J . Phys. Chem. 1994, 98, 6212-6215
6212
Composite Pd-Ag Particles in Aqueous Solution Matthias Michaelis and Amim Henglein' Hahn-Meitner- Institut, Abteilung Kleinteilchenforschung, I4109 Berlin, FRG
Paul Mulvaney School of Chemistry, University of Melbourne, Parkville VIC 3052, Australia Received: March 16, 1994" Colloidal composite particles consisting of a nucleus of palladium and a mantle of silver are prepared via the reduction of silver ions on the surface of Pd particles (mean radius: 4.6 nm) with formaldehyde. The absorption spectra of the particles coated with Ag shells of different thicknesses are compared to the calculated ones (using Mie theory and the dielectric data of the bulk materials). The particles possess a surface plasmon absorption band close to 380 nm when more than 10 monolayers are deposited. Where the shell thickness is less than 10 monolayers, the absorption band is located at shorter wavelengths. The band disappears below about three monolayers. The calculated position of the band agrees with the observed one, although the intensity of the band is smaller than expected from theory. The electron mean free path in the shell is much smaller than the value predicted by Granqvist and Hunderi. Chemisorbed SH- ions damp the plasmon band, even when the shell is only four monolayers thick. This damping effect is less pronounced than for pure silver particles. Damping also takes place when the iodide ion adsorbs to the colloidal particles.
Introduction During the past few years, increasingattention in small-particle research has been paid to the preparation of composite metal particles as colloids in solution and the investigation of their physical properties. These studies include metal alloys' as well as layered structures formed by two2 or even three3 different metals. The catalytic properties of such composite particles are of interest as well as their optical and nonlinearoptical properties. In the case of the layered structures, the optical spectra can be calculated from the bulk dielectric properties of the metals involved, using extended Mie theory, and these spectra can be compared with the experimental ones. Differences have occasionallybeen observed and attributed to electronicinteractions of the metals or to alloy formation. In the present paper, the preparation of particles is described, which contain a palladium nucleus and silver mantles of different thicknesses. The particles are made by reducing silver ions in a solution of palladium particles. It is importnt to find reduction conditionswhere the formation of individual silver particles does not occur. The absorption spectra are compared with calculated spectra, and a few chemisorption experiments are also reported. It is known from previous studies4 that the surface plasmon absorption band of pure silver particles is strongly damped by adsorbed substances which electronicallyinteract with the metal surface. Experimental Section The palladium colloid was prepared using citrate to reduce palladi~m(II).~Tri(sodium citrate dihydrate) (750 mg) was added to 500 mL of a solution containing 2 X 10-4 M NazPdCl4, 4X M HCl, and 5 X 10-4 M potassium polyvinylsulfate (PVS). The pH was adjusted to 6.1 by adding 3.3 X 10-3 M NaOH. The solution was then boiled under argon for 14 h. It acquired a brown color. Amberlite Resin (Sigma MB 1) (17 g) was added after cooling to remove most of the ionic species; this treatment causes the specific conductivityto decrease from 1750 to 50 pS/cm over 10 min. The solution was then liberated from Amberlite particles by filtration through a G4 glass filter and stored under a nitrogen atmosphere. During these deionization and filtration procedures part of the colloid was lost. The final Abstract published in Advance ACS Abstracts, May 15, 1994.
