Doping-undoping Process Research Articles

Roughened gold electrode for in situ SERS analysis of structural changes accompanying the doping process of polyaniline in acidic aqueous media

Electropolymerization of aniline on suitably roughened gold electrodes enabled us to record in situ surface-enhanced Raman scattering (SERS) spectra of polyaniline (PAni) during the doping-undoping process. Indeed, SERS-activation pretreatment of gold working electrode, using a specific electrochemical oxidation-reduction procedure, leads to the formation of countless surface asperities. The intergranular sites, known as “hot spots”, thus generated by the surface roughness are the seat of a strong electromagnetic coupling which leads to a huge SERS effect. Contrary to the case of platinum or mirror-polished gold electrodes, the use of roughened gold leads to high quality in situ SERS spectra of PAni, allowing the observation of important changes in the spectral features of its different oxidation states. In this study, we relied on spectral decomposition procedures to monitor and try to quantify these transformations accompanying the variation in the oxidation state of the polymer in different acidic aqueous solutions. In particular, the transition between the different forms of PAni (leucoemeraldine → emeraldine → pernigraniline), the drastic passage from benzenoid to quinoid structures of some aniline nuclei and the evolution of the charge-carrying structural defects are investigated despite the limitations imposed by the challenging conditions of the in situ analysis.

Electrochemically roughened silver surface versus fractal leaf-shaped silver crystals for surface-enhanced Raman scattering investigation of polypyrrole

In surface-enhanced Raman scattering (SERS) analyses, the surface condition of the metal substrate plays a decisive role in the quality of the recorded Raman spectra. In this contribution, we will examine the SERS spectra of polypyrrole (PPy) in contact with two types of silver substrates prepared according to two different protocols: (i) electrochemical, leading to Ag surface with coral-like structure, and (ii) photochemical, giving fractal leaf-shaped Ag clusters. On the one hand, PPy films were electrochemically synthesized on massive silver electrodes previously SERS-activated according to the well-known oxidation-reduction cycle (ORC) procedure. The Raman spectra of the obtained samples exhibit significant SERS effect with high signal-to-noise ratio in comparison with those recorded in the same conditions on platinum. This made it possible to perform an ex situ rovibrational characterization of PPy, to determine the orientation and anchoring mode of the polymer chains in the coating/electrode interfacial region, and to monitor, in situ, some structural changes during the electrochemical doping-undoping process. On the other hand, to further improve the SERS enhancement factor, a second photochemical approach has been developed to prepare PPy chains embedded in a fractal structure made of leaf-shaped silver crystals. Here, the size and growth mode of the silver clusters give rise to a larger number of “hot spots” that are responsible for the huge Raman intensity enhancement.

In situ surface-enhanced Raman scattering investigation of poly(5-amino-1,4-naphthoquinone) using roughened electrodes and metal colloids

Electropolymerization of 5-amino-1,4-naphthoquinone on electrochemically roughened gold electrode has been performed by anodic oxidation of the monomer in CH3CN+0.1M LiClO4 electrolytic medium. The cyclic voltammograms recorded during the first stages of electropolymerization clearly demonstrate the competition between the oxidation reactions of the monomer and the working electrode. Thanks to the surface asperities of the roughened electrode and to gold particles embedded within the inner layers of the polymer, the Raman analyses with red laser line led to the observation of surface-enhanced Raman scattering effect. The in situ SERS spectra of poly(5-amino-1,4-naphthoquinone) obtained during the gradual variation of the electrode potential between the reduced and the oxidized states in both directions in CH3CN+LiClO4 medium confirmed the electroactivity and electrochemical stability of the polymer during the doping-undoping process, as well as the reversibility of the transition between the oxidized (CO) and reduced (CO−⋯Li+) forms of the quinone groups. Moreover, SERS analyses were also carried out using metal colloids. In this case, despite the very low polymer concentration (traces) and the mild experimental conditions (weak laser power) used in these experiments, a large amplification of the Raman signal took place. Thanks to the suspension of silver or gold particles, the polymer displays a SERS effect, which greatly improves the signal-to-noise ratio of the Raman spectra, thus allowing to perform a much better vibrational analysis and to determine the orientation and the fixation mode of the polymer on the metal surface.

