In Situ Electrochemical Scanning Tunneling Microscopy of Ni(111), Ni

Itaya Electrochemiscopy Project, ERATO/JRDC, 2-1-1 Yagiyama-minami, ... The dissolution of Ni from the S-modified Ni(100) was found to proceed in the ...
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J. Phys. Chem. 1996, 100, 8954-8961

In Situ Electrochemical Scanning Tunneling Microscopy of Ni(111), Ni(100), and Sulfur-Modified Ni(100) in Acidic Solution Takeshi Suzuki,† Taro Yamada,† and Kingo Itaya*,‡ Itaya Electrochemiscopy Project, ERATO/JRDC, 2-1-1 Yagiyama-minami, Taihaku-ku, Sendai 982, Japan, and Department of Applied Chemistry, Faculty of Engineering, Tohoku UniVersity, Aoba, Sendai 980, Japan ReceiVed: September 22, 1995; In Final Form: February 21, 1996X

Single-crystal Ni electrodes were investigated by in situ electrochemical scanning tunneling microscopy (STM) under potential control in 0.05 M Na2SO4 at pH 3.0. On Ni(111), a Ni(111)-(1×1) structure was observed in high-resolution STM images acquired at cathodic potentials. Under passivation by forming oxide layers, a hexagonal lattice with an interatomic distance of 0.29 nm was discerned, which was attributed to oxide layers of NiO(111) or Ni(OH)2(0001). On Ni(100), a square lattice with a p(2×2) of oxygen was observed even at cathodic potentials. The p(2×2) structure was converted to a quasi-hexagonal structure in the passive region due to the formation of bulk oxide films. Sulfur adatoms chemically attached on Ni(100) formed a c(2×2) adlattice, which was consistently observed even under the anodic dissolution of the underlying Ni substrate. The dissolution of Ni from the S-modified Ni(100) was found to proceed in the layer-by-layer mode, forming atomically flat terraces extending over large areas.

Introduction Numerous electrochemical works have been reported on Ni electrodes to understand their anodic dissolution, passivation, and oxide formation, because Ni is one of the most important electrode materials used for industrial purposes such as electrocatalysis, batteries, and electroplating.1-5 A variety of experimental techniques such as voltammetry, ellipsometry, X-ray analysis, and Raman and infrared spectroscopies have been used to characterize the oxides formed on polycrystalline and single-crystal Ni substrates.5 Nevertheless, atomic-scale elucidation of electrochemical processes occurring on Ni metal still remains to be an important subject to be investigated. Although atomically well-defined surfaces must be exposed to solution to understand structure-reactivity relationships on an atomic scale, it has been difficult to prepare well-defined Ni single-crystal surfaces in previous electrochemical studies. Chemical and electrochemical etching have been most frequently used in preparing Ni surfaces for electrochemical studies.1-5 Yau et al. recently reported on their investigation by in situ scanning tunneling microscopy (STM) of Ni(100) in an alkaline solution, in which the surfaces were initially prepared by chemical etching and finally by an in situ cathodic polarization which was performed to remove oxide layers.6 They presented an atomic STM image with a quasi-hexagonal lattice in an early stage of the oxidation of Ni(100), which was attributed to the (111) plane of NiO or the (0001) basal plane of Ni(OH)2. An attempt to image an oxide-free Ni(100) surface was unsuccessful probably due to the difficulty of removing spontaneously formed oxide layers in alkaline solutions.6 An ex situ STM study in air also recently demonstrated the capability of STM to reveal atomic structures of electrochemically formed Ni oxide layers.7 Although the bare Ni(111) surface was anticipated to be unstable in alkaline solutions even under cathodic polarization,6 it was firmly concluded by MacDougall and Cohen that complete removal of Ni oxides was possible in acidic solutions of pH less than ca. 3.3,4 * To whom correspondence should be addressed. † Itaya Electrochemiscopy Project, ERATO/JRDC. ‡ Tohoku University. X Abstract published in AdVance ACS Abstracts, April 15, 1996.

