Crystallization-Induced Morphological Tuning Toward Denim-like Graphene Nanosheets in a KCl-Copolymer Solution Yaxin Chen,†,‡ Liluo Shi,‡ Qiong Yuan,‡ Ang Li,‡ Shaozhuan Huang,§ Hui Ying Yang,§ Xiaohong Chen,‡ Jisheng Zhou,‡ and Huaihe Song*,†,‡ †
Beijing Advanced Innovation Center for Soft Matter Science and Engineering and ‡State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China § Pillar of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore S Supporting Information *
ABSTRACT: Although nucleation and crystallization in solution-processed materials synthesis is a natural phenomenon, the morphology design of graphene nanosheets by controlling the dual crystallization has not been established. In this work, we systematically demonstrate how the dual crystallization of ice and potassium chloride induces the morphological variation of the freeze-dried scaffold from fractal structure toward stepped sheet-like structure. A denim-like graphene nanosheet (DGNS) has been fabricated by annealing the F127-coated stepped sheet-like scaffold in nitrogen. DGNS shows parallel and straight stripes with an average stripe spacing of 10 nm. When used as a lithium-ion battery anode, DGNS possesses a superhigh reversible capacity of 1020 mAh g−1 at the current density of 1 A g−1 after 600 cycles. This work reports the control of dual crystallization of ice and salt crystals and provides an efficient way to design the morphology of two-dimensional materials by adjusting the crystallization. KEYWORDS: crystallization, morphological tuning, denim-like graphene nanosheets, stripes, lithium-ion battery materials.17−22 Carbon nanosheets can be fabricated on the surface of a cubic salt template. For example, porous carbon nanosheets with a thickness of tens of nanometers were prepared using glucose as the carbon precursor and sodium chloride (NaCl) as the template.23 A composite of carbon nanocages and carbon nanosheets was synthesized by using 1hexadecylamine as the carbon precursor and potassium chloride (KCl) as the template with the catalysis of CoCl2·6H2O.24 In our previous work, we also prepared carbon nanosheets with the thickness of ca. 6 nm using F127 as the carbon precursor and NaCl as the template.25 Although the salt crystals have been used to fabricate 2D materials, the thickness of most of these products is quite high. Moreover, the crystal growth has not been well controlled, and the mechanism of the salt template on the 2D carbon material formation is still lacking.
T
he graphene nanosheet (GNS) is an ideal twodimensional material with a series of advantages, such as high electrical and thermal conductivity, chemical inertness, and high flexibility.1−3 Since its debut,2 the GNS has attracted great interest for the potential application in many fields, especially energy storage devices and catalysts.4−7 In this regard, mass production of GNSs is critical to meet the growing demands. The fabrication of GNSs has been developed into mainly two strategies: top-down and bottom-up approaches.3,8 Top-down approach separates the GNS from the bulk graphite by breaking the bonds between the graphite layers.2,6,8−10 Conversely, bottom-up approach assembles small molecule units to a graphene framework.11 Many templates or substrates can be chosen to fabricate GNSs.12 For example, epitaxial growth on silicon carbide and metal substrate has been widely reported to fabricate GNSs.12−16 However, the low deposition rate and the difficult removal of substrate impede mass production. Recently, salt crystals have been developed to be used as the template to enrich the preparation of 2D © 2018 American Chemical Society
Received: March 6, 2018 Accepted: April 2, 2018 Published: April 2, 2018 4019
DOI: 10.1021/acsnano.8b01708 ACS Nano 2018, 12, 4019−4024
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and local nanocrystallites. It can further be corroborated by the broad and dispersive (002) diffraction peak at ca. 23° in the Xray diffraction (XRD) pattern (Figure S1a) and the ID/IG of 0.92 from the Raman spectrum (Figure S1b). As revealed in the N 2 adsorption/desorption isotherms (Figure S1c), the desorption curve has a typical H4-type hysteresis loop, which is associated with the existence of narrow slit-like mesopores.38 The slit-like mesopores are ascribed to a stack of the curled 2D GNSs. Imaging the area marked by the yellow dotted line at a high magnification, one can see that parallel stripes are displayed evenly on the DGNS like denim (Figure 1b). More clearly, high-resolution transmission electron microscopy (HRTEM) image (Figure 1e) presents the parallel and straight stripes with an average stripe spacing of 10 nm. After extra long-term sonication in