Fabrication, characterization and application of metal tetraaminophthalocyanine polymer nanostructures

A protocol combining size-confinement with the electrochemical polymerization technique may offer a versatile way for the fabrication of useful organometallic conducting polymer nanostru

Chapter 1 Introduction

Introduction

The discovery of carbon nanostructures (fullerenes and carbon nanotubes) in the last century brought a whole perspective to the semiconductor industry, making the development applications of silicon-based nanostructures possible Such nanostructured materials are often produced in a metastable state, and they are unique compared to individual atoms/molecules on the one hand, and macroscopic bulk materials on the other Their detailed atomic configuration depends sensitively on the kinetic processes by which they are fabricated Therefore, the properties of nanostructures can be widely adjustable by changing their size, shape and processing conditions Nanostructures of metals, semiconductors, and inorganic and organic polymers have drawn a high level of attention not only because they can be widely employed in the industry, but also for their novel characteristics which could facilitate the understanding of fundamental theories and mechanisms, for instance quantum and surface/interface effects

A protocol combining size-confinement with the electrochemical polymerization technique may offer a versatile way for the fabrication of useful organometallic conducting polymer nanostructures Interesting properties may conceivably appear in these polymer nanostructures which hold potential for applications in electrocatalysis,

as electrochemical sensors, electronic and photovoltaic devices In the following sections, the fabrication strategies, characterizations and potential applications of

conducting polymers of metal tetraaminophthalocyanines (MTAPc) in the nanostructural level will be introduced in detail

1.1 Conducting Polymers

Conducting polymers (CPs) possess a unique combination of properties as organic polymer semiconductors, and they are attractive alternatives for materials currently used in microelectronics The field of conducting polymers could be traced back to the mid 1970’s when Shirakawa et al found the first polymer, polyacetylene, which was capable of conducting electricity [1, 2] This work eventually resulted in the award of the Nobel Prize (2000) in Chemistry Within a few years of the discovery, progress in the study of conjugated poly-heterocycles [3] was dramatic, and it motivated the material science community to explore increase in conductivity by doping the polymers chemically or electrochemically In these studies, efforts were made to produce novel materials which combine properties of environmental stability, mechanical flexibility, and lightness in weight, thereby capitalizing on the easily processable advantages of an organic polymer with the available electrical properties

of a metal

1.1.1 Bandgap theory and conduction mechanism

With regard to conduction properties, materials can be classified into three broad categories: Conductors (or metals), Semiconductors and Insulators The most frequently used mechanism to explain these conduction properties may be the band

theory The highest occupied band is the valence band (VB), while the lowest unoccupied band is the conduction band (CB) The energy spacing between these two bands is known as the bandgap energy (Eg) For conductors, the VB and CB overlap and the intrinsic conductivity is attributed to the zero band gap For semiconductors, the band gap energy is small and the electrons may be excited by vibrational, thermal

or photon excitation to jump to the CB to render the material conductive For insulators, the energy separation is too large for electrons to jump to the CB at room temperature In terms of their bandgaps, these three classes are demonstrated in Figure 1.1

Fig 1.1 Band structure of three categories of materials based on conducting properties

Conducting polymers have always been treated as semiconductors from the point of traditional solid-state physics, though conduction in CPs is different from that in conventional inorganic materials such as doped silicon or GaAs The bandgap

view-in CPs is usually above 1.5eV, and thus the view-intrview-insic conductivity is rather low (Table 1.1 shows several important conducting polymers and their conductivities)

Table 1.1 Conductivities of several important conducting polymers

Conducting polymer Conductivity (S cm-1)

103

In order to achieve conductivities comparable to metals, doping process is performed (chemically or electrochemically), and consequently the polaron-bipolaron model has been widely applied to explain the band structure changes In chemistry, a polaron is defined as a radical cation that stabilizes itself by polarizing, while bipolaron is a

bound pair of two polarons By way of an example, these properties are explained for polypyrrole in Figure 1.2

Figure 1.2 Band structure evolution and actual structure for polypyrrole upon doping

At zero doping level, the polymer is neutral and its band structure is that of a standard semiconductor Upon the oxidative doping of polypyrrole, an electron is removed from the backbone chain to produce a polaron which is a combination of a charge site and a radical The polaron state of polypyrrole is symmetrically located about 0.5 eV from the CB and VB edges [4] The partial delocalization of polaron across several

monomer units leads to structural distortion in the polymer On removal of an additional electron by further oxidation, the free radical of the polaron is removed, creating a bipolaron The bipolaron levels are located symmetrically with a band gap

of 0.75 eV from both band edges

During this process, the entire polymer chains would firstly be saturated with polarons before bipolaron formation At even higher doping level, the formation of individual bipolarons would lead to a continuous bipolaron band Their band gap also consequently increases as newly formed bipolarons are made at the expense of the band edges The bipolaron bandwidths are about 0.4 to 0.45eV At such high doping levels, these spinless bipolarons exhibit high mobility under electrical field and this explains the generation of high conduction property from doped conducting polymers Meanwhile, it is expected that the two bipolaron bands would gradually merge with the CB and VB to produce near metallic conductivity through partially filled bands

