Polypyrrole electrochemistry: Environmentally friendly corrosion protection of steel: (im)possibilities

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Polypyrrole Electrochemistry

Environmentally friendly corrosion protection of steel: (im)possibilities

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Polypyrrole Electrochemistry

Environmentally friendly corrosion protection of steel: (im)possibilities

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus, prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 7 november 2005 om 15:30 uur

door

Wouter Jan HAMER

materiaalkundig ingenieur geboren te Delft

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Prof. dr. J.H.W. de Wit Prof. dr. R. van der Linde Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Prof. dr. J.H.W. de Wit Technische Universiteit Delft, promotor Prof. dr. R. van der Linde Technische Universiteit Eindhoven, promotor Prof. dr. ir. J. van Turnhout Technische Universiteit Delft

Prof. dr. ir. L.A.I. Kestens Technische Universiteit Delft Prof. dr. R. Boom Technische Universiteit Delft

Prof. dr. W. Bogaerts Katholieke Universiteit Leuven, Belgi¨e

Dr. L. Koene Technische Universiteit Delft

Dr. L. Koene heeft als begeleider in belangrijke mate aan de totstandkoming van dit proefschrift bijgedragen.

ISBN 90-9019603-X

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The research described in this thesis was financially supported by the IOP Heavy Metals & Environmental Technology research programme with project number izw98101.

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LIST OF FIGURES

1.1 Factors influencing PPy performance on steel substrates. . . 3 2.1 Conductivity comparison between several materials. . . 8 2.2 Molecular structures of Polyacetylene (PA), Polyaniline (PAni), Polypyrrole (PPy)

and Polythiophene (PTh) . . . 9 2.3 Two extremes of the polypyrrole resonance structure: aromatic (top) and quinoid

(bottom) structures. . . 10 2.4 A simplified concept of semiconductor doping: The removal of one square from

the box on the left enables the movement of the other squares on the right. . . . 11 3.1 Free electron kinetic energy E vs. wave vector k . . . 16 3.2 Energy of a quasi–free electron vs. wave vector . . . 17 3.3 Electron energy vs. wave vector in a semiconductor . . . 18 3.4 quasi–Fermi levels at equilibrium (left), under illumination (middle) and under

illumination at the surface (right) . . . 21 3.5 Surface potential diagrams of clean, uncharged surfaces . . . 22 3.6 Energy diagram of a n-type semiconductor–vacuum interface in the presence

of Shockley states. . . 23 3.7 Energy diagram of a metal–semiconductor interface before and after contact. . 24 3.8 Energy diagram of a metal–semiconductor interface with electron transfer

ac-cording to the thermionic emission model. . . 25 3.9 Depletion and accumulation layers in the space charge region of p–type

semi-conductors in contact with electrolytes. . . 30 4.1 Factors influencing PPy performance on steel substrates. . . 33 4.2 Generic potential transient for galvanostatic polypyrrole electrodeposition on

steel in an aqeous solution of oxalic acid and pyrrole monomer. . . 37 4.3 Formation of FeC2O4·2 H2O crystals on steel. Current density i = 0.56 mA/cm2

at pH 1.4. Processing time indicated, total induction time = 598 s. Substrate coverage by oxalate crystal increases with time. . . 38

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4.4 AFM image of polypyrrole nodules formed on polycrystalline platinum. Depo-sition in acetonitrile medium using a potentiostatic pulse method. Substrate is homogenously covered with polypyrrole nodules. . . 39 4.5 SEM image of polypyrrole nodules formed on iron. Deposition in 0.1M pyrrole

/ 0.05M potassium tetraoxalate aqueous solution using galvanostatic electrode-position (1 mA/cm2, total charge 2.7 C/cm2). . . 39 4.6 Parameters and characteristics related to the deposition of polypyrrole on steel. 40 4.7 PPy deposits on low–carbon steel Q–panels, magnification 10x. Deposition in

0.1M pyrrole + 0.1M oxalic acid solution, 1200 seconds at (a) 1 mA/cm2, (b) 5 mA/cm2_{and (c) 20 mA/cm}2_{. . . .} ₄₁

4.8 Potential transient of oxalate layer deposition on steel using a low (45 µA/cm2) current density in 0.1M oxalic acid + 0.1M pyrrole. Discontinuities in this plot show where no potential data was recorded as EIS experiments were taking place. 43 4.9 Bode and Nyquist representations of galvanostatic EIS experimental data

per-formed during oxalate deposition. The data plotted spans the entire induction time of 3 hours. Current density i = 45 µA/cm2. . . 45 4.10 Equivalent circuit for galvanostatic deposition of ferrous oxalate on steel. See

text for details. . . 45 4.11 Fitted resistance RCT (—) and CPE power () as a function of charge input.

Model deposition experiment on steel using a low (45 µA/cm2) current density in 0.1M oxalic acid + 0.1M pyrrole. . . 45 4.12 Potential transients for depositions 3600A & 3600B in 0.1M oxalic acid and 0.1M

pyrrole. Current density i = 1 mA/cm2. . . 46 4.13 Potential transient for dual–layer depositions 4800A & 4800B in 0.1M oxalic

acid and 0.1M pyrrole, and 5 g/l NaPSS and 0.1M pyrrole. Current density i = 1 mA/cm2_. _{. . . .} ₄₆

4.14 Cross–section of sample 3600A (Supporting nickel layer on top). Layer thick-ness averaged from several micrographs equals 6.5 µm. . . 47 4.15 Cross–section of dual–layer sample 4800B. Total layer thickness averaged from

several micrographs equals 25.6 µm. The dual layer configuration cannot be discerned from the optical micrographs. . . 47 4.16 Nyquist plots of PPy on steel in pH=5 HNO3with 0.1M NaNO3. Bias potentials

plotted are -0.528 VNHE(◦) and -0.628 VNHE(4) . . . 51

4.17 Equivalent circuits designed to model the electrolyte/PPy/steel system with and without an inductive element. . . 51 4.18 Mott–Schottky plot: 2.5µm PPy on Armco sample, measured in 0.01mMol HNO3.

Frequencies plotted are 9.0 (), 11.6 (•) and 15.0 kHz (_N) . . . 52 5.1 Barrier layer obstructing oxidant flow . . . 61 5.2 Damaged barrier layer no longer obstructs oxidant flow . . . 61

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List of Figures

5.3 Schematic band diagram of the p-type semiconductor/metal interface before (left) and after (right) contact. Reference values taken from literature for

polypyr-role and iron. . . 62

5.4 Passivating oxide layer prevents corrosion . . . 64

5.5 The damage to the passive layer can be repaired . . . 64

5.6 Input E and output I signal with shifted phase φ indicated. . . . 68

5.7 OCP transients for polypyrrole/oxalate–coated steel in 0.1M Na2SO4(—) and 0.1M NaCl (- - -). Samples deposited with approx. 4.5 C/cm2charge input. . . 70

5.8 OCP transients for polypyrrole/NaPSS–coated steel in 0.1M Na2SO4(—) and 0.1M NaCl (- - -). Samples deposited with approx. 4.5 C/cm2charge input. . . 70

5.9 Cracked blister on dual layer sample exposed to 0.1M Na2SO4. . . 71

5.10 OCP transients for polypyrrole/oxalate–coated steel in 0.1M Na2SO4(—) and 0.1M NaCl (- - -). Samples deposited with approx. 4.5 C/cm2 charge input. Time scale adjusted to match fig. 5.11 . . . 72

