Generation of platinum microstructures on non-conducting surfaces by means of the scanning electrochemical microscope (SECM)

The scanning electrochemical microscope (SECM) was used to form platinum microstructures. For this purpose, a thin layer of platinum dichloride was deposited as precursor on different substrates by evaporating it in high vacuum. For the reduction of this precursor the SECM provided methyl viologen radical cations locally as reducing agent. The mechanism of the reduction is discussed.     

Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy

In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM) utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of microscopically small precursor sites for localised corrosion.     

Scanning microelectrochemical characterization of the anti-corrosion performance of inhibitor films formed by 2-mercaptobenzimidazole on copper

The aim of this work is to explore the applicability of the scanning electrochemical microscope (SECM) to characterize the inhibiting effect of 2-mercaptobenzimidazole against the corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times. The kinetic changes in the corrosion processes were monitored over time from the Z-approach curves. Furthermore, inhibitor-modified copper samples presenting various surface finishes were imaged by SECM and the scanning vibrating electrode technique (SVET), allowing changes both in the surface activity of metal-inhibitor films and in the extent of corrosion attack to be spatially resolved. Differences in the local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample were successfully monitored.     

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.     

Shielding effect of double microelectrode tips in a scanning electrochemical microscope

Electrochemical interaction of coaxial double microelectrodes, in which a ring microelectrode was surrounded by another ring microelectrode, was investigated. Mass-transfer reactions that occurred on both inner and outer microelectrodes interfered with each other and showed a “shielding” effect depending on potentials and geometries of microelectrodes. Application of the inner microelectrode of the double microelectrodes for a probing tip of a scanning electrochemical microscope (SECM) revealed that the shielding effect by the outer microelectrode affected the electrochemistry on the inner microelectrode in the vicinity of the substrate surface. The effect was intensified above the insulator but attenuated above the conductor as the microelectrodes approached in feedback mode of the SECM. Approach to a critical interelectrode distance also intensified the shielding effect in the substrate generation/tip collection mode. An SECM line-scan using a platinum/epoxy resin-model substrate was carried out to investigate the shielding effect on current sensitivity and lateral resolution of an SECM image.     

Scanning electrochemical microscopy for the investigation of corrosion processes: Measurement of Zn2+ spatial distribution with ion selective microelectrodes

Ion-selective microelectrodes can be employed as tips in scanning electrochemical microscopy (SECM) for chemical imaging of corrosion processes. They present higher chemical selectivity than conventional amperometric microdisks, and may be the only effective option to visualize the dissolution of metals with negative redox potentials in aqueous environments when the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions. A robust micro-sized ion selective electrode has been developed which allows the spatial distribution of Zn²⁺ during galvanic corrosion of a model Fe/Zn couple to be investigated using SECM. Owing to the low internal contact potential achieved with the novel design, the resistance of the micropipette electrodes is only fractions of the resistance of conventional micropipette electrodes of the same size. As a result, no special shielding of the microelectrodes is required and higher scanning rate can be used for scanning in the potentiometric modes using these micropipette tips. Concentration profiles over corroding surfaces measured with this technique will be presented.     

Patterns of functional proteins formed by local electrochemical desorption of self-assembled monolayers

Patterned self-assembled monolayers (SAMs) were formed using the scanning electrochemical microscope (SECM). The procedures is based on the local electrochemical desorption of an alkanethiolate monolayer by applying a 5 kHz square-wave voltage of 2 V (peak-to-peak) to a two-electrode configuration consisting of an ultramicroelectrode (UME) of 10 μm diameter placed about 5 μm above a macroscopic SAM-covered gold electrode. Desorption occurs on well-defined regions with a diameter of (12.8±2.8) μm. These regions of bare gold are able to chemisorb a ω-functionalized thiol or disulfide such as cystamine to form patterns of amino-terminated surfaces. Functional proteins can be coupled to the amino groups present at the modified regions of the monolayer. This approach was demonstrated by imaging the activity of horseradish peroxidase bound to the patterned SAMs in the generation–collection mode of the SECM. A considerable improvement of the procedure could be achieved by performing the desorption in a solution containing a millimolar concentration of the ω-functionalized thiol/disulfide ensuring effective refilling of the monolayer by the desired molecules and hence high concentration of the immobilized proteins. The method is discussed with respect to prospective application in the field of chip-based bioanalytical assays.     

