Next generation of conducting materials for organic electronics

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. I 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 II 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 III 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 IV 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 V 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 pressure…………………………………………………………….……………....91 4.4 Conclusions…………………………..………………………………………………………92 Chapter Five Conclusions and Future Work………………………...............................................98 VI 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. VII Nomenclatures 1D 1-Dimension 2D 2-Dimension 3D 3-Dimension CA Contact Angel CP Conducting Polymer CTAB Cetyltrimethylammonium Bromide EB Emeraldine Base EM Emeraldine ES Emeraldine Salt FTIR Fourier Transform Infrared Spectroscopy LM Leucoemeraldine NA Nigraniline PANI Polyaniline PNA Pernigraniline PT Polythiophene PPY Polypyrrole SEM Scanning Electron Microscopy TOAC Trioctylmethylammonium Chloride TOA Trioctylamine UV-vis Ultraviolet-visible VIII List of Figures Figure 1.1…………………..2 Octameric structures of polyaniline in various intrinsic redox states Figure 1.2……………..……3 Inter-conversions among different oxidation states and protonated (ES)/deprotonated (EB) states in PANI Figure 2.1…………………30 SEM images (a, b) and TEM images (c, d) of PANI sub-microspheres Figure 2.2…………………31 Energy-dispersive X-ray spectrum of PANI sub-microspheres Figure 2.3…………………32 X-ray powder diffraction pattern of PANI sub-microspheres Figure 2.4…………………33 Schematic diagram illustrating the formation of PANI sub-microspheres Figure 2.5………………....34 SEM of PANI/Au powder synthesized at different monomer concentration Figure 2.6……………..…..35 SEM images of PANI/Au powder synthesized at different HAuCl4 concentration Figure 2.7…………..……..36 SEM images of PANI sub-microspheres at different reaction conditions Figure 2.8…………………37 Schematic diagram and optical image of experimental setup for electrical measurement of PANI sub-microspheres with two electrodes IX Figure 2.9………………....38 I-V characteristics of PANI sub-microspheres at different pressures Figure 2.10…………….….40 (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. Figure 2.11………………..41 FTIR spectra of PANI sub-microspheres before (a) and after (b) reduced pressure Figure 2.12……………..…42 UV-vis spectra of PANI sub-microspheres before (a) and after (b) reduced pressure Figure 3.1………………....53 SEM images with TEM insets of the PANI structures at different [HAuCl4] with fixed [Aniline]/ [HAuCl4] at 33 Figure 3.2………………....55 SEM images with TEM insets of the PANI microstructures with fixed [Aniline]/ [HAuCl4] at 16 Figure 3.3………………....56 SEM images with TEM insets of the PANI microstructures with fixed [Aniline]/[HAuCl4] at 5 Figure 3.4…………………57 Diameter distributions of spheres for sample C1 Figure 3.5……………..…..58 SEM images of the PANI microstructures with fixed [Aniline]/ [HAuCl4] at 1.67 Figure 3.6…………………59 SEM images with TEM insets of the PANI microstructures with fixed [HAuCl4] at 12 mM; varying the [Aniline]/[HAuCl4] X Figure 3.7…………...…….61 Schematic diagram of synthesis locations: microplates were adhered to reactor wall; tubes and spheres were produced in solution Figure 3.8…………………63 SEM images with TEM insets of the PANI microstructures with different temperatures Figure 3.9…………………64 SEM images of dopant effect on the PANI microstructures with [HCl]/[Aniline] molar ratio fixed at 0.5 Figure 3.10……………….65 Electron diffraction (a) sample A5; (b) Au aggregates in the background Figure 3.11………………..66 SEM images with corresponding Energy-dispersive X-ray (EDX) spectra Figure 3.12……………..…67 FTIR and Uv-Vis spectra of different PANI structures Figure 3.13………………..68 FTIR and Uv-Vis spectra of PANI structures produced at different [Aniline]/ [HAuCl4] molar ratios Figure 3.14……….……….70 Shapes of a water droplet on different films and their contact angles Figure 4.1…………………79 Schematic diagram of synthesis locations: microplate structures were adhered to the glass wall; other structures were produced via reverse micelles in the solution Figure 4.2…………………82 SEM images with SEM insets (b, d) and TEM insets (c, f) of the PANI micro and nano structures when [HAuCl4] is at12mM XI Figure 4.3…………………83 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 Figure 4.4…………………84 FTIR and Uv-Vis spectra of different PANI structures Figure 4.5…………………85 FTIR and Uv-Vis spectra of PANI microplates Figure 4.6…………………86 (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 Figure 4.7…………………87 I-V characteristics of an individual PANI microplate plotted on a log-log scale Figure 4.8………………....89 (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 Figure 4.9…………………90 Schematic diagram of the experimental setup and optical image of the sample for electrical measurement of PANI macroscopic films with two electrodes Figure 4.10………………..91 I-V characteristics for the macroscopic PANI film of microplates at different pressures XII List of Tables Table 3.1............................51 Synthesis details for PANI structures Table 3.2………………....53 Morphologies of the PANI A-series products Table 3.3………………....54 Morphologies of the PANI B-series products Table 3.4………………....56 Morphologies of the PANI C-series products Table 3.5…………………57 Morphologies of the PANI D-series products Table 3.6…………………59 Effect of [Aniline]/[HAuCl4] ratio on morphologies of the PANI samples Table 3.7………………....62 Effect of temperature on morphologies of the PANI samples A5 Table 4.1…………………78 Synthesis details for PANI products Table 4.2…………………81 Effect of [Aniline]/[HAuCl4] ratio on morphologies of the PANI products Table 4.3…………………83 Effect of [HAuCl4] on morphologies of PANI products XIII 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 1 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). 2 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.5,6 Inter-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 3 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 4 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 1.2.1 Nanomaterials 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.8 Nanoscience 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. 5 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 6 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 7 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 dopants23 and 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 8 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 organic/inorganic composites with spatially controlled composition such as MnO2- 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 9 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.65 Typically 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 both ammonium persulfate and cetyltrimethylammonium bromide (CTAB)67-69 or 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 10 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 11 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 12 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 13 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 14 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 15 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. 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For example, Wan have reported the synthesis of hollow PANI spheres by a self-assembly method.36-38 Li synthesized hollow PANI colloidal spheres under hydrothermal conditions.39 Travas-Sejdic obtained hollow nanosphere composites of PANI and Au nanoparticles.40 The above works focused on the production of hollow spheres. In this chapter, the one-pot synthesis of mono-dispersed solid PANI sub-microspheres was conducted using chloroaurate acid as the oxidant. The influence of the synthetic conditions on the specific morphology and size of PANI spheres was investigated. The morphologies of the reaction products were characterized by SEM and TEM, and the chemical and electronic structures of the PANI spheres were studied by Fourier transform IR (FTIR) and UV-vis spectroscopies, respectively. 2.2 Experimental Section 2.2.1 Chemicals Aniline (An) was purchased from Sigma (Singapore). Chloroaurate acid was 27 purchased from Alfa Aesar. Aniline monomer was freshly distilled under reduced pressure. Other chemicals were used as received without further treatment. 2.2.2 Preparation of solid PANI spheres The 10 mM HAuCl4/CTAB/toluene solution was prepared by dissolving 1 mmol of cetyltrimethylammonium bromide (CTAB) and 1 mmol of HAuCl4 into 100 mL toluene. Then 0.1 mmol of aniline and 5 ml of the 10 mM HAuCl4/CTAB/toluene solution were rapidly dissolved in 5 ml of toluene. The mixture was strongly stirred for several minutes and left to react at room temperature for 16 h. The resulting precipitate was centrifuged and washed with distilled water and ethanol several times to remove residual surfactants and reactants. A series of samples were prepared under different conditions in order to study the effect of reaction time, temperature and monomer concentration on the structures of the products. 2.2.3 Characterizations The morphology of the samples was examined by a JEOL JSM-6701F field emission scanning electron microscope (SEM) and a JEOL JEM 3010F transmission electron microscope (TEM). In the SEM experiments, Synchronous Energy Dispersive X-ray (EDX) analysis was also conducted using a LINK ISIS300 instrument. X-ray diffraction (XRD) patterns were taken with a Bruker AXS D8 Advance X-ray diffraction instrument using CuKα irradiation. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 2000 IR spectrometer in the range of 400-4000 cm-1 on sample pellets made with KBr. The absorption spectra of the PANI products in ethanol were recorded with an UV-vis spectrophotometer (UV-1700PC, Shimadzu). 