Fauze Jacó Anaissi

Electrochemical conditioning of vanadium(V) pentoxide xerogel films

Abstract The electrochemical behavior of layered vanadium(V) pentoxide xerogel films has been studied, focusing on the lithium intercalation and reorganization effects accompanying the electrode conditioning process. The films at the initial condition consist of electrochemically heterogeneous regions, which collapse into a uniform band structure as the lithium ions are inserted into the interlamellar space. At the saturation point, a single broad intervalence-transfer band is observed in the visible–near infrared region, and the electrochemical behavior becomes independent of the diffusion process.

Electrochemical properties of assembled polypyrrole/V2O5 xerogel films

Abstract Mixed vanadium pentoxide xerogel (VXG)/polypyrrole (PPy) films were assembled onto fluorine doped thin oxide glass electrodes (FTO), according to three different sequential coating procedures, as represented by A=FTO/VXG/PPy, B=FTO/(VXG+PPy) and C=FTO/PPy/VXG. Type A films exhibited maximum conductivity in the 500–1000 mV range and relatively fast electrochemical response. No memory effect was observed by chronoamperometry, after alternating their exposition to reducing species such as ferrocene, and then to the pure electrolyte solution. Type B films consisted of a composite material where PPy was generated inside the interlamellar space of VXG. The electrochemical response in this case was intermediary fast, and the conduction window was shifted to the 0–500 mV range. This type of film exhibited a good memory, responding to ferrocene ions in open circuit. Type C films consisted of VXG deposited onto an electropolymerized film of PPy. The conductivity properties in this case exhibited a narrow window around 500 mV and a much slower electrochemical response, with little memory effect. In this case the charging of the external VXG layer is controlled by the intermediate PPy film.

Long-term aging of vanadium(V) oxide xerogel precursor solutions: structural and electrochemical implications

Abstract Long-term aging of polyvanadic acid precursor solutions gives rise to dramatic effects on the structural and electrochemical characteristics of the corresponding V 2 O 5 · n H 2 O xerogel products. Among these, an 880 mV shift of the redox waves has been observed in the cyclic voltammograms of the modified electrodes, prepared by using 1-year-old precursor solutions. The aging effects have also been evaluated from the systematic changes in the FTIR spectra, especially for the VO and VOV vibrational peaks in the 1130–950 and 900–700 cm −1 range, respectively. Based on light scattering measurements, it has been shown that the polymerization of the polyvanadic acid precursor solutions does not stop even after 1 year. A condensation process takes place in parallel, leading to the formation of aggregates involving approximately four polymeric units. The polymerization and aggregation processes have been correlated with the changes in the X-ray diffraction and EIS patterns, as well as with the changes in the electrochemical behavior of the VXG films before and after Li + ion intercalation.

Hybrid polyaniline/bentonite–vanadium(V) oxide nanocomposites

Abstract This work focuses on the preparation and properties of novel ternary composites generated from the redox polymerization of aniline inside the lamellar bentonite–vanadium(V) oxide (BV) matrix. These materials are stable in water and usual organic solvents, and their good electrical conductivity ensures potential applications as electrode modifiers, for analytical and sensor purposes. The incorporation of polyaniline (pani) into the BV matrices, leads to the decay of the charge transfer band at 450 nm and to the rise of a strong band around 650 nm, reflecting the reduction of V V sites, concomitant with the formation of polyaniline, in the emeraldine form. The modest expansion (∼2.5 A) observed in the pani intercalated composites, is consistent with the orientation of the polyaniline chains parallel with the interlamellar planes. On the other hand, the presence of intercalated polymer seems to stabilize the BV framework, minimizing the structural reorganization usually required for the insertion of lithium ions into the matrix. Interestingly, in small amounts, e.g. in BV(pani) 0.7 , polyaniline dramatically increases the conductivity and charge-capacity of the BV matrix; while, increasing amounts of polyaniline lead to an opposing effect.

Iron oxyhydroxide nanostructured in montmorillonite clays: Preparation and characterization.

Akaganéite is a very rare iron oxyhydroxide in nature. It can be obtained by many synthetic routes, but thermohydrolysis is the most common method reported in the literature. In this work, akaganéite-like materials were prepared through the thermohydrolysis of FeCl(3).6H(2)O in water and suspensions containing clay minerals. X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) data show that the clays determine the crystal phase and size of the iron oxyhydroxide crystals. According to XRD and FTIR data, beta-FeO(OH) (akaganéite) is the main metal oxyhydroxide phase. Considering the small basal spacing (d(001)) displacement observed when comparing the XRD patterns of pristine clays with the composites containing beta-FeO(OH), the iron oxyhydroxide should be mostly located on the basal and edge surfaces of the clay minerals. UV-Vis electronic absorption spectra indicate that the preferred phase of the iron oxyhydroxide is determined by the nature of the clay minerals.

