The decomposition of TaCl 5-graphite flakes by long-time exposure in water and air

Abstract

Natural graphite flakes from Madagascar and Kropfmühl, Bavaria, were intercalated with anhydrous tantalum (V) chloride (TaCl 5). X-ray diffraction patterns showed that second stages were formed. The graphite intercalation compounds were stored in water for 2 h at 90 °C. After this treatment, the flakes were exposed to air for 2 months. One part of the flakes was embedded in epoxy resin, in such a way that the prismatic faces were sealed and the basal planes still had contact with the surrounding air. The samples were stored again in air for 2 months. A line scan, with an electron microprobe across the embedded sample, showed a high variation of Cl/Ta ratio. In the center of the intercalated flakes the original Cl/Ta ratio (above 5) remained. The Cl/Ta ratio decreased near cracks of the basal planes and near edges of the flakes. At one edge the Cl/Ta ratio was 1 and 3; near cracks, this ratio was 3-5. This different hydrolysis behavior showed two different products of hydrolysis. It is likely that tantalum oxychloride (TaOCl 3) was formed between the graphene layers as a first step. With increasing exposure time, the intermediate phase formed the final compound, amorphous tantalum oxide (oxopolymer). Non-embedded flakes deintercalated after some months. The oxopolymer cannot deintercalate from sealed prismatic faces. The basal planes of such embedded flakes showed many cracks after a few months. Water molecules can penetrate through these cracks into the host lattice and the hydrolysis can continue in the center of the flakes. The number of cracks on the basal plane increased with exposure time.