Determination of the Potential of Zero Charge of Au (111) Modified

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Anal. Chem. 2007, 79, 6473-6479

Determination of the Potential of Zero Charge of Au(111) Modified with Thiol Monolayers Pablo Ramı´rez,† Rafael Andreu,*,† A Ä ngel Cuesta,‡ Carmen J. Calzado,† and Juan Jose´ Calvente*,†

Departamento de Quı´mica Fı´sica, Universidad de Sevilla, 41012-Sevilla, Spain, and Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, 28006-Madrid, Spain

A new method is proposed for the determination of the potential of zero charge of gold electrodes modified with thiol monolayers. It makes use of the immersion technique, in combination with a vapor deposition protocol to build the thiol monolayers. As compared to previous methods, the present approach provides more accurate results, particularly in the case of long-chain alkanethiol monolayers, and it is applicable to any monolayer irrespective of its degree of hydrophilicity. Results are presented for a series of 12 alkanethiol monolayers and for 11-mercaptoundecanol and 11-mercaptoundecanoic acid monolayers. Good agreement is found between the variation of the potential of zero charge along the alkanethiol series with the corresponding change of the surface work function. The potential of zero charge of the 11-mercaptoundecanoic acid monolayer is shown to depend on the extent of dissociation of the acid, thus opening the possibility of applying this type of measurements to the study of surface ionization processes.

Molecular tailoring of interfacial properties has been boosted in the last two decades with the advent of thiol self-assembled monolayers.1 These monolayers are currently being applied in a variety of scientific areas, such as molecular electronics,2 corrosion protection,3 nanolithography,4 electrochemical sensing,5 or cellular * Corresponding authors. Phone: +34-954557177. Fax: +34-954557174. E-mail: [email protected]; [email protected]. † Universidad de Sevilla. ‡ Instituto de Quı´mica Fı´sica “Rocasolano”. (1) (a) Love, J. C.; Estroff, L. A.; Kriebel, J. C.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (b) Witt, D.; Klajn, R.; Barski, P.; Grzybowski, B. A. Curr. Org. Chem. 2004, 8, 1763. (c) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (2) (a) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (b) Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. Adv. Mater. 2003, 15, 1881. (c) Xiao, Y.; Patolsky, F.; Katz, E.; Hainfeld, J. H.; Willner, I. Science 2003, 299, 1877. (3) (a) Scherer, J.; Vogt, M. R.; Magnussen, O. M.; Behm, R. J. Langmuir 1997, 13, 7045. (b) Whelan, C.; Kinsella, M.; Carbonell, L.; Ho, H.; Maex, K. Microelectron. Eng. 2003, 70, 551. (4) (a) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (b) Kraemer, S.; Fuierer, R. R.; Gorman, C. B. Chem. Rev. 2003, 103, 4367. (c) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (5) (a) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894. (b) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003, 15, 81. 10.1021/ac071341z CCC: $37.00 Published on Web 08/04/2007

© 2007 American Chemical Society

adhesion.6 In spite of all these multidisciplinary applications, some of their fundamental properties remain poorly understood. One of these properties is the potential of zero charge, whose determination with a reasonable accuracy constitutes the main goal of this work. The potential of zero charge (Epzc) is directly related to the surface work function, and its value is determined by the presence of a net dipole moment along the surface normal.7,8 Careful tuning of metal work functions plays a critical role in the development of optoelectronic devices,9 and analogously, an adequate control of the Epzc is required in order to favor (or to hinder) a given adsorption10 or charge-transfer process11 in an electrochemical environment. Thiol monolayers offer the possibility of adjusting the Epzc value by an appropriate choice of the monolayer composition, thereby imparting predesigned Epzc variations of the order of hundreds of millivolts in a controllable manner. Only a few attempts have been made to determine the Epzc of thiol-modified electrodes.12-17 Previous measurements were based on the location of either the electrocapillary maximum,12,14,15 the minimum in the diffuse layer capacitance,13 or the influence of the ionic strength on the Stark effect of fluorescent probes embedded in the monolayer.16 These studies provided initial Epzc estimates, but most of them were rather inaccurate (reliability was typically worse than 0.1 V) and were determined using polycrystalline gold substrates, whose interfacial properties are likely to be less reproducible than those of well-oriented singlecrystal surfaces. More reliable Epzc values, obtained using Au(111) electrodes modified with 1-propanethiol, 1-undecanethiol, 1-octadecanethiol, and 1H,1H,2H,2H-perfluorodecanethiol monolayers were recently reported by Iwami et al.17 They observed a strong (6) (a) Luk, Y. Y.; Kato, M.; Mrksich, M. Langmuir 2000, 16, 9604. (b) Chapman, R. G.; Ostuni, E.; Takayama, S.; Holmlin, R. E.; Yan, L.; Whitesides, G. M. J. Am. Chem. Soc. 2000, 122, 8303. (7) Trassati, S.; Lust, E. Mod. Aspects Electrochem. 1999, 33, 1. (8) Fawcett, W. R. Liquids, Solutions and Interfaces; Oxford University Press: New York, 2004. (9) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. Adv. Mater. 2005, 17, 621. (10) Kuznetsov, B. A.; Byzova, N. A.; Shumakovich, G. P. J. Electroanal. Chem. 1994, 371, 85. (11) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (12) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560. (13) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233. (14) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Electroanal. Chem. 1994, 367, 49. (15) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 2237. (16) Pope, J. M.; Buttry, D. A. J. Electroanal. Chem. 2001, 498, 75. (17) Iwami, Y.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Electroanal. Chem. 2004, 564, 77.