Simulation of pH-controlled dissolution of aluminium based on a modified Scanning Electrochemical Microscope experiment to mimic localized trenching on aluminium alloys

Some constituent intermetallic (IMPs) particles at the surface of aluminium alloys are considered as preferential sites for the initiation of structural corrosion resulting in localized trenching around the particles and the surrounding Al matrix. In this work, a modified scanning electrochemical microscope (SECM) experiment was used to induce such phenomena via a local alcalinisation on 200 nm thick aluminium coatings promoting their local dissolution in an aerated 0.1 M NaCl electrolyte. The local alcalinisation was induced by the oxygen reduction reaction on the tip of a SECM which mimics the surface of an isolated IMP. From a phenomenological point of view, reproducible cylindrical damage develops in front of the platinum ultramicroelectrode (UME). Using a 2D finite element modelling to simulate the SECM experiments, the role of the local alcalinisation was validated and the calculated Al dissolution rate was found in agreement with the experimental evaluation.     

Application of the boundary element method numerical simulations for characterization of heptode ultramicroelectrodes in SECM experiments

Quantitative analysis of the scanning electrochemical microscope (SECM) experiments with heptode ultramicroelectrodes (UME) as a probe is performed by means of the boundary element method (BEM). The method is used to calculate the amperrometric steady-state response of the heptode UME in multiple electrode working mode, including the disk-ring competition mode, within the SECM feedback mode. SECM approach curves and line scans with the heptode UME are measured and analyzed by means of numerical simulations. Simulations are performed in the 3D space, thus enabling treatment of non-symmetrical systems, which the heptodes themselves and the line scan experiments are. The approach curves and the line scans are simulated using the real heptode geometry obtained from CCD camera images. The experimental parameters are determined and adjusted as a result of simulations. In particular, the focus was made on the analysis of such system imperfections as electrode/sample tilts, electrode scan heights and heterogeneous reaction rates at the UME. It is shown how the simulations featuring the real system geometry can be used further for the analysis of the SECM line scan and imaging experiments.     

Studies on the hydration of tricalcium silicate pastes I. Scanning electron microscopic examination of microstructural features

A comprehensive examination of the early hydration and microstructural development in tricalcium silicate pastes has been undertaken using scanning electron microscopy. Pastes have been studied for up to 28 days using both fracture surfaces and randomly sawn surfaces that have been polished and etched. Two distinct morphologies of calcium silicate hydrates have been observed: an acicular hydration product and an inner product which is not resolved by the scanning electron microscope. The relationships in the microstructure between the various hydration products and their influence on strength development is discussed.     

Simulation in electrochemistry using the finite element method part 2: scanning electrochemical microscopy

The previously introduced adaptive finite element (AFE) algorithm for use in electrochemistry is applied to the simulation of selected multidimensional problems: steady state simulation, chronoamperometric simulation, cyclic voltammetry at microelectrodes, and simulation of arbitrarily shaped scanning electrochemical microscope (SECM) tips. It is shown that the algorithm is suitable for this kind of problems and can be easily extended to the simulation of many types of electrochemical experiments.     

The application of nanopipettes to conducting polymer fabrication, imaging and electrochemical characterization

The emergence of conducting polymer nanostructures, with their important and wide-ranging applications in sensors, displays and coatings, has not been accompanied by an emergence of appropriate electrochemical nanoscale characterization tools. Herein we show that nanopipettes, as implemented in variants of the scanning ion conductance microscope, have the potential to address numerous needs of the conducting polymer nanostructure community. Specifically, nanopipettes can fabricate freestanding conducting polymer nanowires, map electroactivity and conductivity, deliver doses of reagents with nanoscale precision, perform highly localized cyclic voltammetry and characterize ion flux from actuators. Additionally, nanopipette innovations already demonstrated in biological and analytical fields - such as individually controlled double-barreled nanopipette setups, voltage-controlled deposition and functionalized surfaces - open the door to new approaches to conducting polymer nanostructure fabrication and characterization.     

Adherent natural soils on fiber surfaces

Adherent natural soils on fiber surfaces have been studied with the scanning electron microscope. The soils on the fiber surface resemble oily soil.     

Electrochemical nanostructuring with an STM: A status report

Over the last decade, electrochemists have made increasingly the use of an STM as a tool for nanostructuring electrode surfaces. The various approaches include: (i) a mechanical impact of the tip on the surface, e.g., creating reaction centres by a tip crash or by locally removing tarnishing overlayers, (ii) the spatially confined metal deposition or dissolution via the tip and (iii) down-scaling of the SECM to nanometer dimensions. Examples for each of the different strategies are given. Nanostructuring electrode surfaces by metal clusters, which are generated by a jump-to-contact between tip and substrate is described in more detail with Cu and Pd on Au(1 1 1) as examples. Preliminary results of tunneling spectroscopy are also presented.     

