Langmuir 1994,10, 3018-3022
3018
Reduction and Aggregation of Silver Ions in Aqueous Gelatin Solutions S. Kapoor,? D. Lawless,? P. Kennepohl,tj* D. Meisel,*??and N. Serpone*J Argonne National Laboratory, Chemistry Division, Argonne, Illinois 60439, and Concordia University, Montreal, Quebec, Canada H3G 1M8 Received March 25, 1994. In Final Form: June 28, 1994@ Radiolytic reduction of silver ions and the subsequent formation of silver clusters were studied in aqueous gelatin solutions and are compared with the parallel processes in aqueous solutions. The presence of gelatin in the solution affects the early processes,via complexation of&+ ions with the amino acid moieties ofthe gelatin. The ratio of&+ to gelatin chains predeterminesthe kinetic consequencesto the agglomeration processes. This ratio may accelerate or inhibit any of the processes that involve silver ions (reduction as well as growth). The complexationreduces somewhatthe rate ofreductionby hydrated electrons. However, when all the ions are complexed to the gelatin, the agglomeration may become very fast; at the extreme the agglomeration rate is determined by the rate of reduction. Some of the small AgQm+bind to the gelatin stronger than Ag+ions. Excess silver ions enhance the stability of smaller transient clusters in the presence of gelatin. Three long-livedaggregatesof different sizes are stabilizedby the gelatin upon complete reduction of the silver ions.
Introduction The reduction of Ag+ ions in aqueous solutions leads to the formation of A$ atoms. The Ag atoms initially form various complexes with Ag+ ions and subsequently agglomerate to form oligomeric clusters. These clusters eventually lead to the formation of metallic colloidal silver particles of a diameter of several nanometers. It is well established that these silver particles show an intense sharp surface plasmon absorption band in the 380-400 nm regi0n.l Recently, these processes have been investigated in some detail to determine the mechanism by which colloidal silver is formed.2-6 In some studies, stabilizers such as polyacrylic acid' or polyphosphate8were used to stabilize small silver clusters and it was shown that the absorption bands of the silver clusters are sensitive to the species adsorbed at the cluster s ~ r f a c e . ~ The reactivity of the primary radicals from water radiolysis ( O H and e-aq)with gelatin has been examined recently.1° The effect of high concentrations of gelatin on the diffusion of small radicals, when a rigid matrix is formed, was also reported.1° These studies have shown that the macrorigidity of the matrix does not affect the diffusion rates of these species. Earlier studies have established that gelatin complexes to Ag+ in basic media, probably through methionine.11-13 Such complexation of the gelatin to Ag+ may be important in controlling both
Table 1. Physical Parameters of Inert Gelatin I A G . KG4322 Used throughout the Present Study
physical property
or chemical content setting point 10%("C)
analysis result
transmittance (%) PH
moisture content (%) sulfur dioxide (ppm) calcium (ppm) chloride (ppm) isoionic point
26.7 93 5.8 10.7 nil 5100 740 4.88
the reduction rate and the agglomeration processes of silver in the photographic processes. Similar to the aqueous gelatin solutions, Ag+ ions a t the surface of silver halide grains bind to the pendant groups ofthe polypeptide chain in the gelatin.12 These interactions are known to modify the absorption spectra of the developed silver grains and are expected to affect the generation of silver clusters upon reduction. In the present work, an attempt is made to reduce the gap between neat aqueous systems and the photographic emulsion system. The effects of relatively small concentrations of gelatin on Ag+ reduction in aqueous solutions, and on the subsequent aggregation of silver, are investigated as models for the more complex photographic emulsion. The results are compared with similar experiments in gelatin-free aqueous solutions.
t Argonne National Laboratory.
* Concordia University. @
Abstract published in Advance ACS Abstracts, September 1,
1994. (1) Henglein, A. Chem. Reu. 1989,89,1861-73. (2) Gutierrez, M.; Henglein, A. J . Phys. Chem. 1993,97,1136811370 _.
