4182
J . Phys. Chem. 1990, 94, 4182-4188
reduction of quenching ability due to the presence of the aggregates. For both the NBD-DPPE/RhB-DPPE and HHC/RhB-DPPE systems the experimental values of Ro given in Table I1 agree favorably with that calculated theoretically via eq 2. The energy-transferred fluorescence intensity also seems to agree favorably with that given by eq 9. It is also important to note that there is far less scatter in these systems than in the DilCls(3)/DilCls(5) systems. All of these points seem to indicate that these monolayers, at least with respect to the acceptor molecules, are homogeneous. The linearity of RhB-DPPE fluorescence shown in Figure 10 also suggests that no self-quenching is occurring between RhB-DPPE molecules at the concentrations used in these experiments. This in turn is further evidence for the homogeneity of RhB-DPPE in DOPC monolayers. The intensity measurements over time needed to calculate the quenching curves for both these systems shown in Figures 8 and 9 were also constant and showed less (about 5%) variation than that of the DilC,,(3)/DilCI,(5) systems. This suggests that if either NBD-DPPE or HHC is aggregated in the systems studied, the aggregates must be much smaller than the area illuminated by the optical fibers. In light of the above discussion, it is interesting to look at the a-A isotherms given in Figures 1 and 6. For the DilC18(3)/ DilC,,(5) systems all of the components, including the diluent molecules, exhibit isotherms with either plateau regions or some type of phase transition. The NBD-DPPE/RhB-DPPE/DOPC and HHC/RhB-DPPE/DOPC systems, on the other hand, with the exception of NBD-DPPE, all show continuous isotherms with no plateaux or inflections. Since at least DilC,,(5) is inhomogeneouslq mixed in eicosanol and DPPC, this suggests that if the isotherms of individual components in a monolayer of more than one component show inflections or plateaux, the components may not mix together. On the other hand, as RhB-DPPE at least seems to be homogeneously mixed in DOPC, then this suggests that multicomponent monolayers, whose components exhibit smooth expanded type isotherms, may mix. This behavior was also ob-
served by Tweet et al.,24who looked at the concentration quenching of chlorophyll a in a variety of monolayer diluents. They found that chlorophyll a (which exhibits an expanded type isotherm) was concentration quenched in stearyl alcohol (a condensed film) to the extent that no fluorescence could be observed from the monolayer. However, when chlorophyll a was diluted in either phytol, oleyl alcohol, or triolein (all of which are expanded films), fluorescence could be observed from chlorophyll a in the monolayer. These workers attributed this behavior to the miscibility of chlorophyll a with diluents which possessed, like itself, expanded isotherms and its immiscibility in monolayers in which the diluents possessed condensed-type isotherms. These statements are in line with our observations described above. Conclusion
In summary, the observed quenching seen in this study can be attributed to FET in all the systems studied. FET also seems to be a method by which aggregation or immiscibility can be detected in monolayer systems, down to an aggregate size which may not be accessible by other methods. This technique is far more sensitive than that of F A isotherms as systems with only a few mole percent of molecules can be tested for homogeneity. As one of the prerequisites for the use of Langmuir-Blodgett films in many industrial applications is that the films must be homogeneous, this method may be a way of determining whether or not the precursor air/water monolayers are homogeneous, and thus provide a monitor by which homogeneous Langmuir-Blodgett films may be attained.
Acknowledgment. R.S.U. acknowledges support from a Commonwealth Postgraduate Research Award. This project was supported by an Australian Research Council grant. Registry No. NBD-DPPE, 92605-64-6; RhB-DPPE, 1261 11-99-7; DOPC, 4235-95-4; H H C , 26038-83-5; DilC,,, 41085-99-8; DPPC, 6389-8; eicosanol, 629-96-9.
Long-Lived Nonmetallic Silver Clusters in Aqueous Solution: A Pulse Radiolysis Study of Their Formation Paul Mulvaney and Arnim Henglein* Hahn- Meitner- Institut Berlin GmbH, Bereich Strahlenchemie, IO00 Berlin 39, FRG (Receiued: December 5, 1989)
Deaerated solutions of AgCIO4 containing (1-5) X IO4 M sodium polyphosphate and 0.1-1 M alcohol are irradiated with single electron pulses or trains of pulses and the intermediates of silver ion reduction detected by optical absorption measurements. Polyphosphate is found to exert a drastic effect on the reduction as small nonmetallic silver clusters are stabilized. In fact, when single pulses are applied, in which only a few percent of the Ag' ions are reduced, the cluster Ag,2+ is the final product which lives for hours. The elementary steps leading to this cluster are investigated. They include ( I ) reduction of Ag' ions by hydrated electrons, (2) complexation of the Ago atoms formed to yield Ag2', and (3) dimerization of the Ag2+complexes. Reaction 1 occurs rather slowly when Ag+ ions adsorbed on polyphosphate chains are involved. Under the experimental conditions chosen, reactions 1 and 2 occur mainly in bulk solution. Reaction 3 is accelerated by a factor of 10 in the presence of polyphosphate. The effect is understood in terms of the reduction in the dimensionality of reaction space as the Ag2+ complexes are attracted by polyphosphate chains. A small fraction of the Ag2+ escapes dimerization by stabilization on the polyphosphate chains. When pulse trains are applied, up to 100% Ag+ reduction can be achieved. The first clusters, i.e., Ag2+ and Ag4*+,are further reduced to yield larger clusters Ag, of nonmetallic silver having absorption bands at 300 ( n = 3) and 330 nm ( n > 3) and in the 345-360-nm range ( n >> 3). A small amount of colloidal metallic silver is also formed.
