Article pubs.acs.org/Langmuir
Effects of Anions on the Electrodeposition of Cobalt on Pt(111) Electrode Yenchung Kuo, Weicheng Liao, and ShuehLin Yau* Department of Chemistry, National Central University, Jhongli, Taiwan 320, Republic of China S Supporting Information *
ABSTRACT: Voltammetry and in-situ scanning tunneling microscopy (STM) were used to examine electrodeposition of cobalt (Co) on a stationary Pt(111) electrode in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4 (or the sulfate solution) without and with 10 mM chloride (the chloride solution). Under- and overpotential deposition (UPD and OPD) of Co resulted in reduction peaks at −0.52 and −0.74 V (vs Ag/AgCl), respectively. Hydrogen evolution was the major obstruction to Co2+ reduction, which limited the efficiency of Co deposition at ∼63% in both solutions. UPD of Co resulted in a highly ordered honeycomb structure in the sulfate solution, whereas that formed in the chloride solution was clearly disordered. Multilayer Co deposit formed by OPD at −0.74 V in the sulfate medium was crystalline, forming moiré structures for the first eight layers, followed by pyramids made of stacked triangles. These results suggested face-centered cubic stacking of the Co deposit. Co film produced in the chloride solution was also layered, except perimeters of Co layers were mostly rugged. Distinct screw dislocations and spiral defects were seen in the Co thin films produced in both solutions.
1. INTRODUCTION Perpendicular magnetic anisotropy (PMA) is an interfacial phenomenon which can be exemplified by cobalt (Co) thin films deposited on Pt(111) substrate. Studies reported thus far have addressed electronic perturbation of Pt substrate on Co adatoms and the degree of ordering and packing habit (facecenter cube or hexagonal close packed) of Co thin film.1,2 The interfacial structure of Co deposited on ordered Pt(111) substrate has been studied mainly in a vacuum using structuresensitive tools of low-energy electron diffraction (LEED), X-ray diffraction techniques, and scanning tunneling microscopy (STM).3−5 In comparison, electrodeposition of Co on Pt(111) single crystal bead electrode prepared by the annealing-andquenching method has been relatively unexplored.6 STM capable of imaging in real time and real space has been used to examine electrodeposition Co on Au(111),7−9 Pt(111) precoated with a Cu thin film,10 and Pt(111).11 In all cases, a smooth Co thin film made of ordered Co structures is observed. Previous studies show that anions in the supporting electrolyte can control not only the occurrence of underpotential deposition but also the growth mode of the subsequent Co deposition on Au(111).9 This issue however has not been addressed for Co deposition on Pt(111). Furthermore, chemical composition, particularly sulfate or chloride, in bath used for electrowinning of cobalt can determine the deposition efficiencies of Co.12,13 In this study in-situ STM revealed distinct differences in the deposition process and atomic structures of Co deposit obtained in electrolytes containing sulfate and chloride. A highly ordered UPD layer of Co assuming honeycomb spatial arrangement was © 2014 American Chemical Society
observed in a pH 3 sulfate medium but changed to disordered state upon the addition of 10 mM KCl. For multilayer Co thin film formed by overpotential deposition, moiré patterns were seen for the first eight layers, followed by prominent fcc twins and spiral growth. These features were not seen with the Co film prepared in chloride electrolyte.14
2. EXPERIMENTAL SECTION The Pt(111) electrodes used to conduct STM and voltammetric experiments were single crystal beads made out of a polycrystalline Pt wire.6,15 The Pt electrode was first annealed with a hydrogen torch for 5 min, followed by quenching in hydrogen-saturated Millipore water (resistivity 18.2 MΩ·cm). Being covered by a thin film of water, the Pt(111) electrode was removed from the quenching tube and transferred quickly into an electrochemical or STM cell. The electrode was immediately brought under potential control. This procedure was effective in making an electrochemically clean, contamination-free Pt electrode. The STM cell was equipped with a Pt counter and a Pt quasi-reference electrode. All potentials reported here are converted into a scale of a Ag/AgCl electrode. It is shown that the as-prepared Pt electrode would have a monolayer thick oxide, which was reduced by sweeping the potential to −0.3 V in 0.1 M K2SO4 + 1 mM H2SO4 before proceeding to Co deposition experiment. The resultant CV profile would have characteristic peaks that can be used to confirm well-ordered state of the Pt(111) electrode. The STM scanner used was a A-head (Veeco, Santa Barbara, CA) with a maximal scan size of 500 nm. The tip was a tungsten tip etched by AC in 6 M KOH. After thorough rinsing with Millipore water and Received: September 2, 2014 Revised: October 30, 2014 Published: November 5, 2014 13890
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Figure 1. CVs recorded at 50 mV/s with Pt(111) in 0.1 M sulfate solution (0.1 M K2SO4 + 1 mM H2SO4) (a) and sulfate solution + 50 mM KCl (b). The electrolytes used for (c) and (d) were the same as those of (a) and (b), except 0.01 M CoSO4 was added. The dotted lines marked under A3 peaks in panels c and d denote the baselines used to integrate the amount of charge under these peaks.
