3710
Langmuir 1993,9, 3710-3716
Two-Dimensional Silver Electrocrystallization under Monolayers Spread on Aqueous Silver Nitrate Nicholas A. Kotov, M. Elisabete Darbello Zaniquelli,? Fiona C. Meldrum, and Janos H. Fendler' Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received June 18,1993. In Final Form: September 2 7 , 1 9 9 P
Electrolysis of aqueous silver nitrate solutions coated by monolayers, prepared from dihexadecyl phosphate, dipalmitoylphosphatidicacid, dioleoylphosphatidic acid methyl ester, dipalmitoylphosphatiester, has resulted dylethanolamine,and phosphoricacid n-hexadecylN- (0-vinylbenzoy1)-11-aminoundecyl in the two-dimensional electrocrystallization of 50- to 70-nm-diameter silver particles at potentials more negative than those used for silver ion reduction at electrode surfaces. In contrast, no silver particle formation was observed under monolayers prepared from surfactants having positively charged or bulky headgroups. Chronoamperometry and cyclic voltammetry have established the importance of strong silver ion complexation in kinetically controlled electroreduction under monolayers.
Introduction Nanodimensional metallic particles and particulate films are important industrial ~atalysts.l-~ They exhibit sizeand morphology-dependent chemical, electrochemical,and spectroscopic properties.&g The formation and stabilization of nanosized colloidal metallic particles demands careful attention to the preparation conditions and to the presence of stabilizers; polyphosphates, polyacrylic acid, and other polyelectrolytes have been frequently employed for the stabilization of colloidal metal particles. Reversed micelles or micro emulsion^^^^ and, more recently, monolayers9 have been utilized as templates for the in situ generation and subsequent stabilization of nanosized metallic particles and particulate films. Silver particulate films have been generated, for example, under negatively charged monolayers floating on an aqueous silver nitrate solution by the slow infusion of f~rmaldehyde.~ Importantly, the silver particulate film could be transferred, at any stage of its growth, to a solid substrate. A preliminary communication has also been published on the electrochemical generation of a silver particulate film under monolayers formed from a polymerizable dialkyl phosphate surfactant or from arachidic acid.1° Electrical connection was made between a silver electrode (1.0-mm diameter) immersed into the aqueous silver nitrate subphase and a platinum electrode (20-pm diameter) touching the monolayer surface. Application of 1.8-1.9 V potential across these electrodes resulted in silver ion reduction at the monolayer-aqueous solution interface. The initially nucleated silver particles effectively extended the cathode t Permanent address: Departamento de Qufmica-F.F.C.L.R.P., Universidade de SHo Paulo, Avenida Bandeirantes, 3900/14049-901 RibeirH Preto-SP, Brazil.
* Abstract published in Advance ACS Abstracts, November 15, 1993. (1)Anderson, J. R. Structure of Metallic Catalysis; Academic Press: London, 1975. (2)Somorjai, G.A. Science 1985,227,902. (3)BBnnemann, H.;Brijoux, W.; Brinkman, R.; Dinjus, E.; Joussen, T.; Korall, B. Angew. Chem., Znt. Ed. Engl. 1991,30,1312. (4)Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discus. 1991,92, 31. (5)Mulvaney, P. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.;VCH Publishers: New York, 1992;p 345. (6)Henglein, A. Zsr. J. Chem. 1993,33,77. (7) Fendler, J. H. Chem. Reu. 1987,87,877. (8)Fendler, J. H. Membrane-Mimetic Approach t o Advanced Materials; Springer-Verlag: Heidelberg, 1993. (9)Yi, K. C.; Fendler, J. H. Unpublished work, 1992. (10)Zhao, X.K.; Fendler, J. H. J.Phys. Chem. 1990,94,3384. 0743-7463/93/2409-3710$04.00/0
along the monolayer interface and resulted in the formation of a silver particulate film. This process could be considered to be a two-dimensional electrocrystallization, resembling underpotential deposition of metals on ele~trodes.ll-'~ The electrochemical investigation of silver electrocrystallization under monolayers formed from structurally different surfactants is the subject of the present report. The progress of silver-ion reduction has been monitored by chronoamperometry and cyclic voltammetry under a variety of experimental conditions. The presence of partially interconnected, 50-70-nm particles in samples of electrocrystallized silver particles transferred from the monolayers to solid substrates was established by transmission electron microscopy.
