Synthesis and Characterization of Various-Shaped C60 Microcrystals

Jul 13, 2010 - Here, we report the systematic synthesis and characterization of ... microcrystals using alcohols as antisolvents in drowning-out cryst...
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Synthesis and Characterization of Various-Shaped C60 Microcrystals Using Alcohols As Antisolvents Jinyoung Jeong,† Woo-Sik Kim,‡ Sang-Im Park,† Tae-Sung Yoon,† and Bong Hyun Chung*,† BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology, 111 Gwahangno, Yuseong, Daejeon 305-600 Korea, and School of Chemical Engineering, Molecular Separation Research Center, Kyunghee UniVersity, Yongin, Kyungki-Do 449-701 Korea ReceiVed: April 29, 2010; ReVised Manuscript ReceiVed: June 28, 2010

Solvent-based synthetic methods of fullerene nano/microstructures are known to enhance and utilize unique optical and electrical properties of fullerene structures. Here, we report the systematic synthesis and characterization of various-shaped fullerene microcrystals using alcohols as antisolvents in drowning-out crystallization. The microcrystals are formed in one-, two-, and three-dimensional structures depending on the alcohol type, and the size and shape of the microcrystals are also varied by the C60 concentration and the volume ratio of the solvents. X-ray diffraction patterns demonstrate that the crystalline structures differ from the chain lengths of alcohols. It is suggested that the formation mechanisms are driven by supersaturation related to the C60 solubility in alcohols. This crystallization could allow for production of C60 microcrystals with the desired shape and crystalline structure, leading to potential applications in optoelectronics and photoconducting devices. 1. Introduction Fullerene (C60) continues to attract attention as a promising material due to its structure and unique optical and electrical properties, which make it a promising material in versatile applications in semiconductors and optoelectrical devices.1,2 In particular, such applications utilize C60 in a self-assembled onedimensional (1D) or 2D structure that promotes its electronic and optical properties. To enhance the utility of C60, various synthetic approaches have recently been explored to develop 1D or 2D C60 structures with different rod, tube, or plate morphologies. Wang et al. introduced a slow evaporation method for producing C60 nanorods that emit highly enhanced photoluminescence originating from a change in the electronic level during C60 self-assembly.3,4 Shin et al. developed a vapor-solid process to produce disk-type C60 structures that can be used in optical devices due to their photoconductivity.5 Miyazawa and co-workers developed a liquid-liquid interfacial precipitation (LLIP) method to produce C60 nanowhiskers and nanotubes for use in nanoelectronics.6,7 They also extended this method to produce porous nanowhiskers and hexagonal nanosheets by altering the organic solvents used during processing.8,9 However, despite these extensive efforts to produce C60 structures with various sizes and shapes, the actual mechanism of formation is still unclear. In general, solvents play an important role in controlling the size and shape of the C60 crystals. Most 1D C60 nanostructures (e.g., nanorods or nanowires) have been prepared by slow evaporation of a solution of C60 in organic solvents, such as toluene, xylene, 1,2,3-trimethylbenzene, and dichlorobenzene, and multiple twinned C60 nanoparticles with decahedron or icosahedron shapes have been produced by aerosol-droplet drying a C60 solution in toluene at high temperatures (400-700 * To whom correspondence should be addressed. Tel: +82-42-860-4444. Fax: +82-42-879-8594. E-mail: [email protected]. † Korea Research Institute of Bioscience and Biotechnology. ‡ Kyunghee University.

