Sara E. C. Dale

Generator-collector double electrode systems: A review

A variety of generator-collector systems are reviewed, from the original rotating ring-disc electrodes developed in the 1950s, to very recent developments using new geometries and microelectrodes. An overview of both theoretical and experimental aspects are given, and the power of these double electrode systems in analytical electrochemistry is illustrated with a range of applications.

Long-range intramolecular electronic communication in bis(ferrocenylethynyl) complexes incorporating conjugated heterocyclic spacers:Synthesis, crystallography, and electrochemistry

A new series of bis(ferrocenylethynyl) complexes, 3-7, and a mono(ferrocenylethynyl) complex, 8, have been synthesized incorporating conjugated heterocyclic spacer groups, with the ethynyl group facilitating an effective long-range intramolecular interaction. The complexes were characterized by NMR, IR, and UV-vis spectroscopy as well as X-ray crystallography. The redox properties of these complexes were investigated using cyclic voltammetry and spectroelectrochemistry. Although there is a large separation of ∼14 Å between the two redox centers, ΔE(1/2) values in this series of complexes ranged from 50 to 110 mV. The appearance of intervalance charge-transfer bands in the UV-vis-near-IR region for the monocationic complexes further confirmed effective intramolecular electronic communication. Computational studies are presented that show the degree of delocalization across the Fc-C≡C-C≡C-Fc (Fc = C5H5FeC5H4) highest occupied molecular orbital.

Dual band electrodes in generator–collector mode: Simultaneous measurement of two species

Abstract A computational model for the simulation of a double band collector–generator experiment is applied to the situation where two electrochemical reactions occur concurrently. It is shown that chronoamperometric measurements can be used to take advantage of differences in diffusion coefficients to measure the concentrations of both electroactive species simultaneously, by measuring the time at which the collection efficiency reaches a specific value. The separation of the electrodes is shown to not affect the sensitivity of the method (in terms of percentage changes in the measured time to reach the specified collection efficiency), but wider gaps can provide a greater range of (larger) absolute values of this characteristic time. It is also shown that measuring the time taken to reach smaller collection efficiencies can allow for the detection of smaller amounts of whichever species diffuses faster. The case of a system containing both ascorbic acid and dopamine in water is used to exemplify the method, and it is shown that mole fractions of ascorbic acid between 0.055 and 0.96 can, in principle, be accurately measured.

A gold-gold oil microtrench electrode for liquid-liquid anion transfer voltammetry

Two flat gold electrodes are placed vis‐à‐vis with an epoxy spacer layer that is etched out to give a ca. 100 μm‐deep electrochemically active trench. A water‐insoluble oil phase, here the redox system N,N‐diethyl‐N′N′‐didodecyl‐phenylenediamine (DDPD) in 4‐(3‐phenylpropyl)‐pyridine (PPP), is immobilized into the trench to allow anion transfer upon oxidation of DDPD (oil) to DDPD+ (oil). In “mono‐potentiostatic mode” quantitative transfer/expulsion of anions into the trench oil phase occurs. However, in “bi‐potentiostatic mode” feedback currents dominated by rapid plate‐to‐plate diffusion normal to the electrode surfaces are observed. Comparison of “normal” diffusion and “lateral” diffusion shows that the rate of diffusion–migration charge transport across the oil film is anion hydrophobicity dependent.

Shape-controlled electrodeposition of tin crystals from Sn(II)-fluoroborate solutions

The shape evolution of mesoscopic tin crystals electrodeposited on boron-doped diamond (BDD) substrates is described as a function of the ion concentration and deposition potential. Concentrations of 5–100 mM Sn(II) in 1 M fluoroboric acid have been explored and electron micrographs of the deposited crystals used to characterise the different shapes and sizes of the crystals obtained. Low concentrations and low overpotentials result in faceted cuboid-shaped crystals while higher concentrations and deposition potentials yield highly complex and fractal-like structures with high surface energies. These crystals can be used as model systems for studying mesoscopic superconductivity, or as a template for core–shell structures if plated with a second metal.

