© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 36, SEPTEMBER 4, 1997
LETTERS Sonochemical Preparation of Bimetallic Nanoparticles of Gold/Palladium in Aqueous Solution Yoshiteru Mizukoshi,† Kenji Okitsu,† Yasuaki Maeda,† Takao A. Yamamoto,‡ Ryuichiro Oshima,§ and Yoshio Nagata*,§ Research Institute for AdVanced Science and Technology, Osaka Prefecture UniVersity, 1-2, Gakuen-cho, Sakai, Osaka 593, Japan, Department of Applied Materials Science, College of Engineering, Osaka Prefecture UniVersity, 1-1, Gakuen-cho, Sakai, Osaka 593, Japan, and Department of Nuclear Engineering, College of Engineering, Osaka UniVersity, 2-1, Yamadaoka, Suita, Osaka 565, Japan ReceiVed: NoVember 14, 1996; In Final Form: June 3, 1997X
Colloidal dispersions of bimetallic nanoparticles composed of gold and palladium were prepared by a sonochemical method, in which Au(III) and Pd(II) ions in an aqueous solution of sodium tetrachloroaurate(III)dihydrate and sodium tetrachloropalladate(II) were reduced by ultrasound irradiation in the presence of sodium dodecyl sulfate (SDS). In addition to the stabilizing effect, SDS remarkably enhanced the reduction rate, probably due to the thermal decomposition that occurs at the interfacial region between cavitation bubbles and bulk solution and provides reducing radicals. Transmission electron microscopy (TEM) photographs showed spherical particles whose size had a fairly narrow distribution with a geometric mean diameter about 8 nm and a geometric standard deviation of 1.1. Analyses with UV-vis spectra indicated that Au(III) ions were first reduced and after their consumption reduction of Pd(II) ions set in. A core-shell structure of the particles, a core of gold and a shell of palladium, was confirmed by high-resolution TEM and X-ray diffraction.
Introduction In recent years, many studies have been carried out on nanoparticles of noble metals, because their interesting physicochemical properties attracted much attention of researchers from scientific and technological viewpoints. The bimetallic particles are also an attractive target of catalytic research, because their potential properties, such as selectivity of catalytic reactions and chemical and physical stability, are distinct from those of the component monometallic particles.1 Various methods for nanoparticle preparation have been reported so far; they are chemical reduction,2,3 photolytic reduction,4 radiolytic reduction,5 metal evaporation,6 etc. However, the sonochemical †
Department of Applied Materials Science. ‡ Department of Nuclear Engineering. § Research Institute for Advanced Science and Technology. X Abstract published in AdVance ACS Abstracts, August 15, 1997.
S1089-5647(96)03809-6 CCC: $14.00
preparation method of noble metal particles has not been tried as much except for the work at a few places,7-12 although many applications of high-power ultrasound have been tried nowadays in various fields of chemistry.13 In this Letter, we report the first sonochemical formation of gold/palladium bimetallic nanoparticles by simultaneous reduction of Au(III) and Pd(II) ions. The mechanism of formation and the morphology of the particles have been investigated by UV-vis spectra measurements, elemental analyses, XRD measurements, and TEM analyses. Experimental Section A multiwave ultrasonic generator and a barium titanate oscillator of 65 mm diameter were used for ultrasonic irradiation and operated at 200 kHz with an input power of 200 W. A sample solution of 60 mL was sonicated in a cylindrical glass © 1997 American Chemical Society
7034 J. Phys. Chem. B, Vol. 101, No. 36, 1997
Letters
vessel of 50 mm inside diameter with a total volume of 150 mL. The vessel had a side arm with a silicone rubber septum for gas bubbling and sample extraction without exposing the sample to air. The vessel was mounted at a constant position relative to a nodal plane of the sound wave (3.75 mm: half a length of ultrasound wave from the oscillator). The bottom of the vessel was planar and made as thin as possible (1 mm), because transmission of ultrasonic waves increases with decreasing thickness of the bottom. The argon-saturated aqueous solution of NaAuCl4‚2H2O, PdCl2‚2NaCl‚3H2O, and sodium dodecyl sulfate (SDS) was sonicated. During the irradiation, the vessel was closed. The sonication was carried out in a temperature-controlled water bath. Gold and palladium concentrations, both ionic and zero-valent species in the sample solution, should be simultaneously determined as a function of sonication time to investigate the formation kinetics of particles. The improved colorimetric method14 was applied to the simultaneous determination of both Au(III) and Pd(II) ion concentrations by dual-wavelength spectrophotometry.15 Addition of saturated NaBr solution to the sample solution containing the ionic species of noble metals, metallic particles, and SDS makes the particles aggregate by the salting-out effect of sodium cations, and it becomes feasible to remove them by filtration. Noble metal ions also become bromo complexes through the coordination of bromide ions to noble metal ions via a ligand exchange reaction.16,17 These complexes formed according to eqs 1 and 2 have relatively large absorption coefficients in the near-ultraviolet region.
Figure 1. Change in the absorption spectra of a solution containing 0.5mM NaAuCl4, 0.5mM PdCl2, and 8mM SDS with the duration of sonication.
AuCl4- + 4Br- f AuBr4- + 4Cl-
(1)
PdCl42- + 4Br- f PdBr42- + 4Cl-
(2)
Figure 2. Time profile of gold(III) (O) and palladium(II) (b) ions with sonication. The experimental conditions are the same as in Figure 1.
