Langmuir 1994, 10, 4566-4573
4566
Alkanethiol Monolayers on Preoxidized Gold. Encapsulation of Gold Oxide under an Organic Monolayer Hannoch Ron and Israel Rubinstein* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
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Received August 1, 1994® The adsorption of dodecyl mercaptan (DM) monolayers onto plasma preoxidized gold was studied. The degree of preoxidation was determined quantitatively using electrochemical stripping analysis. It is shown that (i) alkanethiols form oriented monolayers on gold oxide surfaces, (ii) The contact angles and ellipsometric parameters of DM monolayers on gold vary systematically with the degree of preoxidation of the evaporated gold substrate, (iii) The monolayer effectively isolates the oxide from its surrounding, thus preventing chemical and electrochemical reduction of the inherently unstable gold oxide. The existence of a layer of encapsulated gold oxide under the organic monolayer is verified by X-ray photoelectron spectroscopy (XPS) and combined electrochemical-depth profiling measurements.
Introduction
or refer to, a certain procedure of surface treatment prior to monolayer absorption. Some of the common procedures for cleaning gold surfaces include exposure to powerful oxidizing agents, such as oxygen plasma,7 8UV/ozone,43 and a mixture of concentrated sulfuric acid and hydrogen peroxide (“piranha” solution).21 (Electrochemical oxidation pretreatment is different in that the oxidation is usually followed
Self-assembled (SA) monolayers chemically adsorbed onto solid surfaces have been of growing interest in recent years, from both basic and technological points of view. Organosulfur amphiphiles form SA monolayers on gold1™50 as well as on silver47™57 and copper48,57-58 surfaces; of these, gold has the unique advantage of chemical inertness. Indeed, gold is by far the most commonly used metal for self-assembly. Of particular interest is the fact that a gold surface is not oxidized under ambient conditions,598 while its high surface energy is lowered by adsorption of organic molecules from its surrounding.60 Hence, the cleanliness of the gold surface plays a major role in the self-assembly process, and most published studies contain, *
®
Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, November
Hautman, J.; Klien, M. L. J. Chem. Phys. 1989, 91, 4994. Ulman, A. Introduction to Ultrathin Organic Films; Academic New York, 1991. Ulman, A. Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; ACS Symposium Series 447, American Chemical Society: Washington, DC, 1990; pp 144-159. (19) Ulman, A. J. Mater. Educ. 1989,11, 205. (20) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147. (21) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (22) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem.Soc. 1990, (16) (17) Press: (18)
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(1) Nuzzo, R. G.; Aliara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (3) Li, T.-T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984,106, 6107. (4) (a) Aliara, D. L.; Nuzzo, R. G. Langmuir 1985,1, 45. (b) Aliara, D. L.; Nuzzo, R. G. Langmuir 1985,1, 52. (c) Nuzzo, R. G.; Fusco, F. A.; Aliara, D. L. J. Am. Chem. Soc. 1987,109, 2358. (d) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (e) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570. (f) Nuzzo, R. G.; Dubois, L. H.; Aliara, D. L. J. Am. Chem. Soc. 1990,112, 558. (g) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Am. Chem. Soc. 1990, 93, 767. (5) Porter, M. D.; Bright, T. B.; Aliara, D. L.; Chidsey, C. E. D. J.Am. Chem. Soc. 1987, 109, 3559. (6) Harris, A. L.; Chidsey, C. E. D.; Levinson, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987, 141, 350. (7) (a)Sabatani, E.; Rubinstein, I.;Maoz, R.; Sagiv, J. J.Electroanal. Chem. 1987,219, 365. (b) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (c) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (8) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Aliara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (9) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988,110, 3665. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988,110, 6560. (c) Bain, C. D.; Whitesides, G. M. Science 1988,240,62. (d) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (e)Bain, C. D.; Whitesides, G. M.; Evall, J. J. Am. Chem. Soc. 1989, 111, 7155. (f) Bain, C. D.; Whitesides, G. M.; Evall, J. J. Am. Chem. Soc. 1989, 111, 7164. (g) Bain, C. D.; Whitesides, G. M. Langmuir 1989,5, 1370. (h) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989,93,1670. (i) Bain, C. D.; Biebuyck, . A.; Whitesides, G. M. Langmuir 1989, 5, 723. (10) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (11) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (12) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (13) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (14) Stefely, J.; Markowitz, M. A.; Regen, S. L. J. Am. Chem. Soc. 1988, 110, 7463. (15) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, . P. J. Electroanal. Chem. 1988, 241, 199.
