Combined EIS and FTIR–ATR study of water uptake and diffusion in polymer films on semiconducting electrodes

Electrochemical impedance spectroscopy (EIS) and Fourier Transform Infrared Attenuated Total Reflection (FTIR–ATR) Spectroscopy were combined as a spectroelectrochemical method to analyze water uptake of thin polymer films on semiconducting optical crystals.

An oxide covered n-doped silicon crystal was used as optical crystal and an epoxy-amine adhesive was used as polymer film. The combined in-situ analytical approach allowed the analysis of diffusion coefficients of water with two independent measurement techniques, the quantification of the water uptake and its influence on the interphasial composition. The analytical approach is shown to be suitable for defect free polymer films and for defect containing polymer films. The corresponding semiconductor electrochemistry of silicon was evaluated to analyze the film capacitance in series with the semiconductor space charge capacitance.     

Evolution of non-stoichiometric iron sulfide film formed by electrochemical oxidation of carbon steel in alkaline sour environment

Non-stoichiometric iron sulfide films (Fe_xS_y) were formed electrochemically on a 1018 carbon steel/1 M (NH₄)₂S, 500 ppm CN⁻ interface, using cyclic chronoamperometry for different time intervals. The films showed great stability in medium typical of the catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN⁻ as NaCN, pH 8.8). Characterization of the films by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show two different behaviors depending on the growth time. For films grown at times <15 min, oxidation of the film was continuous, while oxidation ability for the film formed at times >15 min was lower. Film characteristics were more clearly defined by EIS experiments, as the Nyquist diagrams show depressive loops with high, real impedance values (Zr>1 kΩ cm²) for films grown at longer times. Structural characterization by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) correlated electrochemical behavior with topographical changes and chemical composition of the films formed. The abundance of sulfur and pyrrhotite is evident in the samples grown for increasing times, and is likely due to electrochemical/chemical oxidation of iron sulfides during film growth. The sulfur-rich layer is responsible for the passive character of these films. The equivalent electrical circuit describing the EIS spectra for films formed over longer times has fewer elements than that used to model EIS spectra for films grown at shorter times. In particular, diffusion of atomic hydrogen is not apparent in sulfur-enriched films, and the charge transfer is carried out at the metal/film interface with values that are insensitive to film thickness and chemical nature.     

The effect of thickness on the composition of passive films on a Ti-50Zr at% alloy

Anodic films were grown potentiodynamically in different electrolytes (pH = 1-14) on a Ti-50Zr at% cast alloy, obtained by fusion in a voltaic arc under argon atmosphere. The thickness of the films was varied by changing formation potential from the open circuit potential up to about 9 V; growth was followed by 30 min stabilization at the forming potential. Films having different thicknesses were characterized by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Moreover, film composition was analyzed by X-ray photoelectron spectroscopy (XPS).Regardless of the anodizing conditions, passive films on the Ti-50Zr at% alloy consist of a single layer mixed oxide phase containing both TiO₂ and ZrO₂ groups. However, an enrichment of Ti within the passive film, increasing with the film thickness, is detected both by PCS and XPS. This leads to concentration profiles of Ti⁴⁺ and Zr⁴⁺ ions along the thickness, and to different electronic properties of very thin films (more insulating) with respect to thicker films (more semiconducting), as revealed by the photocurrent-potential curves.     

Li-intercalation behaviour of vanadium oxide thin film prepared by thermal oxidation of vanadium metal

In order to produce thin films of crystalline V₂O₅, vanadium metal was thermally oxidised at 500 °C under oxygen pressures between 250 and 1000 mbar for 1-5 min. The oxide films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). The lithium intercalation performance of the oxide films was investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). It was shown that the composition, the crystallinity and the related lithium intercalation properties of the thin oxide films were critically dependent on the oxidation conditions. The formation of crystalline V₂O₅ films was stimulated by higher oxygen pressure and longer oxidation time. Exposure for 5 min at 750 mbar O₂ at 500 °C resulted in a surface oxide film composed of V₂O_5, and consisting of crystallites up to 200 nm in lateral size. The thickness of the layer was about 100 nm. This V₂O₅ oxide film was found to have good cycling performance in a potential window between 3.8 and 2.8 V, with a stable capacity of 117 ± 10 mAh/g at an applied current density of 3.4 μA/cm². The diffusion coefficients corresponding to the two plateaus at 3.4 and 3.2 V were determined from the impedance measurements to (5.2 and 3.0) × 10⁻¹³ cm² s⁻¹, respectively. Beneath the V₂O₅ layer, lower oxides (mainly VO₂) were found close to the metal. At lower oxygen pressure and shorter exposure times, the oxide films were less crystalline and the amount of V⁴⁺ increased in the surface oxide film, as revealed by XPS. At intermediate oxygen pressures and exposure times a mixture of crystalline V₂O₅ and V₆O₁₃ was found in the oxide film.     

