Langmuir 1993,9, 1664-1667
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Preparation of Bimetallic Ag-Pd Colloids from Silver(I) Bis(oxalato)palladate(11) Kanjiro Torigoe and Kunio Esumi' Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received November 23, 1992. I n Final Form: March 22,1993 Bimetallic silver-palladium colloids have been prepared by reduction of silver(1)bis(oxalato)palladate(11) with 253.7-nm UV light in the presence of poly(N-vinyl-2-pyrrolidone).The average particle size ranged from 3 to 11nm in the 5 X 103 mol dm3 PVP system. The EDX profides reveal that each particle is bimetallic Ag-Pd, although the composition is not uniform. The average composition shows a dependence on the concentration of Agz[Pd(C20&]; silver becomes rich in colloid particlea with an increase in the concentration. Optical spectra of these colloids show a plasma rwonance band at 330-375 nm. The peak shift with changing composition of the bimetallic colloids is in qualitative agreement with Mie theory.
Introduction Bimetallic colloids of noble metals have been extensively studied due to their superior properties in catalytic reactions. Several methods are used for the preparation, eg., chemical reduction14 or W photoreduction7of mixed solutions of two metal salts. Toshima et al. synthesized polymer-protected bimetallic Pd-Ptl and Pd-Au2 particles by reduction of ethanovwater solutions and studied their crystal structure and catalytic activity for hydrogenation of dienes. Michel et aL3 and Liu et aL4 independently inveatigated Pd-Au bimetallic colloids and discussed their catalytic activities. However, bimetallic colloids containing Ag have not been studied in detail except for the AgAu s y ~ t e m . ~As J possible reasons, firstly, as is very wellknown,it should be noted that Ag+ ion readily reacts with halogen ions to provide water-insoluble silver halides. Due to these reactions, noble metal compounds to be mixed with a silver salt are very limited, since many noble metal compounds except halides are not or hardly soluble in water. Secondly, many other water-soluble noble metal compounds such as cyanides and ammine complexes are less active than halides for elimination or substitution reactions of ligands which should take place before reduction of metal ions, due to strong binding between the ligands and metal ions.* Besides, the standard electrode potential of Ag+/AgO is relatively high, so it often arrives that Ag+ is reduced much more rapidly than the other metal ions and hence bimetallic particles are not formed. However, it appears that silver-containing bimetallic colloids are very attractive in studying optical spectra, SERS activity, or photoelectrochemical reactions. It is known that Ag colloid shows a sharp absorption band in the visible region due to surface plasma resonance.c11 The (1)Harada. M.: Asakura.. K.:. Ueki. Y.: Toshma, N. J . Phvs. Chem. 1992,96,9730. (2) Toehima.N.:Harada.M.:Yamazaki.Y.:Asakura.K. . . , . . . J. Phvs. Chem. 19i2; 96,9927.. . (3) Michel, J. B.; Schwartz, J. T. In Preparation of Catalysis IV; Delmon,B., Jacobs, P. A., Poncelet, G.,Eds.;Elsevier: Amsterdam, 1987; p 669. (4) Wang, Y.; Liu, H. Polym. Bull. 1991, 26, 139. (5) Esumi, K.;Shiratori,M.;Ishizuka,H.;Tano,T.;Torigoe, K.;Meguro, K. Langmuir 1991, 7,457. (6) Sato, T.; Kuroda, S.; Takami, A.; Yonezawa, Y.; Hada, H. Appl. Organomet. Chem. 1991,5,261. (7) Teo, B. K.; Keating, K.; Kao, Y.-H. J. Am. Chem. SOC.1987,109, 3494. - ._ ..
(8) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th
ed.; John Wiley & Sons: New York, 1988.
(9) Creighton,J. A.; Eadon, D. G. J. Chem. SOC., Faraday Trans. 1991, 87, 3881.
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plasmon band is considered to be responsible for the SERS activity.12-14 The other SERS active metals, Le., Cu and also have a plasmon band in. the visible region when they are in the colloid state. On the other hand, Pd, Pt, and Rh colloids which do not have a plasmon band in the visible region are SERS inactive. However, to our knowledge, little is known about the SERS activity of bimetallic colloids composed of two elements in the different groups. On the other hand, Henglein et al.1G18 have studied in detail the electrochemistry of small silver particles. They observed spectral change of silver colloids by adsorbed ions or by deposition of another metals (Pb,In) and elucidated it by the change in the surface charge with electron or hole injection. In the present paper, we have prepared bimetallic AgPd colloids from silver(1) bis(oxalato)palladate(II). This complex is slightly soluble in water (Ica. 3 mmol dm4) and the oxalato ligand rapidly decomposes by light irradiation or heating. The bimetallic colloids have been prepared by UV irradiation and characterized by TEM, EDX, and UV-vis spectra.
