Structure and Electrochemical Characterization of 4-Methyl-4 '-(n

J. Phys. Chem. C 2007, 111, 17409-17419

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Structure and Electrochemical Characterization of 4-Methyl-4′-(n-mercaptoalkyl)biphenyls on Au(111)-(1 × 1) R. Aguilar-Sanchez,† G. J. Su,† M. Homberger,‡ U. Simon,‡ and Th. Wandlowski*,†,§ Institute of Bio- and Nanosystems (IBN3) and Center of Nanoelectronic Systems for Information Technology (CNI), Research Center Ju¨lich, D-52425 Ju¨lich, Germany, Institute of Inorganic Chemistry, RWTH Aachen, D-52074 Aachen, Germany, and Department of Chemistry and Biochemistry, UniVersity of Berne, CH-3012 Berne, Switzerland ReceiVed: June 8, 2007; In Final Form: September 1, 2007

4-Methyl-4′-(n-mercaptoalkyl)biphenyl (CH3-C6H4-C6H4-(CH2)n-SH, n ) 1-6, BPn) monolayers selfassembled on Au(111)-(1 × 1) electrode surfaces were studied by scanning tunneling microscopy, cyclic voltammetry, and chronoamperometry. Distinct odd-even effects were found for the adlayer structures, the interfacial adlayer capacitances, as well as for the potentials and charges of reductive and oxidative desorption. The potential of zero charge, Epzc, of BPn- modified gold electrodes was estimated by a controlled immersion technique in hanging meniscus configuration to Epzc ) -(0.30 ( 0.05) V, rather independent of the length of the alkyl spacer. The reductive desorption of BP3 and BP4 adlayers were quantitatively described by models based on hole nucleation and growth mechanisms, such as the exponential law of one-step hole nucleation (BP3), or hole nucleation according to a power law (BP3, BP4) in combination with a linear law of growth. No odd-even characteristics were found for the kinetic currents of the Fe(CN)64-/Fe(CN)63- redox reaction in the presence of BPn’s. For the dependence on alkyl chain length, a structure sensitive attenuation parameter β ) (11.5 ( 1.0) nm-1 was derived, which is interpreted according to a “through-bond” tunneling mechanism.

1. Introduction The understanding of the relationships between molecular structure of tailored organic molecules, their hierarchical organization in assemblies, and their functionality represents a fundamental topic of current interest.1,2 Examples include Langmuir monolayers at the air-water interface,3 LangmuirBlodgett films,4 lipid membranes,5 liquid crystals,6 and selfassembled monolayers (SAM).7-11 SAMs are ordered molecular assemblies formed by adsorption of an active surfactant on a solid surface. They offer design flexibility both at the individual molecular level and at the materials level (“surface engineering”), and represent model systems to explore specific interactions and reactions at surfaces and interfaces.7-11 Assemblies of thiol-based organic molecules self-assembled on metal substrates are a special class of SAMs. These adsorbates are composed of one or two thiol-anchor groups, custom-designed alkyl or phenyl spacer units and, in some cases, incorporated tailored functionalities.7,12-14 The application of SAMs covers diverse areas such as adhesion,15 lubrication,16 microfabrication,13 bioenergetics,17 corrosion,18 fundamental electrochemistry,19 and metal deposition,20 as well as the emerging field of nanoelectronics.21-23 Of particular interest is their ability to control charge transfer in metalmolecular hybrid systems.24,25 * To whom correspondence should be addressed. E-mail: th. [email protected], Web: http://www.fz-juelich.de/isg/index.php? index)239. Also, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse3, 3012 Bern, Switzerland. † IBN3 and CNI. ‡ Institute of Inorganic Chemistry. § University of Berne.

Although most fundamental studies on electrochemical and electrical properties of organosulfur compounds focused on aliphatic thiols,7,10,19,21-32 aromatic thiols have been investigated far less.7,14,31,33-35 However, the vision of molecular level electronics has triggered an active research interest in aromatic thiol-based molecular rods and functional building blocks of single molecules and tailored assemblies (see refs 23,24,30,32-36). The rigid aromatic system offers a unique control of structure and local molecular functionality. Furthermore, the absence of conformational disorder (employing the concept of commensurability of intra-assembly planes37) may lead to stable and low-defect, molecularly engineered model surfaces. With reference to the unique structure properties of 4,4′-dialkylsubstituted biphenyls,38 4-methyl-4′-(n-mercaptoalkyl)biphenyls (BPn) represent a particularly interesting group of molecular building blocks for the construction of well-defined hierarchical molecular assemblies at surfaces. The molecules consist of an aromatic biphenyl moiety with an aliphatic spacer unit of variable length between the annular system and the sulfur anchoring group. Buck, Zharnikov, and Wo¨ll described the structure and assembly of BPn adlayers on polycrystalline and, in some cases, on single crystalline gold and silver surfaces in a series of spectroscopic39,40 and scanning tunneling microscopy (STM) studies41-44 under UHV conditions and in air. The three groups reported a pronounced odd-even effect with a denser molecular packing for BPn with n ) odd ((2x3 × x341-43) unit cell), in comparison to the less favorable packing with n ) even ((5x3 × 3) and (6x3 × 2x3) unit cells41-43). They also found that these structural differences led to a periodic odd-even alteration of the reductive desorption behavior,45,46 as well as alterations in the stability against organic exchange by other thiols.47

10.1021/jp0744634 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/01/2007

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Aguilar-Sanchez et al.

Figure 1. (A) Large scale and (B) high-resolution STM image of the (x3 × 2x3) adlayer of BP3 on Au(111)-(1 × 1) in mesitylene. Tunneling conditions: Ebias ) -0.55 V, iT ) 70 pA. The unit cell is indicated in (B). Panel (C) illustrates a top view of the structure model of the (x3 × 2x3) overlayer of BP3 (ellipsoids) on Au(111)-(1 × 1) (gray circles). Complete details of the structure analysis are given in ref 48.