not seen at −0.82 V in the sulfate solution. C1 evidently arose from reduction of protons, while C2 and C3 were due to UPD and OPD of Co2+ on the Pt(111) electrode. These features are seen also in pH 3 ClO4− + Cl− electrolyte.11 Since C3 peaks were located at the same potentials in both solutions and had the same peak shape, it is likely that they were due to reduction of the same species. Chloride resulted in a minor process at −0.82 V. CV measurements examined the possibility of Co UPD on Pt(111) in chloride solution. We have obtained more CV results to address the nature of C2 peak. These results shown in the Supporting Information (Figure s1) suggest strongly that C2 peak arose from Co UPD. The charges associated with stripping of Co (A3 at −0.38 V) were determined to be 2.88 and 3.80 mC/cm2 for the sulfate and chloride solutions, respectively. Assuming oxidation of a monolayer of Co from the Pt(111) electrode requires 0.48 mC/cm2 charges; these charges indicate roughly 6 and 7.9 layers of Co were deposited on Pt(111) in these solutions. Because hydrogen oxidation overlapped with Co stripping in potential, as revealed by the tailing seen at the positive end of the A3 peak, particularly in the case of chloride solution, the amount of Co deposit could be less than those mentioned above. We also evaluated current efficiencies ζ of Co deposition under these experimental conditions, where ζ is defined as the ratio between the amount of charges involved in stripping Co deposit and the overall charges consumed in the entire reduction process. According to the CV results shown in Figures 1c and 1d, the overall charges consumed in the reduction processes were 4.62 and 6.21 mC/cm2, respectively. These results then yielded ζ ∼ 63% for both cases. The 50 mM chloride added to the solution had little influence over current efficiency. However, it is noteworthy that previous studies conducted galvanostatically in solutions containing much higher concentration of Co2+ (∼1 M) and chloride (4 M) can yield ζ as high as 96%. Electrodeposition of Co via potential dynamic means was complicated by the kinetics of Co2+ and H+ reduction, potential scan rate, potential window, etc. 3.2. STM Characterization. 3.2.1. Electrodeposition of the First Monolayer of Co. Switching the potential from −0.4 to −0.45 V in 0.1 M K2SO4 + 1 mM H2SO4 (pH 3) + 10 mM CoSO4 resulted in a progressive and marked increase of spots
dried with acetone, it was insulated by applying an Apeazon wax coating. The imaging conditions of bias voltage and tunneling current were important to the stability of STM imaging, possibly because the double layers of the tip and the Pt electrodes could overlap.16 In addition, the tip could physically block the flux of Co2+ to the Pt electrode, which caused Co to deposit in a somewhat different manner under the tip. A bias voltage of 150 mV seemed to be optimal for having stable STM imaging. All STM images reported in this study were acquired with the constant-current imaging mode with the slow scan direction always running in parallel to the y-axis of STM images. The electrochemical cell used for voltammetry had a three-electrode configuration, including a Ag/AgCl reference electrode and a Pt counter electrode. The potentiostat was a CHI 600 (Austin, TX). Suprapure sulfuric acid (H2SO4) was purchased from Merck (Darmstadt, DFG). Potassium sulfate (K2SO4) and potassium chloride (KCl) were purchased from Showa (Tokyo, Japan) and Sigma-Aldrich (St. Louis, MO), respectively. Cobalt(II) sulfate heptahydrate (CoSO4·7H2O, 99.999% purity) was obtained from Afra Aesar (Ward Hill, MA). Triple-distilled water (Lotun Technology Co., Taipei) was used to prepare all electrolytes. All chemicals were used as received.
