A Sensitized Nb₂O₅ Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb 2 O 5 nanocrystalline films functionalized with [Ru(bpy) 2 (4,4′-(PO 3 H 2 ) 2 bpy)] 2+ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key properties—conduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiency—were undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb 2 O 5 nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO 2 . Anatase TiO 2 has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb 2 O 5 is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage ( V oc ) for orthorhombic Nb 2 O 5, compared to anatase TiO 2 . In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA 4– ) added as a reductive scavenger, H 2 quantum yield and photostability measurements show that Nb 2 O 5 is comparable, but not superior, to TiO 2 .