Next generation of conducting materials for organic electronics

Various PANI micro and nano structures, including 1D open-ended microtubes, 3D solid microspheres and 2D novel solid microplates were controllably produced.. PANI films with interesting

NEXT GENERATION OF CONDUCTING MATERIALS

FOR ORGANIC ELECTRONICS

WEN TAO (B.Eng Tianjin University)

A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE

DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE

2010

Acknowledgements

It is a great pleasure to thank my supervisor Prof Hardy Sze On Chan in Department of Chemistry, and co-supervisor AP Chorng-Haur Sow in Department of Physics, for their patient guidance, invaluable suggestions and constant encouragement

I gratefully acknowledge the kind assistance from Dr J H Shi, who is now an Associate Professor in Henan University China, for his hands-on help with the synthesis and characterizations of nanostructures I would like to thank other seniors in my group, Dr S Zhang, Dr H J Che, C H

Xu, D M Fan and colleagues from Department of Physics, Dr Binni Varghese, M R Zheng, Y L Xie and K K Lee I also owe my special thanks to Dr X N Xie, of the Nanoscience and Nanotechnology Initiative (NNI) for his inspiring discussion

My gratitude also goes to the National University of Singapore (NUS) for the financial award of research scholarship and the generous support of The Agency for Science, Technology and Reserach

in the provision of the TSRP-PMED Grant

Most important of all, this is the most precious opportunity to thank my parents, who devoted themselves to raising me up to an educated adult They have been and will always be my emotional corner stone whenever I meet any difficulty in the life Best wish to my parents

TABLE OF CONTENTS

Acknowledgements……….I Table of Contents……… II Summary……… VII Nomenclatures……… VIII List of Figures……… IX List of Tables……… … XIII

Chapter One

Introduction……… 1

1.1 Conducting Polymers……… ……… ……… 1

1.1.1 Classification……… ……… 1

1.1.2 Background of Polyaniline………… …… 1

1.1.3 Applications of Polyaniline… ……… …3

1.1.3.1 Reduction of precious metal……… …4

1.1.3.2 Rechargeable batteries……… …4

1.1.3.3 Light-emitting devices……… ……….……….………4

1.1.3.4 Solar cells……… ……4

1.2 Nanomaterials……… ……… ……… ……5

1.2.1 Background……… ……… …5

1.2.2 General fabrication methods…… ………6

1.2.2.1 Lithography in microelectronics……… …6

1.2.2.2 Manipulation and lithography with SPM……… …7

1.2.2.3 Molecular beam epitaxy……….………7

1.2.2.4 Self-assembly……… ……….……….…7

1.3 Synthetic methods of micro and nano structured conducting polymers 8

1.3.1 Hard template method… ……….……… ………8

1.3.2 Seeding method ………9

1.3.3 Soft template method… ……….……10

1.3.4 Template-free method……… ……….……11

1.4 Applications of micro and nano structured conducting polymers…… 13

1.4.1 Hydrophobic surfaces……… ……….…13

1.4.2 Chemical sensors……… ………14

1.4.3 Photothermal effects……… ……… ………14

1.4.4 Biomedical applications………… ……….14

1.4.5 Organic electronics……… ……….……15

1.4.5.1 Electrochromic display devices……… ……15

1.4.5.2 Organic field effect transistors… ……… …16

1.5 Objectives and scope……… ……… 16

Chapter Two

Synthesis and Electrical Characteristics of Solid Polyaniline

Sub-Microspheres………27

2.1 Introduction……… ……….… 27

2.2 Experimental Section……… ……… ……… 27

2.2.1 Chemicals…… ……… ……27

2.2.2 Preparation of solid PANI spheres…… ……….…28

2.2.3 Characterizations……… ………28

2.2.4 Electrical Measurements… ………29

2.3 Results and Discussion……… ……… 29

2.3.1 Synthesis and characterizations of PANI sub-microspheres 29

2.3.2 Electrical Properties……….……… 36

2.3.2.1 Current-Voltage (I-V) characteristics of PANI films at different pressures……….36

2.3.2.2 Current-Voltage (I-V) characteristics and calculated conductivity of an individual sub-microsphere… 38

2.4 Conclusions……… ………42

Chapter Three Morphology Evolution of Polyaniline Microstructures via Reverse Micelles and Intrinsic Hydrophobicity……… ….48

