Studies of electrodes modified with zeolites and poly(4 nitro 2 phenylenediamine) and their composite

Chemically modified electrode is defined as "an electrode made of a conducting or semiconducting material that is coated with a selected monomolecular, multimolecular, ionic, or polymeri

CHAPTER I

Introduction

Since the 1960s, electrochemists have shown interest in the occurrence and consequence of adsorption of ions and molecules on electrode surfaces Adsorption can have both desirable and deleterious consequences, and adsorption research has been adjunct to numerous fundamental insights into the electrical double layer and the kinetics and mechanisms of electrochemical reactions A great deal of information has accumulated from what species adsorb on various electrodes in different solvents and electrolyte media In some instances, adsorption phenomena are easily explained based on chemical reactivity or solubility grounds, the adsorption of simple metal complexes on mercury electrodes being a case in point However, to a substantial extent, the discovery of an adsorption phenomenon is an empirical event, and the exploitation of it for useful purposes has had few systematic or fundamental studies Chemically modified electrodes (CMEs) as discussed here diverge sharply from the traditional field of adsorption on electrode surfaces The most essential difference is that one deliberately seeks in some hopefully rational fashion to immobilize a chemical on an electrode surface so that the electrode thereafter displays the chemical,

electrochemical, optical, and other properties of the immobilized molecule(s) [1-4]

Recently, the terminology, chemically modified electrodes, has been clearly delineated and a short lexicon of related terms provided Chemically modified electrode is defined as "an electrode made of a conducting or semiconducting material that is coated with a selected monomolecular, multimolecular, ionic, or polymeric film of a chemical modifier and that by means of faradaic (charge transfer) reactions

or interfacial potential difference (no net charge transfer) exhibits chemical, electrochemical, and/or optical properties of the film" [3]

CMEs are a relatively modern approach in electrode systems that find utility

in a wide spectrum of basic electrochemical investigations, including the

relationship of heterogeneous electron transfer and chemical reactivity of electrode surface chemistry, electrostatic phenomena at electrode surfaces, and electron and ionic transport phenomena in polymers; the design of electrochemical devices and systems for applications (e.g chemical sensing, energy conversion and storage, molecular electronics, electrochromic displays, corrosion protection, and electro-organic syntheses, etc) are the ideas and motivations associated with many of the recent and current researches on electrodes bearing immobilized chemicals

In this chapter, Section I discusses the chemical and physical routes for deliberate, hopefully stable immobilization of molecular systems on electrodes and the electrochemical and other consequences of this Section II provides a discussion

of the techniques that have been used or invented to detect the electroactivity, chemical reactivity, and surface structure of electrode-immobilized molecules and films In Section III, applications in both applied science and fundamental are presented These descriptions are brief plus a few virtues, promises, and limitations Finally, the objectives of this thesis are discussed

1.1 Preparation of chemically modified electrodes

Molecular species for preparing chemically modified electrodes fall into three broad categories: monomolecular layers, multimolecular layers, and spatially defined, molecularly heterogeneous layers [2] The manners of preparation and the uses of these modifiers are generally distinctive to each category, as will be indicated in the sections that follow

1.1.1 Electrodes modified with monomolecular layers

Monomolecular layers modified electrodes can be further classified according

to the principal routes used to immobilize the substrates, which can be grouped as

chemisorption, covalent bonding and hydrophobic layers [1,2]

1.1.1.1 Chemisorption

Chemisorption is an adsorptive interaction between a molecule and a surface

in which electron density is shared by the adsorbed molecule and the surface [5,6] Chemisorption requires direct contact between the chemisorbed molecule and the electrode surface; as a result, the highest coverage achievable is usually a monomolecular layer In addition to this coverage limitation, chemisorption is rarely completely irreversible In most cases, the chemisorbed molecules slowly leach into the contacting solution phase during electrochemical or other investigations of the chemisorbed layer For these reasons, electrode modification via chemisorption was quickly supplanted by other methods, most notably polymer-coating methods

1.1.1.2 Covalent bonding

Reagents can also be attached covalently to surfaces On the electrode with oxide surfaces, metal hydroxyl is a natural terminator of the oxide phase For carbon, the edges of basal plane sheets tend to be terminated with oxidized sites including

carboxylic acid groups These two kinds of surface functionalities lead to a fairly versatile monomolecular layer surface bonding chemistry [7-9]

