Solids at the liquid–liquid interface: Electrocatalysis with pre-formed nanoparticles

Magnetic particles (Fe3O4) magnify ion transfer processes at the electrified liquid-liquid interface. Case study: Levamisole detection

This article describes the effect of non-stabilized magnetic particles Fe3O4 (nanoparticles aggregates) addition to the aqueous phase of the polarized liquid-liquid interface (LLI) on the interfacial ion transfer processes. LLI was formed between 1,2-dichloroethane and water solutions (1,2 DCE)|water. The synthesis of Fe3O4 magnetic particles (MPs) was achieved by the co-precipitation method, after which their appearance, size of aggregates, and zeta potential were assessed. All electrochemical measurements reported in this study were performed using cyclic voltammetry (CV). We evaluated the effect of pH and the presence of different concentrations of magnetic Fe3O4 nanoparticles aggregates always initially added to the aqueous phase on tetramethylammonium cation (TMA+), and 4-octylbenzenesulfonic acid (OBS-) ion transfer. We have found that the addition of Fe3O4 MPs followed by their precipitation and LLI interface modification leads to pH dependent magnification of the recorded ionic currents attributed to the cation and anion transfer from the aqueous to the organic phase and vice versa. As such, we have plotted the calibration curves of TMA+ and OBS- in the concentration range of (10-200 μM) revealing that Fe3O4 MPs have a significant effect on detection sensitivity, which is dependent on the interaction between Fe3O4 MPs and the analyte being studied. Finally, we assessed the electrochemical behavior of levamisole at the 1,2-dichloroethane|water interface in the presence and absence of Fe3O4 MPs.

Electrochemical photonics: a pathway towards electrovariable optical metamaterials

This review article focuses on the latest achievements in the creation of a class of electrotuneable optical metamaterials for switchable mirrors/windows, variable colour mirrors, optical filters, and SERS sensors, based on the voltage-controlled self-assembly of plasmonic nanoparticles at liquid/liquid or solid/liquid electrochemical interfaces. Practically, these experimental systems were navigated by physical theory, the role of which was pivotal in defining the optimal conditions for their operation, but which itself was advanced in feedback with experiments. Progress and problems in the realisation of the demonstrated effects for building the corresponding devices are discussed. To put the main topic of the review in a wider perspective, the article also discusses a few other types of electrovariable metamaterials, as well as some of those that are controlled by chemistry.

Molecular Precursor Routes for Ag-Based Metallic, Intermetallic, and Metal Sulfide Nanoparticles: Their Comparative ORR Activity Trend at Solid|Liquid and Liquid|Liquid Interfaces

The electrochemical conversion of oxygen to water is a crucial process required for renewable energy production, whereas its first two-electron step produces a versatile chemical and oxidant─hydrogen peroxide. Improving performance and widening the limited selection of the potential catalysts for this reaction is a step toward the implementation of clean-energy technologies. As silver is known as one of the most effective catalysts of oxygen reduction reaction (ORR), we have designed a suitable molecular precursor pathway for the selective synthesis of metallic (Ag), intermetallic (Ag₃Sb), and binary or ternary metal sulfide (Ag₂S and AgSbS₂) nanomaterials by judicious control of reaction conditions. The decomposition of xanthate precursors under different reaction conditions in colloidal synthesis indicates that carbon-sulfur bond cleavage yields the respective metal sulfide nanomaterials. This is not the case in the presence of trioctylphosphine when the metal-sulfur bond is broken. The synthesized nanomaterials were applied as catalysts of oxygen reduction at the liquid-liquid and solid-liquid interfaces. Ag exhibits the best performance for electrochemical oxygen reduction, whereas the electrocatalytic performance of Ag and Ag₃Sb is comparable for peroxide reduction in an alkaline medium. Scanning electrochemical microscopy (SECM) analysis indicates that a flexible 2-electron to 4-electron ORR pathway has been achieved by transforming metallic Ag into intermetallic Ag₃Sb.

