Surface Specific Adsorption of Saccharin on Iodine-Modified Pt(111

Sep 1, 1997 - Department of Chemistry, Indiana University Purdue UniversitysIndianapolis,. 402 North Blackford Street, Indianapolis, Indiana 46202-327...
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Langmuir 1997, 13, 5197-5203

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Surface Specific Adsorption of Saccharin on Iodine-Modified Pt(111) Electrodes S. Singh, D. H. Robertson, Qiyuan Peng, and J. J. Breen* Department of Chemistry, Indiana University Purdue UniversitysIndianapolis, 402 North Blackford Street, Indianapolis, Indiana 46202-3274 Received April 4, 1997. In Final Form: July 10, 1997X Saccharin (o-benzoic sulfimide) preferentially adsorbs onto platinum (111) electrodes modified with the (3×3) iodine adlayers (θI ) 0.44). This is in contrast to platinum (111) electrodes modified with the (x7×x7)R19.1° iodine adlayer (θI ) 0.43) where no adsorption is detected. Saccharin adsorption was monitored through changes in the voltammograms associated with the oxidative desorption of the strongly bound iodine adlayer and Ag underpotential deposition on the iodine-modified electrode surfaces in acidic solutions. Simple modeling calculations support the experimental observation and suggest that the preferential adsorptive behavior of saccharin is due to the enhanced van der Waals interactions associated with the symmetric Pt(111)(3×3)-I adlayer structure. The Pt(111)(3×3)-I adlayer structure is one of two (3×3) iodine adlayer structures found to a comparable extent of coverage on flame-annealed Pt(111) electrodes exposed to iodine for a longer duration than that leading to the formation of the Pt(111)(x7×x7)R19.1°-I adlattice.

Introduction Small- to medium-sized organic molecules such as saccharin, thiourea and coumarin are commonly used as leveling and brightening agents in electroplating.1,2 These and other additives can enhance the morphology and physical properties of metal electrodeposits as well as affect the composition of metal alloy films and the rate of metal film deposition.3 While some additives are known to affect the deposition process through ion complex formation and ion pairing with the depositing metal ions, other plating additives interact with the substrate surface or the depositing metal film and act either by blocking particular surface sites or by electrode filming.4,5 Despite the widespread use of plating additives, an atomic level understanding of their role in the deposition process is not very advanced nor is there an understanding of the relationship between an additives’ molecular structure and the finishing performance of the additive for a particular metal deposited. In this paper we report cyclic voltammetry experiments and supporting calculations examining the effects of the atomic scale surface structure of iodine-modified Pt(111) electrodes on the adsorption of saccharin (o-benzoic sulfimide) (I). Saccharin is a commonly used brightening agent in nickel electroplating,1,2 and it has been suggested that it will work similarly for Ag deposition.5 The choice of the iodine-modified Pt(111) electrodes allows the adsorption to be investigated with well-ordered, structurally varied, and air stable electrodes which can be easily prepared. These iodine-modified Pt(111) electrodes can be selectively prepared either with a Pt(111)(x7×x7)R19.1°-I adlattice (θI ) 0.43) or with a surface covered with domains of two different Pt(111)(3×3)-I (θI ) 0.44) X Abstract published in Advance ACS Abstracts, September 1, 1997.

(1) Bockris, J. O. M.; Kahn, S. U. M. Surface Electrochemistry: A Molecular Level Approach; Plenum: New York, 1993. (2) Harrison, J. A.; Thirsk, H. R. The Fundamentals of Metal Deposition; Harrison, J. A., Thirsk, H. R., Eds.; Marcel Dekker: New York, 1971; Vol. 5, pp 67-148. (3) Lowenheim, F. A. Modern Electroplating, 2nd ed.; Lowenheim, F. A., Ed.; John Wiley: New York, 1963, pp 769. (4) Franklin, T. C. Plat. Surf. Finish. 1994, 81, 62-67. (5) Oniciu, L.; Muresan, L. Rev. Appl. Electrochem. 1991, 29, 565574.

S0743-7463(97)00349-1 CCC: $14.00

adlattices which coexist at comparable coverages. All three Pt(111)-I adlattice structures are depicted in Figure 1.