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Alkanethiol Monolayers
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by reduction.298) These cleaning procedures have proven efficient in removing organic contaminations; what is hardly ever considered is the fact that the oxidizing treatments tend to oxidize the gold surface itself. For example, a gold surface is known to oxidize when treated with even mild oxygen plasma.2-61 Gold oxide is a thermodynamically unstable compound and is decomposed in the ambient.596 Nonetheless, some attempts to self-assemble monolayers on gold surfaces oxidized prior to adsorption have been reported. Finklea et al.62 studied octadecyltrichlorosilane monolayers adsorbed onto anodically preoxidized gold surfaces. On the basis of cyclic voltammetry measurements, they concluded that the gold oxide is reduced by S1CI3 moieties. Similar observations based upon XPS measurements were reported by Fischer et al.63 in an attempt to adsorb 1,1'ferrocenediyldichlorosilane onto anodized gold surfaces. Nuzzo and Aliara1 assembled monolayers of disulfides on zero-valent gold, which were then characterized by water contact angles, ellipsometry, and grazing angle Fourier transform infrared (GFTIR) spectroscopy. When adsorbed onto surfaces pretreated with oxygen plasma, the presence of a monolayer could not be detected. In another study, Hickman et al.64 reported successful formation of disulfide monolayers on oxygen plasma pretreated gold surfaces. XPS measurements could not detect any oxidized gold after monolayer adsorption. Bain et al.2 were unable to generate good re-alkanethiol monolayers consistently on gold surfaces cleaned by oxygen plasma, presumably due to the formation of a metastable gold oxide. It is therefore clear that, while oxidizing pretreatments are commonly applied in the study of SA monolayers on gold, the nature and consequences of adsorption of organosulfur monolayers onto gold surfaces pretreated by such procedures are not well understood. We have used dc oxygen plasma successfully for cleaning gold substrates prior to monolayer adsorption, as previously reported.7 Reproducible results were obtained only upon using short plasma cleaning under mild conditions. The ellipsometric parameters were measured before and after oxygen plasma cleaning, as well as after adsorption of the SA monolayer. After short oxygen plasma treatment the optical parameters of the gold slightly change and the water contact angle (CA) is effectively zero. This was interpreted as reflecting gold surface cleaning from organic contaminants. The monolayer thicknesses calculated using the gold parameters before and after mild plasma treatment were comparable and reproducible. When the exposure to oxygen plasma is prolonged, significant changes in measurable quantities are observed. (48) Laibinis, P. E.; Whitesides, G. M.; Aliara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991,113, 7152. (49) Laibinis, P. E.; Whitesides, G. M. J.Am. Chem. Soc. 1992,114, 1990. (50) Widrig, C. A.; Chinkap, C.; Porter, M. D. J. electroanal. Chem. 1991, 310, 335. (51) Walczak, . M.; Chinkap, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991,113, 2370. (52) Sandorf, C. J.; Garoff, S.; Leung, K. P. Chem. Phys. Lett. 1983, 96, 547. (53) Arndt, Th.; Schupp, H.; Schrepp, W. Thin Solid Films 1989, 178, 319. (54) Harris, A. L.; Tothberg, L.; Dubois, L. H.; Levinson, N. J.; Dhar, L. Phys. Rev. Lett. 1990, 64, 2086. (55) (a) Sobocinski, R. L.; Bryant, . A.; Pemberton, J. E. J. Am. Chem. Soc. 1990,112,6177. (b) Bryant, . A.; Pemberton, J. E. J.Am. Chem. Soc. 1991,113, 3629. (c) Bryant, . A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (56) Renter, P.; Eisenberger, P.; JunLi; Camillone, N., Ill; Bemasek, S.; Scoles, G.; Ramanarayanan, T. R.; Liang, K. S. Langmuir 1991, 7, 2013. (57) (a) Blackman, L. C. F.; Dewar, M. J. S. J. Am. Chem. Soc. 1957, 171. (b) Blackman, L. C. F.; Dewar, M. J. S.; Hampson, H. J. Appl. Chem. 1957, 7, 160.
