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Langmuir 1993,9, 2914-2981
Thioaromatic Monolayers on Gold: A New Family of Self-Assembling Monolayers Eyal Sabatani,t*$Joseph Cohen-Boulakia,+Merlin Bruening,s and Israel Rubinstein'9t Department of Materials and Interfaces and Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel Received March 1,1993. I n Final Form: June 29,199P Self-assembliig (SA) monolayers of the aromatic compounds thiophenol (TP), p-biphenyl mercaptan (BPM), and p-terphenyl mercaptan (TPM) were prepared on gold electrodes. Contact angle (CAI measurements and ellipsometry indicate that TP forms poorly defiied layers, while BPM and TPM form monolayers with reproducible CAS and ellipsometric thicknesses. BPM and TPM films are substantially more stable than TP layers toward various electrochemical conditions and replacement by alkanethiols as shown by the combined CAS,ellipsometricthicknesses, and the electrochemical measurements of the corresponding monolayer-covered electrodes. The surface coverages of the three monolayer systems, determined from the electrochemicalresponses of the respectiveelectrodes, show that the blocking efficiency increaseswith the number of phenyl rings in the molecule. Three electrochemicaltechniques, namely gold oxide formation/removal,cyclicvoltammetry (CV),and ac impedancewere used to determinethe monolayer surface coverage. The ac impedance technique proves to be superior for surface coverage determination, mainly due to the milder conditionsapplied. Molecular mechanics calculations show that BPM and TPM might form (d3xd3)R3O0overlayers on Au(ll1) with the molecules nearly perpendicular to the gold surface. Such overlayers would have favorable intermolecularinteractions and retain the usual structure for sulfur on Au(ll1). variety of experimental techniques, including Fourier transform infrared spectroscopy (FTIR) in the external reflection mode,293 contact angle measurements,2t4~5 ellipsometry,2p5 X-ray photoelectron spectroscopy,6 X-ray diffraction: electron diffraction,' helium diffraction: and scanning tunneling microscopy (STM)? One difficulty in monolayer characterization is determining the extent and nature of the surface coverage. Longchain alkyl monolayers can provide an efficient barrier toward heterogenous electron transfer and toward ion * Author to whom correspondence should be addressed. penetrati~n.~JO-l~ The dependence of the blocking effi+ Department of Materials and Interfaces. ciency on the chain length was rationalized by poorer Present address: Division of Chemistry and Chemical Engiorganization and packing of the shorter alkanethiols. neering, California Institute of Technology, Pasadena, CA 91125. 5 Department of Organic Chemistry. Residual electroactivity of monolayer-covered electrodes, e Abstract published in Advance ACS Abstracts, September 1, which is frequently observed, was assumed to occur at 1993. pinholes in the monolayer1' or via electron tunneling (l)Swalen,J.D.;Allara,D.L.;Andrade,J.D.;Chandroee,E.A.;Garoff, through the monolayer.2 Studies with alkylsilane monoS.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K.J.; Yu, H.Langmuir 1987,3, 932. layers12 and with mixed alkylsilane and alkanethiol ( 2 ) Porter, M. D.; Bright, T. B.; Chidsey, C. E.D. J. Am. Chem. SOC. monolayerslO strongly support the pinholes model, with -1987. - - ., -109.3559. .., - ... the pinholes behaving as microelectrode arrays.l0J3 Such (3) Ulman, A.; Eilers, J. E.;Tillman, N. Langmuir 1989,5,1147. (4) Allara, D. L.; Nuzzo, R. G. Langmuir 1986, 1, 45. systems,with monolayer coverages exceeding 99%, strong(5) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall,J.;Whitesides, ly suppress surface redox reactions (Le. electrode reactions G. M.: Nuzzo, R. G. J. Am. Chem. SOC.1989,111,321. (b) Bain, C. D.; which are not controlled by mass transport); however, Evd.'J.:Witesides. G.M. J. Am. Chem. SOC.1989.111. 7155. dependingupon the pinhole distribution and the time scale (6jSmart, M. G.:Brown, C. A.; Gordon, J. G. Lakgmuir 1991,7,437. (7) Strong, L.; Whitesides, G. M. Langmuir 1988,4, 546. of the experiment, they can maintain almost all the ( 8 ) Camillone, N., III; Chidsey, C. E.D.; Liu, G.