7061
J . Phys. Chem. 1993,97, 7061-7064
Electrochemistry of Multilayer Colloids: Preparation and Absorption Spectrum of Gold-Coated Silver Particles Paul Mulvaney, Michael Giersig, and Arnim Henglein’ Hahn- Meitner-Institut Berlin, Abteilung Photochemie, 1000 Berlin 39, Germany Received: March 23, 1993
Gold was deposited onto colloidal silver (mean particle size 76 A) in aqueous solution, and the composite particles were studied by spectrophotometry and electron microscopy. The deposition took place via electron transfer onto the silver particles by radiolytically generated free radicals and subsequent reduction of dissolved Au(CN)2- by the stored electrons. The method allows one to control the coating thickness. The optical spectra were also calculated using Mie theory and the optical constants of the pure metals. It was found that one monolayer of gold should be sufficient to mask the silver resonance band completely and that the gold plasmon band should develop more quickly than observed. The differences between the calculated and observed spectra are explained by possible alloying of the surface layer.
Introduction
About 12 years ago, a radiolytic method was developed which enabled one to deposit excess electrons in a controlled manner onto colloidal metal particles in aqueous solution.’ The excess electrons could be used to initiate electrochemical reactions on the “microelectrodes” such as the reduction of metal ions.2 During the past few years, it was recognized that the properties of metal adatoms can be studied and that layers of variable thickness of a second metal can be prepared.3 More recently, the first preparation of concentric trimetallic particles (gold nucleus, lead mantle, and cadmium peripheral layer) has been r e p ~ r t e d . ~ In the present work, the optical changes which occur during the deposition of gold onto silver particles are described. Of particular interest are the interaction of the initially deposited gold atoms with the underlying silver lattice, how they perturb the optical properties of the silver particles, and how the transition of the surface deposit from an array of adatoms to a bulk gold mantle takes place. The optical changes are interpreted using the extended Mie theory for coated particles which was originally developed by Aden and Kerkere5 In the calculations of the optical spectrum of the bimetallic colloids, the bulk optical constants of gold are used for the surface phase. Any differences between the calculated and experimental spectra are then interpreted in terms of chemical interactions between the gold and silver phases. It should be emphasized that the radiolytic method employed here leads to a well-defined kinetic pathway for the reduction of the gold complex. Free reducing radicals, (CH&COH, are produced in the silver sol6 (which also contains K(Au(CN)z), acetone, and 2-propanol), and these radicals transfer electrons to the gold particles at a diffusion-controlled rate; the deposited electrons then reduce the gold(1) in the cyano complex. As is shown, the diffusion-controlled accumulation of electrons on the colloidal particles led to the formation of almost monodisperse gold-coated particles, although the initial size distribution of the silver particles was rather wide.