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On the other hand, Wang et al. made an effort to expose a well-defined Ni(111) in an alkaline solution using an ultrahigh vacuum (UHV)-electrochemical system, in which well-defined clean Ni(111) and Ni(111) protected with a CO adlayer were prepared in UHV and transferred to an electrochemical cell.8 The c(4×2)-CO adlayer on Ni(111) was found to protect its surface from the oxidation. Procedures using iodine and CO adlayers have commonly been employed to expose well-defined surfaces such as Pt and Rh in solution.9,10 The protective CO adlayer on Pt and Rh can be removed by an electrochemical oxidation without destroying the substrate structure. Unfortunately, on Ni(111) the electrochemical oxidation of CO takes place in an alkaline solution at rather positive potentials, where the Ni(111) surface is also electrochemically oxidized.8 Oudar and Marcus previously reported on the effect of adsorbed monolayer of S on the rate of anodic dissolution of Ni.11 The active region for the anodic dissolution of Ni was greately extended toward more positive potentials, suggesting that the Ni surface was protected from the oxide formation by the S adlayer.11,12 The technique using a protective layer generally seems to be one of the promising approaches to expose various well-defined metal surfaces. In this study, we carried out in situ STM of Ni(111) and Ni(100) in an acidic solution of Na2SO4 of pH 3.0, where Ni oxides are expected to be reduced by a cathodic polarization, as described above.3,4 The first atomic image of Ni(111)-(1×1) was successfully obtained in a cathodic potential range. A p(2×2) structure was found on Ni(100) as the first monolayer of the adsorbed oxygen. On an S-modified Ni(100) electrode, a c(2×2)-S monolayer was clearly discerned by in situ STM. Anodic dissolution processes of Ni(111), Ni(100), and S-modified (100) were also investigated. Experimental Section Ni(111) and Ni(100) single crystals used were obtained from Mateck Inc., Germany. We preliminarily attempted chemical and electrochemical etching to obtain flat clean surfaces, which consistently yielded STM images of atomically rough surfaces. The so-called flame-annealing-quenching method frequently applied to Pt13 could not be applied to Ni because of the thermal © 1996 American Chemical Society

