Detection of the fractal character of compact adsorbed layers by the dropping mercury electrode

The development of compact adsorbed layers at the dropping mercury electrode is used for the detection of resulting changes in the geometry of the electrode-solution interface from Euclidean to fractal. A simple size scaling method of the Hg drop surface is applied, which focus on the time dependence of the drop area under conditions of constant adsorbate and base electrolyte concentration, constant potential and constant mercury flow rate.It is found that the slope of the log-log plots of capacitance or capacitance current against electrode surface area or electrode radius is increasing upon the formation of a compact adsorbed layer. Under equilibrium conditions the slope values of the linear log-log plots can be reasonably identified with the fractal dimension of the electrode/solution interface. However, due to the limited extent of the power-law range of the feature vs. scale relation, the size scaling method of the dropping mercury electrode can be used to probe fractal adsorbate films on a strictly qualitative level.     

A simple equation for ohmic solution resistance in cells with mercury drop electrodes

A simple analytical expression for the ohmic solution resistance in cells with mercury drop electrodes is presented. This expression is derived using a geometric model in which the bottom of the glass capillary is treated as an infinite insulating plane and equipotentials in solution are considered to be spheres centered about the drop electrode and truncated by the insulating plane. A simple modification of the expression makes it possible to treat the hanging merecury drop electrode as well as the dropping mercury electrode. It is shown that the resistance predicted by the equation derived in the present work closely approximates that obtained from a rigorous solution of the primary potential and current distribution.     

Influence of electrode geometry on the redox switching characteristics of conducting polymers

The dimensions of working electrodes relative to those of conducting polymer films have been found to influence the redox switching characteristics of polymer films. Using microband electrode arrays, the electrodes were operated either altogether as a single electrode or individually as the working electrode, leading to a large difference in the contact area between the electrode and the overlaying polymer film. When the microband electrodes were operated altogether, the voltammetric peak potential was significantly lower and the rate of oxidation was much faster than when the electrodes were operated individually. Each electrode within the array can also be operated individually (i.e. with equivalent contact area) to address the polymer from different positions across the film. An individual electrode located towards the edge of the film gave rise to higher oxidation peak potentials and slower rates of oxidation when compared to an electrode situated towards the centre of the film. Potentiometric measurements of electrodes situated adjacent to the working electrode showed that the rate of the oxidation was controlled by the propagation of the interface between the conducting and insulating phases [1].     

Voltammetric and Scanning Electrochemical Microscopic Studies of the Adsorption Kinetics and Self-Assembly of n-Alkanethiol Monolayers on Gold

The adsorption kinetics and self-assembly of hexadecyl mercaptan on gold have been investigated by scanning electrochemical microscopy (SECM), chronoamperometry, and cyclic voltammetry. The developed methodology allows one to evaluate the surface coverage and the average size of the defects in the monolayer film from the effective rate constant of electron transfer. Two kinetic regimes of self-assembly were identified: a rapid initial adsorption of hexadecyl mercaptan onto a clean gold surface from 5 mM solution (more than 90% coverage obtained in 1 to 5 min), and a slower subsequent annealing of a thiol monolayer resulting in a more compact film. Typically, a long-chain-length thiol-treated gold surface acts as an electronically insulating surface after about 1 h. The SECM images of partially covered gold surfaces were always featureless, suggesting that the defects in the film were smaller than 0.5 μm for any exposure time ≥ 1 min.     

An overview of techniques for characterizing inhomogeneities in organic surface films and underfilm localized corrosion

This paper presents an overview of techniques for characterizing inhomogeneities in various forms of organic surface films including organic coatings, corrosion inhibitor and anti-corrosion oil films, and for understanding the effects of surface film inhomogeneities on underfilm localized corrosion. Particular focus is on technological innovations that have been made over the past two decades, including scanning probe techniques and the wire beam electrode method.     

Corrosion protection of carbon steel by thin films of poly(3-alkyl thiophenes) in 0.5 M H2SO4

Poly(3-octyl thiophene) (P3OT) and poly(3-hexylthiophene) (P3HT) dissolved in toluene were deposited onto 1018 carbon steel and corroded in 0.5 M H₂SO₄. P3OT and P3HT films were chemically deposited by drop casting onto 1018-type carbon steel with two surface finishing, i.e. abraded with 600-emery paper and with alumina (Al₂O₃) particles of 1.0 μm in diameter (mirror finish). Their corrosion resistance was estimated by using potentiodynamic polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy, EIS, techniques. In all cases, polymeric films protected the substrate against corrosion, but the protection was improved if the surface was polished with Al₂O₃ particles of 1.0 μm in diameter. The polymer which gave the best protection was P3HT because the amount of defects was much lower than that for the P3OT films. The polymers did not act only as a barrier layer against aggressive environment, but they improved the passive film properties by decreasing the critical current necessary to passivate the substrate, increasing the pitting potential and broadening the passive interval.     