3.1 Introduction……… ……….………… …48

3.2 Experimental Section………… ……….……… 50

3.2.1 Chemicals…… ……….………… …50

3.2.2 Preparation of HAuCl4/TOAC/toluene solution… ……… … …50

3.2.3 Synthesis of PANI micro and nano structures……… ……… … 50

3.2.4 Characterizations………… ………51

3.3 Results and Discussion… ……… 52

3.3.1 Morphologies evolution… ……….………52

3.3.1.1 Effect of chloroauric acid concentration [HAuCl4]……… ……52

3.3.1.2 Effect of aniline to HAuCl4 molar ratio……… ………58

3.3.1.3Effect of temperature on microplates……… …62

3.3.1.4 Effect of mechanical stirring……… …63

3.3.1.5 Effect of additional acid……… ……64

3.3.2 Structural characterizations………66

3.4 Hydrophilic and hydrophobic properties…… ……… ……….…………68

3.5 Conclusions………… ……… 70

Chapter Four Electronic Transport in Polyaniline Solid Microplates……… 77

4.1 Introduction…… ………77

4.2 Experimental Section……… ……… … 78

4.2.1 Chemicals……… 78

4.2.2 Preparation of HAuCl4/TOA/toluene solution……… 78

4.2.3 Synthesis of PANI……… ……… …78

4.2.4 Structural Characterizations……… ……… …79

4.2.5 Electrical Measurements……… ………79

4.3 Results and Discussion…… ……… 80

4.3.1 PANI Synthesis and Characterizations… ……… ……80 4.3.2 Electrical Measurements……… ………86

4.3.2.1 Current-Voltage (I-V) Characteristics of an individual microplate….…….… …86 4.3.2.2 Current-Voltage (I-V) Characteristics of two stacked microplates……….….……88 4.3.2.3 Current-Voltage (I-V) Characteristics of macroscopic films of microplate

aggregates.……….………… …….89 4.3.2.4 Current-Voltage (I-V) Characteristics of macroscopic films at atmospheric

Summary

One-pot synthesis of PANI micro and nano structures was conducted in toluene, by employing both cationic and non-ionic surfactants to form reverse micelles The reverse micelles of cetyltrimethylammonium bromide (CTAB) led to mono-dispersed solid sub-microspheres When trioctylmethylammonium chloride (TOAC) was used as the cationic surfactant, morphology evolution was readily observed Various PANI micro and nano structures, including 1D open-ended microtubes, 3D solid microspheres and 2D novel solid microplates were controllably produced In addition, the non-ionic surfactant trioctylamine (TOA) was used to produce PANI microstructures for the first time

The electrical properties of the prepared PANI solid sub-microspheres and microplates were

investigated at room temperature by measuring their current-voltage (I-V) curves The I-V curves of

both an individual sub-microsphere and its macroscopic film showed semiconducting characteristics

I-V curves were also obtained for an individual microplate, two stacked microplates and the

macroscopic film For an individual plate, the current followed Ohm’s law at low voltages and power-law with exponent of 3/2 at high voltages Large and non-Ohmic contact resistance between structures was shown to be the dominating factor in determining electrical properties of stacked microplates and microplate aggregates

PANI films with interesting hydrophobic properties were prepared by controlling the surface roughness due the co-existence of nano and micro spherical structures

Cetyltrimethylammonium Bromide

Emeraldine Base Emeraldine Emeraldine Salt Fourier Transform Infrared Spectroscopy Leucoemeraldine

Nigraniline Polyaniline Pernigraniline Polythiophene Polypyrrole Scanning Electron Microscopy Trioctylmethylammonium Chloride Trioctylamine

Ultraviolet-visible

List of Figures

Figure 1.1……… 2

Figure 1.2……… ……3

Figure 2.1………30

Figure 2.2………31

Figure 2.3………32

Figure 2.4………33

Figure 2.5……… 34

Figure 2.6……… … 35

Figure 2.7………… …… 36

Figure 2.8………37

Octameric structures of polyaniline in various intrinsic redox states

Inter-conversions among different oxidation states and protonated

(ES)/deprotonated (EB) states in PANI

SEM images (a, b) and TEM images (c, d) of PANI sub-microspheres

Energy-dispersive X-ray spectrum of PANI sub-microspheres X-ray powder diffraction pattern of PANI sub-microspheres

