Electrochemical Deposition and Nanostructuring of As at Pt (111)

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Electrochemical Deposition and Nanostructuring of As at Pt(111) Xiaoyin Xiao† and Helmut Baltruschat* Institut fu¨ r Physikalische Chemie, Universita¨ t Bonn, D 53117 Bonn, Germany Received October 25, 2002. In Final Form: April 24, 2003 The deposition of As on a Pt(111) electrode has been studied by cyclic voltammetry and scanning tunneling microscopy (STM). The shape of the redox peaks of As is very much influenced by its surface coverage: At partial coverages, which can be achieved by spontaneous deposition at open circuit, a single sharp, reversible peak is observed at 0.57 V in addition to the hydrogen and sulfate adsorption peaks, which somewhat broadens when hydrogen adsorption is completely suppressed; further deposition of As leads to an additional pair of peaks at 0.38 or 0.45 V (cathodic or anodic sweep direction, respectively). Whereas a monolayer of As deposited at open circuit or above 0.45 V can be dissolved, again leaving behind a smooth Pt(111) surface, deposition below 0.38 V, and in particular at potentials where bulk As deposition occurs, leads to surface roughening. In cyclic voltammetry, the typical shape of hydrogen and sulfate adsorption peaks is not regained, and STM images reveal a rough surface after dissolution of As. We interpret this as being due to a surface alloying process which takes place when the As coverage exceeds a certain, critical value. Bulk As deposition leads to 3D deposits. Tip-induced nanostructuring of As at Pt(111) can be achieved at more positive potentials in an As3+-containing solution where no surface roughening takes place. The structures are formed due to local alloy formation when the tip is scanning at a very short distance above the Pt(111) electrode.

Introduction From the beginning of scanning tunneling microscopy (STM), it has been tried to use STM also for nanostructuring of the surface. Among such approaches at electrode surfaces are the introduction of surface defects, which then are used for local metal deposition.1-3 Local deposition was achieved according to the “jump to contact” mode for Cu and Pd on Au substrates4,5 and the method of Schindler,6,7 who uses the fast dissolution of Co previously deposited at the tip in order to achieve a locally high Co oversaturation, leading to a local deposition at the substrate. We have recently shown that a localized deposition of Cu on Pt single crystals is also possible by scanning with the tip very close to the surface in the potential region of Cu underpotential deposition (UPD), that is, in a potential range when normally only a Cu monolayer is present on the surface.8 The stability of the nanostructures thus produced suggests that they consist of a Cu/Pt alloy formed by the force interaction of the tip with the Cu UPD atoms. We have furthermore demonstrated for Cu and Pd on Au that the tip-substrate “distance” at which nanostructuring can be achieved is that for which the conductance between tip and surface is given by that of a quantum nanowire.9 † Permanent address: Chemistry Department, Xiangtan Normal University, Xiangtan, 41100, P.R. China.

(1) Li, W.; Virtanen, A.; Penner, R. M. J. Phys. Chem. 1992, 96, 65296532. (2) Li, W.; Hsiao, G. S.; Harris, D.; Nyffenegger, R. M.; Virtanen, J. A.; Penner, R. M. J. Phys. Chem. 1996, 100, 20103-20113. (3) Nyffenegger, R. M.; Penner, R. M. J. Phys. Chem. 1996, 100, 17041-17049. (4) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097-1099. (5) Engelmann, G. E.; Ziegler, J. C.; Kolb, D. M. J. Electrochem. Soc. 1998, 145, L33-L35. (6) Schindler, W.; Hofmann, D.; Kirschner, J. J. Appl. Phys. 2000, 87, 7007-7009. (7) Schindler, W.; Hofmann, D.; Kirschner, J. J. Electrochem. Soc. 2001, 148, C124-C130. (8) Berenz, P.; Xiao, X.; Baltruschat, H. J. Phys. Chem. B 2002, 106, 3673-3680. (9) Nielinger, M.; Baltruschat, H. Submitted.