ethanol, the DGNS was disrupted into nanobelts, as shown in Figure 1f,g. It implies that DGNS can be considered as splicing of parallel nanobelts. The morphology of GNS (Figure 2, Figure S2) can be controlled by variation of the KCl concentration. A sequential set of AFM images (Figure 2a,b,d,e, Figure 1d) displays the thickness variation of products with the increase of KCl concentration. More than 20 sites in one product were chosen to calculate the average thickness (Figure 2c). The average thickness of GNS shows a “V-shape” with the lowest value of ∼2.8 nm when the KCl concentration is 150 g L−1. More significantly, the surface morphology can be well tuned from messy points (Figure 2f, 40 g L−1) over short curved stripes (Figure 2g, 100 g L−1) to long-range straight parallel stripes (Figure 2h, 150 g L−1). The morphology tuning of these GNSs draws our attention to explore the transformation mechanism. The KCl crystals scaffold acts as a template for the fabrication of GNS. Hence the morphology of GNS is greatly influenced by the KCl crystals scaffold. Figure 3a−e shows the morphologies of the scaffolds after freeze-drying with the increasing KCl concentration. The F127 concentration is fixed at 10 g L−1. When KCl is not added, the scaffold without uniform morphology is produced after freeze-drying (Figure S4). At a low KCl concentration (40 g L−1), the scaffold is a fractal structure with many slim secondary- and tertiary-grade arms (Figure 3a). When increasing the KCl concentration to 100 g L−1, the thirdgrade branches almost disappear. The secondary branches become thicker and the number becomes less (Figure 3b). Notably, further increase of KCl concentration to 150 g L−1 completely changes the scaffold into a sheet-like structure with no dendrites (Figure 3c,f). The higher the KCl concentration, the larger the sheets size (Figure 3c,d). If the KCl concentration is increased to an extremely high level of 300 g L−1, a brick-like scaffold (P-10K300) would be produced, as shown in Figure 3e. Notably, the fractal scaffold is small-sized and not uniform. The brick-like scaffold possesses a much lower specific surface area than the sheet-like scaffold for a given mass of KCl (Figure S3g). There is a proper KCl concentration to produce large-sized uniform salt scaffold with high surface area to benefit the formation of GNS with low thickness. The effectiveness of KCl in controlling the dual crystallization of ice and KCl crystals is exhibited in Figure 3g and Figure S5. At a low KCl concentration (top of Figure 3g), ice crystals grow as a dendritic structure with developed and compacted secondary- and even tertiary-grade arms.39,40 Salt atoms are pushed by the freezing ice interface and concentrate at the freezing interfaces, resulting in the increase of the
Therefore, it is important to clarify the salt crystals growth mechanism to control the salt phase morphology and further modulate the products with special morphologies. Materials crystallization from solution is an interesting phenomenon in nature.26,27 Among them, ice crystals growth is the most common process. Great scientific efforts have been devoted to applying this phenomenon to prepare materials.28,29 During the ice growth, the insoluble particles are expelled from the growing ice crystals interface. Then the ice sublimates in the freeze-drying process, and the scaffold with a negative replica of the ice crystals can be obtained. Many materials can be used as the expelled building blocks, such as ceramic,30 polymer,31 inorganic particle,32 carbon nanotube,33 graphene oxide,33,34 and silica gel.35 By controlling the ice growth and freeze-drying, ice crystallization can be applied to fabricate porous36 and aligned scaffolds37 in water-based systems. But the crystals growth is still lacking in a dual-crystallization system. Both competition and cooperation exist during the growth of the two crystals. It is possible to guide the dual crystals growth to achieve scaffolds with complex morphology. Hence, it is quite interesting to understand and then control the dual crystallization to design materials. In this work, we systematically investigated the dual crystallization process of ice and KCl crystals in a KClcopolymer solution. The freeze-dried scaffolds varied from a fractal structure to stepped sheet-like structure. With the assistance of the stepped KCl crystals scaffold, denim-like graphene nanosheet (DGNS) with ordered and parallel stripes was fabricated after annealing in nitrogen. DGNS showed excellent electrochemical performance when used as anode materials of LIBs. This effective approach could be used on the morphology design of 2D materials by controlling the dual crystallization.
