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Acetate Stabilization of Metal Nanoparticles and Its Role in the Preparation of Metal Nanoparticles in Ethylene Glycol J. Yang,† T. C. Deivaraj,‡ Heng-Phon Too,‡,§ and Jim Yang Lee*,†,‡ Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore-MIT Alliance, 4 Engineering Drive 3, National University of Singapore, Singapore 117576, and Department of Biochemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received November 10, 2003. In Final Form: February 9, 2004 Acetate-stabilized ruthenium nanoparticles were prepared by the NaBH4 reduction of the metal precursor salt at room temperature. Nanoparticles with a mean diameter of 2.20 nm and a standard deviation of 1.03 nm could be obtained under experimental conditions. The Ru nanoparticles so obtained could be easily extracted to a toluene solution of alkylamine, giving rise to alkylamine-stabilized Ru nanoparticles with a mean diameter of 2.96 nm and a standard deviation of 0.92 nm. The new found role of acetate stabilization was used to formulate a mechanism for the formation of metal (Pt, Ru) nanoparticles in ethylene glycol. In this mechanism metal nanoparticles are stabilized in ethylene glycol by adsorbed acetate ions, which are produced as a product of the OH- catalyzed reaction between the metal precursor salt and ethylene glycol.
Introduction There is presently immense interest in exploring the unusual physical and chemical properties of nano-objects in a diverse range of applications.1-5 Ruthenium metal nanoparticles, in particular, are renowned for their catalytic activity.6-9 Many methods have been developed to prepare ruthenium nanoparticles using stabilizers such as polymers,10 ligands,11 cellulose derivatives,12 and even simpler ones such as methanol.13 Most of these preparations are carried out in an organic medium, and the nanoparticles prepared as such are not water-dispersible. One of our current research interests is the bimolecular guided assembly of Ru and Pt nanoparticles, which necessitates the use of metal nanoparticles dispersible in water. The nonaqueous preparatory routes are therefore of no little use to us. In our search for methods of preparing water-based Ru nanoparticle dispersions, we discovered * To whom correspondence may be addressed. Fax: 65 6779 1936. Tel: 65 6874 2899. E-mail:
[email protected]. † Department of Chemical and Environmental Engineering. ‡ Singaport-MIT Alliance. § Department of Biochemistry. (1) Alivisatos, A. P. Science 1996, 271, 933-937. (2) Fendler, J. H. Chem. Mater. 1996, 8, 1616-1624. (3) Colvin, V. L.; Schlamp, M. P.; Alivisatos, A. P. Nature 1994, 370, 354-357. (4) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. Rev. 2000, 29, 27-35. (5) Wang, Z. L. Adv. Mater. 1998, 10, 13-30. (6) Solorza-Feria, O.; Duron, S. Int. J. Hydrogen Energy 2002, 27, 451-455. (7) Miyazaki, A.; Balint, I.; Aika, K.-I.; Nakano, Y. J. Catal. 2001, 204, 364-371. (8) Balint, I.; Miyazaki, A.; Aika, K.-I. J. Catal. 2002, 207, 66-75. (9) Balint, I.; Miyazaki, A.; Aika, K.-I. Chem. Commun. 2002, 630631. (10) Yu, W.-Y.; Liu, M.-H.; Liu, F.-H.; Ma, X.-M.; Liu, Z.-J. Colloid Interface Sci. 1998, 208, 439-444. (11) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante, P.; Casanove, M.-J. J. Am. Chem. Soc. 2001, 123, 7584-7593. (12) Duteil, A.; Queau, R.; Chaudret, B. Chem. Mater. 1993, 5, 341347. (13) Vidoni, O.; Philippot, K.; Amiens, C.; Chaudret, B.; Balmes, O.; Malm, J.-O.; Bovin, J.-O.; Senocq, F.; Casanove, M.-J. Angew. Chem., Int. Ed. 1999, 38, 3736-3738.