Langmuir, Vol. 10, No. 12, 1994 4567 The differences in the ellipsometric parameters before and after plasma cleaning increase, and hence the monolayer thicknesses calculated using the parameters of the gold before and after oxygen plasma cleaning become substantially different. The CAs for the solvents used (water, bicyclohexyl, hexadecane) also change, and the ellipsometry and CA measurements become less reproducible. Hence, oxidizing pretreaments of gold prior to self-assembly, with all their clear merits, have one major drawback: The distinction between the conditions which produce an apparently “clean” surface and those which lead to erroneous results is not always clear and easy to
identify.
The cited literature reports and our own observations imply that the degree of gold oxidation prior to adsorption may influence strongly the characterization and possibly the properties of the SA monolayer, a topic well worth investigating. Moreover, adsorption of organosulfur monolayers on natively oxidized silver and copper surfaces has been receiving increasing attention recently.47-50 The influence of the oxide layer on the quality and the
characterization of monolayers on active metals is far from being understood47-48 and presents substantial hindrance to such investigations. A quantitative study on the role of the oxide in monolayer formation and characterization in the case of metals such as silver, copper, or iron is complicated by the fact that controlling the amount of oxide on the surface of these metals under ambient conditions is practically impossible. Thus, absorption of a simple organothiol monolayer on a gold surface oxidized in a controlled manner may serve as a useful model and a starting point for the understanding of SA processes on natively oxidized surfaces. The first objective of the present work is therefore to investigate quantitatively the formation of SA monolayers on gold surfaces pretreated by oxidizing procedures prior to adsorption, and particularly the effect of surface preoxidation on monolayer characterization. The second objective concerns the fate of the thin gold oxide film following monolayer adsorption, i.e. whether or not the inherently unstable oxide persists under a single monomolecular layer. The latter question is of fundamental interest, as it would demonstrate (i) the exceptional blocking capability of alkanethiol monolayers and (ii) the possibility to “encapsulate” an unstable compound under a single monolayer.
Experimental Section Materials. Dodecyl mercaptan (DM) (Merck, >98%) was distilled under reduced pressure. Chloroform (Biolab, AR), bicyclohexyl (BCH) (Aldrich, AR), and hexadecane (HD) (Sigma, AR) were passed through a column of activated basic alumina (Alumina B, Akt. 1, ICN). Water was triply distilled. Ethanol (Merck, AR), acetone (Biolab, AR), H2SO4 (Merck, 95-97%), and LÍCIO4 (Merck, AR) were used as received. Gases used were argon (99.996%), oxygen (99.5%), and dry purified air. (58) Laibinis, P. E.; Whitesides, G. M. J.Am. Chem. Soc. 1992,114, 1990. (59) (a) Trapnell, B. M. W. Proc. R. Soc. London 1953, A218,566. (b) Somorjai, G. A. Chemistry in Two Dimensions·. Surfaces; Cornell University Press: Ithaca, NY, 1981. (60) Smith, T. J. Colloid Interface Sci. 1980, 75, 51. (61) (a) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Caudano, R. Surf. Sci. 1984,141, 211. (b) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Caudano, R. Surf. Sci. 1984, 141, 221. (62) Finklea, . O.; Robinson, L. R.; Blackburn, A.; Richer, B.; Aliara, D. L.; Bright, T. B. Langmuir 1986, 2, 239. (63) Fischer, A. B.; Wrighton, M. S.; Umana, M.; Murray, R. W. J. Am. Chem. Soc. 1979, 101, 3442. (64) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. L; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (65) Cobum, J. W. Plasma Etching and Reactive Ion Etching; American Vacuum Society Monograph Series, IBM Research Laboratory: San Jose, CA, 1982; pp 1-38
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Preparation of Gold Substrates. (Ill) textured gold films
