Chronoamperometric study of the reductive desorption of alkanethiol self-assembled monolayers

Chemistry of cysteine assembly on Au(100): electrochemistry, in situ STM and molecular modeling

Cysteine (Cys) is an essential amino acid with a carboxylic acid, an amine and a thiol group. We have studied the surface structure and adsorption dynamics of l-cysteine adlayers on Au(100) from aqueous solution using electrochemistry, high-resolution electrochemical scanning tunnelling microscopy (in situ STM), and molecular modelling. Cys adsorption on this low-index Au-surface has been much less studied than Cys adsorption on Au(111)- and Au(110)-electrode surfaces. Chronopotentiometry was employed to monitor the adsorption dynamics at sub-second resolution and showed that adsorption is completed in 30 minutes at Cys concentrations above 100 μM. Two consecutive steps could be fitted to these data. Two separate reductive desorption peaks of Cys adlayers on Au(100) with a total coverage of 2.52 (±0.15) × 10-10 mol cm-2 were observed. In situ STM showed that the adsorbed Cys is organized in stripes with "fork-like" features which co-exist in (11 × 2)-2Cys and (7 × 2)-2Cys lattices, quite differently from Cys adsorption on Au(111)-electrode surfaces. Stripe structures with bright STM contrast in the center suggest that a second Cys adlayer on top of a first adlayer is formed, supporting the dual-peak reductive desorption of Cys adlayers. In addition, monolayers of both pure l-Cys and pure d-Cys and a 1 : 1 racemic mixture of l- and d-Cys on Au(100) were studied. Virtually identical macroscopic electrochemical features were found, but in situ STM discloses many more defects for the racemic mixture than for the pure enantiomers due to structural mismatch of l- and d-Cys. Density functional theory (DFT) calculations combined with a cluster model for the Au(100) surface were carried out to investigate the adsorption energy and geometry of the adsorbed monomer and dimer Cys species in different orientations, with detailed attention to the chirality effects. Optimized DFT geometries were used to construct model STM images, and kinetic Monte Carlo simulations undertaken to illuminate the growth of adsorbate rows and the mechanism of the adlayer formation as well as the Cys adsorption patterns specific to the Au(100)-electrode surface.

Modulation of electrochemical hydrogen evolution rate by araliphatic thiol monolayers on gold

Electroreductive desorption of a highly ordered self-assembled monolayer (SAM) formed by the araliphatic thiol (4-(4-(4-pyridyl)phenyl)phenyl)methanethiol leads to a concurrent rapid hydrogen evolution reaction (HER). The desorption process and resulting interfacial structure were investigated by voltammetric techniques, in situ spectroscopic ellipsometry, and in situ vibrational sum-frequency-generation (SFG) spectroscopy. Voltammetric experiments on SAM-modified electrodes exhibit extraordinarily high peak currents, which di er between Au(111) and polycrystalline Au substrates. Association of reductive desorption with HER is shown to be the origin of the observed excess cathodic charges. The studied SAM preserves its two-dimensional order near Au surface throughout a fast voltammetric scan even when the vertex potential is set several hundred millivolt beyond the desorption potential. A model is developed for the explanation of the observed rapid HER involving ordering and pre-orientation of water present in the nanometer-sized reaction volume between desorbed SAM and the Au electrode, by the structurally extremely stable monolayer, leading to the observed catalysis of the HER.