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(>1 V) Epzc shift upon replacing these alkanethiol monolayers with the perfluorinated thiol monolayer, and they reported an increase of the Epzc value as the alkanethiol chain length was shortened. However, it is not evident from their results whether the Epzc values increase linearly with the alkanethiol chain length, in line with the surface potential variations determined by the Kelvin probe technique,18-20 or whether they tend to reach a long-chain limiting value, in agreement with the behavior displayed by the surface work function.21 In this work, we describe a new experimental approach to the determination of the Epzc of gold electrodes modified with thiol monolayers. Our method makes use of the immersion technique,22,23 as implemented previously by one of us.24 As compared to the potential-controlled sessile drop method of Iwami et al.,17 the immersion technique offers some advantages, since it only requires common electrochemical instrumentation and it is not restricted to hydrophobic surfaces, so that valuable information can be obtained from surfaces that expose hydrophilic groups toward the solution. To avoid any interfacial charge buildup during the monolayer formation stage, thiol monolayers were deposited from the vapor phase. In this way, absolute values of the charge on the metal were obtained by integrating the current transients recorded during the potential-controlled immersion experiments. Vapor deposition produces monolayers of similar, or somewhat better, quality than those formed by thiol adsorption from solution, as judged from the monolayer capacitance values, and it can often be carried out under ambient conditions. We have determined the Epzc values for Au(111) surfaces modified with a series of 12 alkanethiols and 2 hydroxy and carboxy derivatives of 1-undecanethiol and compared our results with previously published Epzc and work function values. As a further consistency check, we also confirmed that the observed Epzc variations follow the theoretical trend that is predicted on the basis of the molecular dipole values of the neutral radical thiols. Experimental and Computational Aspects. Reagents and Gold Substrate. 1-Ethanethiol (ETAT), 1-propanethiol (PROT), 1-butanethiol (BUT), 1-pentanethiol (PET), 1-hexanethiol (HEXT), 1-heptanethiol (HEPT), 1-octanethiol (OCT), 1-nonanethiol (NOT), 1-decanethiol (DET), 1-dodecanethiol (DODET), 1-pentadecanethiol (PDET), 1-octadecanethiol (ODET), 11-mercapto-1-undecanol (MUOL), 11-mercaptoundecanoic acid (MUA), and perchloric acid were purchased from Aldrich. Sodium hydroxide and sodium perchlorate were purchased from Fluka. Chemical reagents were used as received. Water was purified with a Millipore Milli-Q system. To prepare the Au(111) surface, a gold wire (99.999% from Advent Research Materials) was melted in a propane-oxygen flame to obtain a small gold bead (d. ∼2.5 mm). Then, a (111) facet of the gold bead was identified from the reflection pattern (18) Evans, D. E.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462. (19) Lu ¨ , J.; Eng, L.; Bennewitz, R.; Meyer, E.; Gu ¨ ntherodt, H. J.; Delamarche, E.; Scandella, L. Surf. Interface Anal. 1999, 27, 368. (20) Ichii, T.; Fukuma, T.; Kobayashi, K.; Yamada, H.; Matsuhige, K. Nanotechnology 2004, 15, S30. (21) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Colorado, R.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690. (22) Kim, S. H. J. Phys. Chem. 1973, 77, 2787. (23) Hamm, U. W.; Kramer, D.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 85. (24) Cuesta, A. Surf. Sci. 2004, 572, 11.

