7843
J . Phys. Chem. 1991, 95, 7843-7846 to molecular recognition by monolayers13 and formation of mixed monolayers having directed configurations! In the present study, we chose a glucolipid, SGP, and its methylated (MeSGP) and acetylated derivatives (AcSGP) as the partners of LPSC. The ability to form intermolecular hydrogen bond decreases in the order of SGP, AcSGP, and MeSGP. Comparative study on the *-A isotherms and fluorescence spectral changes during compression suggests a possibility of hydrogen-bonding interaction in the paring of LP8C and SGP. GMS as well as ARA, which may have the ability to form hydrogen bond, does not show any effect on ordering of the pyrene chromophores of LPSC. The monolayer behavior of the LP8C-GMS and LP8C-ARA systems is the same as that of the LP8C-MeSGP and LP8C-AcSGP mixed systems. On the basis of these results, it can be said that the glucopyranose residue without chemical modification has novel properties. One of the novel properties of SGP is found in the DLA fluorescence. The fluorescence maximum of DLA in the S G P monolayer continuously shifts to shorter wavelengths as the monolayer is compressed, while that in the monolayer of MeSGP, AcSGP, ARA, or GMS is scarcely affected by the compression. This
means that water molecules around the headgroups of SGP are released from the surface, giving more hydrophobic environment of the monolayer surface. Probably, hydration at the SGP head groups is replaced by hydrogen bonding between the saccharide residues as speculated by Hinz et a1.6 The glucopyranose residue has a hydrophobic property to some extent which may make it possible to form hydrogen bonds at the water interface.” If such assumption is correct, it is really likely that the LP8C molecules associate with the SGP molecules through hydrogen bonding, leading to a well-ordered mixed monolayer where the pyrene chromophores are oriented in the face-to-face configuration. Acknowledgment. We thank Nippon Fine Chemical Co. Ltd. for the generous gift of SGP. This work was supported by the Grant-in-Aid for Science Research on Priority Areas, “Multiplex Organic Systems” (No. 01649006), and by the Grant-in-Aid for Science Research No. 01550657 from the Ministry of Education, Science and Culture, Japan. (17) Yamamoto,
K.;Nishi, N. J . Am. Chem. Soc. 1990, 112, 549.
Surface Chemistry of Coiioidai Silver in Aqueous Solution: Observations on Chemisorption and Reactivity Paul Mulvaney, Thomas Linnert, and Arnim Henglein* Hahn-Meitner-Institut Berlin GmbH, Bereich S.1000 Berlin 39, FRG (Received: March 1 1 , 1991) Colloidal silver particles (3-nm diameter) are oxidized in the absence of air by organic and inorganic electron acceptors (such as nitrobenzene, methylviologen, nitropyridine oxide, and hexacyanoferrateII1) when nucleophilic reagents (such as CNand SH-)are present in the solution. The mechanism of catalyzed metal oxidation proposed previously: according to which the interaction of surface atoms with the nucleophiles leads to an excess negative charge in the metal interior which can be picked up by the electron acceptors, explains the observed phenomena. This mechanism contains reversible steps. An experiment is reported in which the reverse sequence of events is observed: chemisorption of Ag(CN)2- on silver particles, producing an ex- positive charge inside the particles, followed by complete reduction of the Ag+ ions of the adsorbed complexes by excess electrons that are deposited by a reducing reagent (i.e., organic radicals generated radiolytically). AO(CN)~-ions in solution are not reduced by radicals.
