Nitrogen/fluorine-codoped rutile titania as a stable oxygen-evolution photocatalyst for solar-driven Z-scheme water splitting

Nitrogen/fluorine-codoped rutile TiO2 (R-TiO2:N,F) was newly synthesized, and its photocatalytic activity for water oxidation was evaluated. R-TiO2:N,F could be prepared by nitridation of the rutile TiO2 (R-TiO2) and (NH4)2TiF6 mixture at 773 K. The prepared samples produced O2 from aqueous AgNO3 solution under visible light irradiation, while R-TiO2 nitrided at the same temperature without any fluorine source showed negligible activity. The highest activity was obtained with the sample prepared at the (NH4)2TiF6/R-TiO2 ratio of 15/85, exhibiting water oxidation activity even in the presence of a reversible electron acceptor such as IO3− or Fe3+ with the aid of a RuO2 cocatalyst. Stoichiometric water splitting into H2 and O2 was achieved using a mixture of Ru/SrTiO3:Rh and RuO2/TiO2:N,F in the presence of [Co(bpy)3]3+/2+ (bpy = 2,2′-bipyridine) as a shuttle redox mediator without noticeable degradation of activity under visible light and even under AM1.5G simulated sunlight. Transient absorption spectroscopy revealed that appropriate nitrogen/fluorine codoping reduces the density of mid-gap states working as deep traps of photogenerated electrons, and increases the number of free electrons compared to only nitrogen-doped R-TiO2. Experimental results highlighted that the photocatalytic activity of R-TiO2:N,F could be enhanced by improving visible-light absorption capability through N/F codoping while suppressing the density of deep trap sites.