8817
J. Phys. Chem. 1995, 99, 8817-8823
Structure of Electrochemically Deposited Iodine Adlayer on Au(ll1) Studied by Ultrahigh-Vacuum Instrumentation and in Situ STM Taro Yamada,? Nikola Batina? and Kingo Itaya*3* Itaya Electrochemiscopy Project, ERATO/JRDC, 2-1-I Yagiyama-Minami, Taihaku-ku, Sendai 982, Japan, and Faculty of Engineering, Tohoku University, Aoha-ku, Sendai 980, Japan Received: January 3, 1995; In Final Form: March 20, 199.5@
The structure of an iodine adlayer electrochemically deposited on a gold( 111) single crystal in potassium iodide solution was investigated as a function of electrode potential by means of ex situ Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and in situ electrochemical scanning tunneling microscopy (STM). All LEED patterns obtained after emersion from 1 mM KI indicated rectangular adlattices which are all described as c(px2/3R-30°), where p continuously decreased from 3 (c(3xd3R-30”) ( 4 3 x 2/3)R30°) to 2.49 with increasing positive potential. The decrease of p indicates compression of the rectangular adlattice only in one direction. In situ STM also revealed the c(px2/3R-30°) adlattices except on the positive side of the potential region, where rotated hexagonal adlattices were observed. The rotated hexagonal adlattices were analyzed in detail by simulation, which indicated isometric compression of the adlattice. These results by ex situ LEED and in situ STM were compared with previously reported data by surface X-ray scattering.
1. Introduction The adsorption of halogens on well-defined gold single crystals is one of the most intensively studied electrochemical processes not only by traditional voltammetry but also by the recently developed microscopic methods. Particular attention has been paid to the adsorption of iodine on Au( 111) because of the variation in its adlayer structure and its relative tolerance to experimental handling. Apart from the electrochemical environment, Cochran and Farrell’ fist investigated dissociative adsorption of 12 molecules onto Au(ll1) in ultrahigh vacuum (UHV). They found a series of LEED patterns of YAu( 111) that changed from ( 4 3 x d3)R3Oo, via patterns with the 2/3 spots splitting into “triads”, and finally to “rosette” patterns with increasing 12 exposure. It should be noted that they observed continuous increase in the splitting distance of the triad spots with increasing I coverage. The recent electrochemical in-situ surface X-ray scattering (SXS) studies by Ocko et a1.* revealed a series of YAu( 111) adlattices, which is in good agreement with Cochran and Farrell’s LEED observation. The two-dimensional phases designated by Ocko et al., the rectangular (pxd3) phase and the rotated hexagonal (“rot-hex”) phase, correspond to Cochran and Farrell’s triad phase and rosette phase, respectively. Ocko et al. claimed that the adlattice constants vary continuously in each of the two phases with the electrode potential and hence with the coverage of the I adatoms. As the electrode potential is shifted in the positive direction, the nearest 1-1 distance in the adlattices is shortened in each phase. Ocko et al. called this phenomenon “electrocompression”, which was originally observed in the case of underpotential deposition of metals by Toney et al.3.4 Our present study was carried out to establish the structure of I monolayers on the Au( 111) electrode surface in greater detail and more precisely by means of ultrahigh-vacuum instrumentation and in situ electrochemical STM (ESTM).
* To whom correspondence should be addressed. Itaya Electrochemiscopy Project.
* Faculty of Engineering, Tohoku University. +
@
Abstract published in Advance ACS Abstracts, May 1, 1995.
0022-3654/95/2099-88 17$09.00/0
Results of a number of ex situ and in situ studies on YAu( 111) have already been reported. Bravo et aL5 investigated Au( 111) emersed from CsI solution by LEED and found a ( 4 3 x 4 3 ) R30” lattice at low coverages and a (5 x J3) structure at more positive potentials. Ex situ STM results were reported by Haiss et aL6 who found that the YAu(ll1) adlattice varies from (d3x43)R3Ooto ( 5 x 4 3 ) and to (7x7)R21A0. Recent in situ studies brought notable results. Gao and Weaver’ studied Au(1 11) in a solution of HC104 KI and confirmed the findings by Haiss et aL6 Tao and Lindsay8reported a potential-dependent transition of ( 4 3 x 43)R3Oo to (3 x 3) structure. We also reported ( 5 x d 3 ) and (7x7)R21.8” structures in pure HC104 solution in the absence of KL9 As seen above, the structural variation of iodine adlattices on Au( 111) in KI solution as a function of the electrode potential has not been fully described. Our aim was to establish the structures precisely and to confirm the continuous compression of the adlattice (“electro~ompression”~-~) by using surface structure-sensitive techniques of LEED and in situ STM.
