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3.4 Effect of Self-Assembled Monolayers on Deposition

Self-assembled monolayers (SAM) of pyrrole-tagged reagents as a template for the polymerization of polypyrrole have been used^60-63 on a variety of substrates to anchor the polymer covalently in place as well as to induce morphological changes in polypyrrole.  Recently, it has been shown that molecules with an amine functional group can be adsorbed irreversibly to form persistent layers atop high-temperature superconductors such as YBa₂Cu₃O_7-x, Y_0.6La_0.4Ba_1.6Ca_0.4Cu₃O_7-x, and the Tl-based superconductors.^{64, 65}  Previously, this work has been completed with a number of redox active probes, and very efficient and rapid electron exchange has been found for redox tagged amines.  Based on this chemical handle, we now have a very effective strategy for coupling molecular reagents to high-temperature superconductors.  This methodology has been used previously not only for electrochemical studies of superconductor surfaces, but also for controlling the interfacial properties of superconductor systems and molecular barrier corrosion layers.  Here, the SAM method is extended to the study of polymer nucleation atop electroactive functional groups adsorbed to high T_c superconductors which contain a polymerizable monolayer as the end group.

This research investigates the electrochemical growth of pyrrole on the surface of the high-temperature superconductors and provides important insight into the local surface electroactivity and conductive properties.  Cyclic voltammetry and chronoamperometry are used with 1% pyrrole monomer (by volume) in acetonitrile with 0.1M Et₄NBF₄ supporting electrolyte.  High-temperature superconductor samples of YBa₂Cu₃O_7-x and its derivatives were studied with and without pre-treatment of the self-assembled monolayer, N-(3-aminopropyl) pyrrole, a pyrrole terminated alkyl amine.

Grazing angle FT-IR was used to characterize the self assembly of N-(3-aminopropyl) pyrrole onto the surface of a large YBa₂Cu₃O_7-x pellet.  This method has been successfully employed in the characterization of self-assembled monolayer on high-temperature superconductors⁶⁶ as well as other materials such as Al,⁶⁷ Ag,⁶⁸ Au,^{69, 70} SiO₂,⁷¹ GaAs,⁷² and glassy carbon.⁷³  Since bulk pellets of YBa₂Cu₃O_7-x are not smooth and have poor reflectivity in the infrared compared with that of conventional metals, the signal-to-noise is poor for monolayer coverages.  However, with the aquisition of a large number of scans, adequate signals can be obtained so that the C-H vibrations can be visualized.  Figure 3.5 shows the infrared spectra of N-(3-aminopropyl) pyrrole.  Here, the major peak is at 2924 cm^-1 which corresponds to the anti-symmetric methylene stretch.  This value deviates from the expected value of 2916 cm^-1 which is seen for crystalline hydrocarbon reagents.  The value of 2924 cm^-1 is reminiscent of the behavior noted for liquid parafin samples, suggesting that alkyl tether is disordered under these circumstances.  It has been noted previously⁶⁶ that polycrystalline samples of YBa₂Cu₃O_7-x have monolayers that lack the crystallinity seen in monolayers deposited on thin films.

 

Figure 3.5: Grazing angle FT-IR spectra of N-(3-aminopropyl) pyrrole adsorbed onto a YBa₂Cu₃O_7-x ceramic pellet.  Shown here is an expanded view of the C-H stretching region.

Electrochemical deposition of polypyrrole onto bare ceramic electrodes occurs so that the surface morphology of the polymer is similar to polypyrrole deposited on Au and Pt.  Illustration 3.6(a) shows polypyrrole grown on a ceramic pellet of Y_0.6La_0.4Ba_1.6Ca_0.4Cu₃O_7-x without pre-treatment of the N-(3- aminopropyl) pyrrole solution.  The surface of the film is covered by ~5-10 µm diameter globular features.  In this case, the surface bears no resemblance to the underlying grains of the ceramic pellet.  Illustration 2.7 of Chapter 2 shows a scanning electron micrograph of the surface of YBa₂Cu₃O_7-x for comparison purposes.  Another ceramic pellet was treated for 18 hours in a 5 mM N-(3-aminopropyl) pyrrole / acetonitrile solution.  Then, pyrrole was polymerized onto the surface of that pellet using the same conditions.  Interestingly, the resulting surface morphology mimics that of the underlying superconductor as in Illustration 3.6. 

The differences in the samples treated with and without N-(3-aminopropyl) pyrrole suggest variability in their nucleation.  Since the morphology of the SAM treated sample has such similarities to that of the underlying superconductor, rapid nucleation is believed to occur over a large portion of the exposed superconductor surface.  This nucleation is then followed by two-dimensional layer-by-layer growth of polypyrrole. 

 

Figure 3.6: Chronoamperimetry of polypyrrole growth at 1.4 V vs. Ag wire on YBa₂Cu₃O₇ polycrystalline electrode (A) with no monolayer treatment (B) with a self-assembled monolayer of N-(3-aminopropyl) pyrrole.

The surface of the sample not treated with the SAM does not reflect the characteristics of the underlying superconductor.  This would be characteristic of nucleation in isolated islands, possibly at defect sites or grain boundaries.  This nucleation is followed by three-dimensional growth resulting in the nodules seen in Illustration 3.6 (b).  The SAM-treated sample displays layer-by-layer growth.  The sample not pre-treated with SAM displays growth characteristics that are three dimensional.  Other evidence that points to two distinct growth patterns is the growth rates as seen by chronoamperometry in Figure 3.6.   After the initial charging in the SAM-treated sample, the current increases in a slow, controlled manner characteristic of a growth pattern that is primarily two dimensional.  The sample not pre-treated with SAM has faster growth even though it appeared to fluctuate.  The fluctuation in current can be explained as nodules that grow into one another (showing a decrease in current) and then nucleate the growth of another three dimensional nodule.