RESULTS AND DISCUSSION As shown in Figure 1a, DGNS is a microsized 2D material with wrinkles. Further confirmed by the atomic force microscopy (AFM) image (Figure 1d), DGNS is ultrathin nanosheets with the thickness of ca. 2.8 nm. As shown in Figure 1c, DGNS exhibits a disordered carbon structure containing both defects
Figure 1. (a, b) SEM and (c) HRTEM images of DGNS. (d) AFM image with the corresponding height profile along the black path. (e) HRTEM image of DGNS with the corresponding spacing profile along the marked cyan line. (f) SEM and (g) HRTEM images of DGNS after long sonication periods. 4020
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Figure 2. AFM images with the corresponding height profile along the black path of (a) C-10K40, (b) C-10K100, (d) C-10K200, and (e) C10K300. (c) The variation of average thickness with the increase of KCl concentration. Transformation process of stripes displayed by TEM images of (f) C-10K40, (g) C-10K100, and (h) DGNS.
of the fractal ice is obtained. When the KCl concentration is further increased (middle of Figure 3g), abundant KCl particles would fill the channels between ice crystals and inhibit the ice crystal growth toward fractal structure, resulting in a decrease of both the size and the number of secondary branches of ice crystals. As a negative replica, a larger scaffold with stout and smooth branches is obtained after freeze-drying. As shown in the bottom of Figure 3g, further raise of the KCl concentration would cause a supersaturation of KCl in the unfrozen solution during the primary growth of the ice crystals, and the KCl particles accumulate at the freezing interface. Ice crystal growth could only occur at the preferred direction (102 to 103 times higher than the orthogonal limited growth direction, ice front rate).30 As a result, lamellar ice crystals are produced instead of ice dendrites. The growth of KCl crystals would take place within the lamellar channels. After freeze-drying, a sheet-like scaffold is produced. Due to the velocity of KCl crystals growth, the scaffold is thicker at a higher KCl concentration.41 If the KCl concentration is excessively high to create a highly supersaturated environment, the growth of KCl crystals would occur before the ice crystallization. As a result, a brick-like scaffold instead of sheet-like structure would be produced. To further investigate the formation of the stripes of GNS, we removed the copolymer F127 from these scaffolds to obtain pure KCl crystals by annealing in air (700 °C for 2 h). The KCl crystals vary from flat surface with little bumps and dips (Figure 4a,b) over an uneven surface with “hills” and “valleys” (Figure 4c,d) to an orderly stepped surface (Figure 4e, f), with the increasing KCl concentration. Correspondingly, the surface morphologies of the GNS change from discrete points, over short-range curved stripes, to long-range parallel and straight stripes (Figure 4g). Besides, KCl crystals with a better stepped surface result in the product with more obvious stripes over the GNS (Figure S6). The stripes of GNS are highly related to the nature of the KCl crystals. The KCl concentration greatly influences the formation of the stepped KCl crystals and then guides the morphologies of products in the form of a template.
Figure 3. Transformation of the freeze-dried scaffolds. SEM images of (a) P-10K40, (b) P-10K100, (c) P-10K150, (d) P-10K200, and (e) P-10K300. (f) Microscopy image of P-10K150. (g) Schematic representation of the transformation process.
supersaturation of KCl. To reduce the supersaturation, KCl crystallization takes place at the freezing interface of the fractal ice crystal branches. Afterward, the ice sublimates in the freezedrying process, thus the leaf-like scaffold with a negative replica 4021
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Figure 4. SEM images of (a, b) annealed P-10K40, (c, d) annealed P-10K100, and (e, f) annealed P-10K150. (g) Schematic illustration of the morphology change with the increase of KCl concentration.
