ABSTRACT
Gold-sulphur bonds holding self-assembled monolayers (SAMs) on their gold substrate can be broken by electrochemical reduction, which typically occurs in an electrode potential range where the electrochemical hydrogen evolution reaction (HER) is thermodynamically possible. This work uses an in situ coupling between cyclic voltammetry and spectroscopic ellipsometry to compare the interfacial structure after desorption of the aliphatic thiols 1-Dodecanethiol (DDT) and 1-Octadecanethiol (ODT), and the ω-hydroxythiol 11-Mercapto-1-undecanol (MUD). For MUD and DDT, the data can only be explained by the presence of a substance with a significantly lower refractive index than the aqueous electrolyte in the interfacial region. This substance is likely to be H2. The hypothesis is put forward here that for MUD and DDT, desorbed molecules stabilise "nanobubbles" of H2. The resulting aggregates form as initial stages of the process of complete disintegration of the SAMs, i.e. the loss of the SAM-forming molecules into solution. On the other hand, desorption and readsorption of ODT are fully reversible - the presence of a layer with low refractive index can neither be excluded nor confirmed in this case. The results indicate that different SAM-stabilities are a consequence of solubility of the thiolates.
ABSTRACT
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.
ABSTRACT
Vibrational sum-frequency-generation (SFG) spectroscopy experiments at electrified interfaces involve incident laser radiation at frequencies in the IR and near-IR/visible regions as well as a static electric field on the surface. Here we show that mixing the three fields present on the surface can result in third-order effects in resonant SFG signals. This was achieved for closed packed self-assembled monolayers (SAMs) with molecular groups of high optical nonlinearity and surface potentials similar to those typically applied in cyclic voltammograms. Broadband SFG spectroscopy was applied to study a hydrophobic well-ordered araliphatic SAM on a Au(111) surface using a thin-layer analysis cell for spectro-electrochemical investigations in a 100 mM NaOH electrolyte solution. Resonant contributions were experimentally separated from non-resonant contributions of the Au substrate and theoretically analyzed using a fitting function including third-order terms. The resulting ratio of third-order to second-order susceptibilities was estimated to be [Formula: see text](10-10) m/V.
ABSTRACT
Palladium nanoparticles have been deposited electrochemically onto self-assembled monolayers (SAMs) of 4-(4-(4-pyridyl)phenyl)phenylmethanethiol. A pronounced correlation between the kinetics of the complexation between pyridine nitrogens and Pd cations and the sample potential has been observed. The amount of the Pd deposit significantly increases by adjusting the sample potential during the complexation step to values below the point of zero charge. The size of the spherical shaped Pd nanoparticles varies within a certain limit according to the amount of Pd(2+) ions initially coordinated on top of the SAM. The metallic state of these particles was confirmed by X-ray photoelectron spectroscopy and infrared reflection-absorption spectroscopy. Moreover, CO adsorption on the clean Pd deposit revealed further information about the crystallographic orientation of the nanoparticles.
ABSTRACT
Based on electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), Au(111) electrodes modified by self-assembled monolayers (SAMs) of a homologous series of pyridine-terminated thiols with aromatic backbones have been investigated. An important correlation between the chain structure and film integrity in electrolytic media was found. Monolayers with odd numbers of methylene spacers in the molecular chain showed superior barrier properties compared to even numbered counterparts. A positive influence of an increase in the number of attached phenyl rings on the integrity of SAMs was observed. Furthermore, cathodic desorption of the investigated SAMs is characterized by multiwave desorption peaks and extraordinarily large cathodic charges indicating an unusual desorption process. Moreover, protonation behavior of the SAMs has been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Protonation has been found to be reversible and surface pK(a) values have been determined to be around 5 for all investigated monolayers.