Here we want to show that this kind of nanostructuring can be extended to the semimetal As. It turned out that the behavior of As on Pt(111) is complicated, and therefore a large part of this paper will deal with the spontaneous and electrochemical deposition of this As layer on Pt(111). As can be spontaneously deposited at Pt(111) at an open circuit from As3+-containing sulfuric acid solution. The maximum surface coverage is believed to be 1/3 ML (1 ML is 1 As/1 Pt).10-12 The cyclic voltammogram at complete suppression of hydrogen adsorption in sulfuric acid solution shows two reversible redox peaks at 0.53 and 0.56 V versus reversible hydrogen electrode (RHE),11 respectively, with a total charge of 240 µC cm-2, which are possibly due to the different As adsorption sites, that is, bridge and 3-fold hollow sites.12 It was found that only the positive redox peak pair is observable when the surface coverage of As is decreased. The cyclic voltammogram after a complete stripping of adsorbed As is similar to that of a bare Pt(111), indicating that Pt(111) stays stable when As is spontaneously adsorbed. The ex situ STM picture seems to reveal a (x3 × x3)R30° lattice,11 however, with the smallest atomic distance of 0.4 nm being smaller than the theoretical value of 0.48 nm. It was assumed in the above papers that the valence state of As adatoms changes from As3+ to As0 at the electrode potential, positive and negative of the redox peaks. On the other hand, the As adatom may also stay in its metallic (i.e., zerovalent) state, and thus the reversible redox peaks may be due to the adsorption of OH, which is similar to the irreversibly adsorbed Bi.13-15 (10) Feliu, J. M.; Fernandez-Vega, A.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1988, 256, 149-163. (11) Orts, J. M.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 1997, 434, 121-127. (12) Blais, S.; Jerkiewicz, G.; Herrero, E.; Feliu, J. M. Langmuir 2001, 17, 3030-3038. (13) Ball, M.; Lucas, C. A.; Markovic, N. M.; Murphy, B. M.; Steadman, P.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Langmuir 2001, 17, 5943-5946. (14) Schmidt, T. J.; Grgur, B. N.; Behm, R. J.; Markovic, N. M.; Ross, P. N. Phys. Chem. Chem. Phys. 2000, 2, 4379-4386.

10.1021/la026750z CCC: $25.00 © 2003 American Chemical Society Published on Web 07/25/2003

Electrochemical Deposition of As at Pt(111)

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The adsorption state of Bi adatoms was examined by X-ray photoelectron spectroscopy (XPS) and in situ surface X-ray scattering. Coadsorption of Bi and As was examined by cyclic voltammetry; it was concluded that both elements form separate domains.16 In the present paper, we will show that As forms surface alloys on Pt below a certain potential. From the literature, it is known that As forms alloys and intermetallic phases with Pt.17 Furthermore, we will show that at more positive potentials also a tip-induced alloy formation can be achieved. Experimental Section The Pt(111) single crystal was prepared by flame annealing followed by cooling in a hydrogen-containing argon atmosphere. Adsorption of As on Pt(111) was performed by applying the immersion technique as used before for obtaining a high surface coverage of tin;18 that is, the Pt(111) electrode was immersed in a 4 × 10-4 M As3+ + 0.5 M H2SO4 solution at open circuit and transferred to a pure sulfuric acid solution at a controlled potential with the protection of a drop of As-containing solution, followed by electrolyte exchange with the pure sulfuric acid solution.10,19 Due to additional deposition of As3+ from the droplet after transfer of the electrode, coverages thus achieved are somewhat higher than after usual spontaneous deposition when the electrode is rinsed before insertion into sulfuric acid.11,12 Electrochemical deposition of As on Pt(111) started at an initial potential, where only an irreversible adsorption of As took place. The setup of the electrochemical scanning tunneling microscopy can be found in our previous paper.8,20 Briefly, the STM imaging was performed with an STM head from Molecular Imaging and a Nanoscope E controller from Digital Instruments; the STM cell was maintained under an argon atmosphere; the Pt/Ir (90:10) tips were etched in KSCN + KOH solution and coated with anodic electrophoretic paint to reduce the area in contact with the electrolyte. All STM measurements were done at room temperature in constant current mode in the electrolyte. A Pt/H2 reference electrode (RHE) was used for obtaining cyclic voltammograms in the conventional cell; a Pt wire immersed in sulfuric acid was used as a reference electrode in the STM cell, which is connected to the bulk solution by a glass frit. All electrode potentials quoted herein are versus the RHE. As3+ solutions were prepared from As2O3, Millipore water, and concentrated sulfuric acid. The electrolyte was deaerated by bubbling argon.