(3)Ershov, B. G.;Janata, E.; Henglein, A. J.Phys. Chem. 1993,97,
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R3R-AR
(4)Ershov, B. G.; Janata, E.; Henglein, A.; Fojtik, A. J . Phys. Chem. 1993,97,4589-94. ( 5 ) Mostafavi, M.; Delcourt, M. 0.;Picq, G. Radiat.Phvs. Chem. 1993, 41,453-9. (6)Platzer, 0.; Amblard, J.; Marignier, J. L.; Belloni, J. J . Phys. Chem. 1992,96,2334-40. (7)Mostafavi, M.; Keghouche, N.; Delcourt, M.-0.; Belloni, J. Chem. Phys. Lett. 1990,167,193-7. (8)Mulvaney, P.; Henglein, A. J . Phys. Chem. 1990,94,4182-8. (9)Henglein, A.Zsr. J. Chem. 1993,33,77-88. (10)Zagorski, 2. Radiat. Phys. Chem. 1989,34,839-847. (11)Rose, P. I. In The theory ofthe photographic process, 4th ed.; James, T. H., Ed.; MacMillan Publishing Co.:New York, 1977;pp 5167.
Experimental Section All chemicals were of the highest purity commercially available
and were used as received. Silverperchlorate(JohnsonMatthey > 99.9%)was the source of&+ ions. Nanopure water was used throughout this study. Gelatin, from limed ossein and HzOz treated (Konica Corporation's KG 4322), was a generous gift from Dr. S.Ehrlich (Kodak). The properties of this specificgelatin are shown in Table 1. The gelatin was allowed to swell by soaking in water for 1530 min at ambient temperature. It was then dissolved by warming the solution for 2-3 min in a 45-50 "C bath with continuous stirring. All solutions were freshly prepared and kept in the dark to prevent possible photochemical reactions. They all contained 1mM sodium borate buffer (pH 9.2) and either (12)Stumer, D. M.; Marchetti, A. P. In Imaging Processes Materials, 8th ed.; Sturge, J. M., Walworth, V., Shepp, A., Eds.; Van NostrandRheinhold: 1989;pp 71-109. (13)Russel, G.J . Photogr. Sci. 1967,15, 151-157.
0743-746319412410-3018$04.50/00 1994 American Chemical Society
Reduction and Aggregation of Silver Ions
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Vol.10,No.9,1994 3019
8 m
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a
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Figure 1. Transientabsorptionspectra following4-11s electron pulse irradiation ([e-,,] = 3.9 x M) of solutions containing 1x M AgC104, 0.1 M tert-butyl alcohol, 0.01 M NaC104 at pH 9.5. 0.1 or 1 M tert-butyl alcohol to scavenge O H radicals. It is well
established that t-BuOH does not react with Ag+ or the various clustersthat evolve from its reduction. Unless otherwise stated, all solutions were deaeratedby bubblingargonusing the syringe technique. Absorption spectra were recorded on a Varian Cary 5 spectrophotometer. For pulse radiolysis experiments,electron pulses of 4 ns to 3 ps duration generatede-,, in the concentration range of (2-100) x M per pulse. For dosimetry, a N2O saturated 0.01M KSCN solution was used [G(OH.)= G(SCNk*= 6.0 molecules/100 eV, €480 = 7600 M-l cm-ll.
Results and Discussion Reduction of Ag+ Ions in the Absence of Gelatin. The reduction of Ag+ in aqueous solutions in the absence of gelatin has been studied For comparison purposes with the gelatin-containing solutions, we show the spectral evolution of silver clusters in the absence of gelatin in Figure 1. This sequence of time-resolved transient absorption spectra was collected following 4-11s electron pulses in aqueous solution containing 1 x M AgC104 and 0.01 M NaC104. A broad band centered a t 360 nm develops during the first 0.5ps following the pulse. This correspondsto the well-established band 0fAgO.l The atoms complex with Ag+ ions to generate Ag2+ species, whose spectrum is fully developed 3 pus after the pulse. The bimolecular rate constant for the formation of Ago was 3.3 x 1Olo M-' s-l, and for the formation of Ag2+ it was 1.4 x 1O1O M-l s-l, in reasonably good agreement with the literature value.4 As can be seen in Figure 1,the band commonly assigned to Ag2+ with a maximum at 310 nm is actually composed of two bands peaking a t 308 and 315 nm. The splitting ofthe 310-nm absorption band into two bands was recently observed in the presence of p01yacrylate.l~ It is clear from Figure 1that several absorption bands, a t 290,308,315, and 325 nm, can be assigned to Agz+ even in the absence (14) Mostafavi, M.; Delcourt, M. 0.;Keghouche,N.; Picq, G. Radiat. Phys. Chem. 1992,40,445-50.
