A comparison of the photocatalytic activity of six tunneled titanates

The photocatalytic behavior of six tunneled titanates—Na0.7Ga4.7Ti0.3O8, Na0.8Ga4.8Ti1.2O10, Na0.8Ga4.8Ti2.2O12, K1Ga17Ti15O56, K1.5Ga1.5Ti6.5O16, and BaTi4O9—was investigated using methylene blue as an indicator and a xenon arc lamp as the radiation source. Powders prepared by solid state reaction had surface areas ranging from 0.40 to 1.58 m2/g and particle sizes ranging from 1 to 25 μm. Bandgaps, as measured from diffuse reflectance data, ranged from 2.84 to 4.15 eV. Two of the tunneled titanates—Na0.8Ga4.8Ti1.2O10 and Na0.7Ga4.7Ti0.3O8—exhibited negligible photocatalytic activity. The activities of Na0.8Ga4.8Ti2.2O12, KGa17Ti15O56, and K1.5Ga1.5Ti6.5O16 were similar to each other, exhibiting apparent first-order reaction rates of 0.275 to 0.31 h−1 using 100 mg of powder in 125 ml of 20 μM methylene blue at room temperature. The BaTi4O9 sample exhibited the highest photocatalytic activity with an apparent first-order reaction rate of 0.53 h−1 under the same conditions. The addition of a RuO2 co-catalyst improved the photocatalytic activity of Na0.7Ga4.7Ti0.3O8, decreased the activity of BaTi4O9, and had little effect on the activity of the other powders. A comparison of the structural features in the six materials shows that photocatalytic activity is strongly related to the density of TiO6 octahedra in the different crystal structures. Among the gallium-containing tunneled titanates, photocatalytic activity increased with decreasing band gap.