J. Phys. Chem. 1995,99, 15120-15128
15120
Stable Hydrosols of Metallic and Bimetallic Nanoparticles Immobilized on Imogolite Fibers Luis M. Liz-MarzBn*J and Albert P. Philipse Van? Hoff Laboratory for Physical and Colloid Chemistry, University of Utrecht, Padualaan 8, Postbus 80.051, 3508 TB Utrecht, The Netherlands Received: December I , 1994; In Final Form: March 24, 1995@
It is shown that reduction of metal salts in aqueous, optically transparent host dispersions of inorganic (imogolite) fibers provides a general and convenient preparation technique for stable dispersions of metallic nanoparticles. Stable hydrosols are obtained for gold and silver particles and various bimetallic colloids (AdAg, A u P t , and A m ) . Colloidal stability is achieved by adsorption of particles on fibers; no stabilizing polymer is required. The optical properties of the obtained colloids are studied and compared with theoretical predictions according to Mie theory. This shows that the bimetallic particles consist of layers of the metals, rather than a mixture at the atomic level, except in the case of A d A g .
Introduction The synthesis of metallic colloids in the nanometer size range (“nanoparticles”) was already performed by Faraday,2 who studied for the first time gold sols in a systematic way. Nowadays, this type of synthesis is still raising a great deal of interest, due to the very small dimensions that can be achieved, which give properties to the particles that are very different from those of bulk metals. Many reports have been published3s4about the special electronic properties of metallic nanoparticles, which are mainly due to a decrease in the density of states in the valence band and the conductivity band when the particle size is decreased. Particles which exhibit such “quantum effects” represent a transition state between the quasi-continuous density of states of the bulk metal and the discrete energy level structure characteristic of atoms and molecules. The surface chemistry of such metallic particles changes with the adsorption of ions5 or other metals.6 A variety of preparation methods have been proposed, such as photored~ction,~ chemical reduction in aqueous medium with7 or stabilizing polymers, chemical reduction in reverse or thermal decomposition in organic solvents.I2 The prepared colloids are often deposited on support^'^ or used for the preparation of films.I4 In a previous paper,’ we reported on the synthesis of platinum nanoparticles in an aqueous dispersion of imogolite. Imogolite is an aluminosilicate with net composition (H0)3Al203SiOH which consists of hollow tubes with an external diameter of about 2 nm and a length in the range 400- IO00 nm. The tubes contain curved gibbsite sheets with silicate groups replacing hydroxyl groups on the inner surface. AlOH groups are located on the outer surface. Imogolite is found in volcanic ash and other soil^'^-'^ and was first prepared by Farmer et a1.I8 Imogolite tubes have aspect ratios which are extremely large for an inorganic colloid. A stable imogolite dispersion can therefore be envisaged as a very mobile network of long fibers which virtually continuously interact, even at low number densities. Near the isoelectric point, at pH = 9-10,19 the fiber contacts become “sticky”, and the dispersion turns into a transparent, space-filling gel at volume fractions as low as q =
* To whom correspondence should be addressed.
’Permanent
address: Department of Pure and Applied Chemistry, Physical Chemisay Section, University of Vigo. As‘Lagoas, Marcosende, Spain. Abstract published in Advance ACS Abstracts, August 15, 1995. @
0022-3654/95/2099- 15120$09.00/0
I
\
\
Figure 1. Schematic representation of the structure of an imogolitemetallic nanoparticle dispersion. The very small metallic particles are attached to the fibers and are therefore stabilized by them. Dashed lines represent the electrical double layer present around the rods due to their positive charge.
