Potential-driven Changes Research Articles

Time-Resolved Product Observation for CO2 Electroreduction Using Synchronised Electrochemistry-Mass Spectrometry with Soft Ionisation (sEC-MS-SI).

The mechanistic understanding of electrochemical CO2 reduction reaction (CO2 RR) requires a rapid and accurate characterisation of product distribution to unravel the activity and selectivity, which is yet hampered by the lack of advanced correlative approaches. Here, we present the time-resolved identification of CO2 RR products by using the synchronised electrochemistry-mass spectrometry (sEC-MS). Transients in product formation can be readily captured in relation to electrochemical conditions. Moreover, a soft ionisation (SI) strategy is developed in MS for the direct observation of CO, immune to the interference of CO2 fragments. With the sEC-MS-SI, the kinetic information, such as Tafel slopes and onset potentials, for a myriad of CO2 RR products are revealed and we show the hysteresis seen for the evolution of some species may originate from the potential-driven changes in surface coverage of intermediates. This work provides a real-time picture of the dynamic formation of CO2 RR products.

Quantifying the Surface Diffusion of Oxygen Adspecies on Gold By Nanoelectrode Voltammetry

Surface diffusion of oxygen adspecies on noble metals plays crucial roles in heterogeneous catalysis, e.g., the anti-poisoning mechanism in fuel cells. However, quantitative measurements of surface diffusion in a solid/liquid environment remains challenging. Nanoelectrode voltammetry provides an easy but quantitative method to study the surface electrochemical behaviors of electroactive adspecies on metallic catalysts, which is valuable for the understanding of electroactalytic mechanisms and the rational design of electrocatalysts.In this work [1] a gold nanodisk electrode was employed to study surface diffusion of oxygen adspecies. Surprisingly, we found that when the size of electrode was decreased to nanometer scale cyclic voltammograms (CV) behavior were drastically different from those observed at macrodisks gold electrodes. Indeed, instead of expected bell shaped CV characteristic to surface controlled process a seemingly diffusion controlled peak was observed during the forward scan. On the contrary, a very sharp almost symmetrical peak was recorded during the backward scan. A quantitative check on the charge of the adspecies during deposition and stripping phases revealed their equality and linear dependence with the reciprocal of the square root of scan rate, again suggesting a mass transport control in contradiction with the expectations. As was suggested previously [2, 3] the adspecies may apparently diffuse from the electrode-solution interface towards the surface of the electrode shaft which is in contact with the insulator. In agreement with this view, the amount of the adspecies stored by the system is drastically increased in comparison to the quantity of adspecies which can be accommodated on the disk electrode-solution interface only. In order to check consistency of these views we resorted to modeling by assuming a diffusion-equivalent transport of adatoms through site hopping [4-6] in agreement with the experimental observation of an Arrhenius’ dependence of the apparent diffusion coefficient. In addition, due to the unconventional behavior during the desorption stage we incorporated potential driven changes into the kinetics of the site hopping (i.e., leading to a larger apparent surface diffusion coefficient) to account for an apparent faster diffusion in agreement with the increasing instability of the metal-adatom bonds in the increasingly cathodic region. This rather simple model provided an excellent qualitative and quantitative agreement between experimental data and theoretical predictions. This, in particular, allowed to estimate the apparent potential-dependent surface diffusion coefficient of oxygen adspecies on Au surface.

Potential-driven changes in hydration of chitosan-derived molecular films on gold electrodes

Chitosan-derived molecular films self-assembled on the gold surface were characterized using electrochemical methods combined with quartz crystal microbalance, atomic force microscopy and surface enhanced infrared absorption spectroscopy. We have observed potential-dependent structural transitions, which involved a reorientation of the oligosaccharide molecules and either depletion or accumulation of water molecules within the film. The nature of water accumulated within the chitosan-derived film varies with the potential but it is dominated by hydrogen-bonded species. The content of water entrapped within the oligosaccharide film can be controlled by the potential applied to the electrode, which means that its hydration/dehydration can be triggered by an electric field.

The utility of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) in surface and in situ studies: new data processing and presentation approach.

