S.I. Córdoba de Torresi

Electrochromic Reactions in Manganese Oxides I . Raman Analysis

Like nickel oxide, manganese oxide is a widely studied material in the primary batteries field. The reactions taking place during voltametric cycling of manganese oxides can be determined using in situ Raman spectroscopy. The main difficulty for the oxide identification is to obtain relevant Raman reference spectra because of the many possible compounds and, for some of these compounds, of their instability in the laser beam. As a consequence, several modifications of different tetra-, tri- and divalent manganese oxides and oxyhydroxides were carefully studied. The electrochromic behavior of three types of manganese oxides, two prepared by thermal oxidations and the other by electrochemical deposition, were then compared. The presence of nonstoichiometry in the pristine material was necessary to obtain a reversible electrochromic effect. The reaction during electrochromic cycling is more complicated than a simple passage from MnO[sub 2] to MnOOH.

Secondary doping of polyaniline studied by resonance Raman spectroscopy

Secondary doping of polyaniline (PANI) was explained by a change in the conformation of the polymeric chains from a coil-like to an extended structure, leading to an increase in the conductivity and in the crystallinity of polyaniline doped with functionalized organic acids. In this work, resonance Raman spectroscopy is used to characterize the secondary doping process aiming to understand the transformations in the different types of charged segments. Raman results give complementary data about the secondary doping process showing that together with the structural change of the polymeric chains there is also a chemical transformation of quinoid segments into semiquinone units. This fact was put in evidence by the careful analysis of Raman spectra at different excitation lines where different vibrational modes of chromophore groups are enhanced. This phenomena was also characterized by electron spin resonance (ESR) measurements that evidences an increase in the number of spins of PANI/CSA samples after the secondary doping process. Experimental results obtained by both techniques, resonance Raman spectroscopy and ESR, indicate the transformation of diamagnetic quinoid segments into radical cation semiquinone units.

Electrochromism of WO3 in acid solutions: An electrochemical, optical and electrogravimetric study

Abstract The electrochromic reaction of a tungsten trioxide electrode prepared by electrochemical deposition was studied. Chronopotentiometric experiments associated with transmittance measurements were carried out for several film thicknesses. The dependence of electrochromic efficiency on the coloration/bleaching current density, the wavelength of the light source and the film thickness were analysed. The mass and structural changes due to the electrochromic reaction were followed using a quartz crystal microbalance and stress experiments respectively. Film formation and dissolution processes for WO 3 in acidic media are also discussed.

The effect of Cd, Co, and Zn as additives on nickel hydroxide opto-electrochemical behavior

Abstract Nickel hydroxide films were coprecipitated with the additives Cd(OH) 2 , Co(OH) 2 , and Zn(OH) 2 by galvanostatic electrodeposition process. The effects of the Cd(OH) 2 , Co(OH) 2 , and Zn(OH) 2 additions on electrode properties such as charge–discharge, reversibility of the electrode reaction, espectroelectrochemical behavior, superficial morphology by atomic force microscopy (AFM) images, were studied. An anomalous coprecipitation of Zn(OH) 2 was observed. The different concentration of zinc nitrate on the solution affects the amount of zinc hydroxide found on the film; this behavior must be attributed to the differences on the solubility products ( K ps ) of the nickel and zinc hydroxides and also to the remarkable zinc amphoteric character. All additives studied displaced the oxygen evolution reaction (OER) to more positive values, and reduced the difference between the oxidation and reduction peaks (Ni(II)/Ni(III)), leading to an augmentation of the process reversibility. However, a significant loss on the electrode efficiency was observed for composite films with zinc. Lithium hydroxide solution as electrolyte showed an augmentation of the difference between the peak potential for the oxidation process and the oxygen evolution reaction, and also increased the reversibility of the redox nickel process. AFM images showed similar superficial morphologies for nickel hydroxide films and films with cobalt; on the other hand, films containing cadmium and zinc showed an inhomogeneous morphology. Nickel hydroxide film shows a differentiated spectroelectrochemical behavior when compared with composite films.