Potential-Modulated Ion Distributions in the Back-to-Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer

Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2-electron O2 reduction reaction can proceed by single-step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O2. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential differenceΔ o w φ provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region. We show that the onset potential for a biphasic single-step IET reaction does not correlate with the thermodynamically predicted standard Galvani IET potential and is instead closely correlated with the potential of zero charge at a polarised L|L interface. We outline that the appliedΔ o w φ required to modulate the interfacial ion distributions, and thus kinetics of IET, must be optimised to ensure that the aqueous and organic redox species are present in substantial concentrations at the L|L interface simultaneously in order to react.

Template-Free and Spontaneous Formation of Vertically Aligned Pd Nanofiber Arrays at the Liquid-Liquid Interface between Redox-Active Ionic Liquid and Water

Vertically aligned Pd nanofiber arrays (NFAs) have been prepared at the liquid-liquid interface between redox-active ionic liquid (RAIL) and water (W) via a template-free manner. The RAIL with high hydrophobicity, (ferrocenylmethyl)dodecyldimethylammonium bis(nonafluorobutanesulfonyl)amide, plays dual roles of reducing agent for Pd precursor ions and the hydrophobic liquid phase simultaneously, and the RAIL|W interface has been utilized as the formation site for the spontaneous growth of Pd NFAs. The Pd NFAs consist of three parts: layers formed by partly connected particles on the top, NFAs in the middle, and firm sheetlike layers on the bottom. Because of the top and bottom supporting layers, the antideformation ability and durability of the Pd NFAs with a length reaching several micrometers are enhanced. A possible mechanism for the formation of the Pd NFAs has been discussed. The Pd NFAs show a good stability and a higher electrocatalytic activity toward the ethanol oxidation reaction than a commercial Pd/C catalyst. The present study provides a new strategy for the template-free and spontaneous formation of Pd NFAs.

Catalytic applications of β-cyclodextrin/palladium nanoparticle thin film obtained from oil/water interface in the reduction of toxic nitrophenol compounds and the degradation of azo dyes

A supramolecular catalyst of Pd/β-cyclodextrin thin film is synthesized via a facile and one-pot procedure at an oil–water interface and applied for the reduction of toxic nitrophenol compounds and the degradation of harmful azo dyes.

Closed bipolar electrochemistry in a four-electrode configuration

The thermodynamic theory underpinning closed bipolar electrochemistry in a 4-electrode configuration is presented; a technique applicable to spectro-electroanalysis, energy storage, electrocatalysis and electrodeposition.

Mediated water electrolysis in biphasic systems

The concept of efficient electrolysis by linking photoelectrochemical biphasic H₂ evolution and water oxidation processes in the cathodic and anodic compartments of an H-cell, respectively, is introduced. Overpotentials at the cathode and anode are minimised by incorporating light-driven elements into both biphasic reactions. The concepts viability is demonstrated by electrochemical H₂ production from water splitting utilising a polarised water-organic interface in the cathodic compartment of a prototype H-cell. At the cathode the reduction of decamethylferrocenium cations ([Cp₂*Fe^(III)]⁺) to neutral decamethylferrocene (Cp₂*Fe^(II)) in 1,2-dichloroethane (DCE) solvent takes place at the solid electrode/oil interface. This electron transfer process induces the ion transfer of a proton across the immiscible water/oil interface to maintain electroneutrality in the oil phase. The oil-solubilised proton immediately reacts with Cp₂*Fe^(II) to form the corresponding hydride species, [Cp₂*Fe^(IV)(H)]⁺. Subsequently, [Cp₂*Fe^(IV)(H)]⁺ spontaneously undergoes a chemical reaction in the oil phase to evolve hydrogen gas (H₂) and regenerate [Cp₂*Fe^(III)]⁺, whereupon this catalytic Electrochemical, Chemical, Chemical (ECC') cycle is repeated. During biphasic electrolysis, the stability and recyclability of the [Cp₂*Fe^(III)]⁺/Cp₂*Fe^(II) redox couple were confirmed by chronoamperometric measurements and, furthermore, the steady-state concentration of [Cp₂*Fe^(III)]⁺ monitored in situ by UV/vis spectroscopy. Post-biphasic electrolysis, the presence of H₂ in the headspace of the cathodic compartment was established by sampling with gas chromatography. The rate of the biphasic hydrogen evolution reaction (HER) was enhanced by redox electrocatalysis in the presence of floating catalytic molybdenum carbide (Mo₂C) microparticles at the immiscible water/oil interface. The use of a superhydrophobic organic electrolyte salt was critical to ensure proton transfer from water to oil, and not anion transfer from oil to water, in order to maintain electroneutrality after electron transfer. The design, testing and successful optimisation of the operation of the biphasic electrolysis cell under dark conditions with Cp₂*Fe^(II) lays the foundation for the achievement of photo-induced biphasic water electrolysis at low overpotentials using another metallocene, decamethylrutheneocene (Cp₂*Ru^(II)). Critically, Cp₂*Ru^(II) may be recycled at a potential more positive than that of proton reduction in DCE.

