Electrochemical impedance studies of the undoping process of platinum phthalocyanine charge transfer microcrystals

Abstract

Abstract Platinum phthalocyanine (PtPc) microcrystal films undergo three successive electrochemical oxidations. Each of these processes is associated with anion insertion or doping. The reverse process of anion insertion, undoping, has been investigated using electrochemical impedance spectroscopy and in-situ UV–vis spectroscopy. The impedance theory of conductive polymer films developed by Vorotyntsev et al. is applicable to this process. The kinetics of the undoping process depend upon the previous oxidative treatment, and thus the doping level. Three different states of the film can be demarcated, depending on the degree of oxidation (and thus the degree of doping) of the PtPc film. These are called the lightly doped, the conductive and the over-doped state, respectively. For lightly doped films, the film conductivity, the redox capacitance, the diffusion coefficient for charge transport and the rate of electrochemical reaction all decrease with decreasing potential. The film conductivity depends upon the concentration of free charge carriers. For the more highly doped conductive film, all of the above parameters are greatly enhanced, and the electrochemical reaction is accelerated and proceeds at a very high rate. The potential dependence of the redox capacitance and the diffusion coefficient depends on the type of anion. During undoping at 0 V, unusually high diffusion coefficients with a magnitude of order 10−2 cm2 s−1 are observed and are attributed to the strong interactions between the electronic and ionic carriers during the phase transformation. For the over-doped film, undoping leads to an increase in the film conductivity and electrochemical reaction rate. The potential dependence of the redox capacitance and diffusion coefficients for charge transport implies strong interactions within the film. Hypsochromic shifts in UV–vis spectra with decreasing potential indicate conformational relaxation during the undoping process. SEM investigation confirms that the doped film swells during the de-doping process.