Synthesis and Characterization of N,N-Dimethyldodecylamine

In this paper a convenient route for synthesizing Aucore−Pdshell bimetallic nanoparticles in toluene has been reported as a result of co-reduction o...
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Langmuir 2005, 21, 10405-10408

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Synthesis and Characterization of N,N-Dimethyldodecylamine-Capped Aucore-Pdshell Nanoparticles in Toluene Sudip Nath, Snigdhamayee Praharaj, Sudipa Panigrahi, Sujit Kumar Ghosh, Subrata Kundu, Soumen Basu, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India Received June 26, 2005. In Final Form: August 6, 2005 In this paper a convenient route for synthesizing Aucore-Pdshell bimetallic nanoparticles in toluene has been reported as a result of co-reduction of gold(III) and palladium(II) precursors in toluene. N,NDimethyldodecylamine was used as a capping agent for the core-shell particles, which not only imparts stability to the organosol but also controls morphology of the evolved particles. The particles were characterized using UV-visible, transmission electron microscopy, and X-ray diffraction measurements. All results substantiate the formation of core-shell structure of the synthesized particles.

Introduction In recent years, much attention has been paid to the preparation and properties of bimetallic colloids due to their unique catalytic, electronic, and optical properties different from those of the corresponding monometallic components.1-8 Nanosized bimetallic particles have been the subject of intensive research of the surface chemistry and catalytic properties9 because the physical and chemical properties of such particles can be modulated by altering the composition of parent components. It is postulated that their importance lies in their catalytic activity superior to that of the parent monometallic components.4 Since most of the reactions are carried out in organic phase, it is desirable to fabricate such a catalyst in organic solvent. Moreover, the high surface energy of the nanoparticles10 and consequently the tendency for agglomeration hamper their preparation in high concentration in an aqueous system. The use of organic solvents, instead of an aqueous one, might be a promising solution to this problem, where the particles suffer fewer encounters between themselves due to the presence of giant hydrophobic stabilizing ligands.11 Therefore, synthesis of bimetallic nanoparticles in organic solvent is in the forefront of several research activities and is the basis of numerous fundamental investigations * Corresponding author. E-mail: [email protected]. (1) Ascencio, J. A.; Mejia, Y.; Liu, H. B.; Angeles, C.; Canizal, G. Langmuir 2003, 19, 5882. (2) Toshima, N.; Harada, M.; Yamazaki, Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (3) Aihara, N.; Torigoe, K.; Esumi, K. Langmuir 1998, 14, 4945. (4) Ghosh, S. K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Appl. Catal., A 2004, 268, 61. (5) (a) Huang, C.-C.; Yang, Z.; Chang, H.-T. Langmuir 2004, 20, 6089. (b) Boucher, A.-C.; Alonso-Vante, N.; Dassenoy, F.; Vogel, W. Langmuir 2003, 19, 10885. (6) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (7) Luciano, E. M.; Howard, H.; Nguyen, L.; Giblin, S. R.; Tanner, B. K.; Terry, I.; Hughes, A. K.; Evans, J. S. O. J. Am. Chem. Soc. 2005, 127, 10140. (8) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090. (9) Hendricks, H. A. C. M.; Ponec, V. Surf. Sci. 1987, 192, 234. (10) Nath, S.; Ghosh, S. K.; Panigahi, S.; Thundat, T.; Pal, T. Langmuir 2004, 20, 7880. (11) Praharaj, S.; Ghosh, S. K.; Nath, S.; Kundu, S.; Panigrahi, S.; Basu, S.; Pal, T. J. Phys. Chem. B 2005, 109, 13166.