nm. Deaerated KAu(CN)2 solution was then added to the sol and the solution reirradiated. The rate of reduction of Au(CN)2- could be followed by monitoring the rate of disappearance of the 238-nm absorption of the cyanoaurate anion (2850 M-l cm-l) or by measuring the conductivityof the solution. The samples were irradiated in glass vessels equipped with either a quartz cuvette for optical measurements or a side arm with two carbon electrodes when monitoring conductivity changes. The solutions were deaerated by evacuating the vessels or bubbling the solutions with pure argon. Samples for electron microscopy were prepared in a nitrogenfilled glovebox (maximum 0 2 pressure 0.2 ppm). A drop of the colloidal solution was placed onto a carbon-coated copper mesh grid and allowed to dry. The grid was then transferred in an air-free container into the Phillips CM 12 microscope, which was equipped with a 9800 EDAX analyzer. Several grids were prepared from each colloid to ensure that the procedure yielded reproducible samples for analysis. Results
Figure 1 shows the absorption spectrum of a 5 X M silver sol before and after the addition of KAu(CN)*. The plasmon band of silver is red-shifted and damped after the addition. The spectra observed after the solution had been y-irradiated for various lengths of time are also shown. The silver band almost disappears after a few minutes, when only a very small amount of gold(1) has been reduced, and a tail develops at longer wavelengths. After 30 min, a peak appears below 500 nm which then becomes more pronounced and red-shifted as the irradiation is continued. When all the gold has been reduced, the initially yellow solution is orange-red and perfectly transparent. Figure 2 shows that the conductivity of the solution decreases as the gold is reduced. During the reduction, hydrogen ions are consumed. The net reaction is (CH,),COH
Experimental Section
+ H+ + Au(CN);
-
(CH,),CO
The silver sols were prepared by y-irradiation of deaerated solutions containing 50 or 100 p M AgC104, 0.1 M 2-propanol, 0.01 M acetone, and 0.1 mM sodium polyphosphate, (NaPOs),, as stabilizer. In some experiments where conductivity changes were recorded, sodium poly(viny1 sulfate) was employed since this does not buffer the solution. The irradiation produced a silver sol with an absorption coefficient at the band maximum of 1.9-2.0 X 104 M-1 cm-1. The band maximum was close to 380 0022-365419312097-7061$04.00/0
+ Au + 2 H C N
(1)
(The specific rate of the direct reaction of the organic radicals with the complex in solution is very low.’ The reaction is therefore catalyzed on the surface of the silver particles as described above). The accompanying change in conductivity is AK = -(A(H+)
+ A(Au(CN),-))AC,,
(2) where A is the molar conductivity and AC (mol cm-’) the 0 1993 American Chemical Society
Mulvaney et al.
.
7062 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
520
\
Au plasmon band
- 480
(u
2 1.0
E
m
I
n
3440
c 0
x
v)
n
0.5
400
360 /j 300
400
500
600
A Inml Figure 1. Absorption spectrum of a sol before (a) and after (b) addition of4 X 1 V M KAu(CN)zandaftervariouslengthsoftimeofy-irradiation. Solution: 5 X 10-5 M silver colloid, 0.1 M 2-propano1,O.Ol M acetone, M polyphosphate;pH = 5 . The rateof free radical formation and 3 X was 1 X 1 P M min-1 at the applied dose rate of 9 X lo4 rad/h.
I
I
1
I
100
200 300 IAul IpMl
400
Figure 3. Wavelength of the silver-gold plasmon band as a function of the concentration of deposited gold for two concentrationsof the silver sol.
-
500
E
5 450 X 0 x
400
350
-200
t
100 t tminl Figure 2. Change in conductivity as a function of irradiation time for two Au(CN)z- concentrations. Initial pH of the solution: 3.3. The solution contained poly(viny1 sulfate) as stabilizer.
I
0
0.2
I
I
I
0.4
0.6
0.8
I
1.0
mole fraction Au Figure 4. Wavelength of the silver-gold plasmon band as a function of
the mole fraction of gold for two silver concentrations.