1.1.2 Fabrication of conducting polymers

The polymerization of a conducting polymer, such as polypyrrole, has been studied in depth CPs have been synthesized mainly by three methods: electrochemical polymerization, chemical polymerization, and chemical vapor deposition The electrochemical and chemical polymerization methods will be introduced in this chapter while the chemical vapor deposition method will be presented extensively in the next chapter Chemical polymerization is mostly done through condensation

polymerization, while nearly all electrochemical methods are based on addition polymerization

1.1.2.1 Electrochemical polymerization

Polypyrroles were produced by oxidative electropolymerization and they are the first materials synthesized by electrochemical methods in the fabrication of CPs Very shortly after that, electrochemical polymerization drew particular interest since it allows for control of the structure, thickness, conductivity, and electrochemical properties of the resulting polymer by variation of the experimental conditions The conditions that could be varied included film-growing rates (applied potentials and current densities), supporting electrolytes, temperatures and solvents Since then, electrochemical polymerization using a variety of aromatic compounds (as monomers), such as indole, aniline, phenylene, furan, azulene, fluorene, pyrene and thiophene [5-9] had been investigated Figure 1.3 lists some of these monomers employed for electropolymerization

Electropolymerization is usually initiated by oxidation via an applied potential to generate the radical ion in the first step (Figure 1.4; polypyrrole is again taken as the example here) The population of radical ions far exceeds that of neutral monomer in the vicinity of the electrode surface This is in contrast to the situation in typical chemical polymerizations where the concentration of monomer takes the majority The electropolymerization is terminated through the exhaustion of reactive radical species or other chain termination processes This is a useful technique for quick and rapid fabrication of CPs However, not all monomers undergo electrochemical

polymerization Two main key factors are the potential for the generation of radical

ions and the stability of these ions in the first step

Pyrrole

NH2

Aniline

H N

Figure 1.3 Examples of some monomers employed for electropolymerization

H N

H

H

H N

N H

polymer fabrication, the potential is scanned typically in the range slightly beyond the monomer oxidation potential and the experiment is performed in a classical three-electrode electrochemical cell These conditions result in consistent morphology as well as desirable control of the thickness by number of cycles The three electrodes comprise the working electrode (WE, also called the indicating electrode), reference electrode (RE) and counter electrode (CE, also known as the auxiliary electrode) Commonly used reference electrodes are the saturated calomel electrode (SCE, Hg/HgCl/KCl) and the silver chloride electrode (Ag/AgCl/KCl), while many materials could be used as counter electrodes Indium tin oxide (ITO) glass or Pt wires are preferred working electrodes in electrochemical polymerization for conducting polymers The former provides a transparent substrate for electrochromic and initial characterization studies, while the latter demonstrates excellent adhesion of most CPs To prepare free-standing polymer films, graphite or glassy carbon electrodes may be preferred The schematic diagram in Figure 1.5 shows a conventional three-electrode electrochemical cell set up

Figure 1.5 Typical electrochemical cell setup for cyclic voltammetric electropolymerization

During electropolymerization, it is also noted that the net charge transfer is a little in excess of the indicated stoichiometric value This is explained as the additional oxidation or doping occurring at the same time during polymerization For example,

in an electropolymerization requiring 2 electrons per monomer molecule, an experimental charge order of 2.5 would mean that the polymer has undergone 50% doping in the process More details relating to such doping will be presented in Section 1.1.3

1.1.2.2 Chemical polymerization

Among all the chemical polymerization methods, the most well known is the Shirakawa method This is an adapted Ziegler-Natta polymerization used for the synthesis of stereospecific polymers Most coupling methods have been adapted from methods ranging from traditional organic synthesis to condensation polymerization, e.g in the formation of poly-phenylene and poly-phenylene vinylene[10]

On the other hand, most chemical polymerization strategies for CPs are based on addition polymerization After the initial generation of the radical ion, coupling occurs between radical and monomer, unlike the radical-radical coupling in electrochemical polymerization The second difference is that many chemical polymerizations are precursor polymerizations This means that initially a processable and usually soluble polymer needs to be synthesized, which subsequently yields the final CP through a few relatively simple chemical steps Common chemical polymerization methods are usually simple, involving a monomer, an oxidant (which could be the dopant as well) and a suitable reaction system (solvent and temperature)

For instance, as shown in Figure 1.6, polythiophene can be obtained by polymerization in the presence of oxidizing agents such as Cu(ClO4)2, FeCl3 and NOBF4[11-13] Polypyrroles have been fabricated by chemical oxidation of the monomer with ferric salts[14, 15] Polyanilines can be prepared through polymerization by specific oxidizing agents e.g (NH4)2S2O8 or common ones such as Fe(ClO4)3 or Cu(BF4)2[16]

Figure 1.6 A Proposed Chemical Polymerization Mechanism for Polythiophene [11]

