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Subnanometer Silver Clusters Exhibiting Unexpected Electrochemical Metastability on Graphite Kwok H. Ng, H. Liu, and R. M. Penner* Institute for Surface and Interface Science, and Department of Chemistry, University of California at Irvine, Irvine, California 92697-2025 Received November 9, 1999. In Final Form: January 19, 2000 The anodic dissolution of silver particles on highly oriented pyrolytic graphite (HOPG) electrode surfaces is investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and atomic force microscopy (AFM). As a discontinuous and granular silver electrodeposit is anodically stripped from HOPG surfaces at a potential of +500 mV vs Ag0/Ag+, the number and diameter of silver particles larger than 20 nm in diameter visible by SEM decreases from ∼109 cm-2 to zero in 10 min. On these same surfaces, AFM and TEM images acquired at longer times show that silver nanoparticles continue to diminish in size until a high density (∼109 cm-2) of clusters having a diameter of 0.3-0.6 nm are obtained within 30 min. The coverage of the anodically poised HOPG surface by these subnanometer silver clusters gradually decreases to zero over the course of an hour. SAED analysis confirms the composition of the clusters seen by AFM is FCC silver. A decrease in the coverage of the surface by silver nanoparticles is correlated with an increase in the nucleation overpotential for silver as measured by cyclic voltammetry. Possible origins for the metastability of silver clusters at positive potentials are discussed.
I. Introduction The electrochemistry of nanometer-scale metal particles should differ from the electrochemistry of bulk metal electrodes. The best reason for believing this is that the physical properties of metal nanoparticles (smaller than 15 nm or so) differ from those of bulk metal, and these properties depend on the particle size. In the case of silver, for example, the lattice constant of the FCC unit cell is compressed by as much as 9%1-4 for nanoparticles, and the melting point is depressed by as much as 700 °C.5,6 Electronic properties including the photoelectron yield7-9 and the energy of the plasmon resonance absorption10-17 also exhibit a particle size dependence in this size regime. Collectively, these observations lead to the expectation that the electrochemical properties of silver nanoparticles, such as the standard electrode potential, E°, should also be size dependent. Is this expectation reasonable from a theoretical standpoint? The answer provided by the calculations of * Address correspondence to:
[email protected]. (1) Wasserman, H. J.; Vermaak, J. S. Surf. Sci. 1970, 22, 164. (2) Apai, G.; Hamilton, J. F.; Stohr, J.; Thompson, A. Phys. Rev. Lett. 1979, 43, 165. (3) Montano, P. A.; Schulze, W.; Tesche, B.; Shenoy, G. K.; Morrison, T. I. Phys. Rev. B 1984, 30, 672. (4) Montano, P. A.; Purdum, H.; Shenoy, G. K.; Morrison, T. I.; Schulze, W. Surf. Sci. 1985, 156, 228. (5) Buffat, P.; Borel, J.-P. Phys. Rev. A 1976, 13, 2287. (6) Castro, T.; Reifenberger, R.; Choi, E.; Andres, R. P. Phys. Rev. B 1990, 42, 8548. (7) Mu¨ller, U.; Schmidt-Ott, A.; Burtscher, H. Z. Phys. B 1988, 73, 103. (8) Schleicher, B.; Burtscher, H.; Siegmann, H. C. Appl. Phys. Lett. 1993, 63, 1191. (9) Schmidt-Ott, A.; Schurtenberger, P.; Siegmann, H. C. Phys. Rev. Lett. 1980, 45, 1284. (10) Doremus, R. H. J. Chem. Phys. 1964, 40, 2389. (11) Doremus, H. J. Chem. Phys. 1965, 42, 414. (12) Genzel, L.; Martin, T. P.; Kreibig, U. Z. Physik. B 1975, 21, 339. (13) Smithard, M. A. Sol. State. Commun. 1974, 14, 407. (14) Dupree, R.; Smithard, M. A. J. Phys. C 1972, 5, 408. (15) Heath, J. R. Phys. Rev. B 1989, 40, 9982. (16) Schimmel, T.; Bingler, H.-G.; Franzke, D.; Wokaun, A. Adv. Mater. 1994, 6, 303. (17) Mulvaney, P. Langmuir 1996, 12, 788.