Experimental Section Dihexadecyl phosphate (l),arachidic (eichosanoic)acid (3), dipalmitoylphosphatidicacid sodium salt (4), dioleoylphosphatidic acid methyl ester monosodium salt (51, dipalmitoylphosphatidylethanolamine (61, dipalmitoylphosphatidylcholiie (8), dipalmitoylphosphatidicacid diphenyl ester (S), l-monooleoylrac-glycerol (lo), dioctadecyldimethylammonium bromide (1l), n-octadecylamine bromide (12), and n-heptadecyltrimethylammonium bromide (13) were purchased from Sigma and used as received. Phosphoric acid n-hexadecyl N-(O-vinylbenzoyl)-llaminoundecyl ester (2) and 2-n-hexadecylpropanodioicacidN,Nbis(2-aminoethy1)diaide (7) were synthesized according to procedures described elsewhere.I6 AgNOs, Ca(NOs)*,Pb(NO&, Cu(NO&,andHgz(NO&were purchasedfromFisher. Platinum/ iridium (0.5 mm) and silver wire (0.5 mm, 0.126 mm) and HAuC4 were purchased from Aldrich. Monolayers were formed on aqueous silver nitrate (10-2 M) solutions,either in a crystallizing dish or in a commercial Lauda film balance. The monolayer was prepared by depositing a chloroform solution of the selected surfactant on the surface of the aqueous subphase to give an initial surface area of 150-700 AZ/molecule. After evaporation of the solvent (5-10 min), the monolayer was compressed to the desired area (A). All electrochemical experiments were carried out under air. The (11)Budevaki, E. B.In Comprehensiue Treatise of Electrochemistry; Part 7: Kinetics and Mechanisms of Electrode Processes; Conway, B. E., Ed.; Plenum Press: New York, 1984;p 399. (12)Milchev,A.; Chierchie, T.; JClttner,K.; Lorenz, W. J. Electrochim. Acta 1987,32,1039. (13)Milchev, A,; Chierchie, T.;Jtittner, K.; Lorenz,W. J. Electrochim. Acta 1987,32,1043. (14)Hottenhuis, M. H.J.; van den Berg, A. L. M.; van der Eerden, J. P. Electrochim. Acta 1988,33,1519. (15)Yuan, Y.; Tundo, P.; Fendler, J. H. Macromolecules 1989,22,29.
0 1993 American Chemical Society
Langmuir, Vol. 9, No. 12,1993 3711
Two-Dimensional Silver Electrocrystallization
2
4 hr
cy
- 4
cy
h c
OCH3
5
I
n-ClsH31-
C -O-CH
II
0
I
CH2-0-P
0
II I
y
hr
cy
cv
Figure 1. Experimentalsetup: 1,working electrode; 2, reference electrode; 3, auxiliary electrode; 4, Teflon tubing; 5, silver particulate film; 6, monolayer of surfactant; 7, potentiostat; 8, computer.
