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Can Temperature Control the Size of Au Nanoparticles Prepared in Ionic Liquids by the Sputter Deposition Technique? Yoshikiyo Hatakeyama, Satoshi Takahashi, and Keiko Nishikawa* Graduate School of AdVanced Integration Science, Chiba UniVersity, Chiba 263-8522, Japan ReceiVed: March 27, 2010; ReVised Manuscript ReceiVed: May 10, 2010
It was recently discovered that sputter deposition of metal onto the surface of an ionic liquid generates nanoparticles in the liquid with no additional stabilizing agents. We performed small-angle X-ray scattering (SAXS) experiments to investigate the temperature effect on the structure and formation process of Au nanoparticles synthesized by this method. We selected 1-buthyl-3-methylimidazolium tetrafluoroborate as the capture ionic liquid and obtained SAXS intensities of the nanoparticles generated at various temperatures from 20 to 80 °C. The SAXS results revealed that the particle size was relatively uniform for a fixed temperature, and that it strongly depended on the temperature of the capture ionic liquid. Temperature change causes drastic changes in the viscosity of the liquid and the diffusive velocities of the scattered Au particles. Therefore, it is concluded that the collision frequency of the Au particles is one of the important factors in determining their size, as well as the stabilization effect for once generated nanoparticles by the constituent ions of the ionic liquid. The ions work to prevent further particle-aggregation in balance with the collision frequency of particles. Most sputtered Au particles are Au atoms in our experimental condition, and the formation of the nanoparticles occurs in the beginning of dispersion in the ionic liquid. 1. Introduction Room temperature ionic liquids are expected to fit into several fields of application,1-8 because they have many unique physicochemical properties. Among these, the extremely low vapor pressures9,10 of ionic liquids are attractive, because they can be treated under high vacuum conditions and are regarded as fruitful liquid media for vacuum sciences and technologies.7,11-14 Torimoto et al. reported a novel method using a sputter deposition technique to synthesize metal15,16 and alloy17 nanoparticles in ionic liquids. It is an extremely clean and simple method, since metal nanoparticles can be synthesized with only ionic liquids as capture media without any additional stabilizing agents. Using a small-angle X-ray scattering (SAXS) technique, we investigated the structure of Au nanoparticles synthesized by the above-mentioned method and extracted the properties of the ionic liquid that would affect the formation process.18 For a systematic study, we selected imidazolium-based ionic liquids with different alkyl chain lengths fixing the anion of BF4-. The SAXS results revealed that Au nanoparticles with 0.75-3.5 nm diameter are generated under various experimental conditions, and that the particle size is relatively uniform for a fixed condition. It was also demonstrated that the particle size depends on the type of ionic liquid and on the concentration if the ionic liquid is fixed. We summarized that the important factors to determine the size and its distribution are surface tension and viscosity of the capture ionic liquid and Au concentration. However, the temperature of the ionic liquid was not controlled in our previous experiments18 or in those by Torimoto et al.15-17 In our experimental conditions, it took a relatively long sputtering time (30-50 min) to generate sufficient Au nanoparticles in the ionic liquids. During the sputtering and deposition, the temperature of the capture ionic liquid rose and * To whom correspondence should be addressed. E-mail: k.nishikawa@ faculty.chiba-u.jp.
consequently its properties were thought to change. In the present study, therefore, we prepared Au nanoparticles in 1-buthyl-3-methylimidazolium tetrafluoroborate (C4mim+/BF4-) in the constant-temperature condition and changed the temperatures in the range of 20-80 °C. The characterization of the prepared Au nanoparticles with SAXS experiments revealed remarkable relationships between the particle size and temperature of the ionic liquid as the capture medium. 2. Experimental Section We selected 1-buthyl-3-methylimidazolium tetrafluoroborate (C4mim+/BF4-) as the capture medium, which was purchased from Kanto Chemical. It was colorless, and its purity was guaranteed to be better than 98%. For most ionic liquids, adventitious water immensely affects their physical and chemical properties.19 Therefore, the sample was dried for 24 h at 333 K under a vacuum of about 10-3 Pa and kept under an Ar atmosphere before the sputter deposition. The water content was determined by Karl Fischer titration and was less than 20 ppm. Au nanoparticles were prepared following the method of sputter deposition onto ionic liquids.15 For temperature control of the capture ionic liquid during the sputter deposition, a sputter coater (SC-704, SANYU Electron) was remodeled by attaching a circulating device of temperature-regulated water into the base of the deposition. This remodeling made it possible to keep the sample temperature constant in the range of 20-80 °C within the deviation of (1 °C. The deposition was performed at a voltage of 1 kV and current of 20 mA under Ar pressure of 12-13 Pa at a constant temperature. The ionic liquid (2 cm3) was spread on a stainless plate (15.9 cm2) that was horizontally set in the sputter coater. The liquid surface was located at a distance of 25 mm from the gold foil target (99.99% in purity). The typical sputtering time was 50 min. Small-angle scattering intensities were measured with a laboratory scale SAXS apparatus (NANO-Viewer, RIGAKU).