Results As Adlayer in Pure Sulfuric Acid Solution. Figure 1A shows a cyclic voltammogram (CV) starting at an immersion potential of 0.71 V. It shows in the first sweep two reduction peaks with their corresponding oxidation peaks at 0.45 and 0.53 V, respectively. Their peak height is about the same. In the second sweep, the peak current at 0.53 V increases and the other at 0.45 V nearly disappears due to some dissolution of As at the positive potential limit. The peak at 0.45 V (in addition to that at 0.53 V) was also observed before but was not further discussed.10,19 A similar peak pair appears on Pt(111) with bismuth coverages above 0.33 ML.21 The cyclic voltammograms recorded during and after further stripping of (15) Schmidt, T. J.; Stamenkovic, V.; Attard, G. A.; Markovic, N. M.; Ross, P. N. Langmuir 2001, 17, 7613-7619. (16) Dollard, L.; Evans, R. W.; Attard, G. A. J. Electroanal. Chem. 1993, 345, 205-221. (17) Massalski, T. B. Binary Alloy Phase Diagrams; ASM International, The Materials Information Society: Materials Park, OH, 1992. (18) Xiao, X.; Tillmann, S.; Baltruschat, H. Phys. Chem. Chem. Phys. 2002, 4, 4044-4050. (19) Clavilier, J.; Feliu, J. M.; Fernandez, A.; Aldaz, A. J. Electroanal. Chem. 1990, 294, 193. (20) Xiao, X. Y.; Berenz, P.; Baltruschat, H.; Sun, S. J. Electroanal. Chem. 2001, 500, 446-452. (21) Smith, S. P. E.; Abruna, H. D. J. Phys. Chem. B 1998, 102, 3506-3511.

Figure 1. (A) The first two potential cycles and (B) the further potential cycles with increasing anodic potential limits during the stripping process of an As-covered Pt(111) surface in 0.05 M H2SO4. The Pt(111), still protected by the As3+ containing electrolyte, was immersed in pure 0.05 M H2SO4 at 0.71 V. (‚‚‚) Bare Pt(111) for comparison. Sweep rate: 50 mV/s.

As (cf. Figure 1B) indicate that the Pt(111) surface is still long range ordered. On one hand, the changes of the reversible redox peaks of As are very much similar to those reported on the spontaneously deposited As: for example, the voltammogram shows a sharp peak at 0.53 V and a shoulder at 0.57 V when the hydrogen adsorption is completely suppressed and only a single sharper peak at 0.57 V at small coverages; on the other hand, the “spike” for long range ordered sulfate adsorption is increased with the decrease of As coverage, and at a final state when the upper potential limit is increased to 1.6 V the oxygen adsorption gives rise to a sharp peak at 1.37 V, which is similar to that on bare Pt(111). Figure 2 shows another example when the initial potential was set at 0.45 V. The cyclic voltammograms show that the initial potential does not have an influence on the shape of the CV, as long as it is between 0.45 and 0.75 V. Still, the same redox peaks at 0.45 and 0.53 V appear in the first potential cycle. However, with the anodic potential limit staying below 0.8 V, one could observe a gradual decrease of the peak current at 0.45 V instead of its sudden disappearance in Figure 1A with an equal increase of the peak current at 0.53 V. Finally the peak at 0.45 V disappears and the peak at 0.53 V reaches a maximum value, at which stage the CV shows a sharp peak at 0.53 V and a shoulder at 0.57 V. The hydrogen adsorption is completely suppressed, and the total oxidation charge remains constant at around 240 µC cm-2 in

