engleski

Mixed metal oxides: single-step preparation and structural studies

The synthesis of nanoparticles has been studied extensively in order to understand physics and chemistry at a ‘small’ scale and because of promising applications due to their size-dependent properties.1 There are two major reasons why nanocrystals and their bulk counterparts differ from each other: (i) in nanocrystals the number of surface atoms is a large fraction of the total, (ii) the size of the nanocrystal interior causes a systematic transformation in the density of electronic energy levels.2 Namely, in any material the surface atoms make a distinct contribution to the free energy, so the changes in thermodynamic properties of the nanocrystals compared to the crystalline bulk material of same composition can appear (e.g. melting temperature depression, solid-solid phase transition elevation). Also, changes in thermodynamic stability associated with size can induce modification of cell parameters and/or structural transformations. To display mechanical or structural stability, a nanoparticle must have a low surface free energy. As a consequence of this requirement, phases that have a low stability in bulk materials can become very stable in nanostructures. This structural phenomenon has been detected in TiO2, VOx, Al2O3 or MoOx oxides. For (semi)conductors, as the particle get smaller an additional effect, the so-called quantum-size effect or confinement effect which essentially arise from the presence of discrete, atom-like electronic states which influences the energy shift of exciton levels and optical band gap3. Therefore, synthesis of nanoparticles with controlled size and shape has been found to be demanding for tailoring the desired materials properties since the thermal, electric, optical, catalytic and magnetic properties are strongly composition-, structure- , but also, size- and shape-dependent4. Our work is focued on development of new synthetic routes for oxide materials preparation, namely, direct molecular-to-materials pathways which include heterometallic complexes as single-molecular precursors. Generally, preparation method from single-molecular precursor5, as compared to conventional methods, has several advantages: (i) the obtained material is more homogeneous because the metals are mixed at the molecular level ; (ii) the resulting materials are characterized by relatively high specific surface areas because the they are usually formed under milder ; and (iii) bridging or chelating ligands in the precursor can prevent unwanted metal separation during the oxide formation. Structural and microstructural characterisation of Ba4Nb2O9, Ba4Ta2O9, CoMn2O4 and CaCr2O4 will be discussed in details.

Nanomaterails; mixed oxides; XRD

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