Cathodic Step Research Articles

Designing bipolar anodizing towards white anodic aluminum oxide (AAO)

Obtaining white anodic aluminum oxide (AAO) is a major challenge in the surface finishing industry. The present work presents a cost-effective route to design light-scattering morphologies through submicrometric hollows homogeneously dispersed in the depth of the oxide layer, created by localized hydrogen evolution. To this end, cathodic steps are added in alternance with anodic steps in a bipolar pulse anodizing process performed in 2 M sulfuric acid. The mechanisms of nucleation and growth of the targeted hollows within the AAO are highlighted by combining in situ electrochemical measurements and high magnification SEM observations. The aesthetic performances in terms of white color, evaluated by reflectance spectroscopy in comparison with a white reference, are directly related to the size, shape and distribution of the hollows, controlled by both the parameters of the cathodic step and the thickness of the unit anodic oxide layers.

Bipolar pulse anodizing of aluminum: Understanding the fundamental electrochemical mechanisms

Anodic aluminum oxides (AAO) with attractive morphological properties might be obtained by pulse anodizing via controlled hydrogen release during the cathodic step. In order to enhance such processes currently relying on empirical studies, the present paper aims to investigate basically the AAO formation on pure aluminum in sulfuric acid during a bipolar pulse anodizing process, by combining in situ electrochemical measurements and cross-section scanning electron microscopy. Results show that the hydrogen evolution at the metal/oxide interface during the cathodic step can be easily managed by current control. Actually, under cathodic galvanostatic conditions, successive mechanisms occur, depending on both the duration and the current density: a first stage corresponds to the reorganization of the electric charges and the migration of H+ through the AAO barrier layer and is followed by hydrogen evolution at the metal/oxide interface, which induces the formation of local or more extended defects at the metal/oxide interface until the detachment of the AAO layer.

Hybrid Cathodic/Anodic Electrosynthesis of Phase Pure Ag4V2O7 Thin Films

Here, we demonstrate a two-step electrosynthesis approach for the preparation of silver pyrovanadate, Ag4V2O7 in thin-film form. In the first, cathodic step, polycrystalline Ag was deposited on fluorine doped tin oxide (FTO) substrate from a non-aqueous bath. Aqueous pyrovanadate species were then generated by aging of a CO2-infused sodium orthovanadate (Na3VO4) solution for three weeks. Silver ions were subsequently generated in situ in this medium using anodic stripping of the Ag/ITO films from the first step. Interfacial precipitation of the Ag+ ions with the pyrovanadate species afforded the targeted product in phase pure form. The various stages of the electrosynthesis were monitored in situ via the combined use of voltammetry, electrochemical quartz crystal nanogravimetry (EQCN), and coulometry. The Ag4V2O7 thin films were characterized by a variety of experimental techniques, including X-ray diffraction, laser Raman spectroscopy, diffuse reflectance spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements afforded information on the energy band structure of the p-type Ag4V2O7 semiconductor. Finally, the electrochemical and photoelectrochemical properties of the electrosynthesized Ag4V2O7 thin films were studied in both aqueous and non-aqueous electrolytes.

Fabrication of Pyramid-Like Structured Cu Coatings by Pulse-Reverse Current Electrodeposition

Copper (Cu) coatings with a high surface area have attracted significant attention for functional devices due to their high thermal and electrical properties. Pulse-reverse current (PRC) electrodeposition has been introduced to fabricate Cu coatings for a long history, but compared to the individual anodic step, little attention has been paid to the synergistic effects of anodic and cathodic steps. The new parameter “artificial exchange current density” (j A) has been demonstrated to quantify the effects of anodic and cathodic currents on the morphology in our previous work. Herein, a key metric−the relative current amplitude−that is positively correlated with j A, is used to further clarify the role of j A in the electrodeposition process. We fabricate Cu coatings with well-dispersed pyramid-like structures and find the relative current amplitude dominates the formation of a larger raised structure at the initial nucleation stage. Moreover, it is speculated that the anodic current can dissolve high-energy planes and achieve a highly (111)-oriented texture. Afterward, screw dislocation drives the spiral growth of grains, resulting in pyramid-like structures. This study not only enriches our understanding of the artificial exchange current density in PRC electrodeposition but also guides us to achieve Cu coatings with high surface area.