3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammograms. Shown in Figure 1a,b are CV recorded at 50 mV/s with Pt(111) electrode in 0.1 M K2SO4 + 1 mM H2SO4 (sulfate solution) and sulfate solution +10 mM chloride (chloride solution). Both profiles had a pair of peaks near −0.45 V due to the hydrogen evolution reaction (2H+ + 2e− → H2). In addition, weak and yet distinct features associated with adsorption of anions, such as bisulfate in the sulfate solution and OH in the chloride solution, were also noted at 0.25 and 0.5 V, respectively. The hydrogen reduction peak occurred at the same potential with a negligible difference in the current densities. Reduction of Co2+ (Eo = −0.45 V) certainly occurred at potential more negative than hydrogen evolution. The current efficiency of Co deposition would be affected greatly by this H+/H2 reduction reaction. Shown in Figure 1c,d are CVs recorded with Pt(111) electrode immersed in chloride (50 mM in this case) and sulfate solutions with 10 mM CoSO4 added. In chloride solution, the negative potential scan resulted in first a reduction peak at −0.45 V (C1), followed by another weak peaks at −0.55 V (C2). A major reduction peak was seen at −0.74 V (C3), followed by a weak peak at −0.82 V (C4) and a progressive increase of cathodic current. The weak peak C4 was 13891
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Figure 2. A series of STM images recorded consecutively with Pt(111) electrode at −0.4 V (a) and −0.45 V (b−e) in 0.1 M K2SO4 + 1 mM H2SO4 (pH 3) + 10 mM CoSO4. The acquisition time of each image is indicated. The inset (30 × 30 nm2) of panel e reveals that the moiré pattern was misaligned with the neighboring step (likely the close-packed direction of Pt(111)) by 6°. The cross-section profile shown in panel f reveals corrugation heights along the dotted line marked in panel d. All images are 200 × 200 nm2. The bias voltage and set point current were 150 mV and 0.8 nA, respectively.
Figure 3. In-situ STM images obtained with Pt(111) electrode at −0.45 V in 0.1 M K2SO4 + 1 mM H2SO4 (pH 3) + 10 mM CoSO4. These images reveal the spatial structure of the first monolayer of cobalt deposited on the Pt electrode. Panels b and c reveal atomic resolution STM images of the Co monolayer. The bias voltages and set point currents were 200 mV and 1 nA for (b) and 250 mV and 2 nA for (c), respectively.
predominantly monatomic high in sulfate and in chloride solutions.11 The inset of Figure 2e is a close-up STM scan revealing the misalignment of the moiré pattern and a neighboring step (likely the close-packed atomic direction of the Pt(111) substrate) by 6°. Deposition of the first Co adlayer ended with an organized monolayer on the Pt(111) electrode, as seen in Figure 3a. The ordered domain spanned more than 50 nm, despite numerous vacancy and misalignment defects, denoted by the dotted line marked in Figure 3a. The degree of ordering hinged greatly on the quality of the Pt(111) substrate and the electrochemical environment. As revealed by Figure 3b, this ordered structure appears as honeycomb defined by two sets of mounds exhibiting slightly different corrugation heights. Two equilateral triangles are marked to indicate these two sets of mounds. Edges of these triangles measured 2.2 nm long were aligned close to the ⟨110⟩ directions of the Pt(111) substrate. The STM appearance of this structure varied with the imaging conditions. For example, Figure 3b was acquired with imaging conditions of 200 mV and 1 nA, but changing the conditions to 250 mV and 2 nA led to different STM appearance, where depressions of honeycombs became less pronounced (Figure 3c). These STM results are similar to those found with CoO
on the terraces seen by the STM (Figure 2), which are ascribed to UPD of Co on Pt(111). Step defects found at the upper end of the imaged area were one Pt atom high (Δz = 0.25 nm). They appeared to be smooth or free of Co nuclei, which implies uniform Co nucleation of Co, not preferentially at steps on the Pt(111) electrode. Ultimately, the nucleation mode was determined by strength of interaction between Co adatoms and the Pt substrate. The same phenomenon was also noted in chloride-containing solution.14 In addition, hydrogen could be adsorbed also on the Pt electrode and competed with Co adatoms for surface sites. Co nuclei grew laterally with time, leading to a smooth film in 7 min, as revealed by the sequenced STM images shown in Figure 2c−e. (This series of STM images suffered from a slight upward thermal drift.) Co nuclei were on average 0.15 nm higher than the Pt substrate, as revealed by the corrugation profile (Figure 2f) taken along the dotted line in Figure 2d. This value is smaller than the physical size of 0.22 nm expected for Co adatom. We reconcile this result by citing that STM was tracing the electronic state, not solely the topography of a sample, as noted also with nucleation of Co on Au(111).7,17 However, in strong contrast to bilayer Co clustering on Au(111) and Cu(111),18 Co nuclei found on Pt(111) were 13892
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Figure 4. In-situ STM images obtained with Pt(111) electrode at −0.4 V (a, b) and −0.72 V (c) in 0.1 M K2SO4 + 1 mM H2SO4 (pH 3) + 10 mM KCl + 10 mM CoSO4. These images reveal the structures of the first monolayer (UPD layer) and bilayer cobalt deposited on the Pt(111) electrode. The bias voltage and feedback current were 100 mV and 1 nA, respectively.
