Interface Chemistry Engineering for Stable Cycling of Reduced GO

Letter pubs.acs.org/NanoLett

Interface Chemistry Engineering for Stable Cycling of Reduced GO/ SnO2 Nanocomposites for Lithium Ion Battery Lei Wang,†,‡ Dong Wang,† Zhihui Dong,† Fengxing Zhang,‡ and Jian Jin†,* †

i-LAB and Nano-bionics Division, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China ‡ Key Laboratory of Synthesis and Natural Functional Molecular Chemistry (Ministry of Education), College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710069, China S Supporting Information *

ABSTRACT: From the whole anode electrode of view, we report in this work a system-level strategy of fabrication of reduced graphene oxide (RGO)/SnO2 composite-based anode for lithium ion battery (LIB) to enhance the capacity and cyclic performance of SnO2-based electrode materials. RGO/ SnO2 composite was first coated by a nanothick polydopamine (PD) layer and the PD-coated RGO/SnO2 composite was then cross-linked with poly(acrylic acid) (PAA) that was used as a binder to accomplish a whole anode electrode. The cross-link reaction between PAA and PD produced a robust network in the anode system to stabilize the whole anode during cycling. As a result, the designed anode exhibits an outstanding energy capacity up to 718 mAh/g at current density of 100 mA/g after 200 cycles and a good rate performance of 811, 700, 641, and 512 mAh/g at current density of 100, 250, 500, and 1000 mA/g, respectively. Fourier transform IR spectra confirm the formation of cross-link reaction and the stability of the robust network after long-term cycling. Our results indicate the importance of designing interfaces in anode system on achieving improved performance of electrode of LIBs. KEYWORDS: Interface chemistry engineering, RGO/SnO2 nanosheets, polydopamine, lithium-ion battery

D

performance.21−30 But still, the performance of SnO2-based anode is not very satisfactory, especially the decay of capacity with long-term cycling, due to disintegration of the whole electrode. Materials science has evolved over the past decades. However, most of research on electrode for energy storage has been focused on active material itself. It is clear that investigating isolated active materials is no longer sufficient to solve all kinds of technological challenges for the development of modern battery infrastructure.31 We should have to pay considerable amounts of attention on the entire electrode system where studying the interface between individual components within the system is of paramount importance. As for an anode system, it usually consists of active materials, buffer layer (nonessential elements but beneficial to the whole system), binder, current collector, and conductive additive. It is expected that the further improvement of energy-storage performance relies on the optimization of the entire system and emphasizes the study of the interactions between these individual components, and how these interactions give rise to the function and performance of the final system. Therefore, it is necessary to investigate the behavior of the entire system

eveloping anode materials with high energy density and long-term cyclic stability is one of the hot topics in lithium ion battery (LIB) research.1−4 Tin oxide (SnO2) is a promising high energy density material with theoretical capacity up to ∼750 mAh/g, which considerably exceed that of commercial graphitic anodes (∼372 mAh/g). However, during charging and discharging process, reversible intercalation of lithium ions between lattices of SnO2 crystals occurs through the so-called conversion reaction mechanism, which causes great volume change and crystallinity decrease of SnO2, thus degrading anode performance.5−8 Designing nanostructured SnO2 has been demonstrated to be an effective solution to partially overcome the aforementioned obstacles.9−14 Nanostructured SnO2 could withstand mechanical strain during lithium ion insertion/desertion better than bulk and provide shorter path length for the transport of electron and lithium ion, improving conductivity, and charge/discharge rate. As a result, pulverization and electrical connectivity loss caused by large and uneven volume change of SnO2 could be weakened to a certain extent. However, due to high surface-to-volume ratio and high-surface free energy of nanomaterials undesirable side reactions can occur easily including electrolyte degradation, which also causes poor cyclic performance. To address this issue, a series of studies on carbon/SnO2 nanoparticles (NPs) hybrid materials were performed in the past few years,15−20 among which various reduced graphene oxide (RGO)/SnO2 NPs hybrid materials have shown promising electrochemical © 2013 American Chemical Society

Received: January 21, 2013 Revised: March 6, 2013 Published: March 11, 2013 1711

dx.doi.org/10.1021/nl400269d | Nano Lett. 2013, 13, 1711−1716

Nano Letters

Letter

Figure 1. Characterization of RGO/SnO2 composite. SEM image (a, inset shows optical image), C 1s (b) and Sn 3d (c) XPS spectra of as-prepared RGO/SnO2 aerogel. TEM (d, inset shows SAED), HRTEM (e) and AFM (f) images of dispersed RGO/SnO2 aerogel after treated by ultrasonication.