28 2.2.4 Electrical Measurements To measure the electrical properties of the PANI sub-microspheres films, current-voltage (I-V) curves were collected using an electrical probe station. A quartz substrate was first cleaned and sonicated in alcohols solution. A thin strip of glue of about 7 µm wide was used as a mask on the surface of quartz substrate and Au film (~150 nm) was deposited on the quartz surface using an E-beam evaporator. After removal of the strip of glue the two Au pads were separated by a gap of about 6.5 µm. Two copper wires as two electrodes were mounted on the surface of the two Au pads via silver paste. The PANI sub-microspheres were ultrasonically dispersed in ethanol and deposited as a film across the gap between two Au pads and dried. Hence the samples, the Au pads, copper wires, and the current sensor of an Alesi REL-2100 analytical probe station form a complete circuit. In order to measure the electrical properties of an individual PANI structure, PANI sub-microspheres were ultrasonically dispersed in ethanol and placed on an insulating SiO2 substrate. Two-probe electronic transport measurements were carried out in a field emission scanning electron microscope (FE-SEM, JSM7401-F, JEOL) equipped with nano-manipulators (sProber Nano-M, Zyvex Instruments) at room temperature under a reduced pressure of ~10-3 Pa. 2.3 Results and Discussion 2.3.1 Synthesis and characterizations of PANI sub-microspheres The mono-dispersed solid PANI sub-microspheres (Fig. 2.1a, 2.1b) were synthesized when the polymerization was carried out under the best reaction conditions: [An] = 0.01 M, [An]/[HAuCl4] = 2, reaction time = 16 h, at room temperature, without stirring. SEM pictures 29 showed mono-dispersed and regular spherical polyaniline particles with diameters of 320-480 nm (Fig. 2.1a). At a higher magnification (Fig. 2.1b), it was revealed that the PANI sub-microspheres were covered with a granular structure on their surfaces. A sharp contrast between the dark edge and the pale center in TEM images is commonly adopted to show the existence of hollow PANI spheres41-43. The solid structure of the spheres synthesized here is thus revealed by TEM (Fig. 2.1c, 2.1d). The granular structures on the surfaces of spheres are due to the presence of short fibers. A few dark spots of gold nanoparticles and large gold aggregates on the surface of PANI sub-microspheres are also observed from the TEM images. Electron diffraction (ED) pattern (Fig. 2.1c inset) shows the diffraction rings and spots corresponding to the amorphous PANI spheres and gold, respectively. Figure 2.1 SEM images (a, b) and TEM images (c, d) of PANI sub-microspheres: (a, c) at low magnification (inset: an electron diffraction pattern); (b, d) at high magnification. Reaction conditions: [An] = 0.01 M, [HAuCl4] = 5 mM, without stirring, 25 oC, t = 16 h. 30 The EDX spectrum in Fig. 2.2 reveals the presence of chlorine and bromine originated from the dopant and CTAB, respectively. The presence of Au elemental can be also shown. Figure 2.2 Energy-dispersive X-ray spectrum of of PANI sub-microspheres. Reaction conditions: [An] = 0.01 M, [HAuCl4] = 5 mM, without stirring, 25 oC, t = 16 h. The X-ray diffraction (XRD) patterns are shown in Fig. 2.3. The four main peaks at 38o, 44o, 65o, 78o correspond to (111), (200), (220), and (311) Bragg reflections of gold respectively. The broad peak centered at 2θ = 29 o is ascribed to PANI. The results further confirm the co-existence of gold and PANI sub-microspheres. 31 Figure 2.3 X-ray powder diffraction pattern of PANI sub-microspheres. Reaction conditions: [An] = 0.01 M, [HAuCl4] = 5 mM, without stirring, 25 oC, t = 16 h. The proposed mechanism for the formation PANI spheres using HAuCl4 as oxidant is as follow. It is possible that the fibrous solid spheres are formed by the co-operative polymerization of aniline from two soft templates in the form of the CTAB reverse micelles and the anilinium cation micelles respectively.24,38,45 Fig. 2.4 depicts the formation of mono-dispersed solid PANI sub-microspheres. Firstly, CTAB form the reverse micelles readily in toluene because of its amphiphilic property. The reverse micelles adopt spherical shapes corresponding to the configuration with lowest surface energies.44 Aniline monomers diffuse into the reverse micelles and get into contact with the oxidant HAuCl4. Oxidative polymerization then takes place inside the spherical and mono-dispersed reverse micelles. These spherical reverse micelles therefore function as soft templates for aniline polymerization. In addition, the anilinium cation micelles can also serve as soft templates to form PANI nanofibers on the surfaces. As a result, PANI spheres covered with short nanofibers are formed. The elemental gold, reduced from HAuCl4 in the reaction with aniline, may be protected by polyaniline or the surfactant CTAB. 32 Figure 2.4 Schematic diagram illustrating the formation of PANI sub-microspheres. In order to test the proposed mechanism, we investigated the influence of concentration of monomer and oxidant on the morphologies and sizes of PANI products. SEM images for PANI structures synthesized at different monomer concentrations are shown in Fig. 2.5. A transition from spherical to granular morphology can be observed by simply adjusting the concentration of aniline from 0.01 M to 0.1 M. As described above, the amount of HAuCl4 was kept constant, whereas the monomer to oxidant ratio was varied. At a given concentration of oxidant, lower concentration of monomer favors the sub-microsphere formation due to the formation of spherical reverse micelles. At higher monomer concentration, however, the excess amount of aniline destroys the formation of spherical reverse micelles or the primordial spheres become scaffolds for the overgrowth of polyaniline, which finally leads to irregularly shaped agglomerates.16 33 Figure 2.5 SEM of PANI/Au powder synthesized at different monomer concentration: (a) 0.01 M, (b) 0.02 M, (c) 0.05 M, (d) 0.1 M. Other reaction conditions: [HAuCl4] = 5 mM, 25 oC, time = 16 h. The morphologies and sizes of PANI spheres can also be affected by the concentration of the oxidant. When the amount of aniline was kept at 0.01 M and the concentration of HAuCl4 was changed from 20 mM to 0.5 mM, it was found that the spherical morphology remained the same as what is shown in Fig. 2.4. The findings indicate that the influence of the concentration of the oxidant on the morphology is small. However, the average sizes of spheres decrease while reducing the concentrations of HAuCl4 (Fig.2.6a-2.6d), because the corresponding lowering of surfactant CTAB concentration leads to a decrease in the size of spherical reverse micelles.44,45 Interestingly, the PANI spheres formed at low concentration of HAuCl4 (and meanwhile the low concentration of CTAB) became connected into a string of beads to form a 1D structure (Fig. 2.6d). 34 Figure 2.6 SEM images of PANI/Au powder synthesized at different HAuCl4 concentration: (a) 20 mM, (b) 5 mM, (c) 2.5 mM, (d) 0.5 mM. Other reaction conditions: [An] = 0.01 M, 25 oC, t = 16 h To further investigate the validity of the proposed formation mechanism, some control experiments were also carried out. It was found that the morphologies and sizes of PANI products were affected by the presence or absence of mechanical stirring, reaction temperature and the polymerization time. Under stirring, it was difficult to fabricate the intact spheres because of disruptive effect of stirring on the stability of the spherical CTAB reverse micelles (Fig. 2.7a, compared to 2.7d). At room temperature and 10 oC, mono-dispersed PANI spheres could be prepared at high yield (Fig. 2.7c, compared to 2.7d). The formation of PANI spheres with a smaller size formed at 10 oC may be attributed to the effect of temperature on the size of reverse micelles.32 The as-synthesized PANI after 1 h polymerization formed spheres with smaller diameters (Fig. 2.7b, compared to 2.7d). The results from control experiments suggest that the solid PANI spheres undergo a growth process from tiny spheres to larger ones without stirring at room temperature, in 35 consistent with the proposed formation mechanism. Figure 2.7 SEM images of PANI sub-microspheres at different reaction conditions: (a) under stirring, 25 oC, t = 16 h; (b) without stirring, 25 oC, t = 1 h; (c) without stirring, 10 oC, t = 16 h; (d) without stirring, 25 oC, t = 16 h. Reaction conditions: [An] = 0.01 M, [HAuCl4] = 5 mM. 2.3.2 Electrical Properties 2.3.2.1 Current-Voltage (I-V) characteristics of PANI films at different pressure levels For electrical measurements, the samples were deposited across the gap of several micrometers between two planar gold pads. The samples were transferred into an enclosed chamber in which the pressure level could be controlled by pumps. Fig. 2.8 shows the schematic diagram and optical image of the experimental setup for electrical measurement of PANI sub-microspheres films with two electrodes. 36 Figure 2.8 Schematic diagram and optical image of experimental setup for electrical measurement of PANI sub-microspheres with two electrodes. The current-voltage measurements of PANI sub-microspheres were conducted in an electrical probe station at different pressure levels from the normal atmospheric to 8.0×10-5 Pa (Fig. 2.9). At the normal atmospheric pressure, the I-V curve of PANI sub-microspheres films showed a nearly ohmic behavior (Fig. 2.9 a). The linearity of the plot demonstrated a typical metallic behavior of highly doped PANI sub-microspheres, due to the formation of polaron lattice.46,47 When the pressure in the chamber was reduced to 7.5×10-1 Pa for one day, the samples displayed a quite different I-V curve and exhibited semiconducting characteristic. The resistance also increased two orders of magnitude compared with that measured at the normal atmospheric pressure (Fig. 2.9 b). Furthermore, the conductivity measured at 8.0×10-5 Pa was five orders of magnitude lower than that measured at the normal atmospheric pressure. All the I-V results above can be repeated perfectly. The conductivity of PANI is well known to be dependent on its doping level48. At a reduced pressure level, hydrochloric acid as the dopant can easily escape from the PANI matrix. As a 37 result, PANI sub-microspheres became partially de-doped. The I-V characteristics of PANI sub-microspheres therefore changed from linear at atmospheric pressure to nonlinear at reduced pressure levels. Figure 2.9 I-V characteristics of PANI sub-microspheres at different pressure levels: (a) at atmospheric pressure; (b) at 7.5×10-1 Pa; (c) at 1.6×10-4 Pa; (d) at 8.0×10-5 Pa. 2.3.2.2 Current-Voltage (I-V) characteristics and calculated conductivity of an individual sub-microsphere In order to confirm the intrinsic conductivity of PANI sub-microsphere, we directly