In ref 1, we examined and explained the influence of the imogolite host dispersion on the colloidal properties of the resulting platinum particles. It turns out that the metallic particles are immobilized on the imogolite fibers, making them very effective stabilizers for the platinum colloids which by themselves do not form stable dispersions in water. Such a stabilizing effect is partly due to the large surface area of imogolite, which provides many adhesion sites for the metallic nanoparticles. A schematic picture of the metal-imogolite system is shown in Figure 1. Various characterization techniques confirmed the conclusion that the platinum particles adsorb irreversibly on the fibers, which prevents (significant) platinum aggregation. Apart from van der Waals attractions, electrostatic attractions also possibly contribute to this adsorption. The adsorption of a metallic particle to a second fiber is hindered by the electrical double layer around the imogolite fibers, as sketched in Figure 1. So the “loaded” fibers remain in stable suspension, unless of course the pH is too close to the isoelectric point. The fibers (either as a gel or suspension) can be deposited on an inorganic surface which becomes coated with finely divided metal particles.’ The abundant literature on the synthesis of (bi)metallic particles shows that the colloidal stability and size strongly depend on the method and experimental conditions followed. Preparation of particles of different metals with the same method also usually leads to quite different results, since nucleation and
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 41, 1995 15121
Hydrosols of (Bi)metallic Particles on Imogolite growth rates as well as surface properties are specific to each material. In this paper we intend to establish the use of imogolite dispersions as a general tool for the synthesis of very small and fairly monodisperse metallic particles, which remain stable in the form of a hydrosol. The main advantages of the method are (i) easy to perform (no special care for dust-free glassware or solutions is necessary), (ii) no requirement of the addition of protective polymers to stabilize the particles, (iii) it is performed at room temperature, and (iv) the particles are deposited on an inorganic carrier from the moment of their preparation. We focus our attention on the noble metals gold, silver, and platinum, to conclude that for every metal stable dispersions composed of quite monodisperse nanoparticles can be prepared. The method also permits the synthesis of stable sols of composite bimetallic particles. Due to the differences in the reduction rate of the various metals, these particles appear to have a core-shell structure, rather than a mixture at the atomic level, except in the case of AdAg. The optical properties of both the pure metals and the composite particles are studied in some detail and compared with calculations based on the Mie theory for core-shell bimetallic particles. Here we make use of the important fact that imogolite hardly scatters light and does not absorb in the visual wavelength region; thus, it is optically transparent.
Experimental Section Materials. Tetraethoxysilane (TES, Fluka) was distilled before use. Aluminum see-butoxide (ASB, Fluka), hydrazine monohydrate (Fluka), perchloric acid (Merck, p.a. 70%), ammonia (Merck, p.a. 25%), H2PtC16 (Baker analyzed reagent), AgN03 (Baker analyzed reagent), HAuC4 (Janssen Chimica), and NaBH4 (Janssen Chimica) were used as received. Freshly double distilled water was used in the preparation of all solutions. Carrier Synthesis. The preparation of imogolite mainly comprises the hydrothermal treatment of a mixture of TES and ASB in the stoichiometric ratio (1:2) at 95-100 “C for 3 days. An extensive description of this synthesis and the influence of synthesis conditions can be found in ref 1, and further information is contained in references therein. Prior to their use for metal particle preparation, the stable, transparent imogolite dispersions are purified from alcohol and ions by dialysis for 2 days against a flow of deionized water. The imogolite weight concentration of the employed dispersions is typically about 4 g dm-3. Metallic Particles Synthesis. Following the method in ref 1, the metallic particles are prepared in the dialyzed imogolite dispersion by the addition of the corresponding metallic salt (or salts, in the case of bimetallic particles) and N a B h solutions (in this order) under vigorous stirring at room temperature. The various salts are specified under “materials”. The order of addition of the salts for the synthesis of bimetallic particles does not seem to have any influence on the preparation results. The borohydride solutions were freshly prepared before each experiment, so that degradation of borohydride20 into B02- is minimized. The addition of the borohydride solution should be quick, for we have observed that dropwise addition favors the formation of bigger clusters of particles. Vigorous stirring is also very important to achieve a homogeneous reaction in the whole sample. According to Mal’tseva et a1.,2’ the reduction of metals by borohydride can take place via the following two reactions:
+ nH2B03- + 4nH’ + 2nH2 (1) 8M”’+ nBH4- + 3nH20- 8M + nH2B03- + 8nH’ (2) 4M
In ref 1,the use of the stoichiometric ratio of the f i s t reaction was shown to be adequate for the formation of platinum particles with a narrow size distribution. Nevertheless, to ensure the completion of the reaction, an excess of borohydride was used for the syntheses reported in this paper. In the syntheses of bimetallic particles, we kept in every case a constant metal concentration in the final dispersion of 2 x M for the sake of comparison, because metal salt (and reductant) concentrations influence the size and stability of the obtained particles.’ Quick coloration of the solution, manifesting the formation of the colloids, was always observed upon addition of borohydride. It should be emphasized that all the preparations have been performed such that the final volume was 10 mL, using magnetic stirring only. When the reaction is performed at a larger scale, the stirring should be more vigorous; otherwise larger particles are formed. In most cases, the dispersions are stable for months without the need of any stabilizing “protective” polymer. Nevertheless, in cases when the salt content derived from the reduction reaction is too high, flocculation or gelation may occur. Then dialysis should be performed after the metal synthesis to obtain a stable system. In some of the preparations (see next section), a small number of particles grew until they reach a dimension which is much larger than the diameter of imogolite fibers. In such cases, the system segregated into a black sediment, and a supematant containing the remaining metallic particles attached to the imogolite fibers, on a time scale of hours to days, in a sort of self-cleaning of the system. Experimental Techniques. Transmission electron microscopy (TEM) was performed with a Philips CMlOH STEM microscope, and particle size distributions were calculated by image analysis (IBAS), averaging over more than 100 counts in every case. UV-visible spectra were measured with a Spectronic 200 W spectrophotometer and subsequently digitalized into a computer.