Introduction
When silver ions are reduced radiolytically in aqueous solution, silver atoms are generated which subsequently agglomerate to yield colloidal silver It has been found recently that small ( I ) Henglein, A . Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 556.
0022-3654/90/2094-4 182$02.50/0
clusters, which do not yet possess the properties of metallic silver, can be stabilized for a while when sodium polyphosphate is present during the r e d u c t i ~ n .These ~ ~ ~ clusters live for a few hours until (2) Tausch-Treml, R.; Henglein, A,; Lilie, J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1335. (3) Henglein. A,; Tausch-Treml. R. J . Colloid Interface Sci. 1981,80, 84.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4183
Nonmetallic Ag Clusters in Aqueous Solution they yield colloidal metal. The kinetics of silver salt reduction in the absence of a stabilizer has been studied previously by using the method of pulse radi~lysis.~,~ In the present paper these studies are resumed with solutions containing polyphosphate in order to find evidence for the stabilization of small silver aggregates. To date, metal clusters sufficiently long-lived to study their physical properties and chemical reactivity have not been prepared in solution. Silver clusters have been observed in vacuo,6 in matrices at low temperature,' in photographic plates,8 and in zeolite^.^ Their electrochemical potential in aqueous solution has also been calculated.1°
2.0 1
Experimental Section Pulse radiolysis was carried out with 3.6-MeV electrons from a Van de Graaff generator. The transient signals of the optical absorption were processed and recorded as described previously.'l The results are presented as specific absorbance, e, by dividing the absorbance signals by the concentration of reducing species generated and by the path length of the cell. All solutions were deaerated by bubbling with argon prior to irradiation. The application of the method is based on the fact that the incident electron radiation is mainly absorbed by the aqueous solvent and that the yields of the reactive species from the radiolysis of water are well-known.I2 These species are hydrated electrons, eaq-, hydroxyl radicals, OH, and a small amount of H atoms. They can attack solutes. In the present work, the hydrated electrons are used to reduce silver ions:
I
1
I
, '2'40'
' 3b0'
' ' ' 400 ' ' h lnml
'
' 460
Figure 1. Absorption spectrum of a solution at various times after the pulse. 1 X lo4 M AgCIO, and 0.1 M methanol; absence of polyphosphate. 2,0
i
F A
The oxidizing OH radicals are scavenged by a dissolved alcohol. In these scavenging processes, free organic radicals are formed, such as the hydroxymethyl radical in the presence of methanol: CH30H
+ OH
-
CH2OH
+ H20
(2)
These organic radicals also have reducing properties. However, they cannot directly reduce silver ions because of the high negative standard redox potential of the system Ag+/Ago.' The radicals may contribute to reduction in a later stage by reducing clusters formed as a result of the ea; + Ag+ reaction.
Results Ag' Reduction in the Absence of Polyphosphate. The silver atoms formed in reaction 1 are known to react rapidly with Ag+
ions to yield Ag2+: Ago + Ag+
-
Ag2+
(3)
(4) Henglein, A. Chem. Phys. Lett. 1989, 154, 473. (5) Henglein, A. Chem. Reo., in press. (6) (a) Hilpert, K.; Gingerich, K. A. Ber. Bunsen-Ges. Phys. Chem. 1980, 844, 739. (b) Woodward, R.; Le, P. N.; Temmen, M.; Gole, J. L. J . Phys. Chem. 1987, 91, 2637. (c) Yamada, I.; Usui, H.; Tagaki, T. In Metal Clusters; Trager, F., zu Putlitz, G., Eds.; Springer: Berlin, 1986; p 37. (d) Fayet, P.; Waste, L. Ibid., p 77. (7) (a) Kolb, D. M. In Matrix Isolation Spectrscopy; Barnes, A. J., Orville-Thomas, W. J., Muller, A., Gaufres, R., Eds.; Reidel: Dordrecht, 1981; p 447. (b) Kolb, D. M.; Forstmann, F. Ibid., p 347. (c) Schulze, W.; Frank, F.; Charle, K.-P.; Tesche, B. Ber. Bunsen-Ges Phys. Chem. 1984,88,263. (d) Bechthold, P. S.;Kettler, U.; Schober, H. R.; Krasser, W. In Metal Clusters; Trager, F., zu Putlitz, G. Eds.; Springer: Berlin, 1986; p 163. (e) Bennemann, K. H.; Reindl, S. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 278. (8) Fayet, P.; Granzer, F.; Hegenbart, G.; Moisar, E.; Pischel, B.; Woste, L. In Metal Clusters; Trager, F., zu Putlitz, G., Eds.; Springer: Berlin, 1986; p 199. (9) (a) Gellens, L. R.; Mortier, W. J.; Schoonheydt, R. A,; Uytterhoeven, J . B. J . Phys. Chem. 1981, 85, 2783. (b) Ozin, G. A,; Hugues, F. J . Phys. Chem. 1983.87, 94. (IO) (a) Henglein. A. Top. Curr. Chem. 1988, 143, 1 1 3 . (b) Henglein, A. In Modern Trends of Colloid Science in Chemistry and Biology; Eicke, H.-F., Ed.; Birkhauser: Basel, 1985. (1 I ) Kumar, A.; Janata, E.; Henglein, A. J . Phys. Chem. 1988, 92, 2587. (12) See, for example: Henglein, A,; Schnabel, W.; Wendenburg, J . Einjiuhrung in die Strahlenchemie; Verlag Chemie: Weinheim, 1969.