measured the I-V characteristics of a single PANI sub-microsphere. All the electrical measurements were carried out in the FE-SEM chamber at room temperature at a reduced pressure of 9.0×10-4 Pa. Two probe electronic transport measurements were carried out using tungsten tips with the size ~50 38 nm. A Keithley source measurement unit (model: 4200SCS) connected to the tungsten probes was used to measure the I-V characteristics of the sub-microspheres. A typical SEM image of electrical measurement of a single PANI sub-microsphere with two electrical probes is displayed in Fig. 2.10a. The electrical characteristics of single PANI sub-microsphere were investigated by recording the tip current (I) as a function of the bias voltage (V). The typical I-V characteristic obtained from this sphere is shown in Fig. 2.10b. The nonlinear I-V curve demonstrated the typical semiconducting characteristic of partially doped PANI. The conductivity, σ, was estimated from the I-V curve using equation (1). σ = L/(S·R) (1) Where σ is the conductivity, L is the diameter of the sub-microsphere (325 nm), and S is the largest section area of the sub-microsphere (calculated as πr2). Although the I-V characteristic of a semiconducting sub-microsphere is affected by the contacts between the metal electrodes and sub-microsphere, these contact effects may be decoupled from experimental data.49,50 When the current passing through the sub-microsphere is sufficiently large, the applied bias increment is mainly distributed on the sub-microsphere. Since the resistance of the two electrical probes is much lower than that of the sub-microsphere, the resistance (R) for the sub-microsphere was estimated from the linear fit of the I-V curve at large bias (from 4V to 5V). The conductivity of the single PANI sub-microsphere is calculated to be 9.3×10-3 S/cm. At the same reduced pressure levels, the measured conductivity of a single sub-microsphere is around six orders greater than that of sub-microspheres films (~10-6 S/cm), due to the removal of inter-spherical contact resistance. 39 Figure 2.10 (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. Upon partial de-protonation under reduced pressure levels, the conductivity of PANI sub-microspheres can be lowered by as much as five orders of magnitude. In the doped form, the nitrogen atoms are protonated to form a delocalized polaron structure. Under the reduced pressure level, transition from a highly doped form to a partially doped form will strongly affect the chemical structure and the electronic band structure of the polymer. For further investigations, the chemical and electronic structures of the solid PANI spheres subjected to different pressure levels were studied by FTIR (Fig. 2.11) and UV-vis spectroscopy respectively (Fig. 2.12). The FTIR spectra of PANI spheres before and after reduced pressure levels are presented in Fig. 2.11. The characteristic PANI peaks at 1573 cm-1 and 1493 cm-1 (the C=C stretching of quinoid and benzenoid rings, respectively), 1300 cm-1 (the C-N stretching of the secondary aromatic amines), 1146 cm-1 (the in-plane bending of the aromatic C-H), and 814 cm-1 (the out-of-plane bending of C-H in the 1,4-disubstituted benzene ring) are observed (Fig. 2.11a), which are in good agreement with the emeraldine salt form of PANI.18,19,40 After exposure to the reduced pressure, all characteristic peaks shift to higher frequency by about 10-20 cm-1 (Fig. 2.11b). Compared to the intensity of the C=C stretching vibration of benzenoid rings (1510 cm-1), the 40 intensities of C=C stretching vibration of quinoid rings (1593 cm-1), the C-N stretching of the secondary aromatic amines (1310 cm-1) and the in-plane bending of the aromatic C-H (1166 cm-1) all decreased relatively when the as-prepared PANI was treated by the reduced pressure. The 1166 cm-1 band in particular is reported as a measure of the degree of delocalization of electrons in PANI and therefore is directly related to the degree of doping and electrical conductivity of PANI51,52. The IR spectra therefore support the dedoping process under the reduced pressure levels. Figure 2.11 FTIR spectra of PANI sub-microspheres before (a) and after (b) the reduced pressure. The presence of the conductive emeraldine salt of PANI sub-microspheres is also supported by the UV-vis absorption spectrum of PANI spheres dispersed in ethanol, as shown in Fig. 2.12. The peak centered around 346 nm and a shoulder at 443 nm are attributed to the π-π* transition of the benzenoid rings and the polaron/biplaron transition respectively. A strong peak centered at 41 about 950 nm with a free-carrier tail extending into the near-infrared region is also observed, in accordance with the reported results of highly doped as-prepared PANI sub-microspheres.18,19 After exposure to environment with reduced pressure, the peak at 443 nm disappears and the 346 band shifts to 366 nm due to the deprotonation which lowers the formation of polarons and bipolarons. The peak at 950 nm with a free tail shifts to 773 nm which is ascribed to exciton absorption of the quinoid rings along dedoped PANI main cains.53,54 The results also confirm a significant degree of dedoping under reduced pressure. Figure 2.12 UV-vis spectra of PANI sub-microspheres before (a) and after (b) reduced pressure. 2.4 Conclusions In summary, the mono-dispersed solid PANI sub-microspheres were prepared directly using chloroaurate acid as the oxidant in toluene. These spheres were covered with short fiber on 42 their surface. It was found that the morphology and size of PANI spheres were affected by the concentration of aniline and HAuCl4, reaction temperature, polymerization time and mechanical stirring. It was proposed that the spherical reverse micelles of CTAB and rod-like micelles composed of anilinium cation might act as soft templates in the formation of rough solid spheres. This method provides a simple route to prepare solid spheres of conducting polymers. The current-voltage (I-V) characteristics of an individual PANI sub-microsphere and the macroscopic film were measured. It was also found that the conductivity of the products was significantly influenced by different pressures in measurement system. 43 References 1. Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. 2. Liang, L.; Liu, J.; Windisch, C. F.; Exarhos, G. J.; Lin, Y. Angew. Chem. Int. Ed. 2002, 41, 3665. 3. Jang, J. Adv. Polym. Sci. 2006, 199, 189. 4. 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Macromolecules 1995, 28, 2858. 47 Chapter 3 Morphology Evolution of Polyaniline Microstructures via Reverse Micelles and Intrinsic Hydrophobicity 3.1 Introduction Conducting polymer nanostructures, such as one-dimensional (1D) nanofibers, nanotubes, nanorods, nanoneedles and nanowhiskers1-8 and three-dimensional (3D) microspheres9-11 have been intensively studied. Two-dimensional (2D) nanoflakes, nanosheets and nanodisks are only recently reported.12-14 Polyaniline (PANI) has attracted much attention since its discovery, due to its simple preparation, good environmental and thermal stability, structural versatility.15 PANI micro and nano structures have been synthesized for many applications.1, 2, 11 Highly oriented arrays of PANI structures, as such flake and fiber arrays, have high application potentials in micro sensors and micro electronics.16 Chemical synthesis methods are commonly employed to synthesize PANI micro and nano structures.1 The methods are generally classified as template-directed growth and template-free growth. The former typically employs hard-templates such as anodized aluminum oxide (AAO), inorganic seeds such as V2O5 and MnO2 nanofibers or soft templates such as surfactants. For surfactant soft templates, four micelle models have been proposed to describe the formation of nanotubes, fibers and rods.17 On the other hand, the immiscible organic/aqueous biphasic systems and rapid-mixing methods are mainly used in template-free synthesis. Oriented arrays of PANI short nanofibers can also be produced on appropriate substrates, in absence of any template.1 Template-free growth is a promising way to produce polymer nanostructures in bulk 48 quantities without the need to remove templates. However, the preparation of polymer micro or nano structures with controlled morphologies and dimensions without templates is still a challenge. In particular, PANI synthesized in aqueous solution exhibits a wide variety of morphologies, because the reaction systems are particularly sensitive to slight variations in synthetic conditions. For example, strong acid, weak acid and alkali solutions would lead to conducting PANI nanogranules, nanotubes and non-conducting microspheres composed of ortho- and para- coupled units, respectively.18, 19 Salicylic acid doped PANI could be changed from nanotubes to hollow microspheres by simply altering the molar ratio of the dopant to the monomer, due to the different nature of hydrogen bonding.10 In addition, it is reported that nanofibers could be altered to nanosheets by simply increasing reaction humidity.13 It is therefore important to control the self-assembly process in order to induce morphological evolution in aqueous reaction systems by judiciously adjusting the reaction parameters. Nanoflakes, nanorods or nanospheres were obtained by changing the selenious acid to aniline molar ratio, leading to changes in micelles morphologies.12 Morphology evolution of PANI structures prepared under hydrothermal conditions was also reported20 which is attributed to the charged property and reactivity of the semiquinone radical cations that are sensitive to the concentration of the doping acids.20, 21 pH-stat chemical oxidation was recently reported to induce morphological evolution of self-assembled PANI nanostructures.22 Previous studies on morphology evolution were carried out mainly in aqueous systems, such as template-assisted evolution of the nanostructure morphology via direct emulsions.23 So far only a few experiments have been conducted using reverse micelles to synthesize PANI structures.24-26 In this chapter we will demonstrate that controllable morphology evolution of both 49 PANI micro and nano structures in various dimensions and shapes can be realized in toluene by adjusting reaction conditions. In particular, we have been able to produce solid PANI plates in micron size. The PANI micro and nano morphologies were examined by SEM and TEM. Their chemical and electronic structures were determined by FTIR and UV-vis studies. Hydrophilic and hydrophobic properties of PANI products were investigated by contact angle measurements. 