-Y.; Putvinski, T. M.; reactivity of the electrode toward reactions controlled by Scoles, G. J. Chem. Phys. 1991, 94, 8493. diffusion.l0 The effectiveness of monolayers in inhibiting (9) (a) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC. 1991,113,2805. (b) Sun, L.; Crooks, R. M. J. Electrochem. SOC.1991, electrode kinetics can be effectively used to evaluate the 138, L23. (c) Kim, Y. T.; Bard, A. J. Langmuir 1992,8, 1096. quality of the monolayer. The convenience and sensitivity (10) (a)Sabatani, E.; Rubinstein, I.;Maoz, R.; Sagiv,J.J.Electroanal. of electrochemical probing methods using cyclic voltamChem. 1987,219,365. (b) Sabatani, E.;Rubinstein, I. J.Phys. Chem. 1987,91,6663. metry (CV)2J0-13and ac impedance measurementdo have (11) Finklea, H. 0.;Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, been demonstrated. 3, 409. Several recent reports have been devoted to the study (12) Finklea, H. 0.;Robinson,L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239. of electron tunneling at electrodes covered by alkanethiol (13) Finklea, H. 0.;Snider, D. A.; Fedyk, J. Langmuir 1990,6, 371. based monolayer^.'^ Heterogeneous electron-transfer rate (14) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, constants were found to decay exponentially with the alkyl A. M. J.Am. Chem. SOC.1990,112,4301. (b) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991,95,877. (c) Finklea, H. 0.;Hanshew, chain length.14bc It would be of great interest to compare D. D. J. Am. Chem. SOC.1992,114,3173. the electrode kinetics observed with alkanethiol based (15)(a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv,J. monolayers to that of analogous aromatic monolayers. Nature 1988,332,426. (b)Steinberg, S.; Tor,Y.; Sabatani, E.; Rubinstein, I. J.Am. Chem. SOC.1991,113, 5176. While monolayers of saturated hydrocarbons are highly
Introduction Akanethiol self-assembling (SA) monolayers are probably the most intensively studied SA monolayers on gold surfaces, mainly due to their stability, organization, and potential application as basic building blocks for novel microstructure^.^-^^ The various aspects of monolayer formation, structure, packing, orientation, wetting properties, and stability have been examined using a large
0143-1463/93/2409-2914$04.00/0
0 1993 American Chemical Society
Thioaromatic Monolayers on Gold
Langmuir, Vol. 9, No. 11, 1993 2975
chart I resistive and slow the electrode reaction significantly, monolayers of aromatic nature are expected to present a OM (24 A) considerably lower barrier due to the high density of delocalized electrons. The dependence of the electrode reaction rate on the number of repeating units in aromatic monolayers is expected to be somewhat complicated, since the degree of delocalization itself is a function of the number of the repeating units.16 Well-characterized thioaromatic monolayers may, therefore, add a new dimension to the study of electron tunneling phenomena at monolayer covered electrodes and may allow examiTPM (14 A) nation of theoretical models describingelectron tunneling in organic materials, including conducting polymers. In addition to the potential use of aromatic monolayers as electron and ion barriers at electrode surfaces, there is an increasing interest in the use of thioaromatic derivatives as electrode surfacemodifiers for a variety of other possible app1ications.l' In contrast with the wide research on SA monolayers of saturated hydrocarbon thiols, little attention has been given to thioaromaticmonolayers. The first reported study described adsorption of aromatic thiols onto single crystal platinum.18 Auger electron spectroscopy (AES) and electron-energy loss spectroscopy (EELS)were used to determine the packing density of thiophenol and benzyl chlorosulfonic acid (Anal&) in chloroform (Bio-Lab).= Submercaptan on a Pt(ll1) surface and showed that the sequently, the potassium salt of biphenylsulfonic acid was adsorption of aromatic thiols on platinum is associated precipitated from a saturated aqueous solution of potassium with a proton loss. Recently the study of adsorbed carbonate (Merck,AR).2%The saltwas reacted with phosphorus aromatic thiols was extended to other metal pentachloride (Merck) in a solid phase reactionaabat 60 OC. The and more evidence for cleavage of the S-H bond and product, p-biphenylsulfonyl chloride, was separated from the formation of a metal-thiolate bond upon adsorption was reaction mixture by sublimation. It was then reduced using a presented.lS2l Surface-enhanced Raman spectroscopy zinc (Fluka)amalgam (AR HgCl2 was purchased from Merck) in proved to be a powerful tool for determination of the concentrated hydrochloric acid (Bio-Lab).