Organosilane chemistry, commonly used to prepare chromatographic stationary phases, can also be used to modify surfaces containing hydroxyl groups [10,11] The covalent schemes include procedures for attaching both monomolecular layer and (in polymeric form) multimolecular layer quantities of electroactive sites by the natural or inducible functional groups available on the electrode surface Because

of this interesting synthetic strategy, covalent attachment of functional groups remains

an attractive approach to modifying electrode surfaces

1.1.1.3 Hydrophobic Layers

The "stiff" model theme has become important in recent works in which hydrophobic chain interactions have been invoked to produce structurally organized monolayers on electrodes [2] These monolayers are of two broad types, those relying for molecular organization on hydrophobic effects plus compression as Langmuir-Blodgett (LB) films [12,13] and those based on self-assembly of hydrophobic chains with a chemisorbable terminus [14,15] In the LB experiment, a long-chain hydrophobic target molecule, which may have an electron donor or acceptor group at one terminus, is spread and compressed as a monolayer at the air-water interface in a Langmuir trough, and then transferred to the electrode Self-assembly is also a powerful method for electrode modification It has the virtue of operational simplicity, but lacks the molecular layer compression variable (Langmuir trough surface pressure) of the LB experiment In self-assembled films, chemisorption occurs onto the electrode from the solutions of functionalized hydrophobic molecules, such

as fatty thiols, sulfides, disulfides, silanes onto Au surfaces, and nitriles on Pt

1.1.2 Electrodes modified with multimolecular layers

Typically, the following materials are employed for multimolecular layers modified electrodes: polymer and inorganic film

1.1.2.1 Polymers

By using polymeric modifying layers, fairly thick films, containing many more electroactive sites than a monolayer, can be formed on an electrode surface The introduction of electrochemically reactive polymer materials was an important development in molecularly designed electrode surfaces [1,2] A further categorization for polymeric multilayers can be made with respect to the electronic character of the electron donor-acceptor sites Polymeric multilayer species with

delocalized electronic states, such as poly(aniline), are usually referred to as

electronically conducting polymer; Molecular layers in which the donor-acceptor sites

are electronically well defined and localized as molecular states, such as ferrocene,

are referred to as redox polymer; Polymeric ion-exchange materials with redox ion,

such as Nafion®, are usually referred to as ion-exchange polymer (loaded ionomer)

[4] Various methods are used to prepare polymer-modified electrodes [1,2]:

A Dip coating This procedure consists of immersing the electrode in a solution of

the polymer for a period sufficient for spontaneous film formation to occur by adsorption The film quantity in this procedure may be augmented by withdrawing the electrode from the solution and by allowing the film of polymer solution to dry on the electrode

B Solvent evaporation A droplet of a solution of the polymer is applied to the

electrode surface and the solvent is allowed to evaporate A major advantage of this approach is that the polymer coverage is immediately known from the original polymer solution concentration and droplet volume

C Spin coating The electrode is set spinning after a drop of the polymer solution is

placed on the surface Excess solution is spun off the surface and the remaining thin polymer film is allowed to dry Multiple layers are applied in the same way until the desired thickness is obtained

D Electrochemical deposition This relies on the dependence of polymer

solubility with oxidation (and ionic) state, so that film formation will occur, often irreversibly when a polymer is oxidized or reduced to its less soluble state

E Organosilanes This is a particularly useful chemical basis for polymer films

because bonding to the electrode (SnO2, Pt / PtO, etc.) as well as polymer cross-linking (-SiOSi-) can occur Organosilane monomers can be polymerized under dip coating or droplet evaporation conditions as mentioned above, or a vinyl copolymer can first be formed between the vinyl monomer of interest (vinylferrocene)

or styrene sulfonic acid and a silane monomer

F Radiofrequency polymerization Forming polymeric materials by exposing vapors

of monomers to a radio-frequency plasma discharge is a well-known polymer-filming method Upon exposure to air, plasma films typically take up oxygen and contain other unknown functionalities as a result of chemical damage in the RF discharge

G Electrostatic binding of redox ion When the film is to be employed as an ion as

exchanger, scavenging ionic redox species from solutions, it is of course deposited first by one of the procedures described above

H Electropolymerization A solution of monomer is oxidized or reduced to

intermediates which electropolymerize sufficiently rapidly as to form a polymer film directly on the electrode Electropolymerization presents several advantages which make itself a unique tool for electrochemical studies as well as for some specific electrochemically oriented technological applications of conducting polymers

Rapidity is probably the most immediate feature of electropolymerization The growth of a polymer film of a few hundred nanometers thickness, which is generally convenient for most electrochemical and spectroscopic characterizations, requires only a few seconds This is of course nothing compared to the several hours and tedious work-up required by chemical methods Simplicity is another evident advantage Besides further time saving, this specific one-step process leads to more heavily and more homogeneously doped materials than post-polymerisation doped chemically synthesized polymers Perhaps the most attractive feature of electropolymerization is that it represents one of the simplest and most

straightforward methods for the preparation of modified electrodes

Electronically Conducting Polymer

Electronically conducting polymer (ECP) is an exciting new class of modifying materials with unique electronic, electrochemical, and optical properties Because of these unusual and useful properties, ECP is the focus of massive international research efforts [1,2,4]

