Structural and electrochemical characterization of fullerene-based surfaces of C60 mono- or bis-adducts grafted onto self-assembled monolayers

Single layers of C₆₀ mono- and bis-adducts have been obtained by functionalizing an acid-terminated self-assembled monolayer (SAM) of thiols on gold. X-ray photoelectron spectroscopy demonstrated the grafting onto the SAM by covalent bonding, via the formation of an amide bond, while cyclic voltammetry and electrochemical impedance spectroscopy provided information on the redox properties of the C₆₀-films, as well as on structural characteristics.     

Identification of inductive behavior for polyaniline via electrochemical impedance spectroscopy

Polyaniline (PANI) film electrodeposited in HCl medium using cyclic voltammetry (CV) with an upper potential limit of 0.90 V, exhibited an inductive behavior. PANI films deposited with different conditions were subjected to various applied potentials and the impedance characteristics were recorded through electrochemical impedance spectroscopy (EIS). The impedance results clearly reveal the existence of inductive behavior to PANI. Inductive behavior was observed for PANI films deposited with conditions which favor benzoquinone/hydroquinone (BQ/HQ) formation and further evidenced by X-ray photoelectron spectroscopy (XPS). A comparative analysis of the EIS and XPS results of PANI films prepared under similar conditions with the upper potential limits of 0.75 and 0.90 V, respectively, clearly documented that the presence of BQ/HQ, the degradation product of PANI, formed during the electrochemical polymerization at the upper potential limits causes inductive behavior to PANI.     

Estimation of kinetic parameters of the passive state of carbon steel in mildly alkaline solutions from electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data

The unambiguous interpretation of electrochemical impedance spectra of complex systems such as passive metals and alloys in terms of an unique kinetic model is often hampered by the large number of adjustable modeling parameters. In this paper, a combination of in situ electrochemical data and ex situ surface analytical information is employed to validate the estimates of kinetic and transport parameters of the passive state of carbon steel. For the purpose, electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data for the oxidation of carbon steel in mildly alkaline solutions are quantitatively compared with the predictions of the Mixed-Conduction Model for oxide films that represent the passive oxide as an intermediate phase between magnetite and maghemite. Estimates of the kinetic rate constants at the film interfaces, as well as the diffusion coefficients and field strength in the film are obtained and their relevance for the corrosion mechanism of carbon steel is discussed.     

Effect of electroless nickel interlayer on the electrochemical behavior of single layer CrN, TiN, TiAlN coatings and nanolayered TiAlN/CrN multilayer coatings prepared by reactive dc magnetron sputtering

The electrochemical behavior of single layer TiN, CrN, TiAlN and multilayer TiAlN/CrN coatings, deposited on steel substrates using a multi-target reactive direct current (dc) magnetron sputtering process, was studied in 3.5% NaCl solution. The total thickness of the coatings was about 1.5 μm. About 0.5 μm thick chromium interlayer was used to improve adhesion of the coatings. With an aim to improve the corrosion resistance, an additional interlayer of approximately 5 μm thick electroless nickel (EN) was deposited on the substrate. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to study the corrosion behavior of the coatings. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the corroded samples. The potentiodynamic polarization tests showed lower corrosion current density and higher polarization resistance (R_p) for the coatings with EN interlayer. For example, the corrosion current density of TiN coated steel was decreased by a factor of 10 by incorporating 5 μm thick EN interlayer. Similarly, multilayer coatings of TiAlN/CrN with EN interlayer showed about 30 times improved corrosion resistance as compared to the multilayers without EN interlayer. The porosity values were calculated from the potentiodynamic polarization data. The Nyquist and the Bode plots obtained from the EIS data were fitted by appropriate equivalent circuits. The pore resistance (R_pore), the charge transfer resistance (R_ct), the coating capacitance (Q_coat) and the double layer capacitance (Q_dl) of the coatings were obtained from the equivalent circuit. Multilayer coatings showed higher R_pore and R_ct values as compared to the single layer coatings. Similarly, the Q_coat and Q_dl values decreased from uncoated substrate to the multilayer coatings, indicating a decrease in the defect density by the addition of EN interlayer. These studies were confirmed by examining the corroded samples under scanning electron microscopy.     