Phase Pure Silver Pyrovanadate (Ag4V2O7) Thin Films: Synthesis via a Hybrid Cathodic/Anodic Approach and Characterization

Combining two or more binary compounds to generate a multinary compound is a strategy for enhancing the photoelectrochemical (PEC) performance and properties of the parent binary compounds. In this vein, multinary compound semiconductors have recently come under the spotlight in a variety of device applications. In particular, the low energy band gap of the Ag-V-O ternary family (Ag3VO4, Ag4V2O7, AgVO3, Ag2V4O11, and AgV7O18) has recently sparked interest in using these visible light active semiconductor for photovoltaic solar cells, solar water splitting, CO2 photoreduction, and environmental remediation applications. In particular, Ag4V2O7, a visible light active semiconductor with a low band gap (2.49 eV) has been synthesized in powder form using solid state, hydrothermal, and precipitation methods.1,2 However, for device applications, deposition techniques that facilitate direct synthesis of semiconductor in film form, are attractive.In this vein, electrochemical deposition is a fast, low temperature, and scalable method to prepare large active area films of a given semiconductor. Therefore, this paper presents a hybrid cathodic and anodic route for the electrodeposition of Ag4V2O7 thin films and its subsequent physical and optoelectronic characterizations. Ag4V2O7 thin film on fluorine-doped tin oxide (FTO) substrate was synthesized using a hybrid cathodic and anodic route previously developed in our laboratory.3 In the cathodic step, a metallic silver thin film was electrosynthesized from silver ions in a nonaqueous medium on fluorine-doped tin oxide (FTO) support. In the anodic step, the generated silver ion (Ag+) from anodic stripping of the silver thin film in a pyrovanadate ion (V2O7 4-)containing-solution resulted in an in situ precipitation reaction with V2O7 4- to form Ag4V2O7 thin film on FTO. Electrochemical quartz crystal nanogravimetry (EQCN) confirmed one and four electron stoichiometry for the cathodic and anodic steps respectively.Energy-dispersive X-ray analysis (EDX) of as-prepared sample gave the Ag/V ratio: 1.95 ± 0.02, close to the expected stoichiometric Ag/V ratio for Ag4V2O7. X-ray diffraction analyses of the as-synthesized sample significantly confirmed that room temperature electrodeposited sample was crystalline and phase-pure. To measure the direct and indirect energy band gaps, Tauc plots were constructed from diffuse reflectance spectroscopy (DRS) data. The estimated direct and indirect optical transition energies of Ag4V2O7 thin films were 2.58 ± 0.04 and 2.52 ± 0.02 eV respectively. A valence band energy position as -4.88 ± 0.03 eV (on the vacuum energy scale) was estimated for Ag4V2O7 using atmospheric photoemission spectroscopy (APS) results. By combining the DRS and APS results, the calculated conduction band edge position of Ag4V2O7 was determined to be -2.36 eV on the vacuum energy scale. Kelvin-probe measurement located the Fermi level of silver pyrovanadate thin films at -4.82 ± 0.02 eV on the vacuum energy scale. The corresponding EF for Ag4V2O7 was only 0.06 eV above the valence band position consistent with a moderately-doped p-type semiconductor. These new results will be discussed in the framework of the application of silver pyrovanadate for PEC reduction processes such as hydrogen evolution or CO2 reduction.

Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide.

