Crystallization-Induced Morphological Tuning Toward Denim-like

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Crystallization-Induced Morphological Tuning Towards DenimLike 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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01708 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Crystallization-Induced Morphological Tuning Towards Denim-Like Graphene Nanosheets in a KCl-Copolymer Solution

Yaxin Chen,a,b Liluo Shi,b Qiong Yuan,b Ang Li,b Shaozhuan Huang,c Hui Ying Yang,c Xiaohong Chen,b Jisheng Zhou,b Huaihe Songa,b,*

a

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, PR China b

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 c

Pillar of Engineering Product Development, Singapore University of Technology and

Design, 8 Somapah Road, Singapore 487372, Singapore. *Corresponding author E-mail: [email protected] 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 towards stepped sheet-like structure. 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 lithium-ion batteries anode, DGNS possesses a super-high 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 1

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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.

Graphene nanosheet (GNS) is an ideal two-dimensional material with a series of advantages, such as high electrical and thermal conductivity, chemical inertness and high flexibility.1-3 Since the debut,2 GNS has attracted great interests for the potential application in many fields, especially energy storage devices and catalyst.4-7 In this regard, mass production of GNS is critical to meet the growing demands. The fabrication of GNS has been developed into mainly two strategies: top-down and bottom-up approaches.3,8 Top-down approach is to separate GNS from the bulk graphite by breaking the bonds between the graphite layers.2,6,8-10 Conversely, bottom-up approach is the way by assembling the small molecules units to graphene framework.11 Many templates or substrates can be chosen to fabricate GNS.12 For example, epitaxial growth on silicon carbide and metal substrate has been widely reported to fabricate GNS.12-16 However, the low deposition rate and the difficult removal of substrate impedes the mass production. Recently, salt crystals have been developed to be used as the template to enrich the preparation of 2D materials.17-22 Carbon nanosheets can be fabricated on the surface of cubic salt template. For example, porous carbon nanosheets with a thickness of tens of nanometers was 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 1-hexadecylamine as the carbon precursor and potassium chloride (KCl) as the template with the catalysis of 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, 2


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the crystal growth has not been well controlled, and the mechanism of the salt template on the 2D carbon material formation are still lacking. Therefore it is important to clarify the salt crystals growth mechanism to control the morphology of salt phase 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 it is still lacked on the crystals growth 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 KCl-copolymer solution. The freeze-dried scaffolds varied from 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 3


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Figure 1. (a, b) SEM and (c) HRTEM image 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. As shown in Figure 1a, DGNS is micro-sized 2D materials with wrinkles. Further confirmed by the atomic force microscopy (AFM) image (Figure1d), 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 and local nanocrystallites. It can further be corroborated by the broad and dispersive (002) diffraction peak at ca. 23° in the X-ray diffraction (XRD) pattern (Figure S1a) and the ID/IG of 0.92 from the Raman spectrum (Figure S1b). As revealed in the N2 adsorption/desorption isotherms (Figure S1c), the desorption curve owns 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 the stack of the curled 2D GNS. 4


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Imaging the area marked by yellow dotted line at a high magnification, one can see that parallel stripes are displayed evenly on DGNS like the 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, DGNS was disrupted into nanobelts, as shown in Figure 1 (f, g). It implies that the DGNS can be considered as splicing of parallel nanobelts.

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) C-10K300. (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. 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) display 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 5


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average thickness of GNS shows a “V-shape” with the lowest value of ~2.8 nm when 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).

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.

The morphology tuning of these GNS draw our attention to explore the transformation mechanism. The KCl crystals scaffold acts as template for the fabrication of GNS. Hence the morphology of GNS is greatly influenced by the KCl crystals scaffold. Figure 3a-e show 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 6


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low KCl concentration (40 g L-1), the scaffold is fractal structure with many slim secondary and third-grade arms (Figure 3a). When increasing the KCl concentration to 100 g L-1, the third-grade 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 sheet-like structure with no dendrites (Figure 3c, f). The higher KCl concentration, the larger size of sheets (Figure 3c, d). If the KCl concentration is increased to an extremely high level of 300 g L-1, 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 much lower specific surface area than 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 dendritic structure with developed and compacted secondary and even third arms.39,40 Salt atoms are pushed by the freezing ice interface and concentrate at the freezing interfaces, resulting in the increase of the supersaturation of KCl. To reduce the supersaturation, KCl crystallization takes place at the freezing interface of the fractal ice crystal branches. Afterwards, the ice sublimates in the freeze-drying process, thus the leaf-like scaffold with a negative replica 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 towards 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 7


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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, 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, brick-like scaffold instead of sheet-like structure would be produced.

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. 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℃ for 2 h). The KCl crystals vary from flat surface with little bumps and dips (Figure 4a, b) over uneven surface with “hills” and “valleys” (Figure 4c, d) to 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 8


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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 influence the formation of the stepped KCl crystals, and then guide the morphologies of products in the form of template. Further investigation on the effect of copolymer F127 to the morphology tuning is expounded in the Supporting Information (Figure S7-S11).

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.

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∑ ∯ γ

(1)

where γ is the unit surface free energy, which is a function of direction for unit area 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 flat surface.42,44,45 In 9


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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 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 (a function of the concentration) is given as:41 1 −

) (

(2)

where and are the rates of atoms to reach and leave the upper plane edge, respectively. and are the rates of atoms to reach and leave the lower plane edge, respectively. , , and are the concentrations of the inclusions at edge of the step, the lower plane and the upper plane, respectively. The growth probability of steps ( ) 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, which is close to zero would result in a poor 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 are 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 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. 10


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Figure 6. Lithium storage performance of DGNS at the current density of 1 A g-1. Featuring the 2D structure, DGNS owns the potential application as high-rate anode materials for LIBs. As shown in Figure S12c, DGNS electrode shows the reversible capacities of 1072, 1046, 901, 771, 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 V to 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 nanosheets electrodes (Table S2). The results reveal that the DGNS electrode exhibits much better electrochemical performance than these electrodes in the reviewed reports. The excellent lithium storage performance further prove 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.

CONCLUSION The dual crystallization of ice and KCl in a KCl-copolymer solution controlled the morphological turning of scaffold. KCl and F127 of different concentrations influenced the crystallization process significantly. By adjusting these variables, various scaffolds 11


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from fractal structure to stepped sheet-like structure were successfully fabricated. 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.

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 freeze-drying to obtain 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 concentration (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), 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 12


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microscopy (carried out in tapping mode, AFM, DMFASTSCAN2-SYS, Germany) and N2 sorption-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 (China-Land Co. Ltd.). Cyclic voltammetry (CV) measurements were carried out on a ZAHNER ENNIUM electrochemical workstation within the voltage range between 0.01 V and 3 V at the scan rate of 0.1 mV s-1.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51272016, 51272019 and 51672021). ASSOCIATED CONTENT Supporting Information Experimental details. XRD pattern, Raman spectrum and N2 adsorption/desorption isotherms of 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. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author 13


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*E-mail: [email protected].

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Crystallization-Induced Morphological Tuning Toward Denim-like

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