Henglein18-20 and Plieth21,22 is yes. If it can be assumed that the surface free energy, γ, is the same for metal nanoparticles and bulk metal surfaces, then the Kelvin equation23 predicts that metal nanoparticles will exhibit an E° that is shifted negatively from the E° of the bulk metal.21,22 Superimposed on the monotonic decrease in E° with size are smaller quantum mechanical effects that cause E° to oscillate: Metal clusters with even numbers of atoms are stabilized with respect to clusters with odd numbers of atoms. The net effect of this negative shift in E° for small silver clusters is not subtle: A single silver atom, for example, predicted24,25 to exhibit an E° of -1.8 V vs NHE (vs +0.799 V for bulk silver) and the silver trimer (i.e., Ag3) is predicted to have a potential near -1.0 V.26 These silver clusters are therefore predicted to be stronger reducing agents than zinc (E°Zn/Zn2+ ) -0.76 V)! Do experiments confirm this prediction? The existing experimental evidence must be classified as indirect. For example, the E° for silver nanoparticles can be approximated based on their ability to dissociatively reduce organic chlorides in reactions of the type:
Agn + RCl h Agn-1 + Ag+ + Cl- + R
(1)
For example, the dissociative reduction of both ethyl chloride and chloroacetic acid by Ag10 has been reported and Ag2+ has been reported to reduce chloroform.27 The (18) Henglein, A. Chem. Rev. 1989, 89, 1861. (19) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (20) Henglein, A. Top. Curr. Chem. 1988, 141, 113. (21) Plieth, W. J. J. Phys. Chem. 1982, 86, 3166. (22) Plieth, W. J. Surf. Sci. 1985, 156, 530. (23) Fisher, L. R.; Israelachvili, J. N. J. Colloid Interface Sci. 1981, 80, 528. (24) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Dis. Chem. Soc. 1991, 92, 31. (25) Henglein, A. In Modern Trends of Colloid Science in Chemistry and Biology; Eicke, H.-F., Ed.; Berkha¨user-Verlag: Basel, Switzerland, 1985; p 126. (26) The quantum chemical calculations of Henglein and co-workers predict that the silver dimer (Ag2) exhibits a much more positive redox potential of near 0.0 V vs NHE. (27) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1977, 81, 556.
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E° values estimated in this way are consistent with the E° values obtained for the various silver species predicted by theory. The behavior of silver grains in photographic emulsions has also been attributed to negative shifts of E° with decreasing grain size.28,29 However, some of this reactivity data is contradictory. Inexplicably, the same silver clusters that reduce organic chlorides are incapable of reducing water, for example.30 For metal nanoparticles on electrode surfaces we believe there is no experimental data that either confirms or refutes the predictions of Henglein and Plieth. In a first attempt to investigate this phenomenon, we report here investigations of the anodic dissolution of silver microand nanoparticles on graphite surfaces. Silver has been selected for these initial investigations because of its nobility, because the reduction and oxidation of silver are kinetically facile,31 and because a large quantity of physical data for silver nanoparticles exists for purposes of comparison. We find that the oxidative dissolution of “coarse” silver electrodeposits32 consisting of micron-scale particles (and 5-20 equivalent atomic layers of metal) occurs until silver clusters, having a diameter of 0.3-0.6 nm, remain on the surface. Ex situ atomic force microscope images document the dissolution and eventual disappearance of these silver clusters in a process that, at an applied potential of +500 mV vs Ag0/Ag+, requires more than an hour. It is demonstrated that the presence of these metastable silver clusters on the surface accounts for the ubiquitous hysteresis that has been observed in electrodeposition and stripping cyclic voltammograms at graphite electrode surfaces. II. Experimental Methods II. A. Electrochemistry. A glass and Kel-F cell was employed for the electrodeposition of silver on freshly cleaved highly oriented pyrolytic graphite (HOPG). The working electrode in this cell employed an O-ring to isolate a 0.1412 cm2 circular area of the graphite basal plane. This circular working electrode was exposed to a N2-sparged plating solution containing either 1.0 mM silver perchlorate (Alfa, 99.9%) and 0.10 M lithium perchlorate (Aldrich, 99.99%), or 1.0 mM silver nitrate (Aldrich, 99.99%) and 0.10 M potassium nitrate (Aldrich, 99.9%). The solvent for all experiments was acetonitrile (Burdick & Jackson, >99.9%). The deposition of silver was accomplished using a silver wire reference electrode (Ag0/Ag+) immersed directly in the silver plating solution; silver-free control experiments were carried out using a saturated calomel electrode (SCE) as a reference. A platinum wire counter electrode was employed for all experiments. A computer-controlled EG&G Princeton Applied Research 273A potentiostat was employed for all experiments. II. B. Microscopy. Noncontact AFM experiments in air were performed using a commercial instrument (Park Scientific Instruments (PSI Inc.) AutoProbe LS). This instrument operates in the slope-detected mode. Cantilevers were 2.0 µm thick Ultralevers (PSI Inc.) having a force constant of 18 N/m, a resonance frequency near 300 kHz, and a nominal tip radius of 10 nm. The cantilever excitation frequency employed for these measurements was adjusted to be near the maximum slope of the cantilever resonance response curve. Following electrochemical deposition of silver on a graphite surface, the surface was removed from the plating solution, and rinsed briefly in a stream of pure acetonitrile. The surface was then allowed to dry in a vacuum desiccator prior to examination with the NC-AFM. (28) Konstantinov, J.; Malinowski, G. J. Photogr. Sci. 1975, 23, 1. (29) Konstantinov, J.; Panov, A.; Malinowski, G. J. Photogr. Sci. 1973, 21, 250. (30) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1335. (31) The exchange current density for the reaction: Ag+ + e- f Ag0 at polycrystalline silver surfaces is I0 ) 24 + 5 A cm-2 (cf., Gerischer, H.; Tischer, R. P. Z. Electrochem. 1957, 61, 1159). Thus, the interfacial rate of electron transfer for silver is greater than for any other metal. (32) Zoval, J. V.; Stiger, R. M.; Biernacki, P. R.; Penner, R. M. J. Phys. Chem. 1996, 100, 837.
Figure 1. Cyclic voltammetry of highly oriented pyrolytic graphite in a silver plating solution containing 1.0 mM Ag+ in 0.10 M LiClO4 in acetonitrile at a scan rate of 20 mV s-1. (a) First three cycles from an initial potential of +0.5 V vs Ago/Ag+. (b) Detail of the cathodic branch of the CVs shown in a illustrating the definition of the nucleation overpotential, ηdep, employed in this paper. TEM and SAED data were acquired using a Philips EM-200 TEM and an accelerating voltage of 200 keV. SAED patterns were obtained at a camera length of 1350 mm using a selected area aperture having a diameter of 10 µm. SEM images were obtained using a Philips XL30-SEM and an accelerating voltage of 25 keV. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM) data were all acquired on silver electrodeposits without removal from the graphite basal plane surface. In the case of TEM and SAED measurements, this was accomplished by preparing a HOPG working electrode consisting of thin (10-40 nm thick) HOPG flake (∼1 mm2) supported on a carbon coated gold grid (Ted Pella). This electrode was transparent to the 200 keV electrons employed for the acquisition of TEM and SAED data. The electrodeposition and dissolution of Ag were carried out on this electrode using the same procedure as described above.
III. Results and Discussion III. A. Electrochemical Investigations. Figure 1a shows a familiar phenomenon associated with the electrodeposition and stripping of a metal from a graphite electrode surface by cyclic voltammetry. A large nucleation overpotential, ηdep,33 on the first, negative-going voltammetric scan is replaced by a much smaller nucleation overpotential on the second and subsequent scans. As shown in Figure 1b, for example, ηdep for scans 1 and 2 are (33) In this manuscript we shall define ηnucl to equal the overpotential at a cathodic current density of 200 µA cm-2.
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Figure 2. (a) Linear scan voltammograms for an HOPG surface in the silver plating solution as a function of the equilibration time at +500 mV, τ+500, as labeled. Plots of ηdep as a function of τ+500 for four different HOPG surfaces, and for two different electrolytessnitrate and perchloratesas indicated. Along the right hand side of this plot is indicated the ηdep observed for the first scan.