Influence of the surface chemistry of modified activated carbon on its electrochemical behaviour in the presence of lead(II) ions

Cyclic voltammetric studies of the influence of surface chemistry on the electrochemical behaviour of granulated and powdered activated carbon samples in the presence of lead(II) ions both in bulk solution and pre-adsorbed on carbon were carried out. Variety in surface chemical character was achieved through modification of carbon samples by heat treatment in vacuum, ammonia and ammonia-oxygen atmospheres, as well as by oxidation in moist air and with concentrated nitric acid. For the samples obtained, the surface area (BET), acid–base neutralization capacities and sorption capacity towards Pb²⁺ ions were estimated. The states of the deposited Pb species were assessed by means of FTIR and XPS spectra as well as cyclic voltammetry. The importance of the surface chemistry of the carbon electrode materials are discussed in terms of their electrochemical properties and the mechanism of adsorption processes. The Cπ-metal and heteroatom-metal interaction are dominant in amphoteric and basic carbons, but in oxidized samples adsorption take place mainly by ion-exchange. Other forms of adsorption, such as the formation metal hydroxide species, are also covered buy this paper. Various forms of adsorbed lead species exhibit different electrochemical activities.     

Effects of sulfuric acid treatment on the microstructure and electrochemical performance of a polyacrylonitrile (PAN)-based carbon anode

To examine whether acid treatment of a non-graphitizing hard carbon influences positively or negatively its electrochemical anodic performance, this study reports the effects of sulfuric acid treatment on the microstructural changes and electrochemical performance of PAN-based hard carbons prepared at various temperatures. It was found that PAN-based hard carbons heat treated at 900 °C (TAN9) exhibit an increased reversible capacity by up to ∼20% and a decreased irreversible capacity by up to ∼8% following the sulfuric acid treatment (TAN9S series). Since small changes in microstructure, except for slight reductions in surface area and crystallite size (L_a) value, were observed after sulfuric acid treatment, it was speculated that the capacity responses of samples in series TAN9S were due to the introduction of new functional groups such as SO₃H and SO₄H. As the newly introduced functional groups are strong acids but their conjugates ( and ) are weak bases, those conjugates were thus considered to be able to react with Li⁺ ion relatively weakly and reversibly. For PAN-based hard carbons heat treated at 1100 °C (TAN11), there were much smaller changes, as compared with series TAN9S samples, in surface chemistry and microstructure by sulfuric acid treatment. Consequently, we observed an analogous but smaller influence of sulfuric acid oxidation on electrochemical performance. The samples from series TAN11S exhibited very stable cycling behavior. It was suggested that the acidity/basicity of surface functional groups may be an influential factor for improving the electrochemical performance of hard carbon-based anodic materials for lithium ion batteries.     

Preliminary evaluation of the anticorrosive properties of aircraft coatings by electrochemical methods

Strict regulations concerning the content of volatile organic compounds (VOCs) and heavy metals (Cr⁶⁺) in aircraft coating systems have increased the economic burden of the United States Air Force (USAF) in the area of coating maintenance. To this end, it is critical to have methods to characterize new coating systems in such a manner that the data can be used to predict accurately and reliably the expected lifetime of the coatings in service. Electrochemical noise method (ENM) and electrochemical impedance spectroscopy (EIS) are two techniques used to monitor extent and rate of corrosion. The USAF is currently employing these methods in order to supplement data acquired from traditional salt-spray methods. ENM and EIS are used to evaluate each component of the coating system and its contribution to corrosion prevention. Preliminary evaluations of an aircraft coating system on aluminum substrate (Al 2024-T3) produces resistance noise values of 10⁶ to 10⁷ Ω/cm². It is hoped that these results will form the basis of coatings that give increased USAF fleet service life and reduction in maintenance manpower and materials costs.     

Self-assembled monolayers of C60-perylenetetracarboxylic diimide-C60 triad on indium tin oxide surface

Indium tin oxide (ITO) electrodes modified chemically with self-assembled monolayers (SAMs) of C₆₀-perylenetetracarboxylic diimide-C₆₀ triad have been designed for the first time to act as an efficient light-to-current converter in molecular devices. The monolayers were characterized using UV-Vis, X-ray photoelectron spectroscopy and atomic force microscopy. Photoelectrochemical measurement of the SAMs of C₆₀-Perylenetetracarboxylic diimide-C₆₀/ITO indicated prompt, steady, and reproducible photocurrent generation when irradiated by white light.     

Preparation of photocatalytic TiO2 coatings of nanosized particles on activated carbon by AP-MOCVD

Anatase TiO₂ coatings on highly porous activated carbon were prepared by a novel method—atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). At a source temperature of 423 K, the TiO₂ particles were mostly coated on the external surface of activated carbon. These particles were well dispersed with their sizes ranging from 10 to 50 nm. The optimum loading of TiO₂ was found to be 12 wt%. The TiO₂ photocatalysts so prepared behave similarly to that of the pure commercial TiO₂ powder. The activated carbon supported TiO₂ catalyst could be easily separated from the treated water with its catalytic performance maintained even after 10 cycles, indicating that the TiO₂ coating was stable. It was observed that TiO₂ supported on activated carbon had a high capacity to mineralize pollutants. Consequently, activated carbon supported TiO₂ by AP-MOCVD is a promising photocatalyst for the photodegradation of pollutants in water.     

