Theoretical Modelling of the Surface Oxidation of a Silicon Carbide Nanopowder, Based on the v (SiH) Frequency Evolution
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MARIE-ISABELLE BARATON*, SYLVETTE BESNAINOU** *LMCTS - URA 320 CNRS, Faculty of Sciences, 123 Av. Albert Thomas, F-87060 Limoges, France, baraton@unilimfr "**Laboratory of Physical Chemistry, URA 176 CNRS, University of Paris VI, 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France.
ABSTRACT Due to hydrolysis by atmospheric water, the first atomic layer of silicon carbide contains oxygen atoms. The oxygen content on the surface dramatically increases for nanosized SiC powder and strongly affects the overall properties. The presence of SiH groups on the SiC surface, already proven by Fourier transform infrared spectrometry, can be used to investigate the oxidation mechanism. Indeed, the v(SiH) stretching frequency known to strongly depend on the surrounding electronegativity of the silicon atom is directly related to the oxidation degree. Based on our infrared spectroscopic experimental data, ab initio calculations were performed on models representing the Sill surface species in different environments. Using the Hartree-Fock method with 3-21G and 6-31G basis sets, the v(SiH) frequencies were calculated for each model versus the number of substituted oxygen atoms. The comparison with the experimental values led to a discussion of our proposed models and of the SiC surface oxidation mechanism. INTRODUCTION As it is the case for all non-oxide ceramics, 1,2 silicon carbide surface is partly hydrolyzed by atmospheric water.3 Consequently, oxygen atoms are present in the first atomic layer. They can be considered as standard impurities in a silicon carbide powder as long as the grains are of micron size. But when the specific surface area increases, the role played by the surface becomes non-negligible and the presence of oxygen on the surface may have drastic consequences on the SiC overall properties. 4-7 Therefore, the need for a careful investigation of the first steps of the oxidation mechanism appears critical. According to our previous studies, the SiC nanosized powder presents Sill surface species.8,9 The v(SiH) stretching band, although weak, is of special interest since its absorption frequency is very sensitive to the electronic environment of the silicon atom.10-14 As we already demonstrated it, this frequency can be a good sensor of the oxidation degree of the SiC surface.15 In this work, models are proposed to represent the Sill species on the SiC surface according to the oxidation degree. Ab initio calculations are performed on these models whose validity will be checked by comparison between experimental and calculated v(SiH) frequencies. EXPERIMENTAL RESULTS The chemical species on a SiC surface were identified by Fourier transform infrared (FT-IR) spectrometry according to a procedure we described earlier. 16 When the surface is activated, that is heated at 873 K under dynamic vacuum, the SiC nanosized powder' 7 presents SiOH, CH. and 327
Mat. Res. Soc. Symp. Proc. Vol. 405 0 1996 Materials Research Society
Sill groups.3 ,18 Their location was proved to be on the surface by isotopic exchange H/D.3,
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