Marie-Claude Bernard

Protonation and oxidation processes in polyaniline thin films studied by optical multichannel analysis and in situ Raman spectroscopy

Abstract In addition to the optical characterization of the different forms displayed by polyaniline (PANI) thin films during their polarization in media of different acidities and in a wide range of potentials, the use of the optical multichannel analysis technique (OMA) allowed us to reach the vry fast kinetics of equilibration in a new form following the change of polarization potential. The formation of either excitons or polarons as a function of pH and the progressive passage from single to bipolaron or large polaron were stidied in particular. The results were compared with those of Raman spectroscopy; it was shown that Raman spectroscopy is particularly suitable to describe the oxdiation mechanisms (passage from benzenoid to quinoid rings), but that the protonation phenomena (passage from base to salt forms) are better analysed by OMA. We were also interested in the electronic properties of PANI. This polymer is generally used in very acidic electrolytic media; at pH = 0 or 1, the colour change mechanisms are essentially related to the chain protonation-deprotonation. The use of a less acidic medium (pH = 3) than those generally employed is a good choice for electronic devices and the properties of films polarized in this medium were compared with the results in very acidic media. Polymer degradation mechanisms during cycling were also studied and explained by the gradual formation of head-to-head couplings.

Characterisation of new nickel hydroxides during the transformation of α Ni(OH)2 to β Ni(OH)2 by ageing

Abstract The aim of this paper is to characterise the compounds formed by the ageing of α NI(OH) 2 in 5 M KOH at 70 °C. The final product of this treatment is known as the β Ni(OH) 2 phase, but with the ageing period, a progressive change of electrochemical reactivity as well as a textural and structural evolution are observed. The products obtained are stable in time once the ageing is stopped and are not a mere mixture of α and β but single phase nickel hydroxides whose properties are intermediate between those of α and β Ni (OH) 2 . Intermediate forms of nickel hydroxide, whose specific features seem to be the crystallites size and the quantity of absorbed nitrate ions, will be called “badly crystallised β” or β bc as proposed by Delmas et al .

Elaboration and study of a PANI/PAMPS/WO3 all solid-state electrochromic device

An all solid-state electrochromic device (ECD) was formed by anodically colored polyaniline (PANI) and cathodically colored tungstic acid, WO3, 2H2O, separated by a protonic conductor solid electrolyte: poly 2-acrylamido 2-methylpropane sulfonic acid, (PAMPS). PANI as well as tungstic acid films were electrochemically deposited on SnO2. The transmission spectra of ECD were displayed using near IR/visible/near UV spectroscopy as well as optical multichannel analysis which allows, in addition, to obtain the kinetics of coloration/bleaching. They were checked after up to 2·104 electrochromic cycles extending from −1.2 to +1.2 V; the change of optical density, ΔOD, the coloration yield and the stability during the cycling were determined. The influence of the experimental parameters (electrolyte pH in the range 0.65–7.65, coloration potential) was studied; good coloration reversibility and memory effect can be obtained up to pH=7.65. A typical device presented a coloration time of ≈30 s and a shorter (<10 s) bleaching time, with ΔOD=0.4 at 2.23 eV (human eye maximum sensitivity), falling to 0.25 after 2·104 cycles. The choice of PAMPS as a solid electrolyte has many advantages: its pH can be set to the best value, rather high, and the structure of the PANI/PAMPS interface allows to explain the presence of conductivity even at high pH. The excellent reversibility of ECD is associated to the presence of an optical free-carriers tail.

Raman spectroscopy for the study of polyaniline

Abstract Raman spectroscopy (RS) is generally used for a qualitative discrimination of the different polyaniline (PANI) forms. The theoretical assignment of the main bands was done satisfactorily in the case of insulating forms, but it is not the same in the case of conductive forms. A new band at ≈ 1330 cm−1 empirically observed during protonation is assumed to correspond to the stretching of C-N bonds. This band, as well as the C=N stretching one must be modified in function of the polaron delocalization. The electropolymerization of PANI allows to form films with variable protonation and/or oxidation levels; the corresponding modifications of the Raman spectra can be scrutinized to progress in the interpretation of these spectra, in order to use this technique for a better analysis of conductive polymers.

