on the Morphology of Platinum Electrodeposited on Highly Oriented

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Effect of Pb(II) on the Morphology of Platinum Electrodeposited on Highly Oriented Pyrolytic Graphite Adam C. Hill, Ryan E. Patterson, James P. Sefton, and Michael R. Columbia* Chemistry Department, Indiana UniversitysPurdue University Ft. Wayne, Ft. Wayne, Indiana 46805-1499 Received September 21, 1998. In Final Form: March 1, 1999 Platinum films electrodeposited from 10 mM H2PtCl6 have been probed using scanning electron microscopy (SEM) and ex situ scanning tunneling microscopy (STM) to examine morphological changes produced when Pb(II) is added to the deposition solution. Deposition was carried out potentiostatically at -250 and -1000 mV (vs Ag/AgCl) from solutions containing three different Pb(II) concentrations: 0, 0.05, and 0.4 mM. The presence of Pb(II) produced an extremely rough, spiky morphology on the nanometer scale and suppressed dendritic aggregation on the micrometer scale. These observations are interpreted in terms of Pb(II) counteracting enhanced surface diffusion of Pt atoms resulting from interaction between them and chloride ions at the solution/electrode interface.

Introduction In the past 10 years several papers have appeared in the literature dealing with the electrodeposition of platinum. Some of these1-5 have been part of a larger body of work, in which researchers have exploited the recent availability of scanning probe microscopy to study the morphology of electrodeposited metals on the nanometer scale. Others,6,7 motivated by the role dispersed platinum films play in biosensors and fuel cells, have used or augmented longer-established electrochemical techniques to characterize various aspects of the electrodeposition process. These articles signal a renewal of interest in a process first reported nearly a century ago and studied extensively from the mid-1930s until approximately 1970; Feltham and Spiro reviewed the main points in the field at the end of that period.8 As the focus for this overview, they summarized the various reported plating bath recipes and how their compositions affected the appearance and properties of the electrodeposited film. A dominant issue throughout this discussion is the role which additives in the plating solution, namely Pb(II) as its acetate or nitrate salt, play in producing high-surface-area films of platinum black; these had a far rougher morphology than films deposited from additive-free solutions. The formation of platinum black was observed for a wide range of Pt/Pb atomic ratios and current densities with an undefined interplay between the two. Also, several effects are attributed to Pb(II), such as changing the Pt lattice constant, suppressing hydrogen evolution, and enhancing * Corresponding author. (1) Itaya, K.; Sugawara, S.; Higaki, K. J. Phys. Chem. 1988, 92, 67146718. (2) Aindow, M.; Farr, J. P. G. Trans. Inst. Met. Finish. 1992, 70, 171-176. (3) Zubimendi, J. L.; Va´zquez, L.; Oco´n, P.; Vara, J. M.; Triaca, W. E.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. 1993, 97, 5095-5102. (4) Layson, A. R.; Columbia, M. R. Microchem. J. 1997, 56, 103-113. (5) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1998, 102, 1166-1175. (6) Garrido, P.; Go´mez, E.; Valle´s, E. J. Electroanal. Chem. 1998, 441, 147-151. (7) Kelaidopoulou, A.; Kokkinidis, G.; Michev, A. J. Electroanal. Chem. 1998, 444, 195-201. (8) Feltham, A. M.; Spiro, M. Chem. Rev. 1971, 71, 177-193 and references therein.

nucleation rates, but no consensus was reached regarding what role Pb(II) plays in roughening the film. More recent work has ignored the role of additives, instead choosing to deal with the electrodeposition from solutions free of them. In general, the electrodeposition of Pt proceeds by an irreversible two-step reduction of the hexachloroplatinate ions:8

PtCl62- + 2e- f PtCl42- + 2Cl-

(1)

PtCl42- + 2e- f Ptad + 4Cl-

(2)

