J. Phys. Chem. 1995, 99, 14149-14155
14149
Electrochemical Scanning Tunneling Microscopy and Ultrahigh-Vacuum Investigation of Gold Cyanide Adlayers on Au(ll1) Formed in Aqueous Solution Takahiro Sawaguchi, Taro Yamada, and Yutaka Okinaka Itaya Electrochemiscopy Project, ERATO/JRDC, 2-1 -1 Yagiyama-Minami, Taihaku-ku, Sendai 982, Japan
Kingo Itaya* Department of Engineering Science, Faculty of Engineering, Tohoku University, Aoba-ku, Sendai 980, Japan Received: April 28, 1995@
The adsorption of aurous cyanide (AuCN) on the surface of Au( 111) from solutions containing KAu(CN)2 has been investigated using cyclic voltammetry (CV), in-situ electrochemical scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The cyclic voltammogram exhibits a characteristic couple of reversible peaks at +0.15 V vs SCE attributable to the adsorption of AuCN. The in-situ STM study revealed two different structures for the AuCN adlayer, i.e., incommensurate structures of p( 1.15x &R-30") and p( 1.41x 2&R-3Oo) on the positive and negative sides of the peak potential, respectively. The results of ex-situ LEED and AES analyses were consistent with the in-situ STM results. Furthermore, the reversible transformation between the two adlayer structures was consistently observed in potential scans near the peak. These results show that the well-ordered AuCN adlayers are formed at potentials 0.8 V more positive than the bulk gold deposition potential and that the adlayer structures are transformed reversibly at the phase transition peak. This study is believed to represent the first in-situ STM observation of the structures of AuCN adlayers formed from dicyanoaurate(1) anion, Au(CN)2-.
Introduction Understanding the electrochemical and electroless deposition processes of gold is of both fundamental and practical importance.' Gold deposition is currently an indispensable process for the fabrication of electronic circuitry, and the control of the Au deposition processes is essential for obtaining desirable chemical and physical properties of the Au films. Extensive effort has been made toward understanding electrochemical, chemical, and physical behaviors involved in the deposition processes and the Au deposits formed.'i2 Application of new methods of surface science is expected to bring unprecedented knowledge on the mechanism of Au deposition. The techniques in ultrahigh vacuum (UHV), such as low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES),have in general provided valuable information on the surface structures of adsorbates formed iq solution^.^-^ A breakthrough for direct observation of electrode surfaces has been recently achieved by the development of scanning probe methods, such as electrochemical scanning tunneling microscopy (ESTM) and atomic force microscopy (AFM).These in-situ techniques have served as powerful and promising tools to capture atomic-scale structures of the electrolyte-electrode interfaces under electrochemical potential controL6 The electrochemical deposition of Au is generally performed in solutions containing KAu(CN)z. Nakahara and Okinaka7-8 examined electrodeposited gold by transmission electron microscopy (TEM) and found that the gold contained a minute amount of aurous cyanide (AuCN), which is known to form a polymeric structure with infinite linear hai in.^.'^ It was noted that their results support the proposed mechanism of cathodic gold deposition from Au(CN)2-, in which the reaction starts with a chemical step prior to the charge transfer step, involving the formation of AuCN as an adsorbed
* To whom correspondence should be addressed. @
Abstract published in Aduunce ACS Abstracts, August 1, 1995.
0022-365419512099-14149$09.0010
The adsorption of cyanide ions (CN-) has also been studied as it is closely related to fundamental stages of chemical and electrochemical etching and dissolution processes. On Pt(111) surface^,'^-^^ the structure of a cyanide adlayer has been atomically characterized by ex-situ LEED and AESI9 and more recently by STM and infrared reflection-absorption spectroscopy On other substrates, mostly Au and Ag,24-29 several investigators applied infrared spectroscopy and surfaceenhanced Raman spectroscopy (SERS). In those studies, however, ordered structures of the adsorbed cyanide layer in solution were not discussed because the Au and Ag electrodes employed were polycrystalline. McCarley and Bard30recently made an STM investigation using an Au(ll1) substrate in aqueous cyanide solution and reported that adsorbed cyanide enhances the mobility of Au atoms. Thus, the adsorption of cyanide and related compounds on the surface of single-crystal electrodes of Au has not been fully investigated on an atomic scale. Particularly, detailed atomic structures of adsorbed aurous cyanide (AuCN) are still unknown. Our present goal is to characterize the adsorption process of Au(CN)2- and to determine the structure of the adlayer on Au(111) by electrochemistry, in-situ STM, and ex-situ LEED methods. We report here atomically resolved in-situ STM images and LEED patterns of AuCN adlayers. Two different incommensurate structures of p( 1.15x h R - 3 0 " ) and p( 1.41x 2 h R - 3 0 " ) were observed at positive and negative potentials with respect to the characteristic CV peaks found at +0.15 V vs SCE on Au( 111).
