Photocatalytic Hydrogen Evolution from Hexaniobate Nanoscrolls and Calcium Niobate Nanosheets Sensitized by Ruthenium(II) Bipyridyl Complexes

Hexaniobate nanoscrolls (NS-H4Nb6O17) and acid-restacked calcium niobate nanosheets (R-HCa2Nb3O10) were compared as oxide semiconductors in photocatalytic assemblies for H-2 production using ethylenediaminetetraacetic acid (EDTA) as a sacrificial electron donor and platinum (Pt) nanoparticles as catalysts. Ru(bpy)(3)(2+) and Ru(bpy)(2)(4,4'-(PO3H2)(2)bpy)(2+) (bpy = 2,2'-bipyridine) were employed as visible light sensitizers (abbreviated as Ru2+ and RuP2+, respectively). RuP2+, which is anchored by a covalent linkage to the NS-H4Nb6O17 surface, functions more efficiently than the electrostatically bound Ru2+ complex, because of more efficient electron injection from the excited sensitizer to NS-H4Nb6O17. RuP2+-sensitized NS-H4Nb6O17 and R-HCa2Nb3O10 both produce H-2 photocatalytically using visible light (lambda > 420 nm) with initial apparent quantum yields of 20-25%. At the optimum sensitizer concentration and Pt loading, the photochemical hydrogen evolution process is primarily limited by the efficiency of light absorption and charge injection from the photoexcited sensitizer into the oxide semiconductor particles. The dependence of the hydrogen evolution rate on Pt loading suggests that the scavenging of conduction band electrons by Pt is substantially faster than charge recombination or EDTA reduction of the oxidized sensitizer under optimized conditions.