Figure 5. In-situ STM images collected consecutively at −0.72 V in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4, revealing the deposition process of bilayer Co. Panel f highlights a boundary (broken line) between the second and third Co adlayers. This area is the same as that marked by square in panel b after multilayer Co was deposited. The bias voltage and set point current were 350 mV and 1 nA, respectively. Scan size: 150 nm (a−e) and 50 nm (f).
deposited on Pt(111) in a vacuum.4 Thus, STM imaging operated under different conditions probed different electronic states of a sample. Close examination of the structure of the Co monolayer reveals a hexagonal array with two nearest neighbors separated by 0.31 nm, which is substantially larger than the ideal value of 0.255 nm anticipated for Co adatoms. One way to explain this result is that this structure was a UPD layer of Co, which usually acquired a partially charged state. This view bears out from the fact that it was observed at potential more positive than the Nernst potential of Co2+ and prolonged potential holding at −0.45 V would not produce more Co deposit on the electrode. The structure seen here differs greatly from that seen by STM in a vacuum, which shows mostly an epitaxial Co monolayer with adatoms occupying mainly fcc hollow sites and occasionally hcp and other sites.4 In light of the partially charged state of metal adatoms deposited via UPD,19 anions are frequently needed to compensate for the charge particles such as Co2+ at the electrode. As the STM results described above were obtained in the sulfate solution, the capping anionic layer was likely sulfate or bisulfate. It is peculiar to note that the nearest-neighbor spacing seen in this honeycomb pattern (Figure 3b) was merely
0.31 nm, much smaller than that (∼0.46 nm) of the typical (√3 × √7) bisulfate structure observed on the (111) plane of Au, Pt, Rh, Pd, etc.20,21 The rather small intermolecular spacing of 0.31 nm can be reconciled by proposing that a CoOH was produced on the Pt(111) electrode. STM results shown in Figure 3 are similar to those reported with a CoO bilayer generated by oxidation of Co adlayer deposited on Pt(111) in a vacuum.22 We propose that a layer of Co adsorbed on Pt(111) and an OH layer sitting atop the Co layer. Both layers were hexagonal with nearest-neighbor spacing of 0.31 nm. Given the unlike lattice constants between Co adlayer (0.31 nm) and Pt substrate (0.278 nm), Co adatoms expectedly occupied different sites and thus yielded the long-range intensity modulation seen by the STM (Figure 3b). 3.2.2. Anion and Potential Effects on the Structure of the First Co Layer. The effect of adding chloride on the honeycomb Co monolayer was examined next. Being affected greatly by the Pt substrate, the first Co adlayer could exhibit the most drastic change of physical properties such as the magnetism.23 In electrochemistry the first Co layer was deposited via the UPD process, which usually involved anions to compensate the charged metal adatoms. One of the most commonly used components in electrodepositionchloridewas added to the 13893
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Figure 6. High-resolution STM image collected with bilayer (a, b) trilayer (c) Co deposit on Pt(111) in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4. Two moiré patterns misalignment by were observed on a terrace. Panel b highlights a hollow mound. Lines marked in panel c indicate mismatch domains. The protrusion seen in the middle of (c) is likely a nucleus for the fourth Co layer. The bias voltage and set point current were 250 mV and 2 nA, respectively.