prepared in a cryogenically-pumped resistive evaporator. Gold (1000 Á) (99.99%) was evaporated from a tungsten boat at 4 x 10-6 Torr, at a deposition rate oflA/s, onto glass microscope cover slides (Deckglass, Superior).66 Slides (22 x 11 mm) were chemically cleaned with acetone followed by mild Ar plasma cleaning prior to gold evaporation. Monolayer Preparation on Bare Gold Substrates. Following gold evaporation, the chamber was opened by letting argon in. The ellipsometric parameters of no more than three slides were measured, and the slides were immediately immersed into the adsorption solution (2 mM solution of DM in ethanol) for 2 h. After removal from the solution the slides were rinsed twice with chloroform and ethanol and dried under a stream of dry air. were
Monolayer Preparation
on
Oxidized Gold Substrates.
The plasma treatment was carried out in an Edwards S150B sputter-coater using oxygen plasma at 1 x 10-1 Torr, 4 mA. Prior to oxidation the chamber was altematingly purged 5 times with argon and oxygen. The amount of oxidation was controlled by controlling the duration of the plasma treatment. An identical degree of oxidation was observed with slides that were successively oxidized under the same conditions. In a typical experiment two identical gold-coated slides were oxidized by O2 plasma under the same conditions and their optical constants measured. One of the slides was then used for DM adsorption, while the other slide was electrochemically stripped (see below) to determine the amount of oxide on the slides. The adsorption was carried out as above. Ellipsometry. Measurements were carried out before and immediately after plasma oxidation of the gold substrate (the gold oxide refractive index used to estimate the oxide thickness is nf = 3.24, kf = 1.09, calculated from ref 67), as well as before and immediately after monolayer adsorption (the monolayer refractive index used to obtain the monolayer thickness is nt = 1.5, kf = 0).2 A Rudolph Research Auto-EL IV ellipsometer with a monochromated tungsten-halogen light source was used, at an angle of incidence = 70° and a wavelength A = 6328 Á. The same three points were measured on the bare and the monolayer covered slide. Contact Angle Measurements. Contact angles (advancing and receding) of H2O, BCH, and HD were measured within 10 min after removal of the slides from the adsorption solution. Three measurements at different spots were carried out with each solvent. A Rame-Hart NRL Model 100 contact angle goniometer was used. The error in the contact angle measurements was ±2° (for BCH and HD) and ±3° (for water). Electrochemical Measurements. The electrochemical system included a potentiostat and an electrochemical programmer, both from the Department of Chemistry, Technion, Haifa, and a Houston Instruments Model 100 x-y recorder. The counter and reference electrodes were, respectively, a Pt flag (lxl cm2) and a mercurous sulfate reference electrode (MSE, +0.400 V vs KC1 saturated calomel electrode, SCE). For the sake of clarity, all potentials are reported with respect to a SCE. Two types of measurements were carried out: (i) Electrochemical stripping of gold oxide: Immediately following ellipsometric characterization the slide was mounted as a working electrode in a three-electrode cell containing 0.1 M H2SO4. The gold oxide was reduced by applying a potential scan from +1.025 to +0.200 V (vs SCE) at 0.1 V/s (the same method was previously used to evaluate the amount of oxide produced by electrochemical oxidation of gold).62·68·69 The stripping was usually followed by an additional scan in the same voltage range, to verify the total removal of the oxide in the first scan, (ii) Open-circuit potential decay (OCPD) of preoxidized slides: This technique involves the measurement of the potential of an electrode (vs a reference electrode) as a function of time at open circuit (no applied potential).70 The open-circuit potential of treated electrodes was measured vs a SCE reference electrode (through a salt bridge, (66) Golan, Y.; Margulis, L; Rubinstein, I. Surf. Sci. 1992,264,312. (67) Kim, Y. T.; Collins, R. W.; Vedam, K. Surf. Sci. 1990,233, 341. (68) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 31, 29. (69) Oesch, U.; Janata, J. Electrochim. Acta 1983, 28, 1237. (70) Celdran, R.; Gonzalez-Velasco, J .J. Electrochem. Soc. 1986,132, 2373.