300 400 h [nml Figure 2. Absorption spectrum of the solution of Figure 1 at longer times after the pulse. '230
The bimolecular rate constant of this reaction is 5 . 9 X lo9 M-' s - ~ . The ~ fate of the Ag2+ species is also known: it dimerizes 2Ag2+ ---* Ag42+
(4)
the rate constant being 3 . 0 X 1O8M-I sAg42+is a form of dimeric silver, which possibly is in equilibrium with other forms: Ag42+~1 Ag,+
+ Ag+ e Ag, + 2Ag+
(5)
As we do not know the positions of these conceivable equilibria, we will continue calling the product of reaction 5 A&+. A reason based on experimental observations is given below for Ag42+being the preferred structure of dimeric silver in aqueous solutions containing excess Ag+ ions. Figure 1 shows the absorption spectrum of a pulsed solution containing 1 X low4M AgC104 and 0.1 M methanol a t various times after the pulse. In this solution, hydrated electrons and hydroxymethyl radicals are formed. Only the hydrated electrons react with silver ions under the conditions of pulse radiolysis. The absorption coefficient, t, was therefore calculated by using the radiation chemical yield of the hydrated electron, G(ea,-) = 2.7 electrons per 100 eV of absorbed radiation energy. Reaction 1, the rate constant of which is 3 . 6 X 1Olo M-' s-I, occurs within 1 qs under the concentration conditions chosen. The first spectrum in Figure 1, taken at 0 . 6 3 ks, is due to Ago and a small contribution from hydrated electrons that had not yet reacted. After 1 . 9 qs, reaction 1 was over. However, a small fraction of the silver atoms were already consumed as reaction 3 is fast. Reaction 3 is completed after 6 . 9 p s in Figure 1, the spectrum at this time representing the known spectrum of Ag2+.2 This spectrum disappears in the millisecond range as shown by Figure 2. After 3 . 8 ms, when all the Ag2+ ions have disappeared, the solution has the
4184
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
1
-2
-1
@' 0
----L'L-.-l
':
Mulvaney and Henglein
i
1
I
0.5
1.5
1.0
[Ba!CIO,l,l
2
2.0
I 2.5
ImMl
:PF . ' ! A g I
10s
Figure 3. Rate constant of the reaction of eaq-with Ag+ as a function of the concentration ratio polyphosphate/silver ion. The AgC104 concentration was 1 X M, and polyphosphate was added in various concentrations (upper scale; polyphosphate molarities refer to the formula NaPO,). The solution contained 0.1 M methanol.
31
01 0
i
1
'
,
LO
20
60 80 [mMl Figure 5. Upper part: rate constant of the hydrated electron-silver ion reaction as a function of Ba2+concentration. The solution contained 1 X M AgC104 and 6 X IO4 or 2 X IO-) M polyphosphate. Lower part: rate constant as a function of added sodium perchlorate; 1 X IO4 M AgCIO, and 6 X M polyphosphate. INaCIO,]
'\'.
f,,,'Ag',
32:
- 2- -.i
~
-'
-L
0
^
log PP/As+
Figure 4 Fraction fads of adsorbed Ag+ ions and fraction &, of silver atoms formed i n bulk solution as a function of the concentration ratio polyphosphate/Ag+ Data from Figure 3
spectrum of Ag42f. It has previously been reported that the maximum of Ag42f absorption lies at 260 nm;2 we have to correct this on the basis of the present experiments as the maximum was found now to be at 275 nm. At longer times than shown in Figure 2, the 275-nm absorption of Ag42+decreases and the 380-nm absorption band of colloidal silver is built up.233 Reaction ea; Ag' A$ in the Presence of Polyphosphate. The rate constant of reaction 1 is 3.6 X 1Olo M-I s-I in the absence of polyphosphate.') In the experiments of Figure 3, the pseudo-first-order decay of the 700-nm absorption of the hydrated electron was recorded after application of an electron pulse to a M AgC10, and various amounts of solution containing 1 X polyphosphate. The rate constant k is plotted as a function of the concentration ratio polyphosphate/Ag+. It is seen that k decreases as more and more polyphosphate is added until a limiting value of 4.2 X IO9 M-] s-I IS ' reached. These results are understood in terms of an adsorption of Ag' ions on the polyphosphate chains, the complex formed being less reactive toward ea; than free silver ions. The overall negative charge of the polyphosphate chain repels the electrons. The two curves in Figure 4 were derived from the data in Figure 3 as follows. First, the fraction of silver ions adsorbed on the polyphosphate chains is calculated. The observed rate of reaction 1 is
-
+
kobs[Ag+jo = kads[Ag+lads + kb[Agflb
(6)
where [Ag'], is the overall silver ion concentration, kobs is the observed rate constant from Figure 3, and the indexes ads and b designate the corresponding entities in the adsorbed state and bulk solution, respectively. Letting fads = [Ag+]ads/[Ag+],, we can rearrange eq 6 to obtain fads
=
(kobs
- kb)/(kads
-
kb)
(7)
for the fraction of adsorbed silver ions. Using the above values of 3.6 X IOio and 4.2 X lo9 M-I s-l for kb and kads, respectively, (13) Anbar, M.; Bambenek, M.; Ross, A. B. Narl. Stand. Ref. Data Ser. ( U S . . Natl. Bur. Stand.) 1973. 43.