Results Influence of Imogolite (Experiments in Pure Water). The effect of the presence of imogolite on the result of the reduction of the metallic salts becomes very clear by a comparison with reductions in pure water. Let us recall here that the imogolite dispersion was dialyzed against deionized water previous to the reduction experiments. This means that virtually no ionic species are present in the dispersion apart from those involved in the formation of the metallic particles, so only the fibers determine the difference with respect to pure water. All experiments reported in Table 1 were repeated in double distilled water without imogolite. In virtually all cases aggregates andor polydisperse particles were formed, evidenced by (more or less) rapid flocculation in most cases and by TEM observations. The presence of larger particles and aggregates can also be detected by comparison of the UV-visible spectra of the mentioned aqueous dispersions and the equivalent metalimogolite dispersions (see Figure 2). For platinum, the presence of aggregates (in TEM observed to be formed by individual particles linked into chains) can be confirmed by the finding of a smaller slope of a log-log plot of the spectrum.’*23For gold,
15122 J. Phys. Chem., Vol. 99, No. 41, 1995
Liz-Marzin and Philipse
TABLE 1: Description of Single Metal Dispersiom? Ptl (mom)
sample Imti 1wt2 IWAu 1 IWAu2 IWAu3 IWAu4 IWAuH IWAg 1 IWAg2 IWAg3 IWAg4 IWAg5 IMIAg6 IWAgH
[Aul (mol/L)
LBH4-1
[Agl (mom)
(mom)
4 x 10-4 2 x 10-4
[NzHsOHl (mom)
4 x 10-4 10-3 4 x 10-4 2 x 10-4 1.2 x 10-4 10-3
4 x 10-4 2 10-4 1.2 x 10-4 2 x 10-4 2 x 10-4
10-3 4 8 3 4 4 4 2
10-3
x x x x x x x
10-3 2x 7.5 x 10-5 10-4 5 x 10-4 10-3
10-4 10-4 10-4 10-4 10-4
10-3
d (nm)
u
2.66 1.98 6.31 5.31 4.76 3.37 8.65 9.30 7.91 4.51 7.42 5.99 4.71 24.02
0.32 0.31 2.20 0.88 1.58 0.91 5.58 4.27 2.35 2.07 2.96 5.03 4.09 6.86
Anax
PW
(nm)
(nm)
color
68 56 63 62 72 92 119
light brown light brown red red light red light red pink dark yellow yellow light yellow light yellow light yellow light yellow yellowlorange
531 527 521 510 526 392 399 401 401 405 409 398
d is the most probable particle diameter, and u is the standard deviation of the size distribution measured by TEM. I., position, and pw is the peak width at half-height.
-IM/Pt2 ........ Ptlaq2
0.8-
-
0.21.6. 1.4
-
, . , .
I
.
v
.
I
.
I
.
I
,
I
.