Several novel alternative polymerization methods have been developed For instance,

a polypyrrole was prepared by the direct photosensitization of Ru(bpy)33+ using a 490

nm dye laser in a matrix of Nafion [17] By using a solvent evaporation method (the solvent was evaporated to increase the concentration of oxidant), the same polypyrrole was formed using FeCl3 in a polyvinyl acetate matrix in methanol solvent [18] A semi-soluble sulfonated polyaniline was “enzymatically” synthesized by treatment with enzyme horseradish peroxidase [19] It was also noted that this polymer could be self-assembled into multilayer structures Polyaniline was also reported to be synthesized by plasma polymerization using radio-frequency (RF) glow discharges with resistive coupling between stainless steel electrodes at a

frequency of 13.5MHz and pressure around 2×10-2 Torr [20] Other unique polymerization methods also include oriented polymerization [21], chemical vapor deposition [22] and chemical template polymerizations [23]

Chemical polymerized CPs have been obtained with comparable or even better conductivity than those prepared by electropolymerization However, it is more difficult to control the morphology, conductivity, doping and composition in chemical polymerization due to the need for more careful control of reaction conditions like the reaction temperature, concentration and many other factors Occasionally, even the same synthetic procedure may not consistently yield exactly the same polymer

1.1.2.3 Doping Techniques

Borrowing strategies from traditional p/n type doping semiconductors, it is deduced that oxidation would generate a positively charged CP with an associated anion, while reduction produces a negatively charged CP and its associated cation These associated ions are called dopants Typical dopants are listed in Table 1.2, and they could be small anions like I- , or bulk species such as polyvinyl sulfonic acid The extent of a doping process is called doping level and this is usually measured as the proportion of dopant incorporated per monomer unit Increased doping level leads to increased conductivity by increasing the number of mobile charges Nevertheless, it is generally impossible to have a CP with 100% doping level because of various constraints in polymers

Table 1.2 Typical dopants for CPs

-Trifluoromethane sulfonated (Trifl) CF3SO3

-p-toluene sulfonated (Tos) CF3-C6H5-SO3

Besides these two traditional main methods, alternative doping techniques have also been developed in the last two decades Photochemical doping uses irradiation such

as UV or X-ray radiation to treat pristine CPs [25, 26] In methods widely used in microelectronics, ion implantation is introduced to CPs to produce doped polymers by bombardment with high energy ions Another interesting method is heat treatment doping [27] Ladder-type CPs such as the benzimidazolebenzophenanthroline-type ladder (BBL) has been formed by heat treatment at different temperatures, yielding an enhancement of conductivity [27] Such results are attributed to the improved structure order and thermally excited charge carriers Just as in the case of photochemical doping, “dry doping” is developed combining heat treatment by using heat-generated active dopants (thermally decomposed) to incorporate CPs [28] This latter method could be carried out without the assistance of solvent, electrochemical environment or radiation

Although many techniques have been developed for the fabrication of novel CPs, there is a lack of information on the morphology and theoretical studies related to their conductivity In certain cases, even models for disordered materials from condensed matter physics could hardly be applied to CPs While some attention has been given to bulk properties of such materials, the same cannot be said of studies on

a micro-scale level apart from the one reported by Martin et al in 1990 on a series of

CP nanostructures [29-34]

1.2 Nanostructured Materials

The word "nano" means a billionth (10-9) part of a unit in general Nanostructures or nanostructured materials refer to material systems with length scale in the range of around 1 to 100 nm in at least one dimension Nanostructures are unique materials as compared with individual atoms/molecules on the one hand or the macroscopic bulk materials on the other Interesting properties may arise from nanoscale structures that

do not exist in the same materials in the other scale ranges

1.2.1 Background and quantum mechanics

For common materials in the bulk such as a gold brick or water, their intrinsic physical properties are independent of the sizes For instance, if the gold brick was cut into small pieces, its density, conductivity or chemical reactivity remains the same However, if the cutting process is repeated again and again until gold atoms are obtained for instance, then these properties could not be kept unchanged Significant changes show up as the materials get down to the nanoscale level The properties of nanostructured materials are affected by several effects: quantum confinement, quantum coherence and surface/interface effects [35]

In a nanostructure, electrons are confined in the nanoscale dimension(s), but are freely moving in other dimension(s) This results in quantization of energy and momentum, and reduced dimensionality of electronic states From the perspective of

quantum confinement, nanostructures could be classified based on the dimensions in which electrons are free to move:

Quantum well: electrons are confined in one dimension (1D), free in the other two

dimensions (2D) It can be realized by sandwiching a narrow-bandgap semiconductor layer between the wide-gap ones A quantum well is often called a 2D electronic system

