J. Phys. Chem. C 2010, 114, 849–856
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Nanoparticles of Fullerene C60 from Engineering of Antiquity Shigeru Deguchi,*,† Sada-atsu Mukai,†,‡ Tomoko Yamazaki,†,§,| Mikiko Tsudome,† and Koki Horikoshi† Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan, and Institute for AdVanced Study, Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan ReceiVed: September 28, 2009; ReVised Manuscript ReceiVed: December 2, 2009
Nanoparticles of fullerene C60, including those as small as 20 nm, were obtained by simple hand-grinding of bulk solids with an agate mortar and pestle. Crystalline structure, size distribution, and surface characteristics of the hand-ground C60 nanoparticles were studied. In addition, the C60 nanoparticles having different sizes were prepared successfully by fractionating the as-prepared nanoparticles by filtration or centrifugation, and size-dependent optical properties were determined for the C60 nanoparticle. A linear relationship was found between the position of the peak at around 350 nm and the diameter of the C60 nanoparticles. Introduction
Experimental Section
When the size of particles becomes smaller than 100 nm, the particles exhibit properties that are not observed for molecules or bulk counterparts.1 Such particles, the so-called nanoparticles, are building blocks for nanotechnology-derived applications including single-electron devices, ultra dense recording media, bioelectronic devices and sensors, bioimaging, optoelectronic devices, catalysis and chemical sensors, and energy conversion and storage.1,2 The most obvious approach to prepare small particles is a top-down approach, in which the bulk solid is reduced to small particles by mechanical forces. Indeed, particle-size reduction dates back to 8000 B.C. when the Egyptians started grinding wheat by mortar and pestle,3 and the technology for the particlesize reduction has seen remarkable developments since then.4 When a physical force is applied to a bulk solid, the solid undergoes plastic deformation until strain reaches a threshold value, above which fracture results.5 Smaller particles are generated as the process is repeated. However, when the size of the solid becomes small enough, energy is dissipated primarily as heat and the size reduction becomes increasingly difficult.5 Consequently, this approach only gives particles on the order of micrometers in size. It is possible to produce nanoparticles in this manner, but very high energy has to be applied using a special device such as a high-energy ball mill.6,7 Instead, nanoparticles are usually produced by bottom-up approaches, in which molecules are allowed to assemble into nanoparticles in solutions or gas through chemical reactions.1,2 In the case of fullerene C60, however, we found that nanoparticles including the ones as small as 20 nm were generated readily by simple hand-grinding of a bulk solid in an agate mortar with an agate pestle.8,9 In this paper, preparation and characteristics of C60 nanoparticles by hand-grinding are described.
Materials. C60 (>99.9% pure) was purchased from Tokyo Kasei, Co., Ltd. (Tokyo, Japan), and used as received. Sodium dodecylsulfate (SDS) was purchased from Nacalai Tesque, Inc. (Kyoto, Japan). Millipore water was used throughout the work. Water was deaerated by purging gaseous nitrogen for at least 30 min before use. Sample Preparation. In a typical procedure, solid C60 (ca. 10 mg) was placed in an agate mortar and ground with an agate pestle for 1-2 min. Upon grinding, black particles of C60 turned to brownish fine powder. Inhalation of the fine powder during grinding was not an issue, as the ground powder of C60 adhered onto the surfaces of the mortar and pestle. The fine powder was scraped off by using a spatula and ground again. The process was repeated for several times. Finally, ground C60 (7 mg) was placed in a vial, and 5 mL of an aqueous solution of SDS (40 mM) was added, after which the headspace gas was replaced with argon and the vial was tightly sealed with a screw cap. The mixture was subjected to ultrasonic treatment for 30 min using Astrason Ultrasonic Processor XL2020 (20 kHz output frequency, Misonix, Inc., Farmingdale, NY) with a cup-horn attachment. Temperature of the sample was kept between 9 and 12 °C by an external