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generated by a He-Ne laser. Extensive polishing parallel to the (111) facet produced a hemispherical gold sample, whose flat circular basis had the desired crystal orientation and was used as substrate for thiol adsorption. A surface area value of 0.072 cm2 was determined by comparing the differential capacity curve obtained in the presence of aqueous 0.01 M HClO4 with that reported by Kolb and Schneider.25 This comparison was also used as a check of the surface crystal orientation. Vapor-Phase Deposition of Thiols. Thiol monolayers were formed just before the measurement of each immersion transient. The freshly flame-annealed gold surface was exposed to a thiolsaturated atmosphere under ambient conditions (i.e., ∼1 atm pressure and ∼23 °C temperature), except in the case of PDET, ODET, MUA, and MUOL, which are solid at room temperature, and whose deposition was carried out inside a furnace at 75 °C. The gold surface was placed 1 cm above 3 mL of the neat thiol in a sealed glass vial. Under these conditions, exposure times between 15 and 60 min, depending on the hydrocarbon chain length of the thiol, were enough to produce compact thiol monolayers. No significant changes in monolayer properties, such as differential capacity values or desorptive voltammetric transients, were observed either upon further increasing the exposure time or by performing the deposition process under a nitrogen atmosphere. Electrochemical Measurements. Electrochemical measurements were carried out in a three-electrode electrochemical cell (Figure 1). A Ag|AgCl|KCl (saturated) electrode and a Pt wire were used as reference and auxiliary electrodes, respectively. The reference electrode was housed in a separate compartment, connected to the cell via a Luggin capillary. The thiol-modified gold surface was used as working electrode. In the case of voltammetric and impedance measurements, contact of the working electrode with the cell solution was made by forming a hanging meniscus. To perform charge measurements, the reference electrode was inserted into its compartment, with the stopcock (SC in Figure 1) in the closed position to create an overpressure, the gold surface was placed ∼2 mm above the tip of the Luggin capillary, and the desired potential value was applied between the working and reference electrodes. By opening the stopcock of the reference electrode compartment, a thin jet of electrolyte solution was allowed to flow through the Luggin capillary, so that it flushed the gold surface and, simultaneously, closed the cell electrical circuit. Electrode charge values were then determined by numerical integration of the recorded current transient (Figure 2). To check our experimental setup, we reproduced the potential of zero charge value reported by Kolb et al.23 (0.33 V) of a bare Au(111) electrode in contact with a 0.1 M HClO4 solution. Measurements were performed at room temperature, 23 ( 2°C. Electrolyte solutions were deaerated with a presaturated argon stream prior to the measurements. Voltammetric, chronoamperometric, and cell impedance measurements were carried out with an Autolab PGSTAT30 (Eco Chemie). All potential values are referred to the Ag|AgCl|KCl (saturated) electrode. Dipole Moments of the Neutral Radical Thiols. Recent theoretical calculations26,27 have shown that the Au-S bond is formed without (25) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929. (26) Rousseau, R.; De Renzi, V.; Mazzarello, R.; Marchetto, D.; Biagi, R.; Scandolo, S.; del Pennino, U. J. Phys. Chem. B 2006, 110, 10862. (27) Rusu, P. C.; Brocks, G. J. Phys. Chem. B 2006, 110, 22628.

Figure 1. Schematic representation of the electrochemical cell used to determine absolute charge values of the thiol-modified electrodes by the impinging jet method. The stopcock, in the right-hand arm of the reference electrode, controls the flow of the electrolyte solution toward the gold electrode surface. Table 1. Dielectric Characteristics and Epzc Values of the Au(111)/Thiol Monolayer Interfaces

Figure 2. Capacitive current transients recorded after flushing the thiol-modified electrode surface with the electrolyte solution flowing through the Luggin capillary (see Figure 1). Transients a-c correspond to butanethiol and d-f to pentadecanethiol-modified gold electrodes at the following potential values: (a) -0.5, (b) -0.1, (c) 0.1, (d) -0.6, (e) -0.2, and (f) 0.0 V vs Ag|AgCl|KCl (sat.). The origin of the time axis is arbitrary.