Figure 1.
(a) (-) Cyclic voltammogram for the electrochemical
oxidation-reduction of a gold electrode in 0.1 M H2SO4 (scan rate, 100 mV/s, electrode area, 1.3 cm2). (--) Same, in pH 7.0 potassium phosphate buffer, (b) Linear potential scan in 0.1 M H2SO4 for the stripping of gold oxide produced by exposure of a gold electrode to (-) oxygen plasma for 2 min; (--) UV/ ozone treatment; (· ·) no pretreatment (scan rate, 100 mV/s; =
electrode area, 1.1 cm2).
0.1 M LÍCIO4) in either ethanol or in 0.1 M H2SO4, using the potentiostat (in an open-circuit mode) and the recorder (in a y-t mode).
X-ray Photoelectron Spectroscopy (XPS). Measurements
carried out at the Facultes Universitaires Notre-Dame de la Paix, Laboratoire LISE, Namur, Belgium, using a Surface Science SSX-100 ESCA instrument. A beam size of 1000 µ . was used for the survey scans and 600 µ for the narrow scans. Monolayer Etching. Argon ion etching of monolayers controlled by Auger electron spectroscopy (AES) was carried out at the Surface Analysis Laboratory, Technion, Haifa, using a scanning Auger spectrometer (Perkin-Elmer PHI model 590-A). UV/Ozone Treatment of Bare Gold. Gold-covered slides were treated 10 min with ozone in a UV/ozone cleaner (UVOCS, Inc., Model TlOx 10/OES-E). In the present study the amount of resultant oxide is qualitative and varies with the distance from the UV radiation source.
were
Results and Discussion
Quantitative Analysis of Gold Oxide Using Controlled Electrochemical and Chemical Decomposition. Cyclic voltammetry (CV) and open-circuit potential decay (OCPD) were used to study quantitatively the oxidation of gold surfaces by reactive oxygen plasma. Figure la (solid line) shows a voltammogram obtained with a gold electrode in 0.1 M H2SO4 upon cycling the potential between +0.20 and +1.41 V (vs SCE) at a scan rate of 0.10 V/s. In this solution the anodic oxidation of gold takes place at potentials positive of +1.03 V, while the reduction of the gold oxide is observed upon reversing the scan direction, between +1.03 and +0.63 V. In pH 7 buffer the anodic oxidation of gold occurs at potentials positive of +0.65 V, while the reduction of the gold oxide takes place between +0.65 and +0.20 V, as shown in Figure la (dashed line). The amount of oxide produced in the anodic oxidation of gold can be quantitatively determined by integrating the area under the reduction peak.68’69 Figure lb (solid line) shows a voltammogram for a similar gold-coated slide pretreated in a reactive oxygen =
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Langmuir, Vol. 10, No. 12, 1994 4569
plasma, while the gold in Figure lb (dashed line) was pretreated in a UV/ozone chamber. Here the potential scan was initiated at 1.025 V, in the negative direction. When the potential is scanned from +1.025 to +0.200 V, sizable reduction peaks are observed, corresponding to reduction of gold oxide formed by the oxygen plasma and
UV/ozone treatments. Hence, gold oxide produced as a result of O2 plasma, as well as UV/ozone treatment, can be quantitatively reduced electrochemically (“stripped”). Note that when the slide is not preoxidized, or when it is exposed to argon plasma, no reduction peak is observed upon applying a stripping cycle, and a curve similar to the one shown in Figure lb (dotted line) is observed. Electrochemical stripping was not possible when goldcoated slides used in the present work were pretreated in “piranha” solution, as the gold films peeled off. Nevertheless, OCPD measurements described below indicate that the gold surface is oxidized to some extent following treatment in “piranha” solution. The