Letter pubs.acs.org/NanoLett
NaxMV(PO4)3 (M = Mn, Fe, Ni) Structure and Properties for Sodium Extraction Weidong Zhou,† Leigang Xue,† Xujie Lü,‡ Hongcai Gao,† Yutao Li,*,† Sen Xin,† Gengtao Fu,† Zhiming Cui,† Ye Zhu,*,§ and John B. Goodenough*,† †
Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong ‡
S Supporting Information *
ABSTRACT: NASICON (Na+ super ionic conductor) structures of NaxMV(PO4)3 (M = Mn, Fe, Ni) were prepared, characterized by aberration-corrected STEM and synchrotron radiation, and demonstrated to be durable cathode materials for rechargeable sodium-ion batteries. In Na4MnV(PO4)3, two redox couples of Mn3+/Mn2+ and V4+/V3+ are accessed with two voltage plateaus located at 3.6 and 3.3 V and a capacity of 101 mAh g−1 at 1 C. Furthermore, the Na4MnV(PO4)3 cathode delivers a high initial efficiency of 97%, long durability over 1000 cycles, and good rate performance to 10 C. The robust framework structure and stable electrochemical performance makes it a reliable cathode materials for sodium-ion batteries. KEYWORDS: Sodium cathodes, framework oxide, manganese oxidation, Na+ transport, STEM
T
the crystal structure. Although Na3V2(PO4)3 shows potential promise in terms of applications, the substitution of V with other lower-cost and earth-abundant active redox elements remains desirable due to the relatively high cost of elemental V. Moreover, the replacement of V allows for a possibility to improve the cell voltage. Here, we report an investigation of NASICON structured Na4MnV(PO4)3, Na3FeV(PO4)3, and Na4NiV(PO4)3 as cathode materials for sodium-ion batteries. Both Na4MnV(PO4)3 and Na3FeV(PO4)3 materials deliver an initial capacity of over 100 mAh g−1 and show stable capacities over 1000 cycles. Na4MnV(PO4)3 exhibits two flat voltage plateaus located at about 3.6 and 3.3 V; Na3FeV(PO4)3 shows two voltage plateaus at 3.3 and 2.5 V, both with good capacity retention at 10 C, showing they are highly durable cathodes for sodium-ion batteries with cost-effective elements for large-scale energy storage applications.
he commercialization of large-scale rechargeable batteries for distributed stationary electric power storage and/or for powering electric vehicles is projected to place a too large demand on lithium deposits since there is insufficient lithium conveniently available.1 Therefore, there is broad interest in the development of alternative sodium-ion batteries due to the wider availability and lower cost of Na, which is two hundred times more abundant than lithium in nature.2−7 To make the sodium batteries competitive, a critical component is the cathode material. Three types of sodium-insertion components have been identified: layered oxides or sulfides,8−12 cyanoperovskites,13 and framework oxides.14,15 The layered sodium cathode materials commonly exhibit multiple phase changes and strongly sloping discharge voltages, and the cyanoperovskites have potential environmental problems due to the cyanide anions. Among the framework structures, Na3V2(PO4)3 with the NASICON type structure delivers the best electrochemical performance,16−19 showing a highly reversible capacity of 110 mA h g−1 with a flat voltage plateau of 3.3− 3.4 V associated with the V4+/V3+ redox couple. The initial Coulombic efficiency can reach as high as 97% with good cycling and rate performance owing to the fast Na+ transfer in © XXXX American Chemical Society
Received: September 27, 2016 Revised: November 3, 2016 Published: November 7, 2016 A
DOI: 10.1021/acs.nanolett.6b04044 Nano Lett. XXXX, XXX, XXX−XXX