+
2. Experimental Section 2.1. Ex Situ Measurement. The ex situ experiment was performed in an ultrahigh-vacuum system consisting of analysis. preparation, evaporation, and electrochemical (EC) chambers. Figure l a illustrates the vacuum parts of the apparatus. The analysis chamber is equipped with an STM, a retarding-field optics for LEED and Auger electron spectroscopy (AES), and a sample manipulator with a heating stage (Omicron Vacuumphysik, Germany). The EC chamber houses a Teflon-based electrochemical cell driven by a magnetic coupler. Figure l b shows a schematic view of the EC chamber. The EC chamber can be backfilled with ultrapure Ar prepared in a hightemperature Ti getter chamber. The Teflon electrochemical cell is connected with a solution delivery system, which can store, deaerate, inject, and eject solutions by pressure of ultrapure Ar. A computer-controlled potentiostat is connected to these electrodes and the sample holder. A Pt foil (8 cm2 surface area) was employed as the counter electrode and a Ag wire as the reversible Ag/AgI reference electrode. The potential of the Ag/ 0 1995 American Chemical Society
8818 J. Phys. Chem., Vol. 99, No. 21, 1995
a.
Sample Transfer Rod
Sample Manipulator
Yamada et al. 10
1
b.
Ion Gun
I
I
I
t I
UHV STM 1
I
I \-
1
w
Ultrapure Ar
Preparation Chamber -10
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
Electrode Potential I V vs. AgiAgl
Figure 2. Cyclic voltammogram for Au( 111) in 1 mM KI. Scan rate = 20 mV s-I. Arrows a, b, and c indicate the potentials of emersion after which the LEED and AES data in Figures 3 and 4 were recorded. Dellvery Syeiem Magnetlc Unear Drive
II’ Y
Figure 1. Schematic views of (a) the entire UHV apparatus and (b)
the EC chamber. AgI reference electrode was calibrated in 1 mM KI solution with respect to a saturated calomel electrode (SCE). It was found that the Ag/AgI (1 mh4 KI) is equivalent to -0.21 V vs SCE and hence to +0.03 V vs NHE. An Au( 111) single-crystal disk (8 mm in diameter and 2 mm in thickness) was metallographically polished, and two slits were made on the edge in which Ta wires, 0.3 mm in diameter, were inserted. The wires were used for mechanical support of the crystal and for heating the crystal by applying a direct current. To prepare a clean surface, cycles of Ar-ion bombardment and annealing up to 900 “C were repeated. Only one face of the crystal was exposed to the electrolyte. The experimental procedure was as follows: (1) Transfer the clean Au( 111) crystal into the EC chamber. Fill the EC chamber with ultrapure Ar. (2) Insert the KI solution container, and immerse the Au(ll1) surface. (3) Start cyclic voltammetry (CV). Scan a few cycles until a stable CV is obtained. (4) Stop scanning of CV at the desired electrode potential. Wait for 10 min for equilibration. ( 5 ) With the electrode maintained under potential control, remove the Au(ll1) crystal from the KI solution. Blow away droplets left on the surface by an Ar blower installed in the EC chamber. (6) Evacuate the EC chamber, and transfer the sample into the analysis chamber. (7)Record LEED and then AES to avoid possible beam damage. 2.2. In Situ Measurement. In situ STM measurements were carried out with a Nanoscope I11 electrochemical STM (Digital Instruments). The tips used were electrochemically etched W or commercial Pt-Ir (80:20). To minimize the background current, the tips were coated with nail polish and maintained at a constant voltage vs the reference; thus, the bias voltage varied depending on the applied electrode potential. STM images were recorded in the constant current mode.