Schematic diagram illustrating the formation of PANI sub-microspheres

SEM of PANI/Au powder synthesized at different monomer concentration

SEM images of PANI/Au powder synthesized at different HAuCl4 concentration

SEM images of PANI sub-microspheres at different reaction conditions

Schematic diagram and optical image of experimental setup for electrical measurement of PANI sub-microspheres with two

electrodes

Figure 2.9……… 38

Figure 2.10……….….40

Figure 2.11……… 41

Figure 2.12……… …42

Figure 3.1……… 53

Figure 3.2……… 55

Figure 3.3……… 56

Figure 3.4………57

Figure 3.5……… … 58

Figure 3.6………59

I-V characteristics of PANI sub-microspheres at different

pressures (a) Typical SEM image of electrical measurement of single PANI sub-microsphere with two electrical probes

(b) I-V characteristics of single PANI sub-microsphere

FTIR spectra of PANI sub-microspheres before (a) and after (b) reduced pressure

UV-vis spectra of PANI sub-microspheres before (a) and after (b) reduced pressure

SEM images with TEM insets of the PANI structures at different [HAuCl4] with fixed [Aniline]/ [HAuCl4] at 33

SEM images with TEM insets of the PANI microstructures with fixed [Aniline]/ [HAuCl4] at 16

SEM images with TEM insets of the PANI microstructures with fixed [Aniline]/[HAuCl4] at 5

Diameter distributions of spheres for sample C1 SEM images of the PANI microstructures with fixed [Aniline]/ [HAuCl4] at 1.67

SEM images with TEM insets of the PANI microstructures with fixed [HAuCl4] at 12 mM; varying the [Aniline]/[HAuCl4]

Figure 3.7………… …….61

Figure 3.8………63

Figure 3.9………64

Figure 3.10……….65

Figure 3.11……… 66

Figure 3.12……… …67

Figure 3.13……… 68

Figure 3.14……….……….70

Figure 4.1………79

Figure 4.2………82

Schematic diagram of synthesis locations: microplates were adhered to reactor wall; tubes and spheres were produced in solution

SEM images with TEM insets of the PANI microstructures with different temperatures

SEM images of dopant effect on the PANI microstructures with [HCl]/[Aniline] molar ratio fixed at 0.5

Electron diffraction (a) sample A5; (b) Au aggregates in the background

SEM images with corresponding Energy-dispersive X-ray (EDX) spectra

FTIR and Uv-Vis spectra of different PANI structures FTIR and Uv-Vis spectra of PANI structures produced at different [Aniline]/ [HAuCl4] molar ratios

Shapes of a water droplet on different films and their contact angles

Schematic diagram of synthesis locations: microplate structures were adhered to the glass wall; other structures were produced via reverse micelles in the solution

SEM images with SEM insets (b, d) and TEM insets (c, f) of the PANI micro and nano structures when [HAuCl4] is at12mM

Figure 4.3………83

Figure 4.4………84

Figure 4.5………85

Figure 4.6………86

Figure 4.7………87

Figure 4.8……… 89

Figure 4.9………90

Figure 4.10……… 91

SEM images with TEM insets (b, c) and a SEM inset (b) of the PANI micro and nano structures at different [HAuCl4] with fixed [Aniline]/ [HAuCl4] molar ratio at 33

FTIR and Uv-Vis spectra of different PANI structures FTIR and Uv-Vis spectra of PANI microplates

(a) Typical SEM image of electrical measurement of an individual PANI microplate with two electrical probes

(b) I-V characteristics of an individual PANI microplate

I-V characteristics of an individual PANI microplate plotted on a

log-log scale (a) Typical SEM image of electrical measurement of two stacked PANI microplates with two electrical probes

(b) I-V characteristics of two stacked PANI microplates

Schematic diagram of the experimental setup and optical image

of the sample for electrical measurement of PANI macroscopic films with two electrodes

I-V characteristics for the macroscopic PANI film of microplates

at different pressures

List of Tables Table 3.1 51

Table 3.2……… 53

Table 3.3……… 54

Table 3.4……… 56

Table 3.5………57

Table 3.6………59

Table 3.7……… 62

Table 4.1………78

Table 4.2………81

Table 4.3………83

Synthesis details for PANI structures Morphologies of the PANI A-series products Morphologies of the PANI B-series products Morphologies of the PANI C-series products Morphologies of the PANI D-series products Effect of [Aniline]/[HAuCl4] ratio on morphologies

of the PANI samples Effect of temperature on morphologies of the PANI samples A5

Synthesis details for PANI products Effect of [Aniline]/[HAuCl4] ratio on morphologies