Experimental Section Chemicals. The NaC104 solution (pH 6.8) was prepared by mixing equimolar amounts of perchloric acid (61.4 wt %, Cica Merck, Ultrapur grade) and sodium hydroxide (11.O wt %, Cica Merck, Ultrapur grade). Potassium dicyanoaurate (KAu(CN)2, A.C.S. Grade, Kanto Chem.) was used without further purifica0 1995 American Chemical Society
Sawaguchi.et al.
14150 J. Phys. Chem., Vol. 99, No. 38, 1995 tion. All solutions were prepared with ultrapure water (Millipore Milli-Q, > 18 MQ cm). Substrates. For in-situ STM observations, Au single-crystal beads were prepared at the ends of Au wires (99.99% purity) by the method described previou~ly.~' Atomically flat terracestep structures were consistently observed on the (111) facets formed on the single-crystal beads with octahedral configuration. One of the (111) facets was directly used as an STM scanning plane without further polishing. For electrochemical measurements, mechanically polished (111) surfaces were carefully prepared with an angular error within f0.1". For the final treatment in both cases, the single crystals were annealed in a hydrogen-oxygen flame at about 600 "C and then quickly brought into a hydrogen gas stream to cool to ca. 300 "C, followed by immersing into ultrapure water saturated with hydrogen. The electrode was quickly transferred to an electrolyte solution for further investigation with a droplet of water to protect the surface from contamination. For ex-situ measurements, an Au( 111) crystal disk (8 mm in diameter and 2 mm in thickness) with minor-finished surface was used. The crystal was fixed on a sample holder by fine Ta wires placed in two mechanically grooved slits on the edge, which were also used to heat the crystal by applying a dc current. The Au(ll1) surface was cleaned by Ar ion sputtering and annealing at temperatures up to 900 "C until LEED and AES indicated a clean and well-ordered (111) surface.32 Electrochemical Measurement. A PAR 273A (Prinston Applied Research) potentiostat connected to an extemal potential scanner (HAB 111, Hokuto Denko Co.) was used for cyclic voltammetry. The instruments were remote-controlled with an IBM computer. A bright Pt wire was used as a counter electrode. All electrode potentials are reported with respect to a saturated calomel electrode (SCE). All solutions were deoxygenated by bubbling purified N2, and electrochemical measurements were conducted at room temperature (20 f 1 "C). In-Situ STM Measurement. STM images were obtained with an SPI 3600 scanning probe microscope (Seiko Instruments, Inc.) under electrochemical potential control. All images shown here were taken in the constant-current (topometric) mode. Images were slightly filtered using a low-pass filter option of the SPI 3600 software to reduce the noise when necessary. The tunneling tip was made from a pure Pt wire (15 pm in diameter) insulated by molding in a soft-glass capillary. The end (5-15 pm) of the tip was polished by a turning grindstone. The tip electrode was cleaned in concentrated H2S04 just before use. The electrochemical cell used for STM experiments was described previou~ly.~' Ex-Situ Measurement. The ex-situ measurements were performed in an ultrahigh-vacuum system purchased from Omicron Vacuumphysik, Germany, consisting of several chambers where LEED, AES, and STM observations and electrochemical treatments can be carried out. This system was linked to a separate chamber filled with ultrapure Ar gas, containing a Teflon electrochemical cell connected to a solution delivery system and a computer-controlled potentiostat. This setup was described in detail elsewhere.32
Results and Discussion Electrochemistry of Au(CN)z- at Au(ll1). Previous electrochemical investigations of Au(CN)2- were carried out using polycrystalline Au electrodes. To our knowledge, no work using single-crystal Au( 111) has been reported. Figure 1 shows cyclic voltammograms (CV's) obtained at a well-ordered Au( 111) electrode in the presence (solid line) and absence (dashed line) of 1 mM KAu(CN)2 in 0.1 M NaC104. The Au( 111) electrode
100
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,. .._____.._._.__.__ _._...._..._... ,,_....: ~
L .1
-0.5
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1
1 .5
E l V vs. SCE Figure 1. Cyclic voltammograms (CV's) of an Au(ll1) electrode in 0.1 M NaC104 in the presence (solid line) and absence (dashed line) of 1 mh4 KAu(CN)z. Scan rate was 50 mV/s. The CV's were recorded in (a) wide potential ranges and (b) narrow potential ranges.
