engleski

Heterometallic complexes with (oxalato)tantalate anions as molecular precursors for the preparation of nanocrystalline oxides

The aqueous chemistry of tantalum is mainly related to certain forms of carboxylates, such as oxalate, tartarate, citrate, glycolate, lactate and polyaminocarboxylate, as well as peroxide compounds. Usually these species have been utilized as starting materials for the sol-gel or polymeric precursor methods for the preparation of oxide materials. Structurally defined heterometallic complexes of tantalum obtained from aqueous solutions are very scare, despite of great potential to produce mixed-metal oxides via thermal decomposition. Metal tantalates represent very common functional inorganic materials with various properties and applications, in particular photocatalytic and dielectric. It is known that the effect of crystallinity, particle size and morphology of oxides can highly affect their photocatalytic activity. These factors can be tuned in part by changing the synthesis method. It is widely recognized that a large surface area of photocatalyst is desired, since it can promote substrate adsorption to the surface and increase the number of active sites for catalysis. However, in order to obtain materials of good crystallinity high temperature treatments are often employed, inducing increase of the particle size and decrease of the surface area. Thus, it is difficult to harmonize both in the one-step synthesis approach. The thermal decomposition of heterometallic complexes give rise to (nano)crystalline oxides with relatively high specific surface area due to milder conditions than those employed in traditional solid-state syntheses. Also, due to the mixing of metal ions at the molecular level better control of metal stoichiometry and higher homogeneity is achieved. Relatively low thermal stability of the heterometallic oxalate compounds (oxalate ligand decomposes at  400 °C to gaseous CO2 and CO) makes them suitable for the use as molecular precursors for the mixed-metal oxides. As a continuation of our previous studies on the oxalate chemistry of tantalum, we have prepared a series of heterometalllic compounds of the formula [M(phen)3][Ta(OC2H5)(C2O4)3]·H2O [M = Fe, Co, Ni ; phen = 1, 10-phenanthroline] that were used as precursors for mixed-metal oxides: FeTaO4, CoTa2O6, Co4Ta2O9 and NiTa2O6. Up to now, such mixed-metal oxides have been synthesized by the solid-state reactions that required long-term heating at relatively high temperatures and repeating grinding procedures. The influence of heating treatment in studied Fe-Ta, Co-Ta and Ni-Ta systems on the composition and crystalline size of oxides was examined by powder X-ray diffraction. Microstructural characterization showed that all thermal decomposition residues were nanocrystalline, in general indicating that this method is very convenient way for obtaining mixed-metal oxides.

Oxalate ; Tantalum ; Molecular Precursor ; Mixed-Metal Oxide ; X-ray Diffraction

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