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diameter of 100 nm stack together with some interparticle pores. In this nanostructure, the small particle dimension and generous interparticle void space allows the good contact with carbon particles and so improves the electrical conductivity since the transition metal phosphates usually suffer from low electrical conductivity as cathode materials. High-resolution scanning transmission electron microscopy (HR-STEM) images in Figure 1 show that both Na4MnV(PO4)3 and Na3FeV(PO4)3 have an ordered single phase crystal structure, and the particle maintains this structure all of the way to the surface. Electron energy loss spectroscopy (EELS) analysis gives strong signals of Mn, V, O and Fe, V, O, indicating a welldistributed existence of elemental Mn, V, O and Fe, V, O in Na4MnV(PO4)3 and Na3FeV(PO4)3 (Figure S1 of the Supporting Information), respectively. Rietveld analysis of the X-ray diffraction (XRD) pattern confirmed the existence of a pure single phase (in Figure 2, Figure S2, and Table S1). The lattice parameters obtained for the Na4MnV(PO4)3 nanoparticles were a = 8.9647 Å and c = 21.4938 Å with a R3̅c trigonal structure; these values compare well with the existing structure of Na4Fe2+Fe3+(PO4)3.21 Na4MnV(PO4)3 has a corner-shared Mn/VO6 octahedral and PO4 tetrahedral units to establish the anion framework [MnV(PO4)3]4− (Figure 2b). Two types of independent Na cations are located in the interstitial space of the framework with different oxygen environments, one for 6-fold coordination (Na1), and three have 10-fold coordination (Na2) per formula unit.22 Owing to a stronger bonding force between O and Na at Na1 sites, the Na2 cations are more easily extracted and inserted in the electrochemical redox process of energy storage. The structure of Na3FeV(PO4)3 is a little different from that of the Na4MnV(PO4)3; a cooperative distortion of the
Na4MnV(PO4)3 and Na3FeV(PO4)3 were synthesized using a sol−gel method followed by calcination of the precursor at 650 °C in argon.20 The scanning electron microscopy (SEM) images in Figure 1 show that worm-like nanoparticles in a
Figure 1. (a) Large-scale SEM image and (b) STEM images of Na4MnV(PO4)3. (c) SEM image and (d) STEM images of Na3FeV(PO4)3.
Figure 2. Synchrotron radiation pattern and Rietveld refinement of (a) Na4MnV(PO4)3 and (c) Na3FeV(PO4)3 with λ = 0.4246 Å. Structural illustration of (b) Na4MnV(PO4)3 and (d) Na3FeV(PO4)3. B
DOI: 10.1021/acs.nanolett.6b04044 Nano Lett. XXXX, XXX, XXX−XXX
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diameter of the Na+, which allows the fast transport of Na+ in these channels (Figure 3c−d and Figure S3). Figure 4a and b shows a clear atomic arrangement in the aberration-corrected STEM images of Na3FeV(PO4)3. Com-
FeO6 octahedron distorts the crystal structure to monoclinic (Figure 2d).23 Nevertheless, Na cations in both are located in two different interstitial positions of the framework with multidimensional Na+ migration pathways. Aberration-corrected STEM, Figure 3 and Figure S3, was employed to obtain a direct vision of the atomic-scale
Figure 4. Aberration-corrected STEM images of Na3FeV(PO4)3 and crystal structure viewed from the directions of (a) [001], (b) [101], (c) [201], and (d) [103]. The superimposed atomic array indicating the locations of each atom [Na (green-blue), P (red), and Fe/V (yellow)].
Figure 3. Aberration-corrected STEM images and crystal structure of Na4MnV(PO4)3 viewed from the (a) [−111], (b) [210], (c) [5−21], and (d) [48−1] crystallographic directions. The superimposed atomic array indicates the locations of each atom [Na (green-blue), P (red), and Mn/V (green)].