nucleated, yielding a rather smooth Co deposit. Figure 5f shows a 50 × 50 nm2 STM scan revealing a boundary between the second and third layers of Co. The broken line marked there denotes a section of the monatomic step edge seen in Figure 5a−c. However, STM imaging did not discern any distinct defect between these two domains. The area on the left was higher than the right by 0.02 nm, which corresponds to the difference of the physical size of Pt and Co atoms. The STM tip could block mass transfer of Co2+ toward the electrode, resulting in delay in Co deposition. A few high-resolution STM images are presented in Figure 6 to reveal the internal structure of the second and third layers of the Co thin film, which exhibited clear moiré patterns (Figure 6a,b). The long-range intensity modulation undulated close to the ⟨110⟩ direction of the Pt(111) electrode in a periodicity of 3.1 nm. Most mounds in this moiré pattern were 0.05 nm higher than valleys, although hollowed mounds highlighted in Figure 6b were also noted. The former is more typical for Co and Ni deposited on Au(111) and Pt(111) electrodes,7,11,28 but hollow mounds were also observed with Co deposited on Pt(111) in a vacuum.4 This moiré pattern stemmed from the a 7.9% reduction of the in-plane lattice constant of the second Co layer, as compared with the first Co layer. This model likely holds for the moiré pattern observed in this study. Defects in the Co film were always present; for example, two domains misaligned by 10° are delineated in Figure 6a. Figure 6c shows a similar moiré structure seen with the third Co layer. Mounds were still aligned close to the ⟨110⟩ directions of Pt(111) with two neighboring mounds separated by 3.5 nm, which is notably larger than the 3.1 nm found with the second layer of Co. These values were used to calculate the in-plane lattice constant via the moiré formula, which yielded 0.257 and 0.256 nm, which are both less than 1% larger than the idea value of 0.255 nm determined for bulk Co. All mounds exhibited the same intensity of 0.022 nm, as compared with the 0.05 nm observed for the second layer. The particularly bright spot seen in the middle of Figure 6c is ascribed to a nucleus of the fourth layer of Co, forming preferentially at a protruding mound. Lines are marked in Figure 6c to indicate a misalignment defect in the moiré pattern. This moiré structure was important to guide Co deposition from the second all the way up to the eighth layers. As stated, nucleation commenced preferentially at mounds, followed by adding more Co atoms at the perimeters to nuclei, as reported in our earlier study of Co deposition on Pt(111) in perchlorate media.14 The amplitude in the corrugation height and periodicity of the Co moiré patterns varied with thickness. Two Co planes seen by the STM differed by 0.22 nm in height,
sulfate solution to see how it would affect the structure of the Co monolayer assuming the honeycomb structure. Resultant STM images are shown in Figure 4. Despite the smooth surface morphology seen Figure 4a, no ordered structure was observed upon switching to higher resolution scans as shown in Figure 4b. In other words, the prominent honeycomb pattern described above was no longer present. Here we propose that chloride displaced coadsorbed anions of (bi)sulfate or OH, thus yielding drastic changes in the lateral structures of the adlayer. It seems likely that chloride interacted more strongly with Co than OH− (or HSO4−), although the CoCl bilayer was disordered. Lattice mismatch between adlayer and substrate is frequently cited as the most important factor in guiding the growth of thin film.20 To our knowledge, many metal oxides such as CoO,22 FeO,24 and CeO25 form ordered structures on the Pt(111) substrate. In the case of metallic halides, CuCl is known to be adsorbed in highly ordered structure on Pt(111).26 Although Cu+ and Co+ have roughly the same diameters, crystal structures of CuCl and CoCl could be different enough to yield different spatial structures at the electrified interface. This view is in line with a number of reports addressing the effect of anion on the structure of metal UPD.26,27 In spite of the lack of ordered packing in the first Co adlayer, the second Co layer was ordered, as revealed by Figure 4c. This structure had a long-range intensity modulation or a moiré pattern in a periodicity of 3−3.2 nm. Details of this system are already reported.11 3.2.3. Bulk Cobalt Deposition. Shifting the potential negatively to −0.72 V in 0.1 M K2SO4 + 1 mM H2SO4 (pH 3) + 10 mM CoSO4, yielded marked changes at the Pt(111) electrode. As revealed by Figure 5, protrusions emerged on terraces (solid circles) and at lower ends of steps (marked by dotted circles). These features are ascribed to nuclei of the second Co adlayer. In particular, the one at the corner of the upper terrace grew much faster than the others, yielding a notable patch in 110 s (Figure 5b,c). Meanwhile, nucleation of the third layer of Co occurred preferentially at the lateral boundaries between the Pt substrate and Co deposit (marked by arrow D in Figure 5c). The inset in Figure 5c highlights the moiré pattern seen with the second Co layer. At this stage, Co deposition appeared to occur preferentially at the step edge, along with nucleation of the third layer. The former process could result from a high density of kink sites at the step edge (marked by arrow K), which facilitated interlayer mass transfer for incoming Co adatoms. Expanding islands eventually coalesced with their neighbors to form a full monolayer of Co (Figure 5d,e). The second Co layer deposited on terrace was nearly 90% completed before the third Co layer 13894