3.2 3.2.1 Experimental Section Chemicals Aniline and trioctylmethylammonium chloride (TOAC) were purchased from Sigma (Singapore). Chloroaurate acid was purchased from Alfa Aesar. Aniline was freshly distilled under reduced pressure. Other chemicals were used as received. 3.2.2 Preparation of HAuCl4/TOAC/toluene solution 1.2 mmol HAuCl4 and 1.8 mmol TOAC were co-dissolved in 100 mL toluene. The mixture was then strongly stirred to form a clear yellow HAuCl4/TOAC/toluene solution at a concentration of 12 mM. 3.2.3 Synthesis of PANI micro and nano structures In a typical synthesis, 1 mmol of aniline (An) and 2.5 mL of the 12 mM HAuCl4/TOAC/toluene solution were rapidly dissolved in 250 ml of toluene. The mixture was strongly stirred for several minutes and left to react at room temperature for 12 h. The resulting precipitate was centrifuged and washed with distilled water and ethanol several times to remove residual surfactants and reactants. A series of samples products was prepared under different 50 conditions by changing HAuCl4 concentration, molar ratio of HAuCl4 to aniline. A summary of the amount of reactants used for each sample is provided in Table 3.1. Table 3.1 Synthesis details for PANI structures Samples Aniline mole (mmol) HAuCl4/TOAC/toluene solution (12mM) volume (ml) Toluene (ml) A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 C1 C2 C3 C4 D1 D2 D3 D4 E F 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2.5 2.5 2.5 2.5 2.5 2.5 5 5 5 5 5 5 16.7 16.7 16.7 16.7 50 50 50 50 150 200 0 2.5 7.5 50 250 350 0 2.5 10 50 150 250 0 33.3 83.3 183.7 0 150 250 250 0 0 Reaction time is 12 h at room temperature. The effects of temperature, mechanical stirring and additional acid on the morphology of the product were also studied. 3.2.4 Characterizations The morphology of the samples was examined by a JEOL JSM-6701F field emission scanning electron microscope (SEM) and a JEOL JEM 3010F transmission electron microscope (TEM). In the SEM experiments, Synchronous Energy Dispersive X-ray (EDX) analysis was also 51 conducted using a LINK ISIS300 instrument. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 2000 IR spectrometer in the range of 400-4000 cm-1 on sample pellets made with KBr. The absorption spectra of the PANI products in ethanol were recorded with an UV-vis spectrophotometer (UV-1700PC, Shimadzu). PANI films for contact angle measurements were prepared by spin-coating an ethanol solution of PANI on Au films coated on glass slides, at a speed of 200 rpm for 3 min. More than 10 determinations were carried out across the surface of the films with an accuracy of ±3°. 3.3 Results and Discussion 3.3.1 Morphologies evolution SEM and TEM images clearly reveal the morphology evolution of PANI products, from 1D microtubes, to 2D microplates, finally to 3D micro and nano spheres, depending on reaction conditions. They will be discussed in turn. 3.3.1.1 Effect of chloroauric acid concentration [HAuCl4] Fig. 3.1 shows the morphology evolution of PANI products prepared at different [HAuCl4] and the results are summarized in Table 3.2. Incomplete (sample A1, Fig. 3.1a) and complete microtubes (sample A2, Fig. 3.1b) were produced at high [HAuCl4]. At lower [HAuCl4], spheres (sample A3, Fig. 3.1c) and plates (sample A4, Fig. 3.1d) were formed, together with microtubes. At very low [HAuCl4], only plates (sample A5, Fig. 3.1e) and small nanospheres (sample A5, Fig. 3.1f) were produced on the wall and at the bottom of the glass beaker and in the solution, respectively. The nanospheres formed in solution were quite uniform, with an average diameter of 275 nm. The dimensions of plates are generally above tens of micrometers, and their thicknesses are 52 below 1μm. The adhesion of microplates to the glass was so strong that some of them could only be completely removed under ultrasonication. Table 3.2 Morphologies of the PANI A-series products Sample SEM image [HAuCl4] Morphology A1 Fig. 3.1a 12 mM Broken tubes + spheres/particles (minority) A2 Fig. 3.1b 6 mM Tubes + spheres/particles (minority) A3 Fig. 3.1c 3 mM Tubes + spheres + plates A4 Fig. 3.1d 0.57 mM Plates + sphere clusters (minority) A5 Fig. 3.1e/3.1f 0.12 mM Plates (on the wall) + spheres (in the solution) A6 Not shown 0.08 mM No definable structure [Aniline]/ [HAuCl4] =33; reaction time is 12 h at room temperature. Figure 3.1 SEM images with TEM insets of the PANI structures at different [HAuCl4] with fixed [Aniline]/ [HAuCl4] at 33. (a) 12mM; (b) 6mM; (c) 3mM; (d) 0.57mM; (e) 0.12mM on the glass beaker wall; (f) 0.12mM in the solution. 53 Morphology evolutions were observed if [Aniline] to [HAuCl4] ratio was changed to 16, 5 and 1.67. The evolutions details are listed in Table 3.3, 3.4 and 3.5, respectively. Fig. 3.2 shows the morphology evolution of PANI products at the ratio of 16. Microtubes, microspheres and solid microplates formed when [HAuCl4] was reduced. At the lowest [HAuCl4], fused hemispheres (sample B5, Fig. 3.2e) and separated hemispheres (sample B6, Fig. 3.2f) were produced. The bottom of the hemisphere was covered with tubercles, extending a spine 30 nm in average. These tubercles were similar to the reported rambutan-like hollow PANI spheres surfaces. These tubercles formation was thought to be related with the oxidative polymerization at micelle interfaces.11 Table 3.3 Morphologies of the PANI B-series products Sample SEM image [HAuCl4] Morphologies B1 Fig. 3.2a 12 mM Tubes +spheres/particles (minority) B2 Fig. 3.2b 6 mM Tubes + spheres/particles (minority) B3 Fig. 3.2c 4 mM Spheres + tubes B4 Fig. 3.2d 1 mM Plates + spheres B5 Fig. 3.2e 0.4 mM Fused hemispheres B6 Fig. 3.2f 0.2 mM hemispheres [Aniline]/ [HAuCl4] at 16; reaction time is 12 h at room temperature. 54 Figure 3.2 SEM images with TEM insets of the PANI microstructures with fixed [Aniline]/ [HAuCl4] at 16; concentration of HAuCl4: (a) 12 mM; (b) 6 mM; (c) 4 mM; (d) 1.0 mM; (e) 0.4 mM, (f) 0.2 mM. A mixture of micro and nano spheres, ranging from several hundred nanometers to more than 10 micrometers, was obtained at the ratio of 5. (Fig. 3.3a). Its diameter distribution is shown in Fig. 3.4. These spheres tended to aggregate into planar structures with many crevices at intermediate [HAuCl4] (Fig. 3.3b). Microplates began to appear and coexist with spheres if the [HAuCl4] was further lowered (Fig. 3.3c). Plates became the dominating morphology at the lowest [HAuCl4] (Fig. 3.3d). 55 Table 3.4 Morphologies of the PANI C-series products Morphologies Sample SEM image [HAuCl4] C1 Fig. 3.3a 12 mM Spheres + plates (minority) C2 Fig. 3.3b 4 mM Fused spheres into planar structures C3 Fig. 3.3c 2 mM Spheres + plates (minority) C4 Fig. 3.3d 1 mM Plates + particles (minority) [Aniline]/ [HAuCl4] at 5; reaction time is 12 h at room temperature. Figure 3.3 SEM images with TEM insets of the PANI microstructures with fixed [Aniline]/[HAuCl4] at 5; concentration of HAuCl4: (a) 12 mM; (b) 4 mM; (c) 2 mM; (d) 1 mM. 56 Figure 3.4 Diameter distributions of spheres for sample C1. When the ratio was further reduced to 1.67, only microplates and particles were produced. Similar to sample A5, plates formed on the glass beaker wall (Fig. 3.5c); and particles were produced in the solution (Fig. 3.5d). Table 3.5 Morphologies of the PANI D-series products Sample SEM image [HAuCl4] Morphologies D1 Fig, 3.5a 12 mM Plates + particles D2 Fig. 3.5b 3 mM Particles + plates D3 (on the wall) Fig. 3.5c 2 mM Plates + particles (minority) D3 (in the solution) Fig. 3.5d 2 mM Particles + plates (minority) [c] [Aniline]/ [HAuCl4] at 1.67; reaction time is 12 h at room temperature. 57 Figure 3.5 SEM images of the PANI microstructures with fixed [Aniline]/ [HAuCl4] at 1.67; concentration of HAuCl4: (a) 12 mM; (b) 3 mM; (c) 2 mM on the glass wall; (d) 2 mM in the solution. So far the few reports on 2D solid large plate-like structures have been limited to works carried out in aqueous systems.12, 13, 20 For example, ladder-like and grid-like planar structures were produced in highly acidic aqueous media, due to crosslinking between radical semiquinone cations along adjacent chains.20 It is well known that fibrous structures are preferred in acidic aqueous systems due to the semi rigid-rod PANI backbone.1 Almost no fibers were observed in our work which indicates that spheres and their aggregates are the dominating forms in toluene, because reverse micelles would mainly adopt spherical morphology in thermodynamic equilibrium. 3.3.1.2 Effect of aniline to HAuCl4 molar ratio Morphology control can also be achieved by changing the molar ratio of aniline to HAuCl4, while keeping [HAuCl4] constant at 12 mM. The different PANI products obtained are summarized in Table 3.6. As the aniline to HAuCl4 molar ratio decreases, hollow microtubes, solid 58 microspheres and microplates were formed respectively (Fig. 3.6). Table 3.6 Effect of [Aniline]/[HAuCl4] ratio on morphologies of PANI products Sample SEM image [Aniline]/[HAuCl4] ratio Morphology A1 Fig. 3.1a/3.6a 33 Broken tubes + spheres/particles (minority) B1 Fig. 3.6b 16 Tubes + particles (minority) C1 Fig. 3.6c 5 Spheres + plates (minority) D1 Fig. 3.6d 1.67 Plates + particles E Fig. 3.6e 0.57 Plates + irregular clusters F Fig. 3.6f 0.40 Plates + particles [HAuCl4] =12 mM; reaction time is 12 h at room temperature. Figure 3.6 SEM images with TEM insets of the PANI microstructures with fixed [HAuCl4] at 12 mM; varying the [Aniline]/[HAuCl4]: (a) 33; (b) 16; (c) 5; (d) 1.67; (e) 0.57; (f) 0.40. Polymerization of aniline is generally considered to proceed via two stages: 59 nucleation at the induction stage and polymer chains growth in the polymerization stage. In our work, nucleation can take place on the glass beaker wall or in solution, as reported in aqueous electrochemical synthesis27 and chemical synthesis.28 The nucleation sites compete with each other at the induction stage. It is known that the nucleation rates for aniline are strongly correlated with reagent concentrations.6, 28-30 The nucleation rate in solution is faster than that on substrates under concentrated condition. In dilute conditions, however, aniline typically first heterogeneously nucleates on solid surfaces, and minimizes interfacial energy barriers for subsequent growth. 