The standard reduction potentials for these half-reactions are reported to fall between 560 and 450 mV (vs Ag/AgCl), but between 450 and 390 mV, only eq 1 is reported to proceed across the entire electrode, while eq 2 occurs only at defects on its surface.3 At more negative voltages, the half-reactions collapse into a single step (eq 3), with deposition at nondefect sites increasing as the voltage is made more negative.3,8

PtCl62- + 4e- f Ptad + 6Cl-

(3)

On the basis of modeling by Scharifker and Hills,9 Kelaidopoulou and co-workers posit that film growth proceeds by progressive nucleation at potentials anodic of hydrogen reduction; subsequently, the nuclei grow into hemispherical clusters.7 This is consistent with deposition requiring an overpotential, as well as microscopic imaging of films deposited over a range of potentials.1-7 Returning to the effects of Pb(II), results previously reported from this laboratory described the morphology of a Pt black film deposited at 100 mV.4 The following report is an extension of that research and compares the morphology of Pt films deposited at more negative voltages. Changes in morphology observed on both micrometric and nanometric scales, produced by varying deposition potential and Pb(II) concentration, are reported. An explanation for these observations based on surface diffusion of Pt atoms is offered. Experimental Section Platinum films were prepared using a three-electrode electrochemical cell described elsewhere.4 ZYH-grade highly oriented pyrolytic graphite (HOPG) obtained from Advanced Ceramics (9) Scharifker, B.; Hills, G. Electrochim. Acta 1983, 28, 879-889.

10.1021/la981291g CCC: $18.00 © 1999 American Chemical Society Published on Web 04/28/1999

4006 Langmuir, Vol. 15, No. 11, 1999 Corporation was used as the working electrode material. (HOPG is easily cleaved to reproducibly provide a flat, relatively inert substrate for deposition and imaging of the films.) The counter electrode was a polycrystalline Pt disk, and the reference was a Ag/AgCl electrode, both obtained from BioAnalytical Systems. (All reported voltages are referenced to the Ag/AgCl electrode.) Control of the applied potential and monitoring of the deposition current were performed with a BioAnalytical Systems 100 electrochemical analyzer. A 25 mM H2PtCl6 stock solution was prepared by dissolution of 99.95% purity Pt metal in aqua regia according to a method described by Hills and Ives10 and subsequent dilution with deionized water. To prepare the Pb(II)-free deposition solutions, aliquots of the H2PtCl6 stock solution were diluted to 10 mM (This will be referred to as solution A). For Pb(II)-containing deposition solutions, a 4 mM Pb(II) stock solution was prepared by dissolving Pb(C2H3O2)2 (Mallinckrodt, analytical reagent grade) in deionized water. Aliquots of this were mixed with the aliquots of the H2PtCl6 stock solution prior to dilution to give deposition solutions which were either 0.05 mM Pb(II)/10 mM H2PtCl6 (solution B) or 0.4 mM Pb(II)/10 mM H2PtCl6 (solution C). The Pt/Pb atomic ratios in these solutions are 200:1 and 25:1, respectively. (These ratios were chosen because they are near the extremes of the critical range (500:1 to 10:1) needed for the production of Pt black.8) No supporting electrolyte was added to these solutions in order to eliminate any influence which it might exert on the morphology of the Pt films. Following deposition, the HOPG working electrode was removed from the electrochemical cell and dried using a stream of pressurized N2 gas. Imaging of the Pt films was performed on the micrometer scale using an ISI Mini SEM, and that on the nanometer scale, using a Burleigh ISTM. The tips used to acquire the ST micrographs were cut from 0.5 mm diameter 90% Pt/10% Ir wire using wire cutters provided with the microscope’s sampling kit; only those tips which could achieve atomic resolution on a freshly cleaved HOPG surface were used to image the platinum deposits. These micrographs (256 samples × 256 samples) were acquired in constant height mode using scan delays of 1-2 ms per sample followed by filtering for high-frequency noise using Fourier transform software.

