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The Morphology of Platinum Black Electrodeposited on Highly Oriented Pyrolytic Graphite Studied with Scanning Electron Microscopy and Scanning Tunneling Microscopy
MICROCHEMICAL JOURNAL ARTICLE NO.
56, 103–113 (1997)
MJ961440
The Morphology of Platinum Black Electrodeposited on Highly Oriented Pyrolytic Graphite Studied with Scanning Electron Microscopy and Scanning Tunneling Microscopy A. R. Layson and M. R. Columbia Chemistry Department, Indiana University—Purdue University Ft. Wayne, Ft. Wayne, Indiana 46805-1499 Scanning electron and scanning tunneling microscopies have been used to study the morphology of Pt films electrodeposited on highly oriented pyrolytic graphite (HOPG); deposition was from solutions containing K2PtCl6 and Pb(C2H3O2)2 in order to produce films of Pt black. The resulting films consisted of mixed regions of Pt black and Pt gray surrounding bare patches of HOPG. Microscopic analyses reveal that the gray areas contain Pt islands which form patterns consistent with deposition preferentially at defects on the electrode surface. These islands grow as large as 1 mm in diameter before coalescing. The black areas exhibit a greater density of Pt with deposition occurring both at defects and on the flat areas between them. Pt islands in the black areas grow to only ca. 0.2 mm before coalescing. The aggregates formed in the black region have a rougher morphology due to the smaller size which islands reach before they coalesce. The presence of both gray and black areas is attributed to two factors: the role of adsorbed Pb2/ ions and the deposition rate gradients created by pockets of N2 gas trapped on the HOPG surface. q 1997 Academic Press
INTRODUCTION
High surface area films of Pt black have been produced by electrodeposition for over a century (1). Feltham and Spiro reviewed this topic, discussing the parameters which control the appearance and characteristics of Pt films deposited on Pt electrodes (2). One parameter identified as key to the formation of the Pt black is the presence of additives; Pb, in particular, is effective. (Without additives, Pt deposits have lower surface areas and appear gray.) One recommendation they proposed for future research is probing the structure of the Pt black deposits to better discern the role Pb plays in their formation. The development of scanning tunneling microscopy (STM) by Binnig and Rohrer has provided a powerful tool for the study of surface morphology down to the atomic level (3). Coupled with scanning electron microscopy (SEM), a wide range of magnifications (from 110 to 1108) is available. Indeed, STM has been widely used to study model electrodeposition systems using highly oriented pyrolytic graphite (HOPG) as a substrate. Fresh HOPG surfaces with low defect densities and large, flat regions can easily be prepared by cleavage with adhesive tape; this produces ideal ‘‘palettes’’ for observing the early stages of metal electrodeposition (4). Figure 1 is a typical STM image of a freshly cleaved HOPG surface showing wide terraces separated by a step formed by the termination of one of the terraces. The regularity of this surface allows easier microscopic identification of Pt deposits than films deposited on Pt electrodes. Some STM studies of Pt electrodeposition on HOPG have already been performed (5–7). Itaya et al. claim deposition of Pt on HOPG produces a random distribution 103 0026-265X/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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˚ 1 7500 A ˚ area of freshly cleaved highly oriented FIG. 1. Scanning tunneling micrograph of a 7500 A pyrolytic graphite.
of Pt islands (6) while Arvia and coworkers report preferential deposition at surface defects (7); such defect-driven deposition has also been reported for Pb (8) and Ag (9) on HOPG. Neither group pursued the deposition of Pt in the presence of lead to produce Pt black. In this paper, we present our results from the study of Pt electrodeposition in the presence of lead on HOPG. Both STM and SEM have been exploited to probe the morphology of these deposits. MATERIALS AND METHODS
Voltammetric measurements and electrodeposition were performed using a BAS100 electrochemical analyzer. The electrochemical cell, illustrated in Fig. 2, was fabricated from a Nalgene container and a modified Ace-Thred #15 connector; its configuration allowed a 9 mm 1 9 mm piece of highly oriented pyrolytic graphite (Advanced Ceramics Corporation, ZYH grade) to be held against an opening in the container, allowing contact with the electrodeposition solution within. A leak-tight seal was made by placing a 0.25-in. ID O-ring between the opening and the HOPG
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FIG. 2. Illustration of the electrochemical cell used to deposit Pt on graphite electrode.