amount of oxide produced upon exposure of gold to oxygen plasma and ozone treatment can be quantitatively determined using electrochemical stripping, as shown above (Figure lb) and previously reported (for O2 plasma) by Armstrong and White.71 Note that Sondag-Huethorst and Fokking43 concluded, based upon cyclic voltammetry measurements, that a gold surface is not oxidized upon exposure to UV/ozone. This seemingly erroneous observation appears to be due to the fact that they started the potential scan at a potential negative of the gold oxide reduction, where the oxide is reduced instantaneously. Figure lb shows unequivocally that gold surfaces treated under UV/ozone conditions are oxidized. Gold oxide is known to be an unstable compound,61 and it decomposes rapidly in the ambient unless special care (such as reduced temperature or controlled environment) is taken. It should be noted, however, that the amount of oxide (as measured by voltammetric stripping) remains practically unchanged during the time needed to perform our experiments (i.e. within 15 min of the gold oxidation). The potentials where oxidation/reduction of gold takes place depend upon the pH of the solution (Figure la). In addition, the peak shape and position vary with the composition of the gold oxide.69·72’73 Comparison of the voltammetric shape for the stripping of gold oxide (in 0.1 M H2SO4) produced by exposure to O2 plasma and UV/ ozone (Figure lb) with that for the anodic gold oxide (Figure la, solid line) shows a marked similarity, suggesting that the oxide produced by the former treatments is also AU2O3. The latter conclusion is supported by XPS studies of gold oxide produced by anodization of gold in an aqueous solution, carried out by Dickinson et al.74 as well as by Peuckert et al.,75 and by Amstrong and White’s study71 concerning the exposure of gold to oxygen rf plasma. Comparison with XPS measurements of gold oxides produced upon exposure to O2 plasma by Pireaux et al.61b shows again that gold oxidation by O2 plasma produces AU2O3.
The thickness of the gold oxide produced by O2 plasma measured ellipsometrically, as listed in Table 2 for various exposure times. The oxide thicknesses in Table was
(71) Amstrong, N.
121.
R; White, J. R. J. Electroanal. Chem. 1982,131,
(72) Bruke, L. D. In Electrodes of Conductive Metallic Oxides, Part A; Trasatti, S., Ed.; Elsevier: Amsterdam, 1980; p 141. (73) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429. (74) Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1975, 71, 298. (75) Peuckert, M.; Coenen, F. P.; Bonzel, . P. Surf. Sci. 1984,141, 515.
Figure 2. Open-circuit potential decay of gold electrodes after the following treatments: (1)2 min in oxygen plasma (measured in pH 7.0 potassium phosphate buffer); (2) 2 min in oxygen plasma (measured in ethanol); (3) 6 min in oxygen plasma (measured in ethanol); (4) 0.5 min in “piranha” solution (measured in ethanol); (5) 2 min in argon plasma (measured in ethanol). =
2 were calculated using the refractive index of anodically grown gold oxide, derived from the results of Kim et al.42 In the above publication a thickness of 4 Á corresponds to the stripping of 1400 /¿C/cm2, in noted disagreement with our present results, where a thickness of 4.2 Á corresponds to 600 /iC/cm2. Previous, unsuccessful attempts to determine optical properties of gold oxide were reported by Shironi and Genshaw76 as well as by Horkans et al.77 Our thicknesses are in good agreement with those calculated for the electrochemically-grown gold oxide in ref 69, where 4.7 Á oxide (a monolayer of oxide) corresponds to 723 /