Hill et al.

Figure 1. Cyclic voltammograms of an HOPG electrode in contact with (A) 0.5 M H2SO4/10 mM H2PtCl6 and (B) 10 mM H2PtCl6. Scan rate is 10 mV/s.

Results and Discussion Electrochemical Measurements. Platinum films were deposited using a single-potential-step method described by Gunawardena et al.11 and utilized by Zubimendi et al.3 and Kelaidopoulou et al.7 in their studies of Pt electrodeposition. To determine the deposition potentials, the electrochemical behavior of PtCl62- was first characterized by cyclic voltammetry. Figure 1 shows voltammograms produced for solution A with 0.5 M H2SO4 added as a supporting electrolyte (A) and for solution A with no supporting electrolyte (B). Voltammogram 1A exhibits separate features on the cathodic sweep for the deposition of Pt and the adsorption of H adatoms prior to the bulk reduction of H+ to H2 gas. In voltammogram 1B, the lack of supporting electrolyte leaves the Pt deposition feature relatively unchanged, but the feature for H adsorption is shifted cathodically by over 200 mV. Additionally, the sharp current increase produced by bulk H+ reduction in 1A is far less abrupt in 1B. Voltammograms produced for solutions B and C (no supporting electrolyte) had the same general appearance as that of 1B, but with higher current levels after the onset of Pt deposition. To follow up the cyclic voltammetry results, a series of current versus time profiles (current transients) was obtained after stepping the potential from a rest value of +500 mV to voltages between +100 and -1500 mV. Figure (10) Hills, G. J.; Ives, D. J. G. Reference Electrodes; Academic Press: New York, 1961. (11) Gunawardena, G.; Hills, G.; Montenegro, I.; Scharifker, B. J. Electroanal. Chem. 1982, 138, 225-239.

Figure 2. Current versus time profiles after stepping the potential at the working electrode from +500 mV to the following voltages: (a) 100 mV; (b) 0 mV; (c) -100 mV; (d) -250 mV; (e) -300 mV.

2 shows the transients obtained for steps to voltages between +100 and -300 mV using solution A. The most anodic voltage in this series, +100 mV, produced a transient which starts at a current less than 50 µA and then continues to decrease. The transient produced at 0 mV started at a slightly higher current (52 µA) and then decreased until it reached a minimum after 45 s followed by a slow recovery. This recovery was more pronounced in the transients obtained at -100 and -250 mV, with shorter times needed to observe it (10 and 3 s, respectively); these transients also continued to exhibit small increases in initial current (76 and 97 µA, respectively). The transient obtained at -300 mV showed a marked increase

