J. Phys. Chem. C 2008, 112, 10089–10094
10089
Au@Pd Core-Shell Nanoparticles through Digestive Ripening Deepa Jose and Balaji R. Jagirdar* Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India ReceiVed: March 29, 2008; ReVised Manuscript ReceiVed: April 18, 2008
Colloids of gold and palladium nanoparticles have been prepared by the Solvated Metal Atom Dispersion (SMAD) method. Digestive ripening, a process involving mixing of the as-prepared gold and palladium colloids consisting of polydisperse nanoparticles and refluxing the mixture in the presence of a surfactant resulted in highly monodisperse Au@Pd core-shell nanoparticles with an average diameter of 6.6 ( 0.5 nm. The synthesis has been successfully carried out reproducibly on gram scale. The core-shell structure was established by using UV-visible spectroscopy, high-resolution electron microscopy, energy filtered electron microscopy, energy dispersive X-ray analysis, high angle annular dark field imaging, and powder X-ray diffraction methods. Introduction There has been burgeoning interest in the bimetallic nanoparticles either as core-shell structures or alloys from both the scientific as well as the technological standpoint.1 This is due to their interesting physical and chemical properties resulting not only from the size effects but also from the cooperative effects of both metals.2 Bimetallic clusters are also of great interest in catalysis.2 They typically fall in the 1-5 nm size regimes. In most cases, the catalyst preparation involves placing the support, e.g., alumina, in contact with an aqueous solution of the precursors of the two metals. Among the various bimetallic nanoclusters, the Au/Pd system has been intensively investigated. The studies have been directed toward the synthesis of core-shell structure,3 cluster-in-cluster structure,4 threelayered onion-shaped structure,5 and random alloy.6 Although various synthetic methods have been reported for the Au/Pd system, they suffer from certain problems related to the scale up, reproducibility, and high monodispersity of particle size. A synthetic method that has broad appeal for the preparation of stable metallic colloids in organic solvents is based on the growth of metal atom clusters at low temperatures.7 This method, known as Solvated Metal Atom Dispersion (SMAD hereafter), usually employing organic solvents as the matrix host material to prepare colloidal suspensions of transition metals, has been effectively used by Klabunde and his co-workers for realizing highly monodisperse gold, silver, and copper nanoparticles on gram quantities.8 The synthetic procedure involves codeposition of metal atoms and a stabilizing solvent on the walls of a reactor maintained at -196 °C. Warm up of the matrix affords a slurry comprised of metal particles that grow in size. An organic ligand/surfactant added to this mixture halts the growth process. This colloid consisting of polydisperse metal nanoparticles is subjected to a process of digestiVe ripening,9 wherein the polydisperse sample is transformed into a highly monodisperse colloid by heating the as-prepared colloid at or near the boiling point of the solvent. Although not fully understood, in the digestive ripening process, smaller particles grow whereas the larger ones erode until the mixture becomes homogeneous in size and a dynamic equilibrium is established. The SMAD process offers several advantages compared to other methodologies for the preparation of colloids of metal nano* Corresponding author. E-mail:
[email protected].