entailing the catalytic and optical phenomenon of metallic colloids. Among the various bimetallics, core-shell nanoparticles are in the frontier of advanced materials chemistry.12-17 The importance stems largely from the diverse attributes of the core-shell particles as model building blocks toward functional materials. The combination of gold and palladium is one of the most popular examples, which is miscible at any ratio as can be seen from their phase diagram.18 A few methodologies have been devised pertaining to the synthesis of gold-palladium bimetallics, where most were based on their generation in aqueous phase using suitable capping agents.2,19-27 However, the synthesis of gold-palladium bimetallics in organic solvent and their stabilization through a judiciously selected capping agent are still under investigation and are the basis of several fundamental researches. Herein, we have reported the synthesis of goldpalladium bimetallic nanoparticles in the form of coreshell morphology in toluene. The present article also reports the convenient way to synthesize gold and pal(12) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (13) Mandal, M.; Jana, N. R.; Kundu, S.; Ghosh, S. K.; Panigrahi, M.; Pal, T. J. Nanopart. Res. 2004, 6, 53. (14) Caruso, F. Adv. Mater. 2001, 13, 11. (15) Pal, T.; Pradhan, N.; Mandal, M.; Mallick, K. Nano Lett. 2001, 1, 319. (16) Damle, C.; Biswas, K.; Sastry, M. Langmuir 2001, 17, 7156. (17) Schnmeider, J. J. Adv. Mater. 2001, 13, 529. (18) Okamoto, H., Subramanian, R. P., Kacpraz, L., Eds. Binary Alloy Phase Diagrams, 2nd ed.; ASM International: Materials Park, OH, 1992; Vol. 1. (19) Harpeness, R.; Gedanken, A. Langmuir 2004, 20, 3431. (20) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033. (21) Mandal, M.; Kundu, S.; Ghosh, S. K.; Pal, T. J. Photochem. Photobiol., A 2004, 167, 17. (22) Kim, Y.-G.; Garcia-Martinez, J. C.; Crooks, R. M. Langmuir 2005, 21, 5485. (23) Sinfelt, J. H. Bimetallic Catalysis: Discoveries, Concepts and Applications; Wiley: New York, 1983. (24) Wu, M.-L.; Chen, D.-H.; Huang, T.-C. Langmuir 2001, 17, 3877. (25) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (26) Scott, R. W. J.; Wilson, O. M.; Oh, S.; Kenik, E. A.; Crooks, R. M. J. Am. Chem. Soc. 2004, 126, 15583. (27) Liu, H. B.; Pal, U.; Medina, A.; Maldonado, C.; Ascencio, J. A. Phys. Rev. B 2005, 71, 075403.

10.1021/la051710r CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005

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Figure 1. UV-visible spectra of 5 mM aqueous solution of HAuCl4 and K2PdCl4.

ladium organosol along with the Aucore-Pdshell particles, employing a suitable stabilizing agent, N,N-dimethyldodcylamine (N,N-DDA). The particles were characterized with UV-visible, transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. To, the best of our knowledge, this is the first report for the synthesis of Aucore-Pdshell bimetallic nanocluster in organic solvent. Experimental Section Reagents. All the reagents used were of AR grade. Double distilled water was used throughout the reaction. HAuCl4 and K2PdCl4 were purchased from Merck. Phase-transfer reagent tetraoctylammonium bromide (TOAB) was received from Aldrich. N,N-Dimethyldodecylamine was used as received from Merck. Toluene was purchased from Merck and was distilled before use. Instruments. All the absorption spectra were recorded on a Spectrascan UV 2600 spectrophotometer (Chemito, India) taking the solutions in a 1 cm quartz cuvette. The spectra were taken with reference solvent in a reference cell in the double beam spectrophotometer to subtract solvent background. XRD was done in a PW1710 diffractometer, Philips, Holland. The XRD data were analyzed from the JCPDS database. TEM technique was involved to characterize the solution phase bimetallic particles. TEM was done in a Hitachi H-9000 NAR instrument at a magnification of 100K. The sample was prepared by placing a drop of solution on a carbon-coated copper grid. Procedure. The synthesis of organosols was carried out by exploiting a two-phase (water-toluene) extraction procedure. An aqueous solution of HAuCl4 and K2PdCl4 was prepared individually in water and used as a stock solution. Explicitly, aqueous HAuCl4 and K2PdCl4 (1 mL, 5 mM) were transferred to the toluene phase separately by shaking with a toluenic solution of TOAB (10-2 M) in small portions. The toluene phase (25 mL) was subsequently collected, and N,N-dimethyldodecylamine (0.1 mM as final concentration in toluene) was mixed into it. Then the toluenic mixture was mixed with water (25 mL). Finally, the entire mixture was treated with 10 mg of solid NaBH4 and shaken gently in round-bottom flask for 10 min. Afterward the biphasic system was allowed to stand for an hour, and finally the toluene layer was collected to obtain the desired amine-stabilized organosol. The high solubility of NaBH4 in water results in the removal of excess reducing agent from organosol. Then methanol was added to the solution, to increase the polarity of the medium, and consequently, solid nanoparticles were selectively precipitated out. Then the resulting mass was centrifuged, decanted, and washed several times with methanol to remove free capping agents. Finally, the as-synthesized nanoparticles were dried (∼40 °C, 150 mbar, 1 h) and redispersed in toluene to get desired amine-capped bimetallic organosol in toluene.