Experimental Section Synthesisof Silver(I) Bis(oxalato)palladate(II). Siver(I) bis(oxalato)palladate(II) was synthesized according to the 1iterat~re.l~ A 1.24-g portion (7.0 "01) of palladium chloride (Wako Pure Chemicals) was dissolved in about 60 cma of concentrated HC1 (Wako). A 1.05-gportion (14.1 "01) of KC1 (Wako)was added to this solution and heated to vaporize HC1, then dark green Ka[PdC&]was crystallized. Subsequently to 1.80g (5.5 mmol) of K2[PdC&]dissolved in 50 cms of water, 2.03 g (11.0 "01) of K2C204.HzO (Wako) was added at 60 "C with stirring. Immediately the solution turned to orange and K2[Pd(C20d21.4H20 was precipitated, which was recrystallized seven times from cold water to completely remove the byprodud KC1.
u.;
(10) Kreibig, Zacharias, P.2.Phy8. 1970,231, 128. (11) Doremus, R. H. J. Chem. Phys. 1965, 42, 414. (12) Blatchford, C. G.; Siiman,0.;Kerker, M. J. Phys. Chem. 1983, 87, 2503. (13) Fornasiero, D.; Grieser, F. J. Chem. Phys. 1987, 87, 3213. (14) Blatchford, C. G.; Campbell,J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435. 116) Heard, S. M.; Grieser, F.; Barraclough, C. G. Chem. Phys. Lett. 1983,95, 154. (16) Henglein,A.; Mulvaney,P.; Holzwarth, A.; Soeebee, T. E.; Fojtik, A. Ber. Bunsen-Gee. Phy8. Chem. 1992,96, 754. (17) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. 1991, 92, 31. (18) Henglein, A. Chem. Rev. 1989,89,1861. (19) Gmelim Handbuch deranorganischen Chemie,Palladium,1942,
320.
0 1993 American Chemical Society
Preparation of Bimetallic Ag-Pd Colloids Finally, to 50 cms of aqueous solution dissolving 1.0 g (2.3 "01) 1.27 g (2.3mmol)of AgNOs (KokusanKagaku) of K2[Pd(C201)~], was added at 60 "C with stirring. On cooling in an ice water bath, yellow needle crystalsof Ag2[Pd(C20&1.3H20were precipitated, which were washed with cold water and stored in the dark. The averageyield from Ka[PdC&]was about 30%. The final product was identified by IR spectra. All reagents were reagent grade and used as received. Water was deionized with a Milli-Q water purification system. Preparation of the Metal Colloids. A 3.5-cm3 portion of aqueous Ag2[Pd(CaO&l (0.1-3.0 mmol dm-9) containing PVP (MW = 3.6 X lV, Tokyo Kasei) was placed in a rectangular quartz vessel. Concentration of the polymer was 5.0 mmol dm-9 in monomer units unless otherwise noticed. The sample was irradiated for a maximum of 3 h at room temperature by UV light (A = 253.7 nm) with a 40-W low-pressure mercury lamp, SUV-40GL (Sen Tokushu Kogen),and a UlrB-X power supply. The 184.9-nmemission was cut off by the lamp jacket made of synthesized quartz. The incident light intensitywas determined to be 1.34 X 1016 quanta cm-2 8-l by a potassium ferrioxalate chemical actinometer." As reference samples, AgClO, (Wako) and synthesized K2[Pd(C20,)&4H20 were irradiated under the same condition as Agz[Pd(CzO4)zl. Characterizationof the Metal Colloids. Electron micrographs were taken with a Hitachi H-800 and a JEOL 2000-FX2 transmission electron microscopy, operating at 150 and 200 kV, respectively. The samples were mounted on carbon-coated copper grids and allowed to reach dryness in a desiccator. Their particle sizes were determined from the maximum length of the particles. The size distributionswere tabulated by counting about 200 particles. Energy dispersive X-ray (EDX) analysis was performed for several particles in each diluted sample with a Philips EDAX system. The spatial resolution was about 5 nm. The X-ray counts for Ag K lines (Ka, 22.124 key KO, 24.930 keV) and Pd K lines (Ka, 21.137 keV; KO,23.805 keV) were measured. Optical spectra were measured with a Hitachi 220A double beam spectrophotometer at 0.1 or 1cm of path length. Reduction yields for Ag and Pd were separately determined as follows. Aliquota (3.0 cm3)of the samples after irradiation were dialyzed in a semipermeabletube (2.4-nmpore) against 7 X 1 0 - 2 mol dm-9 sucrose for 4 days, and the colloid particles remained were dissolved in 30 cms of 3.6 mol dma HN03. Concentrations of the two metal ions were determined by using a Hitachi Z-8O00 Zeeman polarized atomic absorption spectrophotometer. Results and Discussion Transmission electron micrographs and the size distributions of metal particles prepared from various concentrations of Ag2[Pd(C204)21 are shown in Figure 1. As can be seenin the micrographs, the morphology of particles somewhat deviates from spherical, especially remarkable in higher concentrations. However, since their axis ratios are close to unity, the irregularity of the particle shape would not affect much their optical absorption spectra which are discussed later. The micrographs also indicate that most of the particles are isolated from each other. On the other hand, the size distribution becomes broader with an increase in the concentration. The broadening was found to be little affected by increasing polymer concentration, thus it is probably due to increase in the growth rate with time. The average sizesof the colloid particles shown in Figure 1 are plotted in Figure 2. They ranged from 3.2 to 11.3 nm. The increase in particle size is remarkable in the concentration of Agz[Pd(C204)21 being lower than 5 X 10-4 mol dm3. In higher concentrations than 1 X 103 mol dmd, the increase is less pronounced. A similar trend is found in the systems containing 15 mmol dm4 PVP, though the particle sizes are smaller. (20) Calvert, J. G.; Pitts, N. J., Jr. Photochemistry; Wiley L Sons: New York, 1966.