230 mV and 80 mV, respectively; a reduction of 150 mV or 65%. Qualitatively similar cyclic voltammetry has been reported at graphite electrodes34 for silver,35 palladium,36 and other metals. Although the behavior seen in Figure 1 is “binary” (a large ηdep is obtained for scan #1; a much smaller ηdep on scans 2, 3, etc.), the nucleation overpotential for silver can be adjusted to any intermediate value, and even to larger values, by pausing at the positive turning potential (+500 mV in Figure 1) and equilibrating the electrode at this potential for various times. The linear scan voltammograms (LSVs) shown in Figure 2 document this effect. Each of these LSVs was obtained following the deposition and stripping of silver from the HOPG surface, and the equilibration of the surface at +500 mV vs Ag0/Ag+ for a duration, τ+500, ranging from 0 min to 5 h. The smallest ηdep value was obtained for τ+500 ) 0.0 min and this scan is, of course, coincident with scan 2 in the experiment of Figure 1. As τ+500 was increased (up to a maximum of 5 h), the value of ηdep also increased monotonically by as much as 350 mV. This increase in ηdep is caused by the elimination from the surface of nucleation sites for silver created by the initial deposition and stripping silver. In most experiments, the nucleation overpotential observed (34) Including vitreous carbon and glassy carbon as well as HOPG. (35) Gunawardena, G.; Hills, G.; Montenegro, I. J. Electroanal. Chem. 1982, 138, 241. (36) Bell, M. F.; Harrison, J. A. J. Electroanal. Chem. 1973, 41, 15.
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after 1 to 5 h was greater than that seen at the freshly cleaved HOPG surface (i.e., scan #1 in Figure 1). This means that fewer nucleation sites for silver exist on a HOPG surface on which silver has been deposited and stripped and then equilibrated for several hours, than at a freshly cleaved graphite surface on which silver has never been deposited. The increase of ηdep as a function of τ+500 is plotted for four different experiments (involving different HOPG surfaces, and two different anions) in Figure 2b. It is apparent that the magnitude of the increase in ηdep is different for different HOPG surfaces. This variability likely derives from the cleave-to-cleave variability of the defect density of the HOPG surface. The HOPG surface used to obtain the data of curve 4, for example, possessed a relatively large defect density (as evidenced by the small value of ηdep seen in scan #1 of 140 mV) and the effect of equilibration at +500 mV on ηdep was also minimal (a maximum ηdep of 150 mV for τ+500 ) 5 h). On this surface, defects associated with the cleaved HOPG surface facilitated silver nucleation, and the removal of nucleation sites by the phenomena operating at +500 mV had a relatively small effect on the overall “ability” of the surface to nucleate silver. The data of curve 1 represents the opposite extreme: The freshly cleaved HOPG surface is relatively free of defects as evidence by a first scan ηdep of 303 mV. Equilibrating this surface at +500 mV after silver stripping resulted in a large decrease of ηdep by over 350 mV for τ+500 ) 5 h. The data of Figure 2 suggest the existence of two types of nucleation sites on HOPG surfaces: intrinsic nucleation sites are those present on a freshly cleaved graphite surface. On graphite, we’ve shown32 that intrinsic nucleation sites for silver exist both at step edges and on atomically smooth terraces. Extrinsic nucleation sites are defined as those created by the deposition and stripping of silver from the surface. Our data suggests that the phenomenon occurring at +500 mV affects predominantly extrinsic nucleation sites because in most experiments, the increase in ηdep that is seen for long equilibration times approximately equals the ηdep measured for the freshly cleaved surface (indicated by the arrows in Figure 2b). This is never exactly true, however, and we have already noted that in most experiments, long equilibration times yielded ηdep values that were somewhat greater than the ηdep measured for the freshly cleaved surface. This implies that some intrinsic nucleation sites present on the ascleaved graphite surface are removable at +500 mV. The data of Figures 1 and 2 provoke two questions: First, what are extrinsic nucleation sites, and second, what is the nature of the process acting at +500 mV to remove them? With regard to the first question, two explanations