Contrasting 1D tunnel-structured and 2D layered polymorphs of V2O5: relating crystal structure and bonding to band gaps and electronic structure

New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel zeta-phase and the orthorhombic a-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 +/- 0.20 eV in the alpha-phase to 1.50 +/- 0.20 eV in the zeta-phase, accompanied by the loss of the alpha-phase's characteristic split-off d(xy) band in the zeta-phase. States of d(xy) origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. zeta-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to alpha-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the alpha- and zeta-phase.