Immobilization of amines at carbon fiber surfaces

The reaction between amines acting as nucleophiles and the C=C bonds on the carbon fiber surface acting as electrophilic vinyl groups has not yet been explored. In this contribution it is demonstrated that both thermal reactions and electrochemical oxidation of amines at carbon fibers allow the covalent bonding of these molecules directly to the carbon fiber surface, presumably via nucleophilic attack of the amine at electrophilic C=C sites at the surface and subsequent formation of C–N bonds between the surface and the amine. A novel strategy for a quantitative assay of the number of amines attached to the surface is developed in which Fe(CN)₆³⁻ is electrostatically bound to the protonated, cationic amine sites, followed by electrochemical determination of the amount of bound Fe(CN)₆³⁻ as a function of its concentration in solution. Analysis of the isotherm for this electrostatic binding process then provides a measure of the number of interfacially immobilized amines. The composition of the amine layer is also probed using X-ray photoelectron spectroscopy (XPS). Mechanisms are discussed by which attachment of amines at the electrophilic vinyl groups of the carbon fibers can occur. The likely influence that this type of reaction has on the interfacial shear strength in carbon fiber/epoxy composite materials is also discussed.     

Anisotropic crack propagation behavior for the silicon-bond coat layer in a multilayer coated system

This study sought to examine the relationship between the degradation mechanism, thermal stress, and crack propagation behavior in environmental barrier coating (EBC) systems. An EBC system composed of a mullite topcoat (TC), Si-bond coat (BC), and SiC substrate was prepared by atmospheric plasma spraying. Heat exposure tests were conducted to evaluate the microstructure of the EBC system at 1300°C for 1, 10, 50, and 100 h. The fracture resistance of the Si BC for the in-plane (direction parallel to each layer, 0.4–0.6 MP) and through-thickness directions (direction from the TC to substrate, 1.7–2.1 MP) differed because a thermal compressive stress was induced for the in-plane direction owing to the mismatch of the thermal expansion coefficients for each layer, which acted as a barrier for crack propagation. However, cracks tended to propagate in the in-plane direction because they were not affected by the in-plane compressive stress. These results clearly showed that Si BC exhibited in-plane anisotropy and crack propagation after heat exposure, which were the major sources of delamination of the EBC system.     

Environmental studies of smart/self-healing coating system for steel

Smart/self-healing micro-capsulated inhibitor incorporated in epoxy primer before painting on a steel surface was evaluated for its corrosion protection effectiveness on exposure to ASTM D 5894 electrolyte in laboratory and natural tropical sea-shore environment. The “healant” inhibitor was industrial custom-made and non-chromate organic-based microcapsules which were mixed into the primer before applying a polyurethane topcoat layer on steel surface. The results indicate that the active components in ruptured embedded inhibitor microcapsules were released into an inflicted scribe primer and topcoat film on steel surface on exposure to inhibit development of an electrochemical cell. Undamaged surface film of the test and control specimens exposed in the environments demonstrated excellent corrosion-inhibition performance as reflected by both visual inspection and electrochemical impedance spectroscopy experimental data. The results obtained on the performance of self-healing inhibitors should provide an understanding of the fundamental material-property relationships of smart inhibitor coatings. And, thus, should facilitate the development of optimized paint compositions in order to extend the useful service life of steel-infrastructure applications.     

Study on the corrosion behavior of reinforcing steel in simulated concrete pore solutions using in situ Raman spectroscopy assisted by electrochemical techniques

In situ Raman spectroscopy, electrochemical impedance spectroscopy (EIS) and polarization curves were used to study the corrosion behavior of reinforcing steel in simulated concrete pore (SCP) solutions (saturated Ca(OH)₂ solutions). Results indicated that the reinforcing steel remained passive in chloride-free SCP solutions. However, the anodic polarization curve of the steel did not exhibit a stable passive region in the SCP solution with 0.5 M NaCl, the corrosion current density exceeded 0.1 μA cm⁻², the steel surface was unstable with chloride attack and localized corrosion appeared on it with FeCO₃ and Fe₂O₃ as the main corrosion products.     

Selective reduction and extraction of Gd@C82 and Gd2@C80 from soot and the chemical reaction of their anions

Gd@C₈₂ and Gd₂@C₈₀ were selectively reduced and extracted from the raw soot in a high yield by using Al-Ni alloy in toluene/tetrahydrofuran (THF). By tuning the ratio of toluene and THF, Gd₂@C₈₀ with purity higher than 95% was isolated directly from the raw soot without high performance liquid chromatography (HPLC) purification. Furthermore, we found that the anions of Gd@C₈₂ and Gd₂@C₈₀ could react with aqueous NaOH to form water-soluble Gd-metallofullerols, which are potential magnetic resonance imaging contrast agents.     

