VOLUME 100, NUMBER 27, JULY 4, 1996
© Copyright 1996 by the American Chemical Society
LETTERS Bimetallic Colloids: Silver and Mercury Lynne Katsikas, Maritza Gutie´ rrez, and Arnim Henglein* Hahn-Meitner Institut, Abteilung Kleinteilchenforschung, 14109 Berlin, FRG ReceiVed: February 5, 1996; In Final Form: April 8, 1996X
Mercury ions, Hg2+, are radiolytically reduced in an aqueous silver sol. The plasmon absorption band of silver is blue-shifted, which is explained by the formation of a solid mercury layer around the silver particles. The metal deposition stops when the silver particles carry about two monolayers of mercury and the additionally reduced mercury forms colloidal Hg particles. Chemisorbed SH- and I- ions damp the plasmon absorption band of silver and silver-mercury particles, the effect being less pronounced with increasing mercury content of the particles. It is concluded that the collective excitation of the electron gas in the silver particles still strongly contributes to the optical absorption, even when they carry a mercury shell. The chemisorption equilibrium is shifted to the desorption side upon the deposition of excess electrons on the silver-mercury particles.
Introduction Calculations of Creighton and Eadon have shown that colloidal mercury particles in water, the size of which is much smaller than the wavelengths of light, should have a rather narrow absorption band at 275 nm.1 A sharp band in the nearUV is well-known for colloidal silver; it is produced by a collective oscillation of the electron gas in the particles. This silver absorption band has a remarkable property: it is strongly damped upon the adsorption of tiny amounts of nucleophilic anions, such as I- and SH-.2 One of the reasons for undertaking the present work was to check whether the absorption band of mercury is also affected by adsorbed substances. In addition, the changes in the shape of the silver absorption band were to be investigated when small amounts of mercury were chemisorbed. In the beginning, it was intended to prepare a stable sol of mercury particles smaller than 20 nm and to carry out optical measurements and adsorption experiments. Aqueous mercury sols were prepared in the early days of colloid science.3,4 They * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, June 15, 1996.
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had a dark color and contained rather large particles, which condensed rapidly to yield a precipitate. We have not yet been successful in improving the synthesis: using γ-rays for reduction of Hg2+ ions in the presence of various stabilizing polymers, such as polyacrylamide, polyarylate, and polyphosphate (which in recent years have been quite useful in the preparation of colloids of other metals),2b only solutions of rather large particles were obtained which were stable for just a few hours. The direction of the experiments was then changed: mercury ions, Hg2+, were radiolytically reduced in a solution of silver particles, hoping that the latter would be covered with a mercury layer to yield bimetallic particles which are accessible to optical measurements and adsorption experiments. The deposition of mercury on the silver particles could be followed by observing the changes in the shape and position of the plasmon band of the latter. Experimental Section The silver colloid was prepared by γ-irradiation of a deaerated AgClO4 solution (about 1.0 × 10-4 M), 0.1 M 2-propanol, and 5.0 × 10-4 M polymer. The polymer used was strongly carboxylated polyacrylamide (70% carboxyl content: PAA-70), © 1996 American Chemical Society
11204 J. Phys. Chem., Vol. 100, No. 27, 1996
Letters
Figure 2. (1) Spectrum of a silver-mercury sol (containing the metals in the ratio 1:0.2). (2) Spectrum after the addition of 0.2 × 10-4 M Hg(ClO4)2. (3) Spectrum after subsequent reduction of the added Hg2+ ions by γ-irradiation. Figure 1. Spectrum of a silver sol(0) obtained by γ-irradiation of a solution containing 1.0 × 10-4 M AgClO4, 0.1 M 2-propanol, and 5.0 × 10-4 M PAA-70, and spectra of the solution after adding and reducing 2.0 × 10-5 M Hg(ClO4)2 stepwise to the silver sol. The numbers on the curves give the total concentration of reduced mercury.
purchased from Acros Organics. After irradiation for 20 min at a dose rate of 3 × 105 rad/h, all the silver ions were reduced.5 The solution contained approximately spherical particles with a broad size distribution around 6 nm. The glass vessel had sidearms carrying a septum and an optical cuvette. Mercury ions in the form of an aqueous Hg(ClO4)2 solution were then injected into the silver sol, and the solution was again exposed to γ-radiation. In some experiments, the solution already contained mercury ions from the very beginning, and in other experiments additional mercury ions were added in several steps as described in more detail below. The absorption spectrum was recorded at various times until no more changes occurred, i.e., when all the mercury ions had been reduced. Hydrogen sulfide or iodide anions were introduced into the solution by injecting H2S gas or an aqueous KI solution, respectively. The radiolytic reduction mechanism is well-known.2b,5 Hydrated electrons and organic radicals, (CH3)2COH, are generated in solutions containing 2-propanol. The electrons and radicals have reducing properties. They reduce Ag+ ions to yield Ag0 atoms, which form colloidal silver particles via various growth processes.6 The electrons and radicals can also reduce Hg2+ ions to yield Hg+ and finally Hg0. The Hg0 atoms may finally settle on the silver particles. In another route of reduction, the radicals transfer electrons to the colloidal silver particles, and the stored electrons then reduce Hg2+ directly on the surface of the particles. In the present letter, the reduction mechanism will not be elucidated in great detail. In the future, pulse radiolysis experiments will be carried out, to recognize the various elementary processes that occur in the reduction of Hg2+ in the presence silver particles. Results and Discussion Colloid Preparation and Absorption Spectra. The absorption spectrum of a 1.0 × 10-4 M silver sol obtained by γ-irradiation is shown in Figure 1. It contains a rather broad plasmon absorption band of silver at 380 nm. Electron microscopy revealed that particles of different sizes between 4 and 10 nm were present.