Quantum wire: confined in 2D and free in 1D Thus, it is usually called a 1D

electronic system Quantum wires include polymer chains, nanowires and nanotubes

Quantum dot (QD): electrons are confined in all dimensions, as in clusters and

nano-crystallites

For electrons moving in a nanostructure, certain phase relation of wave function is preserved and wave interference effect has to be considered Nevertheless, in nanostructures, the quantum coherence generally is not maintained perfectly compared with atoms or molecules The coherence is often partially discontinued by defects in the nanostructures Therefore, both quantum coherent and de-coherent effects have to be considered and consequently the description of electronic motion in

a nanostructure becomes more complicated

Many nanostructured materials are exposed to environmental conditions as part of their application Surface effects such as surface chemistry, oxidation and adsorption may dominate the behavior of these materials because a significant fraction of atoms

in nanostructure is located at or near the surfaces/interfaces The mechanic,

thermodynamic, electronic, magnetic, optical and chemical states of these atoms can

be quite different from those of the interior atoms[36] Slight modification at the surface may result in considerable changes in the mechanical properties of nanostructures such as resonance frequency and quality factor On the other hand, defects generated at the surface may cause the rearrangement of existing bulk defects which consequently produces unique properties These changes can have dramatic consequences for the stability and reliability of devices utilizing these properties

These factors all play important roles but to different extents Nanostructured materials are often in a metastable state and their detailed atomic configuration depends sensitively on the kinetic processes in which they are fabricated Therefore,

by controlling their size, shape and processing conditions the properties of nanostructures can be widely adjusted

1.2.2 Fabrication of Nanostructures

In the past 20 years, inorganic nanostructures have been applied extensively in microelectronic, optical, and mechanic devices, and in biomaterials and materials for drug delivery [37-39] Development in more advanced fabrication strategies of nanowires and nanotubes to improve this nano technology is ongoing, since every strategy used presently has its limits and are only suitable for fabricating certain kinds

of nanostructure material Self-assembly is one popular strategy which has shown great success [40], although for this process, chemicals, reaction environment and impurities control are relatively critical Apart from their use in the imaging of nano-

scale materials, STM (Scanning tunneling microscopy) and AFM (Atomic force microscopy) have also been applied in fabricating nanostructures based on manipulation of individual atoms [35, 41] However two disadvantages have hindered their utilization - they are time consuming methods and require relatively stringent environment Patterning lithography is another popular technique, and it has been widely used in industry because of its high productivity [42, 43], although diffraction effect limits its further improvement since the critical dimension has already reached

100nm

1.2.2.1 Lithography and manipulation

Photolithography is an important process in microelectronic processing by lithography for making small features on the printed circuit board[44] Firstly, the circuit design is converted to patterns using computers and these patterns reflect particular device and interconnection layout Then, the patterns are physically created

as masks using lithography such as optical or electron-beam pattern generator Lastly the patterns on the masks are transferred to Si wafers This process is realized by using a layer of photoresist material coated on the wafer As shown in Figure 1.7, the photoresist material in the areas not covered by the mask is exposed to UV radiation, and certain changes occur after the exposure With the positive resist layer (resist to

UV radiation and protect the layer underneath) that is usually used today, the exposed part of this layer becomes easily dissolvable in the developer solution to leave behind the uncovered silicon oxide Consequently, processing steps such as etching, film deposition and ion implantation can be performed in these regions However, the

resolution in the printing process is also limited by the diffraction effect, especially when UV light is used as radiation source[45-47] In order to further decrease the minimum feature size, shorter-wavelength radiation (such as deep UV, X-ray, e-beam and ion beam) may be introduced [48] Unfortunately, the lack of effective optical systems and suitable resist materials are the main obstacles for the applications of the X-ray and deep UV radiation sources For e-beam and ion beams, the problem arose from the strong interaction between the beams and resist material; such interaction leads to the relatively rough edges of the lithographic pattern Additionally, mask alignment uncertainty also imposes a limit on the scale resolution, especially when more than one mask is used

Figure 1.7 Stages of Photolithography.

The most remarkable example of manipulation is the construction of a quantum corral consisting of 48 iron adatoms on a copper(111) surface [49, 50] In the imaging mode, the STM tip is usually positioned far away from the adatoms and thus the

interaction is weak When an increased tunneling current or a reduced bias voltage is applied, the distance between the tip and the sample decreases and the interaction then becomes strong enough for the manipulation of the atoms When an even higher bias voltage is applied, it is possible to pick up the adatoms and even the atoms in surface layer by means of the extremely strong attractive force [51] Such atomic lithography is normally controlled by bias voltage pulses and thus atoms on the tip can be selectively deposited on the surface On the other hand, local chemical or electronic states of the surface or adsorbates can be modified by the electric field and/or electron beams generated by the SPM tip [52, 53] For example, local oxidation can be induced by the electric field generated with a SPM tip on substrates such as silicon, and oxide patterns can be written on these substrates Nevertheless, from the industry viewpoint, these processes are extremely time-consuming and often require ultra-high vacuum (UHV) conditions Thus, real applications from them could hardly be realized