One of the most interesting and potentially useful aspects of this polymer is that it can be reversible “switched” between electronically insulating state and electronically conductive state This switching reaction involves either oxidation or reduction of a nonionic and electronically insulating parent polymer to form a conductive polycationic or polyanionic daughter polymer Further, ECP is conjugated, and the cationic sites created upon oxidation are delocalized along the polymer chain The delocalization in the ECP causes this polymer to be electronic conductor (i.e similar to metals) This conductivity is imparted due to the addition of dopants in relatively large quantities into the polymer matrix [4] The deliberate and controlled

modification of the electrode surface with ECP can produce electrodes with new and interesting properties [16-19]

Redox Polymer

Redox polymer consists of electronically locally electron donor and acceptor sites that are bonded to a polymer chain, or linked together to form a polymeric chain [4] In this material, electron transfer occurs via a process of sequential electron self-exchange between neighboring redox groups It is in contrast to ECP whose backbone

is extensively conjugated, which results in considerable charge delocalization

Redox film can be preassembled and then applied as a film to the electrode surface, or it can be assembled from monomer directly as a film on the electrode Both approaches have been widely researched and each offers certain advantages Generally, larger amounts of materials are available through direct synthesis of the redox polymer, which allows a relatively better analytical and structural characterization On the other hand, fabrication of very thin, uniform films from preassembled redox polymers can be difficult, since they are often multiply charged and reluctantly soluble In situ assembly of redox polymer films by hydrolytic or electrochemical polymerization of monomers can yield superb thin film forming characteristics, but usually at the expense of a less thorough analytical characterization, since only ultra-thin film is available [2,20-27]

complex via coordination of the metal to the polymer-bound pyridine [28] Likewise, ion-exchange polymer incorporates electroactive counterions via an ion-exchange reaction The most extensively investigated polymer of this type is Du Pont’s perfluorosulfonate ionomer, Nafion® Nafion® is a strong acid ion-exchange polymer

A large number of electroactive cations can be incorporated into Nafion® films at electrode surfaces Since the procedure for dissolving the film polymer of Nafion®was developed by Martin’ group [29], Nafion® film-coated electrodes have become the most extensively investigated modified electrodes [30-33]

1.1.2.2 Inorganic films

This section deals with the preparation of electrodes modified by forming films of inorganic materials on a conductive substrate surface These inorganic materials are of interest because they are ion exchangers, like ion-exchange polymers; however, unlike polymers, zeolites and clays can withstand high temperatures and highly oxidizing solution environments Furthermore, these inorganic materials have well-defined microstructures [34] Different types of inorganic materials, such as metal oxides, zeolites and clays, can be deposited on electrode surfaces In the following, a few examples are described

Metal oxides

A wide variety of metal oxide materials can be employed to modify electrode surfaces through sol-gel technique In this approach, metal oxides undergo the hydrolysis followed by a cascade of condensation and polycondensation reactions in solution at room temperature Sol-gel processes can be divided into two synthetic routes: aqueous-based methods, which originate with a solution of a metal salt, and alcohol-based methods, which employs an organometallic precursor that is dissolved

in the appropriate alcohol Murray and co-workers [35] were the first to apply redox modified siloxane-based cross-linked films on silicon, platinum, and other metal electrodes Schmidt [36] exploited the fact that Si-C bonds are very stable and do not hydrolyze during sol-gel processing in order to develop the organically modified ceramic and silicate materials, using organofuntional silane precursors such as

methyltrimethoxysilane Avnir et al [37] showed that it was possible to immobilize

organic compounds by mixing them with the sol-gel precursors

Zeolites and Clays

Zeolites are crystalline aluminosilicates organized into regular three dimensional networks with intracrystalline void spaces consisting of channels and cages which may be interconnected [38] Such pores and channels allow the ingress and egress of molecular and ionic species controlled by factors such as size, charge and shape Thus, zeolites possess interesting properties such as molecular sieving, analyte preconcentration, and ion-exchange, etc

In the context of electrochemistry, the first reported instance of a zeolite modified electrode was in 1983 when Ghosh and Bard modified a tin oxide electrode with a thin layer of clay (a material related to zeolite) [39] Since then, due to the advantageous properties of zeolite, there have been keen interests in zeolite modified electrodes (ZMEs) [40]

1.1.3 Electrodes modified with spatially defined and heterogeneous layers

In addition to the modified electrodes described above in the previous sections, which usually involve a conductive substrate and a single film of modifying

material, more complicated structures are also described here A wide assortment of electrode coatings have been devised based on electroactive films that are heterogeneous in some manners (non-uniform in composition or structure), or are based on the multiple layers of uniform composition or multiple contacting electrodes,

or both [2] Typical examples include the films with particles deliberately added during film preparation (e.g zeolites and clays into the polymer), or formed in situ within a polymer film (e.g metal particles into the polymer layer), multiple layers of different polymers (bilayer structures), polymers sandwiched between electrodes (sandwich structures), or resting in the interelectrode gaps of interdigitated array electrodes, etc [2] These often show different electrochemical properties than the simpler modified electrodes