palladium concentration was 1.2 X 10-4 M as determined by atomic emission spectroscopy. In the silver deposition experiments, the palladium solution was diluted by roughly a factor of 4. Experiments were also carried out to reduce Pd(I1) in the absence of the stabilizing PVS. Under these circumstances, the loss of reduced palladium was much greater than in the presence of PVS. This polymer is not removed by the treatment of the solution with Amberlite (in fact, when a pure PVS solution was treated with Amberlite, the conductivity actually increased, because the K+ ions of PVS were exchanged for free H+ ions). In order to coat the Pd colloids, two methods were used to reduce Ag+ ions. First, the radiolytic method was tried, which in many other cases has been quite useful for preparing concentric bimetallic particles. NaAg(CN)2 was added to the palladium colloid as well as 0.5 M propanol-2 and 0.1 M acetone. As described previously,4b exposure to y-rays leads to the formation of organic radicals that transfer electrons to the Pd particles. The Pd particles are polarized cathodically, and the stored electrons reduce the silver-cyano complexon thesurface of the Pd particles, which leads to a silver mantle. However, it was found that part of the radicals reduced the complexed silver ions directly in solution, the result being that individualsilver particles were also formed. Second, formaldehyde was used as the reducing agent. When Ag+ ions are reduced by formaldehyde in the absence of colloidal palladium, an induction period of several minutes is observed. However, when Pd particles are present, the reduction occurs practically instantaneously and individualsilver particles are not produced. This method was then applied in all experiments. In detail, the procedure was as follows: 2 X 10-4 M formaldehyde was added to 75 mL of Pd solution (at pH = 11). The solution was in a round flaskunder nitrogen, which had a side arm carrying a 1-cm optical cuvette and a septum. Several milliliters of deaerated AgC104 solution ( 5 X 10-5 to 2 X 10-3 M) was slowly injected (less than 1 mL per minute) under strong stirring. The total injection time was about 8 min. During the injection, the spectral changes took place immediately. This means that the stationary concentration of Ag+ ions was low during the injection, which is the reason why practically no spontaneous nucleation of individual Ag, particles occurred. The palladium concentration in the final solution was 2.9 X le5 M, and the total concentration of added silver varied between 2.2 X 10-6 and 2.0 X 10-4 M.
0022-3654/94/2098-62 12%04.50/0 0 1994 American Chemical Society
Composite Pd-Ag Particles in Aqueous Solution
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6213 ,-,..,
Figure 1. Electron micrographs of the palladium particles before (left) and after deposition of two concentrations of silver (middle and right). The overall concentration of palladium was 2.9 X 10-5 M, those of silver were 7.7 X M (middle) and 2.0 X 1 0 4 M (right). 3.0
1
150 I
2.5 u
al c
2.0
aJ U
t 50 1.5 180nm
0
5
10 radius [nml
m=13
1.0
15
Figure 2. Size distributions of the samples of Figure 1.
c 0.5
Samples for electron microscopy were prepared on a carbon copper grid in a nitrogen-filled glovebox; the dried grid was then transferred to a vacuum holder (Gatan Model 648) from the glovebox to a Phillips CM 12 microscope, equipped with an EDAX 9800 analyzer. Absorption spectra were calculated from the bulk dielectric data for palladium6 and silver.' The scattering coefficients, am and b,, for coated particles were originally derived by Aden and Kerker.* We calculated them by using the equations in the form given by Bohren and Huffmann9 (these authors also include a detailed program for computing the coefficients; note that this program for coated particles takes into account retardation effects, Le. higher terms in the Mie equations).
n
9nm\
md.5
t
Results and Discussion Electron Microscopy of Composite Particles. Figure 1 shows an electron micrograph of the palladium colloid. It can be seen that the particles are not exactly spherical. The size distribution is shown in Figure 2. A mean radius of 4.6 nm is obtained from this distribution. Figure 1 also shows pictures for composite colloids. These colloids were obtained by reducing different amountsof Ag+ ions. As can be seen, the particles become larger and deviations from spherical shape become more pronounced with increasing degrees of coating. The size distributions of the composite particles are also shown in Figure 2. (In the calculation of the sizedistribution, the particles were assumed to be spherical, using a medium diameter for each particle.) The standard deviation is about 10% for the three distributions shown in Figure 2. The elementary composition of the Pd-Ag particles was determined by energy dispersive X-ray spectroscopy. The sample, which contained less silver in Figure 2, should have had a composition of 27 mol % Pd:73 mol 3' 6 Ag. The ratio found, i.e. 2275, is close to the expected value. This ratio was not
n 5
0 0.3 0.2 0.1 n "200
300
400
500
A [nml Figure 3. Absorption spectrum of the palladium colloid before (0) and after deposition of various amounts of silver. m: number of monolayers of silver.