1.2.2.2 Self-assembly strategies

Nanostructures can also be formed by self-assembly methods which do not require the use of masks or fine-focused beams Self-assembly has numerous advantages as a strategy in nanotechnology [7, 54, 55] Firstly, it takes the advantages of some energetic, kinetic or geometric effects in the growth process to carry out many difficult steps in nanofabrication Secondly, in the atomic-level modification of structures, it preferentially forms a thermodynamically stable product with fewer defects Thirdly, numerous examples could be derived from biological systems to

help in the design of self-assembly strategies which produced structures that are biologically incorporation-ready Lastly, self-assembly is generally a parallel process since many nanostructures are formed simultaneously during their fabrication process This makes it one of the most promising methods in industrial nanofabrication

Multi-layers of QDs (Ge with a pyramidal structure on silicon surface) are fabricated with uniform size and distribution by taking the advantage of the Stranski-Krastanov growth mode [56] A stacked structure of self-assembled GaSb/GaAs QDs has been grown by molecular beam epitaxy in the same mode [57] QDs in different layers are correlated in position due to elastic interaction between them Also, there are many self-assembly processes for the fabrication of nanostructures by chemical methods Self-assembled monolayers (SAMs) of thiolates have been fabricated with noble metals making use of the high affinity of thiols towards these metals [58] The SAMs can act as surfactants for other nanostructures as chemical templates Hexagonal array

of pores with high aspect ratio can be formed on alumina during the anodized process

of aluminum [59-62], and similar structures of porous silicon have been produced by immersing the silicon wafer as the anode in hydrogen fluoride (HF) electrolyte[63, 64] Subsequently, pure metal nanostructures have been synthesized using anodic aluminum oxide (AAO) membranes as physical templates [65-69] These templates have also been used in the fabrication of a whole array of other materials [70-80]

The key issues that make self-assembly a useful strategy are the effective controls of size, shape and positioning of individual nanostructures fabricated Such controls can

be achieved by the proper selection of the process condition which allows for taking advantage of some intrinsic material properties On the other hand, using self-assembled nanostructures as template to combine other fabrication strategies of nanomaterials should provide a more versatile and convenient way to fabricate other nanomaterials

1.2.2.3 Template-assisted strategies

Template-assisted strategies, especially those involving the use of chemical templates, are widely used to prepare hollow nanostructures [81] The chemical template strategy allows the formation of a polymer shell around a preformed template particle which can be removed subsequently Therefore, it is one of the most promising approaches in the production of polymeric hollow nanospheres due to its relative ease

of operation Inorganic nanoparticles are used as templates, and the polymerization reaction can be either catalyzed by an initiator or by the colloidal particles themselves [82-84] Through layer-by-layer deposition, oppositely charged polyelectrolytes are deposited step-by-step to produce the well-known polyelectrolyte self-assembly at charged surfaces and this process has been applied as a template to prepare a series of nanomaterials [85-91] Melamine-formaldehyde particles have been widely used as template particles without affecting the layered polyelectrolyte shells [92-95] They have the advantage of being soluble when exposed to an acidic solution

On the other hand, physical template-assisted fabrication (mostly using AAO template) has a number of interesting and useful features First of all, the uniform

hexagonal channels make it convenient to introduce various materials into the template by general approaches, such as vaporization, evaporation, microemulation and electropolymerization A series of electronically conductive polypyrrole nanostructures have been fabricated by electrochemical methods in template and characterized by the pioneer Martin’s group since 1989 [29-32] Interesting electronic, electrochemical and optical properties have been studied [33] Metal nanowire arrays, metal oxides and sulfides nanostructures such as Ag, TiO2 and CdS have been fabricated by electrodeposition [69, 72, 75] in the last ten years Highly-aligned nanotubes arrays have been synthesized by a few groups recently using chemical vapor deposition techniques based on the template [73, 78, 80, 96] Moreover, extraordinarily small diameters (less than 10nm) of the template can be achieved by a relatively simple process of adjusting different reaction conditions, e.g acid types and temperatures [60, 61] The length of the channels and pore density could be well-controlled in the same manner Wu and Bein [97] reported the fabrication of polyaniline nanostructures with controlled diameters of 3nm, which is difficult even for usual lithography process Lastly, the template itself could be easily removed either chemically (dissolved in acid or base) or physically (laser etching) which is greatly advantageous in industrial application All in all, these advantages make it a versatile strategy in nanostructured materials fabrication and CPs nanostructures have received particular benefits from the method because of this inorganic, well-structured and easily-removable template

1.3 Metal phthalocyanine (MPc) and substituted MPc polymeric nanostructures

While CPs were intensively studied and such systems had been applied in numerous fields of study, organometallic polymers have up to now not drawn much attention as attractive areas of investigation because of their extremely active nature, their chemical instability, their need for storage in a vacuum environment, as well as their weak mechanical properties

1.3.1 MPc group and tetraamino-substituted MPc (MTAPc)

On the contrary, interesting properties also may arise from these identified

“weaknesses”, for example in the photovoltaic property, corrosion resistance,

catalysis, and chemical or biochemical sensitivity of these materials [98-104] Metal

phthalocyanine is one example of an interesting organometallic material that is

relatively stable in the atmospheric environment [105]

