Films electrochemical generation

D. Tsiplakides, S. Neophytides, and C.G. Vayenas, Investigation ofthe state ofthe electrochemically generated adsorbed O species on Au films interfaced with Y203-doped-ZrOj, Ionics, submitted (2001). 

Electrochemically generated nickei(lll) oxide, deposited onto a nickel plate, is generally useful for the oxidation of alcohols in aqueous alkali [49]. The immersion of nickel in aqueous alkali results in the formation of a surface layer of nickel(ll) oxide which undergoes reversible electrochemical oxidation to form nickel(lll) oxide with a current maximum in cyclic voltammetry at 1.13 V vj. see, observed before the evolution of oxygen occurs [50]. This electrochemical step is fast and oxidation at a prepared oxide film, of an alcohol in solution, is governed by the rate of the chemical reaction between nickel oxide and the substrate [51]. When the film thickness is increased to about 0.1 pm, the oxidation rate of organic species increases to a rate that is fairly indifferent to further increases in the film thickness. This is probably due to an initial increase in the surface area of the electrode [52], In laboratory scale experiments, the nickel oxide electrode layer is prepared by prior electrolysis of nickel sulphate at a nickel anode [53]. It is used in an undivided cell with a stainless steel cathode and an alkaline electrolyte. 

Fig. 91. Schematics of the circular trough (not drawn to scale) used for the electrochemical generation of silver particulate films at monolayer interfaces...

Importantly, no silver particulate film formation could be observed at the air/liquid interface, either in the chemical or in the electrochemical generation, in the absence of surfactants or under monolayers prepared from positively charged surfactants. 

Rapid polymerization of electrochemically generated species such as organic radicals can cover an electrode with a polymeric film. Such films are sometimes impenetrable and difficult to remove, which results in passivation of the electrode surface. A typical case is the oxidation of 1,2-diaminobenzene [9]. The modification of electrode properties by coating with thin polymer films is currently an area of active investigation [10— 12]. 

Chronoamperometric transduction can be applied to electroinactive analytes as well as electroactive, which are sorbed by the MIP film and then undergo an electrochemical reaction [25]. In the latter case, the analyte should be able to diffuse freely both towards and away from the electrode surface for the current to flow. The primary requirement of chronoamperometric sensing is a linear relationship between the current measured at the constant potential and the concentration of the analyte. Moreover, the electrochemically generated species should readily diffuse away from the electrode surface coated by the sensing film. By way of example, a few representative chronoamperometric sensors based on MIPs are presented below. 

Figure 2. UV-Vis spectra for a 0.5 mil thick Kapton film after a 5 sec immersion in a solution containing 0.0475 M benzil/0.0025 M benzil radical-anion electrochemically generated in 0.1 M TBAFB/ACN solution.

Amperometric glucose biosensors based on co-immobiUzation of GOx with [Os(bpy)2(4-VP)ioCl]Cl in electrochemically generated polyphenol film [72]. A bilayer electrode in which Os-pol3nmer is first adsorbed on a Pt electrode and then electrochemically deposited on polyphenol-GOx fihn showed sensitivity of 1.63-1.79 /jA/cm /mM to 20 mM glucose at 0.4 V and 6-7 mM. Low background current and less interference by common electroactive compounds were noted. 

A problem in the practical application of these electrodes is their sensitivity to oxidation in air molecular oxygen was found to induce oxidation of the polymer into monomeric Ru(II) species and disintegration of the cathode. To circumvent this problem, the polymer material was electrochemically generated within preformed films of polypyrole containing functionalized bipyridine ligands. The polypyrole apparently acts as a barrier to oxygen and the stability of... 

An example of the protective value of polypyrrole is to be found in the work of Skotheim et al (43) who stabilized n-Si with thin films of this material. These conductive films were photo-electrochemically generated and they extended the operational lifetime of illuminated n-Sl from about four hours to 6 days in the presence of l2 /I couple in aqueous solution. The cell showed no decay after running continuously at anodic current densities of 9 ma/cm2 for this long period. Noufl et al (42) found similar enhanced stability of n-Si coated with polypyrrole in the presence of the Fe3+/2+ couple, and for n-GaAs in CH3CN solutions (45). 

Heterojunctions formed by electrodepositing polypyrrole or polyaniline on retype silicon were studied with PVS [756]. Experimental observations were explained invoking photosensitization by electrochemically generated polarons in the polymer film. The effects of nano-sized particles of Fe203 that were incorporated into films of polypyrrole were investigated with PVS [757]. 

The photo-electrochemical generation of a conducting film on the photo-... 

Z. Peng, E. Wang, and S. Dong, Incorporation of surface-derivatized gold uanoparticles into electrochemically generated polymer films, Electrochem. Commun., 4, 210-213 (2002). 

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