5.11 Evolution of OCP with time for polypyrrole/oxalate coated iron in 3% NaCl (—) and 3% H2SO4(- - -) solutions. Samples deposited with approx. 2 C/cm2 charge input. . . 72

5.12 Equivalent circuit for polypyrrole–coated steel. The meaning of the elements is detailed in the text. . . 75

5.13 Nyquist (left) and Bode (right) plots for PPy/NaPSS coated steel in 0.1M Na2SO4 after 300h of immersion. . . 76

5.14 Nyquist plots for PPy/oxalate coated steel in 0.1M NaCl. Immersion time indi-cated both in hours and in % of time to failure tf. . . 77

5.15 Nyquist plots for PPy/PSS coated steel in 0.1M NaCl. Immersion time indi-cated both in hours and in % of time to failure tf. . . 77

5.16 Nyquist plots for PPy/PSS coated steel in 0.1M NaCl. Immersion time indi-cated both in hours and in % of time to failure tf. Plots cover 105% to 150% tf in detail. . . 77

5.17 Nyquist plots for PPy/oxalate coated steel in 0.1M Na2SO4. Immersion time indicated both in hours and in % of time to failure tf. . . 78

5.18 Nyquist plots for PPy/PSS coated steel in 0.1M Na2SO4. Immersion time indi-cated both in hours and in % of time to failure tf. . . 78

5.19 Nyquist plots for PPy/PSS coated steel in 0.1M Na2SO4. Immersion time indi-cated both in hours and in % of time to failure tf. Plots cover 60% to 110% tfin detail. . . 78

5.20 Refined equivalent circuit for polypyrrole–coated steel without ‘mf’ Voigt cir-cuit. The meaning of the elements is detailed in the text. . . 78

5.21 Schematic representation of blister developing as a result of corrosion under a coating. . . 84

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5.22 Low–frequency resistance response of single and dual layer samples in sodium chloride (left) and sodium sulphate (right). Values plotted represent the real component of the complex impedance data measured at 6.55 mHz. . . 85 5.23 Schematic anodic polarisation curve of a ferrous alloy . . . 90 5.24 Schematic polarisation curves of three systems. . . 91 5.25 e–pH diagram for the iron—water system at 25◦C (iron ionic activities = 10−6,

evs. NHE). . . 92

6.1 Coatings with different pigment volume concentrations (PVC). . . 100 6.2 SEM micrographs of PPy/PBMA latex material showing the conductive shell

in light contrast. Magnifications 7.000x (left) and 10.000x (right). . . 101 6.3 Schematic representation of surface resistivity measurement between two

elec-trodes. . . 105 6.4 Schematic representation of electrochemical cell setup. . . 105 6.5 OCP transient of tosylate (pH=4) doped coatings with different binder

poly-mers. Coated steel samples were immersed in 0.1M LiClO4. . . 108

6.6 OCP transient of PSS–doped coatings with two different Uralac binder poly-mers. Coated steel samples were immersed in 0.1M LiClO4. . . 108

6.7 OCP transients of tosylate doped coatings with different titration endpoint pH’s. Coated steel samples were immersed in 0.1M LiClO4. . . 109

6.8 Molecular structures of selected dopant compounds used. Top: NaPSS. Middle: p-TSA (left) and CSA. Bottom: Briquest 301-50A (left) and Briquest ADPA-60A. 111 6.9 OCP transient of Uralac SN808 based coatings with different dopant anions.

Coated steel samples were immersed in 0.1M LiClO4. . . 111

6.10 OCP transients of PSS doped Uralac SN808 coatings between EIS measure-ments. Coated steel samples were immersed in 0.1M LiClO4. . . 112

6.11 Nyquist (left) and Bode (right) plots for PU core / PPy(PSS doped) coated steel in 0.1M LiClO4. Immersion time indicated both in hours and in % of time to

failure tf. . . 113

failure tf. The dashed line in the right panel represents the data at t=0h. . . 114

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LIST OF TABLES

4.1 Determination of polypyrrole layer thickness (dPPy), standard deviation (σd)

and deposition rate as a function of deposition charge input. . . 47 4.2 Literature references on polypyrrole deposition rates (given in C·cm−2/ µm) . 48 4.3 Flatband potentials and charge carrier densities compared . . . 52 5.1 Work functions eφMof selected metals and theoretical Schottky barrier heights

eφSBin metal/polypyrrole junctions. . . 63

5.2 The time to failure tffor single and dual layer systems. . . 71

5.3 The time to failure tffor single and dual layer systems and 25%, 50% and 75%

fractions. . . 75 5.4 Values for the parameters of the R_Ω(RQ) circuit obtained from the best NLLS fit

to the impedance data. Data for single layer samples at different tffractions. . . 81

5.5 Values for the parameters of the modified Randles part obtained from the best NLLS fit to the impedance data. Data for dual layer samples at different tf

frac-tions. . . 82 6.1 DSM resins used as binder polymers in core–shell latex coatings. . . 103

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1. INTRODUCTION

1.1 Corrosion — an inevitable fact of life?

Ever since man has started to construct and engineer tools, objects and buildings, corrosion has played an important role. Despite the vast number of constructions built in the past, only a few remain, most of which are cyclopean in size (e.g. the Egyptian pyramids, The Great Wall of China).

Corrosion, especially that of metals, has a significant impact on a nation’s econ-omy. This has become more apparent since the Industrial Revolution introduced all kinds of equipment, most of which were made out of metals. The significance of cor-rosion can be demonstrated by expressing the economic loss inflicted: an industrial country loses several percent of its GNP∗to corrosion damage every year [1, 2].

In short, corrosion occurs because materials are used in a non–stable form. Met-als are produced from metal ores from mines around the world. These ores consist of metal oxides or other stable metal compounds that must be decomposed to obtain pure metals. Almost all metals will react spontaneously when in contact with oxy-gen, except for ‘noble’ metals like platinum and gold. The reverse reaction, known as the reduction of the metal oxide, is non–spontaneous and it will therefore effort is required to obtain the pure metal from the ore. For instance, the steel used in a car becomes more stable when it corrodes. As a result, a driving force exists for it to re-vert to its oxidised (corroded) state. This driving force is a fact of nature (a matter of thermodynamics) and can therefore not be taken away. However, the rate at which the oxidation process progresses can be reduced, often to a point where corrosion is virtually non–existent. Many ways have been developed to achieve this goal, the most prominent of which is the use of paints and varnishes.

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1.2 Paint

Historically, the development of paints was not geared towards corrosion protec-tion, but more towards decorative and religious purposes. Many cave paintings have been found, some dating back more than 32,000 years [3]. The oldest paintings are more carved than painted, using chalk and charcoal. The first paints were biolog-ical in origin, possibly using e.g. blood as a binding agent. The pigments used were mainly natural minerals containing oxides of iron [4]. No protective effect was in-tended at that time, and the paint was therefore more a means of applying colourful pigments.

An interesting change in the use of paint came about in the Ancient Egyptian Era (5000 BC). Egyptian mummy cases were varnished with egg white varnish, but some seem to be varnished with an oil. However, the first written evidence of the use of fast-drying vegetable oils in painting would not be until early sixth century A.D. by Aetius Amidenus†. In a text on the medical virtues of oils Aetius (502-575 AD) recommends nut oil varnish for protecting gilt surfaces.