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3′-DHBPY)(CO)3Cl] (3,3′-DHBPY = 3,3′-dihydroxy-2,2′-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO2 to CO. The combined techniques of cyclic voltammetry and IR/UV–vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4′-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O–H···O–. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4′-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3′-DHBPY-H+)(CO)3Cl]− under ambient conditions occurs at a cathodic potential close to that of the Re/4,4′-DHBPY-H+ derivative. However, for the stabilized 3,3′-DHBPY-H+ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e–-reductive cleavage of the second O–H bond, triggering dissociation of the Cl– ligand from [Re(3,3′-DHBPY-2H+)(CO)3Cl]2–. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3′-DHBPY-2H+)(CO)3]3–, the equivalent of classical 2e–-reduced [Re(BPY)(CO)3]−. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm–1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl– dissociation from [Re(3,3′-DHBPY-2H+)(CO)3Cl]2– was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3′-DHBPY-2H+)(CO)3(PrCN)]3–. Useful reference data were obtained with substituted parent [Re(3,3′-DHBPY)(CO)3(PrCN)]+ that also smoothly deprotonates by the initial reduction to [Re(3,3′-DHBPY-H+)(CO)3(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3′-DHBPY-2H+)(CO)3(PrCN)]3– via a counterintuitive ETC step generating the 1e– radical of the parent complex, viz., [Re(3,3′-DHBPY)(CO)3(PrCN)]. The same alternative reduction path is also followed by [Re(3,3′-DHBPY-H+)(CO)3Cl]− at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3′-DHBPY)(CO)3Cl]•– further reducible to [Re(3,3′-DHBPY-2H+)(CO)3]3– as the CO2 catalyst.

Ionic direct current modulation evokes spike-rate adaptation in the vestibular periphery

Recent studies have shown that ionic direct current (iDC) can modulate the vestibular system in-vivo, with potential benefits over conventional pulsed stimulation. In this study, the effects of iDC stimulation on vestibular nerve fiber firing rate was investigated using loose-patch nerve fiber recordings in the acutely excised mouse crista ampullaris of the semicircular canals. Cathodic and anodic iDC steps instantaneously reduced and increased afferent spike rate, with the polarity of this effect dependent on the position of the stimulating electrode. A sustained constant anodic or cathodic current resulted in an adaptation to the stimulus and a return to spontaneous spike rate. Post-adaptation spike rate responses to iDC steps were similar to pre-adaptation controls. At high intensities spike rate response sensitivities were modified by the presence of an adaptation step. Benefits previously observed in behavioral responses to iDC steps delivered after sustained current may be due to post-adaptation changes in afferent sensitivity. These results contribute to an understanding of peripheral spike rate relationships for iDC vestibular stimulation and validate an ex-vivo model for future investigation of cellular mechanisms. In conjunction with previous in-vivo studies, these data help to characterize iDC stimulation as a potential therapy to restore vestibular function after bilateral vestibulopathy.

Dissolution and passivation of aluminide coatings on model and Ni-based superalloy

Electrolytic dissolution of aluminide coatings on model (pure Ni, Ni20Cr) and René 125 Ni-based superalloy was conducted by alternated cathodic and anodic polarizations. The pure nickel aluminide dissolved homogeneously. In contrast, the inhomogeneity of dissolution increased with the incorporation of Cr in the coating. The coatings were however uniformly dissolved when the cathodic step was eliminated. XPS and TEM demonstrate that the occurrence of a passive layer containing Cr and Al oxy-hydroxides blocked dissolution.

Controlled electrochemical doping of graphene-based 3D nanoarchitecture electrodes for supercapacitors and capacitive deionisation.

Chemically-doped graphenes are promising electrode materials for energy storage and electrosorption applications. Here, an affordable electrochemical green process is introduced to dope graphene with nitrogen. The process is based on reversing the polarity of two identical graphene oxide (GO) electrodes in molten KCl-LiCl-Li3N. During the cathodic step, the oxygen functional groups on the GO surface are removed through direct electro-deoxidation reactions or a reaction with the deposited lithium. In the anodic step, nitrogen is adsorbed onto the surface of graphene and subsequently reacts to form nitrogen-doped graphene. The doping process is controllable, and graphene with up to 7.4 at% nitrogen can be produced. The electrochemically treated electrodes show a specific capacitance of 320 F g-1 in an aqueous KOH electrolyte and maintain 96% of this value after 10 000 cycles. The electrodes also display excellent electrosorption performance in capacitive deionisation devices with the salt removal efficiency reaching up to 18.6 mg g-1.