significant charge transfer between these two atoms. Therefore, in order to estimate the surface potential generated by a thiol monolayer, we have modeled the array of adsorbed alkanethiol molecules as a set of point dipoles, each dipole value being given by that of the corresponding alkanethiol neutral radical. The dipole moments listed in Table 1 have been obtained from unrestricted spin B3LYP28 calculations on the doublet state. Similar trends have been found when the Becke exchange functional29 combined with the Perdew and Wang gradient-corrected correlation functional30 was used. Basis sets of quality 6-31G(d) have been chosen for all (28) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (29) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (30) Burke, K.; J. P. Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998.

thiol

na

δb/nm

Cmc/µF cm-2

Epzc/V

r

µd/D

ETAT PROT BUT PET HEXT HEPT OCT NOT DET DODET PDET ODET MUA (HClO4) MUA (NaClO4) MUA (NaOH) MUOL

1 2 3 4 5 6 7 8 9 11 14 17 10

0.42 0.51 0.65 0.74 0.87 0.96 1.10 1.19 1.32 1.54 1.86 2.21 1.58

7.2 ( 1.1 5.0 ( 1.4 3.7 ( 0.6 2.5 ( 0.3 2.2 ( 0.2 1.86 ( 0.07 1.66 ( 0.13 1.44 ( 0.07 1.34 ( 0.03 1.17 ( 0.12 0.98 ( 0.04 0.8 ( 0.1 2.07 ( 0.13

-0.09 ( 0.03 -0.16 ( 0.07 -0.31 ( 0.04 -0.46 ( 0.03 -0.44 ( 0.03 -0.45 ( 0.02 -0.51 ( 0.03 -0.51 ( 0.02 -0.53 ( 0.01 -0.53 ( 0.04 -0.51 ( 0.01 -0.51 ( 0.05 0.12 ( 0.01

3.4 2.9 2.7 2.1 2.2 2.0 2.1 1.9 2.0 2.0 2.0 2.0 3.7

1.866 1.930 1.997 2.009 2.051 2.044 2.077 2.062 2.090 2.098

10

1.58

2.0 ( 0.2

-0.03 ( 0.01

3.6

10

1.58

2.0 ( 0.6

-0.42 ( 0.02

3.6

10

1.58

1.2 ( 0.3

0.07 ( 0.03

2.2

1.808

1.956

a Number of methylene units in the HS-(CH ) -X thiol molecules. 2 n Monolayer thickness values taken from refs 33 and 37, by assuming a 30° tilt of the adsorbed thiol molecules with respect to the surface normal. c Capacitance values for MUOL and MUA monolayers have been corrected for solution creeping, as indicated in the text. d Dipole moment value of the neutral thiol radical (1 D ) 3.336 × 10-30 C m).

b

atoms contained in the system. All the calculations have been performed by means of the GAUSSIAN 98 code.31 In the case of nonderivatized alkanethiols, the total dipole moment has been evaluated for the fully optimized geometry, which corresponds to the all-trans configuration. In the case of MUOL, depending on the relative position of the hydroxyl group with respect to the molecular plane, there exists a set of conformations close in energy, all of them thermally accessible. For this reason, it seems appropriate to use in our model not the dipole moment corresponding to the global energy minimum, but a mean value that takes into account the possibility of free rotation of the H atom of the hydroxyl group. With respect to the MUA, the fully optimized geometry locates the COOH group in the same plane as the backbone of carbon atoms, and gives a H-O-C-O (31) Frisch, M. J.; et al. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.

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torsion angle of 0°. The alternative conformation with a torsion angle of 180° also corresponds to a local minimum, with a close energy value. However, in contrast with the MUOL case, now the H rotation activation barrier is significantly higher than the thermal energy, preventing free rotation of the H atom at room temperature. Then, for this system, the total dipole moment listed in Table 1 corresponds to the global minimum energy conformation. RESULTS AND DISCUSSION (a) Alkanethiol Monolayers. Some typical capacitive current transients, measured by using the impinging jet protocol, are illustrated in Figure 2. They originate from the current flow required to build the interfacial structure at a given potential. Their irregular shape and the time (∼10 ms) necessary for completion of the charging process are determined by the progressive wetting of the electrode surface by the electrolyte jet. The sign and magnitude of the peak current give a straightforward indication of the location of the potential of zero charge with respect to the applied potential. For instance, the Epzc values for butanethiol and pentadecanethiol monolayers (see Table 1) lie between the potentials corresponding to transients a - b and d - e in Figure 2, respectively, where the capacitive currents change their sign. By assuming that the vapor-phase formation of the monolayer does not involve any charging of the surface, absolute charge density values on the metal σM can readily be obtained by integrating the current density transient at each potential, i.e.:

σM(E) )

∫

∞

0

i(E,t) dt

(1)

where i is the capacitive current density and t is the time elapsed along the transient. Figure 3 shows some representative plots of σM as a function of the applied potential in the presence of 0.1 M NaOH or 0.1 M HClO4 aqueous solutions. In all cases, a linear relationship is obtained, indicating that the monolayer capacitance Cm remains constant over the explored potential range. In fact, systematic deviations from linearity at either more positive or negative potential values were interpreted as an indication of the onset of faradic surface processes and, therefore, were used to set the limits of the ideally polarizable range. Epzc values were directly available from the σM versus E plots obtained in basic solutions only, but the good overlap between the plots corresponding to acid and basic solutions provides strong evidence for the independence of the Epzc values with respect to both pH and electrolyte nature. Good agreement was found between the Cm values obtained in this work (see Table 1) and those reported previously from either voltammetric32-34 or impedance14,35,36 measurements. Figure 4a illustrates the linear dependence of C-1 m on the alkanethiol chain length, which can be rationalized in terms of (32) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (33) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y. J. Phys. Chem. 1995, 99, 13141. (34) Hagenstro ¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (35) Protsailo, L. V.; Fawcett, W. R. Electrochim. Acta 2000, 45, 3497. (36) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222.

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Figure 3. Electrode charge density as a function of the electrode potential for Au(111) electrodes modified with HS-(CH2)n-CH3 monolayers. (a) (b, O) ethanethiol, (2, 4) butanethiol, and (9, 0) heptanethiol. (b) (b, O) decanethiol, (2, 4) pentadecanethiol, and ((, )) octadecanethiol. Open and filled symbol data were obtained in 0.1 M HClO4 and 0.1 M NaOH solutions, respectively. Solid lines are linear least-squares fits to the filled symbols. The number of methylene groups in the corresponding thiol molecule is indicated beside each plot. Vertical arrows help to locate the value of the potential of zero charge.

an ideal capacitor whose parallel plates are located at the metallic surface and at the solution side of the monolayer,32 i.e.:

Cm ) 0r/δ

(2)

The use of the expressions for the film thickness (δ) proposed by Smalley et al.33 leads to relative permittivity (r) values close to 2, as expected for a dielectric medium made up of hydrocarbon chains. It may be observed in Figure 4a that only the three shortest thiol monolayers deviate significantly from the linear plot, in contrast with previous reports32,33 where these deviations were already evident for longer alkanethiols (with δ e 1 nm). At this point, the improvement in the dielectric behavior of these monolayers can tentatively be ascribed to the vapor-phase preparation procedure, which does not require the removal of interfacial solvent molecules during the self-assembly process. It may be observed in Figure 4b how the Epzc values of alkanethiol monolayers display a saturating effect as the molecular chain length increases. Initially, they shift toward more negative values, but once a limiting value of -0.52 ( 0.01 V is reached for the octanethiol monolayer, Epzc remains insensitive to further increases in the molecular chain length. Earlier measurements12-16 made use of polycrystalline gold substrates and were restricted to a few long-chain (i.e., with n g 7) alkanethiols. They provide an average estimate for the long-chain Epzc limit of -0.40 ( 0.05 V, that is ∼100 mV more positive than our estimate for thiol monolayers deposited on (111) single-crystal gold surfaces. More recently, Iwami et al.17 determined the Epzc values of Au(111)

subjected to a wide potential scan. These two factors have been shown before37,38 to set up suitable conditions for ion permeation through thiol monolayers, particularly in the case of short-chain alkanethiols where lateral interchain interactions are rather weak. In order to rationalize its behavior, it is convenient to split the potential of zero charge into its components:7,8

Epzc )

[

WAu(111) e0

]

+ ∆χmonolayer + ∆χthiol/solution - Eref

(3)

where WAu(111) is the work function of the Au(111) surface, e0 is the elementary charge, ∆χmonolayer is the change in surface potential due to the adsorption of the thiol monolayer, ∆χthiol/solution is the change of surface potential originated at the thiol/solution boundary, and Eref is the absolute value of the reference electrode potential. The sum of the two terms within the brackets represents the work function (divided by e0) of the Au(111) surface modified with the thiol monolayer. The variation of the Epzc upon increasing the alkanethiol chain length (Figure 4b) is expected to be mainly determined by that of the molecular dipole of the thiol,39 according to