where k+u and k−u are the rates of atoms to reach and leave the upper plane edge, respectively, k+l and k−l are the rates of atoms to reach and leave the lower plane edge, respectively, and ns, nl, and nu are the concentrations of the inclusions at edge of the step, the lower plane, and the upper plane, respectively. The growth probability of steps (Pg) enhances with the increase of the concentration (σ).41 During the growth of KCl crystals, the system would produce more orderly and clear steps on the surface of KCl crystals at a higher KCl concentration. Conversely, at a low KCl concentration, Pg which is close to zero would result in a poorly stepped morphology of KCl crystals. These results allow us to clarify the formation mechanism of DGNS. As shown in Figure 5c, KCl crystals form a sheet-like structure with straight parallel steps on the surface, meanwhile, F127 is pushed out from the solution and adsorbed on the surface of KCl crystals. It is worth noting that a physical enrichment of F127 occurs at the corners of the stepped KCl crystals. As a result, for the final products after carbonization, the thickness of the region which forms at the corners is slightly larger than that formed on the upper and lower plane of the stepped KCl. Therefore, parallel stripes are produced over GNS. Featuring the 2D structure, DGNS owns the potential application as a high-rate anode material for LIBs. As shown in Figure S12c, DGNS electrode shows the reversible capacities of 1072, 1046, 901, 771, and 610 mAh g−1 at 0.1, 0.2, 0.5, 1, and 2 A g−1, respectively. Figure 6 shows the cycling performance of DGNS at 1 A g−1 within the voltage window between 0.01 and 3 V. A high reversible capacity of 1025 mAh g−1 can be maintained after 600 cycles with a Coulombic efficiency of 99.5%, indicating the superior cycling stability of DGNS. We also compared the lithium storage performance of DGNS with various carbon nanosheet electrodes (Table S2). The results reveal that the DGNS electrode exhibits much better electrochemical performance than the electrodes in the reviewed reports. The excellent lithium storage performance further proves that the 2D structure with ultrathin thickness of DGNS could both improve the contact between the electrolyte and active materials and provide abundant reaction sites for Li+ to enhance the reversible lithium storage capacity.
Further investigation on the effect of copolymer F127 to the morphology tuning is expounded in the Supporting Information (Figures S7−S11). We next give the growth mechanism of stepped sheet-like KCl crystals. Total surface free energy (γ∑) plays an important role in the formation of surface structure for crystal with a certain volume. γ∑ is given by the formula:42 γ∑ = ∮∮S γndS
(1)
where γn is the unit surface free energy, which is a function of direction n for unit area dS of anisotropic bodies. A low γ∑ builds up a stable crystal system.42,43 Thus, crystals would turn into the optimal equilibrium morphology to minimize γ∑ of eq 1. Notably, the scaffold with stepped interfaces is more stable than that with a flat surface.42,44,45 In order to decrease γ∑, KCl crystals tend to act as steps with the minimum height instead of planes (Figure 5a). The cross section of KCl crystal face is
Figure 5. Schematic illustration of (a) the three-dimensional KCl crystal template, (b) the cross section of KCl crystal face with the corresponding potential energy, and (c) the preparation of DGNS.
revealed in Figure 5b. The sharp increase at the upper plane of the potential energy builds barriers for the unordered adsorption of KCl particles, stabilizing the stepped structure of KCl crystals during the growth process.41,44,46 The differing conditions at the edge and on the plane influence the growth kinetics of the parallel steps train on the surface of KCl crystals significantly.41,45 The growth probability of steps Pg (a function of the concentration) is given as41 Pg = 1 −
(ku− + kl−)ns kl+nl + ku+nu
CONCLUSION The dual crystallization of ice and KCl in a KCl-copolymer solution controlled the morphological turning of scaffold. KCl and F127 in different concentrations influenced the crystal-
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Figure 6. Lithium storage performance of DGNS at the current density of 1 A g−1.