In this paper, we present a comprehensive electrochemical study employing cyclic voltammetry, chronoamperometry, and immersion experiments to explore preparation strategies and fundamental interfacial properties of BPn’s (n ) 1-6) immobilized on well-defined Au(111)-(1 × 1) single-crystal electrodes. The structures of the respective BPn adlayers were investigated by high-resolution STM. For a comprehensive characterization, we refer to a recent STM/STS study48 aimed to addressing structure and electric, specifically conductance properties, of the fabricated adlayer assemblies. On the basis of this knowledge, in the present report we address double layer properties, such as interfacial capacitance and potential of zero charge, of the BPn modified Au(111)-(1 × 1) electrodes in 0.1 M NaOH and 0.1 M HClO4. Subsequently, we explored the stability limits of the self-assembled adlayers quantitatively, with focus on reductive (cathodic limit) and oxidative (anodic limit) desorption. Finally, we employed Fe(CN)63-/Fe(CN)64- as a redox probe to extract electron transport properties of the highly ordered BPn monolayers. The dependence of the (macroscopic) electrochemical properties on adlayer structure, and specifically on the length of the alkyl spacer, will be described in detail. 2. Experimental Section 2.1. Synthesis. The 4-methyl-4′-(n-mercaptoalkyl)biphenyls (CH3-C6H4-C6H4 (CH2)n-SH, n ) 1-6, BPn) were prepared via a Grignard C-C coupling reaction of the corresponding phenyl- or alkylbromides followed by the conversion of the bromide to the thiol group with thiourea.39 2.2. Electrochemical Measurements. The electrolyte solutions were prepared with Milli-Q water (18 MΩ, 2 ppb TOC), HClO4 (Merck, suprapure), NaOH (Merck, suprapure), Na2SO4 (Merck, suprapure) or KClO4 (Fluka, pro analysi, twice recrystallized from water). K4[Fe(CN)6]‚3 H2O and K3[Fe(CN)6] (Fluka, puriss p.a.) were used in the electron-transfer studies. All electrolytes were deareated with high purity argon before and during the experiment. The measurements were carried out at (20.0 ( 0.5) °C. The glassware was cleaned either in caroic acid or in a hot 1:1 mixture of H2SO4 (95-97%, pro analysis, Merck) and HNO3 (65% purissimum, Riedel-de-Haen) followed by extended rinsing with Milli-Q water. The electrochemical measurements were performed with an Autolab PGSTAT 30 work station employing a lab-built three-electrode glass cell. The counter electrode was a platinum wire, a saturated mercury sulfate electrode (MSE, neutral, or alkaline pH) or a trapped hydrogen electrode (acidic pH) served as reference. However, all potentials in this paper are quoted with respect to a saturated calomel electrode (SCE). 2.3. Electrode and Sample Preparation. The Au(111) single crystals used in this work were massive cylinders of 4 mm height

and 4 mm diameter (EC), or 2-mm height and 10-mm diameter (STM), nominal miscut angle e0.1°. A gold wire was attached to the rear for mounting purposes. Before each experiment, the electrodes were annealed in a butane flame at bright red heat for ∼10 min and then cooled in argon atmosphere to room temperature. Island free Au(111)-(1 × 1) surfaces were prepared by immersing a flame-annealed Au(111)-(p × x3) electrode under potential control at 0.50 V (vs SCE) into deareated 0.05 M HCl.49 These electrodes exhibit defect-free terraces significantly larger than 500 × 500 nm. After a waiting time of 60 s, the electrodes were emersed, rinsed with copious amounts of Milli-Q water, dried in a stream of argon, and subsequently used in studies of the bare electrodes in supporting electrolyte. The organic monolayers were prepared by immersing the dry Au(111)-(1 × 1) electrodes with the polished and oriented surface in a 0.1 mM ethanolic solution of the respective BPn thiols. The samples were annealed in a sealed, oxygen-free stainless steel container for 12 h at 90 °C. Subsequently, the modified (equilibrated) electrodes were removed from the assembly solution, rinsed with warm ethanol to dissolve physisorbed BPn molecules, and transferred to the electrochemical or STM cell. Contact with the electrolyte was always established under potential control in a hanging meniscus configuration and in the absence of oxygen. 2.4. Scanning Tunneling Microscopy. The STM measurements were carried out with freshly prepared, unreconstructed Au(111)-(1 × 1) electrodes in mesitylene using a Molecular Imaging Pico SPM. The STM tips were electrochemically etched Pt/Ir (70%/30%, 0.25 mm diameter) electrodes. All measurements were carried out in constant current mode employing low tunneling currents (10 to 50 pA) and a bias voltage ranging from -0.70 to 0.70 V. 3. Results and Discussion 3.1. STM Measurements. The structure properties of the prepared BPn monolayers on Au(111)-(1 × 1) were monitored by high-resolution STM studies in mesitylene. The work of Cyganik et al.41-44 inspired us to first optimize the assembly conditions. In the following, we report key results obtained after assembly of BPn from 0.1 mM ethanolic solution at 90 °C for 12 h. The results of the complete STM/STS studies were published in ref 48. Figure 1A shows a large scale STM image of BP3, a typical representative of the investigated molecular class with an odd number of (CH2)- spacer units. Individual terraces are covered by large domains of a uniform adlayer. Within the terraces, monatomically deep depressions with a typical size of 5-10 nm and a depth of 0.25 nm were identified. They are evenly distributed on flat terraces. Depletion was found in the

4-Methyl-4′-(n-mercaptoalkyl)biphenyls

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17411

Figure 2. (A) Large scale and high-resolution STM image of the coexisting (B) (3 × 5x3) and (C) (2x3 × 5x3) adlayer of BP4 on Au(111)-(1 × 1) in mesitylene. Tunneling conditions: Ebias ) -0.5 V, iT ) 40 pA. The panels (D) and (E) represent top view structure models of the (3 × 5x3) and (2x3 × 5x3) overlayers of BP4 (ellipsoids) on Au(111)-(1 × 1) (gray circles). The unit cells are indicated. The parameters are discussed in the text, and more details are presented in ref 48.

neighborhood of step sites due to coarsening and thermal annealing processes.50 High-resolution experiments (Figure 1B) revealed a hexagonal pattern of molecular size protrusions arranged in alternating dark and bright rows. The dimensions of the unit cell were estimated from height profiles to a ) (0.52 ( 0.02) nm, b ) (1.03 ( 0.05) nm, and R ) (58 ( 4)° corresponding to a (x3 × 2x3) structure with two BP3 molecules per unit cell (Figure 1B and Figure 1C). The resulting area per molecule is obtained as Aex ) 0.23 nm2 (Amodel ) 0.22 nm2) which gives a coverage of Γex ) 7.3 × 10-10 mol cm-2 (Γmodel ) 7.7 × 10-10 mol cm-2). This observation is in agreement with LEED and ex-situ STM experiments recently reported by Azzam et al.41-43 Careful inspection of the entire organic adlayer revealed no additional coadsorbed structures. Similar results were also obtained with BP5.48 In contrast, the adlayers fabricated with BPn’s having an even number of (CH2)- units exhibit coexisting phases. Figure 2 demonstrates, as an example, a typical large scale scan of BP4 on Au(111)-(1 × 1). On the same terrace, one observes areas of bright contrast (phase I) and areas of dark contrast (phase II), occasionally coexisting with monatomically deep holes. Domain boundaries between the two phases are not sharp, but no large regions of disorder exist between them. High resolution measurements reveal that phase I is represented by a regular array of protrusions, which are assigned to individual BP4 molecules. The suggested repeat pattern is estimated to a ) (0.86 ( 0.05) nm, b ) (2.45 ( 0.12) nm separated by an enclosed angle R ) (93 ( 5)°, which is consistent with a (3 × 5x3) commensurate unit cell composed of 8 BP4 molecules,41-43,8 Γex ) 6.3 10-10 mol cm-2 (Γmodel ) 6.1 10-10 mol cm-2; Figure 2B and Figure 2D). The analysis of phase II gives a ) (1.04 ( 0.07) nm, b ) (2.46 ( 0.10) and R ) (61 ( 4)°, which leads to a (2x3 × 5x3) unit cell of 8 BP4 species and an area per molecule Aex ) 0.28 nm2 (Amodel ) 0.27 nm2, Figure 2C and Figure 2E).48 The corresponding coverage is calculated to Γex ) 5.9 × 10-10 mol cm-1 (Γmodel ) 6.1 × 10-10 mol

