Sensitized layered metal oxide semiconductor particles for photochemical hydrogen evolution from nonsacrificial electron donors

Layered alkali-metal titanates (Na₂Ti₃O₇ and K₂Ti₄O₉), niobates (KNb₃O₈ and K₄Nb₆O₁₇), and titanoniobates (KTiNbO₅ and CsTi₂NbO₇) were internally platinized, acid-exchanged, sensitized with ruthenium polypyridyl complexes, and studied as photocatalysts for the production of H₂ and I₃^- from acidic alkali-metal iodide solutions. The titanates were inactive as photocatalysts, whereas the niobates and titanoniobates were active with quantum efficiencies up to 0.3% for HI photolysis with visible light. Calculations based on electronegativity showed that the conduction band edge potentials of the acid-exchanged titanates were too positive to prevent semiconductor-mediated recombination of photogenerated H₂ and I₃^-. Laser flash photolysis/transient diffuse reflectance spectra established that iodide reduces the oxidized sensitizer, forming I₂^•-, which subsequently decays in a bimolecular reaction to form I₃^-. The inefficiency of HI photolysis can be attributed to charge recombination between I₃^- and conduction band electrons for the niobates and titanoniobates. Modulation of the layer spacing in the hexaniobate, A_4-xH_xNb₆O₁₇, by exchange with different alkali metals (A), showed that the hydrogen evolution rate decreased sharply as the average layer spacing increased. This result suggests that the competition between charge recombination and electron tunneling between layers determines the efficiency of the HI photolysis reaction.