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Notes Comparative Study of Dodecanethiol-Derivatized Silver Nanoparticles Prepared in One-Phase and Two-Phase Systems So Young Kang and Kwan Kim* Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received June 30, 1997. In Final Form: October 16, 1997
Introduction Ever since Brust et al.1 reported the synthesis of thiolderivatized gold nanoparticles in a two-phase liquid/liquid system (and later in a one-phase system2), numerous studies have been performed to characterize their physicochemical properties.3-8 These substances were found to be readily soluble, air stable, and isolable, and the black solids exhibited significant electron-hopping conductivity. Since the substances can be easily characterized by solution-based techniques and are amenable to further functionalization, applications for their use can be found in many areas of science and technology, including optoelectronics, chemical sensors and biosensors, drug delivery, and catalysis.9 In conjunction with the above implications, alkanethiolderivatized silver nanoparticles have also been prepared by numerous investigators, i.e., via an aerosol processing approach,10 by using a two-phase liquid/liquid method,11 and by extraction from a hydrosol12 and inverse micelles.13 Herein we hope to report the preparation of dodecanethiolderivatized silver nanoparticles by a one-phase method in ethanol and a two-phase method in water/toluene, similar to those developed for gold particles,1,2 and to describe their structural similarity and dissimilarity. In fact, it was observed that the average particle sizes of the two synthesized samples were very comparable to each * To whom all correspondence should be addressed. E-mail:
[email protected]. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (3) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchinson, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (4) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (5) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (6) Whetten, R. L.; Khoury, J. T.; Alcarez, M. M.; Murphy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (7) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (8) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51. (9) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (10) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (11) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (12) Sarathy, K. V.; Kulkarni, G. U.; Rao, C. N. R. Chem. Commun. 1997, 537. (13) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950.
other at around 7.7-7.9 nm. The dodecanethiolate species bound to the silver nanoparticles that had been prepared via the one-phase synthetic route appeared, however, to assume more close-packed all-trans zigzag alkyl chains than those bound to the silver nanoparticles prepared via the two-phase synthesis. Experimental Section Nanoscale silver particles were prepared via either a onephase or a two-phase synthetic route. In the one-phase synthesis, 0.1 mL of dodecanethiol was added dropwise to 30 mL of 3 × 10-2 M AgNO3 in ethanol with vigorous stirring. To the resulting solution, 60 mL of a saturated NaBH4 solution in ethanol was added dropwise, and then after stirring for 2 h, the solution was kept in a refrigerator at -18 °C for 4 h. Thereafter, the cooled solution was dried with an aspirator, and the remaining brown, fine powders were washed at first briefly with toluene and then several times with ethanol and acetone to remove any remaining free thiols. In the two-phase synthesis, 30 mL of 3 × 10-2 M aqueous AgNO3 solution and 50 mL of 5 × 10-2 M hexadecanesulfonic acid in toluene were added together into a separatory funnel and shaken vigorously. After the aqueous phase was decanted, the organic phase was collected in a beaker. To the latter phase was added 0.2 mL of neat dodecanethiol dropwise with stirring, and subsequently 25 mL of 0.4 M aqueous NaBH4 solution was added for 1 h. After the mixture was stirred further for 2 h, the organic phase was concentrated to a 10 mL solution in a rotary evaporator. The resulting solution was diluted 20 times with ethanol and then kept in a refrigerator at -18 °C for 4 h. Thereafter, the cooled solution was dried with an aspirator, and the remaining brown, fine powders were washed briefly with toluene and then several times with ethanol and acetone. UV/vis spectra of prepared samples were taken in polar and nonpolar solvents with a Beckman DU-68 UV/vis spectrometer. Infrared spectra were taken with a Bruker IFS 113v spectrometer after pelleting the fine powders with KBr. Transmission electron micrographs (TEMs) were obtained with a JEM-200CX transmission electron microscope at 160 kV after placing a drop of sonicated ethanolic sample solution on carbon-coated copper grids (150 mesh). XPS measurements were made using a PerkinElmer PHI-558 spectrometer. A monochromatic Mg KR X-ray source at 1253.6 eV was used, and the system was calibrated with respect to the Au 4f peak from a standard sample.