300
350
400
450
Wavelength, (nm)
Figure 2. Transientabsorptionspectrafollowing40-11selectron pulse irradiation ([e-,,] = 2.0 x M) of solutions containing 1x M Agc104,O.l M tert-butyl alcohol 0.01 M NaC104 at pH 9.5.
of any stabilizer. These bands, in particular a t 308 and 315 nm, are very narrow and required a 2-nm resolution to be observed. They appear as a single band at 5-nm resolution. The two bands decay a t identical rates, indicating that they belong to the same species. Figure 2 shows the transient spectra obtained in aqueous solution containing 1x M AgC104 and 0.01 M NaC104 at pH 9.5. A 40-ns electron pulse was employed for this reduction, to generate 2 x M e-aqin aqueous solution. A broad band centered around 355 nm, due to Ago, was observed at -1.0 ps after the pulse. However, as this band decays, the known bands at 310 and 290 nm do not seem to grow in. Large Agn-Agm+clusters do not appear either, in spite of the complete reduction of Ag+. Rather, it can be seen in Figure 2 that the spectra contain a near isobestic point a t approximately 295 nm during the time window of 3-15 ps. This indicates that under these experimental conditions only two species are involved: Ago atoms converting to Ag2O. The latter have absorption maxima at 275 and 310 nm. At later times (up to 200 ps), the dimer decays with little new features in the accessible spectral range. Results presented below clearly show that the complete reduction eventually leads to colloidal silver formation, indicating that the lack of new spectral features as the dimer decays is not due to its oxidation. Examination of the spectra in Figure 2 accordingly leads to the conclusion that 6295Ag" 2c295k2. Similarly the extinction coefficient at 275 nm per silver atom remains nearly constant as the clusters grow during the first 5-298 pus following the complete reduction of Ag+. However, the spectrum of the clusters does change during this time period. Effect of Gelatin. No reduction of gelatin by e-tq was observed in the time scale and gelatin concentration of our experiments. Figure 3 shows the effect of gelatin on the rate of the reaction of e-aq with Ag+. It can be seen that the rate constant decreases as the concentration of
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3020 Langmuir, Vol. 10,No. 9,1994
Kapoor et al.
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: :
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Figure 3. Dependence of the rate constant for the reaction of e-aqwith Ag+ ions on gelatin concentration at pH 9.5. Gelatin
and AgC104 concentrations are shown in the figure; all other concentrationsare the same as in Figure 1. Insert: Reciprocal fraction of complexed Ag+ vs reciprocal gelatin concentration. gelatin increases. The reduction in rate is attributed to complexation of Ag+ with the gelatin. Previous studies have demonstrated binding of Ag+ to methionine groups in the gelatin.11J3 The reactivity of the complexed ions toward e-aq is lower than that of free Ag+. The observed rate of formation of AgO can be expressed as
where [Ag+]ois the initial Ag+ concentration, kobsis the observed rate constant, and the indexes f and b denote free and gelatin-boundspecies, respectively. From Figure 3 we obtain kf = 3.3 x 1O1O and k b = 1.1 x 1O1O M-l s-l. The fraction of bound Ag+ is given by
which can be rearranged to give
(3) Assuming a simple equilibrium for binding Ag+ to gelatin (i.e., binding is independent of Ag+ loading), the equilibrium constant can be estimated from the results in Figure 3 and using eq 4
1
-=1+
f
1 mgelatin]
(4)
These results, shown in the insert of Figure 3, agree well with eq 4. The slope of the line yields K = 9 (wt %>-l.
Wavelength (nm)
Figure 4. Transient absorption spectra following 4-11s electron pulse irradiation of solutions containing 0.1 wt % gelatin; all other conditions and concentrations were as in Figure 1.