°C).10,11 While these aromatic solvents have a strong affinity for the π electrons in C60, other subtle properties, such as the polarity and solubility of the C60 aggregates in solvents, also have an impact on the size and shape of the resulting C60 crystals. Moreover, it was recently demonstrated that solvent geometry determined the shape of C60 crystals by a drop-drying process.12,13 On the other hand, the combination of two solvents (i.e., good solvent and poor solvent) was revealed to control the size and shape of C60 crystals by a reprecipitation method.14-16 However, regardless of the progress in controlling the size and morphology of C60 nano/microstructures, a systematic approach is still needed to facilitate the morphologically controlled synthesis of C60 crystals. Herein, we report the synthesis of micrometer-sized 1D, 2D, and 3D C60 microcrystals using drowning-out crystallization with alcohols as antisolvents. The morphologies of the resulting C60 crystals were found to be determined by alcohol type and controlled by varying the volume ratio of the solvents (toluene and alcohols) and the C60 concentration. Therefore, this study investigates the mechanism of formation of C60 microcrystals and chemical and structural properties using Fourier transform infrared (FT-IR), Raman spectroscopy, thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). 2. Experimental Section To synthesize shape-controllable C60 microcrystals, a stock solution of C60 dispersed in toluene (C60/tol) was prepared based on a concentration of 2 mg/mL. The C60 microcrystals were prepared by immediately adding a solution of C60/tol into alcohols, including ethanol, 1-propanol, and 1-butanol, with designed volume ratios (i.e., 1:9, 2:8, 4:6 toluene and alcohols) and given concentrations (i.e., 0.05, 0.1, 0.2 mg/mL in final concentration) of C60 at room temperature. Scanning electron microscopy (SEM) images were obtained using an FEI Sirion (Netherlands) operated at an accelerating

10.1021/jp103875s  2010 American Chemical Society Published on Web 07/13/2010

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Figure 1. SEM images of C60 microcrystals prepared by drowning-out crystallization using ethanol, 1-propanol, and 1-butanol with drowning-out ratios of 9.0 (a-c), 4.0 (d-f), and 1.5 (g-i), respectively. Insets: high-magnification images of C60 microcrystals. The scale bars indicate 5 µm in the large images and 2 µm in the inset images.

voltage of 10 kV. The solution samples were taken using a micropipet, dropped on a silicon wafer, and dried at room temperature. Field emission TEM images were then obtained using a JEOL JEM-2100F electron microscope (Japan) operating at 200 kV. The alcohol-suspended samples were deposited on a carbon-coated copper grid. FTIR spectra were recorded on an IF66 Bruker Optics spectrometer (U.S.A.) using KBr pellets, and Raman spectra were obtained from LabRam Horiba Jobin Yvon (France) using an argon ion laser at 514 nm excitation and 0.2 mW/mm2. The solid-state 13C nuclear magnetic resonance (NMR) spectrum was recorded by a Bruker AVANCE 400WB, Bruker Science (U.K)., at an NMR frequency of 400 MHz. The spectrum was recorded using 1H-13C cross-polarization/magic angle spinning (CP/MAS) at a 4.5 µs pulse and 4 s recycle delays. For accurate measurement, the C60 crystals were prepared by precipitating from the solution and drying by evaporation overnight to remove remaining solvent. TGA was then performed using a Netzsch TG209 F3 (Germany) under nitrogen (N2) gas. The maximum temperature achieved was 1000 °C with an increase rate of 10 °C/min, and the microbalance resolution was 0.1 µg. XRD analysis was also carried out using a Rigacu multipurpose attachment X-ray diffractometer (Japan) with Cu KR in a range of 2-35 2θ using the θ-2θ method. The operating power of the XRD was set to 40 kV and 300 mA, while the scan speed was 0.5 degree/min at 0.02 s intervals. 3. Results and Discussion Drowning-out crystallization occurs during addition of an antisolvent that induces supersaturation by decreasing the solubility of the solute.17 When compared to evaporation methods, drowning-out crystallization is easy to perform, highly