Results and Discussion Addition of AgNO3 solution to thiol in a 1:1 molar ratio is known to result in layered compounds.14 In contrast, under conditions of higher molar ratio (>10∼20) and in the presence of a reducing agent such as NaBH4, thiolderivatized fine particles are usually produced.1,2 Hereafter, the fine particles prepared via the one-phase and the two-phase synthetic routes will be called ‘sample 1’ and ‘sample 2’, respectively. In fact, both samples were dissolved readily in nonpolar solvents such as toluene, tetrahydrofuran, chloroform, and cyclohexane (although they were usually insoluble in polar solvents, they became slightly soluble in ethanol and 2-propanol when heated). Free thiols were not detected in the 1H NMR (in CDCl3) and the infrared spectra of those samples. (14) Fijolek, H. G.; Grohal, J. R.; Sample, J. L.; Natan, M. J. Inorg. Chem. 1997, 36, 622.
S0743-7463(97)00696-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/06/1998
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Figure 2. Size distributions determined from the TEM images for the dodecanethiolate-stabilized silver nanoparticles corresponding to (a) ‘sample 1’ and (b) ‘sample 2’.
Figure 1. TEM images of dodecanethiolate-stabilized silver nanoparticles prepared via (a) one-phase (‘sample 1’) and (b) two-phase (‘sample 2’) synthetic routes.
Parts a and b of Figure 1 show the TEM images taken for ‘sample 1’ and ‘sample 2’, respectively. For both samples, most of the particles are not agglomerated, and their sizes are mostly less than 10 nm. This indicates that the samples prepared in this work are in fact nanosized silver cluster compounds. To get more detailed data, the size distribution of each sample was determined by image analysis, using at least 100 counts. Parts a and b of Figure 2 represent the particle size distribution of ‘sample 1’ and ‘sample 2’, respectively. For both samples, the size of silver clusters ranges around 4-13 nm, but the 7-9 nm-sized particles are obviously dominant; the average size was 7.9 nm for ‘sample 1’ and 7.7 nm for ‘sample 2’. The present analysis indicates that the size of silver clusters prepared via the one-phase synthesis should be comparable to that of silver clusters prepared via the two-phase synthesis. Nonetheless, comparing the mean and median values, the particles prepared via the former method seems to be more homogeneously distributed than those prepared via the latter method. In fact, Heath et al.11 reported recently that dodecanethiolderivatized Ag nanocrystals exhibited a quite broad size
Figure 3. UV/vis absorption spectra of dodecanethiolatestabilized silver nanoparticles in ethanol and cyclohexane media. To take spectra in ethanol, samples were dissolved in hot ethanol.
distribution when they were prepared by a two-phase liquid/liquid method. Silver metal is known to have an intense plasmon absorption band in the visible region.15 In fact, as shown in Figure 3, distinct peaks are observed at 400-420 nm in the UV/vis spectra of the two samples prepared in this work (to take spectra in ethanol, samples were dissolved in hot ethanol). They must arise from a surface plasmon absorption of silver clusters. The fact that the peak positions of the two samples are nearly the same suggests that their particle sizes are comparable to each other, as evidenced by the TEM analysis. It is intriguing, however, that the peak maxima of the two samples are affected (15) Henglein, A. J. Phys. Chem. 1993, 97, 5457.
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Figure 4. XPS spectra of (a) carbon 1s, (b) sulfur 2p, and (c) silver 3d regions for ‘sample 1’.
