Behavior of acid species during heat treatment and re-anodizing of porous alumina films formed in malonic acid

In the present work infrared spectroscopy, photoluminescence spectral measurements and the potenthiodynamic technique for studying the effect of treatment temperature on compositional and electronic properties of malonic acid alumina films were used. In the course of our studies, it has been proven that heat treatment of malonic acid films at temperatures from 250 up to 400 °C leads to considerable changes in the photoluminescence properties and voltammetric response during their potentiodynamic re-anodizing. We suggest that defects, such as electron traps, in this type of porous anodic films are caused by the atoms of hydrogen (one or two) escaping from the CH₂ groups of the malonic acid species as a result of the heat treatment. The sites of such defects provide pathways for easy electron migration under a high electric field increasing electroconductivity of anodic alumina films. On the contrary, no structural defects responsible for enhanced electroconductivity are observed during thermal splitting of oxalate groups in the oxalic acid alumina films.     

An EQCM investigation of charging RuO2 thin films prepared by the polymeric precursor method

An electrochemical quartz crystal microbalance (EQCM) study of RuO₂ thin films, prepared by the sol-gel precursor method, is presented. The X-ray diffraction (XRD) analysis demonstrates that RuO₂ films were crystallized in the rutile phase and scanning electron microscopy investigations indicated the formation of a smooth surface. Cyclic voltammetry and EQCM studies were performed simultaneously in order to investigate the charging processes of the RuO₂ films in 0.1 M HClO₄. The voltammetric and mass versus potential responses present three well-defined regions associated with the RuO₂ redox couples. Based on these results and on the mass-charge relationships, the corresponding charging mechanisms are proposed. In the potential region governed by the Ru³⁺/Ru⁴⁺ redox couple, the mass-charge relation can be associated with the double-injection of protons and electrons. The other regions correspond to water release and oxyhydroxide species formation during charging.     

Surface modification of gold electrodes with anthraquinone diazonium cations

Electrochemical grafting of anthraquinone (AQ) groups to gold electrodes was carried out in acetonitrile (ACN) and in aqueous acidic media (0.05 M H₂SO₄). For the first time, the covalent attachment of AQ to gold is demonstrated. Electrochemical quartz crystal microbalance (EQCM), atomic force microscopy (AFM) and cyclic voltammetry (CV) were used to characterise the AQ-modified Au electrodes. Electrografting from both solutions containing the corresponding diazonium salt yielded a strongly attached AQ layer. AFM examination showed that a multilayer AQ film was formed. The CV behaviour of the modified electrodes was tested in 0.1 M KOH and a quasi-reversible response was observed.     

Oxygen evolution: the mechanism of formation of porous anodic alumina

Abstract  Aluminium anodization behavior in ammonium sebacate solution (w = 4%) in ethylene glycol, and in several H₃PO₄-containing electrolytes, has been investigated. A new mechanism is proposed for the formation of porous anodic films. The model emphasizes the close relationship between pore generation and oxygen evolution. PO₄ ³⁻ ions incorporated in the anodic films behave as the primary source of avalanche electrons. It is the avalanche electronic current through the barrier film that causes oxygen evolution during anodization. When growth of anodic oxide and oxygen evolution occur simultaneously at the aluminium anode, cavities or pores are formed in the resulting films. Accordingly, the mechanisms of growth of barrier and porous films are not very different in nature. These findings are a decisive new step towards full understanding of the nature of anodic alumina films. Graphical abstract        

Surface-modified anodic TiO2 films for visible light photocurrent response

N-doped titania thin films were prepared by anodic oxidation of titanium sheets and subsequent heat treatment in the presence of urea pyrolysis products at 400 °C. The resulting films are modified predominantly at the surface. They exhibited a significant photocurrent response upon visible light irradiation inducing an incident photon-to-current efficiency of 1.5% at 400 nm. The flatband potential was anodically shifted by 0.2 V as compared to the unmodified film. Photocurrent transients revealed that nitrogen-centered intra-bandgap states, responsible for visible light response, induce also enhanced recombination as indicated by a cathodic “overshoot” after turning off the light. This recombination can be inhibited by the presence of iodide.     