0
concentration of gold produced. The molar conductivities used were A(H+) = 350 and and A(Au(CN)2-) = 53 S cm2 mol-'. When all the gold is reduced, a final value of the conductivity decrease is reached. The final values for the two gold concentrations in Figure 2 (-84 and -170 S cm2 mol-I) are in good agreement with the theoreticalones(-80.4and-161 Scm*mol-l). From the slope of the curve in Figure 2,a yield of 3.6 Au atoms per 100 eV of absorbed radiation energy is calculated. This is somewhat less than the theoretical value of 4.4/100eV and is probably due to a loss of radicals because of radical-radical combination before they reach the silver particles. In Figure 3, the position of the plasmon absorption band is shown as a function of the concentration of deposited gold for 50 and 100 pM silver sols. As the band is only a weak inflection a t small deposits (Figure l ) , the exact position is difficult to determine. It then develops into a true maximum, shifting steadily to longer wavelengths. However, even for a gold to silver ratio of 8: 1, the band only reaches 500 nm and is still a long way from the value of 520 nm for pure colloidal gold. This is in strong contrast to the situation found with Pb- or Cd-coated Ag sols, where even at a ratio of 1:1 the plasmon band of the surface metal was seen at the wavelength where colloids of the pure shell metal absorb.2 In Figure 4, the wavelength is plottedvs the mole fraction of Au for two silver concentrations. The data fall onto a common line. In Figure 5 , electron micrographs of the coated sols are shown, taken at a constant magnification of 120 000. The particles remain well separated even on the copper grids. The silver sol (left) has a broad size distribution with a mean particle diameter of 76 A. The silver particles arecrystalline, and the characteristic
lattice plane spacing of 2.36 A is readily seen a t higher magnifications. The middle and the right-hand micrographs of Figure 5 show Ag colloids with two gold deposits. The average particle size is clearly larger, but most remarkably, the particles become more monodisperse. The size distributions shown in Figure 5 are based on the measurement of 250 particles in each case. The mantle of the gold-coated particles had a lattice plane spacing of 2.35 A which corresponds to the gold( 111) reflex. This is clearly seen in the high-resolution micrograph in Figure 6. The EDAX spectra of the three sols in Figure 5 are shown in Figure 7. Peak integration and normalization were carried out to measure the ratio of Au to Ag in the individual particles, or data were collected over larger areas to measure the average ratio in the sol. The results were always the same to within 5%. Furthermore, the EDAX spectra of those parts of the grids where no particles were present showed no gold and silver signals. These results prove that there was no nucleation of colloidal gold in the bulk solution. The particle sizes derived from the EDAX analysis and from the composition of the solutions are in good agreement, as can be seen from Table I. Electron diffraction showed that both the silver colloids and the gold-coated silver particles possessed the face-centered cubic structure.
Discussion Au(CN)2- is not reduced directly by organic radicals? although the standard redox potential of the Au(CN)z-/Aubyk system is more positive (-0.6 V) than that of the (CH3)zCO + H+/(CH&COH system (-1.5 V).* The reason is that the gold atom would be formed free in solution. Taking into account the fact that the free enthalpy of sublimation of gold is 3.2 eV, the standard potential of the system Au(CN)z-/AuO (AuO: free gold atom) is seen to be -3.2-0.6 = -3.8 V. Thus, the reduction by the organic radicals would be endoergic by 2.3 eV. In the presenceof colloidal
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 7063
Electrochemistry of Multilayer Colloids
1- ,
- .."a-
~
,-
.-
I
Particle Diameter
(A)
Figure 5. Electron micrographs and particle size distributions of the silver sol (left) and two silver-gold sols (middle and right). (The bar in the third micrograph represents 150 A.)
TABLE I Au:Ag particles (EDAX analysis) 0
sol 0
2.0 4.0 a
2.0 5.1
diameter (A) e.m. calcd
75*25 96i 1 1 145f22
Taking R A =~ 1.6 A and m = ( R t a l -
no. of monolayer9 (m)
108
-
127
10.9
3.3
&)/2RAu.
of the coated sol in comparison with that of the pure silver sol. Obviously, the smaller silver particles grow faster than the larger particles. The rate at which the volume of a particle increases is proportional to the number of radicals scavenged per unit time: Figure 6. High-resolution micrograph of a gold-coated silver particle showing lattice planes (2.35A).
d V / d t = 4 d ( d r / d t ) = k(r) [R] V,,,
(3)
where [R]is the stationary radical concentration and V, the molar volume of the metal being deposited. The electron-transfer reaction between a radical and a silver particle may in principle be controlled by activation or by diffusion. We regard first the case of activation control. The rate constant k(r) will be proportional to the surface area. Writing the proportionality factor as a,one obtains k(r) = 4 d a Combining eqs 3 and 4 yields
0
5
10 15 ENERGY IKeV1
20
25
Figure 7. EDAX spectra of the three sols of Figure 5. Copper signals are due to the grid.