Electrochemical study of effect of the concentration of azole derivatives on corrosion behavior of stainless steel in H2SO4

In order to improve corrosion resistance of the stainless steel structures exposed to acidic media, a variety of corrosion inhibitors particularly organic ones have been examined. In this work, the corrosion inhibition performance of two azole derivatives namely benzotriazole and benzothiazole on stainless steel in 1 M sulfuric acid was studied through taking advantage of electrochemical techniques as well as SEM surface analysis. Revealing effectiveness of the two inhibitors, the AC impedance spectra indicated no change in corrosion mechanism. The noise resistance and average current density as parameters extracted from electrochemical noise measurements revealed the direct proportion of inhibition function to the inhibitor concentration. In accordance with the polarization curves, benzotriazole and benzothiazole appeared to act as mixed type inhibitors. The adsorption of the two corrosion inhibitors was shown to obey Langmuir isotherm. Moreover, it was deduced from the isotherm that the type of adsorption can be physical and chemical in nature. The corrosion damage mitigation was also confirmed through SEM in the presence of benzothiazole.     

In situ electrochemical impedance spectroscopy/synchrotron radiation grazing incidence X-ray diffraction—A powerful new technique for the characterization of electrochemical surfaces and interfaces

A marriage of electrochemical impedance spectroscopy (EIS) and in situ synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) has provided a powerful new technique for the elucidation of the mechanistic chemistry of electrochemical systems. In this study, EIS/SR-GIXRD has been used to investigate the influence of metal ion buffer calibration ligands, along with natural organic ligands in seawater, on the behaviour of the iron chalcogenide glass ion-selective electrode (ISE). The SR-GIXRD data demonstrated that citrate - a previously reported poor iron calibration ligand for the analysis of seawater - induced an instantaneous and total dissolution of crystalline GeSe and Sb₂Se₃ in the modified surface layer (MSL) of the ISE, while natural organic ligands in seawater and a mixture of ligands in a mimetic seawater ligand system protected the MSL's crystalline inclusions of GeSe and Sb₂Se₃ from oxidative attack. Expectedly, the EIS data showed that citrate induced a loss in the medium frequency time constant for the MSL of the ISE, while seawater's natural organic ligands and the mimetic ligand system preserved the medium frequency EIS response characteristics of the ISE's MSL. The new EIS/SR-GIXRD technique has provided insights into the suitability of iron calibration ligands for the analysis of iron in seawater.     

Group 15 element-graphite composites synthesized by reduction of chlorides by KC8: Characterization and electrochemical lithiation

Group 15 element-graphite M/C composites were prepared by reduction of MCl_x chlorides (AsCl₃, SbCl₅, BiCl₃) by KC₈ in THF. Arsenic and antimony were both amorphous: antimony appeared as a film-like material, formed of aggregated nano-sized particles covering parts of the graphite surface; arsenic was present as large graphite supported particles. On the contrary, bismuth was present as crystalline metal nanoparticles distributed on the graphite surface. Amorphous As and Sb-graphite composites displayed stable reversible capacities of 310 and 420 mA h/g, respectively while that of crystalline Bi-graphite materials decreased regularly upon cycling. Although these practical capacities were lower than the expected theoretical ones corresponding to the formation of Li₃M compounds, it appeared that, in the presence of graphite, amorphous solids exhibiting a (partly) covalent character like As and Sb gave better long life cycling properties than the crystalline and metallic bismuth. It was very likely that our one step synthesis could generate bonds between graphite and (partly) covalent solids reducing consequently the volume expansion effects occurring during cycling. On the contrary, metallic solids like bismuth that were not inclined to bond with graphite behave, upon cycling, as corresponding massive metals.     

Investigations on the electrochemical properties of new conjugated polymers containing benzo[c]cinnoline and oxadiazole moieties

A four-step route was designed to synthesize 3,8-benzo[c]cinnoline dicarboxylic acid (4). New conjugated polymers, POXD (I) and POXD (T), containing benzo[c]cinnoline and oxadiazole moieties, were obtained by thermal cyclodehydration of their soluble polyhydrazide precursors PHA (I) and PHA (T), respectively. Two reduction peaks were observed for these new conjugated polymers during CV cathodic scan. From the CV voltammograms combined with the results from molecular simulation, we concluded that the first reduction occurred at oxadiazole moiety and benzo[c]cinnoline moiety was responsible for the second reduction. It indicates that oxadiazole has stronger electron affinity than benzo[c]cinnoline. We proposed a mechanism to explain this two-stage reduction process. Due to the planar and electron-accepting ability of benzo[c]cinnoline and oxadiazole moieties, POXD (I) and POXD (T) exhibited very low LUMO (−3.42 and −3.45 eV) and HOMO (−6.23 and −6.27 eV) energy levels. They can be used as hole-blocking or electron-injection layers for OLED applications.