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Study of palladium nanoparticles prepared from water-in-oil microemulsion
Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 119–124
Study of palladium nanoparticles prepared from water-in-oil microemulsion Meng Chen a,b,∗ , Yong-gang Feng a , Li-ying Wang a , Lu Zhang a , Jun-Yan Zhang b a
b
Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, PR China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR China Received 23 September 2005; received in revised form 12 February 2006; accepted 16 February 2006 Available online 6 March 2006
Abstract Pd nanoparticles capped by thiol derivatives were prepared in AOT reverse micelles. Studies of UV–vis spectra have indicated that nitrogen purging plays an important role in the formation of Pd nanoparticles while without nitrogen purging results in the formation of Pd(II)–S complexes. Experimental results have also shown that thiolated poly(ethylene glycol) and tiopronin, which could be used to form water-soluble gold nanoparticles, did not favor the formation of isolated water-soluble Pd nanoparticles. However, tiopronin-capped Pd nanoparticles are found to be able to self-organize into nearly spherical aggregates. © 2006 Elsevier B.V. All rights reserved. Keywords: Palladium; Reverse micelle; Self-organization; UV–vis spectrum
1. Introduction Palladium nanoparticles have been extensively used as one of the primary catalysts for many organic reactions, such as Heck, Suzuki, and hydrogenation of alkenes and allylamines [1–5]. Therefore, the stabilization of palladium nanoparticles is currently receiving much attention and has been studied intensely. In addition, stabilization and size control of Pd nanoparticles by ligand coatings can enable facile manipulation, analysis and facilitate their application to catalysis and waste-water treatment. The usual route for the preparation of Pd nanoparticles capped by (typically) functionalized long-chain organic molecules is the chemical reduction of the palladium salt in the presence of capping agents. Uniform Pd nanoparticles have been first synthesized in the presence of sodium polyacrylate with sodium formate as reducing agents [6]. Colloidally stable palladium nanoparticles of various sizes have been prepared in a variety of micellar systems [7–10], through the thermal decomposition of a Pd–surfactant complex [11], within the presence of dendrimers or block copolymer or linear polymer [1,3,12,13]. Very recently, Kim et al. [11] reported that thermal decomposition of a Pd–surfactant complex could produce monodisperse Pd
∗
Corresponding author. Tel.: +86 21 55664181; fax: +86 21 55664192. E-mail address: [email protected] (M. Chen).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.02.024
nanoparticles with particle size less than 10 nm, and their size could be controlled by varying the concentration of stabilizing surfactant. A thorough understanding of possible chemical reactions involved in the formation of Pd nanoparticles has been given by Xiong et al. [14,15] who prepared Pd cubooctahedral nanoparticles and Pd nanocubs using a modified polyol process. As of other metal nanoparticles, various capping agents play an important role in the stabilization of Pd nanoparticles and made it possible to make large amounts of Pd nanoparticles of reproducible quality under mild conditions. A variety of protective agents have been successfully employed, including n-alkanethiols [7,8,16], phosphines [11,17], polymer such as poly(N-vinyl-2-pyrrolidone) [18] and dendrimers [1,3,12], but a few examples of Pd nanoparticles coated with cationic isocyanides [19], thiolated -cyclodextrin [4], N,N,N-trimethyl(8mercaptooctyl)ammonium chloride [20], mercaptoammonium [21] have also been reported. Reverse micelle represents a powerful technique to prepare nanoparticles and control their sizes. There exist a number of studies about AOT reversed micelles to metal nanoparticles [22–26]. Recently, we have obtained nearly monodisperse Pd nanoparticles in AOT reverse micelles with the reduction of sodium hypophosphite [9]. In the study, we first employ inverse micelle of AOT surfactants in isooctane to prepare Pd nanoparticles, and then add capping agents to the suspension during the purified process. To extend this work and to better understand the effect of synthetic conditions on the formation of Pd
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nanoparticles, we present a systematic study of palladium synthesis by analyzing the UV–vis spectra of the solution at different time after reaction under different conditions. We also show the self-organization of Pd nanoparticles coated with thiolated poly(ethylene glycol). 2. Experimental 2.1. Materials Potassium tetrachloropalladate(II) (K2 PdCl4 ), sodium borohydride (NaBH4 ), hydrazine hydroxide (N2 H4 ·H2 O), sodium hypophosphite (NaH2 PO2 ), sodium bis(2-ethylhexyl)sulfosuccinate (AOT), spectrometer-grade isooctane supplied by Fisher Co. were used without treatment. A-methoxy-mercapto-poly(ethylene glycol) (MWavg = 5000, PEG-SH, Fluka) and N-(2-mercaptopropionyl)glycine (tiopronin, 99%, Ardrich) were used as received. The water used through out the work was purified by Milli-Q ultrapure water purification system (>18.2 M). 2.2. Preparation Pd nanoparticles are synthesized using the technique described earlier [9]. Briefly, inverse micelles containing K2 PdCl4 and a suitable reducing agent in an AOT solution of isooctane are mixed by injection. The resulting reverse micelles are crashed by ethanol and then centrifugation-dispersion is used to wash off the surfactants and the remaining residues. Finally, a capping agent is injected into the suspension and further stirred for several minutes. With ethanol crashing and centrifugation the final pellet is dried with agentle nitrogen and then dissolved in toluene and pure water. Fig. 1 shows a simple process of preparation of Pd nanoclusters in AOT reverse micelle and color change of the reactant solutions and the resulting solution.