Plant derived cationic dye as an effective corrosion inhibitor for 7075 aluminum alloy in 3.5% NaCl solution

The inhibition of the corrosion of 7075 aluminum alloy in 3.5 wt.% NaCl solution by 5,6-dihydro-9,10-dimethoxybenzo[g]-1,3-benzodioxolo[5,6-a]quinolizinium (berberine) has been studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning electrochemical microscopy (SECM) and scanning electron microscopy (SEM). Inhibition efficiency was found to increase with increasing concentration of berberine. The adsorption of the berberine on the 7075 aluminum alloy surface obeyed the Langmuir adsorption isotherm. The adsorbed film on 7075 aluminum alloy surface containing inhibitor was confirmed by the SEM, and SECM. The results obtained showed that the berberine could serve as an effective inhibitor of the corrosion of 7075 aluminum alloy in 3.5% NaCl.     

Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM)

Transition metal hexacyanoferrates are versatile inorganic compounds widely employed for the assembling of sensors and biosensors in a variety of different electroanaytical applications. A modified version of the recently introduced redox competition mode of scanning electrochemical microscopy (RC-SECM) was exploited to visualize the local electrocatalytic activity of microstructured Prussian blue (PB) films towards the reduction of H₂O₂ with improved lateral resolution. The PB films were electrochemically deposited in a spot on glassy carbon surfaces using a droplet cell. The influence of the potential applied to the PB modified surface on the current at the SECM tip was evaluated when both the sample and the SECM tip were competing for H₂O₂ in solution. Thus, high local electrocatalytic activity is indicated by low currents at the SECM tip. The same strategy was successfully employed for the characterization of the performances of a biosensor employing the enzyme glucose oxidase (GOx) immobilized within a polymer hydrogel matrix on the top of PB-modified glassy carbon electrodes.     

Assessment of the corrosion protection of aluminium substrates by a Mg-rich primer: EIS,SVET and SECM study

Magnesium-rich coatings have the capability of providing sacrificial corrosion protection to aluminium substrates and therefore present a new and challenging field of development. In this work the mechanism of protection of an aluminium substrate by a Mg-rich coating was investigated using the scanning vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM). The SVET has shown the evolution of the pit activity with time under sacrificial protection, whereas the SECM allowed indirect sensing of the cathodic activity above the electrodes. The study was complemented by classical electrochemical techniques, namely electrochemical impedance spectroscopy and open circuit potential measurement.     

SECM imaging of electrocatalytic activity for oxygen reduction reaction on thin film materials

The oxygen reduction reaction (ORR) on sputtered Pt thin films in acidic solution was successfully studied by scanning electrochemical microscopy (SECM) in a modified tip generation-substrate collection (TG-SC) mode. SECM images of ORR activity in different sample areas were obtained and it is shown that this TG-SC SECM technique can be used to screen electrocatalytic activity of continuous thin film samples efficiently and quickly for the ORR in an acidic medium. It is observed that this technique is not very sensitive to the tip-substrate separation within a certain range. The SECM images obtained are strongly dependent on the substrate potential. The advantages of this technique for studying ORR electrocatalysts are discussed.     

Fracture failure processes in polymers. II: Fractographic evidence

Tensile fracture and punch shear failure surfaces have been observed in a scanning electron microscope. In most cases tensile cracks were visible in the shear fracture surfaces and crack initiation points could be found in the tensile fracture surfaces. The punch test imposes very large shear strains on the materials (many thousand percent) and this appears to reorient the chains so that, so long as the chain length is great enough (>4000 repeat units for polyethylene), chain scission is observed, and the failure process at the molecular level is tensile rather than shear. The exceptions are the shorter chain polyethylenes and polypropylene, which are probably truly liquid-like, with little or no chain scission being involved in tension or shear.     

Effects of silane coupling agents on mica-filled styrene-butadiene rubber

The effects of silane coupling agents on styrene-butadiene rubber (SBR)-mica composites have been studied with special reference to the technical properties and polymer-filler interaction. Silane-treated mica improves polymer-filler interaction, which subsequently enhances the tensile and other technically important mechanical properties. Failure surfaces of tensile, tear, and wear specimens have been examined under scanning electron microscope (SEM) and the fractographs have been correlated with the strength and failure mode of the composites.