Notes
Figure 5. Infrared transmission spectra of dodecanethiolatestabilized silver nanoparticles, in the 2700∼3100 cm-1 region, corresponding to (a) ‘sample 1’ and (b) ‘sample 2’. (c) Infrared transmission spectrum of neat liquid dodecanethiol.
differently when the medium is changed from a polar to a nonpolar solvent; for ‘sample 1’, the plasmon absorption peak appearing at 408 nm in ethanol is observed at 403 nm in cyclohexane, i.e., blue-shifted by 5 nm, but for ‘sample 2’, the band appearing at 412 nm in ethanol is observed at 419 nm in cyclohexane, i.e., red-shifted by 7 nm. The exact origin of this difference is uncertain, albeit the metal plasmon band is known to be in general very susceptible to the surface/interface effect.15 According to Mie’s theory,16 the plasmon band should be red-shifted, as for ‘sample 2’, upon the solvent exchange from ethanol to cyclohexane, due to the higher refractive index of cyclohexane with respect to that of ethanol. On the other hand, the fact that the intensity of the plasmon peak for ‘sample 1’ is much weaker than that for ‘sample 2’ may indicate that the damping effect on the plasmon band by the alkanethiol overlayers is much greater for ‘sample 1’ than for ‘sample 2’. According to the XPS measurements, ‘sample 1’ was much the same as ‘sample 2’. Figure 4a shows the XPS spectrum of the carbon 1s region for ‘sample 1’. A single band is seen at 284.5 ( 0.1 eV, and this should correspond to the C-C bonded atoms of dodecanethiolate. The S 2p signal was somewhat weak due to the small scattering cross-section of the S atom and the low amount of material present.8 Nonetheless, as shown in Figure 4b, this region had a distinct doublet at 162.0 ( 0.1 and 162.8 ( 0.1 eV which could be assigned, respectively, to the S 2p3/2 and S 2p1/2 peaks of thiolate species. Any peak due to sulfinate (165.5 eV)17 as well as sulfonate (168 eV)17 species was completely absent. The XPS spectra for the C and S regions are thus all consistent with the presence of dodecanethiolate on the silver clusters.
Figure 4c shows a XPS spectrum of the Ag 3d region for ‘sample 1’; two silver bands, i.e., Ag 3d5/2 and Ag 3d3/2, are identified at 368.6 ( 0.1 and 374.6 ( 0.1 eV, respectively. One would expect that a significant proportion of the outer silver atoms should be oxidized from Ag0 to Ag+ upon thiolate formation. Such an oxidation is expected to contribute to the Ag 3d5/2 peak to shift it to higher binding energies by 0.7 eV.8 However, since the sample consists overall of metallic particles, the net charge of Ag should be negligible or at least far below a resolution limit. The peak positions observed here are somewhat different from the handbook values of zero valent silver (367.9 and 373.9 eV),18 but the difference between the two peaks (Ag 3d5/2 and Ag 3d3/2) observed is exactly the same as the difference (6 eV) in the handbook values. Furthermore, our separate experiment showed that the Ag 3d peak positions were much the same in the vacuum-evaporated silver film and the silver nanoparticles. This suggests that the majority of the silver atoms in the clusters prepared in this work must be in the Ag0 state. Similar observations have been made for the alkanethiol-derivatized gold nanoparticles.8 Parts a and b of Figure 5 show the infrared spectra, in the 2700∼3100 cm-1 region, for ‘sample 1’ and ‘sample 2’, respectively, dispersed in KBr matrices. The corresponding spectra in the 1000∼1500 cm-1 region are shown in Figure 6a and b. For comparison, the infrared spectra of neat liquid dodecanethiol are shown in Figure 5c and 6c. The spectral features of ‘sample 1’ and ‘sample 2’ are overall very similar to that of neat dodecanethiol. The detailed spectral features are nonetheless quite different in the spectra of clusters and neat dodecanethiol. First of all, CH2 symmetric and asymmetric stretching bands
(16) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (17) Mekhalif, Z.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Langmuir 1997, 13, 2285.
(18) Wagner, C. D.; Riggs, W. M.; Davis, C. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corporation: Eden Praire, MN, 1979; p 122.
