Polyaniline film thickness

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

On both Pt and polyaniline-coated electrodes, diffusion-limited currents are observed at <250 mV for Fe(CN)500 mV for Fe(CN)g- oxidation. Potentiostatic EHD impedance was measured on both diffusion plateaux (50 mV and 550 mV), using Pt electrodes coated with polyaniline films of various thickness 50 and 130nm. As an example, the results obtained on the cathodic plateau are shown in Fig. 6-13 those observed on the anodic plateau were very similar [93]. 

Consider the polymer-on-metal interface, which might be prepared by coating a thin metal film with polymer in a polymer-based LED. The case of the counter electrode, formed by vapor-deposition, is discussed subsequently. First, assume that the substrates have clean surfaces hydrocarbon and oxide free, or naturally oxidized but still hydrocarbon free (pointed out as necessary). Typically, in connection with polymer-based LEDs, the metallic substrate could be gold, ITO (indium tin oxide) coated glass, the clean natural oxide of aluminum ( 20 A in thickness), the natural oxide which forms upon freshly etched Si( 110) wafers ( 10 A), or possibly even a polyaniline film. Dirt , which may be either a problem or an advantage, will not be taken up here. Discussions will alternate between coated polymer films and condensed model molecular solid films, as necessary to illustrate points. 

In a recent paper, Coutanceau et al. made a detailed kinetic study of the ORR on Pt particles dispersed in a polyaniline film with several Pt loadings ranging from a fewpgcm to a few hundred pgcm [20]. The PAni film (0.5-pm thickness) was... 

Figure 10.5 Electrocatalytic oxidation of 1 M CH3OH in 0.5 M H2SO4 at a polyaniline film (1 /an thickness) grown on a gold electrode and modified by different amounts of dispersed platinum v=5mV s T = 25°C . (...) no platinum (—) 33 fig cm (—) 69 fig cm . (Reprinted with permission from ref 62)...

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

Figures. Vis/uv spectra of polyaniline.HCl deposited (dipping time 5.5 minutes) on (A) a hydrophobic glass surface (film thickness 1200 A) and (B) a hydrophilic glass surface. (Both spectra were recorded vs. as-receiv glass microscope slides in the spectrometer reference beam).

In an extension of their Prussian Blue study, Ogura and coworkers [110] prepared bilayers in which a polyandine film was overlaid onto a Prussian Blue inner layer. The interesting feature here is that, on charge grounds, polyaruline (see Sect. 2.7.3.7.2) is a barrier to the normal cation-transfer-based electroneutrahty mechanism exhibited by the Prussian Blue/Everitt s salt redox chemistry (see above). For thin polyaruline outer layers, the cation-transfer mechanism is achieved, but thick polyaniline films preclude this. At intermediate outer layer thicknesses, there is a more complex mechanism in which redox-driven K" " cation expulsion (on oxidation of the Everitt s salt form of the irmer layer) from the inner layer to the outer layer is electrically balanced by Cl anion entry into the outer layer. Further oxidation of the bilayer (oxidizing the polyaniline) results in expulsion of the K" " ( inherited from the inner layer), the Cl normally incorporated in this polyaniline oxidation process already being present to neutralize... 

Fig. 24 Variation of EQCM resonant frequency with simultaneously ellipsometrically determined film thickness during deposition of polyaniline. Electrode Ron 5-M Hz AT-cut quartz crystal. Solution ...

The sensitivity of conventional polyaniline films to vapors is strongly thickness-dependent. Generally thinner films result in better performance. As shown in Figure 7.27, when the thickness is decreased fi-om 1.0 to 0.3 p,m, the response time increases, and the magnitude of the response at a fixed time increases by more than five orders of magnitude. Since only the outermost surface is likely to interact with the vapor molecules, thicker films have more inactive material that does not contribute to tbe sensing process at short times. Therefore, the performance of conventional polyaniline thin film sensors is limited by the thickness of the underlying polyaniline film [56,62]. 

However, if the film is composed of long nanofibers (Figure 7.30b), is determined by the diameter of the nanofibers, rather than the film thickness. This may explain the thickness effects observed in the experiments. Since the surface area per unit mass of the nanofiber films is generally much greater than that of the conventional films (unless the thickness is reduced to less than the nanofiber diameter), a nanofiber sensor should have much better sensitivity than a conventional polyaniline sensor. 

HCl doped polyaniline films can be used as base sensors. When these fully HCl doped polyaniline films are exposed to ammonia vapor, a drop in conductivity is observed. As expected, a nanofiber film outperforms a conventional polyaniline film. Again the nanofiber films are not dependent on thickness, whereas the sensitivity of conventional polyaniline films is completely dependent on thickness [62]. 

According to the theory of metastable adsorption of de Gennes [216], when an adsorbed polymer layer is in contact with a pure solvent, the layer density diminishes with increasing distance to the srrbstrate (e g., metal) sirrface. The behaviors of several polymer film electrodes, such as poly(tetracyanoqirinodimethane) [133], poly(vinylferrocene) [148,217], polypyrrole [218] and polyaniline [69,219], have been explained by asstrming that the local film density decreases with film thickness that is, from the metal strrface to the poly mer solution interface. 

Optical techniques have been used to clarify the electronic conduction mechanism and the electrochemical film conversion process [142-144]. Redondo et al. [143] employed ellipsometry [144] to optically probe films grown on platinum electrodes in hydrochloric acid solutions. The optical information may be used to draw conclusions on film density variations as a function of polymerization conditions and film thickness. More important, by potential cycling, films could be examined while undergoing conversions from the reduced (insulating) forms to the oxidized (conducting) forms. Redox reactions carried out on polyaniline-coated electrodes suggest that polyaniline conducts solely in its oxidized emeraldine salt state. 

Ftg. 7. Characteristic of the device represented by Scheme using a polyaniline film (cu. 5 microns thick) in 0.5 M sodium hydrogen sulfate solution. The versus plots were measured by varying... 

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