2.3. Characterization Ultraviolet visual absorption spectra are carried out at room temperature on a Varian cary 5000 UV–vis–NIR spectrophotometer. Transmission electron microscopy (TEM) is performed on JEOL JEM 2010 operating at 200 kV. Particle size analysis is performed by manually digitizing the TEM image with Image Tool, from which the average size and standard deviation of particles are generated. 3. Results and discussion 3.1. UV–vis spectra of thiol-capped Pd nanoparticles In our experiments, formation of Pd nanoparticles requires strict free-oxygen condition. Before addition of reducing agents, both micellar solutions are purged with high pure nitrogen for at least 20 min and then mixed by injection. Reduction under air results in light yellow solution and stable micellar solution. But after purification with ethanol-washing and centrifugation, and dissolution of the resulting pellet in toluene, a few precipitates could be found in toluene 3 days or so later. With N2 purging, the reduced micellar solution is black and the obtained solution of Pd nanoparticles in toluene is very stable for several months. In addition, investigation of UV–vis spectra further reveals the difference of the products synthesized with or without purging. Fig. 2 shows the UV–vis spectra of the toluene solution 3 days after reduction by sodium borohydride without and with nitrogen purging. Obviously, there is no surface plasmon resonance band found in the spectrum of the solution prepared under N2 purging, which consists with most reports about the solution of thiolated Pd nanoparticles in toluene [16,27] or in water [21], and theoretical calculation [28], but not with the observation of 302 nm surface plasmon band for octadecanethiolate-protected palladium nanoparticles [29]. This spectrum also indicates that
Fig. 1. Schematic illustration of the route to soluble Pd nanoparticles.
M. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 119–124
Fig. 2. UV–vis spectra of the toluene solutions 3 days later after prepared with and without N2 purging. Reference: toluene.
the reduction by NaBH4 was completed and the size of the obtained Pd nanoparticles is below 10 nm [14]. In contrast, for the solution prepared without oxygenpurging, two absorption peaks can be observed at 340 and 380 nm, which could be ascribed to the formation of complexes of Pd(II) and dodecanethiol molecules. Two peaks around 340 and 380 nm have been observed before for S–Pd(II) complexes that correspond to metal-to-ligand and ligand-to-metal charge transfer band, respectively [16,30,31]. Interestingly, no obvious absorption peak can be observed for the solutions immediately after the preparation either with N2 purging or without N2 purging. But after more than 1-haging, surface plasmon bands around 340 and 380 nm appeared for the solution generated under air condition (Fig. 3). In addition, the intensities of absorption peaks increase as the time extends. On the other hand, no absorption peak in the range
Fig. 3. UV–vis spectra of the toluene solutions against the time after the preparation in air. Reference: toluene.
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Fig. 4. UV–vis spectra of the toluene solutions including different starting materials. Reference: toluene.