we calculated the curve forfads in Figure 4. The fraction of Ago atoms formed in the bulk is obtained as @b
=
-fads)kb/kobs
(8)
It can be seen from Figure 4 that under the conditions normally M Agf and 6 X lo4 M polyphosphate, employed, Le., 1 X loW4 90% of the Ag' ions are adsorbed but about 65% of the Ag atoms produced are generated in bulk solution. In Figure 5, upper part, solutions containing 1 X M AgCIO, and two different polyphosphate concentrations were used, in which the reaction between eaq- and Ag' is slow. Various amounts of barium perchlorate were added, and the rate constant of reaction 1 was determined. It is recognized that k increases with increasing Ba2+ concentration. At a concentration ratio Ba2+/polyphosphate of about 0.5, the rate of reaction in the absence of polyphosphate is reached. Barium ions are known to be strongly adsorbed on the polyphosphate chains,I4 one cation occupying two adsorption centers, Le., anion sites along the chain. With increasing Ba2+ concentration, the equilibrium Ba2+ + Ag+-PP
-
Ag+
+ Ba2+.-PP
(9)
is shifted to the right; more Ag+ ions are in the bulk solution, and the rate of reaction 1 is increased. In the lower part of Figure 5, the effects of added sodium perchlorate are shown. The rate constant again increases with increasing salt concentration, although higher concentrations are necessary than in the case of added barium salt. The final value of k does not reach 3.6 X 1Olo M-l s-I, i.e., the value in the absence of polyphosphate, which is ascribed to the high ionic strengths of the solutions containing large NaC10, concentrations. The normal kinetic salt effect then slows down the bulk reaction between eaq- and Ag'. In all these experiments, the buildup of the 360-nm absorption due to the silver atoms was also investigated. With increasing barium or sodium salt concentration in the polyphosphate-containing solution, the absorption of the hydrated electron disappeared more quickly and the 360-nm buildup became correspondingly faster. The Later Steps of Reduction in the Presence of Polyphosphate. Figures 6 and 7 show how the absorption spectrum of a solution containing 6 X IO-, M polyphosphate besides AgC104 and methanol changes after the pulse. The only difference between ~
(14) Van Wazer, J R , Callis, C F Chem Rec 1958, 58, 101 1
The Journal of Physical Chemistry, Vol. 94, No. IO, 1990 4185
Nonmetallic Ag Clusters in Aqueous Solution
.. :._
t
-
I
\
0
A
/
250
r
300
350
\
\
400
I
u 0_ t
-
450
X [nml Figure 6. Absorption spectrum of a solution containing 6 X lo-, M polyphosphate besides 1 X IO-, M AgCIO, and 0.1 M methanol at various times after the pulse. 1 X IO" M hydrated electrons generated in the pulse.
0
50
PS
Figure 8. Oscilloscope curves of the 310-nm absorption. 1 X lo4 M AgCIO,; different polyphosphate concentrations. Curve e': Ba(C104)2 added to solution e.
la1 - 0
1.0
0
2.0
[PPI IIO-'MI 0 225 250
350
300
X
400
450
[nml
Figure 7. Solution as in Figure 6 . High dose: 5 X 10" M hydrated electrons per pulse.
these two figures is the fact that pulses of different dose were applied. A pulse produced 1 X 10" M hydrated electrons in the experiments of Figure 6 and 5 X 10" M in those of Figure 7. At 1.9 ps after the pulse the absorption spectrum of Agocan be seen. This spectrum disappears within the next few microseconds as in Figure I . However, the spectrum of Ag2+ (peaking at 310 nm) appears only weakly and in a transitory manner and the spectrum of Ag42+(peaking at 275 nm) is present after less than 20 ps. It thus seems that reaction 4 is tremendously accelerated by polyphosphate. The time profile of the 310-nm absorption a t different polyphosphate concentrations is shown in Figure 8. In the absence of polyphosphate (a), the absorption decayed according to second-order kinetics as reaction 4 takes place. With increasing polyphosphate concentration (b-e), the maximum 310-nm absorption shortly after the pulse became smaller, the following decay faster, while the final absorption increased slightly. The decay followed first-order kinetics in the presence of polyphosphate. In Figure 9a, the reciprocal half-life is plotted as a function of polyphosphate concentration. The effects of barium perchlorate added to the polyphosphate-containing solution were also studied. The comparison of curves e and e' in Figure 8 shows that the addition of Ba2+ causes the 310-nm absorption to decrease more slowly; i.e., addition of Ba2+ has an effect similar to a decrease in polyphosphate concentration. In Figure 9b, the reciprocal half-life is plotted vs the concentration of Ba2+. One can see that even at high BaZ+concentrations the rate of the 310-nm absorption decay amounts to more than twice the rate in the absence of polyphosphate. Practically all the Ag2+ ions are formed in bulk solution under these circumstances. The fact that reaction 4 is still accelerated shows that formation of Ag2+on the polyphosphate chains is not a prerequisite for acceleration. Figure 9b also contains the results of experiments in which sodium perchlorate
--
3
ibi
"t
,
\
O
Y
o
without PP
0
0 0
0.5 50
-
BalClO,l, ,
,
I
1.0 mM Ba(CIO,), 100 mM NaCIO,
Figure 9. (a) Reciprocal half-life of 310-nmabsorption as a function of polyphosphate concentration (1 X lo4 M AgCIO, and 0.1 M CH30H). (b) Reciprocal half-life as a function of added Ba(C10J2 and NaCIO, ( 6 X IO-, M polyphosphate).