pared with the crystal structure projections of Na3V2(PO4)3 with space group R3̅c and Na3Fe2(PO4)3 with C12c1 (Figure S4),27 the STEM images of Na3FeV(PO4)3 are a better match to that of the Na3Fe2(PO4)3 in the directions of [001] and [101]. The other two STEM images also match well with the Na3Fe2(PO4)3 in the direction of [201] and [103] (Figure 4c,d), further demonstrating that the Na3FeV (PO4)3 adopts a monoclinic phase in good agreement with the results of XRD refinements in Figure 2. It is interesting that, although Fe3+ and V3+ have similar radii of 0.64−0.65 Å, the Na3V2(PO4)3 adopts an R3̅c structure while Na3Fe2(PO4)3 adopts a C12c1 structure and the Na3FeV(PO4)3 takes the same monoclinic structure as Na3Fe2(PO4)3. It would appear that the combination of an Fe3+ ion and a Na3 sodium content stabilizes the monoclinic distortion to occur above room temperature. Electrodes prepared from Na4MnV(PO4)3 and Na3FeV(PO4)3 were examined in coin-type half-cells with sodium metal as a counter electrode. The electrochemical behaviors of two materials were first determined by cyclic voltammetry (CV) scans (Figure S5). Both Na4MnV(PO4)3 and Na3FeV(PO4)3 show two redox couples, which can be assigned to the multistep reduction mechanism of the different transition metals. According to previously reported results, the redox peaks centered at about 3.6−3.5 V, 3.4−3.2 V, and 2.7−2.4 V can be attributed, respectively, to the equilibrium voltage of the Mn3+/ Mn2+, V4+/V3+, and Fe3+/Fe2+ redox couples.16,28,29 In agreement with the CV data, two well-defined charge/discharge plateaus were observed in both Na4MnV(PO4)3 and Na3FeV(PO4)3 (in Figure 5a,b). The Na4MnV(PO4)3 delivers a
arrangement in the crystal structure of Na4MnV(PO4)3 for a better understanding of the sodium extraction mechanism. Various projections, including [−111], [210], [001], [110], [42−1], [5−21], and [48−1], were adopted for observation, because separated columns of Na, P, O, and Mn/V ions are aligned in different crystal directions and the atomic displacements are easily missed in a single direction.24−26 As Mn and V share an identical position in Na4MnV(PO4)3, they are nearly indistinguishable in STEM images. The intensity of STEM images is sensitive to the atomic number, and therefore the image contrast is dominated by heavy elements (Mn/V and P). The contrast of Na and O in STEM is much weaker and not clearly discernible in most projections owing to the relatively low atomic number; the only exception is at the [210] zone axis where the Na columns show up next to the P columns (Figure 3b). As shown in Figure 3a−b, the framework structure with void space in the atomic alignment for sodium migration along the [−111] and [210] directions can be clearly observed. More projected lattice images along other directions of the [001], [110], and [42−1] planes in Na4MnV(PO4)3 are shown in Figure S3, where an individual Mn/V surrounded by six PO4 at a regular hexagon can be observed in a [001] direction, in good agreement with the atomic arrangement of the NASICON structure. In four directions of [−111], [110], [5−21], and [48−1], clear pictures of Na-ion transportation channels can be identified, and the dimension of the channel is larger than the C
DOI: 10.1021/acs.nanolett.6b04044 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. Charge and discharge voltage profiles of Na4MnV(PO4)3 (a) and Na3FeV(PO4)3 (b) at different C-rates. (c) Cycling and C-rate performance of Na4MnV(PO4)3 and Na3FeV(PO4)3.
Figure 6. Electrochemical profiles (a) and corresponding ex situ XRD patterns (b) recorded during first charge−discharge of Na4MnV(PO4)3.