a simple procedure in which acetate functions satisfactorily as the stabilizing agent. An ensuing literature survey revealed that Viau and co-workers14 had previously used sodium acetate to control the size of Ru nanoparticles prepared in liquid polyol. However, the preparation of acetate-stabilized Ru nanoparticles in a 100% aqueous environment has not been reported. Perhaps more importantly, from the observation that metal (Pt, Ru) nanoparticles could indeed be stabilized by acetate, a more credible mechanism for explaining the preparation of “unprotected” metal nanoparticles in ethylene glycol could be put forward. Experimental Section Ruthenium(III) chloride hydrate, hydrogen hexachloroplatinate(IV) hydrate, ethylene glycol (99+%), and dodecylamine (98%) from Aldrich; sodium acetate (99%), sodium hydroxide (pellet, GR), and ethanol (99%) from Merck; sodium borohydride (98%) from Fluka; and toluene from Baker were used as received. Deionized water was distilled by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven. In a typical experiment, 1 mL of 1 M aqueous sodium acetate solution was added to 10 mL of 2 mM aqueous RuCl3 solution, and under vigorous stirring, 1 mL of 112 mM of aqueous NaBH4 solution was introduced dropwise to prepare a Ru hydrosol in which acetate served as the stabilizer. A molar ratio of NaBH4 to RuCl3 greater than 5 was used to ensure the complete reduction of Ru to its zerovalent state. The Ru hydrosol thus obtained was dark brown in color and very stable. No precipitation occurred even after a few days of storage. Acetate-stabilized Ru and Pt nanoparticles were also prepared in ethylene glycol by microwave heating using a CEM Discover system. Briefly, 20 mL of 2 mM of metal salt solution (Ru3+ or PtCl62-) in ethylene glycol was added to 1 mL of 1 M aqueous solution of sodium acetate and subjected to 300 W of microwave radiation for 3 min where temperature was controlled at 160 °C. A fairly stable brown (for platinum) or dark brown (for ruthenium) metal organosol was formed within minutes. (14) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; FievetVincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486-494.
10.1021/la0361159 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/16/2004
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A Waters ion chromatography system was used for the analysis of ions in the solution. An injection loop 20 µL in volume was used for sampling. The anion-exchange column was a 4.6 × 75 mm IC-Pak Anion HR (Waters T9 3652 A06, part no. WA T026765). All standard solutions and eluent were prepared from GR grade chemicals using distilled water. The mobile phase solution was prepared by diluting a mixture of 16 mL of 100 mM NaHCO3 and 14 mL of 100 mM Na2CO3 to 1 L and degassing the latter in an ultrasonic bath before use. Transmission electron microscopy (TEM) using a JEOL JEM2010 microscope at a magnification of 1500000× was used to size the particles. For TEM measurements a drop of the nanoparticle solution was placed on a 3 mm copper grid covered with carbon film. Excess solution was removed by an adsorbent paper. The mean particle size and particle size distribution were obtained from a few randomly chosen areas in the TEM image containing approximately 150 nanoparticles each. A VG ESCALAB MKII spectrometer was used for the X-ray photoelectron spectroscopic (XPS) characterization of the Ru nanoparticles. Samples were obtained by centrifuging the Ru hydrosol and were copiously washed with distilled water before the analysis. Narrow scan photoelectron spectra were recorded in the Ru 3p region.
Results and Discussions Without the addition of sodium acetate, the Ru nanoparticles prepared by NaBH4 reduction of RuCl3 in water would easily agglomerate and precipitate out from the hydrosol when the pH of the hydrosol reached 4.9. In the presence of sodium acetate, however, the Ru hydrosol was highly stable, and no precipitation occurred even after several days of storage, demonstrating the role of acetate as a stabilizer. The adsorption of acetate anions on Ru nanoparticles and the resulting electrostatic repulsion between the nanoparticles had adequately prohibited the latter from getting close enough for irreversible particle growth. Figure 1 shows the TEM image and the size distribution of acetate-stabilized Ru nanoparticles in the hydrosol. The particle size ranged from 1.2 to 5.1 nm, with a mean value of 2.2 nm and a standard deviation of 1.03 nm. XPS was used to analyze the surface composition of the Ru nanoparticles recovered from the hydrosol. Unfortunately, the overlap of the Ru3d3/2 peak with the C1s peak interfered with the unambiguous analysis of the nanoparticle surface composition, and the Ru3p3/2 signal had to be used instead (Figure 2). The Ru3p3/2 XPS signal could be deconvoluted into two peaks of different intensities at 461.8 and 463.6 eV respectively. According to the reference literature, the peak at 461.8 eV corresponds to the Ru zerovalent state while the peak at 463.6 eV may be assigned to the higher oxidation states of Ru such as RuIV in RuO2 15,16 The presence of Ru oxides on the surface of Ru is a known phenomenon17 since Ru nanoparticles are easily oxidized in air. The acetate adsorbed on the surface of the Ru nanoparticles could be easily displaced by stronger coordinating ligands serving as replacement stabilizers. This property makes the acetate-stabilized Ru nanoparticles a good starting point for constructing complex Ru clusters by using the same Ru core particles and by exchanging acetate with progressively stronger, and/or multifunctional stabilizing agents, in one or more steps. Here we present an improved phase transfer technique to modify the Ru nanoparticles with dodecylamine as an illustration. (15) Zhang, X.; Chan, K.-Y. Chem. Mater. 2003, 15, 451-459. (16) Arico, A. S.; Creti, P.; Kim, H.; Mantegna, R.; Giordano, N.; Antonucci, V. J. Electrochem. Soc. 1996, 143, 3950-3959. (17) McClune, W. F. Powder Diffraction File Alphabetical Index Inorganic Phase; JCPDS: Swarthmore, PA, 1980.