The development of drying oils in paints progresses from Aetius onwards, and these oils are still used in some of today’s environmentally friendly solvent–free paints. The Industrial Revolution (approx. 1750–1830 AD) introduced many new and exciting machines, methods of transportation and constructions. Many of these new developments were made of cast iron or steel, requiring protection against cor-rosion. It is in this era that protection against corrosion with painted coatings became really important.

1.3 Conversion layers

Although a coating may protect the substrate on which is was applied for a long time, it will not last forever. The polymers in the coating may deteriorate under influence of the ultra–violet components in sunlight or physical damage may occur in the form of cracks and pits. When this happens, corrosion of the substrate will eventually occur. Long ago, it was discovered that a treatment of the metal object with a chromium compound called chromate will protect the object in the event of coating failure.

Chromate conversion layers are top–performers in long term corrosion protection of aluminium, zinc and (galvanised) steel. A great amount of research has been

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1.4 Research objective and topical break–down

ducted to improve the protective qualities of chromate conversion layers, resulting in a better understanding of the mechanisms protecting e.g. aluminium [5] and zinc [6]. In spite of the tremendous benefits of chromate conversion layer application, there are serious health and safety concerns. The hexavalent Cr6+that is present in chromate is known to pose a serious health risk [7–10], and therefore a reduction of chromate usage is called for. Indeed, the use of hexavalent chromium in vehicles will be banned by the European Union as of July 2007 [11, 12]. The functions chromate conversion coatings perform are still necessary however, which explains the search for environmentally friendly alternatives. Alternatives under investigation include molybdate and cerate conversion layers, silane treatments and conducting polymer coatings.

The use of intrinsically conducting polymers to replace chromate conversion lay-ers is based on the complex electrochemical behaviour of these polymlay-ers. Many intrinsically conducting polymers possess noble (high) electrochemical potentials, possibly enabling anodic protection of suitable substrates (most notably steels).

1.4 Research objective and topical break–down

The research described in this Ph.D. thesis is part of the search for a more environ-mentally friendly corrosion protection. The objective has been to assess the require-ments and possibilities of corrosion protection using intrinsically conducting poly-mers. In accordance with the research objective, this thesis concerns the corrosion

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protective performance of an intrinsically conducting polymer (polypyrrole or PPy) on steel substrates. In this context, performance relates to the protection that layer can offer the substrate it is applied to. Obviously, the performance must be as high as possible, while keeping the economic and environmental costs minimal. Several factors influence the performance of a PPy coating on steel, as illustrated in figure 1.1. Although thoroughly investigating each of these factors would be the preferred approach, it is impossible to accomplish. The factors indicated in figure 1.1 can each be divided further. Even with the addition of only one level of detail, the number of potential research topics increases dramatically. This thesis will therefore cover only a few of these topics, but where relevant, relations between them will be discussed.

1.5 Thesis outline

A concise introduction to intrinsically conductive polymers (ICPs)will be given in chapter 2. This includes a brief historical perspective, the general molecular structure of ICPs and the basics of electric conductivity in these polymers.

ICPs are known to be extrinsic semiconductors (i.e. they are made conductive by introducing impurities, a process known as doping). Chapter 3 introduces the electrochemistry of semiconductors, which differs significantly from ‘regular’ elec-trochemistry. Some related experimental results are presented in chapter 4 on page 48.

The deposition process of PPy layers on steel substrates is described in chapter 4 from page 33 onwards. Initially, the deposition process described by Beck [13] and Su [14] is investigated. A refined mechanism for the deposition of PPy on steel in oxalic acid is proposed. Additionally, the relations of deposited layer thickness to the charge input as described by Diaz [15] and Pickup [16] are evaluated experimentally. The electrochemical properties of PPy model coatings on steel substrates are ex-amined in chapter 5 on page 59. The mechanisms mentioned in literature concern-ing corrosion protection usconcern-ing intrinsically conductconcern-ing polymers are discussed in section 5.2 on page 60. The layers examined in chapter 5 are model layers that are most suited for fundamental research. A different approach using commercial pro-totype coatings is followed in chapter 6 on page 97. Based on larger scale commercial research, the results presented in chapter 6 have been obtained from dozens of dif-ferent coating compositions. Experimental methods in both chapter 5 and 6 include potential monitoring and EIS. The difficulties encountered while developing a new corrosion protective coating based on polypyrrole are briefly discussed in chapter 6

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References

as well.

Chapter 7 at the very end of this thesis presents all conclusions drawn from the work in described in this thesis. In a general discussion of these results, correlations will be pointed out between the conclusions and the research objective presented here in this introductory chapter.

Acknowledgement

The research described in this thesis is part of a research programme of the Dutch Government. The Department of Education, Culture & Science, the Department of Economic Affairs and the Department of Housing, Planning & Environmental Af-fairs have devised a programme that both researches the impact of heavy metal us-age and searches for ways to reduce emissions to the environment. This IOP En-vironmental Technology / Heavy Metals research programme has sponsored the re-search described here under project number izw98101. It has been conducted at Delft University of Technology, Eindhoven University of Technology and TNO Science & Industry.

References

[1] R. Winston Revie (editor). Uhlig’s Corrosion Handbook. The Electrochemical Society Series, second edition. Wiley, New York, U.S.A., 2000. ISBN 0-471-15777-5.

[2] J. O. Bockris and A. K. N. Reddy. Electrodics in Chemistry, Engineering, Biology, and Environmental Science, volume 2B of Modern Electrochemistry. Second edition. 542 pages, Plenum Press, New York, U.S.A., 2001. ISBN 0-306-46324-5.

[3] M. Balter. “ARCHAEOLOGY: Paintings in Italian Cave May Be Oldest Yet.” Science 290(5491) 419– 421, 2000.

[4] J. D. J. van den Berg. Analytical chemical studies on traditional linseed oil paints. Ph.D. thesis, University of Amsterdam, 2002.

[5] P. Campestrini. Microstructure-related Quality of Conversion Coatings on Aluminium Alloys. Ph.D. thesis, Delft University of Technology, 2002.

[6] X. Zhang. Cr(VI) and Cr(III)-Based Conversion Coatings on Zinc. Ph.D. thesis, Delft University of Technology, 2005.

[7] D. Bagchi, S. J. Stohs, et al. “Cytotoxicity and oxidative mechanisms of different forms of chromium.” Toxicology 180(1) 5–22, 2002.

[8] J. Singh, D. E. Pritchard, et al. “Internalization of Carcinogenic lead chromate particles by cultured normal human lung epithelial cells: formation of intracellular lead-inclusion bodies and induction of apoptosis.” Toxicology and Applied Pharmacology 161(3) 240–248, 1999.

[9] K. Kondo, N. Hino, et al. “Mutations of the p53 gene in human lung cancer from chromate-exposed workers.” Biochemical and biophysical research communications 239(1) 95–100, 1997.

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[10] C. Pellerin and S. M. Booker. “Reflections on Hexavalent Chromium: Health Hazards of an Industrial Heavyweight.” Environmental Health Perspectives 108(9) A402–A407, 2000.

[11] European Communities. “Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 on end-of-life vehicles.” Official Journal of the European Communities 43(L269) 34–42, 21-10-2000.

[12] European Communities. “Commission decision 2002/525/EC of 27 June 2002, amending Annex II of Directive 2000/53/EC (...).” Official Journal of the European Communities 45(L170) 81–84, 29-6-2002. [13] F. Beck, M. Oberst, and R. Jansen. “On the mechanism of the filmforming electropolymerization of

pyrrole in acenitrile with water.” Electrochimica Acta 35(11-12) 1841–1848, 1989.