The iodine-modified Pt(111) electrodes are important in electrochemical surface science both as chemically modified electrodes and for fundamental studies of the electrochemical double layer.6 Iodine adatoms are strongly bound to the Pt(111) surface in specific coordination sites resulting in the passivation of the electrode surfaces against oxidation and also rendering the surfaces hydrophobic.7,8 The surface structures have been investigated by a wide array of in situ and ex situ surface science techniques including low-energy electron diffraction (LEED),7-9 angular distribution Auger microscopy (ADAM),10,11 surface-extended X-ray absorption fine structure (SEXAFS),12 optical second harmonic generation,13 and scanning tunneling microscopy.14-17 In addition the irreversible adsorption of iodine from iodide solutions onto (6) Hubbard, A. T. Chem. Rev. 1988, 88, 633-656. (7) Wieckowski, A.; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T. Surf. Sci. 1984, 146, 115-134. (8) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J. Electroanal. Chem. 1987, 222, 305-320. (9) Felter, T. E.; Hubbard, A. T. J. Electroanal. Chem. 1979, 100, 473-491. (10) Frank, D. G.; Chyan, O. M. R.; Golden, T.; Hubbard, A. T. J. Phys. Chem. 1993, 97, 3829-3837. (11) Frank, D. G.; Chyan, O. M. R.; Golden, T.; Hubbard, A. T. J. Phys. Chem. 1994, 98, 1895-1903. (12) Abruna, H. D. X-Ray Absorption Spectrocopy in the Study of Electrochemical Systems; Abruna, H. D., Ed.; VCH Publishers, Inc.: New York, 1991; pp 1-54. (13) Corn, R. M. Proc.sElectrochem. Soc. 1992, 92-11, 264-77. (14) Yau, S.-L.; Vitus, C. M.; Schardt, B. C. J. Am. Chem. Soc. 1990, 112, 3677-3679. (15) Schardt, B. C.; Yau, S.-L.; Rinaldi, F. Science 1989, 243, 10501053. (16) Chang, S.-C.; Yau, S.-L.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 4787-4794. (17) Vogel, R.; Kamphausen, I.; Baltruschat, H. Ber. Bunsenges. Phys. Chem. 1992, 96, 525-530.

© 1997 American Chemical Society

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Figure 1. Ball models depicting the three different iodine-modified Pt(111) adlattice structures prepared for this study: (a) Pt(111)(x7×x7)R19.1°-I; (b) symmetric Pt(111)(3×3)-I; (c) asymmetric Pt(111)(3×3)-I. The dark open circles represent the iodine adatoms and the white, gray, and black circles represent the top three layers of Pt atoms in the crystal.

bare polycrystalline Pt (55% Pt(111) and 45% Pt(100)) electrodes has also been used to determine the coverage and orientation of a variety of adsorbed aromatic molecules.18 In this work the extent of saccharin adsorption is monitored through changes in the voltammogram associated with the oxidative desorption of the surface bound iodine atoms leading to the formation of IO3- 18 and changes in the voltammograms associated with Ag underpotential deposition (UPD). UPD refers to the reductive deposition of metals on foreign surfaces at potentials more positive than the thermodynamically reversible potential.19,20 The process is usually limited to a single monolayer and is thought to occur due to a stronger attractive force between the depositing metal adatom and the foreign atom of the electrode than between the like atoms of the bulk atoms being deposited. It is a phenomenon commonly observed for a wide variety of adatom/ electrode systems, and the potential and acuteness of the deposition peaks are a sensitive measure of the overall structural order of the electrode surface. Recently a number of investigations studying the effects of a variety of additives on the UPD of Cu and Ag on well-ordered Pt(111) single crystal electrodes have been reported.21-25 The differences revealed in the underpotential deposition voltammograms in the presence of the platting additives are indicative of the relative strength of interaction of the organic additive and the metal surface and between the deposited metal monolayer and the organic additive. In these studies molecular adsorbates, which are chemisorbed to the Pt(111) electrodes, produce the most dramatic changes in the UPD voltammograms in terms of anodic shifts in the deposition potential and in some cases complete inhibition of metal deposition. Our experiments reveal that saccharin adsorbs more strongly and to a much greater extent on electrodes with the mixed Pt(111)(3×3)-I adlattices as compared to the (18) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 2742-2747. (19) Kolb, D. M. Physical and Electrochemical Properties of Metal Monolayers on Metallic Substrates; Kolb, D. M., Ed.; John Wiley: New York, 1978; Vol. 11, p 125. (20) Juttner, K.; Lorenz, W. J. Z. Phys. Chem. (Munich) 1980, 122, 163-185. (21) White, J. H.; Abruna, H. D. J. Electroanal. Chem. 1991, 300, 521-542. (22) Bhatt, D. P.; Twomey, T.; Plieth, W.; Schumacher, R.; Meyer, H. J. Electroanal. Chem. 1992, 322, 279-288. (23) Wunsche, M.; Nichols, R. J.; Schumacher, R.; Beckmann, W.; Meyer, H. Electrochim. Acta 1993, 38, 647-652. (24) Dakkouri, A. S.; Batina, N.; Kolb, D. M. Electrochim. Acta 1993, 38, 2467-2472. (25) Taylor, D. L.; Abruna, H. D. J. Electrochem. Soc. 1993, 140, 3402-3409.