cm-2). The unit cell is smaller than the (2x3 × 6x3) structure recently proposed by Azzam et al.41-43 The bright protrusions define molecular rows, which align with the short direction of the unit cell in both phases. Interestingly, rows representing adjacent neighboring phases are often perpendicular with respect to each other. Coexistence of a majority (2x3 × 5x3) phase with a more complex (3 × nx3) structure was also observed for adlayers of the even numbered molecules BP2 and BP6 assembled at 90 °C for 12 h from ethanolic solution.41-43,48 3.2. Electrochemical StudiessOverview. General Aspects. On the basis of the developed preparation strategies (see paragraph 3.1) and the knowledge on the BPn structures, we carried out a comprehensive electrochemical study of BP1 to BP6 on Au(111)-(1 × 1). Figure 3A shows, as an example, a series of current-voltage curves for BP4 in 0.1 M NaOH and 0.1 M HClO4, respectively. Three characteristic potential intervals labeled I, II, and III, which are separated by the current peaks P1 and P2/P2′, can be readily distinguished. The stability region of I, which is assigned to the double layer range, is delimited at negative potentials by the onset of reductive desorption (peak P1) and at positive potentials by the oxidative desorption of BP4 (peak P2). Double Layer Properties. In a typical experiment, a dry, freshly prepared Au(111)-(1 × 1) electrode modified with a BP4 monolayer was brought in contact with the electrolyte at E ) 0.0 V (vs SCE) establishing a hanging meniscus configuration in an oxygen free environment. Scanning the electrode potential in -0.50 V e E e 0.10 V (in 0.1 M NaOH) or in -0.10 V e E e 0.50 V (in 0.1 M HClO4) revealed a capacitance of 2.4 µF cm-2, rather independent of the electrolyte. As compared to bare Au(111)-(1 × 1), the BP4 covered electrode exhibits a markedly reduced charging current, and an almost potential independent behavior in a wide potential range, which is characteristic for thin organic adlayers with a low dielectric constant blocking ion transfer and solvent penetration.


Figure 3. (A) Typical single scan current vs potential curves for the reductive desorption (black traces, region labeled II) and for the oxidative desorption (gray traces, region labeled III) of a monolayer of BP4 assembled on Au(111)-(1 × 1) in 0.1 M NaOH and 0.1 M HClO4, respectively. Scan rate 10 mVs-1. Contact with the electrolyte was always established under strict potential control in hanging meniscus configuration at E ) 0.00 V. The dashed black and gray curves represent voltammograms in the double layer region as recorded after immersion and stabilization either in the alkaline or in the acidic electrolyte. The dotted gray trace in potential region III illustrates the second anodic scan measured after the first oxidative desorption trace. The open and filled circles symbolize the charges derived from immersion experiments in 0.1 M NaOH (O) and 0.1 M HClO4 (b) at various potentials. The solid line was obtained from a linear regression analysis (see text for further details). (B) Typical immersion i-t traces as recorded in 0.1 M NaOH for a freshly prepared and dry BP4 monolayer on Au(111)-(1 × 1) at E ) -0.40 V (solid line) and E ) -0.20 V (dotted line).

An alternative way to determine the interfacial capacitance is based on the so-called “immersion method”.51-53 The method was already successfully applied to estimate Epzc of the reconstructed Au(111)-(p × x3) surface in 0.01 M HClO454. We adopted this technique here to explore thermodynamic properties of a well-defined Au(111)-(1 × 1) electrode modified with an aromatic self-assembled monolayer. The following strategy is applied: The dry, BP4 covered Au(111)-(1 × 1) electrode is brought under potential control and in the absence of oxygen in contact with the electrolyte forming a hanging meniscus without wetting the walls of the electrode. Figure 3B illustrates two current transients recorded at E ) -0.40 V and at E ) -0.20 V. The current decreases (increases) steeply, reaches a minimum (maximum) and subsequently decays exponentially to zero. The charging current through the interface is either negative or positive and varies systematically with the electrode potential chosen. The charge consumed is attributed to building up the electrochemical double layer at the respective immersion potentials. The full and open circles in Figure 3A represent the charge density vs potential curve, as obtained by

Aguilar-Sanchez et al. integration of a series of systematically recorded current-time transients for two different electrolytes in -0.70 V e E e 0.70 V. The plot is linear with a slope of (2.2 ( 0.2) µF cm-2, which is in excellent agreement with the voltammetric data reported above. The zero crossing at E ) (-0.30 ( 0.01) V represents an estimation of the potential of zero charge (Epzc) of the BP4 covered Au(111)-(1 × 1) electrode. This value is significantly more negative than Epzc of the bare supporting electrolyte (see Epzc ) 0.240 V published for Au(111)-(1 × 1) in 0.01 M HClO455). For comparison, Sondag-Huethorst and Fokkink56,57 reported Epzc ) -0.450 V for self-assembled monolayers of alkanethiols on Au(poly), independent of the length of the alkyl chain, employing the Wilhelmi plate technique. Becka and Miller et al. found for similar systems Epzc ) (-0.40 V ( 0.20) V based on impedance measurements in dilute electrolytes.58 Iwami et al. determined Epzc of alkanethiol-modified Au(111) single-crystal electrodes applying the sessile drop method.59 These authors reported values of Epzc ranging between -0.34 V and -0.56 V (vs SCE). Their experiments with self-assembled monolayers of propanethiol, undecanethiol, and octadecanethiol revealed that Epzc decreases with increasing length. This trend, which was not observed with polycrystalline gold substrates,57 was attributed to the difference of the dipole moment of the adsorbed alkanethiols. All three groups also demonstrated that Epzc shifts toward more positive values with increasing polarity of the terminal functional group (OH, CN, Cl) of the alkanethiol monolayer.58-60 We comment that Epzc of an unmodified Au(poly) electrode is at least 0.1 V more negative than Epzc of a carefully prepared Au(111)-(1 × 1) single-crystal electrode in the absence of specific adsorption.49,55 Accordingly, the difference in the positions of Epzc of the thiol modified electrodes is expected to be of the same order of magnitude. The additional small difference might come from a higher local disorder of the SAM adlayers due to the use of Au(poly) as substrate. The unavoidable surface roughness might lead to solvent and electrolyte penetration into the adlayer. This effect could be ruled out in our work due to the preparation technique chosen. We may conclude that there is reasonable agreement between the estimated values of Epzc for the thiol-based monolayers, as obtained with different techniques by Sondag-Huethorst and Fokkink,56,57 Miller and Becka,58 Iwami et al.,59 and us. We also note that the above values of Epzc of gold electrodes modified with alkanthioles and aromatic thiols are 0.60 to 0.80 V more positive than recent data published by Laredo et al. for octadecanthiol on Au(111)61 using an approach based on chronocoulometry.62 The reason for this discrepancy is not clear at present (see also below). ReductiVe Desorption. The stability of the BP4 adlayer at negative potentials is limited by the so-called “reductive desorption”. The first potential scan (in alkaline electrolyte 0.1 M NaOH) with a rate of 10 mV s-1 leads to a sharp, slightly asymmetric cathodic current peak at -1.053 V (fwhm ≈ 0.024 V). Decreasing the scan rate shifts the peak potential to more positive values. The pronounced peak was only observed during the first potential scan with a freshly prepared sample, and exhibits no anodic counterpart. Subsequent multiple cycling of the electrode potential in -1.20 V < E e -0.20 V did not reveal any reductive desorption features. The organic monolayer is entirely removed and does not re-adsorb. Similar observations were reported previously for alkanethiols of moderate chain lengths on gold electrodes.63-65 Current integration yields a charge QBPred ) (73 ( 5) µC cm-2, estimated from five independent measurements. This reductive desorption charge was corrected by fitting to the baseline before and after the



