11995
J. Phys. Chem. 1993,97, 11995-11998
Observation of Intermediate CuCl Species during the Anodic Dissolution of Cu Using Atomic Force Microscopy Jack Y. Josefowicz,’ Like Xie, and Gregory C. Famngton Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6391 Received: June 1, 1993; In Final Form September 4, 19938
Electrochemical oxidation and reduction reactions of Cu were studied in chloride-containing media by observing changes to the surface morphology of graphite and Cu working electrodes using in-situ electrochemistry atomic force microscopy (ECAFM). AFM images combined with cyclic voltammograms identified three overpotential ( q ) regions corresponding to the anodicdissolution of Cu in low-concentration (0.002 M HC1) chloride solutions. At low q, “the Tafel region”, AFM images showed the dissolution of Cu, which was accompanied by an exponential increase in current density. At higher q, the current-limiting region, AFM images showed the growth of CuCl crystals, which was accompanied by an abrupt decrease in current density. As q was increased in the currentlimiting region, AFM images showed the simultaneous electrodissolution of both CuCl and exposed Cu, which suggested electrochemical mechanisms involving both the formation of (CuClz)-, which reacts to form Cuz+ and 2C1-, and the direct conversion reaction of Cu to C U ~ + .
Introduction Copper is a metal used widely because of its good resistance to corrosion caused by sea water combined with mechanical workability, excellent electrical and thermal conductivity, and ease in soldering and brazing. Vast quantities of Cu and Cu alloys are used in ship machinery and fittings in power stations, where one of the most important applicationsis for tubular-type heat exchangers. Due to its extensive use in chloride solutions, the electrodissolutionof Cu has received considerableattention.’“ In an early report by Cooper and Bartlett,’ the anodic dissolution of Cu at low overpotentials(q) in chloride-containing solutions was described by a mechanism where C1- is adsorbed on the surface and reacts with Cu to form an intermediate CuCl species. In this low-q region, often referred to as “the Tafel region”, it was proposed that C1- reacts with the intermediate to form (CuC12)- ; this reaction and the rate of mass transfer of (CuC12)- were concluded to be the rate-controlling steps. At higher 9 , where the current becamelimiting,CuCl precipitated on the Cu. Cooper and Bartlett, and other investigators,g proposed that in the current-limiting region C1- is mass transfer limited, leading to the precipitation of CuCl on the Cu surface. At even higher q, it was suggested that the dissolution of CuCl on the surface can proceed by a transfer of C1- through pores in the CuCl and that there may also be a reaction which converts Cu directly to Cu2+. The purpose of this study was to analyze the oxidation and reduction reactions of Cu by making in-situ observations of morphology modifications on the working electrode surface by using electrochemistry atomic force microscopy (ECAFM). In this study, AFM images were recorded in the Tafel region as well as in thecurrent-limiting regions. The AFM results provide direct in-situ confirmation for the anodic dissolution models proposed by Cooper and Bartlett and by other investigators.l4 Atomic force microscopy is a relatively new surface probe technique which has the capability of determining the surface structure of both conducting and insulating with atomic resolution in both gas and liquid environments. AFM can be combined with electrochemical cycling techniques Visiting professor from Hughes Research Laboratories, Malibu, CA, and author to whom correspondenceshould be addressed at the University of Pennsylvania. Abstract published in Aduance ACS Absrracrs. October 15, 1993.
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Figure 1. Current versus voltage oxidation-reduction behavior for Cu on a highly oriented graphite working electrode.