[14] W. C. Su and J. O. Iroh. “Formations of polypyrrole coatings onto low carbon steel by electrochemical process.” Journal of Applied Polymer Science 65(3) 417–424, 1997.

[15] A. F. Diaz, J. I. Castillo, et al. “Electrochemistry of conducting polypyrrole films.” Journal of Electro-analytical Chemistry 129(1-2) 115–132, 1981.

[16] P. G. Pickup and G. L. Duffitt. “Enhanced Ionic Conductivity of Polypyrrole due to Incorporation of Excess Electrolyte during Potential Cycling.” Journal of the Chemical Society - Faraday Transactions 88(10) 1417–1423, 1992.

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2. INTRINSICALLY CONDUCTING

POLYMERS: AN OVERVIEW

2.1 Introduction

Most common polymers are non-conductive. They are used not only for structural purposes, but for electrical isolation as well. Electrically conductive particles like car-bon black and metal fibres are sometimes added to a polymer to form a conductive polymer blend. These blends are often called extrinsically conducting polymers and may be used in applications where electrostatic discharge (ESD) must be prevented, for instance, cleanrooms for IC production.

In contrast, the intrinsically conducting polymers or ICPs described in this the-sis possess electronic conduction abilities within their own molecular structure. The conductive behaviour of these ICPs has a strong resemblance to semi-conductors and is likewise governed by charge carrier surplus or shortage. A significant difference with metallic semiconductors is the lack of long–distance three–dimensional order-ing, as the polymer chains have a limited length (typically tens of monomer units for polypyrrole).

2.2 Historical overview

Conductance in non–metallic substances has been known about for more than a hun-dred years. In 1862, H. Letheby of the College of London Hospital produced a partly conductive material by anodic oxidation of aniline in sulphuric acid. The material produced was probably a form of polyaniline [1]. Somehow, the unique nature of Letheby’s findings slipped into oblivion for many decades [2]. In 1916, Angeli re-ported on the chemical polymerization of pyrrole [3], which was known to form a conductive pyrrole black under influence of ambient oxygen. Conduction in poly-mers was also reported by Hatano and co-workers in 1961 [4]. Their polyacetylene

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Fig. 2.1: Conductivity comparison between several materials. Taken from [1].

had a conductance of 10−5S/cm, which is a significant number for a material class which, at that time, was thought to consist of isolators. The now extensively used electropolymerisation of pyrrole was first reported by dall’Ollio in 1968 [5].

The research into conducting polymers really took off with the publications re-garding the polyacetylene doping process by Shirakawa, MacDiarmid and Heeger in 1977 [6–8]. This process resulted in a tremendous shift of the conductivity over many orders of magnitude (figure 2.1), producing a highly conductive polyacety-lene. Their research has earned them the Nobel Prize in Chemistry 2000 [1].

The discovery of conducting polyacetylene was a discovery by chance [9]. An at-tempt to prepare polyacetylene failed in the Japanese laboratory of Prof. Shirakawa. Instead of the expected black powder, a bright and glistening silvery film was ob-tained. It turned out that molar quantities of a catalyst were used by mistake, as mil-limolar amounts were sufficient for polyacetylene preparations. When MacDiarmid visited Shirakawa’s lab in 1975, experiences were exchanged on the golden (SN)x

in-organic polymer and the silvery (CH)xpolyacetylene. Heeger and MacDiarmid had

already shown that the conductivity of (SN)xcould be dramatically improved by

ex-posure to bromine vapour. It was decided to expose polyacetylene films to bromine vapour at room temperature, resulting in the now–famous dramatic change in con-ductivity. Doping polyacetylene could make the material change from insulating to semi–conducting and even metal–like in conduction. The polymer allowed new the-ories about conduction in materials and it could be both p- and n-doped via redox reactions. A myriad of opportunities evolved from the sheer happenstance of the mistake in Shirakawa’s lab – a prime example of serendipity [10].

Several polymers have been found to show the same behaviour as polyacetylene, including polypyrrole (PPy), polythiophene (PTh) and polyaniline (PAni). These polymers all show conjugation in their molecular structures (figure 2.2), which is

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2.3 Polypyrrole

Fig. 2.2: Molecular structures of Polyacetylene (PA), Polyaniline (PAni), Polypyrrole (PPy) and

Polythio-phene (PTh)

the basis for their conductive behaviour. The ICPs depicted in figure 2.2 are in-fusible (they have no melting point but decompose at high temperatures) and are very poorly soluble in both organic and inorganic solvents. As a result, the processi-bility of these polymers is very limited [11]. Some techniques have been developed to improve the processibility of the ICPs, including block copolymerisation and the incorporation of flexible parts in the chain [11].

Applications of ICPs can be found in many areas, including transparent conduct-ing films [11] (i.e. anti–static coatconduct-ings), all–organic electronic devices like field–effect transistors [12–14], electromagnetic shielding [15] and corrosion protection [16, 17]. Many of these applications are relatively new, and as a result tangible improvements can be expected in the near future.

2.3 Polypyrrole

2.3.1 Conjugated structure

Polypyrrole is the polymerisation product of the monomer pyrrole (see figure 2.2): in polypyrrole, the repeating unit is pyrrole (C4H5N). Pyrrole is a heterocyclic aromatic

compound, i.e. it possesses a heterogeneous ring–like structure in which electrons can move around. The pyrrole liquid used in the work described by this thesis was produced synthetically, but pyrroles are not just esoteric synthetic compounds. For instance, they contribute significantly to the aroma of one of the most popular bev-erages in the world: brewed coffee [18, 19].

Most of the more common polymers are hydrocarbons — they consist of long chains of linked carbon atoms and contain hydrogen atoms bonded to these carbon atoms. Uncharged carbon atoms contain six electrons, configured as 1s2 _2s2 _2p2_.

The outer four (valence) electrons are available for chemical bonding. In saturated polymers all four valence electrons of each carbon atom are paired to the valence electrons of the four atoms it forms bonds to, e.g. two hydrogen atoms and two carbon atoms in polyethylene. In these saturated polymers, all valence electrons

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are tightly bound in covalent σ–bonds: they are localised. Saturated polymers are the electrically insulating polymers used everywhere where electrical insulation is required, as no mobile electrons are available to transport charges. If a polymer contains double or triple bonds, it is unsaturated.

In a simple unsaturated molecule like ethylene (C2H4), each of the two carbon

atoms has a total of three σ–bonds: one to each of two hydrogen atoms and one to the other carbon atom. The two remaining valence electrons (one for each carbon atom in the molecule) form a second carbon-carbon covalent bond, in which the electrons are less tightly bound than in the first (σ–)bond. The resulting bond is known as a

π–bond and may be considered less localised than the σ–bonds [20].

In conjugated polymers, the carbon chain consists of an alternating sequence of singly and doubly bound carbon atoms. The overlap of the quantum–mechanical wave–functions of the individual π–bonds in the carbon chain causes the electrons in these bonds to be delocalised into a molecular π–orbital. The mobility of charges along this molecular ‘charge highway’ provides the conduction observed in doped conjugated polymers.

Fig. 2.3: Two extremes of the polypyrrole resonance structure: aromatic (top) and quinoid (bottom)

struc-tures.

Similar to benzene, the electron structure in conjugated polymers is highly dy-namic. In polypyrrole, the structure rapidly resonates between the two extremes depicted in figure 2.3. The quinoid structure can be considered as a state higher in energy (excited) of the aromatic structure [21].

