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In Situ Atomic Force Microscopy of the Electrochemical Dissolution of a Copper Grain H. S. O. Chan, P. K. H. Ho, L. Zhou, N. Luo, S. C. Ng, and S. F. Y. Li* Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore Received October 23, 1995X Dynamic evolution of the submicrometer surface morphology of a copper grain undergoing electrodissolution in the “electroetching regime” has been monitored by in situ atomic force microscopy. Images obtained for a nominal current density of 400 µA cm-2 indicate rapid etching into the surface to reveal well-defined crystallographic faces. The thermodynamically most stable {111} facets develop first, forming the initial primary dissolution faces; but as dissolution progresses, they are replaced by stably dissolving {211} and {221} facets. Hence, surface morphology can either be thermodynamically or kinetically controlled. Local current density is distributed inhomogeneously at the submicrometer level, being 1 order of magnitude larger than the global average at some locations. Identical crystallographic facets do not etch at the same rate and the dissolving facets typically evolve in a complex temporal-spatial manner. This behavior may be related to nonlinear pattern formation. Images obtained for a lower current density of 20 µA cm-2 provide unequivocal evidence of a surface recrystallization phenomenon concurrent with the anodic dissolution process. The surface reordering extends up to the submicrometer length scale and leads to development of smooth facets.

Introduction Metal electrochemical processes such as electrodissolution and electrocrystallization have generated much theoretical interest1 and some important practical applications2 in electropolishing, electroplating, and electrochemical machining. Most of our understanding of such processes at the metal-electrolyte interface has been derived from electrochemical studies, such as voltammetry and chronoamperometry, and spectroscopic techniques, like Raman scattering, reflectance spectroscopy, and laser light scattering. The thermodynamic and kinetic information obtainable is averaged over the entire sampling area and over all sampled grain orientations for polycrystalline surfaces. Such methods are useful in providing a macroscopic understanding without addressing in detail local events and variations. To achieve the latter, surface imaging techniques have to be employed. Until recently, these have essentially been confined to ex situ electron microscopies which normally yield static information and are incompatible with many in situ studies. Development of the scanning probe microscopies (SPM),3 on the other hand, provides an exciting opportunity for real-time study of surface changes in real spacesin vacuum, air or under liquidsand with the potential for atomic resolution. Copper has been the subject of many studies4 because of its commercial importance and unique position in the electrochemical series. As a result, its surface chemistry and anodic behavior in various electrolytes are relatively well-documented.4 Several SPM studies5 have also been undertaken to clarify some aspects of its surface evolution * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) (a) Winand, R. Electrochim. Acta 1994, 39, 1091. (b) Furtak, T. E. Surf. Sci. 1994, 299-300 , 945. (2) (a) Ullman’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Cambridge, 1987; Vol. A9, p 231. (b) McGraw-Hill Encyclopedia of Science and Technology, 7th ed.; McGraw-Hill: New York, 1992; Vol 6, p 265. (3) (a) Bard, A. J.; Fan, F.-R. F. In Scanning Tunneling Microscopy and Spectroscopy: Theory, Techniques and Applications; Bonnell, D. A. Ed.; VCH: New York, 1993; p 287. (b) Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy: Methods and Applications; Cambridge University Press: Cambridge, 1994; p 468. (c) Sarid, D. Scanning Force Microscopy: With Applications to Electric, Magnetic and Atomic Forces; Oxford University Press: New York, 1994; p 129.