31, 32 There would be a competition between the wall of the glass beaker and the solution for the reactive intermediates1 in the chain growth process. Reaction in solution In the TOAC/HAuCl4 /toluene solution, the [(CH3N[(CH2)7CH3]3) + -- AuCl4-] complex is expected to form reverse micelles.33 Neutral anilines are considered protonated even for the sample at the highest pH in our experiments, due to the presence of surfactants. 25 The remaining neutral anilines can be protonated during subsequent oxidation.18 These resultant anilinium cations or radical oligomers can also function as cationic surfactants to participate in reverse micelle construction, as proposed by Wan10, 11, 17, 34 and others.12, 18, 21 When the initial aniline concentration is high, large amount of protonated aniline would enter into the reverse micelle cores, and elongate the sphere micelles to tube-like micelles.35, 36 When the initial aniline concentration is low, micelles would maintain sphere shape due to the reduced surface energy. Aniline concentration therefore plays a decisive role in morphologies of final products.12, 17, 37 The exact change point of tube to sphere depends on the specific reaction Generally, microtubes with one end open instead of microrods are obtained in our 60 experiments. We propose that at high concentrations of aniline, the protonated aniline and oligomers are pushed out into the surrounding reaction media due to the exothermic nature of PANI polymerization, leaving hollow tubes with an open end, as previously reported.18 Reaction on glass PANI films were reported adhered to the walls of glass beakers38 and a wide range of conducting and non-conducting substrates28, 39 in acidic aqueous reactions. In our experiment, microplates were formed on the beaker wall in large quantities while only a few nanospheres were produced in the solution at very low [HAuCl4]. Fig. 3.7 shows a schematic diagram which depicts the formation of the two PANI structures. We believe that the formation of microplates may result from the preferential nucleation on substrates under dilute condition28 and the strong adsorption of pernigraniline onto the glass beaker wall38 which causes an increase in the concentration of polymerizing species (anilinium cations, oligomers) locally39, both leading to the epitaxial growth of PANI on the glass beaker wall. Figure 3.7 Schematic diagram of synthesis locations: microplates were adhered to the reactor wall due to absorption polymerization; tubes and spheres were produced by employing reverse micelles as templates in the solution. 61 3.3.1.3 Effect of temperature on microplates In this experiment, synthesis of sample A5 was carried out under the same conditions except for the temperature. Results at different temperatures are listed in Table 3.7. At 0℃ and 10℃, ring-like structures (Fig. 3.8a) and truncated spheres (Fig. 3.8b) were produced in solution, respectively, with very few microplates formed on glass beaker wall. In contrast, many more microplates appeared on glass beaker wall at higher temperatures of 30℃ (Fig. 3.8c) and 45℃ (Fig. 3.8d) which formed the majority of the products. Table 3.7 Effect of temperature on morphologies of the PANI sample A5 Temperature SEM figure Morphologies 0℃ Fig. 3.8a Truncated spheres + plates (minority) 10℃ Fig. 3.8b Ring-like structures + plates (minority) Room Temperature Fig. 3.1e/3.1f Plates (on the beaker wall) (20℃) + spheres (in the solution) 30℃ Fig. 3.8c Plates + spheres (minority) 45℃ Fig. 3.8d Plates + spheres (minority) Sample A5: [Aniline]/ [HAuCl4] =33, [HAuCl4] =0.12 mM; reaction time is 12 h at room temperature. 62 Figure 3.8 SEM images with TEM insets of the PANI microstructures with different temperatures; [HAuCl4] is 0.12mM, and the [Aniline]/[HAuCl4] molar ratio is 33: (a) 0℃in solution; (b) 10℃in solution; (c) 30℃ on the glass wall; (d) 45℃ on the glass wall. At low temperatures, there are not enough oligomers and anilinium cations in solution locally to form complete spheres due to low diffusion rates.40 On the other hand, at high temperatures the reactants can evenly diffuse to the glass beaker wall from solution during polymerization. Thus more microplates are formed on glass beaker wall. 3.3.1.4 Effect of mechanical stirring Sample A1 and A5 were chosen to investigate the mechanical stirring effect on PANI morphologies. Irregular aggregations were produced under both conditions. The results for both samples indicate a disruptive function of stirring on the reversed micelles in solution and adsorption layers on reactor wall. 63 3.3.1.5 Effect of additional acid Hydrochloride acid was also transferred from water to toluene by TOAC. [TOAC] to [HCl] and [HCl] to [Aniline] ratios were maintained at 1.5 and 0.5, respectively. In either case, granular morphologies were produced instead. (Fig. 3.9) When HCl/TOAC solution is mixed into the reaction system, they will disturb the synthesis condition. First, mass transfer can happen between TOAC-HCl and TOAC- HAuCl4 reverse micelles,41 which lowers the initial acidity inside TOAC-HAuCl4 reverse micelles. This acidic environment resembles the aqueous acidic condition and thus results in granular morphologies. Second, more small micelles of anilinium cations can be formed when neutral anilines in solution are protonated by additional HCl and finally leads to the network of nanoparticles. Moreover, the increased ionic strength is reported to favor isotropic polymer growth as well.6 Figure 3.9 SEM images of dopant effect on the PANI microstructures with [HCl]/[Aniline] molar ratio fixed at 0.5. (a) Sample A5: [HAuCl4] = 0.12 mM, and the [Aniline]/[HAuCl4] ratio is 33; (b) Sample D3: [HAuCl4] = 2 mM, and the [Aniline]/[HAuCl4] ratio is 1.67. 64 Note that very few gold nanoparticles (AuNPs) were found in our samples. Only AuNPs aggregates might be observed dispersed in the background. Electron diffraction shows no sign of gold inside PANI microplates, (Fig. 3.10a) compared with AuNPs aggregate in the background (Fig. 3.10b). Au contents in the whole test area are well below 3% mass percents (Fig. 3.11e). Therefore, we believe AuNPs did not play an important role in forming various morphologies in our work. However, considering that AuNPs have been shown to direct structural formation of conducting polymers and composites,42-44 we can not certainly exclude the possibility that nucleation could happen on AuNPs at the induction stage. Figure 3.10 Electron diffraction (a) sample A5; (b) Au aggregates in the background. 65 Figure 3.11 SEM images with corresponding Energy-dispersive X-ray (EDX) spectra (a) SEM images and (b) EDX spectrum of AuNPs aggregate in the background; (c) SEM images (d) EDX spectrum and (e) element distribution of the sample C1. 3.3.2 Structural characterizations FTIR spectra are quite similar for different samples prepared under different synthesis conditions (Fig. 3.12). The typical peaks around 1600 cm-1 and 1500 cm-1 are ascribed to the C=C stretching of quinoid and benzenoid rings, respectively; peaks around 1300 cm-1 are ascribed to the C-N stretching of the secondary aromatic amines; peaks around 1140 cm-1 are ascribed to the in-plane bending of the aromatic C-H; and peaks around 820 cm-1 are due to the out-of-plane bending of C-H in the 1,4-disubstituted benzene ring. These peaks are in good agreement with those obtained from the emeraldine form of PANI.15, 18 The 1140 cm-1 peak in particular is considered as a measurement of doping level and is therefore an indication of electron delocalization in PANI. In our experiments, this peak shifts to around 1600 cm-1 in all spectra, indicating that all of our samples are not in the fully doped state. 66 The partially doped emeraldine form is further supported by UV-vis studies of PANI dispersion in ethanol as shown in Fig. 3.12. The absorption peaks centered around 350 nm and 440 nm are attributed to the π-π* transition of benzenoid rings and the polaron/biplaron transition respectively. The highly doped PANI would show a strong peak at around 850 nm along with a free-carrier tail extending into the near-infrared region.12, 15, 20 However, this peak blue shifted to around 750 nm without the rising tails for our samples, which confirms partial doping. Fig. 3.13 shows the FTIR and UV-vis spectra for other samples in Table 3.3-3.5. The results indicate that they are also in the partially doped emeraldine form of PANI. Figure 3.12 FTIR (left) and Uv-Vis (right) spectra of different PANI structures: (a) [Aniline]/[HAuCl4] molar ratio is fixed at 33; (b) [HAuCl4] was fixed at 12 mM; (c) sample A5 synthesized at different temperatures. 67 Figure 3.13 FTIR (left) and Uv-Vis (right) spectra of PANI structures produced at different [Aniline]/ [HAuCl4] molar ratios: (a) Sample series B: [Aniline]/ [HAuCl4] at 16; (b) Sample series C: [Aniline]/ [HAuCl4] at 5; (c) Sample series D: [Aniline]/ [HAuCl4] at 1.67. 3.4 Hydrophilic and hydrophobic properties PANI films usually show hydrophilic properties, whose CAs are less than 40° depending on the dopant incorporated.17 The smallest CA for doped PANI films is less than 5°.28 Almost all our samples are hydrophilic, for example, A5 has a CA of about 37°±3° (Fig. 3.14b). The only exception is sample C1 prepared from spheres whose CA is 129°±2° (Fig 3.14a). In comparison, CAs for glass slide and Au film are around 52°±3° and 65°±2° respectively, in 68 agreement with the reported values within 6°45. Hydrophobic PANI films have been obtained by adding hydrophobic substances, such as TiO2 nanoparticles46, perfluorooctance sulfonic acid (PFOSA)11, perfluorosebacic acid (PFSEA)47 or treated with CHF3 or CF4 plasma28 However, PANI film in our work shows the intrinsic hydrophobicity without any additional treatment. It is theoretically possible that an intrinsic hydrophilic material can be changed to hydrophobic by adjusting its micro and nano structures,48 such as lotus and rice leaves49 or water strider legs.50 Hydrophobic PANI films were also experimentally obtained, due to rough surfaces created by the micro and nano scale hierarchical structures.11, 34, 47 Compared with smooth surfaces, air is trapped between hierarchical structures on rough surfaces; thus the air/water interface fraction increases and the solid/water interface fraction decreases. According to Cassie and Baxter’ equation51, this change in interface fractions can lead to a great change in CA. Therefore, the hydrophobic film in our experiment may be attributed to its rough surface created by the co-existence of micro and nano structures for sample C1. 