2.3.2 Doping polypyrrole

Like many semiconductors, polypyrrole is barely conductive in the default config-uration. It is through the abstraction or the addition of a charge that conduction becomes possible. In this sense, it is much like the situation in figure 2.4: the squares

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2.4 Electric conduction

Fig. 2.4: A simplified concept of semiconductor doping: The removal of one square from the box on the

left enables the movement of the other squares on the right.

in the box cannot be moved around until one of them is removed from the box. The concept described in figure 2.4 is known as semiconductor doping and it can be done by either adding or removing charges from the semiconductor. In polypyr-role oxidative or p–doping is used, where electrons are removed from the polymer backbone in order to make it conductive. To maintain electroneutrality (Kirchhoff’s first law), the removed charges are compensated by anions situated close to the poly-mer molecule. Polypyrrole is a linear molecule, so the charge introduced by doping can only move easily within the physical extend of the polymer chain.

2.4 Electric conduction

Conductivity in solids is defined by Ohm’s Law:

U=I·R (2.1)

This equation states that the application of a voltage U (measured in Volts, V) over a resistor of constant resistance R (unit Ohms,Ω) results in a current of magnitude I (unit Amperes, A). Equation 2.1 is obviously linear and many real–world materials only follow this linearity in a small range of potentials or currents. Some materials even refuse to conform to Ohm’s Law at all. Among those more or less disobedi-ent materials are semiconductors and electrolytic solutions, both of which play an important role in this thesis.

In Ohmic materials, the resistance R of a resistor of length l is equal to: R= ρl

A (2.2)

in which ρ is the specific resistivity of the resistor (inΩ·m) and A equals the cross– sectional area of the resistor. The inverse of the resistivity ρ is the conductivity σ, which is measured inΩ−1m−1(sometimes expressed as Siemens/m or S/m).

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The conductivity of a solid σ is proportional to the charge carrier concentration n and mobility µ (in cm2/Vs):

σ=nµe (2.3)

This equation pertains to electrons, hence the presence of the electron charge e. Non–Ohmic materials require the addition of additional terms to this equation, e.g. to account for additional positively charged carriers in semiconductors (known as ‘holes’).

The electrical properties of a material are governed by the electronic structure it possesses. In metals the electron orbitals of the atoms in the crystal lattice over-lap with neighbouring siblings, similar to molecular orbitals in isolated molecules. The amount of orbitals in a metal is equal to the amount of atoms in the lattice (N). As N becomes very large even for tiny pieces of metal (typically 1022 _{for 1 cm}3_of

metal), the orbitals form continuous bands of energies. The theory on energy bands in solids is described in detail in section 3.2.1 on page 15. In doped intrinsically conducting polymers the molecular π–orbital is split up in bonding and anti–bonding energy levels, thus producing analogues of the valence and conduction bands in inorganic semiconductors. The conductivity of semiconductors is governed by the amount of mobile charge carriers available for conduction. In intrinsically conduct-ing polymers, the conductivity can be controlled by the degree of dopconduct-ing applied.

2.5 Concluding remarks

In this chapter, a brief historical overview of the serendipitious discovery of intrin-sically conducting polymers is given, followed by the introduction of polypyrrole and its doping process. A concise introduction of electric conduction in materials concludes this chapter.

References

[1] Royal Swedish Academy of Sciences. “Laudatio Nobel Prize Chemistry 2000: Conductive poly-mers.”, 2000.

[2] F. von Kieseritzky. New Oligothiophenes. Ph.D. thesis, Kungliga Tekniska H ¨ogskolan, Stockholm, 2004.

[3] A. Angeli. Gazzetta Chimica Italiana 46 279, 1916.

[4] M. Hatano, S. Kambara, and S. Okamoto. “Paramagnetic and electric properties of polyacetylene.” Journal of Polymer Science 51(156) S26–S29, 1961.

[5] A. dall’Ollio, G. Dascola, et al. “Resonance paramagnetique electronique et conductivit´e d’un noir d’oxypyrrol electrolytique.” Comptes Rendus d’Acad´emie des sciences C267 433, 1968.

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References

[6] H. Shirakawa, E. J. Louis, et al. “Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x.” J. Chem. Soc. Chem. Commun. (16) 578–580, 1977.

[7] C. K. Chiang, C. R. Fincher, et al. “Electrical Conductivity in Doped Polyacetylene.” Physical Review Letters 39(17) 1098–1101, 1977.

[8] C. K. Chiang, M. A. Druy, et al. “Synthesis of highly conducting films of derivatives of polyacetylene, (CH)x.” Journal of the American Chemical Society 100(3) 1013–1015, 1978.

[9] N. Hall. “Twenty-five years of conducting polymers.” Chemistry Communications 1(1) 1–4, 2003. [10] R. M. Roberts. Serendipity: accidental discoveries in science. The Wiley Science Editions. 270 pages, John

Wiley & Sons, 1989. ISBN 0-471-60203-5.

[11] F. M. Huijs. Thin Transparent Conducting Films based on Core-Shell Latexes. Ph.D. thesis, Rijksuniver-siteit Groningen, 2000.

[12] D. van Delft. “Gooi- en smijtelektronica: Philips zet grote stap richting flexibele displays.” NRC Handelsblad 16-9-2000(Wetenschapsbijlage) 49, 2000.

[13] C. Jongeneel. “Slap in Glas.” de Volkskrant 16-9-2000(Wetenschapsbijlage) 1W, 16-9-2000.

[14] D. Hohnholz and A. G. MacDiarmid. “Line patterning of conducting polymers: New horizons for inexpensive, disposable electronic devices.” Synthetic Metals 121(3) 1327–1328, 2001.

[15] R. V. Gregory, W. C. Kimbrell, and H. H. Kuhn. “Conductive textiles.” Synthetic Metals 28(1-2) 823– 835, 30-1-1989.

[16] D. E. Tallman, G. Spinks, et al. “Electroactive conducting polymers for corrosion control. Part 1. General introduction and a review of non-ferrous metals.” Journal of Solid State Electrochemistry 6(2) 73–84, 2002.

[17] G. Spinks, A. Dominis, et al. “Electroactive conducting polymers for corrosion control. Part 2. Ferrous metals.” Journal of Solid State Electrochemistry 6(2) 85–100, 2002.

[18] M. D. Fuster, A. E. Mitchell, et al. “Antioxidative Activities of Heterocyclic Compounds Formed in Brewed Coffee.” Journal of Agricultural and Food Chemistry 48(11) 5600–5603, 2000.

[19] K. Yanagimoto, K.-G. Lee, et al. “Antioxidative Activity of Heterocyclic Compounds Found in Coffee Volatiles Produced by the Maillard Reaction.” Journal of Agricultural and Food Chemistry 50(19) 5480– 5484, 2002.

[20] A. P. Monkman. “Physics of Conductive Polymers.” In M. C. Petty, M. R. Bryce, and D. Bloor (editors), An introduction to Molecular Electronics, book chapter 7, pages 142–167. Edward Arnold, London, U.K., 1995. ISBN 0-340-58009-7.

[21] R. C. G. M. van der Schoor. Applying Polypyrrole on Plastics for the Electrodeposition of Copper. Ph.D. thesis, Delft University of Technology, 2000.