I
-IWAu4
........ Aulaq4 -
f; 0.4
-
0.20.0-
300 350 400 450 500 550 600 650 700
Wavelength [nm]
Figure 2. Comparison between the UV-visible spectra of stable aqueous dispersions of platinum, gold, and silver (1 day after their preparation), with the correspondent dispersions prepared in the presence of imogolite fibers.
the aqueous sample gives after 1 day a spectrum with a plasmon absorption band slightly red-shifted (A,,,= = 5 15 nm) and clearly broader than the one corresponding to the imogolite sample. This reflects both a larger average particle size and a high polydispersity. On the other hand, the spectrum of aqueous
is the plasmon peak
silver (see dashed line in Figure 2, lowest) presents a maximum at 385 nm and a shoulder starting at 425 nm, 1 day after its preparation. This is a clear indication of polydispersity and aggregation, confirmed by the flocculation observed a few days later. Notice that the shift of the plasmon absorption band goes in opposite directions for gold and silver. This effect will be discussed in detail later. Single Metal Dispersions. Elsewhere, we have shown' that, for the synthesis of platinum nanoparticles in imogolite dispersions, an increase of the initial concentrations of both the platinum complex and reducing agent results in an increase of polydispersity and average particle size together with a decrease of colloidal stability. It was also shown' that as a consequence of such an increase of particle size, the slopes of log-log plots of the W-visible spectra decrease (see Figure 7 of ref 1).Thus, absorption measurements allow a qualitative comparison between average particle sizes of various systems. In the cases of silver and gold, this comparison is even more rewarding, due to the presence of a plasmon absorption band in their spectra, the position and shape of which strongly depend on average particle size (see discussion on next section). UV-visible spectroscopy, combined with TEM, allows to check the influence of the preparation conditions on the size distribution of the final colloids. Using these two techniques, we find that also for silver and gold an increase in the concentrations of the metallic salts, at constant [reductant]/ [metal] ratio, leads to larger average particle size and polydispersity (see Figures 3 and 4, Table 1). Furthermore, we have also observed for all three metals that the use of larger [borohydride]/[metal]ratios results in smaller and more monodisperse particles (see Figures 3 and 4, Table 1). From Table 1, it becomes immediately apparent that the size of the particles depends on the reduced metal. In general, the tendency is Ag > Au > R. It is also remarkable that for silver colloids with TEM frequently a small population is found of particles which are considerably larger (20-50 nm) than the average diameter. In the case of gold, such big particles are also occasionally found, although with smaller sizes (10-20 nm). No such large particles have been observed in any of the platinum sols studied. With respect to the colloidal stability of the samples, it is a general observation that the metal-imogolite dispersions did not show any sign of flocculation or gelation for a period of weeks to months. Nevertheless, in the case of some silver colloids, after a few days, a very small black precipitate was detected. As mentioned before, this is due to the aggregation of a small population of big particles. An exception to the
J. Phys. Chem., Vol. 99, No. 41, 1995 15123
Hydrosols of (Bi)metallic Particles on Imogolite 1.2
,
.
,
.
,
.
,
-IWAgl
,
.
I
.
-IWAUH
1.0-
w
.
.
0.0L'
400
1.0
450
"
500
"
550
"
600
"
850
" 700
-
300
350
450
400
Wavelength [nm] Figure 3. UV-visible spectra of silver/imogolite colloids. The upper part shows the influence of the starting concentration of silver salt and reductant. The lower part shows the influence of the ratio [NaB&]/ [AgN03]. Normalization of the data has been performed to make the comparison easier. See text and Table 1 for details. 1.2-
0.2
"
I
,
I
,
1
,
I
.
I
.
I
.
1
-IMlAul .
-
0.0'
"
400
' . " 450 500 550
"
"
600
' " 650 700
Wavelen,& [nm] Figure 4. UV-visible spectra of goldimogolite colloids. The influence of the starting concentration of gold salt and reductant and
the ratio [NaBH4]/[HAuC14] is shown. Normalization of the data has been performed to make the comparison easier. See text and Table 1 for details. general colloidal stability of the dispersions was the sample with the largest reagent concentration (IM/Agl), which showed signs of a very slow flocculation after several weeks of its preparation. Influence of the Kind of Reductant. We have checked the influence of the use of a different reductant, namely hydrazine hydrate, on the reduction of gold, silver, and platinum in the presence of imogolite. For this purpose, we prepared samples containing 2 x M salt and M hydrazine hydrate. A general observation is that the color formation proceeds much more slowly than when using borohydride as reductant. With gold, a pink color appears in a few seconds, but with silver, about 5 min is required for a noticeable yellow/orange color. In the case of platinum, no visible coloration developed, and after 2 days, a black deposit was observed.