ASSOCIATED CONTENT
lization process significantly. By adjusting these variables, various scaffolds from a fractal structure to a stepped sheetlike structure were successfully fabricated. An ultrathin graphene nanosheet with ordered parallel stripes was fabricated by annealing the sheet-like scaffold in nitrogen and exhibited excellent electrochemical performance when used as anode materials of LIBs. Our work reports the dual crystallization of ice and KCl and supplies a strategy on the morphology design of 2D materials by controlling the crystals growth.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b01708. Experimental details. XRD pattern, Raman spectrum and N2 adsorption/desorption isotherms of DGNS. Microscopy, SEM and TEM images of products. Cartoon showing the dual crystallization. Electrochemical lithium storage properties of DGNS. Comparison of the properties of DGNS with various carbon nanosheets electrodes (PDF)
MATERIALS AND METHODS Materials. Pluronic F-127 (Mn = 12600, Sigma-Aldrich) was purchased from Sigma-Aldrich. KCl and sucrose were purchased from Aladdin with the purity of >99.5%. All reagents were used without preprocessing. Methods. To prepare DGNS, F127 (1 g) and KCl (15 g) were mixed and dissolved absolutely in deionized water (100 mL). Then the solution was kept at −27 °C for 24 h in the freezer, followed by freezedrying to obtain a soft powder. The resulting powder was annealed at 750 °C for 2 h in N2 flow. The obtained product was washed with deionized water and dried at 80 °C overnight. A series of experiments were performed to investigate the morphology tuning of the graphene nanosheets. The obtained powder after freeze-drying and before annealing was marked as P-xKy. The final product was named as C-xKy, where x and y are the concentrations (g L−1) of carbon precursor and KCl, respectively. The details are shown in the Supporting Information. Materials Characterizations. The samples were characterized by scanning electron microscopy (SEM, ZEISS SUPRATM 55 field emission microscope), transmission electron microscopy (TEM, Hitachi 7700), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The structure information was collected by X-ray diffraction (XRD, Rigaku D/max-2500B2+/PCX, Cu Kα, λ = 1.54056 Å), Raman spectroscopy (collected using a 532 nm laser, Aramis, Jobin Yvon), atomic force microscopy (carried out in tapping mode, AFM, DMFASTSCAN2-SYS, Germany), and N2 adsorption/ desorption measurements (ASAP2020-M, Micromeritics, USA). Electrochemical Measurements. The working electrodes were prepared by mixing 80 wt % of active materials, 10 wt % of acetylene black, and 10 wt % of polyvinylidene difluoride (PVDF) on copper foil. The loading weight was about 0.75 mg cm−2. The batteries were assembled into 2025 coin-type cells with pure lithium metal as the counter electrode, 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 by volume) as the electrolyte, and polypropylene membranes as the separator, respectively. The charge−discharge performance, including the cycling and rate performances, was tested by CT2001A Battery Test System (ChinaLand Co. Ltd.). Cyclic voltammetry (CV) measurements were carried out on a ZAHNER ENNIUM electrochemical workstation within the voltage range between 0.01 and 3 V at the scan rate of 0.1 mV s−1.