The Au(ll1) electrode was prepared by evaporating a 200 nm thick Au film on “Robax” glass (AF, Berliner Glas KG) undercoated with Cr (2 nm) for better adhesion. Such films possess the morphology of the Au(ll1) surface.I0 Before each experiment, the Au film was annealed in a H2 flame for 1 min, cooled partially in air, and then immersed in 1 mM aqueous KI solution for 3 min. The sample was then quickly transferred to the STM electrochemical cell. Pt wires were used as the quasi-reference and the counter electrodes. All electrode potentials reported here are with respect to the Ag/AgI reference. Although the KI solution was not deaerated, as the STM-EC cell was open to air,it was ascertained that the lack of deaeration had little influence on the results. Aqueous solutions were prepared with potassium iodide (Kanto Chemicals, Japan) and ultrapure water (Milipore-Q). 3. Results 3.1. Cyclic Voltammetry. The Au(ll1) samples were initially examined by cyclic voltammetry in 1 mM KI for comparison with published data.2*5A typical current-potential curve for Au(ll1) in 1 mM KI is shown in Figure 2. This particular curve was obtained in the electrochemical cell of the EC-UHV setup in Ar-deaerated KI solution with the UHVcleaned single-crystal disk. Almost identical cyclic voltammograms were obtained in the electrochemical cell of the in situ STM with the Au-on-glass samples. Iodine adsorption started at potentials more positive than -0.4 V. At potentials more positive than +OS V, oxidation of iodide anions in the solution occurs. It has been suggested* that the small peak at f0.4V observed on the positive scan is associated with the existence of two different phases of the iodine adlayer. As described below, this peak actually represents the transition between the two phases. 3.2. AES and LEED Results. Each experiment was initiated with characterization of a clean and well-ordered Au(111)by LEED and AES. LEED pattems were recorded at three different emersion potentials between -0.2 and +0.7 V. Typical LEED pattems in this course are presented in Figure 3. A genuine ( 4 3 x d3)R3O0 was observed (Figure 3a) upon emersion at -0.2 V. At more positive emersion potentials, such
J. Phys. Chem., Vol. 99, No. 21, 1995 8819
Iodine Adlayer on Au( 111)
Figure 3. Typical LEED patterns on Au( 111) immediately after emersion from 1 mM KI solution at given potentials: (a) -0.20 V vs Ag/AgI, LEED incident electron energy = 44.2 eV; (b) +0.15 V, 50.9 eV; and (c) +0.30 V, 60.3 eV. 1
I
I
a.
b. n L.
1.
200
300
400
500
600
Auger Electron Energy /eV Figure 4. Typical Auger electron spectra on Au( 111) immediately after emersion from 1 mM KI solution at given potentials: (a) -0.20 V vs Ag/AgI; (b) +0.15 V; (c) +0.30 V; and (d) of clean Au(ll1). The incident electron energy = 2.0 keV. Each of spectra a, b, and c was recorded right after observation of the respective LEED pattern in Figure 3.
as +0.15 V (Figure 3b) and +0.3 V (Figure 3c), the LEED patterns underwent mainly splitting into "triads" and shifting of the original (43xd3)R30° spots. As the potential was increased, the subspots moved away further from the center and the distances between the split spots increased. Occasionally, for example in Figure 3c, additional weak spots were seen near the split spots. The purity of the iodine adlayer was confirmed by AES. Figure 4 shows AES of Au( 111) emersed at the above three different emersion potentials, as well as of clean Au( 111). Only the iodine signal at about 520 eV was added on the clean Au(1 11) spectra in each case. Quantitative analysis was not carried
out because of considerable background noise in the retardingfield optics. A signal for K (250 eV) was not clearly seen overlapping one of the Au signals. When 10 mM KI solution was used, a sharp peak of K was clearly seen. Quantitative analysis of these LEED patterns was performed to summarize their- continuous changes. In Figure 5a, the fundamental reciprocal unit cell vectors al* and a2* (lal*I = la2*l, Lal*a2* (angle between al* and a2*) = 60")and suitable adlattice cell vectors bl* and b2* were overlaid on the LEED pattern from Figure 5c. A rectangular centered lattice for bl* and b2* was taken for visual convenience. The lengths of these vectors were taken as the average of all equivalent symmetric vectors. The LEED patterns were recorded at slightly different incident electron energies (50 f 10 eV) to render the greatest number of spots visible. Correction for the change in the incident energy was made by using the ratios Ibl*l/lal*l and lb2*l/la2*I, indicated in Table 1. It is seen that the ratio Ibl*l/ la1 * I increases with increasing potential, while Ib2* I/la2* I remained constant at 1.O. The ratio Ibl*l/lal*l is defined here as p* (a variable parameter), and b2* = a2* with Lbl*b2* = 90". The real centered lattice vectors bl and b2 are derived from the relationships bl = pal (where p = 2/p*), Ib21 = 431a21, and Lblb2 = 90". The parameter p represents the length of bl, and the values of p are also given in Table 1. Figure 5b illustrates bl, b2, the fundamental real lattice vectors a1 and a2, and their relationships. This real adlattice was designated as c(pxd3R-3Oo), on the basis of the complete method of description by Wood's notation mentioned by Clarke. To verify c(px d3R-30") as the proper structure, the LEED patterns were regenerated as reciprocal lattice plots for various p values by taking into consideration a three-domain symmetric structure and double scattering via the six fundamental spots nearest to the center spot. Figure 5c shows one of these plots. The series of such plots completely reproduces the series of experimental LEED patterns. As can be expected, the doublescattering spots were weak and often missing in the observed LEED patterns. The observed increase only in Ibl* I indicates a decrease only in Ibl I. The variation of the adlattice constants is only along the basal Au atomic rows. As the largest value of p is equal to 3, at which the adlattice is identical to ( 4 3 x 43)R30", a value of p smaller than 3 signifies compression of the ( 4 3 x 4 3 ) -
8820 J. Phys. Chem., Vol. 99, No. 21, 1995
a.
-
Yamada et al.
b.
go9
'0 0' 0
Figure 5. (a) Observed LEED pattern (reproduced from Figure 3c) with fundamental reciprocal unit vectors al*, a2* and centered adlattice reciprocal unit vectors bl*, b2* overlaid. As defined in the text, Ibl*l = p*lal*l (p* = 0.784 here), b2* = a2*, and Lbl*b2* = 90". (b) illustration of the real lattice with real fundamental unit vectors al, a2 of Au(ll1) (1x1) and c(2.5xd3R-30") adlattice vectors, bl and b2, overlaid. (c) LEED pattern of the c(2.5 x 43R-30") adlattice calculated by taking into consideration (1) 3-fold symmetrical domain structure and (2) electron double scattering via the six fundamental spots nearest to the center spot: large open circles, fundamental Au( 111) spots; medium open circles, c(2.5 x 43R30") single-scattering spots; small filled circles, c(2.5 x 43R-30") double-scattering spots. TABLE 1: Change of the Adlattice Reciprocal Unit Vector Lengths Ibl*l/lbl*l, lbz*!/la2*1 and the Value of the Parameter p as a Function of Emersion Potential emersion potentiaW vs Ag/AgI Ibl*l/lal*l Ib2*l/la2*l p -0.20 0.667 1.01 3.00 -0.10 0.699 0.97 2.86 +o.oo 0.722 0.99 2.77 +o. 11 ' 0.735 0.98 2.72 +0.15 0.749 1.00 2.67 +0.21 0.97 2.57 0.778 +0.30 1.03 2.55 0.784 +0.4 1 0.778 1.03 2.57 +0.49 0.791 1.01 2.53 +0.61 0.797 0.99 2.51 +0.70 1.00 2.49 0.803 R30" structure only in the direction of the Au( 111) atom row. Hence, the variation of p is referred to as "uniaxial compression". The LEED patterns at selected emersion potentials are described by c(pxd3R-30") (3.00 2 p 2 2.49). The splitting ~
of the LEED spots are directly related to the uniaxial compression that varies with the emersion potential. In this study, only the c(p x d3R-30") structure was observed even at potentials more positive than +0.4 V, at which a rotated hexagonal structure had been expected.2 This is discussed below. 3.3. In Situ STM Results. Collection of STM images was also started at -0.2 V, followed by collection at intervals of 0.04-0.05 V up to 0.54 V. Atomic-resolution images were obtained at all potentials. In order to monitor the distances between the individual iodine atoms, a very small area such as 5 x 5 or 10 x 10 nm2 was usually scanned. Two different types of images were obtained in the potential range from -0.2 to +0.54 V, as shown in Figures 6 and 7. The transition between these two phases occurred at ca. +0.4 V, where a small reversible peak appears in the voltammogram in Figure 2. The obvious difference between these phases is in the height of the adatoms. As shown in Figure 6, all adatoms were at the same height, with atomic corrugation about 0.03-
~~
C.