In a first series of experiments (not shown in Figure 1), various concentrations of Hg(ClO4)2 were added to a silver sol and all metal ions reduced until no further changes in the absorption spectrum occurred. It turned out that the silver plasmon band was blue-shifted, which is taken as an indication that mercury was deposited on the silver particles. However, no more than 2.0 × 10-5 M mercury could be used to yield a transparent solution. When larger amounts of mercury ions were reduced, dark solutions were obtained, from which a precipitate sedimented within a few hours. It was then found that greater amounts of mercury could be reduced on the silver particles to yield solutions stable for about 1 day, when the addition of mercury was carried out in small steps with subsequent irradiation. The results of such an experiment are shown in Figure 1. Always 2.0 × 10-5 M Hg(ClO4)2 was added and the silver sol and the solution were subsequently irradiated for 10 min. It can be seen from Figure 1 how the plasmon band shifted. In the inset of the figure, the wavelength of the absorption maximum is plotted versus the mercury concentration. It is recognized that the band shifts less and less with increasing amount of reduced mercury, until above a total mercury concentration of about 8.0 × 10-5 M practically no more shifts are observed. However, the absorption band became broader, and, especially at the last mercury additions, there was a great increase in the absorption at longer wavelengths and a weak shoulder at 290 nm appeared (see arrow). Various concentrations of PAA-70, between 5.0 × 10-4 and 5.0 × 10-3 M, were investigated. The same shifts of the absorption bands were observed as in Figure 1; only the width of the band became larger with increasing polymer concentration. All further experiments were therefore carried out with the lowest PAA-70 concentration of 5.0 × 10-4 M. An additional fact has still to be mentioned. Upon the stepwise addition of Hg(ClO4)2 the absorption spectrum of the sol always underwent the remarkable changes, which are illustrated in Figure 2. The absorption band decreased, and a stronger absorption at long wavelengths was present. However, upon the subsequent reduction of the added Hg2+ ions by γ-irradiation, the long-wavelength absorption decreased again, and the plasmon band appeared at a shorter wavelength. Only in the case where a larger amount of mercury had already been deposited, the increased long-wavelength absorption remained after subsequent reduction (Figure 1). A large fraction of roughly 25% of the atoms in the 6 nm (mean size) silver nanoparticles are located at the surface. At
Letters the relatively small mercury concentrations reduced in the experiment of Figure 1, only about two atomic monolayers of mercury could have been deposited. We explain the above effects in the following way: In the beginning of the mercury deposition, the Hg atoms are strongly bonded to the silver surface, possibly by forming a solid amalgam-like structure. When the amount of deposited mercury exceeds about two monolayers, the deposited metal has a structure resembling liquid mercury; the mercury from these outer layers can then easily be detached into the solution, the result being the formation of larger colloidal mercury “drops”. The slight opalescence and long-wavelength absorption of the solution, when larger amounts of mercury are reduced, are attributed to these mercury “drops”. The shoulder at 290 nm, mentioned above, is probably also caused by the “drops”, the expected 275 nm wavelength position of the maximum being masked by the strong underlying absorption of the silver-mercury particles present. When cations are adsorbed on silver particles, the plasmon band is often red-shifted and broadened. This effect has been described in some detail in the case of adsorbed Ag+ ions and explained by the withdrawal of electron density from the metal particles.7 It is well-known that the plasmon band shifts to longer wavelengths as the density of the electron gas decreases.8 The effect of Hg2+ ions in Figure 2 is explained in this way. An experiment was also carried out, in which a solution containing both AgClO4 and Hg(ClO4)2 was γ-irradiated until all the metal ions were reduced. The concentrations of AgClO4, 2-propanol, and PAA-70 were the same as in the experiment of Figure 1, and the Hg(ClO4)2 concentration was 0.2 × 10-4 M. An absorption spectrum resulted, which was identical with the spectrum obtained in Figure 1 when the silver ions were separately formed and then 0.2 × 10-4 M Hg2+ were added and reduced. It thus seems that in the presence of both Ag+ and Hg2+ ions, silver is first formed and subsequently Hg deposited, although in the early stages both metals are attacked by the reducing radicals. Details of the mechanism in such common solutions of the two metals will be investigated in the future. Chemisorption of I- and SH-. Figure 3 shows how the absorption band of the pure silver colloid and of two silvermercury colloids is affected upon addition of different amounts of hydrogen sulfide. It can be seen that the absorption band is damped in all cases, although the effect is less pronounced the higher the mercury content. As has previously been found in time-resolved adsorption experiments on pure silver, the changes in the optical absorption are rather complex.2c The adsorption itself (which occurs in some 100 ms) is accompanied by a damping of the plasmon band, and one observes often an isobestic point with increasing amounts of the adsorbates. However, the particles may subsequently agglomerate (within seconds) to form small particle clusters, whereby further damping and a red-shift of the band occur. In the case of the pure silver particles in Figure 3, the overall effect consists indeed of very strong damping and a shift to longer wavelengths. In the case of the mercury carrying colloids in Figure 3, the second part of the optical change, i.e., the red-shift due to particle-cluster formation, seems to be absent, as the isobestic points at 420 nm (middle) and 375 nm (bottom) can clearly be seen. The blue-shift of the plasmon band is explained by the transfer of electron density by the adsorbed nucleophilic SH- into the colloidal particles. It thus seems that the mercury-covered particles have less tendency to agglomerate upon SH- adsorption than the pure silver particles. Damping of the absorption band of the silver-mercury
J. Phys. Chem., Vol. 100, No. 27, 1996 11205
Figure 3. Absorption spectrum of the pure silver colloid (top) and two silver-mercury colloids (middle and bottom) before and after addition of various concentrations of H2S to the solution.