of the PANI products

products

Chapter One Introduction

1.1 Conducting polymers

1.1.1 Classification

Generally, electrically active polymer-based systems are broadly classified into four primary types Each type has its own distinctive conduction mechanism The first type comprises composites of insulating polymer matrixes and conductive fillers Carbon and metal particulates or fibers are the common fillers used to increase conductivity.1 The second type is the ionic conducting polymers utilized in the battery industry Mobile ions such as the lithium ions in polyethylene oxide render electrical conductivity The third type is known as the redox polymers, such as the insulating polymer backbone with ferrocene branches as redox centers In contrast to the free ions, electrons transfer among immobile redox centers by hopping, thus a significantly large amount of redox centers must be present.2

The last type is the conjugated polymers which consist of alternating single and double bonds along the polymer chain Their extended π-conjugated network leads to the intrinsically conducting polymers (CPs) Conductivity could be readily achieved through an oxidation-reduction doping process, because CPs usually have a low ionization potential and a high electron affinity

1.1.2 Background of Polyaniline

The first discovered CP is the doped polyacetylene However, its poor stability and

processability render the material unsuitable for practical applications.2

Polyaniline (PANI), polypyrrole (PPY) and polythiophene (PT) are the three most widely researched CPs PPY has the merits of facile synthesis and good environmental stability, suitable for the application in gas separation.3 PT and its derivatives are most intensively investigated nowadays due to the easy modification of the monomer This structural manipulation affects many properties such as their bandgap.4

Polyaniline (PANI) has attracted much attention since its discovery, due to the large-scale supply of the monomer aniline, simple preparation, good environmental and thermal stability, structural versatility and many potential applications.5,6 In this project, we will focus our research on PANI

The chemical structures of PANI in different oxidation states are well studied and generally accepted as shown in Fig.1.1

Figure 1.1 Octameric structures of polyaniline in various intrinsic redox states.5

(a) the fully reduced leucoemeraldine (LM); (b) the 50% oxidized emeraldine (EM);

(c) the 75% oxidized nigraniline (NA); (d) the fully oxidized pernigraniline (PNA)

Different from other CPs, chemical and physical properties of PANI are controlled by both oxidation (redox doping) and protonation (acid doping) Oxidation or reduction doping involves the partial addition or removal of electrons to or from the polymer backbones, respectively

In contrast, the acidic doping results in the formation of a delocalized poly-semiquinone radical cation, without changing the number of electrons on the polymer backbone Typically, the insulating Emeraldine base (EB) form and the conducting Emeraldine Salt (ES) form could be reversibly switched when exposed to strong bases or acids, respectively.5,6Inter-conversions among different oxidation states and protonated/deprotonated states in PANI are summarized in Fig.1.2

Figure 1.2 Inter-conversions among different oxidation states and protonated (ES)/deprotonated (EB) states in PANI. 5

1.1.3 Applications of Polyaniline

The reversible charge transfer reactions among different stable oxidation states and the unique inter-conversions between protonated/deprotonated states of PANI have made PANI a very versatile CP with many potential applications.2

1.1.3.1 Reduction of precious metals

The reversible redox processes of PANI make it possible for applications in the electroless reduction of precious metals from acid solution Protonation, de-protonation, oxidation, re-protonation and subsequent reduction of PANI in acid solution could realize spontaneous and sustained reduction of precious metals For example, in the chloroauric acid solution, the imine nitrogens of EB are first protonated to ES By coupling the reduction of gold ion to its element form, spontaneous de-protonation results in an increase in the oxidation state of NA or PNA The highly oxidized PANI is subsequently re-protonated and reduced to ES in acid medium.5 This principle will

be used in this project to synthesize micro and nano structures of PANI

1.1.3.2 Rechargeable batteries

The ability of PANI to store charges through redox processes leads to its applications

in recharge batteries PANI is used as cathode materials when combined with lithium or zinc and as anode materials when combined with lead oxide It was also proposed to employ two different oxidation states of PANI as cathode and anode in a rechargeable battery.6

1.1.3.3 Light-emitting devices (LEDs)

EB form of PANI exhibits colors under various excitations and thus can be used as the emitting layer in LEDs.2 Moreover, EB is applied as two redox polymer layers sandwiching another emissive polymer layer in symmetrically configured LEDs This configuration enables LEDs

to work under both forward and reverse direct-current bias, as well as in alternating-current mode.5