electrode. The exposed area of the HOPG electrode was then 0.32 cm2. An Ag/AgCl electrode was used as the reference and Pt wire was used as the counter electrode. Electrodeposition solutions were prepared fresh daily using nanopurified water with the following reagents: K2PtCl6 , Spectrum Chemical (40% min. Pt assay); HNO3 , Fisher Chemical (ACS Certification grade); Pb(C2H3O2)2 , Mallinckrodt (Analytical reagent grade). For each deposit, a fresh HOPG surface was created by cleavage with tape and the electrodeposition solution was deaerated by bubbling N2 gas through it for 5 to 10 min prior to deposition. Scanning electron microscopy was performed using an ISI SuperMini microscope and scanning tunneling microscopy was performed using a Burleigh ISTM. RESULTS AND DISCUSSION
Figure 3 is a typical cyclic voltammogram produced by a freshly cleaved HOPG electrode surface immersed in a quiescent solution which contains 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 . The supporting electrolyte was 1 M HNO3 and the sweep rate was 50 mV/s. During the cathodic sweep, deposition of Pt by the reaction 0 0 PtCl20 6 / 4e r Pt(ad) / 6Cl
(1)
commences between 500 and 400 mV and reaches a diffusion limit by ca. 300 mV. The slight increase in current beyond 0 mV is most probably due to the adsorption of hydrogen on the deposited Pt followed by its bulk reduction beyond 0200 mV:
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H/ / e0 r H(ad)
(2)
2H/ / 2e0 r H2,(g) .
(3)
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FIG. 3. Cyclic voltammogram of a HOPG electrode immersed in a solution which has 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 at a scan rate of 50 mV/s. The supporting electrolyte was 1 M HNO3 and a Pt wire was used as the counter electrode.
The anodic sweep desorbs the residual hydrogen with the current level returning to zero beyond 100 mV. No current is produced from production or stripping of an oxide layer since the potential sweep was not taken to sufficiently anodic values to oxidize the Pt film. The voltammogram is similar to those produced by solutions containing no Pb2/; however, those voltammograms usually exhibited lower current levels with onset of Pt deposition at more cathodic potentials. This is in agreement with previously reported results (2). To prepare thin films of Pt black for microscopic analysis, 10 to 20 mC of Pt was deposited at /100 mV (vs Ag/AgCl). This potential was chosen to prevent any reduction of hydrogen from contributing to the deposition charge. At this level of deposition charge, sufficient Pt is deposited to produce thin films 33 to 66 layers thick, if each layer is uniform and densely packed (1.5 1 1015 Pt atoms/cm2). Visual inspection revealed films that had little uniformity. The films produced consisted of mixed regions of Pt black and Pt gray encircling bare patches where no Pt was present. Figure 4 is a scanning electron (SE) micrograph of such a film at low magnification (130). The Pt film appears lighter in contrast to the graphite which appears darker. The large dark region on the left side is the portion of HOPG surface outside the Oring, while the smaller dark regions are the bare patches. Within the film, the regions of Pt gray are all located adjacent to these patches, effectively separating them from the Pt black regions. Pockets of N2 gas trapped on the surface are the most plausible source of these patches. Occasionally, these gas pockets have been large enough to be visible to the naked eye; they seem to be produced during deaeration when N2 gas
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FIG. 4. Scanning electron micrograph of Pt deposited on an HOPG electrode immersed in a solution which has 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 at / 100 mV vs Ag/AgCl. The deposition charge was 15 mC. Magnification is 130.