Figure 4. Spectrum of a silver-mercury colloid before and after the addition of various concentrations of KI.
colloids also occurs when potassium iodide is added to the solutions. It was found again that the damping was less pronounced for the colloids carrying a mercury deposit than for the pure silver colloid. Figure 4 shows typical spectra. Note by comparison with Figure 3 (middle) that much higher iodide concentrations are required to produce the same damping as SH-. Iodide seems to be much less efficiently adsorbed than SH-. In fact, the 225 nm charge-transfer-to-solvent band of free I- can be seen in Figure 4, which indicates that only part of the added iodide is adsorbed (chemisorbed iodide does not possess this band9). The absorption band of the particles that carry a mercury
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Figure 5. Spectrum of a silver-mercury colloid before and after the addition of 10 µM SH- and after deposition of excess electrons by γ-irradiation.
deposit is broader than that of the pure silver particles. Obviously, some damping of the plasmon oscillation by the mercury shell occurs. This is possibly the reason why the band is less damped by adsorbed SH- or I- ions than in the case of the pure silver particles. The position of adsorption-desorption equilibria on silver particles depend on the position of the Fermi level in the particles.2a,7 When excess electrons are stored on the particles, the equilibrium is shifted to the desorption side. Figure 5 shows the result of such a desorption experiment using a solution of silver-mercury particles that carry adsorbed SH-. Excess electrons are deposited by generating organic radicals in the solution by γ-radiolysis. The solution in Figure 5 was irradiated for 10 min after H2S addition. It can be seen that the plasmon absorption band of the particles is to a large extent restored as SH- ions are desorbed. Final Remarks The formation of a mercury layer around silver particles is accompanied by a moderate broadening of the plasmon absorp-
Letters tion band and a blue-shift of the band. The damping of the band is less pronounced than in other cases, such as in the deposition of cadmium on silver particles.10 This is attributed to the relatively small 2 values (imaginary part of the dielectric constant) of mercury in the near UV. The absorption band of silver particles carrying a thin mercury layer is damped by adsorbed nucleophilic anions. Such damping is generally not observed when the absorption is caused by interband transitions in the metal. The occurrence of damping is taken as an indication that a collective electron excitation contributes significantly to the absorption in the silver-mercury particles. Only preliminary experiments could be carried out in the present work to elucidate the nature of the mercury “drops” that are formed when too much mercury is reduced in the silver sol. It seems that they are crystalline and also contain silver. These amalgam-like colloidal particles will in the future be investigated in more detail. Acknowledgment. The authors thank Dr. Su for supporting electron microscope measurements. References and Notes (1) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (2) (a) Mulvany, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843. (b) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (c) Strelow, F.; Henglein, A. J. Phys. Chem. 1995, 99, 11834. (3) Amberger, K. Kolloid Z. 1911, 8, 88. (4) Gmelins Handbuch der Anorganischen Chemie; 8, Auflage, Quecksilber, Teil A1, Syst. Nr. 34, 1960, p 222. (5) For the radiation chemical foundations, see, for example: (a) Henglein, A.; Schnabel, W.; Wendenburg, J. Einfu¨hrung in die Strahlenchemie; Verlag Chemie, Weinheim, 1969. (b) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (6) Ershov, B. G.; Janata, E.; Henglein, A. J. Phys. Chem. 1993, 97, 339. (7) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. 1991, 92, 31. (8) Doremus, R. H. J. Chem. Phys. 1965, 42, 414. (9) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (10) Henglein, A.; Mulvaney, P.; Linnert, T.; Holzwarth, A. J. Phys. Chem. 1992, 96, 241.
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