RESUMO

Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between -0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm-2 and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe3 O4 bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction.

RESUMO

Well-ordered molecular films play an important role in nanotechnology, from device fabrication to surface patterning. Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) on the Au(100)-(1 × 1) and Au(111)-(1 × 1) have been used to understand the interplay of molecule-substrate interactions for heterocyclic thiols capable of binding to the surface by two anchors, which spontaneously form a highly disordered film on Au(111). Our results reveal that for the same surface coverage the simple change of the substrate from Au(111)-(1 × 1) to Au(100)-(1 × 1) eliminates molecular disorder and yields well-ordered SAMs. We discuss these findings in terms of differences in the surface mobility of 6MP species on these surfaces, the energetics of the adsorption sites, and the number of degrees of freedom of these substrates for a molecule with reduced surface mobility resulting from its two surface anchors. These results reveal the presence of subtle molecule-substrate interactions involving the heteroatom that drastically alter SAM properties and therefore strongly impact on our ability to control physical properties and to build devices at the nanoscale.

RESUMO

A combination of Polarization Modulation Infrared Reflection Absorption Spectroscopy (PMIRRAS) under electrochemical control, Electrochemical Scanning Tunneling Microscopy (ECSTM) and Molecular Dynamics (MD) simulations has been used to shed light on the reductive desorption process of dodecanethiol (C12) and octadecanethiol (C18) SAMs on gold in aqueous electrolytes. Experimental PMIRRAS, ECSTM and MD simulations data for C12 desorption are consistent with formation of randomly distributed micellar aggregates stabilized by Na(+) ions, coexisting with a lying-down phase of molecules. The analysis of pit and Au island coverage before and after desorption is consistent with the thiolate-Au adatoms models. On the other hand, PMIRRAS and MD data for C18 indicate that the desorbed alkanethiolates adopt a Na(+) ion-stabilized bilayer of interdigitated alkanethiolates, with no evidence of lying down molecules. MD simulations also show that both the degree of order and tilt angle of the desorbed alkanethiolates change with the surface charge on the metal, going from bilayers to micelles. These results demonstrate the complexity of the alkanethiol desorption in the presence of water and the fact that chain length and counterions play a key role in a complex structure.

Assuntos

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Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on the Au surfaces by forming thiolate-Au bonds. We found that the J-aggregates are preferentially adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot organize into aggregates on the substrate surface. These results indicate that the CNN-Au interaction is not able to disorganize the large J-aggregates stabilized by π-π stacking to optimize the S-Au binding site but it is strong enough to hinder the π-π stacking when CNNs are chemisorbed as monomers. The optical properties of the J-aggregates remain active after adsorption. The possibility of covalently bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au distance opens a new path regarding their improved stability and the wide range of biological applications of both CNN and AuNP biocompatible systems.

Assuntos

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A detailed polarization modulation infrared reflection absorption spectroscopy, scanning tunneling microscopy, and electrochemical study on methylene blue (MB) incorporation into alkanethiolate self-assembled monolayers (SAMs) on Au(111) is reported. Results show that the amount of MB incorporated in the SAMs reaches a maximum for intermediate hydrocarbon chain lengths (C10-C12). Well-ordered SAMs of long alkanethiols (C > C12) hinder the incorporation of the MB molecules into the SAM. On the other hand, less ordered SAMs of short alkanethiols (C < or = C6) are not efficient to retain the MB incorporated through the defects. For C12 the amount of incorporated MB increases as the SAM disorder is increased. This information is essential to the design of efficient thiol-based Au vectors for transport and delivery of molecules as well as thiol-based Au devices for molecular sensing.

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Attractive combination: Biopolymer-modified nanoparticles which combine magnetic properties with biocompatibility are prepared and delivered following a three-step strategy (see figure): i) Adsorption of thiol-capped metal nanoparticles on graphite, ii) electrochemical modification, iii) potential-induced delivery of the modified nanoparticles to the electrolyte. Thiol-capped gold nanoparticles modified with iron-melanin are attractive because they combine magnetic properties and biocompatibility. The biopolymer modified nanoparticles are prepared and delivered following a three step strategy: i) adsorption of thiol-capped metal nanoparticles on graphite, ii) electrochemical deposition of melanin-iron, iii) potential-induced delivery of the modified nanoparticles to the electrolyte.

Assuntos

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Electrodes modified layer-by-layer by self-assembly of redox active polyelectrolytes comprised of osmium bipyridine-pyridine derivatized poly(allyl-amine) and poly(vinyl) sulfonate have been studied by EQCM, ellipsometry, cyclic voltammetry and electrochemical impedance spectroscopy in aqueous solutions of different anions and cations. Redox driven swelling by solvent exchange during oxidation, in excess to the hydration number, occurs by perturbation of the equilibrium between the osmotic and elastic forces as a result of the electrochemical injection of charge into the film. The exchanged mass and volume change during redox switching strongly depends on the nature of the anion under anion Donnan permselectivity conditions.

Assuntos

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A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.

RESUMO

The sulfonation of polyaniline (PANI) films by nucleophilic addition of sulfite ion has been controlled through the polymer oxidation state under electrochemical control. The process was monitored by in situ electrochemical quartz crystal microbalance (EQCM), and the polymer oxidation was accomplished by electrode potential steps in sulfite aqueous solutions. The nucleophilic addition of sulfite to PANI only takes place on the oxidized polymer. From the ratio of added mass to the injected charge, the degree of sulfonation has been obtained with a yield as high as 50%. It has been observed that the ion-exchange mechanism during the oxidation-reduction process in the resulting sulfonated polymer is analogous to the polymer produced by electrophilic sulfonation of polyaniline or by copolymerization of aniline with aminosulfonic acids, unlike the ionic exchange observed for unmodified PANI.

RESUMO

Probe beam deflection during chronoamperometric oxidation-reduction of osmium complex in layer-by-layer self-assembled redox active polyelectrolyte multilayers has shown that the nature of the charge in the topmost layer determines the ion flux that balances the redox charge.