gave no characteristic peak in the CV in pure 0.1 M NaC104 over the wide potential range between -0.9 and f 0 . 2 V. The anodic peaks at potentials more positive than f 0 . 7 V are due to the oxidation of Au( 111) surface. The cathodic peaks at ca. 0 and f 0 . 2 V correspond to the reduction of oxide layers formed during the positive scan as described previou~ly.~'~ The electrochemical behavior of the Au( 111) electrode changed remarkably when the solution contained KAu(CN)2. In Figure la, the CV (solid line) shows a reversible couple of peaks at -0.65 V, which is due to the gold deposition from Au(CN)2- and the dissolution of deposited gold according to the following electrochemical reaction: Au(CN),-
+ e-
Au
+ 2CN-
The deposition and dissolution peaks increased proportionally with the square root of scan rate, but the ratio of the dissolution peak to the deposition peak decreased at low scan rates. The bulk deposition process of Au is controlled by the diffusion of Au(CN)2- in solution at potentials more negative than -0.8 V. The scan rate dependence of the deposition-dissolution peaks can be explained by the diffusion process of CN- ions in solution. When Au is deposited from Au(CN)2-, CN- ions must be liberated at the electrode surface as shown in eq 1 and expected to diffuse away from the surface. A partial dissolution of Au in the subsequent positive scan is expected to occur by the back-diffusion of CN- ions to the surface. Therefore, most of the CN- ions diffuse away from the surface at lower scan rates, resulting in smaller dissolution peaks. To avoid damaging the well-defined Au( 111) surface, the potential scan was limited to +0.4 V in 1 mM KAu(CN)2/0.1 M NaC104, beyond which the formation and reduction of gold oxides take place as described above. It is interesting to note that the characteristic sharp peaks appear on the Au( 111) at f0.15 V vs SCE in the presence of KAu(CN)2. A magnified CV of the peaks is shown in Figure lb, overlaid on that for a bare Au(ll1) electrode (dashed line). It is seen that the characteristic peaks are very sharp with only a small peak separation. Both anodic and cathodic peak currents increase linearly with the potential sweep rate. These results strongly suggest that the characteristic peaks at +0.15 V are due to a structural transformation of adsorbed species on the Au( 111) surface.
J. Phys. Chem., Vol. 99, No. 38, 1995 14151
Gold Cyanide Adlayers on Au( 111) Harrison and Thompson12 proposed a mechanism involving the formation of "AuCN" species on the Au surface as shown in eq 2. Such adsorbed species is reduced to form the bulk Au (eq 3). Au(CN),AuCN
f
"AuCN
+ CN-
+ e- -Au + CN-
(slow)
c2) (3)
3.
MacArthur" presented evidence for the adsorption of AuCN intermediate at a low overpotential from detailed cyclic voltammetry. At high overpotentials, the direct decomposition of Au(CN)2- to form Au and 2CN- (eq 1) was thought to be the mechanism for the bulk Au deposition. EisenmannIs formulated a reaction scheme from his kinetic study of the deposition that the reaction proceeds with two equilibrated adsorption-desorption processes (eqs 4 and 6) preceding and following the electron transfer step (eq 5): Au(CN),-
* (AuCN),, + CN-
0
2
1
3
Xlnm
(4)
0.2
. I
McIntyre and Peck16observed a distinct cathodic prewave in a voltammogram which was attributed to the reduction of an insoluble film of AuCN. Note that small AuCN crystals as an inclusion were observed at grain boundaries of electrodeposited gold films in TEM e~aminations.