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3. SEMICONDUCTOR ELECTROCHEMISTRY

3.1 Introduction

The field of semiconductor electrochemistry is a combination of solid state physics and electrochemistry. A thorough understanding of both physical and chemical as-pects is required to appreciate the differences between metal electrodes and semicon-ductor electrodes. In this chapter, an introduction to solid state physics is given first, followed by sections on semiconductor electrochemistry. Experimental work on this topic is discussed in chapter 4. In an attempt to provide a concise and self–contained coverage of semiconductor electrochemistry, some of the sections in this chapter are less applicable to the topics covered in this thesis. Many literature sources have been consulted, of which the excellent books by Sato[1], Tanner[2] and Memming[3] and a review article by Gerischer[4] were exceptionally useful.

3.2 Solid state physics

3.2.1 Energy levels in solids

The electronic structure of solids is part of the physical model of the solid state. It is built upon basic quantities of free electron physics, which will be introduced here first. A free electron in space can be described both in classic mechanical terms and in quantum mechanical terms. A combination of these descriptions leads to the relation of the electron’s wavelength λ, mass me, momentum p and velocity v derived by De

Broglie in 1924:

λ= h

p = h

mev (3.1)

in which h is Planck’s constant. The electron wave may also be described by its wave vector~k:

~_k₌ 2π

λ =

2π

h p (3.2)

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Fig. 3.1: Free electron kinetic energy E vs. wave vector k E= 1 2mv 2₌ h2 8π2_m~k 2 _(3.3)

A plot of this parabolic energy equation is depicted in figure 3.1. In a metal, the electrons are not completely free as they are bound to the crystal’s dimensions. Ac-cordingly, the kinetic energies and corresponding wave vectors are bound to these dimensions as well. Therefore, the energy levels (and the corresponding wave vec-tors~k) allowed for the electrons can no longer be described by equation 3.2. A quan-tum mechanical treatment (described in detail by Tanner [2]) leads to an equation in which the dimensions of the crystal are imposed on the wave vector of quasi–free electrons:

~_k₌ πn

L (3.4)

In this equation, L is the length of a crystal cube’s rib and n is a non–zero integer number known as the quantum number. Substituting equation 3.4 into equation 3.3 yields an expression for the kinetic energy of quasi–free electrons:

E= h

2_n2

8mL2 (3.5)

This equation excludes certain energy levels, indicated in figure 3.2 as forbidden gaps. Inside these gaps, no valid wave vectors exist. The dimensions of a crystal are usu-ally such that the allowed k values (cf. equation 3.4) are quite small, yielding a quasi– continuum in the E−k plot.

A treatise of the electronic band structure of solid state matter requires an ex-act description of the electron’s wave function. This wave function can be obtained by solving one of the basic equations in quantum mechanics: the Schr¨odinger tion. Introduced by the Austrian physicist Erwin Schr ¨odinger in 1926, this equa-tion defines electron wave vectors in terms of standing waves. For a metallic solid,

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3.2 Solid state physics

Fig. 3.2: Energy of a quasi–free electron vs. wave vector

the Schr ödinger equation describes the movement and energy of the vast amount of electrons it contains. However, semiconductors and insulators have very few of these (quasi–)free electrons, rendering the solutions provided by the Schr ödinger equation invalid. Fortunately, this incompatibility has been solved by applying the Bloch theorem: The potential energy functions U(~r)in crystalline solids have the same periodicity as the crystal lattice. The Schr ödinger equation for this situation is given in equation 3.6:

E= h

2

8mL2n

2 _(3.6)

and a wave function solution is given by

ψ_~k(~r) =ej~k~rUn(~k~r) (3.7)

In this equation, Un(~k~r) has a periodicity of~r and n represents the ‘band index’.

In a uni–dimensional case the periodicity equals one of the lattice vectors~a,~b or~c. For N uni–dimensional unit cells of dimension a, the maximum k value of k= π/a

is obtained at n = N. This maximum value marks the edge of the Brillouin zone representing all k values in reciprocal space up to π/a. As larger k values lead to a repetition of the first Brillouin zone, only k values of π/a and below are considered in the solid’s band structure.

Using Bloch’s theorem, a solution of the Schr ¨odinger equation for a semiconduc-tor yields two bands of energy values, separated by a ‘forbidden zone’ known as the band gap Eg. A schematic plot is given in figure 3.3. In this plot, the upper

(conduc-tion) band has a parabolic appearance similar to that of figure 3.1, but in practice there may be significant deviations from parabolic behaviour. To allow free electron relations to be applied to semiconductor crystals, the energy bands must be fitted to the (parabolic) free electron energy profile. This can be achieved by introducing an

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Fig. 3.3: Electron energy vs. wave vector in a semiconductor

effective mass m∗in the energy model. This mass is given by equation 3.8: m∗= h 2 4π2 1 d2_E dk2 (3.8) This equation relates the effective mass m∗to the second derivative of the E vs. k plot. For the valence band (depicted in figure 3.3), a negative effective mass would be obtained. Physically, this cannot be readily accepted, hence it must be assumed that the conduction in this band is due to positively charged electron holes instead of electrons. Experimentally, the positive charge of valence band charge carriers has been verified by Hall measurements.

In figure 3.3, the extremes of band energy values coincide at k=0. This situation is known as a direct band gap. In many cases the conduction band minimum does not occur at the same~k as the valence band maximum, a situation known as an indirect band gap. Such indirect band gaps can give rise to several effects, including non– linear optical behaviour and the induction of acoustic lattice vibrations (phonons).

3.2.2 Density of states

Given a uniform distribution of points in E−k space and the assumption that sur-faces of equivalent energy are spherical, the volume between the sursur-faces of energy E and E+∆E equals 4πk2_{dk. The volume of a sphere of energy E equals 8π}3_/V,

and each energy level may be occupied by two electrons of opposite quantum spin (Pauli’s principle). For this situation, the density of states D(E)dE per unit volume is given by: D(E)dE= 8πk 2 8π3 dk= k2 π2dk (3.9)

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When this equation is substituted into the Schr ¨odinger equation, taking into account the definition of effective mass (equation 3.8), the density of states equation for both valence and conduction bands in semiconducting materials is obtained:

D(E)dE= 1

2π2_h3(2m ∗₎3/2

E1/2dE (3.10)

The energy E is measured with respect to the band edge, and the density of energy states up to a certain energy level is given by:

N(E) = 1

3π2_h3(2m ∗₎3/2

E3/2 (3.11)

Equation 3.11 is obtained by integration of equation 3.10.

Intrinsic semiconductors

Using equation 3.11, the number of electrons occupying levels in the conduction band n can be calculated by

n=

Z ∞

Ec

N(E)f(E)dE (3.12)

in which f(E)is the Fermi-Dirac distribution given by

f(E) = 1

1+expE−EF

kT

(3.13)

where EFis the Fermi level. No analytical solutions exist to the integral defining n, but

by imposing additional restrictions a solution may be obtained. Here, it is assumed that(E−EF)/kT1, allowing the following formulation for n:

n=Ncexp −Ec−EF kT (3.14) in which Ncis the density of energy states within a few kT from the conduction band

edge:

Nc= 2

(2πm∗ekT)3/2

h3 (3.15)

Using an effective mass m∗e equal to the electron mass me, a density of states of Nc≈

5×1019 _cm−3 _{is obtained within 1 kT of the conduction band lower edge. Most}

semiconductors have a charge carrier density that is orders of magnitude less, so most energy levels cannot be occupied.

