A. Robert Hillman

Nucleation and growtn of polythiophene films on gold electrodes

Abstract The nucleation and growth of polythiophene films on gold electrodes has been studied using potentiostatic steps. The mechanism has been deduced and estimates made of the kinetic parameters. Dissolution of the gold substrate at potentials where thiophene polymerisation occurs is suppressed by the initial rapid formation of a monolayer of polymer. The data indicate that formation of bulk film occurs by the instantaneous nucleation and three-dimensional growth of polymer on top of this monolayer. Rate constants for growth parallel to the surface on the bare gold substrate and the covering polymer layer are surprisingly very similar. Growth perpendicular to the surface is slightly more rapid, typically by a factor of 1.5–3, although it is less dependent on potential. The high density of nuclei results in their overlap at an early stage, after which growth is only possible perpendicular to the surface. Within a narrow potential range, the observation of maxima and minima in current-time transients is interpreted in terms of the “death” and “rebirth” of growing centres.

Ion and solvent transfer accompanying polybithiophene doping and undoping

Abstract The electrochemical quartz crystal microbalance (EQCM) has been used to monitor mass changes accompanying polybithiophene (PBT) doping and undoping in acetonitrile solutions containing tetraalkylammonium salts. The observed mass changes are close, but not identical to those anticipated if counter ion (anion) were the only transferred species. More detailed consideration shows that the data can be rationalised if there is also salt and solvent transfer. The motions of these two species are in opposite directions, resulting in nearly compensatory mass changes. Comparison of the mass and charge fluxes show that the two are correlated during PBT oxidation, but not during reduction. Reduction of doped PBT occurs in two stages. In the first stage, at more positive potentials, the mass change per unit charge is less than the average value for the overall redox process. In the second stage, at more negative potentials, it is larger. This could not be accommodated within a model involving a single mobile species. Interpretation of the overall mass change must be consistent with mass and charge fluxes observed throughout the redox switching process.

Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques

Abstract A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the mobile species transfers associated with the redox processes of thin ( Γ ≅100–150 nmol cm −2 ) α- and β-nickel hydroxide films exposed to aqueous LiOH solution. A comparison of the measured PBD signal with the predicted PBD profiles, calculated by temporal convolution analysis of the current and mass responses, enabled the contributions to redox switching of anion (OH − ) and solvent (H 2 O) transfers to be discriminated quantitatively. The responses from the combined instrument are reconciled in terms of H + deintercalation/intercalation within the nickel hydroxide structure as OH − ions enter/exit the film. Hydroxide ion movement is associated with a counterflux of water. Thin nickel hydroxide films show a gradual α→β phase transformation with continuous voltammetric cycling, especially when the films are exposed to high concentrations of electrolyte. α-Films are characterised by OH − transfers that dominate the H + and H 2 O movements; β-films are characterised by an increased participation of water and protons to the exchange dynamics.

Viscoelastic characterization of electroactive polymer films at the electrode/solution interface

We describe and explore a new general transmission line model that describes the viscoelastic characteristics of thin films in terms of their shear moduli. The model contains within it the commonly used lumped-elementmodel as a limiting case, and we delineate the conditions under which thesimpler lumped-element model can be applied without significant error. We apply our analysis to the interpretation of crystal impedance spectra acquired dynamically during the electrodeposition of poly(2,2′-bithiophene) (PBT) conducting polymer films. In the very early stages of deposition, PBTentrapped within surface features behaves as a rigidly coupled mass. Beyondthis, it is a viscoelastic material behaving as a Maxwell fluid. PBT storage and loss moduli are initially polymer coverage dependent, rising to limiting values G′≈G″≈5×106 dyn cm-2. These values imply an acoustic decay length of ca. 0.3 µm. Accordingly, films significantly thicker than this behave as bulk material. For thick films there is also evidence for resonance effects.

Thermodynamic changes in ion and solvent populations accompanying redox switching in polyvinylferrocene films

Abstract The electrochemical quartz crystal microbalance (EQCM) was used to study redox induced changes in equilibrium ion and solvent populations of polyvinylferrocene (PVF) films. The effects of solution concentration and the identities of the cation (co-ion) and anion (counter ion) were investigated in aqueous sodium and perchlorate salt solutions. The redox behaviour of these PVF films is dependent on the bathing electrolyte composition. At low concentrations, the film is permselective: only counter ions (and solvent) participate during film oxidation. At higher concentrations, permselectivity fails, and oxidation is accompanied by the ingress of salt (both co- and counter ions). Potentiometric data agree with this model. Regardless of the concentration and the electrolyte studied, some solvent transfer occurs as a consequence of solvent activity coefficient changes that accompany redox switching.