b e
t
t
t
Figure 6. In situ STM images (5 x5 nm2)of the iodine adlayer on Au( 111) in 1 mM IU solution at (a) -0.07 V vs Ag/AgI, (b) +0.2 V, and (c) +0.33 V. Tunneling current was between 2 and 3 nA,and the potential of the tip was between -0.6 and -0.3 V vs Ag/AgI. The adlayers can be described as (a) c(3xd3R-30") (=(43xd3)R3Oo) with the nearest 1-1 distance 0.50 nm; (b) c(2.6x43R-30") with 0.47-0.48 nm; and (c) c(2.45xdR-30") with 0.42-0.43 nm. The 4 3 direction in each image is indicated by an arrow.
Iodine Adlayer on Au( 111)
J. Phys. Chem., Vol. 99, No. 21, I995 8821
b.
a.
/
/ d.
C.
Figure 7. STM images ( l o x 10 nm2) of the rot-hex iodine monolayer on Au( 1 11) recorded in situ in 1 mM IU solution, at (a) +0.4 V vs Ag/AgI and (c) 4-0.54 V, with corresponding simulated models b and d. Experimental conditions: (a) tunneling current 4.8 nA,tip potential +O.O6V vs Ag/AgI; (c) 28.5 nA, 4-0.4 V. Simulation patterns are characterized by the following parameters: (b) the nearest 1-1 distance = 0.45 nm, the rotation angle between the adlayer atom row and imaginary (d3xd3)R3Oo atom row = 2.9"; (d) 0.43 nm and 3.3". The direction parallel to the Au( 1 1 1) atom row [(' 1101 direction) in each image is indicated by an arrow.
0.04 nm. In Figure 7a,c, and on the other hand, STM images at potentials >+0.4 V contain a well-ordered lattice modulated with periodically arranged surface features. The I adlattice obtained at -0.07 V possessed ( 4 3 x 4 3 ) R30" symmetry with a characteristic interatomic distance of 0.50 nm (Figure 6a). A positive shift of the electrode potential resulted in the formation of a more densely packed adlattice. For example, the number of atom rows in the x-direction of Figure 5b is 14, whereas it is equal to 15 in Figure 6c. As can be recognized in all three images presented in Figure 6, the spacing of atoms in 4 3 direction (marked for each image with an arrow) remained unchanged, regardless of the electrode, potential. Images obtained at more positive potentials contain more atoms in two of three main directions of the iodine rows in the same scanning area. These uniaxially compressed incommensurate adlattices correspond to what was described earlier as c(px43R-30") in the LEED result. Accurate determination of the interatomic distance was rather difficult. Several factors such as the regime of the potential change and the duration in which the electrode was kept at the
'
given potential influenced our measurements. Very often it seemed that complete adsorption equilibrium was not achieved. The best series of results were obtained when the potential was kept at each step for 10 min and the direction of the potential scan was fixed. To determine the adlattice constant precisely, STM images were recorded under two different tunneling currents (ca. 1 and 40 nA)consecutively. The first scan imaged the iodine adlayer only, and the second scan only the underlying Au( 111)-( 1x 1). The lattice constants were more accurately determined by overlapping these two images. It should be mentioned that this approach of imaging both the adlayer and the substrate layer is based on the previously published 0bservations.7~~~~~~ The evaluation p in c(px J3R-30") was based on the nearest iodine-iodine distance. The distance between two nearest iodine atoms in the compressed rows varied from 0.50 nm ((43x43)R3O0 in Figure 6a) to 0.43 nm ( ~ ( 2 . 43R-30") 4~ in Figure 6c). In the phase observed at potentials more positive than +0.4 V, shown in Figure 7, the atomic adlattices of I possess a true 6-fold symmetry. The nearest 1-1 distance was smaller than
8822
J. Phys. Chem., Vol. 99, No. 21, 1995
Yamada et al.
