J . Phys. Chem. 1992,96, 10419-10424
10419
Surface Chemistry of Colloidal Gold: Deposition of Lead and Accompanying Optical Effects Paul Mulvaney, Michael Giersig, and Arnim Henglein' Abteilung Photochemie, Hahn-Meitner-Institut Berlin, IO00 Berlin 39, FRG (Received: July 2, 1992; In Final Form: September I , 1992)
Lead ions were reduced on the surface of colloidal gold particles (190 A in diameter) and the resulting bimetallic particles were investigated using spectrophotometryand electron microscopy. Deposition of lead adatoms causes the plasmon absorption band of gold to be shifted to shorter wavelengths, which is explained by Pb Au electron donation and double layer charging. Three monolayers of lead are sufficient to produce the plasmon band of lead. Mie theory calculations of the optical spectrum (using the bulk optical constants of Au and Pb) yielded excellent agreement with the observed spectra for lead mantles thicker than 1 monolayer. The deposited lead is oxidized by oxygen, except for the first monolayer, which is produced by underpotential deposition. Similarly, lead atoms in the first layer are not oxidized by methyl viologen, whereas the atoms in the subsequent layers readily react. Experiments on the preparation of trimetallic particles (gold nucleus, lead layer, and outer cadmium layer) are also reported.
-.
Introductioa Surface chemical studies on colloidal metal particles have mainly been limited to silver particles in aqueous solution.' Chemisorbed substances cause changes in the electronicproperties of the metal particles, and these changes lead to increased chemical reactivity of the particles and to changes in their optid properties. Silver is especially suitable for such studies because its intense plasmon absorption band at 380 nm (in water) is very sensitive to changes in the electronic structure of the particles. In the present paper, similar experiments with gold particles are described. An advantage of the gold colloids is that they can be prepared with a much narrower size distribution than the silver sols used in the previous studies. Colloidal gold has an absorption band around 520 nm which is caused by a surface oscillation of the electron gas. Unlike silver,interband transitions also contribute to the absorption in this wavelength region. Nevertheless, the plasmon band is also found to change its shape and position when the surface is chemically modified. We report here on the effects of chemisorbed lead atoms on the optical properties of colloidal gold. Such atoms can be deposited on the gold particles by the reduction of adsorbed W+ions, using a radiolytic method: organic free radicals, generated by y-irradiation, diffuse to the gold particles and transfer electrons to them. A colloidal particle can store a great number of electrons. The accumulated electrons are used to reduce Pb2+ ions on the surface of the particles. This method, which allows one to carry out electrochemical experiments with "microelectrodes" of nanometer dimensions in a reproducible and controlled manner, has previously been described in detail.* It is also shown that a layer of cadmium can be deposited on a gold-lead particle using this radiolytic method.
Experimental Section Colloidal gold was prepared by the method of Turkevich et aL3 using citrate as the reducing agent. Typically, 36 mg of KAuCI, was added to 285 mL of water, and the solution heated to boiling point. A 15-mL aliquot of 1% sodium citrate solution (80 "C) was added rapidly to the stirred gold solution. The yellow color of the AuC1,- anion disappeared immediately, and after about 1 min the solution slowly became violet and then deepened to a wine-red color. The solution was boiled for a further 20 min and then allowed to cool. The mean particle diameter was 180 A (varying about 10% from batch to batch). The sols contained predominantly round, crystalline particles with a small fraction of hexagonally-shaped crystals, especially when the solution was allowed to boil for more than 30 min. The absorption spectrum showed a maximum at 519.5 nm with an absorption coefficient of 3910 M-'cm-I. The solutions were stable for about 2 weeks in the absence of a stabilizer. Addition of poly(acry1ic acid) or