Electronic bandstructure and van der Waals coupling of ReSe2 revealed by high-resolution angle-resolved photoemission spectroscopy

ReSe2 and ReS2 are unusual compounds amongst the layered transition metal dichalcogenides as a result of their low symmetry, with a characteristic in-plane anisotropy due to in-plane rhenium ‘chains’. They preserve inversion symmetry independent of the number of layers and, in contrast to more well-known transition metal dichalcogenides, bulk and few-monolayer Re-TMD compounds have been proposed to behave as electronically and vibrational decoupled layers. Here, we probe for the first time the electronic band structure of bulk ReSe2 by direct nanoscale angle-resolved photoemission spectroscopy. We find a highly anisotropic in- and out-of-plane electronic structure, with the valence band maxima located away from any particular high-symmetry direction. The effective mass doubles its value perpendicular to the Re chains and the interlayer van der Waals coupling generates significant electronic dispersion normal to the layers. Our density functional theory calculations, including spin-orbit effects, are in excellent agreement with these experimental findings.

Real-time in situ atomic force microscopy imaging of bismuth crystal growth

We have performed real-time atomic force microscopy (AFM) imaging of bismuth crystals that were grown under electrochemical control at low overpotentials to ensure a slow growth rate and allow in situ observation of the growth. A two step chronoamperometric potential was applied to a boron-doped diamond (BDD) working electrode with a short high overpotential, -0.4 V (2 s), to nucleate the bismuth, and then a long low overpotential for slow growth, -0.32 V (4.4 h). Growth rates of individual crystals and detailed growth mechanisms could be followed in real time because of the slow crystal growth. The close proximity of the AFM tip and tip holder to the working electrode appears to hinder the diffusion of bismuth to the BDD surface, as evidenced by the significantly lower density of crystals under the cantilever as compared to the rest of the electrode, therefore slowing down the growth process.

Enhanced TiO2 surface electrochemistry with carbonised layer-by-layer cellulose-PDDA composite films

In this report we demonstrate a versatile (and potentially low-cost) cellulose nano-whisker-based surface carbonisation method that allows well-defined films of TiO(2) nanoparticles surface-modified with carbon to be obtained. In a layer-by-layer electrostatic deposition process based on TiO(2) nanoparticles, cellulose nano-whiskers, and poly(diallyl-dimethylammonium) or PDDA are employed to control the ratio of surface carbon to TiO(2). Characterisation based on optical, AFM, XRD, and XPS methods is reported. Electrochemical measurements suggest improved access to surface states, dopamine binding at the anatase surface, and surface redox cycling aided by the thin amorphous carbon film in mesoporous TiO(2). In future, the amorphous carbon layer method could be applied for surface processes for a wider range of semiconductor or insulator surfaces.

Chapter 4:Electrochemistry within nanogaps

Electrochemical processes in micro-gap and nano-gap electrochemical systems are of increasing interest and may be regarded as the “missing link” between tunnel processes ( 1μm gap). This is the length scale of charge hopping processes (with typically Dapp=10−15 m2s−1) and therefore for processes related to charge storage, photoelectrochemistry, bioelectrochemistry, and sensing. A range of important electrochemical phenomena and processes are encountered in nano-gap electrode systems including the direct observation of slow charge hopping at surfaces, coupled surface-chemical processes in mesoporous oxides, fast feedback current measurements for low concentration analysis down to single molecules, chemical signal filtering for sensor, and stochastic effects due to single molecule/low concentration fluctuations. There are interesting future applications in particular in progressing fundamental science and in enabling new types of micro-fabricated electroanalytical sensing devices.

Interdigitated ring electrodes: Theory and experiment

Abstract The oxidation of potassium ferrocyanide, K4Fe (CN)6, in aqueous solution under fully supported conditions is carried out at interdigitated band and ring electrode arrays, and compared to theoretical models developed to simulate the processes. Simulated data is found to fit well with experimental results using literature values of diffusion coefficients for Fe ( CN ) 6 4 - and Fe ( CN ) 6 3 - . The theoretical models are used to compare responses from interdigitated band and ring arrays, and the size of ring array required to approximate the response to a linear band array is investigated. An equation is developed for the radius of ring required for a pair of electrodes in a ring array to give a result with 5% of a pair of electrodes in a band array. This equation is found to be independent of the scan rate used over six orders of magnitude.