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Figure 2. Cyclic voltammogram of an As-covered Pt(111) surface in 0.05 M H2SO4 solution starting at the immersion potential of 0.45 V. Sweep rate: 50 mV/s. The arrows indicate the direction of the changes of the peaks with increasing cycle number.

these potential cycles. The charge of 240 µC cm-2 is essentially the same as that observed on spontaneously deposited As at Pt(111).11,12 An STM image recorded at this stage reveals that in addition to some small monatomic islands on the terraces, especially all the step edges are rough and decorated by 2D nuclei (cf. Figure 3). Interestingly, the islands at the terraces have a similar size as those at the step edges. But the surface between the clusters is still atomically smooth. The high-resolution STM image shows the disordered As adlayer with very small ordered domains (possibly a x3 × x3 adlattice, because the atomic distance is around 0.48 nm). The disorder of the As adlayer may be due to the mobility of As adatoms, similar to the Bi adatoms,13 and probably to the surface roughening which starts at this critical surface coverage. In the case of Sn on Pt(111), the high mobility of the Sn adatoms even prevents the observation of the adsorbate structure.18 Electrochemical Deposition of As. Figure 4 shows cyclic voltammograms on a Pt(111) surface in a 4 × 10-4 M As3+ + 0.5 M H2SO4 solution. The immersion potential of Pt(111) is set at 0.7 V; the polarization starts in the negative direction. Since this immersion potential is more negative than the open circuit potential at which As can be spontaneously adsorbed, the surface is already covered by As at this starting potential. The first deposition peak appears at 0.38 V with a corresponding charge of around 360 µC cm-2. Further deposition starts at 0.26 V and yields a second peak at 0.10 V. A large oxidation peak appears at 0.5 V. The charge in this peak increases with the decrease of the negative limit of the potential sweep (cf. Figure 4B), indicating that this peak corresponds to the stripping of the bulk deposited As (cf. the standard electrode potential of As3+/As in acidic solution is ∼0.24 V). In the second sweep, the first deposition peak is no longer visible, and the bulk deposition of As starts at a more positive potential. The deposition peak then stays stable at 0.16 V in the subsequent potential sweeps. The difference between the total deposition charge and the stripping charge in the first potential sweep is around 360 µC cm-2, exactly the same charge as measured in the first deposition peak. The stripping charge therefore exactly corresponds to the charge deposited in the second cathodic peak. Also, in the second sweep shown in Figure 4A, essentially the same amount of As is stripped. Figure 4C shows the cyclic voltammograms with a much more negative potential limit. The hydrogen evolution is

Figure 3. STM images of a Pt(111) surface fully covered by As in 0.05 M H2SO4. The corresponding cyclic voltammogram is the same as the last sweep in Figure 2. (A) Es ) 0.4 V, Et ) 0.45 V, It ) 1 nA. (B) Es ) 0.4 V, Et ) 0.45 V, It ) 2 nA.

inhibited and shifted to a more negative potential below -0.56 V. The reduction of As to AsH3 may also occur along with the hydrogen evolution, because the oxidation peak at 0.05 V can be observed only when the potential is swept negatively below -0.56 V. The oxidation peak at 0.05 V may therefore be related to the oxidation of AsH3; of course most of the AsH3 formed is diffusing away from the electrode along with the hydrogen evolution. The oxidation of hydrogen may be excluded because the Pt surface is deactivated by the adsorbed As. The oxidation current at potentials above 0.9 V is related to the oxidation of As/ As3+ to As5+.19,22 The disappearance of the first deposition peak and the potential shift of bulk deposition in the subsequent cycles indicate that the property of the substrate surface is irreversibly changed. Figure 5 further shows the effect of the lower potential limits on the initial deposition process. In the first anodic sweep after a lower limit of 0.38 V (i.e., the peak potential at the first deposition peak), a small oxidation current can be observed; the charge is much (22) Shibata, M.; Kobayashi, T.; Furuya, N. J. Electroanal. Chem. 1997, 436, 103-108.


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Figure 5. Effect of potential reversal within the deposition peak: The first five potential cycles on Pt(111) with decreasing lower potential limits in a 4 × 10-4 M As3+ + 0.5 M H2SO4 solution. Sweep rate: 50 mV/s.