Figure 2. Characterization of PD-coated RGO/SnO2 composite. TEM image (a), TG and DTG curves (b), DF-STEM image and corresponding element mapping images of C, N, O, and Sn (c).

from the viewpoint of interface designing between individual components within the anode electrode. We report herein a system-level strategy of designing RGO/ SnO2 composites based anode electrode aims at enhancing the energy-storage performance of RGO/SnO2-based materials,

especially their cyclic performance. The RGO/SnO2 composite is first coated by polydopamine (PD) layer which function as a buffer. A cross-link reaction is then built between buffer and polyacrylic acid (PAA) binder to integrate individual RGO/ SnO2 composites into a whole system through forming a robust 1712

dx.doi.org/10.1021/nl400269d | Nano Lett. 2013, 13, 1711−1716

Nano Letters

Letter

(Supporting Information Figure S4). Thermogravimetry (TG) and derivative thermogravimetry (DTG) analysis were used to quantify the amount of PD in the composite (Figure 2b). The mass loss between 300 and 650 °C is due to the oxidation of RGO (Supporting Information Figure S5a). The mass loss appearing between 200 and 600 °C is attributed to the oxidation of PD (Supporting Information Figure S5b). It is calculated that the weight percentage of PD in the composite is 9 wt %. Correspondingly, the weight percentage of SnO2, RGO, and PD is 68, 23, and 9 wt %, respectively. Dark-field scanning transmission electron microscopy (DF-STEM) image and corresponding elemental mapping images demonstrate that the elements Sn, O, and N are evenly distributed in the sample (Figure 2c). These results clearly show that PD layer uniformly coats on RGO/SnO2 composites and that there are nearly no free PD and bare RGO/SnO2 composites in the sample. In this work, PD was chosen as a buffer layer to coat RGO/ SnO2 composites. On the one hand, as a relatively soft and elastic polymer, PD could endure a large volume change coming from contraction and expansion of SnO2 NPs through adjusting its own elastic deformation, thus releasing partial pressure of SnO2 NPs during cycling. On the other hand, the existence of PD layer could avoid direct contact between electrolyte and RGO/SnO2 composites. During lithiation process, lithium ions have to penetrate through the PD layer before reaching RGO/SnO2, which is helpful to diminish the occurrence of side reaction at the electrode−electrolyte interface.36,37 Most importantly, by means of PD layer, a cross-link reaction could be designed between PD-coated RGO/SnO2 composite and binder through forming amide bond to further enhance the cycling stability of the whole anode system. To achieve cross-link reaction between PD and PAA, the working electrode containing PD-coated RGO/SnO2 composite, carbon black, and PAA (weight ratio of 80:10:10) pasted on copper foil as current collector was treated at 150 °C under vacuum condition. The chemical equation between PD and PAA is shown in Figure 3a. Fourier transform infrared spectroscopy (FTIR) was used to confirm the formation of cross-link reaction (Figure 3b). Without cross-link, hydrogen bond dominates the range from 3700 to 2800 cm−1, among which the broad absorption band between 3200 to 2800 cm−1 belongs to the hydrogen bond between −COOH groups of PAA and a relatively strong peak around 3400 cm−1 is ascribed to N−H/O-H (or N−H/N−H, O−H/O−H) hydrogen bond of PD. The bands at 1716 and 1400 cm−1 are the characteristic vibrations of CO stretching and O−H bending of carboxyl group in PAA, respectively. The strong absorption at 1570 cm−1 is due to N−H bending vibration of secondary amine in PD. For the spectrum with cross-link, the broad band between 3200 to 2800 cm−1 almost disappears, which indicates the hydrogen bond between −COOH groups of PAA is weakened greatly after cross-link reaction. Meanwhile, the bands at 1716, 1570, and 1400 cm−1 are all decreased obviously. Instead, a new peak at 1631 cm−1 appears which is attributed to CO stretching vibration of amide. This result provides a concrete proof for the occurrence of cross-link reaction between binder and buffer through the dehydration of carboxyl group of PAA and amino group of PD to produce tertiary amide. As a complement, the other bands below 750 cm−1 are associated with Sn−O vibration. We therefore conclude that covalent amide bond is formed between PAA and PD under this condition. To quantitatively examine the effect of cross-link on