(ECAFM) to investigate electrochemical reactions.lOJ This new technique provides a powerful probe for investigations of surface reactions such as passivation, pitting, and electrochemical12J3 and stress corrosion.l4-’6 There are reports of in-situ scanning tunneling microscopyl’ (STM) imaging of electrochemically modified electrode surfaces,1*J9some involving AgCl salt precipitation.2O.21 However, the presence of electric field and Faradaic currents at the STM tip can interfere with and modify an electrode surface.22 Furthermore, when electrochemically altered surfaces contain regions of both insulating and conducting materials, which was the case in our study, the interpretation of STM images becomes uncertain since the tunnelingcurrent between the tip and the sample surface is influenced strongly by the conductivity of the surface. Such problems are avoided with ECAFM since the tip is insulating and does not participate in the electrochemical reactions at the sample surface. Experimental Section The AFM and electrochemical wet cell used in our studies were commercially available models.23 The AFM probing tip is attached to an insulating cantilever which has a low springconstant (-0.1-1 N/m) and is deflected in response to the forces between the probe tip and the sample. The deflections of the cantilever in our AFM were monitored by using a laser deflection method.24 The working electrodes used in this study were freshly cleaved, highly oriented graphite wafers25 and high-purity Cu. These 0 1993 American Chemical Society
11996 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
Josefowicz et al.
Figure 2. (a) AFM image of a freshly cleaved graphite working electrode surface at the start of the CV scan. (b) AFM image showing the onset of Cu electrochemical deposition on the graphite step defect (q = -80 mV, Figure 1). The deposited crystals were confirmed to be pure Cu (99.5%) by EDS spectroscopy.26 (c) AFM image showing Cu crystals deposited on graphite at the end of the cathodic deposition cycle ( q = 0 V, Figure 1). (d) AFM image showing the growth of crystals at sites previously occupied by Cu. The formation of CuCI, which has a unit cell more than 3 times larger than that of Cu,*' would account for the increased crystal size. (e) AFM image of the graphite surface at q = 120 mV showing that the size of the crystals began to decrease as the potential increased. (f) Image of the graphite surface at the end of the CV scanning cycle ( q = 400 mV). The surface was free of all crystalline deposits and had an appearance similar to that at the start of the CV scan (Figure 2a).
electrodes were masked with Teflon tape to produce a microelectrode area of 0.01 cm2. The counter and reference electrodes were Cu. The electrolyte solution was composed of 2 X 10-4 M CuS04 and 2 X 10-3 M HCl in distilled water. Analytical grade
HCI, CuSO4, and deionized water were used. Triangular potential wave cyclic voItammetry'2J3 (CV) was performed with a scan rate of 2 mV/s. A typical applied potential scanning sequence was 0 V, to +400 mV, to -400 mV, and back to +400 mV.
Electrodissolution of Cu
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11997
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Figure 4. Auger spectrum for the surface of crystals deposited on the Cu working electrode during the electrochemicaldissolution of Cu; both CI and Cu are present. Similar results were obtained after Ar ion sputter etching to a depth of -3000 A, indicating that the composition was uniform.
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Figure 3. (a) AFM image of a Cu working electrode at the start of a CV experiment. Note the ‘X-shaped” polishing marks at the center of the image as a reference. (b) AFM image of the Cu electrode surface with q = 240 mV. Pitting of the Cu along the “X-shaped” polishing marks was evidence of the direct reaction converting Cuo to Cu2+. Also shown is thesimultaneousgrowth of large CuCl crystalson thecusurface.
