J. Phys. Chem. 1993,97. 679-682
679
Surface Chemistry of Colloidal Silver: Surface Plasmon Damping by Chemisorbed I-, SH-, and
cass-
Thomas Llnaert, Paul Mulvaney, and Arnim Henglein' Hahn-Meitner-Institut Berlin, lo00 Berlin 39, Germany Received: August 18, 1992
The optical changes that occur upon the addition of iodide, iodine, and colloidal silver iodide to a colloidal solution of silver particles (6-nm mean diameter) were monitored spectrophotometrically. For small concentrations of added I- and I2 (formation of less than a monolayer of adsorbate), the changes in the shape of the silver plasmon absorption band are practically identical. Silver iodide particles in contact with silver particles affect the plasmon band only slightly. It is concluded that the AgI molecules formed by the surface oxidation of the Ag particles by iodine do not possess the properties of bulk AgI. The structure Agd+I- is proposed as in the case of adsorbed iodide. With increasing amounts of AgI on the silver particles, the optical properties of bulk AgI appear. The coadsorption of I- and SH- was also investigated. SH-is adsorbed more strongly than I-. In fact, SH- displaces I- even a t coverages smaller than one monolayer, which is explained by an electronic mechanism (SH- donating electron density into the silver particle to shift the adsorption/desorption equilibrium of I-). The coadsorption of SH- and CaH& was also investigated. The two anions are about equally strongly bound to the silver surface. I-, SH-, and C6HsS- ions lose their CTTS absorption bands when they are adsorbed.
Introduction The intense 380-nm plasmon absorption band of colloidal silver in aqueous solution is very strongly damped when a nucleophilic reagent is adsorbed on the surface of the colloidal particles. The adsorption of a nucleophile also causes the Fermi level in the silver particles to shift to a more negative potential, the result being an increased sensitivityof the silver particles toward electron acceptors.'-3 In the present study, the effects of adsorbed iodide on the shape of the surface plasmon absorption band of silver particles are described. Adsorbed iodide may be regarded as a precursor" to AgI on the surface of the metal particles. Experiments were also performed in which AgI was generated in situ on the surface of the colloidal silver particles by stepwise addition of iodine, the question being whether small particles of bulk AgI are formed in this way or whether a homogeneous layer of adsorbed AgI having the properties of the precursor" is formed. The adsorption of SH-ions and the coadsorption of SH-with I- and C6H5S(thiophenolate) were also investigated. Experimental Section
The silver sol was prepared by yirradiating a solution of 1 X M AgC104, 0.1 M 2-propanol, and 3 X 10-5 M sodium polyphosphate as described previously.] The ionizing radiation generates reducing free radicals and hydrated electrons in the solution, which reduce the Ag+ ions. The solution was irradiated in a glass vessel carrying a side arm with an optical cuvette (1-cm optical path length) and a septum. Before irradiation, the solution was deaerated by bubbling with argon. Reagents were added to the sol by injectinga known amount of deaerated solution through the septum. Excess electrons were deposited on the silver particles by adding 10-2 M acetone to the solution and irradiating it with monochromatic 265-nm light from a 450-W xenon lamp (Kratos monochromator). In the presence of 2-propanol and acetone in the solution, organic free radicals are generated by UV light that diffuse to the silver particles and transfer an electron to them.3 A silver particle can store many electrons and is cathodically polarized by the electron transfer. A M silver iodide sol was prepared by precipitationof Ag+ ions with a slight excess of KI. This solution was added in small amounts to the silver sol in some experiments. 0022-3654/58/2097-0679$04.00/0
Results Figures 1 and 2 show how the absorption spectrum of a 1 X
IO4 M silver sol changes when NaSH and KI are added in different amounts. In both cases, the plasmon band decreasesin intensityand becomes broader. The decrease as a function of the concentrationof added adsorbate is shown in Figure 3 (left part). Initially, both SH-and I- have about the same effect, whereas SH-is more effective at higher concentrations. Above about 20 pM of the additives, little change is observed. This is interpreted as being theconcentrationat which monolayer coverage is reached, the SH- and I- ions added in excess of 20 pM remaining in the solution. In fact, based on the known mean diameter (60 A) of the silver particles and using an atomicdiameterof (3 Vm/4?rNA)I/3 (Vm: molar volume of silver), one calculates a concentration of surface atoms of 20 rM. This conclusion is corroborated by the absorptionchanges that take place in the 220-230-nm region where SH- and I- in aqueous solution have charge transfer to solvent (CTTS) bands. As long as less than 20 pM is added, these bands are not present. They appear, however, at the higher concentrations of additives. It is concluded that SH- and I- lose their CTTS bands upon chemisorption. The wavelength of the maximum of the plasmon band also changes upon the adsorption of anions. This is shown on the right-hand side of Figure 3. In the case of the pure silver sol, the band is located at 380 nm. With increasing I-concentration, the maximum is red-shifted until it reaches about 420 nm, when one monolayer of anions is adsorbed. In the case of SH-, there is a blue-shift at the beginning, followed by a red-shift at higher concentrations. Note also that the width of the band is especially great in the case of added SH-. It has been shown previously that the SH-ions are desorbed when excess electrons are deposited on the silver parti~les.~ This can also be brought about in the case of adsorbed iodide. Figure 4 shows the absorption spectrum of a silver sol (pH = 10.5). which contained 20 pM iodide, before and after deposition of excess electrons. One can see that electron deposition causes the narrow absorption band of silver to develop. At the same time, the CTTS band at 225 nm of free iodide appears. After 10 min of electron deposition, all the iodide was desorbed. When the solution was now aged for several hours, the plasmon band became 0 1993 American Chemical Society
Linnert et al.