Introduction
Although colloidal metals in aqueous solution are systems with a huge amount of interface, there have been relatively few surface chemistry studies of these systems. In the case of silver, two kinds of spectral changes have been used to investigate surface reactions: ( I ) surface-enhanced Raman scattering by chemisorbed molecules,’ which, however, is applicable only in certain cases, and (2) the observation of the changes in the shape of the surface plasmon absorption band at 380 nm that is caused by an oscillation of the electron gas in the particles.2 It was shown recently that small concentrations of nucleophilic molecules in a silver sol can drastically change the shape and the position of the plasmon absorption band.* The silver atoms on the surface are coordinatively unsaturated. One may expect that unoccupied orbitals exist on the surface into which a nucleophilic reagent can donate an electron pair. The consequence is not only a change in the optical absorption of the colloidal particles but also a change in their reactivity. A mechanism has been proposed which is schematiclly depicted in the upper part of Figure 1 : A surface atom carrying a nucleophilic molecule N acquires a small ( I ) (a) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J . Chem. Soc., Furuduy Trans. 2 1979. 75.790. (b) Wiesner, J.; Wokaun, A.; Hoffmann, H. Prog. Colloid Polym. Sci. 1988, 76. 271. (2) Henglein, A,; Linnert. T.; Mulvaney, P. Ber. Bunrenges. Phys. Chem. 1990, 94, 1449.
positive charge 6+ (’preoxidation” of the surface atom) and the interior of the colloidal particles receives a corresponding negative charge 6-. Only a certain number of molecules N can be chemisorbed until an equilibrium is reached as the accumulated negative charge on the particle prevents further donation of charge. The figure also shows that the Fermi potential at equilibrium is shifted to a more negative value. Silver particles carrying nucleophilic adsorbates on their surface are very reactive toward oxygen. O2picks up the excess negative charge on the particle, thus allowing further complexation of surface atoms until the whole particle is oxidized, i.e., dissolved in the form of AgN molecules.2 It is the purpose of the present paper to report some new observations which corroborate the mechanism of Figure 1 for the oxidation of silver. Experimental Section
All experiments were carried out under anaerobic conditions as described previously.’ y-Radiation was used to prepare the silver sols, making use of the reducing hydrated electrons and organic radicals that are formed when solutions containing AgClO, ( 1 or 2 X 1 0-4 M) and 0.5 M propanol-2 are irradiated.’ This (3) (a) Henglein, A. Chem. Phys. Le??.1989, I54 473. (b) Mulvaney, P.; Henglein, A . J . Phys. Chem. 1990, 94,4182. (c) Linnert, T.; Mulvaney, P.; Henglein, A,; Weller, H. J . Am. Chem. Soc. 1990, 112,4657. (d) Henglein, A.; Mulvaney, P.;Linnert, T. Furuduy Discuss. 1991, in press.
0022-3654f9 1 f 2095-7843%02.50/0 .., . 0 199 I American Chemical Society I
,
~
Mulvaney et al.
7844 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 A&A~,
6' X N
N
-m .L
C
c 0,
N
0
n
.-
N
EW
U
300
200
/
N
Fermi level
tA+
6*
Ag,- XAg XN Figure 1. (Upper) Schematic presentation of surface complexation
(preoxidation) by a nucleophilic reagent N and final oxidation of the silver particle by oxygen. (Lower) Chemisorption of a complex AgN and final reduction of the silver ions in chemisorbed AgN by excess electrons deposited by reducing radicals. I n both parts of the figure, the changes in the position of the Fermi level in the colloidal silver particles are indicated. kind of reduction of silver ions is most reproducible, the advantages being that (i) the number of reducing radicals generated is well-known, (ii) homogeneous reduction in the solution is possible, Le., local concentration gradients which are introduced by mixing the solution with the solution of a reductant are avoided, and (iii) a minimum number of disturbing impurities are introduced. The solutions contained lO+-I O4 M sodium polyphosphate (Riedel de Haen). The pH of the solutions was 6-7 (at these pH values and the low polyphosphate concentrations, the reduction of Ag+ leads to metallic particles without formation of oligomeric silver clusters). ?-Radiation was also used to generate reducing radicals in the silver sols themselves. These radicals are able to transfer an electron to the colloidal particles. It was shown previously that a colloidal particle can store several hundred electrons. In these previous studies the stored electrons were used to initiate multielectron reduction processes such as the reduction of water and various s ~ l u t e s . ~Stored J electrons shift the position of the Fermi level in the colloidal particles. Thus, by letting silver particles, which carry chemisorbed species, react with reducing radicals, one can investigate how the chemisorption changes when the position of the Fermi level is varied.