was added. Although rather large concentrations were used here, the rate of the 3 10-nm decay of the Ag2+changed little. However, the initial absorption at 310 nm was increased substantially by adding NaCIO,, demonstrating that Agz+ was formed more quickly due to the release of adsorbed Ag+ ions by added Na+ ions. Again, together with the results shown in Figure 5, lower part, one has to conclude that Agz+was almost exclusively formed in bulk solution at higher NaCIO4 concentrations. Nevertheless, the polyphosphate chains still had a significant accelerating effect on reaction 4. The spectra shown in Figures 6 and 7 did not change further up to the longest time of 15 s accessible in our pulse radiolysis setup. No absorption at 380 nm of colloidal silver appeared. It can thus be concluded that very early reduction products of Ag+ are captured and efficiently stabilized by polyphosphate. A closer inspection of the final spectrum (after 37 fis in Figure 6 and 16 ps in Figure 7) and comparison with the spectrum of A g t + in Figure 2 (3.8 ms) reveal that the spectrum has a shoulder at 350 nm in the case of the polyphosphate-containing solutions (Figures 6 and 7). This shoulder is more pronounced (as compared to the 275-nm absorption of Ag,Z+) at lower pulse doses. The species absorbing at 350 nm is formed during the decay of Ag2+. This was shown in an experiment, in which a sol containing a much
4186
The Journal of Physical Chemistry, Vol. 94 No. 10, 1990
Mulvaney and Henglein
5 Z 0.6
-
(D
fl
5; 0.4 3 m
0.2 0
25C
300
350
400
:5@
v
X [nmj
Figure 10. Irradiation of a solution with a pulse train .4bsorption spectrum after various pulse numbers. I X IO4 M AgClO,; 6 X IO4 M polyphosphate; 0. I M methanol.
higher AgC104 concentration (2 X M) was used. Under these conditions, reactions 1 and 3 are faster than reaction 4 even in the presence of polyphosphate. The absorption spectrum of Ag2+ could be clearly seen immediately after the pulse. During the decay of Ag2+, the 275- and 350-nm bands both built up, showing that a second species besides Ag,2+ (which is always the main product of Ag2+ decay) is formed. “Multiple Pulse” and Combined “y-Irradiation plus Pulse” Experiments. In the experiments described above only a few percent of the silver ions were reduced. In order to obtain larger conversions up to the complete reduction of the Ag+ ions, the experiment of Figure 10 was carried out. A 1 X IO4 M AgCIO, solution containing 0.1 M methanol and 6 X IO4 M polyphosphate was irradiated with a pulse train, the interval between the 1 ps pulses, which produced 1 p M hydrated electrons and radicals, being 3.3 ms. The spectrum after application of various pulse numbers is shown in the figure. Most of the absorption produced at low conversions lies in the 250-300-nm range. At greater conversions two relatively sharp bands appear at 300 and 330 nm. After 160 pulses the absorption spectrum did not change further, practically complete silver reduction had been reached here. The spectrum at larger pulse numbers also contains a shoulder at 350-360 nm. The spectra are quite similar to those observed in the y-irradiation of silver ion solution^.^ In comparing these spectra, one finds that the spectra in Figure 10 show relatively little absorption in the 380-400-nm range where colloidal silver metal absorbs. The development of the sharp bands at 300 and 330 nm was already observed in the previous y-irradiation experiment^.^ As these bands appear. the 275-nm band of Ag42+disappears. This indicates that the Ag+ : cluster undergoes reaction with the hydrated electrons and/or organic radicals to yield other clusters absorbing a t 300 and 330 nm. These effects can be studied in more detail and with greater reproducibility in y-irradiation experiments which will be described in a forthcoming paper. In Figure 1 I , the results of a combined y-irradiationlpulse experiment are shown. It was carried out to study the rate of the rather sudden formation of the 300- and 330-nm bands. A solution at pH = 9 containing 2 X 1 M AgCIO,, 0.03 M methanol, and 1.2 X M polyphosphate was y-irradiated for 8.5 min at a dose rate of I .2 X I O 5 rad/h. After this time, the solution had acquired the absorption band at 275 nm of Ag42+,although weak absorptions at longer wavelengths were also present (Figure 1 I . lower part). Continued y-irradiation would have caused the 275-nm band to disappear within 1 min and the 300- plus 330-nm bands to appear. Indeed, the onset of these bands is just evident in the spectrum in the lower part of Figure 11. However, instead of y-irradiating, the solution was now exposed to a single highM hydrated energy electron pulse which produced 2 X electrons in the solution and about the same amount of hydroxymethyl radicals. Figure 11, upper part, shows the temporal development of the difference spectrum after the pulse. Two steps contribute to this development, the first one occurring with a
i I
3co
ioC
scc
Xhvl Figure 11. Lower part: spectrum of a y-irradiated solution (2 X M AgCIO,; 0.03 M methanol; 1.2 X M polyphosphate; pH = 9).