during anodic dissolution at both the nanometer and micrometer levels. Micrometer-scale differential grain etching of polycrystalline Cu (j ) 0.1-11 mA cm-2, H2SO4) has been observed.5a Nanometer-size nucleation and cluster growth during electrodissolution of a Cu polycrystal in HClO4 has been imaged.5b Step-edge and kink dynamics on a Cu(111) single crystal (j ) 1 µA cm-2, HCl) have also been reported.5c In this paper, we focus however on nanometer to submicrometer topology changes accompanying the electrodissolution of a copper grain in H2SO4. We study intermediate current densities of 20 and 400 µA cm-2 which fall under the “electroetching regime”.2 Undertaken at higher overpotentials and higher dissolution rates than conveniently accessible to atomic resolution studies, the present study reveals interesting new behavior and bridges the gap between previous nanometer- and micrometerscale studies. With a large “depth of field”, high spatial resolution, and the ability to repeatedly scan the same area, and under liquid, the atomic force microscope (AFM) is well suited to the study of dynamic processes. This has already been established by in situ studies of the solution recrystallization of organic compounds6 and surface erosion of biodegradable polymers.7 Experimental Section A mechanically-polished polycrystalline Cu foil immersed in 50 mM H2SO4 was oxidized with controlled current steps in an AFM electrochemical cell. In situ electrodissolution images were (4) (a) Jenkins, L. H. J. Electrochem. Soc. 1966, 113, 75. (b) Becerra, J. G.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1988, 33, 613. (c) da Costa, S. L. F. A.; Agostinho, S. M. L. Corrosion 1989, 45, 472. (d) Ehlers, C. B.; Stickney, J. L. Surf. Sci. 1990, 239, 85. (e) Fonseca, I. T. E.; Marin, A. C. S.; Sa, A. C. Electrochim. Acta 1992, 37, 2541. (f) Souto, R. M.; Gonzalez, S.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1994, 39, 2619. (5) (a) Cruickshank, B. J.; Gewirth, A. A.; Rynders, R. M.; Alkire, R. C. J. Electrochem. Soc. 1992, 139, 2829. (b) Stimming, U.; Vogel, R.; Kolb, D. M.; Will, T. J. Power Sources 1993, 43-44, 169. (c) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725. (6) (a) Ohnesorge, F.; Binnig, G. Science 1993, 260, 1451. (b) Hillier, A. C.; Ward, M. D. Science 1994, 263, 1261. (7) (a) Shakesheff, K. M.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Shard, A.; Domb, A. Macromolecules 1995, 28, 1108. (b) Shakesheff, K. M.; Davies, M. C.; Heller, J.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Langmuir 1995, 11, 2547.

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Figure 1. Steady-state cyclic voltammogram for polycrystalline Cu in 50 mM H2SO4; scan rate, 20 mV s-1. obtained in ambient conditions (22 °C) on a TMX 2000 Explorer atomic force microscope (TopoMetrix, Santa Clara, CA) equipped with a 2.3 µm × 2.3 µm × 1.3 µm scanner attached with a Si3N4 cantilever (force constant, 0.032 N m-1) and integral pyramidal tip. The electrochemical cell (0.3 mL) consisted of the Cu foil working electrode (2.25 cm2) as the base, a Pt ring counterelectrode (1.6 cm2), and a home-built capillary Ag/AgCl reference electrode. With the tip under feedback control, the desired current step was applied for a preselected time interval. Images were then immediately acquired in constant-force mode with the working electrode returned to open-circuit potential. Another current step was applied and the cycle repeated until a sufficient number of images was collected. Such a galvanostatic method has the advantage of automatic potential adjustment to maintain a constant redox activity even as the electrolyte composition drifts and alters the rest potential, as sometimes encountered in microvolume AFM cells. Surface preparation of the Cu foil (BDH, 99.9+% pure) working electrode involved mechanical-polishing with 12 µm and 3 µm Al2O3 on a Metcloth polishing cloth mounted on a Buehler grinder-polisher, rinsing copiously with and storing under Millipore water (σ ) 18.5 MΩ cm-1). This procedure consistently gave a mirror-like finish with a typical root mean square roughness (1 µm × 1 µm) of 20-30 nm. AFM examination of chemically-etched surfaces (5:5:2 NH4OH-H2O-H2O2, 30 s) indicated the Cu grain size was typically larger than 10 µm while the average dislocation density intercepting the surface was less than 0.1 µm-2. The electrolyte solution was reagent grade H2SO4 diluted to 50 mM in Millipore water. All emf values are reported vs SCE. Root mean square (rms) roughness values were computed by software (TopoMetrix) from the square root of the power spectral density evaluated over all 200 × 200 pixel points in the image.

Results and Discussion Figure 1 shows a representative cyclic voltammogram for the Cu working electrode in 50 mM H2SO4 at 20 mV s-1. Both the multicycle voltammogram and the initial negative sweep from the rest potential of -100 mV to the hydrogen evolution region do not reveal any peak associated with surface oxide formation.8 The anodically polarized surface may further be free from SO42- adsorption.9 At 400 µA cm-2, the etching rate allows progressive but dramatic changes in surface topology to be imaged over 20 min of total dissolution time (Figure 2A-O). Indexing of the crystallographic etch facets can be performed by detailed dihedral angle analysis, which is facilitated to a large extent by high symmetry of the Cu face-centered cubic structure.10 (8) (a) Cruickshank, B. J.; Sneddon, D. D.; Gewirth, A. A. Surf. Sci. Lett. 1993, 281, L308. (b) Ikemiya, N.; Kubo, T.; Hara, S. Surf. Sci. 1995, 323, 81. (9) Brown, G. M.; Hope, G. A. J. Electroanal. Chem. 1995, 382, 179. (10) Hammond, C. Introduction to Crystallography, rev ed; Royal Microscopical Society: Oxford, 1992.