Enhanced morphology, crystallinity and conductivity of poly (3,4-ethyldioxythiophene)/ErGO composite films by in situ reduction of TrGO partially reduced on PEDOT modified electrode

In this work, novel composites obtained by electrochemical reduction of graphene oxide (ErGO)/poly(3,4-ethylenedioxythiophene), PEDOT, have been prepared by partially reduced (TrGO) graphene oxide reduction inside the polymer. Composites were characterized using different techniques namely, RAMAN, XRD, FESEM, HR-TEM and XPS. The results showed that the conductivity of PEDOT/ErGO significantly improves on TrGO reduction. In fact, doping-undoping process increased 10-fold and the electrochemical response showed a 2-fold increase as compared with PEDOT and PEDOT/TrGO. RAMAN, XRD and XPS allowed corroborating that CO bonds disappeared in PEDOT/ErGO composites suggesting a partial removal of oxygen-containing groups. FESEM and HR-TEM images exhibited a uniform and porous structure of the PEDOT/ErGO composite. On the other hand, the reduction of TrGO, improves the composite specific surface area. As a result, the PEDOT/ErGO composite presents much higher electrochemical response in comparison with PEDOT and PEDOT/TrGO. The electrochemical properties put them into a promising position as a potential material for energy storage devices, e.g. super capacitors, batteries and biosensors, due to the permeable film with defined pore sizes that PEDOT/ErGO showed.

Facile synthesis of amorphous Ni(OH)2 for high-performance supercapacitors via electrochemical assembly in a reverse micelle

The specific and unique amorphous nickel hydroxide nanostructures were synthesized via a simple and straightforward electrochemical way. By simply tuning the soft templates and applying a suitable current density, a specific and unique microstructure is obtained. The diameter of the Ni(OH)2 nano-sphere is greatly influenced by the current density, the sample obtained at 1 mA•cm−2 is about 200nm while the size of that assembled at 6 mA•cm−2 is less than 20nm. However, the Ni(OH)2 film prepared at the current density of 2 mA•cm−2 showed the best electrochemical performance for the highly porous surface morphology which contributed to the uniform morphology obtained at this current density. The maximum specific capacitance can be achieved as high as 1460F•g−1 in 2M NaOH at the scan rate of 2mV•s−1. Such impressive electrochemical properties can be ascribed to the highly porous microstructures and disorder of the amorphous phase which was considered for the better accommodation of repeated volume changes associated with doping-undoping process. The high specific capacitance and remarkable rate capability of amorphous transition metal oxides nanostructures show broad prospect for potential applications in energy conversion and storage devices.

Preparation of PPy/CNT Composite Applications for Supercapacitor Electrode Material

In this paper, the CNT was used as the reinforcing material to overcome the PPy swelling and shrinkage during the doping-undoping process. The electrochemical capacitance is about 25 F/g in organic electrolyte. The PPy/CNT composite (2:1, m/m) was prepared by depositing the polypyrrole onto functionalized multi-walled CNT in aqueous-methanol solution via chemical polymerization. The obtained composite material was characterized by scanning electron microscopy and infrared spectrum. And the electrochemical performance of the composite material was tested by cyclic voltammetry and galvanostatic charging–discharging cycling. The results showed that the obtained polymer was the PPy and sheet; moreover, the PPy was coated on surface of the CNT. The electrochemical capacitance was about 154.5 F/g (organic electrolyte), which was about 6 times that of pure PPy (about 25 F/g).

Polyaniline nanowires on Si surfaces fabricated with DNA templates.

It is essential to put individual, free-standing nanowires onto insulating substrates and integrate them to useful devices. Here we report a strategy for fabrication of conducting polymer nanowires on thermally oxidized Si surfaces by use of DNA as templates. The direct use of stretched and immobilized DNA strands as templates avoids the agglomeration of DNA caused by shielding of charges on DNA when polyaniline/DNA complexes formed in solution. Most importantly, the oriented DNA strands immobilized on the Si surface predetermine the position and the orientation of the nanowires. The approach described here is the first step toward uniting the programmable-assembly ability of DNA with the unique electronic properties of conducting polymers for high-density functional nanodevices. The conductivity of the nanowires is very sensitive to the proton doping-undoping process, suggesting that the nanowires hold great promise for sensitive chemical sensor applications.

Polypyrrole/Polymer Electrolyte Composites Prepared by In Situ Electropolymerization of Pyrrole as Cathode/Electrolyte Material for Facile Electron Transfer at the Solid Interface

Polypyrrole (PPy)/ion-conducting polymer electrolyte (PE) composites are prepared by in situ electropolymerization of pyrrole in a solvent-free PE matrix which is formed by photocross-linking of triacrylated poly(ethylene oxide-co-propylene oxide) (TA). X-ray photoelectron spectroscopy measurements reveal the gradient structure at the PPy/PE interface where PPy is formed with the concentration gradient. For the composite film, a facile electron transfer across the PPy/PE interface is demonstrated by a cyclic voltammogram for the doping-undoping process, and a decrease in the charge-transfer resistance at the interface is proved by an electrochemical impedance analysis. This is in marked contrast to a combined bilayer film of PPy and TA-based PE for which an electron transfer across the interface between two polymer films is not clearly observed. The composite exhibits charge-discharge performance, which suggests a possibility of fabricating solid-state batteries composed of conductive polymer electrode/ion-conductive PE/Li by one-step electrolysis. © 2001 The Electrochemical Society. All rights reserved.