Notes
Figure 6. Infrared transmission spectra of dodecanethiolatestabilized silver nanoparticles, in the 1000∼1500 cm-1 region, corresponding to (a) ‘sample 1’ and (b) ‘sample 2’. (c) Infrared transmission spectrum of neat liquid dodecanethiol.
appeared, respectively, at 2853 and 2924 cm-1 for neat dodecanethiol, but the corresponding bands are observed at 2847 and 2916 cm-1 for ‘sample 1’ and at 2850 and 2916 cm-1 for ‘sample 2'’(see Figure 5). It is well-known that the peak positions of the symmetric and asymmetric CH2 stretching vibrations can be used as a sensitive indicator of the ordering of the alkyl chains.19-21 Lower wavenumbers are characteristic of highly ordered conformations with preferential all-trans characteristics; for all-trans zigzag conformations, the νs(CH2) and νas(CH2) modes are usually observed below 2850 and 2920 cm-1, respectively. Therefore, the higher νs(CH2) and νas(CH2) values observed for neat dodecanethiol have to be attributed to a greater proportion of gauche conformations. On the other hand, the lower frequencies observed for ‘sample 1’ and ‘sample 2’ suggest that the dodecanethiolate species are bound to the cluster surface, assuming all-trans zigzag conformations. The fact that the νs(CH2) frequency in ‘sample 1’ is lower by 3 cm-1 than that of ‘sample 2’ may reflect the fact that the alkyl chains of dodecanethiolates are more closely packed on cluster particles those prepared via the one-phase synthesis rather than the two-phase synthesis. This may be evidenced further by the spectral pattern in the 1150-1400 cm-1 region, where twisting-rocking (Tx) and wagging progression bands (Wx) appear. It is welldocumented that the presence of these progression bands as a series of well-resolved peaks is a strong indicator of crystallinity.22,23 As can be seen in Figure 6a, the (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (20) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (21) Ulman, A. An Introduction to Ultrathin Organic Films; Academic: New York, 1991. (22) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6273. (23) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032.
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progression bands appeared very distinctly for ‘sample 1’. The corresponding bands are comparatively far less distinct in Figure 6b, suggesting that the silver clusters prepared via the one-phase synthesis should be more crystalline than those prepared via the two-phase synthesis. The position of the scissoring band of the methylene groups [δ(CH2)] is known also to give insight into the chain packing;24 for an all-trans methylene chain, the band is known to appear distinctly at 1467 cm-1. In this regard, the sharp band at 1469 cm-1 in Figure 6a and that at 1468 cm-1 in Figure 6b can be attributed to the δ(CH2) mode of the all-trans zigzag chain. Further, the peak position implies that the alkyl chains of dodecanethiolate assume a hexagonal subcell,25 usually giving rise to a band around 1468 cm-1. At this moment, it is informative to know that the infrared spectral features of ‘sample 2’ are nearly the same as those of dodecanethiolate-stabilized gold particles prepared similarly by Hostetler et al.5 via a two-phase synthetic route. The size of the gold nanoparticles (2.362.82 nm) was about three times smaller than that of the silver nanoparticles, but the peak positions of the νs(CH2) and νas(CH2) modes were the same for both samples. Besides, the Tx and Wx progession bands of the gold cluster compounds were not as distinct as those of the silver cluster compounds. This may suggest that the detailed structure of the alkanethiolate residing on the nanosized metal clusters would be more susceptible to the method of synthesis actually employed rather than to the kind of metal. It would also be informative to briefly mention the characteristics of silver nanoparticles reported in the literature. According to Liz-Marzan and Lado-Tourino,26 nanosized silver clusters could be formed in ethanol by reduction of AgNO3 with nonionic surfactants such as Brij 97 (poly-(10)-oxyethylene oleyl ether) and Tween 80 (polyoxyethylene-(20)-sorbitan monooleate). The yield was, however, very low (around 1%). Although the nanoparticles