- The product, packing and orientation of the molecules in such films.20*21 p-biphenylmercaptan (BPM), was obtained by steam distillation from the reaction flask using a stream of boiled hydrochloric High surface density is usually found,la21 but only with acid. High-purity BPM was then obtained by reduced pressure Ag(ll1) has long range order been reported.lg However, sublimation. The melting point of the BPM is 100-102OC. TLC, no electrochemical characterization of benzyl mercaptan proton NMR, 13C NMR, mass spectroscopy, and elemental and thiophenol monolayers has been performed, and, to analysis were used to verify the structure and purity of the BPM, our knowledge, no studies of longer thioaromatic monowhich was then kept in a desiccator under a positive pressure of layers exist. argon. This paper reports the first systematic study of the p-Terphenyl mercaptan (TPM, see Chart I) was synthesized preparation and characterization of thioaromatic monoin four steps. Firat,p-terphenylchlorosulfonicacid was prepared using the published procedures for biphenyl,= except that layers on gold electrodes. Monolayers of thiophenol (TP) concentrations were decreased 10-fold and 1,Bdichloroethane and of its longer analogs p-biphenyl mercaptan (BPM) was substituted for chloroform. A 150-mLportion of 1M NaOH and p-terphenyl mercaptan (TPM) were prepared on gold was added to the p-terphenylchlorosulfonic acid to precipitate electrodes by the self-assemblytechnique. Contact angle sodium terphenylsulfonate. The salt was washed with water and (CA) measurements, ellipsometry and several electrolyophilized. Next, 30 g of the salt was reacted with 30 g of PCL chemical techniques were used to study the properties in tetrachloroethane for 5 h at 70 OC. After fitration, p-terand stability of these monolayer systems and to determine phenylsulfonyl chloride was precipitated from the solution on the electrode coverage by the monolayers. Molecular addition of hexane and subsequently dried in vacuum. Finally, mechanics calculations were used to show the feasibility the sulfonyl chloride was reduced to the thiol using the known of the formation of a densely packed overlayer, commenprocedure forp-biphenylmercaptama Theyellow solid obtained was rinsed with water to achieve a neutral pH and the compound surate with known S overlayers on Au(ll1).
I[
Experimental Section Materials. Octadecyl mercaptan (OM,see Chart I) (Aldrich) was recrystallized from hexane. Thiophenol (TP, see Chart I) (Merck) was vacuum distilled. p-Biphenyl mercaptan (BPM, see Chart I) was synthesized in a four-step protocol, following published procedures. Biphenyl (Aldrich) was para-sulfonized by a dropwise addition of 10.0 M (16) Ford, W. K.;Duke, C. B.; Paton, A. J.Chem. Phys. 1983,78,4734. (17) (a) Rubinstein, I.; Rishpon, E.; Sabatani, A.; Redondo, A.; Gottesfeld, S.J. Am. Chem. SOC. 1990,112,6135. (b) Sun,L.; Johnson,B.; Wade, T.; Crooks, R. M. J.Phys. Chem. 1990,99,8869. (c) Kwan,W.4. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7,1419. (18) Hubbard, A. T. Chem. Rev. 1988,88,633, and references therein. (19) Gui, J. Y.;Stem, D. A.; Frank, D. G.;Lu, F.; Zapien, D. C.; Hubbard, A. T. Longmuir 1991, 7,955. (20) Carron, K.T.; Hurley, L. G. J.Phys. Chem. 1991,95,9979. (21) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Longmuir 1992,8,753.
was vacuum dried. Mass spectra of the compound gives the appropriate parent peak at MW 262. The melting point is above the decomposition temperature of 250 OC. NMR of TPM could not be measured because the compound was insufficientlysoluble in all the solventa tried (methanol, ethanol, ether, acetone, chlorocarbons, benzene, toluene, and hexane). The N M R of the precursor p-terphenylsulfonyl chloride, however, shows that the sulfonyl chloride is para and,thus, the thiol should also be para. The 4M-MHz NMR spectrum of the terphenylsulfonyl chloride is described below. Labeling of the a t o m is the same as shown for p-terphenyl mercaptan in Chart I (a, triplet, 6 = 7.363 ppm, J = 0.018; b, triplet, 6 = 7.456, J = 0.020; c, doublet, 6 = 7.860, J = 0.015; d,e, AB system, 6 = 7.704; f,g, AB system, 61 = 7.760, 62 = 7.858, J = 0.022; hyperfine splittings are not included).