Ni(111), Ni(100), and Sulfur-Modified Ni(100) oxidation of Ni in the hydrogen-oxygen flame. Therefore, the Ni single crystals were subjected to thermal annealing in a H2 stream in this study. The surfaces of crystals were polished metallographically, finished with 0.25 µm diamond paste, and sonicated in trichloroethylene, acetone, methanol, and pure water. Then the crystals were annealed in a quartz tube under a H2 (1 atm) atmosphere at 1237 K for 3-10 h. The furnace was slowly cooled down to room temperature also under H2 stream. To avoid oxidation of surfaces by oxygen in air, the crystals were dropped into hydrogen-saturated water under a H2 stream and mounted on an STM cell filled with a H2saturated Na2SO4 solution. The electrode was immediately brought under potential control at potentials near or negative with respect to the open circuit potential (OCP) to avoid further oxidation in the electrochemical cell. A similar procedure to protect the surface by hydrogen-saturated solution was applied to Pt and Rh electrodes after the flame-annealing, as described in our previous papers.13,14 The S-modified Ni(100) was prepared in a H2S stream. After the annealing procedure in the H2 atmosphere as described above, the cooling was halted at 773 K. Hydrogen gas (1 atom) containing 200 ppm H2S was introduced for 1 min, and the cooling was resumed in pure H2 flow. This procedure is similar to that described in the previous paper.11 Both electrochemical and STM measurements were carried out in 0.05 M Na2SO4, the pH of which was adjusted to 3.0 using 0.1 M H2SO4 and 0.1 M NaOH of ultrapure grade (Cica MERCK). Voltammograms were obtained with a PAR Model 273A potentiostat. STM imaging was performed with a Nanoscope III STM from Digital Instruments Inc. Atomic images were taken in the constant height mode with scan rates of 10-40 Hz. The potential of tip was usually held at -0.10 V vs a saturated calomel electrode (SCE). Two Pt wires were used as the reference and the counter electrodes for STM measurements. STM tips were made of W wires etched electrochemically, and their side walls were coated with a clear nail polish.15 The piezoelectric tube was carefully calibrated using highly ordered pyrolytic graphite, and STM images were acquired under the condition for minimizing thermal drift, which usually causes a distortion in STM atomic images. During STM measurements, the STM head-sample assembly was covered by a cylindrical glass vessel purged with Ar gas. The potential of the Pt quasi-reference electrode was calibrated against the SCE. All potentials are hereafter reported with respect to the SCE. Results and Discussion Cyclic Voltammetry. Figure 1 shows cyclic voltammograms (CV) of Ni(111) recorded at a scan rate of 20 mV s-1 in 0.05 M Na2SO4 (pH 3). The OCP was found to be -0.37 V. The first scan was made in the negative direction from the OCP, and then it was reversed at -0.85 V. A small cathodic current can be seen during the first scan, which might be due to a slow hydrogen evolution reaction. In the following anodic scan, the anodic current abruptly increased at about -0.4 V, formed a peak at -0.05 V, and rapidly decreased at more positive potentials. In the subsequent cathodic scan from 0.25 V, no corresponding anodic current was found, indicating that the Ni(111) surface was passivated by the formation of oxide layers. However, the cathodic current observed in the second cathodic scan was clearly larger than that found in the first cathodic scan, suggesting that the oxide layer formed at positive potentials is electrochemically reduced to metallic Ni. The difference in the cathodic charges between the first and the second scans was ca. 1.9 mC cm-2, which is expected to be the charge consumed for reducing the oxide film. If it is assumed that NiO(111) is formed on Ni(111), 1.9 mC cm-2 corresponds to the charge for

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Figure 1. Cyclic voltammograms on bare Ni(111) in 0.05 M Na2SO4 (pH 3.0). The scan rate was 20 mV s-1. The first scan, indicated by the broken curve, was started from the OCP (-0.37 V) in the negative direction. The second scan, indicated by the full curve, was started from the OCP in the positive direction and reversed at 0.25 V.

the reduction of ca. 4.5 layers of NiO(111), because a charge of 0.43 mC cm-2 is required for the reduction of a monolayer of NiO, which was calculated on the basis of the transfer of two electrons per Ni2+. After the reduction of the oxide layer, the anodic dissolution peak appeared again in the subsequent anodic scan with a similar current density. MacDougall and Cohen reported that the oxide film formed on polycrystalline Ni in Na2SO4 solutions was composed of NiO with a nearly constant thickness (0.9-1.2 nm) in the pH range 2-8.4 and that the oxide film could be completely removed by a negative scan in solutions of pH less than ca. 3.3,4 Our estimation of the film thickness of ca. 1.08 nm is in good agreement with their results. According to the result shown in Figure 1, the anodic current at least in the rising portion of the peak is expected to be due to a simple anodic dissolution of Ni to form soluble Ni2+ species in solution. Figure 2 shows a CV of Ni(100) obtained at a scan rate of 20 mV s-1. The experimental procedure was identical to that for Figure 1. The overall behavior of Ni(100) is similar to that of Ni(111) with a slightly smaller anodic peak current. The difference in the cathodic charges between the first and second scans is ca. 1.0 mC cm-2, which corresponds to the reduction of ca. 2.7 atomic layers of NiO(100). A charge of 0.36 mC cm-2 is expected for the reduction of one monolayer of NiO(100). Although the charge found on Ni(100) is about half of that on Ni(111), the equivalent thickness of 2.7 layers of NiO(100) is ca. 1.12 nm, which is again in good agreement with the previous data obtained on polycrystalline Ni.3,4 Note that the anodic current at 0.25 V was 120 µA cm-2 on Ni(100), which is greater than 60 µA cm-2 on Ni(111), as was also pointed out by Oudar and Marcus.11 This difference in the anodic currents at 0.25 V suggests that the anodic oxide film on Ni(111) had a better crystallinity than that on Ni(100). The larger anodic current observed on the (100) surface can be attributed to the oxidation of an additional amount of Ni taking place at defects in the oxide layer. Figure 3 shows a CV of a S-modified Ni(100) electrode obtained in S2--free 0.05 M Na2SO4 (pH 3.0). The broken line is the CV of the bare Ni(100) from Figure 2, which is shown here for comparison in the same current scale. The initial rising part of the anodic dissolution current on the S-modified