Phthalocyanines (Pcs) are 18 π-electron aromatic macrocycles with four isoindole units linked together through the 1, 3-positions by aza bridges (Figure 1.3) The two-dimensional π-electron delocalization in such molecules gives rise to a great number

of unique properties Varying the substituents at the substituted ligand on benzene rings and the central metal ion is a useful way to construct new materials, and thereby allowing process to fine-tune the physical properties of such materials Phthalocyanines are capable of incorporating more than 70 different metallic and nonmetallic cations in their ring cavity A wide variety of substituents could be

attached at the periphery of the macrocycle [103, 104], which can alter the electronic structure of the system When bulky or long-chain substituted groups are present, the solubility of Pcs in common organic solvents could be highly improved Additionally, Pcs and their analogues hold the ability to form a wide range of condensed phase materials with controlled molecular architectures, such as liquid crystals, polymer and thin films[106-108]

Figure 1.8 Structure of a metal tetraaminophthalocyanine (MTAPc) molecule

The amine derivatives of metal phthalocyanines (Figure 1.8) have been previously synthesized through reduction from their nitro-substituted analogues [109-111] and are mostly used for the preparation of inks, dyes and pigments Amino group substituents not only increase the electrical conductivity but also improve the solubility of the molecule in a few organic solvents [112, 113] Intensive studies have been carried out in the field of synthesis, catalysis and photovoltaic properties[99, 105, 114], while special interest has been placed on the electropolymerization of MTAPc monomers

1.3.2 Characterization and application of electropolymerized poly-MTAPc

MTAPc was first electropolymerized by Li and Guarr [115] on the surface of carbon glass electrodes Extended work on Co, Fe, Ni and Zn TAPcs were also done by this group in continuous cyclic voltammetry (CV), scanning over potential ranges which involve amino oxidation [116, 117] The effect of experimental conditions on the morphology of the polymers was investigated by F Xu in further work by the same group [118] These conditions included solvent, monomer concentration and scan rate, relevant to the total electropolymerization rate

However, intensive studies of polymerized film properties were not started until 1991 The reversible electrochromism of poly-MTAPc (Cp, Ni and Pd) was demonstrated

by Li and Guarr [117] Ni and Pd TAPc polymers behaved similarly with the color changing from green to blue over a broad potential range centered near -1.1V vs SCE Poly-CoTAPc showed a large variety of colors from blue-green (intrinsic state) to yellow-brown (-0.5V) and finally to deep pink (-1.65V) CuTAPc polymer showed analogous results with Ni; however, an unusual near-colorless state was found when the additional 0.5 e- was applied per monomer site [119, 120] As a “sandwich” Pc, Lu(TAPc)2 polymer thin film was deposited on ITO glass and it showed two broad reduction waves, with color changing from green to gray and gray to blue respectively [121] Nevertheless, electrolytes have been utilized in most of these studies and the results show poor stability on repeated cycling over a relatively wide potential window in aqueous solvents

Besides being widely investigated in their electrochromic properties and corrosion studies [122], poly-MTAPcs have been studied as sensors and electrocatalysts Sun and Tachikawa investigated the use of poly-CoTAPc and poly-CuTAPc films as biochemical sensors for the amperometric detection of glucose [123] At about the same time, kinetics of heterogeneous electron transfer across films of electronically conducting NiTAPc polymer were studied [124] The film formed on the electrode surface was found to act as an unusual n-doped conductor Two years later, Tse reported the finding that poly-CoTAPc films held catalysis properties for the electrochemical reduction of O2 in solution environment [120, 125] The same reduction properties were also found a few years later for FeTAPc polymers on glass carbon electrode [126] More recently, Kang et al reported a rare example of the voltammetric behavior of electropolymerized NiTAPc towards dopamine (DA) in dimethylformamide (DMF) [127] Following this finding, Goux et al developed an improved poly-NiTAPc modified electrode through electrochemical treatment in alkaline aqueous[128] The polymer film exhibited a much larger electrocatalytic behavior and this phenomenon was explained as the formation of interconnected O-Ni-O oxo bridges as well as the changes in the staking structure upon the treatment These results provide evidence of the possibility of utilizing MTAPcs as the active material in the fabrication of sensors towards a series of oxidizing gases and biomaterials Upon electrochemical impedance spectroscopy (EIS) investigation, these electropolymerized NiTAPc and their drop-dry films showed great corrosion protection ability [122] Nevertheless, the comparison of sensitivity between drop-dry and polymeric samples in these works was done with different amounts of sensing

materials and consequently the results obtained were not completely accurate The surface morphology of these two kinds of materials was not compared as well And further, the exact nature of drop-dry material is not completely studied yet, since it is still not clear whether this material is a polymer or the aggregation of dried monomer

Poly-FeTAPc modified electrode was found to be useful as an amperometric sensor

of organic peroxides in 1996 [129], while MgTAPc and NiTAPc polymer were reported to be useful for peroxynitrite (PON) and nitrite determination respectively [130, 131] Following this routine, the determination of oxidizing gases (such as NO and NO2) was studied with high sensitivity and selectivity using poly-CuTAPc as the modified film [132-135] Such a routine which is solely based on traditional electrochemical analysis of thin film modified electrodes cannot be extended to nanostructured materials Meanwhile, similar active layer of uniform nanostructures could hardly be formed to meet the requirement of this method Thus, the results do not provide full information on their sensing properties Spectral investigation of the sensitivity towards NO2 by Raman spectrum was investigated by T.V Basvoa et al

and Brozek-Pluska et al in 2005 [136, 137] Another spectral determination study by