(22) (a) Clark, R. F.; Simons, J. H. J. Org.Chem. 1961,26,6197. (b) Niwa, H. Chem. Abstr. 1958,52,7234%. (c) Lester, C. T.; Rodgem, G.~F.; Reid, E. E. J. Am. Chem. SOC.1944,66, 1674.
2976 Langmuir, Vol. 9, No. 11, 1993
Bicyclohexyl (BCH) (Aldrich) and chloroform (Bio-Lab)were passed through basic alumina (ICN Biochemicals) to remove residual water. The chloroform was stabilized with 1% ethanol. Absolute ethanol (Bio-Lab) was used as received. KJ?e(CN)B (Fluka, AR), Kpe(CN)e (Fluka, AR), and KCl (Merck, AR) were used as received. Acid solutionswere prepared from concentrated H2SO4 (Palacid). Water used in aqueous solutions, rinsing, and contact angle measurements was triply distilled (typical resistivity in the M i h m range). Purified argon was used to deaerate the solutionswhen needed. For the plasma sputtering treatments, purified argon and oxygen were used. Preparation of Electrodes and Monolayers. Gold electrodes were prepared by resistive evaporation of gold (- lo00 A) onto glass microscope slides. The glass slides were treated by Ar plasma prior to gold evaporation, to improve the gold adhesion to the glass. A systematic study has been conducted toward the preparation of high-quality monolayers. A wide spectrum of preparation conditions has been examined. Parameters such as substrate pretreatment, adsorption solution, and adsorption time were systematicallyvaried and the resultant monolayerproperties compared. This rather elaborate part of the study will not be described in any detail. We report here only the procedures which gave reproducible monolayerswith the highest CAS, lowest hysteresis between the advancing and receding CAS,and reproducible ellipsometricthicknesses. TP monolayerswere prepared by immersion of gold electrodes for 15 min in a 1.0 mM aqueous solution of Tp. BPM monolayers were prepared by immersion for 20 min in a 1.0 mM solution of BPM in ethanol. TPM monolayers were prepared by immersion for 3 h in a saturated solution of TPM in ethanol. Octadecyl mercaptan (OM) adsorption was carried out by immersion for 2 h in a 1.0 mM solution of OM in BCH. All electrodes were treated with a mild (- 1.0 W/cm2) oxygen plasma for 1 min to remove contaminations immediately before immersion into the adsorption solutions. Contact Angles (CAS) of water were measured within a few hours of monolayer preparation using a Ram6-Hart 100 goniometer. The advancing CAS were measured immediately after placing a drop of water on the substrate with a micropipet. The receding CASwere measured after the drop volume was reduced by suction. Ellipsometric measurements were performed using a null ellipsometer (Rudolph, Model AutoEl IV) in the ambient. The ellipsometric parameters 9 and A were measured before and after monolayer adsorption. A standard program was used to calculate the monolayer film thicknesses. For the calculation, we assumed that all monolayers studied here are transparent (kf = 0) and that the original refractive index of the substrate is not altered by the adsorbed layer. The real part of the index of refraction, q,assigned to all monolayers was 1.5.2p Varying q between 1.4 and 1.6 does not significantly change the calculated thickness. Electrochemical measurements were performed using a three-electrode cell comprising the gold working electrode, a platinum flag counter electrode, and a commercial reference electrode. The reference electrodes used werea saturated calomel electrode (SCE), used in chloride solutions, and a mercurous sulfate electrode (MSE, +0.400 V vs SCE), which was used in sulfate solutions. For cyclic voltammetry (CV) measurements, a Solartron potentiostat (Model 1286)was used with a Houston 100 x-y recorder. For ac measurements the same potentiostat, connected to a Solartron frequency response analyzer (Model 1250), was used, at an amplitude of 5 mV (rms). Molecular Mechanics Calculations. Molecules were positioned with Macromodel,” while the intermolecular interaction energies were calculated usingVAX-Model,u which has a specific aromatic carbon atom type. Crystal structure coordinates were obtained from the Cambridge Data Base.% In the postulated overlayer, each molecule has two “herring bone” interactions with
Sabatani et al. its nearest neighbors and one parallel interaction with its next nearest neighbor. Longer range interactions and interactions with the thiol group were neglected and all molecules were assumed to have the same conformation and tilt. Terphenyl was treated as a rigid structure and librations were neglected. Energy minimizations of the overlayer were performed first with respect to rotation about the Czaxis. During this minimization, molecules were tilted 15O from the surface normal about the axis (axis p ) connectingthe bottom carbons of neighboringparallel molecules. Adjacent moleculesin the “hemng bone”were rotated in opposite directions. Next, the energy was minimized with respect to rotation of each molecule about the axis perpendicular to axis p and in the plane containing the bottom carbon atoms of all molecules. Finally, the energy was minimized with respect to rotation about axis p.