69 Figure 3.14 Shapes of a water droplet on different films and their contact angles (CA): (a) Hydrophobic PANI film of sample C1; (b) Hydrophilic PANI film of sample A5 (c) Hydrophilic glass slide; (d) Hydrophilic gold film on glass slide. 3.5 Conclusions Morphology evolution of PANI has been observed via a reverse micelle method using TOAC as the surfactant and chloroauric acid as the oxidant in toluene. Various structures, including 1D open-ended micro tubes, 2D solid micro plates and 3D solid micro spheres were controllably prepared in the same reaction system by adjusting the chloroauric acid concentration or the molar ratio of aniline to chloroauric acid. In less concentrated solutions, solid micro plates were mainly formed on the glass beaker wall due to adsorption polymerization while tubes and spheres were obtained in solution. Other experimental parameters, such as reaction temperature, mechanical stirring and the addition of more acid were found to influence PANI morphologies. FTIR and UV-vis spectra indicated that our PANI products were in the partially doped emeraldine form. The PANI film 70 prepared from spheres showed intrinsic hydrophobicity due to the rough surface caused by the co-existence of micro and nano spheres. 71 References 1. Tran, H. D.; Li, D.; Kaner, R. B. AdV. Mater. 2009, 21, 14-15, 1487. 2. Huang, J. X. Pure Appl. Chem. 2006, 78, 1, 15. 3. Martin, C. R. Science 1994, 266, 5193, 1961. 4. Martin, C. R. Acc. Chem. Res. 1995, 28, 2, 61. 5. Huang, J. X.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. 6. Zhang, X. Y.; Kolla, H. S.; Wang, X. H.; Raja, K. Adv. Funct. Mater. 2006, 16, 1145. 7. 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Faraday Soc. 1944, 40, 0546. 75 Chapter Four Electronic Transport in Polyaniline Solid Microplates 4.1 Introduction Morphology is one of the decisive factors in determining properties of polymeric materials.1 Micro and nano structures of conducting polymers have attracted much attention due to their possible applications in many fields, such as surface with special properties2, chemical sensors3, vehicle for drug release4, organic solder at the nanoscale5, and organic electronics.6,7 Various structures were classified according to their dimensions, including 1D fibers, tubes, rods, needles and whiskers,3,8-14 3D spheres,15-17 and the recently reported 2D structures.18-24 There are basically three general chemical methods to synthesize structured conducting polymers: (i) the hard-template method was first used to produce controlled structures;11 (ii) the template-free method is the most convenient, by employing the immiscible biphasic system,3 by using functional acids as dopants,25 or by polymerizing under special conditions26-28 and (iii) the soft-template method, which utilizes structure directing templates such as dyes, micelles and ionic surfactants, has demonstrated considerable flexibility to produce a variety of micro and nano structures.29 Morphology of conducting polymers in the solid state is largely determined by the reaction media.30 Instead of the widely used direct micelles employed in water31-33, reverse micelles in non-polar solvents were used to prepare polymers with a more ordered structure.34,35 It is reported that polypyrroles36 and polythiophenes37 of different morphologies and electrical properties were obtained, depending on the type of surfactant used. Although anionic34,38-40 or polymeric surfactants30,41 were used to produce polypyrrole nanofibers42 and poly(3,4-ethylenedioxythiophene) 76 (PEDOT) nanotubes43,44, only nanoparticles or granular products of PANI were produced. The cationic surfactant, cetyltrimethylammonium bromide (CTAB), was recently used to produce PANI nanospheres in cyclohexane.35 It would therefore be interesting to investigate the influence of surfactant type on the PANI morphologies and electrical properties. However, as far as we know non-ionic surfactants have not been used to synthesize PANI structures in reverse micelles. In this chapter, we will demonstrate that different micro and nano structures of PANI can be produced in toluene using trioctylamine (TOA), a non-ionic surfactant which forms reverse micelles. In the past, electrical properties of micro and nano structures were commonly measured in the form of pressed pellets or thin films. However, such results represented the collective properties of aggregates, rather than those of individual structures,10,11 For applications in molecular electronic and nano devices, it is important to measure electrical properties of individual micro and nano structures. The conductivity of a single PANI or polypyrrole tube/fiber has been measured using patterned electrodes by focused ion beam deposition,45-48 or by electron-beam lithography49,50 In contrast, no electrical property of 2D structures has been discussed,23 although their morphological and chemical characterizations have recently been reported. In this chapter, we will conduct electrical tests on synthesized microplates for the first time. In addition, it is acknowledged that influence of interfaces between organic layers on electrical properties should not be neglected, but few reports have provided direct proofs.46 In this work, electrical properties of two stacked microplates and films prepared from microplate aggregates are also measured and compared with those of an individual microplate, in an attempt to understand better the role of interfaces in electrical properties of multidimensional nanostructures. 77 4.2 Experimental Section 4.2.1 Chemicals Aniline and Trioctylamine (TOA) were purchased from Sigma (Singapore). Chloroaurate acid was purchased from Alfa Aesar. Aniline was freshly distilled under reduced pressure. Other chemicals were used as received. 4.2.2 Preparation of HAuCl4/TOA/toluene solution 1.2 mmol HAuCl4 and 1.8 mmol TOA were co-dissolved in 100 mL toluene. The mixture was then strongly stirred to form a clear yellow HAuCl4/TOA/toluene solution at a concentration of 12 mM. 4.2.3 Synthesis of PANI In a typical synthesis, 1 mmol of aniline was rapidly dissolved in 2.5 mL 12 mM HAuCl4/TOA/toluene solution. The mixture was strongly stirred for several minutes and left to react at room temperature for 12 hours. The resulting precipitate was centrifuged and washed with distilled water and ethanol several times to remove residual surfactant and reactants. A series of products were prepared under different conditions by changing reagents concentrations (aniline, HAuCl4/TOA/toluene solution) and total reaction solution volumes. A summary of the amount of reactants used for each sample is provided in Table 4.1. Samples A B1 B2 B3 B4 C/D E/F Table 4.1 Synthesis details for PANI products Aniline mole HAuCl4/TOA/toluene solution (mmol) (12mM) volume (ml) 1 1.67 1 2.5 1 2.5 1 2.5 1 2.5 1 16.7 1 111.1 78 Toluene (ml) 0 0 2.5 7.5 247.5 0 0 Reaction time is 12 hours at room temperature. In a single reaction, both spheres (sample E) and microplates (sample F) were produced. Spheres were mainly obtained from the solution, while microplates were found adhered to the wall and bottom of the glass beaker. Fig. 4.1 shows a schematic diagram which depicts the formation of the PANI structures. The adhesion of microplates was rather robust that they were only completely removed under ultrasonication. Figure 4.1 Schematic diagram of synthesis locations: microplate structures were adhered to the glass wall; other structures were produced via reverse micelles in the solution. 4.2.4 Structural Characterizations The morphology of the samples was examined by a JEOL JSM-6701F field emission scanning electron microscope (SEM) and a JEOL JEM 3010F transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 2000 IR spectrometer in the range of 400-4000 cm-1 on sample pellets made with KBr. The absorption spectra of the PANI products in ethanol were recorded with an UV-vis spectrophotometer (UV-1700PC, Shimadzu). 4.2.5 Electrical Measurements 79 Current-Voltage (I-V) curves of an individual microplate, two stacked microplates and the macroscopic film of microplate aggregates were measured. For microscopic measurements, PANI microplates were ultrasonically dispersed in ethanol and drop cast onto an insulating SiO2 substrate. Two-probe methods were carried out in a field emission scanning electron microscope (FE-SEM, JSM7401-F, JEOL) equipped with nano-manipulators (Prober Nano-M, Zyvex Instruments) and Keithley 4200 Semiconducting characterization system at room temperature and at the reduced pressure of ~10-3 Pa. After finding an individual microplate, two tungsten microprobes were tightly placed on it during measurement. For macroscopic film of PANI microplate aggregates, current-voltage (I-V) curves were collected by an electrical probe station. First, a thin strip of glue of about 7 µm wide was used as a mask on the surface of a clean quartz substrate; Au film of ~100 nm thick was then deposited on the quartz surface. After removal of the glue strip, two Au pads were separated by the gap. Two copper wires were mounted on the Au pads via silver paste as two electrodes. The PANI microplates were ultrasonically dispersed in ethanol and deposited as a film across the gap between two Au pads. Hence the PANI films, the Au pads, copper wires, and current sensor unit of an Alesi REL-2100 analytical probe station formed a complete circuit. All the I-V results, for an individual microplate, two stacked microplates and the macroscopic film of microplate aggregates were highly reproducible. 