Several phases (α, ε2, and γ) of aluminum fluoride were prepared from precursors such as
NH4AlF4, (NH4)3AlF6, and β-AlF3·3H2O using various synthetic strategies. The precursors
and the aluminum fluoride phases obtained were characterized by X-ray diffraction (XRD),
X-ray fluorescence, scanning electron microscopy (SEM), and infrared spectra (FT-IR). The
structural evolution of the precursors to the different phases of aluminum fluoride during
thermal treatment was studied by dynamic XRD experiments, differential scanning
calorimetry (DSC), and thermogravimetric analysis (TGA). α- and γ-AlF3 phases were
obtained with high BET surface areas (>120 and 30 m2/g, respectively). These aluminum
fluorides with high BET areas are of potential interest as supports or catalysts in
hydrodechlorination and fluorination reactions.
Thin Al films sputtered on n-silicon wafers were anodized in two potentiodynamic steps in 0.4 M tartaric acid at 210 V and 0.4 M malonic acid at 95 V to prepare nanoporous alumina templates of type A and B, respectively. Processes at the Al/n-Si interface were closely examined to avoid field-assisted alumina dissolution during anodizing. WO 3 semiconductor layers were radiofrequency magnetron sputtered onto the as-anodized and pore-widened templates. It was revealed that the sputtered WO 3 layers covered the entire surface of type A templates while only partly penetrating into the widened pores of type B templates. The WO 3 layers crystallized at 350°C, with monoclinic phase of P2 1 /n symmetry and an average crystallite size of 28.0 nm ͑type A͒ and 32.5 nm ͑type B͒. Amorphous alumina in the Al 2 O 3 /n-Si systems did not crystallize up to 1200°C. The common trend for the WO 3 /Al 2 O 3 /n-Si systems was a preferential growth of WO 3 nanocrystallites in the ͗100͘ direction, the texture increasing with further pore widening. Besides, the crystal texture in the WO 3 layer was systematically higher on type B templates. The link between the structural features and the gas-sensing ability of the WO 3 /Al 2 O 3 /n-Si systems has been discussed.
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