network of covalent bond in the anode. By such a design, two stable interfaces between active material (RGO/SnO2 composite) and buffer layer and between buffer layer and binder are built to improve the stability of electrode. Our results indicate the effectiveness of system engineering to the final performance of anode electrode, especially its long-term cycle stability. As a result, the designed anode system exhibits an outstanding energy capacity up to 718 mAh/g at current density of 100 mA/g after 200 cycles. Results and Discussion. RGO/SnO2 aerogel was first prepared using hydrothermal method. Figure 1a shows the scanning electron microscopy (SEM) and optical images of asprepared RGO/SnO2 aerogel. A well-defined and interconnected three-dimensional (3D) porous network with hierarchical pores can be clearly recognized. Pores with diameters in the range from several hundred nanometers to tens of micrometers are embedded within the ultrathin layer of aerogel matrix. The RGO/SnO2 aerogel exhibits a high surface area of 161 m2/g based on BET measurement (Supporting Information Figure S3). Through forming RGO/SnO2 aerogel, it effectively prevents the aggregation of RGO/SnO2 composite, which is advantageous to carry out a complete and uniform buffer coating on RGO/SnO2 composite in the following step. Figure 1b,c displays C 1s and Sn 3d X-ray photoelectron spectroscopy (XPS) spectra of RGO/SnO2 composites. The C 1s XPS spectrum of as-prepared graphene oxide (GO) is also given in Figure 1b as a comparison. The as-prepared GO gives three C 1s peaks located at 284.6 eV attributed to CC of sp2 hybridized carbon atoms, 286.8 eV to C−OH and/or CO bond, and 288.9 eV to COOH bond, respectively.32 As for RGO/SnO2 aerogel, the oxygenate species of GO almost disappear, revealing the reduction of GO to RGO. The two characteristic peaks at 487.6 eV (Sn 3d5/2) and 496.1 eV (Sn 3d3/2) are observed in Figure 1c, confirming the formation of SnO2. To effectively coat a uniform PD layer on RGO/SnO2 composite, RGO/SnO2 aerogel was dispersed in water by ultrasonic treatment to produce a homogeneous RGO/SnO2 suspension. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high resolution TEM (HRTEM) were used to examine dispersed RGO/SnO2 composites as shown in Figure 1d,e. It shows that RGO/SnO2 aerogel has been successfully dispersed into RGO/SnO2 nanosheets. SnO2 NPs with average size of 5 nm densely distribute on the surfaces of RGO nanosheets. SAED shows two bright diffraction rings which are attributed to (110) and (101) crystal planes of SnO2 and several weak diffraction dots which are attributed to RGO. The lattice distance of 0.33 nm obtained from HRTEM images agrees with the lattice spacing of (110) crystal plane of SnO2. Atomic force microscopy (AFM) image shows that the thickness of an individual RGO/ SnO2 nanosheet is around 14 nm, which means SnO2 NPs grow on both the two faces of RGO (Figure 1f). PD has been proved to be a powerful building block for spontaneously coating a wide variety of materials surfaces in the form of polydopamine films.33 Recent results also demonstrated that PD could improve a lot of critical properties such as electrolyte wetting, the electrolyte uptake, and the ionic conductivity.34,35 In situ coating of PD layer on RGO/SnO2 nanosheets was carried out via self-polymerization of dopamine in weak base condition according to previous reports.33 Compared to bare RGO/SnO2 nanosheets, the PD-coated RGO/SnO2 composites are quite blurry due to PD covering (Figure 2a) and their surface area sharply reduces to 9 m2/g 1713

