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J. Phys. Chem. C 2010, 114, 3568–3574
Formation of 1,8-Octanedithiol Mono- and Bilayers under Electrochemical Control Daniel Garcı´a-Raya, Rafael Maduen˜o, Manuel Bla´zquez, and Teresa Pineda* Departamento de Quı´mica Fı´sica y Termodina´mica Aplicada, UniVersidad de Co´rdoba, Campus de Rabanales, Ed. Marie Curie, E-14071 Co´rdoba, Spain ReceiVed: October 22, 2009; ReVised Manuscript ReceiVed: January 17, 2010
We present results of the formation of 1,8-octanedithiol (ODT) monolayers on the Au(111) single-crystal surfaces by oxidative deposition from alkaline solutions under electrochemical control. Cyclic voltammetry shows the presence of two well-separated oxidative peaks that are assigned to the formation of the S-Au bond (peak A1) and the oxidation of the thiolate species to give the disulfide dimer either in solution or in the adsorbed state (peak A2). The formation of a disulfide species can take place with the participation of two neighboring ODT molecules or through the formation of a bilayer in the adsorbed state. The reductive desorption of the layers formed under these conditions gives us some information about its nature and allows us the choice of the experimental conditions to carry out a potentiostatic method to build the layers of ODT with determined properties. Electrochemical techniques, such as cyclic voltammetry, differential capacitance-potential, and chronocoulometry curves, are used to discriminate between the monolayer and bilayer formation. Moreover, XPS data are used to confirm the electrochemical results. It is concluded that very reproducible layers that contain mainly standing-up ODT molecules are formed by the potentiostatic method and that they are built in a shorter time than those formed by the spontaneous assembly from an ethanolic solution. Introduction Alkanethiolate self-assembled monolayers (SAMs) form welldefined structures on Au(111) single-crystal electrodes due to the spontaneous formation of strong gold-sulfur bonds and attractive van der Waals forces between adjacent alkyl chains.1 The terminal group of the alkyl chain confers the desired chemical and physical properties to the layer.2,3 Thus, -SHterminated thiol SAMs efficiently bind metallic nanoparticles, making these ordered structures essential in many of the “bottom-up” methods proposed to build a variety of devices and nanostructured materials. The control of the organization of these architectures is of great technological importance, and the formation of well-organized layers is needed. In these layers, the individual molecules are bonded through one thiol to the metal and the second unreacted thiol moieties are exposed to the outer surface. However, the high lateral mobility of the long alkane chain together with the presence of the second -SH functional group in the alkanedithiol molecules brings up the question of whether only one or both sulfur groups bind to the gold surface in the SAM. In this sense, laying-flat dithiol molecules that bind to the surface through the two -SH groups have been observed at low surface coverages.4-7 To obtain compact dithiol monolayers of standing-up molecules, different preparation methods, which include protection of the second thiol group8 or using a mixture of monothiol and dithiol at the appropriate relative concentrations that results in an almost pure dithiol film,9,10 have been reported. A standing-up orientation has been found for assembled dithiol molecules that are treated with a reductor as tri-nbutylphosphine that prevents the formation of multilayers or intralayer disulfide bond formation.11 These complex organizations have been found to be formed by the oxidation of the * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +34-957-218646. Fax: +34-957-218618.
