Article pubs.acs.org/crystal
Experimental Observation of Fullerene Crystalline Growth from Mesocrystal to Single Crystal Hongbian Li,# Mingyu Guan,# Guoxing Zhu, Gui Yin, and Zheng Xu* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *
ABSTRACT: Fullerene hierarchical mesocrystals were first prepared by antisolvent induced precipitation method. Their morphologies and sizes can be controlled by adjusting the antisolvent type and the ratio between the solvent (toluene) and antisolvent (ethyl acetate or tetrahydrofuran). The formation of fullerene mesocrystals and their transformation to single crystal were observed by time-dependent experiments with SEM and TEM. Fullerene mesocrystals can be separated from the solution and are stable for several months. HRTEM revealed that mesocrystals were made up of highly oriented nanoparticles. The formation of fullerene mesocrystals and their transformation to single crystals provide a new way for the construction of fullerene nanostructures with different applications.
1. INTRODUCTION Single crystal usually forms by two routes: one is the classical mode, where ions or molecules deposit onto the crystal nucleus directly, and the other is nonclassical one, which involves the self-assembly of primary nanocrystals, crystallographic reorganization in the self-assembly process to generate mesocrystals and their fusion to single crystal.1−8 Mesocrystals, as the intermediate for the nonclassical mode,9 are arranged by highly oriented nanocrystals, showing unique properties such as uniform crystallinity and high surface area.10,11 Many mesocrystals have been synthesized in the past few years for their applications in photocatalysis,12,13 environmental cleanup,14 and energy storage.15,16 However, most of these mesocrystals are inorganic nanostructures with ions as the building blocks for the primary nanocrystals, and their formation process usually needs high temperature and pressure. Unlike inorganic nanostructures, organic single crystal forms easily during solvent evaporation or antisolvent introduction. Therefore, organic mesocrystals can be prepared under a much milder condition. Although various organic nanostructures have been fabricated, the study of the organic mesocrystals and their transformation into single crystal is quite limited. For example, Cölfen et al. prepared D,L-alanine mesocrystals and reported a transition from a mesocrystal to a compact single crystal in a solution with time-resolved small angle neutron scattering and dynamic light scattering.9 Li et al. fabricated a peony-flower-like hierarchical mesocrystal from diphenylalanine, and studied their transformation with XRD.17 Therefore, the study of organic mesocrystals and their behavior should be highly developed for the construction of functional organic nanostructures and their applications. © XXXX American Chemical Society
Fullerene, as an important member of the carbon family, has attracted attention from researchers due to its unique properties, such as excellent redox, optoelectronic, and catalytic properties.18−20 With its molecular state in solvents like toluene, fullerene nanostructures can be easily obtained by adding an antisolvent for the self-assembly of the molecules. Many fullerene nanostructures with different morphologies, such as one-dimensional rods,21−23 nanowires,24 nanotubes,25 and 2D fullerene sheets,26 have been prepared. However, fullerene mesocrystals and hierarchical micro- or nanostructures were seldom obtained, let alone their formation mechanism. In this paper, we prepared fullerene hierarchical structures consisting of mesocrystals and single crystals by the antisolvent induced precipitation method (see Scheme 1 for illustration of pathways). With scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the existence of the fullerene mesocrystal stacked by highly oriented nanocrystals was found for the first time. Furthermore, as the reaction time is extended, the transformation from mesocrystal to single crystal was obtained through time-dependent experiments, which provide guidelines for the preparation of fullerene nanostructures.