migrates down the narrow portion of the deposition cell (see Fig. 2). The presence of such pockets would prevent contact between the deposition solution and underlying portions of the electrode surface. It could also explain the Pt gray regions as well. Figure 5 shows two SE micrographs of a gray region at higher magnifications. Figure 5A is at a magnification of 1000. It reveals that many of the Pt deposits form line-like structures. Interspersed between them are more random Pt islands which have no long-range order comparable to the line structures. Figure 5B shows these islands at a higher magnification (120,000). There seems to be a mixture of individual islands and larger aggregates formed from coalescence of some of the islands. The diameters of the individual islands measure between 0.5 and 1 mm with the largest aggregates formed by the merger of as many as 10 of the islands. The islands and aggregates to the far left side appear to be part of one of the line structures, signifying that these structures are comprised of a linear arrangement of islands. A possible explanation is, in these regions, Pt deposits primarily at steps on the HOPG surface created during cleavage. This would be consistent with deposition which originates from nucleation and growth of Pt islands along the ‘‘line-like’’ steps illustrated in Fig. 1. Figure 6 shows two SE micrographs of a black region at higher magnifications. Figure 6A is at a magnification of 5000; it shows a region much more densely packed with Pt than the gray area. There are still some observable line structures, but they are much harder to discern due to a higher density of random Pt islands between them. In Fig. 6B, higher magnification (120000) of these islands shows them to have a greater variety of shapes and sizes than those in the gray area; indeed, the smallest individual islands have a diameter of ca. 0.2 mm, only 20% to 50% of the diameters of the individual islands in Fig. 5B. As in the gray region, these islands appear to
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FIG. 5. Scanning electron micrographs at (A) 11000 and (B) 120000 of a Pt gray region in the deposit depicted in Fig. 2.
coalesce to form larger aggregates. The greater density of random Pt islands coupled with the smaller size of the individual islands indicates that significant nucleation occurs on the flat portions of the HOPG surface existing between the step defects. ˚ 1 10,000 A ˚ square Figure 7 is a scanning tunneling (ST) micrograph of a 10,000 A within a gray region; Fig. 8 is a similar micrograph of a black region. The maximum ˚ . This contrast in height is confined height variation within the gray region is 7000 A to one area just right of center while the rest of the Pt deposit is more even. The maximum height variation for the black region is slightly less than the gray region ˚ ), yet the this deposit is rougher with several sharp, localized variations in (6760 A height. These locales of height variation could be viewed as ‘‘valleys’’ formed between
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FIG. 6. Scanning electron micrographs at (A) 15000 and (B) 120000 of a Pt black region in the deposit depicted in Fig. 2.
two Pt islands as they grow together. The existence of only one such valley in the gray region micrograph reflects the larger diameters of the islands identified in Figure 5B. The rougher surface in the black region results from coalescence of smaller islands. Based on the SE and ST micrographs, the primary difference between the black and gray areas seems to be where the Pt initially deposits on the HOPG surface. Within the gray areas, Pt deposits at defects on the HOPG surface forming small islands. Further deposition takes place preferentially at these islands rather than on the flatter regions. This permits the islands to reach diameters of up to 1 mm before they encounter each other and coalesce. Within the black regions, Pt deposition occurs at both defects and on the flatter areas. With no restrictions on where the initial
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˚ 1 10,000 A ˚ area of Pt gray. FIG. 7. Scanning tunneling micrograph of a 10,000 A
deposition occurs, far more islands form. Each island grows to a smaller diameter (0.2 mm) than those in the gray regions before they run into each other. The coalescence of these smaller islands produces rougher Pt surfaces than those in the gray areas. Two factors contribute to the way these Pt films look. The first is the presence of Pb in the deposition solution. We have never been able to produce Pt black without Pb being present; in some way it promotes the deposition of Pt on the flat areas of the HOPG surface. Although the deposition potential is too anodic for the reduction of Pb2/ to deposit metallic Pb on HOPG (8), Pb2/ ions adsorbed on the surface in the double-layer might play a role. Bernard proposed that the role of adsorbed Pb is as an inhibitor to the growth of Pt islands (2, 10); however, this would not explain the large amount of deposition on flat areas of the HOPG when comparing the black and gray deposits are compared. A more probable explanation is that the adsorbed Pb ions act as sites for electron transfer between the surface and the PtCl20 ions. 6 These Pb ions could be considered ‘‘pseudo-defects’’ acting as nucleation centers for Pt islands on the flat areas of the HOPG surface. (Recent work reported by Srinivasan and Gopalan on the electrodeposition of Cu on HOPG utilized in situ STM to identify Cu ions adsorbed at the HOPG surface prior to their reduction to Cu atoms; similar probing of the Pb/HOPG interface might also show similar adsorption (11).) The second effect involves variations in local deposition rates. (The rate of deposition is another key parameter identified in the review by Feltham and Spiro (2).) Although no method exists to measure localized deposition rates, variation in these rates is consistent with production of both black and gray areas. Since Pb2/ is not reduced at the applied potential, its concentration should be constant throughout the solution; in other words, all areas of the electrode in contact with the solution should
