High-throughput synthesis and characterization of vanadium mixed metal oxide pigments using synchroton radiation

UNA range of inorganic vanadium mixed metal oxides, with potential applications as<br/>inorganic pigments, have been synthesised and characterised in terms of their crystal<br/>structure, and band gap energies using powder X-ray diffraction (PXD), synchrotron-based<br/>X-ray absorption fine structure (XAFS) spectroscopy, solid state UV-Visible and Raman<br/>spectroscopy, Scanning and Transmission electron microscopy (SEM and TEM).<br/><br/>Vanadium mixed metal oxide BiMeVOX with Me = Mo, Cu and Fe, and calcium<br/>pyrovanadate with Me = Pb, Cd, Sr, doped systems, have been synthesised in sequential<br/>and high throughput array by peroxo sol-gel methods and calcined at 700°C in an oxygen<br/>atmosphere. Powder X-ray diffraction studies show the synthesised BiMoVOX<br/>(Bi1-x/3Mo1-xVxO4) to have a tetragonal crystal structure in the space group I 41/a, BiCuVOX<br/>and BiFeVOX (Bi2V1-xMexO5.5-?) to have a tetragonal crystal structure in the space group<br/>I 4/mmm, and pyrovanadate ((Ca2-x,Mex)V2O7) to have triclinic in the space group P-1.<br/>PXD profiles have displayed phase transitions from monoclinic to tetragonal crystal phase<br/>together with peak positions shift, reflecting the changes in d-spacing and consequent<br/>volume cell growth by incorporation of the different ions into BiMeVOX; the calcium<br/>pyrovanadate kept their triclinic crystal structure.<br/><br/>Raman and XAFS spectroscopy data analysis have shown isolated metal-oxygen<br/>tetrahedra for both vanadium and molybdenum cations in the BiMoVOX materials.<br/>Vanadium coordination in both the BiFeVOX and the BiCuVOX materials is more complex<br/>with three different cases: octahedral distorted, trigonal bipyramid and tetragonal ; these<br/>structure consists of alternating layers of [Bi2O2]2+ and [VO3.5?0.5]2-, where ? represents oxide<br/>ion vacancies. Calcium pyrovanadate materials display vanadium-oxygen polyhedra for a<br/>tetrameric chain comprising two edge-shared VO5 pyramids, and each also sharing a corner<br/>with a VO4 tetrahedron.<br/><br/>UV-Visible spectroscopy of these materials reveals broad reflectance bands and a<br/>steep absorption edge, which is directly related to the band gap energy of the compound. By<br/>varying the doping levels on the vanadium and calcium site with different cations the colour<br/>of the compounds can be seen to visibly alter. This can be attributed to a modification in the<br/>band gap energies as direct result of the fine-tuning of the electronegativity difference<br/>between cations and anions, their overall contribution to the molecular orbitals of the<br/>materials, and the total concentration of oxygen vacancies.<br/>SEM and TEM analysis showed a more round particle shape for samples made by the<br/>peroxo sol-gel reaction compared to samples made by calcinations reaction in the BiMoVOX<br/>series.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Russu, Sergio

Advisor dc:contributor.advisor

• Evans, John