CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 369–371
Communications Methodological Features of the Emulsion and Reprecipitation Methods for Organic Nanocrystal Fabrication Kento Ujiiye-Ishii,* Eunsang Kwon, Hitoshi Kasai, Hachiro Nakanishi, and Hidetoshi Oikawa Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan ReceiVed July 27, 2007; ReVised Manuscript ReceiVed NoVember 13, 2007
ABSTRACT: The specific methodological features in the emulsion and the reprecipitation methods to fabricate organic nanocrystals were examined using perylene. The size-control of perylene nanocrystals was accomplished for the first time using the emulsion method, which has not been achieved so far by the reprecipitation method. The standard deviation of size distribution of nanocrystals in the emulsion method became smaller than that in the reprecipitation method. Although the residual good solvent may influence the nanocrystal growth process in the reprecipitation method, no such influence was detected in the emulsion method. Nanosized materials have been extensively studied from the viewpoints of fundamental interests and their application.1–3 Owing to novel and interesting features such as peculiar size-dependent optical and electronical properties, organic nanocrystals have especially attracted much attention in the past few years.4,5 However, because of the thermal instability in organics, the fabrication process6–8 for inorganic nanoparticles could not be applied to organic compounds. We have established the reprecipitation method,9 named the “rmethod”, which was devised from the necessity of using the fabrication method of organic nanocrystals possibly at room temperature. The r-method, which is based on the difference in solubility of the target molecules in two miscible good and poor solvents, allowed us to fabricate π-conjugated organic nanocrystals. This method, however, is only applicable to compounds having high solubility in some good polar solvents such as acetone and tetrahydrofuran that are infinitely diluted in water as a poor solvent. Namely, there was no method to prepare organic nanocrystals from nonpolar organic solvents such as toluene and carbon tetrachloride, which commonly dissolve organic compounds, until we reported the stabilizer-free emulsion method,10,11 named the “e-method”. In the e-method, an emulsion was first prepared by dispersing a hot organic solvent including target compounds into an aqueous medium at the same temperature as the organic solvent, and then nucleation and crystal growth proceeded in the emulsion by decreasing the temperature. A significant feature of this method is that the resulting nanocrystals can be transferred and redispersed in an aqueous medium. Although these two kinds of different but complementary approaches will lead to a great deal of research fields for organic * To whom correspondence should be addressed. Tel: 81-22-217-5645. Fax: 81-22-217-5645. E-mail:
[email protected].
nanocrystals, the nanocrystallization process is still unclear regarding the e-method as well as the r-method. In the present study, we demonstrate the most characteristic features of both methods in the nanocrystallization process, that is, size-control of perylene nanocrystals, the redispersion process of nanocrystals into aqueous medium in the e-method. Experimental Procedures. Materials. Perylene powder used was purchased from Wako Pure Chemicals, Osaka, Japan. Acetone (Wako Pure Chemicals, dehydrated), toluene (Wako Pure Chemicals, dehydrated), and o-xylene (Kanto Chemicals, Tokyo, Japan, dehydrated) were commercially available and used without further purification. Ultrapure water (18.2 MΩ · cm-1) was also used as a dispersion medium. Fabrication of Perylene Nanocrystals Using the Reprecipitation Method (r-Method). A 0.1 mL of acetone solution of perylene (2 mM) was rapidly injected into vigorously stirred water (10 mL) at 0 °C using a microsyringe. The resulting dispersion liquid was heated to 25 °C and then was irradiated by microwave (2.45 GHz, 50 W) for 5 min. The obtained perylene nanocrystals dispersion liquid was apparently pale-yellow. Fabrication of Perylene Nanocrystals Using the Emulsion Method (e-Method). Perylene was first dissolved in the mixed solvent of toluene/o-xylene (1:4, vol/vol), whose concentrations were 15 and 30 mM. A 15 mL of perylene solution previously heated around 90 °C was quickly added into hot water (150 mL, 90 °C). A stable emulsion was produced in an aqueous liquid by mechanically stirring at 3000 rpm and irradiating ultrasound (45 kHz, 100 W), and then this emulsion dispersion liquid was gradually cooled at a given cooling rate within 2 h to 30 °C through mechanically stirring under ultrasound irradiation. Subsequently, 5 mL of diethylether as an antifoaming agent was further added to break emulsion and stirred for 20 min at room temperature, and then the separated organic solvent layer was removed. To completely remove the residual organic solvent in an aqueous liquid
10.1021/cg700708g CCC: $40.75 2008 American Chemical Society Published on Web 01/09/2008
370 Crystal Growth & Design, Vol. 8, No. 2, 2008
Figure 1. Schematic model of the organic nanocrystal fabrication using the e- and r-methods.
