Figure 4d). With increasing current density, the capacity of the HDC electrode decreases more rapidly than that of the GTC electrode, which further supports that the graphene template is essential for the excellent electrochemical performance of GTC. To reveal the mechanism for the ultralong cycling stability of the GTC electrode, the cell after different cycles was first investigated using electrochemical impedance spectroscopy (EIS). The tested cell was disassembled after 2000 cycles in fully desodiation conditions, and the morphology and structure of the hybrid were examined using SEM, STEM, and EDS elemental mapping techniques. The EIS analysis of the GTC electrode suggests that the gradually stabilized impedance upon cycling can probably be assigned to the delayed infiltration of electrolyte into the hybrid, in accord with the cycling performance (Supporting Information, Figure S7, and Figure 4c). The SEM images of the electrode before and after cycling test show the production of SEI film on the surface of the electrode without an obvious crack (Figure 5a and 5b), indicating that the integrity of the GTC electrode can be

Figure 5. (a) SEM image of the fresh GTC electrode. (b) SEM image of the GTC electrode after a cycling test. (c) C K-edge XANES spectra of GTC and HDC. (d) SAED pattern of GTC after cycling test.

maintained during cycling. Combined with the STEM image, the elemental mapping images reveal that the SEI layer was evenly formed on the surface of GTC (Supporting Information, Figure S8). In order to disclose the interactions between graphite nanocrystallites and the graphene template in GTC, we conducted XANES measurements (Figure 5c).40 Compared to HDC, GTC shows an obvious decrease of carbon K-edge peak intensities at 286.1 and 289.1 eV, corresponding to carbon atoms in graphitic nanocrystallites bonded with oxygen atoms. This indicates the possible formation of new C−O−C bonding between graphite nanocrystallites and a graphene template in the GTC hybrid, which weakens the electron density of the C− O bonds in the graphite nanocrystallites. Besides, the SAED observation of the cycled GTC also displays a ring-like mode, which can be indexed to graphitic nanocrystallites with an interlayer distance of 0.37 nm, thus confirming that GTC is very stable even after undergoing 2000 cycles of sodiation/ desodiation processes (Figure 5d). These results suggest that the sodiation-induced stress formed during sodium uptake and release processes could be effectively alleviated through the C− O−C bonding and the large layer distance of 0.37 nm, thus leading to the ultralong cycling stability of GTC. The ultralong cycle life and excellent rate performance of GTC can be attributed to the following reasons. First, the C− O−C bonding between graphitic nanocrystallites and the graphene template guarantees the continuous and rapid electron transport. Second, the large interlayer distance of the graphite nanocrystallites promotes sodium-ion transport and storage between the graphene layers.27,29,31,33 Last, but not least, the flexible graphene template along with the strong coupling effect between graphitic nanocrystallites and graphene sheets as well as the large interlayer distance of the graphite nanocrystallites could effectively relieve the sodiation-induced stress during cycling, thus giving the outstanding electrochemical stability of GTC.30,32,34 22429