obtainable, and allows crystal separation at ambient temperature. These advantages have resulted in the widespread use of drowning-out crystallization in research areas varying from pharmaceutical chemistry to the synthesis of organic nanoparticles/nanostructures.18,19 Solvent-induced crystallization, including the LLIP method and reprecipitation method, is similar to the drowning-out crystallization. In this study, C60 crystals of various shapes were prepared by drowning-out crystallization using different alcohols, such as ethanol, 1-propanol, and 1-butanol, as the antisolvents. When the alcohol antisolvents were added to the C60 solution in toluene with various antisolvent/solvent volume ratios (drowning-out ratios), the transparent pinkish color of the solution immediately changed to light brown in ethanol, dark brown in 1-propanol, and black in 1-butanol. A rapid precipitation of aggregated C60 molecules ensued within several minutes. The precipitates were collected from the mother liquor using centrifugation, the crystals were dried at room temperature, and then their size and shape were observed using SEM. Figure 1 shows SEM images of the C60 microcrystals resulting from the crystallization with a final C60 concentration of 0.1 mg/mL and drowning-out ratio of 9.0. The shapes of the microcrystals were popcorn, hexagonal, and rod with ethanol, 1-propanol, and 1-butanol, respectively, (Figure 1a-c). The sizes of the popcorn-like C60 microcrystals ranged from 0.6 to 1 µm (rough average size ∼ 0.83 ( 0.12 µm), the hexagon-shaped microcrystals had a diagonal length of 3.2 µm and a height of 1.5 µm, and the multiple rod-shaped microcrystals, which grew from a single point in random directions, measured 5-7 µm in length and 1.5 µm in diameter. Different shapes of microcrystals were obtained when the drowning-out ratio was varied. When

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TABLE 1: Summary of the Length and Diameter of the Microcrystals Obtained from Three Alcohols and the Drowning-Out Ratios in Figure 1 BuOH drowning-out ratio

EtOH (diameter, µm)

PrOH (diameter, µm)

length (µm)

diameter (µm)

9.0 4.0 1.5

0.83 ( 0.12 1.37 ( 0.26 1.88 ( 0.53

3.21 ( 0.4 3.83 ( 0.69 4.01 ( 0.38

5.89 ( 0.78 10.08 ( 2.12 16.04 ( 2.94

1.58 ( 0.31 2.14 ( 0.89 2.67 ( 0.76

the drowning-out ratio using ethanol was decreased from 4.0 to 1.5, the irregular popcorn-like microcrystals gradually changed into regular polygons (e.g., decahedrons, octagons, and truncated octagons) and the size of the microcrystals increased (Figure 1d,g). Using 1-propanol with a drowning-out ratio of 4.0 led to hexagonal microcrystals that had the appearance of a six-petal flower. The petals became larger and more distinct when a drowning-out ratio of 1.5 was employed (Figure 1e,h). In 1-butanol, the C60 microcrystals were only hollow at the top of the rod when a drowning-out ratio of 4.0 was used, yet this hollowness was extended and the shell thickness of the rod reduced when the drowning-out ratio was decreased to 1.5. Moreover, the length of the rod-shaped microcrystals increased from 5 to 8 µm when the drowning-out ratio was decreased from 4.0 to 1.5 (Figure 1f,i). The length and diameter of the microcrystals shown in Figure 1 are summarized in Table 1 with statistical analysis from about 30 samples each. To investigate the effect of the alcohol type on the formation of the shape-selective C60 microcrystals, binary mixtures of alcohols were used as the antisolvent in the drowning-out crystallization (Figure 2). The shape of the microcrystals was determined by the dominant alcohol fraction; that is, irregular popcorn-like microcrystals were formed when the volume ratio

of ethanol/1-propanol was 2:1, whereas hexagon-shaped microcrystals were formed when the volume ratio of ethanol/1propanol was 1:2 (Figure 2a,c). Similarly, the popcorn-like microcrystals gradually shifted to a hybrid form and then to multiple rods growing from a single point when the 1-butanol fraction of a binary mixture of ethanol and 1-butanol was increased (Figure 2d-f). From this result, we inferred a competition between the two alcohol species to dictate crystal growth and crystal morphology. In addition to the effect of the alcohol fraction, it was also found that the alcohol chain length had an influence on the crystal morphology based on observing the microcrystals produced in a 1:1 volume ratio of two alcohols. For example, mostly hexagon-shaped microcrystals were obtained from a binary mixture of ethanol/ 1-propanol (Figure 2b), whereas multiple rods were obtained from a binary mixture of ethanol/1-butanol (Figure 2e) or 1-propanol/1-butanol (Figure 2h). When considering the C60 concentration and drowning-out ratio of the C60 crystallization, the crystal morphology was found to change based on a combination of three factors: the alcohol type, drowning-out ratio, and C60 concentration (Figure S1, Supporting Information). When the drowning-out ratio and C60 concentration were decreased using ethanol, the microcrystals

Figure 2. SEM images of C60 microcrystals prepared using solvent mixtures of toluene and two alcohols: (a-c) ethanol and 1-propanol, (d-f) ethanol and 1-butanol, and (g-i) 1-propanol and 1-butanol at volume ratios of 2:1, 1:1, and 1:2, respectively. The scale bars indicate 2 µm in all images.