300–800 has shown for the solution of capped-Pd nanoparticles in toluene even 3 days after the preparation with N2 purging. To further understand where the absorption peaks around 340 and 380 nm origin from, several control samples were prepared and characterized by UV–vis spectra. No absorption peak in the range of 300–800 nm is observed for the solution of 0.1 M AOT in isooctane, 1 mM K2 PdCl4 in AOT solution, and even those above solutions aged for several days. For the solution of 1 mM K2 PdCl4 and 1 mM dodecanethiol in AOT solution, and the solution after reduction in air followed by addition of 1 mM dedecanethiol, no peak can be found immediately. However, two peaks appeared for both solutions just letting them sitting for more than 1 or 3 days (Fig. 4). These phenomena might further prove formation of complexes of Pd(II)–S in the above solution. In the freshly-made solution of K2 PdCl4 and dodecanethiol, no absorption peak is seen because those Pd(II) ions exist in the complexes of PdCl4 − . As time proceeds, Pd(II)–S complexes were formed by replacing Cl− with –SC12 H25 , which results in the appearance of two absorption peaks around 340 and 380 nm in their UV–vis spectra. For the case of the reduction carried out under air, the freshlyformed Pd nanoparticles have very high reactivity because of the large surface area and the higher density of defects on their surfaces, and are readily oxidized by oxygen in the air. In this stage, the absorption profile of the solution does not show any welldefined surface plasmon band. However, after several hours, oxidized compound of Pd(II) have slowly formed complexes with thiol molecules, leading to the appearance of two absorption peaks. There were also several reports showed the higher reactivity of Pd nanoparticles and their oxidative etching induced by oxygen or FeCl3 , by which Pd@PdO core-shell nanostructures could be formed [14,15]. The complexes of Pd(II) with thiols may have a strong bond between S and Pd atoms, since only yellow micellar solution
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was obtained by adding strong reducing agents such as sodium borohydride after addition of thiol. In addition, if the micellar solution was crashed with ethanol and then centrifuged, nothing could be obtained. A similar observation has also been reported by Murray and coworkers [16]. Although different surfactant system and method from those in this study were used in the literature, two absorbance peaks can still be seen at ca. 350 and 400 nm when the thiol:Pd synthesis ratio of more than two was employed, from which the conclusion was inferred that some form of Pd(II) thiolate complex or oligomer exists. 3.2. Pd nanoparticles reduced by NaBH4 , N2 H4 to NaH2 PO2 A number of works about the application of reverse micelle, especially AOT system, in nano-synthesis have already been done by Pileni and coworkers [22–24]. For example, size controlled nanocrystallites of CdS, Cu and others have achieved in water-in-oil microemulsion system by altering water content and reducing agents [22]. In our experiments, the size of Pd nanocrystals is also controlled but just in the range of less than 5 nm by changing reaction conditions such as water content, reactant kind and concentration. Because if the molar ratio of water to AOT is increased to above 10, the resulting micellar solution was not stable and clear enough for further reaction steps. The samples held at the overall K2 PdCl4 concentration of 1 and 4 mM, are shown in Fig. 5. Image analysis yields an average size of 2.58 nm ± 23% for low concentration and 3.28 nm ± 14% for high concentration. Both samples show large polydispersity, which prevents the formation of particle ordered array [8,26]. The reducing agents were also varied from NaBH4 , N2 H4 to NaH2 PO2 . For hydrazine as a reducing agent, the particles are similar to that produced by NaBH4 . But for the Pd nanoparticles generated by sodium hypophosphite, better TEM images with good size distribution are seen. With increasing the K2 PdCl4 concentration varied from 1 to 4 mM, larger size (from 2.75 to 3.49 nm) and narrower size distribution (from 15.6 to 9.3% standard deviation) are obtained. The detailed results about the reason of variation of size and size distribution and their selforganization by changing supports for TEM carbon grids have been shown in our previous work [9].
Fig. 5. TEM images and size distribution of palladium nanoclusters prepared in AOT-isooctane reverse micelles by NaBH4 with K2 PdCl4 concentration of 1 mM (A, B) and 4 mM (C, D). Other conditions: [AOT] = 0.1 M, [NaBH4 ] = 0.01 M, w0 = 7).
of hexanethiol, decanethiol, mercaptoacetic acid and mercaptopropanoic acid as coating agents in the preparation of Pd nanoparticles and their solubility control have already been described [9]. In this study, two specific capping agents for preparing water-soluble gold nanoparticles were applied in the present study, expecting the formation of stable water-soluble Pd nanoparticles. 3.3.1. Tiopronin Murray and coworkers [32] have used tiopronin to prepare water-soluble gold nanoparticles in the mixture of acetic acid and methanol, in which tetracholorauric acid and tiopronin are readily dissolved and reduced by sodium borohydride. Tiopronin has four polar groups, such as –SH and –CO2 H groups, which can provide further reactivity with surface metallic atoms.
3.3. Pd nanoparticles coated by thiol derivatitives Most reports about the preparation of soluble metal nanoparticles focus on the solubility of particles just in either non-polar solvent or waters. In addition, the methods employed in most case are just for one type of solvents, either toluene or water. In this report, we employ inverse micelles of AOT surfactant in isooctane to prepare Pd particles with size less than 5 nm. Through purification of redispersion–centrifugation process of the resulting nanoparticles followed by simple addition of different kinds of capping agents, Pd nanoparticles soluble in either toluene or water could be controlled. Details about the use
According to our experience of using AOT method for synthesizing water-soluble Pd nanoparticles, Tiopronin was added to the alcoholic suspension during the purification process and the suspension was stirred overnight. We failed to get water-soluble Pd particles even after adjusting pH value with HCl solution.