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Figure 2. Cyclic voltammograms on bare Ni(100) in 0.05 M Na2SO4 (pH 3.0). The scan rate was 20 mV s-1. The potential scan was the same as for Figure 1.

Figure 4. (a, top) Unfiltered STM current image of Ni(111) in a 7.5 nm × 7.5 nm frame at -0.5 V in 0.05 M Na2SO4 (pH 3.0). (b, bottom) Filtered in situ STM current image in a 5 nm × 5 nm frame at -0.50 V in 0.05 M Na2SO4 (pH 3.0). The lattice constant of the adlattice seen is 0.25 ((0.01) nm. The arrows indicate the direction of atom rows of Ni(111).

Figure 3. Cyclic voltammograms of S-modified Ni(100) in 0.05 M Na2SO4 (pH 3.0). The scan rate was 20 mV s-1. The solid and broken curves are for S-modified and bare Ni(100), respectively.

Ni(100) was not very different from that on the bare Ni(100). However, it is surprising that the anodic dissolution current continued to increase with potential beyond 0 V, where the surface was already passivated in the case of bare Ni(100), as shown in Figure 2. Clearly, the existence of the S adlayer on Ni(100) retarded the formation of oxide films. Oudar and Marcus reported on their first intensive electrochemical study on the effect of S adsorbed on Ni(111), -(100), and -(110).11 Our result shown in Figure 3 is consistent with their results. It is reasonably expected that the S adlayer exists on the outermost surface of Ni substrate even during the anodic dissolution at a rate higher than 5 mA cm-2, because almost identical anodic currents were observed in both anodic and cathodic scans, as shown in Figure 3. No hysteresis is seen in the passive region. A similar effect was discovered and characterized by Soriaga and co-workers for iodine-modified Pd electrodes.16-18 The

anodic dissolution of Pd is known to be catalyzed by the presence of an iodine adlayer. In Situ STM of Ni(111). In general, STM images acquired on freshly prepared Ni(111) showed atomically flat terrace-step features, indicating that the experimental procedure described in the Experimental Section successfully produced well-defined surfaces of Ni. However, we usually found some small islands on the atomically flat terrace at OCP, STM images of which showed a hexagonal atomic structure with a lattice constant of 0.29 ((0.02) nm. Our STM images of these islands were similar to those reported previously using an ex situ STM in air for electrochemically passivated Ni(111).7 Nevertheless, it was found in this study that these islands disappeared when the electrode potential was set at potentials more negative than -0.5 V, where the oxide layer is reduced, as expected from Figure 1. Figure 4 shows two examples of in situ STM images with atomic resolution obtained at -0.50 V in 0.05 M Na2SO4. These images were obtained at the constant height mode (current image). It is clearly seen in the image shown in Figure 4a that the surface of Ni(111) in the solution has an atomically flat terrace-step structure. It was also found in topographic images obtained at the constant current mode that the height of each