Platinum Electrodeposited on HOPG

in initial current (225 µA) relative to the transient obtained at -250 mV but exhibited no observable current recovery. For transients produced at voltages more cathodic than -300 mV, initial currents continued to increase up to approximately 600 µA, observed at -1000 mV. Recovery in current transients has been attributed to the establishment and growth of metal nuclei on an electrode surface;11 this phenomenon is easily observed using scanning electron micrography (vide infra). It is interesting to note that the onset of current recovery in the transients coincides with the applied voltage necessary to produce a visibly observable film on the HOPG electrode. Between 0 and -250 mV, visibly observable films are produced after a deposition time of 5 min. No film was ever observed visually following deposition for the same period of time at more anodic voltages, although ex situ inspection of the electrode by scanning electrode microscopy revealed the presence of platinum islands. (No detailed morphological study was performed on platinum deposited at these more positive potentials, but the islands appeared to have roughly spherical shapes and were unevenly dispersed across the surface.) Heavier films were produced at more negative voltages. These films were distinctly gray in color. When solutions which contained the Pb(II) additive were used (solutions B or C), transients obtained for the same voltages exhibit similar behavior to those in Figure 1. The primary difference occurred in the transients obtained for +100 mV. Current recovery was observed after 70-80 s for about half of the transients. For those runs which exhibited current recovery, films sufficiently thick to be seen by the naked eye were produced after 15 min; for those not exhibiting current recovery, deposits could only be observed using SEM. Micrography. On the basis of the results from the electrochemical measurements, Pt films were prepared at voltages positive of bulk H+ reduction to avoid any complications resulting from H2 gas generation. These were then probed using microscopic techniques to determine the effect of Pb(II) on the film’s morphology. Figure 3 shows two SE micrographs taken of Pt deposited from solution A at -250 mV for 5 min. Visually, films produced at this and more positive voltages were never uniformly distributed with large portions of the electrode surface remaining uncovered. The micrograph in Figure 3A clearly demonstrates that this phenomenon is also present on the micrometer scale. It displays a 45 µm × 45 µm area on the HOPG electrode supporting three dendritic platinum aggregates. Boundaries between these aggregates and the bare substrate are distinct. The micrograph in Figure 3B reveals a completely different type of morphology. In this 18 µm × 18 µm area several roughly circular aggregates, ranging in diameter from 0.3 to 1 µm, are spatially distributed in a random fashion. Several of these aggregates appear to be coalescing into larger agglomerates. This area, unlike that imaged in Figure 3A, was part of a wider swath of the surface which exhibited the same random arrangement of circular aggregates. When deposition was carried out from solutions containing Pb(II), only the morphology depicted in Figure 3B was observed. A commonly invoked picture describing overpotential deposition of a metal envisions the process commencing with instantaneous or progressive nucleation of the substrate by reduced metal adatoms. This is followed by diffusion of more metal ions from the solution to stable nuclei on the surface. Continued deposition under limited diffusion results in the two- or three-dimensional growth of the nucleus and eventual coalescence with other nuclei

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Figure 3. Scanning electron micrographs of Pt deposited from 10 mM H2PtCl6 for 5 min at -250 mV: (A) dendritic aggregates magnified ×1700; (B) hemispherical aggregates magnified ×4200.

to form a continuous film covering the entire substrate (ref 9 and references therein). The morphology observed in Figure 3B is consistent with this picture; the isolated structures observed in Figure 3A are not. To explain such a morphology, diffusion of metal adatoms across the surface is considered to play an important role in its formation. (Indeed, growth models such as diffusionlimited aggregation (DLA) have simulated deposit morphologies almost identical to those in Figure 3A12.) Diffusion of both individual atoms and clusters of atoms of noble metals on HOPG has been reported following deposition in a vacuum. The extent of this surface diffusion displayed a complex dependence on temperature, surface step height, and cluster size.13,14 Recently, Martı´n et al. used diffusion of electrodeposited Au adatoms on HOPG (12) Baraba´si, A.-L.; Stanley, H. E. Fractal Concepts in Surface Growth; Cambridge University Press: Cambridge, U.K., 1995. (13) Francis, G. M.; Goldby, I. M.; Kuipers, L.; von Issendorff, B.; Palmer, R. E. J. Chem. Soc., Dalton Trans. 1996, 665-671. (14) Francis, G. M.; Kuipers, L.; Cleaver, J. R. A.; Palmer, R. E. J. Appl. Phys. 1996, 79, 2942-2947.

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Figure 5. STM cross section profile of a short-range spiky morphology.

Figure 4. Scanning tunneling micrographs of Pt (50 nm × 50 nm) deposited for 5 min at -250 mV from (A) 10 mM H2PtCl6, (B) 10 mM H2PtCl6/0.05 mM Pb(II), and (C) 10 mM H2PtCl6/0.4 mM Pb(II).