substantially dependent on the position of the particles on the grid. The sample, which contained the higher silver concentration in Figure 2, was expected from the overall concentrations of the two metals in the solution to have the molar ratio 12:88 (Pd:Ag). The ratio found was again close to this value, Le. 11:89 mol %. Absorption Spectra. The optical changes which m u r upon the deposition of various amounts of silver can be seen from Figure 3. The spectra were recorded 4 days after the silver addition. It was observed that a strong increase in the intensity of the absorption spectrum occurs practically immediately after the injection of the Ag+ ions. However, smaller increases follow over hours and days. A typical example is shown in Figure 4. At first sight, it could be supposed that the reduction of the Ag+ ions is not complete at the end of the injection. In order to find out whether the solution still contained some Ag+ ions, 0.1 M propanol-2 was added after the injection and the solution exposed to y-rays for 60 min (dose rate: 3.5 X 105 rad/h). Under these conditions, reducing organic radicals are generated, which would reduce the nonreduced silver ions. It was found that this treatment did not lead to an additional increase in the absorption spectrum of the solution. It is thereforeconcluded that the thermal reduction
Michaelis et al.
6214 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 0.3
250
1
300
350
400
450
X lnml Figure 4. Spectrumof the palladium colloid before and after the addition M Ag+ ions and after various tima of aging. of 2.2 X
0.5 0.4
1 t
n
.
390
, .
.
0
. . ,
. .
, .
.
1 1.5 c(Ag+) I [10"M]
0.5
.
.
,
2
. .
,
2.5
Figure 6. Position of the absorption maximum as a function of the conccntrationofdepositedsilver. Points: experimental. Line: calculated. I
1
m6.4
0
1
2
3
4
5
silver shell thickness [nml Figure 7. Comparisonof experimentaland theoretical MFP for variously coated Pd colloids.
h lnml Figure 5. Calculated spectra of 9.2-nm Pd particles coated by a silver
mantle of various thicknesses.
of the Ag+ ions by formaldehyde is fast and complete after the injection. The optical changes occurring at longer times are possibly due to a reorganization of the silver mantle formed. The number of deposited silver monolayers, m, is given on the curves in Figure 3. It was calculated from the formula
which is derived from simple geometric considerations, assuming that the original Pd particles are spherical and the composite particles are concentric. R is the mean radius of the Pd particles, the Vs are the molar volumes of the two metals, and d is the diameter of an Ag atom calculated from the relation d = ( V A ~ / (6.03 X 1023))1/3. It is seen that the spectrum of palladium, which has an increasing absorption toward the UV, increases a t all wavelengths upon the deposition of oneor two silver monolayers. At m = 4.2, a maximum at 347 nm appears which is substantially shorter than that at 380 nm, where the surface plasmon band of pure silver particles is located. Upon further silver deposition, the maximum becomes stronger and gradually shifts toward 380 nm. Note that the absorption changes little a t 320 nm, where a deep minimum is formed. At shorter wavelengths, a t which 4d 5sp interband transitions occur in silver, the absorption increases strongly with increasing Ag deposition. The absorption spectra of the composite particles were first calculated using Granqvist's formula to correct for the reduced mean free path (MFP) of the electrons in the silver 1ayer:'O
-
where the d's are the diameters of the composite particles and the Pd core. The results are shown in Figure 5 (upper and lower parts). A comparison with the experimental spectra in Figure 3 shows good agreement. To further illustrate this agreement,