Ni(111), Ni(100), and Sulfur-Modified Ni(100) step was in accord with the monatomic step of 0.176 nm on Ni(111). The monatomic steps shown in Figure 4a are in parallel with the atomic rows of Ni(111), but they sometimes formed in a long twisted line. The direction of atomic rows of Ni was determined by the orientation of the single crystal with respect to the scan direction of the piezoelectric tube. Attention was paid to determining lattice parameters as precisely as possible on Ni(111) at -0.5 V, because the oxide formed on Ni(111) also exhibited a hexagonal lattice in STM images with an interatomic distance of ca. 0.29 nm, which is slightly larger than that (0.25 nm) of the Ni(111)-(1×1) structure. Figure 4b shows a typical atomic image acquired on a relatively large terrace under such conditions. Nearly perfect hexagonal lattices are seen with an angle of 60° ((2°) between the atomic rows. It is also confirmed that the atomic rows are parallel to the closepacked directions of Ni(111) determined from the orientation of the crystal on the STM stage shown by the arrow in Figure 4b. The nearest neighbor distance averaged in all atomic directions is between 0.23 and 0.25 nm. Therefore, we conclude that the surface structure of Ni(111) at -0.5 V is Ni(111)-(1×1). This is the first atomic image of bare Ni(111)-(1×1) obtained in solution. It is noteworthy that, to our knowledge, no STM image of clean Ni(111) has been presented in the past even in UHV,19 while images of Ni(100) and Ni(110) have been shown.20,21 The Ni(111)-(1×1) structure was observed in the potential range between -0.6 and -0.4 V, indicating that no oxidation including the preadsorption of oxygen species takes place in this range. However, it is interesting to note that the (1×1) structure was also consistently observed even at potentials near the foot of the rising portion of the anodic dissolution current. The anodic dissolution simply occurred with the same substrate structure (1×1) being maintained at least at the potentials corresponding to the foot of the anodic dissolution peak. We acquired a series of STM images of the same area at -0.33 V, where the anodic dissolution takes place slowly. The images shown in Figure 5 were acquired at intervals of 50 s. The arrangement of atoms as well as the step lines can be clearly discerned in these images. It is seen that the step edge on the right corner shown in Figure 5a extends in the downward direction, exposing the underlying Ni(111)-(1×1) plane. All step lines seen in Figure 5a,b disappeared completely, and a widely extended terrace with atomic flatness was observed in Figure 5c. Note that no pit was found to form on the terrace, indicating that the anodic dissolution took place only at the step edge under the condition of Figure 5. The result shown in Figure 5 clearly demonstrates that the anodic dissolution proceeds in the “layer-by-layer” mode at the onset of the passive region. A more detailed study of the potential dependence of the dissolution mechanism is now of our special interest. Oxide films are expected to be formed when the anodic current starts to decrease at -0.15 V in Figure 1. Figure 6 shows a typical STM image obtained at -0.15 V. Again, the image clearly shows an almost perfect hexagonal structure. However, the observed lattice constant is distinctly different from that of Ni(111) and is in a range 0.28-0.29 nm. The direction of atom rows is parallel to that of the underlying Ni(111). The observed lattice constant is close to 0.295 nm of NiO(111) or 0.312 nm of Ni(OH)2(0001). It is not easy to determine whether the structure observed in Figure 6 corresponds to NiO or Ni(OH)2 because the difference between these lattice parameters is too small to discriminate by STM. The Ni(111)-(1×1) terrace was uniformly covered by an oxide layer, because the surface at -0.15 V appeared with an atomically flat-step feature, suggesting that the oxide formed at -0.15 V was the first monolayer of oxide. The fact of the parallel

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Figure 5. Successively recorded unfiltered STM current images of Ni(111) being dissolved at -0.33 V in 0.05 M Na2SO4 (pH 3.0). Imaging of (a), (b), and (c) (top, middle, bottom) was started at 50 s intervals. The scan for each 7.5 nm × 7.5 nm frame was completed in 10 s. The arrows indicate the direction of atom rows of Ni(111).

appearance of the atomic rows without Moire´ pattern, which would result from lattice mismatch, suggests that the first oxide layer is formed uniformly and two dimensionally to maintain the same symmetry of the underlying Ni(111). The oxide film

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Figure 6. In situ filtered STM current image in a 3 nm × 3 nm frame of the hexagonal structure observed on Ni(111) at -0.15 V in 0.05 M Na2SO4 (pH 3.0). The hexagonal lattice constant is 0.29 ((0.02) nm. The arrow indicates the direction of atom rows of Ni(111).