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˚ 1 10,000 A ˚ area of Pt black. FIG. 8. Scanning tunneling micrograph of a 10,000 A
produce only Pt black. The presence of gray areas indicates that deposition rates are not uniform across the electrode. As mentioned already, trapped gas pockets could explain the bare patches of HOPG observed in Fig. 4; they could also produce deposition rate gradients following the formation of a static diffusion layer. After the onset of PtCl20 reduction, depletion 6 near the electrode surface causes diffusion of more PtCl20 from the bulk solution. A 6 diffusion layer is formed which continues to increase in thickness until a static value is reached. Trapped pockets of gas would increase this thickness for regions of the electrode surface around their periphery. This is illustrated in Fig. 9. In the upper portion, an ion is shown as it enters the diffusion layer adjacent to a gas pocket. The ion must traverse a distance greater than the static thickness to reach the HOPG surface at the periphery of the gas pocket. Pt deposition in this peripheral region occurs at a slower rate than elsewhere on the surface. The lower portion of Fig. 9 correlates the different surface conditions following deposition with the position of the gas pocket during deposition. SUMMARY
Thin films of Pt have been produced by potentiostatic reduction of PtCl20 6 on HOPG electrodes; the deposition solution contained 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 and the reduction potential was /100 mV vs Ag/AgCl. Deposition of 10 to 20 mC of Pt produced nonuniform films which contained areas of Pt gray and black, as well as patches of uncovered HOPG. Scanning electron and scanning tunneling microscopic analysis shows the gray area consists of Pt islands formed at defect sites on the HOPG surface; these islands grow to diameters of 0.5 mm and 1 mm before beginning to
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FIG. 9. Illustration showing (A) the effect of a gas pocket on ion diffusion to the graphite electrode and (B) the resulting appearance of the electrode surface after deposition.
coalesce. The resulting aggregates are smooth when compared to the morphology of the black areas. Similar microscopic analysis of the black areas reveals a denser film with Pt islands present on the flat areas of the HOPG surface. These islands grow to only ca. 0.2 mm before they coalesce to form aggregates. A rougher morphology results from the higher density of valleys produced between the coalescing islands. There are two factors which explain the appearance of the films: the presence of Pb2/ in the deposition solution and pockets of nitrogen gas trapped on the electrode surface. The Pb2/ promotes the deposition of Pt on the flat HOPG terraces; one possible reason for this effect is the ability of Pb2/ ions in the double layer at the electrode surface to act as ‘‘pseudo-defects.’’ The trapped gas pockets block portions of the HOPG surface, preventing deposition of Pt on these areas; they also create decreases in the Pt deposition rate on adjacent portions of the surface, resulting in Pt gray forming between the bare patches and the black regions. Elimination of trapped gas pockets and the deposition rate gradients they produce will be sought in future
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experiments, so that the effect of deposition rate on Pt film morphology may be probed in a systematic fashion. ACKNOWLEDGMENTS This research was supported by an award from Research Corporation. We also gratefully acknowledge the support of the Indiana University—Purdue University Ft. Wayne Chemistry Department and the assistance of A. C. Hill in the acquisition of Fig. 1.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Lummer, O.; Kurlbaum, F. Sitzungsber. K. Preuss. Akad. Wiss., 1894, 229. Feltham, A. M.; Spiro, M. Chem. Rev., 1971, 71, 177. Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett., 1982, 49, 57. Park, S.-I.; Quate, C. F. Appl. Phys. Lett., 1986, 48, 112. Itaya, K.; Sugawara, S. Chem. Lett., 1987, 1927. Itaya, K.; Sugawara, S.; Higaki, K. J. Phys. Chem., 1988, 92, 6714. Zubimendi, J. L.; Vasquez, L.; Ocon, P.; Vara, J. M.; Triaca, W. E.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem., 1993, 97, 5095. Hendricks, S. A.; Kim, Y.-T.; Bard, A. J. J. Electrochem. Soc., 1992, 139, 2818. Vazquez, L.; Hernandez Creus, A.; Carra, P.; Ocon, P.; Herrasti, P.; Palacio, C.; Vara, J. M.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem., 1992, 96, 10,454. Bernard, C. Electrochim. Acta, 1970, 15, 271. Srinivasan, R.; Gopalan, P. Surf. Sci., 1995, 338, 31.
