Oxygen Adsorption on Tin Oxide Nanosized Powders Characterized by FTIR Spectrometry and Relation with the Sensor Propert
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evaporation of compressed micronsized SnO2 powder with the pulse radiation of a 1200W Nd : YAG-laser and subsequent condensation of the vapor in a controlled atmosphere [6]. The powder referred to as SOI was obtained by condensation in atmosphere whereas the powder referred to as S02 was obtained under 0.01 bar of Argon. In order to increase the powder yield, the resulting aerosols were pumped at a controlled rate through an induction pipe into a mechanical filter system. The nanoparticles were collected on a glass fibre filter with an average pore size of several micrometers. The separation mechanism of the nanoparticles is largely governed by diffusion and interception of the particles onto the surface of the fibres. A TEM analysis revealed that the nanoparticles are of spherical shape. The size distribution of the particles is logarithmic normal with a median particle size of 14 nm for S01 and 8 nm for the S02 sample. RESULTS AND DISCUSSION Surface activation and oxygen adsorption The infrared spectrum of the raw SOI pellet is shown figure la between 800 and 4000 cm1 corresponding to the range of transparency of this sample. The broad band centered around 3300 cm-' corresponds to water hydrogen-bonded to surface hydroxyl groups [7]. In order to remove all physisorbed and weakly chemisorbed species, the sample was activated, that is heated at 400'C under dynamic vacuum for one hour and subsequently cooled to room temperature (rt) while still under vacuum. The spectrum of the activated sample (Fig. lb) shows that this activation treatment modifies the transparency of the sample because of a loss of stoichiometry [8], thus increasing the electrical conductivity of the powder. When oxygen is adsorbed (Fig. 1c) the sample partially recovers its transparency. The broad band around 3150 cm-1 and the band at 3652 cm1 are assigned to the v(OH) stretching vibration of perturbed and free OH groups respectively. Several bands are noted at 924 (shoulder), 964, 1068, 1155, 1182, 1262, 1348, 1427, 1480, 1531 and 1680 cm-'. The last five bands decrease when the sample under oxygen is heated at 150'C and cooled at rt (Fig. 1d). When the temperature is increased up to 400'C, these five bands totally disappear (Fig. le). These 1348, 1427, 1480, 1531 and 1680 cm' bands are assigned to adsorbed molecular oxygen [9]. Since these species are removed by heating under oxygen in excess, the corresponding adsorption sites are unstable and further destroyed by an oxidation treatment above 150°C [10]. The bands in the 3000-4000 cm-' range, hardly visible at room temperature (Fig. Ic), can be attributed to isolated (3660 cmn1) and perturbed OH groups via hydrogen bonds (3602, 3552, 3520 and 3483 cm-'). According to the literature [11], the frequencies at 950 (shoulder), 970, 1145, 1180 and 1260 cm' might be assigned to the corresponding 5(OH) bending modes. However, coordinated 022- oxygen species (either bent end-on "superoxo" or symetrical side-on "peroxo" species), 02 radical-ion and ionic forms (02. and O-) absorb in the same range as th
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