silver, the organic radicals transfer electrons at a diffusioncontrolled rate to the metal particles. The stored electrons can reduce Au(CN)*- to yield Au atoms, the reaction becoming exoergic due to the binding energy of the Au atoms to the silver surface. Particularly remarkableis the narrowing of the sizedistribution
(4)
d r / d t = a[R]Vm (5) The increase in radius of a particle is independent of its size, which means that there will not be preferential growth of the smaller particles. On the other hand, in the case of a diffusioncontrolled reaction, the rate is given by Smoluchowski's equation: k ( r ) = 47rrD (6) where D is the diffusion coefficient of the radical. Thus, one obtains d r / d t = ( D / r )[R] Vm
(7)
i.e., the particle radius increases more slowly for larger particles; the metal deposition will now lead to a narrowing of the size distribution.
Mulvaney et al.
7064 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
01
1.0
U
c m n
L
0
VI
n
m
n"
200
300
400
500
600
X [nml Figure 8. Theoretical s ectra of gold-coated silver colloids. Radius of the silver particles: 30 Total silver concentration: 1 X 10-4 M.The
1.
opticalconstants were taken from Johnston and Christy.Io The dielectric constants were corrected for the effect of the mean free path using the method proposed by Kreibig."
The case of diffusion-controlled electron accumulation on the colloidal particles is fulfilled for the highly reactiveorganic radicals generated by radiation, as has been shown previously for silverlb and gold.1c The extent to which a particle grows depends on the rate at which it picks up electrons from the radicals and not on the rate of the subsequent gold reduction. There might be other cases, such as colloid-catalyzed chemical reduction of metal ions, where an activation-controlled step is present; under these circumstances, the narrowing of the size distribution would not be expected. Figure 8 shows calculated spectra of silver particles coated with various thicknesses of gold. The scattering coefficients, a, and bn, for coated particles were originally derived by Aden and Kerker.5 We calculated them using the equations in the form given by Bohren and H ~ f f m a n n .These ~ authors also include a detailed progam for computing a, and b,. For the small particles used here, no more than five iterations were generally required to generate the absorption coefficients a t each wavelength. The optical constants for silver and gold were taken from Johnston and Christy."-' In the case of silver, the dielectric constants were corrected for the reduction in mean free path using the procedure outlined by Kreibig.11 The dielectric constants in the region around the surface plasmon absorption band obey €1
= t, - opZ/(w2
= (dp20d/(d(w2
+ w2) +w l )
(8)
(9) where t- is the value of €1 at infinite frequency, upis the bulk plasma frequency (1.4 X 10l6s-l), and wd is the rate of damping in the bulk metal (2.7 X 1013 s-l). For particles smaller than the mean free path in the bulk metal (520 A), the plasma oscillations are additionally damped by diffuse surface scattering. The damping rate, Wd, becomes size dependent: €2
where uf is the Fermi velocity (1.4 X 106 m s-1) and r is the particle radius. We took r = 30 A, which was the average radius found by electron microscopy. This yielded a maximum absorption coefficient of 1.6 X lo4 M-' cm-1, only a little less than the observed value of 1.9 X lo4 M-1 cm-1. As can be seen from the calculated spectra in Figure 8, the surface plasmon band is rapidly damped by the presence of even