capacity around 101 mA h g−1 at 1 C with two charge/ discharge plateaus at 3.6 and 3.3 V, which are associated with the redox behavior of Mn3+/Mn2+ and V4+/V3+. Na3FeV(PO4)3 gives a capacity around 103 mAh g−1 at 1 C with two charge/ discharge plateaus at 3.3 and 2.5 V, corresponding to the redox behavior of V4+/V3+ and Fe3+/Fe2+. Neither the Na4MnV(PO 4 ) 3 nor the Na 3FeV(PO4 ) 3 electrodes experienced significant capacity loss as the C-rate was increased from 1C to 10C (from 101 mA h g−1 to 90 mA h g−1), demonstrating excellent rate performance, which can be attributed to the uniform nanosize of the particles aggregated with a porous morphology and to high speed Na+ migration in the NASICON framework structure.30 Both compounds show a high initial Coulombic efficiency of around 96−97% followed by a stable Coulombic efficiency around 99%, indicating a low initial sodium loss in the formation cycles, which benefits their
application as commercial cathodes since there is no need for an excess sodium source in the full cells. On longer cycling at 1C, stable discharge capacities were observed for both structures, as is shown in Figure 5c. After 1000 cycles, capacities of 90 mA h g−1 and 100 mA h g−1 were obtained for Na4MnV(PO4)3 and Na3FeV(PO4)3, respectively, corresponding to the capacity retentions of around 89% and 95%. Generally, the manganese-based polyanion compounds suffer from some structural distortion in the high oxidation state because of the appearance of a cooperative Jahn−Teller distortion at the Mn3+.31 The good long-term cycling performance of Na4MnV(PO4)3 indicates the presence of V4+ suppresses the cooperative geometric distortion and ensures the structural stability of the Na4MnV(PO4)3 framework during long-term extraction and insertion of sodium ions. D
DOI: 10.1021/acs.nanolett.6b04044 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 7. (a) Charge and discharge voltage profiles and (b) cycling performance of Na4NiV(PO4)3 at different C-rates.
stable structure during long-term sodium extraction-insertion cycling. The XPS spectra show that the bonding energy of V increased and the bonding energy of Ni did not change when charged to 3.5 V, corresponding to the exclusive oxidation of V3+/V4+ at this stage (Figure S9).35 Interestingly, the bonding energy of both V and Ni slightly increased after charge to 4.0 V, meaning that both V and Ni were oxidized during the charge process. Since less than two sodium ions were extracted in Na4NiV(PO4)3, the Ni2+ and V4+ were both partially oxidized to Ni3+ and V5+ during charge from 3.5 to 4.0 V. In conclusion, we prepared three cathode materials for a sodium-ion battery that not only show an acceptable capacity and good initial efficiency but also have long durability and excellent rate performance needed for real applications. In Na4MnV(PO4)3, the introduction of Mn lowers the cost of materials and improves the voltage compared with the Na3V2(PO4)3; the presence of V suppresses a cooperative Jahn−Teller distortion of Mn3+ during charge and stabilizes the structure over long cycling. Therefore, the combination of Mn and V integrates the benefits from each and addresses the problems of each. In addition, these materials exhibit fast ionic transport paths for facile sodium ion diffusion that, coupled with the nanosized morphology, are capable of charge and discharge in a matter of minutes. Compared with the lithiumion batteries presently ubiquitous, these cathodes offer a promising low-cost alternative for stationary electric power storage.