prepared from Brij 97 could be redispersed in a nonpolar solvent such as cyclohexane, the particles prepared with Tween 80 were not dispersed in a nonpolar solvent. According to TEM, the particles prepared with Brij 97 had polyhedral shapes, with sizes in the range 3-10 nm. This size distribution is seen to be comparable to that of present samples. The position of the silver plasmon band peaked around 425 nm, about 6-12 nm above that of our samples. The red-shift of the plasmon band in the Brij 97 stabilized silver nanoparticles seems to be due to the presence of a relatively large concentration of Ag+ ions. On the other hand, Pal et al.27 prepared nanosize silver particles by reduction of aqueous AgNO3 solution with NaBH4 in the presence of surfactants such as CTAB (cetyltrimethylammonium bromide), SDS (sodium dodecyl sulfate), and TX-100 (poly(oxyethylene) isooctylphenyl ether). A very stable, sharp and narrow plasmon absorption band was observed in 10-4 to 5 × 10-3 M CTAB, 10-4 to 8 × 10-3 M SDS, and 10-4 to 5 × 10-4 M TX-100 solutions. The maxima of the plasmon band were located at 410, 392, and 394 nm for CTAB, SDS, and TX-100 solutions, respectively, in optimum concentrations. The value (24) Weers, J. G.; Scheuing, D. R. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; ACS Symposium Series 447; American Chemical Society: Washington, DC, 1991; p 87. (25) Lin, B.; Bohanon, T. M.; Shih, M. C.; Dutta, P. Langmuir 1990, 6, 1665. (26) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (27) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481.
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observed in CTAB solution (410 nm) is in fact comparable to that of our samples. Recalling that adsorption of nucleophile onto the particle surface increases the Fermi level of silver particles due to its donation of electron density to the particle, resulting in a blue-shift of the plasmon band, and vice versa,15 dodecanethiol may be viewed to be less nucleophilic to silver than SDS and TX100 surfactants. According to the TEM images, most of the CTAB-, SDS-, and TX-100-stabilized silver particles were agglomerated, with their average particle sizes around 65, 80, and 55 nm, respectively. These particle sizes are obviously ∼10 times larger than those of ‘sample 1’ and ‘sample 2’ prepared in the present work. As mentioned briefly in the Introduction, Harfenist et al.10 prepared ca. 5.0 nm sized dodecanethiol-derivatized silver nanoparticles via an aerosol processing approach. After depositing a highly concentrated drop of a toluene solution of nanoparticles on an ultrathin amorphous carbon film substrate, they could observe by TEM high molecular orientational order in nanocrystal lattices. On the other hand, Sarathy et al.12 claimed recently that dodecanethiol-derivatized silver nanoparticles (diameter 1-10 nm) prepared by extraction from a hydrosol resembled the faceted Ag particles prepared under vacuum by Harfenist et al.10 Our sample was not spherical at all but appeared to have a somewhat faceted polyhedral
Notes
shape. In this sense, we will test in the near future whether the nanoparticles prepared in this work assemble into a superstructure. In summary, we could prepare alkanethiol-derivatized nanosized silver clusters via a one-phase as well as a twophase synthetic route. Silver nanoparticles prepared via either method are dissolved well in nonpolar solvents (usually they were insoluble in polar solvents, but they could be dissolved slightly in ethanol and 2-propanol when heated). Nonetheless, infrared and TEM analyses dictate that the one-phase synthetic route is better than the twophase synthetic route, at least from the standpoint of preparing nanosized silver particles onto which alkanethiolates are bound with all-trans zigzag alkyl chains in the crystalline state. Acknowledgment. This work was supported in part by the Korea Research Foundation through the Non Directed Research Fund (1995), by the Korea Science and Engineering Foundation through the Specified Basic Research Fund (95-0501-09), by the Center for Molecular Catalysis at Seoul National University (1997), and by the Ministry of Education, Republic of Korea, through the Basic Science Research Fund (1997). LA970696I