10.1021/jp102763n 2010 American Chemical Society Published on Web 06/03/2010
Controlling the Size of Au Nanoparticles With use of a multilayer mirror, X-rays emitted from a rotating Cu target of an X-ray generator were monochromatized to λ ) 0.154 nm (λ: wavelength of the X-ray) and were focused to a beam with a 0.4 mm diameter at the sample position. The camera length was set to 400 mm. All X-ray paths except the sample position were maintained in a vacuum of about 10 Pa to avoid the X-ray scattering from air. The SAXS intensity was measured with a two-dimensional detector (imaging plate), and the positional resolution of the imaging plate was 50 µm. The observable q-region in the measurements was 0.24-5.5 nm-1, where the scattering parameter q (absolute value of scattering vector q) is defined as q ) (4πsinθ)/λ (2θ: scattering angle of X-rays). The intensity of the incident X-rays was monitored with a microionization chamber (REPIC). To determine the absorption factor of a sample experimentally, the intensities of the transmitted X-rays of the Au-dispersed ionic liquid and the pure liquid were measured every time before the SAXS intensity measurements by an ionization chamber (OKEN), which was set instead of the imaging plate. For SAXS measurements, each sample was packed in a holder with windows of polyetherimide thin films. The inner size of the sample holder was 6 mm in diameter and 0.3 mm in thickness. The thickness was adjusted by a Teflon spacer. Because of the hygroscopic property of ionic liquids, all sampling operations were performed under an Ar atmosphere. The accumulation time for measurement of scattering intensity from a sample was 1800 s. The details of the data corrections were reported in our previous report.18 To further investigate the temperature-rising effect on the size of the nanoparticles after generation in the lower temperature, we designed a temperature-controllable sample cell for the SAXS measurements and performed in situ SAXS measurements during the rise of the sample temperature. 3. Results and Discussion In the previous paper,18 we reported that the size and its distribution of Au nanoparticles were dependent on the concentration of Au. At that time, however, we did not control the temperature of the ionic liquids. In the present investigation, to check the inherent concentration dependence, we prepared various Au-dispersed C4mim+/BF4- solutions, changing the sputtering time, which is linearly related to Au concentration, and keeping the temperatures of the capture ionic liquid constant at 20 and 50 °C, individually. As shown in the Supporting Information, it is suggested that the particle size is not dependent on Au concentration in the present concentration range but on the temperature of the ionic liquid. The apparent concentration dependence of Au particle size reported previously18 was due to the temperature rising during sputtering. Then we focused on studying the temperature effect on the size and its distribution. We prepared the nanoparticles in constant-temperature conditions and performed SAXS experiments to determine the structure of the nanoparticles in the ionic liquid without any preparation for measurements. Eleven types of samples were synthesized in the temperature range of 20-80 °C. The sputtering time was fixed at 50 min for all the samples, whose Au concentrations were about 27 mmol/dm3 as Au atoms. These particles were stably dispersed in the ionic liquid and further cohesion among the nanoparticles was not observed.20 Each ionic liquid with Au nanoparticles was naturally cooled to room temperature, and SAXS experiments were performed without any temperature control. This treatment is reasonable, because the temperature change after formation of nanoparticles, if it is not high, does not affect the particle size as shown later.
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Figure 1. Representative scattering profiles of Au nanoparticles generated in C4mim+/BF4- at different temperatures.