Upper part: difference spectrum at various times after the pulse. Inset of upper part: temporal development of the 300-nm absorption after the pulse. half-life of about 3 1 s and producing about 30% of the total absorption change. The second step extends into the 100-ps range (see also inset of upper part). The 700-nm absorption of eaqdecayed in this solution with a half-life of 3 ps (this decay being mainly due to reaction of ea; with Ag+ ions still present in low concentration and with hydrogen ions produced during the yirradiation). We attribute the first step to the attack of the clusters being present after the y-irradiation by part of the hydrated electrons generated and the second step to the reaction of the organic radicals with these clusters. Note that the difference spectrum contains mainly the 300-nm band while the 330-nm band, which is much stronger at higher degrees of reduction (Figure IO), appears only as a shoulder. This indicates that the two bands belong to different clusters. One can also see that in the 360-500-nm range, where the metallic or almost metallic particles formed during the y-irradiation absorb, positive (360-430 nm) and negative (430-500 nm) absorption changes take place. In another experiment, the solution was preirradiated with y-rays for 1 1 min. Its absorption spectrum now contained well-developed bands at 300 and 330 nm. Upon pulsing, no increase in the 300-nm absorption was observed, although a substantial increase occurred a t 330 nm. It occurred in one step with a half-life time of 72 ps. These observations corroborate the above conclusions: the 300-nm band did not grow by pulsing as the precursor cluster of the 300-nm cluster had already been consumed during the prolonged y-irradiation. However, the solution still contained the precursor of the 330-nm cluster. The fact that the formation of the 330-nm cluster occurred in one step is explained by the scavenging of the hydrated electrons generated by the H+ ions being present a t a higher concentration after the 1 I-min y-irradiation. Only the organic radicals produced the 330-nm cluster. A multipulse irradiation experiment was also made with a solution that contained propanol-2 instead of methanol. Figure 12 gives the spectrum of the solution after application of 160 pulses (0) i.e., after complete Ag+ reduction. It contains a lot of absorption in the 380-400-nm range where colloidal silver metal absorbs besides the bands of silver clusters below 360 nm. The figure also shows the spectrum at 8.6 s after the pulse or pulse train. Note the rather fast decrease in the intensities of the bands at 300 and 330 nm. This decrease is accompanied by an increase
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 4187
Nonmetallic Ag Clusters in Aqueous Solution 0.8 I
'
I
2;O
3b0
3kO X [nml
4b0
450
Figure 12. Irradiation of a solution with 160 pulses (0) and spectrum 8.6 s afterward. The solution contained 0.1 M propanol-2 besides 1 X M AgC104 and 6 X M polyphosphate. 0.3
I 5
A
I
A
c160
0.2
aul c m n L
v) 0
n
0.1
0
350 400 450 X [nml Figure 13. Irradiation of a solution containing tert-butyl alcohol with a train of pulses. Absorption spectra after various pulse numbers. 1 X IO4 M AgCIO,: 6 X M polyphosphate; 0.5 M tert-butyl alcohol. 250
300
in the absorption at longer wavelengths. It thus seems that the nonmetallic clusters are thermally rather unstable when formed under the irradiation conditions of Figure 12 and are rapidly converted into larger particles. However, the 360-nm absorption maximum of these larger particles is still at shorter wavelength than the 380-nm plasmon absorption band of metallic silver. One concludes that most of the larger particles present at 8.6 s after the pulse have not yet fully attained metallic properties. Figure 13 shows the spectra obtained after application of various pulses to a solution containing tert-butyl alcohol. The most characteristic effect is the preferential formation of clusters absorbing at 275 nm at the lower doses. In comparison with Figures I O and 1 2, less colloidal silver is produced. One can also recognize that the increase in absorption is slower in the presence of tert-butyl alcohol. The experiments demonstrate once more that the course of reduction of Ag+ is strongly dependent on the nature of the alcohol added (or, better, the nature of the organic radical formed from the alcohol).