/
0 CI
-0CH2-CH2NH2
-
CV
u
-
h
300 -
OH 6
0
II
,C-NH
-CH~-CH~-NHZ
C16H33CH
C‘
II
-NH
--CHz--CH2--NH2
0 7
reference (standard calomel, S C E Fisher) and counter (0.5 mm rectangular or circular silver wire) electrodes were immersed in the subphase prior to spreading the monolayer. The working electrode (a 0.5-mm-diameter silver or platinum/iridium alloy wire) was then brought into contact with the water surface using a mechanical lifting device and reequilibration of the monolayer was allowed (some surfactants exhibited a drop in pressure of ca. 30%). The configuration of electrodes adopted in the experimental cell was similar to that used in conventionalpolarography (Figure 1). The working electrode was immersed into the
0
2
1
3
4
DEPTH OF IMMERSION, m m
Figure 2. Calibration curve for the determination of the
-
immersion depth using the silver working electrode: [AgNOJ = 10-2 M; potential scan +800 -600 mV; scan rate = 20 mV/s. The slopes of cyclic voltammograms us electrode immersion depths are plotted. subphasesuch that the electrodejust touched the solution surface; this corresponded to an immersion depth of 0.3-0.5 mm. The immersion depth was readily estimated since the portion of the electrode tip in contact with the solution darkened during the electrolysisprocess. Additionally, the cyclic voltammetric curves, taken by using the silver working electrode, were determined to be linear for electrode potentials larger than +500 mV (vs SCE) and the slope was found to be proportional to the depth of electrode immersion. The obtained calibration curve (Figure 2) was used to estimate the depth of electrode immersion. A potential was then applied to the working electrodeand the electrical current through the system was recorded. Cyclic voltammetry (CV) was generally performed in the +800 to -600 mV (us SCE) range with a sweep rate of 5-20 mV/s. Chronoamperometry (CA) curves were recorded at 30-9intervals. After use, the electrode was cleaned with ultrafine sandpaper and acetone. Twenty-five points on the surface were regularly used to take CA and CV curves in order to obtain statistically correct data for a certain surface concentration of surfactant molecules. All of the applied potentials were measured with a saturated calomel reference electrodeand are reported with respectto SCE. An EG&G Princeton Applied Research potentiostat/galvanostat (Model 273 interfaced with an IBM PC-XT computer) was used to perform all electrochemical experiments. The resistivity of the silver film was measured by supporting it on a glass slide coated with a conductive I T 0 layer (Meadowlark Optics Co.). The IT0 layer was etched by concentrated HC1 to produce a stripe pattern. The nonconductive gap between the conductive stripes was 2 mm. The silver particulate film was deposited on this conductive stripe by horizontal lifting through the monolayer-supportedsilver particulate film. TEM images were taken using a JEOL 2000EX transmission electron microscope, operatingat 120kV. Sampleswere mounted on carbon-coated, Formvar-covered copper grids (400 mesh; obtained from Structure Probes, Inc., Westchester,PA) and were prepared by gently lifting a grid through the silver film from the
3712 Langmuir, Vol. 9, No. 12, 1993
Kotou et al.
Figure 3. TEM images of silver films: (a, top left) a representative image of the center of the film; (b, top right) the edge of the spot; (c, bottom left) enlarged image of Ag particles recorded a t higher electron magnification; (d, bottom right) diffraction pattern from the silver film. Parts a and b show particles generated under dicetyl phosphate monolayer, 45 A2/molecule;?r = 51 mN/m, AI3 = -200 mV; [AgN03] = 0.01 M. Scale bars = 200 nm (a and b) and 100 nm (c). solution phase. Care was taken not to disrupt the structure of the film. We consider that minimal disruption of the in situ film structure occurs during the preparation of TEM samples. Upon transfer of the silver film formed under the monolayer to a glass slide, no change in size of the circular silver spot formed around the electrode was observed. Also, no change in the appearance of the film on drying was noted by optical microscopy. The film, thus, behaves as though fairly rigid. Additionally, the images obtained after investigation of a great number of TEM samples were reproducible and uniform over the entire grid; slight variations in such factors as drying rate were found to be unimportant. This suggests that artifacts induced in silver films upon drying and transfer are minimal.
Results Morphologyof the Silver Particulate Films Formed under Monolayers. Application of a negative potential ( 10-4M. These concentration levels correlate well with the binding constants of the metal ions to phosphate headgroupsm and with the solubility products of the corresponding phosphates;21the lower the solubilityproduct (orthe higher the binding constant), the more pronounced is the effect of the cation. The efficiency of the formation of the silver layer was also dependent on the pH of the subphase (Figure 11).An increase in the acidity of the subphase reduced the rate of silver formation. The location of the pH-dependence curves correlates with the PKa constants of surfactants. The S-curves for phosphatidic acid, 4 (pKa = 2.3),22and hexadecyl phosphate, 1 (PKa = 2.51, are of similar form, with the phosphatidic acid (4) curve being shifted 0.2 pH unit toward lower pH values with respect to that of hexadecyl phosphate (1). In contrast, the curve for dipalmitoylphosphatidylethanolamine,6, is much broader and significantly shifted toward more basic solutionsthan the latter two curves. This can be attributed to the ionization of the amino group (p& = 10.5) present in this surfactant, although the phosphate group also present is of the same acidity as in the other surfactants (pK, = 0.32,232.7%).