Figure 1 displays some of the scattering profiles of the Au nanoparticles that were generated in C4mim+/BF4- at different temperatures, after the corrections18 for the intensity fluctuation of incident X-rays, background intensities, and absorption effects. As we chose the scattering intensity of pure C4mim+/ BF4- as the background, the scattering profiles shown in Figure 1 are the scattering intensities of Au nanoparticles themselves. The profiles drastically change depending on the temperatures of the capture ionic liquid, which would reflect the size of Au nanoparticles. It is noted that the scattering curve of 80 °C swells a little at about 1 and 2 nm-1. The curve fitting by theoretical scattering curves was performed, assuming that the nanoparticles are spherical, and that the size distribution is expressed by Γ distribution, the details of which are described in our previous paper.18 Although not shown, the theoretical curves could simulate the experimental intensities very well, except the one at 80 °C. The experimental curve at 80 °C with small swellings could be simulated only by an assumption of interference between the nanoparticles.21 This shows that the nanoparticles, at that temperature, start gathering without cohering. The particle size distributions are shown in Figure 2. The curves indicate the distributions referring to the number of particles22 vs the diameter, and they are normalized by the area to highlight the widths of dispersion. The value at the peak-top corresponds to the diameter of the most abundant nanoparticles. In Figure 3, the values of the diameter at the peak-top (dpeak) and full width at half maxima (WFWHM) of the distribution curves are plotted against the temperature of the capture ionic liquid. The error bars in dpeak and WFWHM refer to those estimated from the fitting functions. To experimentally check the reproducibility for the results, we repeated three times the SAXS experiments at several measuring points for individually prepared samples, and confirmed that the experimental errors were contained within the error bars from the fitting functions. The value of WFWHM is hereafter expressed as the dispersion of the particle size. From Figures 2 and 3, we summarize the SAXS results for the Au nanoparticles as follows. First, the sizes of Au nanoparticles are relatively uniform at constant temperatures. Second, the sizes are strongly dependent on the temperature of the capture ionic liquid. The particle sizes increase as the temperature rises. In
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Figure 4. The plot of dpeak vs T/η.
Figure 2. Particle-size distributions of Au nanoparticles generated in C4mim+/BF4- at different temperatures.
Figure 3. Temperature dependences of the diameter of peak-top (dpeak) and the full width at half maxima (WFWHM) of the distribution curve. The sizes of cuboctahedrons with closed-shell structures are shown by the broken lines in the left side.
detail, the values of dpeak are almost constant at about 0.6-0.7 nm at 20-30 °C, then gradually and continuously increase with the rise of temperature, and at last, reach a saturated value of 3.4-3.5 nm. Third, the values of WFWHM increase with temperature rise. In detail, at lower temperatures such as 20 and 30 °C, WFWHM values are almost the same as dpeak values. The values gradually increase with a rise in the particle size, form a plateau at 40-70 °C, and then increase sharply with temperature rise. It is well-known that the Ar+ bombardment of Au causes the physical ejection of Au atoms and/or small clusters.23 As shown in Figures 2 and 3, nanoparticles increase their diameter as a function of temperature of the ionic liquid. These facts indicate that the growth of the nanoparticles occurs by aggregation of the sputtered Au atoms and clusters after touching the temperature-controlled ionic liquid. As shown in the Supporting Information, the final concentration after dispersion of Au into the ionic liquid is not the ultimate factor to determine the particle size. We also checked in the previous study that nanoparticles,
once generated, do not change size and distribution by dilution of the solution.18 As a result, it is interpreted that aggregation of the sputtered Au atoms and small clusters occurs on the surface of the ionic liquid and/or beginnings of the Au dispersion into the ionic liquid. The Au atoms and small clusters generated by the Ar+ bombardment first deposit on the surface of the ionic liquid. Therefore, the surface tension of the ionic liquid seems to influence the formation of Au nanoparticles to some extent. Let us now consider the effect of surface tension of ionic liquids. The surface tension of C4mim+/BF4- linearly decreases from 43.5 to 40.2 mJ/m2 with the temperature change from 20 to 80 °C.24,25 The change seems too small to cause the drastic change in the nanoparticle size. It is considered that the surface tension will determine the staying time of the deposited Au atoms and small clusters, which will remain longer on the surface of the ionic liquid with higher surface tension. If the staying time were the most significant factor to aggregate the deposited Au atoms and small clusters on the surface of the ionic liquid, the nanoparticles generated at higher temperature (lower surface tension) would become smaller. However, the experimental results for the temperature dependence of the size are completely inverse. This shows that the surface tension is not a significant factor in determining the Au particle size. After the deposition on the surface of the ionic liquid, Au atoms and/or small clusters start dispersing into the liquid. At that time, their diffusion velocity related to ionic liquid’s viscosity will influence the aggregation processes. The viscosity of C4mim+/BF4- drastically changes from 155 to 10 cP depending on the temperature change from 20 to 80 °C, as shown in the Supporting Information.26 According to the Stokes-Einstein relationship, the diffusion constant D of a particle in a medium is given by