reduction peak in EP1 - 0.10 V < E < EP1 + 0.10 V, or by subtracting the current of the corresponding second potential scan. Referring to the coverage of BP4, Γex ) (5.9-6.3) × 10-10 mol cm-2 as estimated from our STM studies and assuming the commonly accepted one-step one-electron process19,63

RS-Au + e f Au° + RS-

(1)

to be valid, a charge of 57 to 61 µC cm-2 is predicted. This value is smaller than QBPred ) (73 ( 5) µC cm-2, the charge obtained by integration of the prominent desorption peak, which indicates a substantial double layer contribution of (14 ( 5) µC cm-2 to the total charge balance. The latter could be estimated roughly by referring to the shift in Epzc due to BP4 adsorption from 0.24 V66 to -0.30 V, which leads to dQ ≈ 15 µC cm-2, a value close to the one observed in our experiment. The importance of this additional contribution was already pointed out by Schneider and Buttry in elegant experiments employing the electrochemical quarz crystal microbalance.67 Additional efforts to account for double layer charging were also reported in chronoamperometric,61,62,68 cyclic,69 and linear sweep voltammetry.70,71 Quantitative treatments based on the Frumkin model61,62 or on a sticky site model71 demonstrated that coverages estimated by the reductive desorption method (without correction for double layer charging currents) are too high by 15% to 30%. This is in agreement with the present study. However, the approaches developed by Kakiuchi et al.59,71 and the group of Lipkowski61,62 are based on or lead to distinctly different values of Epzc for rather similar SAMs on Au(111). Our data are in agreement with the results of Kakiuchi’s group. The apparent contradictions to results and conclusions published by Lipkowski et al. will require a comprehensive and focused study on this specific topic, which shall be communicated in a separate contribution. OxidatiVe Desorption. The stability limit of the BP4 monolayer on Au(111)-(1 × 1) at positive potentials was investigated in 0.1 M HClO4. The electrode potential was scanned to positive values after immersion at 0.00 V and establishing a hanging meniscus configuration. The first cycle revealed a constant capacitive charging current up to 1.20 V, implying the blocking of the onset of gold surface oxidation (Figure 3A). The inhibition effect of the aromatic BP4 adlayer is much stronger than previously reported for aliphatic thiols in acidic solution.72 At E > 1.20 V a broad anodic peak P2 with a maximum at 1.40 V develops (fwhm ≈ 70 mV, scan rate 10 mV s-1). The latter is followed by an additional increase in current, which might be related to the formation of a complex gold oxide film.73 The charge corresponding to P2 as derived from three independent measurements amounts to Qox ) (2050 ( 80) µC cm-2. This value accounts for contributions from the oxidative desorption of the BP4 adlayer as well as from gold oxidation. Reversing the direction of the potential scan gives rise to a cathodic peak P2′ at 0.832 V with Qre ) (740 ( 30) µC cm-2. The latter is only slightly higher, 10% to 15%, than typical charges of the reduction of the gold surface oxide for a well-prepared Au(111)-(1 × 1) electrode in 0.1 M HClO4.74 The second potential scan (dotted gray line in Figure 3A) leads to the voltammetric profile of a bare Au(111) electrode having a low density of surface defects with typical charges for surface oxidation and reduction.65 We may conclude that the BP4 adlayer is completely desorbed during the first oxidation scan, and no significant amount of BP4 or reaction products could re-adsorb. In consequence, the upper limit of the charge attributed to the oxidative desorption of BP4 is estimated as QBPox ) Qox - Qre

Figure 4. Single voltammetric scans of the reductive desorption for BP1 to BP4 (A) and BP5 and BP6 (B) monolayers on Au(111)-(1 × 1) in 0.1 M NaOH after immersion at 0.00 V, scan rate 10 mVs-1 (only traces at E < -0.20 V are shown). The insets illustrate the chain length dependence of the potentials (A) and charges (B) of reductive desorption for the investigated organic adlayers (see Table 1).

≈ (1310 ( 80) µC cm-2. This value is larger than data for aliphatic thiols previously reported.63,75 The positive oxidative desorption potential P2 of the aromatic BP4 monolayer may involve contributions of a rather complex gold oxidation process. We notice that the above charge does not account for anion readsorption and gold oxide dissolution. The latter may contribute substantially.76 Additional complexity results from the chemistry of the oxidative desorption process. On the basis of FTIR measurements in 0.1 M KOH75 and 0.1 M HClO4,77 breaking of the C-S bond, oxidation of the sulfur to (H)SO42-, formation of RCOO-, and further oxidation to CO2 are suggested. The multistep reaction mechanism involves 1166 or up to 1868 electrons. The more detailed understanding of the mechanism of oxidative desorption of BP4 on Au(111)-(1 × 1) in 0.1 M HClO4 requires a qualitative and quantitative product analysis based on spectroscopic and DEMS (differential electrochemical mass spectrometry) experiments, which is beyond the scope of the present work. Dependence on Alkyl Spacer Lengths. The voltammetric properties of BP4 monolayers in 0.1 M NaOH and 0.1 M HClO4 were compared with a series of BPn’s with n ranging between 1 and 6 (Figures 4 and 5 and Table 1). The adlayer capacitance C, as estimated from potentiodynamic and immersion experiments in the double layer region I, is decreasing with increasing chain lengths of the alkyl spacer group. The plot 1/C vs number of CH2- units yields a reasonably straight line with a slope of (0.023 ( 0.004) cm2 µF-1 and an intercept (0.34 ( 0.02) cm2 µF-1 (σ ) 0.95). This linear correlation quantitatively agrees with previously reported capacitance data for alkanethiols on Au(poly) electrodes.59 However, a more detailed inspection reveals that the adlayer capacities of BPn’s with n ) odd (even) are slightly lower (higher) than the theoretical values predicted by the results of the linear regression analysis described above. This trend is in agreement with the higher (lower) packing density and the lower


Figure 5. Single voltammetric scans of the oxidative desorption for BP1, BP3, and BP5 monolayers on Au(111)-(1 × 1) in 0.1 M HClO4 recorded after immersion at 0.00 V, scan rate 10 mVs-1. The inset illustrates the chain length dependence of the potentials of oxidative desorption (see Table 1).