~~J~ According to the previously proposed mechanisms described above, it is reasonably expected that (AuCN)~exists on Au(1 11) at potentials more positive than the potential for the bulk deposition of Au. Furthermore, it can be expected that the characteristic peaks at +0.15 V might be due to a phase transition of ordered adlayers of (AuCN)~. Backgroundcorrected charges consumed for the characteristic peaks were determined to be 17.4 and 15.0 pC/cm2 for the cathodic and anodic peaks, respectively. These small values indicate that the peaks are not due to an electron transfer reaction of the adsorbates on Au(ll1). This issue will be discussed later. It must be noted that these sharp peaks at +O. 15 V are observable only on the Au( 111) surface. We found no such sharp peaks with a polycrystalline Au electrode or with other single-crystal surfaces such as Au(100), suggesting that the structure of adsorbed (AUCN)ad strongly depends on the crystallographic orientation of the substrate. In-Situ STM Observation of the AuCN Adlayer. Figure 2a shows an example of high-resolution STM images obtained at +0.2 V vs SCE which is just positive with respect to the characteristic peaks. The image was treated using a low-pass filter to reduce noise. It can be seen in Figure 2a that the two atomic rows, denoted as A and B, are almost perpendicular to each other. The observed adlattice is rectangular and evidently different from the hexagonal lattice of bare Au( 111) surface, in which atomic rows cross each other at an angle of 60" as expected for the surface 3-fold symmetry. Figure 2b,c presents cross sections of the corrugations in the two orthogonally crossing rows A and B. The atomic row labeled A gives the periodic distance of 0.33 f 0.02 nm and the corrugation with an almost uniform height of ca. 0.07 nm. On the other hand, the cross-sectional curve along the atomic row B exhibits a shoulder on the ascending portion of each wave, and the periodic distance is 0.52 f 0.02 nm and the corrugation height ca. 0.19 nm. By considering crystallographic orientations of the single-
0.15 0.1 0.05 n "0
0.5
1
1.5
2
2.5
3
3.5
Xlnm
Figure!2. (a) An in-situ STM top view (3.0 x 3.0 nm2)of the AuCN adlayer at Au(ll1) electrode obtained at 0.2 V in 1 mM KAu(CN)2/0.1 M NaC104. The potential of the tip was 0.125 V. The tunneling current was 10 nA. The superimposed rectangle indicates the p( 1.15x fiR-30') unit cell. (b) and (c) indicate the corrugations along the atomic rows labeled A and B, respectively.
crystal bead employed, the direction of atomic row B is direction, determined as being nearly [Ti21 or the so-called and its periodicity is roughly identical to & times 0.2885 nm, the lattice constant of Au( 111). Note that the corrugation in atomic row B (ca. 0.19 nm) is considerably larger than that in atomic row A or the typical atomic corrugation of 0.03-0.04 nm for the bare Au( 1 11) surface.33 A surface complex such as AuCN might cause a greater response of the STM tip than the surface atoms of clean gold. Nakahara and Okinaka8 concluded from their TEM results that AuCN codeposits in electroplated hard gold. The structure of AuCN is known to be polymeric with infinite parallel linear chains,!)*10 whose crystallographic structure was hexagonal with lattice constants of a = 0.340 nm and c = 0.509 nm. The atomic distances measured in Figure 2b,c are in good agreement with the lattice constants within the experimental error. The two-dimensional adlattice in Figure 2a is identical to the (1070) net plane imbedded in the AuCN crystal. Below, a crucial evidence for the adspecies being AuCN will be described. The atomic structure shown in Figure 2a was consistently obtained over the wide terraces when the electrode potential was scanned slowly between +0.2 and +0.4 V, i.e., on the positive side of the characteristic peaks. The configuration of the atomic rows in different areas of the STM image was almost the same as that shown in Figure 2a. Interestingly, molecular defects were sometimes observed in the AuCN adlayer. Figure 3 shows an example of the STM image of the defects, in which several AuCN molecules are missing in a few different areas
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14152 J. Phys. Chem., Vol. 99, No. 38, 1995
6-
4-
9 G 2-
I
I
I 1 f
r
I
!I
0
-
0
I
I
I
2
4
6
X/nm
Figure 3. An in-situ STM top view (6.0 x 6.0 nm2) of the AuCN adlayer at Au( 1 1 1) electrode obtained at 0.2 V in 1 mM KAu(CN)2/ 0.1 M NaC104. The potential of the tip was 0.125 V. The tunneling current was 7.3 nA.