Commercial polymer films as calibration standards for EIS measurements

The current protocol regarding calibration and troubleshooting of potentiostats used to conduct electrochemical impedance spectroscopy (EIS) measurements on coated metals involves dummy cells made of simple electrical components. This concept has two major limitations: first, the instrument is used at an impedance range it has not been calibrated for—the instrument is calibrated for impedances in the order of ohms, but corrosion protective coatings usually have impedance values in the range of GΩ cm² or higher; second, commercial coating systems do rarely behave like ideal equivalent circuits of electronic components. In view of these limitations, we propose the use of thin polymer films such as PVF or poly ethylene terephthalate (PET) as new calibration standards for EIS measurements. These films are well defined in thickness and composition and give a highly reproducible response when subjected to EIS measurements. In addition, they closely mimic commercial coating systems with regard to target impedance range and electrical characteristics.     

Redox behavior and optical response of nanostructured poly(3,4-ethylenedioxythiophene) films grown in a camphorsulfonic acid based micellar solution

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been electropolymerized from an aqueous micellar solution comprising camphorsulfonic acid (CSA), lithium trifluoromethanesulfonate (LiCF₃SO₃) and EDOT. The inclusion of the dopants CS⁻ and CF₃SO₃⁻ in the polymer structure and an unusually high doping level of 0.54 have been ascertained by the X-ray photoelectron spectroscopy. Transmission electron microscopy and atomic force microscopy studies show that the micellar effect of CSA leads to a morphology wherein polymer particles link together to form elongated shapes and also endows the film with a surface roughness of 25-30 nm. These nanostructures permit a facile intercalation-deintercalation of anions in the film during redox cycling. Electrochemical impedance spectroscopy show that the charge transfer phenomenon at the PEDOT-electrolyte interface is dominant in the high frequency region and diffusion controlled ionic movement prevails in the low frequency regime. The use of these films as potential cathodes in electrochromic windows is rationalized not only on the basis of their high scalability and ease of processing but also due to their large coloration efficiency (123 cm² C⁻¹) and transmission modulation (50%) at a photopic wavelength of 550 nm. But further improvement in color-bleach kinetics and reproducibility of redox behavior is desirable to broaden their spectrum of utility.     

Characterization of conducting poly(3‐methylthiophene) films prepared under sono‐electrochemical conditions

Poly(3‐methlthiophene) films were prepared under “silent” and “sono‐electrochemical” potentiostatic (SEP) conditions. A three‐electrode one‐compartment sono‐cell was used with a working platinum disc electrode. The sono‐electrochemically formed polymer films were deposited with different working electrode‐to‐horn distances. The composition, electrochemical, spectroscopic, and morphological characteristics of the resulting polymer films were determined. Elemental analysis, FTIR‐spectra, and X‐ray photoelectron spectroscopy (XPS) data proved that the polymer films prepared under SEP conditions have predominant α‐α′‐couplings between the 3MT units, and the aromatic ring integrity is maintained in the film. Scanning electron microscopy showed that those films are more compact and less porous compared to the films prepared under silent conditions. The use of sono‐irradiation during electropolymerization enhanced the diffusion of the monomer units towards the electrode surface and resulted in relatively less doped polymers with less conductivity. Electrochemical impedance spectroscopy (EIS) data for films prepared under silent and SEP conditions were collected in a monomer‐free solution. The results show that the impedance of SEP films is relatively higher than those prepared under silent conditions, and a combination of charge transfer kinetics with diffusion‐controlled conduction mechanism within the films. The diffusion was found to be a function of the porosity of the film. Conductivity measurements are in good agreement with EIS, elemental analysis, and XPS data. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2416–2425, 2006     

Influence of the solution cation mobility on the water uptake estimation of PVC Plastisol freestanding films by EIS