Raman characterization of polyaniline induced conformational changes

Abstract Conformational changes induced by temperature and secondary doping process, were followed by Raman resonance spectroscopy and ESR measurements. Raman results give complementary data about the secondary doping process showing that together with the structural change of the polymeric chains, there is also a chemical transformation of quinoid segments into scmiquinoid units. When the temperature is raised, changes in Raman spectra arc coincident with those observed in secondarily doped samples, showing in this way, the importance of polymer conformation in the conductive behaviour. ESR measurements confirm the increasing amount of free radicals when the PANI/CSA films are treated with m-cresol in good agreement with Raman results.

Electrochromic behaviour of manganese dioxide electrodes in slightly alkaline solutions

Abstract Manganese dioxide films prepared by electrochemical deposition onto transparent substrates were studied in borate solutions (pH = 9.2). In this paper emphasis will be laid on the electrochromic properties of such electrodes. Potetiostatic experiments associated with transmittance measurements were carried out for both monochromatic and white light sources. The dependence of the electrochromic efficiency parameter on the potential and the wavelength of the incident light was analysed. The loss of the electrochromic properties related to the value of the bleaching potential is also discussed.

Influence of dopant, pH and potential on the spectral changes of poly(o-methoxyaniline): relationship with the redox processes

Abstract Poly( o -methoxyaniline) films doped with functionalized acids show high solubility in conventional organic solvents and a high optical contrast between the reduced and the oxidized forms. They are also more stable towards thermal treatment than when they are doped with HCl. These characteristics make them suitable for use in electrochromic displays. The electrochromic behavior of this soluble derivative of polyaniline was investigated in detail by comparing the curves obtained by the differentiation of absorbance at fixed wavelengths with simultaneously obtained voltammograms. This analysis allows the chromophore species producing color changes to be related to the redox processes taking place during the electrochemical cycling. Molar absorptivities were calculated and the effect of pH on the absorption speetra of the emeraldine form of the polymer is also reported.

On the electrochemical and spectroscopic properties of a soluble polyaniline parent copolymer

Abstract Copolymers of aniline and 2-ethylaniline can be easily formed by electrochemical or chemical oxidation in acidic medium, but the composition of the polymeric chain differs from the composition of the synthesis solution. Copolymers formed from precursor solutions containing less than 30% (in moles) of 2-ethylaniline show intermediate properties between those of the homopolymers, and these properties vary gradually with the amount of 2-ethylaniline units in the copolymeric chain. The properties of those copolymers have been studied by UV–Vis and resonance Raman spectroscopies, cyclic voltammetry, two-probe conductivity and solubility measurements. Results obtained by these techniques corroborate the formation of a block copolymeric material.

Quartz crystal microbalance characterization of electrochemical doping of polyaniline films

Abstract The electrochemical ionic exchange of polyaniline film-coated electrodes was studied in detail by electrogravimetry using an electrochemical quartz crystal microbalance. The influence of film thickness, electrolyte composition and potential sweep rate is discussed. The data obtained from these measurements establish the dependence of ion population inside the film on the degree of oxidation from −0.25 to 0.5 V (versus SCE). Analysis is made by calculating the relative contribution of anions and cations to the polymer electroneutrality from the electrochemical quartz crystal microbalance and electric charge data.

On the Template Synthesis of Nanostructured Inorganic/Organic Hybrid Films. Suitable Platforms for Biosensors

Prussian Blue has been introduced as a mediator to achieve stable, sensitive, reproducible, and interference-free biosensors. However, Na + , Li + , H + , and all group II cations are capable to block the activity of Prussian Blue and, because Na + can be found in most human fluids, Prussian Blue analogs have already been developed to overcome this problem. These analogs, such as copper hexacyanoferrate, have also been introduced in a conducting polypyrrole matrix to create hybrid materials (copper hexacyanoferrate/polypyrrole, CuHCNFe/Ppy) with improved mechanical and electrochemical characteristics. Nowadays, the challenges in amperometric enzymatic biosensors consist of improving the enzyme immobilization and in making the chemical signal transduction more efficient. The incorporation of nanostructured materials in biosensors can optimize both steps and a nanostructured hybrid CuHCNFe/Ppy mediator has been developed using a template of colloidal polystyrene particles. The nanostructured material has achieved sensitivities 7.6 times higher than the bulk film during H 2 O 2 detection and it has also presented better results in other analytical parameters such as time response and detection limit. Besides, the nanostructured mediator was successfully applied at glucose biosensing in electrolytes containing Prussian Blue blocking cations.