first layer thiol-exposed groups, depending on the presence of the oxidants in solution.12-19 The existence of intralayer disulfide bonds and its dependence on the dithiol chain length has been evidenced by electrochemical means.7,20,21 Recently, it has been demonstrated that the addition of a disulfide reducing agent as dithiotreitol gives homogeneous dithiol monolayers that are able to bind gold nanoparticles in a controllable fashion.22 On the other hand, Morin et al.23-25 have reported an electrochemical method for the sequential formation of monolayers and bilayers of R-ω dithiols by controlling the electrode potential in the presence of the monomers in solution. The preparation of thiolate monolayers under electrochemical control was first reported by Porter et al.,26 who found that these monolayers had structures and interfacial properties similar to those of their self-assembled analogs and that the extent of deposition could be controlled by the reaction thermodynamics. The monolayer deposited under potentiodynamics conditions are typically slightly less than a monolayer and are determined by deposition kinetics. However, a full monolayer can be formed by holding the applied voltage at a value positive of the deposition wave. More recently, Uosaki et al.27,28 have studied the electrochemical oxidative formation and reductive desorption processes of a decanethiol (DT) SAM on a Au(111) surface in KOH ethanol solution. They found that the positions of the peaks for the formation and desorption of the layer depend on the concentration of DT and the reductive charge increased with the DT concentration and with the decrease of the sweep rate. Moreover, they investigated the potentiostatic SAM formation by holding the potential at a value positive to the SAM formation process and found that the surface coverage increased with time and reached a saturation value corresponding to a full monolayer. In a recent work, we have reported the characterization of a self-assembled monolayer of octanedithiol (ODT) formed from an ethanolic solution. Electrochemical techniques, such as cyclic voltammetry and electrochemical impedance spectroscopy, have
10.1021/jp910113n 2010 American Chemical Society Published on Web 02/04/2010
ODT Layers been used to follow the changes in the electron-transfer processes of the redox probe ferricyanide upon increasing modification time. It has been found that a homogeneous and almost defect-free monolayer can be formed (24-48 h modification time), provided that the molecular oxygen is completely removed from the dithiol solution.29 These experimental conditions are hard to accomplish in order to carry out a subsequent functionalization of the interface by modification with gold nanoparticles. However, other authors30,31 have found that wellorganized nonanethiol monolayers can be prepared from degassed n-hexane solutions of the dithiols in the absence of ambient light at lower modification times. Moreover, they have demonstrated the key role of the chain length as well as the experimental procedure in controlling the surface structure and chemistry of the SAMs of dithiols on Au(111) surfaces.31 These facts have encouraged us to study new methods for the formation of dithiol layers in shorter times and under less restrictive conditions. In this work, we present a study of the formation of monolayers and bilayers of ODT on a Au(111) single-crystal electrode under electrochemical control from an aqueous alkaline solution. The layer composition is evaluated by electrochemical techniques, such as cyclic voltammetry, chronocoulometry, and differential capacity measurements as well as by X-ray photoelectronic spectroscopy. The results demonstrate that, under these conditions, either ODT monolayers or ODT bilayers can be formed in a time shorter than this necessary for the spontaneous self-assembling process. Experimental Section Chemicals. 1,8-Octanedithiol, octanethiol, and semiconductor-grade purity potassium hydroxide were purchased from Aldrich-Sigma. All solutions were prepared with deionized water produced by the Millipore system. Methods. A conventional three-electrode cell comprising a platinum coil as the counter electrode, a saturated calomel electrode as the reference electrode, and a gold single crystal (111-face) as the working electrode was used. The Au(111) single-crystal electrode was a homemade hemisphere with a diameter of approximately 2 mm with a gold wire, mounted at its far tip, that allows easier handling of the crystal. The single crystal was grown by melting a high-purity Au wire (99.9998%), and subsequently oriented, cut, and polished, following the method developed by Clavilier.32,33 Before each electrochemical measurement, the electrode was annealed in a natural gas flame to light red melt for about 20 s and, after a short period of cooling in air, quenched in ultrapure water. The electrode was then transferred into the electrochemical cell with a droplet of water adhering to it to prevent contamination. The configuration of the sample consists of the contact of the (111) gold face with the solution by the hanging meniscus method. The surface condition was confirmed by a cyclic voltammogram in 0.01 M HClO4, and the real surface area was determined from the reduction peak of oxygen adsorption on the Au electrode. This surface treatment was the most appropriate to produce a surface that is clean, ordered, and highly reproducible. Cyclic voltammetry (CV), chronocoulometry, and capacitance-potential (C-E) curves were recorded on an Autolab (Ecochemie model Pgstat20) instrument attached to a PC with proper software (GPES and FRA) for the total control of the experiments and data acquisition. X-ray photoelectronic spectroscopy (XPS) analyses were performed with a SPECS Phoibos 150 MCD spectrometer using nonmonochromatized (12 kV, 300 W) Mg KR radiation (1253.6 eV). The ODT-modified single-crystal electrode was mounted
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Figure 1. Cyclic voltammetry of a Au(111) single-crystal electrode in a 1 mM ODT in 0.1 M KOH solution at different switching potentials in the high-potential region. The reductive desorption potentials are Es ) -0.80 V, Ep ) -1.05 V, qRD ) 52.7 µC/cm2; Es ) -0.60 V, Ep ) -1.05 V, qRD ) 66.2 µC/cm2; Es ) -0.44 V, Ep ) -1.06/-1.09 V, qRD ) 74.5 µC/cm2; Es ) -0.38 V, Ep ) -1.135 V, qRD ) 76.7 µC/ cm2; V ) 20 mV/s.
on a steel sample holder and introduced directly into the XPS analytical chamber. The working pressure was
