Henry D. Tran

The oxidation of aniline to produce “polyaniline”: a process yielding many different nanoscale structures

The number of different nano- and micro-scale structures produced from the chemical oxidation of aniline into “polyaniline” is rivaled by few other organic materials. Nanoscale structures such as fibers, tubes, aligned wires, flowers, spheres and hollow spheres, plates, and even those resembling anatomical organs, insects, and sea animals have been observed for the products produced when aniline is oxidized. This feature article examines these different structures and the small and subtle changes in reaction parameters that result in their formation. These changes can often result in drastic differences in the polymers nanoscale morphology. Because a nanomaterials properties are highly dependent on the type of morphology produced, understanding polyanilines propensity for forming these structures is crucial towards tailoring the material for different applications as well as improving its synthetic reproducibility. The different approaches to commonly observed polyaniline nanostructures are presented in this article along with some of the highly debated aspects of these processes. The article ends with our approach towards resolving some of these contentious issues and our perspective on where things are headed in the years to come.

Nanoscale Morphology, Dimensional Control, and Electrical Properties of Oligoanilines

While nanostructures of organic conductors have generated great interest in recent years, their nanoscale size and shape control remains a significant challenge. Here, we report a general method for producing a variety of oligoaniline nanostructures with well-defined morphologies and dimensionalities. 1-D nanowires, 2-D nanoribbons, and 3-D rectangular nanoplates and nanoflowers of tetraaniline are produced by a solvent exchange process in which the dopant acid can be used to tune the oligomer morphology. The process appears to be a general route for producing nanostructures for a variety of other aniline oligomers such as the phenyl-capped tetramer. X-ray diffraction of the tetraniline nanostructures reveals that they possess different packing arrangements, which results in different nanoscale morphologies with different electrical properties for the structures. The conductivity of a single tetraaniline nanostructure is up to 2 orders of magnitude higher than the highest previously reported value and rivals that of pressed pellets of conventional polyaniline doped with acid. Furthermore, these oligomer nanostructures can be easily processed by a number of methods in order to create thin films composed of aligned nanostructures over a macroscopic area.

Toward an Understanding of the Formation of Conducting Polymer Nanofibers

Introducing small amounts of additives into polymerization reactions to produce conducting polymers can have a profound impact on the resulting polymer morphology. When an oligomer such as aniline dimer is added to the polymerization of aniline, the nanofibers produced are longer and less entangled than those typically observed. The addition of aniline dimer can even induce nanofiber formation under synthetic conditions that generally do not favor a nanofibrillar morphology. This finding can be extended to both the synthesis of polythiophene and polypyrrole nanofibers. The traditional oxidative polymerization of thiophene or pyrrole only produces agglomerated particles. However, when minute amounts of thiophene or pyrrole oligomers are added to the reaction, the resulting polymers possess a nanofibrillar morphology. These results reveal important insights into a semirigid rod nucleation phenomenon that has hitherto been little explored. When polyaniline nucleates homogeneously, surface energy requirements necessitate the formation of ordered nuclei which leads to the directional polymerization of aniline. This ultimately leads to the one-dimensional nanofibrillar morphology observed in the final product. The synthetic procedures developed here are simple, scalable, and do not require any templates or other additives that are not inherent to the polymer.

Versatile solution for growing thin films of conducting polymers.

The method employed for depositing nanostructures of conducting polymers dictates potential uses in a variety of applications such as organic solar cells, light-emitting diodes, electrochromics, and sensors. A simple and scalable film fabrication technique that allows reproducible control of thickness, and morphological homogeneity at the nanoscale, is an attractive option for industrial applications. Here we demonstrate that under the proper conditions of volume, doping, and polymer concentration, films consisting of monolayers of conducting polymer nanofibers such as polyaniline, polythiophene, and poly(3-hexylthiophene) can be produced in a matter of seconds. A thermodynamically driven solution-based process leads to the growth of transparent thin films of interfacially adsorbed nanofibers. High quality transparent thin films are deposited at ambient conditions on virtually any substrate. This inexpensive process uses solutions that are recyclable and affords a new technique in the field of conducting polymers for coating large substrate areas.