D ) kBT/6πrη
(1)
where η is the viscosity of the medium, kB is the Boltzmann constant, T is the absolute temperature, and r is the hydrodynamic radius of the particle. The plots of dpeak against T/η are shown in Figure 4. The graph shows a positive relationship between dpeak and T/η. As the atoms and small clusters disperse faster in a medium with lower viscosity, they easily collide with each other to form nanoparticles. In such a way, larger nanoparticles are considered to be generated in the lower
Controlling the Size of Au Nanoparticles viscosity (higher temperature) ionic liquid. Namely, the positive relationship between dpeak and T/η suggests that one of the main mechanisms for the nanoparticle growth is the collision and following aggregation of Au atoms and small clusters in the ionic liquids. The intercept, which corresponds to the averaged size of the first-deposited Au atoms and small clusters, is 0.3-0.4 nm. This value refers only to the diameter of an Au atom. As a result, we can speculate that most of the firstdeposited Au atoms are not small clusters but atoms. During the beginning of the dispersion in the ionic liquid, Au atoms collide and aggregate to form nanoparticles. As mentioned below, the nanopartcles generated once are stabilized by constituent ions of the ionic liquid and prevented from further aggregating in the balance of the collision frequency of Au particles. The cuboctahedron or icosahedron clusters are stable for Au.27,28 The sizes of the cuboctahedron clusters with the closedshell structures are shown in Figure 3. The sizes of the nanoparticles generated at 20-30 °C are comparable with or less than the size of Au13. Therefore, from the viewpoint of size, the present nanoparticles can be expressed as nanoclusters. It is considered that these clusters with the closed-shell structures are more stable and more easily formed compared to the clusters with unclosed-shell structures. For smaller clusters, the abovementioned tendency seems to be more conspicuous. The curve of WFWHM against temperature forms a plateau in the temperature range of 45-70 °C (Figure 3). In this region, it is speculated that the Au atoms are apt to form stable clusters with closedshell structures,29 so that the dispersion of the particle size (WFWHM) is relatively small. Over 75 °C, the values of WFWHM increase abruptly. The distribution curve for 80 °C (Figure 2) only has a longer tail toward the larger diameter. Adding the fact shown in the later section, the cohesion of the nanoparticles seems to start at a temperature higher than 75 °C. The mechanism of growth of Au nanoparticles generated at 75 and 80 °C is supposed to be different from that at lower temperatures: namely, after nanoparticles are formed in the first step, they start gathering with each other in the second step. So far, we have discussed the factors affecting the size of Au nanoparticles, focusing on the aggregation process of sputtered Au atoms. Since ionic liquids themselves work as stabilizers, they should also determine the size and its distribution. The surface of a metal nanoparticle is mostly electron deficient.30 It has been suggested that even for chemically synthesized metal nanoparticles, their stabilization in ionic liquids is due to the electric double layer on the surface of the paricles.30-32 In the present study, the surface-adsorbed anion of the ionic liquids is fixed as BF4-. The second coordination shell seems to be formed by the cation of C4mim+. Provably, the formation of nanoparticles is mainly determined by the balance between the collision frequencies of sputtered Au atoms, stabilization of the formed nanoparticles with the closed-shell structures, and stabilization by coordination of the anions and cations. We are now studying the coordinating-ion effect on the size of Au nanoparticles, changing systematically the type of constituent ions of ionic liquids, the results of which will be reported in the near future. To investigate the temperature effect on the Au nanoparticles that were generated once, we performed a temperature-rising experiment. Namely, the ionic liquid dispersed with the Au nanoparticles prepared at 20 °C in the concentration of 27 mmol/ dm3 as Au atoms was placed in a SAXS sample holder and the temperature was raised in steps of 5 °C. After the temperature
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Figure 5. Heating-treatment effect on dpeak and WFWHM for Au nanoparticles which are once generated at 20 °C. With heating up to 55 °C, the division into two components is observed as shown by solid and open symbols, which suggests the cohesion between smaller nanoparticles generated at 20 °C.