Characterization of the barrier layer of nanoporous alumina films prepared using two different contact configurations

We investigated the structure of the barrier layers of nanoporous alumina formed by anodization of aluminum films laid over platinum electrodes by cyclic voltammetry technique and scanning electron microscopy. Two methods of anodization were employed in this study—the conventional sub-surface anodization method and a novel pipette anodization method. Measurements of the electron transfer rate constant values at these alumina-modified electrodes demonstrated very different alumina microstructures at the barrier layer region for these two types of alumina-modified electrodes. The sub-surface anodized electrode consists of a barrier layer, which forms a kinetic barrier for the electrochemical reaction of a redox probe at the underlying platinum electrode. In contrast, the quasi-reversible electrochemical behaviour of a redox probe at the pipette-anodized electrodes closely resembles the reaction observed at a bare platinum electrode. Scanning electron microscopy revealed pore structures along the underside surface of the alumina overlayer of a pipette-anodized electrode. These results indicate alumina modified eletrodes fabricated using the pipette anodization method comprise channels which penetrate the barrier layer, giving rise to ‘barrier layer-free’ alumina films.     

High surface area diamond-like carbon electrodes grown on vertically aligned carbon nanotubes

Electrochemically active diamond-like carbon (DLC) electrodes featuring high specific surface area have been prepared by plasma-enhanced chemical vapour deposition (CVD) onto densely packed forests of vertically aligned multiwall carbon nanotubes (VACNTs). The DLC:VACNT composite film exhibits a complex topography with web like features and ridges generated by partial coalescence of the DLC over the CNT arrays. DLC:VACNT electrodes exhibit low background responses over a large potential window, low uncompensated resistance, as well as low charge-transfer impedance in the presence of ferrocyanide as a redox probe. The interfacial capacitance associated with the DLC:VACNT electrode is in the range of 0.6 mF cm⁻², a value two orders of magnitude larger than in conventional flat carbon electrodes. DLC films grown onto single-crystal Si(1 0 0) under identical conditions resulted in essentially insulating layers. Conducting-atomic force microscopy studies reveal that the film electro-activity does not arise from specific topographic features in the highly corrugated film. The ensemble of experimental results suggests that the enhanced electrochemical responses are not connected to areas in which the CNT support is exposed to the electrolyte solution. This is remarkable behaviour considering that no dopants have been included during the DLC film growth.     

Surface properties of self-assembled monolayer films of tetra-substituted cobalt, iron and manganese alkylthio phthalocyanine complexes

Self-assembled monolayer (SAM) films of iron (SAM-1), cobalt (SAM-2) and manganese (SAM-3) phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the non-peripheral positions, were formed on gold electrode in dimethylformamide (DMF). Electrochemical, impedimentary and surface properties of the SAM films were investigated. Cyclic voltammetry was used to investigate the electrochemical properties of the films. Ability of the films to inhibit common faradaic processes on bare gold surface (gold oxidation, solution redox chemistry of [Fe(H₂O)₆]³⁺/[Fe(H₂O)₆]²⁺ and underpotential deposition (UDP) of copper) was investigated. Electrochemical impedance spectroscopy (EIS), using [Fe(CN)₆]^3−/4− redox process as a probe, offered insights into the electrical properties of the films/electrode interfaces. Surface properties of the films were probed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films were employed for the electrocatalytic oxidation of the pesticide, carbofuran. Electrocatalysis was evidenced from enhanced current signal and less positive oxidation potential of the pesticide on each film, relative to that observed on the bare gold electrode. Mechanism of electrocatalytic oxidation of the pesticide was studied using rotating disc electrode voltammetry.     

Analysis of evaporating thick liquid films on solids

The diffusion-controlled evaporation of small circular volatile liquid films from solid surfaces was monitored by employing video microscopy from a plan view and then applying digital image analysis techniques. The decrease of the liquid-solid contact area of these films during the last stages of the evaporation was found to be linear with time. This paper presents experimental results of four organic liquid films (n-nonane, n-octane, toluene, n-butanol) on three substrates poly(methyl methacrylate), poly(ethylene terephthalate) (Mylar), and glass. The linear decreases of the surface areas of hanging drops from a polypropylene fiber for the same liquids were also monitored using both plan and side view video cameras for comparison. Analyses of optically recorded liquid film and drop shapes were carried out and a diffusion model depending on the presence or absence of the substrate was developed. By combining the experimental area decrease of a spherical drop due to the diffusion-controlled evaporation with that of a small spherical cap shaped liquid film resting on a solid surface, it is possible to calculate the small contact angles (less than 10°) of the wetting thick liquid films on solids. The relationship between film evaporation rate and the solid-liquid interfacial interactions is also discussed.     