1.1.3.4 Solar cells

PANI can be used as p-type semiconductor in a p-n heterojunction because it has the

reversible electron-donating/accepting properties The heterojunction is sensitive to sunlight, and thus can convert light energy into electricity.2 Moreover, PANI is also used as the protection coating against photo-corrosion of inorganic semiconductor electrodes to enhance stability of photo-current.6

1.2 Nanomaterials

1.2.1 Background

Nanoscience and nanotechnology as a research area has grown very rapidly in the last

30 years Why do they attract such intense global interests? It all started with R P Feynman’s visionary 1959 lecture ‘There is plenty of room at the bottom’ (Feynman 1959), but the following statement from the US President’s Advisor for Science and Technology summarizes the widely perceived potential of nanoscale science in the coming decades: ‘If I were asked for an area of science and engineering that will most likely produce the breakthroughs of tomorrow, I would point

to nanoscale science and engineering’ (A Lane, from the introduction to National Nanotechnology Initiative: Leading to the Next Industrial Revolution, US National Science and Technology Council, February 2000).7

Nanoscale science, engineering and technology are concerned with the manipulation

of matters on the nanometer length scale, which is now generally taken as the 1 to 100 nm range.8Nanoscience is not simply a natural and necessary progression from the microscale towards higher miniaturization but instead a discovery of a wealth of novel physical, chemical and biological behaviors on the nano-scale However, fabrication of nanomaterials is absolutely essential to any research and practical application Four major synthesis methods are discussed below

1.2.2 General fabrication methods

Current fabrication methods are roughly divided into two main classes, top-down and bottom-up.8,9 Four methods are generally employed in the fabrication of nanomaterials They are lithography in microelectronics, molecular beam epitaxy, manipulation and lithography with STM/AFM, and self-assembly Each has its own strength and weakness and the choice would depend

on the ultimate goals

1.2.2.1 Lithography in microelectronics

Lithography, the printing process developed in microelectronic industry, is a typical top-down method Circuit patterns of sub-micro structures can be produced on silicon wafers with photoresists and masks Although this method has very high productivity, further improvement of the critical dimensions down to less than 100 nm meets several obstacles For example, the lack of effective optical systems and suitable resist materials for shorter wavelength than UV, the proximity effect for electron/ion-beams, and the mask alignment uncertainty.10-12

Direct writing can use e-beam or ion-beam to directly write patterns onto wafers without masks Although able to achieve a high spatial resolution, this sequential process is comparatively slow and is only suitable for research purposes or for fabricating molds in nano-lithography.13

Nano-lithography is a recently established lithography method A pre-fabricated mold

is pressed into a thin thermoplastic polymer film on a substrate to transfer the mold pattern Etching

or deposition is then carried out as a general lithographic process Nano-lithography has achieved a sub-10 nm resolution and thus can potentially be used to fabricate high-density magnetic storage media However, this technology is only applicable for simple nanostructures because layer-by-layer

NIL is difficult and too imprecise to control.14,15

1.2.2.2 Manipulation and lithography with SPM

Scanning Tunneling Microscope (STM) and Atomic Force Microscopy (AFM) are powerful tools not only to image nanomaterials, but also to fabricate nanostructured patterns In normal imaging mode, the tip-sample interaction is weak In manipulation mode, adatoms and surface atoms could be picked-up and selectively deposited following bias voltage changes The most remarkable demonstrations are the placing of atoms in a particular location16 and the construction of a quantum corral of 48 Fe atoms by STM. 17 The resolution of SPM tip is so high that this method is often called the atomic lithography However, the process is extremely time-consuming and solely restricted to working in ultrahigh vacuum

1.2.2.3 Molecular beam epitaxy

Molecular Beam Epitaxy (MBE) is a typical bottom-up growth technique to control the film thickness to sub-atomic-layer scale, while maintaining crystallinity and purity Thus the advantage of precise atomic composition is mostly exploited to make heterojunction-based nanostructures such as quantum wells, wires and superlattices, for the study of quantum effects To achieve a high growth rate, metal-organic chemical vapor deposition (MOCVD) is sometimes also used for quantum structural fabrication when less abrupt composition changes are acceptable.18,19