Wavelength [nm] Figure 5. UV-visible spectra of goldimogolite (upper) and silver/ imogolite (lower) sols. Solid lines correspond to colloids prepared by reduction of the respective salts with hydrazine hydrate. Broken lines correspond to reduction with sodium borohydride in the same concentration.
Electron microscopy and W-visible spectroscopy results are shown in Table 1 and Figure 5. It is clear that both particle size and polydispersity are increased by the use of hydrazine hydrate as a reductant. This is most probably related to the well-known factz2that hydrazine retards the nucleation rate. As a consequence, fewer nuclei are formed, and more metal atoms are left for growth, leading to larger final particles. Bimetallic Dispersions. Simultaneous reduction of two metallic salts with borohydride, in the presence of imogolite rods, was also investigated. Depending on the particular metals and on their relative concentrations, both the size distribution and the optical properties of the obtained dispersions change noticeably. Samples have been prepared for all three possible pairs with platinum, silver, and gold, with the following molar ratios for every pair: 10:0, 8:2, 6:4,4:6, 2:8,0:10. In every preparation, M, the total molar concentration of metal ions was 2 x and excess reductant ( low3M) was added. For every dispersion, the size distribution (measured by image analysis of TEM negatives) as well as the W-visible spectrum was measured. Table 2 summarizes the measured parameters, and the spectra are shown in Figures 6-8. The majority of the dispersions are stable for months without any sign of flocculation, again illustrating the stabilizing role of imogolite fibers. However, samples IM/AgAu4:6, IM/ AgAu6:4, and IM/AgAu8:2 showed a black deposit after a few days, while the supematant remained colored and apparently stable for months. It should be pointed out, in addition, that TEM observation of sample IM/AgPt8:2 revealed the presence of a very small population of big particles, similar to what was mentioned for silver sols.
Liz-Marzfin and Philipse
15124 J. Phys. Chem., Vol. 99, No. 41, I995
TABLE 2: Description of Bimetallic DispersiomF sample d (nm) (T A,,, (nm) pw (nm) color 409 148 light yellow 4.71 4.09 IMIAg 382 217 light brown 1.54 0.24 IM/AgPt8:2 light brown 370 268 1.80 0.46 IMIAgPt6:4 light brown IM/AgPt4:6 2.06 0.41 light brown IM/AgPt2:8 2.47 0.50 light brown IM/pt 1.98 0.31 light brown IM/AuPt2:8 2.38 0.53 light brown IM/AuPt4:6 2.66 0.71 light brown IM/AuPt6:4 2.94 0.63 reddish brown IM/AuPt8:2 2.62 0.67 light red 122 517 IMIAu 2.63 1.00 448 222 dark yellow IMIAgAu8:2 1.87 0.45 495 171 light orange IM/AgAu6:4 2.41 0.60 510 115 orange IM/AgAu4:6 2.99 0.85 light redorange 515 93 IM/AgAu2:8 3.08 0.73
-IM/Au .
IM/8Au+2Pt IM/6Au+4Pt IM/4Au+6Pt
a d is the most probable particle diameter, and u is the standard deviation of the size distribution measured by TEM. Am is the plasmon peak position, and pw is the peak width at half-height. The composition is expressed in the sample code, where the ratio is always of the first metal with respect to the second.
1,o
-lM/Pt
-
IM/AgPt8:2
...... IM/AgPt6:4
.
....... IM/AgPt4:6 ...... IM/AgPl2:8
300
400
500
600
700
Wavelength [nm]
Figure 7. UV-visible spectra of A u P t bimetallic imogolite sols prepared by simultaneous reduction of the metallic salts with the molar ratio expressed in the name (upper) and mixture of dispersions of the
pure metals in the expressed ratio (lower).
-IM/Pt
-
...... IM/EP1+2Ag
...... IMI6Pt+4Ag
0,o-
, 300
,
,
,
400
,
,
500
, 600
,
.
,
-
700
Wavelength [nm] Figure 6. UV-visible spectra of Ag/Pt bimetallic imogolite sols prepared by simultaneous reduction of the metallic salts with the molar ratio expressed in the name (upper) and mixture of dispersions of the pure metals in the expressed ratio (lower).