In this work, the influence of the outer solution cation mobility on the electrochemical behaviour of PVC Plastisol freestanding films was studied. For that purpose, the paint films were immersed in 0.5 M and 0.05 M chloride solutions with different cations (Na⁺, K⁺, Cs⁺). The electrochemical impedance spectroscopy (EIS) was the technique chosen for this study and estimated values of film capacitance and resistance were calculated.For PVC Plastisol films, it was observed that the ion diffusion depends on the outer solution concentration. To explain such results different mechanisms for the ionic diffusion were considered. For higher concentration solutions the ions of the outer solution must diffuse into the film, while for lower concentration solutions the dissolution inside the film occurs followed by ionic diffusion from the film to the outer solution.The water uptake was determined by EIS and gravimetry techniques. The Brasher and Kingsbury (BK) and the low frequency solution (LF) models were used to estimate this parameter by the EIS technique. A direct dependency between water uptake and solution concentration was obtained by this technique, while by gravimetry an inverse relation was obtained. However, the LF model presented water uptake values better correlated with the gravimetric ones comparatively to the BK equation. For the more concentrated solutions, the cation mobility influenced the water uptake estimated by EIS, whereas, such dependency was not observed with the gravimetric technique.The results obtained in this work suggest that the outer solution resistivity and consequently the film resistance influence the water uptake measurements and should take part in any improved model used for water uptake determination.     

Plasma-polymerized coatings of trimethylsilane deposited on cold-rolled steel substrates Part 2. Effect of deposition conditions on corrosion performance

Trimethyl silane (TMS) plasma-polymerized films were deposited on cold-rolled steel (CRS) under different conditions. The films were characterized by angular-dependent X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and time-of flight secondary ion mass spectrometry (TOFSIMS). The complementary information generated by these surface techniques gave detailed information on the film composition. The corrosion performance of the plasma films was estimated by electrochemical impedance spectroscopy (EIS) and by exposure in a humidity test. All films were Si-based and their composition was a function of the deposition conditions and the plasma cleaning of substrate prior to deposition. A reducing plasma for metal surface treatment resulted in a film with the highest impedance. The plasma film surfaces were highly oxidized. The contact angle was the lowest for plasma films deposited from a mixture of TMS and oxygen and their corrosion performance was the poorest.     

Electrochemically grown passive films on carbon steel (SAE 1018) in alkaline sour medium

Corrosion films were prepared by applying cyclic potential pulses to the 1018 carbon steel-sour medium interface (1 M (NH₄)₂S, 500 ppm CN⁻) for 1 min. Electrochemical behavior and surface morphology of these films were determined using electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and scanning photoelectrochemical microscopy (SPECM). EIS diagrams and SPECM images show the passive properties and homogeneity of the films. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize their chemical nature and structure. XPS results show that different oxide and sulfur structures were developed during the electrochemical oxidation of carbon steel in concentrated sour media. The analysis of O 1s data indicated that, during film growth, H₂O and/or hydroxyl groups are incorporated into the film structure. The XPS spectra of Fe 2p show iron bonds with S as iron sulfide (FeS₂ and FeS) and the corresponding peak of O 1s shows those bonds with oxygen as Fe₂O₃ and/or FeO. XPS depth profile analyses for the film showed that the ratio of FeS and FeO increases from film surface to film-carbon steel interface. This corroborates the diffusion of iron ions through the film during its electrochemical growth. The chemical shift through the film for the peak associated with Fe 2p signal proves that transport mechanism of iron ions through the film is carried out by chemical diffusion.     

Spectroelectrochemical characterisation of copper salen-based polymer-modified electrodes

Electrogenerated polymers based on copper salen-type complexes were characterised electrochemically and by in situ UV-vis and ex situ EPR spectroscopy. The films, poly[Cu(salen)] and poly[Cu(saltMe)], exhibit reversible oxidative electrochemical behaviour in a wide potential range (0.0-1.5 V). Different regimes for charge transport behaviour are accessed by manipulation of film thickness and experimental time scale: thin films (surface concentration, Γ < ca. 80 nmol cm⁻²) show thin-layer/surface behaviour in the scan rate range used (0.020-2.0 V s⁻¹), whereas thicker polymers (Γ > ca. 90 nmol cm⁻²) exhibit a changeover from thin-layer to diffusion control regime at a critical scan rate that depends on polymer and film thickness: 0.15-0.20 V s⁻¹ for poly[Cu(salen)], 90 < Γ < 130 nmol cm⁻² and 0.20-0.30 V s⁻¹ for poly[Cu(saltMe)], 170 < Γ < 230 nmol cm⁻².UV-vis and EPR spectroscopies have allowed the characterisation of electronic states in the reduced and oxidised forms. The role of the copper atom during film oxidation was probed by combining UV-vis data with EPR on copolymers of the copper and nickel complexes. Data from both techniques are consistent and indicate that polymerisation and redox switching are associated with ligand-based processes. EPR of Ni-doped Cu polymers provided evidence for the non-involvement of the metal centre in polymer oxidation; like the analogous nickel polymers, copper polymers behave like delocalised π-system (‘conducting’) rather than discrete site (‘redox’) polymers.     