Light-controlled ion transport through spiropyran-modified nanoporous silica colloidal films

The transport of positively charged redox active species through spiropyran-modified nanopores in silica colloidal films was controlled using light. The silica colloidal films were comprised of 18 layers of face centered cubic (fcc)-packed 170 nm silica spheres. The surface of the films was first modified with amines, which were then used to attach the spiropyran moiety to the surface. The limiting current of a positively charged redox active species through the spiropyran-modified nanopore decreased after irradiation with UV light at pH 6.6. When the silica colloidal film was subsequently irradiated with visible light the initial limiting current was restored.     

十八硫醇自组装膜对银的缓蚀作用

在银电极表面制备了十八硫醇自组装膜,采用极化曲线、交流阻抗等电化学方法研究了不同浓度的十八硫醇对银的缓蚀作用的影响。结果表明:在温度50℃,自组装时间4h,十八硫醇0.05mol/L的条件下,缓蚀效率达到97.1%,覆盖度达到97.8%,十八硫醇在银电极表面形成了致密的自组装膜,有效地抑制了银的腐蚀。     

Comparative study of the electrochemical response of poly (alkyl thiophene) derivatives deposited on platinum and titanium electrodes

Summary A comparative investigation of the electrochemical response of poly(alkyl thiophene) derivatives (3-methylthiophene and 3-octylthiophene) on two different electrode materials (platinum and titanium) was performed using cyclic voltammetry. By analyzing the voltammetric data, a slight potential shift of the oxido-reduction peaks in polyoctylthiophene film deposited on titanium electrode compared to platinum has been observed. The calculated surface concentrations of the electroactive centers were found to decrease significantly in poly(octylthiophene) grafted onto titanium surface, while in poly(methylthiophene) these changes were negligible. The influence of the nature of the electrode material and the alkyl substituent length on the redox properties has been discussed. The reduced forms of the synthesized polymer films were characterized by FTIR spectroscopy. The FTIR spectra indicate decrease of the conjugation length, the degree of polymerization and the presence of defects (2,4 linkages and C=O groups) with the increase of the alkyl group length. Received: 16 July 2002/Revised version: 4 February 2003/ Accepted: 10 March 2003 Correspondence to Ljubomir Arsov     

Influence of mercury on hydrogen overvoltage on solid metal electrodes—III. Adsorption and evolution of hydrogen on poisoned platinum

The adsorption capacity for hydrogen as well as the exchange cd of hydrogen evolution and the double-layer capacitance of a platinum electrode depend on the degree of poisoning by mercury. Mercury electrodeposited with simultaneous hydrogen evolution on smooth platinum forms droplets, the curvature radius of which is of the order 10^?6 cm, and for complete inhibition of hydrogen adsorption, an amount equivalent to several monolayers of mercury is necessary. By submerging a pure platinum electrode in a dilute solution of mercuric compounds without external polarization, a monolayer is formed and one atom of mercury replaces just one atom of hydrogen. On platinum-black electrodes, electrodeposited drops of mercury cover the orifice of the pores and one mercury atom shields on an average 2–3 active sites of the electrode surface.     

Studies on slag deposit formation in pulverized coal combustors. 1. Results on the wetting and adherence of synthetic coal ash drops on steel

As part of a study of slag deposit formation in pulverized-coal-fired boilers, apparent contact angles and adhesion strengths of molten mineral drops contacted with cooler oxidized steel substrates have been investigated. High-speed photography indicated that freezing of the interface between the molten drop and metal surface occurred in milliseconds. Adherence occurred between an oxide film on the metal and the drop, adherence was weak on stainless steels, and particles of oxide film were broken away from the substrate when the drops were sheared off. Higher substrate temperatures gave increased adhesion, with a larger area of strong interaction between the oxide and the drop interface and less area of weak interaction. Addition of compounds to lower the liquidus temperature of the drop gave increased adherence. Pyrite drops were converted to mainly pyrrhotite on melting and showed a high degree of wetting and adherence to the oxidized steel even at low temperatures.     

Stochastic phenomena during phase transition of lecithine films at the mercury solution interface

1,2-Dipalmitoyllecithine forms adsorbed films at electrodes in two distinct potential regions. At temperatures below the phase transition temperature the film at negative potentials is highly ordered and blocks the surface for electrode reactions. Potential steps between both regions enable us to monitor the random nucleation during the film transition by its influence upon the charge transfer process. The time distribution of current pulses obeys the Poisson distribution. The current power spectrum is given.     