Discussion Stability. First we want to emphasize the crucial role of the imogolite fibers regarding the stability of the metallic hydrosols. In the previous section we showed that the metallic particles prepared by borohydride reduction at room temperature and standard (not extremely low) concentrations are basically unstable. On the other hand, when imogolite fibers are present, a wide range of concentrations can be used to prepare dispersions with a long-time stability. The experiments performed with all these systems confirm the stability mechanism proposed in ref 1, Le., the irreversible adsorption of the metallic nanoparticles on the (much longer) fibers which leads to a stable composite dispersion. (This adsorption also allows an easy
immobilization of the particles through gelation or deposition onto inorganic substrates.') The stability provided by the fibers is a prerequisite here for the study of the optical properties of the metal particles, which are discussed next. Single Metal Dispersions. Optical Properties. In general, the precise relationship between the shape of the absorption spectra of metallic sols and metal particle size is not yet completely understood. Even for the most widely studied metals (silver and gold), there is little agreement between the (interpretation of the) results of various different preparation methods. In the case of silver, Heard et al.24studied colloids prepared according to several standard recipes and found that a 4-fold increase in particle size leads to a 17 nm increase in the maximum of the plasmon absorption band, I,,,. Schultze et aLZ5found a similar behavior for silver nanoparticles prepared by the gas aggregation technique, together with a peak broadening when decreasing particle size. Andrews and Ozin26found an increase of Amax when increasing the thickness of silver nanolayers deposited on several substrates. In this case, the peak broadening behavior depends on the substrate. On the other hand, Petit et al. show" that, for their silver clusters prepared in reverse micelles, there is an increase of Amax, as well as a peak broadening, with decreasing particle size. It should be mentioned that the size range studied in refs 25 and 11 is about the same (2-10 nm) and that in ref 24 the particles are generally larger (6-30 nm). This suggests that the occurrence of blue or red shifts is related to the particle size range. In addition, Barnickel et aL2' report good agreement between the spectra of silver colloids prepared in reverse micelles and calculations which take into account not only the average particle size but also the whole size distribution and include the presence of an ionic layer around the surface. All these studies demonstrate indeed the strong influence of the environment, which constitutes the main difference between
Hydrosols of (Bi)metallic Particles on Imogolite 1.o
J. Phys. Chem., Vol. 99, No. 41, 1995 15125
-IWAg
___ ___
IM/AgAu2:8. IM/AgAu4:6IM/AgAu6:4.
I -
in the nucleation stage, less is left for growth. This idea is supported by the results obtained when using hydrazine as reductant. When in turn we compare particle sizes of the different metals, we see that the sequence is Ag > Au > Pt. We can try to correlate this sequence to the redox potentials. The standard redox potentials E" for the relevant redox couples at 1 M arez9 AuC1,-
-
Ag Ag' -I- ePtC1:4e-
+
1.o
-IM/Au
___ ___ 7
IMI2Ag+8Au IM/4Ag+6Au-
a
0.01
-
+ 3e-
, 300
,
,
,
400
, 500
,
,
600
,
, j
700
Wavelength [nm]
Figure 8. UV-visible spectra of Ag/Au bimetallic imogolite sols prepared by simultaneous reduction of the metallic salts with the molar ratio expressed in the name (upper) and mixture of dispersions of the pure metals in the expressed ratio (lower).
