Periodic density functional study on structural and vibrational properties of vanadium oxide aggregates

We present periodic density-functional calculations within the generalized gradient approximation (Perdew-Wang 91) on structures and vibrational properties of different vanadium oxide aggregates, namely, bulk ${\mathrm{V}}_2{\mathrm{O}}_5$ and its (001) surface, as well as thin vanadium oxide films supported by $\alpha$-alumina. Vanadium is differently coordinated by oxygen in the different systems. The calculated vibrational frequencies of bulk ${\mathrm{V}}_2{\mathrm{O}}_5$ are in good agreement with observed IR and Raman frequencies, for stretching modes the rms deviation is $40{\mathrm{cm}}^{-1}.$ The calculations for the ${\mathrm{V}}_2{\mathrm{O}}_5$(001) surface suggest modifications of previous assignments of high-resolution electron-energy-loss spectroscopy (HREELS) data. In agreement with HREELS, vanadyl frequencies shift to higher wave numbers on surface formation. The calculated frequencies for bulk ${\mathrm{Al}}_2{\mathrm{O}}_3$ are systematically lower than the observed IR data (by about $30{\mathrm{cm}}^{-1}).\mathrm{}$ Models for ${\mathrm{V}}_2{\mathrm{O}}_3$ supported on ${\mathrm{Al}}_2{\mathrm{O}}_3$ are obtained when in the outermost layers of ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) slabs Al is replaced by V. These films do not show vibrations above $930{\mathrm{cm}}^{-1}.$ Oxygen adsorption on top of the vanadium sites on these supported films creates very stable vanadyl groups with binding energies of about 450 kJ/mol $\left(\frac{1}{2}{\mathrm{O}}_2\right).\mathrm{}$ Bond distances, vibrational frequencies, and oxygen binding energies are compared with those of vanadyl groups at the ${\mathrm{V}}_2{\mathrm{O}}_5$(001) surface and in $({\mathrm{V}}_2{\mathrm{O}}_5{)}_n$ clusters $(n=2,$4). The relevance of the findings for experiments on vanadia particles supported on alumina is discussed.