have been advanced: At graphite surfaces the intercalation of metal ions into interlayer interstices can occur. This process is facilitated at edge-oriented HOPG, and it can also occur at other carbons that expose graphitic edges at their surfaces including vitreous graphite and glassy carbon. It is not clear, however, how the intercalation of metal into graphite will facilitate nucleation at the surface of the graphite electrode, and the details of this process have not been explored by those who have proposed this explanation. A second hypothesis is that metal nuclei persist on the electrode surface following the first voltammetric scan. Since this scan can involve an excursion of 100 mV or more to potentials positive of E0′, this explanation requires that silver nuclei be stable for several seconds or more at these positive potentials. As already indicated in the Introduction Section, this is a radical proposal in view of what has been postulated regarding the reactivity
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of metal nanoparticles. To our knowledge, no direct evidence exists in the literature that confirms or refutes either of these hypotheses. We shall show that at HOPG surfaces, however, the second explanation is correct. III. B. Electron Microscopy, Electron Diffraction, and Atomic Force Microscopy. Investigations. In this study we have used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in conjunction with selected area electron diffraction (SAED) to observe phenomena on the graphite surface associated with Figures 1 and 2 above. All of these studies were performed ex situ following the emersion of the graphite electrode from the silver plating solution, rinsing of the electrode with Nanopure water, and drying. In the case of TEM and SAED studies, we have performed all of the required electrochemical manipulations on thin graphite flakes immobilized on gold TEM grids, as previously described.37-41 In all of these studies, the graphite electrode was first cycled from +500 mV to -500 mV and back to a final potential of -100 mV (Figure 3 a,b only), or to +500 mV (all other experiments) at a scan rate of 20 mV s-1. In experiments in which the final potential was +500 mV, this scan caused the electrodeposition of several monolayers of silver. It is the stripping of this silver electrodeposit at +500 mV, that is of interest in this study. For experiments terminating at +500 mV, microscopy data was then acquired as a function of the equilibration time at this potential, τ+500. SEM images of a graphite surface at two magnifications are shown in Figure 3. Images have been acquired of the silver electrodeposit before stripping (Figure 3 a,b), following stripping, but after no equilibration at +500 mV (i.e., τ+500 ) 0 min, Figure 3 c,d), for τ+500 ) 10 min (e,f), and τ+500 ) 1 h (g,h). The discontinuous nature of the initial silver electrodeposit is shown in the images of Figure 3a,b which were obtained for an HOPG surface which was scanned at 20 mV s-1 to -500 mV and back to the foot of the stripping wave at -100 mV. The distribution of particle sizes visible by SEM on these surfaces were clearly bimodal consisting of micron scale particles at a coverage of ∼106 cm-2 (Figure 3a), and a much higher coverage (∼109 cm-2) of 20-100 nm particles (Figure 3b). The SEM used in this study was not capable of detecting silver clusters smaller than ∼20 nm. The HOPG surface shown in Figure 3c and d is that obtained when the graphite electrode is scanned through the silver stripping wave and disconnected at +500 mV; immediately upon completion of a single voltammetric scan (i.e., τ+500 ) 0 min). At a scan rate of 20 mV s-1, this means that the surface was subjected to oxidizing potentials (i.e., Eappl > E0′Ag0/Ag+) for 25 s. The SEM images of Figure 3c and 3d show that the dissolution of silver from the graphite surface was not completed during this interval. However the number density of silver micron-scale particles was reduced by about one-half, and the coverage of 20-100 nm diameter silver particles was reduced by 70-80%. In the case of high magnification images such as that of Figure 3d, it is tempting to conclude that the missing sub-100 nm silver particles have been stripped from the surface. We shall see that these particles are still present on the surface,