Size-controlled electrochemical growth of PbS nanostructures into electrochemically patterned self-assembled monolayers

1-Hexadecanethiol self-assembled monolayers (HDT SAMs) on Au(111) were used as a molecular resist to fabricate nanosized patterns by electrochemical reductive partial desorption for subsequent electrodeposition of PbS from the same solution simultaneously. The influences of potential steps of variable pulse width and amplitude on the size and the number of patterns were investigated. The kinetics of pattern formation by reductive desorption appears to be instantaneous according to chronoamperometric and morphological investigations. PbS structures were deposited electrochemically into the patterns on HDT SAMs by a combined electrochemical technique, based on the codeposition from the same saturated PbS solution at the underpotential deposition of Pb and S. Scanning tunneling microscopy measurements showed that all of the PbS deposits were disk shaped and uniformly distributed on Au(111) surfaces. Preliminary results indicated that the diameter and the density of PbS deposits can be controlled by controlling the pulse width and amplitude of potential applied at the reductive removal stage of HDT SAMs and the deposition time during the electrochemical deposition step.

Reorientation of thiols during 2D self-assembly: interplay between steric and energetic factors

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.

A new method to evaluate the surface dipole potential of thiol and disulfide self-assembled monolayers and its application to a disulfidated tetraoxyethylene glycol

A procedure to evaluate the surface dipole potential chi of thiol and disulfide self-assembled monolayers (SAMs) is described. The procedure consists of self-assembling the monolayers on a hanging mercury drop electrode and of measuring the charge involved in a progressive expansion of the mercury drop. This measurement is then combined with an estimate of the charge density q experienced by diffuse layer ions, obtained by measuring the diffuse layer capacitance of the SAM at different electrolyte concentrations by electrochemical impedance spectroscopy. These chi measurements, combined with chronocoulometric measurements of the total charge density sigma(M) against potential, indicate that SAMs of tetraoxyethylene glycol-D,L-alpha-lipoic acid ester (TEGL), 2,3-di-O-phytanyl-sn-glycerol-1-tetraoxyethylene glycol-D,L-alpha-lipoic ester (DPTL), and trioxyethyleneoxythiol (EO3) on mercury may undergo a reversal in the surface dipole potential of their polyoxyethylene chain with a change in the interfacial electric field. Moreover, TEGL and EO3 form stable SAMs without electron transfer to the metal, while no such conclusion can be drawn for DPTL.

Alkanethiol monolayers at reduced and oxidized zinc surfaces with corrosion protection: a sum frequency generation and electrochemistry investigation

In this work, octadecanethiol (ODT) was demonstrated to form ordered monolayers at either electrochemically reduced or oxidized Zn surfaces, by means of sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG spectra of ODT-modified Zn electrodes featured three methyl group resonances in the C-H vibrational region (2800-3100 cm(-1)). A significant decrease in interfacial capacitance and an increase in charge-transfer resistance were observed in EIS measurement after ODT modification. The alkane chain tilt angle of ODT within a monolayer at the Zn surface was estimated as 0 degrees with respect to the surface normal by interfacial capacitance measurement via EIS. CV and SFG investigation revealed that ODT monolayers undergo reductive desorption from the Zn electrode in 0.5 M NaOH at -1.66 V (vs SCE) and in 0.5 M NaClO4 at -1.62 V. The integrated charge consumed to the desorption of ODT is determined as 87 mC/cm2 from the reductive peak on CV curve, resulting in a coverage of 9.0 x 10(-10) mol/cm2 (5.4 x 10(14) molecules/cm2) if assuming the reduction follows a one-electron process. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt, and acid media. However, the protection efficiency was reduced with immersion time due to the presence of defects within the monolayers.

Electrochemical desorption of n-alkylthiol SAMs on polycrystalline gold: studies using a ferrocenylalkylthiol probe

The reductive voltammetric desorption of n-alkylthiol self-assembled monolayers (SAMs) was studied using ferrocenyldodecanethiol (FcC12SH) as a probe. A short (10 s) incubation in a 2 mM FcC12SH solution labels the pre-existing defect sites present in a tetradecanethiol (C14S-Au) SAM. Additional defects in a C14S-Au SAM are then created by voltammetric cycling to reductive potentials. The effects of changing the desorption potential and the hold time at the desorption potential were investigated by monitoring the change in the two characteristic FcC12S-Au peaks. These two peaks are associated with FcC12S-Au filling individual single site defects (peak I at 260 mV) and pinhole defects (peak II at 380 mV). The reductive desorption potential applied to a binary (FcC12S-/C14S-Au) SAM results in a partial desorption and a potential "remixing" of the phase-separated state.