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(5) Kim, S. W.; Seo, D. H.; Ma, X.; Ceder, G.; Kang, K. Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 710−721. (6) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.; Kim, S.; Ma, X.; Ceder, G. Voltage, Stability and Diffusion Barrier Differences Between Sodium-Ion and Lithium-Ion Intercalation Materials. Energy Environ. Sci. 2011, 4, 3680−3688. (7) Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J. Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Adv. Mater. 2011, 23, 3155−3160. (8) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-Ion Batteries. Adv. Funct. Mater. 2013, 23, 947−958. (9) Gao, X.-P.; Yang, H.-X. Multi-Electron Reaction Materials for High Energy Density Batteries. Energy Environ. Sci. 2010, 3, 174−189. (10) Chen, J.; Cheng, F. Combination of Lightweight Elements and Nanostructured Materials for Batteries. Acc. Chem. Res. 2009, 42, 713− 723. (11) Cao, A.-M.; Hu, J.-S.; Wan, L.-J. Morphology Control and Shape Evolution in 3D Hierarchical Superstructures. Sci. China Chem. 2012, 55, 2249−2256. (12) Abel, P. R.; Lin, Y.-M.; de Souza, T.; Chou, C.-Y.; Gupta, A.; Goodenough, J. B.; Hwang, G. S.; Heller, A.; Mullins, C. B. Nanocolumnar Germanium Thin Films as a High-Rate Sodium-Ion Battery Anode Material. J. Phys. Chem. C 2013, 117, 18885−18890. (13) Jian, Z.; Han, W.; Lu, X.; Yang, H.; Hu, Y.-S.; Zhou, J.; Zhou, Z.; Li, J.; Chen, W.; Chen, D.; Chen, L. Superior Electrochemical Performance and Storage Mechanism of Na3V2(PO4)3 Cathode for Room-Temperature Sodium-Ion Batteries. Adv. Energy Mater. 2013, 3, 156−160. (14) Park, Y.-U.; Seo, D.-H.; Kwon, H.-S.; Kim, B.; Kim, J.; Kim, H.; Kim, I.; Yoo, H.-I.; Kang, K. A New High-Energy Cathode for a NaIon Battery with Ultrahigh Stability. J. Am. Chem. Soc. 2013, 135, 13870−13878. (15) Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B. A Superior Low-Cost Cathode for a Na-Ion Battery. Angew. Chem., Int. Ed. 2013, 52, 1964−1967. (16) Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-Type Nax[Fe1/2Mn1/2]O2 Made from Earth-Abundant Elements for Rechargeable Na Batteries. Nat. Mater. 2012, 11, 512−517. (17) Wang, X.; Liu, G.; Iwao, T.; Okubo, M.; Yamada, A. Role of Ligand-to-Metal Charge Transfer in O3-Type NaFeO2−NaNiO2 Solid Solution for Enhanced Electrochemical Properties. J. Phys. Chem. C 2014, 118, 2970−2976. (18) Zhu, C.; Mu, X.; van Aken, P. A.; Yu, Y.; Maier, J. SingleLayered Ultrasmall Nanoplates of MoS2 Embedded in Carbon Nanofibers with Excellent Electrochemical Performance for Lithium and Sodium Storage. Angew. Chem., Int. Ed. 2014, 53, 2152−2156. (19) Sun, Y.; Zhao, L.; Pan, H.; Lu, X.; Gu, L.; Hu, Y.-S.; Li, H.; Armand, M.; Ikuhara, Y.; Chen, L.; Huang, X. Direct Atomic-Scale Confirmation of Three-Phase Storage Mechanism in Li4Ti5O12 Anodes for Room-Temperature Sodium-Ion Batteries. Nat. Commun. 2013, 4, 1870. (20) Xu, Y.; Zhu, Y.; Liu, Y.; Wang, C. Electrochemical Performance of Porous Carbon/Tin Composite Anodes for Sodium-Ion and Lithium-Ion Batteries. Adv. Energy Mater. 2013, 3, 128−133. (21) Qian, J.; Wu, X.; Cao, Y.; Ai, X.; Yang, H. High Capacity and Rate Capability of Amorphous Phosphorus for Sodium Ion Batteries. Angew. Chem., Int. Ed. 2013, 52, 4633−4636. (22) Rudola, A.; Saravanan, K.; Mason, C. W.; Balaya, P. Na2Ti3O7: an Intercalation Based Anode for Sodium-Ion Battery Applications. J. Mater. Chem. A 2013, 1, 2653−2662. (23) Liu, Y.; Xu, Y.; Zhu, Y.; Culver, J. N.; Lundgren, C. A.; Xu, K.; Wang, C. Tin-Coated Viral Nanoforests as Sodium-Ion Battery Anodes. ACS Nano 2013, 7, 3627−3634.

4. CONCLUSIONS In summary, a durable GTC hybrid has been fabricated through a facile two-step strategy involving a GO-directed self-assembly process and subsequent pyrolysis treatment. When used as an anode material for SIBs, the GTC electrode exhibits an ultralong cycling stability and retains 190 mA h g−1 after 2000 cycles at a current density of 200 mA g−1 (92% capacity retention). Even at a high current density of 10 A g−1, the electrode still can deliver a reversible capacity of 45 mA h g−1. The superior electrochemical performance could be attributed to the strong coupling effect between graphitic nanocrystallites and the graphene template and the large interlayer distance of the graphitic nanocrystallites, both of which can not only effectively alleviate the sodiation-induced stress and maintain the electrode integrity during the sodium insertion/extraction processes but also facilitate the electron and sodium-ion transport. Such a high-performance hybrid structure with large-scale fabrication probably holds great potential for the production of long cycle life and high powder density sodiumion batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Nitrogen adsorption/desorption isotherms, SEM/TEM/STEM images, elemental mappings, and Nyquist plots of GTC, HDC, or RGO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(X.Z.) E-mail: [email protected]. Tel./Fax: +86-2585891027. *(Z.D.) E-mail: [email protected]. Tel./Fax: +86-2585891051. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21175069 and 21171096), the Scientific Research Foundation for Advanced Talents of Nanjing Normal University (2014103XGQ0073), the financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Program of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials. The authors thank Prof. Li-Jun Wan for the fruitful discussions.

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REFERENCES

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dx.doi.org/10.1021/jp5064403 | J. Phys. Chem. C 2014, 118, 22426−22431