Pt(111)(x7×x7)R19.1°-I adlattice. The adsorption is evident through the inhibition of both iodine desorption and Ag deposition. Model calculations will show that sites present on the symmetric Pt(111)(3×3)-I adlattices favor adsorption on this surface. This work demonstrates that atomic scale changes in an electrode’s surface structure can strongly effect molecular adsorption and that a physisorbed additive can act in a similar fashion to many chemisorbed additives with regard to the inhibition of metal deposition. Experimental Section Experiments were conducted using an oriented, cut, and polished Pt(111) single crystal (1 cm diameter, 2 mm thick) purchased from Aremco Products (Ossining, NY). A 0.025 in. Pt wire spot welded to the back of the crystal functioned as a handle for the crystal during the annealing process and as an electrical contact for electrochemical measurements. The iodine-modified electrode surfaces were prepared following repeated oxidationreduction voltammetric cycling in 0.1 M HClO4 by annealing the crystal in a hydrogen flame and then holding the crystal 1 cm above a glass cup of iodine crystals over which argon is passed.7 The different iodine adlattice structures are obtained by cooling the hot crystal in the iodine/argon vapor either for 1 min, to obtain the Pt(111)(x7×x7)R19.1°-I surface structure, or for 5 min to obtain the mixed Pt(111)(3×3)-I symmetric and Pt(111)(3×3)-I asymmetric adlattice. These latter two (3×3)-I on platinum surface structures cannot be prepared exclusive of one another and coexist to a comparable extent on the Pt(111) surface.15 Preparation of the different iodine-modified electrode surfaces was confirmed through both the electrochemical measurements presented in this paper and by imaging experiments using an Nanoscope III electrochemical scanning tunneling microscope (STM) (Digital Instruments, Santa Barbara, CA). The iodine-treated crystal was transferred in air to a closed electrochemical cell similar to that described in ref 6. A Ag/AgCl reference electrode (Bioanalytical Systems, West Lafayette, IN), isolated from the main chamber by a porous glass salt bridge, and a Pt wire auxiliary electrode were employed. Electrolyte solutions were made using MilliQ water (Millipore, Bedford, MA), AgClO4 and HClO4, doubly distilled from Vycor (both from GFS Chemicals, Powell, OH), and saccharin (Aldrich Chemical Co., Milwaukee, WI). All solutions were thoroughly degassed with argon, and all chemicals were used as received. Electrochemical experiments were conducted using a Cypress Model CYSY-I computer-based potentiostat (Cypress Systems, Inc., Lawrence, KS) and primarily with saccharin added to the electrolyte solution in varying amounts. Voltammograms were obtained using the hanging meniscus method after waiting 5 min following immersion of the iodine-treated electrode at 0.925 V (all potentials quoted are versus Ag/AgCl). For the Ag UPD experiments the potential is scanned to and from the onset of bulk Ag deposition, the potential of which is used to correct for small differences observed in the reference potential on different days. Throughout the course of these experiments the charac-

Saccharin Adsorption

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Figure 3. Cyclic voltammograms depicting the underpotential deposition of Ag onto the electrodes with the Pt(111)(x7×x7)R19.1°-I adlattice in the presence of various amounts of saccharin. The scans were initiated at 0.925 V at a rate of 2 mV/s. The electrolyte is 1 mM AgClO4 in 0.1 M HClO4 with saccharin concentrations of 1 × 10-8, 1 × 10-5, or 1 × 10-3 M. All scans are first scans.

