Martin Vrňata

Polypyrrole nanotubes: mechanism of formation

This article presents a contribution to better understanding of the processes which take place during the synthesis of polypyrrole nanotubes using a structure-guiding agent, methyl orange. Polypyrrole was prepared by oxidation of pyrrole monomer with iron(III) chloride. In the presence of methyl orange, the formation of nanotubes was observed instead of the globular morphology. Two reaction schemes with reversed additions of oxidant and monomer have been tested and they show remarkable influence on the produced morphology. Nanotubes with circular or rectangular profiles and diameters from tens to hundreds of nanometres have been obtained. FTIR and Raman spectra were used to assess the molecular structure of polypyrrole and detect residual methyl orange in the samples. The conductivity of nanotubes compressed into pellets was as high as 68 S cm−1. The mechanism of nanotubular formation starting at the nucleus produced with the participation of organic dye is proposed. The growth of a nanotube, however, proceeds in the absence of a template. An alternative mechanism for the formation of nanotubes, the coating of solid templates with a polypyrrole overlayer, is also discussed.

Polypyrrole–silver composites prepared by the reduction of silver ions with polypyrrole nanotubes

Polypyrrole nanotubes were prepared by the oxidation of pyrrole with iron(III) chloride in the presence of methyl orange. They were subsequently used for the reduction of silver ions to silver nanoparticles. The nanotubular form of polypyrrole is compared with the classical globular morphology in its ability to reduce silver ions. Both polypyrrole salts and bases were used in the experiments. The content of metallic silver in the resulting composite, determined by thermogravimetric analysis, was 21–31 wt%. Elemental composition is also discussed on the basis of energy-dispersive X-ray spectroscopy. Contrary to the expectation, the conductivity of polypyrrole nanotubes in salt form, 35.7 S cm−1, was reduced to 20.9 S cm−1 after the incorporation of silver. The presence of silver had generally little effect on the conductivity. The temperature dependence of conductivity reveals that the composites maintain the semiconducting character of polypyrrole and their conductivity increased with increasing temperature. The conductivity of the composites surprisingly increased when the samples were placed in vacuo.

The response of tin acetylacetonate and tin dioxide-based gas sensors to hydrogen and alcohol vapours

The aim of this work is to investigate the properties of gas sensors with active layers prepared by pulsed laser deposition (PLD) technology. The active layers were deposited on planar sensor chips with interdigital platinum electrodes. The deposition was carried out from tin dioxide and tin acetylacetonate (SnAcAc) targets by KrF excimer laser. In some cases Pd catalyst was sputtered on the surface of the active layer. The ‘as-deposited’ sensors were submitted to heat treatment. The chemical composition of heat treated active layers was studied by XPS method. This method revealed the oxidation state of palladium and the distribution between organic and inorganic phase during the deposition of SnAcAc. The DC responses of the sensors to a reducing atmosphere containing 1000 ppm of hydrogen, methanol, ethanol, n-propanol and n-butanol were also measured. The maximum sensitivity S (ratio of sensor resistances SaRair/Rgas) achieved 67 for hydrogen, 21 for methanol, 50 for ethanol, 71 for n-propanol and 44 for n-butanol. The temperature of maximum sensitivity (Tmax )t o distinct gases, the influence of molecular weight of detected gas on Tmax and the influence of molecular weight on the sensor response speed are also discussed. # 2000 Elsevier Science B.V. All rights reserved.