poly(viny1 sulfate) to the sols rendered them indefinitely stable against coagulation. In all experiments, the sols were used within a week of preparation. In order to coat the particles with a second metal, poly(viny1 sulfate) (PVS) was first added to the sol together with acetone and 2-propanol. The sol was then stirred for 5-10 min to ensure complete contact with the polymer, and then the second metal was added as the perchlorate salt. If no stabilizer was present, the sol coagulated upon addition of the metal ions. (The gold sols are stabilized by free citrate ions, which are readily neutralized by the heavy metal ions). In the presence of the stabilizer the absorption band of the gold shifted by at most 1 nm to longer wavelengths when the second metal was added. The solutions were deaerated by evacuation or bubbling with pure argon and then exposed to the y-rays of a 'Wo-source. In the presence of the dissolved acetone and 2-propanol radiolysis generates reducing 2-hydroxypropyl radicals, (CH,),COH. The rate of free radical production was 9 X 1od M m i d at the applied dose rate of 8.7 X 104 rad/h. Because of their low specific rate of reaction with Pb2+,4these radicals cannot react directly with the lead ions at the low concentrations usad; instead they diffuse to the gold particles and polarize them cathodically by transferring electrons to them. The lead ions are then reduced by the accumulated electrons on the surface of the gold particles. For preparation of samples for electron microscopy, a drop of the metallic sol was placed onto a carbon coated aluminum grid and allowed to dry. The dried grids were then transferred in a nitrogen filled container to a Phillips CM 12 microtape, equipped with an EDAX 9800 analyzer. The sample preparation was carried out in a nitrogen filled glovebox (maximum oxygen pressure 0.2 ppm) to prevent air oxidation of the samples. A number of grids were prepared from each sample in order to check the reproducibility of the preparative procedure and to ensure that no oxidation of the coated particles had taken place. Bright-field images were taken under ConditioIlSof minimum phase contrast.5 Quantitative EDAX analysis of individual metal particles was carried out with the dectron microscope in nanoprobe mode [beam spot size reducible down to 20 A] using a spot size equal to the particle diameter. RWultS
In Figure 1,the spectral changes are shown to the plasmon band of gold when a 1.25 X 10-4 M gold sol is irradiated in the presmce of 1 X 10-4M lead ions. The band is shifted to shorter wavelength with increasing irradiation time and decreases in intensity. At longer wavelengths, the intensity increases. After 20 min, the plasmon band is located at 490 nm, and, after 40 min, it disappears. Upon continued irradiation, the UV absorption increases;
0022-365419212096-10419S03.00lO Ca 1992 American Chemical Societv
10420 The Journal of Physical Chemistry, Vol. 94, No. 25, 1992
E
E 0.4 Q) U
c m
f
x
0.2
500
600 X
J 700
I
I
I
O'
600
lnml
Figure 1. Spectrum of an evacuated 1.25 X lo4 M Au sol containing 1 X 10-4 M Pb2+,0.1 M acetone, 0.1 M Zpropanol, and 2 X 10-4 M PVS after various times of ?-irradiation. Dose rate: 8.7 X 10" rad/h.
2.0
-5 c Q)
z 1.0
U
n
0 L v)
n
m
O200
400 X
600
lnml
Figure 2. Spectrum of a 9 X IV5M Au sol containing 1 X 10-4 M Pb2+ before and after complete reduction. Other concentrations as in Figure 1.
