FTIR Analysis of the Mechanisms of NO x Detection by Semiconductor Metal Oxide Nanoparticles
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FTIR Analysis of the Mechanisms of NOx Detection by Semiconductor Metal Oxide Nanoparticles Marie-Isabelle BARATON 1 and Lhadi MERHARI 2 1 SPCTS - UMR CNRS, Faculty of Sciences, 87060-Limoges, France ([email protected]) 2 CERAMEC R&D, 87000-Limoges, France ABSTRACT The chemical reactions occurring at the surface of tin oxide nanoparticles under NOx adsorption have been investigated by Fourier transform infrared spectroscopy and correlated with the variations of the electrical conductivity of tin oxide. These surface reactions have been found dependent on the particle size and on the presence of oxygen. The strong coordination of the newly formed chemical species onto the nanoparticle surface may result in a poor reversibility of the sensor response. INTRODUCTION Nitrogen oxides (NOx) are particularly harmful to the environment because these primary pollutants are responsible for generating ozone. In previous works [1-3], we have already improved the performance of the chemical sensors in terms of sensitivity and detection thresholds by using nanoparticles in an optimized screen-printing fabrication process. However, the fundamental mechanism of the NOx detection is still unclear. We earlier also reported the contribution of Fourier transform infrared (FTIR) spectroscopy to the understanding of the chemical reactions at the origin of the gas detection mechanism by semiconductor nanoparticles [4,5]. Through in situ experiments in transmission mode, it is indeed possible to identify the chemical species at the nanoparticles surface and to monitor the reactions or interactions in presence of the gas to be detected while simultaneously analyzing the variation of the free carrier infrared absorption corresponding to the variations of the electrical conductivity. We report here the surface FTIR analysis of tin oxide nanoparticles when NOx is adsorbed under the operating conditions of real sensors. These surface reactions are correlated with the variations of the electrical conductivity in the nanoparticles. EXPERIMENTAL The tin oxide (SnO2) nanoparticles used for this study were synthesized by laser evaporation of compressed micron-sized powder [6]. The material is mainly in the rutile phase and, depending on the synthesis conditions, the average particle diameter is either 15 or 8 nm. To simulate the gas sensors, 30 mg of nanopowder were slightly pressed into a thin gridsupported pellet. A vacuum cell, specially designed to fit in the sample chamber of the spectrometer, allowed in situ thermal treatment of the pellet under dynamic vacuum or under various gases whose pressures were adjusted through a valve system [7]. In situ experiments made possible the direct comparison of the spectra, and whenever needed, spectral differences could be performed to emphasize modifications of the absorption bands during an experimental step. Like the real sensors, the nanoparticles were pre-treated at 350°C under 50 mbar oxygen. A procedure was then systematically followed to test the sensing properties of the nanoparticles. It
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