4.3 Results and Discussion 4.3.1 PANI Synthesis and Characterizations SEM and TEM clearly reveal that various micro and nano structures were produced 80 when either the molar ratio of aniline to HAuCl4 (Fig. 4.2) or the HAuCl4 concentration ([HAuCl4]) (Fig. 4.3) was varied. They will be discussed in turn. The molar ratio of aniline to HAuCl4 was changed to investigate its effect on PANI morphologies as shown in Fig. 4.2. The results are summarized in Table 4.2. Only nanoscale granular particles (Fig. 4.2a) and hollow microspheres (Fig. 4.2b) were formed in solution when the molar ratio was high. Hollow microspheres are shown in the enlarged SEM images (Fig. 4.2b inset). When the ratio was lowered, hollow microtubes (Fig. 4.2c) and irregular particle aggregates (Fig. 4.2e) were produced in solution and at the same time solid microplates (Fig. 4.2d and 4.2f) were formed on the bottom and the wall of the glass beaker. The hollow microtube structure is shown by the sharp contrast between the dark edge and relatively transparent center in the TEM images (Fig. 4.2c inset). Solid microplates are shown by TEM images (Fig. 4.2f inset). Table 4.2 Effect of [Aniline]/[HAuCl4] ratio on morphologies of the PANI products Sample SEM image [Aniline]/[HAuCl4] ratio Morphologies A Fig. 4.2a 50 Submicro-granular particles B1 Fig. 4.2b 33 Hollow microspheres C Fig. 4.2c 5 Hollow microtubes D Fig. 4.2d 5 Microplates (majority) E Fig. 4.2e 0.75 Submicro-particles F Fig. 4.2f 0.75 Microplates [HAuCl4] =12mM; reaction time is 12 hours at room temperature. 81 Figure 4.2 SEM images with an enlarged SEM inset (b) and TEM insets (c, f) of the PANI micro and nano structures with fixed [HAuCl4] at 12mM. Varying the [Aniline]/[HAuCl4] ratio. (a) ratio of 50 in the solution; (b) ratio of 33 in the solution; (c) ratio of 5 in the solution; (d) ratio of 5 on the glass beaker wall; (e) ratio of 0.75 in the solution; (f) ratio of 0.75 on the glass beaker wall. Different [HAuCl4] were then used to investigate its effect on PANI morphologies with the [Aniline]/ [HAuCl4] ratio fixed at 33. The results are summarized in Table 4.3. Granular particles (Fig. 4.3a and 4.3d) and hollow microtubes (Fig. 4.3b and 4.3c) were formed in the solution, while microplates (Fig 4.3b and 4.3c right insets) were produced on the bottom and the wall of the glass beaker. Hollow microtubes are also shown by the TEM images (Fig. 4.3b and 4.3c left insets). 82 Table 4.3 Effect of [HAuCl4] on morphologies of PANI products Morphologies Sample SEM image [HAuCl4] B1 Fig. 4.3a/4.2b 12 mM Hollow submicro-spheres B2 Fig. 4.3b 6 mM Short hollow microtubes + Microplate B3 Fig. 4.3c 3 mM Long hollow microtubes + Microplate B4 Fig. 4.3d 0.12 mM Submicro granular particles [Aniline] / [HAuCl4] =33; reaction time is 12 hours at room temperature. Figure 4.3 SEM images with an enlarged SEM inset (a) and TEM insets (b, c) of the PANI micro and nano structures at different [HAuCl4] with fixed [Aniline]/ [HAuCl4] molar ratio at 33. (a) 12 mM; (b) 6 mM; (c) 3 mM; (d) 0.12 mM. In this work, polymerization at two locations (Fig. 4.1) led to distinctively different morphologies. Microplates are formed on the bottom and the wall of the glass beaker and different micro and nano structures were produced in the solution. Reverse micelles in toluene, made up of TOA, anilinium cations, radical oligomers and AuCl4- act as templates to direct PANI structures growth.34,41,51 The formation of microplates is believed to result from the preferential nucleation on 83 substrates in dilute conditions52 and the strong adsorption of pernigraniline onto the glass beaker wall53 which causes a relative increase in concentration of polymerizing species (anilinium cations, oligomers) near the glass wall,54 both leading to the epitaxial growth of PANI. We have observed similar phenomena in Chapter 3, using TOAC as the cationic surfactant in toluene. A detailed mechanism was given in last chapter. Chemical and electronic structures of PANI products obtained in solution were characterized by FTIR and UV-vis, respectively (Fig. 4.4). The results indicate that the PANI structures produced in solution are mostly in the emeraldine form, but not in the fully doped state17,27,55. Figure 4.4 FTIR (left) and Uv-Vis (right) spectra of different PANI structures: (a) [HAuCl4] was fixed at 12 mM; (b) [Aniline]/ [HAuCl4] molar ratio was fixed at 33; The reaction proceeded at room temperature for 12 hours. 84 PANI microplates could be doped into full emeraldine salt state after post-synthesis treatment with hydrochloric acid. The signature peaks of FTIR and UV-vis spectra are marked in Fig. 4.5. The presence of 1556 cm-1 peak is indicative of protonation of PANI emeraldine base.56,57 Figure 4.5 FTIR (left) and Uv-Vis (right) spectra of PANI microplates. The application of nanomaterials in complex devices demands precise positioning of nanostructures. However, current approaches mostly involve a two-step synthesizing-and-then-positioning procedure. The last step of pick-and-place process is now very time-consuming and arduous.58,59 The “grow-in-place” fabrication technology was recently proposed to avoid post-synthesis treatments.60,61 In our work, microplates were found robustly adhered to glass beaker wall and bottom or other immersed substrates, such as glass slides or gold films. They are expected to grow to the desired dimensions, shapes and positions on well designed substrates. We believe our polymerization process is quite suitable for the “grow-in-place” technology. This technology has been applied to PANI nanoribbons, where electrochemical polymerization was used and ohmic current between nanoribbons and electrodes was measured.62 Our work has the advantage of producing larger quantities of products. 85 4.3.2 Electrical Measurements 4.3.2.1 Current- Voltage (I-V) Characteristics of an individual microplate Electrical measurements for an individual microplate were first conducted. In the FE-SEM chamber, two probes were tightly placed on an individual microplate. A typical SEM image is displayed in Fig. 4.6a. In our experiments, two tungsten tips should not form Schottky contacts with the p-type semiconductor PANI, because the work function of metal tungsten (4.55 eV) is higher than that of PANI (4.1-4.45 eV).63 Fig. 4.6b shows a typical I-V curve of an individual microplate over a large applied voltage range without breakdown. A symmetric curve, instead of rectifying behavior at positive and negative voltages, suggests an Ohmic contact between tungsten tips and the PANI microplate in our measurement. Figure 4.6 (a) Typical SEM image of the electrical measurement of an individual PANI microplate with two electrical probes. (b) I-V characteristics of an individual PANI microplate. Transport mechanism in an individual microplate is further studied by plotting the data on a log-log scale. Two regions of linearity with different slopes are shown in Fig. 4.7. 86 Figure 4.7 I-V characteristics of an individual PANI microplate plotted on a log-log scale. Both the best fit curves have correlation coefficient of greater than 0.9995, with a transition point around 5V. This linear curve indicates a power-law relationship between current and voltage, which can be expressed as I ∝V m (1) The power-law indicates the dependence of charge carrier mobility on applied voltage, in which the exponent m has two different values below and above a transition voltage. At low voltages, the I-V curve obeys Ohm’s law with m equal to 1. At high voltages, it follows the space-charge-limited current (SCLC) model, with m equal to 2 with no trap.64 The SCLC model has been observed in bulk PANI films and the space charge accumulation during charge injection is due to the low 87 mobility of charge carriers.65 In our experiments, the exponent m is determined to be 1.03 in the low voltage regime. Considering the value is so close to unity, the I-V relationship in fact follows Ohm’s law, where the carriers are mainly thermally activated when the voltage drop between metallic domains in doped PANI is smaller than the thermal energy.46,63 In the high voltage regime, the exponent m is determined to be 1.48, or approximately 3/2. If the voltage is above some critical field, the drift velocity has been reported to be proportional to the square-root of the applied field in the presence of a distribution of shallow traps.63 Our result fits the 3/2 power-law of the SCLC mechanism quite well, indicating the existence of shallow traps inside an individual microplate. 4.3.2.2 Current- Voltage (I-V) Characteristics of two stacked microplates The electrical properties of two stacked microplates were also investigated. In this measurement, one probe was firmly placed on the top microplate while the other on the bottom microplate. A SEM image is displayed in Fig. 4.8a. Fig. 4.8b compares their I-V curves with that for an individual microplate in a low voltage regime. As compared to the individual plates, the I-V curve of the stacked plates is non-linear with an eight-fold reduction in current. The results demonstrate the existence of a large and non-Ohmic contact resistance between the stacked microplates. 88 Figure 4.8 (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. 4.3.2.3 Current- Voltage (I-V) Characteristics of PANI macroscopic films The electrical measurements were also carried out for a macroscopic film prepared from microplate aggregates. Fig. 4.9 shows the schematic diagram of the experimental setup and optical image of the sample studied. In order to conduct measurements under the same condition, the setup was placed in an enclosed chamber, in which the pressure was set at around 10-3 Pa. 89 Figure 4.9 Schematic diagram of the experimental setup and optical image of the sample for electrical measurement of PANI macroscopic films with two electrodes. The I-V curve in Fig. 4.10 is slightly asymmetric. At high voltages, the current value is five orders of magnitude smaller than that for an individual microplate. At small voltages, the current value is more than three orders smaller than that for stacked plates. As discussed above, there are great differences in electrical properties between two stacked plates and an individual plate, due to the plate to plate contact. We therefore consider the numerous plate to plate contacts inside the macroscopic film as the dominating factor that determines the electrical properties of bulk samples. It has been reported that intertubular contact resistance between PANI nanotubes in compressed pellets decreases the conductivity by four orders of magnitude compared with a single nanotube.45 Similar conclusions were also drawn for PANI hollow microspheres46 and polypyrrole microtubes.47 In our work, the influence of inter-structure contact is more significant due to the use of drop-cast film instead of pressed pellets. 