Modified electrodes based on mixed bentonite vanadium(V) oxide xerogels

A novel type of composite xerogel can be obtained by the combination of polyvanadic acid and bentonite clay, displaying a characteristic green colour, associated with the presence of vanadium(IV) sites and intervalence-transfer bands in the near-infrared region. The slow evaporation of the xerogel suspensions leads to lamellar solids containing vanadium(V) oxide and silicate layers. In contrast to the vanadium-oxide films, the composite xerogels are not soluble in water, showing a good electrochemical response when applied onto electrode surfaces. Enhanced electrochemical signals are observed in the presence of electroactive species such as the hexaammineruthenium(II) complexes, reflecting their intercalation into the interlamellar space.

Da cor à cor inexistente: uma reflexão sobre espectros eletrônicos e efeitos cromáticos

FROM COLORS TO INEXISTENT COLORS: A REFLECTION ON ELECTRONIC SPECTRA AND CHROMATIC EFFECTS.Inexistent colors have been the inspiring theme of investigation by Israel Pedrosa, a Brazilian artist, who has devoted his life tocreative painting, exploiting the chemical and physical effects associated with light, especially those generated in the light refractiondomain. In this article, by focusing on the electronic spectra of phthalocyanines and gold nanoparticles, we discuss how such effectscan influence the spectroscopic measurements, leading to inexistent bands and transitions.Key words: inexistent colors; refraction colors; Israel Pedrosa.

Characterization and properties of mixed bentonite-vanadium(V) oxide xerogels

Polyvanadic acid interacts with bentonite clay in aqueous colloidal suspension, yielding a green, flocculent material. The slow evaporation of the xerogel suspensions leads to insoluble lamellar solids, exhibiting basal distances of 13.0 A, and a strong intervalence transfer band around 1150 nm. Scanning electron microscopy images show the existence of interconnected nodules containing a uniform distribution of vanadium ions and aluminum-silicates. Evidence of polyvanadate-bentonite interaction is provided by the FTIR spectra, from the appearance of a strong peak at 835 cm-1, tentatively ascribed to a V-O-Si vibration. The constitution of the green composite depends on the relative amounts of bentonite and polyvanadic acid employed in the synthesis, following the approximate composition bentonite.y(V2O5).z(H2O), where y = 1.5–4.0, and z = 5–12, and bentonite ≈ Nax(Al2-xMgx)(OH)2Si4O10. The dramatic changes in the solubility and aggregation properties corroborate the spectroscopic evidence of strong interactions between polyvanadic acid and the bentonite particles.

Preparation and properties of polypyrrole/bentonite/vanadium (V) oxide ternary composites

Abstract The preparation and characterization of new ternary composites formed by the redox/intercalative reaction of pyrrole with the lamellar bentonite/V 2 O 5 composite are reported. The generation of a conducting polymer combined with a mixed semiconductor oxide-clay material provided an interesting material where the conductivity depends upon the polypyrrole content, and on the particular way it influences the Li + ions migration in the films.

Porphyrin doped vanadium pentoxide xerogel as electrode material

Abstract The lamellar composite material, VXG-TMPyP, obtained from the combination of cationic, water-soluble meso-(tetra-4-methylpyridinium)porphyrin (TMPyP) and vanadium pentoxide gel was investigated and employed as electrode modifying material. This material was isolated as a xerogel and characterized by X-ray diffraction, UV-Vis spectroscopy, cyclic voltammetry, spectroelectrochemistry and TG analysis. According to the X-ray diffraction data, the original VXG lamellar matrix framework is kept in the composite, evidencing a topotatic reaction. UV-Vis spectra indicated a strong interaction between VXG and TMPyP leading to the protonation of the porphyrin ring. In contrast with the vanadium oxide xerogel the new material is stable in water. The presence of the cationic porphyrin species in its structure turns it able to incorporate negatively charged ions, such as ferrocyanide and I−. The presence of the I2/I− couple gives rise to a dramatic increase in the reversibility of the VV/IV process and in the charge capacity of the material.