Monitoring of electroinitiated polymerization of aniline by Raman microprobe spectroscopy

It was observed that some water soluble products were formed during the electrochemical synthesis of polyaniline, and Raman spectroscopy can be used to detect those products. The amount of water soluble products increases with increasing electrolysis potential and the peak observed at 1620 cm −1 is characteristic for these products. Rinsing the polyaniline coated electrode with water even once removes the soluble products from the polymer, which indicates that care must be taken in polyaniline characterization studies during washing, solvation and doping-undoping processes. The transition of H 2SO 4-doped polyaniline to undoped form by treating with 1.0 M NaOH takes place very rapidly in high yields which can be followed by in situ Raman spectroscopy.

EQCM and Quartz Crystal Impedance Measurements for the Characterization of Thiophene-Based Conducting Polymers

AbstractEQCM was extensively used to investigate ion-transport phenomena during dopingundoping processes of electronically conducting polymers. Several early studies assumed that the polymer films were rigidly coupled to the quartz crystal so as to relate the mass change to the quartz crystal resonant frequency change via the Sauerbrey equation. However, the rigidity of electronically conducting polymer films is doubtful and it has to be demonstrated. Quartz crystal impedance analysis near the resonance is of paramount importance to get insight into the viscoelastic properties of the film [1] and to avoid misleading in the interpretation of EQCM data.This contribution presents and discusses quartz crystal impedance measurements, performed with a Frequency Response Analyzer in the 9MHz region, and EQCM data collected for a dithienothiophene based polymer.

Redox transformations of electronic polymers as heterogeneous structures

The anodic and cathodic doping-undoping processes of poly-3-methylthiophene (P3MT) and polybithiophene (PBT) were studied in acetonitrilic solutions of tetraethyl- and tetrabutylammonium tetrafluoroborate. It was shown that most of polymer doping charge is involved in quasi-reversible redox transformations: both the doping and undoping processes occur close to the corresponding standard potentials. However, a residual portion of positive or negative doping charge was shown to be conserved in the polymers during their undoping and their redox transformations are strongly hindered. The residual charge formation within non-conducting matrices of the neutral polymers was attributed to the polymer heterogeneity.

Gas transport properties of polyaniline membranes

The transport properties of He, H2, CO2, O2, N2, and CH4 gases in solvent cast, HCl doped, and undoped polyaniline (PANi) membranes were determined. Measurements were carried out at 40 psi pressure from 19°C to 60°C. An excellent correlation was found between the diffusion coefficients and the molecular diameters of gases. The solubility coefficients of gases were found to correlate with their boiling points or critical temperatures. The sepa-ration factors for CO2/N2 and CO2/CH4 are dominated by the high solubility of CO2. These correlations enable us to predict the permeability, diffusion, and solubility coefficients of other gases. After the doping-undoping process, the fluxes of gases with kinetic diameters smaller than 3.5 A increased but those of larger gases decreased. This results in a higher separation factor for a gas pair involving a small gas molecule and a larger one. © 1996 John Wiley & Sons, Inc.

Mixed electronic and ionic conductors: a new route to Nafion®-doped polyaniline

Abstract Nafion® 117-doped polyaniline films were prepared by two chemical methods. The preparation procedures were very similar, differing only in the timing of the doping step, i.e. before or after film preparation. The method involving direct doping of neutral polyaniline in N-methylpyrrolidone by the acid form of Nafion® is better. It enables the preparation of homogeneous films with electronic conductivities up to 0.3 S cm−1 and a doping level of sulfonate groups of up to 30%. Electrochemical characterization reveals an improvement in the polyaniline doping-undoping process kinetics compared with that of pure polyaniline films. The increased metal-insulator transition kinetics is explained by a high lithium diffusion coefficient, of the order of 10−7 cm2 s−1, and a molecular entanglement of both polymeric networks deduced from chronoamperometric experiments. The high cycle life and charge retention of this promising electrode material are demonstrated.