Electroactive bilayers employing conducting polymers: Part 1. Suitability of thiophene- and pyrrole-based systems

Surface bilayer structures have been studied using slow scan cyclic voltammetry and in situ transmission spectroscopy. The bilayers were prepared by sequential deposition of two films chosen variously from polythiophene (PT), polybithiophene (PBT), polypyrrole (PP) and polyxylylviologen (PXV). The influence of inner/outer film redox couple energetics and polymer segregation on outer film charge “trapping” “untrapping” has been studied. PT/PP (inner film/outer film) and PBT/PP “bilayers” have inappropriate energetics. Pyrrole electropolymerisation within the inner layer also denies adequate segregation. PT/PXV and PBT/PXV bilayers do not suffer from either of these problems, although defects in thin PT films are deleterious to bilayer behaviour. PBT/PXV bilayers show sharp responses for viologen reduction. PXV0 was identified spectroscopically as the primary outer film species. Thin inner PBT films maintain stable outer film charge storage over a wide potential range. PBT+ mediated oxidation of PXV0 restores the original bilayer state.

Dynamic quartz crystal impedance measurements of polyvinylferrocene film deposition

Dynamic crystal impedance measurements were used to study the electrochemically-driven deposition of polyvinylferrocene (PVF) films from dichloromethane solutions of PVF onto Au electrodes of a quartz crystal microbalance. Under all the conditions studied, the PVF films were non-rigid, to an extent that depended upon several experimental factors. Films deposited from solutions containing perchlorate salts of tetraalkylammonium salts departed less from rigidity than those deposited under otherwise identical conditions from tetrafluoroborate media. In contrast, the use of different tetraalkylammonium cations, or the use of a potential sweep vs. a potential step from 0 V to the deposition potential (0.7 V), made no significant difference. The departure from rigid characteristics varied with coverage. For the interval of time during deposition that the coverage was 10–160 nmol cm−2 (of monomeric ferrocene units), the films showed no additional departure from rigidity. On the basis that the upper figure corresponds closely to a “monolayer” of terminally attached chains of mean length, we attribute this behaviour to the presence of a dense film, within which chain motion is considerably restrained.

Counter-ion specific effects on charge and solvent trapping in poly(vinylferrocene) films

Abstract This study determined the effect on the electrochemical quartz crystal microbalances (EQCMs) response of substituting tetrafluoroborate and hexafluorophosphate as counter ions into a poly(vinylferrocene) (PVF) film that had been prepared initially with perchlorate ion as the counter ion in methylene chloride. This PVF+ClO4− film was redox cycled successively in aqueous 0.1 M sodium perchlorate, 0.1 M sodium hexafluorophosphate and 0.1 M sodium tetrafluoroborate solutions. Cyclic voltammetric EQCM experiments determined the changes in film mass and charge during the first and subsequent potential cycles. In all bathing solutions, the total mass change and charge consumed during the first oxidation cycle were larger than in subsequent potential cycles; thereafter there were negligible changes. This is rationalized by the trapping of one or more oxidized forms of PVF produced in the first oxidation cycle, and two possible detailed mechanisms are given. The extent of film oxidation was controlled kinetically, and it changed in the counter ion order tetrafluoroborate>perchlorate>hexafluorophosphate. Less film oxidation occurred at high potential scan rates for all counter ions. At the highest potential scan rate (0.080 V s−1) the amount of water entering the oxidized film from solution was greater in tetrafluoroborate, than in perchlorate, or than in hexafluorophosphate solutions. At slower potential scan rates these differences still existed but were less obvious. Free volume restraints within the film limited the total amount of counter ion and solvent that could be introduced under kinetically controlled conditions.

An ellipsometric study of the nucleation and growth of polythiophene films

The electrodeposition of polythiophene films has been monitored ellipsometrically. The species that are generated immediately after application of the polymerization potential form in solution near the electrode. Deposition of material on the electrode commences a short time later, in agreement with the point at which current‐time data indicate nucleation of growth sites. Previous analysis of current‐time data suggested three‐dimensional growth, but calculation of film thickness as a function of time from ellipsometric data shows that the process is more complex. Films formed at different potentials require different thicknesses of polymer to be deposited before the film properties approach those characteristic of the bulk film. There is, furthermore, a marked distinction between the optical properties of films as grown at potentials above and below approximately 2.0V. The presence of excess water in the polymerization solution results in the production of a film about 1.5 nm thick which is optically dissimilar from that produced in the absence of water.

EQCM studies of polypyrrole films. 1. Exposure to aqueous sodium tosylate solutions under thermodynamically permselective conditions

We describe electrochemical quartz crystal microbalance (EQCM) studies of polypyrrole films exposed to aqueous sodium tosylate solutions. The films are thermodynamically permselective in both doped and undoped redox states only for electrolyte concentrations at or below 0.1 mol dm−3. Under kinetically controlled conditions, permselectivity failure occurs transiently (even in dilute electrolyte) for the undoped form. These phenomena result in complex, non-monotonic film mass changes during redox cycling. This is a consequence of competing counter and co-ion transfer mechanisms for instantaneously maintaining electroneutrality, along with solvent transfer that attempts to maintain a constant film volume. The observed responses are a function of time scale; initial transfer of the faster moving cation ultimately gives way to the thermodynamically favored transfer of the slower moving anion. The interconversion of the variously solvent and salt populated forms of the two film redox states can be visualized by an extension of the scheme-of-cubes model, represented as a prism. This new prism visualization helps rationalize the participation of the competing elementary steps and gives insight into film history and experimental time scale effects.