Dispersion of ruthenium oxide on barium titanates (Ba6Ti17O40,Ba4Ti13O30,BaTi4O9and Ba2Ti9O20)and photocatalytic activity for water decomposition

Abstract

Ruthenium oxide supported on barium titanates (Ba6Ti17O40, Ba4Ti13O30, BaTi4O9 and Ba2Ti9O20) was employed as a photocatalyst for water decomposition. The RuCl3-impregnated titanates were subjected to either reduction or reduction–oxidation. High-resolution electron microscopic images demonstrated that ruthenium metal and ruthenium oxides were uniformly dispersed on BaTi4O9 with an average particle size of 2.6 nm. Similar uniform ruthenium oxide dispersions were observed for the other barium titanates; the average particle sizes were 4.7 nm for Ba6Ti17O40, 2.3 nm for Ba4Ti13O30, and 4.4 nm for Ba2Ti9O20. Particle size distributions were narrower for BaTi4O9 and Ba4Ti13O30, and slightly larger for Ba6Ti17O40 and Ba2Ti9O20. Stoichiometric production of oxygen and hydrogen occurred for a RuO2/BaTi4O9 photocatalyst. A small amount of hydrogen and no oxygen were produced from the other barium titanates (Ba6Ti17O40, Ba4Ti13O30 and Ba2Ti9O20) combined with ruthenium oxides. EPR spectra at 77 K in He or O2 with UV irradiation demonstrated that a strong signal, assigned to a surface O- radical, appeared for BaTi4O9 but not for the other barium titanates. These produced small complicated signals, indicating that only BaTi4O9 has a high efficiency for photoexcited charge formation. Raman spectra showed that a strong single peak at a high wavenumber of 860 cm-1, characteristic of BaTi4O9, was absent in the rest of the barium titanates. The different photocatalytic properties among these titanates are discussed on the basis of structure differences of the barium titanates, and the presence of internal fields; a long Ti–O bond in the distorted TiO6 octahedra is proposed to be important in photocatalysis.