Figure 1. TEM image of sodium acetate stabilized Ru nanoparticles: d ) 2.20 nm, σ ) 1.03 nm, σ j ) 0.47.
Figure 2. Ru 3p3/2 XPS spectrum of Ru anoparticles recovered from the hydrosol.
It was difficult to transfer the acetate-stabilized Ru nanoparticles from water to toluene by directly mixing the Ru hydrosol with a toluene solution of dodecylamine. Prolonged stirring only produced a milky mixture of metal hydrosol and toluene, but no particle transfer took place after the mixture was settled down into two immiscible
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of mutual solubility, which restrained the exchange between alkylamine and acetate ions to the small interfacial area between water and toluene. Ethanol, which is water-miscible and a good solvent for dodecylamine, was therefore used to increase the interfacial contact between acetate-stabilized metal nanoparticles and alkylamine, before the nanoparticles were extracted to toluene in the last step. The phase transfer technique not only produced Ru nanoparticle of high stability but also led to the refinement of particle size distribution. By comparison of the histograms in Figure 1 and Figure 3, a significant change in the relative standard deviation from 0.47 to 0.31 was observed. This ability to generate a narrower size distribution through some sieving action is definitely a desirable and welcome feature. Some Ru nanoparticles in this work were also prepared by the “polyol process”, an established method for obtaining organosols of metal nanoparticles. Wang and co-workers have used this method to prepare “unprotected” nanoparticles of Pt, Rh, and Ru in ethylene glycol.18 In that work an ethylene glycol solution of NaOH was added to a glycol or aqueous solution of the metal salt (Pt, Rh, Ru) under stirring to obtain a transparent hydroxide or oxide colloidal solution which was subsequently heated at 160 °C for 3 h. A flowing Ar stream passing through the reaction mixture was used to remove the organic byproducts. A transparent homogeneous colloidal solution was obtained without any precipitate. The stabilization of the metal nanoparticles was attributed to the adsorption of ethylene glycol and simple anions such as OH- on the surface of the metal nanoparticles. Later, Komarneni et al.19 proposed the following mechanism for the formation of metal nanoparticles in ethylene glycol:
HOCH2CH2OH f CH3CHO + H2O 2CH3CHO + M(OH)2 f CH3-CO-CO-CH3 + 2H2O + M Figure 3. TEM image of dodecylamine-stabilized Ru nanoparticles: d ) 2.96 nm, σ ) 0.92 nm, σ j ) 0.31.
layers. The metal hydrosol would retain its original color and the toluene layer was completely colorless. However, phase transfer was made possible using a slight variation of the technique. Briefly, for the preparation of dodecylamine-stabilized Ru nanoparticles, the Ru hydrosol obtained above was mixed with 10 mL of ethanol containing 100 µL of dodecylamine and stirred for 2 min. Five milliliters of toluene was then added, and the mixture was stirred for a further 3 min. The extraction of the dodecylamine-stabilized Ru nanoparticles to toluene occurred rapidly, leaving behind a colorless aqueous solution. The transfer of protection control from acetate to dodecylamine appeared to be complete and efficient as the resulting dodecylamine-stabilized Ru nanoparticles were more stable (no agglomeration was observed even after weeks of storage). A control experiment was carried out in the absence of dodecylamine to confirm that ethanol alone was unable to transfer the Ru nanoparticles from water to toluene. A TEM image of the dodecylamine-stabilized Ru nanoparticles is shown in Figure 3. The slight growth in the average particle size was most probably caused by particle agglomeration that occurred during the stabilizer exchange, in which acetate had to be removed first before dodecylamine could approach the nanoparticle surface. The failure in directly transferring Ru nanoparticles from hydrosol to toluene could be attributed to the lack
The mechanism did not explain the stability of metal nanoparticles in ethylene glycol. In their experiments Komaneni and co-workers had actually added poly(vinylpyrrolidone) (PVP) as a steric stabilizer, although it was not necessary since nanoparticles are stable in alkaline solutions (see later). We have observed in our own experiments that the addition of OH- was not necessary for metal nanoparticle formation; the reduction of RuCl3 and H2PtCl4 was equally complete in the absence of OH-. However, the organosols so obtained were unstable, and precipitates would appear several minutes later. It is therefore apparent that OH- contributed toward the stabilization of metal nanoparticles in ethylene glycol. Once the metal nanoparticles had been stabilized, we found experimentally that neutralization of the metal organosol with acid would not bring about the agglomeration of the metal nanoparticles. On the basis of our observation that sodium acetate could stabilize Ru nanoparticles in the aqueous environment and that acetate is a common product from aldehyde oxidation, we surmise that acetate may be produced as an intermediate product with stabilizing function during the formation of metal nanoparticles in ethylene glycol. To verify this, an ion chromatographic analysis was carried (18) Wang, Y.; Ren, J.; Deng, K.; Gui, L.; Tang, Y. Chem. Mater. 2000, 12, 1622-1627. (19) Komarneni, S.; Li, D.; Newalkar, B.; Katsuki, H.; Bhalla, A. S. Langmuir 2002, 18, 5959-5962.
