Electron Spectroscopy and Electrochemical Scanning Tunneling

Chapter 19

Downloaded by STANFORD UNIV GREEN LIBR on October 6, 2012 | http://pubs.acs.org Publication Date: February 28, 1997 | doi: 10.1021/bk-1997-0656.ch019

Electron Spectroscopy and Electrochemical Scanning Tunneling Microscopy of the Solid—Liquid Interface: Iodine-Catalyzed Dissolution of Pd(110) 1

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Manuel P. Soriaga , W. F. Temesghen , J. B. Abreu , K. Sashikata , and Κ. Itaya 2

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Department of Chemistry, Texas A&M University, College Station, TX 77843 Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai 980-77, Japan

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This article showcases the unique capabilities afforded by the combi nation of electron spectroscopy, scanning tunneling microscopy, and electrochemistry in the study of complex processes at the electrode -electrolyte interface. The interfacial reaction investigated was the anodic dissolution of a Pd(110) single-crystal surface catalyzed by a single chemisorbed layer of zerovalent iodine atoms. Previous work had shown that dissolution of the two other low-index planes, Pd(100) and Pd(111), altered neither the adlattice structures nor the interfa cial coverages. In comparison, while the anodic dissolution of the Pd(110)-I surface did not lead to changes in the iodine coverage, it resulted in a disordered surface. The in situ S T M work provided valu able insight as to why the initial and post-dissolution structures were not identical. Major advances in the study of the electrode-electrolyte interface at the atomic level were ushered in two decades ago by the integration of traditional electrochemical methods with modern surface-sensitive analytical techniques [1]; such a strategy, now commonly referred to as the ultrahigh vacuum-electrochemistry (UHV-EC) approach, made possible critical correlations between interfacial structure, interfacial composition, and interfacial reactivity [2]. The subsequent development, less than a decade later, of procedures for the fabrication, regeneration, and verification of single-crystal electrode surfaces without recourse to expensive surface science equipment [3] also had a profound influence in the progress of electrochemical sur face science. The recent resurgence in research in surface electrochemistry has been instigated by the evolution of in situ methods [4] that allow the simultaneous imple mentation of electrochemical and surface-analytical experiments; the most promi nent of these techniques is scanning tunneling microscopy (STM) [5]. While the impressive strides in electrochemical surface science brought about by the combination of STM and electrochemistry (STM-EC) may prompt the aban donment of other surface characterization methods, a lesson learned by the pio© 1997 American Chemical Society

In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


19. SORIAGA ET AL.

Iodine-Catalyzed Dissolution of Pd(l 10)

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neers of modern surface science need only be recalled: the sheer complexity of het erogeneous processes cannot be unraveled by just one technique [6], The singular strength of STM-EC lies in its integration with other methods such as those that make possible the determination of interfacial energetics, composition, and elec tronic structures; the combination of STM-EC with UHV-EC serves as an example. The primary motivation of this paper is to illustrate the use and to showcase the power of UHV-STM-EC, even in a less-than-ideal scenario in which the U H V and S T M experiments were performed separately in two different laboratories. The case studied involves the dissolution of Pd(llO) that occurs only when a monolayer of iodine is present on the surface [7,8]. The structural and compositional features of the halogen-metal interface that characterize the adsorbate-catalyzed corrosion were first explored with low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES); initial- and final-state measurements were quite easily obtained. Certain questions, however, such as the mechanism for dissolution, necessitated an in situ technique for answers; it was in this regard that the STM-EC work was prompted. Experimental The experimental procedures specific to the UHV-EC [7] and STM-EC [5c,8] investigations have been described i n detail elsewhere. UHV-EC work, carried out at Texas A & M University (College Station, TX) employed a commercially oriented and metallographically polished, 99.9999%-pure Pd(llO) single-crystal electrode. STM-EC studies, undertaken at Tbhoku University (Sendai, Japan) were done with Pd(llO) single-crystal surfaces prepared from 99.995%-pure polycrystalline Pd wires by the method of Clavilier [3] modified to compensate for the unique chemical prop erties of Pd metal. Commercial instruments were used in the UHV-EC (PerkinElmer, Eden Prairie, MN) and STM-EC (Digital Instruments, Santa Barbara, CA) experiments. Results a n d Discussion E l e c t r o n Spectroscopy. Figure 1 shows current-potential curves for an untreated (clean) and an iodine-coated Pd(llO) facet, formed on a single-crystal bead, in halide-free 0.05 M H 2 S O 4 . As was demonstrated previously [6], the exceedingly large anodic peak at about 1.1 V (vs. RHE) represents the Pd°( )-to-Pd ( q) anodic strip ping that occurs only when interfacial iodine is present; in the absence of iodine, only the passivating oxide layer is formed, as indicated by the solid curve. If the potential applied to the I-coated surface is held below the peak potential, the cur rent, as expected from a material-limited dissolution process, does not decay but remains essentially constant. The data in Figure 1 bear strong similarity to those obtained for Pd(lll)-(V3 χ V3)R30°-I [7a] and Pd(100)-c(2 χ 2)-I [7b], cases for which layer-by-layer dissolution has been demonstrated [8], Such mode of dissolution has been exploited for electrochemical "digital etching" in which disordered P d ( l l l ) and Pd(100) surfaces were rendered atomically smooth by dissolution of the (un stable) disordered domains [9]. It may be mentioned that the reactivity of the Icoated Pd(110) towards Pd dissolution is considerably higher than those of the Itreated P d ( l l l ) and Pd(100) surfaces [10]. Indicated in Figure 1 are the potentials at which the UHV-EC surface characterization and the STM-EC experiments were performed. 2+