Zeolite and clay particles can be coated on electrodes through casting from colloidal dispersions, adsorptive effects, using various polymeric materials as binders, and electrophoretic deposition One of the earliest methods involves applying a drop

of a polymer (usually polystyrene) solution containing suspended zeolite particles on the electrode surface Evaporation of the solvent leaves behind a thin polymer film containing the suspended zeolite particles Such modified electrodes are reported to be mechanically weak and do not possess good stability [40,41] A more successful fabrication method in term of reproducibility employs a mixture of conductive (carbon) powder / binder / zeolite If the binder is oil (e.g Paraffin, Nujol), a zeolite-modified carbon paste electrode is the result [42] If the binder is a polymer (e.g styrene/divinylbenzene), a composite electrode, which is mechanically stronger than a carbon paste electrode, is formed [43]

More interesting structures have been produced with polymer films combined with inorganic particles / films Aniline was included in the zeolite particles and then

electropolymerized; the polymer is present on the surface of the zeolite and in the intergallery region [44,45] Zeolite nanoparticle and zeolite nanowire composites have been formed and evaluated electrochemically [46-48] Further, metal oxide pillared zeolite films have been formed on electrodes and shown to sustain good electroactivity in solutions of cationic transition metal complexes [49] The electrochemistry of zeolite-encapsulated transition metal complexes has also been studied [50]

The incorporation of metal particles into polymer films on electrodes aims at gaining the catalytic activity of such particles in certain electrochemical reactions

The tactic was introduced by Wrighton et al [51] They formed the metal particles by

ion exchanging a metal complex into a redox polymer (e.g a viologen polymer), and then used the redox polymer to reduce the complex to the metal form by reducing the redox polymer Metal particles in films on p-type Si semiconductor electrodes were exploited for the photoelectrocatalytic reduction of hydrogen ion [52,53], and films on conductive electrodes for the electrocatalytic reduction of bicarbonate [54]

Bilayer electrodes are prepared by coating the electrode first with a layer of a redox polymer and then with a second layer of a different redox polymer There is no contact between the electrode and the second polymer layer, except by the way of the electrons that are transported to it by the first innermost layer When in contact with

an electrolyte solution, the redox polymer-polymer interface acts as a junction because it consists of redox species with different formal potentials on opposing sides of the interface [2,56-58] For example, a typical system consists of a

rectifying-Pt substrate with an electrodeposited film of polymerized [Os(vbpy)3]2+ on which a film of poly[Os(vbpy)2(bpy)2]2+ is electrodeposited [58]

Typically, sandwich structure involves a pair of closely spaced electrodes such

as in an electrode array, bridged by a polymer film Alternatively, a different polymer can be deposited on each electrode of an array pair to form a bilayer-like arrangement having a junction where the films meet Three-electrode devices of this type can produce a structure functionally equivalent to a field effect transistor (FET) [55]

1.2 Characterization and analysis of chemically modified

electrodes

Any discussion of attaching molecules to electrodes surfaces is incomplete without a description of how the success of an immobilization procedure manifests itself in some electrochemical or other responses This section describes the methods which have been used or invented to detect the electroactivity, chemical reactivity, and surface structure of electrode-immobilized molecules and films

1.2.1 Electrochemical Methods

The most routine method to characterize and analyse modified electrodes is electrochemical methods Cyclic voltammetry is one of the most reliable electrochemical approaches to elucidate the nature of electrochemical processes, and

to provide insights into the nature of processes beyond the electron-transfer reaction Several investigations have extended this method to the study of the chemical kinetics for chemical processes that precede or follow the electron-transfer process [59,60], as well as for the study of various adsorption effects that occur at the electrode surface [61-65]

It has been revealed that the realization of the synthesis and/or charging/ discharging experiments in solutions of large-size ions led to a strong modification of the cyclic voltammetry curves, so that this technique has become a valuable tool for characterizing the dynamic behavior of polymer films as a function of the ionic strength, or the ionic size Shinohara et al [66] and Lapkowski et al [67] have studied conducting polymers with large ions in the electrolyte, and concluded that two steps

in the doping process could be distinguished The first one was interpreted as "a quasi-irreversible doping" which occurs preferentially and is independent of the

concentration of small ions, whereas the second step was reversible, its amplitude being increased with the small-ion concentration added to the large-size ion electrolyte But, it is well known that cyclic voltammetry alone is usually unable to provide information separately for the electronic and ionic processes, since the shape

of the current-voltage response is retained qualitatively the same for different synthesis conditions, or different compositions of the solution in contact, despite a drastic change of the ionic-exchange properties of the film during the charging process [68]

In voltammetric methods the potential is scanned between selected potentials with the responding current recorded In contrast, for potential-step methods, the potential of the working electrode is changed instantaneously between selected potentials, and either the current-time (chronoamperometry) or charge-time (chronocoulometry) curve is recorded Chronocoulometry is equivalent to the integral

of the chronoamperometric signal As such, the chronocoulometric curve contains no more information than that from chronoamperometry, but its interpretation is simpler [61]