particles: easy scale up, high reproducibility, and avoidance of tedious purification procedures. Klabunde et al. recently reported on the low-temperature alloying of copper and silver nanoparticles with gold nanoparticles via digestive ripening.10 The alloying reaction was carried out at 198 °C, the boiling point of 4-tert-butyltoluene, which was employed as the solvent. Hodak et al. showed that irradiation of Au@Ag core-shell structures with laser light induces melting and the subsequent erasing of the core-shell structure via interparticle diffusion of atoms resulting in AuAg alloys.11 Suzuki and Ito reported that annealing of R-brass@γbrass core-shell particles at 260 °C results in β-brass.12 Prompted by the report of Klabunde and co-workers,10 we carried out the digestive ripening of colloids of Cu and Zn nanoparticles in a solvent with a lower boiling point, butanone (bp 80 °C), and obtained highly monodisperse Cu@ZnO core-shell nanoparticles with an average diameter of 3.0 ( 0.7 nm.13 Our finding of the Cu@ZnO core-shell nanoparticles suggests that kinetically stable core-shell particles could be obtained via digestive ripening of a mixture of separately prepared colloids at fairly low temperatures. In this process, atoms of one particle are transferred to the surface of a particle with different elemental composition. In an attempt to test our hypothesis, we carried out the digestive ripening of gold and palladium nanoparticles. Herein, we report the synthesis and characterization of highly monodisperse Au@Pd core-shell nanoparticles obtained through digestive ripening of the asprepared Au and Pd colloids by the SMAD process. Experimental Section Materials. Gold foil (0.3 mm thickness) (99.99%) and palladium foil (0.3 mm thickness) (99.95%) were purchased from Arora-Matthey, Kolkata, India. Tungsten crucibles were obtained from R. D. Mathis Company, California. 2-Butanone (HPLC grade), dodecanethiol, and 4-tert-butyltoluene were purchased from Aldrich. 2-Butanone was dried over K2CO3 and degassed by several freeze-pump-thaw cycles. Dodecanethiol and 4-tert-butyltoluene were used after purging with Ar for 30 min before each experiment. Preparation of Gold and Palladium Colloids by the Solvated Metal Atom Dispersion (SMAD) Method. The SMAD setup is described in detail in ref 7. The gold and the palladium colloids were prepared by the SMAD method,
10.1021/jp802721s CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
10090 J. Phys. Chem. C, Vol. 112, No. 27, 2008 the details of which have been reported elsewhere.8,14 The asprepared Au-butanone-4-tert-butyltoluene-dodecanethiol colloid was purple whereas the analogous Pd colloid was blackish brown. Preparation of Au@Pd Core-Shell Nanoparticles. The asprepared Au-butanone--4-tert-butyltoluene-dodecanethiol colloid (8 × 10-4 g of Au/mL) and the as-prepared Pd-butanone-4tert-butyltoluene-dodecanethiol colloid (8 × 10-4 g of Pd/mL) colloids were mixed in a 3:1, 1:1, and 1:3 molar ratio (Au:Pd) and the 2-butanone was removed in vacuo. Then the mixture was diluted with 4-tert-butyltoluene to keep the total volume at 10 mL. The reaction mixture was refluxed under Ar for 6 h during which time the colloid became transparent and turned brown. The progress of the reaction was monitored by UV-visible spectroscopy. Addition of 10 mL of absolute ethanol followed by centrifugation resulted in precipitation of a powder that was used for powder XRD measurements. Sample Preparation for TEM. To 0.5 mL of the colloid was added 10 mL of absolute ethanol and then the mixture was centrifuged at 2500 rpm for 1 h. The supernatant liquid was poured out and the residue collected at the bottom of the centrifuge tube was washed with ethanol several times to remove the excess thiol. Finally, the residue was dried under vacuo. It was then redispersed in ethanol by sonication (90 W, 1 min). A 1 µL drop of the colloid was casted on a carbon-coated copper grid and the grid was dried under a table lamp for 12 h. Instrumentation. UV-visible spectra were recorded with a Perkin-Elmer Lambda 35 UV/vis spectrometer. The TEM bright field images, electron diffraction patterns, HRTEM, EF-TEM, and HAADF images were obtained by using a TECNAI F30 transmission electron microscope. The powder X-ray diffraction measurements were carried out with a Philips powder X-ray diffractometer. For annealing the samples, the powders were filled in 0.5 mm glass capillaries, flame sealed, and annealed for 12 h. The XPS measurements were carried out by employing a Mg KR source at room temperature at a base pressure of 3 × 10-10 Torr, using a custom-built high-resolution photoelectron spectrometer.
Jose and Jagirdar
Figure 1. (a) UV-visible spectrum of Au-butanone-4-tertbutyltoluene-dodecanethiol colloid (black trace, as-prepared; red trace, after digestive ripening); (b) TEM bright field image of the as-prepared Au colloid; (c) HRTEM image of Au nanoparticles; and (d) FFT pattern of the Au nanoparticle.