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MJ961440
The Morphology of Platinum Black Electrodeposited on Highly Oriented Pyrolytic Graphite Studied with Scanning Electron Microscopy and Scanning Tunneling Microscopy A. R. Layson and M. R. Columbia Chemistry Department, Indiana University—Purdue University Ft. Wayne, Ft. Wayne, Indiana 46805-1499 Scanning electron and scanning tunneling microscopies have been used to study the morphology of Pt films electrodeposited on highly oriented pyrolytic graphite (HOPG); deposition was from solutions containing K2PtCl6 and Pb(C2H3O2)2 in order to produce films of Pt black. The resulting films consisted of mixed regions of Pt black and Pt gray surrounding bare patches of HOPG. Microscopic analyses reveal that the gray areas contain Pt islands which form patterns consistent with deposition preferentially at defects on the electrode surface. These islands grow as large as 1 mm in diameter before coalescing. The black areas exhibit a greater density of Pt with deposition occurring both at defects and on the flat areas between them. Pt islands in the black areas grow to only ca. 0.2 mm before coalescing. The aggregates formed in the black region have a rougher morphology due to the smaller size which islands reach before they coalesce. The presence of both gray and black areas is attributed to two factors: the role of adsorbed Pb2/ ions and the deposition rate gradients created by pockets of N2 gas trapped on the HOPG surface. q 1997 Academic Press
INTRODUCTION
High surface area films of Pt black have been produced by electrodeposition for over a century (1). Feltham and Spiro reviewed this topic, discussing the parameters which control the appearance and characteristics of Pt films deposited on Pt electrodes (2). One parameter identified as key to the formation of the Pt black is the presence of additives; Pb, in particular, is effective. (Without additives, Pt deposits have lower surface areas and appear gray.) One recommendation they proposed for future research is probing the structure of the Pt black deposits to better discern the role Pb plays in their formation. The development of scanning tunneling microscopy (STM) by Binnig and Rohrer has provided a powerful tool for the study of surface morphology down to the atomic level (3). Coupled with scanning electron microscopy (SEM), a wide range of magnifications (from 110 to 1108) is available. Indeed, STM has been widely used to study model electrodeposition systems using highly oriented pyrolytic graphite (HOPG) as a substrate. Fresh HOPG surfaces with low defect densities and large, flat regions can easily be prepared by cleavage with adhesive tape; this produces ideal ‘‘palettes’’ for observing the early stages of metal electrodeposition (4). Figure 1 is a typical STM image of a freshly cleaved HOPG surface showing wide terraces separated by a step formed by the termination of one of the terraces. The regularity of this surface allows easier microscopic identification of Pt deposits than films deposited on Pt electrodes. Some STM studies of Pt electrodeposition on HOPG have already been performed (5–7). Itaya et al. claim deposition of Pt on HOPG produces a random distribution 103 0026-265X/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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˚ 1 7500 A ˚ area of freshly cleaved highly oriented FIG. 1. Scanning tunneling micrograph of a 7500 A pyrolytic graphite.
of Pt islands (6) while Arvia and coworkers report preferential deposition at surface defects (7); such defect-driven deposition has also been reported for Pb (8) and Ag (9) on HOPG. Neither group pursued the deposition of Pt in the presence of lead to produce Pt black. In this paper, we present our results from the study of Pt electrodeposition in the presence of lead on HOPG. Both STM and SEM have been exploited to probe the morphology of these deposits. MATERIALS AND METHODS
Voltammetric measurements and electrodeposition were performed using a BAS100 electrochemical analyzer. The electrochemical cell, illustrated in Fig. 2, was fabricated from a Nalgene container and a modified Ace-Thred #15 connector; its configuration allowed a 9 mm 1 9 mm piece of highly oriented pyrolytic graphite (Advanced Ceramics Corporation, ZYH grade) to be held against an opening in the container, allowing contact with the electrodeposition solution within. A leak-tight seal was made by placing a 0.25-in. ID O-ring between the opening and the HOPG
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FIG. 2. Illustration of the electrochemical cell used to deposit Pt on graphite electrode.