Figure 2. Cyclic voltammograms depicting the oxidative desorption of iodine from the iodine-modified Pt(111) electodes: (A) Pt(111)(x7×x7)R19.1°-I; (B) Pt(111)(3×3)-I. In each plot the solution for the thin trace is 0.1 M HClO4 and the electrolyte for the thick trace is 0.1 M HClO4 with 5 mM saccharin. The voltammograms were initiated at 0.925 V and scanned first in the negative direction at a rate of 10 mV/s. All scans are the first scans. teristic Ag UPD voltammogram associated with either the Pt(111)(x7×x7)R19.1°-I or Pt(111)(3×3)-I electrodes was reproduced in saccharin-free solutions immediately prior to each voltammetry experiment in solutions containing saccharin.7

Results Pictured in Figure 2 are the cyclic voltammograms obtained in experiments involving the iodine-modified Pt(111) electrodes in 0.1 M HClO4 electrolyte with and without 0.5 mM saccharin. The electrode is introduce into the cell at 0.925 V and held at this potential for 5 min, and then the scan is initiated toward -0.25 V. At the most positive potentials the observed current is due to the oxidative desorption of adsorbed iodine according to

Iads + 3H2O h IO3- + 6H+ + 5eThe charge contained under the curve can be used to quantitatively measure the amount of chemisorbed iodine.18 In the experiments with electrodes prepared with the (x7×x7)R19.1°-I adlattice, only small shifts in the sharp desorption features are observed. In contrast, for electrodes prepared with the Pt(111)(3×3)-I adlattices the onset of desorption is shifted more positive by ≈0.1 V. In addition the sharp anodic feature in Figure 1B is both diminished and shifted more positive due to the presence of saccharin. No attempt was made to identify the origin of the small cathodic peaks at ≈0.2 V in both traces of Figure 1B. Saccharin, which is reduced at -1.94 V at pH 10.0,26 is expected to be electrochemically inactive in the potential window of the experiments described herein. (26) Momose, T. J. Pharm. Soc. Jpn. 1944, 64, 155-156.

Pictured in Figure 3 is a selection of voltammograms taken from a series of experiments examining the UPD of Ag on a Pt(111) electrode prepared with the (x7×x7)R19.1°-I adlattice. The experiments were conducted in electrolytes containing 1 mM AgClO4 in 0.1 M HClO4 with varying concentrations of saccharin. The voltammogram in solutions containing 10-8 M saccharin is unchanged from voltammograms obtained in saccharin-free solutions and similar to those reported in the literature.7,27 Deviations in the shape and potential of the sharp deposition feature at ∼0.825 V are not discernible until the saccharin concentration is increased to 10-3 M. At 10-3 M the Ag UPD voltammogram reveals a broadened feature which appears to be two merged features, at a potential shifted about -0.03 V with respect to the voltammograms in solutions with lesser concentrations of saccharin. The remaining features in the UPD voltammograms which appear prior to the onset of bulk deposition reflect little or no change due to the presence of saccharin at all concentrations investigated. Pictured in Figure 4 is a selection of voltammograms taken from a series of experiments similar to those described above examining the UPD of Ag onto a Pt(111) electrode prepared with the higher coverage Pt(111)(3×3)-I adlattices. Again at a saccharin concentration of 10-8 M the voltammograms are similar to those obtained in saccharin-free solutions and those reported in the literature.7 As the saccharin concentration is increased from 10-8 to 10-5 M, the characteristic doublet feature at 0.78 V merges into a single broadened feature which is shifted to more negative potentials. As the concentration is further increased from 10-5 to 10-3 M the broadened feature continues to shift to more negative potentials and the pair of reductive prepeaks at 0.44 V is almost completely converted to a single larger feature at the more negative potential. Throughout these experiments the charge associated with the initial Ag deposition step and the calculated Ag surface coverage remains consistent with the established value of θAg ) 0.44. (27) Gibson, N. C.; Saville, P. M.; Harrington, D. A. J. Electroanal. Chem. 1991, 318, 271-282.

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Figure 4. Cyclic voltammograms depicting the underpotential deposition of Ag onto the electrodes with the mixed Pt(111)(3×3)-I adlattices in the presence of various concentrations of saccharin. The scans were initiated at 0.925 V at a rate of 2 mV/s. The electrolyte is 1 mM AgClO4 in 0.1 M HClO4 with saccharin concentrations of 1 × 10-8, 1 × 10-5, or 1 × 10-3 M. All scans are first scans.