Figure 7. In situ STM topographic image of bare Ni(100) in a 200 nm × 200 nm frame at the OCP()-0.37 V) in 0.05 M Na2SO4 (pH 3.0).

formed at -0.15 V was easily reduced at negative potentials, as evidenced by the fact that the image shown in Figure 4b was restored with the lattice constant of 0.25 nm when the electrode potential was scanned back to -0.5 V. Formation of NiO and Ni(OH)2 on Ni surfaces has also been studied in UHV enviroment.22-26 When the electrode potential is further scanned from -0.15 V in the positive direction, the thickness of oxide layer is expected to increase up to ca. 1 nm at 0.25 V, as can be inferred from the result shown in Figure 1. During the above scan, the atomically flat terrace was roughened with many newly formed step lines. The averaged step width was in the range 2-5 nm, although clear atomic images were also obtained on narrow terraces. The hexagonal oxide plane was tilted up by an angle between 6° and 15° from the substrate Ni(111). Such morphological changes might be due to lattice relaxation during the formation of thick oxide layers with a lattice constant different from that of Ni(111). A similar observation of the formation of stepped surface was described in the previous ex situ STM study performed in air.7 In Situ STM of Ni(100). Figure 7 shows a typical example of STM images acquired at the OCP in a large area of 200 nm × 200 nm, revealing many islands with heights of 1-3 nm. Pits with similar depths be can also seen on relatively flat areas.

Figure 8. (a, top) In situ unfiltered STM current image of bare Ni(100) in a 10 nm × 10 nm frame at -0.5 V in 0.05 M Na2SO4 (pH 3.0). The lattice constant seen is 0.50 ((0.01) nm, representing a p(2×2) adlattice. The arrows indicate the direction of atom rows of Ni(100). (b, bottom) Model for p(2×2) adlayer of oxygen on Ni(100).

When the electrode potential was scanned to -0.5 V, the islands seen in Figure 7 became smaller and lower in height. The islands seem to be oxides formed during the sample preparation. Although the surface of Ni(100) contained a large number of defects, it was found in this study that fairly clear atomic images can be acquired on relatively flat areas. Figure 8a shows an in situ STM image acquired with atomic resolution at -0.5 V. It is surprising to find a well-ordered atomic arrangement with 4-fold symmetry. Atomic scale pits and clusters are also clearly seen, indicating a rather slow mobility of surface atoms. The lattice constant of 0.50 ((0.01) nm observed in the ordered domains is twice as large as that of Ni(100). The rows of adatoms are parallel to the substrate Ni(100) atom rows. The adlattice structure is designated as p(2×2), as shown in Figure 8b. No Ni(100)-(1×1) lattice was found to be included in the STM images acquired at -0.5 V. It is well demonstrated by LEED24 and more recently by STM21,27 in UHV that oxygen adatoms on Ni(100) form p(2×2) and

Ni(111), Ni(100), and Sulfur-Modified Ni(100)

Figure 9. In situ unfiltered STM current image of bare Ni(100) in a 4.8 nm × 4.8 nm frame at -0.25 V in 0.05 M Na2SO4 (pH 3.0). The observed lattice is a distorted hexagonal. The arrows indicate the direction of atom rows of Ni(100).