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Figure 3. (a) FTIR spectra of C60 microcrystals obtained from ethanol, 1-propanol, and 1-butanol with a fixed C60 concentration of 0.1 mg/ mL and drowning-out ratio of 9.0. (b) Raman spectra of pristine C60 and the microcrystals prepared with ethanol, 1-propanol, and 1-butanol at a drowning-out ratio of 9.0 and a C60 concentration of 0.1 mg/mL.

Figure 4. (a) NMR spectrum of the C60 microcrystals prepared with ethanol at a drowning-out ratio of 9.0 and a C60 concentration of 0.1 mg/mL. (b) TGA analysis of C60 crystals with a temperature increase rate of 20 °C/min in nitrogen. Inset: TGA analysis of pristine C60 powder.

formed had a regular octahedral instead of a popcorn-like shape. This is to be expected if the relationships between the supersaturation levels and the factors, such as the drowning-out ratio and C60 concentration, are taken into account. That is, a higher supersaturation level is induced with a higher drowning-out ratio and higher C60 concentration. This results in smaller and more irregular microcrystals due to a higher nucleation and growth rate. However, in 1-propanol, different optimal conditions were identified for hexagonal microcrystals. Whereas a high drowning-out ratio of 9.0 produced mostly hexagonal crystals with a C60 concentration of 0.1 mg/mL, the optimal drowning-out ratio for regular hexagonal crystals shifted to 4.0 with a C60 concentration of 0.2 mg/mL. This result stands in contrast to those obtained when ethanol was used as an antisolvent. One possible explanation for this observation is that there is a complicated interaction between the supersaturation of C60 and the 1-propanol molecules involved in the crystal nucleation and growth. Unique crystal morphology behavior was also found in the case of 1-butanol, where the crystal morphology remained consistently rod-shaped throughout the whole range of drowning-out ratios and C60 concentrations. However, the rods did become hollow when the drowning-out ratio was reduced, presumably due to the concentration depletion of C60. To investigate the chemical composition of the microcrystals, FTIR and Raman spectroscopies were conducted on the microcrystals produced with a drowning-out ratio of 9.0 and C60 concentration of 0.1 mg/mL. The FTIR spectra for all samples consistently showed four strong and sharp absorption peaks at 525, 576, 1182, and 1429 cm-1, corresponding to the characteristic bands of pristine C60 (Figure 3a).20 Moreover, no new absorption bands were seen in the 700-800 cm-1 range, which is characteristics of polymerized C60, resulting in a subsequent reduction of the Ih symmetry of C60.21 We also carried out Raman spectroscopy analysis, where the Raman spectra of the

crystals were found to be similar to that of pristine C60 (Figure 3b). The “pentagonal pinch” mode of pristine C60 at 1468 cm-1 was slightly shifted to 1467, 1465, and 1465 cm-1 in the C60 microcrystals from ethanol, 1-propanol, and 1-butanol, respectively. This appears to be caused by photoinduced polymerization by the probing laser during the measurement.22,23 Interestingly, the FT-IR spectra revealed additional peaks at 1050 (νs C-O), 2860 (νs CH2), 2920 (νas CH2), and 3430 (νs OH) cm-1, indicating the existence of alcohol molecules in the microcrystals. Similar bands arising from residual antisolvents have been reported in the spectra of C60 structures prepared using solvent-induced self-assembly methodologies.3,24 The entrapment of solvents in the microcrystal was also confirmed by solid-state 13C NMR spectroscopy (Figure 4a). The lower and broadened lines at 128.6, 58.78, and 20.35 ppm corresponded to aromatic carbon, primary alcoholic carbon, and methyl carbon, respectively, whereas the strong and sharp line at 143.5 ppm indicated pristine C60.25 To further investigate the entrapment of solvent molecules in the microcrystals, the crystals were heated to 1000 °C in a nitrogen atmosphere (Figure 4b). In the case of solvents remaining on the surface of the microcrystals, weight loss was expected to occur below the boiling temperatures of solvents (below approximately 110 °C). However, the crystals only started to lose weight at temperatures over 120 °C, mainly in three stages: 149, 430, and 800 °C. Whereas the third stage accounted for the highest weight loss (80-90%), due to the sublimation of C60, the two other weight losses were assumed to be caused by the release of the solvents trapped in the crystals (i.e., alcohol during the first stage and toluene during the second).10,26 The estimated weight percentages of the solvents were 6.39, 11.75, and 8.97% in ethanol, 1-propanol, and 1-butanol, respectively. When converting the percentages, the molar ratio of the solvents to C60 was approximately 1:1, which was consistent with previous reports