that of (2/3xd3)R30° (0.50 nm), and the whole lattice was simultaneously rotated by several degrees with respect to the ( 4 3 x 2/3)R30". This type of adlattice has been denoted as rothex by Ocko et a1.* The adlattices are furthermore modulated with periodically arranged surface features. These features, namely, groups of slightly elevated I atoms, are interpreted as Moire patterns resulting from the mismatch between the adlattice and the (1x1) lattice of Au(ll1). In the image obtained at +0.40 V (Figure 7a), the groups of atoms (1-1 distance 0.45 nm, 10-12 members) are arranged approximately 2.1 nm apart from each other, and they are situated 0.03-0.04 nm higher than the lowest iodine atoms. At +OS4 V (Figure 7c), the groups are smaller (1-1 distance 0.43 nm, 6-7 members), separated by 1.8 nm, and elevated by 0.05-0.06 nm. Since attempts to image the underlying Au( 111) were not successful in this potential region, we took advantage of analyzing the Moire pattern by simulation to determine the adlattice constant precisely. The STM images were simulated by computer calculation based on a simple "hard-ball contact model" with the assumptions that (1) the underlying Au( 111) is kept invariant, (2) the projection of the rot-hex iodine adlattice onto the substrate plane is kept invariant with the lattice constant and the rotation angle fixed, and (3) the height of each iodine atom is determined by contact with the nearest Au atom with contact radii of I and Au (van der Waals radius of I = 0.215 nmI4 and a half of the nearest 1-1 distance of Au(ll1) = 0.144 nm, respectively). The heights of all adatoms in a frame were calculated and displayed in gray scale. Various com ression ratios (0-15% subtracted from the values for ( 4 3 x 3)R30") and rotation angles (0"-5" from ( 4 3 x 43)R30") were tested. The similarity between computer-generated and experimentally obtained images can be seen by comparison of Figure 6b,d with Figure 6a,c, respectively. The rot-hex 1-1 distance obtained by this simulation varied from 0.45 to 0.43 nm and the rotation angle from 2.9" to 3.3", the variations corresponding to the change of potential from +0.40 to +OS4 V.
Jp
4. Discussion Our study has indicated that the structure of the well-ordered iodine monolayer formed on Au( 111) in KI solution strongly depends on the electrode potential (in situ) and the emersion potential (ex situ). Figure 8 summarizes the value of p as a function of the potential obtained by both LEED and in situ STM. The LEED data revealed that the parameter p varied continuously from 3 to 2.49 in the broad range of emersion potential from -0.2 to +0.7 V. The continuous structural variation of c(pxJ3R-30") and rot-hex have not been reported in earlier STM studies. The ( 4 3 x d3)R30° adlattice was found to be the most open structure of iodine adlattice. The so-called ( 5 x 4 3 ) structure, an incomplete notation often found in the literature, is equivalent to what is here refined as c(2.5xd3R-30"). These two structures, the most relaxed and the most compressed, have very often been reported as the only existing The essential difference in our ex situ and in situ results is that the LEED pattern after emersion from KI at potentials >+0.4 V did not indicate rot-hex but still co)x2/3R-30°) adlattices. Our ex situ results can be compared with the results by Cochran and Farrell.' They obtained the rosette pattern in I2 atmosphere at Pa at room temperature, which was reverted to a triad pattern when the gas phase was evacuated. This was due to thermal desorption of the rosette I adlayer into vacuum. The rosette LEED pattern can be constructed on the basis of the rot-hex adlattice. Therefore, the rot-hex adlattice in our case was immediately converted into c@x43R-30°)
2.8
P
i
-
2.7 2.6
-
2.5 2.4 -0.4
I
-0.2
I
0
I
0.2
I
0.4
0.6
0.8
EmersioniElectrode Potential / V vs. AgiAgl
Figure 8. Value o f p in the c(pxJ3R-30')
unit cell as a function of the emersiodelectrode potential f r o d i n 1 mM KI solution: (0)plots of the LEED data in Table 1: (m) averaged values from in situ STM images; ( R ) a plot converted from Ocko et al.'s 1-1 distances2 in 0.1 M KI solution, p = 2.5 at f0.29 V vs Ag/AgI.