c(2×2) structures. In the model presented in Figure 8b, it is assumed that oxygen adatoms are located on the 4-fold hollow sites. It is not clear in this study whether bright spots or dark holes correspond to the oxygen adatom. Oxygen adatoms on Ni(100) were often imaged as dark holes by STM in UHV owing to an electronic effect in the process of imaging rather than the exact geometric position of oxygen atoms, which should be located 0.08 nm above the Ni surface.20,21,27 Nevertheless, the image shown in Figure 8 is the first evidence that the p(2×2) structure exists on Ni(100) in aqueous solution even at negative potentials. It is noteworthy that the p(2×2) lattice was consistently observed in the potential range between -0.40 and -0.60 V. The p(2×2) oxygen adlayer seems to be a thermodynamically stable phase, which is anticipated to be difficult to reduce electrochemically under the present condition (-0.6 V). We further attempted to acquire an atomic image at a more negative potential of -0.8 V, but failed to achieve atomic resolution because of the hydrogen evolution reaction. When the electrode potential was set at -0.25 V, the anodic dissolution current gradually decreased and eventually reached zero. Although the p(2×2) structure was seen in the beginning at this potential, the atomic image became unclear, and finally a completely different image appeared when the anodic current was almost completely diminished. Figure 9 shows an STM image acquired at -0.25 V. This STM image shows a quasihexagonal symmetry with an average interatomic distance of ca. 0.30 nm. One of the atomic rows is almost parallel and the other is rotated by ca. 35° with respect to the atom rows of the substrate Ni(100). Such distorted hexagonal structures were also reported by Yau et al. on Ni(100) in alkaline solution.6 They proposed a ball model for NiO(111) or Ni(OH)2(0001) formed on Ni(100) (see Figure 4 in ref 6). The present image shown in Figure 9 seems to be consistent with their model. When the potential was scanned more positively, the surface was first covered with the same hexagonal lattice and then atomically roughened, and eventually it became difficult to acquire STM images with atomic resolution. The surface roughening caused by the formation of thicker oxide layers on Ni(100) is consistent with the previously reported finding that the oxide formed on Ni(111) was well crystallized, but not the oxides on (100) and (110).11 The layer-by-layer mode observed for the anodic dissolution of Ni(111) was not the case for Ni(100). The surface of Ni(100) became rougher by further anodic dissolution. The Ni(100) surface is expected to be

J. Phys. Chem., Vol. 100, No. 21, 1996 8959 covered partially or fully by thicker oxides depending on the electrode potential even in the active region, as shown in Figure 9. Under such conditions, relatively fast dissolution might occur on the Ni(100)-p(2×2) surfaces, and slow chemical dissolution of the oxide might take place on that part of the Ni(100) covered by the hexagonal oxide layer, followed by either the fast dissolution of Ni or the formation of additional oxide layers. This heterogeneous dissolution seems to result in the roughening of Ni(100) by the anodic dissolution. In Situ STM of S-Modified Ni(100). As shown in Figure 3, the presence of the monolayer of S on Ni(100) greatly affects the anodic dissolution and the passivation of Ni(100). Figure 10a shows an atomic STM image of an area of 7.5 nm × 7.5 nm obtained at -0.37 V. It is clear that the surface is composed of monatomic steps and atomically flat terraces. In general, the surface of Ni(100) modified by S prepared by the procedure described in the Experimental Section showed a well-defined terrace-step structure. The terraces observed usually extended over more than 50 nm with an atomically flat feature. More importantly, an atomic structure with 4-fold symmetry is clearly seen on both upper and lower terraces. The atomic rows are rotated by 45° with respect to the close-packed direction of the underlying Ni(100). The observed lattice constant of ca. 0.35 nm corresponds to x2 times the lattice constant of Ni(100) of 0.25 nm. This structure is designated as c(2×2), as shown in Figure 10c. This c(2×2)-S adlayer is well ordered and can be seen over the entire area of the terraces, even at the end of the upper terrace. It has been clearly demonstrated by LEED in UHV23-26 that the c(2×2) structure is one of the various structures of the S adlayers on Ni(100). Although STM determined various atomic structures of S in UHV on Ni substrates, mainly (111) and (110),19,20 our result shown in Figure 10 is the first in situ STM image to prove that the S-modified Ni(100)-c(2×2) structure is maintained in an aqueous solution. Note that the c(2×2) structure was consistently observed in the potential range between -0.3 and -0.6 V. Oudar and Marcus pointed out that the adsorbed S on Ni(100) was stable even after a prolonged hydrogen evolution.11 Indeed, it was found in this study that the c(2×2) structure persisted after a potential cycle up to -1.0 V. We pursued an investigation on the effect of adsorbed S on the dissolution at potentials near the onset of the anodic current. Soon after the acquisition of the image shown in Figure 10a, the electrode potential was slightly polarized positively to follow the movement of the step line resulting from the anodic dissolution. The image shown in Figure 10b was acquired after ca. 30 s. It is remarkable that the zigzag step at the center of the image of Figure 10a retracted downward and became straight along the direction of the atomic row of S. The step along the S row seems to be more stable than other steps with different orientations. It is notable that the c(2×2) pattern was continuously observed even on the newly appeared lower terraces. This indicates that the S adatoms always stayed on the topmost layer of Ni(100) surface during the dissolution of Ni atoms at the step sites. At potentials near the onset of the dissolution, only the step movement as shown in Figure 10 was observed for a long period of time, more than 1 h. During our STM observation, no pit formation was found on the terrace, indicating that the anodic dissolution of Ni(100) with the S adlayer proceeded by the layer-by-layer mode, similar to that of bare Ni(111). Our recent paper described an in situ STM study of a similar process of the anodic dissolution of Pd with an iodine adlayer.18 Further details of the dissolution of Ni in the presence of S adlayer are now under investigation.