Figure 4. Cyclic voltammograms of Pt(111) in a 4 × 10-4 M As3+ + 0.5 M H2SO4 solution. Sweep rate: 50 mV/s. (A) First three cycles. (B) Further potential cycles with decreased lower potential limits. (C) Potential cycle into the hydrogen evolution region.

lower than the corresponding deposition charge. In the subsequent three potential cycles, still a small deposition current is observable; however the oxidation current is negligible. In the fifth cycle, the oxidation current is increased due to the bulk deposition of As. However, the reversible redox peaks at 0.45 and 0.53 V observable in pure sulfuric acid (cf. Figure 1 and Figure 2) are not observed in Figure 5, which may indicate that the surface

coverage of As is higher; the anodic peak at 0.48 V (cf. Figure 4) is missing because bulk deposition is negligible. The above results show that the first deposition peak corresponds to a further irreversible UPD process of As (the first irreversible adsorption process is the spontaneous adsorption occurring at more positive potentials). However, when the first deposition process is incomplete due to a more positive lower potential limit, a small amount of As can still be dissolved, indicating that this surface process consists of two consecutive steps: a fast, reversible process and a very slow, irreversible one. Assuming a three-electron transfer in this deposition process, the charge of 360 µC cm-2 corresponds to a coverage of around 0.5 ML (i.e., 360/3/240 ) 0.5 ML). Since 0.33 ML of As has already been spontaneously deposited at the initial potential as generally assumed in the literature, then 0.83 ML As in a total stays at the surface, which cannot be stripped in these potential sweeps with an upper potential limit of 0.75 V. To examine the surface states of Pt and As, the electrode was taken out of the solution at 0.2 V and transferred to a pure sulfuric acid solution at the same potential. As shown in Figure 6A, the bulk As is stripped in the first positive sweep; the charge of 1760 µC cm-2 indicates that more than one monolayer of As is stripped. Some redeposition of As gives rise to a broad peak at 0.2 V. In the subsequent potential sweeps, a very small broad redox peak appears at 0.45 V. Figure 6B shows the change of the cyclic voltammogram when the positive sweep limit is afterward increased to 0.9 V. A gradual positive shift of the peak potential appears, together with a gradual increase of the magnitude of the peak current. The redox peak becomes sharper and has a maximum charge of around 240 µC cm-2. This peak is similar to that observed when As is spontaneously adsorbed at Pt(111) at open circuit. By a further stripping in this potential region, this peak becomes smaller. The hydrogen adsorption current becomes larger and resembles that at poly-Pt; no spike for sulfate adsorption is observed when after complete stripping the hydrogen adsorption change is negligible (cf. Figure 6C). Note that the width of the As redox peaks is gradually decreased and that the peak at 0.45 V is not simultaneously observed with the peak at 0.53 V (cf. Figure 6B). Also it may be important to mention that the redox peaks would not become sharper in the further stripping process, contrary to a smooth10-12 or only slightly roughened surface

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Figure 7. STM image of a Pt(111) surface covered by As in 0.05 M H2SO4 solution. The stripping of As was performed in a conventional glass cell until the appearance of the maximum peak current at 0.53 V, similar to the solid curve in Figure 6C. The STM images are essentially the same at potentials positive and negative of the peak potential. Es ) 0.45 V, Eb ) 20 mV, It ) 1 nA.

Figure 6. Cyclic voltammograms of stripping of bulk As on Pt(111) in a 0.05 M H2SO4 solution. (A) First four cycles. (B) Subsequent cycles; the arrows indicate the direction of changes. (C) Comparison between the CVs with high coverage (s), low coverage (- - -) of As, and bare Pt(111) (‚‚‚). Sweep rate: 50 mV/s.