was held for about 10 min at each measuring point, the SAXS intensities were measured. The values of dpeak (red square) and WFWHM (blue square) are plotted in Figure 5. For comparison, the regression curve for dpeak, for which the nanoparticles were generated in constant-temperature conditions (Figure 3), is shown by the pale red curve. As shown in Figure 5, the values of dpeak and WFWHM are almost constant up to 50 °C. However, drastic change occurred when the ionic liquid was heated to 55 °C. The SAXS profile of 55 °C could be simulated only by the assumption of two components. The values of dpeak and corresponding WFWHM for the newly appearing component are shown by open red and blue squares, respectively. At 60 °C, definite cohesion occurred and deposited Au masses adhered on the windows of the SAXS sample holder. These observations show that the Au nanoparticles generated at lower temperatures are stable and keep the same sizes and their distributions, even if the temperature is increased afterward up to 50 °C. At higher temperatures, cohesion of smaller nanoparticles occurs easily, and larger particles are generated. Recently, Torimoto’s group reported the heat-treatment effect on Au nanoparticles which were generated in C4mim+/PF6-.33 Using vis-UV spectrum measurements and TEM observation, they studied time dependence of the growth of Au nanoparticles keeping the temperature of the solution at 100 °C, and concluded that the growth of the particles was caused by coalescence of smaller nanoparticles in the time scale of several ten minutes. Although anions are different such as BF4- and PF6-, the growth of the nanoparticles33 corresponds to our phenomena observed at higher temperature than 55 °C. Due to the difference of stabilization ability of nanoparticles between BF4- and PF6-, the beginning temperature and growing speed for the coalescence seem to be different. Our results for the Au particles generated in C4mim+/PF6- will be reported in the near future. 4. Conclusion By SAXS measurements, we studied the structures of Au nanoparticles prepared in C4mim+/BF4- by the sputter deposition technique. We focused on the temperature effect on the formation of Au nanoparticles. The SAXS intensities were analyzed by profile-fitting, using theoretical scattering curves in the assumption that their particles are spherical.
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The results showed that the particle size and its distribution were greatly affected by the temperature of the capture ionic liquid. The temperature change caused a drastic change in the viscosity of the liquid and the diffusive velocities of the sputtered Au particles. The collision of the sputtered Au particles was the ultimate factor to determine the particle size and its distribution. In addition to the diffusion factor, the stabilities of the formed Au nanoparticles and the coordination and stabilization by the constituent ions of the ionic liquid, especially the anion, influenced the generating process of the nanoparticles. Once prepared and stabilized by ions, Au nanoparticles were stably dispersed in the ionic liquid at lower temperatures. This study indicates that we can control the size and dispersion of Au nanoparticles only by regulating the temperature of the capture ionic liquid and can prepare very small nanoparticles of less than 1-nm diameter. Acknowledgment. This study was supported by a Grandin-Aid for Scientific Research (No. 17073002 for K.N.) on Priority Area “Science on Ionic Liquids” (Area no. 452) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We also thank the New Energy and Industrial Technology Development Organization (NEDO) for partial financial support. Supporting Information Available: Figures showing the concentration dependence of the Au-particle size and the temperature dependence of viscosity of C4mim+/ BF4-. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Greaves, T. L.; Drummond, C. J. Chem. ReV. 2008, 108, 206–237. (2) Plechkova, N. V.; Seddon, K. R. Chem. Soc. ReV. 2008, 37, 123– 150. (3) Wei, D.; Ivaska, A. Anal. Chim. Acta 2008, 607, 126–135. (4) Angell, C. A.; Byrne, N.; Belieres, J. Acc. Chem. Res. 2007, 40, 1228–1236. (5) Han, X.; Armstrong, D. W. Acc. Chem. Res. 2007, 40, 1079–1086. (6) Ohno, H., Ed. Electrochemical Aspects of Ionic Liquids; WileyInterscience: Hoboken, NJ, 2005. (7) Torimoto, T.; Tsuda, T.; Okazaki, K.; Kuwabata, A. AdV. Mater. 2009, 21, 1–26. (8) Mo, Z.; Yu, J.; Dai, S. AdV. Mater. 2010, 22, 261–285.