electrode. The exposed area of the HOPG electrode was then 0.32 cm2. An Ag/AgCl electrode was used as the reference and Pt wire was used as the counter electrode. Electrodeposition solutions were prepared fresh daily using nanopurified water with the following reagents: K2PtCl6 , Spectrum Chemical (40% min. Pt assay); HNO3 , Fisher Chemical (ACS Certification grade); Pb(C2H3O2)2 , Mallinckrodt (Analytical reagent grade). For each deposit, a fresh HOPG surface was created by cleavage with tape and the electrodeposition solution was deaerated by bubbling N2 gas through it for 5 to 10 min prior to deposition. Scanning electron microscopy was performed using an ISI SuperMini microscope and scanning tunneling microscopy was performed using a Burleigh ISTM. RESULTS AND DISCUSSION
Figure 3 is a typical cyclic voltammogram produced by a freshly cleaved HOPG electrode surface immersed in a quiescent solution which contains 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 . The supporting electrolyte was 1 M HNO3 and the sweep rate was 50 mV/s. During the cathodic sweep, deposition of Pt by the reaction 0 0 PtCl20 6 / 4e r Pt(ad) / 6Cl
(1)
commences between 500 and 400 mV and reaches a diffusion limit by ca. 300 mV. The slight increase in current beyond 0 mV is most probably due to the adsorption of hydrogen on the deposited Pt followed by its bulk reduction beyond 0200 mV:
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H/ / e0 r H(ad)
(2)
2H/ / 2e0 r H2,(g) .
(3)
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FIG. 3. Cyclic voltammogram of a HOPG electrode immersed in a solution which has 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 at a scan rate of 50 mV/s. The supporting electrolyte was 1 M HNO3 and a Pt wire was used as the counter electrode.
The anodic sweep desorbs the residual hydrogen with the current level returning to zero beyond 100 mV. No current is produced from production or stripping of an oxide layer since the potential sweep was not taken to sufficiently anodic values to oxidize the Pt film. The voltammogram is similar to those produced by solutions containing no Pb2/; however, those voltammograms usually exhibited lower current levels with onset of Pt deposition at more cathodic potentials. This is in agreement with previously reported results (2). To prepare thin films of Pt black for microscopic analysis, 10 to 20 mC of Pt was deposited at /100 mV (vs Ag/AgCl). This potential was chosen to prevent any reduction of hydrogen from contributing to the deposition charge. At this level of deposition charge, sufficient Pt is deposited to produce thin films 33 to 66 layers thick, if each layer is uniform and densely packed (1.5 1 1015 Pt atoms/cm2). Visual inspection revealed films that had little uniformity. The films produced consisted of mixed regions of Pt black and Pt gray encircling bare patches where no Pt was present. Figure 4 is a scanning electron (SE) micrograph of such a film at low magnification (130). The Pt film appears lighter in contrast to the graphite which appears darker. The large dark region on the left side is the portion of HOPG surface outside the Oring, while the smaller dark regions are the bare patches. Within the film, the regions of Pt gray are all located adjacent to these patches, effectively separating them from the Pt black regions. Pockets of N2 gas trapped on the surface are the most plausible source of these patches. Occasionally, these gas pockets have been large enough to be visible to the naked eye; they seem to be produced during deaeration when N2 gas
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FIG. 4. Scanning electron micrograph of Pt deposited on an HOPG electrode immersed in a solution which has 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 at / 100 mV vs Ag/AgCl. The deposition charge was 15 mC. Magnification is 130.
migrates down the narrow portion of the deposition cell (see Fig. 2). The presence of such pockets would prevent contact between the deposition solution and underlying portions of the electrode surface. It could also explain the Pt gray regions as well. Figure 5 shows two SE micrographs of a gray region at higher magnifications. Figure 5A is at a magnification of 1000. It reveals that many of the Pt deposits form line-like structures. Interspersed between them are more random Pt islands which have no long-range order comparable to the line structures. Figure 5B shows these islands at a higher magnification (120,000). There seems to be a mixture of individual islands and larger aggregates formed from coalescence of some of the islands. The diameters of the individual islands measure between 0.5 and 1 mm with the largest aggregates formed by the merger of as many as 10 of the islands. The islands and aggregates to the far left side appear to be part of one of the line structures, signifying that these structures are comprised of a linear arrangement of islands. A possible explanation is, in these regions, Pt deposits primarily at steps on the HOPG surface created during cleavage. This would be consistent with deposition which originates from nucleation and growth of Pt islands along the ‘‘line-like’’ steps illustrated in Fig. 1. Figure 6 shows two SE micrographs of a black region at higher magnifications. Figure 6A is at a magnification of 5000; it shows a region much more densely packed with Pt than the gray area. There are still some observable line structures, but they are much harder to discern due to a higher density of random Pt islands between them. In Fig. 6B, higher magnification (120000) of these islands shows them to have a greater variety of shapes and sizes than those in the gray area; indeed, the smallest individual islands have a diameter of ca. 0.2 mm, only 20% to 50% of the diameters of the individual islands in Fig. 5B. As in the gray region, these islands appear to
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FIG. 5. Scanning electron micrographs at (A) 11000 and (B) 120000 of a Pt gray region in the deposit depicted in Fig. 2.