Figure 5 summarizes results from a larger set of UPD experiments conducted with the two different iodinemodified electrode surfaces. Plotted in these two graphs are the potential shifts observed for the most positive deposition peaks relative to the potential of the most positive deposition peak in saccharin-free solutions. As shown in Figure 3, the potential shift for the Pt(111)(x7×x7)R19.1°-I electrodes is negligible except for the most concentrated saccharin solutions where the deposition peak shifts and begins to split. In the case of electrodes with the mixed (3×3)-I adlattices, the potential of the deposition peak shifts rapidly over the 10-6 to 10-5 M range and then continues to shift to more negative values as the saccharin concentration is further increased to 10-3 M. In each plot two symbols are used to describe the observation of two partially resolved depsosition features in the voltammograms.

Figure 5. Plots revealing the changes in the Ag depositon potential for the most positive deposition peaks on the two iodine-modified Pt(111) electrodes for a wide range of saccharin concentrations in solutions containing 1 mM AgClO4 in 0.1 M HClO4. The potentials plotted as ∆E are relative to the deposition potential in saccharin-free solutions. The squares represent the most positive deposition while the circles represent a less positive and partially resolved feature adjacent to the main depostion peak. Table 1. Saccharin Partial Charges (6-31G* Level)

Model Calculations We have modeled the adsorption of saccharin on the various Pt(111)-I adlayers using a simple, solvent-free, electrostatic model. Electrostatic charges on each of the atoms in saccharin were determined from an ab initio calculation at the 6-31G* level using Spartan.28 These atomic charges are depicted in Table 1 and reveal that the nitrogen and oxygen atoms associated with the fivemembered ring are all negatively charged with the bulk of the negative charge residing on the two oxygen atoms attached to the sulfur atom. Saccharin is a weakly acidic molecule (pKa ) 11.7), and consequently only the neutral molecule was used in our calculations since the electrolyte used in this study is strongly acidic.29 The electrode surface is modeled as a single layer of 300 or 400 iodine atoms (10 × 10 unit cells). The atoms are arranged using Cerius2 30 with the relative unit cell atom positions (x, y, z) as determined in the ADAM experiments (28) Spartan, Wavefunction Inc, Irvine, CA . (29) Weast, R. C. CRC Handbook of Chemistry and Physics, 59th ed.; Weast, R. C., Ed.; CRC Press, Inc.: West Palm Beach, FL, 1978. (30) Cerius2, Molecular Simulations Inc., Burlington MA.

atom

charge (au)

C1 C2 C3 C4 C5 C6 H7 H8 H9

-0.151 -0.038 -0.120 -0.092 -0.102 -0.104 0.158 0.142 0.145

atom

charge (au)

H10 S11 O12 O13 N14 O15 H16 C17

0.165 1.404 -0.624 -0.623 -0.792 -0.544 0.452 0.725

and reproduced in Table 2. The charge of each of the iodine atoms is set in accordance with a fixed surface charge density of 92 µC/cm2. These particular values were chosen to set a surface charge density well positive of the potential of zero charge for a Pt(111)-I electrode. Minimization calculations determining the lowest energy configuration of a single saccharin molecule adsorbed onto the surface were performed using QUANTA/CHARMm.31

Saccharin Adsorption

Langmuir, Vol. 13, No. 19, 1997 5201 Table 2. Electrode Surface Model Descriptors

Pt(111)-I adlayer structure (x7×x7)R19.1° symmetric (3×3)

asymmetric (3×3)

a

assigned charges (au)

adatom typea

unit cell coordinates x, y, z (Å)a

model 1

model 2

model 3

1 fold atop 3 fold hcp 3 fold fcc 1 fold atop 2 fold bridged 2 fold bridged 2 fold bridged (cntr) 3 fold fcc 1 fold bridged (asym) 1 fold bridged (asym) 1 fold bridged (asym)

0, 0, 0.8 2.776, 3.205, 0.6 5.552, 6.411, 0 0, 0, 0.8 4.164, 0, 0 6.246, 3.606, 0 2.082, 3.606, 0 0, 0, 0 4.164, 0, 1 2.082, 3.606, 1 6.246, 3.606, 1

0.090 0.090 0.090 0.087 0.087 0.090 0.090 0.087 0.087 0.090 0.090

0.099 0.087 0.084 0.096 0.084 0.084 0.084 0.078 0.090 0.090 0.090

0.081 0.093 0.096 0.078 0.090 0.090 0.090 0.096 0.084 0.084 0.084

Frank, D. G.; Chan, O. M. R.; Golden, T.; Hubbard, A. T. J. Phys. Chem. 1993, 97, 3829.