(cf. Figure 1). The results indicate that a surface alloying process is involved during the electrochemical deposition of As. After the stripping of As, the absence of the spike indicates that the Pt(111) is rough. The absence of the peaks due to hydrogen adsorption at defects implies that some residual As is decorating these surface defects. Figure 7 shows a Pt(111) surface after stripping of bulk As, but when the hydrogen adsorption is still completely suppressed. The preparation of this surface is essentially similar to that shown in Figure 6A,B; that is, the Pt(111) was first subjected to a bulk deposition of As at 0.2 V in an As3+-containing solution, and then a stripping process was performed up to the last cycle of Figure 6B in which a maximum amplitude of the redox peak at 0.54 V was obtained. Finally, the surface was transferred to the STM cell with the protection of sulfuric acid solution. The STM

image shows that the surface is rough, however, with the step edges of the substrate still discernible. Figure 8 shows a series of STM images showing the As deposition in the first deposition peak at 0.35 V. The tip potential was kept constant at 0.72 V. Figure 8A is an image recorded at the immersion potential of 0.6 V versus RHE, showing smooth, small terraces separated by monatomic steps. Figure 8B shows a drastic change when the electrode potential was swept from 0.6 to 0.35 V at 20 mV/s. The bad quality in the middle of the image may be due to processes at the tip or unresolved processes at the surface. The surface can be clearly observed at the upper part of image C, but the surface is blurred at the bottom. This area becomes smaller in the subsequent images. The surface coverage of the new adlayer stays almost constant in the upper part of images C and D. The height of the islands is measured to be 0.23 nm (cf. Figure 9A,B). The islands have a small corrugation and random distribution. All step edges are modified. Obviously, the underlying process is slow and starts at the step edges. No significant change is observable when the electrode potential is increased positively to 0.7 V and held there for 4 min (cf. Figure 9C,D), indicating that As deposited in the first deposition peak forms an alloy with substrate Pt and therefore is not dissolved in the As3+-containing solution. When the electrode potential was decreased to 0.10 V, in a large image area, a few big clusters without any regular shape and cluster edges can be observed (cf. Figure 10). The height of the clusters ranges from 1 to 3 nm. The clusters were very slowly dissolved at more positive potentials, which may be due to passivation of bulk As or the formation of As/Pt alloy. The surface area between the large clusters is rough and becomes rougher and rougher with the increase of deposition and stripe sequence; please note that several monatomic steps are observable in the area shown in Figure 10B before As deposition (cf. Figure 8A), but now they are no longer observable because of the surface roughness. The width of the islands between the large clusters is around 2-4 nm, much smaller than the size of the 2D islands deposited in the first deposition peak (cf. Figure 9). Such a surface remains rough after bulk As is stripped at 0.6 V.


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Figure 8. STM images of As electrochemical deposition in the first deposition peak at 0.35 V in 4 × 10-4 M As3+ + 0.5 M H2SO4 solution. The black arrow in image B indicates the change of the electrode potential, and the white arrow indicates the scan direction of the tip. 90 s per image. Et ) 0.65 V, It ) 1 nA. (A) At the immersion potential at 0.6 V. (B) Potential changing from 0.6 to 0.35 V; see the black arrow. (C,D) Potential staying at 0.35 V.

Nanostructuring of As at Pt(111). Nanostructuring of the Pt(111) surface was performed at 0.60 V, at which potential only spontaneously deposited As is present on Pt(111). It was first shown by recording an STM image that the Pt(111) surface is smooth at the immersion potential of 0.6 V with a tip potential at 0.65 V. The nanostructuring process was performed by tip scanning at a close tip-substrate distance in a certain surface area. The tip potential was 2 mV more negative of the Pt(111), and the tunneling current was 15 nA. The tip was scanned for 30 s under these conditions. Figure 11 shows images recorded after the nanostructuring process at a tip potential of 0.65 V and a tunneling current of 1 nA. Figure 11A shows two nanospots obtained when the tip was scanned in 10 nm × 10 nm during the nanostructuring process. These nanospots are of around 11 nm in diameter and 0.18 nm in height. Figure 11B shows another three nanospots that were obtained when the tip was scanned in 50 nm × 6 nm (scan ratio X:Y ) 8:1). The nanostructures thus obtained are similar to stripes; their length, however, is somewhat smaller than given by the settings. They have the same height as the two clusters produced first and are