water-circulating bath (TRL-C10, Thomas Kagaku Co., Ltd., Tokyo, Japan) during the treatment. A stock dispersion was obtained by filtering the dispersion with a membrane filter (Millex-SV, nominal pore size 5 µm, Millipore). Working dispersions for further analysis were prepared by diluting the stock dispersion with water. Characterization. Particle size distribution of as-received and hand-ground C60 was evaluated by using Partica LA-950 Laser Diffraction Particle Size Analyzer (Horiba, Ltd., Kyoto, Japan). Solid particles were dispersed in water containing 0.1 wt % SDS by agitating the mixtures by hands, and the dispersions were introduced to the measuring unit. The average size of the C60 nanoparticle in the dispersions was measured by dynamic light scattering (DLS) on an FDLS-1200 (Otsuka Electronics Co., Ltd., Osaka, Japan) equipped with a He/Ne laser (λ ) 632.8 nm, 10 mW) and a solid-state laser (λ ) 532 nm, 100 mW). The measurements were done at 25.0 ( 0.1 °C and at a fixed scattering angle of 90°. Average hydrodynamic diameter (dH) was calculated by using a cumulant method,10 while CONTIN11
* Corresponding author. Tel: +81-46-867-9679. Fax: +81-46-867-9715. E-mail:
[email protected]. † JAMSTEC. ‡ Kyushu University. § On leave from Department of Biological Applied Chemistry, Toyo University. | Present address: DJK Corporation Co., Ltd.
10.1021/jp909331n 2010 American Chemical Society Published on Web 12/28/2009
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was employed to obtain size distribution. Concentration of C60 in the dispersions was measured according to the literature.12 Size distribution of the C60 nanoparticles in the dispersions was studied by measuring the decrease in concentration of C60 after filtering the dispersion with membrane filters of different pore sizes (800 nm, 450 nm, 220 and 100 nm). UV-visible absorption spectra were recorded on a UV-2400PC UV-vis recording spectrophotometer (Shimadzu, Kyoto, Japan) at room temperature. All of the spectra reported here are backgroundcorrected. The ζ-potential was measured by electrophoretic light scattering on an ELS-8000 (Otsuka Electronics, Co., Ltd., Osaka, Japan). Powder X-ray diffractograms were recorded on a RINTUltima III (Rigaku Corporation, Tokyo, Japan). Matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopic analysis was performed on an AXIMA-TOF2 (Shimadzu, Kyoto, Japan). Saturated solutions of both ground and as-received C60 in toluene were prepared. The solutions were mixed with a toluene solution of a matrix (trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]-malononitrile, 10 mg mL-1). The mixtures were deposited on a target, air-dried, and subjected to the analysis. Electron Microscopy. Specimens for scanning electron microscopy (SEM) were prepared by spreading the particles onto a plastic weighing boat, and transferring them onto a conductive double-sided adhesive tape mounted on a brass stub. The specimens were coated with osmium (estimated coating thickness, 99.8%, Merck) containing 40 mM of SDS. Average diameter of the C60 nanoparticles in the dispersion was 249.6 nm by DLS. Concentration of C60 after filtration was 6.98 × 10-3 M (5.03 mg mL-1, dispersion efficiency 32 wt %). 1H decoupled 13C NMR spectra were recorded on an Avance 600 (Bruker Biospin GmbH, Rheinstetten, Germany) with a cryoprobe at room temperature. The measurement used 30° pulse width, 0.911 s FID acquisition time, and 5 s recycle delay and took 8000 scans over 13.3 h. The FID was processed with 5 Hz of broadening factor. Chemical shifts were reported relative to the resonance of R-carbon of SDS (72.01 ppm). Chemical shifts of the resonances of SDS were calibrated by separately measuring 13C NMR spectra of the D2O solution of SDS using an aqueous solution of sodium 3-(trimethylsilyl)propane-1-sulfonate (DSS) in a capillary as an external standard. Results Formation of Nanoparticles by Hand-Grinding. Figure 1 shows electron microscopic images of C60 before and after handgrinding. As-received C60 particles were approximately 100 µm in size and had faceted morphologies (Figure 1 a). Handgrinding reduced the particle size to less than 10 µm (Figure 1 c). Very surprisingly, close examinations revealed the presence of significant amounts of particles smaller than 100 nm in
Deguchi et al.