1.2.2.4 Self-assembly

The discovery of self-assembly originates from Langmuir and Blodgett’s observation

of the close-packed arrangements of amphiphilic molecules on liquid and solid surface.20 This technique is a relatively simple bottom-up process, without the need of masks and fine-focused beams The key issues here are the effective control of sizes, shapes, composition and even the final

incorporation of nanostructures in devices.21,22 Self-assembly is a widely used technique to produce organic micro and nano structures It is involved in all of the methods to fabricate nanostructured PANI to be discussed in the next section

1.3 Synthetic methods of micro and nano structured conducting polymers

Generally, CPs can be prepared by the chemical method and the electrochemical method Traditionally, PANI is produced by oxidation polymerization of aniline monomers with a strong oxidant in acidic media Common mineral acids such as HCl and H2SO4 are used as dopants23and the products are usually in the form of powders in bulk polymerization Recently, micro and nano structures of CPs, (including PANI and its derivatives) with different morphologies 24-117 and their hierarchical assemblies118-122 have been reported The array of synthetic methods for CPs micro and nano structures is discussed below

1.3.1 Hard template method

Hard template method is the most straightforward method for producing CPs micro and nano structures.24 Hard templates are porous membranes, typically the anodized aluminum oxide porous (AAO) membrane made by the electrochemical techniques or the polycarbonate (PC) membrane fabricated by the ‘track-etch’ method.25,26 Hard templates guide the growth of micro and nano structures within the pores This process was pioneered by Martin27 and a range of pore sizes down to 5 nm have been reported.28-38 In addition, other hard templates such as nanochannel array glass membranes,39 porous alumina silicate MCM-41,40 mesoporous zeolites,41 microporous polymeric filtration membranes,42 carbon nanotubes,43 lipid tubule edges,44 electro-spun polymer fibers,45 highly oriented pyrolytic graphite,46 DNA,47-52 tobacco mosaic virus53 and other biological

templates54 have also been employed

Solid rods/wires and hollow tubes are the common structures synthesized by the hard template method Electrostatic and solvophobic interactions induce CPs to nucleate and grow preferentially along pore walls to form tubular structures CPs will grow inwardly to form solid structures if polymerization proceeds further The limit of monomer diffusion rate is considered as the key factor at sufficiently high oxidation potentials: (i) solid structures are formed under a high monomer concentration and a slower polymerization rate, in which monomers will have enough time

to diffuse toward the pore center; (ii) hollow structures are produced under a low monomer concentration and a fast polymerization rate In this case, monomers are not sufficient to completely fill into the pores during the limited time.34-37

The hard template method is particularly useful for the fabrication of

-Poly(3,4-ethylenedioxythiophene) (PEDOT) core-shell structures.38 Recently, PPy-CdS p-n junction nanowires55 have been obtained, showing a strong photo-dependent rectifying effect Unusual structures such as hollow octahedrons of PANI can only be obtained by the hard template method.56

Hard template method has its limitations such as laborious and cumbersome post-synthesis purification steps to remove the templates and difficulties in scalability For example, ordered nanorods in an AAO matrix tend to collapse during the template removal process, mainly due to the harsh conditions Novel templates such as cuprous oxide56 and certain porous diblock copolymers57 have therefore been developed for easy removal

1.3.2 Seeding method

CPs nanofibers, wires and tubes can be formed on existing nanomaterials, mostly

oxidative inorganic nanofibers/wires such as V2O5 and MnO2.58-62 It is believed that the monomer undergoes “pre-polymerization” reactions on the V2O5 nanofiber surfaces and the nanofibers would transfer their morphology to the growing CPs during the polymerization when extra oxidizing agents are added In this way, pure CPs nanotubes could be obtained by etching the V2O5-CPs core-shell structures with HCl during purification.61 MnO2 can be used as both the template and the oxidant to produce PANI nanotubes without special purification steps.24

1.3.3 Soft template method

Soft templates are the mesophase structures formed by self-assembly of external structure-directing agents,63 such as crown ether derivatives.64 Driving force for the assembly includes hydrogen bonding, π-π stacking, van der Waals forces, and electrostatic interactions.65Typically micellar structures act as soft templates when the surfactant concentration reaches the critical micelle concentration This technique is quite versatile for the preparation of many different CPs, not only producing sphere-like structures, but also fibers and tubes Surfactant micelles would undergo a sphere to rod transition when surfactants achieve the second critical micellar concentration.66 These anisotropic micelles are believed to direct the growth of CP 1D structure Mechanistic studies also reveal that cationic surfactants with long chains are more efficient than anionic or non-ionic surfactants.24 Moreover, some oxidants can assist surfactants in soft templates formation For example, insoluble lamellar precipitate as a soft template can be formed by adding

hexadecyltrimethylammonium bromide into the pyrrole solution, resulting in PPY nanowires and ribbons.70

