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

Layered alkali-metal titanates (Na[sub 2]Ti[sub 3]O[sub 7] and K[sub 2]Ti[sub 4]O[sub 9]), niobates (KNb[sub 3]O[sub 8] and K[sub 4]Nb[sub 6]O[sub 17]), and titanoniobates (KTiNbO[sub 5] and CsTi[sub 2]NbO[sub 7]) were internally platinized, acid-exchanged, sensitized with ruthenium polypyridyl complexes, and studied as photocatalysts for the production of H[sub 2] and I[sub 3][sup [minus]] 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[sub 2] and I[sub 3][sup [minus]]. Laser flash photolysis/transient diffuse reflectance spectra established that iodide reduces the oxidized sensitizer, forming I[sub 2][sup [sm bullet][minus]], which subsequently decays in a bimolecular reaction to form I[sub 3][sup [minus]]. The inefficiency of HI photolysis can be attributed to charge recombination between I[sub 3][sup [minus]] and conduction band electrons for the niobates and titanoniobates. 29 refs., 16 figs., 1 tab.