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Figure 7. In-situ STM images obtained at −0.74 V in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4, revealing triangular texture of Co deposit amounting more than 8 layers thick on Pt(111). As revealed by the marked arrows, edges of triangles were roughly aligned in the close-packed directions of Pt(111). Trilayer islands are highlighted in panels b and c. The bias voltage and set point current were 100 mV and 1 nA, respectively.
f. Given the observation of twinned fcc packing in the Co film (Figure 7), it is plausible that step dislocations could derive from variations in the packing habits of the Co deposit. These defects further developed into screw dislocations or spirals seen in Figure 9c−f with two of these indicated by two arrows in Figure 9c. Both spirals were rotated counterclockwise, but it could rotate the other way. Spirals could comprise singular or double threads seen with the upper and lower ones, respectively. The upper one was annealed after two Co layers were deposited, but the lower one persisted in this 13 min imaging period. The triangular pyramids are also seen with Co deposited on Pt(111) in vacuum,4 from which fcc packing in the Co metallic deposit is determined. The packing habit of Co thin film produced by electrodeposition in aqueous solution is found to vary with pH, overpotential, temperature, etc. The preference toward fcc packing seen in pH 3 solution could result from adsorption of hydrogen or formation of metal hydride. It is however difficult to substantiate these possibilities using STM alone. Shown in Figure 10 are STM images acquired with Pt(111) at −0.74 V in chloride solution containing 10 mM CoSO4. The deposition process and atomic structure of the Co thin film were largely the same as those seen with the sulfate solution. However, when the Co film grew to 10 layers thick, the surface morphology was notably different. The presence of multilayer Co (Δz = 0.22 nm) at the same time speaks to the 3D growth, similar to those seen with sulfate medium. All Co layers had evidently rugged perimeters, which contrast sharply with the triangular pyramids seen in sulfate solution. We attribute this disparity to the adsorption of anions on the Co deposit, which can determine the orientation of steps of Cu single crystal electrode. Chloride adsorbed on Cu(111) and Cu(100) electrodes carve out steps in the ⟨121⟩ and ⟨110⟩ directions, respectively, which mark the close-packed direction of adsorbed chloride.30−33 In contrast to chloride adsorbed on Cu electrode, where STM imaging results in ordered chloride adlayer, we were unable to resolve atomic structures on the multilayer Co deposit at −0.74 V. However, judged from the very different step morphology seen in sulfate and chloride media, one would reason that (bi)sulfate and chloride were adsorbed on the Co film, although they were not “seen” with the STM. Although the potential of −0.74 V may be too negative to have anions adsorbed on the Co film, one needs to consider the strength of adsorption. For example, chloride adlattice was imaged on Cu(111) at potential as negative as −0.55 V in 10 mM HCl. Similar to those found in sulfate media (Figure 8), spirals were observed in the Co film 10 layers thick, as revealed by
which is the ideal value for monatomic step height of metallic Co(111). Starting from the ninth layer, STM imaging revealed well-defined triangular mesas with atomically smooth top plane (Figure 7a). Two kinds of equilateral triangles rotated by 180° to each other were observed, as denoted by the triangles marked there. They appeared ∼0.66 nm high as they were stacks of trilayer Co, which agrees with the three-tier morphology seen at the lower end of the mesa in Figure 7b and similarly at the left end of mesa seen in Figure 7c. These triangular features are observed in a number of studies4,18 and explained by the preferential formation of certain atomic structures between adatoms and the fcc(111) substrate. This is illustrated by the schematics shown in Figure 8a,b, where triangular islands comprising hexagonal arrays are
Figure 8. Schematics showing triangular adlayers (in red) stacking on fcc(111) planes (in blue), yielding (100) and (111) oriented microfacets (panels a and b). These triangles were rotated by 180° with respect to each other.