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a


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SOLID-LIQUID ELECTROCHEMICAL INTERFACES


-i—Γ /\ /

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UHV-EC *

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Ε ο

0.05MH SO 2

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Pd(110) Pd(110)-I

200 STM-EC 100

ο μ _L 0.5

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E/V v s . R H E

Figure 1. Current density-vs-potential curves in the surface-oxidation region i n 0.05 M H 2 S O 4 for a Pd(110) facet on a single-crystal bead, clean (solid curve) and I-coated (dashed curve). Potential sweep rate, r = 10 mV s" . 1


19. SORIAGA ET A L

Iodine-Catalyzed Dissolution of Pd(110)

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Photographs are shown in Figure 2 of L E E D patterns for UHV-prepared Pd(llO) before (A) and after (B) immersion in 0.05 M H 2 S O 4 containing 1 m M K l . Figure 2(A) is the familiar ( l x l ) L E E D pattern for the clean, unreconstructed Pd(110) surface. The L E E D pattern in Figure 2(B) for the KI-immersed Pd(110) surface has been referred to as a distorted-hexagonal or pseudohexagonal structure that results when the adsorbate coverage Θι (s Γι/Tpd, where Γ is the surface pack ing density expressed in units of mole cm ) exceeds 0.5 [11]. When the latter adlattice is heated above 700 K, a fraction of the interfacial iodine is desorbed and a stable Pd(110)-c(2 χ 2)-I adlattice of coverage Θι = 0.5 is formed; as detailed elsewhere [12], the Pd(110)-c(2 χ 2)-I structure can also be produced directly, under electro chemical conditions, without the ex situ heat treatment. For the present manu script, only the anodic dissolution of the easier-to-form Pd(110)-pseudohexagonal-I adlattice was investigated. Figure 3 shows a real-space model of a (high-coverage) Pd(110)-pseudohexagonal-I interface; the actual coverage depends upon how dis torted the overlayer structure is from perfect hexagonal symmetry [11]. The L E E D pattern obtained after the removal of the equivalent of 10 mono layers (ML) of Pd, each monolayer represented by the Faradaic charge (0.30 mC) required for the 2-electron anodic dissolution of 9.36 χ 1 0 Pd(110) atoms cm" , is shown in Figure 2(C). The degradation of the post-dissolution L E E D pattern is ob vious. Thus, in contrast to the cases of Pd(100)-c(2 χ 2)-I and Pd(lll)-(V3 χ V3)R30°I [6], the iodine-catalyzed corrosion of Pd(110) results in a disordered adlayer. l b ascertain whether or not the disorder is due to removal of the chemi sorbed iodine during the dissolution process, Auger electron spectra were obtained prior to and after the 10-ML dissolution. The results are shown i n Figure 4 where it can be seen that, identical to those obtained for the Pd(100)-c(2 χ 2)-I and P d ( l l l ) (V3 χ V3)R30°-I adlattices [6], the iodine coverage has remained essentially un changed. In other words, the final-state disorder of the Pd(110)-I interface is not due to unwanted side reactions that may have led to iodine removal.


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E l e c t r o c h e m i c a l S c a n n i n g Tunneling Microscopy. The results from the in situ S T M experiments are summarized in terms of the (low-resolution) images shown in Figure 5; the top image [Figure 5(A)] was acquired at the start of the dissolution reaction (at the potential shown in Figure 1), whereas Figure 5(B) was obtained after four minutes of dissolution. The STM image of the Pd(110)-I surface prior to the dissolution experiments was similar to that given in Figure 5(A) except that the pits were not present. The pre-dissolution image was characterized by the absence of wide terraces, despite metallographic polishing and thermal annealing, and the predominance of steps that run parallel to the {110} direction. Upon initiation of the iodine-catalyzed dissolution, circular-shaped pits sev eral nm in diameter and about 0.5 nm in depth immediately appeared as shown in Figure 5(A). After four minutes of further dissolution, dramatic changes resulted in the S T M image, as can be viewed i n Figure 5(B). The notable changes are as fol lows: (a) The pits, originally minuscule, have, individually or collectively, increased considerably in size, (b) The shapes of the pits have been transformed from circular to rectangular with straight edges (steps) that run parallel to the {110} and {100} directions, (c) The rectangular terraces are preferentially elongated towards the {100} direction, (d) New (smaller) pits are formed at the bottom of the rectangular pits; consequently, the surface becomes progressively roughened, as was initially indicated by the L E E D data [Figure 2(C)]. The changes in the S T M images i n Fig ures 5(A) and 5(B), in particular the formation of preferentially-elongated rectan gular pits, point to a selective dissolution mechanism.


SOLID-LIQUID ELECTROCHEMICAL INTERFACES


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Figure 2. Low-energy electron diffraction (LEED) patterns for clean (iodinefree) Pd(llO) (A) and Pd(110)-pseudohexagonal-I before (B) and after (C) anodic dissolution of approximately 10 monolayers of Pd(110) surface atoms. Beam energy = 60 eV; beam current = 2mA.



19. SORIAGAETAL.

Iodine-Catalyzed Dissolution of Pd(l 10)

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Figure 3. Real-space model for a (high-coverage) Pd(110)-pseudo-hexagonal-I adlattice; the actual coverage depends upon how different the overlayer struc ture is from perfect hexagonal symmetry [11].

AES of Pd(110)-Pseudohexagonal-I τ £ Έ

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ST C o

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Electron Spectroscopy and Electrochemical Scanning Tunneling

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