Spectroscopic investigation of vanadium speciation in vanadium-doped nanocrystalline anatase

The speciation of vanadium doped into anatase particles of 50-90 Angstrom diameter has been studied by a variety of techniques including solid state V-51 NMR, EPR, XPS and Raman spectroscopy. EPR spectra of V-doped anatase are extremely complex showing up to five different V4+ centres (species A-E). Partial spin Hamiltonian parameters for these species have been determined and ascribed to interstitial (species A) and substitutional V4+ (species B-D), as well as surface V4+-rich clusters (species E). Solid state V-51 NMR also shows the presence of several V5+ species that exhibit both tetrahedral and octahedral coordination and which are attributed to structural and residual surface V5+. It is shown by NMR, Raman spectroscopy and XPS that the dissolution of this residual surface vanadium component into the anatase structure becomes increasingly difficult as the vanadium concentration increases.