MTAPc was reported based on the fluorescence enhancement of a red-region dye towards four strong acids in strongly acidic medium [104] The experimental results show satisfactorily that the relative fluorescence intensity was proportional to the logarithmic value of the acid concentration Although these two are non-destructive methods and suitable for nanostructure characterization, the sensitivity of these

methods was limited by the nature of the film structure because of the relatively low surface area provided by the film morphology

1.3.3 Pc and substituted Pc nanostructures

Although not as popular as polypyrrole, Pc nanostructures, on the other hand, could

be powerful tools for designing new materials As poly MPc of MTAPc nanostructures with high surface area would be interesting materials for sensing oxidizing gases and other novel properties, several groups have recently been successful in fabricating MPc nanostructures

Carbon paste electrodes (CPE) modified with nano-sized CoPc particles were fabricated in 2004 [138] Their enhanced electrocatalytic oxidation ability and better electrocatalytic activity towards theophylline (THP) were investigated by means of differential pulse voltammetry (DPV) The response current was enhanced about 3.4 times compared to bulk CoPc-modified CPE The size of MPc particles was also physically reduced by ultrasound treatment of different frequencies [139] More interestingly, when high frequency was applied to the MPc, rod-shaped nanotubes with extremely thin walls were formed Unless the ultrasound process is carefully controlled, the final nanostructures produced could hardly be utilized in further studies

Recently, MPc (Cu, Ni, Fe, Co and Zn) nanowires and nanoribbons have been grown

by organic vapor deposition on different substrates (glass, Si, indium tin oxide,

fluorine doped tin oxide) Their morphology, nanowire size and crystal structure were mainly determined by the substrate temperature [140, 141] The change in central ion does not seem to affect the resulting nanostructures Among these MPc nanowires, CuPc showed acceptable field emission (FE) properties [142] compared with several polymer nanostructures An important drawback is that their FE performance is largely dependent on the sample preparation, especially the substrate temperature Alternatively, MPc nanoflowers and nanoparticles have been grown by the same method at room temperature on gold coated quartz substrates [143] The formed nanostructures were found to be mainly determined by templates used rather than the local temperature From a different but simpler synthesis routine, Xu et al fabricated CuPc nanowires by electrophoretic deposition (EPD) on AAO templates [144] These nanowires exhibit smooth surface and relatively uniform diameters However, both types of nanowire structures exhibited poor mechanic property probably because of the methods themselves In addition, low uniformity in the diameter and length was observed, arising from difficulties in the control of these parameters during the whole fabrication process Lastly, these studies did not include any investigation into the sensing abilities of these materials

Insufficient attention has so far been given to research in the field of CP nanostructures, especially those involving organometallic polymers Hence we have identified the following problems for our studies:

• General and versatile methods suitable for the fabrication of organometallic polymer structures More importantly, the method should also be amendable

for follow up analysis with techniques like electrochemistry and spectrometry (IR, vis-UV and Raman) after the fabrication process

• Investigation of materials at the nanoscale level The successful fabrication of nanostructured MTAPc has not been reported Scaling down to a nanometer level is expected to result in a high surface area, together with a higher level

of sensitivity

• Lack of intensive studies in the electronic properties of polymeric MTAPcs, especially at the nanoscale level This would be potentially useful for exploring the applications of such materials in microelectronics in the form of field emitters and high ability capacitors

• The nature of drop-dry MTAPc materials is still unclear A good understanding of the chemical structure and the surface morphology is necessary for understanding the differences in sensitivity

• The mechanism of the MTAPc electropolymerization and the actual molecular linkage in polymerized structures are not yet fully understood It would be very useful to contribute to our understanding of such systems

1.4 Research Objectives

Bearing in mind the above, the main objectives of this thesis are to:

1) Develop versatile and non-destructive fabrication methods for organometallic polymer nanostructures

2) Synthesize poly-CuTAPc nanostructures by template-assisted electropolymerization and analyze their properties chemically and physically

3) Study Electrochemical Impedance Spectroscopy (EIS) analysis and field emission properties of MTAPc nanowires and nanotubes for their potential application in microelectronics

4) Investigate the chemical structure and surface morphology of drop-dried MTAPc materials; study their sensibility towards a series of oxidizing gases by Raman spectroscopy

5) Study the sensing ability of poly-MTAPc nanostructures towards oxidizing gases such as NO and NO2 as well as chemical or biochemical molecules

6) Explore the possible mechanisms of the nanostructure polymerization process chemically and electrochemically