stacked on a fcc(111) planes, yielding (100) and (111) oriented microfacets between the adlayer and the substrate. These two preferred structures then yield triangular mesas rotated by 180° with respect to each other, as seen in Figure 7a.29 These features are similar to those observed with Co film deposited on Pt(111) in a vacuum, which is determined to characteristics of twinned fcc stacking sequence (ABC... vs CBA...).4 Holding potential at −0.74 V substantiated Co deposition, causing triangular mesas to grow laterally before they coalesced to form larger polygonal mesas with sharp edges aligned roughly in the ⟨110⟩ directions of the Pt(111) substrate, as revealed by Figure 9a. Most steps seen here were one Co atom high (Δz = 0.22 nm). With more than five layers seen simultaneously on the surface, deposition of Co seemed to proceed in 3D at this stage. In addition to step defects, dislocation defects, as highlighted in Figure 9b, were also found in the subsequently obtained STM images shown in Figure 9c− 13895
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Figure 9. A series of in-situ STM images revealing the deposition process of a Co film >10 layers in thickness. The acquisition time of each image with respect to that of panel a is indicated. The potential pf Pt(111) was held at −0.74 V in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4. Panel b highlights one of the slip planes found on the Co deposit. This kind of defect was responsible for the spiral growth found in the subsequent images c−f. Scan size 100 nm, except (b) 20 nm. The bias voltage and set point current were 100 mV and 1 nA, respectively.
Figure 10. In-situ STM images obtained with Pt(111) at −0.74 V in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM KCl + 10 mM CoSO4. Layered surface morphology is evident in panel a, where at least two spirals were found within this scan area, as shown in panels b and c.
Figure 10b,c. These spirals found at nearby sites rotated in opposite directions. Co adatoms were deposited preferentially at these defects, affording their continuous growth with time. More STM experiments are needed to unravel the formation mechanism of these spirals. Meanwhile, this kind of defect was also noted in copper thin film deposited on Pt(111), as illustrated by the STM images shown in the Supporting Information (Figure s2).
identified by the STM, which produced step dislocations starting from the ninth layer, leading to screw dislocations as the Co deposit grew thicker. By contrast, chloride could be codeposited with the first Co adlayer in disarray. The subsequent deposition up to the eight layers was largely the same as those observed in sulfate media. Step and screw dislocations were also observed in the chloride media, except the step morphology was clearly rugged, whereas it was mostly sharp and well-aligned in the sulfate media.
4. CONCLUSIONS In-situ STM has provided molecular insights into electrodeposition of Co on Pt(111) electrode in pH 3 sulfate and chloride media. The spatial structure of the first Co adlayer generated by the UPD process varied greatly with the anions present in the electrolyte; a highly ordered honeycomb pattern was seen in the sulfate solution, whereas it was rough in the presence of chloride. Regardless of the anion, (bi)sulfate or chloride, in the electrolyte, bulk Co deposit grew in layers assuming hexagonal atomic structure exhibiting moiré characteristics for the first eight layers. The in-plane lattice constant of Co layers decreased progressively with thickness. Strain built up at the Co/Pt interface with the thickness, which directed 3D Co deposition to produce triangular pyramidsan indication of fcc stacking of the Co thin film. Twinned fcc grains were
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ASSOCIATED CONTENT
S Supporting Information *
Figures s1 and s2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel 886-3-4279573; Fax 886-3422-7664 (S.Y.). Notes
The authors declare no competing financial interest. 13896
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ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology of Taiwan (Contract NSC 103-2120-M-008-002).
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