Printable Hecl Electrode Materials for Disposable Bioaffinity Assay Cartridges

We have demonstrated earlier that hot electrons can be injected into aqueous electrolyte solution from thin insulating film coated cathodes [1, 2]. Insulating film have typically been fabricated from an insulator material with a band gap of ca. 5 eV or preferably higher, with the Fermi level of the insulator somewhere in the mid band gap region. The hot electrons are emitted to aqueous electrolyte solution by direct field-assisted tunneling at these devices [2, 3]. After injection into the conduction band of water the hot electrons are thermalized and solvated to form hydrated electrons which finally act as reducing mediators in the Hot Electron-induced Electrochemiluminescence (HECL) systems. HECL is utilized mainly in immunoanalysis and in DNA-probe assays in which electrochemiluminescent labels are utilized [ 2 - 5 ]. Best labels are certain aromatic lanthanide chelates which allow sensitive time-resolved HECL detection of the labels [3, 6]. Silicon electrodes or silicon chips on which the working electrode/electrodes and counter electrode have been integrated have been shown to be excellent functional components of disposable assay cartridges [5,7]. Recently, we have been able to develop printable HECL electrode materials that show almost equal performance and are based on conductor particles embedded in polystyrene or in some other organic insulating materials.

Role of typical soil particle‐size distributions on the long‐term corrosion behavior of pipeline steel

The electrochemical behavior of X70 pipeline steel in a 1.5 wt% NaCl sandy soil environment with two typical particle size distributions (PSD), continuous gradation (CG), and gap gradation (GG) was investigated using electrochemical measurements, scanning electron microscopy (SEM), and X‐ray diffraction (XRD). The results showed that, as the burial time increased, the corrosion rate of X70 pipeline steel increased in the initial corrosion stage, decreased at an intermediate stage, and then increased in the final stage in these two typical soil PSD environments. However, compared with the soil PSD pertaining to the CG environment, the corrosion rate of steel increased significantly in the GG environment, as evinced by the severe corrosion spots observed on the steel surface, attributed to the fact that, in the soil PSD for the GG environment, both the extent of liquid dispersion, and the number of galvanic cells increased, accelerating the cathodic step, i.e., oxygen‐reduction reaction in the three‐phase boundary (TPB) zone, and anodic step, i.e., the metal dissolution stage, of the X70 pipeline steel corrosion process, respectively. Both the anodic and cathodic processes were activation controlled. Thus, the corrosion of X70 pipeline steel was considerably dependent on the soil PSD in sandy soil environments.

Stochastic electrochemistry and photoelectrochemistry of colloidal dye-sensitized anatase nanoparticles at a Pt ultramicroelectrode.

We report the stochastic interactions between dye sensitized anatase nanoparticles, suspended in a colloid, and a Pt ultramicroelectrode (UME) that result in step-wise behavior in the current vs. time response. The stochastic currents are observed in the dark and under illumination. In the dark, the currents are anodic, consistent with the oxidation of the dye N719 at the Pt surface. The electrochemical behavior of the dye was investigated in MeOH and MeCN with a quasireversible cyclic voltammogram (CV) observed at 1 V s-1. The anodic currents observed in the dark due to nanoparticles (NPs) at the Pt surface are consistent with the CVs in MeOH and MeCN. Under illumination cathodic steps are observed and assigned to the reduction of the oxidized form of the dye generated after electrons are injected into the TiO2 NPs. The colloidal behavior is a strong function of the history of the colloid with illumination time increasing the size of the agglomerates and with larger agglomerates being less photoelectrochemically active. Agglomerates of ca. 100 nm in diameter are proposed to be photoactive entities with a higher probability of detection that contribute to the staircase photocurrent response.