to explain a potential dependent shift in the morphology of aggregates from quasi-hemispherical to a dendriticlike pattern.15 It would appear that surface diffusion and its dependence on surface conditions is a good candidate for explaining the difference in micrometer scale morphologies observed in these experiments, but a detailed determination of this phenomenon is beyond the scope of this paper. The major point gleaned from this comparison is the suppression of dendritic structures when Pb(II) is present in the deposition solution with a possible link to decreased surface diffusion of Pt atoms. This idea will be developed further after discussion of the morphology observed on the nanometer scale. Figure 4 displays the nanometer-scale morphology of the films deposited at -250 mV. Figure 4A shows a 50 nm × 50 nm area of a Pt film deposited from solution A. The morphology is almost cloudlike in appearance with lobes having dimensions of several nanometers. The addition of 0.05 mM Pb(II) to the deposition solution does little to change this morphology (Figure 4B); however, when the Pb(II) concentration is increased to 0.4 mM, spiky features dominate the surface (Figure 4C). These spikes are 1-2 nm thick at the base and 10-20 nm high, as illustrated (15) Martı´n, H.; Carro, P.; Herna´ndez Creus, A.; Gonza´les, S.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1997, 13, 100-110.

Figure 6. Scanning tunneling micrographs of Pt (50 nm × 50 nm) deposited for 5 min at -1000 mV from (A) 10 mM H2PtCl6, (B) 10 mM H2PtCl6/0.05 mM Pb(II), and (C) 10 mM H2PtCl6/0.4 mM Pb(II).

by a cross-sectional profile in Figure 5. Similar isolated features were reported by Itaya et al. on the surface of a Pt hemisphere.1 To probe the influence of H+ reduction, films were also deposited from the same three solutions at -1000 mV. Figure 6 displays 50 nm × 50 nm areas of these films. The film deposited from solution A differs little from the one deposited from the same solution at -250 mV (Figure

Platinum Electrodeposited on HOPG

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Figure 7. STM cross section profile of long-range morphology of Pt deposited at -1000 mV from (a) 10 mM H2PtCl6 and (b) 10 mM H2PtCl6/0.4 mM Pb(II). Table 1. Average Zrms Values avg Zrms (nm) deposition potential (mV)

[Pb(II)] ) 0.0 mM

[Pb(II)] ) 0.05 mM

[Pb(II)] ) 0.4 mM

-250 -1000

1 1

1 8

5 10

4A), but those containing Pb(II) differ significantly. Films produced from both solution B (Figure 6B) and solution C (Figure 6C) are considerably rougher on two scales. First, both films exhibit the same spiky morphology observed in the film deposited at -250 mV from solution C. In addition, these spikes are superimposed on an underlying surface characterized by greater height variations than any observed on the films deposited at -250 mV. This difference is easily discerned by comparing crosssectional profiles as illustrated in Figure 7. For films deposited at -250 mV, the boundaries between lobes produced maximum height variations of 10-20 nm; this was also true for the film deposited from 10 mM H2PtCl6 at -1000 mV. For films deposited at -1000 mV from solutions containing Pb(II), height variations of 50-100 nm are observed. To better quantify the difference in shorter-range roughness for the different films, the root-mean-square of the height variation (Zrms) was measured for 10-12 arbitrary 10 nm2 regions on each. The position of these regions was chosen to avoid straddling boundaries between the lobes which comprise the longer-range morphology. The results for this analysis are listed in Table 1. They corroborate the qualitative trends mentioned previously, namely morphological roughness increases as Pb(II) concentration increases and as deposition potential is made more negative. The morphological differences on the nanometer scale can also be discussed in terms of surface diffusion; however, the surface now under consideration is not HOPG but that of the growing Pt aggregates. Initial growth of Pt nuclei should be primarily parallel to the substrate, but as each nucleus grows to become an aggregate of several atoms, its expanding area will become an ever larger target for the deposition of new Pt atoms. The behavior of these new Pt atoms determines the morphology of the aggregate as it begins to grow perpendicular to the substrate.12 If these new atoms cannot diffuse across the aggregate, random deposition results in an extremely disordered surface and produces a rough morphology; however, if the atoms can diffuse rapidly relative to the deposition rate, each will migrate to lower energy sites.