Results Wettability and Thickness Measurements. The wettability of surfaces covered with monolayers can be correlated with the quality of the m~nolayers.~fi*Therefore, advancing and receding CAS of water were the preliminary tool in the investigation of the thioaromatic monolayers. Since there is essentially no information regarding the wettability of surfaces covered with aromatic monolayers or CA values for water on such monolayers, we have to rely on the reproducibility of our results and the hysteresis between the advancing and receding CAS% to evaluate the monolayer quality. Table I summarizes the advancing and receding CAS typical of high-quality monolayers of TP,BPM, and TPM. The CAS for Au/BPM and Au/TPM are either higher (BPM) or have a smaller hysteresis (TPM) than those obtained for Au/TP, suggesting a better packing or different orientation of the former monolayers. When subjecting the electrodes to a subsequent adsorption of a long chain aliphatic thiol, such as octadecyl mercaptan (OM),the CAS for water are altered. The most pronounced change in the CAS is observed at Au/TP; the CAS of Au/ (TP + OM) are closer to the CAS of pure OM monolayer (Au/OM)lp4than to the CAS of Au/TP. In contrast with this observation, the CAS of Au/(TPM + OM) differ only slightly from the CASof Au/TPM. The change in the CAS of BPM shows an intermediate response after subjecting Au/BPM to OM adsorption. The thickness of the monolayer can also be an indication of its quality. It can serve to estimate the monolayer compactness and the orientation of the adsorbed molecules. To get an idea regarding expected thickness values, Chart I presents the relevant compounds and theoretical monolayer thicknesses when the molecules are fully extended and oriented perpendicular to the electrode surface. The experimental thicknesses reported below were measured using ellipsometry, which has been widely used for monitoring monolayer t h i c k n e ~ s e s . ~ ~ s * ~ The ellipsometric thicknesses measured a t Au/TP, Au/
BPM,Au/TPM,andatAu/(TP+OM),Au/(BPM+OM),
and Au/(TPM + OM) are also shown in Table I. The thickness obtained for a TP monolayer is significantly smaller than the expected value (see Chart I), possibly due to a nonperpendicular orientation of the phenyl ring with respect to the surface19 and/or poorer packing of the molecules in the monolayer. It is also possible that a certain change (induced by the adsorbed thiol) of the substrate index of refraction is contributing to the low (23)Still, C., Department of Chemistry, Columbia University, New apparent thickness. Contrary to Au/TP, the thicknesses York, NY. (24) Steliou, K. Department of Chemistry, University of Montreal, obtained for Au/BPM and Au/TPM are quite close to the Montreal, Canada. expected values (see Chart I). The thicknesses obtained (25)AUen,F.H.;Bellard,S.;Brice,M.D.;Cartwright,B.A.;Doubleday,
A.; Higgs, H.; Hummelink, T.; Hummelink-Peters, B. G.; Kennard, 0.; Motherwell, W. D. 5.;Rodgers, J. I. R.; Watson, D. G. Acta Crystollogr. Sect. B 1979,35, 2331.
(26) Ulman,A. Anlntrodoction to Ultrathin OrganicRlm; Academic Press: New York, 1991, and references therein.