Combining the size-confinement of AAO templates with the electrochemical polymerization technique should offer a versatile way for the fabrication of MTAPc polymer nanostructures These polymer nanostructures may contribute to investigations on conductivity studies in nano-sized materials and may also have applications in microelectronics, chemical sensing, and field emission materials On the other hand, the investigation of the mechanism of MTAPc polymerization process will enhance our understanding of the electropolymerization of a series of amino-substituted organic materials and this will help us to explain their sensing abilities However, in this thesis, dramatic improvements in the sensing properties of MTAPc nanotubes towards various compounds were not observed as they were formed embedded inside the template channels Therefore, the catalysis function will be largely weakened due to diffusion confinement and blockage from the template

In the next chapter, the template-assisted electropolymerization method will be first presented as well as further analysis and characterization methods Details of electrochemical parameters for poly-MTAPc nanostructures fabrication and morphology studies by field emission scanning electron microscopy (FE-SEM) will

be expanded

Chapter 2

Fabrication, Characterization and

Electronic Properties of Poly-MTAPc

Nanowires

2.1 Introduction

Nanostructures of CPs have attracted considerable attention due to their interesting properties and potential applications in the microelectronic industry [35, 36, 145] Although various methods have been developed for fabricating interesting nanostructures [146-148], general strategies for the fabrication of organometallic nanowires have yet to be established [149, 150] Anodic aluminum oxide (AAO) templates with a regular porous structure have been widely used to produce CP nanostructures in the last decade [29, 30] Various materials could be introduced within the highly ordered channels in the AAO templates by processes like electroplating, evaporation and electrodeposition [34, 79, 80, 151] Among multifarious organometallic materials, metal phthalocyanines (MPc) and their derivatives have been extensively studied for their potential application in electrocatalysis, and as electrochemical sensors, electronic and photovoltaic devices [152, 153] The successful preparation of MTAPc polymer thin films on conductive surfaces was also previously demonstrated using electrochemical oxidation [9, 154]

AAO templates together with the electrochemical polymerization technique may offer

a versatile way for fabricating MTAPc polymer nanostructures In this chapter, copper-centered MTAPc was chosen as the monomer to demonstrate this possibility due to its stability and potential application in nanocapacitor materials [155-157] MTAPc with other central metals such as Fe and Ni will also be studied The polymer nanostructures fabricated will be characterized using Field Emission

Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray Spectroscopy (EDX), Transmission Electron Microscopy (TEM), Electrochemical Impedance Spectroscopy (EIS; for MTAPc polymer nanowires incorporating copper and nickel only) and Raman Microscopy The morphology of these electropolymerized nanowires will be investigated under different electrochemical conditions

2.2 Experimental Section

2.2.1 Synthesis of Monomer

All the chemicals used in this study were purchased from Aldrich and were of analytical-reagent grade The procedure for the synthesis of CuTAPc complex was described in Figure 2.1 [158] 2.40 g (9.6ml) of copper sulfate pentahydrate, 7.40 g (35mmol) of 4-nitrophthalic acid, 0.90 g (16.8mmol) of ammonium chloride, 0.10 g (0.008mmol) of ammonium molybdate and 12.0g (0.2mol) of urea were finely ground and mixed After addition of 5 ml of nitrobenzene the mixture was placed in a 100 ml flask and the temperature of the stirred reaction mixture was slowly increased to 185°C and maintained at this temperature for 4.5 h The dark blue-colored solid product was washed thoroughly with ethanol until free of nitrobenzene 100 ml of 1

M hydrochloric acid saturated with sodium chloride was added, and the reaction mixture was boiled for about 5 min, before it was cooled to room temperature and filtered The residue was then treated with 100ml of sodium hydroxide containing 40

g of sodium chloride and heated at 90°C until the evolution of ammonia ceased The product was treated again after filtration with 1 M of hydrochloric acid, 1 M of sodium hydroxide and 1 M of hydrochloric acid consecutively, and then separated by centrifugation The copper tetranitrophthalocyanine (CuTNPc) product was washed with water until it was chloride-free (tested using AgNO3 solution as indicator) and then treated with methanol and acetone for 24 hours The dark blue complex was dried at 125°C 6.0 g (8.2mmol) of CuTNPc was placed in 150 ml of water and 30 g

of sodium sulfide nonahydrate was added to this slurry and stirred at 50°C for 5 h

After filtration and washing with water the residue was twice treated first with 450 ml

of 1 M hydrochloric acid, centrifuged, followed by the addition of 300 ml of 1 M sodium hydroxide and 1 hour of stirring After centrifugation, the product was thoroughly washed with water and methanol, stirred and centrifuged until the material was free from sodium hydroxide (tested with pH paper) and sodium chloride The pure copper complex was dried in vacuum over P4O10 MTAPcs with central metal such as nickel and iron have also been prepared by the same method (by replacement with the corresponding metal salts in the first step) For this study, the final products were characterized by Fourier transform infrared spectroscopy (FT-IR), Varian 3100 FT-IR and Ultraviolet-visible (UV-vis) spectrometry, UV-2550 SHIMADZU

First step

Second step

Figure 2.1 Synthesis Steps for CuTAPc