For potential-step methods, measurements should be made over as long a time period as possible to ensure reliable results For example, a potential step at a monolayer-covered electrode, when electron transfer is fast, causes a relatively uninformative exponentially decaying current-time curve Chronocoulometry and chronoamperometry methods are most useful for the study of adsorption phenomena associated with electroactive species Although less popular than cyclic voltammetry for the study of chemical reactions that are coupled with electrode reactions, these

"chrono-" methods have merits for some situations In all cases, each step (diffusion, electron transfer, and chemical reactions) must be considered For the simplification

of the data analysis, conditions are chosen such that the electron-transfer process is controlled by the diffusion of electroactive species However, to obtain the kinetic parameters of chemical reactions, a reasonable mechanism must be available (often ascertained from cyclic voltammetry) A series of literatures provide details of useful applications for these methods [1,61,62] However, one disadvantage of these

"chrono-" methods is that it requires the independent determination of uncompensated solution resistance This may introduce some limitations to the application for the polymer-modified electrodes [62,69]

Electrochemical impedance spectroscopy (EIS) is generally considered as one

of the very powerful tools of electrochemistry [70-87] It has been applied to numerous systems and tasks ranging from electric double layer studies to kinetic measurements The evaluation of measured impedance data through equivalent circuits yields structural and kinetic data For example, equivalent circuits have been most popular in the studies of conducting polymer-modified electrodes, which are composed of numerous components taking into account the redox electrochemistry of the polymer itself, its highly developed morphology, the interpenetration of the electrolyte solution and the polymer matrix, the extended electrochemical double layer established between the solution and the polymer with locally considerably different properties (e.g degree of oxidation, conductivity etc.) [70-80]

The promise of impedance spectroscopy is that with a single experimental procedure encompassing a sufficiently broad range of frequency, the influence of the governing physical and chemical phenomena may be isolated and distinguished at a given applied potential The critical factor becomes the interpretation of the spectra However, because the ambiguity inherent in the interpretation of the spectra is reflected in the controversial and often divisive questions that arise in the literatures

over the interpretation of spectra, the inability of impedance spectroscopy to serve as the stand-alone method for the identification of a correct model has been addressed experimentally by including additional analysis techniques or by incorporating multiple or more directed forcing functions [81-87] Therefore, other techniques (such

as spectroscopy methods and mass measurements, etc) could be used to support model identification

1.2.2 Spectroscopy and microscopy methods

Electrochemical signals can detect small quantities of surface-confined, electroactive substances, but convey little information about the surface molecular structure To help establish the integrity of surface immobilization schemes, several spectroscopy and microscopy methods have been applied

Molecular-level details of modified electrodes are often difficult to infer from electrochemical methods alone, but do lend themselves to spectroscopic analyses In recent years there has been an explosion of new spectroscopic techniques for characterizing modified electrodes X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) have been the most widely applied spectroscopes The most straightforward use is the detection of elements incorporated onto an electrode surface by a surface derivatization procedure [88,89] Fourier transform infrared spectroscopy (FTIR) can provide a great deal of information on molecular identity and orientation at the electrode surface FTIR has been used in both transmission and reflection modes to study changes in electrode surfaces during electrochemical processes [90,91] Raman spectroscopy can offer vibrational information that is complementary to that obtained by IR Since the Raman spectrum

reveals the “backbone” structure of a molecular entity, it is particularly useful in the examination of polymer film-coated electrodes [92]

Microscopy methods give direct information about the structure and topography of modified electrodes Scanning electron microscopy (SEM) is applied to the thicker, polymeric films but may be resolution limited for monolayer films Kaufman [93] observed substantial surface morphological roughening upon electrochemical oxidation of polymeric films Of interest are the features of electrode roughness protruding through the film, and film topology itself Oyama [94] detected uneven deposition of poly(vinylpyridine) on carbon at 10 pm resolution, and found an increase in unevenness following coordination of a Ru(III)(EDTA) complex to the film Further, scanning tunneling microscopy (STM) was invented two decades ago, and was first applied to the solid-liquid interface in 1986 [95] Since then, there have been numerous applications of STM for in situ electrochemical experiments [96,97] Because STM method is based on tunneling currents between the surface and an extremely small probe tip, the sample must be reasonably conductive Hence, STM is particularly suited to investigate conducting polymer-modified electrodes [98,99]

1.2.3 Quartz crystal microbalance

Since the original work of Sauerbrey [100], quartz crystal microbalance (QCM) has been applied in various contexts for the detection of mass changes at the nanogram level The heart of QCM is a specially cut quartz crystal that oscillates at some resonant frequency when an alternating voltage is applied across its thickness The adsorption of foreign materials on the surface of the crystal leads to minute but detectable changes in the resonant frequency Electrochemical quartz crystal microbalance (EQCM) simply employs one of the two oscillator driving electrodes as

the working electrode [101] EQCM is particularly suited to the modified electrode studies where oxidation or reduction of the film on the electrode surface causes ions

to enter or leave the film [102,103]