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Fig. 6. TEM images of palladium nanoparticles capped with thiolated polymer. TEM sample was prepared by depositing a drop of the Pd aqueous solution on a TEM grid supported by silicon wafer.
At the same time, the resulting Pd pellet after purification was dispersed in the mixture of acetic acid and methanol, in which tiopronin was added. After stirring the suspension, the desired Pd particles soluble in water haven’t been achieved. Murray method has also been simply repeated except for replacing gold salt with palladium salt. K2 PdCl4 was dissolved in the same mixture after stirring for more than 24 h, and then reduced by sodium borohydride. After purification with absolute ethanol and centrifugation, a deep black aqueous solution was obtained. However, TEM investigation hasn’t found anything on the TEM grid, which might be attributed to the formation of Pd(II) complex with tiopronin. 3.3.2. Thiolated poly(ethylene glycol) (PEG-SH) Thiolated polymer, ␣-methoxy--mercapto-poly(ethylene glycol) (PEG-SH) was first used as a capping agent for watersoluble gold particles in a similar Brust method by Murray and coworkers [33]. The procedure for producing the water-soluble Pd nanoparticles is similar to our toluene soluble particles except PEG-SH was used instead of dodecane thiol. Then the obtained Pd nanoparticles are very soluble and stable in water for more than 2 months. Interestingly, TEM inspection reveals that Pd nanoparticles of less than 4 nm self-organized into nearly spherical aggregates with size in the range of 50–400 nm (shown in Fig. 6A). Fig. 6C shows that the dark parts consists of very small Pd nanoparticles, while the gray part is partially crystallized PEG phase because at room temperature PEG-SH and PEG–S–Pd phase can easily be crystalline [33]. There has been only one report of self-organization into spherical aggregates of palladium nanoparticles with size of 4.0 nm [34]. 4. Conclusion In summary, a systematic study of palladium synthesis by UV–vis spectra has been reported in this paper. Size and size distribution of Pd nanoparticles prepared in reverse micelles could be controlled in a narrow range by changing the concentration of the starting materials and the kinds of reducing agents. The effects of varying the capping agents and the self-organization of Pd nanoparticles coated with thiolated poly(ethylene glycol) have also been presented.
Acknowledgment This research work was financially supported from the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry of Education, China. References [1] K.R. Gopidas, J.K. Whitesell, M.A. Fox, Nano Lett. 3 (2003) 1757. [2] J. Liu, J. Alvarez, W. Ong, E. Roman, A.E. Kaifer, Langmuir 17 (2001) 6762. [3] Y. Li, M.A. El-Sayed, J. Phys. Chem. B 105 (2001) 8938. [4] J. Alvarez, J. Liu, E. Roman, A.E. Kaifer, Chem. Commun. (2000) 1151. [5] N.A. Dhas, A. Gedanken, J. Mater. Chem. 8 (1998) 445. [6] J. Turkevich, G. Kim, Science 169 (1970) 873. [7] I. Quiros, M. Yamada, K. Kubo, J. Mizutani, M. Kurihara, H. Nishihara, Langmuir 18 (2002) 1413. [8] J.E. Martin, J.P. Wilcoxon, J. Odinek, P. Provencio, J. Phys. Chem. B 106 (2002) 971. [9] M. Chen, J. Falkner, W.H. Guo, J.Y. Zhang, C. Sayes, V.L. Colvin, J. Colloid Interface Sci. 287 (2005) 146. [10] C.C. Wang, D.H. Chen, T.C. Huang, Colloid Surf. A: Physicochem. Eng. Asp. 189 (2001) 145. [11] S.W. Kim, J. Park, Y. Jang, Y. Chung, S. Hwang, T. Hyeon, Y.W. Kim, Nano Lett. 3 (2003) 1289. [12] R.W.J. Scott, H.C. Ye, R.R. Henriquez, R.M. Crooks, Chem. Mater. 15 (2003) 3873. [13] K. Esumi, T. Itakura, K. Torigoe, Colloid Surf. A: Physicochem. Eng. Asp. 82 (1994) 111. [14] Y.J. Xiong, J.Y. Chen, B. Wiley, Y.A. Xia, Y.D. Yin, Z.Y. Li, Nano Lett. 5 (2005) 1237. [15] Y.J. Xiong, J.Y. Chen, B. Wiley, Y.N. Xia, J. Am. Chem. Soc. 127 (2005) 7332. [16] F.P. Zamborini, S.M. Gross, R.W. Murray, Langmuir 17 (2001) 481. [17] S.U. Son, Y. Jang, K.Y. Yoon, E. Kang, T. Hyeon, Nano Lett. 4 (2004) 1147. [18] T. Teranishi, M. Miyake, Chem. Mater. 10 (1998) 594. [19] T. Yonezawa, K. Imamura, N. Kimizuka, Langmuir 17 (2001) 4701. [20] S.W. Chen, K. Huang, J. Cluster Sci. 11 (2000) 405. [21] D.E. Cliffel, F.P. Zamborini, S.M. Gross, R.W. Murray, Langmuir 16 (2000) 9699. [22] M.P. Pileni, J. Phys. Chem. 97 (1993) 6961. [23] A. Taleb, C. Petit, M.P. Pileni, Chem. Mater. 9 (1997) 950. [24] M.P. Pileni, J. Phys. Chem. B 105 (2001) 3358. [25] D.H. Chen, C.C. Wang, T.C. Huang, J. Colloid Interface Sci. 210 (1999) 123. [26] L. Motte, F. Billoudet, E. Lacaze, J. Douin, M.P. Pileni, J. Phys. Chem. B 101 (1997) 138.