Electrochemical exfoliation in real time of natural graphite deposited onto glassy carbon. Doping and modifying carbons through ultra-thin graphite layers

Natural graphite, deposited onto smooth glassy carbon, when reduced (around −2V vs. Ag/AgCl) in the presence of tetraalkylammonium salts (TAAX) in aprotic polar solvents, leads to spectacular exfoliation processes followed in real time in the course of voltammetric scans. The method permits to get extraordinary stable modified carbon surfaces, with quite simple voltammetric responses (1 to 4 reversible cathodic steps) that strongly depend on the nature of the cations used. Those cathodic steps exhibit currents quasi perfectly proportional to the scan rate. It is expected the formation of a graphene-like substrate. Surface modifications both by ferrocene and anthraquinone could underline the originality of those new materials.

Electrochemical activation of diamond microelectrodes: implications for the in vitro measurement of serotonin in the bowel.

The electrochemical pretreatment of diamond microelectrodes was investigated for the purpose of learning how an anodic, cathodic or a combined anodic + cathodic polarization affects the charge-transfer kinetics for two surface-sensitive redox systems: ferri/ferrocyanide and serotonin (5-hydroxytryptamine, 5-HT). The pretreatments were performed in 0.5 mol L(-1) H2SO4. The anodic pretreatment was performed galvanically for 30 s at 250 mA cm(-2). The cathodic pretreatment was performed for 180 s at -250 mA cm(-2). The combined pretreatment involved application of the anodic step first followed by the cathodic step. The results clearly demonstrate that the best performance for both redox systems is obtained after the cathodic polarization, which presumably activates the electrode by cleaning the surface and removing site-blocking surface carbon-oxygen functionalities. The cathodic pretreatment was found to be effective at activating a fouled microelectrode in situ. This observation has important implication for the measurement of 5-HT in the bowel.

Electro-catalytic scission of carbon–iodine bonds. Building of SAMs at gold surfaces through long chain primary radical adsorption

Primary alkyl iodides (RI) are reduced at smooth gold electrodes in polar organic solvents such as N,N-dimethylformamide (DMF) in the presence of tetraalkylammonium salts (TAAX). Beside a main two-electron step currently observed at potential>−2.0V vs. Ag/AgCl, a bell-shaped step appears with all organic iodides within a potential range between −0.6V and −1.2V. This peak does not depend on the organic substrate concentration, its current is proportional to the scan rate in voltammetry; it disappears at the second scan. These features arise from slow adsorption of the RI at gold, followed by an electro-catalytic reduction induced by the metal. It is expected that this (simple) process allows the formation of alkyl radicals readily adsorbed at the metal surface and thus produced layer strongly inhibits the main cathodic step at more negative potentials. With linear chains in C10, C12, C16 and C18, the coverage of gold leads to self-assembled monolayers (SAMs). The compactness of this coverage was found to be about 10−10molcm−2.

Electrodeposition of Cu–Zn thin films from room temperature ionic liquid

The electrosynthesis of Cu, Zn and Cu–Zn deposits from the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid has been investigated. Anion of this metallic salt (bis(trifluoromethylsulfonyl)imide) is the same as that used as the solvent. The redox potential sequence of copper and zinc in the used ionic liquid was the same as that of aqueous solvent. The obtained voltammograms for single metal deposition exhibits several electrodeposition steps. In the case of copper, the first step does not lead to metal deposition and is attributed to the Cu(II)→Cu(I) transformation. Rest of the steps result in the Cu(0). Contrary, all the steps in zinc deposition, correspond to growth of metallic deposits. In the case of the copper–zinc alloys, their chemical composition exhibits a surprising evolution versus potential and seems to be greatly dependent on the cathodic steps. The morphology and analysis of copper, zinc and their alloys are reported.