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Figure 3. Crystal size distribution histograms of perylene nanocrystals produced in the e-method (a, 30 mM of perylene solution) and in the r-method (b, 2 mM of perylene solution). Table 1. Mean Crystal Size of Perylene Nanocrystals Produced in the e- and r-Methods, Which Were Obtained by SEM Image Analysis mean size/nm
a
run no.
e-methoda
r-methodb
1 2 3
253 ( 29 260 ( 55 250 ( 28
157 ( 44 165 ( 46 145 ( 47
30 mM of perylene solution. b 2 mM of perylene solution.
Figure 2. SEM images of perylene nanocrystals fabricated by the e-method (a, 15 mM of perylene solution; b, 30 mM of perylene solution) and the r-method (c, 2 mM of perylene solution).
layer, microwave irradiation (2.45 GHz, 700 W) was performed for 30 s per one-time irradiation for requisite times. As a result, perylene nanocrystals were obtained as a stable dispersion in an aqueous liquid, which was also apparently pale-yellow. Characterization. The mean crystal size was determined using both the dynamic light scattering technique (DLS: Otsuka Electronics Co. Ltd., DLS-7000) and scanning electron microscopy (SEM: JEOL, JSM-6700F), and crystal structure was evaluated by powder X-ray diffraction measurement (XRD: Mac Science, MXP18). A residual organic solvent (in aqueous dispersion liquid) was investigated by 1H nuclear magnetic resonance spectroscopy (NMR: JEOL, LAMBDA 400 MHz). Results and Discussion. The r- and e-methods are in principle distinguishable in the nanocrystallization and the subsequent process from their experimental procedure as shown in Figure 1.9,10 In the e-method, the nucleation and growth proceed in the emulsion by decreasing the temperature, whereas the nucleation in the r-method may occur in the process of the mutual diffusion between good and poor solvents.12 As discussed later, the perylene nanocrystals fabricated by the r-method became larger with elapsed time in an aqueous medium,13 and the size control has not been completely achieved so far. In the e-method, however, the size of perylene nanocrystals could be stably controlled by changing the concentration of perylene in the emulsion. Thus, the larger nanocrystals were obtained by increasing the solute concentration in the emulsion at a given cooling rate. For example, the perylene nanocrystals with sizes of 180 and 250 nm were fabricated successfully from 15 and 30 mM of perylene solutions, respectively (Figure 2a,b). In addition, the size distribution in the e-method was narrower than that in the r-method (Figures 2 and 3). The mean crystal size and standard deviation (SD) in the r- and e-methods, which were obtained by an SEM image analysis, are summarized in Table 1. It was found that the SD was commonly smaller in the e-method than that in the r-method. The UV–vis absorption maxima of the nanocrystals were shifted to a shorter wavelength region by decreasing their size. The absorption maxima were observed at a wavelength of 474 nm in the crystal size of 250 nm and at a wavelength of 455 nm in the size of 180 nm, respectively. This blue-shift characteristic of the
Figure 4. SEM image of perylene nanocrystals directly filtrated from emulsion. The concentration of perylene solution was 15 mM.
nanocrystals would be originated by a lattice softening, as well as that accumulated from other organic nanocrystals in our previously reported study.14,15 The identical X-ray diffraction (XRD) patterns of perylene nanocrystals fabricated by the r- and e-methods suggested the same crystal structure as the R-form of the bulk type.16,17 A significant feature of the e-method is that the nanocrystals grown in the emulsion are readily transferred and redispersed in an aqueous medium.10,11 As shown in Figure 4, the SEM images of perylene nanocrystals were directly filtrated from emulsion, using a 0.05 µm pore-size membrane filter at room temperature to investigate the morphological difference in perylene nanocrystals before and after the redispersion process. The morphology and the size of these nanocrystals were essentially the same as those in Figure 2a. As mentioned above, perylene nanocrystals fabricated by the r-method continue to grow gradually after the reprecipitation until the size is reached at a certain steady value (around 200 mm).13 This is basically related to a nanocrystallization mechanism, in which the cluster-like nanoparticles essentially are formed at once, and subsequently bring about the nucleation and crystal growth through thermal collision between these clusters.
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Crystal Growth & Design, Vol. 8, No. 2, 2008 371 ca. 10% of good solvent was still residual in the r-method, and it is difficult to completely remove good solvent, due to high solubility in water as a poor solvent. The influence of the residual solvent in the r-method is currently now being investigated.