coalesce to form larger aggregates. The greater density of random Pt islands coupled with the smaller size of the individual islands indicates that significant nucleation occurs on the flat portions of the HOPG surface existing between the step defects. ˚ 1 10,000 A ˚ square Figure 7 is a scanning tunneling (ST) micrograph of a 10,000 A within a gray region; Fig. 8 is a similar micrograph of a black region. The maximum ˚ . This contrast in height is confined height variation within the gray region is 7000 A to one area just right of center while the rest of the Pt deposit is more even. The maximum height variation for the black region is slightly less than the gray region ˚ ), yet the this deposit is rougher with several sharp, localized variations in (6760 A height. These locales of height variation could be viewed as ‘‘valleys’’ formed between
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FIG. 6. Scanning electron micrographs at (A) 15000 and (B) 120000 of a Pt black region in the deposit depicted in Fig. 2.
two Pt islands as they grow together. The existence of only one such valley in the gray region micrograph reflects the larger diameters of the islands identified in Figure 5B. The rougher surface in the black region results from coalescence of smaller islands. Based on the SE and ST micrographs, the primary difference between the black and gray areas seems to be where the Pt initially deposits on the HOPG surface. Within the gray areas, Pt deposits at defects on the HOPG surface forming small islands. Further deposition takes place preferentially at these islands rather than on the flatter regions. This permits the islands to reach diameters of up to 1 mm before they encounter each other and coalesce. Within the black regions, Pt deposition occurs at both defects and on the flatter areas. With no restrictions on where the initial
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˚ 1 10,000 A ˚ area of Pt gray. FIG. 7. Scanning tunneling micrograph of a 10,000 A
deposition occurs, far more islands form. Each island grows to a smaller diameter (0.2 mm) than those in the gray regions before they run into each other. The coalescence of these smaller islands produces rougher Pt surfaces than those in the gray areas. Two factors contribute to the way these Pt films look. The first is the presence of Pb in the deposition solution. We have never been able to produce Pt black without Pb being present; in some way it promotes the deposition of Pt on the flat areas of the HOPG surface. Although the deposition potential is too anodic for the reduction of Pb2/ to deposit metallic Pb on HOPG (8), Pb2/ ions adsorbed on the surface in the double-layer might play a role. Bernard proposed that the role of adsorbed Pb is as an inhibitor to the growth of Pt islands (2, 10); however, this would not explain the large amount of deposition on flat areas of the HOPG when comparing the black and gray deposits are compared. A more probable explanation is that the adsorbed Pb ions act as sites for electron transfer between the surface and the PtCl20 ions. 6 These Pb ions could be considered ‘‘pseudo-defects’’ acting as nucleation centers for Pt islands on the flat areas of the HOPG surface. (Recent work reported by Srinivasan and Gopalan on the electrodeposition of Cu on HOPG utilized in situ STM to identify Cu ions adsorbed at the HOPG surface prior to their reduction to Cu atoms; similar probing of the Pb/HOPG interface might also show similar adsorption (11).) The second effect involves variations in local deposition rates. (The rate of deposition is another key parameter identified in the review by Feltham and Spiro (2).) Although no method exists to measure localized deposition rates, variation in these rates is consistent with production of both black and gray areas. Since Pb2/ is not reduced at the applied potential, its concentration should be constant throughout the solution; in other words, all areas of the electrode in contact with the solution should
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˚ 1 10,000 A ˚ area of Pt black. FIG. 8. Scanning tunneling micrograph of a 10,000 A
