Microrreatores e sensores de gases baseados em nanotubos de carbono

Abstract

This work exploits the fabrication, setup and tests of the micro-reactors and gas sensors based on carbon nanotubes. The configuration of the gas sensors was designed to allow the use of multi-walled carbon nanotubes - as commercially acquired, oxidized or decorated with nanoparticles – as sensor elements which have similar behavior to a sensor element formed of a single-walled carbon nanotube with large diameter (~ 20 nm). In this configuration, the nanotubes were deposited using dielectrophoresis technique with their ends supported on the edges of the electrodes and the central part suspended (bridged) over a gap ~ 1 m wide and ~5 m deep, in order to connect the ends of pre-produced standard metal electrodes. A new approach to improve the electrical and thermal contact between multi-walled carbon nanotubes deposited by dielectrophoresis and metal electrodes was developed by using heating spatially localized, through the use of laser radiation in the microRaman setup. Subsequent to deposition, the nanotubes were directly heated in the ambient atmosphere by a focused laser beam. The Raman signal of the nanotubes was used to determine the temperature reached in the process and this method has been used to improve the electrical contact with different electrodes (W, Ti and Au). The reduction in electrical resistance was up to three orders of magnitude, resulting in contact resistivity as low as ~ 0,1-1 kΩ.μm2, with the lowest values obtained for Au. A new method of simultaneous deposition of nanotubes and decoration with gold nanoparticles grown by electrochemical method is also presented. The method enables the production of hybrid structures with high potential for application in opto-electronic devices such as gas sensors, biosensors or photodetectors. The structural and morphological characterization of each sensor element used in this work was mainly made by scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, Raman spectroscopy and electrical measurements. Sensing elements with different combinations of nanostructured materials (MWCNTs as commercially acquired, oxidized, or decorated with metal nanoparticles) were tested in inert (Ar or N2) and oxidant (O2) atmosphere. Crucial parameters for the proper functioning of gas sensors (specific response time/recovery, sensitivity, stability, reproducibility, etc.) were investigated based on the obtained experimental data.