Results Catalysis of Silver Oxidation. If the above mechanism for the catalyzed oxidation of silver is correct, one should be able to substitute the oxygen, used in the second step of final oxidation, by other inorganic or even organic electron acceptors. In Figures 2-5, experimental results are presented which show that the oxidation by other acceptors is indeed possible. The electron acceptors used were methylviologen (1 ,I'-dimethyl-4,4'-bis(pyridinium chloride), MV2+:standard one-electron transfer potential: -0.44 V). nitrobenzene (-0.40 V), nitropyridinium oxide (>-0.2 V), and potassium hexacyanoferrate(II1) (+0.36 V). These acceptors were added to a silver sol which also contained either CNions (weak nucleophile) or SH- ions (strong nucleophile). The changes in the shape of the surface plasmon absorption band were recorded at various times after the addition of the electron acceptor. It is emphasized that, in all cases, the addition of the electron acceptor to the silver sol in the absence of the nucleophile did not result in the oxidation of the silver particles. ~~
~
~~
~
~~~
~~
(4) (a) Henglein, A. J . Phys. Chem. 1979.83, 2209. (b) Henglein, A. J . Phys. Chem. 1979,83,2858. (e) Henglein, A. Ber. Bunsenges. Phys. Chem. 1980.81,253. (d) Henglein, A. J . Phys. Chem. 1980,81, 3461. ( e ) Henglein, A.; Lilie, J. J . Am. Chem. SOC.1981, 103, 1059. (5) (a) Meiscl, D. J. Am. Chem. Soc. 1979, 101,6133. (b) Miller, D. S.; Bard, A. J.; MeLendon, G.; Ferguson, J. J . Am. Chem. Soc. 1981, 103, 5336.
400 500 600 A Inml Figure 2. Spectrum of a 1 X IO" M silver sol before and after the addition of KCN plus methylviologen. The sol was prepared by y-irradiation of a 1 X IO4 M AgC10, solution containing 0.5 M propanol-2, 1X M polyphosphate, and 0.01 M acetone (pH 6.5). 1.5
7
1.0 a U l t
m
2 0 v)
n m
0.5
0 300
500
400 A
lnml
Figure 3. Spectrum of a silver sol before and after addition of NaSH and at two times after the addition of nitrobenzene. (Sol as in Figure 2.)
In Figure 2, the absorption spectrum of the silver sol is shown before and after the addition of KCN and methylviologen. The two compounds were added simultaneously to the sol. The solution became blue immediately, which is attributed to the formation of the radical cation of half-reduced methylviologen, MV'. The well-known absorption bands a t 395 and around 600 nm can be seen in the absorption spectrum. The intensity of MV+ formation increased with increasing concentration of both CN- and MV2+. The color was weak in the case of the lower concentrations in Figure 2 and faded away within 2 min. A much stronger and permanent color appeared at the higher concentrations. The amount of MV+ formed was equal to the amount of silver present in the sol; Le., all of the metal had been oxidized by methylviologen. When the solution was finally exposed to air, all the absorption above 320 nm disappeared immediately (MV2+starting to absorb at 320 nm). When nitrobenzene was used as electron acceptor in the presence of 1 X lo-' M KCN,a fast disappearance of the silver absorption band was observed. The broad absorption band of a silver sol, which contained 5 X IO4 M SH-, faded away more slowly. The spectra at 30 s and 30 min after the addition of nitrobenzene are shown in Figure 3. The solution was colored yellow, its absorption spectrum being identical with the well-known spectrum of colloidal Ag2S.* A more rapid oxidation of the silver particles, which again carried SH-groups on their surface, was observed when nitropyridine oxide was used. This can be seen from Figure 4, where the broad plasmon absorption band of the silver plus SH-particles decayed practically instantaneously after the addition of the electron acceptor. The spectrum of the solution was that of Ag2S
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1845
Surface Chemistry of Colloidal Silver
1.0 u U c m
U al
c m
-2 0
? 1.0
n
n
0
vl
v)
m
m
0.5
0 300
0 300
500
400
A lnml Figure 4. Spectrum of a silver sol before and after addition of NaSH and immediately after the addition of nitropyridineoxide. (Sol as in Figure
400 500 A Inml IO-‘ M silver sol before and after addition
Figure 6. Spectrum of a 1 X of various amounts of NaAg(CN)2. (Sol as in Figure 2.)
