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

CHAPTER I

Introduction












1
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

2
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.









3
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

4
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.

5
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.

6
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 (SnO
2
, 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.

7
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

8
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].

Ion-Exchange Polymer
Ion-exchange polymer is not electroactive itself, but can incorporate
electroactive guest molecules [4]. For example, Anson’s group showed that the film
of poly(vinylpyridine) can incorporate electroactive coordinatively unsaturated metal

9
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

10
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

11
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

12

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 rectifying-
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
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].

13
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].
















14
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

15
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

16
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

17
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

18
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

19
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].

20
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),

21
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

22
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

23
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].
















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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-

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