Figure 1. (a) SEM image of as-received C60, scale bar, 10 µm; (b) SEM image of small particles (indicated by triangles) found on a surface of an as-received C60 particle, scale bar, 1 µm; (c) SEM image of handground C60, scale bar, 10 µm; (d) SEM image of agglomerated nanoparticles found in hand-ground C60, scale bar, 100 nm; (e) HRTEM image of C60 nanoparticles found in hand-ground C60, scale bar, 20 nm; and (f) close-up of a C60 nanoparticle (square in panel e) found in hand-ground C60. Lattice spacing calculated from a Fourier-transformed image (0.8 nm) agrees with that of (111) plane of bulk crystals of C60 (0.81596 nm). No grain boundary is seen, indicating that the particle is a single crystal. Scale bar, 5 nm. Reproduced from ref 8.
agglomerated forms (Figure 1d). Examination by HRTEM (Figure 1, panels e and f) revealed nanoparticles including those as small as 20 nm (Figure 1f) with distinct fringes. Lattice spacings calculated from Fourier-transformed images agreed with those of bulk crystals of C60,13 indicating that these particles were pristine crystals of C60. Powder X-ray diffractograms of as-received and hand-ground C60 showed that the crystalline structure remained unchanged after hand-grinding (Figure 2). Significantly broader signals from hand-ground C60 suggest the presence of small particles. Line width analysis according to the Scherrer equation (eq 1)
L)
Kλ β cos θ
(1)
where λ is the wavelength of the incident X-ray (Cu KR line, 1.541841 Å), β is the full width at half the maximum intensity of a diffraction peak, 2θ is the diffraction angle, and K is the Sherrer constant (0.94), showed that the average size of the crystallites (L) decreased from >100 nm to 20 - 30 nm by handgrinding. The results also show that the formation process of the nanoparticles of C60 by hand-grinding is completely different from mechanochemical processes in that no change in the crystalline structure including amorphization is associated.6 We propose a new name mechano-assisted reduction of size (MARS) for the process to discriminate it from the conventional processes.14
Nanoparticles of Fullerene C60
J. Phys. Chem. C, Vol. 114, No. 2, 2010 851 TABLE 1: Concentration of C60 in the Dispersion after Filtration with Membrane Filters of Different Pore Sizesa pore size/nm
[C60]/M
5000 800 450 220 100
6.56 × 10-4 4.96 × 10-4 3.99 × 10-4 3.12 × 10-4 9.57 × 10-5
C60 [C60]/µg dispersion mL-1 recoveredb/wt% efficiencyc/wt % 473 357 287 225 69
100 75 61 48 15
34 26 21 16 5
a Reproduced from ref 8. b Calculated as the weight ratio of the amount of C60 in the filtrate to that in the stock dispersion. c Calculated as the weight ratio of the amount of C60 in the filtrate to the amount used for sample preparation.
Figure 2. Normalized powder X-ray diffractograms of as-received (top) and hand-ground C60 (bottom). There is no difference in the poison of the peaks, indicating the crystalline structure of C60 remains virtually unchanged after hand-grinding. Broadening of the peaks of hand-ground C60 suggests the presence of small particles. The diffractograms are shifted arbitrarily for clarity.
Figure 4. 1H decoupled 13C NMR spectra of the C60 nanoparticles in D2O containing 40 mM of SDS. Inset is the same spectra at 20 times magnification. Redrawn from ref 8.
Figure 3. Size distribution of C60 particles before (broken curve) and after (solid curve) hand-grinding.