Infrared spectroscopy is a powerful non-destructive technique for the identification and quantification of organic molecules widely used in scientific studies. For many years, efforts have been made to adopt this technique for the in situ monitoring of reactions. From these efforts, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was developed three decades ago. Unfortunately, because of the complexity of data processing and interpretation, PM-IRRAS had been avoided in lieu of the single potential alteration infrared spectroscopy (SPAIRS) and subtractively normalized interfacial Fourier transform infrared (SNIFTIR). In this work, we present a new approach for PM-IRRAS data processing and presentation, which provides more insight into in situ and surface studies besides dramatically improving the S/N. In this new approach, we recommend three complementary methods of data treatment (eqn (7), (9) and (10)) as the new protocols for presenting PM-IRRAS data. These equations are robust in visualising the surface processes at the solid-liquid and solid-gas interphases. Eqn (7) contrasts the surface adsorbed species with respect to the isotropic background with or without the influence of the applied potential. Eqn (9) highlights the surface potential-driven changes between the sample and the reference spectra. Eqn (10) focuses on the bulk-phase (solution/gas and surface species) potential-driven changes between the sample and the reference spectra, and hence it can be used to track the production of species, which desorb from the surface upon their formation. Examples of ethanol electro-oxidation reaction are provided as a test system for in situ studies and PVP deposited on glassy carbon for thin-film studies to illustrate the utility of the new PM-IRRAS data handling protocol, which is poised to improve the understanding of the chemistry and physics of surface processes.

Submolecular Structure and Orientation of Oligonucleotide Duplexes Tethered to Gold Electrodes Probed by Infrared Reflection Absorption Spectroscopy: Effect of the Electrode Potentials.

Unique electronic and ligand recognition properties of the DNA double helix provide basis for DNA applications in biomolecular electronic and biosensor devices. However, the relation between the structure of DNA at electrified interfaces and its electronic properties is still not well understood. Here, potential-driven changes in the submolecular structure of DNA double helices composed of either adenine-thymine (dAdT)25 or cytosine-guanine (dGdC)20 base pairs tethered to the gold electrodes are for the first time analyzed by in situ polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) performed under the electrochemical control. It is shown that the conformation of the DNA duplexes tethered to gold electrodes via the C6 alkanethiol linker strongly depends on the nucleic acid sequence composition. The tilt of purine and pyrimidine rings of the complementary base pairs (dAdT and dGdC) depends on the potential applied to the electrode. By contrast, neither the conformation nor orientation of the ionic in character phosphate-sugar backbone is affected by the electrode potentials. At potentials more positive than the potential of zero charge (pzc), a gradual tilting of the double helix is observed. In this tilted orientation, the planes of the complementary purine and pyrimidine rings lie ideally parallel to each other. These potentials do not affect the integral stability of the DNA double helix at the charged interface. At potentials more negative than the pzc, DNA helices adopt a vertical to the gold surface orientation. Tilt of the purine and pyrimidine rings depends on the composition of the double helix. In monolayers composed of (dAdT)25 molecules the rings of the complementary base pairs lie parallel to each other. By contrast, the tilt of purine and pyrimidine rings in (dGdC)20 helices depends on the potential applied to the electrode. Such potential-induced mobility of the complementary base pairs can destabilize the helix structure at a submolecular level. These pioneer results on the potential-driven changes in the submolecular structure of double stranded DNA adsorbed on conductive supports contribute to further understanding of the potential-driven sequence-specific electronic properties of surface-tethered oligonucleotides.

Charge fluctuations in nanoscale capacitors.

The fluctuations of the charge on an electrode contain information on the microscopic correlations within the adjacent fluid and their effect on the electronic properties of the interface. We investigate these fluctuations using molecular dynamics simulations in a constant-potential ensemble with histogram reweighting techniques. This approach offers, in particular, an efficient, accurate, and physically insightful route to the differential capacitance that is broadly applicable. We demonstrate these methods with three different capacitors: pure water between platinum electrodes and a pure as well as a solvent-based organic electrolyte each between graphite electrodes. The total charge distributions with the pure solvent and solvent-based electrolytes are remarkably Gaussian, while in the pure ionic liquid the total charge distribution displays distinct non-Gaussian features, suggesting significant potential-driven changes in the organization of the interfacial fluid.