Analog to the derivation of n, the number of holes occupying levels in the valence band p is defined as:

p=Nvexp −Ev−EF kT (3.16)

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with

Nv=

2(2πm∗_hkT)3/2

h3 (3.17)

in which m∗_his the effective hole mass. In an intrinsic semiconductor, charge neu-trality implies equal hole and electron densities. This allows the Fermi level EFto be

calculated: EF = Ec+Ev 2 +kT ln Nv Nc = Ec+Ev 2 + kT 2 ln _m∗ h m∗e 3/2 (3.18) From this equation, it can be verified that for equal charge carrier effective masses, the Fermi level is located exactly in between the band edges. The intrinsic charge carrier density nican be obtained by multiplying n and p:

np=NcNvexp −EG kT =n2_i (3.19)

An estimate of nifor a bandgap of 1 eV while assuming m∗e = meyields ni ≈ 1011

cm−3.

Extrinsic or doped semiconductors

In pure semiconductors crystals, only a few charge carriers are available. As a result, the conductivity of such crystals is low. By introducing charged impurities into the crystal, the charge carrier density is dramatically increased. This process is known as doping and it changes the average charge per atom or unit cell in a structure without changing the structure itself.

The introduction of impurities in semiconductor crystals creates energy levels within the band gap. Two types of impurities each generate their own energy levels: donor levels and acceptor levels. Donor levels have a positive charge when occupied (by a hole) and acceptor levels have a negative charge when occupied (by an elec-tron). Both are neutral when empty.

The inclusion of donor (n–type material) or acceptor (p–type material) impuri-ties influences the charge neutrality of the crystal, forcing the Fermi level to adjust itself accordingly. For a n-type semiconductor, the charge neutrality equation under influence of the ionised donor density N_D+can be expressed as:

n=N_D++p (3.20)

in which N_D+is related to the Fermi function by N_D+= (1− f)ND=ND  1− 1 1+expED−EF kT   (3.21)

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If the amount of impurities is very large, the Fermi level EFmay exceed the

conduc-tion band edge and the semiconductor will be in a state of degeneracy. This means that the semiconductor shows metal–like behaviour, invalidating most relations de-rived for semiconducting material phenomena.

3.2.3 Non-equilibrium conditions

Under equilibrium, the electrochemical potential in a crystal is constant throughout the crystal. It equals the Fermi level in this situation. However, when the charge carrier density is increased by e.g. excitation by incident light, np>n2_i. This implies that the electron and hole densities no longer relate to the same Fermi level. Instead, two quasi–Fermi levels are introduced, one for each charge carrier species:

EF,n = Ec−ln Nc n (3.22) EF,p = Ev−ln Nv p (3.23)

If the light excitation changes both charge carrier densities by the same amount, i.e. ∆n= ∆p in such a way that ∆p_≫ p0and∆n ≪n0, only the hole Fermi level is

changed significantly. The situation described will however only occur at the very surface of the semiconductor, as light cannot penetrate most semiconductors very well. Accordingly, the degree of Fermi level separation is a function of the distance to the crystal’s surface, as depicted in figure 3.4.

Fig. 3.4: quasi–Fermi levels at equilibrium (left), under illumination (middle) and under illumination at the

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Fig. 3.5: Surface potential diagrams of clean, uncharged surfaces

3.3 Semiconductor surfaces and solid state

junctions

3.3.1 Vacuum

For uncharged and clean metal and semiconductor surfaces, schematic plots of sur-face potentials are drawn in figure 3.5. The reference energy level Evac∞ is the energy

of an electron in vacuum at infinitely large distance. The Fermi level is related on the work function eφ in all three cases. The ionisation energy I however is different for metals and semiconductors: in metals, it equals the work function and the electron affinity EA, in semiconductors it equals the energy required to excite a valence band

electron. The electron affinity of semiconductors corresponds to the energy gained when a free electron is placed in the conduction band. For crystals with uncharged surfaces, the Fermi level EFconsists of two contributions:

EF =µe−eχ= −eφ (3.24)

where µeis the chemical potential and eχ represents an electrostatic term that

origi-nates from charged dipoles present in many surfaces.

A phenomenon often encountered at semiconducting surfaces is the presence of surface states. When surface states are present, additional energy levels exist in the band gap, close to the band edges. Two types of surface states exist, intrinsic and extrinsic states. The extrinsic type is a result of external influences like strongly ad-sorbed species, whereas the intrinsic type is a result of the very presence of a surface the marks the end of the crystal. Intrinsic states can be distinguished in covalent and

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3.3 Semiconductor surfaces and solid state junctions

Fig. 3.6: Energy diagram of a n-type semiconductor–vacuum interface in the presence of Shockley states.

ionic subtypes, also known as Shockley and Tamm states, respectively.

Tamm states are attributed to severed bonds in the surface, and these do not in-duce changes in the position of the band edges. Tamm states mainly occur in ionic composition semiconductors like zinc blende (ZnS) and cadmium blende (CdS). Co-valently bound semiconductors like Si and Ge sport Shockley type states, attributed to surface charge buildup in the presence of surface radicals. The charge buildup causes a change in the band structure, often described as band bending in literature. An example of this effect is shown in figure 3.6.

3.3.2 Metal–semiconductor contacts

A contact made between two metals or between a metal and a semiconductor (known as a Schottky junction) equates the Fermi levels of both materials, assuming thermal equilibrium. In absence of surface states and for a n-type semiconductor with a Fermi level exceeding that of the metal, the situation is depicted in figure 3.7. When contact is established, electrons will flow from the material with the highest Fermi level to that with the lowest Fermi level, creating a positive charge in the semicon-ductor displayed in figure 3.7. A charge is built up on both sides of the interface, and because the semiconductor has a low charge carrier density this will result in a posi-tively charged volume known as the space charge layer. The thickness of this layer is proportional to the Fermi level difference between the metal and the semiconductor and to the charge carrier density of the semiconductor. The occurrence of a charge buildup results in band–bending.

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Fig. 3.7: Energy diagram of a metal–semiconductor interface before and after contact.

the semiconductor work function (φs) can be expressed by:

Vk=φm−φs (3.25)

This potential difference cannot be measured directly because any closed circuit will negate the contact potential differences it contains. The potential barrier height present in a metal / n–type semiconductor junction is defined by eφb(n):

eφb(n) =eφm−EA (3.26)

in which EA equals the electron affinity of the semiconductor. The potential

differ-ence in the space charge layer amounts to:

eφSC=eφm− (EC−EF) (3.27)

In junctions between metals and n–type semiconductors, conduction band electrons are transferred. In metal / p–type semiconductor junctions, valence band electrons are transferred and a different equation must be used:

eφb(p) =Eg− (eφm−EA) (3.28)

From these equations it follows that

eφb(p) +eφb(n) =Eg (3.29)

If surface states do exist in the semiconductor, the buildup of space charge is significantly less. The surface states will be able to exchange at least part of the charge with the metal, if not all of it. Accordingly, the energy scheme looks different as the contact and barrier potential differences are now concentrated across a very limited layer. This phenomenon is also known as Fermi level pinning.

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3.3 Semiconductor surfaces and solid state junctions n-type semiconductor EV EC EF efb efsc metal vX

Fig. 3.8: Energy diagram of a metal–semiconductor interface with electron transfer according to the

thermionic emission model.

3.3.3 Majority charge carrier transport

Assuming a thermal equilibrium that is not disturbed by the passing of electric cur-rent and a barrier height eφb > kT, the thermionic emission model can be used to

de-scribe electron transfer across a metal–semiconductor Schottky junction.