similar to the nanostructures obtained when Cu2+ is in the sulfuric acid solution. Discussion When As is irreversibly adsorbed above 0.45 V, but below the open circuit potential, a pair of peaks appears at 0.38 V/0.45 V in addition to the redox peak around 0.53 V. Although under these deposition conditions the As coverage achieved probably is higher than after adsorption at an open circuit, which only leads to one redox peak at 0.53 V (with a shoulder at 0.57 V), the total charge of both redox peaks is not increased. The difference in coverage, however, may be very small, and the appearance of the additional peak may correspond to a phase transition when the coverage is slightly exceeding a critical value; it is reversible when some of the deposited As is dissolved again at higher potentials. This behavior is somewhat similar to that of adsorbed Sn at Pt(111): there, above a threshold coverage which also corresponds to complete suppression of hydrogen adsorption, one of the peaks decreases at a constant redox charge in the complete potential range.18

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Figure 9. STM images of As adlayers deposited at 0.35 V and corresponding section analysis. (A,B) At 0.35 V, Et ) 0.65 V, It ) 1 nA. (C,D) At 0.7 V, Et ) 0.65 V, It ) 1 nA.

Interestingly, when the potential is swept beyond this peak in an As3+-containing solution, this peak disappears after the first sweep. The irreversibility of this process, as well as the slow disappearance when the negative critical limit is at the peak potential, already suggests a 2D alloy formation. The term “irreversible” is used here in two somewhat different meanings: (1) Irreversible, spontaneous deposition of As3+ at open circuit potential gives rise to a reversible redox peak; positive of this peak, however, As stays adsorbed. This process is called an irreversible UPD process, although this As layer can be slowly dissolved. (2) Slow deposition below 0.4 V in the As3+ solution is totally irreversible because no corresponding anodic process occurs; however, as noted before, the first step of this process is reversible as shown in the experiment of Figure 5. The slow formation of a two-dimensional, incomplete layer (cf. Figure 8), which seems to start at step edges (cf. Figure 9) and cannot be dissolved again at more positive potentials, is further very strong evidence for a 2D alloy phase or surface compound. The rough steps in the STM image of Figure 3 show that at the step edges this process already commences in the As3+-free solution. Since the peak at 0.38 V is also visible in the As3+-free solution, it is not simply a deposition peak. Only a part of its total charge of 360 µC/cm2 might correspond to deposition of

As3+ from solution. This additional As might be the origin of the 2D surface alloy. On the other hand, also the more negative potential might directly lead to the alloy formation; the negative potential alone with the coverage of As achieved above 0.45 V, however, certainly is not sufficient. The structure of the images shown in Figure 8 and Figure 9 somewhat resembles that of tellurium on Au(111),23 as well as that of selenium on Au(111),24 for which a roughening transition was observed positively close to the bulk deposition potential. The difference is that the roughening process with tellurium on Au(111) is reversible and therefore was not interpreted as being due to alloy formation. The formation of a surface alloy continues when the potential is swept further negative into the bulk deposition region in As3+-containing solution. The deposition peak in the second sweep is positively shifted due to the facilitated nucleation at a roughened surface. Whereas the small clusters in Figure 10B might be interpreted as being due to As nanodeposits, the fact that the surface is still rough even after dissolution of As proves that a surface alloy has been formed. The fact that the CV of smooth (23) Sorenson, T. A.; Varazo, K.; Suggs, D. W.; Stickney, J. L. Surf. Sci. 2001, 470, 197-214. (24) Lister, T. E.; Stickney, J. L. J. Phys. Chem. 1996, 100, 1956819576.


Figure 10. STM images of As deposition on Pt(111) at 0.10 V in 4 × 10-4 M As3+ + 0.5 M H2SO4 solution. Et ) 0.65 V, It ) 1 nA. (A) Cluster shape, illuminated view. (B) Rough surface between the clusters.