2.)
2.0
1.2 40-4( rJcJs
tc
1.0
al
U
c m
2 1.0
m
n L
0
vl
n
vl
0
m
n n
0.5
0
n
“240
400 500 A Inml Figure 7. Absorption spectrum of a 1 X IO” M silver sol containing 1 X IO4 M NaAg(CN)2 and 1 X IO-‘ M polyphosphate after various 300
500
400
300 A lnml
Figure 5. Spectrum of a silver sol containing KCN before and at two
times after the addition of K3Fe(CN),. (Sol as in Figure 2.)
radical concentrations were generated by 7-irradiation.
at longer wavelengths; the band at 295 nm is attributed to nitropyridine formed in the oxidation of the silver particles by NPO. Although Fe(CN),’- is the electron acceptor with the most positive potential among the ones used, it reacts relatively slowly, Le., not instantaneously, with the surfaceamplexed silver particles. In the experiments of Figure 5, the CN- concentration was 1 X IO-’ M. As can be seen, the silver band at 30 s after the addition of Fe(CN),’- had clearly decreased but a tiny amount of metal was still present after 2 min. Reduction of Ag(CN),-. The Ag(CN),- anion is extremely stable in solution toward reducing organic radicals. For example, I-hydroxyethyl methyl radicals (reduction potential, -1.5 V), which are often used in radiation chemical experiments, do not decompose the silver cyanide complex: when a 1 X IO-’ M solution of NaAg(CN), containing 0.1 M propanol-2 is y-irradiated, no silver is formed. In fact, the potential of the redox equilibrium
NaAg(CN)2. In this experiment a silver sol containing both propanol-2 and nitrous oxide is irradiated (the hydrated electrons from the radiolysis of water react according to ea( + N 2 0 H 2 0 N 2 + OH- + OH, and the O H radicals formed attack the alcohol to produce the organic radicals). It can be seen that the absorption band of the silver particles is increased. The band also becomes narrower and blue-shifted. In fact, the band becomes about 10 times stronger than in the original 1 X M silver sol, which proves that the adsorbed Ag(CN)2- ions are indeed reduced. The quantitative evaluation shows that stoichiometric reduction takes place; Le., one silver atom is formed per reducing radical generated. I n a simple description, one would say that Ag(CN),- can be reduced in the adsorbed state because the silver atom formed is not free in solution but bound to a silver particle. However, the observations made on the changes in the absorption spectrum allow one to interpret the reduction in a more detailed manner. In fact, we postulate below that the reduction does not consist of a direct interaction of a radical with an adsorbed complex but of the interaction of the radical with the silver particle carrying the complex. From this point of view, the changes in the Fermi level of the colloidal particle play an important role for both electron transfer and chemisorption.
Ago + 2CN-
-
Ag(CN),-
+ e-
(1)
is as negative as -2.9 V (Ago, free silver atom in solution). However, the reduction of Ag(CN)2- by the organic radicals is catalyzed in the presence of colloidal silver particles. The changes in the absorption spectrum of the sol when NaAg(CN), is added are shown in Figure 6. The addition causes a strong decrease in the surface plasmon absorption band and a shift of the maximum to longer wavelengths as well as broadening of the band. In Figure 7, the spectral changes are shown which occur upon y-irradiation of a silver sol containing a large excess of
-
+
Discussion
The oxidation of silver by organic and inorganic electron acceptors in the presence of a nucleophilic compound that binds to
7846 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
surface a t o m is an electrochemical process consisting of two steps, in which electron pair donation and electron transfer are involved. The results obtained with colloidal microelectrodes may be compared to the investigations on specific adsorption in conventional electrochemistry; instead of observing changes in electrode potential, the changes in optical absorption are recorded which are caused by chemisorption. The Fermi potential of a silver particle complexed on its surface by CN- ions seems to lie close to 4.44 V, i.e., close to the standard potential of the methylviologen redox system. This explains why a rather high concentration of MV2+ is required to pick up a substantial amount of excess negative charge to yield MV+ (Figure 2). In the case of nitrobenzene as electron acceptor, which has a slightly less negative potential for one-electron acceptance than MV2+,the oxidation is more efficient in the presence of CN-. It might be that this efficiency is due not only to the more positive potential but to the fact that the reduction of nitrobenzene is not