Chemical Characterization by FT-IR Spectrometry and Modification of the Very First Atomic Layer of a TiO 2 Nanosized Pow
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EXPERIMENTAL All the spectra were recorded in transmission mode by means of a Perkin-Elmer Spectrum 2000 FT-IR spectrometer equipped with an MCT cryodetector. The analyzed spectral range extended from 500 to 6500 cm" with a 4 cm-' resolution. The FT-IR experiments were run in situ by using a specially designed heatable vacuum cell [3] placed inside the spectrometer sample compartment. Controlled pressures of gases were adjusted through a precise valve system. The titania powder (P25, Degussa-France) was mainly in the anatase crystalline phase (-70%). The specific surface area measured by the supplier was 50 m2g' and the estimated average particle size was 21 nm. For the infrared analyses, the n-TiO 2 powder was slightly pressed into thin pellets (-50 mg) on a stainless grid (Gantois, France) ensuring a homogeneous thermal distribution. These pellets were systematically kept under vacuum at 673 K prior to be subjected to different gases while keeping the temperature constant. All the gases (Alphagaz, France) were 99% pure, hexamethyldisilazane (HMDS) (Fluka, Germany) was 99.5 % pure and water was bi-distilled and de-ionized. The in situ and ex situ grafting procedures are described below. INFRARED POWDER
SURFACE
ANALYSIS
OF HMDS-GRAFTED
TITANIA
NANOSIZED
As-received n-TiO? powder As it is the case for all oxides, the as-received n-TiO 2 powder is covered with molecular water adsorbed on its surface [4]. A thermal treatment under dynamic vacuum (referred to as activation throughout the text) leads to a surface freed of adsorbed species according to the temperature. The surface is then no longer in an equilibrium state, and as soon as molecules impinge on this activated surface, they adsorb on reactive sites. This activation not only allows one to obtain a good knowledge of the surface species but also to follow the reactions which will eventually take place during the gas-surface interactions. We must underline here that we are not considering the n-TiO 2 pellet as a gas sensor under its standard working conditions. Instead, we are trying to highlight the surface reactions and to understand their mechanism so as to extend this knowledge to the very complex case of a real gas sensor in an atmosphere whose exact composition is unknown. The spectrum of n-TiO 2 surface activated at 673 K is given in Fig. la. The displayed spectral range only extends from 2000 to 4000 cm t where the absorption bands of the surface species which are minority by far even for nanosized powders, are clearly visible. The complex band centered at 3600 cm-1 corresponds to the v(OH) stretching vibrations of different types of surface hydroxyl groups responsible for hydrophilicity. Indeed, according to the cation coordination or/and the number of cations linked to the OH groups, the v(OH) frequencies vary over a large range [5]. When the n-TiO 2 pellet is heated at 673 K under air, a broad feature appears around 3400 cm-' (Fig. 2a) and is assigned to hydrogen-bonded hydroxyl groups. Simultaneously a lowering of the baseline is observed corre
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