but the diameter of these missing silver particles has been reduced to less than 20 nm and these particles are therefore invisible to the SEM. If such a surface is permitted to equilibrate for 10 minutes at a potential of +500 mV, then SEM images such as that seen in Figure 3e and f are obtained. Here, the coverage and size of micron-scale particles has been further reduced, and we shall see that all of the preexisting 50-100 nm particles have been reduced in diameter to below 20 nm. For τ+500 ) 1 h (Figure 3g,h), no silver particles of any size were observable on the surface by SEM. As we have already indicated, the SEM images of Figure 3 do not tell the whole story. We have used noncontact atomic force microscopy (NC-AFM) in parallel with SEM to document the disappearance of silver particles from the HOPG surface during oxidation at +500 mV. For τ+500 ) 0 min (actually an oxidation time of 25 s, as discussed above) and 10 min (Figures 4a,b, respectively), large area NC-AFM images (not shown here) corroborate the dissolution of micron-scale silver nanoparticles seen by SEM. More importantly, though, the fate of the 50-100 nm silver particles seen by SEM at τ+500 ) 0 min (Figure 3d) but not for τ+500 ) 10 min (Figure 3f) becomes apparent. The HOPG surface oxidized for 10 min (Figure 4b) shows 0.4-1.5 nm silver particles at the same coverage of 109 cm-2 at which the 20-100 nm silver particles were seen in the SEM data. Histograms of the NC-AFM-measured particle height shown in Figure 5 show that for τ+500 ) 0 min (Figure 5a), virtually all of the silver particles observed are larger than 5 nm, whereas after a 10 minute oxidation time (Figure 5b), no particles larger than 5 nm in diameter are observed. As shown in the image of Figure 4c, this blanket of silver “clusters” persists on the HOPG surface for an hour while the mean particle height decreases slightly to 0.5 nm (see histogram of Figure 5c). Only after the surface has been oxidized for 5 h at +500 mV are these smallest silver clusters completely removed (Figure 4d). In principle, the dissolution process documented in the SEM and NC-AFM data of Figures 3 and 4 might also be tracked electrochemically. However, we have found that the anodic currents associated with this process are too small to measure reliably. Two lines of evidence suggest that the unexpected persistence of silver particles on the HOPG surface at +500 mV vs Ag0/Ag+ is a kinetic, not a thermodynamic, effect. First, the data of Figures 3 and 4 clearly show that silver nanoclusters possess a finite half-life on the HOPG surface of approximately an hour. Second, we have attempted to locate a voltammetric wave corresponding to the oxidative stripping of these silver clusters at potentials positive of +500 mV vs Ag0/Ag+ and this waves if it existssis not observable using cyclic voltammetry even though the prompt dissolution of these silver clusters would be expected to generate an observable voltammetric response above background (data not shown). Instead, the data presented above suggests that the kinetics for silver dissolution at HOPG become exceedingly slow for silver clusters having dimensions below 1.0 nanometer at +500 mV. It is not worthwhile at this point to attempt a numerical estimate of this rate since any such estimate would be subject to large uncertainties associated with estimation of the silver surface area. The electrochemical metastability of the silver nanoparticles clusters seen in the microscopy data of Figures 3 and 4 is astonishing. This is especially true in view of the aforementioned enhancement of the electrochemical reactivity expected for nanometer-scale silver particles.
(37) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1998, 102, 1166. (38) Nyffenegger, R. M.; Craft, B.; Shaaban, M.; Gorer, S.; Erley, G.; Penner, R. M. Chem. Mater. 1998, 10, 1120. (39) Hsiao, G. S.; Anderson, M. G.; Gorer, S.; Harris, D.; Penner, R. M. J. Am. Chem. Soc. 1997, 119, 1439. (40) Gorer, S.; Ganske, J. A.; Hemminger, J. C.; Penner, R. M. J. Am. Chem. Soc. 1998, 120, 9584. (41) Anderson, M. A.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1997, 101, 5895.
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Figure 3. Scanning electron micrographs of HOPG surfaces following the deposition and stripping of silver by cyclic voltammetry. Images were acquired at two different magnifications (as indicated) and as a function of τ+500: (a,b) Before stripping (scan halted at -100 mV vs Ag0/Ag+), (c,d) scan halted at +500 mV vs Ag0/Ag+; wait ) 0 min, (e,f) 10 min, (g,h) 1 h.