680 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 2.0 I
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A [nml Figure4. Absorption spectrumof a 1 X M silver sol, which contained 20 pM iodide, 0.1 M 2-propano1, and 0.01 M acetone, before and after electron deposition. To deposit the electrons, the solution was irradiated with 265-nm light (acetone absorption band) for various times. Solution pH was 10.5. 2.0
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Figure 2. Absorption spectrum of a silver sol before and after addition of various concentrations of I-. pH = 6.0.
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Figure 5. Absorption spectrum of a silver sol before and after addition of various amounts of iodine. 2.0
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Figure 3. Decrease in absorbance (at the band maximum) (left) and shift of the wavelength of maximum absorption (right) as functions of the concentration of added SH- and I-. Data from Figures 1 and 2.
broader again. At the same time, the CTTS band at 225 nm decreased in intensity; Le., the iodide ions readsorbed onto the colloidal silver particles. This is due to the loss of the deposited electrons as they react with solvent molecules (to yield hydrogen), and, as the Fermi level is anodically shifted, I- ions are readsorbed. In the experiment of Figure 5, various amounts of iodine were added to a silver sol. At the same equivalent iodine atom concentration, the silver plasmon band is as strongly affected as in the experiment of Figure 2, where iodide anions were added. The steep absorption threshold at 425 nm of bulk AgI appears for the first time after the addition of 20 p M iodine. One should keep in mind that the surface area of the silver particles became smaller with increasing iodine addition as silver atoms were consumed by the oxidation. This effect is not important for the smaller iodine concentrations, but for 40 p M 12, the silver band
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Surface Chemistry of Colloidal Silver
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 681 red-shifted further (Figure 9). Up to about 10 pM of added SH-, the CTTS absorption band at 260 nm of the thiophenolate anion did not develop. This contrasts the results obtained above in the simultaneous adsorption of SH- and I-, where the adsorption of SH- led to the desorption of iodide at an early stage. As can be seen from the first appearance of the 260-nm band; the addition of 15 pM SH- finally causes some desorption of C6HsS-.
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Figure 7. Absorption spectrum of a 1 X M silver sol carrying 7.5 rrM I- after the addition of various concentrations of SH-.pH = 10.5. 2.0
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Figure 9. Absorption spectrum of a silver sol containing 8 pM
thiophenolatebefore and after addition of various concentrationsof SH-. The coadsorption of I- and SH- was investigated in the experiments of Figure 7. Various small amounts of SH- were added to a 1 X lo4 M silver sol carrying 7.5 pM I-. At this concentration of iodide, only about one-third of the silver surface was covered (compare to Figures 2 and 3). Upon the addition of 2.5 pM SH-, the silver absorption band was decreased slightly and no sign of free iodide in solution was seen. However, after the addition of 7.5 pM SH-, some I- desorbed (increased absorption a t 225 nm); the silver band was further damped and blue-shifted (which is typical for SH- adsorption, Figure 1). Further desorption of I- took place when the SH-concentration was increased to 10 and finally 15 pM. The coadsorption of CsHsS- and SH- was investigated in the experiment of Figures 8 and 9. The adsorption of small amounts of thiophenolate causes broadening and a red-shift of the plasmon absorption band. The highest concentration in Figure 8 was 8 pM. The solution was transparent and stable for weeks without any sign of agglomeration of the silver particles, such as an increased absorption at longer wavelengths. Solutions that contained thiophenolate at concentration above 15 pM became slightly opalescent. When SH- ions were added to the sol containing 8 pM thiophenolate, the plasmon band decreased and
Discussion The shape of the plasmon absorption band of colloidal silver is very sensitive to the presence of adsorbed substances. By observing the changes in the shape of the absorption band, one can determine whether adsorption of an additive has occurred. The changes can be brought about either because of changes in the refractive index of the medium surrounding the metal particles or because the dielectric properties of the metal particles themselves are changed by chemisorption. It is expected from Mie theory6 that a change in refractive index would lead solely to a shift in the position of the band maximum and would not significantly influence the width of the band. This mechanism cannot explain the drastic damping of the plasmon oscillation observed. Nothing is known at the present time about the nature or cause of this damping. Various mechanisms are conceivable. If the surface silver atoms together with the adsorbed iodide ions form an absorbing surface complex, then the