Study by optical multichannel analysis of the electrochromic phenomena in polyaniline doped with camphorsulfonic acid

Abstract The possibility of protonation of polyaniline (PANI) by functionalized acids such as camphorsulfonic acid (HCSA) is one of the most outstanding reecnt results about this conducting polymer. Here, optical multichannel analysis (OMA) is used to characterize the optical properties of PANI—CSA during its polarization in an electrochemic cell. Compared with PANI doped in usual mineral acid solutions. PANI—CSA displays a remarkable stability during the electrochromic cyclings.

Analysis of corrosion products beneath an epoxy-amine varnish film

In situ Raman analysis was applied to identify the corrosion products formed on iron coated with epoxy-amine varnish and exposed to sodium chloride solution saturated by hydrogen sulfide or carbon dioxide. The results showed the formation of iron sulfide or iron carbonate layer while electrochemical impedance spectrum predicted the polymer coating exhibiting an almost perfect protecting barrier property. It is concluded therefore, that the corrosion process involves the permeation of water and hydrogen sulfide at molecular state through polymer interstices whereas no ionic conduction is allowed to take place in the coating film. The corrosion process induces the complete delamination of the exposed coating area but as far as its integrity is respected (no macroscopic pores or crevices) the corrosion rate is very low, far from that expected from permeation measurements of H2S.

Structural, optical, and electrical properties of epitaxial titanium oxide thin films on LaAlO3 substrate

Titanium oxide thin films were prepared by pulsed-laser deposition on LaAlO3 single crystal substrate at 700 °C. Pure anatase films are obtained at high oxygen pressure (10−1 mbar), while the rutile phase is evidenced at low oxygen pressure (10−5 mbar) despite a large oxygen deficiency (O/Ti=1.75). From asymmetric x-ray diffraction measurements, the in-plane epitaxial relationships be0tween the substrate and the titanium oxide phases are highlighted. Optical constants (refractive index n and extinction coefficient k) were deduced from ellipsometric measurements. The optical band gap energies of the anatase and rutile films are found to be 3.4 and 3.3 eV, respectively. Since the nearly stoichiometric anatase films are resistive (>103 Ω cm), the large oxygen deficiency in rutile films leads to noticeable increase in the conductivity due to the Ti3+ species, which supply electrons in the conduction band. At low temperature (T<200 K) the resistivity of rutile films versus temperature may be explained by a var...

Electrochromic phenomena in polyaniline films. Effect of pH and influence of the sweep range on cycling lifetimes studied by in situ Raman spectroscopy

Abstract Due to fast color changes and easy preparation and use, polyaniline seems to be a promising material for electrochromic devices, as far as long cycling lifetimes can be obtained. A proper coloration was only observed in acidic media. The influence of pH (between 0 and 3) in a HCl+KCl solution was investigated, using Raman spectroscopy. Samples were cycled with various potential ranges and the part played by the anodic reverse potential on the material lifetime was emphasized. The degradation mechanism can be explained by the gradual formation of head-to-head coupling.

Spectroelectrochemical Characterization of Aniline/Metanilic Acid Copolymers

Films of aniline/metanilic acid copolymers ensure a better protection of iron against corrosion with respect to the protection given by polyaniline. Raman and optical spectroscopies are used to characterize this new material. The confrontation with the polyaniline properties allows to understand the improvement of protection.

Spectroelectrochemical characterization (OMA and Raman) of sulfonic acids : doped polyanilines

Polyaniline films doped with camphorsulfonic acid (PANI-CSA) were prepared by electropolymerization and characterized by in situ Raman spectroscopy. Optical Multichannel Analysis (OMA) was used to characterize the changes of optical properties of PANI-CSA during the cycling in an electrochromic cell. The properties of PANI-CSA colored in 1 M HCSA were compared with those of PANI-CSA and PANI colored in 1M HCl. The striking fact is the increase of the polaron band (isolated polaron at 2.95 eV) during the coloration of PANI-CSA in HCSA. Secondary doping of these films after dissolution in m-cresol can be obtained.