The expectation, based on the calculations of Henglein and Plieth, is that silver nanoclusters should dissolve at more negative potentials as compared with bulk silver, or that at a particular overpotential for oxidation the dissolution of nanoscopic silver particles should proceed more rapidly. Since our observations are at odds with these expectations, it is tempting to conclude that the particles we observe are composed of something other than silver metal, or at least that the 0.3-0.6 nm clusters seen in the NC-AFM images of Figure 4b and 4c are not Ag0. Two logical alternatives for the identity of these clusters are
a silver oxide or a silver salt with the anion of the supporting electrolyte (either ClO4- or NO3-). Since we viewed either of these as reasonable possibilities, we used electron diffraction to probe the structure of our silver clusters. Figure 6 shows transmission electron micrographs and electron diffraction patterns for graphite surfaces prepared in exactly the same way as those shown in Figures 3 and 4. Data for two equilibration times are shown: 10 min (Figure 6a,b) and 30 min (Figure 6c,d). In Figure 6a, silver nanoparticles ranging in size from 5 to 50 nm were seen, and in the SAED pattern for this surface
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Figure 4. Noncontact atomic force micrographs showing a 3.0 × 3.0 µm area of HOPG surfaces following the deposition and stripping of silver by cyclic voltammetry. Images were acquired at different equilibration times: (a) τ+500 ) 0 min, (b) τ+500 ) 10 min, (c) τ+500 ) 1 h, (d) τ+500 ) 5 h.
(acquired from a 10 µm diameter area), all diffraction rings can be assigned to FCC silver (see Table 1 for these assignments). In this SAED pattern, closely spaced diffraction lines, such as [311] and [420] were resolvable and this is consistent with a Debye-Scherrer particle diameter of approximately 2 nm. Following oxidation for 30 min, TEM images of the electrode revealed the presence of silver clusters as expected. In the image of Figure 6c, defocusing (as required to achieve the necessary contrast) amplifies the size of these particles somewhat, however the largest particles seen on the surface are approximately 2 nm in diameter; marginally larger than the particles seen in Figure 4b and 4c. SAED analysis of this surface yielded a pattern having broader lines, as expected for silver nanoparticles smaller than 20 Å in diameter, on average. However all prominent diffraction lines could
again be assigned to d spacings for silver, or to a pair of nearly identical d spacings (e.g., [331] and [420]) that are not resolved. These SAED data show conclusively that both the nanoparticles seen by SEM and NC-AFM, as well as the smaller and more persistent clusters, are composed of FCC silver. These results do not rule out the possibility that a strongly adsorbed but disordered anion layer exists at the surface of silver clusters, and the existence of such a passivating anion layer is certainly one of the most plausible explanations for the data presented earlier. It is also worth noting that SAED patterns for silver particles of all sizes (from 1 to 10 nm) consisted of continuous rings of diffracted electrons, even when diffraction was collected from within a single graphite grain (as in Figure 6d, for example). This means that silver
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Figure 5. Histograms of the AFM-measured height of silver nanoparticles following equilibration at +500 mV vs Ag/Ag+ for three times: (a) τ+500 ) 0 min, (b) τ+500 ) 10 min, (c) τ+500 ) 1 h. Figure 7. Plots of the mean NC-AFM measured particle height, and ηdep, in one set of experiments (Figure 2, curve 3), as a function of the oxidation time at +500 mV vs Ag0/Ag+. Table 1. Comparison of Measured d-Spacings for Silver Clusters on HOPG with Expected d-Spacings for FCC Silver hkl
111
200
220
311
222
331
420
FCC silver, Å 2.359 2.044 1.445 1.231 1.180 0.938 0.914 τ+500 ) 10 min, Å 2.360 2.029 1.438 1.232 1.172 0.936 0.909 τ+500 ) 30 min, Å 2.358 2.042 1.448 1.203 1.203 0.918 0.918
Figure 6. Transmission electron micrographs and selected area electron diffraction patterns for HOPG surfaces following the deposition and stripping of silver by cyclic voltammetry. Two sets of data acquired following two equilibration times are shown: (a,b) τ+500 ) 10 min, (c,d) τ+500 ) 30 min. At the top of each diffraction pattern is shown a simulated powder pattern for FCC silver. The line widths in these two simulated patterns are appropriate for 50 Å diameter crystallites (b), and 20 Å diameter crystallites (d).