1.3 Applications of chemically modified electrodes

The applications involving modified electrodes are multiple and widespread: chemical sensing, energy conversion and storage, and electrochromic displays, etc

In addition, modified electrodes have always been used as a tool in fundamental scientific investigations

1.3.1 Chemical sensors

A chemical sensor is a device that provides the concentration of a particular chemical species (called the analyte) in a sample solution The development of chemical sensors continues to be a rapid area Improvements in the stability, selectivity, and the scope of such sensors are highly desirable to meet new challenges posed by clinical and environmental samples

Traditionally, the utility of solid-based sensors is often hampered by a gradual fouling of the surface due to the adsorption of large organic surfactants or of reaction products This offers a great potential for alleviating the above problems; hence, the tailoring of modified electrodes to deliberately control and manipulate the properties

of electrode surface can meet the needs of many sensing problems This field of modified electrodes, which has experienced a period of rapid growth over the past decades, has now reached a level of maturity that allows the used of these electrodes for routine sensing applications [104-108]

Chemical sensors based on modified electrodes are still in the early stages of their life cycle Many exciting developments are expected in the near future based on the diversity of potential (chemical and biological) surface modifiers It is also expectable for more powerful sensing probes based on polishable and robust modified surfaces, arrays of micoelectrodes (each coated with a different modifier),

multifunctional films (based on the coupling of several moieties), and intimate integration of biological and chemical entities

1.3.2 Energy-producing devices

There are two primary types of electrochemical energy-producing devices: battery and fuel cell Both of these devices convert chemical energy via electrochemical reactions The difference between a battery and a fuel cell is that a battery contains all of the chemicals required for the energy-producing reaction within the device package Hence, the advantage of a battery is that it is a completely self-contained energy-producing device In contrast, a fuel cell does not store its chemical reactants within the device itself; the reactants are supplied from external tanks Therefore, the advantage of the fuel cell is that it will run continuously as long as it is supplied with the appropriate chemical fuels

The concepts of modified electrodes have contributed tremendously to battery and fuel cell development For example, following the gap of the applications of electronically conducting polymer as active electrode materials for energy-producing devices during the 1980-90 period, the emergence of electrolytic supercapacitors has triggered a renewed interest in the applications of ECP in electrochemical energy storage [109,110] The development of such capacitors or more generally of electrochemical devices for charge storage applications requires polymers with a high doping level and good reversibility in both doped states

1.3.3 Electrochromic devices

Electrochromism is the ability of a material to change color upon a change in its oxidation state Electrochemists are interested in using electrochromism to make

electrochromic devices that are electrochemical cells showing color changes upon a change in the cell voltage Most of the electrochromic devices are sandwich-type two-electrode electrochemical cells Usually, they are assembled using a liquid electrolyte, which requires a perfect sealing in order to avoid leakage and evaporation of the solvent, and contamination with impurities, etc Also, both the electrochromic active materials are commonly deposited as film on transparent glass electrodes However, the use of glass electrodes brings other restrictions related to its fragility and form and shape limitations [111,112]

Motivated by technological advantages that can be attained if these problems were overcome, the research for developing flexible and solid-state electrochromic devices has recently increased This became possible, after the large scale production

of flexible transparent electrodes, like the film of poly(ethylene terephtalate) coated with indium doped tin oxide (i.e ITO–PET), and the advances that were attained in the field of solid electrolytes (e.g polymer electrolytes) [113]

1.3.4 Fundamental chemistry

In addition to leading to new types of electrochemical devices, modified electrodes have been used as a tool in fundamental scientific investigations The objective of many investigations is simply to obtain fundamental scientific information A good example is the use of modified electrodes to study the fundamentals of electron transfer (ET) reactions For example, Chidsey has used self-assembled monolayers with terminal ferrocene functions to probe ET processes at the electrode-solution interface [114] Because the electroactive sites are bound to the electrode, there is no need to separate kinetic and diffusional components of the measured current Also, because the electrode potential can be varied, the driving

force ∆G° for the reaction can be changed easily In homogeneous ET, one member of the donor/acceptor pair must be changed to change the driving force Hence, Chidsey

is able to quantitatively evaluate Marcus’ theory [115], which postulates a quadratic relation between ∆G° and the activation Gibbs energy ∆G* Finally, the donor-acceptor distance can be varied simply by changing the length of the alkane group [114]