∆Epzc ) ∆(∆χmonolayer) ) -

Figure 4. (a) Reciprocal of the electrode capacitance of alkanethiol monolayers as a function of the estimated monolayer thickness (see Table 1). The straight line describes the expected behavior of a parallel plate capacitor filled with a dielectric having a relative permittivity value of 2. (b) Potential of zero charge of Au(111) electrodes modified with thiol monolayers as a function of the number of carbon atoms in the thiol molecule. Open symbols correspond to values measured in this work: (O) alkanethiol monolayers, (4) MUOL monolayer, (]) MUA monolayer in 0.1M HClO4, (0) MUA monolayer in 0.1M NaClO4, and (3) MUA monolayer in 0.1M NaOH. Filled circles (b) correspond to values reported in ref 11 for alkanethiol monolayers. The inverted triangle (1) reproduces the potential of zero charge of Au(111) in contact with a nonadsorbing aqueous electrolyte from ref 23, for an arbitrary abscissa value. (c) Variations of the potential of zero charge, and of the work function, upon increasing the alkanethiol chain length: (O) experimental ∆Epzc values, computed with respect to the Epzc of 1-octadecanethiol monolayers, (b) experimental ∆W/ e0 values from ref 18, computed with respect to the ∆W/e0 of 1-octadecanethiol monolayers, and (+) theoretical ∆Epzc values derived from eq 4 with i ) 1, computed with respect to the Epzc of 1-dodecanethiol monolayers. The solid line (s) is a smoothed fit of the theoretical values.

electrodes modified with pentanethiol, undecanethiol, and octadecanethiol monolayers, and to facilitate a comparison with our own values, their results have been included in Figure 4b. A good agreement is obtained for the Epzc values of the last two monolayers, but their estimate for the pentanethiol monolayer is significantly more positive than ours. At present, we cannot offer an explanation for this discrepancy, though it may be worth mentioning that their contact angle measurements were carried out on a longer time scale and that their monolayers were

N ∆µ⊥ 0i

(4)

where N is the surface density of thiol molecules in the monolayer, µ⊥ is the component of the dipole moment normal to the surface (pointing away from the gold crystal), i is the effective permittivity of the monolayer, and 0 is the permittivity of free space. To compute µ⊥ from the dipole moments listed in Table 1, these molecular dipoles were assumed to be tilted 30° with respect to the surface normal. The effective permittivity i is introduced to account for the screening of the aligned dipoles by the local environment. To derive eq 4, it has been assumed that both N and i are independent of the thiol chain length. The area under the reductive desorption voltammetric peak provides an estimate of N.40,41 By comparing the values of this area for all the monolayers listed in Table 1 (typically 80 µC cm-2), the first assumption concerning the constancy of N was verified experimentally. Though there is not direct access to the i value, it may be observed in Table 1 that r (the expected upper limit for the i value) takes a low value and is largely independent of the thiol chain length. In order to estimate ∆Epzc from eq 4, we used N ) 5 × 1018 m-2, as derived from reductive desorption experiments. Figure 4c shows a good agreement between experimental ∆Epzc values and theoretical estimates derived from eq 4, by assuming that i ) 1. This last value appears to be too low, since it implies that the adsorbed thiol molecules do not feel any depolarizing influence from their dipolar neighbors in the monolayer. Based (37) Calvente, J. J.; Lo´pez-Pe´rez, G.; Ramı´rez, P.; Ferna´ndez, H.; Zo´n, M. A.; Mulder, W. H.; Andreu, R. J. Am. Chem. Soc. 2005, 127, 6476. (38) Boubour, E.; Lennox, R. B. J. Phys. Chem. B 2000, 104, 9004. (39) Vogel, V.; Mo ¨bius, D. Thin Solid Films 1988, 159, 73. (40) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 355. (41) Calvente, J. J.; Kovacova, Z.; Sanchez, M. D.; Andreu, R.; Fawcett, W. R. Langmuir 1996, 12, 5696.