2. EXPERIMENTAL SECTION 2.1. Reagents. C60, C70, toluene, ethyl acetate, and tetrahydrofuran (THF) were purchased from the Sinopharm Chemical Reagent Company. All chemical reagents were of analytical grade and were used without further purification. Received: October 4, 2015 Revised: December 14, 2015
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DOI: 10.1021/acs.cgd.5b01418 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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3. RESULTS AND DISCUSSION SEM images in Figure 1 show the influence of the fabrication parameters on the morphologies of the C60 crystals. When adding equal volume of ethyl acetate into the C60 stock solution (2.4 g·L−1) under magnetic stirring for 20 min, as shown in Figure 1a, a product of 3D flower-like C60 structure composed of prisms was obtained. The diameter of the prisms is about 1 μm and their length is about 2.2 μm. The diameter of the prism increased to 1.5 μm when the C60 concentration decreased to 1 g·L−1 and their length increased to 3−5 μm, which is shown in Figure 1b. As the stock concentration decreased to 0.75 g·L−1, the diameter and length further increased to about 2 and 9 μm, respectively (Figure 1c). In Figure 1d, no 3D flower could be found when the concentration was further reduced to 0.4 g·L−1, and only crude prisms with large diameter (about 4 μm) were obtained. The above results indicate that C60 solution concentration is an important influencing factor on diameter and length of the prisms. Based on the principle of chemical kinetics, the increase of the reactant concentration would increase the formation rate of C60 monomers, and high monomer concentration was favorable for the formation of highly branched structures and small prisms. The diameter and length of prism could also be adjusted by changing the ratio (R) between ethyl acetate and toluene. As shown in Figure 1e and f, the length of the prisms increased from ∼2.2 μm to ∼5.4 μm when R decreased from 1 to 0.25 in 1.0 g·L−1 C60 solution. This is understandable: the formation rate of C60 monomer becomes slow in an antisolvent poor solution, and fewer crystalline nuclei could be obtained. In this case, C60 monomer will deposit on the crystalline nuclei in a small number, which is favorable for formation of the longer prism. To reveal the formation mechanism of the 3D flower-like microcrystal, time-dependent experiments were carried out. The morphology evolution of the samples was observed at various times after adding an equal volume of ethyl acetate into the 1 g·L−1 C60 stock solution. The samples were separated from the solution at different reaction time. Figure 2a presents SEM image of the product after two solutions were mixed for 1
Scheme 1. Schematic Illustration of Formation Pathways of Single Crystal by Classical Crystal Growth (Ion-by-Ion or Molecule-by-Molecule Addition) and Nonclassical Crystal Growth (Primary Nanocrystals Firstly Form Mesocrystals by Oriented Alignment and Then Mesocrystal Intermediates Fuse into a Single Crystal)
2.2. Synthesis of Fullerene Microcrystals. C60 or C70 was dissolved into toluene to get the stock solutions. Then, ethyl acetate was poured into the C60 stock solution under stirring. At different time intervals, C60 precipitate can be obtained by centrifuging at 4000 rpm for 2 min. C60 precipitate was washed with absolute ethanol and then dried in a vacuum oven at 50 °C. Plate-like C70 crystals were obtained in the same way except using THF as the antisolvent. 2.3. Characterization. Products were characterized by X-ray powder diffraction (XRD) (Shimadzu XD-3A X-ray diffractmeter with Cu Kα radiation, λ = 0.15418 nm). TEM images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns were obtained on a FEI Tecnai G2 20 S-TWIN highresolution transmission electron microscope, using an accelerating voltage of 200 kV. SEM images were taken with a JEOL JSM 5610LV apparatus.
Figure 1. (a−d) SEM images of the fullerene microcrystals obtained by adding an equal volume of ethyl acetate into C60 solution with various concentrations: (a) 2.4 g·L−1; (b) 1.0 g·L−1; (c) 0.75 g·L−1; and (d) 0.4 g·L−1. (e, f) Influence of the ratio R of ethyl acetate/toluene on the 3D flower-like C60 microcrystals: (e) R = 1; (f) R = 0.25 (concentration of C60 stock solution: 1 g·L−1). B
DOI: 10.1021/acs.cgd.5b01418 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. SEM tracing the formation process of fullerene oriented aggregates in various stages with different mixing time: (a) 1 min; (b) 10 min. (c) XRD pattern of the sample obtained after mixing time of 1 min. (d) Raman spectrum comparison of the C60 mesocrystal and pristine C60. (R = 1:1, CC60 = 1 g·L−1.)