Reverse micelles have recently been used to form dynamic templates to direct 1D CPs

nanostructure growth.71-74 Polymerization would occur along the outside of the template, because water soluble oxidants such as FeCl3 are solvated inside the nanometer-sized water domains Fe3+ is supposed to be able to migrate to the outside of the template to oxidize the monomer, due to the dynamic nature of reverse micelles.71,72 In this technique, product morphologies are particularly sensitive to polymerization conditions For example, short nanorods and longer nanotubes could be produced by slight variations of reaction conditions.74

In general, post-synthetic steps under mild conditions are required to remove soft templates However, if functional dopants can direct PANI growth, no further purification is needed This concept was pioneered75 and summarized76 by Wan This method has been generally adopted

to synthesize a variety of micro and nano structures, including fibers,77 tubes,78-88 tube junctions89 and hollow spheres.90-92 These structures and their dimensions can be adjusted by changing synthetic conditions,93 dopants structures94,95 and redox potentials of the oxidants.96 Micelles composed of the dopant and dopant/aniline salt have been shown to function as soft templates on the basis of dynamic light scattering97 and freeze-fracture transmission electronic microscopy.98 Similar

to the surfactant micelle-assisted growth by accretion99 and elongation,100 reactions occur at micelles/water interfaces which leads to the formation of nanoparticles and 1D structures.76

1.3.4 Template-free method

Template-free method was first proposed to produce PANI nanofibers and nanotubes without any external agents The formation process is based on the preference of PANI to form 1D self-assemblies101 Two main approaches have been proposed, the interfacial approach and the rapid mixing approach

In the interfacial process102,103 which adopts an immiscible organic/aqueous biphasic

system, aniline monomer and a water soluble oxidant are dissolved in the organic phase and the strong acidic aqueous phase, respectively Several minutes after mixing the two solutions in a beaker, PANI nanofibers start to appear at the organic-aqueous interface and gradually migrate into the aqueous phase Finally, an entangled mat of PANI fibers can be filtered and collected

The fast mixing approach in an all aqueous media was discovered later, indicating that

a phase interface was not necessary to produce 1D nanostructures.104,105 In a typical fast mixing process, the monomer and oxidant solution are quickly mixed The oxidant is rapidly consumed to depletion just after producing nanofibers This approach bears the assumption that polymerization is supposed to stop as soon as the nanofibers are formed, in order to effectively suppress secondary overgrowth.101 The growth of insoluble PANI in aqueous solution is accompanied by a precipitation process, so the product morphology is related with its nucleation mode, i.e., homogenous nucleation leads to nanofibers while heterogeneous nucleation results in granular particulates.106,107 This explains why accelerating the polymerization or reducing mechanical agitation is preferred for 1D structure Although this approach could be directly applied to some PANI derivatives, only irregular, micrometer-sized shapes could be produced for other CPs The problem was recently solved by adding a small amount of the appropriate oligomers into reaction solutions.108-111 The exact mechanism is still not clear, but is believed that the predisposition is critical for directing anisotropic growth

Oriented PANI nanofibers of a very low aspect ratio have been grown on a solid surface, without any external template.112,113 Although the aniline monomer first nucleates heterogeneously on the solid surface, the competition with bulk solution polymerization limits the extent of PANI growth on the surface, resulting in vertically ordered arrays of short PANI

nanofibers

Steiska’s group recently reported the polymerization of aniline in high pH aqueous solutions and the production of nanotubes in the absence of any template.114-117 Compared with the dominating head-to-tail coupling in traditionally strong acidic media, ortho-coupling leads to phenazine-containing fragments in high pH solutions Columns or stacks of self-assembled flat phenazine cycles by π-π interaction could direct PANI nanotubes formation when the pH reaches a sufficiently low value, because heterogeneous growth on available nucleates is energetically more favorable The conductivity, however, is modest since phenazine-containing oligomers are not conjugated

1.4 Applications of micro and nano structured conducting polymers

Lately micro and nano structured CPs have attracted much attention Compared with their continuous films prepared form bulk materials, they can render improved performance or demonstrate innovative properties Some of their applications are introduced below

