David C. Loveday

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.

Observation of kinetic effects during interfacial transfer at redox polymer films using the quartz crystal microbalance

The object of this preliminary note is to demonstrate, using the quartz crystal microbalance (QCM), the existence of kinetic barriers affecting mobile species transfer into and out of redox polymer films. In particular, we highlight differences in the rates of electron and mass transfer. In previous papers [l-3] we investigated the redox switching process in surface immobilized redox polymers and explained the accompanying changes in equilibrium populations of both charged and neutral mobile species. This involved a consideration both of the thermodynamic activities of the various species and of the requirements of electroneutrality. We used the QCM to measure in situ film mass changes. We studied the polythionine system in both strong acid (HCl, H,SO,, HClO,, and p-CH3C6H4S03H) [2] and buffered weak acid solutions (CH,CO,H) [3]. A central feature of the theoretical approach and its experimental testing was that it concerned equilibrium parameters. Electrochemical and mass measurements were therefore made at fixed (or sufficiently slowly scanned) potential. At higher scan rates, sometimes the mass responses were non-monotonic, involved considerable hysteresis, and did not track the charge. These facts demonstrate unequivocally that there are kinetic barriers to the movement of one or more of the mobile species participating in the overall redox process, over and above those controlling the charge injection rate. Qualitatively, one might view this behavior as resulting from one (or more) kinetic step(s) following electron transfer.

A general approach to the interpretation of electrochemical quartz crystal microbalance data. II. Chronoamperometry: Temporal resolution of mobile species transport in polyvinylferrocene films

Abstract A diagnostic procedure is developed for interpreting the response of the electrochemical quartz crystal microbalance (EQCM) to the application of a potential step. The criteria necessitate consideration of mass and a derived function, Φ, as functions of charge. The procedure for applying these criteria is illustrated by application to EQCM data for polyvinylferrocene (PVF) redox switching in aqueous NaClO 4 solutions. At high and low electrolyte concentrations, electroneutrality is satisfied mainly by counter ion motion at short times. The slowest species transferred is water. At high electrolyte concentrations, when activity constraints require salt motion, salt is transferred at a rate intermediate between that of counter ion and water. Water ingress into reduced PVF is slower than its expulsion from oxidised PVF.

Transport of neutral species in electroactive polymer films

A new procedure has been applied to the analysis of electrochemical quartz crystal microbalance data for transfer of mobile species in electroactive polymer films. The procedure enables the distinction between global equilibrium and rate-limiting transfer of electrons, ions and neutral species during film redox conversion. For polyvinylferrocene, polythionine and polybithiophene films (under the experimental conditions employed), transfer of a neutral species was found to be rate limiting. In the first two cases, solvent (water) was unequivocally identified as the slow species. In the third case, solvent (CH3CN) transfer was tentatively identified as the rate-limiting step. Slow transfer of neutral species is due to the absence of migration effects, which can aid ion transfer. Transfer rates depend on direction of transfer in all three systems.

Charge Transport in Electroactive Polymer-Films

The electrochemical quartz crystal microbalance (EQCM) and ellipsometry have been used to study directly the movement of ions and solvent into/out of electroactive polymer films. The systems studied were polyvinylferrocene (PVF), polybithiophene (PBT) and polythionine (PTh). The overall mass changes accompanying oxidation/reduction indicate that film sources of counter-ions (required to maintain electroneutrality) can be significant. The extent of participation of these species depends on the nature and concentration of the bathing electrolyte solution. In the case of PVF, optical data also indicate a structural change: reduced PVF appears to be a homogeneous compact film, whilst oxidised PVF+ is a more diffuse, inhomogeneous film, whose polymer content decreases with distance from the electrode. Voltammetric experiments at rapid (and in some cases even moderate) scan rates show that transport of mobile species can be quite slow. It was generally observed that ingress into the polymer was slower than egress of the same species from the polymer. Charged species, notably proton in hydrated systems, move faster than neutral species, such as solvent, due to the influence of the field.

Criteria governing ion and solvent transport rates in electroactive polymers: the existence of kinetic permselectivity

Ion and solvent transport in polythionine and polyvinylferrocene films have been monitored under transient conditions using the electrochemical quartz crystal microbalance. Film sources of required species are utilized before solution-phase sources. Potential gradients within the film influence ion motion, resulting in a diversity of transport rates which can give rise to transient permselectivity.

A general approach to the interpretation of electrochemical quartz crystal microbalance data. Part II. Chronoamperometry: temporal resolution of mobile species transport in polyvinylferrocene films

Abstract A diagnostic procedure is developed for interpreting the response of the electrochemical quartz crystal microbalance (EQCM) to the application of a potential step. The criteria necessitate consideration of mass and a derived function, Φ, as functions of charge. The procedure for applying these criteria is illustrated by application to EQCM data for polyvinylferrocene (PVF) redox switching in aqueous NaClO 4 solutions. At high and low electrolyte concentrations, electroneutrality is satisfied mainly by counter ion motion at short times. The slowest species transferred is water. At high electrolyte concentrations, when activity constraints require salt motion, salt is transferred at a rate intermediate between that of counter ion and water. Water ingress into reduced PVF is slower than its expulsion from oxidised PVF.

Rectifying behaviour of n-titanium dioxide–polybithiophene semiconductor–polymer bilayers: an electrochemical quartz crystal microbalance investigation

We report the first electrochemical quartz crystal microbalance (EQCM) study of an inorganic semiconductor–conducting polymer bilayer. The bilayers were prepared by electrochemically depositing polybithiophene (PBT) onto TiO2 thin films supported on Ti. TiO2 films prepared by electrochemical oxidation of titanium and by sputtering gave indistinguishable results. The current–voltage characteristics are those of a rectifying interface. Interpretation of the EQCM data for bilayer redox switching under voltammetric conditions reveals qualitative and quantitative information on the fate of electronic and ionic charge injected into the TiO2–PBT bilayer. The charge transfer mechanism in the depletion layer at the semiconductor/polymer interface was investigated with respect to anion ingress into and egress from the cation PBT film. Comparison with a previous EQCM investigation on single PBT films deposited on platinum suggests the participation of impact ionization processes at the TiO2/PBT interface.

Faraday communications. In situ electron paramagnetic resonance spectra of n- and p-doped poly(benzo[c]thiophene) films

In situ EPR spectra for both p- and n-doped poly(benzo[c]thiophene)(PBCT) thin films are reported. Simplistic interpretation of the spectra, based on spin–orbit coupling constants, suggests negligible participation of sulfur orbitals in both the HOMO and LUMO. In the latter case, this is at variance with prediction. The spin concentration shows a maximum with doping in either sense. This is anticipated from spectroelectrochemical data for p-doped, but not n-doped, PBCT.

Quartz crystal microbalance study of the adsorption of ions onto gold from non-aqueous solvents

This work investigates the adsorption of ions onto a gold electrode surface using an electrochemical quartz crystal microbalance. It was found that in anisole solutions adsorption of a monolayer of tetrafluoroborate anions or tetrabutylammonium cations occurred at different potentials, whereas in ethanol only tetrabutylammonium cations were adsorbed. The implications that this has upon electron-transfer processes are discussed.