SENSITIZED LAYERED METAL-OXIDE-SEMICONDUCTOR PARTICLES FOR PHOTOCHEMICAL HYDROGEN EVOLUTION FROM NONSACRIFICIAL ELECTRON-DONORS

Layered alkali-metal titanates (Na2Ti3O7 and K2Ti4O9), niobates (KNb3O 8 and K4Nb6O17), and titanoniobates (KTiNbO5 and CsTi2NbO7) were inter nally platinized, acid-exchanged, sensitized with ruthenium polypyridy l complexes, and studied as photocatalysts for the production of H-2 a nd I3- from acidic alkali-metal iodide solutions. The titanates were i nactive as photocatalysts, whereas the niobates and titanoniobates wer e active with quantum efficiencies up to 0.3% for HI photolysis with v isible light. Calculations based on electronegativity showed that the conduction band edge potentials of the acid-exchanged titanates were t oo positive to prevent semiconductor-mediated recombination of photoge nerated H-2 and I3-. Laser flash photolysis/transient diffuse reflecta nce spectra established that iodide reduces the oxidized sensitizer, f orming I2.-, which subsequently decays in a bimolecular reaction to fo rm I3-. The inefficiency of HI photolysis can be attributed to charge recombination between I3- and conduction band electrons for the niobat es and titanoniobates. Modulation of the layer spacing in the hexaniob ate, A4-xHNb6O17, by exchange with different alkali metals (A), showed that the hydrogen evolution rate decreased sharply as the average lay er spacing increased. This result suggests that the competition betwee n charge recombination and electron tunneling between layers determine s the efficiency of the HI photolysis reaction.