1.4 The objectives of the thesis

Early fabrication of zeolite modified electrodes (ZMEs) generally involved the immobilization of zeolite particles in a polymeric binder, traditionally nonconductive polystyrene Such coating invariably have been plagued by poor reproducibility, lack of mechanical robustness in a stirred solution, and nonideal electrochemical behavior due to large regions of the metal electrode surface being in contact with insulating binder To date, some other researchers have made zeolite films using conductive binder, spin, and covalent linker, etc Despite this, controllable formation of zeolite thin film needs new processing schemes to improve quality and reproducibility Electrophoretic deposition (EPD) is a technique where charged particles suspended in a solution are deposited onto a substrate under influence of an electric field The performance of the EPD method has provide an attractive method

to effectively manufacture ordered structures of colloidal systems, including metals, polymers, carbides, oxides, nitrides, and glasses However, the EPD of zeolite particles on electrode surfaces was not introduced until recently Stimulated by this great potential, we systematically investigated the controllability, uniformity and reproducibility of zeolite coatings on ZMEs fabricated by EPD The effect of the EPD process on the bare electrode surface was also studied The dc EPD process was compared to a novel pulsed voltage EPD method

Conducting polymers, in particular electrodes modified with conducting polymer film, have enjoyed initial success and recently stimulated extensive activities The nitro-group substituent of a benzene ring is an easily reduced group, and is reported to undergo an irreversible reduction in various media However, there have been few reports of the electropolymerization of nitro-substituted monomers Our interest stems from the reported difficulty of electropolymerization of nitro-

substituted benzenes Further, the electropolymerization, if feasible, could be expected to give rise to a polymer structurally different from poly(1,2-

phenylenediamine) because of site-blockage at the para-position Of even more

interest is the reduction of the nitro-group on the benzene ring Therefore, we carried

out the anodic electropolymerization of 4-nitro-1,2-phenylenediamine (4NoPD) in

different supporting electrolytes at different pH The feasibilities of forming the

polymer poly(4-nitro-1,2-phenylenediamine) (P4NoPD) on gold and glassy carbon

electrodes were shown The pH of the electropolymerization medium, the nature of the background electrolyte and the number of cycles used were studied how to strongly influence the amount of polymer deposited The reduction of nitro-groups of

the P4NoPD films was also investigated by the effect of solution conditions,

especially pH

Although the initial study has provided valuable information on the P4NoPD

film, much less is known about the film properties, such as film resistance, charge transfer resistance, low frequency capacitance and charge-carrier diffusion coefficients, etc Therefore, we applied the electrochemical impedance spectroscopic

studies of P4NoPD films on glassy carbon electrodes under different conditions The

Kramers-Kronig transformation was employed to validate the impedance data, which was then analyzed on the basis of selected models

Composites of conducting polymers and inorganic materials hold considerable promise for the construction of highly selective chemical sensors, efficient chromatographic stationary supports, specific binding assays, and controlled-transport membranes Inorganic particles (e.g zeolite particles) can be introduced into the matrix of a host-conducting polymer either by some suitable chemical routes or by electrochemical techniques Here, we prepared the composite film from controllable

zeolite EPD coated by P4NoPD The properties of the P4NoPD/zeolite composite film

under various conditions were systemically investigated

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CHAPTER II

Studies of zeolite modified electrodes fabricated

by electrophoretic deposition

2.1 Introduction

The controlled assembly of colloidal particles has received much attention in recent years because of the potential applications of nano- and micro-structured materials in many fields [1] The construction of ordered/patterned well-defined colloidal systems on surfaces has only emerged in the last ten years or so Nevertheless, an impressive range of construction techniques, building blocks and architectures has been reported [2] The construction of two-dimensional colloidal arrays on surfaces has been developed by different techniques, including microlithography, self-assembly, and electrophoretic deposition (EPD) [3-5] This has led to more complex, patterned structures [6] Such controlled construction of colloidal particles on surfaces can be used to create a variety of sensors and microelectronic components [7,8]

In the context of controlled deposition of colloidal particles, we focused the present work on the EPD of zeolite particles on electrode surfaces, particularly glassy carbon electrodes (GCEs) EPD is a practically simple, yet effective method for coating of charged particles on surfaces, which has gained acceptance for coatings in various industrial applications, including the automative, appliance and industrial organic coating industries Its advantages include uniform deposition, control of deposit thickness, low levels of contamination and continuous processing [9].In spite

of these advantages and the extensive previous usage of EPD, it was only recently that EPD has been employed for the fabrication of clay [10] and zeolite [11] coated electrodes Zeolites have also been coated on various substrates as thin films but not for the purposeful fabrication of zeolite modified electrodes [12] Zeolites are crystalline aluminosilicates organized into regular three dimensional networks with intra-crystalline void spaces consisting of channels and cages which may be

interconnected [13] Such pores and channels allow the ingress and egress of molecular and ionic species controlled by factors such as size, charge and shape Thus, zeolites are charged particles possessing interesting properties including sieving, analyte preconcentration, ion-exchange with applications which mesh with the ability to form organized zeolite assemblies on surfaces, e.g sensors In recent years, there has been much interest on the applications of zeolite modified electrodes and, while various methods of coating of zeolite films on electrodes have been used [14], the search for new approaches/directions are ongoing Since zeolites are charged particles, EPD should be a suitable method for their deposition on electrode surfaces Our literature search indicated that, to-date, there have only been four reports [10,11,12,15] on the deposition of zeolite films on electrodes by EPD Of these, three