Assembly dynamics and detailed structure of 1-propanethiol monolayers on Au(111) surfaces observed real time by in situ STM

1-Propanethiol is chosen as a model alkanethiol to probe detailed mechanisms of the self-assembled monolayer (SAM) formation at aqueous/Au(111) interfaces. The assembly processes, including initial physi- and chemisorption, pit formation, and domain growth, were recorded into movies in real-time with high resolution by in situ scanning tunneling microscopy (STM) under potential control. Two major adsorption steps were disclosed in the propanethiol SAM formation. The first step involves weak interactions accompanied by the lift of the Au(111) surface reconstruction, which depends reversibly on the electrochemical potentials. The second step is chemisorption to form a dense monolayer, accompanied by formation of pits as well as structural changes in the terrace edges. Pits emerged at the stage of the reconstruction lift and increased to a maximum surface coverage of 4.0 +/- 0.4% at the completion of the SAM formation. Well-defined triangular pits in the SAM were found on the large terraces (more than 300 nm wide), whereas few and small pinholes appeared at the terrace edge areas. Smooth edges were converted into saw-like structural features during the SAM formation, primarily along the Au(111) atomic rows. These observations suggest that shrinking and rearrangement of gold atoms are responsible for both formation of the pits and the shape changes of the terrace edges. STM images disclose a (2 square root 3 x 3)R30 degrees periodic lattice within the ordered domains. Along with electrochemical measurements, each lattice unit is assigned to contain four propanethiol molecules exhibiting different electronic contrasts, which might originate in different surface orientations of the adsorbed molecules.

Assembled monolayers of Mo3S44+ clusters on well-defined surfaces

A class of inorganic monolayers formed by assembling the molybdenum-sulfur cluster, Mo3S4(4+), onto a well-defined Au(111) surface is presented. The monolayers have been comprehensively characterized by electrochemistry, X-ray photoelectron spectroscopy (XPS), and in situ scanning tunneling microscopy (in situ STM). The voltammetric data show strong reductive and oxidative desorption signals from Au-S interactions, supported by the presence of both S and Mo signals in XPS. In situ STM shows many small pits in the dense adlayers uniformly spread over the surface, which is a typical feature of self-assembled monolayers (SAMs) of alkanethiols. The density of the pits is ca. 23 (+/-5)% and is significantly higher than for straight-chain alkanethiol SAMs with a single -SH group. The pit shapes are irregular, suggesting multiple Au-S interactions from Mo3S4(4+). High resolution images disclose bright round spots of ca. 8 A diameter representing individual molecules in the SAM. This is the first example of in situ monolayer formation by a metal-chalcogenide cluster directly anchored onto the gold surface through core ligands and offers a simple way to prepare a new class of functionalized inorganic monolayers.

Electrochemical detection of nanoscale phase separation in binary self-assembled monolayers

Developing methods to probe the nature and structure of nanoscale environments continues to be a challenge in nanoscience. We report a cyclic voltammetry investigation of the internal, hydrogen-bond-driven phase separation of amide-containing thiols and alkanethiols. Amide-containing thiols with a terminal ferrocene carboxylate functional group were investigated in two binary monolayers, one homogeneously mixed and the other phase separated. The electrochemical response of the ferrocene probe was used to monitor adsorbate coverage, environment, and phase separation within each of these monolayers. The results demonstrate that the behavior of ferrocene-containing monolayers can be used to probe nanoscale organization.