Results and Discussion Transfer of Au(III) and Pd(II) Ions to the Organic Layer. Aqueous solution of HAuCl4 displays a strong absorption band at 290 nm (Figure 1) corresponding to the metal-to-ligand charge-transfer (MLCT) transition.28 On the other hand, aqueous K2PdCl4 shows two strong

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Figure 2. UV-visible spectra of (a) [TOA]+[AuBr4]- and (b) [TOA]22+[PdBr4]2- ion pairs in toluene.

MLCT bands at 420 and 302 nm (Figure 1) corresponding to the hydrolysis product PdCl3(OH)-. Conversely toluenic dispersion of TOAB shows no characteristic peak in the same spectral range. Now addition followed by mixing of TOAB to the aqueous solution of HAuCl4 and K2PdCl4 results out the formation of corresponding bromo complexes due to their higher stability as compared to the chloro one.29 Generally, noble metal chloro complexes are converted to bromo complexes through the coordination of bromide ions to noble metal ions via a ligand exchange reaction. These complexes formed according to eqs 1 and 2 have relatively large absorption coefficients in the nearultraviolet region.

AuCl4- + 4Br- ) AuBr4- + 4Cl-

(1)

PdCl42- + 4Br- ) PdBr42- + 4Cl-

(2)

Now upon shaking with TOAB, the AuBr4- and PdBr42ions were transferred from aqueous phase to the organic layer due to the in-situ formation of [TOA]+[AuBr4]- and [TOA]22+[PdBr4]2- ion pairs, respectively. A similar type of ion-pair formation for gold30 and palladium31 has been reported earlier. Due to the presence of the long hydrocarbon chain of a tetraoctylammonium group, these ion pairs possess hydrophobic character within them and become soluble in toluene and show strong absorption bands at 394 and 347 nm, which can be ascribed to the metal-to-ligand charge-transfer band in [TOA]+[AuBr4]and [TOA]22+[PdBr4]2- ion pairs, respectively (Figure 2). Synthesis of Gold Organosol. Gold organosol was synthesized by the reduction of Au(III) ion in [TOA]+[AuBr4]- by sodium borohydride in the presence of stabilizing agent N,N-dimethyldodecylamine. Unlike another two-phase extraction procedure,32 the extracted toluenic dispersion is again mixed with water before treating with reducing agent. Upon addition of NaBH4 into the reaction mixture, the yellow color of the solution gradually disappears, and after a certain time, the solution becomes completely colorless, indicating the formation of AuO2- species in the alkaline condition.33 On further shaking, the appearance of the pinkish tinge within the solution indicates the onset of the evolution of the gold particles. The color of the solution gradually changes from light pink to red to wine red upon completion of the reaction. Absorption measurement of this resulting solution shows a new absorption band with a maximum at (28) Roy Mason, W., III.; Gray, H. B. Inorg. Chem. 1968, 7, 55. (29) Feldberg, S.; Klotz, P.; Newman, L. Inorg. Chem. 1972, 11, 2860. (30) Nakao, Y. J. Chem. Soc., Chem. Commun. 1994, 2067. (31) Veisz, B.; Kiraly, Z. Langmuir 2003, 19, 4817. (32) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (33) Vogel, A. I. A Text Book of Quantitative Inorganic Analysis, 4th ed.; Longman: London, 1978.

Aucore-Pdshell Nanoparticles in Toluene

Figure 3. TEM image of N,N-DDA-stabilized gold nanoparticles.

Figure 4. TEM image of palladium organosol stabilized by N,N-DDA.