the various methods. It seems that the nature of the material in contact with silver particles plays an important role in the optical properties of the dispersion. This agrees with the studies by Henglein and c o - ~ o r k e r s ~on . ~ .the ~ changes of optical properties due to the adsorption of different species. In our case, from Figure 3 and Table 1, a behavior similar to that reported by Petit et al." is observed: a decrease in particle size promotes a slight red shift and a noticeable broadening of the plasmon absorption band. We should mention that the presence of the (optically transparent) imogolite fibers, at which silver particles are attached, may very well influence the absorption spectra. In the case of the sample labeled IM/Agl, several shoulders are present on the band, which are probably due to the high polydispersity of the metal particles in that system. For gold colloids, early work by Turkevich et al.** shows that particle sizes between 7.5 and 33 nm would correspond to a spectrum with a peak at 522 nm. From 33 nm upward, a red shift and broadening of the peak would appear, resulting in violet and blue dispersions. More recently, Wilcoxon et al.Io reported on gold particles synthesized in reverse micelles, where decreasing particle size from 14 to 2.4 nm leads to a blue shift from 522 to 480 nm, accompanied by peak broadening. We also observe a blue shift of the plasmon band with decreasing particle size, although the relationship with peak width is not very clear. Effect of [Reductant]/[Metal] ratio. From Table 1 and Figures 3 and 4, it is clear that for a constant metal concentration an increase in the concentration of the reductant leads to the formation of smaller particles. This is probably due to a faster metal reduction rate when a larger amount of reductant is present. This higher rate leads to the formation of a larger amount of nuclei in the early stage of the reaction, on which the other metal atoms will precipitate. As more material is used
Au
R
+ 4C1-
+ 6C1-
E" = +0.99 V Eo = +0.80 V Ea = +0.74 V
They do not follow the above-mentionedsequence. However, this trend is a thermodynamic one, which does not include any kinetics. Here we assume a kinetic control, and therefore different arguments should be used to explain the sequence. For silver, only a one-electron reduction is performed, which is kinetically much easier than a multielectron reduction, in which several unstable valence states are passed through, slowing the rate down. Comparison between Au and Pt leads to the same argument, given that we have Au3+ in AuCL- and Pt4+ in
Ptchj-. The fast reduction of silver, together with the largest size of silver particles as compared to gold and platinum, implies that nucleation for silver is relatively slow, whereas the rate of growth is relatively high. If the rate-determining step in the formation of nuclei is the electron transfer to Ag+ to yield free Ago, the high negative electrochemical potential of Ago of - 1.8 V would make the low nucleation rate understandable. However, there is another thermodynamic component in addition to redox potentials. According to La Mer's homogeneous nucleation and growth model,30 there will be a critical nucleus size which is characteristic of the specific material considered. Such a critical nucleus size is not known for most materials. Perhaps for the metals considered here the critical sizes do not correlate with their standard redox potentials, but instead with the experimentally observed sequence in final particle size. Bimetallic Dispersions. Optical Properties. From the comparison between the upper and lower parts of Figures 6-8 it becomes evident that the simultaneous reduction of two kinds of metallic salts leads to the formation of composite particles, rather than to a mixture of single-metal particles. Here we use the term "composite" to denote particles which contain two metals, without specifying their structure. When the sols of the pure metals are mixed in any proportion, the spectra of the resulting dispersions (lower part of the figures) are simple combinations of the spectra of the corresponding metal sols. That is not the case for the dispersions of bimetallic particles. A second evidence for the formation of composite particles is constituted by the size distributions of the systems studied. In Table 2 one can see that particle size is significantly smaller when a second metal is present. Furthermore, no bimodal size distributions have been found, which should be the case for mixtures of single-metal particles. As an example, Figure 9 shows electron micrographs of samples IWAg6, IM/Pt2, and IM/AgPt8:2. The difference in size is obvious. Now that it is clear that particles are composite, the question is whether they are formed by a mixture at the atomic scale or whether they consist of separate phases containing each of the metals. There are reports in the literature where bimetallic particles are prepared, and structures for them are proposed. Almost all the authors propose a structure where one of the metals occupies the core of the particles and the other forms a surrounding shell.