The spectral evolution of the various reduced silver species under conditions identical to those of Figure 1, but in the presence of 0.1 wt % gelatin, is shown in Figure 4. Comparison with Figure 1reveals that the absorbances are approximately half their intensity in the absence of gelatin. Since gelatin does not compete for e-aq a t the concentration used, we conclude that the molar absorptivity of Ago and its successor cluster, Ag2+, is reduced upon complexation to gelatin. As can be recognized upon comparing the spectra at longer times, for larger clusters this effect diminishes, presumably because the ratio of [AgOHbound gelatin] decreases. The spectrum 5ps after the pulse is predominantly that ofAg2+(Figure 4). It is red-shifted from 315nm in aqueous solutions to 320 nm in the gelatin-containing medium. The rate of formation of the Ag2+ species was monitored a t 310 nm at various gelatin concentrations (up to 0.1w t %) and at Ag+ concentrations in the range of (1-10) x M. At 1 x M Ag+, the formation rate of Ag2+ dropped from 1.4 x 1O1O M-l s-l in the absence of gelatin to 1.1 x 1O1O M-l s-l in 0.1 wt % gelatin. The latter is essentially identical to the rate of reduction of Ag+ (see Figure 3). One, therefore, concludes that as the concentration of gelatin (or Ag+)increases, the conversion of Ago to Ag2+ increasingly occurs within a single gelatin microdomain. When both Ago and Ag+ reside within the same microdomain their combination is fast and the ratedetermining step becomes the reduction of Ag+ to Ago. Figure 5 shows the dependence of the decay traces at 310 nm on gelatin concentration. The decay at this wavelength has been shown to describe the conversion of Ag2+ to Ag42+(Amm = 265 nm) in the absence of gelatin.4 It was previously reported, and confirmed here, that in the absence of gelatin the absorption of Ag2+ at 310 nm decays essentially to zero within -500 As can be seen in Figure 5, increasing the gelatin concentration increases the rate of the decay of Ag2+. Concomitantly,
Reduction and Aggregation of Silver Ions
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d
Langmuir, Vol. 10, No. 9, 1994 3021
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Wavelength (nm)
Figure 5. Effect of [gelatin] on the decay of Agz+ at 310 nm following 4-ns elecrtron pulse. All other conditions and concentrations were as in Figure 1.
Figure 6. Transient absorptionspectrafollowing40-nselectron pulse irradiation of solutions containing 0.1 w t % gelatin; all other conditions and concentrations were as in Figure 2.
the residual absorption a t 310 nm increases on increasing [gelatin]. As the rate of decay of Agz+ increases with increasing [gelatin], the absorption at 275 nm (attributed to Ag42f) decreases. Both these observations can be rationalized assuming that Agz+decays to a species larger than Ag2’. Indeed, inspection of Figure 4 reveals that a band at 325 nm has already developed at 40 ps in the presence of 0.1wt % gelatin. This band is absent in Figure 1 (no gelatin). We thus conclude that the presence of several silver atoms, formed upon the reduction of several Ag+ ions, in the same gelatin microdomain enhances the aggregation process toward larger Agn clusters. We now try to identify the enhanced residual absorption that is observed at 310 nm (Figure 5). I t was concluded above that under the conditions of Figure 5 only partial complexation of Ag+ ions to the gelatin chain occurs. The question, therefore, arises whether the two bands, at 275 nm and 310 nm (t 2 40 ps), which also appear in Figure 1, are due to a residual fraction of silver clusters that remains free in solution. This cannot be the case since no decay is observed at 310 nm during the first 200 p s in the presence of 0.1 gelatin (Figure 5). If it had been the case that cationic Ag42+clusters were free in the aqueous solution, some decay would have been discernible. Thus, it appears that essentially all of the reduced silver resides on gelatin chains approximately l o p s following the pulse when 0.1 wt % gelatin is present. As is evident in Figure 3, under the same conditions -50% of the silver ions are still free in solution. To achieve such preferential relocation of silver clusters (Ag,*+), one would probably invoke hydrophobic interactions between the clusters and the gelatin. Complete reduction of silver ions in gelatin-containing solutions was done under otherwise identical conditions as in Figure 2 but in the presence of 0.1 wt % gelatin. Spectra recorded are shown in Figure 6. Quantitative analysis of these spectra was difficult because aggregation of clusters is substantial at times when solvated electrons still have not completely disappeared. Qualitatively,
