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

Abstract

UNA range of inorganic vanadium mixed metal oxides, with potential applications as
inorganic pigments, have been synthesised and characterised in terms of their crystal
structure, and band gap energies using powder X-ray diffraction (PXD), synchrotron-based
X-ray absorption fine structure (XAFS) spectroscopy, solid state UV-Visible and Raman
spectroscopy, Scanning and Transmission electron microscopy (SEM and TEM).

Vanadium mixed metal oxide BiMeVOX with Me = Mo, Cu and Fe, and calcium
pyrovanadate with Me = Pb, Cd, Sr, doped systems, have been synthesised in sequential
and high throughput array by peroxo sol-gel methods and calcined at 700°C in an oxygen
atmosphere. Powder X-ray diffraction studies show the synthesised BiMoVOX
(Bi1-x/3Mo1-xVxO4) to have a tetragonal crystal structure in the space group I 41/a, BiCuVOX
and BiFeVOX (Bi2V1-xMexO5.5-?) to have a tetragonal crystal structure in the space group
I 4/mmm, and pyrovanadate ((Ca2-x,Mex)V2O7) to have triclinic in the space group P-1.
PXD profiles have displayed phase transitions from monoclinic to tetragonal crystal phase
together with peak positions shift, reflecting the changes in d-spacing and consequent
volume cell growth by incorporation of the different ions into BiMeVOX; the calcium
pyrovanadate kept their triclinic crystal structure.

Raman and XAFS spectroscopy data analysis have shown isolated metal-oxygen
tetrahedra for both vanadium and molybdenum cations in the BiMoVOX materials.
Vanadium coordination in both the BiFeVOX and the BiCuVOX materials is more complex
with three different cases: octahedral distorted, trigonal bipyramid and tetragonal ; these
structure consists of alternating layers of [Bi2O2]2+ and [VO3.5?0.5]2-, where ? represents oxide
ion vacancies. Calcium pyrovanadate materials display vanadium-oxygen polyhedra for a
tetrameric chain comprising two edge-shared VO5 pyramids, and each also sharing a corner
with a VO4 tetrahedron.

UV-Visible spectroscopy of these materials reveals broad reflectance bands and a
steep absorption edge, which is directly related to the band gap energy of the compound. By
varying the doping levels on the vanadium and calcium site with different cations the colour
of the compounds can be seen to visibly alter. This can be attributed to a modification in the
band gap energies as direct result of the fine-tuning of the electronegativity difference
between cations and anions, their overall contribution to the molecular orbitals of the
materials, and the total concentration of oxygen vacancies.
SEM and TEM analysis showed a more round particle shape for samples made by the
peroxo sol-gel reaction compared to samples made by calcinations reaction in the BiMoVOX
series.

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Identifiers

ORCID for John Evans: orcid.org/0000-0003-3290-7785

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