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[27] M. Michaelis, A. Henglein, P. Mulvaney, J. Phys. Chem. 98 (1994) 6212. [28] J.A. Creighton, D.G. Eadon, J. Chem. Soc., Faraday Trans. 87 (1991) 3881. [29] C.K. Yee, R. Jordan, A. Ulman, H. White, A. King, M. Rafailovich, J. Sokolov, Langmuir 15 (1999) 3486. [30] S.P. Perlepes, P. Jacobs, H.O. Desseyn, J.M. Tsangaris, Spectrochim. Acta Part A, Mol. Biomol. Spectrsc. 43 (1987) 1007.
[31] C.A. Grapperhaus, M.J. Maguire, T. Tuntulani, M.Y. Darensbourg, Inorg. Chem. 36 (1997) 1860. [32] A.C. Templeton, S.W. Chen, S.M. Gross, R.W. Murray, Langmuir 15 (1999) 66. [33] W.P. Wuelfing, S.M. Gross, D.T. Miles, R.W. Murray, J. Am. Chem. Soc. 120 (1998) 12696. [34] K. Naka, H. Itoh, Y. Chujo, Nano Lett. 2 (2002) 1183.
Study of palladium nanoparticles prepared from water-in-oil microemulsion Meng Chen a,b,∗ , Yong-gang Feng a , Li-ying Wang a , Lu Zhang a , Jun-Yan Zhang b a
b
Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, PR China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR China Received 23 September 2005; received in revised form 12 February 2006; accepted 16 February 2006 Available online 6 March 2006
Abstract Pd nanoparticles capped by thiol derivatives were prepared in AOT reverse micelles. Studies of UV–vis spectra have indicated that nitrogen purging plays an important role in the formation of Pd nanoparticles while without nitrogen purging results in the formation of Pd(II)–S complexes. Experimental results have also shown that thiolated poly(ethylene glycol) and tiopronin, which could be used to form water-soluble gold nanoparticles, did not favor the formation of isolated water-soluble Pd nanoparticles. However, tiopronin-capped Pd nanoparticles are found to be able to self-organize into nearly spherical aggregates. © 2006 Elsevier B.V. All rights reserved. Keywords: Palladium; Reverse micelle; Self-organization; UV–vis spectrum
1. Introduction Palladium nanoparticles have been extensively used as one of the primary catalysts for many organic reactions, such as Heck, Suzuki, and hydrogenation of alkenes and allylamines [1–5]. Therefore, the stabilization of palladium nanoparticles is currently receiving much attention and has been studied intensely. In addition, stabilization and size control of Pd nanoparticles by ligand coatings can enable facile manipulation, analysis and facilitate their application to catalysis and waste-water treatment. The usual route for the preparation of Pd nanoparticles capped by (typically) functionalized long-chain organic molecules is the chemical reduction of the palladium salt in the presence of capping agents. Uniform Pd nanoparticles have been first synthesized in the presence of sodium polyacrylate with sodium formate as reducing agents [6]. Colloidally stable palladium nanoparticles of various sizes have been prepared in a variety of micellar systems [7–10], through the thermal decomposition of a Pd–surfactant complex [11], within the presence of dendrimers or block copolymer or linear polymer [1,3,12,13]. Very recently, Kim et al. [11] reported that thermal decomposition of a Pd–surfactant complex could produce monodisperse Pd
∗
Corresponding author. Tel.: +86 21 55664181; fax: +86 21 55664192. E-mail address: [email protected] (M. Chen).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.02.024
nanoparticles with particle size less than 10 nm, and their size could be controlled by varying the concentration of stabilizing surfactant. A thorough understanding of possible chemical reactions involved in the formation of Pd nanoparticles has been given by Xiong et al. [14,15] who prepared Pd cubooctahedral nanoparticles and Pd nanocubs using a modified polyol process. As of other metal nanoparticles, various capping agents play an important role in the stabilization of Pd nanoparticles and made it possible to make large amounts of Pd nanoparticles of reproducible quality under mild conditions. A variety of protective agents have been successfully employed, including n-alkanethiols [7,8,16], phosphines [11,17], polymer such as poly(N-vinyl-2-pyrrolidone) [18] and dendrimers [1,3,12], but a few examples of Pd nanoparticles coated with cationic isocyanides [19], thiolated -cyclodextrin [4], N,N,N-trimethyl(8mercaptooctyl)ammonium chloride [20], mercaptoammonium [21] have also been reported. Reverse micelle represents a powerful technique to prepare nanoparticles and control their sizes. There exist a number of studies about AOT reversed micelles to metal nanoparticles [22–26]. Recently, we have obtained nearly monodisperse Pd nanoparticles in AOT reverse micelles with the reduction of sodium hypophosphite [9]. In the study, we first employ inverse micelle of AOT surfactants in isooctane to prepare Pd nanoparticles, and then add capping agents to the suspension during the purified process. To extend this work and to better understand the effect of synthetic conditions on the formation of Pd
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nanoparticles, we present a systematic study of palladium synthesis by analyzing the UV–vis spectra of the solution at different time after reaction under different conditions. We also show the self-organization of Pd nanoparticles coated with thiolated poly(ethylene glycol). 2. Experimental 2.1. Materials Potassium tetrachloropalladate(II) (K2 PdCl4 ), sodium borohydride (NaBH4 ), hydrazine hydroxide (N2 H4 ·H2 O), sodium hypophosphite (NaH2 PO2 ), sodium bis(2-ethylhexyl)sulfosuccinate (AOT), spectrometer-grade isooctane supplied by Fisher Co. were used without treatment. A-methoxy-mercapto-poly(ethylene glycol) (MWavg = 5000, PEG-SH, Fluka) and N-(2-mercaptopropionyl)glycine (tiopronin, 99%, Ardrich) were used as received. The water used through out the work was purified by Milli-Q ultrapure water purification system (>18.2 M). 2.2. Preparation Pd nanoparticles are synthesized using the technique described earlier [9]. Briefly, inverse micelles containing K2 PdCl4 and a suitable reducing agent in an AOT solution of isooctane are mixed by injection. The resulting reverse micelles are crashed by ethanol and then centrifugation-dispersion is used to wash off the surfactants and the remaining residues. Finally, a capping agent is injected into the suspension and further stirred for several minutes. With ethanol crashing and centrifugation the final pellet is dried with agentle nitrogen and then dissolved in toluene and pure water. Fig. 1 shows a simple process of preparation of Pd nanoclusters in AOT reverse micelle and color change of the reactant solutions and the resulting solution.
2.3. Characterization Ultraviolet visual absorption spectra are carried out at room temperature on a Varian cary 5000 UV–vis–NIR spectrophotometer. Transmission electron microscopy (TEM) is performed on JEOL JEM 2010 operating at 200 kV. Particle size analysis is performed by manually digitizing the TEM image with Image Tool, from which the average size and standard deviation of particles are generated. 3. Results and discussion 3.1. UV–vis spectra of thiol-capped Pd nanoparticles In our experiments, formation of Pd nanoparticles requires strict free-oxygen condition. Before addition of reducing agents, both micellar solutions are purged with high pure nitrogen for at least 20 min and then mixed by injection. Reduction under air results in light yellow solution and stable micellar solution. But after purification with ethanol-washing and centrifugation, and dissolution of the resulting pellet in toluene, a few precipitates could be found in toluene 3 days or so later. With N2 purging, the reduced micellar solution is black and the obtained solution of Pd nanoparticles in toluene is very stable for several months. In addition, investigation of UV–vis spectra further reveals the difference of the products synthesized with or without purging. Fig. 2 shows the UV–vis spectra of the toluene solution 3 days after reduction by sodium borohydride without and with nitrogen purging. Obviously, there is no surface plasmon resonance band found in the spectrum of the solution prepared under N2 purging, which consists with most reports about the solution of thiolated Pd nanoparticles in toluene [16,27] or in water [21], and theoretical calculation [28], but not with the observation of 302 nm surface plasmon band for octadecanethiolate-protected palladium nanoparticles [29]. This spectrum also indicates that
Fig. 1. Schematic illustration of the route to soluble Pd nanoparticles.