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Shaozhuan Huang: 0000-0002-4188-6421 Hui Ying Yang: 0000-0002-2244-8231 Huaihe Song: 0000-0003-1547-0382 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51272016, 51272019, and 51672021). REFERENCES (1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (3) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (4) Schwierz, F. Graphene Transistors. Nat. Nanotechnol. 2010, 5, 487−496. (5) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109−162. (6) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10451−10453. (7) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611−622. (8) Tour, J. M. Top-Down versus Bottom-Up Fabrication of Graphene-Based Electronics. Chem. Mater. 2014, 26, 163−171. (9) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217−224. 4023
DOI: 10.1021/acsnano.8b01708 ACS Nano 2018, 12, 4019−4024
Article
ACS Nano
Recrystallization of Ice Crystals. Angew. Chem., Int. Ed. 2017, 56, 997− 1001. (29) Knight, C. A. Structural Biology: Adding to the Antifreeze Agenda. Nature 2000, 406, 249−251. (30) Deville, S. Freeze-Casting of Porous Ceramics: A Review of Current Achievements and Issues. Adv. Eng. Mater. 2008, 10, 155− 169. (31) Zhang, H.; Hussain, I.; Brust, M.; Butler, M. F.; Rannard, S. P.; Cooper, A. I. Aligned Two- and Three-Dimensional Structures by Directional Freezing of Polymers and Nanoparticles. Nat. Mater. 2005, 4, 787−793. (32) Munch, E.; Launey, M. E.; Alsem, D. H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Tough, Bio-Inspired Hybrid Materials. Science 2008, 322, 1516−1520. (33) Hwa, Y.; Seo, H. K.; Yuk, J. M.; Cairns, E. J. Freeze-Dried Sulfur-Graphene Oxide-Carbon Nanotube Nanocomposite for High Sulfur-Loading Lithium/Sulfur Cells. Nano Lett. 2017, 17, 7086−7094. (34) Hu, Y.; Luo, B.; Ye, D.; Zhu, X.; Lyu, M.; Wang, L. An Innovative Freeze-Dried Reduced Graphene Oxide Supported SnS2 Cathode Active Material for Aluminum-Ion Batteries. Adv. Mater. 2017, 29, 1606132. (35) Mukai, S. R.; Nishihara, H.; Tamon, H. Formation of Monolithic Silica Gel Microhoneycombs (SMHs) Using Pseudosteady State Growth of Microstructural Ice Crystals. Chem. Commun. 2004, 874−875. (36) Wu, S.; Zhu, C.; He, Z.; Xue, H.; Fan, Q.; Song, Y.; Francisco, J. S.; Zeng, X. C.; Wang, J. Ion-Specific Ice Recrystallization Provides a Facile Approach for the Fabrication of Porous Materials. Nat. Commun. 2017, 8, 15154. (37) Zhang, H.; Cooper, A. I. Aligned Porous Structures by Directional Freezing. Adv. Mater. 2007, 19, 1529−1533. (38) Li, Y.; Zhang, Q.; Zhu, J.; Wei, X.-L.; Shen, P. K. An Extremely Stable MnO2 Anode Incorporated with 3D Porous Graphene-Like Networks for Lithium-Ion Batteries. J. Mater. Chem. A 2014, 2, 3163− 3168. (39) Ishiguro, H.; Rubinsky, B. Mechanical Interactions Between Ice Crystals and Red Blood Cells During Directional Solidification. Cryobiology 1994, 31, 483−500. (40) Flemings, M. C. Solidification Processing. Metall. Trans. A 1974, 5, 2121−2134. (41) Roland, C.; Gilmer, G. H. Kinetics of Nucleation-Dominated Step Flow. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 2931− 2936. (42) Herring, C. Some Theorems on the Free Energies of Crystal Surfaces. Phys. Rev. 1951, 82, 87−93. (43) Wulff, G. XXV. Zur Frage der Geschwindigkeit des Wachsthums und der Auflösung der Krystallflächen. Z. Kristallogr. - Cryst. Mater. 1901, 34, 449−530. (44) Schwoebel, R. L.; Shipsey, E. J. Step Motion on Crystal Surfaces. J. Appl. Phys. 1966, 37, 3682−3686. (45) Burton, W. K.; Cabrera, N.; Frank, F. C. The Growth of Crystals and the Equilibrium Structure of Their Surfaces. Philos. Trans. R. Soc., A 1951, 243, 299−358. (46) Schwoebel, R. L. Step Motion on Crystal Surfaces. II. J. Appl. Phys. 1969, 40, 614−618.