Preparation of π-conjugated poly(thiophene-2,5-diyl), poly(p-phenylene), and related polymers using zerovalent nickel complexes. Linear structure and properties of the π-conjugated polymers

Dehalogenation polycondensation of 2,5-dihalothiophene and 1,4-dihalobenzene affords polythiophene (PTh) and poly(p-phenylene), respectively, with linear structure and high crystallinity in high yields. Vacuum deposition of both polymers gives crystalline films with their main chain essentially perpendicular to the surface of the substrates. Doping of PTh with iodine and FeCl 3 gives semiconducting adducts, and the doping-undoping process of PTh with iodine and hydrazine is reversible from the viewpoint of not only chemical structure but also packing structure in the solid state.

Surface spectroscopic characterization of advanced polymer materials

The potential of a complementary use of X-ray photoelectron spectroscopy and static ToF-SIMS has been exploited with reference to the characterization of electron conducting polymers (polybithiophene) electrochemically synthetized. Particular attention has been focused on the problem of material stability under electrochemical switching. A coherent picture could be drawn showing that irreversible modifications of the surface chemistry occur, strongly dependent on the dryness of the solvent; moreover, the pathway of doping-undoping process can vary noticeably in dependence on the experimental conditions

Evolutionary spectral factor analysis of doping-undoping processes of thin conductive polymer films

Mathematical methods based on evolutionary factor analysis are applied to the doping-undoping processes of poly(3-methylthiophene). Two new methods are developed in this work: differential evolutionary factor analysis (DEFA), which is sensitive to changes in the composition of the mixture during the evolution process, and the projection matrix method, which can be used to annihilate the influence of a known spectrum from a set of measured spectra in order to obtain information about the unknown components.

An in situ infrared study of the electrochemical doping-undoping process of poly(1-naphthol) film

The electrochemical doping-undoping process of poly(1-naphthol) film is studied by potential-step experiments. Infrared spectra are recorded simultaneously. Spectral changes show clearly the structural modifications: oxygen atoms are positively charged during oxidation associated with counterion diffusion from the electrolyte. Reversible behaviour is observed during reduction.

Studies of porous polyacenic semiconductors toward application II. Fundamental electrochemical properties

Fundamental electrochemical measurements on the porous polyacenic semiconductor (PAS) materials reported in our preceding paper I ( Synth. Met., 38 (1990) 169–175) have been performed. It has been found that the charge-storing capacity becomes maximum in the porous PAS sample with a [H]/[C] molar ratio in the range 0.15–0.3. Detailed examination of the electrochemical doping-undoping processes in this sample has revealed a reversibility during these processes utilizing both the p-type and n-type regions.

Dual-Mode Behavior in Doping–Undoping of Polypyrrole with Alkanesulfonate

Abstract Polypyrrole (PPy) films incorporated with a series of 1-alkanesulfonates, RnSO3− (CnH2n+1SO3−, n=3–10), were prepared by the electrochemical polymerization of pyrrole in H2O containing sodium salt of RnSO3−. The mass transfer in the PPy matrix of the RnSO3− incorporated PPy (PPy/RnSO3−) films were investigated by electrochemical measurements. The electrochemical properties of PPy/RnSO3− were affected by the chain length of the incorporated RnSO3−. The incorporation and release of cationic species into the PPy/RnSO3− matrix during an electrochemical oxidation–reduction of PPy were observed and the contribution of cationic species during the doping-undoping process was increased with the dopant size. The PPy/RnSO3− showed a dual-mode behavior during the electrochemical doping–undoping process.

Application of Pyrrole/N-Methylpyrrole Composite Polymer to Cathode Active Material of Lithium Secondary Battery

Abstract An electrochemically prepared polypyrrole(PPy)/poly(N-methylpyrrole)(PMPy) composite polymer film(PPy-PMPy) was used as a cathode active material of a rechargeable lithium battery. The charging-discharging behavior of a Li/LiClO4/(PPy-PMPy) battery was studied and compared with those of Li/LiClO4/PPy and Li/LiClO4/PMPy batteries. In order to rationalize the charging-discharging properties of these batteries, anion doping-undoping processes at PPy, PMPy, and PPy-PMPy electrodes were checked using the usual electrochemical techniques, such as cyclic voltammetry and potential step chronoamperometry. A PPy-PMPy film electrode gave an intermediate conductivity as well as an intermediate redox potential just between those of the PPy and PMPy electrodes. Li/LiClO4/(PPy-PMPy) batteries with thin (0.3 C cm−2-charged) and thick (10 C cm−2-charged) PPy-PMPy films were found to show different charging-discharging characteristics, depending on their film thickness. The IR spectra obtained for a PPy-PMPy film showed that the composition of PPy and PMPy in the resulting PPy-PMPy film was different according to its film thickness and that the PPy content in the PPy-PMPy film became higher with an increase in its film thickness.