527 nm, which corresponds to a typical plasmon band of gold nanoparticles. The excess NaBH4 is removed in water, and hence a completely reducing agent free organosol in toluene is obtained. In Figure 3 the TEM image of the gold particles are shown. From the TEM image it is clear that the particles are spherical in shape, with a size of 5 ( 1 nm. The particles are well-separated due to steric stabilization from others, indicating the complete surface passivation of the particles by N,N-DDA. Synthesis of Palladium Organosol. Palladium organosol was prepared in the same procedure as that employed for the case of gold organosol. The yellow-colored toluenic dispersion of [TOA]22+[PdBr4]2- ion pair shows an absorption maximum at 347 nm. Now upon reduction with sodium borohydride the color changes to black with evolution of a featureless absorption band in the region of 400-700 nm. It is known that the formation of Pd nanoparticles by both chemical and photochemical means is indicated by the appearance of faint black color of the solution. After the complete reduction of Pd(II) ions in organic media there was no further change in the nature and the profile of the spectrum. The consistency of the spectral behavior of the solution indicates the completion of the reduction of Pd(II) ions in toluene. The color of the solution persists over a period of time with unaltered spectral profile. Again, from the TEM image (Figure 4) it is clear that the particles are spherical in shape with an average size of 5 ( 1 nm. Synthesis of Aucore-Pdshell Bimetallic Organosol. The synthesis of Aucore-Pdshell bimetallic organosol was achieved by introducing equal molar amounts of [TOA]+[AuBr4]- and [TOA]22+[PdBr4]2- ion pairs in toluene. The color of the toluenic dispersion containing gold(III) and palladium(II) ions was found to turn from pale yellow, which originated from noble metal complexes, into dark brown as the shaking progresses. It has been reported previously that the reduction of gold and palladium ions does not occur simultaneously but rather sequentially, first for gold and then for palladium, which can be judged

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Figure 5. Absorption spectra of (a) gold, (b) palladium, (c) mixture of gold and palladium, and (d) Aucore-Pdshell in toluene.

Figure 6. Relationship between absorption intensity of the surface plasmon absorption of gold at 527 nm and the molar ratio of gold in both bimetallic and mixture of monometallics.

from the change in the UV-vis absorption spectra of the sample solution.34 UV-Visible Spectroscopy. Comparing absorption spectra of bimetallic particles with those of mixtures of monometallic nanoparticles (Figure 5), it was suggested that gold and palladium interact with each other in the case of bimetallic particles. It has been observed that the bimetallic particles show a completely different spectral profile both from the monometallic particles and from their mixture. It is clear from the spectra that gold-palladium bimetallic mixtures have a much weaker intensity than the mixture of gold and palladium particles with the same metal ratios. The plasmon band of gold nanoparticles disappeared when the palladium ratio exceeded 80% in mole/mole ratio. Figure 6 shows a relationship between the absorption intensity of the surface plasmon absorption of gold nanoparticles at 527 nm and the molar ratio of gold. These facts support the observation that the colloidal dispersion obtained by the reduction of gold(III) and palladium(II) ions are not composed of simple mixtures of pure gold and palladium particles. Some interaction seems to exist between gold and palladium in the system of bimetallic nanoparticles. The dampening and even disappearance of the Au surface plasmon resonance is consistent with the previous observations that the presence of a group 10 metal (d8s2) in bimetallic nanoparticles suppresses the surface plasmon energies of group 11 metal (d10s1),35,36 and implies the formation of Au-Pd bimetallic nanoparticles. Gold and palladium can be easily reduced to their corresponding metallic state because of their positive reduction potential. Thermodynamically it is easier to reduce gold ions because the reduction potential of gold is more positive than that of palladium. The result shows that co-reduction of Au(III) and Pd(II) ions in organic (34) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (35) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179. (36) Liu, H.; Mao, G.; Meng, S. J. Mol. Catal. 1992, 74, 275.

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Figure 7. TEM image of Aucore-Pdshell bimetallic nanoparticles. Figure 9. UV-visible spectra of bimetallic gold-palladium nanoparticles (a) without addition of KCN, (b) after addition of 20 µL of KCN, (c) 40 µL of KCN, and (d) 60 µL of KCN.