Polyaniline as cathodic material for electrochemical energy sources: The role of morphology

Polyaniline layers of different morphologies ranging from open and “sponge-like” structures to compact and “pebble-like” surfaces were synthesized from perchlorate solutions and employed as cathode in the galvanic cell with Zn anode and NH₄Cl/ZnCl₂ electrolyte. Cathodic properties of synthesized layers were investigated by the constant current charging/discharging method in 500 cycles. Specific charge capacities and specific energies obtained form the current-time curves strongly depend on the morphology of investigated layers and discharge conditions. The results unambiguously show that charging/discharging reaction of polyaniline layers is limited to relatively thin layer at polymer/solution boundary. Specific charge capacities are inversely related to both the polymer thickness and the discharge current density. In the limit of zero current densities the specific charge capacity as high as 245 A h kg⁻¹ could be achieved for porous structures of polyaniline layers. Specific capacitance higher than 400 F g⁻¹ obtained at 2 mA cm⁻² current density makes polyaniline a promising material for the application in electrochemical supercapacitors. The electrochemical behaviour of the layers was investigated by cyclic voltammetry and electrochemical impedance spectroscopy before and after 500 cycles of charging/discharging experiments. Both, cyclic voltammetry and electrochemical impedance spectroscopy showed that some polyaniline layers develop an increased charged transfer resistance at the carbon support/polymer interface during charging/discharging process. The increased charge transfer resistance does not affect the overall specific charge of the layers. The low-frequency capacities in impedance spectra are attributed to charging/discharging of polymer/electrolyte interface and seem to be related to the specific charge capacities obtained by extrapolation to zero current density discharge reaction.     

Corrosion protection of sol–gel coatings doped with an organic corrosion inhibitor: Chloranil

Hybrid sol–gel films have been prepared with diethoxydimethylsilane (DEODMS), methyltriethoxysilane (MTEOS) as a source of hydrolysable silane and tetra-propoxyzirconium (TPOZ) as a source of hydrolysable zirconium. In order to improve corrosion protection, an organic corrosion inhibitor: tetrachloro-p-benzoquinone (chloranil), has been incorporated into the sol–gel matrix. The effect of chloranil, added with various concentrations from 3 to 12 × 10⁻⁴ M, on the sol–gel film morphology and composition has been examined by atomic force microscopy (AFM) and glow discharge optical emission spectroscopy (GDOES), respectively. Addition of high additive concentrations (>9 × 10⁻⁴ M) strongly disorganised the sol–gel network. The anticorrosion properties of the doped sol–gel films have been characterised by electrochemical impedance spectroscopy (EIS) in chloride solution and have been compared to salt spray observations. Chloranil additions (<9 × 10⁻⁴ M) have significantly increased the corrosion protection of the sol–gel layers for a long term.     

Influence of counter ions in electrochemical properties and kinetic parameters of poly tyramine electroactive film

This work present that the electrodeposited poly tyramine (PTy) film by using potentiostatic method on the surface of the graphite electrode is fractal object with semiconductor properties. Electrosynthesized polymers in the presence of the different anions have been characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The effect of different anions and electrosynthesis parameters on the morphology, kinetic parameters and fractal dimension of the polymer was evaluated. Diffusion coefficient of counter ions calculated from the slope of the Warburg line in the Nyquist plot. A value of γ that is the characteristic of anomalous diffusion is obtained by EIS method and then the relation between fractal dimension and anomalous diffusion has been investigated.     

In situ capacitance measurements for in-plane water vapor transport in paint films

A capacitance technique has been adapted to study in-plane water vapor transport in paint films. The technique requires an application of electrical contact materials on the paint film surface for capacitance measurements by electrochemical impedance spectroscopy (EIS). The capacitance obtained by EIS using Cu tape and Ag paste as the contact materials are presented. A direct comparison of capacitance and gravimetric measurements demonstrates that the change in the coating capacitance is quantitatively correlated with the total amount of in-plane water vapor transported in paint films. The water vapor diffusion coefficient derived from the capacitance technique agrees with one from the gravimetric method.     