Langmuir, Vol. 9, No. 7,1993 1665 To examine whether the metal colloids are a mixture of Ag and Pd particles or bimetallic ones, composition of each particle was analyzed by EDX. It should be noted that the spatial resolution (ca. 5 nm) was narrow enough to detect X-ray generated from a single particle. Figure 3 shows the results for two particles with different sizes, revealing that both Ag and Pd are contained in the particles. The two elements were detected in all the particles analyzed. This strongly suggests that they are bimetallic particles. On the homogeneity of the composition of the particles, however, relative intensities of Ag to Pd were a little different from one particle to another, which indicates that the composition has a certain distribution. From the results of about ten particles measured, it appears that larger particles are enriched in silver. However, it will be necessary to analyze many particles to determine distribution in the composition of the bimetallic particles. Therefore, only their average compositions are discussed in the present study. The average composition of the bimetallic colloids was determined from reduction yields for the two metal species. Figure 4 showsthe reduction yields of the dialyzed samples. Different behaviors are observed; the reduction yield for Pd monotonically decreased from 85.0 to 37.9% with increasing concentration of Agz[Pd(CzOr)nl, whereas that for Ag showed a maximum value of 84.7% at 1.0 X 10-9 mol dm-3. The average composition of the bimetallic colloids calculated from the reduction yields are shown in Figure 5: The silver content increases from 48.0 to 78.9 atom % with increasing concentration of Ag2[Pd(C204)21. On the other hand, formation of the bimetallic colloids can be semiquantitativelyexamined by measuring the peak wavelength in optical extinction spectra of the colloids. Figure 6 depicts optical spectra of the metal colloids with various concentrations of AgdPd(C204)21. In low concentrations below 5 X 10-4 mol dm-3, the colloids did not show a distinct absorption band a t X 1 300 nm. In 8 X 10-4 mol dm-3, a small absorption peak appeared at 330 nm, and gradually shifted toward 375 nm with increasing concentration of the metal salt. It was found that the peak position was little affected by increasing polymer concentration. On the other hand, Pd colloid showed only a continuous absorption band in X L 300 nm. Ag colloid was not formed in the absence of alcohols or acetone; however those reduced by NaBH4 showed a strong absorption peak at X = 387 nm. According to the Mie theory,10*21 the optical absorption and scattering of metal colloids are due to excitation of surface plasmon of small metal particles by external oscillating electric field. When the particle size is small enough compared to the wavelength of light, their optical spectra are predominantly attributed to the light absorp tion by dipolar polarization of the particles. The maximum absorption wavelength of the surface plasmon band is related to €1,the real part of the complex dielectric function of the metal. For bimetallic colloids composed of a metal a and another metal b, the dielectric function ei is given by concentration average of those for the two metal species: ei = xci" + (1- X)cib (i = 1, 2). Assuming this relationship, we performed a computer simulation of the composition dependence of plasma wavelength using bulk dielectric constants of Ag and Pd.22 Figure 7 showsthe plots of calculated and measured plasma wavelengths against composition of bimetallic particles. (21) Mie, G. Ann. Phys. 1908,25, 377. (22)Zolotarjev, V. M.; Morozov, V. N.; Smirnova, E. V. Optical constants of nutural and artificial media; Himiya: Sanct Peterburg, 1984.
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