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Thesis

Gas-sensing properties of V2O5-nanofibers and carbon nanotubes

In this thesis, the synthesis and characterization of V2O5-nanostructures, as well as the gas sensing properties of V2O5-nanofibers and carbon nanotubes have been investigated. Various modifications of the nanowires have been successfully employed in order to achieve an improved sensitivity and selectivity of the sensors to specific analytes. The V2O5-nanofibers have been synthesized via solution-based chemistry. Two modifications of the standard synthesis route have been attempted in order to reduce the time required to grow fibers of sufficient length. The first modification utilizes silver ions, which allowed for a ten-fold increase in growth speed. In order to determine the role of the silver in the synthesis, energy dispersive X-ray (EDX) analysis has been performed, which revealed the presence of silver-clusters attached to the V2O5-nanofibers, as well as silver incorporated into the fibers. The second modification is based on hydrothermal synthesis performed at 180oC, which yielded VOx-nanobelts rather than V2O5-nanofibers. Most striking is their appearance in the shape of a boomerang with a reproducible angle of 96o. The origin of the kinked structure, as determined with transmission electron microscopy (TEM) and selected area electron diffraction (SAED), was found to be twinning of the crystalline material along the [130]-direction. Raman spectroscopy and temperature-dependent electrical transport measurements on the V2O5-materials revealed their close similarity. In all cases, the electrical transport was found to be dominated by a hopping-like conduction. To assess the gas sensing properties of V2O5-nanofibers and carbon nanotubes, network samples have been investigated at room temperature, using the change of resistance as sensor signal. For the detection of ammonia with the V2O5-nanofibers, the change of resistance has been ascribed to charge transfer. Through evaporation of gold onto the V2O5-nanofibers the sensor response could be improved by a factor of seven. The slower desorption of ammonia from the gold-modified fibers, as compared to the unmodified material, is attributed to a stronger binding of ammonia to gold. The detection of butylamine with V2O5-nanofibers has been studied due to the application relevance, as this compound is produced in rotten food. By investigating different contact configurations, the sensor response ( 500 %) has been found to originate from intercalation of the analyte between electrode and fiber, resulting in an increased contact resistance. Modifying networks of V2O5-nanofibers via deposition of an ultrathin layer of palladium rendered the sensors highly sensitive to hydrogen, resulting in responses of more than 100,000 %. The sensor mechanism, as elucidated through a combined study using Raman spectroscopy and temperature-dependent electrical transport measurements, involved atomic hydrogen, formed within the palladium layer. Its reaction with oxygen ions in the V2O5-lattice leads to oxygen deficiencies and hence the formation of additional polarons, which combine to less mobile bipolarons, as apparent from an increase in the hopping activation energy. Besides V2O5-nanofibers, the sensor properties of carbon nanotubes have been studied. Their gas sensing behavior has been interpreted in terms of charge transfer between the analyte molecules and the p-type semiconducting nanotubes present in network samples. In the specific case of ammonia exposure, the nanotube sensors do not recover completely to their original resistance, but equilibrate at an increased value. This observation has been accounted for by an adapted Langmuir isotherm, which assumes reversible and irreversible adsorption sites on the nanotube surface. The presence of irreversible sites, which require temperatures up to 500 K for the desorption of ammonia, could be experimentally confirmed by thermal desorption spectroscopy (TDS). To render carbon nanotubes sensitive to hydrogen, palladium nanoparticles were electrodeposited. This method offers the advantage of restricting the modification to nanotubes which are contacted to the electrodes, while isolated nanotubes and substrate are not affected. Palladium-modified nanotubes prepared in this fashion showed good responses to hydrogen at room temperature.

    Thèse École polytechnique fédérale de Lausanne EPFL, n° 3104 (2004)
    Section de physique
    Faculté des sciences de base
    Institut de physique des nanostructures
    Jury: Benoît Deveaud-Plédran, Laszlo Forro, Falko Netzer, Louis Schlapbach

    Public defense: 2004-12-10

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    Record created on 2005-03-16, modified on 2016-08-08

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