1.4.1 Hydrophobic surfaces

Super-hydrophobic surface, whose water contact angle is larger than 150°, have many practical applications.123 Although PANI is usually hydrophilic, Wan’s group first proposed to fabricate super-hydrophobic surfaces of PANI films of nanostructures by doping with perfluorooctane sulfonic acid (PFOSA) or perfluorosebacic acid (PFSEA) Their hydrophilic groups act as dopants and soft-templates while perfluorinated carbon chains contribute to super hydrophobicity.119-121 Moreover, hierarchical structures of PANI could create surfaces rough enough

to efficiently trap air inside vacancies and thus becoming hydrophobic,124,125 just like the

hydrophobic natural organisms.126,127

1.4.2 Chemical sensors

Conductivity of CPs film can change significantly by interaction with oxidative or reductive chemicals which forms the basis of chemical sensors.128,129 For example, PANI and its derivatives are claimed to be employed as active elements for chemical sensors.5 Moreover, PANI also responds to acids or bases due to its unique doping and dedoping mechanism A plethora of analytes have already been reported.130-142 The mechanisms are classified into the five established models.130 In particular, films based on nanofibers of PANI are more sensitive than conventionally continuous films not only because of their much larger surface areas, but also due to their shorter diffusion path length for vapor molecules.102

1.4.3 Photothermal effects

Theoretically, absorbed radiation energy is generally dissipated in three ways, radioactive relaxation, charge separation and non-radioactive relaxation The former two has been widely used in organic electronics; while the last was recently developed as a flash welding technique, especially for PANI nanofibers.143 The phonons in the bulk form are easily and rapidly dissipated throughout the materials and the temperature increase is limited In contrast, it is supposed that the scattering of phonons at peripheries significantly trap heat inside nanostructures, and the temperature is reported to exceed 1500 ℃.144,145 Flash welding can easily produce smooth and continuous films, and thus is suitable for selective patterning and even asymmetric films fabrication PANI nanofibers have also been suggested to be an ideal organic solder for welding nanoscale building blocks for complex devices.143

1.4.4 Biomedical applications

Research on CPs for biomedical applications started in the 1980s, and has been expanded to many applications which involve electrical stimulations such as biosensors, tissue engineering, neural probes, drug delivery and bio-actuators Large surface areas of nanofibers can effectively increase the detected signal and thus lower the detection limits.146 One recent publication successfully demonstrated the use of CPs nanotubes as a novel drug release platform PEDOT nanotubes can control the kinetics of drug release by responding, contracting or expanding, to external electrical stimulations.147

1.4.5 Organic electronics

Today, researchers are focusing their attention on reducing the size of semiconductor devices to achieve high-integration density, low power consumption and cheap information processing and storage systems Compared with their inorganic counterparts, organic electronics based on molecular or polymeric materials, has the following advantages: (i) many properties of organic materials can be finely tuned to fit specific requirements, such as solubility in organic solvents and the color of emission and (ii) easy processing of organic materials assists to realize low-cost large-scale fabrication, because the existing coating technology can be applied over large areas and various substrates.148

1.4.5.1 Electrochromic display devices

Electrochromic cells are used to go from opaque to transmissive at selected regions of the electromagnetic spectrum.149 The electrochromic effect of CPs has attracted much attention for fabricating flexible display devices.150-153 For example, the color of a PANI film is reversibly changed to green by oxidation and to transparent yellow by reduction Compared with traditional

CPs continuous films, nanofibers and nanotubes can shorten diffusion path lengths of counterions and thus effectively reduce the redox switching time.35-37,154

1.4.5.2 Organic field effect transistors (OFETs)

OFETs based on CPs as the active element are ready for commercialization155 after decades of R&D156-163 Continuous P3HT film is one of the most intensively investigated active component materials OFETs demonstrate higher field effect mobility and a greater on/off ratio when P3HT nanowire is used instead of continuous P3HT film, because P3HT nanowires are more structurally ordered, and thus perform more efficiently in charge transport.164,165

1.5 Objectives and scope

The purpose of this study is to synthesize various PANI micro and nano structures by chemical methods Their electrical properties and hydrophobicity are also investigated for possible applications The specific objectives of this project are listed as the following:

(i) To synthesize PANI micro and nano structures in toluene using both cationic and non-ionic

surfactants;

(ii) To measure the hydrophobic and hydrophilic properties of PANI films of micro and nano

structures;

(iii) To measure current-voltage (I-V) curves of an individual PANI microplate and

sub-microsphere, as well as their macroscopic films

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