EQCM as a unique tool for determination of ionic fluxes in microporous carbons as a function of surface charge distribution

We have recently discovered (M.D. Levi et al. (2009) [5]) that electrochemical quartz crystal microbalance (EQCM) method can be successfully used to provide unique information on the ions adsorption during reversible cycling of porous carbon electrodes. The present paper extends the proposed approach to the analysis of the ions content in the carbon micropores in contact with binary aqueous CsCl solutions of different concentrations, and with a mixed CsCl + HCl solution. Evidence was found that microporous carbons suitable for supercapacitor applications reveal perm-selectivity failure near the pzc in the binary electrolyte solutions due to competing counter- and co-ions fluxes. Oxygen-containing functionalities are responsible for the development of the positive charge on the carbon surface in contact with mixed acidified solution, imparting the carbon electrode the property of an anion-exchanger, readily revealed by EQCM.     

Underpotential Deposition and Stripping of Lead at Disorganized Monolayer‐Modified Gold Electrodes

Underpotential deposition (UPD) and stripping of Pb²⁺ at thiol‐based disorganized monolayer‐modified gold electrodes was studied by cyclic voltammetry (CV) and electrochemical quartz crystal microgravimetry (EQCM). Electrodes modified with mercaptoacetic acid or mercaptoethane sulfonic acid were studied. Due to the proximity of the potentials for the Pb UPD and thiol reductive desorption, achievement of a UPD‐stripping voltammetry methodology for determination of low concentrations of Pb²⁺ was not successful. However by comparison of the CV and EQCM data and consideration of the possible mass changes per mole electrons transferred in light of the other species present in solution, possible mechanisms are put forward for the deposition and stripping of Pb²⁺ at thiol‐modified electrodes.     

EQCM Study of Iridium Anodic Oxidation in H2SO4 and KOH Solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold‐sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H₂SO₄ and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H₂SO₄ and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V<E<0.8 V followed by regular increase in mass at 0.8 V<E<1.2 V. The cathodic process initially, at 1.2 V>E>0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E<0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F^?1in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F^?1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E<0.8 V is attributable to redox transition Ir(0)→Ir(III), whereas those observed in the case of acidic medium at E>0.8 V should be related to the redox process Ir(0)→Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redox transitions, the gel‐like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.     

Effect of the diffuse double layer on the interpretation of EQCM results in dilute NaClO4 solutions

The response of the electrochemical quartz crystal microbalance (EQCM) in dilute NaClO₄ solutions was studied with gold and iron electrodes during a stepwise increase of the perchlorate concentration. In the range from 10⁻⁴ M to 7.8×10⁻² M, the quartz resonant frequency of the 10 MHz AT cut crystals increased by about 700 Hz, indicating a mass loss on the electrode. A model was developed in which the diffuse double layer and the oscillating bulk electrolyte layer, characterised by the velocity decay length of the damped shear wave in solution, are treated as two independent, superimposed sheets. By assuming a characteristic thickness of the diffuse double layer according to the Gouy–Chapman theory and by treating the diffuse double layer as a rigid sheet, the measured mass loss could be simulated qualitatively. The viscosity changes in the diffuse double layer as well as in the sensed electrolyte bulk layer were found to be negligible in the concentration range investigated. In dilute solutions, the frequency shift following a concentration change is entirely due to thinning of the diffuse double layer with increasing concentration. The results demonstrate the importance of diffuse double layer effects for EQCM measurements in dilute electrolytes.     