In Situ dynamic monitoring of electrochemical oxidative adsorption and reductive desorption processes of a self-assembled monolayer of hexanethiol on a Au(111) surface in KOH ethanol solution by scanning tunneling microscopy

Electrochemical oxidative formation and reductive desorption processes of a self-assembled monolayer (SAM) of hexanethiol on a Au(111) surface in KOH ethanol solutions containing various concentrations of hexanethiol were investigated by in situ scanning tunneling microscopy in real time. The generation and disappearance of vacancy islands (VIs), corresponding to the formation and desorption of the SAM, respectively, were observed as anodic and cathodic current, respectively, flowed when the thiol concentration was higher than ca. 1 microM. When the VIs disappeared after the reductive desorption of the SAMs, the herringbone structure corresponding to the (radical3 x 23) structure of Au(111), was observed on the surface, indicating that a clean reconstructed surface was exposed even in the hexanethiol ethanol solution. During both oxidative adsorption and reductive desorption of the SAMs, the shape of the steps of the gold substrate changed drastically and the step lines became parallel to the 121 direction of the Au(111) surface, suggesting that gold atoms on the surface were extremely mobile during these processes. The coalescence of adjacent vacancy islands and growth of larger islands triangular in shape accompanied with the disappearance of nearby smaller islands were observed, confirming that the VIs grew according to the Ostward ripening model.

Resolution of intermediate adsorbate structures in the potential-dependent self-assembly of n-hexanethiolate on Silver by in situ surface-enhanced Raman spectroscopy

Resolution of the reaction steps and the associated component Raman spectra during the formation or desorption of self-assembled monolayers is challenging because intermediate adsorbate populations are present at low concentrations and their spectral bands overlap. By collecting Raman spectra versus applied potential into a two-dimensional data set, one can utilize multivariate statistical techniques to resolve the component concentration profiles along with their corresponding Raman spectra. In situ surface-enhanced Raman spectroscopy (SERS) spectra were collected during the potential-dependent formation and desorption (-1.50 to -0.70V versus Ag/AgCl) of n-hexanethiolate monolayer at a polycrystalline Ag electrode. Resolution of the pure component spectra from these components was accomplished by using self-modeling curve resolution (SMCR), which does not require a physical model. For monolayer adsorption, the potential-dependent Raman spectra could be described by three significant eigenvectors; the eigenvectors could be rotated into a set of pure component spectra and concentration profiles using a linear least-squares step to find a common plane in the space of the eigenvectors representing the linear combination of the real-component responses. The convex hull surrounding the data in the plane and positive amplitude criteria were utilized to identify the coordinates of the pure component responses. The C-S stretching vibrations of the resolved spectra show that the initial adsorbate is a gauche conformer, which allows the hydrocarbon chain to lie on the metal surface; a second phase arises at higher coverage with trans C-S conformation, where the hydrocarbon chains are oriented off the surface plane, and a final complete monolayer is formed with a well-ordered, all-trans C-S configuration. In contrast, desorption studies showed only two surface phases, the initial well-ordered monolayer and the low-density phase dominated by gauche conformations. The results illustrate the utility of self-modeling curve resolution to unravel interfacial reaction mechanisms and intermediate structures from two-dimensional SERS data, without requiring prior knowledge of a physical model for the process.

A massively parallel electrochemical approach to the miniaturization of organic micro- and nanostructures on surfaces

This paper describes a simple and convenient strategy for reducing the dimensions of organic micro-and nanostructures on metal surfaces. By varying electrochemical desorption conditions, features patterned by dip-pen nanolithography or micro contact printing and made of linear alkanethiols or selenols can be gradually desorbed in a controlled fashion. The process is referred to as electrochemical whittling because the adsorbate desorption is initiated at the exterior of the feature and moves inward as a function of time. The whittling process and adsorbate desorption were studied as a function of substrate morphology, adsorbate head and tail groups, and electrolyte solvent and salt. Importantly, one can independently address different nanostructures made of different adsorbates and effect their miniaturization based upon ajudicious selection of adsorbate, applied potential, and supporting electrolyte. Some of the physical and chemical origins of these observations have been elucidated.