Thioaromatic Monolayers on Gold
Langmuir, Vol. 9, No. 11, 1993 2977
Table I. CAS for Water and Ellipsometric Thicknesses of Monolayer-Covered Electrodes electrode
tme
treatment
AuITP AdBPM
AuITPM Au Au/TP Au/BPM AuITPM
Au/TP
AuIBPM Au/TPM
afterOM adsorption
after repetitive CVin &Fe(CN),j
CA [degl adv rec 80f2 76f2 85fl 82f1 79f2 80f2 112f1 110f3 97 f 1 84f2
112f1 106f2 91 f 1 84k2
monolayer thickness [A] 1f1 9f2 llfl 23f 1 20f4 13f3 16f3 14 f 4 8f2 10f2
for the mixed monolayers show the same trend as the corresponding CA measurements. While OM alters the thickness of TP to a large extent, it has less influence on the thicknesses of BPM or TPM monolayers. The increase in the CAS of water and the alteration of the thickness of the original monolayer due to additional adsorption of OM suggest OM adsorption at the electrode surface and partial replacement of the original aromatic molecules. Subsequent adsorption of OM is, therefore, an effective probe for the compactness and stability of thioaromatic monolayers. From the results presented in Table I, the most resistive monolayer to OM adsorption is TPM, while BPM is also quite stable. TP molecules, on the other hand, are easily replaced by OM. An interesting effect, also shown in Table I, is the influence of repetitive voltammetric oxidation/reduction cycles of Fe(CN)sP/3- (see below) on the ellipsometric thickness of the monolayers. It appears that the electrochemical treatment affects the apparent thickness of TP monolayers to a much greater extent than the thickness of BPM or TPM monolayers. A reasonable explanation would be that the change in the measured thickness is due to incorporation of ferro/ferricyanide ions in the monolayer during the treatment. The apparent increase in the thickness may therefore be an indication of the monolayer permeability and possible interactions of the monolayer with ions, thereby changing the film refractive index. TP monolayers are evidently less compact and more permeable than BPM or TPM monolayers; moreover, BPM and TPM are more hydrophobic and hence more inert and less attractive toward charged species than TP. Electrochemistry at Monolayer-Covered Electrodes. Gold Oxide FormationlRemoval. CV of gold electrodes in acidic aqueous solutions shows a characteristic I-V behavior, corresponding to the gold oxide formation upon scanning the potential positively, and removal of the oxide upon reversing the scan direction. The quantity of gold oxide formed or stripped, and hence the electrical charge passed during its removal, is proportional to the effective electrode area.27a Therefore, comparing the charge under the reduction wave (removal of the oxide) at a monolayer-covered electrode with the respective charge at a bare electrode may serve to measure the electrode coverage by the monolayer. This idea was previously tested and proved to apply to octadecyl-type monolayers.lOJ1 Figure 1 shows voltammetric gold oxide formation/ removal in 0.1 N H2S04 at bare gold, Au/TP, Au/BPM, and AuITPM electrodes. A positive shift and an enhanced oxidation current (at potentials >1.1 V vs SCE) are (27) (a) Gileadi, E.; Kirowa-Eisner, E.; Penciner, J. Interfacial EZectrochemistry;Addison-Wesley: Reading,MA, 1975;pp 434-437. (b)Ibid. pp 448-450. (c) Ibid. p 373.
-
._.
.
,-.
\;I U
t
f
I
H 0.2 v Figure I. Cyclic voltammograms in 0.1 N HzSOc for (a) bare Au, (b) Au/TP, (c) Au/BPM, and (d) Au/TPM in a limited potential range (dashed-dotted line) and in the gold oxide formation/removalpotential range duringthe f i t cycle (dashed line) and after four repetitive cycles (solid line) (A = 1.0 cm2,u = 0.1 V/s).
observed at the three monolayer-coveredelectrodes during the fiist positive scan. The most significant shift in potential and the most pronounced enhancement of the oxidation curren occur a t Au/TPM. In the reverse direction and in subsequent voltammetric cyclesa different behavior is observed at the different types of monolayercovered electrodes when comparing the reduction charge at the monolayer-coveredelectrodes with that at bare gold. While a t Au/TP the charge passed during the first oxide removal wave is 69 % of its value at bare gold, it is 48 76 at Au/BPM and is only 11% at Au/TPM. At Au/BPM, however, after three additional voltammetric cycles the reduction wave closely resembles that at the bare gold. At Au/TPM the reduction charge after three additional cycles also increases (relative to the first cycle) but is still less than 30% of that measured a t steady state at bare gold. Enhanced oxidation currents are common for noble metal electrodes covered with adsorbed organic subs t a n c e ~ . ~The ' ~ enhanced oxidation wave in the first scan at the thiol-monolayer covered electrodes may, therefore, be an indication of the occurrence of another process at the electrode, namely, oxidation of the monolayer itself, as suggested earliere2*The shape of the subsequent cycles indicates that this process is irreversible and results in monolayer removal from the surface. It takes only one gold oxide formation/removal cycle to destroy the TP monolayer almost completely, whereas four such cycles are needed to remove the BPM monolayer. The TPM monolayer is, apparently, more stable, but is also damaged by the positive bias. When the potential scan is confined to more negative potentials (