Figure 6. Experimental i-t traces (dotted lines) of the reductive desorption recorded for freshly prepared monolayers of BP3 (A) and BP4 (B) after immersion at 0.0 V, stabilization, and subsequently stepping the potential from Ei ) -0.90 V (waiting time twait ) 30 s) to various final potentials Ef, which are indicated in the figure. The solid curves represent the numerical fits calculated according to the models represented by eq 3 (A) and eq 5. (B). The parameters of the nonlinear regression analysis are summarized in Table 3 and Table 4.

tilt angle of the biphenyl moiety of BPn for n ) odd (even) adlayers on Au(111) (refs 41-43, paragraph 3.1, and Table 1). The immersion experiments also demonstrated that the values of Epzc ) -(0.31 ( 0.02) V of the BPn-modified Au(111)-(1 × 1) electrodes in contact with 0.1 M NaOH or 0.1 M HClO4 are independent of the electrolyte and on the length of the alkyl spacer (Table 1). The biphenyl moiety seems to mask the increasing length of the alkyl spacer unit, and no chain length dependence of Epzc as reported for alkanethiol SAM’s on single crystalline Au(111) electrodes was observed.59 The reductiVe desorption properties of BP4 were compared with those of a series of BPn’s, n ) 1-6 (Figure 4 and Table 1) Rather identical characteristics, i.e., a single and narrow (fwhm e 35 mV) cathodic peak P1 without anodic counter part,

Aguilar-Sanchez et al. were found for BP1 to BP4 (Figure 4A). Different features were observed for BP5 and BP6 monolayers. A remarkable oxidative readsorption takes place as indicated by the single peak P1′ in Figure 4B as the potential is swept back toward positive values. This result suggests that the diffusion of the described BP5 and BP6 thiols out of the Helmholtz layer into the electrolyte is less effective than for BPn thiols with shorter alkyl chains. The extended oxidative readsorption increases with increasing lengths of the alkyl group because of the decreasing solubility of the thiolate in aqueous electrolyte (“hydrophobic effect!”). Multiple potential cycles lead to a positive shift of the reductive desorption potential, accompanied with a peak broadening and a continuous decrease in coverage (charge). Similar trends were reported in the adsorption/desorption of insoluble surfactants physisorbed on Au(111) single-crystal electrodes in contact with aqueous electrolytes.78 The comparison of the six BPn thiols reveals a pronounced odd-even effect for the position of the potentials P1 as well as for the charges of reductive desorption QBPred (Table 1 and Figure 4 insets). Within the two series BP1, BP3, BP5 (odd), and BP2, BP4, BP6 (even), the potentials of P1 shift linearly in the negative direction by about 18 mV per CH2 unit (compare also Thorn et al. in ref 6). This value is in good agreement with the slope of 15 to 20 mV per CH2 reported in the literature for alkanthiols with n g 6.63-65 The observed negative shift in P1 with increasing lengths of the alkyl spacer is attributed to structure effects related to different packing densities,6 to increasing van der Waals interactions as well as to changes in the potential gradient and permeabilities with the number of CH2 units.3 The experimentally observed reductive desorption charges for BPn (even) vary between 72 and 77 µC cm-2. For BPn (odd) one obtains higher values ranging between 82 and 90 µC cm-2. This difference is rationalized by the higher packing density of BPn (odd) in comparison to BPn (even) assuming a one electron Faradaic process (1) and a comparable double layer contribution to the charge balance. Referring to the coverages of BP4 and BP3 obtained in our STM studies, Γex ) 7.3 10-10 mol cm-2 (BP3) and Γex ) (5.9-6.3) 10-10 mol cm-2 (BP4), we estimate an additional double layer charge of approximately 20% to the total charge of reductive desorption QBPred. Similar contributions are expected for the other four BPn adlayers studied on Au(111) based on the coverages extracted from previous STM studies of Cyganik et al.41-43 and us.48 The oxidatiVe desorption of all BPn adlayers on Au(111)-(1 × 1) in 0.1 M HClO4 is represented by one broad anodic peak P2 (fwhm 65 to 90 mV), which blocks the onset of gold surface oxidation. No readsorption is observed upon multiple scanning. The peak P2 shifts to higher potentials with increasing lengths of the alkyl spacer (Figure 5). Similar as for the potentials of reductive desorption, one observes a characteristic odd-even effect. The peak potentials P2 shift linearly within the two series BP3, BP5 and BP2, BP4, BP6 by ∼18 mV per CH2 unit. BP1 does not fit in this sequence, which could be related to the higher adlayer disorder and defect density of the self-assembled adlayer as reported in a recent STM study.41-43 Typical charges QBPox attributed to the oxidative desorption of BP, without correction for gold dissolution, are estimated as (1375 ( 100) µC cm-2 and (1470 ( 100) µC cm-2 for BPn (even) and BPn (odd), respectively. 3.3. Kinetics of Reductive DesorptionsChronoamperometry. The time dependence of reductive desorption was explored by single potential step experiments (from Ei to Ef). Selected transients (dotted lines) for freshly prepared BP3 and BP4 monolayers on Au(111)-(1 × 1)/0.1 M NaOH are displayed



TABLE 1: Characteristic Electrochemical Data for BPn Monolayers on Au(111)-(1 × 1) in the Double Layer Region, for Reductive (in 0.1 M NaOH) and Oxidative (in 0.1 M HClO4) Desorptiona

BP1 BP2 BP3 BP4 BP5 BP6 a

C µF cm-2

Epzc V (SCE)

EP1a V (SCE)

QBpred µC cm-2

fwhmred V (SCE)

EP2b V (SCE)

Qox µC cm-2

Qre µC cm-2

fwhmox V (SCE)

2.7 ( 0.2 2.6 ( 0.1 2.3 ( 0.2 2.4 ( 0.2 2.1 ( 0.1 2.1 ( 0.2

-0.32 ( 0.02 -0.31 ( 0.02 -0.33 ( 0.03 -0.30 ( 0.01 -0.30 ( 0.02 -0.30 ( 0.01

-1.035 -1.009 -1.103 -1.053 -1.120 -1.086

82 ( 10 72 ( 6 90 ( 10 73 ( 5 85 ( 7 77( 8

0.030 0.027 0.033 0.024 0.040 0.22

1.342 1.389 1.451 1.400 1.496 1.468

2185 ( 100 2159 ( 120 2329 ( 90 2050 ( 80 2415 ( 130 2231 ( 100

740 ( 30 770 ( 80 796 ( 80 740 ( 30 850 ( 120 803 ( 110

0.065 0.085 0.090 0.070 0.065 0.080

All abbreviations are explained in the text. b Standard deviation < (0.003 V. c Standard deviation < (0.005 V.