In these calculations the initial orientation and position of the saccharin molecule above the surface were randomly chosen and the minimum energy surface-adsorbed saccharin structure was found using an adopted-basis Newton Raphson algorithm. While the structure of the saccharin molecule is allowed to vary, the iodine atoms in the surface are held fixed. To accommodate adlattice site dependent charge variations which may be observed in STM experiments, calculations were conducted with three models of each of the three Pt(111)-I electrode surfaces.16 In model 1 the charge on all the iodine atoms is set equal to one another. In model 2 the charge is increased on the highest atoms by an amount corresponding to 10% of the value in model 1 and decreased by a corresponding amount on the lower atoms to maintain the same surface charge density. In model 3 the charge on the highest atoms is decreased by 10% and increased by the corresponding amount on the lower atoms. These assigned charges on the iodine atoms in all the electrode surface models are also given in Table 2. Additional calculations were also conducted to examine the effects of a reduced surface charge density using a model surface with the charge on all iodine atoms set to half the value in model 1 resulting in a surface charge density of 46 µC/cm2. With the exception of an 11% reduction in the electrostatic energy contribution to the binding energy, the results using this model of the electrode surface are not substantially different than those reported for models 1, 2, and 3. Discussion Clearly evident in Figures 2-5 is the much greater extent of saccharin adsorption on electrodes prepared with the (3×3)-I adlattices and the subsequent effect this adsorption has on both iodine adatom desorption and the underpotential deposition of Ag in experiments. Focusing on the Ag UPD experiments, saccharin inhibits the initial Ag deposition at considerably lower concentrations for electrodes with the (3×3)-I adlattices in contrast to electrodes with the (x7×x7)R19.1°-I adlattice. As shown in Figure 5 the reduction in the potential for the initial deposition peak is larger, ∼0.10 V versus ∼0.03 V, on the (3×3) surfaces even at the highest concentration examined indicating that greater energy is required to displace adsorbed saccharin molecules and commence the Ag deposition. For electrodes prepared with the (x7×x7)R19.1°-I adlattice the most positive underpotential deposition peak, at ∼0.82 V has been shown to lead to the formation of mixed Pt(111)(3×3)-Ag-I structure in which the Pt(111) (31) QUANTA/CHARMm, Molecular Simulations Inc., Burlington MA.

surface is covered by a (3×3) Ag layer which is in turn covered by a (3×3)-I layer.7,11,27 In this initial deposition step Ag atoms are deposited beneath the iodine adlayer. The consistency in the charge associated with the initial Ag deposition peak in all experiments with and without saccharin suggests that once displaced from the surface the saccharin has little or no effect on formation of the Ag monolayer. The considerably smaller effect of saccharin on the remaining peaks in the UPD voltammograms indicates the adsorption is either less prevalent or more easily displaced by the subsequent Ag deposition steps leading to the formation of higher Ag coverage domains. In voltammograms examining the second Ag deposition/ removal cycle, the most positive deposition peaks appear at the same potential but they are considerably broader. A summary of the results from our model calculations is displayed in Table 3. In the case of each electrode surface model the binding energy of saccharin favors adsorption on the symmetric (3×3)-I adlattice and to decreasing amounts on the (x7×x7)R19.1°-I and asymmetric (3×3)-I adlattices, respectively. While we do not suggest that the binding energies calculated using these simple surface models are accurate, as we have neglected the important effects of the electrolyte, the differences in binding energies between surfaces are informative. Comparing the symmetric (3×3)-I adlattice with the (x7×x7)R19.1°-I adlattice, calculations using model 1 show a difference in binding energy of ∼0.6 kcal/mol favoring adsorption on the symmetric (3×3)-I adlattice over the (x7×x7)R19.1°-I adlattice. While this difference is about 1% of the calculated binding energy, it is still on the order of kT for measurements at room temperature. Similar results are obtained when site dependent variations in the assigned atom charges are introduced (model 2 and model 3) or when the surface charge density is reduced to a lesser value. The observed differences in binding energy are largely due to the enhanced van der Waals interactions between the molecule and symmetric (3×3)-I adlattice, which relative to the (x7×x7)R19.1°-I was as much as 0.765 kcal/mol. Examination of the other contributions to the total energy of the system reveals that the internal energy remains relatively constant and the difference in the electrostatic energy are of a lesser magnitude relative to the differences in van der Waals energies. Pictured in Figure 6 are the structures corresponding to the lowest energy conformations determined in our calculations using surface atom charge model 1. The saccharin structure pictured is in good agreement with single-crystal X-ray crystallographic data revealing two nearly equivalent oxygen atoms connected to a distorted tetrahedral sulfur atom and protruding above and below the planar aromatic structure of the remainder of the molecule.32,33 The actual lowest energy structure on each