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Figure 4. TEM image of acetate-stabilized Ru nanoparticles prepared in ethylene glycol: average particle size d ) 1.1 nm, σ ) 0.32 nm, σ j ) 0.29.
out to characterize the chemical compositions of the metal organosols of ruthenium and platinum prepared according to the procedure of Wang et al.18 The ion chromatograms (Supporting Information) clearly show the presence of acetate in the organosols of ruthenium and platinum, which could not be explained by Komarneni’s mechanism.19 Stable organosols of Ru and Pt in ethylene glycol could also be obtained by replacing sodium hydroxide with sodium acetate in the polyol process. The TEM images in Figure 4 and Figure 5 show acetate-stabilized Ru and Pt nanoparticles, respectively, prepared in ethylene glycol. The average particle size was 1.5 nm for platinum and 1.1 nm for ruthenium. These numbers are fairly close to the values obtained by Wang and co-workers 18 (1.3 nm for platinum and 1.1 nm for ruthenium) where OH- was explicitly used in the preparation of stabilized metal nanoparticles. Therefore there may be other interpretations for the role of OH- in the polyol process other than serving as a direct stabilizer for the metal nanoparticles. On the basis of the analysis above, we put forward the following modified reaction mechanism,20 which better rationalizes the presence of acetate in the preparation of metal nanoparticles in ethylene glycol.
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Figure 5. TEM image of acetate-stabilized Pt nanoparticles prepared in ethylene glycol: average particle size d ) 1.5 nm, σ ) 0.65 nm, σ j ) 0.43.
For ruthenium: HOCH2CH2OH f CH3CHO + H2O 3CH3CHO + 2Ru3+ + 9OH- f 3CH3COO- + 2Ru + 6H2O For platinum: HOCH2CH2OH f CH3CHO + H2O 2CH3CHO + (PtCl6)2- + 6OH- f 2CH3COO- + Pt + 6Cl- + 4H2O In the proposed modified mechanism, hydroxide ions are needed for the generation of acetate ions which are responsible for stabilizing the metal nanoparticles in ethylene glycol. The hydroxide ions per se may not have a direct stabilization effect. Furthermore, taking platinum as an example, quantitative analysis of the ion chromatographic data yielded a CH3CO2- to Pt mole ratio of 1.94, (20) Finar, I. L. Organic Chemistry, Volume I-The fundamental principles, 6th ed.; Longman: New York, 1973; p 220.
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which agrees well with the stoichiometric relationship in the above equations (the theoretical value should be 2). The results of this work show that the “unprotected” metal nanoparticles in ethylene glycol could actually be acetatestabilized metal nanoparticles. It is noted that water is always present surreptitiously in the polyol process. While metal nanoparticle (Ru, Pt) could still be obtained (amidst poor stability) without OH-, the total elimination of water in the synthesis resulted only in a green solution with no particle existence, even when microwave heating was applied. This shows that the mechanism of the polyol process is more complex than what we understand today. Conclusions Acetate-stabilized Ru nanoparticles have been prepared in aqueous solutions and examined by transmission electron microscopy (TEM). The particle size distribution was characterized by the following parameters: range of 1.2-5.1 nm, mean of 2.2 nm, and standard deviation of 1.03 nm. The acetate-stabilized Ru nanoparticles could be easily transferred to a dodecylamine solution in toluene by using ethanol in a mediating step. The aminestabilized Ru nanoparticles were also stable, with an
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average diameter of 2.96 nm and a relative standard deviation of 0.31 nm for the particle size distribution. On the basis of the facts that acetate was found in the reaction mixture of the polyol process and that acetate could completely replace hydroxide in the polyol process as the nanoparticle stabilizer, a modified reaction mechanism was proposed that rationalizes these findings better than the prevailing views. We conclude that the “unprotected” metal nanoparticles were actually acetate protected, and acetate ions are responsible for the stabilization of metal nanoparticles in the polyol process. Acknowledgment. The authors acknowledge financial support from the Singapore-MIT Alliance. Y.J. acknowledges the National University of Singapore for his research scholarship. Supporting Information Available: Ion chromatograms of a standard solution of sodium acetate, ruthenium organosol in ethylene glycol, platinum organosil in ethylene glycol, and a mixture of standard solution of sodium acetate and platinum organosol in ethylene glycol. This material is available free of charge via the Internet at http://pubs.acs.org. LA0361159