in the well-ordered AuCN adlayer as indicated by the arrows. The depth of the defects was ca. 0.15 nm on the average. In-situ STM imaging was also attempted at potentials more negative than the peak potential. STM images taken in this potential region suffered a larger background noise in the tunneling current. Figure 4 shows an image acquired at +0.07 V vs SCE. A new adlattice structure can be seen as a deformed honeycomb lattice, which is quite different from that shown in Figures 2 and 3. It is obvious that the AuCN adlayer underwent a change in structural arrangement from rectangular to deformed honeycomb upon the potential scan, namely, a “phase transition”. The periodic distances are 0.48 f 0.03 nm in the direction of the atomic row labeled A’ and 0.97 f 0.03 in the row labeled B’. The atomic row B’lies along the & direction of Au( 11l), and its periodic distance is close to 2& times the Au( 11 1) lattice constant. It should be noted that these two atomic structures changes reversibly from one to the other depending on the electrode potential. When the electrode potential was scanned back to +0.2 V from +0.07 V, the honeycomb adlattice shown in Figure 4 disappeared, and the rectangular adlattice shown in Figure 2a was observed again. The reversible change between the two atomic structures was repeatedly observed upon the potential scans over the phase transition peaks at +O. 15 V. We conclude from the STM observations that the phase transition is a reversible process. For the structure of AuCN adlayers, two mGels were derived for the AuCN adlattices observed at the positive and negative sides of the phase transition peak: p( 1.15 x &R-30”) for the rectangular adlattice and p( 1.41x 2AR-30’) for the honeycomb adlattice, respectively. The corresponding structures are shown in Figures 6c and 8c. These two structures were determined from ex-situ surface analyses (see the next section for details). In the p( 1.15x &R-30°) structure, a rectangular unit cell of 0.332 x 0.500 nm2 is adopted for the AuCN adlattice, and each AuCN is positioned on the corner of the unit cell. Thus, one unit cell consists of one Au and one CN. If the Au( 11 1) substrate is fully covered with the AuCN adlayer with p( 1.15 x &R-30’) structure, the fraction of the surface AuCN monolayer with respect to the number of Au(ll1) substrate atoms, Le., the surface coverage of AuCN, is 0.435. The corresponding rectangle of unit cell was overlaid on the STM image in Figure 2a, which resulted in an excellent match
_:I 0
0
2
4
6
X/nm
Figure 4. An in-situ STM image (6.8 x 6.8 nm2) of the AuCN adlayer at Au( 1 11) electrode obtained at 0.07 V in 1 mM KAu(CN)2/ 0.1 M NaC104. The potential of the tip was 0.125 V. The tunneling current was 11 nA. The superimposed rectangle indicates the p( 1.4 1 x 2&R-3Oo) unit cell.
with the observed rectangular adlattice. On the other hand, from the p( 1.41 x 2&R-30’) structure, a different rectangular unit cell must be considered for the honeycomb adlattice. Four AuCN’s are located on the corners of the unit cell with one inside. When the unit cell, i.e., a 0.407 x 1.OOO nm2 rectangle, is overlaid on the STM image in Figure 4, it also matches the honeycomb structure fairly well. This size of unit cell contains two AuCN groups, and the surface coverage is 0.355 for this structure. The lattice parameters of the two different AuCN structures were determined precisely as described in the ex-situ measurement section. Ex-Situ Measurements on the AuCN Adlayer. Each exsitu experiment was initiated with the characterization of a clean and well-defined Au(ll1) surface, yielding a sharp (1 x 1) LEED pattern and AES signals for Au only (notable peaks at 180,230, and 250 eV) as shown in Figure 5a. A dilute solution of 1 mM KAu(CN)2 only was used for immersion of the electrode to avoid retaining excess KAu(CN)2 and other electrolytes such as NaC104 on the electrode surface. When Au(ll1) was immersed in the solution, the CV between -0.1 and +0.3 V vs SCE showed a feature almost identical to that shown in Figure 1b. Then the Au( 1 1 1) crystal was emersed in ultrapure Ar atmosphere under the potential control at +0.2 V vs SCE and subjected to AES, LEED, and STM measurements. The adsorption of AuCN was evidently seen in AES in Figure 5b, giving signals of carbon at 266 eV and nitrogen at 380 eV. Quantitative estimation of the C and N elements was unfortunately not possible at this time because of a relatively large background level of AES signals. Figure 6a presents a LEED pattern of the adsorbed AuCN on Au( 111) after emersion at +0.2 V vs SCE. The diffraction geometry is complicated by the splitting of the (1 x 1) spots and additional subspots. The clear diffraction spots suggest that the adsorbed AuCN adlayer forms a well-ordered structure on Au( 11 1) surface. This LEED pattern was analyzed quantitatively to determine the lattice structure of the overlayer. The unit cell vectors are presented in Figure 6b, where a,* and a2* are fundamental reciprocal vectors of Au( 111) with an angle
Gold Cyanide Adlayers on Au( 111)
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250
350
J. Phys. Chem., Vol. 99, No. 38, 1995 14153
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Auger Electron Energy lev
Figure 5. Auger electron spectra of the AuCN adlayers on Au( 1 1 1) surface under various emersion conditions From 1 mM KAu(CN)2: (a) immediately after Ar ion bombardment and annealing cycles; (b) after emersion at +0.2 V vs SCE; (c) after emersion at +0.07 V vs SCE. For ex-situ measurements in UHV, a solution containing only KAu(CN)2 was employed to avoid retaining excess electrolyte on the electrode surface.