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Figure 5. XRD patterns of C60 microcrystals obtained from ethanol, 1-propanol, and 1-butanol with a fixed C60 concentration of 0.1 mg/ mL and drowning-out ratio of 9.0.

on C60 crystals containing solvent molecules in their crystal structures.3,10 Figure 5 shows a typical XRD pattern for the C60 microcrystals prepared with various alcohols at a drowning-out ratio of 9.0. The peaks for the crystals prepared with ethanol were indexed at (111), (220), (311), (222), (331), (422), and (333), corresponding to the typical face-centered cubic (fcc) structure of pristine C60 bulk crystals.27 Meanwhile, the peaks for the crystals prepared with 1-propanol or 1-butanol were indexed at (100), (002), (110), (103), (112), (004), and (006), corresponding to a hexagonal close-packed (hcp) structure.18 This structure is similar to that reported by Skokan et al. for crystals obtained by evaporation using a solution of C60 in organic solvents.28 The structural dimensions were also identified as a ) 10.02 Å and c ) 16.38 Å, which are in close agreement with an hcp structure. Although we still lack a complete explanation for this effect, subtle differences between the alcohols, such as their solubility, diffusion coefficient, and polarity, may account for the formation of different morphologies and crystal structures.29,30 For example, the crystals prepared with methanol or ethanol, both of which have short alkyl chains, exhibited an fcc structure (Figure S2, Supporting Information), whereas the crystals prepared with 1-propanol or 1-butanol, both of which have relatively long alkyl chains, exhibited an hcp structure. In addition to the alcohol type, the diffraction patterns of the crystals changed according to the volume ratio of toluene and alcohol (Figure S3, Supporting Information). A high index (333) facet with ethanol or (006) facets with 1-propanol or 1-butanol appeared when decreasing the alcohol ratio, whereas the main index (111) with ethanol and (002) with 1-propanol or 1-butanol were significantly decreased. This observation indicated that the crystal growth regardless alcohol type was demonstrated by drowning-out ratios, as seen in the SEM images (Figure 1). The high-resolution TEM images and selected area electron diffraction (SAED) patterns of the crystals were measured in order to investigate the structures of the synthesized microcrystals (Figure 6). Because the electron beam cannot penetrate through them, it is difficult to observe crystals that are more than 100 nm thick. Therefore, this study focused on the thin outer rim of the crystals (indicated by a white circle). The crystals prepared with ethanol, 1-propanol, and 1-butanol were found to display dense packing along the growth axis, and the average center-to-center distances (lattice plane spacing) of C60 molecules were measured as 9.6, 8.5, and 8.9 Å, respectively. The SAED patterns represent the index of the main crystallographic planes for the three types of microcrystals. Thus, the main planes for the crystals prepared with ethanol were (022), whereas those for the crystals prepared with 1-propanol or 1-butanol were (002) and (110), respectively. On the basis of the XRD and TEM analyses, we concluded that the crystals have different structures, such as an fcc structure, when ethanol was used and an hcp structure when employing 1-propanol or 1-butanol.

Figure 6. TEM images of C60 microcrystals prepared from (a) ethanol, (b) 1-propanol, and (c) 1-butanol. SAED patterns and high-resolution TEM images are inserted as upper and lower insets, respectively. The distances marked white arrows are 9.6, 8.5, and 8.9 Å in ethanol, 1-propanol, and 1-butanol, respectively.