Discussion The First Reduction Steps. Polyphosphate exerts a drastic influence on the kinetics of the formation of small silver clusters and stabilizes the clusters. In the experiments with single pulses the percent reduction of the silver ions present is small (1-5%). Under these conditions, only the hydrated electrons contribute to the reduction process (eq 1). The organic radicals cannot directly reduce silver ions because of the high negative redox potential of the Ag+/Ago system (-1.8 V i ) . The first steps of silver reduction and cluster formation depend in a complex manner on the polyphosphate concentration and the concentration of added salts. Reaction 1 is substantially slower when the silver ions are not free but adsorbed on polyphosphate
chains (Figure 3). In fact, by measuring the rate constant of reaction 1 as a function of the polyphosphate concentration, one could elucidate the concentrations of Ag+ ions in the bulk and adsorbed state (Figure 4). Under the conditions used in most of the experiments, Le., [Ag'] = 1 X IO4 M and [PPI = 6 X M, reaction of ea; with Ag+ occurs mainly in bulk solution, and this also means that the Ag2+ species from reaction 3 is mainly formed there. When barium or sodium perchlorate is present, the effects of polyphosphate on reaction 1 are reversed to a certain extent (Figure 3). The rate of reaction 1 increases to finally reach the value of the bulk reaction. However, higher NaCIO, than Bal (C1O.J2 concentrations are required. The effects are explained by the release of adsorbed silver ions upon the addition of salt. As the concentration of bulk Ag+ ions increases in this way, the observed rate of reaction 1 increases. For the same reason, one can expect reaction 3 to become faster, too. The difference in the action of Ba2+ and Na+ ion is due to the fact that Ba2+ is specifically, Le., very strongly, adsorbed, while Na+ ions are electrostatically attracted and form a loose cation cloud around the polyphosphate chains. The adsorption of Ag+ ions seems to be an intermediate case of weak specificity. Adsorbed Ag+ ions can therefore readily be displaced by Ba2+ ions, while substitution by Na+ ions is less efficient. This follows the known order for the binding of metal ions to phosphate^.'^ Most interesting is the strong acceleration of reaction 4 by polyphosphate. As Ag2+ has its absorption maximum at 310 nm, one can see this effect by recording the time profile of this absorption after the pulse. The curves in Figure 8 are explained by taking into account two effects of polyphosphate. First, reactions 1 and 3 are slowed down because the concentration of Ag+ is depleted in bulk solution (Figure 3). Second, the disappearance of Ag2+, reaction 4, is accelerated by increasing polyphosphate concentrations. The rate of disappearance of Agzf becomes almost comparable to that of its formation a t high polyphosphate concentrations, the result being a decrease in the maximum of the 3 10-nm absorption a few microseconds after the pulse. The strong acceleration of reaction 4 by polyphosphate (Figure 9a) is understood in terms of a reduction in the dimensionality of reaction space in the presence of polyphosphate anions. Some time ago, the rate of reaction 4 was studied in solutions containing anionic micelle^.'^ It was found that the rate could be enhanced by a factor of 100, although reaction 4 in pure solution is already almost diffusion controlled. The Ag2+ ions are rapidly attracted electrostatically by the large micelles and react with each other on the micellar surface over which they can readily diffuse. The dimensionality of reaction space is thus drastically reduced. The reaction order is changed from second to first. It seems that polyphosphate exerts a similar influence. Reaction 4 is accelerated in the presence of polyphosphate by a factor of the order of 10. The number-average chain length of the polyphosphate used was 18, as determined by titration. At the overall concentration of 6 X lo4 M mostly used, the concentration of chains was about 3 X M, Le., greater than the concentration of Ag2+ ions generated. If the Ag2+ ions were strongly adsorbed on the polyphosphate chains, single occupancies would have mainly occurred and the rate of dimerization would have been strongly decreased. One must therefore assume that the Ag2+ ions are only weakly adsorbed, diffusion from one chain to another being possible. Barium ions sticking to the polyphosphate chains strongly shield the chains and decrease the rate of the Ag2+ + Ag2+ reaction, while sodium ions as a component of the cation cloud have little effect (Figure 9b). In the irradiation with single pulses (Figures 6 and 7 ) , the spectrum of Ag4*+,peaking a t 275 nm, developed within a few s and remained unchanged up to the longest time of 15 s available in the pulse equipment. It thus is concluded that this (1 5) Henglein, A,; Proske, Th. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 471. (16) (a) Henglein, A. In EIecfroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, p 163. (b) Schwarz, H. A.; Dodson, R. W. J . Phys. Chem. 1989, 93, 409.
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complex of dimeric silver, eq 5, is so strongly bound to polyphosphate that it is unable to undergo further reaction with other complexes to form larger clusters. In the previous study on the formation of long-lived clusters by y-irradiation of silver-polyphosphate solutions, it was already found that the Ag42+absorption survived for at least 1 h.4 Since dications are known to be much more strongly bound to phosphates than monovalent or neutral species, it seems likely that the equilibrium of eq 5 lies on the left-hand side. Thus, the polyphosphate chains accelerate reactions between cations of low charge but stabilize highly charged clusters by strong adsorption. A new cluster is detected in the present investigation, whose 350-nm absorption appears in the single-pulse experiments (Figures 6 and 7) and is stable over the whole I-s time range available. This absorption did not occur in the previous studies on y-irradiation where longer times of observation were i n ~ o l v e d . ~ It must therefore be concluded that this cluster has a limited lifetime, i.e., shorter than a few minutes. The absorption developed within about 10 ps, i.e., a time during which only one or two elementary processes could occur after the pulse; the cluster can therefore not contain many atoms. Its absorption relative to that of Ag4*+ became stronger when it was formed at lower doses in the pulse. This observation makes us ascribe the 350-nm absorption to a species resulting from a reaction of Ag2+ which competes with reaction 4. The most simple explanation would be a strong adsorption of Ag2+ ions onto the polyphosphate chains, although it was shown above that Ag2+ generally is not strongly complexed. However, strong complexing might become possible on the polyphosphate chain ends, which normally are protonated at pH's below IO. The substitution of a proton may require some activation: however, once complexed at a chain end, the Ag2+ may remain adsorbed for a long time. Another explanation would consist of a reaction of Agz+ with Ag+ to form the cluster Ag32+ which has been observed in zeolites.' Formution of Larger Clusters. When trains of pulses are used, higher degrees of silver reduction become possible. As the first long-lived clusters accumulate, the hydrated electrons can attack them. However, the main additional component of reduction comes from the organic radicals, eq 2. In the case of the hydroxymethyl radical the reaction may be formulated as Ag,,"'