Hatakeyama et al. (9) Earle, M. J.; Esperanca, J. M. S. S.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831–834. (10) Esperanca, J. M. S. S.; Lopes, J. N. C.; Tariq, M.; Santos, L. M. N. B. F.; Magee, J. W.; Rebelo, L. P. N. J. Chem. Eng. Data 2010, 55, 3–12. (11) Smith, E. F.; Garcia, I. J. V.; Briggs, D.; Licence, P. Chem. Commun. 2005, 5633–5635. (12) Yoshimura, D.; Yokoyama, T.; Nishi, T.; Ishii, H.; Ozawa, R.; Hamaguchi, H.; Seki, K. J. Electron Spectrosc. Relat. Phenom. 2005, 144147, 319–322. (13) Smith, E. F.; Garcia, I. J. V.; Briggs, D.; Licence, P. Chem. Commun. 2005, 5633–5635. (14) Kuwabata, S.; Kongkanand, A.; Oyamatsu, D.; Torimoto, T. Chem. Lett. 2006, 35, 600–601. (15) Torimoto, T.; Okazaki, K.; Kiyama, T.; Hirahara, K.; Tanaka, N.; Kuwabata, S. Appl. Phys. Lett. 2006, 89, 243117. (16) Tsuda, T.; Yoshii, K.; Torimoto, T.; Kuwabata, S. J. Power Sources 2010. Web-published. (17) Okazaki, K.; Kiyama, T.; Hirahara, K.; Tanaka, N.; Kuwabata, S.; Torimoto, T. Chem. Commun. 2008, 691–693. (18) Hatakeyama, Y.; Okamoto, M.; Torimoto, T.; Kuwabata, S.; Nishikawa, K. J. Phys. Chem. C 2009, 113, 3917–3922. (19) Ries, L. A. S.; do Amaral, F. A.; Matos, K.; Martini, E. M. A.; de Souza, M. O.; de Souza, R. F. Polyhedron 2008, 27, 3287–3293. (20) In fact, SAXS measurement detected no change in the particle size and its distribution for the sample preserved at room temperature for two weeks. From the appearance, no precipitate was observed even after two years, when an Au-dispersed ionic liquid was conserved in a refrigerator. (21) Regnaut, C.; Ravey, J. C. J. Chem. Phys. 1989, 91, 1211–1221. (22) Lindner, P.; Zemb, Th., Eds. Neutrons, X-rays, and Light: Scattering Methods Applied to Soft Condensed Matter; Elsevier: Amsterdam, The Netherlands, 2002. (23) Behrisch, R.; Wittmaack, K., Eds. Sputtering by Particle Bombardment III; Springer: Berlin, Germany, 1991. (24) Freire, M. G.; Carvalho, P. J.; Fernandes, A. M.; Marrucho, I. M.; Queimada, A. J.; Coutinho, J. A. P. J. Colloid Interface Sci. 2007, 314, 621–630. (25) Ghatee, M. H.; Zolghadr, A. R. Fluid Phase Equilib. 2008, 263, 168–175. (26) Harris, K. R.; Kanakubo, M.; Woolf, L. A. J. Chem. Eng. Data 2007, 52, 2425–2430. (27) D’Agostino, G.; Pinto, A.; Mobilio, S. Phys. ReV. B 1993, 48, 14447–14453. (28) Uppenbrink, J.; Wales, D. J. J. Chem. Phys. 1992, 96, 8520–8534. (29) Martin, T. P. Phys. Rep. 1996, 273, 199–241. ¨ zkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796–5810. (30) O (31) Scheeren, C. W.; Machado, G.; Teixeira, S. R.; Morais, J.; Domingos, J. B.; Dupont, J. J. Phys. Chem. B 2006, 110, 13011–13020. (32) Migowski, P.; Dupont, J. Chem.sEur. J. 2007, 13, 32–39. (33) Kameyama, T.; Ohno, Y.; Kurimoto, T.; Okazaki, K.; Uematsu, T.; Kuwabata, S.; Torimoto, T. Phys. Chem. Chem. Phys. 2010, 12, 1804–1811.
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