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

Abstract

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.