(10) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. (11) Ruffieux, P.; Wang, S.; Yang, B.; Sanchez-Sanchez, C.; Liu, J.; Dienel, T.; Talirz, L.; Shinde, P.; Pignedoli, C. A.; Passerone, D.; Dumslaff, T.; Feng, X.; Mullen, K.; Fasel, R. On-Surface Synthesis of Graphene Nanoribbons with Zigzag Edge Topology. Nature 2016, 531, 489−492. (12) Wu, T.; Zhang, X.; Yuan, Q.; Xue, J.; Lu, G.; Liu, Z.; Wang, H.; Wang, H.; Ding, F.; Yu, Q.; Xie, X.; Jiang, M. Fast Growth of InchSized Single-Crystalline Graphene from a Controlled Single Nucleus on Cu-Ni Alloys. Nat. Mater. 2016, 15, 43−47. (13) Zhang, L.; Zhang, Y.; Zhou, C. Review of Chemical Vapor Deposition of Graphene and Related Applications. Acc. Chem. Res. 2013, 46, 2329−2339. (14) Huang, H.; Chen, W.; Chen, S.; Wee, A. T. S. Bottom-Up Growth of Epitaxial Graphene on 6H-SiC(0001). ACS Nano 2008, 2, 2513−2518. (15) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; Mullen, K.; Fasel, R. Atomically Precise Bottom-Up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470−473. (16) Wang, Z. J.; Dong, J.; Cui, Y.; Eres, G.; Timpe, O.; Fu, Q.; Ding, F.; Schloegl, R.; Willinger, M. G. Stacking Sequence and Interlayer Coupling in Few-Layer Graphene Revealed by In Situ Imaging. Nat. Commun. 2016, 7, 13256. (17) Wang, H.; Pan, Q.; Wu, Q.; Zhang, X.; Huang, Y.; Lushington, A.; Li, Q.; Sun, X. Ultrasmall MoS2 Embedded in Carbon NanosheetsCoated Sn/SnOx as Anode Material for High-Rate and Long Life LiIon Batteries. J. Mater. Chem. A 2017, 5, 4576−4582. (18) Shi, Y.; Wang, Y.; Yu, Y.; Niu, Z.; Zhang, B. N-Doped Graphene Wrapped Hexagonal Metallic Cobalt Hierarchical Nanosheet as a Highly Efficient Water Oxidation Electrocatalyst. J. Mater. Chem. A 2017, 5, 8897−8902. (19) Pan, Q.; Zheng, F.; Ou, X.; Yang, C.; Xiong, X.; Liu, M. MoS 2 Encapsulated SnO2-SnS/C Nanosheets as a High Performance Anode Material for Lithium Ion Batteries. Chem. Eng. J. 2017, 316, 393−400. (20) Xu, Y.; Li, W.; Zhang, F.; Zhang, X.; Zhang, W.; Lee, C.-S.; Tang, Y. In Situ Incorporation of FeS Nanoparticles/Carbon Nanosheets Composite with an Interconnected Porous Structure as a High-Performance Anode for Lithium Ion Batteries. J. Mater. Chem. A 2016, 4, 3697−3703. (21) Fu, G.; Liu, Z.; Zhang, J.; Wu, J.; Xu, L.; Sun, D.; Zhang, J.; Tang, Y.; Chen, P. Spinel MnCo2O4 Nanoparticles Cross-Linked with Two-Dimensional Porous Carbon Nanosheets as a High-Efficiency Oxygen Reduction Electrocatalyst. Nano Res. 2016, 9, 2110−2122. (22) He, C.; Wu, S.; Zhao, N.; Shi, C.; Liu, E.; Li, J. CarbonEncapsulated Fe3O4 Nanoparticles as a High-Rate Lithium Ion Battery Anode Material. ACS Nano 2013, 7, 4459−4469. (23) Chen, L.; Wang, Z.; He, C.; Zhao, N.; Shi, C.; Liu, E.; Li, J. Porous Graphitic Carbon Nanosheets as a High-Rate Anode Material for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2013, 5, 9537− 9545. (24) Ma, H.; Jiang, H.; Jin, Y.; Dang, L.; Lu, Q.; Gao, F. Carbon Nanocages@Ultrathin Carbon Nanosheets: One-Step Facile Synthesis and Application as Anode Material for Lithium-Ion Batteries. Carbon 2016, 105, 586−592. (25) Chen, Y.; Shi, L.; Guo, S.; Yuan, Q.; Chen, X.; Zhou, J.; Song, H. A General Strategy Towards Carbon Nanosheets from Triblock Polymers as High-Rate Anode Materials for Lithium and Sodium Ion Batteries. J. Mater. Chem. A 2017, 5, 19866−19874. (26) Piana, S.; Jones, F.; Gale, J. D. Assisted Desolvation as a Key Kinetic Step for Crystal Growth. J. Am. Chem. Soc. 2006, 128, 13568− 13574. (27) Baker, M. B.; Peter, T. Small-Scale Cloud Processes and Climate. Nature 2008, 451, 299−300. (28) Geng, H.; Liu, X.; Shi, G.; Bai, G.; Ma, J.; Chen, J.; Wu, Z.; Song, Y.; Fang, H.; Wang, J. Graphene Oxide Restricts Growth and 4024
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