M. Chen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 281 (2006) 119–124
Fig. 2. UV–vis spectra of the toluene solutions 3 days later after prepared with and without N2 purging. Reference: toluene.
the reduction by NaBH4 was completed and the size of the obtained Pd nanoparticles is below 10 nm [14]. In contrast, for the solution prepared without oxygenpurging, two absorption peaks can be observed at 340 and 380 nm, which could be ascribed to the formation of complexes of Pd(II) and dodecanethiol molecules. Two peaks around 340 and 380 nm have been observed before for S–Pd(II) complexes that correspond to metal-to-ligand and ligand-to-metal charge transfer band, respectively [16,30,31]. Interestingly, no obvious absorption peak can be observed for the solutions immediately after the preparation either with N2 purging or without N2 purging. But after more than 1-haging, surface plasmon bands around 340 and 380 nm appeared for the solution generated under air condition (Fig. 3). In addition, the intensities of absorption peaks increase as the time extends. On the other hand, no absorption peak in the range
Fig. 3. UV–vis spectra of the toluene solutions against the time after the preparation in air. Reference: toluene.
121
Fig. 4. UV–vis spectra of the toluene solutions including different starting materials. Reference: toluene.
300–800 has shown for the solution of capped-Pd nanoparticles in toluene even 3 days after the preparation with N2 purging. To further understand where the absorption peaks around 340 and 380 nm origin from, several control samples were prepared and characterized by UV–vis spectra. No absorption peak in the range of 300–800 nm is observed for the solution of 0.1 M AOT in isooctane, 1 mM K2 PdCl4 in AOT solution, and even those above solutions aged for several days. For the solution of 1 mM K2 PdCl4 and 1 mM dodecanethiol in AOT solution, and the solution after reduction in air followed by addition of 1 mM dedecanethiol, no peak can be found immediately. However, two peaks appeared for both solutions just letting them sitting for more than 1 or 3 days (Fig. 4). These phenomena might further prove formation of complexes of Pd(II)–S in the above solution. In the freshly-made solution of K2 PdCl4 and dodecanethiol, no absorption peak is seen because those Pd(II) ions exist in the complexes of PdCl4 − . As time proceeds, Pd(II)–S complexes were formed by replacing Cl− with –SC12 H25 , which results in the appearance of two absorption peaks around 340 and 380 nm in their UV–vis spectra. For the case of the reduction carried out under air, the freshlyformed Pd nanoparticles have very high reactivity because of the large surface area and the higher density of defects on their surfaces, and are readily oxidized by oxygen in the air. In this stage, the absorption profile of the solution does not show any welldefined surface plasmon band. However, after several hours, oxidized compound of Pd(II) have slowly formed complexes with thiol molecules, leading to the appearance of two absorption peaks. There were also several reports showed the higher reactivity of Pd nanoparticles and their oxidative etching induced by oxygen or FeCl3 , by which Pd@PdO core-shell nanostructures could be formed [14,15]. The complexes of Pd(II) with thiols may have a strong bond between S and Pd atoms, since only yellow micellar solution
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was obtained by adding strong reducing agents such as sodium borohydride after addition of thiol. In addition, if the micellar solution was crashed with ethanol and then centrifuged, nothing could be obtained. A similar observation has also been reported by Murray and coworkers [16]. Although different surfactant system and method from those in this study were used in the literature, two absorbance peaks can still be seen at ca. 350 and 400 nm when the thiol:Pd synthesis ratio of more than two was employed, from which the conclusion was inferred that some form of Pd(II) thiolate complex or oligomer exists. 3.2. Pd nanoparticles reduced by NaBH4 , N2 H4 to NaH2 PO2 A number of works about the application of reverse micelle, especially AOT system, in nano-synthesis have already been done by Pileni and coworkers [22–24]. For example, size controlled nanocrystallites of CdS, Cu and others have achieved in water-in-oil microemulsion system by altering water content and reducing agents [22]. In our experiments, the size of Pd nanocrystals is also controlled but just in the range of less than 5 nm by changing reaction conditions such as water content, reactant kind and concentration. Because if the molar ratio of water to AOT is increased to above 10, the resulting micellar solution was not stable and clear enough for further reaction steps. The samples held at the overall K2 PdCl4 concentration of 1 and 4 mM, are shown in Fig. 5. Image analysis yields an average size of 2.58 nm ± 23% for low concentration and 3.28 nm ± 14% for high concentration. Both samples show large polydispersity, which prevents the formation of particle ordered array [8,26]. The reducing agents were also varied from NaBH4 , N2 H4 to NaH2 PO2 . For hydrazine as a reducing agent, the particles are similar to that produced by NaBH4 . But for the Pd nanoparticles generated by sodium hypophosphite, better TEM images with good size distribution are seen. With increasing the K2 PdCl4 concentration varied from 1 to 4 mM, larger size (from 2.75 to 3.49 nm) and narrower size distribution (from 15.6 to 9.3% standard deviation) are obtained. The detailed results about the reason of variation of size and size distribution and their selforganization by changing supports for TEM carbon grids have been shown in our previous work [9].
