Photocatalytic properties of M2Ti6O13 (M=Na, K, Rb, Cs) with rectangular tunnel and layer structures: Behavior of a surface radical produced by UV irradiation and photocatalytic activity for water decomposition

Abstract

The photocatalytic properties of RuO₂-dispersed M₂Ti₆O₁₃ (M=Na, K, Rb, Cs) were investigated as to the photoexcitation ability of the hexatitanates and the role of the dispersed RuO₂. An EPR signal with g=2.020, g=2.018 and g=2.004 produced by UV irradiation of the hexatitanates at 77 K had a short life time in vacuum but remained stable upon exposure to gaseous molecules such as H₂, He, N₂, O₂ and Ar. These gases brought about different stability and pressure dependence of the signal intensity, by which the radical is assigned to O^- resulting from the surface lattice oxygen. The efficiency of the radical formation is associated with the presence of internal fields in the distorted TiO₆ octahedra forming the rectangular tunnels of M₂Ti₆O₁₃ (M=Na, K, Rb). RuO₂ was dispersed on the hexatitanates by impregnation with either RuCl₃(aq) solution or Ru₃(CO)₁₂ in tetrahydrofuran and the subsequent oxidation in the temperature range 573–823 K. The photocatalytic activity for water decomposition was higher for RuO₂/M₂Ti₆O₁₃ (M=Na, K, Rb) photocatalysts prepared using Ru₃(CO)₁₂ than for RuCl₃. High resolution transmission electron microscopic images showed that uniformly distributed smaller RuO₂ particles were formed for the former photocatalyst, which is considered to be responsible for the higher activity. The correlation between the concentration of the O^- radical and photocatalytic activity is established, thus indicating that the O^- radical is proposed to work as a hole site in the photocatalytic reaction, and the ability to separate photoexcited charges reflects the photocatalytic activity order of M₂Ti₆O₁₃.