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on a comparison of the theoretical estimates of the dipole moments of 1-methanethiol and 1-ethanethiol, when they are in the gas phase and embedded in their respective monolayers,26,27 i values are expected to lie within the 1.5-2 range. Therefore, the quantitative agreement obtained in Figure 4c may be somewhat fortuitous, due to the physical simplifications inherent to eq 4. Though, it still gives support to the contention that the qualitative trend observed in the variation of the potential of zero charge originates from changes in the molecular dipoles of the thiols. In a similar way, the molecular dipole value appears to also determine the variations of the work function of Au(111) surfaces modified with alkanethiol monolayers under ultrahigh vacuum (UHV) conditions. To rationalize the variations of the work function (∆W) upon increasing the alkanethiol chain length, eq 4 can be used after replacing ∆Epzc by ∆W/e0.27 This relationship is nicely illustrated in Figure 4c, which shows a good agreement between the ∆Epzc variations obtained in this work by the impinging jet method and those of ∆W/e0 determined in UHV from UPS measurements.21 However, it should be pointed out that surface potentials, as measured by the Kelvin probe technique, display a sustained variation with alkanethiol chain length of ∼10 mV per methylene unit.18-20 Within this context, our results provide clear support for the existence of a chain length saturation effect, as first reported by Alloway et al.,21 which is also in accordance with the expected electrostatic behavior of the adsorbed alkanethiol molecules. (b) ω-Functionalized Alkanethiol Monolayers. Functionalization of the terminal methyl group with either hydroxy or carboxy groups imparts hydrophilic properties to the thiolmodified surfaces. Besides, carboxy-terminated monolayers have proved to be useful platforms to immobilize, either covalently or electrostatically, proteins on electrodes. The presence of electronegative oxygen atoms in the monolayer is expected to modify the surface dipole and, thereby, the potential of zero charge. Current transients recorded at these hydrophilic surfaces decay over a longer time scale than those obtained with the alkanethiol monolayers considered previously. Whereas the alkanethiol transients typically exhibit two maximums and an abrupt drop to zero current after 10-20 ms (see Figure 2), transients recorded at hydroxy- or carboxy-terminated thiol monolayers showed an additional decaying tail that extends up to 80 ms (see inset in Figure 5a). At the same time, desorptive reduction experiments carried out just after recording the immersion transient indicated an increase in the thiol reduction charge in a percentage equivalent to the ratio between the areas under the tail (shaded area in the inset of Figure 5a) and the full transient. Therefore, it seems that a progressively larger electrode area is being sampled in the last part of these transients, due to the creeping of the solution on the hydrophilic electrode surface. Figure 5a shows a comparison of the electrode charge density versus potential plots obtained before and after correcting for this effect, by subtracting the charge corresponding to the tailing current from the charge associated with the entire transient. It may be observed how this correction significantly lowers our estimate of the monolayer capacitance (by ∼40% in the case of MUOL and by ∼25% in the case of MUA monolayers), while leaving the value of the potential of zero charge unaffected. The capacitance values listed in Table 1 were corrected for the 6478 Analytical Chemistry, Vol. 79, No. 17, September 1, 2007

Figure 5. Electrode charge density as a function of the electrode potential for Au(111) electrodes modified with ω-functionalized alkanethiol monolayers. (a) 11-Mercaptoundecanol monolayers, before (2, 4) and after (b, O) correcting for the creeping effect as indicated in the text; open (O, 4) and filled (b, 2) symbols were obtained in 0.1 M HClO4 and 0.1 M NaOH solutions, respectively. The solid lines are the linear least-squares fit to the filled symbols. The inset shows a typical transient recorded at -0.6 V in the 0.1 M NaOH solution; the shaded area highlights the charge correction due to the creeping effect. (b) 11-Mercaptoundecanoic acid monolayers in (b) 0.1 M NaOH, (O) 0.1 M HClO4, and (4) 0.1 M NaClO4 solutions. Solid lines are linear least-squares fits to each set of symbols. Vertical arrows help to locate the value of the potential of zero charge.

creeping effect and turned out to be significantly smaller than those reported previously for MUOL13,15,42 and MUA42-44 monolayers. Since the consequences of solution creeping were not considered in these experiments, it is difficult to ascertain to what extent our lower capacitance values are due to the benefits of the vapor deposition protocol or to differences in the way data are being processed. It may also be observed in Figure 5a how the results are independent, within experimental error, of the electrolyte nature and pH. More interestingly, the presence of the hydroxy group shifts the value of the potential of zero charge by +0.59 V with respect to the long-chain limit of the alkanethiol series. This value is in qualitative agreement with the 0.23 V shift predicted by eq 4 with i ) 1, though there is some additional uncertainty in this case, due to the unaccounted influence of hydrogen bonding on ∆(∆χthiol/solution) and the lack of detailed structural information. Two different potential of zero charge values of polycrystalline Au electrodes modified with a MUOL monolayer have been reported so far by Becka and Miller13 (-0.11 V) and Sondag-Huethorst and Fokkink15 (+0.45 V). Their estimates differ markedly even after considering the large incertitude (∼ (0.1 V) associated with their (42) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (43) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Nike´, K. Langmuir 2000, 16, 5397. (44) Xing, Y. F.; Li, S. P. Y.; Lau, A. K. H.; O’Shea, S. J. J. Electroanal. Chem. 2005, 583, 124.