To determine the structural change of the Na4MnV(PO4)3 and Na3FeV(PO4)3 cathodes during the extraction and insertion of sodium-ions, ex situ XRD patterns were obtained at different depths of the first charge and discharge process and are plotted in Figure 6 and Figure S6. The growth of new peaks and simultaneous decrease of existing peaks were clearly observed throughout the entire first cycle. The peak at 2θ = 31−32° for both cathodes disappeared completely when charged to 3.8 V and recovered gradually during discharge. The peak at 2θ = 35−36° experienced an obvious continuously right−left shift in the charge−discharge process, which is consistent with a change of crystal lattices when the transition metal in octahedral MO6 is oxidized/reduced, accompanied by sodium ion extraction/insertion. In addition, some peaks experience merging or splitting, but they recover their original shapes after a full cycle. These phenomena are typically observed in two-phase reactions and suggest that both Na4MnV(PO4) 3 and Na3FeV(PO4) 3 undergo successive biphasic transitions that preserve the overall framework,32 which is consistent with the two well-defined charge−discharge voltage plateaus and superior electrochemical stability. X-ray photoelectron spectroscopy (XPS) studies were carried out to examine the oxidation states of manganese and vanadium in Na4MnV(PO4)3 at different charge states (Figure S7). The charge process is expected to be accompanied by the oxidation of vanadium from V3+ to V4+ and then manganese from Mn2+ to Mn3+. Upon charge to 3.5 V, the binding energy of V increased from 523.8 to 525.1 eV, and the peak of Mn 2p does not show any obvious shift in this state, indicating the formation of a higher oxidation state only of V.16,17 After further charge to 3.8 V, the binding energy of Mn 2p3/2 increased to 642.8 eV from 641.9 eV, indicating the formation of Mn3+.33 These results show the successive two-step extraction of two Na + ions from the Na 4 MnV(PO 4 ) 3 accompanied by the successive oxidation of V3+/V4+ and Mn2+/Mn3+. To investigate further these two-metal phosphates, Na4NiV(PO4)3 was prepared through a similar method; but a pure NASICON phase was not obtained after many tries, which may be because of the formation of some pyrophosphate during the heat treatment (Figure S8).34 In a half cell, as shown in Figure 7, Na4NiV(PO4)3 shows a slightly oblique voltage plateau from 3.9 to 3.3 V with capacities near 80 mA h g−1 at 1 C and 70 mA h g−1 at 5 C, indicating that more than one sodium ion can be reversibly extracted. After 500 cycles, a capacity of 67 mA h g−1 at 5 C was retained, around 83% of initial capacity, indicating a
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04044. More detailed experimental procedures, characterizations, and electrochemical data (PDF) Crystallographic representation (CIF) Crystallographic representation (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Sen Xin: 0000-0002-0546-0626 E
DOI: 10.1021/acs.nanolett.6b04044 Nano Lett. XXXX, XXX, XXX−XXX
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(23) Andersson, A. S.; Kalska, B.; Eyob, P.; Aernout, D.; Haggstrom, L.; Thomas, J. O. Solid State Ionics 2001, 140, 63−70. (24) Jian, Z.; Yuan, C.; Han, W.; Lu, X.; Gu, L.; Xi, X.; Hu, Y.-S.; Li, H.; Chen, W.; Chen, D.; Ikuhara, Y.; Chen, L. Adv. Funct. Mater. 2014, 24, 4265−4272. (25) Guo, S.; Liu, P.; Yu, H.; Zhu, Y.; Chen, M.; Ishida, M.; Zhou, H. Angew. Chem., Int. Ed. 2015, 54, 5894−5899. (26) Wang, P.-F.; You, Y.; Yin, Y.-X.; Wang, Y.-S.; Wan, L.-J.; Gu, L.; Guo, Y.-G. Angew. Chem., Int. Ed. 2016, 55, 7445−7449. (27) Masquelier, C.; Wurm, C.; Rodríguez-Carvajal, J.; Gaubicher, J.; Nazar, L. Chem. Mater. 2000, 12, 525−532. (28) Barpanda, P.; Ye, T.; Avdeev, M.; Chung, S.-C.; Yamada, A. J. Mater. Chem. A 2013, 1, 4194−4197. (29) Moreau, P.; Guyomard, D.; Gaubicher, J.; Boucher, F. Chem. Mater. 2010, 22, 4126−4128. (30) Rui, X.; Sun, W.; Wu, C.; Yu, Y.; Yan, Q. Adv. Mater. 2015, 27, 6670−6676. (31) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587−603. (32) Shakoor, R. A.; Seo, D.-H.; Kim, H.; Park, Y.-U.; Kim, J.; Kim, S.-W.; Gwon, H.; Lee, S.; Kang, K. J. Mater. Chem. 2012, 22, 20535− 20541. (33) Carver, J. C.; Schweitzer, G. K.; Carlson, T. A. J. Chem. Phys. 1972, 57, 973−982. (34) Kim, H.; Shakoor, R. A.; Park, C.; Lim, S. Y.; Kim, J.-S.; Jo, Y. N.; Cho, W.; Miyasaka, K.; Kahraman, R.; Jung, Y.; Choi, J. W. Adv. Funct. Mater. 2013, 23, 1147−1155. (35) Nanba, Y.; Iwao, T.; Boisse, B. M.; Zhao, W.; Hosono, E.; Asakura, D.; Niwa, H.; Kiuchi, H.; Miyawaki, J.; Harada, Y.; Okubo, M.; Yamada, A. Chem. Mater. 2016, 28, 1058−1065.