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Table 3. Optimized Binding Energetics of an Uncharged Saccharin with Iodine-Modified Pt(111) Electrode Surfaces Model 1 (+) Charged Surface without Site Dependent Charges energies (kcal/mol)

Pt(111)-I adlayer structure (x7×x7)R19.1° symmetric (3×3) asymmetric (3×3) energy difference Sym(3×3) - (x7×x7)R19.1°

Lennard-Jones

electrostatic

internal

total

-7.128 -7.893 -7.398 -0.765

-83.798 -83.664 -83.204 0.134

24.431 24.466 24.502 0.035

-66.495 -67.091 -66.099 -0.596

Model 2 (+) Charged Surface with Charge Increased for High Adatoms energies (kcal/mol)

Pt(111)-I adlayer structure (x7×x7)R19.1° symmetric (3×3) asymmetric (3×3) energy difference Sym(3×3) - (x7×x7)R19.1°

Lennard-Jones

electrostatic

internal

total

-7.114 -7.896 -7.357 -0.752

-83.816 -83.715 -83.247 0.101

24.442 24.460 24.512 0.018

-66.488 -67.150 -66.092 -0.662

Model 3 (+) Charged Surface with Charge Decreased for High Adatoms Pt(111)-I adlayer structure (x7×x7)R19.1° symmetric (3×3) asymmetric (3×3) energy difference Sym(3×3) - (x7×x7)R19.1°

energies (kcal/mol) Lennard-Jones

electrostatic

internal

total

-7.135 -7.892 -7.428 -0.757

-83.825 -83.615 -83.186 0.210

24.430 24.475 24.502 0.045

-66.530 -67.032 -66.112 -0.502

Figure 6. Illustrations depicting the lowest energy structure obtained using model 1 of an adsorbed saccharin molecule on the three different iodine adlattices investigated in this work: (A) Pt(111)(x7×x7)R19.1°-I; (B) Pt(111)(3×3)-symmetric-I; (C) Pt(111)(3×3)-asymmetric-I. In this figure the darkest circles are the highest iodine atoms and the white circles are the lowest.

of the three different iodine adlattices is not noticeably different for any of the four surface charge density models employed. In each case saccharin interacts with the positively charged electrode surface primarily through the two oxygen atoms attached to sulfur and the hydrogen attached to nitrogen. Coordination is always to the lowest lying iodine atom(s) in the unit cell and orientation of the molecule about the surface normal is determined by the interactions of negatively charged carbonyl oxygen atom. (32) Bart, J. C. J. J. Chem. Soc., B 1968, 376-382. (33) Okaya, Y. Acta Crystaologr. 1969, B25, 2257-2263.

In the case of the symmetric (3×3)-I adlattice, saccharin is coordinated to three equivalent iodine atoms and the planar aromatic structure of the molecule is perpendicular to the surface. In the cases of the asymmetric (3×3)-I and (x7×x7)R19.1°-I adlattices the saccharin is bound to a similar coordination site, inclusive of the lowest iodine atom in the unit cell, but the molecule is canted due to height differences among the iodine atoms forming the coordination site. Overall the coordination sites available on the symmetric (3×3)-I adlattices formed by the three equivalent low lying iodine atoms bring the saccharin molecule further into the iodine adlattice and results in an orientation of the saccharin molecule which maximizes the van der Waals interactions relative to the other iodine surface structures. In addition this perpendicular orientation minimizes the electrode surface area occupied by each saccharin molecule, which is important for conditions leading to high adsorbate coverage34 and favors π-π interactions possible between adsorbed saccharin molecules residing in the two equivalent sites per unit cell (separated by 4.1 Å) and between molecules adsorbed in a bilayer structure. Previous experiments with the iodine-modified Pt(111) electrodes have sought to relate differences in the atomic scale surface structure to the outer-sphere electrontransfer kinetics for the irreversible reduction of a series of cationic CoIII(NH3)5X complexes.16 These experiments were conducted in acidic media at potentials in the range of 0 to -0.2 V vs SCE where it would be expected that the Pt(111)-I electrode surface is negatively charged. When X was an organic carboxylate complex with aromatic substituents, the electron transfer rates (kapp) were as much as 10 times faster on both of the iodine-modified surfaces as compared to a bare platinum electrode. This rate enhancement with respect to bare platinum electrodes was attributed to interactions between the aromatic rings on the ligand and the hydrophobic iodine adlayer. When X was an inorganic ligand, such as -NH3, -F-, or -OSO32-, electron transfer rates were three to five times (34) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. Soc. 1982, 104, 3937-3945.