experimental techniques. Our own estimate of 0.07 V is closer to Miller’s value, but it should be kept in mind that ours applies to a well oriented rather than to a polycrystalline substrate. As might be anticipated intuitively, modification of the Au(111) surface with a mercaptoundecanoic acid monolayer results in electrode charge density versus potential plots that vary with the solution pH (Figure 5b). These plots are parallel to each other in 0.1 M solutions of NaOH, NaClO4, or HClO4, and their slope provides a monolayer relative permittivity value of 3.6, after correcting for the creeping effect, which is somewhat higher than the r value found for MUOL and DODET monolayers. The Epzc value in the acid solution (0.12 V) is very close to that of the MUOL monolayer and is in qualitative agreement with the value (-0.04 V) predicted by eq 4 on the basis of the dipole moment values of undissociated MUA and DODET neutral radicals (see Table 1 and Figure 4b). When the same monolayer is exposed to the basic NaOH solution, the Epzc shifts -0.54 V to a value close to the long-chain limit of the alkanethiol series. The origin of this displacement can be assigned to the appearance of a new dipole along the surface normal, due to the different spatial locations of the negative charges of the dissociated carboxy groups and of the positive charges of the counterions in solution. This particular charge distribution reinforces the dipole of the alkanethiol backbone and tends to offset the contribution of the oxygen atoms to the overall surface dipole. The analysis of the Epzc dependence on solution pH can be taken one step further by assuming that the contributions to the surface potential drop of ionized and unionized carboxy groups are additive, so that

) 0.28 is obtained for the 0.1 M NaClO4 solution with pH 6.8. The degree of ionization and the surface pKa are related by

pKa ) pH -

R 1 ln 2.303 1 - R

(6)

so that a pKa value of 7.2 is derived from the observed Epzc shifts of Au(111) modified with MUA monolayers in these solutions. This pKa value falls within the range of previous estimates,45,46 and it is reported here just to illustrate the potentiality of the Epzc measurement technique to provide quantitative information on surface ionization processes.

(5)

CONCLUDING REMARKS As compared with previous methods for the determination of the potential of zero charge of thiol-modified electrodes, such as the location of the electrocapillary maximum or the diffuse layer minimum in the capacitance, our present approach keeps a reasonable accuracy in the presence of long-chain alkanethiols. The good agreement between the variations of the potential of zero charge and of the work function as the alkanethiol chain length is increased helps to validate our method and lends support to the presence of a molecular dipole saturating effect, which is not observed in the surface potential values derived from Kelvin probe measurements. The potential of zero charge is shown to respond to the presence of oxygen-containing functionalities in the monolayer, but is found to be insensitive to the electrolyte nature and solution pH, with the significant exception of the monolayers of 11-mercaptoundecanoic acid. In this case, deprotonation of the carboxy groups gives rise to a new surface dipole, which shifts the potential of zero charge value by ∼0.5 V. This result opens up the possibility of applying potential of zero charge measurements to the study of surface ionization processes.

where R stands for the fraction of ionized groups in the monolayer at a given pH. If the acid (R ) 0) and basic (R ) 1) limits are reached in the HClO4 and NaOH solutions, respectively, then R

ACKNOWLEDGMENT J.J.C. and R.A. acknowledge financial support from the DGICYT under grant CTQ 2005-01184. Helpful comments from Prof. Claudio Gutierrez and Prof. Willem H. Mulder are warmly acknowledged.

acid Epzc(pH) ) REbasic pzc + (1 - R)Epzc

(45) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F. J. Phys. Chem. B 1999, 103, 1676. (46) Dai, Z.; Ju, H. Phys. Chem. Chem. Phys. 2001, 3, 3769.

Received for review June 25, 2007. Accepted July 12, 2007. AC071341Z

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