Porous germanium multilayers

AbstractWe present the reproducible fabrication of porous germanium (PGe) single‐ and multilayers. Mesoporous layers form on heavily doped 4” p‐type Ge wafers by electrochemical etching in highly concentrated HF‐based electrolytes with concentrations in a range of 30‐50 wt.%. Direct PGe formation is accompanied by a constant dissolution of the already‐formed porous layer at the electrolyte/PGe interface, hence yielding a thinner substrate after etching. This effect inhibits multilayer formation as the starting layer is etched while forming the second layer. We avoid dissolution of the porous layer by alternating the etching bias from anodic to cathodic. PGe formation occurs during anodic etching whereas the cathodic step passivates pore walls with H‐atoms and avoids electropolishing. The passivation lasts a limited time depending on the etching current density and electrolyte concentration, necessitating a repetition of the cathodic step at suitable intervals. With optimized alternating bias mesoporous multilayer production is possible. We control the porosity of each single layer by varying the etching current density and the electrolyte (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrochemistry of the antibacterial and antifungal drug nitroxoline and its determination in bulk form, pharmaceutical formulation and human blood

Nitroxoline has been reduced at the mercury electrode in buffered solutions (pH 2−11) in two irreversible cathodic steps. The first step was attributed to reduction of –NO 2 group to the hydroxylamine stage and the second one to reduction-saturation of the C=N double bond. DC-polarographic and various adsorptive stripping voltammetric methods were developed for determination of nitroxoline in bulk form. Limits of quantitation of 1.02 × 10 −6, 3.05 × 10 −8, 9.01 × 10 −9, and 9.12 × 10 −10 M nitroxoline were achieved by means of the developed DC-polarography, differential-pulse-, linear-sweep-, and square-wave-adsorptive cathodic stripping voltammetric methods, respectively. All these electroanalytical methods were successfully applied for determination of nitroxoline in its Nibiol ® tablets. While only the developed adsorptive stripping voltammetry methods were successfully applied for determination of the drug in spiked human serum and for pharmacokinetic studies in real human plasma. The analysis was carried out without interference from common excipients and without the necessity for prior extraction or interaction with any reagent during the analysis.

Silver electrodeposition from water–acetonitrile mixed solvents. Part III—an in situ investigation by optical second harmonic generation spectroscopy

In this work, we report an investigation based on silver electrodeposition from water–acetonitrile mixed solvents onto a polycrystalline Au electrode, based on in situ optical second harmonic generation (SHG) spectroscopy. This paper is the last one of a series attacking the same topic by cyclic voltammetry and potentiostatic current transients (Mele et al., J Solid State Electrochem in press, 1) and in situ surface-enhanced Raman scattering (Mele et al., J Solid State Electrochem in press, 2). SHG intensity transients following the application of potentiostatic cathodic steps have been measured in order to obtain detailed information on the formation of Ag clusters and nuclei during the electrodeposition process. Our SHG data have been rationalised in terms of a simple optical model accounting for SHG enhancement brought about by Ag cluster formation.

Comparison of simulated pulse plating processes to practical experiments

The development of any pulse sequence is a highly knowledge based process. Beside the knowledge of electrochemical electrolyte properties, the tailoring of the actual field line distribution and the local surface potentials via the applied pulse sequence is the main focus of the scientific development. The combination of pulse plating and numerical simulation offers a very high success rate for definition of pulse plating processes. Simulation tools are the only option to study current distribution and potential fields and are applicable for any possible electrolyte system and cell geometry. For developing a pulse plating sequence it is of major importance that the effect of single pulses within a pulse sequence can be calculated stepwise in high precision, taking into account the change of the substrate surface and geometry after each pulse step. Only in this way can the exact influence of cathodic and anodic pulse steps be retrieved. Additionally, the consideration of shieldings and cell geometry can be performed in parallel. Within this work a detailed study of the correlation between experimental data and the results of numerical simulation was performed focusing on the system Watts Nickel on steel. The conformity revealed by this study is excellent. The numerical simulation tool can be used for development and optimisation of pulse plating processes, if a stepwise, shape alteration including mathematical algorithm is used.