This surface relaxation acts to minimize surface area and produces a comparably smooth morphology. This appears to be the situation for the Pt films deposited from solution A at both -250 and -1000 mV, as evidenced by the relatively smooth morphology and smaller Zrms values. The preceding result is surprising on the basis of the expected rates for deposition and surface diffusion. The diffusion of Pt atoms on Pt electrodes was studied by Alonso et al. by measuring the surface relaxation of the electrode surface roughened by oxidation.16 They reported a diffusivity (D) of 1.5 × 10-16 cm2 s-1 for relaxation under opencircuit conditions in 0.5 M H2SO4 at 323 K. Using the Einstein-Smoluchowski equation, 〈∆x2〉 ) 2Dt, the meansquare displacement equals 1.7 × 10-8 cm/s or about 1 lattice site every 2 s. The smaller deposition current, at -250 mV, is approximately 100 µA; assuming 100% Coulombic efficiency, this translates into a deposition rate of 5 × 1014 Pt atoms cm-2 s-1 or approximately one monolayer every 2 or 3 s. Under these conditions, surface diffusion would have little consequence and growth perpendicular to the surface should be random. Considering this, there should be far less difference in roughness on the nanometer scale when comparing films deposited from solution A with those deposited from solutions B and C. The question must now be asked: What factor might be responsible for enhancing surface diffusion of Pt atoms deposited from solution A? An answer can be found in work reported by several researchers studying the diffusion of Au atoms on Au electrodes still immersed in solution.17-19 They observed a substantial increase in the mobility of gold atoms in the presence of low concentrations (10-5 to 10-4 M) of chloride ions. The reduction of PtCl62releases six chloride ions for every Pt atom deposited; the accumulation of chloride near the electrode surface would very quickly exceed 10-4 M. Given that Pt shows a similar strong attraction to chloride ions, it is reasonable to conclude that the diffusivity of Pt atoms in this circumstance is much greater than that reported by Alonso et al., perhaps by several orders of magnitude. With surface diffusion enhanced, perpendicular growth progresses in a more ordered fashion and the morphology would be smoother than expected. The enhancement is sufficient to prevent an increase in roughness when the deposition potential is more cathodic and the deposition current increases, since no substantial difference in the morphology or Zrms values is seen between deposition from solution A at both -250 and -1000 mV. Additionally, it seems that production of H2 gas at -1000 mV does not impede this diffusion, given no discernible difference in the morphologies of these two films. Returning to the films deposited from solutions B and C, it is rational to speculate that the enhancement in surface diffusion is quenched by the addition of Pb(II) to the deposition solution. This is consistent with the rougher, spiky morphologies observed on the nanometer scale and the suppression of dendritic structures observed at the micrometer scale when Pb(II) is present. The extent of this quenching seems to depend on the Pb(II) concentration at slower deposition rates. This is evidenced by the lack of a spiky morphology for films deposited from solution B (16) Alonso, C.; Salvarezza, R. C.; Vara, J. M.; Arvia, A. J.; Vazquez, L.; Bartolome, A.; Baro, A. M. J. Electrochem. Soc. 1990, 137, 21612166. (17) Wiechers, J.; Twomey, T.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1988, 248, 451-460. (18) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929-932. (19) Honbo, H.; Sugawara, S.; Itaya, K. Anal. Chem. 1990, 62, 24242429.