Sulfide-enhanced electrochemical capacitance of cobalt hydroxide on nanofibered parent substrate

Two approaches—substrate nanostructuring and incorporation of sulfide—were studied with the aim to increase electrochemical capacitance of cobalt (hydro)oxide. A fiber structure of cobalt was deposited electrochemically with the fibers in the order of tens of nanometers in thickness and hundreds of nanometers in length. Cobalt hydroxide film was formed on the nanostructured substrate by anodic polarization in an alkaline solution. The hydroxide formation and its electrochemical capacitance have been studied by cyclic voltammetry in conjunction with the electrochemical quartz crystal microbalance (EQCM). An irreversible behavior was typical of the first anodic polarization cycle; it turned gradually to a reversible one during subsequent cycling. EQCM measurements indicated exponential electrode mass growth during the first cycle, with subsequent transition to a quasipassive state. The redox transitions Co(II) → Co(III) → Co(IV), which determine pseudocapacitance, did not cause remarkable electrode mass change. The electrochemical capacitance of the nanofiber sample was found up to five times higher when compared to that formed on conventional cobalt (abraded surface). Specifics of “per 1 g” evaluation of capacitance performance is discussed. Measurements showed that about 10% of the entire hydroxide structure took part in the capacitive process. The capacitance value determined per 1 g of active Co(OH)₂ was in agreement with the limiting value predicted by the Faraday’s law (2,421 F g⁻¹) sulfide-enhanced system with 18% CoS exhibited up to three times higher capacitance when compared to that of the sulfide-free counterpart. The system shows promise for practical applications due to its low cost and technical simplicity.     

Estimation of electrochemical quartz crystal microbalance frequencies from cyclic voltammetric data:—underpotential deposition of metals as an illustration

The estimation of electrochemical quartz crystal microbalance (EQCM) frequencies from a given cyclic voltammetric data is analysed using underpotential deposition (UPD) of metals as an illustrative process. The crucial role played by the surface roughness of electrodes and electrosorption valency of the depositing species is pointed out. The computed EQCM frequency shifts for the UPD of Cd and Tl on polycrystalline Ag electrodes are in satisfactory agreement with the experimental data.

Electrochemical deposition of poly(trans-[RuCl2(4-vinylpyridine)4]) and its reductive desorption: cyclic voltammetry and electrochemical quartz crystal microbalance studies

The electropolymerization of trans-[RuCl(2)(vpy)(4)](vpy = 4-vinylpyridine) on Au or Pt electrodes was studied by cyclic voltammetry and the electrochemical quartz crystal microbalance (EQCM) technique. Cyclic voltammetry of the monomer in DMSO on Au shows reductions at -2.0 and -2.2 V. Potential cycling over the first wave leads to polymer formation; however, scanning over the second wave leads to desorption of the polymer. These observations were confirmed by EQCM measurements which also revealed a high polymerization efficiency. Electrolysis, EQCM and XPS measurements showed that desorption was associated with substitution of chloride ligands by DMSO when the polymer was in a highly reduced state. The film also showed reversible mass changes due to the oxidation and accompanying ingress of charge-balancing anions and solvent into the film. Measurements on the dried films revealed that large quantities of solvent are trapped in the film during the electropolymerization process.     

Voltammetric and electrogravimetric study of manganese dioxide thin film electrodes. Part 1. Electrodeposited films

Voltammetry and electrochemical quartz crystal microbalance (EQCM) studies of MnO₂ films electrodeposited in acidic media at 85°C show the existence of matter exchange during repeated electrochemical reduction and oxidation in 1 M KOH solution. Usually, these reactions are written without mentioning the exchange of matter. The following are observed: a reversible exchange involving water between the crystal lattice and the solution within each successive sweep potential cycle; an irreversible mass loss caused by low dissolution and slow diffusion of Mn(III) and Mn(II) solubilized species; an irreversible large mass gain during the first cycle due to additional hydration of the material.     

An EQCM study on the influence of saccharin on the corrosion properties of nanostructured cobalt and cobalt-iron alloy coatings

Nanostructured cobalt (Co) and cobalt-iron (CoFe) alloy coatings were electrodeposited from sulfate solutions in the presence and absence of saccharin. The effects of saccharin on the corrosion behavior of Co and CoFe alloy coatings were investigated using the electrochemical quartz crystal microbalance (EQCM) technique coupled with cyclic voltammetry (CV) measurements. Saccharin was added to the electrolyte as a grain refiner and brightener. Interestingly, opposite corrosion behaviors were found for all nanostructured coatings in 0.1 M H₂SO₄ and 0.1 M NaOH. The use of saccharin as an additive in the plating solution accelerated the anodic reaction for all deposits in acidic medium. The mass decreases while dissolution rate increased with higher saccharin concentration. Meanwhile, formation of a thick passive film on the Co electrode surface were enhanced while a hindering effect was observed for CoFe alloy coatings deposited in the presence of saccharin in alkaline solution. The anodic and cathodic curves obtained from potentiodynamic polarization experiments were also in agreement with the EQCM results.     