TABLE 2: Potential Dependence of the Fit Parameters for eq 2 and eq 4 to the Current Transients Recorded for the Reductive Desorption of the BP3 and BP4 Monolayers on Au(111)-(1 × 1) in 0.1 M NaOH, Ei ) -0.90 V as Initial Potential (see Figure 6)

BP3 eq 2 BP4 eq 4

Ef V

k1 µA cm-2 s(k3-1)

k2 s-k3

k3

-1.080 -1.100 -1.110 -1.120 -1.030 -1.040 -1.050 -1.060

-7.43 ( 0.03 -21.7 ( 0.2 -113.5 ( 0.5 -250 ( 1 -2.41 ( 0.05 -13.1 ( 0.1 -36.4 ( 0.4 -77.2 ( 7

0.039 ( 0.001 0.10 ( 0.01 0.538 ( 0.002 1.046 ( 0.005 0.021 ( 0.001 0.086 ( 0.001 0.237 ( 0.002 0.437 ( 0.001

2.17 ( 0.002 2.59 ( 0.005 2.41 ( 0.003 2.61 ( 0.003 1.898 ( 0.008 2.09 ( 0.01 2.248 ( 0.006 2.47 ( 0.01

in Figure 6. The traces are characterized by an initial exponential decay (t < 5 ms, Q < 5 µC cm-2), followed by a well-developed maximum, imax at tmax, which increases and shifts toward shorter times with more negative final potentials Ef. The current decays to zero if Ef is chosen sufficiently far from the onset of hydrogen evolution. No significant dependence on the choice of Ei (in -0.90 V e Ei e -0.40 V) and on the waiting time at Ei was found for ordered and annealed BP3 as well as BP4 adlayers. The initial part of the transient is attributed to the double layer charging of the thiol-coated electrode. The major segment is assigned to the reduction of the organic monolayer. The main cathodic current has two contributions, which are not separated, the Faraday current due to thiol reduction and the capacitive current due to the formation of the double layer of the uncoated gold electrode. The integrated charges of the current-time transients are in good agreement to those obtained from single sweep voltammetry. Maxima in the desorption transients of selfassembled thiol adlayers and anodic sulfide films have been attributed to a hole nucleation and growth process79,80 or to an irreversible desorption based on a Frumkin-type isotherm assuming attractive lateral interaction forces.81 The latter model requires a homogeneous distribution of equal sites, which is not supported by our STM investigations (see paragraph 3.1 and ref 8). Wano et al.82 demonstrated for a self-assembled monolayer of hexanethiol on Au(111) in 0.01 M KOH, based on in-situ STM measurements, that the reductive desorption starts from defects of the adlayer such as domain boundaries or missing rows and edges of vacancy islands, and proceeds by a hole nucleation and growth process with the desorbed thiolate molecules diffusing toward the bulk electrolyte. Morin et al.79 pointed out that the application of the hole nucleation and growth model implies that the reductive removal of the thiol monolayer is assumed to start by the diffusion of cations into the etching centers (“defect sites”), which expand via the reduction of chemisorbed thiols at their edges upon further exposure to solvent molecules and electrolyte counterions. In a first attempt to interpret the reductive desorption transients of BPn based self-assembled monolayers, we analyzed the experimental data displayed in Figure 6 with the diagnostic

k4 µA cm-2 s-(k6-1)

-1.29 ( 0.02 -6.69 ( 0.1 -59.6 ( 0.4 -143 ( 1

k5 s-k6

0.009 ( 0.001 0.044 ( 0.001 0.432 ( 0.002 1.36 ( 0.01

k6

r2

3.61 ( 0.01 4.69 ( 0.02 4.96 ( 0.02 5.12 ( 0.04

0.990 0.980 0.980 0.990 0.990 0.996 0.997 0.997

Avrami eq 2, which is based on the Bewick-FleischmannThirsk model of (hole) nucleation and growth:83,84

i(t) ) k1t(k3-1) exp(-k2tk3)

(2)

where k1 ) QBPredk2k3, k2 is a coefficient which combines the rates of (hole) nucleation and growth, k3 reflects the dimensionality and the nature of the nucleation process, and QBPred corresponds to the total charge involved in the reductive desorption process. The model assumes cylinder-like hole nuclei with a height equal to the thickness of the monolayer, which grow at constant rate radially and parallel to the electrode surface. Nonlinear regression analysis of eq 2 to the experimental reductive desorption transients for BP3 (Figure 6A) results in Avrami exponents k3 ranging between 2 and 3 (Table 2). Noninteger values of k3 can be modeled by the exponential law of one-step nucleation in combination with a linear growth law:75,85

i ) k1′(t - 1/k2′(1 - exp(-k2′t)))* exp(-k3′(t2 - 2t/k2′ + 2/k2′2(1 - exp(-k2t)))) (3) with k1′ ) -2k3′*QBPred, k2′ as the hole nucleation rate and k3′ a constant related to the growth process. The comparison of the experimental data for BP3 (dotted trace in Figure 6A) and the results of the nonlinear regression fit of eq 3 (solid lines in Figure 6A, parameters compiled in Table 3) show excellent agreement. Additional confidence for the validity of the model expressed by eq 3 is given by the charge balance QBPred as derived from chronoamperometric (Table 3) and voltammetric (paragraph 3.2, Table 1) experiments. The values of QBPred estimated with both techniques are nearly identical. We further notice that the nucleation rate k2′ as investigated for final potentials Ef in -1.12 V e Ef e -1.08 V shows no significant potential dependence while the growth parameter k3′ increases significantly with more negative final potentials Ef. This result indicates that preexisting defects within the onecomponent BP3 adlayer (“etching centers”) act as primary hole nucleation centers. Similar trends could be observed with BP5.


Aguilar-Sanchez et al.

TABLE 3: Potential Dependence of the Fit Parameters for the Hole Nucleation and Growth Model Represented by eq 3 to the Current Transients Recorded for the Reductive Desorption of the BP3 Monolayers on Au(111)-(1 × 1) in 0.1 M NaOH (see Figure 6A)

BP3 eq 2

Ef V

k1 µA cm-2 s-1

k2 s-2

k3 s-1

r2

-1.080 -1.100 -1.110 -1.120

-10.38 ( 0.03 -52.4 ( 0.8 -156 ( 1 -374 ( 5

2.61 ( 0.04 1.19 ( 0.03 3.71 ( 0.07 2.62 ( 0.07

0.0607 ( 0.0001 0.31 ( 0.01 0.882 ( 0.001 2.02 ( 0.03

0.991 0.981 0.991 0.992

TABLE 4: Potential Dependence of the Fit Parameters for eq 5 to the Current Transients Recorded for the Reductive Desorption of the BP4 Monolayers on Au(111)-(1 × 1) in 0.1 M NaOH (see Figure 6B)

BP4 eq 5

Ef V

k1 µA cm-2 s(k3-1)

k2

k3 s-k3

k4 µA cm-2 s-4

k5 s-5

r2

-1.030 -1.040 -1.050 -1.060

-2.90 ( 0.01 -16.2 ( 0.08 -48.0 ( 0.3 -122 ( 1

5.7 ( 0.1 5.9 ( 0.4 5.2( 0.1 2.71 ( 0.05

0.021 ( 0.001 0.107 ( 0.001 0.343 ( 0.002 0.844 ( 0.007

-0.23 ( 0.01 -5.26 ( 0.02 -56.3 ( 0.2 -133 ( 1

0.018 ( 0.0001 0.0359 ( 0.0001 0.429 ( 0.001 1.363 ( 0.008

0.97 0.996 0.998 0.997

The situation for the reductive desorption of a self-assembled BP4 (as well as BP2 and BP6) adlayer is more complex (Figure 6B). The experimental transients exhibit a current maximum, imax at tmax, followed by a pronounced shoulder in the decaying section of the trace until zero current is reached. Referring to the structure of BP4 on Au(111)-(1 × 1), which is composed of domains of two different adlayer phases, as being represented by the unit cells (2x3 × 5x3) and (3 × 5x3) (paragraph 3.1), we assume the existence of two (hole) nucleation and growth pathways in parallel, which leads to the following form of the diagnostic Avrami equation (eq 2):

i ) k1t(k3-1) exp(-k2tk3) + k4t(k6-1) exp(-k5tk6)