Fig. 5. TEM images and size distribution of palladium nanoclusters prepared in AOT-isooctane reverse micelles by NaBH4 with K2 PdCl4 concentration of 1 mM (A, B) and 4 mM (C, D). Other conditions: [AOT] = 0.1 M, [NaBH4 ] = 0.01 M, w0 = 7).
of hexanethiol, decanethiol, mercaptoacetic acid and mercaptopropanoic acid as coating agents in the preparation of Pd nanoparticles and their solubility control have already been described [9]. In this study, two specific capping agents for preparing water-soluble gold nanoparticles were applied in the present study, expecting the formation of stable water-soluble Pd nanoparticles. 3.3.1. Tiopronin Murray and coworkers [32] have used tiopronin to prepare water-soluble gold nanoparticles in the mixture of acetic acid and methanol, in which tetracholorauric acid and tiopronin are readily dissolved and reduced by sodium borohydride. Tiopronin has four polar groups, such as –SH and –CO2 H groups, which can provide further reactivity with surface metallic atoms.
3.3. Pd nanoparticles coated by thiol derivatitives Most reports about the preparation of soluble metal nanoparticles focus on the solubility of particles just in either non-polar solvent or waters. In addition, the methods employed in most case are just for one type of solvents, either toluene or water. In this report, we employ inverse micelles of AOT surfactant in isooctane to prepare Pd particles with size less than 5 nm. Through purification of redispersion–centrifugation process of the resulting nanoparticles followed by simple addition of different kinds of capping agents, Pd nanoparticles soluble in either toluene or water could be controlled. Details about the use
According to our experience of using AOT method for synthesizing water-soluble Pd nanoparticles, Tiopronin was added to the alcoholic suspension during the purification process and the suspension was stirred overnight. We failed to get water-soluble Pd particles even after adjusting pH value with HCl solution.
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Fig. 6. TEM images of palladium nanoparticles capped with thiolated polymer. TEM sample was prepared by depositing a drop of the Pd aqueous solution on a TEM grid supported by silicon wafer.
At the same time, the resulting Pd pellet after purification was dispersed in the mixture of acetic acid and methanol, in which tiopronin was added. After stirring the suspension, the desired Pd particles soluble in water haven’t been achieved. Murray method has also been simply repeated except for replacing gold salt with palladium salt. K2 PdCl4 was dissolved in the same mixture after stirring for more than 24 h, and then reduced by sodium borohydride. After purification with absolute ethanol and centrifugation, a deep black aqueous solution was obtained. However, TEM investigation hasn’t found anything on the TEM grid, which might be attributed to the formation of Pd(II) complex with tiopronin. 3.3.2. Thiolated poly(ethylene glycol) (PEG-SH) Thiolated polymer, ␣-methoxy--mercapto-poly(ethylene glycol) (PEG-SH) was first used as a capping agent for watersoluble gold particles in a similar Brust method by Murray and coworkers [33]. The procedure for producing the water-soluble Pd nanoparticles is similar to our toluene soluble particles except PEG-SH was used instead of dodecane thiol. Then the obtained Pd nanoparticles are very soluble and stable in water for more than 2 months. Interestingly, TEM inspection reveals that Pd nanoparticles of less than 4 nm self-organized into nearly spherical aggregates with size in the range of 50–400 nm (shown in Fig. 6A). Fig. 6C shows that the dark parts consists of very small Pd nanoparticles, while the gray part is partially crystallized PEG phase because at room temperature PEG-SH and PEG–S–Pd phase can easily be crystalline [33]. There has been only one report of self-organization into spherical aggregates of palladium nanoparticles with size of 4.0 nm [34]. 4. Conclusion In summary, a systematic study of palladium synthesis by UV–vis spectra has been reported in this paper. Size and size distribution of Pd nanoparticles prepared in reverse micelles could be controlled in a narrow range by changing the concentration of the starting materials and the kinds of reducing agents. The effects of varying the capping agents and the self-organization of Pd nanoparticles coated with thiolated poly(ethylene glycol) have also been presented.
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