Saccharin Adsorption

faster on the (x7×x7)R19.1°-I surfaces as compared to the (3×3)-I surfaces. Overall, there was no clear identification of any particular iodine surface adatom site leading to more facile electroreduction and the increases in kapp for the inorganic ligands on the (x7×x7)R19.1°-I surface was attributed to a preponderance of 3-fold hollow sites relative to the (3×3)-I surfaces. These apparent rate constants are determined from the current measured at a particular cell potential and reflect the transport of the electroactive species to and from the electrode as well as the electron transfer step. Our results suggest that enhanced adsorption on electrodes with the mixed the (3×3)-I adlattices could also be a mitigating factor affecting the measured rate of electron transfer by slowing the rate of product removal relative to that with the (x7×x7)R19.1°-I electrodes. In conclusion, saccharin has been shown to adsorb more strongly on electrodes prepared with the mixed Pt(111)(3×3)-I adlattices in comparison to the electrodes prepared with the Pt(111)(x7×x7)R19.1°-I adlattice. The difference in the extent of saccharin adsorption for the two different iodine-modified electrodes, both of which are hydrophobic, is necessarily due to the arrangement of the iodine adatoms on the electrode surface and the local environment available to the neutral saccharin molecules. Simple modeling calculations reveal that adsorption is most favored for the symmetric Pt(111)(3×3)-I adlayer structure which coexists with an asymmetric Pt(111)(3×3)-I adlattice on the electrodes. When present in the electrolyte, saccharin inhibits the UPD of Ag on electrodes prepared with the mixed Pt(111)(3×3)-I adlattices, reducing the underpotential by amounts related to the saccharin concentration. A reduction in the underpotenial for similar experiments with electrodes prepared with the Pt(111)(x7×x7)R19.1°-I adlattice is only observed in solutions with high saccharin concentrations. Our observation of the effects of saccharin on the

Langmuir, Vol. 13, No. 19, 1997 5203

UPD of Ag onto iodine-treated Pt(111) surfaces is a result of the atomic scale structural differences between the ordered iodine adlayers and a clear example of the inhibition of a metal deposition process by a preferred and structure-sensitive adsorption mechanism. Attempts to probe the structure of the adsorbed saccharin molecules on a Pt(111)(3×3)-I electrode surface in situ with the STM proved unsuccessful. The feasibility of these imaging experiments may be effected by the orientation of the adsorbate with the STM better able to image aromatic molecules adsorbed flat on the electrode surface.35-40 Other experiments such as external reflectance IR absorption spectroscopy measurements would be useful to confirm the extent of saccharin adsorption and establish the orientation of the adsorbed molecules with respect to the electrode surface. Acknowledgment. Acknowledgment is made to the IUPUI Faculty Development Office, the Purdue Research Foundation, and The Petroleum Research Fund, administered by the ACS, for support of this research. The authors also wish to thank Professor F. A. Schultz for helpful discussions and the loan of his Cypress potentiostat as well as Professor David Malik and Dr. Peter J. Mahon for helpful discussions. LA9703499 (35) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795-7803. (36) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547-3553. (37) Dretschkow, T.; Dakkouori, A. S.; Wandlowski, T. Langmuir 1997, 13, 2843-2856. (38) Srinivassan, R.; Gopalan, P. J. Phys. Chem. 1993, 97, 87708775. (39) Srinivassan, R.; Murphy, J. C.; Pattabiraman, N. Ultramicroscopy 1992, 42-44, 453-459. (40) Srinivassan, R.; Murphy, J. C.; Fainchtein, R.; Pattabiraman, N. J. Electroanal. Chem. 1991, 312, 293-300.