Poly(3-Octylthiophene) and Poly(3-Octylthiophene)/TiO2-Coated on Al1050: Electrosynthesis,Characterization and Its Corrosion Protection Ability in NaCl Solution

In this study, 3-octylthiophene (3OT) and its nanocomposites with TiO₂(3OT/TiO₂) were electrochemically synthesized in different initial monomer concentrations (50, 75, 100, and 150 mM) in 0.1 M tetraethylammonium tetrafluoroborate (TEABF₄)/acetonitrile (CH₃CN) on a glassy carbon electrode (GCE). The best modified electrodes, which were taken from the optimum conditions of redox behaviors, were also electropolymerized in 0.5 M oxalic acid/acetonitrile (CH₃CN) solution on Al1050 electrode. The modified Al1050 electrodes were characterized by optical microscope, FTIR-ATR, SEM-EDX, and EIS. The optimum conditions of the electrocoated P(3OT)/Al1050 and P(3OT)/TiO₂ nanocomposites on Al1050 electrode were investigated for corrosion performances against 3.5% NaCl solution. The corrosion tests were performed by EIS and Tafel extrapolation plots together with the equivalent circuit model of R_s(Q_c(R_c(Q_pR_ct))). P(3OT)/TiO₂ nanocomposite films showed higher protection efficiency (PE = 89%) than P(3OT) films (PE = 81%). The P(3OT)/TiO₂ nanocomposite films coated on Al1050 electrodes may be used in industrial applications of corrosion protection against salt water.     

Characterisation of hydrophobic carbon nanofiber-silica composite film electrodes for redox liquid immobilisation

Carbon (50-150 nm diameter) nanofibers were embedded into easy to prepare thin films of a hydrophobic sol-gel material and cast onto tin-doped indium oxide substrate electrodes. They promote electron transport and allow efficient electrochemical reactions at solid|liquid and at liquid|liquid interfaces. In order to prevent aggregation of carbon nanofibers silica nanoparticles of 7 nm diameter were added into the sol-gel mixture as a “surfactant” and homogeneous high surface area films were obtained. Scanning electron microscopy reveals the presence of carbon nanofibers at the electrode surface. The results of voltammetric experiments performed in redox probe—ferrocenedimethanol solution in aqueous electrolyte solution indicate that in the absence of organic phase, incomplete wetting within the hydrophobic film of carbon nanofibers can cause hemispherical diffusion regime typical for ultramicroelectrode like behaviour.The hydrophobic film electrode was modified with two types of redox liquids: pure tert-butylferrocene or dissolved in 2-nitrophenyloctylether as a water-insoluble solvent and immersed in aqueous electrolyte solution. With a nanomole deposit of pure redox liquid, stable voltammetric responses are obtained. The presence of carbon nanofibers embedded in the mesoporous matrix substantially increases the efficiency of the electrode process and stability under voltammetric conditions. Also well-defined response for diluted redox liquids is obtained. From measurements in a range of different aqueous electrolyte media a gradual transition from anion transfer dominated to cation transfer dominated processes is inferred depending on the hydrophilicity of the transferring anion or cation.     

对苯二酚在DMPC仿生膜上的电化学行为

在金电极上构筑一种双肉豆蔻磷脂酰胆碱(DMPC)仿生浇铸膜。经椭圆偏振仪和电化学方法测量证实,DMPC仿生膜是较为致密的多层膜,同时对苯二酚能够透过DMPC仿生膜到达金电极表面进行氧化反应。结果表明,对苯二酚不仅可在脂质膜中进行电子转移,而且是一种重要的生物分子。反应在仿生环境内进行,也为研究生物小分子在真正的生物体内的反应提供有益的帮助。本文探讨对苯二酚在仿生膜修饰电极上的电化学行为,为生物膜中的电子转移过程提供了十分重要的信息。     

Transition behavior of PS-b-PMMA films on the balanced interfacial interactions

The thickness dependence of the order-to-disorder transition (ODT), measured by in situ grazing-incidence small-angle X-ray scattering (GISAXS), has been investigated in thin films of a symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) on a random copolymer (P(S-r-MMA)) grafted to the substrate where the interfacial interactions are balanced. With decreasing film thickness less than 25L₀, the ODT significantly decreases to 193 °C for film of 10L₀ in thickness, because the interfacial interactions by a random copolymer grafted to the substrate provide a surface-induced compatibilization toward two block components. However, a plateau of the ODT at ∼213 °C for films thicker than 25L₀ was observed above the bulk value of 200 °C. The elevation of this ODT indicates a suppression of compositional fluctuations normal to the film surface, more than likely because the dominant orientation of the lamellar microdomains was found to be parallel to the film surface.