Experimental Results A characteristicCV scan for Cu electrochemistryon a graphite working electrodeis shown in Figure 1. Taking into consideration the scanningrates used for AFM imaging and the CV experiments, we collected each AFM image during a time span of -50 s, which corresponds to a 25 mV segment of the CV voltammogram shown in Figure 1. AFM images showing progressive changes on the graphite surface during a complete CV scan are shown in Figure 2a-f. The startingcondition for a freshly cleaved graphite surface at an overpotential of q = 0 V is shown in Figure 2a. The image shows two crystallographic planes separated by a large step defect. During the first phase of the CV sweep (anodiccycle where 7 was swept to +400 mV and then back to 0 V), the graphite surface remained unchanged. At a cathodicapplied potential of 7 = -80 mV, small Cu crystalsnucleated and grew in the vicinity of the step defect, as shown by the AFM image in Figure 2b. The number and size of the Cu crystals increased as 7 continued to sweep more negatively to -240 mV, which corresponded to a current peak. In one such experiment, energy-dispersive X-ray spectroscopy (EDS) analysis26confirmed that these crystalswere composed of pure Cu (99.5%). The decay in current between -240 and -400 mV was assumed to be due to a Cu ion masstransfer limitation13J4at the interface. The AFM image shown
in Figure 2c correspondsto the graphite surface at the end of the Cu deposition cycle, q = -40 mV, where the current became anodic. Between -40 and 40 mV, it was observed that the majority of smaller Cu crystals located on the smooth graphite surface had dissolved. At 7 = 40 mV, a significant growth of crystals was observed at the remaining Cu crystal locations, as shown in Figure 2d. The growth of these large crystals was accompanied by a significant decreasein anodiccurrent. As 7 increased beyond 80 mV, a broad current peak developed which was centered at 240 mV. As this second current peak formed the crystals began to decrease in size, Figure 2e. Finally, as 7 approached 400 mV all the crystals on the graphite dissolved, leaving the surface in its original starting condition, Figure 2f. To determinethe compositionof the crystalswhich grew during anodicoxidation of Cu, ECAFM experiments were repeatedusing a Cu working electrode (all other parameters were unchanged). An AFM image of the Cu working electrode at the start of the experiment is shown in Figure 3a. Notice the polishing marks in the shape of an X. As the potential increased to 7 40 mV, the same 7 reported above for the large crystal growth on Cu for the case of Cu on graphite, AFM images showed two simultaneous phenomena. One was the dissolution of Cu as seen by the formation of pits along the polishing marks, and the other was the growth of crystals on the Cu surface, as shown in Figure 3b (recorded at q 240 mV). In one such experiment,the CV scan was terminated at 240 mV, the Cu electrode was removed, and the crystalswere analyzedby Auger and X-ray diffraction (XRD) surface analyses.26 Auger analysis determined that the crystals were composed of primarily Cu and C1, as shown in Figure 4. XRD analysis verified that the crystals were composed of polycrystalline CuCl, as shown in Figure 5. Since some portion of the surface corresponded to exposed Cu, the XRD spectrum also included Cu diffraction peaks.
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Summary
ECAFM was used to study the surface morphology changes on both graphite and Cu working electrodes during the electrochemical deposition and dissolution of Cu in chloridecontaining media. AFM images of the working electrode surface were correlated with the cyclic voltammogram during oxidation and reduction reactions. On the basis of the CV scan in Figure 1 and correspondingAFM images, including surface analysis of struc-
11998 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
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20 Figure 5. XRD spectrum of the large crystals which precipitated on Cu showing diffraction lines for CuCl and Cu.
tures identified by AFM, the following conclusionscan be drawn about the Cu oxidation and reduction electrochemicalreactions. The reduction peak (at -240 mV) in the CV was associated with the electrodeposition of Cu, as described by
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CU'+ + 2e CU' For the case of the Cu crystals deposited on the graphite surface, AFM images and CV confirmed Tafel-likebehavior at low q. In the Tafel region, our results agree with previous reports,'" which have suggested that at low q the anodic dissolution reactions are described by Cu
+ Cl-+
CuCl
+ c1-
CuCl
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+e
(cUc1,)-
where CuCl is an intermediate species. At higher q, CV exhibited current-limited behavior and AFM images showed the precipitation of CuCl crystals, which indicated a C1- mass-transfer limitation. At even higher v, AFM images showed simultaneous pitting on exposed Cu, as well as the dissolution of CuCl crystals. The CV showed the development of a second current peak in this q region. Two reactions which were suggested by the AFM and CV results in this q region are the direct Cu conversion reaction
-
CU' CU'+ and the dissolution of CuCl CuCl
+ c1-
-
+ 2e
(cuc1,)-
Other investigators1" have speculated that the dissolution of CuCl may proceed at high 7 because of the formation of pores in the
Josefowicz et al. CuCl in which C1- ions may accumulate and take part in the above reactions. Both of the above dissolution reactions include charge transfer, which would account for the current peak at higher t) (peak at 240 mV in the CV, Figure 1). This investigation showed how in-situ electrochemistry AFM can be applied very effectively to find correlationsbetween surface morphology changes during oxidation-reduction reactions and features in a cyclic voltammogram. The ability to monitor reactionsby observing the electrode surface provides insight that may help in the understanding of reactionmechanismsfor systems such as Cu which include the formation of intermediate species.