dx.doi.org/10.1021/nl400269d | Nano Lett. 2013, 13, 1711−1716

Nano Letters

Letter

between them is weak van der Waals interaction and/or hydrogen bond. As for PD-coated RGO/SnO2 composite based electrode, two main interactions including hydrogen bond and catecholic interaction are expected when contact with PAA. The catecholic interaction has been confirmed to be a far stronger interaction than hydrogen bond and other binding forces.38 Furthermore, by means of cross-linking, strong and robust covalent bond is formed between PD and PAA, which enhances the interaction between RGO/SnO2 composite and binder even more. Obviously, the performance of reversible capacity is directly related to the strength of interaction between electrode material and binder. Stronger interaction gives rise to a higher reversible capacity as the whole structure of anode could be stabilized to the most degree after the first discharging.39 Such effect plays a more prominent role in the high-capacity anode materials, such as SnO2 due to their considerable volume expansion during lithium ion insertion/ desertion. Figure 4c,d shows the long-term cyclic and rate performance of PD-coated RGO/SnO2 composite-based electrode without and with cross-link. The electrode without cross-link delivers discharge capacity of 464 and 327 mAh/g at the current density of 100 and 500 mA/g, respectively, after 200 cycles. In rate performance test, it delivers the discharge capacity of 646, 511, 409, and 311 mAh/g at current density of 100, 250, 500, and 1000 mA/g, respectively. The discharge capacity recovers to 322 mAh/g when the current density goes back to 500 mA/g. With cross-link reaction, the electrode exhibits a significantly enhanced cyclic performance compared to the one without cross-link. The discharge capacities of 718 and 514 mAh/g at the current density of 100 and 500 mA/g, respectively, after 200 cycles are delivered. The rate performance of the electrode is also greatly improved. It delivers discharge capacity of 811, 700, 618, and 535 mAh/g when the current density is 100, 250, 500, and 1000 mA/g, respectively, and the discharge capacity recovers to 647 mAh/g when the current density goes back to 500 mA/g. These results clearly demonstrate the importance and validity of covalent bonds between buffer layer and binder on stabilizing the cyclic performance and improving electrode capacity. Electrochemical impedance spectroscopy (EIS) measurement was conducted on the three electrodes (bare RGO/SnO2 composite based electrode, PD-coated RGO/SnO2 composite-based electrode without cross-link, and PD-coated RGO/SnO2 composite based electrode with cross-link) (Supporting Information Figure S10). The Nyquist plots show PD coating and PD/PAA cross-linking have less effect on ohmic resistance and cross-linking between PD and PAA is advantageous to stabilize the charge-transfer resistance of electrode during cycling. Also, the covalent bond is so robust that it still maintain after many cycles of charge/discharge process as confirmed by FTIR spectra (Supporting Information Figure S11). As compared to hydrogen bond or weak van der Waals interaction that usually appears in reported electrodes, the strong covalent bond formed here could effectively integrate the individual components in the electrode into a whole system and greatly improve the stability of the electrode to withstand structure damaging during cycling. Conclusions. In summary, we designed a new RGO/SnO2based electrode composite for LIBs through coating it by a functional buffer layer and cross-linking the buffer layer and binder to address the issue of poor cyclic performance of SnO2based anodes. Our RGO/SnO2 composite exhibited a good cyclic performance with capacity up to 718 mAh/g at current

Figure 3. Characterization of cross-link reaction. FTIR spectra of electrode materials with and without cross-link (a). Stress−strain curves of electrode materials with and without cross-link (b).

mechanical strength of the electrode, stress−strain curves were measured as shown in Figure 3c (see Supporting Information for more details). Obviously, tensile strength increases 66% from 10.9 to 18.1 MPa after cross-linking. A series of electrochemical tests were carried out to investigate the cyclic performance of the electrode. In galvanostatic measurements with the potential range of 0.01− 2.0 V (versus Li/Li+) (Figure 4a,b), two charge−discharge peaks around 0.5 and 1.5 V are observed no matter with or without cross-link, which is accordance with previous reports.7 However, the electrodes with and without cross-link exhibit different reversible capacities. A reversible capacity of 800 mAh/g that corresponds to a Coulombic efficiency (CE) of 57% is obtained in case of without cross-link. It increases to 931 mAh/g in case of with cross-link, which corresponds to a CE of 68%. As comparison, bare RGO/SnO2 composite based electrode without PD coating was also tested. It displays a much lower reversible capacity of 520 mAh/g and CE of 40% (Supporting Information Figure S6). These results demonstrate the importance of PD coating and cross-link interaction between PD and PAA on the first cycle performance. As for bare RGO/SnO2 composite based electrode, PAA directly contacts with RGO/SnO2 composite. The main interaction 1714