The concentration of Pd(II) ions was determined without interference from coexisting ionic gold, because the absorption coefficients of the AuBr4- complex at 332 and 495 nm have insignificant difference. The concentration of Pd(II) ions, [Pd2+], was obtained from
ment and elemental analysis were prepared by adding NaCl to the colloidal dispersion to make aggregations, which were filtrated out, rinsed, and dried in vacuum.
2+
[Pd ] ) ∆Abs(1)/(Pd,332nm - Pd,495nm)/L
(3)
∆Abs(1) ) Abs332nm - Abs495nm
(4)
The determination of [Au3+] was made similarly from
[Au3+] ) ∆Abs(2)/(Au,381nm - Au,308nm)/L
(5)
∆Abs(2) ) Abs381nm - Abs308nm
(6)
In the above equations Abs is the absorbance of irradiated sample solution, (cm-1 M-1) is the absorption coefficient of metal complexes at each wavelength, and L (cm) is the optical path length. Absorption coefficients of metal complexes at the designated wavelengths are listed in Table 1. Specimens for TEM were prepared by drying droplets of colloidal dispersion on a carbon-supported copper mesh in vacuum. Observations were carried out with JEOL-2000EX and Hitachi HF-2000 electron microscopes operated at 200 kV. Sizes of more than 200 particles were measured on micrographs to obtain size histograms, geometric mean diameters, and geometric standard deviations. Specimens for XRD measureTABLE 1: Absorption Coefficient of Noble Metal Complexes AuBr4PdBr42-
308 nm
332 nm
381 nm
495 nm
865 2830
580 11200
4785 2830
580 575
Results and Discussion The color of the solution containing Au(III) and Pd(II) ions and SDS, pale yellow, was found to turn into reddish-violet at first and then into dark brown upon ultrasonic irradiation. Figure 1 shows the time dependence of the UV-vis spectra of the solution reflecting surface plasmon absorptions of nanoparticles when sonicated. It is noticed that an absorption peak due to gold particles located around 520 nm18 appeared, grew, and reached a maximum at about 6 min sonication. A rapid increase of absorbance after the time of the maximum, especially in shorter wavelength regions, is considered to be a tail of a large absorption band due to palladium nanoparticles located at a much shorter wavelength region.18 A peak found around 300 nm at early stages of sonication is due to the AuCl4- complex. The time profile of the spectra mentioned above indicates that the reductions of Au(III) and Pd(II) ions to form the particles do not occur simultaneously, but sequentially, first for gold and then for palladium. The sequence of this process is confirmed in Figure 2, which shows the time profile of the concentrations of Au(III) and Pd(II) ions in the solution containing both the ions and SDS with sonication. It is clear that Au(III) ions begin to be consumed first, and their depletion is followed by Pd(II) ion consumption. This behavior is consistent with that of spectra shown in Figure 1. Figure 3 shows the absorption spectra of the colloidal dispersions with various gold/palladium compositions: (a) simple mixtures of monometallic colloidal dispersions of gold and palladium separately prepared8,10 and (b) simultaneously
Letters
Figure 3. Series of UV-vis specta of (a) a simple mixture of monometallic nanoparticles and (b) bimetallic nanoparticles. Dashed lines show the spectra of pure gold and palladium monometallic nanoparticles. Total concentration of noble metal ions was 1 mM for all sample solutions.
sonicated solutions containing both Au(III) and Pd(II) ions in the presence of SDS. The spectra of pure monometallic gold and palladium colloidal dispersions are also shown by dashed lines for reference. It is found that in (a), the spectra of the mixed solutions are well reproduced by linear combination of those of pure gold and palladium colloidal dispersions, while this is not the case in (b). This fact indicates that the present colloidal dispersion obtained by the simultaneous sonochemical reduction of Au(III) and Pd(II) ions does not consist of a simple mixture of pure gold and pure palladium particles. In Figure 4, the TEM photographs of sonochemically prepared particles are shown: (a) gold, (b) palladium, (c) simple mixture of gold and palladium and (d) gold/palladium bimetallic. Figure 4e is a high-resolution TEM photograph of the same sample as (d). The geometric mean diameters Dg and the geometric standard deviations σg determined from the histograms of the particle sizes were Dg(Au) ) 7.8 nm, σg(Au) ) 1.31, Dg(Pd) ) 7.5 nm, and σg(Pd) ) 1.39. It is found that Dg(Au) > Dg(Pd), σg(Au) < σg(Pd), and palladium particles are more irregular in shape. The histogram obtained from (c) was found to have two peaks corresponding to those obtained from (a) and (b). In micrograph c, features of shapes of the gold and palladium particles were preserved and distinguishable. But the micrograph of the bimetallic particles, d, shows a single distribution of Dg(Au/Pd) ) 8.3 nm, σg(Au/Pd) ) 1.11, and their shape features are more spherical than those of monometallic ones. Especially, much narrower size distribution is to be noted in comparison with those of the monometallic ones. Such uniformity of bimetallic particles was reported for palladium/gold19-21 and palladium/platinum21 systems, etc., prepared by the alcoholic reduction method. The lattice fringes shown in micrograph e clearly indicate the crystalline nature of the present bimetallic particles. It is found that the fringe is close to the {100} spacing of gold. On the other hand, moire´ fringes observed in many particles suggest that they are composed of superimposition of two crystallites that have very similar crystal structure. The ambiguous contrast seen at the outer surface of each particles suggests that they are gold particles covered with thin palladium layers. The carbon
J. Phys. Chem. B, Vol. 101, No. 36, 1997 7035 contents of