Results and Discussion
Figure 2. (a) TEM bright field image of Au-thiol-4-tert-butyltoluene colloid (after digestive ripening); (b) histogram showing the particle size distribution after digestive ripening; and (c) SAED pattern of Au nanoparticles.
(a) Gold Colloids. Thiol-stabilized gold nanoparticles have been prepared and characterized by the SMAD process by Klabunde et al.8c We followed a similar protocol for the preparation of Au-butanone-4-tert-butyltoluene-dodecanethiol colloid. The dark purple as-prepared gold colloid is characterized by a broad absorption band in the visible region with a shoulder at 520 nm (Figure 1a). The TEM micrographs revealed that the sample is polydisperse having necked particles (Figure 1b,c). The HRTEM image (Figure 1c) shows lattice fringes corresponding to the (111) Au plane indicating that the sample is highly crystalline. The FFT pattern (Figure 1d) can be indexed to the diffractions from (111), (200), (220), and (311) planes of Au. The powder XRD pattern also shows diffractions from (111), (200), (220), and (311) planes of Au (see the Supporting Information). The as-prepared Au sample is stable under Ar atmosphere for months. In this sample, 2-butanone is in large excess, therefore it solvates the gold nanoparticles in preference to the dodecanethiol. Removal of 2-butanone under vacuum from the as-prepared gold colloid enables binding of the thiol with the gold nanoparticles and also some ripening of the particles. This is due to the greater solubility of thiol in 4-tert-butyltoluene than in 2-butanone resulting in good dispersion of thiol-bound gold nanoparticles. Refluxing the Au-thiol-4-tert-butyltoluene colloid for 3 h gave a transparent and homogeneous purple colloid
that shows a well-defined Surface Plasmon Resonance (SPR) at 520 nm (Figure 1a) indicating that the sample has undergone digestive ripening. The TEM micrograph revealed spherical Au nanoparticles with a very narrow particle size distribution (Figure 2a,b). The mean particle size is 5.7 ( 0.7 nm. The SAED pattern (Figure 2c) shows diffractions from (111), (200), (220), and (311) planes of Au. Klabunde and co-workers reported that a 1-40 nm polydisperse sample was transformed into a highly monodisperse sample with a mean particle size of 4.5 ( 0.4 nm.8c (b) Palladium Colloids. Living colloidal Pd nanoparticles in acetone with particle size of 8 nm have been prepared by Klabunde et al.14 The Pd-butanone-4-tert-butyltoluene-dodecanethiol colloid synthesized by the SMAD method herein was brownish black and did not exhibit any SPR in the UV-visible absorption spectrum except a trailing increase in absorbance up to 300 nm (Figure 3a). The colloids are stable with respect to precipitation under Ar; however, with time, a Pd-thiolate complex formation sets in (see later), which is apparent from the color change to yellow. The TEM bright field image (Figure 3b) of the as-prepared Pd colloid showed spherical particles with a mean size of 2.8 ( 0.1 nm. The energy dispersive X-ray analysis (Figure 3c) of the sample showed the presence of pure palladium. The powder X-ray diffraction pattern
Au@Pd Core-Shell Nanoparticles
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Figure 4. XPS spectra of Pd metal (blue trace) and Pd nanoparticles (red trace) prepared by the SMAD method.
Figure 5. (a) Progress of the digestive ripening of the as-prepared Au, Pd colloids (1:0, 3:1, 1:1, 1:3, 0:1) as monitored by UV-visible spectroscopy; (b) UV-visible spectral monitoring of the reaction of digestively ripened Au and Pd colloids.
Figure 3. (a) UV-visible spectrum of Pd-butanone-4-tertbutyltoluene-dodecanethiol colloid (black trace, as-prepared; red-trace, after digestive ripening); (b) TEM bright field image of the as-prepared Pd colloid; (c) EDX analysis of Pd nanoparticles; and (d) XRD pattern of Pd nanoparticles.