of 60" between them and bl* and b2* are the superlattice unit cell reciprocal vectors. By averaging the distances measured on several different LEED photographs, Ibl*!/la1* I was evaluated to be 0.755 f 0.005 and Ib2*l/la2*1 to be 0.500 f 0.005. From these values of Ibl*l/lal*l and Ib2*l/la2*l, the real superlattice unit cell vectors are derived as bl = (1.147 f 0.005)al, lb21 = &la21, and bllb2, where a1 and a2 are the fundamental real-lattice vectors. Therefore, Wood's notation for this superlattice should be p( 1.15 x &R-30"). We made LEED observations for more positive emersion potentials up to +0.4 V vs SCE, where the rectangular adlattice as shown in Figure 2a was observed in previous in-situ STM examinations. The LEED patterns obtained were identical to Figure 6a with slightly stronger backgrounds. To verify the LEED analysis results described above, a set of reciprocal lattice points for p( 1.15 x &R-30") was constructed by taking into consideration the 3-fold symmetric domain structure and double scattering of electrons. Figure 6b
a
b
shows the calculated pattern. This pattern completely reproduces the experimental LEED pattern in Figure 6a. Furthermore, by using dAu-Au = 0.2885 nm for the lattice constant of Au( 11l), the notation p( 1.15 x &R-30') is converted to a rectangular unit cell with lattice constants 0.500 nm for the & direction and 0.332 nm for the direction perpendicular to it. This unit cell is in excellent agreement with that observed in in-situ STM images. Figure 6c is a model of the p( 1.15 x &R-30°)-AuCN adlayer in which the relative positioning of Au and CN was determined by referring to the structure of 3-dimensional crystalline A U C N . ~In~this ~ ~model, the AuCN structure is constructed with rectangular unit cells, in which each CN- ion is positioned between two Au' ions along a real vector b2. Along another real vector bl of the unit cell, Aut ions are adjacent to each other and bound to the neighboring CN-. This is actually one rare case in which a monolayer of metal complex compound is formed as adspecies on a solid surface. Atomic resolution of the p( 1.15 x &R-30") adlattice was also achieved in the UHV environment after the same emersion procedure at +0.20 V vs SCE. Three examples shown in Figure 7 are unfiltered STM images taken in UHV. Figure 7a presents the STM image for a flat area of the adlayer with uniform corrugation, indicating that the observed structure of the AuCN was identical to that of in-situ STM. In addition, STM images in Figure 7b,c clearly show some defects in the AuCN adlayer and an intersection of two differently oriented domain boundaries, respectively. The intersection of domain boundaries was not observed clearly in the in-situ STM study, whereas the molecular defects were found as shown in Figure 3. These observations show that the surface complex AuCN adlayer formed a flat lattice of p( 1.15 x &R-30") in a fairly large area in UHV environment, which is stabilized by a rigid planar network of the AuCN adlayer. It can be imagined from the model shown in Figure 6c that this p( 1.15 x &R-30') structure, incommensurate to the substrate Au(l11) along a1 and bl, can exhibit a longer periodicity than bl in the corrugation in STM images. The Au' and CNions along bl are located at slightly different positions with respect to the underlying Au( 11 1) atoms, generating the longer periodicity. Unfortunately, this kind of periodicity was not observed in this UHV-STM study. Although a different or distorted atomic structure was sometimes observed in in-situ STM measurements, it was not resolved clearly. A more C
w w w w Figure 6. (a) LEED pattern of the AuCN adlayer on Au( 1 1 1). Incident beam energy was 48.2 eV. Before transfer to the UHV chamber, the electrode was emersed at 0.2 V vs SCE from 1 mM KAu(CN)2. (b) Calculated LEED pattern of the p( 1.15x &R-30") adlattice. Large open circles, fundamental Au( 1 11) spots; small filled circles, single scattering adlattice spots; small open circles, double scattering adlattice spots. The reciprocal unit cell vectors of Au(ll1) and p( 1.15x&R-3O0) are indicated by ai*, a2* and b,*, b2*, respectively. (c) Illustration of the real lattice of the p( 1.15x &R-30") AuCN adlayer on Au( 1 1 1) with the fundamental real-lattice vectors 81, a2 and the real superlattice vectors bl, b2.