Figure 8. XRD pattern of solid Aucore-Pdshell bimetallic nanoparticles.

solvent leads to the successive reduction of gold followed by the reduction of palladium ions. The Au(0) is formed at first and acts as a nucleation center for the growth of Pd layer, which covers gold and grows to final size. The stability of the nanoparticles toward agglomeration was authenticated from an unaltered spectral profile of its surface plasmon band. The particles are stable toward agglomeration and retain same particle morphology in varied organic solvents. Even the as-synthesized organosol demonstrates exceptional resistivity toward oxidation and shows an unaltered spectral profile for months together. Transmission Electron Microscopy. The TEM image (Figure 7) of the particles shows that they are composed of core-shell morphology having a diameter of ∼6 nm with a tight size distribution. The image shows that the clusters have a core diameter of ∼4 nm and a shell thickness of ∼2 nm. It is also found that each particle is well-separated from the neighboring nanoparticles, indicating that the present nanoparticles are surfacepassivated well and stabilized sterically by amine molecules. It is obvious that the nanoparticles of larger sizes were believed to grow at the expense of smaller ones via Ostwald ripening process, where small nanoparticles dissolved and grew into larger crystals.37 The crystal growth and dissolution of nanoclusters occurred simultaneously. Therefore the formation of uniform and well-shaped particles can be accounted for as a consequence of balance between stabilization and crystal growth in the solvent. Due to a large surface-to-volume ratio of the nanosized particles, the dissolution is much easier and growth rate is comparatively slower because small particles often grow more rapidly than macrocrystals.38 Consequently, a uniform size distribution is obtained at the end. XRD Analysis. The XRD pattern (Figure 8) of the solid bimetallic cluster shows the evidence for the formation of the core-shell structure instead of an alloy. If the products were an alloy, then the diffraction peaks would lie between gold and palladium. The low-angle shift of the diffraction (37) Shen, Z.; Zhao, Z.; Peng, H.; Nygren, M. Nature 2002, 417, 266. (38) Mullin, J. W. Crystallization, 3rd ed., Butterworth Heinemann: Woburn, MA, 1997; p 172.

peak of gold in the bimetallic cluster as compared to the literature value of monometallic gold substantiates the formation of a core-shell in the cluster. The figure depicts the peaks that fit very well with both the gold and palladium diffraction peaks published in the literature. Unlike the previous cases20 here the diffraction peaks of palladium are detected because of the relatively higher thickness of the palladium shell as compared to the previous report where the shell thickness was 0.8 nm. The information from UV-visible spectroscopy, TEM analysis, and the diffraction pattern supports our suggestion of the core-shell structure for the as-prepared sample. Moreover classical knowledge of chemistry was further employed to substantiate the core-shell structure. Cyanide Dissolution Reaction. The formation of core-shell structure in the co-reduction method was authenticated by taking cyanide dissolution reaction into consideration. Upon dropwise addition of toluenic dispersion of potassium cyanide, a gradual fading of the mixture was observed with the appearance of a new peak due to a gold plasmon band (Figure 9). Finally a large treatment of cyanide results in complete decolorization of the mixture due to formation of Au(CN)2- complex. Simultaneous decrease of both peaks due to gold and palladium took place when cyanide was added to the mixture of individual monometallics. This fact corroborates that the bimetallic formed due to co-reduction is not a simple mixture of them but having a core-shell structure. Conclusion This paper describes a convenient and simple approach to design Aucore-Pdshell bimetallic nanoparticles in organic solvent with tight size distribution. Such a core-shell processing strategy promises the production of particles with controlled shape, composition, and surface properties. Transmission electron microscopic analysis indicates that the resulting bimetallic particles had a mean diameter of ∼6 nm with a shell thickness of ∼2 nm. The procedure is simple, is reproducible, and could be a general method for direct synthesis of varied bimetallic organosol systems together with their monometallics also. Such a system might enlight the avenues for heterogeneous catalysts in organic reactions. Finally, the synthesis of pure bimetallic clusters devoid of free capping as well as reducing agent might be a useful one for spectroscopic and optical applications in future. Acknowledgment. The authors are thankful to DST, CSIR, and UGC, New Delhi, and IIT-Kharagpur for financial assistance. LA051710R