(Figure 3d) showed broad reflections. All the reflections match those of pure palladium (JCPDS file no. 461043). The average crystallite size estimated by using the Scherrer equation is about 13 nm. On the other hand, we also prepared Pd-butanone colloids by the SMAD process. The blackish-brown colloids in 2-butanone were found to be stable with respect to precipitation of Pd nanoparticles for months under Ar (see data in the Supporting Information). Samples for XPS measurements were prepared by precipitating Pd from the as-prepared colloid followed by washing the precipitate several times with ethanol and drying under vacuum. The XPS spectra of the Pd nanopowder and the bulk metal corresponding to the 2 spin systems, 3d3/2 and 3d5/2, were examined. For the Pd nanoparticles, peaks at 342.4 and 336.9 eV were observed corresponding to the 3d3/2 and 3d5/2 spin systems, respectively, whereas for the bulk metal the corresponding peaks appear at 341 and 335.7 eV (Figure 4) which match with the literature values for the binding energies of Pd.15 This shows that the binding energy of the 3d electrons of Pd atoms in the case of nanoparticles is greater than that of the bulk metal by 1.2 eV indicating that the surface Pd atoms must be positively charged. Since the probe length of the X-rays used
for the XPS studies was 1.5 nm, it suggests that only the surface atoms are positively charged. The smaller penetration depth compared to the size of the cluster resulted in no interaction of the beam with inner atoms, therefore their electronic state could not be definitively assigned. Digestive ripening of the Pd-butanone-4-tert-butyltoluenedodecanethiol colloid after removal of 2-butanone under vacuum resulted in a Pd-thiolate complex. In fact, even at room temperature, the Pd clusters in the as-prepared colloid disintegrate due to ligand exchange between 2-butanone (original capping agent) and dodecanethiol (capping agent that replaces butanone) resulting in the formation of a Pd-thiolate complex. The FTIR, NMR, and MALDI mass spectroscopy, and finally X-ray crystallography, resulted in the elucidation of the formulation of the Pd-thiolate complex as [Pd(SC12H25)2]6. Klabunde et al. obtained this very complex by refluxing Na2PdCl4 and dodecanethiol in 4-tert-butyltoluene under Ar atmosphere.16 They also reported the structural characterization of this complex. The mechanism of the formation of the thiolate complex in our experiment is unclear. Under the conditions of digestive ripening, the Pd clusters present in the as-prepared colloid actually disintegrate and form a Pd-thiolate cluster rather than affording a monodisperse colloid. (c) Au@Pd Core Shell Nanoparticles. The as-prepared Au and Pd colloids were mixed in 3:1, 1:1, and 1:3 molar ratios. Removal of 2-butanone from these mixtures resulted in the damping of the SPR of gold. Then the samples were subjected to the digestive ripening process. Complete disappearance of the gold SPR was noted in the UV-visible spectrum in the cases of Au:Pd 1:1 and 1:3 after 6 h of digestive ripening; however, the SPR of gold was only partially dampened in the case of Au:Pd 3:1 (Figure 5a). Refluxing together a mixture of digestively ripened Au and Pd colloids (actually Pd-thiolate in the digestively ripened sample, vide infra) at 198 °C under Ar atmosphere resulted in simple mechanical mixtures of the Au nanoparticles and Pd-thiolate complex as evidenced in the UV-visible spectra (Figure 5b): Au SPR at 520 nm and a band for Pd-thiolate complex at 408 nm. The spectra remained unchanged even after 30 h of refluxing.
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Jose and Jagirdar
Figure 7. EDX analysis of Au/Pd bimetallic (1:1) nanoparticles.
Figure 6. (a) TEM bright field image of Au/Pd (1:1) bimetallic nanoparticles (obtained by digestive ripening); (b) histogram showing the particle size distribution; (c) HRTEM image of the Au/Pd bimetallic nanoparticle; (d) SAED pattern of the Au/Pd bimetallic nanoparticle.
The TEM bright field image of the 1:1 Au:Pd sample shown in Figure 6a reveals a narrow size distribution of particles, a consequence of the digestive ripening process. We found in addition to well-separated spherical particles, the presence of