+ CH20H
-
+
Agn(x-')+ CH,O
+ H+
(10)
The radicals generated in solutions containing various alcohols have different redox potentials. CHzOH has less reducing power than the (CH3),COH radical formed in the presence of propanol-2, and the CH2C(CH3),(OH) radical, formed in the presence of tert-butyl alcohol, has the least reducing power.I6 It is therefore not surprising that pulsing solutions containing different alcohols leads to different clusters (Figures IO, 12, and 13). In the case of the propanol-2-containing solution (Figure 12), more absorption in the 360-400-nm range, where larger oligomeric and metallic particles absorb, is produced than in the case of the methanolcontaining solution (Figure IO). In the latter solution, the sharp 300- and 330-nm bands ascribed to smaller clusters are more clusters abundant. I n the tert-butyl alcohol solution, mainly are formed (Figure 13) and the rate of overall reduction is smaller. This indicates that the CH,C(CH,),(OH) radical does not contribute substantially to the reduction under the conditions of pulse radiolysis. At the present time, chemical structures cannot safely be attributed to the clusters absorbing at 300, 330, and 360 nm. The y-irradiation experiments have shown that these clusters live for hours and that they have reducing proper tie^.^ The comparison of their absorption bands with those of silver clusters identified in rare gas matrices' does not lead to clear-cut conclusions. There are several reasons for this comparison not being successful. First, the positions of the absorption bands strongly depend on the nature of the solvent. For example, the Ago atom absorbs in an argon matrix in the 292-3 IO-nm while its absorption maximum in aqueous solution appears at 360 nm71.2 Second, the clusters prepared in matrices are neutral species, while in solutions con-
Mulvaney and Henglein taining excess Ag+ ions equilibria of the type
may exist. Finally, the complexation by polyphosphate may also shift the absorption band of a cluster. The comparison with the absorption spectra of partly dehydrated clusters in zeolites9 and in frozen solutions17 also does not yield a clear-cut correlation. Under the conditions of the experiment of Figure 1 1, 2 X IO6 M hydrated electrons were produced, part of them reacting with the Ag42f clusters and a small amount with the larger metallic particles present from the preceding y-irradiation. A cluster could hardly react with more than one electron under these conditions. The absorption changes appeared during the 3-ps lifetime of the hydrated electron. During this short time period, cluster-cluster interactions, such as cluster dimerizations, could not occur. It is therefore concluded that a single electron transfer converted a cluster being present after the y-irradiation into the cluster absorbing at 300 nm. As the y-irradiation experiments have shown that the 275-nm absorption of Ag?+ disappears when the 300-nm band appear^,^ it is concluded that Ag42+is the precursor of the 300-nm cluster. In other words, the species absorbing at this wavlength must be trimeric silver, various complexes with Ag' ions being conceivable (eq 11). The 330-nm band appeared only heakly in Figure 11. It is probably produced in a single-electron-transfer reaction to a larger oligomer of silver present in small concentration after the y-irradiation. The absorption changes in the 360-500-nm range are attributed to electron transfer to larger metallic or almost metallic clusters. These changes, which also cause a bleaching of the solution a t longer wavelengths, are attributed to a blue shift of the plasmon absorption band of silver as a consequence of the increase in the energy of the Fermi level by electron uptake. This effect will be described in more detail in a forthcoming paper. The reactivity of the clusters toward organic radicals is determined not only by the reduction potential of the latter but also by the redox potential of the clusters. This potential generally becomes more positive with increasing cluster s i ~ e . ~ - ' ~Fur%'~ thermore, with decreasing Ag+ concentration during the reduction process, the potential of the clusters becomes more negative because of decreasing complexation by Agf ions (eq 1 l ) . More detailed experiments and discussion about the complex interplay between various clusters and radicals will soon be reported in a paper in which a number of chemical reactions of the clusters are in~estigated.'~In another paper, the electrochemical potentials of silver clusters will be described in more detail.20
Final Conclusions The complexation of metals by phosphates has thoroughly been studied some decades ago14 after it had in principle been recognized at the end of the past century.2' The only difference between our present work and the earlier, more conventional investigations consists of the idea that polyphosphates could also complex and stabilize intermediates of metal ion reduction and oxidation. The present kinetic studies together with the previous chemical exp e r i m e n t ~show ~ that this concept is valid to explain the formation of long-lived clusters in the reduction of silver ions in the presence of polyphosphate. As mentioned previo~sly,~,'~ new aspects in the electrochemistry of metals may arise as the small metal particles, regarded as microelectrodes, have properties quite different from those of compact metal electrodes. Acknowledgment. The authors thank Dr. Eberhard Janata for advice in the use of the pulse radiolysis equipment. (17) Stevens, A. D.;Symons, M. C. R. J . Chem. Soc.. Faraday Trans. I 1989, 85, 1439. ( 1 8) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Rndiat. Phys. Chem. 1989, 34, 605. (19) Linnert, T.; Mulvaney, P.; Henglein, A,; Weller, H. J . Am. Chem. Soc., in press.
(20) Henglein, A. Ber. Bunsen-Ges. Phys. Chem., in press. (21) Tammann, G J . Prakr Chem. 1892, 45,417.