the wavelength of the maximum is plotted in Figure 6 versus the silver concentration deposited. Onecan see that the experimental points agree well with the calculated curve. It is concluded that there is practically no disturbing interaction between the two metals, for example, by alloy formation. However, two small differences exist: first, the calculated spectra for m = 1 and 2.1 contain a bump and a clear but weak maximum (Figure S), respectively, whereas the experimental spectrajust show a general and unstructured increase in absorption (Figure 3). Second, the experimental absorption coefficients in the maximum are smaller by about 40% than the calculated ones. Both differences can be resolved by introducing smaller values for the reduced mean free path of the electrons in the silver layer. In Figure 7, the electron mean free path which yields agreement with the experimental data is plotted vs the silver shell thickness. The figure also contains the theoretical mean free path as calculated from eq 2. It should be mentioned that a variation in themean free pathdid not change thewavelength ofthemaximum by more than f 3 nm. It can be seen that the "experimental" value of the MFP is much smaller than the "theoretical" one. Imperfections in the silver layer are possibly responsible for the small MFP: the silver initially will be forced to conform to the lattice spacing of the Pd surface, which will cause strain in the silver layers and lead to much greater electron scattering. Since the Pd surface is polycrystalline, the silver layer will also build up with different structures over the Pd. As the different regions fuse, a large number of grain boundaries will be necessary, and these will limit the electron mobility. A contact potential difference must build up a t the silver-palladium interface, which will alter the dielectric properties of the first few monolayers of silver, and this may also contribute to a smaller value of the MFP. The increase in absorption upon aging (Figure 4) is explained by an increase in the MFP as the silver shell relaxes. Chemisorption of SH-and I-. The plasmon band of silver particles is damped by adsorbed anions such as SH- and P . 4 It seems that strong damping occurs only in the case of absorption bands which are caused by surface plasmons, whereas absorption due to interband transitions is not affected by adsorbed substances. Thus, the plasmon character of an absorption band might be tested by damping experiments. The results are shown in Figures
Composite Pd-Ag Particles in Aqueous Solution
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6215 0.3
2.5
1
I
2.0
2 1.5 m
5 n
t
1.0
I
0.5 -
0 250
0 200
400 500 A lnml Figure 8. Absorption spectrum of a Pd-Ag colloid before and after additionof various amountsofNaSH. The SH-conccntrationsaregiven in micromolar units on the curves. The particles were coated with a thick silver shell, m = 19.5.
300
I
350 400 450 A Inml Figure 10. Absorption spectrum of a Pd-Ag colloid before and after the addition of various amounts of SH-. The particles were coated with a very thin silver layer (m = 4). Upon addition of more than 5 pM SH-, no additional changes in the absorption band occurred. 300
of SH-on pure silver particles. The reader is asked to look up Figure 1 in ref 4a and to compare it with Figures 8 and 9. It can be seen that the damping of the plasmon band of pure silver particles is more pronounced. This may be due to the damping of the plasmons by the Pd core, but it is more probable that the effect of chemisorption is much smaller since the electron mean free path is very small in the silver shell. It might finally be mentioned that a few damping experiments with iodideanions were also carried out. The compositeparticles of Figures 9 and 10 were used. In both cases, the band decreased and was broadened. In the case of the thinnest silver layer ( m * 4). the maximum of the band was shifted slightly to shorter and, in the case of the thicker layer ( m = ll), to longer wavelengths. In the case of pure silver particles (Figure 2 in ref 4a), the shift also occurred to slightly longer wavelengths. These details of the change in band shape are not yet understood. A mechanism for damping has recently been proposed, according to which the adsorbed substancesinduce resonance states or virtual levels above the Fermi level in the colloidal particles.!'