Mulvaney et al. Figure 2 shows the spectrum of a 9 X lW5 M gold sol before and after complete reduction of the lead ions. After complete reduction, a rather sharp band at 225 nm can be seen, with a long tail extending across the whole visible part of the spectrum. The solution becomes brown but remains completely transparent. The lead ions normally absorb at 210 nm. Their absorption band in spectrum 0 in Figure 2 lies at 240 nm; this is due to the presence of the PVS stabilizer, which complexes Pb2+. The band at 225 nm formed after complete Pb2+reduction is attributed to the surface plasmon absorption of colloidal Pb. Pure lead colloids have recently been prepared," and the plasmon band is found to be at 215 nm. In Figure 3, electron micrographs of the colloidal particles are shown before (a) and after (b-d) reduction of various amounts of lead ions. The mean particle diameter of the sol used was 190 A (Figure 3a). All micrographs were taken at the same magnification. An increasing ratio of Pb to Au was achieved by adding various amounts of lead perchlorate to the solution and irradiating until complete reduction had taken place. The resulting bilayer particles remained well separated after the deposition. High resolution electron microscopy was used to monitor the structural changes that took place. The results are summarized in Figure 4. The typical lattice plane spacing of 2.35 A (hkl 1 1 1) for cubic gold can be seen in Figure 4a. The subsequent deposition of different amounts of Pb (Figure 4b-d) leads to the appearance of new lattice planes with a spacing of 2.86 A (hkl 1 1 1) characteristic of Pb. It appears that the lead is built up patchwise on the gold surface (as is evident from the lower left and upper right surfaces of the particles shown in Figure 4b), until the whole gold surface is covered by lead (Figure 4c,d). The composition of the bimetallic particles was confirmed by using energy dispersive X-ray analysis (EDAX) to examine individual particles of the colloid samples shown in Figure 4. A typical EDAX spectrum is presented in Figure 5. The aluminum signal in the spectrum is due to the grid. As the amount of lead
Figure 3. Electron micrographs: (a) colloidal Au (0.25 mM); (H) after reduction of 0.1 mM, 0.2 mM, and 0.5 mM lead.
Surface Chemistry of Colloidal Gold
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10421
-
515
E.x 505 495
485
I
I
1
10
20
30
t lminl
F i p 6. Position of the surface plasmon band of Au vs irradiation time for various gold concentrations. 1 X lo-' M Pb was always deposited.
rigure 4. nign resolution micrographs snowing individual particles or the sols in Figure 3.
-
'0
12Pb Monolayers
10
20
30
40
50
60
t Iminl
Figure 7. Yield of oxidized lead (as determined by the MV2+ method) as a function of irradiation time. Au concentration: 2.1 X 10-4 M. Pb2+ concentration: 1 X lo4 M. Other conditions as in Figure 1.
5
10 Energy IKeVl
15
Figure 5. Typical EDAX spectrum of a gold-lead colloid. Au to Pb ratio: 2:l. TABLE I: Bilryer Colloids of PbAu. Comparison of Experimentnl and Theoretical Values of tbe Particle Size and Composition radius (A) Pb/Au Pb/Au in particles electron no. of in sol (EDAX analysis) microscopy calc monolayers 0
0.4 0.8 2.0
0 0.29
95 f 5
1 .oo 1.85
108 f 15 118 f 15 144i 15
114
3.2
127 152
5.8 12.4
deposited increases, there is a proportionate increase in the Pb signal. The quantitative results of the analysis are shown in Table I. The expected particle size of the colloids can be calculated using where V,,,is the molar volume. The results are also tabulated in Table I and can be compared to the values from electron microscopy. The difference is less than lo%, which is within the experimental error, and further confirms that the lead is evenly and homogeneously deposited exclusively on the gold particles. The actual number of monolayers, m,deposited on the gold particles is also shown in Table I. It was approximated using the formula
= (rtotal - rAuan)/2rPb where the radius of a Pb atom is taken to be
(2)
r, = ( 3 V m / 4 ~ ) 1 / 3
(3)
In Figure 6, the position of the plasmon band is shown as a function of the irradiation time (i.e. the amount of deposited lead) for various gold concentrations in the solution. As can be seen, the band is shifted much more for a smaller gold concentration (the rate of Pb formation being the same for all curves). This is consistent with the fact that the gold is homogeneously coated. The extent of the plasmon band shift depends on the Au/Pb ratio, not on the absolute amount of Pb formed. To determine the amount of reduced lead, the methyl viologen method, which was described previously,'" was used. The irradiated sols were made alkaline (pH 12) and MV2+ (2 X lW3 M) was then added under the exclusion of air. The lead is oxidized according to Pb + 2MV2+ + 30H-
-
Pb(OH)