Polypyrrole salts and bases: superior conductivity of nanotubes and their stability towards the loss of conductivity by deprotonation

Polypyrrole nanotubes exhibit conductivity of tens S cm−1 which is one of the highest among the current conducting polymers. They are thus superior to the common globular form with the conductivity of units of S cm−1 or lower. The conductivity of both forms is reduced after treatment with alkalis but still remains high, units of S cm−1 and 10−2 S cm−1, respectively. The deprotonation, which is responsible for conductivity reduction, is discussed on the basis of salt–base transition in polypyrrole. It is not fully reversible, and the reprotonation with acids recovers the conductivity only in part. The role of methyl orange, which was used to support the formation of nanotubes, is proposed to be similar to that of surfactants. FTIR and Raman spectroscopies prove that methyl orange is strongly bound to polypyrrole in its acid form, and an “insertion” mechanism is proposed to explain the resistance towards the deprotonation of nanotubes. The spectra also illustrate that the molecular structure of nanotubular polypyrrole is preserved even under highly alkaline conditions at a pH close to 14, where the globular form becomes damaged. Polypyrrole, especially in its nanotubular form, is of promise in applications requiring electrical conduction even under neutral or alkaline conditions, where other conducting polymers, such as polyaniline, lose their exploitable conductivity.

Polypyrrole Nanotubes and Their Carbonized Analogs: Synthesis, Characterization, Gas Sensing Properties.

Polypyrrole (PPy) in globular form and as nanotubes were prepared by the oxidation of pyrrole with iron(III) chloride in the absence and presence of methyl orange, respectively. They were subsequently converted to nitrogen-containing carbons at 650 °C in an inert atmosphere. The course of carbonization was followed by thermogravimetric analysis and the accompanying changes in molecular structure by Fourier Transform Infrared and Raman spectroscopies. Both the original and carbonized materials have been tested in sensing of polar and non-polar organic vapors. The resistivity of sensing element using globular PPy was too high and only nanotubular PPy could be used. The sensitivity of the PPy nanotubes to ethanol vapors was nearly on the same level as that of their carbonized analogs (i.e., ~18% and 24%, respectively). Surprisingly, there was a high sensitivity of PPy nanotubes to the n-heptane vapors (~110%), while that of their carbonized analog remained at ~20%. The recovery process was significantly faster for carbonized PPy nanotubes (in order of seconds) compared with 10 s of seconds for original nanotubes, respectively, due to higher specific surface area after carbonization.

Dye-stimulated control of conducting polypyrrole morphology

Azo dyes represent important structure-guiding agents which exhibit non-covalent interactions of various types (ionic and hydrogen bonding, π–π stacking, hydrophobic interactions, etc.) allowing for their self-assembly in aqueous solutions and the subsequent formation of seeds or templates for the preparation of supramolecular structures of conducting polymers, especially polypyrrole (PPy). Three azo dyes (Acid Red 1, Orange G and Sunset Yellow FCF) bearing hydrophilic functional groups, with mutually different positions on a hydrophobic naphthylphenyldiazene skeleton, were used as structure-guiding agents in the synthesis of highly organized supramolecular structures of PPy in aqueous media. The synthesized polymers were studied by scanning electron microscopy, energy dispersive X-ray, and Fourier-transform infrared and Raman spectroscopies. Measurement of the conductivity revealed a moderate value of conductivity (around units of S cm−1) and reduced stability indicated by relatively fast conductivity decay. Infrared spectroscopy indicated a lower doping level of all PPy prepared in the presence of tested dyes compared to that of standard globular PPy. In contrast, Raman spectroscopy, which is a surface-sensitive method, indicated a slightly higher protonation level compared to that of standard globular PPy or nanotubular PPy synthesized in the presence of the well-known structure-guiding agent methyl orange. This discrepancy in the obtained doping levels is discussed and some consequences between the doping level of PPy and its conductivity are also pointed out.

Conductive Gas Sensors Prepared Using PLD

This contribution deals with conductive thin film gas sensors fabricated using laser technology. The principles of the pulsed laser deposition (PLD) technology is explained. Hybrid PLD systems, based on a combination of PLD and magnetron sputtering, PLD and RF discharges, and PLD with two laser targets are also presented. The growing layers can be modified by an ion beam. Organic films can be grown using the cryogenic MAPLE technology. Nanocrystalline and nanocomposite thin films for gas sensors can be deposited. Examples of layer fabrication and testing of layer properties for gas sensors are given.