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 H-2 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 H-2 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, A4-xHNb6O17, 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.