dx.doi.org/10.1021/nl400269d | Nano Lett. 2013, 13, 1711−1716

Nano Letters

Letter

Figure 4. Characterization of electrochemical performance. Charge/discharge profiles (a,b) (current density for a and b is 100 mA/g), cyclic performance (c) (current density of the first cycle is 100 mA/g), and rate performance (d) of PD-coated RGO/SnO2 composite based anode without and with cross-link.

supported by the National Basic Research Program of China (Grants 2013CB933000 and 2010CB934700), the National Natural Science Foundation of China (Grant 21004076), and the Key Development Project of Chinese Academy of Sciences (Grant KJZD-EW-M01-3).

density of 100 mA/g after 200 cycles and an excellent rate performance. The great improvement of cyclic and rate performance achieved in this anode is attributed to the introduction of PD buffer layer that effectively integrate the individual components into a whole system and build two stable interfaces between active material (RGO/SnO2) and buffer layer and between buffer layer and binder. Our results show that there is still a plenty of space for improving electrode performance as long as we design the electrode from the view of whole system. The strategy for designing stable interfaces in SnO2-based anode system could be extended to other highcapacity cathode and anode materials systems to address the issue of volume expansion. Our methodology provides a new concept and applicable way to design next-generation LIBs with high performance.

■

■

ASSOCIATED CONTENT

S Supporting Information *

Details for experiment, AFM image of GO, XRD spectrum, N2 adsorption and desorption curves and TG and DTG curves of RGO/SnO2 aerogel, detailed information of cyclic performance, Nyquist plots,and FTIR of cycled electrode. This material is available free of charge via the Internet at http://pubs.acs.org

■

REFERENCES

(1) Chu, S.; Majumdar, A. Nature 2012, 488, 294−303. (2) Scrosati, B.; Grache, J. J. Power Sources 2010, 195, 2419−2430. (3) Armand, M.; Tarascon, J. M. Nature 2008, 451, 652−657. (4) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587−603. (5) Wu, H. B.; Chen, J. S.; Lou, X. W.; Hng, H. H. J. Phys. Chem. C 2011, 115, 24605−24610. (6) Huang, T. Y.; Zhong, L.; Wang, C. M.; Sullivan, J. P.; Xu, W.; Zhang, L. Q.; Mao, S. X.; Hudak, N. S.; Liu, X. H.; Subramanian, A.; Fan, H.; Qi, L.; Kushima, A.; Li, J. Science 2010, 330, 1515−1520. (7) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395−1397. (8) Larcher, D.; Beattie, S.; Morcrette, M.; Edström, K.; Jumas, J. C.; Tarascon, J. M. J. Mater. Chem. 2007, 17, 3759−3772. (9) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascan, J. M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366−377. (10) Wang, Z.; Luan, D.; Boey, F. Y. C.; Lou, X. W. J. Am. Chem. Soc. 2011, 133, 4738−4741. (11) Ding, S.; Chen, J. S.; Qi, G.; Duan, X.; Wang, Z.; Giannelis, E. P.; Archer, L. A.; Lou, X. W. J. Am. Chem. Soc. 2011, 133, 21−23. (12) Subramanian, V.; William, W. B.; Zhu, H. W.; Wei, B. J. Phys. Chem. C 2008, 112, 4550−4556. (13) Kim, C.; Noh, M.; Choi, M.; Cho, J.; Park, B. Chem. Mater. 2005, 17, 3297−3301. (14) Yin, X. M.; Li, C. C.; Zhang, M.; Hao, Q. Y.; Liu, S.; Chen, L. B.; Wang, T. H. J. Phys. Chem. C 2010, 114, 8084−8088. (15) Lin, Y. S.; Duh, J. G.; Hung, M. H. J. Phys. Chem. C 2010, 114, 13136−13141. (16) Lou, X. W.; Li, C. M.; Archer, L. A. Adv. Mater. 2009, 21, 2536− 2539. (17) Sun, X.; Liu, J.; Li, Y. Chem. Mater. 2006, 18, 3686−3494.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Professor Liwei Chen and Ms. Weiling Dong at Suzhou Institute of Nanotech and Nanobionics (SINANO) for providing instrument for battery assembling. This work was 1715