Quantized spectroelectrochemical behaviour of monolayer-protected gold cluster films assessed by reflectance spectroelectrochemical quartz crystal microbalance

The spectral changes occurring in multilayer films of hexanethiolate monolayer-protected Au₁₄₇ clusters (C6–Au₁₄₇ MPCs) as a consequence of quantized MPC core charging have been investigated in aqueous solutions using a multiresponse technique, UV–vis reflectance spectroelectrochemical quartz crystal microbalance (SEQCM). The joint technique, a combination of UV–vis near-normal incidence reflectance spectroelectrochemistry and electrochemical quartz crystal microbalance, has enabled us to follow both reflectance and gravimetric changes taking place in the MPC film concurrently with each single electron transfer event. Reversible film reflectance drops were observed upon anodic MPC charging, which were linearly dependent on the MPC charge state. The values of the formal potential and number of electrons transferred in each charging step, determined from the potential dependence of the reflectance changes, proved that the spectral features were induced by the discrete charging of the MPCs. Simultaneously, the gravimetric signal monitored with EQCM yielded values of the number of MPC-bound electrolyte ions as a function of the MPC redox state, both during voltammetric and potential step charging of the MPC films. Additionally, the dynamics of electron transfers in these multilayer MPC films has been investigated by electrochemical impedance spectroscopy (EIS). Thus, the film capacitance, the resistance to charge transfer, and the electron-transfer rate constant for MPC oxidation have been estimated.     

酸性介质中丙烯基硫脲对铜阳极溶出和阴极沉积过程影响的EQCM研究

采用循环伏安(CV)和电化学石英晶体微天平(EQCM)方法研究了酸性介质中铜阳极溶出和阴极沉积过程以及丙烯基硫脲(AT)对该过程的影响. 结果表明, 铜阳极溶出和阴极沉积过程的M/n分别为32.0和34.2 g/mol, 都是两电子过程, 其间未检测到Cu(Ⅰ)中间产物. AT改变了铜阳极溶出和阴极沉积的历程. 在含AT的溶液中, 铜阳极溶出和阴极沉积过程的M/n分别为61.9和65.4 g/mol, 可指认铜阳极溶出产物为CuAT+, 并提出了AT存在下Cu阳极溶出和阴极沉积过程的反应机理; 从电极表面质量定量变化的角度提供了Cu阳极溶出和阴极沉积过程的新数据.     

Electrochemically induced transformations of heteropolyanions: new electroactive metal oxide films studied by the electrochemical quartz crystal microbalance

New oxide films have been electrodeposited from [P₂Mo₁₈O₆₂]⁶⁻ by potential cycling in mildly acidic aqueous media. To obtain an adherent and persistent film, it is necessary that more than six electrons/molecule be fixed on the framework of the heteropolyanion. The film is then studied in pure supporting electrolyte. In this medium, a remarkable current increase is observed during the potential cycling. Whether the film is deposited on a glassy carbon electrode or on the gold electrode of an electrochemical quartz crystal microbalance (EQCM), exactly the same steady current increase up to a maximum is obtained in cyclic voltammetric measurements. The EQCM reveals a steady mass increase during the continuous cycling of the film in the supporting electrolyte. This behaviour is interpreted as featuring an irreversible water and electrolyte intake into the film, up to a maximum, after which the phenomena observed during reduction and oxidation processes are taken as featuring intercalation/deintercalation, respectively. This behaviour is much the same as described in the literature for WO₃ and MoO₃ bronzes, except that the present films seem very stable and have shown no tendency to dissolve or deactivate. Received: 2 December 1998 / Accepted: 26 January 1999     

Electrochemical and microgravimetric characterization of magnetron-sputtered Fe–Cr–Ni–Ta and Fe–Cr–Ni alloy films in neutral and strongly acidic media