On the basis of these experimental observations and analyses, we propose a possible model to describe the mechanism of formation of C60 microcrystals in a solvent mixture (Figure 7). C60 dissolved in toluene instantaneously forms clusters that serve as nuclei after mixing with alcoholic antisolvents. Because of its low solubility in alcohols, C60 quickly reaches a supersaturation level and crystal growth occurs with different morphologies depending on the type of alcohol. It is widely accepted that crystal morphology is influenced by the supersaturation ratio: 1D structures, such as wires, tubes, and whiskers, are formed under low supersaturation ratio conditions, 2D plates and disks are formed under medium ratio conditions, and 3D polygons (e.g., octahedrons and decahedrons) and even isotropic particles are formed under high ratio conditions.31,32 Therefore, the solubility of C60 in the three types of alcohols with varying drowning-out ratios was investigated by measuring the absorbance of C60 in solution (Figure S4, Supporting Information). The solubility of C60 increased in alcohols with longer chain lengths (ethanol < 1-pro-

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J. Phys. Chem. C, Vol. 114, No. 30, 2010 12981 were attributed to a different preferred growth direction due to a change in the anisotropy energy and subtle difference in the combined degree of each alcohol molecule. Acknowledgment. The authors acknowledge the financial support received from the Fundamental R&D Program for Core Technology (MKE, Korea), the Pioneer Research Center Program (Grant No. 2008-00180, MEST, Korea), and the KRIBB Initiative Program (KRIBB). Supporting Information Available: Experimental details, SEM images, Raman and NMR spectra, and XRD data. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 7. Schematic illustration of the formation mechanism of various-shaped C60 microcrystals depending on the alcohol type and drowning-out ratio of toluene and alcohols.

panol < 1-butanol) and with increasing volume ratios of toluene in a mixture solution. Thus, the low solubility in ethanol induces a high supersaturation point, which leads to the growth of 3D irregular microcrystals, the medium solubility in 1-propanol results in 2D microcrystals, such as hexagonal plates, and the relatively high solubility in 1-butanol induces a low supersaturation point, leading to 1D rod-shaped microcrystals. The drowning-out ratio was also found to play an important role in the growth of the C60 microcrystals. The C60 solubility in all three alcohols increased when increasing the volume of toluene. This increased solubility then decreases the supersaturation level, thereby interrupting the crystal growth at certain sites, such as the corner of the hexagonal cross section in 1-propanol and the top of the rod in 1-butanol, due to the relatively high free energy when increasing the alcohol portion. In contrast, the crystals formed with ethanol create a perfect apex of an octagon. Ji et al. reported that decreasing the volume ratio of the solvent with respect to antisolvent in C60 crystallization encourages the concentration depletion of C60, resulting in site-selective crystal growth.26 Because supersaturation is a function of concentration, the size and shape of C60 microcrystals can be controlled by the drowning-out ratio as well as the concentration of C60 in the initial solution. Changing the C60 concentration also had a significant influence on the size of the microcrystals, while the shape of the microcrystals was only partially changed. The microcrystals produced with 1-butanol were an exception to this trend. Consequently, when varying a combination of factors, such as the C60 concentration, alcohol type, and drowning-out ratio, the resulting shape-selective crystallization of C60 microcrystals was found to be driven by the preferred growth direction and supersaturation rate. Temperature is also known to be one of the key factors to influence the morphology and crystalline structure of crystals.15,33 However, after formation of the crystals in solvent mixtures, the morphology and crystalline structure were just slightly changed after removing the solvents by heating to 100 °C (Figure S5, Supporting Information). 4. Conclusions In conclusion, diverse dimensional C60 microcrystals were prepared by drowning-out crystallization where the shape obtained was controlled by the alcoholic antisolvent. The size and shape of the crystals varied depending on the drowning-out ratio and C60 concentration. XRD and TEM revealed that the crystals had an fcc or hcp structure depending on the type of alcohol used, whereas the FTIR spectra indicated that the crystals were composed of monomeric C60. The different morphologies of the microcrystals

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