Conductometric Hydrogen Gas Sensor Based on Polypyrrole Nanofibers

Polypyrrole nanofibers are synthesized through a template-free chemical route and used as the active component for hydrogen gas sensing at room temperature. The synthesis of polypyrrole nanofibers was achieved by using bipyrrole as an initiator to speed up the polymerization of pyrrole with FeCl as the oxidizing agent. Scanning and transmission electron microscopy studies indicate that the resulting polypyrrole forms a nanofibrous mat with average nanofiber diameter of 18 nm. Fourier transform infrared spectroscopy and elemental analysis confirms that the structure of the nanofibers is comparable to bulk polypyrrole. Gas sensing properties of polypyrrole nanofibers were investigated by depositing nanofiber dispersions on an interdigited conductometric transducer. The sensor performance was tested through programmable exposure towards different concentrations of hydrogen gas diluted in synthetic air in an environmental cell at different temperatures. A short response time of 43 s was observed upon exposure to a concentration of 1% hydrogen with a decrease in film resistance of 312 at room temperature. The sensor sensitivity was analyzed with gradual elevation of the operating temperature.

Applications of oligomers for nanostructured conducting polymers.

This Feature Article provides an overview of the distinctive nanostructures that aniline oligomers form and the applications of these oligomers for shaping the nanoscale morphologies and chirality of conducting polymers. We focus on the synthetic methods for achieving such goals and highlight the underlying mechanisms. The clear advantages of each method and their possible drawbacks are discussed. Assembly and applications of these novel organic (semi)conducting nanomaterials are also outlined. We conclude this article with our perspective on the main challenges, new opportunities, and future directions for this nascent yet vibrant field of research.

A general synthetic route to nanofibers of polyaniline derivatives

Nanofibrous mats of a wide variety of polyaniline derivatives can be synthesized without the need for templates or functional dopants by simply introducing an initiator into the reaction mixture of a rapidly mixed reaction between monomer and oxidant.

Processable colloidal dispersions of polyaniline-based copolymers for transparent electrodes

Copolymerization of aniline with substituted anilines can synergistically combine high electrical conductivity with good solubility and functionality. Here, we report the synthesis of a variety of polyaniline-based copolymer nanofibers with uniform diameters. The relationship between the feed ratio and the final composition is examined by NMR and UV-vis spectra. The conductivity of the copolymers can be tuned over a six order-of-magnitude range by varying the composition of the two building blocks. The copolymer nanofibers exhibit excellent colloidal stability with zeta-potential values as high as 40 mV, which enables them to be spray-coated to form transparent, conductive thin films with good optical properties. This simple process is scalable, and can lead to flexible or patterned films, which may be helpful for applications in organic electronics, optoelectronics, sensors, and energy storage devices.

Hydrogen gas sensor fabricated from polyanisidine nanofibers deposited on 36° YX LiTaO3 layered surface acoustic wave transducer

Polyanisidine nanofibers gas sensor based on a ZnO/36° YX LiTaO3 surface acoustic wave (SAW) transducer was developed and tested at different concentrations of hydrogen gas in synthetic air. Nanofibrous mats of polyanisidine were synthesized without the need for templates or functional dopants by simply introducing an initiator into the reaction mixture of a rapidly mixed reaction between the monomer (anisidine) and the oxidant. The polyanisidine nanofibers are characterized using scanning electron microscopy (SEM) and Ultraviolet-Visible Spectroscopy (UV-vis). Polyanisidine nanofibers were deposited onto the SAW transducer and exposed to different concentrations of hydrogen gas. The frequency shift due to the sensor response was 294 kHz towards 1% of H2. All tests were conducted at room temperature and the sensor performance was assessed for a two day period with a high degree of reproducibility obtained.

A hydrogen gas sensor fabricated from polythiophene nanofibers deposited on a 36°YX LiTaO3 layered surface acoustic wave transducer

A gas sensor was developed by depositing polythiophene nanofibers on the surface of ZnO/36° YX LiTaO3 layered surface acoustic wave (SAW) transducer and tested towards different concentrations of hydrogen gas in synthetic air. Polythiophene nanofibers were synthesized by using a template-free method through the introduction of an initiator into the reaction mixture of a rapidly mixed reaction between the monomer (thiophene) and the oxidant. The yield of the reaction was characterized using scanning electron microscopy (SEM) as well as Ultraviolet-visible (UV-vis) and Fourier Transform Infrared (FTIR) spectroscopies. The frequency shift due to the sensor response was ~17 kHz towards 1% of H2. All tests were conducted at room temperature. The sensor performance was assessed over a two day period and a high degree of repeatability was obtained.