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The “AFM goniometry” applied was refined from a procedure described earlier.6b Faces were first modeled by plane equations after measuring their directional gradients from AFM height data. Then dihedral angles were computed by geometry for all combination pairs of faces and compared to theoretical values10 expected from the proposed crystal planes. Crystal faces were thus assigned iteratively by fitting the set of experimental face dihedral angles, starting with low Miller-index planes. A self-consistent set could be achieved. From repeated measurements and consecutive images, uncertainties in the directional gradients and experimental dihedral angles are estimated to be (1° and (2°, respectively. The derived (hkl) planes agree with all measured dihedral angles to within this experimental uncertainty and are indicated in parts G and H of Figure 2.11 From a survey of AFM height data, we have also determined the effective apical angle of the tip to be 70°. The measured gradients of all indexed faces lie well within this limit. This verifies that no error has been introduced by tip-geometry effects. From the internal consistency, the imaged area is inferred to lie within a single Cu grain oriented as shown in Figure 2P. A grain boundary can be seen to the left of images from Figure 2G onward. Within the first 2 min of electrochemical action (Figure 2 A-E), the specimen surface is considerably roughened by the development of large coarse features. The rms roughness increases steadily from 18 nm at t ) 0 to 83 nm at t ) 2 min, as shown in Figure 3A. This probably arises from the removal of an amorphous overlayer12 followed by selective etching of exposed high-index surfaces. By 4-6 min (Parts F and G of Figure 2), the exposed facets become well-defined and are readily identified as {111} close-packed planes. The characteristic equilateral triangular faces and solid square pyramids,10 first noticed in Figure 2D, are fully developed by then and they constitute the primary dissolution sites. Further electrochemical action (Figure 2 H-O), however, dissolves the {111} faces to give a different set of facets. AFM goniometry indicates that they are best assigned to {211} and {221} planes. These less close-packed planes have higher surface energies13a and are derived from {111} planes by a miscut angle of 19.5° and 15.8°, respectively.12 Calculations based only on nearest neighbor interactions show that the group of {100}, {211}, and {221} planes have ca. 14-16% higher surface energies than {111}.13a They form the next most stable group of low-index planes after {111}. For comparison, the {110} planes are some 22% higher in energy. Therefore, while facet development is thermodynamically controlled in the early stages, kinetic effects take over as electrodissolution progresses. The mechanism triggering the changeover in control is not clear. One possible explanation is that the dissolution may have crossed a low-angle tilt boundary10 between t ) 6 min and t ) 8 min. Continued dissolution could be sustained more easily at these less close-packed surfaces on account of the lower coordination number of the exposed atoms. This may be partly understood by analogy with the predominence of 〈211〉 rather than close-packed 〈110〉 step edges on the dissolving (111) surface of anodically (11) Typical dihedral angles measured by AFM goniometry (theoretical values for face-centered cubic structure), in degrees: Figure 2G, (111)∩(11 h 1) ) 109 (109); (111)∩(1 h1 h 1) ) 72 (71); (1h 1h 1)∩(11h 1) ) 114 (109) Figure 2H, (211)∩(1 h 21) ) 99 (100); (211)∩(12 h 2) ) 106 (106); (211)∩(2h 1h 2) ) 68 (66); (12 h 2)∩(211) ) 106 (106); (12 h 2)∩(1 h 21) ) 68 (66); (12h 2)∩(2h 1h 2) ) 118 (116). (12) Villegas, I.; Ehlers, C. B.; Stickney, J. J. Electrochem. Soc. 1990, 137, 3143. (13) (a) Blakely, J. M. Introduction to the Properties of Crystal Surfaces; Pergamon Press: Oxford, 1973, p 14. (b) Sangwal, K. Etching of Crystals: Theory, Experiment and Application; Elsevier Science Publishers: Amsterdam, 1987; p 85.

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Figure 2. (A-O) 1000 nm × 1000 nm × 950 nm AFM micrographs showing progressive dissolution of a Cu grain in 50 mM H2SO4. The images are presented unfiltered in stereographic projection (viewing latitude 40°) with light illumination from the left to enhance relief contrast. Values beside figure labels indicate dissolution time. Current density is 400 µA cm-2. Applied potential varies between 20 and 200 mV. A small drift occurred between the acquisition of (J) and (K). The images thereafter can be brought back to registry by noting that the (2h 1 h 2) and (211) faces have appeared further right. Crystal face orientations have been indexed by “AFM goniometry” and are given in (G) and (H). (P) Approximate grain orientation relative to scanning axes. The location of the grain boundary (- - -) is more evident in a zoom-out scan which is not shown.