however, the spectra show similar features to the gelatinfree spectra. The isosbestic point at 295 nm is not observed in this case since several clusters are growing simultaneously. Because Ag+ has been completely reduced, the clusters with bands at 305 and 325 nm could not be stabilized (e.g., compare with Figure 4). This is taken as another indication that silver ions are required for stabilization of small silver clusters in the presence of gelatin. Stabilization of small oligomeric clusters was recently achieved in the presence of polyphosphate8 and polyacrylate.’J4 It was proposed that the repulsive forces between the polymeric chains prevent agglomeration of small silver clusters. Since the gelatin chains are negatively charged at pH 9.5, a similar situation can be expected. Long-lived clusters were prepared by reducing Ag+ions by trains of pulses in aqueous solutions containing 1 x MAgC104,l.O M tert-butyl alcohol and 1 wt % gelatin at pH 9.5. Pulse widths of either 40 ns or 3 pus were employed at intervds of 15 or 0.5 s for the two pulse widths, respectively. Similar types of clusters were formed with either of the pulse sequences used. The intensity of absorbance of the various bands was larger for higher doses. A typical case is shown in Figure 7 where -30% of Ag+ has been reduced. None of the bands observed at short time scales could be observed at these slow time scales. However, we do observe several bands at higher wavelengths, which probably correspond to larger aggregates of Ag,. Interestingly, these bands are better resolved when the time interval between consecutive pulses is shortened. This may reflect the higher stability of smaller aggregates when silver ions are present. Varying the gelatin concentration in the range of 0.3-1 wt % had little or no effect on the spectra and intensity of the various aggregates and they remain identical to those shown in Figure 7. Three distinct bands can be discerned in the spectra of Figure 7. It is clear from the differences in these spectra
Kapoor et al.
3022 Langmuir, Vol. 10, No. 9,1994 1.6
+
half height of that band exceeds 100 nm. Clearly these bands represent small particles whose absorption spectra are shifted probably by the presence of gelatin at their surface. Further evidence that the three bands originate from particles of three distinct sizes comes from their stability in the presence of coagulants. For example, the addition of methyl viologen does not lead to oxidation of any of these silver particles (no radicals are formed). Instead, coagulation occurs with the band at higher wavelengths disappearing at the lower viologen concentrations.
Conclusions
Wavelength, nm
Figure 7. Optical absorption spectra ofvarious long-livedsilver clusters. The silver clusters were generated using a train of three pulses, 3 ps wide each. The solution contained 1 x M AgC104, 1wt % gelatin and 1.0 M tert-butyl alcohol at pH 9.5.
at various times that they originate with three different particles of various sizes and stability. From the increase in absorbance at 380 nm at the expense of the bands at 520 and 430 nm during several days (Figure 7), one concludes that the latter is not merely due to particles of larger sizes. Conversion of large particles t o smaller ones in this regime is, of course, thermodynamically forbidden. It can also be recognized that their absorption spectra cannot be described by the Mie theory. For example, for a band at 520 nm (completelydominated by the scattering component),particles of r = 60 nm are required using the Mie theory calculation^.'^ However, with the optical parameters of Johnson and Christy,15the half-width at
The reduction ofAg+ions to the atoms, and the ensuing agglomeration processes, have been studied in gelatincontaining aqueous solutions. The gelatin affects both, the reduction and the agglomeration,relative to their rate in the absence of the gelatin. The former is slower in the gelatin-containing solutions. We attribute this to the binding of Agf to methionine groups in the gelatin. However, increasing the silver ion concentration along the same gelatin chain enhances aggregation to clusters of silver. The rate of the reduction under these conditions may then become the rate-determining step in the aggregation process. The small clusters bind to the gelatin stronger than Ag+ itself indicating that hydrophobic interactions contribute to the binding of the clusters to the gelatin. In accordance with previous observations, excess silver ions were found to enhance the stability of small silver clusters. Whereas the stabilization of very small long lived silver clusters has already been reported, larger clusters of silver are stabilized by gelatin. The current work shows that the gelatin is capable of stabilizing more than one type of silver particle. The coexistence of these types of particles in the same solution for long periods of time may result from the coexistence of several microdomains in the gelatinous regions of the solution.
Acknowledgment. Work at ANL is performed under the auspices of the Office of Basic Energy Sciences,Division of Chemical Science,US Department of Energy, Contract No. W-31-109-ENG-38. We are grateful to Dr. S. Ehrlich (Kodak) for providing samples of gelatin. (15)Johnson, P.B.;Christy, R. W. Phys. Rev. B 1972, 6, 4370-9.