produce only Pt black. The presence of gray areas indicates that deposition rates are not uniform across the electrode. As mentioned already, trapped gas pockets could explain the bare patches of HOPG observed in Fig. 4; they could also produce deposition rate gradients following the formation of a static diffusion layer. After the onset of PtCl20 reduction, depletion 6 near the electrode surface causes diffusion of more PtCl20 from the bulk solution. A 6 diffusion layer is formed which continues to increase in thickness until a static value is reached. Trapped pockets of gas would increase this thickness for regions of the electrode surface around their periphery. This is illustrated in Fig. 9. In the upper portion, an ion is shown as it enters the diffusion layer adjacent to a gas pocket. The ion must traverse a distance greater than the static thickness to reach the HOPG surface at the periphery of the gas pocket. Pt deposition in this peripheral region occurs at a slower rate than elsewhere on the surface. The lower portion of Fig. 9 correlates the different surface conditions following deposition with the position of the gas pocket during deposition. SUMMARY
Thin films of Pt have been produced by potentiostatic reduction of PtCl20 6 on HOPG electrodes; the deposition solution contained 10 mM K2PtCl6 and 0.2 mM Pb(C2H3O2)2 and the reduction potential was /100 mV vs Ag/AgCl. Deposition of 10 to 20 mC of Pt produced nonuniform films which contained areas of Pt gray and black, as well as patches of uncovered HOPG. Scanning electron and scanning tunneling microscopic analysis shows the gray area consists of Pt islands formed at defect sites on the HOPG surface; these islands grow to diameters of 0.5 mm and 1 mm before beginning to
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FIG. 9. Illustration showing (A) the effect of a gas pocket on ion diffusion to the graphite electrode and (B) the resulting appearance of the electrode surface after deposition.
coalesce. The resulting aggregates are smooth when compared to the morphology of the black areas. Similar microscopic analysis of the black areas reveals a denser film with Pt islands present on the flat areas of the HOPG surface. These islands grow to only ca. 0.2 mm before they coalesce to form aggregates. A rougher morphology results from the higher density of valleys produced between the coalescing islands. There are two factors which explain the appearance of the films: the presence of Pb2/ in the deposition solution and pockets of nitrogen gas trapped on the electrode surface. The Pb2/ promotes the deposition of Pt on the flat HOPG terraces; one possible reason for this effect is the ability of Pb2/ ions in the double layer at the electrode surface to act as ‘‘pseudo-defects.’’ The trapped gas pockets block portions of the HOPG surface, preventing deposition of Pt on these areas; they also create decreases in the Pt deposition rate on adjacent portions of the surface, resulting in Pt gray forming between the bare patches and the black regions. Elimination of trapped gas pockets and the deposition rate gradients they produce will be sought in future
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experiments, so that the effect of deposition rate on Pt film morphology may be probed in a systematic fashion. ACKNOWLEDGMENTS This research was supported by an award from Research Corporation. We also gratefully acknowledge the support of the Indiana University—Purdue University Ft. Wayne Chemistry Department and the assistance of A. C. Hill in the acquisition of Fig. 1.
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Lummer, O.; Kurlbaum, F. Sitzungsber. K. Preuss. Akad. Wiss., 1894, 229. Feltham, A. M.; Spiro, M. Chem. Rev., 1971, 71, 177. Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett., 1982, 49, 57. Park, S.-I.; Quate, C. F. Appl. Phys. Lett., 1986, 48, 112. Itaya, K.; Sugawara, S. Chem. Lett., 1987, 1927. Itaya, K.; Sugawara, S.; Higaki, K. J. Phys. Chem., 1988, 92, 6714. Zubimendi, J. L.; Vasquez, L.; Ocon, P.; Vara, J. M.; Triaca, W. E.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem., 1993, 97, 5095. Hendricks, S. A.; Kim, Y.-T.; Bard, A. J. J. Electrochem. Soc., 1992, 139, 2818. Vazquez, L.; Hernandez Creus, A.; Carra, P.; Ocon, P.; Herrasti, P.; Palacio, C.; Vara, J. M.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem., 1992, 96, 10,454. Bernard, C. Electrochim. Acta, 1970, 15, 271. Srinivasan, R.; Gopalan, P. Surf. Sci., 1995, 338, 31.
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