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Figure 3. Variation of rms surface roughness with dissolution time: (A) current density, 400 µA cm-2; image size, 1 µm × 1 µm. (B) current density, 20 µA cm-2; image size, 207 nm × 207 nm.

polarized Cu.5c The rarely observed 〈110〉 step edges retreat quickly to form 〈211〉 step edges, as a result of preferential dissolution from the lower coordination sites at the protruding corners.5c This preferential facet development is clearly related to anisotropic etching of the various crystallographic planes as a result of an interplay between thermodynamic and kinetic factors. The phenomenon is well-known in the literature.13b Dissolution rate differentials have been shown in general to be highly sensitive to etching conditions, such as composition, temperature, and hydrodynamic flow variations of the etchant, as well as the chemical nature of the substrate and the resulting dissolution rate.13b In an early two-circle reflecting goniometric study on Cu single crystals electroetched in 0.2 M CuSO4, 10 µA cm-2, near {100} and {210} facets were preferentially developed after several days.4a Electrodissolution of the large central subcrystallite with (211) and (1 h 21) faces (Figure 2H) shows the process to occur inhomogeneously even on crystallographically equivalent planes. The (211) face retreats nonuniformly along the [2 h1 h1 h ] direction to give a stepped (Figure 2I) and then a polyterraced appearance (Figure 2 J-L). The (1h 21) face, on the other hand, retreats at the same rate as the (211) face between 8-10 min of total dissolution time but becomes “dormant” for the next 4 min and resumes activity again after 14 min. Between 16-18 min, the (211) face becomes “dormant” instead while the (1 h 21) face etches away more rapidly so that the cube-like appearance (Figure 2L) evolves to a cuboid-like appearance (Figure 2M). Other exposed faces, including another (211) face

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at the bottom right of the images, are largely “dormant” for most of this time. This apparently irregular temporal and spatial evolution is interesting because it cannot be fully explained by effects relating solely to crystallographic orientation or geometry of the receding facets, which should lead to a more regular evolution. Although difficult to characterize properly, it is somewhat reminescent of fluctuations or nonlinear pattern formation that may point to dynamic chaos14 at the metal-electrolyte interface.15 Further work needs to be done to clarify these observations. Between 8 and 10 min, the local anodic current density on both the (211) and (1 h 21) faces is found to be 3 mA cm-2, and between 16 and 18 min, 5 mA cm-2 on the (1 h 21) face.16a These values are an order of magnitude larger than the nominal global current density of 400 µA cm-2 and confirms the inhomogeneity of its distribution at the submicrometer level.16b Differential etching of Cu grains or subgrains at the even larger micrometer level has also been reported.5a A remarkably stable series of more than 15 gigantic steps running approximately along the [1 h 12] direction has also been imaged (Figure 2L onward). Each step is 1080 nm high and 15-55 nm wide and involves an ill-defined “bunching”17 of hundreds of atomic steps, giving rise to a gradual variation in surface orientation across the terrace face. They are formed from the preferential electrodissolution of atoms in stacking fault planes, which have an excess energy owing to stacking errors in the 〈111〉 direction. Formation of etched grooves, each marking an emergent fault edge, thus gives rise to a stepped appearance. Similar gigantic steps are imaged in separate experiments also, in agreement with the known facile formation of stacking faults in Cu. As shown in Figure 3A, the rms roughness increases steadily within the first 10 min and thereafter stabilizes at ca. 175 nm. The substantial final surface roughness is attributed to development of deep features and is not indicative of the roughness on individual facets. In fact, the etched facets have a very low rms roughness (250 nm × 250 nm) of ca. 0.3-0.6 nm. Their essentially smooth topology is somewhat “roughened” by the formation of islands less than 1.5 nm (about 10 atomic layers) high and 20-50 nm wide. Atomic resolution was not achieved in the x,y -spatial directions. Upon completion of the study, a repeat scan performed after a 20 min delay at open-circuit potential did not reveal any observable surface reordering at the submicrometer scale. Zooming out to a 2.3 µm × 2.3 µm scan size did not reveal any square-pattern formation either. This confirms the absence of surface modification by the scanning process itself. However, differential shielding of the electrochemical current density, due to close proximity (ca. 5 µm) of the cantilever and its mount to the substrate while (14) Hilborn, R. C. Chaos and Nonlinear Dynamics: An Introduction for Scientists and Engineers; Oxford University Press: Oxford, 1994; p 489. (15) (a) Gu, Z. H.; Chen, J.; Olivier, A.; Fahidy, T. Z. J. Electrochem. Soc. 1993, 140, 408. (b) Li, W.; Nobe, K.; Pearlstein, A. J. J. Electrochem. Soc. 1993, 140, 721. (16) (a) Computed from the equation j ) (2.7 mA cm-2 s nm-1) r where j is the current density and r the etching rate. The equation is derived considering the unit cell volume for Cu to be (0.362 nm)3 ) 4.74 × 10-29 m3, and containing four atoms, each of which is oxidized to the +2 ion. (b) It is clear that this could not have been caused by current density distortions arising from the cell configuration or the presence of the tip. Further, for a truly isotropic dissolution, the computed local current density is expected to be lower than the nominal global value (based on 2.25 cm2) due to a larger ohmic resistance at the center of the cell and a progressive surface roughening which expands real surface area. (17) Williams, E. D. Surf. Sci. 1994, 299-300, 502.