For a given n–type semiconducting material, the exchange current js→m is

de-termined by the amount of electrons with an energy exceeding that of the barrier height, as depicted in figure 3.8.

According to the model, the number of electrons dn having an energy of dE above the barrier height eφb+Ecis calculated. These electrons have an average drift

velocity vx, yielding the net electric current js→m. In absence of an externally applied

potential U, the total current must be zero, so the forward and backward currents are equal in size. The exchange current density j0is given by:

j0=js→m= jm→s= −AT2exp −eφb kT (3.30) In which A equals Richardson’s constant:

A= 4πem0kT h3 m∗ m0 (3.31) And the total net current j as a result of the externally applied potential U is given by: j=j0 exp eU kT −1 (3.32)

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in which the exchange current density j0defined by equation 3.30 can be substituted.

3.3.4 Minority charge carrier transport

Apart from systems in which the current is carried by majority charge carriers, mi-nority charge carrier transport is possible. This may occur when the barrier height approaches the bandgap energy, enabling electrons to transfer via the valence band. For n–type semiconductor / metal junction, holes may be injected into the semi-conductor’s valence band if it is polarised positive with respect to the metal. As a result of this hole injection, the charge carrier density differs from the equilibrium value:

pn>n2_i (3.33)

In n–type semiconductors, the quasi–Fermi level for holes has a lower energy than that of electrons (EF,p < EF,n). Accordingly, the amount of minority charge carriers

increases proportionally to the externally applied potential U:

eU=EF,n−EF,p (3.34)

The current density resulting from this potential is not only dependent on the mag-nitude of this potential, but on charge carrier diffusion as well. In n–type materials, the diffusion of holes dominates the current density. Assuming a linear concentra-tion profile within the diffusion length L, the current density for minority carrier transport conditions is given by:

jp=j0 exp eU kT −1 (3.35) with the exchange current density j0defined by:

j0=

eDpn2_i

n0Lp (3.36)

in which Dpis the hole diffusivity and n0the semiconductor’s bulk electron density.

The fundamental differences between majority and minority charge carrier trans-port are evident: whereas minority charge carrier transtrans-port is governed by mater-ial specific parameters like carrier concentrations and diffusion constants, majority charge carrier transport is influenced by the barrier height, charge carrier mass and temperature.

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3.4 Electrochemistry

3.4 Electrochemistry

3.4.1 Chemical and electrochemical potentials

The chemical potential µ of a species i in phase α is defined by the following equa-tion: µα_i = ∂G ∂ni T,p,nj6=i (3.37) in which G equals the Gibbs free energy, ni the amount of moles in the phase α.

Temperature, pressure and concentration are kept constant. In an ideal case, the chemical potential is proportional to the concentration of the species, as given by:

µi=µ0_i +RT ln ci

c0_i (3.38)

where µ0_i represents the chemical potential of species i at the standard concentration c0

i. Unfortunately, ideal cases are rare and in this case the mutual interaction of the

ions in solution causes a deviation of the former equation. An activity coefficient f , also dubbed the fugacity, is introduced to allow the correction of the equation:

µi=µ0_i +RT ln f ci f c0 i =µ0_i +RT ln ai a0 i (3.39) in which the activity aiapproaches the concentration cifor dilute solutions.

In the case of charged species, the chemical potential must include the additional energy involved with displacing a charge in a potential field. The electrochemical potentialµ¯iis introduced accordingly:

¯

µi =µi+ziFφ (3.40)

In this equation, ziFφ equals the potential energy of the particle, comprised of the

charge number zi, the Faraday constant F and the electric potential φ in Volts.

Equi-librium in a system requires the electrochemical potential of a species to be equal in all phases it exists in.

3.4.2 Potentials of electrodes in electrolytes

The potential difference over an electrode/electrolyte interface is governed by dif-ferences in free energy and charge, as detailed in section 3.4.1. Measured against a chosen reference electrode, the potential of an electrochemical system in equilibrium can be determined by the Nernst equation:

E=Ere f +RT nF ln aox ared (3.41)

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where E and Ere f represent the cell voltage and and the reference cell voltage, R the gas constant, nF the amount of elementary charges transferred times the Faraday number and aoxand aredthe activity of oxidiser and reductor, respectively. A detailed

derivation of this equation is provided by many authors, for instance by Bockris et al. in [5].

3.4.3 Comparing redox potentials to Fermi levels

Redox chemistry describes molecules or ions in solution undergoing oxidation or reduction by the transfer of n electrons:

RedOx+ne− (3.42)

In electrochemical potential representation, this reaction can be written as: ¯

µe,redox=µ¯red−µ¯ox (3.43)

This can be related to the activities of the dissolved species by applying the according equations:

¯

µe,redox =µ0_red−µ0_ox−RT ln cox

cred

+zFφsol− (z+1)Fφsol (3.44) The reference potential for this equation is E∞vac, allowing a direct comparison

be-tween the redox potential of a system and its accordingly referenced Fermi level. However, some ‘rescaling’ is required because chemical potentials are expressed in Joules/mole whereas electronvolts (eV) are used for Fermi levels. This yields the following equation:

EF,redox =

e

Fµ¯e,redox (3.45) If equilibrium is reached in a solid/solution contact, the solid’s Fermi level equals the solution’s Fermi level:

EF,redox =EF (3.46)

As a matter of convenience, potentials in electrochemical cells are measured against the Normal Hydrogen Electrode (NHE), rather than the somewhat esoteric E∞vac. A

relationship between E∞vacand NHE has been determined by Lohmann in his 1967

equation for the absolute electron energy of a redox couple [6]:

Polypyrrole electrochemistry: Environmentally friendly corrosion protection of steel: (im)possibilities

Polypyrrole Electrochemistry

Environmentally friendly corrosion protection of steel: (im)possibilities

Polypyrrole Electrochemistry

Environmentally friendly corrosion protection of steel: (im)possibilities

Proefschrift

CONTENTS

Table of Contents

1. Introduction

2. Intrinsically conducting polymers: an overview

3. Semiconductor electrochemistry

4. Electrochemically deposited polypyrrole model coatings on steel

5. Polypyrrole model coatings on steel substrates

6. Polyurethane / polypyrrole latex coatings on steel

7. Conclusions & General discussion

Summary

Samenvatting

Publications related to this work

NaDankwoord

Curriculum Vitae

LIST OF FIGURES

LIST OF TABLES

1. INTRODUCTION

1.1

Corrosion — an inevitable fact of life?

1.2

Paint

1.3

Conversion layers

1.4

Research objective and topical break–down

1.5

Thesis outline

Acknowledgement

References

2. INTRINSICALLY CONDUCTING

POLYMERS: AN OVERVIEW

2.1

Introduction

2.2

Historical overview

2.3

Polypyrrole

2.3.1

Conjugated structure

2.3.2

Doping polypyrrole

2.4

Electric conduction

2.5

Concluding remarks

References

3. SEMICONDUCTOR ELECTROCHEMISTRY

3.1

Introduction

3.2

Solid state physics

3.2.1

Energy levels in solids

3.2.2

Density of states

Intrinsic semiconductors

Extrinsic or doped semiconductors

3.2.3

Non-equilibrium conditions

3.3

Semiconductor surfaces and solid state

junctions

3.3.1

Vacuum

3.3.2

Metal–semiconductor contacts

3.3.3

Majority charge carrier transport

3.3.4

Minority charge carrier transport

3.4

Electrochemistry

3.4.1

Chemical and electrochemical potentials