Figure 3. (a) TEM and SEM (inset) images of a single bundle with strip-like structure. (b) TEM image of the edge of the bundle. (c) HRTEM image of the spread nanoparticles near the edge of the bundle. (d) HRTEM image of bundle (t = 10 min, R = 1:1, CC60 = 1 g·L−1).
trapping of solvent molecules. Because of the small size of the nanocrystals, the X-ray diffraction patterns are weak and the half-width of the diffraction peaks are larger. As the reaction time increased to 10 min, the triclinic phase disappeared completely, and the half-width of the diffraction peaks became smaller, indicating the removal of the molecules and growth of nanocrystals. For the 20 min sample, a further decrease for the half-width of diffraction patterns appeared, reaching those for pristine C60 (Supporting Information Figure SI-1 and SI-2). Raman spectroscopy was also performed and the comparison between C60 mesocrystals and prisine C60 is shown in Figure 2d. One can see a clear 2 cm−1 downshift of the Hg(1), Ag(1),
min. A lot of bundles with strip-like structure were observed, which is an intermediate with a typical mesocrystalline structure. There is a large void in the central part of bundle and grooves between the strips. With the mixing time increased to 10 min, the void in the center of the bundle disappeared and the grooves between strips became narrow (Figure 2b). X-ray diffraction (XRD) patterns of the mesocrystals obtained at 1 min are shown in Figure 2c: in addition to diffraction peaks of (111), (220), and (311) crystal faces of face-centered cubic (fcc) C60, there are two additional peaks corresponding to (111) and (121) crystal faces of the triclinic phase.27 The same phenomenon was also found in the C60 nanowhiskers prepared by the liquid−liquid interface precipitation method due to the C
DOI: 10.1021/acs.cgd.5b01418 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 4. TEM image (a), SAED pattern (b), and HRTEM image of C60 prism recorded on the marked area with white square in part a (c) (t = 20 min, R = 1:1, CC60 = 1 g·L−1).
Figure 5. (a) SEM image, (b) TEM image, and (c) SAED pattern of C70 microcrystals obtained by using THF as antisolvent after mixing for 60 min. SEM images of the C70 obtained at different mixing time: (d) 1 min; (e) 5 min; (f) 10 min. (The initial concentration of the C70 solution is 1 g·mL−1, and the volume ratio between toluene and THF is 1:1.)
very stable after being isolated from the reaction solution. They did not degrade and the morphology remained constant in air for several months. Figure 4 shows the structural analysis of the highly crystalline microcrystals with mixing time of 20 min. The uniform color in Figure 4a confirms the disappearance of the grooves in the mesocrystals. For the SAED in the white square, which is shown in Figure 4b, the bright dots show the perfect crystalline structure, corresponding to (111) and (220) crystal faces in fcc C60. This phase transition was induced by the removal of the trapped solvent molecules, which was also confirmed by the same phase transition when the mesocrystals were heated to 60 °C in vacuum for 12 h. The highly aligned lattice strips in Figure 4c also show the single crystal nature of the prisms, with axial direction (110) as the preferred growth direction. Based on the results shown above, the formation mechanism of the mesocrystalline and single crystal C60 flower-like structure can be proposed: as is well-known, ethyl acetate is a bad solvent while toluene is a good solvent for C60. When ethyl acetate was added into the toluene solution of C60, the solubility of C60 decreased. A large amount of C60 monomers were rapidly isolated from the concentrated C60 solution in a short time to form nanocrystals for self-assembly. Then, the nanocrystals experienced crystallographic reorganization in the
