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  • ELECTROCHEMICAL CHARACTERISATION OF CONDUCTING POLYMER LAYERS AND THEER

    USE AS GAS SENSORS.

    by

    Timothy Paul McCormac

    A Thesis Presented to Dublin City University for the award of PhD.

    Prepared under the supervision of Dr. John Cassidy, Chemistry Department, Dublin Institute of Technology, Kevin Street and under the direction of Dr. Johannes G. Vos, School of Chemical Sciences, Dublin City University.

    August 1994.

  • DECLARATION

    I hereby certify that this material, which I now submit for assessment on the programme of study leading to the award of Doctor of Philosophy is entirely my own work and has not been taken from the work of others save and to the extent that such work has been cited and acknowledged within the text of my work.

    Signed: /? ) C£c*-rw :ID No.: 9170077915T Timothy Paul McCormac

    Date:

  • ELECTROCHEMICAL CHARACTERISATION OF CONDUCTING POLYMER LAYERS AND THEIR USE AS GAS SENSORS.

    Timothy Paul McCormac.

    ABSTRACT. The main aim of this work was to develop an SO2 sensor based upon a conducting polymer. The methodology and properties of a conducting polymer sensor along with the electrochemical characterisation of various polypyrrole layers is described. It was found that polypyrrole layers containing Copper (11) Phthalocyanine - 3 ,4 ',4", 4"'-tetrasulphonate anions (PPTSP), as the dopant, upon electrochemical switching exhibit cation movement in and out of the polymer matrix. A concentration, pH, temperature and scan rate study was performed on the process of cation movement. Also anions, such as CIO4 ", dodecylbenzenesulphonate (DBS) were employed as the dopants within the polypyrrole matrix. In this work it is described that the low level of non-faradaic current, characteristic of PPDBS layers, was due to the presence of the DBS anion in solution rather than the polymer morphology. Differential Pulse Voltammetry is applied to various polypyrrole layers and sigmoidal differential current outputs were obtained. A model is proposed for this behaviour which includes a set of resistances in parallel. Potential Step experiments were also performed on the polypyrrole layers. A transmission line model is proposed to explain the behaviour of the polymer film. For several electroconducting polymers spectroelectrochemical studies were carried out. The absorption spectra are quantitatively analysed using the Nemst equation by a 'Monomer Unit Model. The reaction between pyrrole and tetracyanoethylene (TCNE) was studied electrochemically and spectroscopically. It was found that TCNE forms a charge transfer complex with pyrrole, and it is present within the PPTCNE layer as the dopant anion. The influence of solvent composition upon the ferricyanide/ferrocyanide system was investigated electrochemically. It was found that in organic solvents ferrocyanide is a strong reducing agent. Polypyrrole layers were deposited across interdigital array electrodes and their responses to various organic vapours and 5 % SO2 were investigated. The use of Prussian Blue films as possible overlayers upon the polypyrrole layers is described. Also solid state voltammetry was performed upon Prussian Blue films, and when these mixed valent films were exposed to certain saturated vapours in nitrogen, good reversible responses were obtained. Also the interaction of organic vapours and polypyrrole films, deposited upon ITO electrodes, was investigated by UV/vis spectroscopy.

  • CHAPTER ONE Introduction 1-8

    CHAPTER TWO Electrochemical Characterisation of Conducting Polymer Layers

    2 . 1 Introduction 9-14

    2 . 2 Theory of Cyclic Voltammetry 15-22

    2.3 Experimental 23 2.3.1 Polymer Preparation 23-24

    2.4 Results and Discussion 25-30 2.4.1 Cation Concentration Study of

    PPTSP Layers 30-45 2.4.2 Scan Rate Study of PPTSP 45-54 2.4.3 pH Study of PPTSP 55-60 2.4.4 Conclusion 61 2.4.5 Cyclic Voltammetry of PDDBS

    Layers 61-68 2.4.6 Conclusion 69

    2.5 References 70-74

    CHAPTER THREE Differential Pulse Voltammetry of Conducting Polymer Layers

    3.1 Introduction 75-79

    3.2 Theory 3.2.1 Theory of Potential Step 80-83 3.2.2 Theory of Differential Pulse

    Voltammetry 83-86

    3.3 Experimental 86-87

  • Results and Discussion 88 3.4.1 Differential Pulse Voltammetry of

    Polymer Layers 88-107 3.4.2 Theory of Proposed Model 107-113 3.4.3 Simulation of D.P.V. for Polymer

    Model 113-128 3.4.4 Potential Step of PPTSP Layer 128-131 3.4.5 DPV of a PPFCN Layer 131-134 3.4.6 Transmission Line Circuit For