particles do not favor a particular azimuthal orientation on the graphite surface. When this observation is coupled with the fact that all diffractions for silver were observed at approximately the expected relative intensities, we conclude that silver nanoparticles interact weakly with graphite, and do not prefer a particular orientation on the surface. Since the lattice mismatch between hexagonal
Ag(111) (with a nearest neighbor spacing of 0.354 nm) and the hexagonal periodicity of the phenyl rings on the graphite surface (which have a x3 spacing of 0.428 nm) is 17%, the absence of epitaxy is not surprising. In prior work, a good match between these two periodicities was required for epitaxial deposition of CdS and CuI nanocrystals on the graphite surface;39-41 for mismatched systems such as platinum and wurzite-phase ZnO on graphite, epitaxial deposition of nanocrystals has not been observed.37 Finally, we hypothesize that the existence of long-lived silver nanoparticles on the HOPG surface is responsible for the renucleation behavior shown in Figure 2. This is reasonable based on the fact that the coverage of the HOPG surface by silver and the nucleation overpotential for silver are inversely correlated as a function of the oxidation time at +500 mV. For example, shown in Figure 7 are plots of ηdep and AFM-measured particle height (one measure of the silver coverage) as a function of the oxidation time at +500 mV. This plot shows that the large initial increase of ηdep (which occurs over the course of a few minutes at +500 mV) correlates with the disappearance of “large” silver nanoparticles from the surface. IV. Summary In this paper we have reported ex situ observations of the dissolution of silver particles from a graphite electrode surface using SEM, AFM, and TEM. This process has also been characterized using electrochemistry, and the structure of long-lived silver particles on the electrode surface has been investigated by electron diffraction. The data support the following conclusions. (1) The anodic dissolution of silver microparticles from a graphite electrode surface potentiostated at +500 mV vs Ag0/Ag+ occurs rapidly until the mean diameter of these particles is reduced to 0.4-1 nm. Because we have deposited an arbitrary amount of silver (the equivalent of several monolayers), the timescale for this initial
Electrochemical Metastability of Ag Clusters on Graphite
Langmuir, Vol. 16, No. 8, 2000 4023
oxidation is meaningless. The main point is that a sharp decrease in the oxidation rate is observed when silver particles are reduced in diameter to 0.4-1 nm. (2) Under these conditions, the remaining 0.4-1 nm clusters are metastable and possess a half-life on the graphite surface of approximately an hour. Characterization of these clusters by electron diffraction shows their composition to be silver metal of FCC crystal structure. (3) The presence of 0.4-1.0 nm silver clusters on the graphite surface is associated with a diminution of the nucleation overpotential for the redeposition of silver onto the surface in cyclic voltammetry experiments. Electrode surfaces on which silver has been electroplated, and from which all silver has been removed (following anodic stripping for more than an hour), exhibit a nucleation overpotential equal to, or greater than, that of a freshly cleaved graphite electrode surface. Consequently, it is reasonable to attribute the “facilitated renucleation” of silver in these experiments to the presence of metastable silver clusters on the electrode surfaces. It is unnecessary to postulate that the intercalation of silver into the graphite electrode surface is responsible for this behavior.
The underlying reason for the slow oxidation kinetics exhibited by subnanometer-sized silver clusters will be the subject of further study. One aspect deserving of closer scrutiny is the possibility that silver particles undergo a metal to nonmetal transition during dissolution, and that interfacial electron transfer to the resulting nonmetallic silver clusters is slow for a variety of reasons. A second possibility involves the existence on these clusters of a strongly coordinated anion layer that impedes their dissolution. Acknowledgment. This work was supported by grants from the National Science Foundation (DMR9876479) and the Petroleum Research Fund of the American Chemical Society (33751-AC5). R.M.P. is grateful for funding from the A.P. Sloan Foundation and the Henry and Camille Dreyfus Foundation. Dr. Art Moore of Advanced Ceramics is acknowledged for generously donating much of the graphite used in this work. LA9914716