presence of an absorbing shell around the particle would lead to damping. However, a shell only one monolayer thick cannot cause such damping, unless unrealistic values are used for the optical constants of the surface complex. More likely, the adsorbate perturbs the optical properties of the silver by increasing €2 (imaginary part of the dielectric constant). Since c2 is inversely proportional to the lifetime of the plasma oscillations, this implies that the adsorbates shorten the lifetime of the plasmon oscillations. It is conceivable that this comes about via coupling of the plasmons with the phonons in the adsorbate layer. Figure 4 illustrates once more that the adsorption/desorption equilibrium and the position of the Fermi level in the silver particles are coupled. Both electron transfer and adsorption/desorption are reversible on thesilver particles without changing the particles themselves. Iodine oxidizes silver: I2 2Ag 2AgI. The silver iodide produced could form either AgI particles, which are more or less attached to the silver particles, or a homogeneous mantle around the silver particles. Attached AgI particles hardly affect the silver plasmon band (Figure 6). However, it was found that added iodine, at low concentrations, had practically the same effect as added iodide (at the same overall concentration of iodine atoms). In the latter case, "precursor" structures Ag,&Agd+Iare postulated;'-' we interpret these findings by postulating that similar structures are also present during the early stages of the oxidation of the silver particles by iodine, although in the case of iodine there should also be a shift of the Fermi level in the silver particles to a more positive potential. When more than about one monolayer of AgI is formed, the bulk properties of AgI such as the absorption threshold a t 425 nm develop. Unusual optical properties due to size quantization of very small AgI particles have been r e ~ r t e d ; in ~ Jthe present work, it is not so much the sizequantization that determines the unusual properties of the first monolayer of AgI but the chemical interaction of AgI molecules with the silver particles. The experiment on the coadsorption of I- and SH- (Figure 7) shows that SH- is more strongly adsorbed than I-. When equal amounts (7.5pM) of both anions are present, the silver plasmon band shows the shape expected for SH-adsorption and free iodide appears in the solution. Although with 7.5 r M I- and 7.5 pM SH- the surface is only covered with about three-quarters of a monolayer of anions, desorption of I- occurs, and the adsorbed SH- ions determine the appearance of the optical spectrum.
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682 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
Linnert et al.
The displacement of the iodide from the surface by SH- before the surface is saturated indicates that chemisorption onto the metal surface is a cooperative phenomenon. If an iodide ion is displaced from a favorable site by SH-, then it should simply be shifted to a site with a lower adsorption energy. That it is desorbed from the surface demonstrates that the adsorption energy at all the surface sites is altered by the adsorption of the hydrogen sulfide anion. This coupling arises through the changes in the position of the Fermi level. In the case of the SH- ion the Fermi level is pushed to such negative values, even when less than a monolayer is present, that adsorption at other sites is no longer favorable for I- ions. Adsorbed anions of thiophenol are not so easily desorbed by added SH-ions (Figure 9). It is concluded that C6H& and SHare about equally strongly adsorbed on the silver particles.
dielectric properties of the surface-adsorbate complex nor the changes to the dielectric properties of the metal are generally known. As mentioned above, various effects can be responsible for the changes in the shape of the plasmon absorption band. For example, the fact that adsorption of small amounts of SH-causes a blue-shift of the band, but adsorption of larger amounts leads to a red-shift (Figure 3), already indicates that more than one effect is responsible for the optical changes. It is hoped that the experimental observations presented in this paper will finally lead to a better understanding of these effects and will enable a theory for the optical properties of surface-modified metal particles to be developed.
Final Remarks
7RA7 ,-
The optical properties of small metal particles can be calculated by using Mie theory when the dielectric properties of both the metal and the surrounding medium are known and when there is no chemical interaction between the two. However, when the particle surface is chemically modified by adsorbates, neither the
References and Notes (1) Henglein, A.; Linnert, T.; Mulvaney, P. Ber. Bunsenges. Phys. Chem. 1990, 94. 1449. (2) Mulvaney, P.; Linnert, T.; Henglein, A. J . Phys. Chem. 1991, 95,
(3) Henglein, A.; Mulvaney, P.; Linnert, T. J . Chem. SOC.,Faraday Discuss. 1991, 92, 31. (4) Schmidt, K. H.; Patel, P.; Meisel, D. J . Am. Chem. Soc. 1988, 110, 4882. (5) Henglein, A.;GutiCrrez, M.; Weller, H.; Fojtik, A.; Jirkovsky, J. Ber. Bunsenges. Phys. Chem. 1989, 93, 593. (6) Mie, G . Ann. Phys. 1908, 25, 377.