and Ag(2) modes, indicating solvent molecules trapped in the mesocrystals.28 We completed further observation on the bundle by TEM and HRTEM to obtain the structure information on the mesocrystals. Figure 3a shows that the diameter of the C60 bundle obtained after mixing 10 min is about 1 μm. It was too thick to see grooves between strips in the SEM image shown in the inset Figure 3a. To acquire the constitutional details of the strips, the bundles were ground gently to get small thin pieces for TEM measurement. The TEM image of the edge of a small piece (Figure 3b) clearly displays that strips are composed of nanocrystals, which have the size of 1−2 nm (Figure 3c). The HRTEM image in Figure 3d shows the continuous and parallel lattice fringes of the nanocrystals, indicating the same crystallographic direction of the nanocrystals in the strips. The above results indicated clearly that strips are mesocrystals composed of solvated C60 nanocrystals in oriented aggregation fashion. The bundle is an even larger mesocrystal composed of mesocrystalline strips in crystallographically oriented fashion. The selected area electron diffraction (SAED) was carried out on the C60 mesocrystals at various parts indicated by differently colored rings (Figure SI-3); only bright dots were obtained, confirming the uniform crystalline structure from the nanocrystals, strips, and the whole prism. These mesocrystals were D
DOI: 10.1021/acs.cgd.5b01418 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Notes
self-assembly process to complete the conversion to the mesocrystal in the same crystallographic direction. In this stage, primary nanocrystals achieved crystallographic alignment although no coherent crystalline materials linked them, leaving many spaces with trapped solvent molecules between the nanocrystals (Figure 2 and Figure 5). The fullerene mesocrystal is an fcc phase structure with parts of triclinic phase. As the aging process proceeds, the trapped molecules are lost and the mesocrystal transformed into compact single crystal. With the same method, the formation and transition of C70 mesocrystals were also observed. THF was preferred as the antisolvent and added into the C70 stock solution. Figure 5a presents plate-like C70 structure by stirring the mixture for 60 min. Parallel and continuous lattice fringe shown in HRTEM images of Figure 5b indicates the single-crystal nature of C70. Distance between adjacent lattices was 9.72 Å, corresponding to the (110) crystal face. Further evidence comes from SAED pattern of C70 plate (Figure 5c), which shows a dot array, with a preferred growth direction of (110). SEM image in Figure 5d clearly shows that the morphology of the C70 structure obtained after mixing for 1 min is an irregular nanoparticle aggregate. After mixing for 5 min, the plate-like morphology in the embryo appears and a plate with smooth surface (indicated by the white arrow) could also be found (Figure 5e). When the mixing time was further increased to 10 min, more plates with smooth surface appeared (Figure 5f). The nanoparticles in the mesocrystals fully fused into a compact single crystal after mixing for 60 min (Figure 5a). Obviously, the whole process includes nanocrystal formation, nanocrystal self-aggregation and further alignment into mesocrystals, and then mesocrystal fusing into single crystals. It suggests that the above mechanism is a general crystallization mechanism in the formation of fullerene crystals.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the financial support from National Major Basic Research Program (973) No. 2007CB936302 and National Nature Science Foundation of China (51202042), Jiangsu Province Natural Science Foundation (No. BK2011260), Jiangsu Overseas Research &Training Program for University Prominent Young & Middle-aged Teachers and Presidents.
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4. CONCLUSION In summary, the formation and transition of the fullerene mesocrystals was experimentally observed with an antisolvent induced precipitation method. The fullerene mesocrystals intermediates are very stable in air when being isolated from the reaction solution. Our work provides evidence for crystalline nonclassic growth mechanism in fullerene crystal. The experimental results further reveal that there is a phase transition from mixture of triclinic and fcc phase to single fcc phase in the transition process from mesocrystal to single crystal, together with the lost of the trapped solvent molecules. It is quite valuable for further understanding the growth mechanism of the single crystal.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01418. XRD patterns comparison of the C60 prisms prepared with mixing time of 1 min, 10 min, 20 min and pristine C60. SAED patterns of the C60 prisms at various parts (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected], Fax: +86-25-83314502. Author Contributions #
These authors contributed equally to this work. E
DOI: 10.1021/acs.cgd.5b01418 Cryst. Growth Des. XXXX, XXX, XXX−XXX