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Suzuki et al. Conclusions On Ni(111), the Ni(111)-(1×1) structure was observed, for the first time, in the potential range between -0.6 and -0.3 V including a part of the region of anodic dissolution. The dissolution proceeded in the layer-by-layer mode at the foot of the anodic peak. The first oxide layer at -0.15 V showed a perfect hexagonal symmetry with the lattice constant of 0.29 nm. At potentials more positive than 0.2 V, stable multilayer oxide films were found to form with the same lattice parameter. The structure of oxide film was close to that of NiO(111) or Ni(OH)2(0001). The p(2×2) monolayer of oxygen was first observed on Ni(100) at cathodic potentials, which was consistent with the structure found previously in UHV. The Ni(100) surface became atomically rough by the anodic dissolution. Multilayer oxide films were formed at potentials more positive than -0.25 V, and a distorted hexagonal lattice close to that of either NiO(111) or Ni(OH)2(0001) was observed. On S-modified Ni(100), the c(2×2)-S monolayer was clearly observed on the well-defined terraces, and it was found to be very stable even during the H2 evolution reaction. The anodic dissolution current was orders of magnitude larger than that found on the bare Ni(100). It was clearly demonstrated that the anodic dissolution of Ni(100) in the presence of S adlayer proceeds in the layer-by-layer mode. The c(2×2)-S monolayer was found even on the newly formed lower terraces during the dissolution. This indicates that the S atoms always remain on the outermost layer of the Ni surface. Acknowledgment. We are grateful to Prof. M. P. Soriaga (Texas A&M University) and Dr. Y. Okinaka for their helpful suggestions and discussion. This work was supported by ERATO-Itaya Electrochemiscopy Project, JRDC. References and Notes

Figure 10. (a (top), b (middle)) Successively recorded unfiltered STM current images of S-modified Ni(100) being dissolved at -0.37 V in 0.05 M Na2SO4 (pH 3.0). Each scan in the 7.5 nm × 7.5 nm frame was completed in 18 s. The scan for (b) was started 30 s after the scan for (a) was started. The superimposed arrows indicate a step edge at which the layer-by-layer dissolution proceeded. The arrows outside the frames indicate the direction of atom rows of Ni(100). (c, bottom) Model for c(2×2) adlayer of sulfur on Ni(100).

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