reversible, the C,H5N0y radical anion first formed undergoing further reaction with another radical. An efficient oxidation is also observed when SH- is present in the solution (Figure 3). Nitropyridine oxide, which is a stronger electron acceptor, oxidizes silver more rapidly than nitrobenzene (Figure 4). The relatively slow oxidation by the Fe(CN)63- ion (Figure 5 ) is possibly due to a kinetic effect, this highly negatively charged reactant being repelled by the polyanion chains on which the silver colloid is stabilized or by the overall negative charge of the SH--loaded silver particles. Two properties of silver determine the position and width of the surface plasmon absorption band for particles where scattering can be neglected:6 ( I ) the density of free electrons in the silver particles (which also determines the position of the Fermi level, an increase in density leading to a blue shift of the band); (2) the effective size of the particle, a decrease leading to a broader band. As is discussed in more detail elsewhere,3d both properties are influenced by chemisorbed species. It should also be emphasized here that the observed changes in the shape of the plasmon absorption band are not due to aggregation or enhanced growth of the silver particles under the influence of the added substances. At the small concentrations of the additives used, such effects are highly improbable. No increase in scattering of the solutions upon addition of the nucleophilic reagents was observed. All solutions were transparent and did not show any opalescence. The reduction of Ag(CN)2- on silver particles is explained by the mechanism which is schematically depicted in the lower part (6) (a) Doyle, W . J. Phys. Reu. 1958. 1 1 1 , 1067. (b) Doremus. R. H. J . Chem. Phys. 1%5,42,414. (c) Hansen, W. N.; Prostak, A. Phys. Reu. 1968, 174, 500.
Mulvaney et al. of Figure 1. Note that the adsorption of AgN is the reverse process to the desorption of AgN from a silver particle whose Fermi level is shifted to a more positive potential as excess negative charge is picked up by O2 (second step in the upper part of Figure 1). Which one of the two processes occurs is determined by the position of the Fermi level in the silver particle. Thus, the adsorbed AgN species can be expected to have similar structures in both cases. In Figure 6,N is 1 or 2 CN-. The red shift and broadening of the surface plasmon absorption band of the silver particle are interpreted as a downward shift or more positive potential of the Fermi level upon the adsorption of Ag(CN);. The silver ion in the adsorbed complex donates the amount A+ of positive charge to the silver particle and retains the charge b+, (A' +)'6 = 1. A dipolar layer A+6+ on the surface is thus produced, similar to the 6-6+ layer formed following the adsorption of a nucleophilic reagent (Figure 1, upper part), possibly even with the opposite electrical moment. The electrons, which, in the subsequent yirradiation, are deposited by organic radicals on the silver particles (Figure 7), cause the Fermi level to move toward more negative potentials. The dipole moment decreases as the charge density due to the transferred electrons increases, until a situation is reached where the silver ions of the adsorbed Ag(CN)y are fully reduced. With further electron deposition on the particles, the remaining adsorbed molecules of the nucleophile are desorbed. Final Conclusions That the oxidation of metal by oxygen is facilitated by nucleophilic reagents is well-known, the extraction of gold from ores by cyanide in the presence of air being a typical example. In the present studies, a kinetic mechanism for such reactions is proposed, using colloidal silver particles, where important optical changes accompany the various steps of the reaction. It is postulated that chemisorption of solute molecules changes the electronic properties of the small metal particles, a dipolar layer on the surface being produced and the potential of the Fermi level being changed. The shift of the Fermi level changes the reactivity of the metal particles. It is also postulated that electron-transfer reactions of chemisorbed molecules do not occur directly but between the surface-modified metal particle as a whole and the substrate. The observed optical and chemical effects are understood by this reaction model. In the present paper, thermal reactions of the surface-modified silver particles are reported. We have also found that the photochemical behavior of colloidal metals is changed by chemisorption; this will be reported elsewhere.' (7) Linnert, T.; Mulvaney, P.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1991, 95. 838.