Y
log([M''I), lg(M)
Figure 10. Effect of the presence of divalent cations M2+on Ag f i i in aqueous 0.01 M AgNOs under monolayers (kept at 50 Az/molecule) prepared from 1: Ca(NOd2(+); Cu(NO& (A);Pb(NO& (W. AE = -200 mV; t = 30 8.
-
Ag+ + e- Ago (7) in homogeneous solution is +330 mV (Figure 9). Thus, the energy required to form the particulate silver film on the monolayer is about 42 kJ/mol higher than that required to reduce silver on the Pt/Ir electrode. As stated earlier, neither Cu2+ nor Pb2+ salts formed metallic layers analogous to those formed by silver. Moreover, investigations showed that even very low concentrations (in the order of 0.001 M) of divalent metal ions in the silver solution subphase can completely inhibit the formation of a surface silver film (Figure 10). The influence of the presence of such divalent metal ions on the size of the silver spot produced by electrolysis at a given applied potential (-200 mV) was studied for Cu2+,
Discussion Examination of the electrochemically-produced silver films at high magnifications revealed that they primarily consisted of a network of interconnected particles. They were found to be conductiveand functioned as an extension of the working electrode. Results of cyclic voltammetry and chronoamperometryhave substantiated this postulate. New particles only formed at the edges of films, as demonstrated by the close particle sizes and particle densities recorded over the entire area of the film. Reduction of silver cations proceeded mainly at the frontiers of the film. These results are explicable in terms of silver cation reduction (eq 7) at the solution/monolayer interface. (20)Marsh, D.Handbook onPhospholipidBilayers; CRCPress: Boca Raton. FL. - _,1990. ~ .. . . (21)CRC Handbook on Physics and Chemistry; CRC Press: Boca ~~~
Raton, FL, 1988/1989. (22)Copeland, B.R.; Andersen, H. C . Biochemistry 1982,21, 3811. (23)Standish, M. M.; Pethica, B.A. Trans. Faraday SOC.1968,64, 1113. (24)Papahadjopoulos, D.Biochim. Biophys. Acta 1968,163, 240.
Kotov et al.
3716 Langmuir, Vol. 9, No.12,1993 A priori, surfactants in the monolayers can be expected to reduce the surfaceenergyof the incipient silver particles. Such a reduction of the free energy for reaction 7 would be expected to result in a shift of the reduction potential toward more positive values (Le,, AG = -nF4 in accord with the analogous underpotential deposition of metals (Le., the metal adatom to substrate binding energy is higher than that to the same bulk metal).11-14 However, the applied potential required to initiate silver deposition at the monolayer interface was found to be more negative than that in bulk water (Figure 9). Therefore, silver cation reduction at monolayers is a kinetically, rather than a thermodynamically, controlledprocess. The overpotential term due to the activation energy of the particle formation connected with the excess of surface energy was calculated by using equation18 7 = 3M,ulzFp r
(8) where 7 is the overpotential, M, is the molecular weight of silver (taken here as 0.108 kg/mol), u is the surface tension of waterlsilver interface, z = 1is the charge of the ion, p is the density of bulk silver (10.50 g/cm3 1, and r is the mean radius of the particles. The calculated 7 was in the order of 10 mV for silver particles formed at the monolayer and ca. 5 mV for those formed on the electrode. This calculation implies an insignificant contribution of the overpotential of that type to the observed silver ion reduction potential at monolayers. It is interesting to note the difference between the mean sizes of silver particles grown under monolayers (ca.50-nmdiameter),those which are in contact with the electrode (ca. 10 nm), and those found in the subphase in the vicinity of the electrode (500lo00 nm). Formation of smaller silver particles under monolayers than in the subphase in the vicinity of the electrodeis consistentwith the larger excesssurface energy in the former location.18 It is well-known that phosphates form insoluble salts with silver cations (the solubility product of Ag3P04 is 10-16 M4/L4)?1@* The surfactants with phosphate headgroups may be anticipated to strongly complex silver cations from the subphase solution and thereby significantly increase the effective silver concentration in the vicinity of the monolayer. The metallic silver film could then form via the reduction of silver cations adsorbed on the monolayer. It would be anticipated that the reduction potential of the silver in this case would be significantly more negative than that for the silver in the bulk phase. Indeed, the rate constant for the reduction of aphosphatesilver complex was shown to be 8.6 times smaller than that for a free silver cation.27*% The importance of the adsorption of silver cations to the surfactant headgroups for film formation was further substantiated by studying the efficiency of silver layer formation with respect to the pH of the subphase and the concentration of M2+cations. Protonation of the phosphate headgroups reduces the equilibrium concentration of the silver/phosphate complexes. Accordingly, curves 1 and 2 in Figure 11 resemble the titration curves of the surfactants forming the monolayers. In the case of dipalmitoylphosphatidylethanolamine(61, the effects of ionization of both the phosphate and amino groups are superimposed. The presence of the positively-charged, (25) Henglein, A. J. Phys.