Redox Electrocatalysis of Floating Nanoparticles: Determining Electrocatalytic Properties without the Influence of Solid Supports

Redox electrocatalysis (catalysis of electron-transfer reactions by floating conductive particles) is discussed from the point-of-view of Fermi level equilibration, and an overall theoretical framework is given. Examples of redox electrocatalysis in solution, in bipolar configuration, and at liquid-liquid interfaces are provided, highlighting that bipolar and liquid-liquid interfacial systems allow the study of the electrocatalytic properties of particles without effects from the support, but only liquid-liquid interfaces allow measurement of the electrocatalytic current directly. Additionally, photoinduced redox electrocatalysis will be of interest, for example, to achieve water splitting.

Bipolar Spectroelectrochemistry

Bipolar electrochemistry is receiving growing attention in the last years, not only because it is an important tool for studying electron transfer processes, but also because it is really fruitful in the development of new analytical sensors. Bipolar electrodes show promising applications as a direct analytical tool since oxidation and reduction reactions take place simultaneously on different parts of a single conductor. There are several electrochemical devices that provide information about electron transfer between two immiscible electrolyte solutions, but to the best of our knowledge, this is the first time that a bipolar device is able to record two spectroelectrochemical responses concomitantly at two different compartments. It allows deconvolving the electrochemical signal into two different optical signals related to the electron transfer processes occurring at two compartments that are electrically in contact. The combination of an electrochemical and two spectroscopic responses is indeed very useful, providing essential advantages in the study of a huge variety of systems. The study of three different electrochemical systems, such as reversible redox couples, carbon nanotubes, and conducting polymers has allowed us to validate the new cell and to demonstrate the capabilities of this technique to obtain valuable time-resolved information related to the electron transfer processes.

Electrochemical Investigation of Adsorption of Single-Wall Carbon Nanotubes at a Liquid/Liquid Interface

There is much interest in understanding the interfacial properties of carbon nanotubes, particularly at water/oil interfaces. Here, the adsorption of single-wall carbon nanotubes (SWCNTs) at the water/1,2-dichloroethane (DCE) interface, and the subsequent investigation of the influence of the adsorbed nanotube layer on interfacial ion transfer, is studied by using the voltammetric transfer of tetramethylammonium (TMA+) and hexafluorophosphate (PF6-) as probe ions. The presence of the interfacial SWCNT layer significantly suppresses the transfer of both ions across the interface, with a greater degree of selectivity towards the PF6- ion. This effect was attributed both to the partial blocking of the interface by the SWCNTs and to the potential dependant adsorption of background electrolyte ions on the surface of the SWCNTs, as confirmed by X-ray photoelectron spectroscopy, which is caused by an electrostatic interaction between the interfacial SWCNTs and the transferring ion.

A facile electrochemical route to the preparation of uniform and monoatomic copper shells for gold nanoparticles

Monoatomic copper shells are electrodeposited onto gold nanoparticles without the presence of a solid substrate.