:
c Y L 0200 300 400 500 O'
A Inml Figure 9. Absorption spectrum of a Pd-Ag colloid before and after the
addition of various amounts of NaSH. The SH-concentrations are given in micromolar units on the curves. The particles were coated with 11 monolayers of Ag. 8-10 for Pd particles carrying silver layers of different thicknesses. The spectral changes upon the addition of various amounts of NaSH can be seen in these figures. The band around 380 nm of the particles which are strongly coated by silver (Figures 8 and 9) decreases rapidly with increasing SH- concentration in the micromolar range. It is also broadened and slightly shifted to shorter wavelengths. At higher SH-concentrations (above about 20 pM in Figure 8, and above about 7 pM in Figure 9), the changes upon further SH- addition are small and, at the same time, the charge-transfer-to-solvent (CTTS) band of free SHdevelops at 230 nm. These results are interpreted as follows: at small concentrations, practically all SH- ions are chemisorbed on the composite particles. T h e adsorbed anions do not have the CTTS absorption band. At higher concentrations, when a monolayer of adsorbed SH- has already been formed, the additional SH- ions remain free in solution. In the experiment of Figure 10, the Pd particles were much less coated with silver ( m = 4) than in Figures 8 and 9. One can see that damping still occurs, although the band is positioned far away from the usual 380-nm position of the plasmon band of silver. It is therefore concluded that the band at 347 nm also originates from a typical surfaceplasmon oscillation. Theseresults may be compared with the results obtained in the chemisorption
Acknowledgment. The authors thank Dr. Recknagel for carrying out the AES analysis and Mrs. U. Bloeck for preparing the electron micrographs. P.M. gratefully acknowledges the receipt of a QEII Research Fellowship. References and Notes (1) (a) Toshima, N.; Takahashi, T.; Hirai, H. J. J. Macromol. Sci., Chem. 1988, A 25, 669. (b) Tmhima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448. (c) Toshima, N.;Harada, M.; Yamazaki, Y.; Asakura, K. J. J . Phys. Chem. 1992,96,9927. (d). Liu, H.; Mao, G.;Meng. S . J. Mol. Catal. 1992, 74,275. (e) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torigoe, K.; Meguro, K. bngmuir 1991,7,457. (9 Marignier, J. L.; Belloni, J.; Delcourt, M. 0.;Chevalier, J. P. Nature 1985,317,344. (2) (a) Henglein, A,; Mulvaney, P.; Linnert, T.; Holzwarth, A. J . Phys. Chem. 199296,241 1. (b) Hmg1ein.A.; Mulvaney,P.; Holzwarth, A.;Sosebce, T. E.; Fojtik. A. Ber. Bunsen-Ges. Phys. Chem. 1992,96,754. (c) Mulvaney, P.; Giersig, M.; Henglein, A. J . Phys. Chem. 1993,97, 7061. (d) Henglein, F.; Henglein, A.; Mulvaney, P. Ber. Bunsen-Ges.Phys. Chem. 1994,98,180. (3) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1992, 96,
10419. (4) (a) Linnert, T.; Mulvaney, P.; Henglein, A. J . Phys. Chem. 1993,97, 679. (b) Henglein, A. J . Phys. Chem. 1993, 97, 5457. (5) Turkevich, J.; Kim, G. Science 1970, 169, 873. (6) (a) Weaver, J. H.; Benbow, R. L. Phys. Reu. B. 1975,12,3509. (b)
Weaver, J. H.; K r a h , C.; Lynch, D. W.; Koch, E. E. Optical Properties of Metals; PhysiceData Scrim 18-2; Fachmformationszentrum Karlsruhe,198 1. (7) Johnston, P. B.;Christy, R. W. Phys. Rev. 1972, 39, 63. (8) Aden, A. L.; Kerker, M. J . Appl. Phys. 1951, 22, 1242. (9) Bohren, C.; Huffmann, D. R.Absorption and Scattering of Light by Small Particles; Wiley, New York, 1983. (IO) Granqvist, C. 0.;Hunderi, 0. 2.Phys. B. 1978, 30, 47. (1 1) Persson, B. N. J. Surf. Sci. 1993, 281, 153.