4. Conclusion In this study, we examined the methodological features in the emulsion method (e-method) and the reprecipitation method (rmethod) to fabricate organic nanocrystals using perylene. Until now, it was difficult to obtain well-defined perylene nanocrystals using the r-method. On the other hand, the size of perylene nanocrystals could be controlled successfully by changing the concentration of perylene in the e-method. Moreover, the size distribution in the e-method was narrower than that in the r-method.
References Figure 5. Relative crystal size changes for perylene nanocrystals in the r-(2) and the e-methods (9), which were measured with DLS. “Time” means the elapsed one after reprecipitation in the r-method, and after redispersion in an aqueous medium in the e-method.
On the contrary, the nanocrystals fabricated by the e-method did not show any such kind of variations in size, after the nanocrystals were redispersed by breaking the emulsion, as shown in Figure 5. This showed that such a collision process for the r-method was not going to happen during the nanocrystallization of the e-method. Furthermore, the droplet size of the formed emulsion determined by DLS was ca. 400 nm, at least, at room temperature, which was independent of the concentration of the solution. However, as mentioned above, the larger nanocrystals were obtained by increasing the solute concentration in the emulsion, and these sizes were smaller than of the droplet size of the emulsion. These results suggest that the crystallization process of the e-method was apparently of the same character as the common emulsion method using a surfactant, in which the homogeneous nucleation occurs at a larger supercooling temperature. The crystallization process, however, in the e-method is a very complex one that depends on the particular set of operation conditions, including organic solvents and solution concentrations, the temperature, and the solubility of the organics in the aqueous phase. Therefore, we next will examine the effect of these condition variables on the crystallization process. The residual organic solvent used in both methods was determined by 1H NMR analysis, which was carried out with deuterium oxide as the disperse medium using an internal standard of 2,2,3,3tetradeutero-3-(trimethylsilyl)propionic acid sodium salt. Organic solvents completely disappeared in the redispersion medium in the e-method by the microwave irradiation treatment. On the other hand,
(1) Cohen, M. L. Mater. Sci. Eng. 2001, C15, 1. (2) Ozin, G. A.; Arsenault, A. C. Nanochemistry; the Royal Society of Chemistry: Cambridge, UK, 2005. (3) Moriarty, P. Rep. Prog. Phys. 2001, 64, 297. (4) Scholes, G. D.; Rumbles, G. Nat. Mater. 2006, 5, 683. (5) Rogach, A. L.; Eychmuller, A.; Hickey, S. G.; Kershaw, S. V. Small 2007, 3, 536. (6) Kimoto, K.; Kamiya, Y.; Nonoyama, M.; Uyeda, R. Jpn. J. Appl. Phys 1963, 2, 702. (7) Townsend, P. D.; Chandler, P. J.; Zhang, L. Optical Effects of Ion Implantation; Cambridge University Press, Cambridge, UK, 1994. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (9) Kasai, H.; Nalwa, H. S.; Oikawa, H.; Okada, S.; Matsuda, H.; Minami, N.; Kakuda, A.; Ono, K.; Mukoh, A.; Nakanishi, H. Jpn. J. Appl. Phys. 1992, 31, L1132. (10) Kwon, E.; Oikawa, H.; Kasai, H.; Nakanishi, H. Cryst. Growth Des. 2007, 7, 600. (11) Kwon, E.; Chung, H.-R.; Araki, Y.; Kasai, H.; Oikawa, H.; Ito, O.; Nakanishi, H. Chem. Phys. Lett. 2007, 441, 106. (12) Onodera, T.; Kasai, H.; Okada, S.; Oikawa, H.; Mizuno, K.; Fujitsuka, M.; Ito, O.; Nakanishi, H. Opt. Mater. 2002, 21, 595. (13) Kasai, H.; Oikawa, H.; Okada, S.; Nakanishi, H. Bull. Chem. Soc. Jpn. 1998, 71, 2597. (14) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Itoh, O.; Nakanishi, H. Jpn. J. Appl. Phys 1996, 35, L221. (15) Katagi, H.; Oikawa, H.; Okada, S.; Kasai, H.; Watanabe, A.; Ito, O.; Nozue, Y.; Nakanishi, H. Mol. Cryst. Liq. Cryst. 1998, 314, 285. (16) Donaldson, D. M.; Robertson, J. M.; White, J. G. Proc. R. Soc. A 1953, 220, 311. (17) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Ito, O.; Nakanishi, H. Chem. Lett. 1997, 1181.
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