14154 J. Phys. Chem., Vol. 99, No. 38, 1995
Sawaguchi et al.
- 5
- 4 - 3 4 \
- 2
Ei
- 1
-0
Figure 7. STM images (4.8 x 4.8 nm2 frames) of the AuCN adlayer on Au( 1 1 1) observed in ultrahigh-vacuum environment. The electrode was emersed at 0.2 V vs SCE from 1 mM KAu(CN)2 before imaging. (a) Hat portion of the adlayer. The superimposed rectangle indicates the p( 1.15x &R-30') unit cell. (b) Defects observed near a domain boundary. (c) Intersection of two domain boundaries with different domain orientations. The bias voltage between the sample and the tip (W) was +0.1 V (tip positive). The tunneling current was 1 nA.
detailed study will .be necessary to clarify whether the longer range periodicity of corrugation is overlaid on the elementary lattice. Ex-situ measurements were also performed at more negative potentials than the phase transition peak at +0.15 V. Figure 5c shows an AES recorded on an Au( 111) immersed in 1 mM KAu(CN)2 and emersed at +0.07 V vs SCE. It is evident from the signal at 260 eV that K is present on the Au(ll1) surface besides C, N, and Au. It is interesting to note that K was retained on the surface after the emersion at this potential. Because K was not detected on the p( 1.15 x &R-3O0)-AuCN adlayer after emersion at potentials on the positive side of the peak, it is unlikely that it originated from residual KAu(CN)2 which accidentally remained on the surface. The phase transition of the AuCN adlayer might involve a submonolayer amount of K+ ions attached to the honeycomb adlattice of AuCN. Figure 8a shows the LEED pattern of the same electrode. The background in Figure 8a is considerably stronger than that in Figure 6a. This pattern of diffraction spots was specific to the Au( 111) emersed at +0.07 V and obviously distinct from that in Figure 6a. This new pattern was subjected to the same quantitative analysis as described above, which yielded p(( 1.410f0.005) x 2&R-3Oo) 2 p( 1.41 x 2&R-30') for this
adlattice. Figure 8b presents a set of calculated reciprocal lattice points for p(1.41x2&R-3Oo). This pattern is again in complete agreement with the experimental LEED pattern in Figure 8a. It must be noted in constructing a structural model that the p( 1.41 x2&R-30') structure is subject to restrictions based on the extinction rules of diffraction, which specify the space group that the adlattice belongs to. All possible diffraction spots expected for genuine p( 1.41 x 2 h - 3 0 ' ) in Figure 8b are found in Figure 8a. It is then concluded that none of the extinction rules apply for this adlayer. Therefore, in constructing a model for this adlayer, it should be avoided to include glide planes and other higher symmetry elements. Based on the insitu STM images in Figure 4 as well as the dimensions and restrictions given by the LEED analysis, a model shown in Figure 8c was constructed. The p( 1.41 x2&R-3Oo) unit cell is composed of two AuCN's, one of which is positioned slightly off all of the mirror-symmetric positions. Now the structures and compositions of the AuCN adlayers have been confirmed: p( 1.15 x &-30') and p( 1.41 x2 &R-30°) on the positive side and the negative side of the CV peaks at +0.15 V, respectively. The CV peaks will be called the "phase transition" peaks between these two structures. Each CN- in the p( 1.15 x &R-30') structure (Figure 6c) is located between Au' ions, forming a linear chain of AuCN along the direction b2. On the other hand, in the p(1.41 x2&R-30°) structure shown in Figure 8c, the model gives the ordered mangement consisting of independently positioned AuCN's, in which each CN- is bound to Au' at the carbon end and there is a tiny open space at the nitrogen end. However, the p( 1.41 x2 &R-30') was recognized as the honeycomb structure in the in-situ STM study, as shown in Figure 4. It is interesting to compare the STM image with the model of p( 1.41 x 2 &R-3Oo) structure, although the STM images originate from an electronic structure rather than a geometric structure of adsorbates. We believe that the nitrogen end of AuCN might interact with Aul of other two AuCN's closer to the nitrogen. It is obvious that further structural information is needed to determine the exact position of CN-. When the phase transition is brought about by the potential scan, the AuCN adlayer structure is transformed between p( l. 15x &?