Chemical Reactions on the Surface of SnO 2 Nanosized Powders at the Origin of the GaS Sensing Properties: FTIR Investiga
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investigation of the surface chemical species modifications under various environments [2], simultaneously with the measurement of the infrared energy transmitted by the semiconductor sample directly related to the variations of the electrical conductivity. According to the DrudeZener theory, the infrared energy (EmR) transmitted by the semiconductor sample is related to the concentration of the free carriers, that is the Em variations are directly related to the variations of the electrical conductivity due to modifications of the gaseous environment of the semiconductor sample, provided that the surface states are not strongly affected [3]. Practically speaking, when a n-type semiconductor is in contact with a reducing gas such as CO, its electrical conductivity increases due to the increase of the free carriers density. This results in a higher absorption of the infrared radiation by the semiconductor sample and, therefore, to a decrease of the infrared energy transmitted by the sample. Conversely, when the n-type semiconductor is subjected to an oxidizing gas such as oxygen, its electrical conductivity decreases and the sample becomes more transparent to the infrared radiation. In summary, when the electrical conductivity decreases, we should observe an increase of the infrared energy transmitted by the sample and vice versa. It is therefore easy to realize the unique contribution of FTIR spectrometry in the understanding of the gas detection mechanism. Although our FTIR experiments were performed on pressed nanosized powders, the surface chemical reactions inducing the sensor response have been demonstrated to be identical on the real sensor fabricated from these nanopowders [4]. 559 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
EXPERIMENTAL The Sn0 2 nanosized powders used in the present experiments were synthesized by evaporation of compressed micron-sized Sn0 2 powder with the pulse radiation of a Nd:YAGlaser and subsequent condensation of the vapor in a controlled atmosphere [5]. Two batches corresponding to the average particle diameters of 15 nm (S 15) and 8 nm (S8) have been studied. XRD analyses essentially showed the quadratic phase. The FTIR measurements were performed by using a Perkin-Elmer Spectrum 2000 spectrometer, equipped with a MCT cryodetector. A specially designed vacuum cell allowed the in situ analyses at different temperatures (from room temperature up to 500'C) under vacuum or controlled atmospheres [2]. The nanopowders to be analyzed were gently pressed into thin pellets and placed inside the furnace of the vacuum cell. The spectra were recorded from 7800 to 450 cm"1 with a 4 cm 1 resolution. For the real sensors prepared from 15 and 8 nm Sn0 2 particles, it has been determined that two sensitivity maxima for CO detection occurred around 120 'C and 350'C [1]. Therefore, our FTIR experiments were performed on the two Sn0 2 nanopowders at these two temperatures. To simulate the gas sensors, the experiments were conducted the following way [6]. The pellet of press
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