(4)

where QBPred ) k1/k2k3 + k4/k5k6, k2 and k5 are the rate coefficients for the (hole) nucleation and growth of the two phases, and k3 and k6 are the respective dimensionality parameters. The nonlinear regression fits of eq 4 yielded good agreement to the experimental transients for BP4 for the transients plotted in Figure 6B. The dimensionality parameters k3 range between 2 and 3, and k5 varies around 5 (Table 2). The values of k3 suggest that one dissolution pathway can be represented by the exponential law of (hole) nucleation in combination with an isotropic linear growth process, e.g., the first term in eq 4 can be detailed by eq 3. The values of k6 ≈ 5 suggest multistep hole nucleation according to a power law.75,86,87 Two scenarios could be anticipated: A stable etching nucleus is formed as a result of (i) three successive decompositions having the same probability or (ii) by the combination of two reactive intermediates.86 On the basis of this phenomenological treatment, we specified the reductive dissolution of self-assembled BP4 adlayers on Au(111)-(1 × 1) as follows:

i ) k1′(t - 1/k2′(1 - exp(-k2′t)))* exp(-k3′(t2 - 2t/k2′ + 2/k2′2(1 - exp(-k2′t)))) + k4t4 exp(-k5t5) (5) The first term is identical to eq 3, whereas the second term stands for hole nucleation with the number of active nuclei increasing with the third power of time. Equation 5 enables an excellent representation of the experimental transients for BP4 (see dotted and solid lines in Figure 6B). The results of the nonlinear regression fits are summarized in Table 4. Considering the potential dependence of the rate parameters one notices that the nucleation rates k2′ do not vary considerably with potential, while the growth rates k3′ are significantly increasing with more negative final potentials. The consistency of the fit is also

supported by the good agreement in the charge balance of the reductive desorption process as derived from chronoamperometric transient data (Table 4) and linear scan voltammetric experiments (see paragraph 3.2 and Table 1). Similar trends were also observed for BP2 and BP6. However, we like to emphasize that the analysis of the experimental i-t transients based on mechanisms, which are expressed by eqs 3-5, is still hypothetical and should be considered with great care. More detailed structure investigations such as attempted in ref 82 are essential to developing a more comprehensive understanding of the mechanisms involved. 3.4. Electron Transfer in the Presence of BPn Adlayers. The electron-transfer properties of the highly ordered selfassembled BPn adlayers on Au(111)-(1 × 1) were explored employing K4Fe(CN)6/K3Fe(CN)6 as a solution-based redox probe28,29 in 0.1 M KClO4. This redox probe was selected since it represents a convenient and electrochemically reversible oneelectron, outer sphere redox couple. Figure 7A illustrates a series of single scan voltammograms for the oxidation of Fe(CN)64to Fe(CN)63- as well as for the reduction of Fe(CN)63- to Fe(CN)64- in the presence of BPn monolayers, n ) 1-6, of various thickness and structure (see paragraph 3.1 and refs 4143). Each individual sweep started at the formal potential of the redox pair E° ) 0.15 V after equilibration in a hanging meniscus configuration. The response of the bare gold surface is added for comparison (dotted line). The measured current decreases and the splitting of the peak potential increases with chain length of the aliphatic spacer unit between the sulfur anchor group and the aromatic biphenyl moiety. The separate single scan voltammograms for the anodic and cathodic redox process were corrected for the effect of reactant depletion due to diffusion by carrying out a convolution analysis (Figure 7B).88,89 The decay constant β ) -(ϑ ln i/ϑ(CH)2)E)const is estimated to be (1.5 ( 0.1) per CH2 unit in the entire accessible potential range between -0.50 and 0.50 V; that is, the ferri and ferrocyanide yield comparable kinetic data for the negative and the positive potential scans, respectively (inset in Figure 7B). The attenuation effect of BPn adlayers employed in the present study is comparable with data for ω-hydroxyalkanethiol coated gold electrodes of similar length at constant overpotential for the Fe(CN)63-/4- redox couple.28 With a change of film thickness of 0.13 nm per CH2 group one obtains β ) (11.5 ( 1.0) nm-1, which corresponds to an effective barrier height Φeff ≈ (1.26 ( 0.04) eV assuming a simple rectangular tunneling barrier and negligible local contributions from defect sites. The value of β depends significantly on the molecular structure of the self-assembled adlayer or molecular bridge, and has therefore


J. Phys. Chem. C, Vol. 111, No. 46, 2007 17417 as suggested for the alkanethiols93 we propose for the present system a molecular “through-bond” tunneling process rather than tunneling through an isotropic dielectric barrier (“throughspace”). Finally, we note that the value of the decay constant β derived in the present study is significantly smaller than the result reported by Felgenhauer et al.47 We propose that the larger values of β in the earlier study are related to different assembly conditions of the BPn adlayers. The strategy chosen in the present study, assembly from 0.1 mM BPn ethanolic solution for 12 h in an oxygen free closed container ensures high quality and low defect density adlayers (see paragraph 3.1), which are distinctly different from those fabricated at room temperature. 4. Summary and Conclusions

Figure 7. (A) Single sweep voltammograms for the oxidation of 1 mM Fe(CN)64- and the reduction of 1 mM Fe(CN)63- in 0.1 M KClO4 in the presence of freshly prepared BPn monolayers on Au(111)-(1 × 1). Each scan started at the formal potential E ) 0.15 V, scan rate 10 mVs-1. The dotted line represents the cyclic voltammograms of 1 mM Fe(CN)64-/1 mM Fe(CN)63- in 0.1 M Na2SO4 in the absence of the organic adlayer. (B) Anodic and cathodic currents from (A) corrected for the effect of reactant depletion due to diffusion by carrying out a convolution analysis following an algorithm as being described in refs 88 and 89. The inset illustrates the chain lengths dependence of the kinetic current for two selected potentials.