-30') and p( l .4 l x 2&R-30°), resulting in a change of the surface coverage as described previously. (The surface coverage is 0.435 for p( 1.15 x &R-30') and 0.355 for p( 1.41 x 2&R-3Oo).) Thus, AuCN adsorption or desorption, corresponding to the difference in surface coverage between the two AuCN adlayers, should occur with the phase transition. However, AuCN is known to be insoluble in aqueous solution. To explain the AuCN adsorption and desorption, the AuCN adlayers on Au(ll1) is considered to interact with free CNions in solution. When a well-ordered AuCN adlayer is formed on Au( 111) through eq 4, one CN- ion is liberated from one Au(CN)2- complex. These CN- ions are associated with the adsorption and desorption processes, and a'certain amount of AuCN is attached to or detached from the surface via eq 4 in the phase transition in the Au(CN)2--containing solution. These processes are considered to result in the consumption of the charges associated with the phase transition peaks, 17.4 pC/ cm2 for the cathodic peak and 15.0 pC/cm2for the anodic peak. It is noted from the AES results that potassium (K) exists on the Au(ll1) surface emersed at +0.07 V, indicating that K+ ions are attached to the AuCN adlayer of p( 1.41x2 &R-30'). The existence of K+ ions is also interesting for complete understanding of the surface structure and composition of the AuCN adlayer in solution, as well as the positioning of
J. Phys. Chem., Vol. 99, No. 38, 1995 14155
Gold Cyanide Adlayers on Au( 1 11)
a
b
C
Figure 8. (a) LEED pattern of the AuCN adlayer on Au( 111). Incident beam energy was 52.3 eV. Before transfer to UHV chamber, the electrode was emersed at 0.07 V vs SCE from 1 m M KAu(CN)2. (b) Calculated LEED pattern of the p( 1.41 x2&R-3Oo) adlattice. Large open circles, fundamental Au( 111) spots; small filled circles, single scattering adlattice spots; small open circles, double scattering adlattice spots. The reciprocal unit cell vectors of Au( 111) and p(1.41 x2&R-30°) are indicated by a!*, a2* and bl*, b2*, respectively. (c) Illustration of the real lattice of the p(1.41 x 2 h - 3 0 " ) AuCN adlayer on Au( 111) with the fundamental real-lattice vectors 81, a2 and the real superlattice vectors b!, b2.
CN- ions as mentioned above. The CN- ions described above might. be adsorbed or attached on the AuCN adlayer with p( 1.41~ 2 h R - 3 0 structure, ~) resulting in the formation of ion complex with K+ ions to preserve electroneutrality of the surface. The larger background noise observed in the in-situ STM imaging and the LEED analysis might be due to random positioning of the K+ ions. Further quantitative information is needed to specify the electrochemical reactions associated with the phase transition peaks and to elucidate the entire reaction mechanism. A more detailed study is now in progress.
Conclusions It has been demonstrated by in-situ STM and ex-situ UHV analyses that two incommensurate surface complex AuCN adlayers are formed on Au( 11 I ) in solutions containing KAu(CN)2 at potentials near the phase transition peaks at +0.15 V vs SCE. The AuCN structures are p(1.15xfiR-30') and p( 1.41~ 2 f i R - 3 0 ~in) the positive and the negative potential regions with respect to the phase transition peaks, respectively. Formation of these monolayer networks of the AuCN has been confirmed even at potentials as much as 0.8 V more positive than the bulk gold deposition. Furthermore, these two structures were repeatedly observed during the potential scans over the phase transition. It has been found that the AuCN adlayer undergoes a reversible structural change in atomic scale depending on the electrode potential. It is considered from our results that the phase transition accompanies the incorporation of cations into the p(1.41 x 2 h R - 3 0 ' ) adlattice. This is believed to be the first study of in-situ formation and structural change of the surface complex AuCN adlayer produced from dicyanoaurate(I) anion, Au(CN)*-.
Acknowledgment. This work was supported by Itaya Electrochemiscopy Project ERATO/JRDC. References and Notes (1) Schmid, G. M.; Curley-Fiorino, M. E. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1975; Vol. IV, Chapter IV-3. (2) (a) Okinaka, Y.; Osaka, T. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: Weinheim, 1994; p 55. (b) Okinaka, Y. In Proceedings of the Symposium on Electrodeposition Technology, Theory and Practice; Romankiw, L. T., Tumer, D. R., Eds.; The Electrochemical Society: Pennington, NJ, 1987; ROC.Vol. 87-17, p 147.
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