15126 J. Phys. Chem., Vol. 99, No. 41, 1995
Liz-MarzBn and Philipse
TABLE 3: Description of Particle Sizes Used for the Calculation of Spectra by Means of Mie Theory for Core-Shell Particles (Figures 6-8)” MAl.2
IM/AgcR8:2 IM/AgcPt,6:4 IM/AgcPt,4:6 IM/AgcPt,2:8 ImAg,2:8 ImAg,4:6 IMmAg,6:4 ImAg,8:2 IM/Pt IM/AQt,2:8 IM/AucP&4:6 IM/AucPt,6:4 IM/Au&8:2 IMmAu,8:2 IMmAu,6:4 ImAu,4:6 IMmAu,2:8 IMfAu
IM/AbAg,8:2 IM/AkAg,6:4 IM/A&Ag,4:6 IM/AkAg,2:8 IM/A&Au,2:8 IM/AgcAu,4:6 IM/AgcAu,6:4 IM/AgcAu,8:2
4.7 1 1.54
1.so
2.06 2.47 1.54 1.80 2.06 2.47 1.98 2.38 2.66 2.94 2.62 2.38 2.66 2.94 2.62 3.63 3.08 2.99 2.4 1 1.87 3.08 2.99 2.4 1 1.87
81.84 62.83 42.9 1 2 1.98 18.16 37.17 57.09 78.02
0.72 0.77 0.78 0.74 0.44 0.65 0.85 1.14
0.05 0.13 0.25 0.495 0.33 0.25 0.18 0.10
21.19 42.73 62.67 81.74 78.81 57.27 37.33 18.26
0.7 1
0.48 0.33 0.2 1 0.09 0.09 0.23 0.4 1 0.57
79.87 59.8 1 39.84 19.89 20.13 40.19 60.16 80.1 1
1.43 1.26 0.89 0.55 0.90 1.10 1.02 0.87
1.oo
1.26 1.22 1.10 1.10
1.06 0.74
0.1 1 0.235 0.315 0.385 0.64
0.395 0.185 0.065
The nomenclature includes first the core metal and second the shell metal (as indicated with subindices). d is the most probable diameter is the volume of the particles in the corresponding syntheses; fraction of the core, calculated from its molar fraction and the mass densities of bulk metals (see eq I); rcm is the corresponding radius of the core; and rSkl1is the corresponding shell thickness. Figure 9. Electron micrographs of samples IMlAg6 (top), IMPt2 (middle), and IM/AgPt8:2 (bottom) (cf. Tables 1 and 2). The background is formed by an agglomeration of imogolite fibers. Scale bar = 45 nm. Toshima and Yonezawa31 prepared Au/Pt colloids protected with poly(vinylpyrro1idone)by the alcohol reduction method. They suggest that platinum covers gold and show that the plasmon absorption band of gold appears in the first stages of the reaction but eventually disappears, due to platinum deposition. The UV-visible spectra they show are very similar to the ones shown in Figure 7. Toshima et al.7 used the same method to prepare polymer-protected Pd/Pt colloids. They show from UV-visible and catalytic activity measurements that Pd covers Pt cores. For Au/Pd clusters, Toshima et al.32propose from EXAFS studies core-shell or cluster-in-cluster structures, depending on the composition. Esumi et a1.12 prepared PdCu particles by thermal decomposition of the metal acetates in organic solvents. They show by means of X-ray diffraction studies that the formed particles contain both metals, but they do not specify the structure. In our case, X-ray diffraction was of no help, because there is a large amount of imogolite in the system, whose peaks overlap those of the metals. Sinfelt and Via33and Davis and BoudartN prepared Mr and Pd/Au bimetallic particles, by means of reduction on respectively dry silica and alumina supports. In both cases, they find a coreshell structure, as demonstrated by EXAFS measurements. All these authors mention as a rule for bimetallic systems that the component with the lower heat of sublimation (surface energy) has a tendency to accumulate on the surface. This rule does not necessarily apply for reactions in solution, where the surface energies also depend on the surrounding fluid and the adsorbed ions, and indeed does not agree with our results.
Of some help at this point can be the comparison of our results with experiments where one metal is specifically reduced on another. This has been done for a number of cases by Henglein and c o - w o r k e r ~ . ~They ~ ~ ~show . ~ ~ that the UV-visible spectra of the coated particles can be qualitatively compared with those calculated by means of the Mie theory. For pure metals, this theory can be directly applied,37but it needs to be adapted for the special case of core-shell particles. In this last case, the equations given by Bohren and H ~ f f m a n ncan ~ ~be used. We used these equations to calculate the spectra of core-shell particles corresponding to our experimental data. This means that the total radii will be those given in Table 2, and the corresponding molar ratios are used to calculate the volume fractions of the components (&), according to the following equation:
where Mi and di are the molar weight and bulk mass density, respectively, of metal i (i = 1, 2) and ni is the molar concentration of metal i in the sample. From the volume fractions and the total radii, it is a simple exercise to calculate the corresponding core radii and shell thicknesses. The results of such calculations are given in Table 3. Introducing those parameters in the equations of Bohren and Huffmann, we obtained the spectra shown in Figures 10-12. For the calculations, we used the bulk dielectric data of gold, silver,39 and platinum.40 Comparison between calculated and experimental spectra is made in what follows. For the silver/platinum system, the evolution of the experimental spectra (Figure 6) follows the same tendency as the
Hydrosols of (Bi)metallic Particles on Imogolite
J. Phys. Chem., Vol. 99, No. 41, I995 15127
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