Optical,Electrochemical, and Structural Properties of Spray Coated Dihexyl Substituted Poly (3,4 Propylene Dioxythiophene) Film for Optoelectronics Devices

The dihexyl substituted poly (3,4-propylenedioxythiophene) (PProDOT-Hx₂) thin films uniformly deposited by cost effective spray coating technique on transparent conducting oxide coated substrates. The electro-optical properties of PProDOT-Hx₂ films were studied by UV-Vis spectroscopy that shows the color contrast about 45% with coloration efficiency of ～ 185 cm²/C. The electrochemical properties of PProDOT-Hx₂ films were studied by cyclic voltammetry and AC impedance techniques. The cyclic voltammogram shows that redox reaction of films are diffusion controlled and ions transportation will be faster on the polymer film at higher scan rate. Impedance spectra indicate that polymer films are showing interface charge transfer process as well as capacitive behavior between the electrode and electrolyte. The XRD of the PProDOT-Hx₂ thin films revealed that the films are in amorphous nature, which accelerates the transportation of ions during redox process.     

Analysis of ion diffusion and charging in electronically conducting polydicarbazole films by impedance methods

An electrochemical impedance analysis of the doping kinetics of polydicarbazole films is reported. Polymer films of varying thickness were analyzed using an impedance model that considers spatially-restricted diffusion of ionic species. The main bulk parameters for diffusion and charge accumulation during doping were determined from fits. These parameters resulted independent of film thickness after considering the experimental error. The equilibrium (bulk) capacitance C₀ varies in the range of 100-800 F cm⁻³. The chemical diffusion coefficient D varies within the range of 10⁻¹⁰ to 10⁻⁸ cm² s⁻¹ and increases as the steady-state potential reaches the oxidation peak potential.     

Corrosion behavior of NiTi alloy in fetal bovine serum

The corrosion behavior of NiTi alloy in fetal bovine serum (FBS) at 37 °C is investigated using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS). The presence of FBS moves the OCP to the negative direction and makes the oxide film thinner and more porous than that in phosphate buffer saline (PBS, pH = 7.4). The impedance of the oxide film formed in FBS is smaller than that in PBS, but the total interface impedance is bigger in FBS because of the formation of a surface bio-film. Pits form on the NiTi alloy after immersion in FBS for 200 h but they are not observed on the sample immersed in PBS. XPS shows that the composition of the oxide film formed in FBS is similar to that formed in PBS and it is composed of mainly Ti oxides with a small amount of Ti hydroxide. Hydrated Ti is observed on the outermost surface of the NiTi alloy. The thickness of the oxide film on the NiTi alloy immersed in PBS is 17 ± 3.0 nm and that on the sample immersed in FBS is 10 ± 3.2 nm. The results are consistent with those obtained by EIS. The presence of FBS can accelerate leaching of Ni ions and the mechanism is investigated and discussed.     

Complex microstructures of ABC triblock copolymer thin films directed by polymer brushes based on self-consistent field theory

The morphology and the phase diagram of ABC triblock copolymer thin film directed by polymer brushes are investigated by the self-consistent field theory in three dimensions. The polymer brushes coated on the substrate can be used as a good soft template to tailor the morphology of the block copolymer thin films compared with those on the hard substrates. The polymer brush is identical with the middle block B. By continuously changing the composition of the block copolymer, the phase diagrams are constructed for three cases with the fixed film thickness and the brush density: identical interaction parameters, frustrated and non-frustrated cases. Some ordered complex morphologies are observed: parallel lamellar phase with hexagonally packed pores at surfaces (LAM₃ ^ll -HFs), perpendicular lamellar phase with cylinders at the interface (LAM^⊥-CI), and perpendicular hexagonally packed cylinders phase with rings at the interface (C₂^⊥-RI). A desired direction (perpendicular or parallel to the coated surfaces) of lamellar phases or cylindrical phases can be obtained by varying the composition and the interactions between different blocks. The phase diagram of ABC triblock copolymer thin film wetted between the polymer brush-coated surfaces is very useful in designing the directed pattern of ABC triblock copolymer thin film.     

单极脉冲电合成聚苯胺膜及其超级电容性能

采用单极脉冲电化学聚合方法在铂基体表面制备了具有三维网络结构的聚苯胺(PANI)纳米纤维膜。采用傅里叶变换红外(FT-IR)光谱和扫描电镜(SEM)分析比较了脉冲聚合和循环伏安聚合膜的组成和形貌,在0.5mol·L-1的硫酸溶液中通过恒电流充放电和电化学阻抗(EIS)等考察了聚合膜的电化学容量性能。研究表明,固定脉冲电压1.0V、苯胺单体浓度0.2mol·L-1时控制单极脉冲频率1.25s-1、占空比50%可获得高达699F·g-1比电容的纳米纤维结构PANI膜;聚合膜呈现均匀的三维有序网络状分布,其离子交换容量、循环稳定性及超级电容性能远高于循环伏安法聚合的PANI膜。