Pt(111) is not regained after As dissolution, whereas it is after dissolution of the As monolayer formed under open circuit conditions, is a further support. A similar surface alloy formation and concomitant roughening had been observed for spontaneously adsorbed antimony on Pt(111),25 but not at low coverages.25,26 In the case of As, such an alloy formation seems to be more difficult and requires more severe deposition conditions. This may be related to a different stress exerted on the Pt substrate or a different interaction strength between Pt and Sb or As. Such an alloy formation involves a certain mobility of the atoms of the Pt substrate. Although on one hand this seems to be a little astonishing because usually Pt is believed to be quite an immobile substrate, there are other examples for the mobility of Pt atoms: well-known is the place exchange with oxygen atoms after adsorption of oxygen.27-29 Another example is the reconstruction of (25) Climent, V.; Herrero, E.; Feliu, J. M. Electrochim. Acta 1998, 44, 1403-1414. (26) Yang, Y. Y.; Zhou, Z. Y.; Sun, S. G. J. Electroanal. Chem. 2001, 500, 233-240. (27) Sashikata, K.; Furuya, N.; Itaya, K. J. Vac. Sci. Technol. 1991, B9, 457-464.

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Figure 11. STM images of nanostructuring of Pt with As at a wide Pt(111) terrace in 4 × 10-4 M As3+ + 0.5 M H2SO4 solution. Fabricating conditions: Es ) 0.6 V; Eb ) -2 mV; It ) 15 nA; scan rate, 36 lines/s; scan time period, 30 s. Imaging conditions: Es ) 0.6 V; Eb ) 50 mV; It ) 1 nA. (A) Two nanospots obtained when the tip was scanned in 10 nm × 10 nm. (B) Another three nanostructures obtained when the tip was scanned in 50 nm × 6 nm.

Pt(100) under special conditions30 and the lifting of the hex-reconstruction.31,32 The nanostructuring of As under a close tip/substrate distance is certainly due to the same mechanism as proposed for Cu on Pt(111), that is, tip-induced alloy formation.8 According to the cyclic voltammogram and STM image, no electrochemical deposition of As takes place at 0.6 V and then no roughening occurs, but with the help of the scanning tip, a local alloy formation occurs and thus nanostructures can be obtained. Conclusion Different adsorption states exist for As on Pt(111). A sharp, reversible peak is observed in cyclic voltammetry for partial coverages, which can be achieved by spontane(28) Tidswell, I. M.; Markovic, N. M.; Ross, P. N. J. Electroanal. Chem. 1994, 376, 119-126. (29) Nagy, Z.; You, H. Electrochim. Acta 2002, 47, 3037-3055. (30) Baltruschat, H.; Bringemeier, U.; Vogel, R. Faraday Discuss. 1992, 94, 317-327. (31) Al-Akl, A.; Attard, G. A.; Price, R.; Timothy, B. J. Electroanal. Chem. 1999, 467, 60-66. (32) Wakisaka, M.; Sugimasa, M.; Inukai, J.; Itaya, K. J. Electrochem. Soc. 2003, 150, E81-E88.

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ous deposition at open circuit. When the coverage approaches a value at which hydrogen adsorption is completely suppressed, this peak broadens. When, after electrochemical deposition above 0.45 V, the coverage exceeds the value necessary for complete suppression of hydrogen adsorption, a second peak appears, pointing toward a distinctly different adsorption state. Except for monatomic steps of the substrate, the surface stays smooth under these conditions. When As is deposited at a potential below 0.38 V, an irreversible change of the surface sets in: the STM images reveal a surface roughening, which becomes even more pronounced at potentials of bulk As deposition. After stripping of As, the surface stays rough as revealed by both STM and the shape of the hydrogen adsorption peaks in cyclic voltammetry. The only possible interpretation is


a place exchange between Pt and As atoms (as is wellknown from oxygen adsorption at Pt) and thus the formation of a 2D alloy phase. Formation of such an alloy can also be induced locally at more positive potentials by scanning the tip of the STM very close above the surface. As in the case of tip-induced local alloy formation of Pt(111) with Cu and of Au(111) with Cu and Pd, a direct mechanical interaction on the atomic level seems to be responsible for this local nanostructuring process. Acknowledgment. Thanks are due to the DFG for financing this work. LA026750Z

Electrochemical Deposition and Nanostructuring of As at Pt (111)

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