Temperature electrochemically prepared films

Han et al. [64, 66] reported the synthesis of highly conductive and thermally stable self-doped mercaptopropanesulfonic-acid-substituted polyanilines by the concurrent reduction and substitution reaction between polyaniline and a nucleophile. These reactions were carried out on both electrochemically generated and free standing polyaniline films prepared from emeraldine base dissolved in N-methylpyrrolidinone. The electrochemically prepared films were dedoped with 5 % aqueous NaiCOs to convert them the into the emeraldine base form. The sulfonated polyaniline was prepared by reaction of a polyaniline emeraldine base film with 0.1 M 3-mercapto-l-propanesulfonic acid sodium salt in methanol under nitrogen at room temperature for approximately 14h [66]. A catalytic amount (0.01 M) of acetic acid was reported to accelerate the reaction. The resulting sulfonated polyaniline film was thoroughly rinsed with methanol, followed by 5 % aqueous NaiCOs to remove reactants. 

Recently, several interesting studies of the electrochemical properties of electrodes coated with thin films of Nafion have been reported. These chemically modified electrodes are prepared using low-EW polymers which are alcohol soluble, or using a solution of a 1100-EW polymer which has been dissolved at high pressure and temperature. Electrochemical studies for cations such as the Ru(bpy)3 couple yielded estimates of ionic diffusion coefficients in the polymer films. However, results also indicate that these films are far more porous than conventional Nafion membranes, so it is not possible to compare values directly with those discussed above. 

These metallic features have been reported in conducting versions of both PAN and PPy. The ability to process PAN doped with camphorsulfonic acid (CSA) from solution [22,76] has resulted in freestanding films with high conductivity 100—400 S/cm) [44,77] that span the IMT even at low temperature [45,75]. Some samples of electrochemically prepared PPy doped with hexafluorophosphate (PFs) are metallic to millikelvin temperatures [29,41,42]. However, when PPy is synthesized using different dopants or at high temperatures, the materials are more disordered and show insulating behavior [28-30]. Similar results are reported in the Kterature for conducting polyacetylene [41,42]. 

The ruthenium oligothienylacetylide complexes 93 (Chart 5.30) [106] and the oligothienylferrocene complexes 94a and b were electrochemically polymerized [107]. The voltammetry of poly-94a and poly-94b films contains redox waves due to both the ferrocene and backbone redox couples. Low-energy absorption bands appear upon oxidation of both the Fe centers and the conjugated backbone in the UV-Vis-near-IR spectrum of the films, and these have been attributed to charge-transfer processes. The poor solubility of 94b prevents electropolymerization at room temperature however, polymer films can be prepared at elevated temperatures. Electropolymerization of 95, in which hexyl chains have been added to increase the monomer solubility, has also been reported [108]. 

The growth process takes place close to thermodynamic equilibrium and can be well controlled. MBE is usually applied to produce single crystals. The deposits obtained by these methods are of a high quality, but such processes are cost-intensive and need vacuum conditions for preparation thus making the semiconductors quite expensive. An alternative process to the above mentioned techniques is electrochemical deposition, which allows an easy control of the process parameters and which is comparably cheap. In general, the electrochemically deposited films do not possess crystalline perfection. However the structure and the size of the deposit can be adjusted by variation of the parameters (e.g., composition of the electrolyte, electrode potential, current density, and temperature) and thus the semiconductors with the required properties can be fabricated. 

Martinez et al. electrochemically prepared (80-1) and obtained a copperblack free-standing film with a room temperature conductivity of 15 Scm . It contained 0.25 Bp4 ion per monomer unit. The neutral film showed a Amax and bandgap at 496 and 689 nm, respectively. Visible-near IR spectra at various applied potentials were explained by the formation of the polaron and bipolaron. Bragadin et al. polymerized the trans conformer of (80-1) chemically and elctrochemi-cally [138]. A pressed pellet of poly(80-l) showed 40Scm . While an electrochemically prepared polymer contained a small amount of the cis form,... 

The Conditions During the Preparation of the Polymer Film. Electro-polymerized PT films have a more compact morphology in contrast to chemically synthesized PT films [146]. Poly[l,2-bis(3-alkyl-2-thienyl)ethylene] prepared chemically is a bulk powder, in contrast to electrochemically prepared polymers which form homogeneous films (see also Sect. 5.3) [147]. The surface of elec-tropolymerized PTT films is also influenced by the current density. PTT films prepared at a current density of 0.4 mA cm (7.5 min) have typically rough surfaces. PTT films prepared at a current density of 0.05 mA cm (60 min), with the same quantity of electricity, have a compact homogeneous surface [146,148]. These characteristics are independent of the material of the electrodes. PTT films electrochemically prepared at room temperature have a more homogeneous and more compact and smooth surface than at — 5 °C, independently of the current density, with the same quantity of electricity [148]. 

In a UPS study by Fujimoto et al,[8I], the effects of air and temperature on the electronic structure of chemically and electrochemically prepared solution-cast poly(3-alkylthiophene) films were investigated. It was found that the threshold energy was almost identical for the different solution-cast films. However, electrochemically, as-prepared films had a smaller threshold value by about 0.3 eV. Changes in the work functions (of about 0.2 eV) of the samples were observed upon heating in vacuum. Subsequent recovery upon exposure to air was attributed to dedoping and doping effects caused by oxygen. 

Successful electrodeposition of Sb2To3 has been reported for the first time by Leimkiihler et al. [229] who prepared polycrystalline thin films of the material on different transparent conductive oxides, as well as CdTe and Mo, from uncomplexed solutions made by mixing stock solutions of SbCb, Te02, and phthalate buffer (pH 4). The electrochemical process was discussed in detail based on results obtained by cyclic voltammetry on ITO/glass. The bath temperature was found to influence... 

For technical purposes (as well as in the laboratory) RuOz and Ru based thin film electrodes are prepared by thermal decomposition techniques. Chlorides or other salts of the respective metals are dissolved in an aqueous or alcoholic solution, painted onto a valve metal substrate, dried and fired in the presence of air or oxygen. In order to achieve reasonable thicknesses the procedure has to be applied repetitively with a final firing for usually 1 hour at temperatures of around 450°C. A survey of the various processes can be found in Trasatti s book [44], For such thermal decomposition processes it is dangerous to assume that the bulk composition of the final sample is the same as the composition of the starting products. This is especially true for the surface composition. The knowledge of these parameters, however, is of vital importance for a better understanding of the electrochemical performance including stability of the electrode material. 

---