Ye Zhu: 0000-0002-5217-493X John B. Goodenough: 0000-0001-9350-3034 Author Contributions
W.Z. and L.X. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation grant number CBET-1438007. Y.Z. was financially supported by The Hong Kong Polytechnic University grant (1-ZE6G). J.G. would also like to thank the Robert A. Welch Foundation Grant number F-1066. Y.Z. thanks Prof. Joanne Etheridge for granting the access of the FEI Titan TEM/STEM at the Monash Centre for Electron Microscopy and Prof. Matthew Weyland for optimizing the Titan microscope for STEM imaging.
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REFERENCES
(1) Tarascon, J. M. Nat. Chem. 2010, 2, 510. (2) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; CarreteroGonzalez, J.; Rojo, T. Energy Environ. Sci. 2012, 5, 5884−5901. (3) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Adv. Funct. Mater. 2013, 23, 947−958. (4) Xiang, X.; Zhang, K.; Chen, J. Adv. Mater. 2015, 27, 5343−5364. (5) Pan, H. L.; Hu, Y. S.; Chen, L. Q. Energy Environ. Sci. 2013, 6, 2338−2360. (6) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. Angew. Chem., Int. Ed. 2015, 54, 3431−3448. (7) Kim, S. W.; Seo, D. H.; Ma, X. H.; Ceder, G.; Kang, K. Adv. Energy Mater. 2012, 2, 710−721. (8) Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. Nat. Mater. 2012, 11, 512−517. (9) Yuan, D.; Liang, X.; Wu, L.; Cao, Y.; Ai, X.; Feng, J.; Yang, H. Adv. Mater. 2014, 26, 6301−6306. (10) Guo, S.; Liu, P.; Yu, H.; Zhu, Y.; Chen, M.; Ishida, M.; Zhou, H. Angew. Chem., Int. Ed. 2014, 53, 8963−8969. (11) Oh, S.-M.; Myung, S.-T.; Yoon, C. S.; Lu, J.; Hassoun, J.; Scrosati, B.; Amine, K.; Sun, Y.-K. Nano Lett. 2014, 14, 1620−1626. (12) Billaud, J.; Clément, R. J.; Armstrong, A. R.; Canales-Vázquez, J.; Rozier, P.; Grey, C. P.; Bruce, P. G. J. Am. Chem. Soc. 2014, 136, 17243−17248. (13) Song, J.; Wang, L.; Lu, Y.; Liu, J.; Guo, B.; Xiao, P.; Lee, J.-J.; Yang, X.-Q.; Henkelman, G.; Goodenough, J. B. J. Am. Chem. Soc. 2015, 137, 2658−2664. (14) Barpanda, P.; Oyama, G.; Nishimura, S.; Chung, S. C.; Yamada, A. Nat. Commun. 2014, 5, 4358. (15) Ellis, B. L.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. Nat. Mater. 2007, 6, 749−753. (16) Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. Adv. Energy Mater. 2013, 3, 444−450. (17) Jian, Z.; Han, W.; Lu, X.; Yang, H.; Hu, Y.-S.; Zhou, J.; Zhou, Z.; Li, J.; Chen, W.; Chen, D.; Chen, L. Adv. Energy Mater. 2013, 3, 156− 160. (18) Goodenough, J. B.; Hong, H. Y. P.; Kafalas, J. A. Mater. Res. Bull. 1976, 11, 203. (19) Masquelier, C.; Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Solid State Chem. 1998, 135, 228. (20) Kang, J.; Baek, S.; Mathew, V.; Gim, J.; Song, J.; Park, H.; Chae, E.; Rai, A. K.; Kim, J. J. Mater. Chem. 2012, 22, 20857−20860. (21) Hatert, F. Acta Crystallogr., Sect. E: Struct. Rep. Online 2009, 65, i30. (22) Song, W.; Cao, X.; Wu, Z.; Chen, J.; Huangfu, K.; Wang, X.; Huang, Y.; Ji, X. Phys. Chem. Chem. Phys. 2014, 16, 17681−17687. F
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