emerged as a characteristic parameter to classify the ability of molecular structures to facilitate tunneling and to infer details of the electron transfer. The result of the present study is comparable with values of the attenuation constant β reported for saturated hydrocarbon bridges in various configurations, such as molecular donor-acceptor systems,90 a mercury junction setup,30 or self-assembled monolayers of alkanethiols on gold as investigated by electrochemical probes,28 CP-AFM or STM.24,91,92 The structure parameter β is higher than typical values reported for π-conjugated aromatic systems (3.5-6.0 nm-1).23,30 The absence of an odd-even effect in the distance dependence of the kinetic current of the Fe(CN)64-/3- oxidation/ reduction with BPn-modified Au(111)-(1 × 1) electrodes and the value of the structure sensitive attenuation parameter β ≈ (1.15 ( 0.10) nm-1 indicate that intermolecular coupling and bonding of the sulfur anchor group to the gold substrate play a minor role in the electron-transfer characteristics of the present system. The different tilt angles of the biphenyl moiety for BPn (odd) and BPn (even) as well as the difference in coverage between the two molecular series are not reflected in the experimental charge-transfer response. This result is supported by barrier height measurements of the BPn-Au(111) system, which were reported in a comprehensive STM/STS study.48 We conclude that the main contribution to the electron transport signature is therefore attributed to the alkyl spacer unit between the chemisorbed sulfur and the biphenyl moiety. The latter just ensures the structural integrity of the BP adlayer, even for short alkyl chains of one or two CH2 units.41-43,48 Similar

(1) 4-Methyl-4′-(n-mercaptoalkyl)biphenyls (CH3-C6H4C6H4- (CH2)n-SH, n ) 1 - 6, BPn) monolayers selfassembled on Au(111)-(1 × 1) electrode surfaces were studied by scanning tunneling microscopy (STM), cyclic voltammetry, and chronocoulometry. (2) High-resolution STM results revealed that the assembly of BPn on Au(111)-(1 × 1) gives rise to well-defined, highquality ordered organic monolayers. BPn with n ) odd form a uniform (x3 × 2x3) structure. The coexistence of a majority (2x3 × 5x3) phase with a minority (3 × px3) phase was found for BPn with n ) even. (3) The BPn-modified Au(111)-(1 × 1) electrodes exhibit capacitive behavior in a wide potential range. The interfacial capacity is rather constant in -0.80 V e E e 0.50 V and decreases with increasing lengths of the alkyl spacer. The plot 1/C vs number of CH2- units yields a reasonable linear correlation, however exhibiting an odd-even fine structure. (4) The potential of zero charge of the BPn-modified Au(111)-(1 × 1) electrodes was estimated based on the series of chronoamperometric immersion experiments in hanging meniscus configuration. The values obtained vary around -(0.31 ( 0.02) V. They are significantly more negative than Epzc of the unmodified Au(111)-(1 × 1) electrode in 0.01 M HClO4 (Epzc ) 0.24 V). No dependence on the chain length was observed. The values of Epzc of the BPn adlayers on Au(111) are also in agreement with data reported in surface tension 56,71 and interfacial capacitance58 measurements. However, the data of the groups of Sondag-Huethorst,56,57 Miller,8 Kakiuchi59,71 and us are 0.60 to 0.80 V small than recent data published by the group of Lipkowski61,62 using chronocouloometry. The reason for the difference is not clear at present, but shall be addressed in a separate focused study on this specific topic. (5) The reductive desorption of BPn’s in alkaline medium exhibits a pronounced odd-even effect in the potential dependencies of peak positions, peak width, and charge densities. The BPn adlayers with n ) odd are more stable than the adlayers with n ) even, clearly reflecting the higher packing densities and the long range order of the former. Referring to the coverages of the BPn adlayers, as extracted from STM experiments, we estimate that the total reductive desorption charge, derived from single sweep voltammetry, contains a double layer charge contribution of approximately 20%. (6) Chronoamperometric experiments with BPn-derivatives of moderate chain lengths revealed no separation between faradaic and capacitive currents in the overall process of reductive desorption. The experimental i-t transients, recorded for BPn adlayers with n ) 3 and n ) 4 were quantitatively described by hole nucleation and growth-based models. The kinetics of desorption of the uniform (x3 × 2x3) BP3 adlayer

17418 J. Phys. Chem. C, Vol. 111, No. 46, 2007 on Au(111)-(1 × 1) follows a mechanism combining one-step hole nucleation according to an exponential law with a 2D linear growth process. Transients of the two-component BP4 adlayer ((2x3 × 5x3) and (3 × 5x3)) were represented by two processes in parallel, hole nucleation (i) according to an exponential law and (ii) according to a power law, both in combination with 2D linear growth. For the two molecular systems, and within the potential range investigated, the hole nucleation rate showed no significant potential dependence, while the growth parameter increases significantly with more negative final potentials Ef. These results indicates that preexisting defects (domain boundaries, etching “holes”, pits, point defects...) within both BPn adlayers act as primary hole nucleation centers. (7) The potentials of oxidative desorption also exhibit a pronounced odd-even effect following the stability of the respective BPn adlayers. The underlying mechanism appears to be rather complex, and may involve up to 18 electrons. A more detailed understanding of the mechanism of oxidative desorption of BPn’s on Au(111)-(1 × 1) in 0.1 M HClO4 requires a qualitative and quantitative product analysis based on spectroscopic and DEMS experiments, which is beyond the scope of the present work. (8) No odd-even effect was detected in charge-transfer experiments employing Fe(CN)64-/Fe(CN)63- as a solution based redox probe. The kinetic current decreases by more than 3 orders of magnitude with increasing lengths of the alkyl spacer in BPn varying from 1 to 6. A constant attenuation parameter β ) (11.5 ( 1.0) nm-1 was found in -0.50 V e E e 0.50 V. The value of β obtained in the present study is compatible with data reported for saturated hydrocarbon bridges in various configurations, and supports a molecular “through-bond” tunneling process. The intermolecular coupling and the bonding of the sulfur anchor group to the gold substrate appear to play a minor role in the electron-transfer characteristics. The main contribution to the transport signature is attributed to the alkyl spacer unit. (9) The unique structure and electrochemical properties of monolayers of self-assembled 4-methyl-4′-(n-mercaptoalkyl)biphenyls on Au(111)-(1 × 1) qualify these molecular systems as promising templates in constructing novel functional metalorganic hybrid structures for a wide range of potential applications. Acknowledgment. R.A-S. acknowledges support from the German Academic Exchange Agency. G.J.S. was supported by an Alexander von Humboldt Research Fellowship. The authors also thank T. Kakiuchi for enlightening discussions, and G. Meszarosz for the help with the convolution analysis of the voltammograms. The work of M.H., U.S. and Th.W. was supported by the Institute of Functional Materials for Information Technology (IFMIT), the Volkswagen Foundation, the RWTH Aachen and the Research Center Ju¨lich. References and Notes (1) Swalen, J. D.; Allara, D. L; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvilli, J.; McCarthy, I. J.; Murray, R.; Pease, R. F.; Rabold, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (2) Iwamoto, M.; Chen-Xu, W. The Physical Properties of Organic Monolayers; World Scientific Publishers: Singapore, 2001. (3) Gaines, G. L. Insoluble Monolayers at Liquid/Gas Interfaces; Wiley: New York, 1966. (4) Petty, M. C. Langmuir-Blodgett FilmssAn Introduction, Cambridge University Press: Cambridge, 1996. (5) Katsaras, J.; Gutberlet, Th. Lipid BilayerssStructures and Interactions, Springer-Verlag: Berlin, 2001.

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Structure and Electrochemical Characterization of 4-Methyl-4 '-(n

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