Other researchers have used SAM  layers as a template for polypyrrole deposition on materials such as Au.  These deposited polymers show remarkable improvement in their adhesion properties.  Polypyrrole deposited without the use of a SAM template can easily be removed by placing adhesive tape on the polymer and then removing it.  When the SAM template was used, the adhesive tape was unable to remove the entire thickness of the polypyrrole.  In one case,⁶⁰ a thick polypyrrole deposited on a SAM treated Au electrode was separated from the electrode and examined by scanning electron microscopy.  The mirror-like sample appeared featureless even to the scanning electron microscope.  The analogous experiment was performed on a sample that was not treated with the SAM template.  In this case, the surface appeared rough, with 50-200 mm pits.  Additionally, it has been noted⁶⁰ that the growth pattern of polypyrrole deposited with a SAM template has two dimensional growth instead of the three dimensional growth that typically occurs.

In this research, the adhesion properties of polypyrrole on YBa₂Cu₃O_7-x were also evaluated by applying adhesive tape to the sample, quickly removing the tape, and imaging the sample before and after the adhesive tape tear with a scanning electron microscope.  Under these conditions, it is noted that the polymer layer is nearly pulled completely off of the sample not treated with SAM, though a few areas appeared to be unharmed.  These areas are likely due to places on the tape where contact was not properly made.  The superconductor modified with the pyrrole amine reagent displays a more complex behavior.  While there is significant removal of the polymer from the surface, a smooth layer of polypyrrole is left behind indicating better adhesion to the superconductor’s surface than to itself.  Polymer-polymer failure is implied from such behavior.  From these studies, it is clear that the presence of the electroactive self-assembled monolayer serves to significantly improve the adhesion of polypyrrole to the superconductor’s surface.

Illustration 3.7: Schematic illustration showing the proposed sequence of steps which may occur at the cuprate surface that is coated with an electroactive monolayer during the electrochemical coating with polypyrrole.  a) In the initial step, the amine-tagged pyrrole monolayer is adsorbed to the cuprate surface.  b) Upon oxidative cycling of the electrode, the adsorbed reagent is rapidly oxidized.  c) On a more gradual time scale, solution dissolved monomer diffuses to the electrode surface.  d) Upon reaching the electrode surface, oxidation of the monomer occurs commensurate with proton loss and coupling of the monomer to the electrode confined template layer whereby growth of the oligomer ensues.  (Adapted from reference⁶⁶)

The large changes in the polymer growth and morphology on the polycrystalline samples suggest that N-(3-aminopropyl) pyrrole serves to alter the polymer nucleation / growth behavior in a significant manner.  Shown in Illustration 3.7 is a proposed sequence⁷⁴ of steps that lead to the deposition of polymer onto the SAM-modified sample: a) In the initial step, the N-(3-aminopropyl) pyrrole monolayer is adsorbed onto the high-T_c surface. b) Upon oxidative treatment of the modified surface, the surface localized reagents are oxidized.  Because of the presence of the amine anchor, the oxidized pyrrole reagent must remain in close proximity to the electrode surface where it can serve as a nucleation site for subsequent polymer growth.  c) On a slower time scale, additional solution dissolved monomer diffuses to the electrode surface where it can be oxidized.  d) Because the oligomers are oxidized more readily than the monomers, some fraction of the solution dissolved monomer couples to the surface localized pyrrole reagent where they foster further polymer growth.

Polymers grown onto the bare cuprate surfaces are expected to couple to the oxide surface in a different manner as seen in Illustration 3.8.  The five steps are described as follows:  a) The initial step involves a diffusion of the electroactive monomer to the surface of the electrode.  b) Once the pyrrole molecules reach the surface, they are oxidized to the radical cation.  c) Two oxidized monomers units can couple to each other, and in the process lose two protons to form a neutral oligomeric compound. This oligomer is more readily oxidized then the isolated monomer units, and again becomes oxidized. In addition, an oxidized monomer can also couple to an oxidized oligomer.  d) The oxidation and coupling steps continue to occur in the solution phase in the region of the electrode until the solubility limit of the organic oligomer is exceeded.  At this stage, polymer nodules begin to collect on the electrode surface at localized spots. e) Continued electrochemical growth which results in a further expansion of the polymer nodules.³⁴

 

Illustration 3.8:     Proposed sequence of steps which may occur at the bare superconductor electrode surface during the coating with an electrochemically deposited layer of polypyrrole. a) First, neutral monomer diffuses to the electrode surface. b) Then, oxidative treatment of the electrode serves to oxidize monomer molecules that are close to the electrode surface. c) Once oxidized, the reactive monomers can couple to other pyrrole units, with simultaneous proton loss, to produce soluble oligomer compounds. d) The oligomer grows until the solubility limit is exceeded at which time the oligomer precipitates onto the superconductor surface. f) Further growth of the polymer can occur at these nucleation sites.  (Adapted from reference⁶⁶)

Polypyrrole deposition onto bare electrodes proceeds through an electrode deposition method where the growing polymer structures couple to the metal surface only after the solubility of the oligomeric compounds is exceeded, as seen in Illustration 3.8.  Under these circumstances, polymer solubility characteristics and precipitation dynamics play an important role in dictating the morphology of the polypyrrole surface.  The large globular features noted in Illustration 3.7 and their similarity to layers grown on noble metal electrodes suggest this type of coupling is occurring at the surface.  The morphology noted for the SAM coated ceramic samples suggests that more rapid and uniform nucleation occurs for these superconductor electrodes. The direct coupling of the polymer chains to the electrode surface is also consistent with an increase in polymer to superconductor adhesion, as described in the adhesion tests.