The Fe–Cr–Ni and Fe–Cr–Ni–Ta alloy films were deposited on quartz substrates by magnetron-sputtering using targets of AISI 316 stainless steel and in combination with pure tantalum. The conventional melting of the Fe–Cr–Ni–Ta alloy formed is virtually impossible because the melting point of tantalum is higher than the boiling points of the other components. Elemental content of the films was determined by XPS analysis. Corrosion behaviour of both alloy films was studied in 5% NaCl and 10 M HCl by electrochemical quartz crystal microgravimetry (EQCM), electrochemical impedance spectroscopy (EIS) and dc-voltammetry. The corrosion resistance of Fe–Cr–Ni–Ta appeared to be significantly higher than that of Fe–Cr–Ni in both neutral (5% NaCl) and strongly acidic (10 M HCl) media. The Fe–Cr–Ni–Ta specimen exhibited an extremely high corrosion resistance in 10 M HCl, where the corrosion rates were about one order of magnitude lower than those of Fe–Cr–Ni in neutral solution. EQCM measurements in NaCl solution indicated accumulation of corrosion products on the Fe–Cr–Ni–Ta surface, which was evident from a distinctive increase in electrode mass. By contrast, the mass of the tantalum-free alloy film decreased with a constant rate, which indicated alloy dissolution to prevail. The corrosion current calculated from the mass decrease was in good agreement with that derived from voltammetric measurements. The EQCM data showed that the corrosion resistance of the Fe–Cr–Ni–Ta alloy film in 10 M HCl was about two orders of magnitude higher than that of the Fe–Cr–Ni.     

Investigation of Electrosorption of Organic Molecules onto Gold and Nickel Electrodes Using an Electrochemical Quartz Crystal Microbalance

The adsorption behaviors of two organic molecules, benzoic acid (BA) and 2-butyne-1,4-diol (BD), on metal electrodes have been studied using an electrochemical quartz crystal microbalance (EQCM) combined with the cyclic voltammetry technique. In the range of potentials studied, BA molecules were adsorbed onto an electrodeposited gold electrode with a saturation concentration of 5.0 × 10¹⁴ molecules/cm². It was found that the Frumkin isotherm model was most suitable to depict the electrosorption behavior. The isotherm parameters by nonlinear fitting, which agreed with the literature values, implied BA was chemisorbed on the gold surface. For BD on an electrodeposited nickel electrode, the equivalent molar mass of the reaction species was calculated on the basis of the voltammetry curve and mass curve, which were obtained simultaneously during the potential scan. The analysis of EQCM data for the electrosorption of BD on gold and nickel electrodes showed an irreversible characteristic; the latter effectively inhibited the hydrogen evolution reaction.     

Electropolymerization of <Emphasis Type="Italic">trans</Emphasis>-[RuCl<Subscript>2</Subscript>(vpy)<Subscript>4</Subscript>] complex—EQCM and Raman studies

The electropolymerization of trans-[RuCl₂(vpy)₄] (vpy=4-vinylpyridine) on Au or Pt electrodes was studied by cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM) technique, and Raman spectroscopy. Cyclic voltammetry of the monomer at a microelectrode shows the typical Ru(III/II) and Ru(IV/III) waves, together with the vinyl reduction waves at −1.5 and −2.45 V and adsorption wave at −0.8 V. Electrodeposition on EQCM technique performed under potential cycling between −0.9 and −2.0 V revealed that the polymerization proceeded well in advance of the vinyl reduction waves. At potentials more positive than −0.9 V, soluble oligomers were deposited irreversibly on the electrode during the oxidative sweep. The film also showed reversible mass changes due to the oxidation and accompanying ingress of charge-balancing anions and solvent into the film. In contrast, potentiostatic growth of the polymer at −1.6 V was slower because the oligomeric material was lost completely from the electrode. Unreacted vinyl groups were detected by in situ Raman spectroscopy for films grown at −0.7, −0.9, and −1.6 V but were absent when the polymerization was carried out at −2.9 V vs Ag/Ag⁺.