a monolayer of gold (radius of a gold atom: 1.6 A), but the silver band remains at almost the same wavelength, simply becoming broader as the coating thickness increases. The gold band is also seen at an early stage of coating as a slight buckle a t 490 nm, which becomes more pronounced as the coating thickness reaches five monolayers. The spectrum of Ag particles coated with 30 A Au looks very similar to that of pure gold, although the dip around 300 nm characteristic of silver is still present. Although the observed spectra in Figure 1 show similarities to the theoretical spectra in Figure 8, important differences remain. In theobserved spectra, the silver band is damped and red-shifted much faster than predicted by the Mie calculations. The gold interband absorption grows steadily for a while without showing any signs of plasmon absorption, and the plasmon band is finally seen at much shorter wavelengths than is predicted. It also shifts much more slowly to longer wavelengths than calculated. The differences are attributed to the chemical interaction of the deposited gold with the silver particles. It has been suggested that electrodeposition of gold onto silver electrodes leads rapidly to alloy formation.12 Silver and gold are in fact miscible in all proportions due to the almost identical lattice constants.13 Consequently, although a lattice plane spacing of 2.35 A is clearly seen in the high-resolution micrograph (Figure 6), this may be attributed to either a pure gold mantle or to an alloy of Ag and Au. The unit-cell size of Ag-Au alloys changes by less than 1% over the entire range from pure Ag to pure Au, and no superlattice reflexes areobserved in thealloys.13 Thus, theelectrondiffraction pattern obtained from individual colloid particles does not allow one to differentiate between these two possibilities. The fact that two plasmon bands are not seen in silver-gold particles whereas in the cases investigated previously (Cd on Ag, Pb on Ag, and In on Ag2) two bands could be seen before the shell material dominated the spectrum suggests that the optical spectrum is affected by alloy formation. References and Notes (1) (a) Henglein, A. J . Phys. Chem. 1979,83,2209. (b) Henglein, A.; Lilie, J. J . Am. Chem. Soc. 1981,103, 1059. (c) Meisel, D.J . Am. Chem. Soc. 1979,101,6133.(c) Westerhausen, J.;Henglein,A.;Lilie, J.Ber.BunsenGes. Phys. Chem. 1981,85,182. (2) (a) Henglein, A. J . Phys. Chem. 1979,83,2858. (b) Henglein, A. J . Phys. Chem. 1980,84,3461. (c) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1980,84,253.(d) Buxton, G.V.; Rhodes, T.; Sellers, R. M. J . Chem. Soc., Faraday Trans. 11982,78,3341. (3) (a) Henglein, A.; Mulvaney, P.; Linnert, T.; Holzwarth, A. J . Phys. Chem. 1992,96,2411.(b)Henglein, A.; Mulvaney,P.;Holzwarth, A.;Sosebee, T. E.; Fojtik, A. Ber. Bunsen-Ges. Phys. Chem. 1992,96,754. (4) Mulvaney, P.; Giersig, M.; Henglein, A. J . Phys. Chem. 1992,96, 10 419. (5) Aden, A. L.; Kerker, M. J. Appl. Phys. 1951,22, 1242. (6) Theorganicradicalsare formedvia the followingelementaryreactions that are initiated by the primary radiolysis products (hydrated electrons, hydroxyl radicals, and H atoms) from water:
e,,
+ (CH,),CO + H++(CH,),COH
OH(H) + (CH,),CHOH
-
(CH,),COH
+ H,O(H,)
Thetotalradiationchemicalyieldofradicalsis6.0/100eVofabsorbedradiation energy. However,as0.8 moleculeofH202isalsogenerated,whichcanreoxidize Au atoms, the effective yield of reducing equivalents under steady-state conditions is 4.4/100eV. (7) Mosseri, S.; Henglein, A.; Janata, E. J . Phys. Chem. 1989,93,6791. (8) (a) Butler, J.; Henglein, A. Radial. Phys. Chem. 1980,25,603.(b) Schwarz, H. A.; Dodson, R. W. J . Phys. Chem. 1989,93,409. (9) Bohren, C.; Huffman, D.R.Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983;p 183. (10) Johnston, P. B.; Christy, R. W. Phys. Reu. 1972,86,4370. (11) Kreibig, U.Z . Phys. 1970,234, 307. (12) Schmidt, E.;Stucki, S . J. Electroanal. Chem. 1972,39, 63. (13) LeBlanc, M.;Erler, W. Ann. Phys. 1933,Z6, 321.