Study of the potential driven changes in a collagen film self-assembled on a polycrystalline gold electrode surface

The influence of the potential applied to the polycrystalline gold electrode on the adsorption state and structure of collagen molecules were studied by means of electrochemistry, in situ ellipsometry and in situ polarization modulation infrared reflection–absorption spectroscopy. At the macroscopic level, potential and the corresponding charge accumulated on the gold electrode determine the adsorption process of the collagen film on the Au electrode surface. The protein film is stable on the electrode surface at potentials close to the potential of zero charge. The protein film sustains a large negative potential drop (ΔϕM|S⩾−0.5V) whereas it is unstable at a small positive potential drop (ΔϕM|S≈0.1V) across the film. Positive net charge accumulated on the Au surface causes electrostatic repulsions of collagen molecules bearing a positive net charge. Loosening of water and destabilization of the protein film reflect repulsions between the Au electrode and the collagen molecules. In contrast, at charge densities between −8<σM<−20μCcm−2 electrostatic attractions between the electrode surface and the protein molecule appear. A stable, well hydrated collagen film is formed on the Au surface. Higher negative charges accumulated on the electrode surface lead to swelling on the protein film by water and lead to the desorption of the protein film from the Au surface. In situ spectroelectrochemical studies show that at the molecular level neither the secondary structure nor the orientation of collagen molecules are affected by electrical potentials. Independently of the applied potential and electric fields acting on the film the collagen molecule maintains its native structure. Collagen molecules adsorbed on the Au surface form a heterogeneous film with well-defined structure and unusual stability at the molecular level.

Electrochemical SERS study of a biomimetic membrane supported at a nanocavity patterned Ag electrode

Lipid bilayers in which two leaflets were made of dimyristoyl-phosphatidylcholine (DMPC) and hybrid bilayers with one leaflet composed of hydrogenated lipid and another leaflet of deuterium substituted molecules (d63-DMPC) were deposited on highly ordered nanocavity patterned Ag electrodes using LB–LS and vesicle fusion techniques. In situ electrochemical surface enhanced Raman scattering (EC-SERS) was then used to study potential driven changes in these model biological membranes. The nanocavity structures provided a SERS active substrate with uniformly distributed surface enhancement. To ensure that the bilayer was separated from the metal surface by a hydrophilic space layer, the electrodes were chemically modified with a self-assembled monolayer (SAM) of β-thioglucose (TG) molecules. The monolayer of TG ensured that the solid substrate surface was hydrophilic. The electrochemical properties of the bilayer were monitored by recording differential capacitance curves. EC-SERS indicated that at the silver surface modified by the monolayer of TG the lower leaflet (in contact with the support) is more ordered than the top leaflet that is contact with solution. However, both leaflets remained in the liquid crystalline (LC) state for the entire range of investigated potentials. The results of this study show that the DMPC bilayer at the nanocavity patterned Ag surface may be used as a good biomimetic membrane model in future SERS studies of membrane proteins. The information concerning the effect of the electrode potential on membrane stability may be useful for the development of biosensors.

The influence of electrolyte concentration on the adsorption of octadecanol on Au(111)

Abstract The influence of electrolyte concentration on the potential dependent adsorption and desorption of octadecanol to/from a Au(111) electrode was investigated utilizing electrochemical and elastically scattered light techniques. The electrolyte concentration was found to influence the potential driven changes of the adsorbed layer (adsorption and desorption). The capacitive changes in the adsorbed layer were found to occur at more negative potentials with lower electrolyte concentration. The changes in the optical measurement, used to measure the characteristics of the desorbed species, or aggregates, were also found to be affected similarly. The magnitude of the overall change in the scattered light intensity was slightly dependent on electrolyte concentration. The re-adsorption of the aggregates was influenced by electrolyte concentration. The scattered light signal for an intermediate adsorbed state (adsorbed aggregate) was more prevalent for higher electrolyte concentration, suggesting that these intermediates were somewhat different compared to lower electrolyte concentrations. The lower electrolyte concentration displayed a larger potential region where this intermediate was stable, but the intensity of the scattered light was much lower. The electrolyte concentration most strongly influenced the potentials of adsorption and desorption, as well as the potential region of stability for the adsorbed intermediates. The sweep rate also has an influence on the scattering characteristics of the desorbed species, suggesting a possible method for measuring the kinetics of the adsorption–desorption process or for controlling the character of the desorbed species. These changes were explained in terms of a mechanism for the wetting or de-wetting of a surface. The influence of electrolyte concentration provides another opportunity for investigating the dynamics of this adsorption–desorption process.