Size Distribution of Hand-Ground C60. The effect of handgrinding on the size reduction of C60 solids was evaluated quantitatively by using a particle analyzer (Figure 3). The median diameter of as-received C60 crystals was 129 µm and agreed well with the SEM images (Figure 1a). C60 after handgrinding gave a bimodal size distribution that was also in good agreement with the SEM images (Figure 1c-f). In addition to a fraction that was centered at around 13 µm, ground C60 contained significant amounts of smaller particles. The size reduction of the particles down to several tens of µm by handgrinding is no surprise, but it appears that nanoparticles are generated with very high efficiency during normal size reduction through fracturing in the case of C60. For the particles having broad size distribution, it is difficult to cover whole size range by the particle analyzer, because information on small particles is masked out by that on larger particles. To obtain more comprehensive information on the size distribution of the nanoparticles of C60 by MARS, we conducted simple filtration experiments (Table 1). A highly turbid dispersion was formed as soon as hand-ground C60 (7 mg) was mixed
with 5 mL of water containing 40 mM of sodium dodecylsulfate (SDS). Ultrasonic treatment followed by filtration with a membrane filter (nominal pore size, 5 µm) gave a turbid and brown dispersion. Concentration of C60 in the dispersion was found to be 6.56 × 10-4 M (473 µg mL-1). Dispersion efficiency, as defined by the weight ratio between the amount of dispersed C60 and the amount used for sample preparation, was 34 wt %. Average diameter of the C60 nanoparticles in the dispersion was 235 nm by DLS. The dispersion was then passed through membrane filters with different pore sizes, and decrease of the C60 concentration was measured. Filtration experiments revealed that 48 wt % of the nanoparticles in the dispersion passed the filter of 220 nm pore size, and 15 wt % of them passed the filter of 100 nm pore size. Characterization by 13C NMR. We found that the dispersion efficiency did not depend on the initial composition, meaning that concentrated dispersions could be obtained simply by using a larger amount of hand-ground C60. We confirmed that a concentrated dispersion containing 5 mg mL-1 of the C60 nanoparticles could be obtained. This concentration is higher than the molecular solubility of C60 in toluene (2.15 mg mL-1),15 which is a representative good solvent for C60. High concentration makes it possible to apply a variety of analytical techniques to study the C60 dispersion, which has not been possible before due to the low C60 content in the dispersions. 13C NMR spectroscopy is one of them, because sensitivity of the technique is poor due to the low natural abundance of 13C nuclei (1.1%).16 The only successful measurements of the C60 nanoparticles used 13C-enriched C60.17 Figure 4 shows 13C NMR spectra of the C60 nanoparticles in the concentrated dispersion showing a single resonance at 145.86 ppm corresponding to C60 and resonances due to SDS (16.45-72.01 ppm). The result shows that all 60 carbons of the C60 molecules are equivalent and the nanoparticles are made of underivatized C60; no significant chemical or mechanochemical reaction took place during hand-grinding or sonication.
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Figure 5. MALDI-TOF mass spectra of (a) as-received and (b) handground C60.
Figure 6. Part of 13C NMR spectra showing the resonance due to C60. The experimental data (dots) can be represented with a sum (solid curve) of two Lorentzian curves (broken curves).
Analysis of ground C60 by MALDI-TOF mass spectroscopy did not show any evidence of significant oxidation either (Figure 5). The chemical shift observed in this work agrees with the reported value for the 13C-enriched C60 nanoparticles in water (146 ppm).17 In contrast, the 13C NMR peak for C60 appears at 142.7 ppm in a molecular solution in benzene18 and at 143 ppm in a solid state.19 Possible reasons for the difference are not clear at present, but it may be related to interactions between water molecules that surround nanoparticles of C60. It is possible that such interaction affects ring current, which is very sensitive to the electronic structure,20 of C60 molecules in nanoparticles. Figure 6 shows a part of the 13C NMR spectra showing the resonance due to C60. The peak shape does not reveal any anistotropic effects. We found that the peak could be represented with a sum of two Lorentzian curves. The full widths at halfmaximum (fwhm) of the narrow and broad components were 0.17 ppm and 1.61 ppm, respectively. NMR signals of solidstate samples are usually broad due to various reasons such as