Chem. 1993,97,5457.
(26) Yi,K.C.; Fender, J. H. Proceedings of the LB6 Conference,Trois
Rivieres, Canada, 1993; p 550. (27) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1977,81,656. (28)Mulvaney, P.; Henglein, A. J. Phys. Chem.1990,94,4182.
Table I. Maximum Value of the Slope of Chronoamperograms for Different Surfactants. name dihexadecyl phosphate (1) dipalmitoylphaphatidic acid sodium salt (4) dioleoylphosphatidic acid methyl ester monosodium salt (5) dipalmitoylphaphatidylethanolamiie (6) phosphoric acid, n-hexadecyl N-(0-vinylbenzoy1)11-aminoundecyl ester (2) 1-monooleoyl-rac-glycerol(10) dipalmitoylphaphatidic acid diphenyl eater (9) dipalmitoylphosphatidylcholine(8) dioctadecyldimethylammoniumbromide (11) a pH = 5.25, hE = -200 mV.
75 74
62
28
64
0.3 0.02
0.01 -0.01
protonated amino group inhibits the formation of a strong silver complex; the pH dependence (see curve 3 in Figure 11)of metal ion binding is thus significantly broader and shifted toward higher pH values, as compared with surfactants bearing only a phosphate moiety. The equilibrium constants of the divalent metal ionsurfactant complexes are much higher than those of the corresponding silver complexes. Consequently, in the investigated systemsin which the subphasecontained both M2+ and Ag+ ions, the M2+ ions were preferentially adsorbed by the monolayer, and no silver film formation was observed. However, it is not immediately apparent why the M2+ ions utilized could not be electrochemically reduced at the monolayer surface. It is conceivable that binding by these metal ions could disturb the conformation of the surfactant from an arrangement favorable for metal layer formation. Indeed, many authors have observed that Ca2+and Ba2+cations significantlychanged the orientation of the phospholipid headgroups at the monolayer/water interface.29JC’ The effect of the surfactant headgroup can be demonstrated by comparing the observed maximum values of dildt for the different surfactants. The results of the chronoamperometry measurements on the different surfactants are summarized in Table I. The most efficient silver particulate film formation was observed under monolayers prepared from surfactants which contained a Ybare”phosphate group (1 and 4). Monolayers formed from surfactants containing bulky phosphatidic acid (91, oleate (lo), and positively charged (11) headgroups did not support two-dimensional silver electroreduction. Strong silver ion complexation provides, therefore, the driving force for the electrocrystallization of 50- to 70-nm-diametersilver particles under monolayers. The process occurs at potentials more negative than those used for silver ion reduction at electrode surfaces and is, therefore, kinetically governed. Note Added in Proof: Subsequent to the submission of this manuscript, the authors became aware of the highly relevant publication: Tail Z.; Zhang, G.; Qian, X.; Xiao, S.;Lu,Z.; Wei, Y.Dendritic Patterns of Two-Dimensional Silver Films Formed with a Series of Amphiphilic Schiff Bases of Diazafluorenone. Langmuir 1993,9,1601-1603.
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. M.E.D.Z. thanks the FundaCgo de Amparo a Perquisa Estado de Sa0 Paulo (FAPESP) for financial support. (29) Toccane, J.-F.; Teissie, J. Biochem. Biophys. Acta 1990, 1031, 111. ’ (30) Eibl, H. Angew. Chem., Znt. Ed. Engl. ISM, 23,257.