[10,11,15] employed dc voltage EPD while Ke et al [12] mentioned briefly that

voltage step application gave more uniform films Also, in a different context, Zhao

et al [16,17] have studied the square wave EPD of gold nanoparticles So far, there is

very little systematic study on the controllability and reproducibility of zeolite coatings deposited on electrode surfaces by EPD for the purpose of ZME fabrication Further, the EPD process may lead to deterioration/damage of the electrode surface [11], resulting in a ZME which may not be functional in its role as an electron sink/source for its target analytes

In view of the potential of ZMEs, we undertook a systematic study to develop methods for assembling non-spherical colloidal (zeolite) assemblies from colloidal suspension by applying an electric field Our initial studies [18] employing dc voltage indicated that while adjustments of solution parameters (pH, zeolite and supporting electrolyte concentration) and other experimental parameters (dc voltage, deposition time) allowed control of the amount of zeolites coated (from sub-monolayer to multi-

layer), electrode surface still functioned well Although the above-mentioned method for EPD by DC has enjoyed successes, we found it inconvenient to control the distribution of EPD by DC while maintaining the activity of electrodes Furthermore,

we report a novel application of pulse for colloidal particles on electrode surfaces [18] This problem can be alleviated under pulse to serve dual purposes, namely controllable deposition and electrode reactivation It is shown that pulse affords control of deposits from submonolayer to multilayers using a single solution by simply tuning the pulse heights, widths and number of pulses

Therefore, the objectives of this work are to systematically investigate the controllability, uniformity and reproducibility of zeolite coatings on ZMEs fabricated

by EPD

2.2 Experimental

2.2.1 Reagents

All chemicals were of analytical reagent grade unless otherwise specified Water was obtained from a Millipore Alpha-Q water purification system (18.2 MΩ; Millipore Corporation, USA) Zeolite 13X was purchased from Aldrich Chemical Co (Milwaukee, WI, USA) The manufacturer’s specifications gave the nominal particle size as 2 µm but our observations showed that there was a wide variation of sizes with

a significant fraction differing substantially from 2 µm After some trials, we developed a procedure to obtain a narrower particle size distribution in the region of 2

µm This involved settling the zeolite in a beaker of water for 10 min, then decanting off the aqueous suspension and discarding the larger particles which had settled to the bottom This was repeated three times The aqueous suspension was then allowed to settle in a measuring cylinder for 2 hrs After this, the aqueous suspension was decanted and discarded while keeping the settled solids, Millipore water was then added and the settling process repeated After sedimentation for four times in the measuring cylinder, the final sedimented zeolite 13X particles were collected and dried in the oven at 140 °C This procedure apparently gave a narrower particle size distribution centred around 2 µm It should be noted that zeolite 13X suspension in water increased the pH of the aqueous phase to between 10 and 11, depending on the amount of suspended zeolite This is due to hydrolysis of aluminium present in the zeolite network [19] This hydrolysis effect led to some difficulties in measurements and adjustments of solution pH and longer times were usually needed to obtain a stable value

2.2.2 Apparatus

Containers (glassware, polyethylene bottles, etc.) were soaked overnight in 10% HNO3 prior to use Electrochemical experiments were performed with an Autolab PGSTAT 30 (Eco Chemie, Netherlands) controlled by a personal computer Two types of electrochemical cells were employed For EPD of zeolite particles, a single compartment cell with a flat, tapering bottom (~200 ml capacity) and a plastic top with holes for electrodes and nitrogen purging was used The distance between counter and working electrode was about 2 cm For stirring during EPD, this cell was placed on top of a magnetic stirrer For other electrochemical experiments, a locally made two-compartment, H-type glass cell of approximately 5 ml capacity was employed Working electrodes were glassy carbon (GC) disks (3 mm diameter) while counter electrodes were platinum disks (3 mm diameter) The reference electrode was Ag/AgCl (saturated KCl) Scanning electron micrographs of the GC electrode surfaces were obtained using a JEOL Model JSM-5200 (JEOL Ltd., Japan) scanning electron microscope (SEM) GC electrodes were coated with different metals (e.g

Au, Pt) film by cool sputter coater (SCD 005, BAL-TEC, Zurich) pH measurements were made with a Hanna Model HI 9318 meter (Hanna Instruments, Woonsocket, RI, USA) while conductivity measurements were carried out with a Model GM-115 conductivity meter (Kyoto Electronics Ltd., Japan)

2.2.3 Procedure

Before each experiment, the glassy carbon electrode (GCE) was polished with alumina (0.3 µm)-water slurry on polishing cloth The electrode surface was then rinsed copiously with Millipore water and wiped with soft tissue wetted with water