    Polymer Modelling 134-141

    Conclusion 141-142

    References 143-146

    CHAPTER FOUR Spectroscopic Investigation of Conducting Layers

    Polymer

    4.1 Introduction 147-154

    4.2 UV Analysis of Polypyrrole Layers 155 4.2.1 Experimental 155-156 4.2.2 Theory of Monomer Unit Model 156-158 4.2.3 Results and Discussion 159-182 4.2.4 Conclusion 182-183

    4.3. Characterisation of the Reaction between Pyrrole and Tetracyanoethylene (TCNE) 4.3.1 Experimental 4.3.2 Results and Discussion 4.3.3 Conclusion

    4.4 A Solvent Study of the Electrochemical Behaviour of the Ferricyanide/Ferrocyanide System. 206 4.4.1. Experimental 206-207 4.4.2. Results and Discussion 207-230 4.4.3. Conclusion 231

    4.5. Infrared Spectroelectrochemistry and its Application to the Study of the Reduction and Reoxidation of TBA3 Fe(CN)6 - 232 4.5.1. Introduction 232-233 4.5.2. Experimental 233-235 4.5.3. Results and Discussion 235-242 4.5.4. Conclusion 242

    183 183-184 184-206 206

  • 4.6 References 243-246

    CHAPTER FIVE Investigation of Conducting Polymer Layers for a Gas Sensor

    Introduction 247-252

    Experimental 253 5.2.1 Fabrication of Gold Interdigital

    Arrays 253-254 5.2.2 Encapsulation of Gold Microband

    Electrodes 254-255 5.2.3 Deposition of Gold Microband

    Electrodes 255-258

    Results and Discussion 259 5.3.1 Characterisation of Gold Interdigital

    Arrays and Screen Printed Carbon Ink Electrodes 259-262

    5.3.2 Preliminary Gas Sensing Experiments 263-266 5.3.3 Response of PP Films deposited across

    Gold Microband Electrodes to Various Vapours 267-272

    5.3.4 Responses obtained with Screen Printed Carbon Ink Electrodes 272-280

    5.3.5 Conclusion 280

    Spectroscopic Investigation of the Effect of Organic Vapours on Polymer Films 281 5.4.1 Introduction 281 5.4.2 Experimental 281 5.4.3 Results and Discussion 282-293 5.4.4 Conclusion 293

    Electrochemical Investigation of Prussian Blue Films as Overlayers on Polypyrrole Layers 294 5.5.1 Introduction 294-295 5.5.2 Experimental 295-296 5.5.3 Results and Discussion 296-316 5.5.4 Conclusion 317

    5.6 References 318-320

  • APPENDIX ONE Fortran Simulation Programme for Differential Pulse Voltammetry of Conducting Polymer Layers. (i) - (iv)

  • Acknowledgements

    Firstly I would like to thank my two supervisors, Dr J.G.Vos and Dr J.F.Cassidy for their help. I am grateful to John for his guidance and encouragement over the past three years.

    I would like to thank Mr E.Rothery and Dr N.R.Russell for the use of facilities within the Chemistry Department, Kevin Street, College of Technology. Also I would like to thank the technical staff of the Chemistry Department in Kevin Street for all their assistance over the past number of years.

    I am grateful to Dr R.Foster of Kevin Street for the screen printed carbon ink electrodes and Dr D.Cameron for the sputtered gold interdigital array electrodes.

    I would like to thank Dr J.F. Cassidy for the Fortran simulation program in Appendix 1, and Mr S.McCormac, Department of Electrical Engineering, Kevin Street, for the modified circuit employed for the gas sensing experiments.

    I am grateful to Ms. M.Brady in Kevin Street for her professionalism and patience in the typing of this thesis.

    Finally I would like to thank my family, friends and the chemistry post graduate students within Kevin Street for all their encouragement and support over the past number of years.

  • CHAPTER 1

    INTRODUCTION

    It is accepted that most polymer systems are insulating in character, as can be

    readily seen from the many applications they serve, for example the coating of

    electrical wires with insulating polymers to prevent shorting.

    However over the past 20 years a novel class of polymers have received

    widespread attention. This new group of polymers has the ability to conduct

    electric current.

    By the late 1970's there was excitement over the possibility of developing new

    materials with electronic and optical properties of metals and semiconductors.

    These conductive polymers are characterised by electronic conductivities ranging

    from 1 0 " 6 Siemens per cm up to 10^ Siemens per cm, a conductivity comparable

    in magnitude to that of iron.

    One of the major discoveries that launched the field of conducting polymers, was

    the high conductivity of polysulphumitride (SN)X, an inorganic explosive

    polymer, which was found to become superconducting at 0.3K. It was also one

    of the first polymeric materials to exhibit metallic properties.

  • However it was polyacetylene, which consists of coupled chains of carbon -

    hydrogen units arranged in a one dimensional lattice, that initiated interest in the

    field of conducting polymers.

    It was in 1974 that a silvery film of poly acetylene was accidently prepared by

    Hideki Shirak