dx.doi.org/10.1021/nl400269d | Nano Lett. 2013, 13, 1711−1716

Nano Letters

Letter

(18) Yu, X.; Yang, S.; Zhang, B.; Shao, D.; Dong, X.; Fang, Y.; Li, Z.; Wang, H. J. Mater. Chem. 2011, 21, 12298−12302. (19) Park, M. S.; Kang, Y. M.; Dou, S. X.; Liu, H. K. J. Phys. Chem. C 2008, 112, 11286−11289. (20) Wang, L.; Wang, D.; Zhang, F. X.; Jin, J. RSC Adv. 2013, 3, 1307−1310. (21) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666−669. (22) Park, S. K.; Yu, S. H.; Pinna, N.; Woo, S.; Jang, B.; Chung, Y. H.; Cho, Y. H.; Sung, Y. E.; Piao, Y. J. Mater. Chem. 2012, 22, 2520−2525. (23) Wang, X.; Cao, X.; Bourgeois, L.; Guan, H.; Chen, S.; Zhong, Y.; Tang, D. M.; Li, H.; Zhai, T.; Li, L.; Bando, Y.; Golberg, D. Adv. Funct. Mater. 2012, 22, 2682−2690. (24) Zhong, C.; Wang, J.; Chen, Z.; Liu, H. J. Phys. Chem. C 2011, 115, 25115−25120. (25) Zhou, X.; Yin, Y. X.; Wan, L. J.; Guo, Y. G. J. Mater. Chem. 2012, 22, 17456−17459. (26) Li, Y.; Lv, X.; Lu, J.; Li, J. J. Phys. Chem. C 2010, 114, 21770− 21774. (27) Liang, J. W.; Wei, W.; Zhang, D.; Yang, Q.; Li, L.; Guo, L. ACS Appl. Mater. Interfaces 2012, 4, 454−459. (28) Xu, C.; Sun, J.; Gao, L. J. Mater. Chem. 2012, 22, 975−979. (29) Wang, X.; Zhou, X.; Yao, K.; Zhang, J.; Liu, Z. Carbon 2011, 49, 133−139. (30) Paek, S. M.; Yoo, E. J.; Honma, I. Nano Lett. 2009, 9, 72−75. (31) Yang, P. D.; Tarascon, J. M. Nat. Mater. 2012, 11, 560−563. (32) Wang, L.; Wang, D.; Dong, X.; Zhang, Z.; Pei, X.; Chen, X.; Chen, B.; Jin, J. Chem. Commun. 2011, 47, 3556−3558. (33) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Science 2007, 318, 426−430. (34) Ryou, M. H.; Lee, Y. M.; Park, J. K.; Choi, J. W. Adv. Mater. 2011, 23, 3066−3070. (35) Ryou, M. H.; Lee, D. J.; Lee, J. N.; Lee, Y. M.; Park, J. K.; Choi, J. W. Adv. Energy Mater. 2012, 2, 645−650. (36) Mcdowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y. Nano Lett. 2011, 11, 4018−4025. (37) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; Mcdowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y. Nat. Nanotechnol. 2012, 7, 310−315. (38) Ryou, M. H.; Kim, J.; Lee, I.; Kim, S.; Jeong, Y. K.; Hong, S.; Ryu, J. H.; Kim, T. S.; Park, J. K.; Lee, H.; Choi, J. W. Adv. Mater. 2012, DOI: 10.1002/adam.201203981. (39) Koo, B.; Kim, H.; Cho, Y.; Lee, K. T.; Choi, N. S.; Cho, J. Angew. Chem., Int. Ed. 2012, 51, 8762−8767.

1716

dx.doi.org/10.1021/nl400269d | Nano Lett. 2013, 13, 1711−1716