Deguchi et al. dipole-dipole interactions and anisotropy of chemical shifts. In the case of C60, however, fast isotropic rotation of the C60 molecules in the crystal makes the line width of NMR signal sharp.19 Still, the fwhm of the signal of C60 in a solid state was reported to be 4.67 ppm at 295 K,19 and the signal obtained from the nanoparticles was significantly narrower. Additional narrowing of the NMR signal observed for nanoparticles of C60 would be ascribed to motional narrowing.21,22 Rapid and random Brownian motion of nanoparticles in solutions suppresses the broadening effect, and provides highresolution NMR spectra. Such effect was demonstrated for nanoparticles of AlF3.21,22 As the extent of the motional narrowing should depend on the particle size, we anticipate that the peak shape analysis may be used to determine the size distribution of the C60 nanoparticles. Dispersibility in Water. Like other nanoparticles, nanoparticles of fullerenes have also been prepared mostly by bottomup approaches by using recrystallization12,23-25 or redox reactions.26 In the recrystallization process, fullerenes are first dissolved molecularly in an organic solvent such as benzene, toluene,23-25 tetrahydrofuran (THF),12 or ethanol.27 The solution is then mixed with a nonsolvent such as water, acetonitrile, and acetone. Upon mixing, fullerene molecules recrystallize to form nanoparticles and remain well-dispersed in the solvent. The most striking systems of such are those that use water as the medium. Fullerenes are hydrophobic carbon allotropes and are not at all soluble molecularly in water.28 However, the nanoparticles of them remain dispersed in water for a long period of time without the aid of a dispersing agent.12,23,24,26 They remain well-dispersed even in water at high temperatures and high pressures up to ∼200 °C and 25 MPa.29,30 The C60 nanoparticles by MARS also dispersed in pure water even without the aid of SDS. In this case, addition of pure water to hand-ground C60 did not lead to a dispersion, and a turbid and brown dispersion was obtained only after sonication. The dispersion contained the C60 nanoparticles, whose average diameter was 229 nm. Concentration of C60 in the dispersion was found to be 3.27 × 10-4 M (235 µg mL-1), giving the dispersion efficiency of 17 wt %. Stirring the mixture by a magnetic stirrer for 1 day also worked, giving a dispersion with a similar dispersion efficiency. The hand-ground C60 nanoparticles showed very similar surface characteristics in pure water to those prepared by the solution processes. For example, dispersibility and stability of the C60 nanoparticles in pure water arise from the electrostatic repulsive force acting between the negatively charged surfaces of the C60 nanoparticles.12,17,31 The hand-ground C60 nanoparticles were also found to bear negative surface charges in pure water and gave ζ-potential of -39.0 ( 1.4 mV. The value compares favorably with those reported for the C60 nanoparticles by the solution processes.12,17,25 In addition, the C60 nanoparticles were not extracted to toluene from water, despite high affinity of C60 to toluene.12,17,24 Furthermore, the hand-ground C60 nanoparticles dispersed in a variety of organic solvents including various alcohols, acetone, acetonitrile, silicone oil, and fluorinated oil,9 all of which do not dissolve C60 molecularly. Size Fractionation. Size is the most important parameter that characterizes various properties of nanostructured materials.1,32 From this perspective, MARS is not an appropriate production method of nanoparticles, because it gives the C60 nanoparticles of the broad size distribution (Figure 3). However, we found that the C60 nanoparticles by MARS could be fractionated without difficulty.
Nanoparticles of Fullerene C60
Figure 7. Size distributions of the C60 nanoparticles fractionated by filtration and centrifugation: (a) stock dispersion; (b) obtained by filtration of (a) with a membrane filter of 0.1 µm pore size; (c) obtained by centrifugation of (a) at 8000g for 1 h.
Figure 8. HRTEM images of C60 nanoparticles found in a dispersion in water. Scale bar, 5 nm.
Size fractionation of the C60 nanoparticles was carried out by either filtration with membrane filters of different pore sizes or centrifugation. The latter was particularly useful for preparing the nanoparticles smaller than 100 nm. Figure 7 shows typical size distributions of the C60 nanoparticles in a stock dispersion and those obtained by fractionation using filtration and centrifugation from the stock dispersion. The stock dispersion was prepared by dispersing hand-ground C60 in water by ultrasonication, followed by filtration with a membrane filter (5 µm pore size). The dh of the C60 nanoparticles in the dispersion was 216 nm by DLS. The distribution was broad ranging from ∼1 µm down to