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at -250 mV; at this slower deposition rate, diffusional quenching induced by 0.05 mM Pb(II) is not sufficient to increase the randomness in perpendicular growth substantially. For 0.4 mM Pb(II), diffusional quenching increases to the point that a spiky morphology is produced. The faster deposition rate at -1000 mV produces a spiky morphology for both concentrations. These films have larger Zrms values than the film deposited from solution C at -250 mV, as well as increased height changes associated with lobe boundaries. Both observations are probably attributable to the fact that these films are thicker than those deposited at -250 mV; however, H2 gas evolution might also play a role. Conclusions Morphological evidence has been presented from scanning tunneling and scanning electron micrographs that Pt films electrodeposited from solutions containing Pb(II) as an additive differ considerably from those produced in its absence. These differences are observed on both the micrometer and nanometer scales. In the absence of Pb(II), SEM reveals both hemispherical and dendritic aggregates are produced at slower deposition rates (-250 mV). The latter structures are inconsistent with a nucleation-and-growth mechanism of film formation but can be attributed to surface diffusion of Pt atoms. They are never present in films deposited from Pb(II)-containing solutions. STM shows a nanographic morphology for the Pb(II)-free films, which is relatively smooth with the largest height variations existing between lobes with diameters of several nanometers. The presence of Pb(II) produces a rougher morphology which is dominated by spikes 1-2 nm at the base and 10-20 nm high superimposed on the longer-range height variations attributed to the lobes. This increased roughness depends on the interplay between deposition rate and Pb(II) concentration. The disappearance of dendritic structures on the micrometer scale suggests Pb(II) reduces or arrests surface diffusion of Pt atoms. When applied to the nanometer scale, the loss of surface diffusion would enhance randomness in aggregate growth perpendicular to the substrate and produce a relatively rough morphology. This scenario relies on the assumption that surface diffusion rates are fast enough to accommodate the deposition rate. Calculation of the diffusion rate at room temperature on the basis of data reported by Alonso et al. shows that it would be inadequate to handle the slower deposition rate at -250 mV. It must be concluded that the surface diffusion rate of Pt atoms deposited from PtCl62- is much greater than that reported for relaxation following surface oxida-

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tion in 0.5 M H2SO4. This discrepancy most probably arises due to interaction between Pt atoms and liberated chloride ions; this would be similar to the interaction responsible for the enhanced mobility of gold atoms observed when low chloride concentrations are added to solutions contacting a gold film. Unfortunately, this effect, while documented, has never been well characterized and no data exist which overtly demonstrate that an additive, such as Pb(II), might interfere with it. The invocation of surface diffusion to interpret the Pb(II)-induced changes in the morphology of electrodeposited platinum is unique but must remain speculative. Ex situ analysis of Pb electrodeposited on Pt from HClO4 solutions reported by Borup et al. revealed small quantities of Clcontaining lead compounds present in the deposit;20 the presence of similar compounds during Pt electrodeposition might disrupt interaction between Pt atoms and chloride ions, which is responsible for enhanced surface diffusion of the former. On the other hand, Martı´n et al. demonstrated that surface diffusion of electrodeposited Au atoms increased as deposition potential was made more negative;15 if this were also the case for Pt electrodeposition, it is less clear how Pb might influence this phenomenon. Another source of uncertainty is the recent discovery of electroless Pt deposition reported by Zoval et al.5 The effect which this type of deposition has on nanometric morphology and the extent to which it is influenced by the presence of Pb have yet to be determined. To address these uncertainties, experiments are planned, based on the method described by Alonso et al.16 Initial work will attempt to reproduce their results and then proceed to measure the effect of chloride concentration on the diffusion rate and, finally, determine whether Pb(II) counteracts this effect. Additionally, it is the authors’ intention to couple this work with an in situ investigation of the deposition using scanning probe microscopy and a quartz crystal microbalance with the hope of developing a model which accommodates observed deposition and diffusion rates and fractal dimensions. Acknowledgment. We wish to acknowledge Patricia Thiel, Jim Evans, and Dennis Johnson, for constructive discussion on various aspects of this manuscript, and Scott Argast, for his generous help in acquiring the scanning electron micrographs. This research was supported by an award from Research Corporation. We also gratefully acknowledge the support of the Indiana UniversityPurdue University Ft. Wayne Chemistry Department. LA981291G (20) Borup, R. L.; Sauer, D. E.; Stuve, E. M. Surf. Sci. 1993, 293, 27-34.