during the emersion and evacuation procedures because of the loss of the chemical potential of I toward adsorption. Our LEED and STM results are in good agreement with the in situ SXS results reported Ocko et a1.* From their results, a value of p = 2.50 (1-1 distance = 0.439 nm) is derived at 0 V vs Ag/AgC1(=+0.20 V vs NHE = -0.04 V vs SCE) in 0.1 M KI. The reference electrode AgIAgI used in the present study corresponds to +0.03 V vs NHE. If the adsorption of I on Au(111) follows the Nemst equation, the p values observed in 0.1 M KI should be shifted positively by 120 mV in 1 mM KI. This data point, plotted in Figure 8, is fairly close to our plots of the c@x1/3R-30") phase. The phenomenon of "electrocompression", which is essentially driven by an increase in the I coverage, is fully supported. It appears that this kind of phase behavior is applicable in relatively weak adsorbates. The distinct difference of our results from Ocko et al.'s is that the commensurate ( 4 3 x 1/3)R3Oo structure was not included in their plots. This might be due to the difference of concentrations of KI solutions used. Since ( 4 3 x 43)R3Oo in our series was seen at the potential out of their potential range, it is not possible to make a true comparison. We have demonstrated the applicability of LEED and in situ STM to the investigation of continuous structural change. As expected from early work by Hubbard, Soriaga, etc.,I5 application of LEED to structural analysis of electrode surfaces has helped to capture small variations in the adlattices. LEED provides the lattice constants of the c@x d3R-30") adlayer precisely enough to describe the structural variation exactly and to confirm the phenomenon of electrocompression. However, its applicability to the rot-hex phase was limited due to the instability of the adlayer in UHV environment. In situ STM provided integrated information on the topography enhanced by simultaneous observation of the adlayer and the substrate. Simulation of STM images by simple geometrical models also helped to obtain accurate structural parameters and to understand the interface structure. Complimentary use of LEED and in situ STM is a powerful technique for determining atomic-level structures of solid-liquid interfaces. Acknowledgment. We are grateful to Dr. B. M. Ocko for his critical comments to our present study. We thank Dr. Y .
Iodine Adlayer on Au( 111) Okinaka for his help in the writing of this manuscript. This work was supported by ERATO-Itaya Electrochemiscopy Project, JRDC.
References and Notes (1) Cochran, S. A,; Farrell, H. H. Surf: Sci. 1980, 95, 359. (2) Ocko, B. M.; Watson, G. M.; Wang, J. J . Phys. Chem. 1994, 98, 897. (3) Toney, M. F.; Gordon, J. G.:Samant, M. G.; Borges, G. L.; Melroy, 0. R.; Kau, L.-S.; Wiesler, D. G.; Yee, D.: Sorensen, L. B. Phys. Rev. B 1990, 42, 5594. (4) Toney, M. F.; Gordon, J. G.; Samant, M. G.:Borges, G. L.: Melroy, 0. R.; Yee, D.; Sorensen, L. B. Phys. Rev. B 1992, 45, 9362. ( 5 ) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P.: Villegas, I.; Suggs, D. W.; Stickney, J. L. J . Phys. Chem. 1991, 95, 5245.
J. Phys. Chem., Vol. 99, No. 21, 1995 8823 ( 6 ) Haiss, W.; Sass, J. K.; Gao, X.; Weaver, M. J. Surf, Sci. Lett. 1992, 274, L593. (7) Gao, X.; Weaver, M. J. J . Am. Chem. SOC. 1992, 114, 8544. (8) Tao, N. J.; Lindsay, S. M. J. Phys. Chem. 1992, 96, 5213. (9) Sugita, S.; Abe, T.; Itaya, K. J . Phys. Chem. 1993, 97, 8780. (10) Batina, N.: Will, T.; Kolb, D. M. Faraday Discuss. 1992, 94, 93. (11) Clarke, L. J. Surface Crystallography: An Introduction to LowEnergy Electron Dzflraction, J. Wiley: New York, 1985. (12) Magnussen, 0. M.; Hagebock, J.; Holtos, J.: Behm, R. J. Faraday Discuss. 1992, 94, 329. (13) Chang, S.-C.; Yau, S.-L.; Schardt, B. C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 4787. (14) Pauling, L. C. The Nature of the Chemical Bond; Come11University Press: Ithaca, NY, 1960. (15) Soriaga, M. P. Electrochemical Surface Science; American Chemical Society: Washington, DC, 1988.
JP950013C