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Figure 4. (A-E) 207 nm × 207 nm × 80 nm AFM micrographs showing surface reordering of a Cu surface in 50 mM H2SO4. The images are presented unfiltered in stereographic projection (viewing latitude 40°) with light illumination from the left to enhance relief contrast. Values beside figure labels indicate dissolution time. Current density is 20 µA cm-2. Applied potential is -80 mV. Markers X, Y, and Z are explained in the text.

on feedback, cannot be ruled out. Such effects have been observed in anodic polarization studies of Cu in alkaline medium.18 Imaging a fresh surface subjected to a lower global current density of 20 µA cm-2 reveals a nanometer-scale surface reordering concomitant with anodic dissolution (Figure 4). Between 0 and 20 s, dissolution occurs at the head marked X and Y (Figure 4 A,B) to reveal an incipient terrace step. However, in the next 30 s (Figure 4 C-E), a recrystallization occurs instead to extend the terrace (18) Our unpublished results.

step toward the top of the image. By the end of the experiment (Figure 4E), the 70 nm high hillock Z has disappeared, the terrace step formed well-defined facets,19 and its height has fallen by 8.0 nm. Since sufficient faradaic charge to remove only an average of 0.3 nm has passed, the features must have evolved through another mechanism. It is highly plausible that driven by a lowering of surface energy, directed mass transport has (19) On the basis of AFM goniometry, the facets can be assigned either to a (100), (110) pair with [001] edge or a (110), (212) pair with [2 h 21] edge. Dihedral angle, 136° (theoretical 135°).

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occurred concurrently from the hillock and other asperities to the terrace floor to smooth out the topology. This process involves also a simultaneous recrystallization of the amorphous overlayer12 produced by mechanical polishing. Figure 3B shows a general decrease in rms roughness from 13 nm at t ) 0 to 6 nm at t ) 40 s. This contrasts with the observed increase in roughness with prolonged dissolution as a result of the deepening of etch features. Concurrent island nucleation and growth on the atomic scale during anodic dissolution has been observed earlier and has been interpreted in terms of a dynamic equilibrium requiring a dissolution-precipitation mechanism.5b An additional contribution could arise from surface migration of adatoms and clusters20 activated by anodic polarization. The images clearly demonstrate these rapid surface reordering effects can extend up to submicrometer length scale at low overpotential. Conclusion In conclusion, our study shows in situ AFM is effective in tracking nanometer scale surface topology changes and could thus provide new, dynamical information on processes occurring at the metal-electrolyte interface. This (20) (a) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929. (b) Moffat, T. P.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1991, 138, 3224. (c) Liu, C.-L. Surf. Sci. 1994, 316, 294.

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could not be accessed previously by other imaging techniques. AFM goniometry allows the primary dissolution faces in an electroetched polyfaceted surface to be indexed. Anodic dissolution of Cu at moderately high current density leads to preferential development of {111} facets initially. These evolve subsequently into stably dissolving {211} and {221} facets. Local current density is distributed rather inhomogeneously at the submicrometer level, and dissolution of “activated” facets proceeds in an apparently irregular manner. The otherwise smooth facets are roughened by the development of nanometer-high islands. Stable gigantic steps that are related to an array of emergent stacking faults have also been imaged. At a lower anodic current density, a potential-induced mass transport leading to rapid surface recrystallization of the amorphous overlayer has been observed. The recrystallized surface exhibits well-defined facets with low surface roughness. These results demonstrate that significant new insights into chemical and electrochemical processes of theoretical or technological interest can be studied on the nanometer scale in real time by the AFM. Acknowledgment. This work was supported by grants from the National University of Singapore. We also thank L. L. Chua for help with the halftones. LA950918H