90 4.3.2.4 Current-Voltage (I-V) Characteristics of macroscopic films at atmospheric pressure In an attempt to investigate the effect of pressure on the doping level in PANI nanostructures, we compare the I-V curves for the macroscopic microplate film measured at 10-3 Pa and at atmospheric pressure. Fig. 4.10 shows a near ohmic I-V curve at atmospheric pressure, similar to the metallic behavior reported for fully doped PANI bulk sample.65,66 The current is also more than two orders of magnitude larger. It is well known that the conductivity of doped PANI is sensitive to environmental humidity.67,68 Removal of environmental moisture in vacuum chamber would significantly reduce the conductivity of doped PANI.69 In addition, dedoping of PANI by the removal of HCl is more pronounced at reduced pressures. It has also been reported that dedoping occurred in nitrogen if thermal treatment was applied.67 Figure 4.10 I-V characteristics for the macroscopic PANI film of microplates at different pressures. 91 4.4 Conclusions Polyaniline (PANI) solid microplates, particle aggregates, spheres and hollow tubes were synthesized in toluene using the non-ionic surfactant TOA. Microplates were formed adhered on glass beaker walls, while other structures were produced in solutions employing reverse micelles as templates. The strong adhesion of microplates may be suitable for the recently proposed “grow-in-place” technology for fabricating complex electric devices. The room-temperature I-V characteristics were measured for an individual microplate, two stacked microplates and macroscopic films prepared form microplate aggregates. For an individual microplate, the current follows Ohm’s law at low voltage and power-law with exponent of 3/2 at high voltage, indicating a space-charge-limited current mechanism in the presence of a distribution of shallow traps. The I-V relationship of two stacked microplates demonstrates that the contact resistance between microplates dominated the electrical properties, which further explains why the current of their macroscopic film is five orders of magnitude smaller than that of an individual microplate under the same testing conditions. For the macroscopic film, its I-V characteristic is also found to be greatly influenced by the external pressure. 92 References 1. Tran-Cong-Miyata, Q.; Nishigami, S.; Ito, T.; Komatsu, S.; Norisuye, T. Nat. Mater. 2004, 3, 448. 2. Ding, H. J.; Zhu, C. J.; Zhou, Z. M.; Wan, M. 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Met. 1988, 26, 1. 97 Chapter Five Conclusions and Future Work 5.1 Conclusions The main purpose of this study was to (i) synthesize PANI micro and nano structures and (ii) measure current-voltage (I-V) characteristics of the prepared structures for possible applications in electronic devices at the small scale. 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 micelle 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 the dilute solution, solid microplates were mainly adhered to the wall and bottom of the glass beaker due to adsorption polymerization, while other structures were mainly formed in solutions via reverse micelles polymerization. The non-ionic surfactant, Trioctylamine (TOA) was also used to produce PANI solid microplates and hollow microtubes. The electrical properties of the prepared PANI solid sub-microsphere and microplate were investigated at room temperature by measuring their current-voltage (I-V) properies. The I-V curves of both an individual sub-microsphere and its macroscopic film showed semiconducting characteristics. The electrical properties of macroscopic films were successfully controlled by the external pressure. The I-V relationships were also measured for an individual microplate, two stacked 98 microplates and the macroscopic film. For an individual plate, the current followed Ohm’s law at low voltage and power-law with exponent of 3/2 at high voltage, indicating a space-charge-limited current mechanism in the presence of a distribution of shallow traps. Large and non-Ohmic contact resistance between structures was shown to be the dominating factor in determining electrical properties for two stacked plates and the macroscopic film of plate aggregates. PANI films with interesting hydrophobic properties were prepared by controlling the surface roughness due the co-existence of nano and micro spherical structures. 5.2 Future Work As reviewed in Chapter one, the nanostructured conducting polymers (CPs) incorporated into devices have wide applications in many fields. The focus is presently on the synthesis of novel composite materials. Hybrid materials of Au and CPs1-5 have attracted much attention for their possible applications in electronics,6 optoelectronics,7 and catalysis.8 Compared with molecular encapsulants for Au nanoparticles (NPs), CPs have several significant advantages, such as producing a more intimate electrical contact between two components, and reducing the complexity of the system.9 Nowadays, micro and nano structures of PANI are not only promising for device miniaturization10, but also in turn influence the material bulk properties.11 Therefore, nanostructured hybrid materials of Au and PANI are likely to exhibit interesting and special properties for applications in electronic devices with superior performances. For example, the nanocomposite of PANI nanofibers and Au NPs demonstrated electrical bistability and have been explored for non-volatile memory devices.12 Among many structures reported, the design of core-shell architecture has attracted 99 increasing interest for its unique properties and various applications.13 Au-PANI core-shell composites were thus synthesized by employing PANI as an effective capping agent to direct the nanostructure growth. 14-16 Au wire-PANI core-shell coaxial nanocable has been recently reported.15, 17 However, its electrical properties have not yet been investigated.18 In the future, we intend to carry out a facile synthesis of Au core-PANI shell structures by the chemical reduction of HAuCl4 and simultaneous polymerization of aniline in water and also to investigate electrical properties of both an individual nanostructures and their macroscopic film. Preliminary results show that negative differential resistance (NDR) behavior can be observed at room temperature. A possible mechanism may be based on charge separation and recombination processes between Au core and PANI shell. 100 References 1. Marinakos, S. M.; Shultz, D. A.; Feldheim, D. L. Adv. Mater. 1999, 11, 34. 2. Kinyanjui, J. M.; Hatchett, D. W.; Smith, J. A.; Josowicz, M. Chem. Mater. 2004, 16, 3390. 3. Sih, B. C.; Wolf, M. O. Chem. Comm. 2005, 3375. 4. Mallick, K.; Witcomb, M. J.; Scurrell, M. S. Gold Bull. 2006, 39, 166. 5. Mallick, K.; Witcomb, M. J.; Scurrell, M. S.; Strydom, A. M. Gold Bull. 2008, 41, 246. 6. Cho, S. H.; Park, S. M. J. Phys. Chem. B 2006, 110, 25656. 7. Englebienne, P.; Van Hoonacker, A. J. Colloid Interf. Sci. 2005, 292, 445. 8. Han, J.; Liu, Y.; Guo, R. Adv. Funct. Mater. 2009, 19, 1112. 9. Smith, J. A.; Josowicz, M.; Engelhard, M.; Baer, D. R.; Janata, J. Phys. Chem. Chem. Phys. 2005, 7, 3619. 10. Jager, E. W. H.; Smela, E.; Inganas, O. Science 2000, 290, 1540. 11. Tran-Cong-Miyata, Q.; Nishigami, S.; Ito, T.; Komatsu, S.; Norisuye, T. Nat. Mater. 2004, 3, 448. 12. Tseng, R. J.; Huang, J. X.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077. 13. Shan, J.; Tenhu, H. Chem. Comm. 2007, 4580. 14. Guo, Z. R.; Zhang, Y.; Huang, L.; Wang, M.; Wang, J.; Sun, J. F.; Xu, L. N.; Gu, N. J. Colloid Interf. Sci. 2007, 309, 518. 15. Sajanlal, P. R.; Sreeprasad, T. S.; Nair, A. S.; Pradeep, T. Langmuir 2008, 24, 4607. 16. Xing, S. X.; Tan, L. H.; Yang, M. X.; Pan, M.; Lv, Y. B.; Tang, Q. H.; Yang, Y. H.; Chen, H. Y. J. Mater. Chem. 2009, 19, 3286. 101 17. Huang, K.; Zhang, Y. J.; Long, Y. Z.; Yuan, J. H.; Han, D. X.; Wang, Z. J.; Niu, L.; Chen, Z. Chem. Eur. J. 2006, 12, 5314. 18. Long, Y. Z.; Huang, K.; Yuan, J. H.; Han, D. X.; Niu, L.; Chen, Z. J.; Gu, C. Z.; Jin, A. Z.; Duvail, J. L., Electrical conductivity of a single Au/polyaniline microfiber. Appl. Phys. Lett. 2006, 88, 162113. 102 [...]... 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. .. 4.9…………………90 Schematic diagram of the experimental setup and optical image of the sample for electrical measurement of PANI macroscopic films with two electrodes Figure 4.10……………… 91 I-V characteristics for the macroscopic PANI film of microplates at different pressures XII List of Tables Table 3.1 51 Synthesis details for PANI structures Table 3.2……………… 53 Morphologies of the PANI A-series products... 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... 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 9 oxidative inorganic nanofibers/wires... 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.65 Typically micellar structures act as soft templates when the surfactant concentration... characteristics of PANI sub-microspheres at different pressures Figure 2.10…………….….40 (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 Figure 2.11……………… 41 FTIR spectra of PANI sub-microspheres before (a) and after (b) reduced pressure Figure 2.12…………… …42 UV-vis spectra of PANI sub-microspheres before... 3.3……………… 54 Morphologies of the PANI B-series products Table 3.4……………… 56 Morphologies of the PANI C-series products Table 3.5…………………57 Morphologies of the PANI D-series products Table 3.6…………………59 Effect of [Aniline]/[HAuCl4] ratio on morphologies of the PANI samples Table 3.7……………… 62 Effect of temperature on morphologies of the PANI samples A5 Table 4.1…………………78 Synthesis details for PANI products Table... 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... 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... image of electrical measurement of an individual PANI microplate with two electrical probes (b) I-V characteristics of an individual PANI microplate Figure 4.7…………………87 I-V characteristics of an individual PANI microplate plotted on a log-log scale Figure 4.8……………… 89 (a) Typical SEM image of electrical measurement of two stacked PANI microplates with two electrical probes (b) I-V characteristics of two ... 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... 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... 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