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3.2 Electrodeposition of Pyrrole on High-T_c Superconductors

The electrochemical polymerization of pyrrole and other related monomers on high-temperature superconductors has been examined by several research studies.^{38-40, 42, 43, 45-48}  The acidic conditions in one early study noted⁵⁹ that while they did form polyaniline on the superconductor electrode, the electrode had a 20% loss of mass.  It is clear that these conditions are too severe for the formation of controlled polymer growth.  Having focused on the conditions used by others in the polymerization of conducting polymers, conditions discussed here will focus on my experimental results for the interfacing of conductive polymers with cuprate superconductors.  My research has focused on experimental conditions which must be optimized for the deposition of the conducting polymers on the superconductor’s surface.  Conditions similar to those discussed in Chapter 2 for obtaining good electrochemical response for redox species in solution need to be used for electrodeposition of conducting polymers.  The key components are the use of dry, non-acidic, non-aqueous solvents, proper resurfacing if needed, and an inert atmosphere for performing electrochemical experiments.

Electropolymerization of pyrrole has been carried out in two different solutions.  The first is a dilute solution of pyrrole in acetonitrile, typically 1-10% by volume.  The second is carried out in neat pyrrole.  Both solutions use 0.1 M tetraethylammonium tetrafluoroborate, Et₄NBF₄, (Aldrich) which was recrystallized from ethyl acetate / ethanol and vacuum dried overnight prior to use.  Acetonitrile was distilled from P₂O₅ under N₂ and passed over activated alumina in an inert atmosphere before use.  Purification of pyrrole is accomplished by passing the liquid over activated alumina in an inert atmosphere prior to  use.  Electrochemical measurements were accomplished with an EG&G PAR 273 potentiostat.  The polymerizations were carried out in an inert atmosphere glovebox to prevent exposure to water. 

Neat pyrrole solutions were generally used to form thick films of polypyrrole.  Here, the pyrrole is used both as the polymerization species and the solvent.  This polymerization route was first studied by Murray and coworkers¹ for both pyrrole and aniline.  Since both pyrrole and aniline are viscous, the diffusion of oligomer away from the electrode is slow.  Also, since there are not additional interactions with solvent molecules, just other pyrroles, there is little chance for side-reactions.  In the case of aniline, it was found that the aniline oligomers are quite soluble in aniline.  Although the formation of polyaniline could be accomplished, there was no precipitation of aniline on the electrode’s surface.  Pyrrole oligomers have different solubility characteristics.  While some brown oligomers are seen moving away from the electrode, a black film of polypyrrole quickly precipitates onto the surface of the electrode.  Polypyrrole formed with dilute pyrrole solutions grows at a much slower rate than that grown in a neat pyrrole solution.  Both have similar morphological features, typically growing to form large nodules across the surface of the electrodes.  The similarities appear to be independent of the substrate, with the exception of work that will be discussed later in section 3.4 of this chapter. 

During the initial stages of polymerization onto the surface of YBa₂Cu₃O_7-x (or one of its derivatives) there is nucleation of polypyrrole as can be implied from the cyclic voltammetry data of Figure 3.1(a) where after initial oxidation of pyrrole further oxidation of the oligomers is made easier.  Note how the current loops back on itself in Figure 3.1(a).  The nucleation loop (i.e., the crossing of the forward and reverse scans) has been seen previously on other non-superconductor electrodes that have supported polypyrrole growth.³⁵  In these cases, the behavior suggests a sluggish polymer coupling initially to the superconductor surface and this kinetically slow step leads to a small amout of current passed in the forward scan.  After the nucleation process begins, the polymer couples to the electrode and serves as a nucleation layer for further polymer growth.  There is a large accumulation of polymerization that subsequently occurs on the back scan.  After several oxidative cycles, nucleation is complete and the charging current has increased by 500% as can be seen in Figure 3.1(b).  At this juncture, the nucleation loop behavior is no longer apparent.

The morphology of polypyrrole is similar even when deposited on various electrodes such as: a bulk pellet of GdBa₂Cu₃O_7-x substrate (Illustration 3.3), a thin film of YBa₂Cu₃O_7-x supported by a sapphire substrate (Illustration 3.4), a thin film of YBa₂Cu₃O_7-x supported by a MgO substrate (Illustration 3.5). 

Figure 3.1:     Cyclic voltammetry of GdBa₂Cu₃O_7-x electrode in 10% pyrrole (by volume) 0.1 Et₄NBF₄ / CH₃CN solution at a scan rate of 50 mV/sec demonstrating a) nucleation of polypyrrole and b) polypyrrole growth after 20 cycles.

Illustration 3.3:     Scanning electron micrograph of polypyrrole (top half) grown on a bulk pellet of GdBa₂Cu₃O_7-x (lower half) by oxidative cycling in a 10% pyrrole solution.

Illustration 3.5:     Scanning electron micrograph showing polypyrrole (top half) grown on a YBa₂Cu₃O_7-x thin film (lower half) electrode by oxidative cycling in a 10% pyrrole solution that is supported by a MgO substrate.

These polymers were grown by cycling the electrode in a 10% pyrrole / acetonitrile solution between 0.0 and 1.0-1.3 Volts vs. Ag wire.  Illustration 3.3 has polypyrrole nodules that are roughly 10-20 mm in diameter, interestly about the same size as the underlying superconductor grains. Illustration 3.4 shows polypyrrole growth on a thin film of YBa₂Cu₃O_7-x supported by sapphire.  The polypyrrole nodules are about 5-20 mm in size. Similar, morphology is observed in Illustration 3.5 grown under analogous conditions.   By looking at these high-T_c electrode templates, it can be seen that polypyrrole grows over the entire area of the exposed superconductor, implying that the majority of the surface is electroactive.  By blocking off part of the underlying electrode with Apiezon wax, polymer growth can be localized in desired regions on the superconductor.