Acknowledgment. We thank Bruce C. Schardt for helpful discussions and advice. We thank Richard A. Reynolds and Arthur N. Chester of Hughes Research Laboratories; the U.S. Offices of Naval Research (L.X., G.C.F.); and the National Science Foundation, MRL Program (Grant DMR91-20668), for support of this work. References and Notes (1) Cooper, R. S.; Bartlett, J. H. J. Electrochem. Soc. 1958, 105, 109.
., (2) Bonfiglio, C. H.; Albaya, H. C.; Cobo, 0. A. Cormion Sci. 1973, -4..
1J Y 1 1 I .
(3) Braun, M.; Nobe, K. J. Electrochem. Soc. 1979,126(10), 1666. (4) Pearlstein, A. J.; Lee,H. P.; Nobe, K. Ibid. 1985, 132 (9), 2159. (5) Lee, H. P.; Nobe, K.Ibid. 1986,133 (IO), 2035. (6) Crundwill, F. K.Electrochim. Acta 1992, 37 (IS), 2707. (7) Bennig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lerr. 1986,56,930. (8) Josefowicz, J. Y.; Maliszewskyj, N. C.; Idziak, S. H. J.; Heiney, P. A.; McCaulev. J. P.: Smith. A. B. Science 1993. 260. 323. (9) Drake, B.;Prater,C: B.;Weisenhorn,A. L.;Gould,S. A. C.;Albrecht, T. R., Quate, C. F.; Cannel], D. S.; Hansma, H. G.; Hansma, P. K. Science B. 1989. 243. 1586. (10) 'Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.;Gewirth, A. A. Ibid. 1991,251, 183. (11) Cruickshank, B. J.; Gewirth, A. A.; Rynden, R. M.; Alkire, R. C. J. Electrochem. Soc. 1992,139 (IO), 2829. ( 12) Bockris, J. 0.; Reddv. A. K. N. Modern Electrochemistrv: _ .Plenum: New York, 1973;Vols. 1, 2,(1 3) Rieger, H. P. Electrochemistry; Prentice-Hall: Englewood Cliffs, NJ. 1987. (14) Song,Q.;Newman,R. C.;CottL,R. ASieradzlri, K. J. Electrochem. SOC.1990,137,435. (15) Williams, D. E.;Newman, R. C.; Song,Q.; Kelly, R. G. Nature 1991, 350,216. (16) Sieradzlri, K.; Newman, R. C. Philos. Mag. A 1985, 51, 95. (17) Binnig, G.; Rohrer, H.; Gerber, C. Phys. Rev. Lerr. 1982,49,57. (18) Dovek, M. M.; Heben, M. J.; Lang, C. A.; Lewis, N. S.; Quate, C. F . Rev. Sci. Instrum. 1988,59 (]I), 2333. (19) Oppenheim, I. C.; Trevor, D. J.; Chidsey, C. E. D.; Trevor, P. L.; Sieradzki, K. Science 1991, 254, 687. (20) Morita, S.; Okada, T.; Mikoshiba, N. Jpn. J . Appl. Phys. 1989,28, 536. (21) Sakamaki,K.; Itoh,K.; Fujishima,A.; Gohshi,Y.J.Vac.Sci.Technol. 1990,A8 (l), 525. (22) Thudat, T.; Nagahara, L. A.; Oden, P. I.; Lindsay, S. M.; George, M. A.; Glaunsinger, W. S . Ibid. 1990,A8 (4). 3537. (23) Digital Instruments, Santa Barbara, CA 93117. (24) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1988,53, 1045. (25) Unioncarbide Advanced Ceramics,Cleveland,OH 44101. Graphite, grade ZYH. (26) Batson, P. E. Analytical Techniques For Thin Films; Tu, K. N., Rosenberg, R., Eds.;Academic Pres: New York, 1988;Chapter 9. (27) Handbook ofChemistry and Physics; Weast, R. C., Ed.; CRC Press: Baco Raton, FL,1990.
