A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) study of a poly(o-toluidine) modified electrode in perchloric acid solution

doi:10.1016/S0022-0728(98)00245-9

Journal of Electroanalytical Chemistry

Volume 454, Issues 1–2, 28 August 1998, Pages 1-8

https://doi.org/10.1016/S0022-0728(98)00245-9 Get rights and content

Abstract

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the effects of film history and redox state of an unhydrated poly(o-toluidine) modified electrode immersed in aqueous perchloric acid (pH=0) solution. The degree of film hydration affected the relative contributions of protons and anions exchanged during the first redox process and the potential of the first anodic voltammetric peak. A distinct proton contribution to the ion exchange during the first redox process was detected during the second and subsequent cycles. Electroneutrality during the second charge transfer step was maintained predominantly by proton expulsion. The determination of the diffusion coefficient of the exchanged species in the first redox process by the time dependence of laser beam deflection delay and by temporal convolution of the current signal are compared. The concomitant changes in the gravimetric and optical signals during film hydration demonstrate the participation of both proton and solvent during redox cycling of poly(o-toluidine).

Introduction

A complete determination of the mobile species populations within a surface-immobilised film requires a knowledge of anion, cation and solvent populations at the electrode-electrolyte interface. Among the methods available for the determination of these three parameters, two in situ techniques, the electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) have proved to be valuable methods for the study of ion and solvent transfer during the redox processes of modified electrodes, including those modified with conducting polymer films. The two techniques have complementary strengths and weaknesses. In particular, (i) refractive index (PBD response) shows essentially the same variation with concentration for protons as for any other ion, whereas the EQCM is “blind” to protons; (ii) PBD is primarily sensitive to solute transfers and essentially “blind” to solvent transfers, whereas the EQCM is sensitive to both according to molecular mass. This combined technique will therefore allow new insight into a redox mechanism where both proton and solvent are suspected to intervene.

The EQCM [1]technique allows monitoring of the electrochemically-driven changes in surface populations of mobile species via the associated electrode mass changes. It has been used to study a number of electroactive polymers, including polypyrrole [2], polyaniline [3]and polyvinylferrocene [4]. PBD (sometimes called the ‘mirage effect’) involves the measurement of a laser beam deflection as it passes close and parallel to an electrode surface, as a result of the electrogenerated refractive index gradient formed in the electrolyte, 5, 6, 7. It has been applied to study the ion fluxes accompanying redox switching of films, including poly(3-methythiophene), [8]polyaniline [9]and polypyrrole [10].

The two techniques were compared recently in the case of ionic transport in polyaniline [9]and poly(N-ethylcarbazole) [11]. Although the determination of all mobile species populations is possible via the electrochemical, frequency and optical responses by the application of both the EQCM and PBD techniques, the structure of a polymer film, at any instant, is a complex function of its prior treatment. The question remains whether sequential experiments and subsequent correlation of the data neglect temporal structural changes, such as relaxation within polymers [12]. Recently, the theoretical framework for a scheme-of-cubes model was proposed to rationalise the influence of history based and experimental time scale effects of electroactive films during redox switching [13]. The three-dimensional representation recognises that the thermodynamic and kinetic descriptions of a system require a knowledge of electron and ion transfer, solvent transfer and polymer reconfiguration. In the present work we report the preliminary experimental results of an investigation of mobile species dynamics during the redox processes of a poly(o-toluidine) film in perchloric acid solution by the application of a combined EQCM and PBD instrument; this avoids any question of film history effects when correlating the gravimetric and optical data.

Section snippets

Film deposition

The poly(o-toluidine) film was electrodeposited by a reported procedure [14]. The film was potentiodynamically deposited onto the exposed gold face of the quartz crystal, from a 1M sulfuric acid solution containing 0.5 M o-toluidine, by cycling the potential between 0 and 800 mV versus SCE (scan rate 100 mV s⁻¹; 30 cycles, polymer coverage Γ=7.66×10⁻⁸ mol cm⁻².) The electropolymerisation was stopped at 0 mV and the polymer film rinsed with 0.1 M sulfuric acid (5×1 ml aliquots), distilled water

Irreversible film hydration studies

The simultaneously recorded cyclic voltammogram, EQCM and probe beam deflection response of the poly(o-toluidine) film from the first and second potential cycles of the series are shown in Fig. 2(a–c) respectively. The currrent response shows the two well-defined quasi-reversible redox processes reported previously 14, 18. The first redox process, denoted step I, may be related to the oxidation of the amine nitrogen atoms to radical cations. The second redox process, step II, is related to the

Conclusions

An important attribute of the combined instrument is that the strengths of the EQCM and PBD are complementary. Thus, solvation phenomena such as the irreversible hydration of the poly(o-toluidine) film are readily detected by EQCM but not directly by beam deflection. The consequence of water uptake, however, affects the relative contributions of protons and anions to the exchange at the first charge transfer, as detected by PBD. The degree of hydration, and therefore the relative contributions

Acknowledgements

Dr S. Ramirez is gratefully acknowledged for helpful discussions. The project was financed by the EPSRC, Grant No. GR/L17597.

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Preliminary noteA combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) study of a poly(o-toluidine) modified electrode in perchloric acid solution

Abstract

Introduction

Section snippets

Film deposition

Irreversible film hydration studies

Conclusions

Acknowledgements

J. Electroanal. Chem.

Synth. Met.

Synth. Met.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

Solid State Ionics

J. Electroanal. Chem.

Chem. Rev.

Preliminary note
A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) study of a poly(o-toluidine) modified electrode in perchloric acid solution