Metastable Layered Oxynitrides for Visible-Light Photocatalysis

Two-dimensional (2D) undoped layered oxynitrides are potential candidates as high performance photocatalysts that work under visible light. However, synthesis of 2D layered oxynitrides is generally difficult because most of them are metastable phases. Nevertheless, our efforts of refining the preparation of oxide precursor and nitridation conditions enabled to synthesize a phase-pure Li₂LaTa₂O₆N with a layered perovskite structure and visible light absorption up to 500 nm. Visible-light-driven CO₂ reduction into formate (<i>λ</i> &gt; 400 nm) proceeded with high selectivity when Li₂LaTa₂O₆N was modified with a binuclear Ru(II) complex, while well-known 3D oxynitride perovskites (e.g., CaTaO₂N and LaTaON₂) did not show functionality. Unfortunately, however, Li₂LaTa₂O₆N is not very stable in aqueous environment, which limits its application as a photocatalyst. Actually, it had been reported that layered oxynitrides have inherent instability in contact with water, accompanied by loss of nitrogen content that finally weakens visible light absorption. An exceptional case is K₂LaTa₂O₆N, which is a Ruddlesden-Popper phase two-layer perovskite analogous to Li₂LaTa₂O₆N and has a band gap of 2.5 eV. This material undergoes <i>in situ</i> H⁺/K⁺exchange in aqueous solution while keeping its visible light absorption capability. The protonated, Ir-modified K₂LaTa₂O₆N (H⁺/Ir/K₂LaTa₂O₆N) exhibited photocatalytic activity for H₂ evolution from aqueous NaI solution under visible light, outperforming Pt/ZrO₂/TaON and Pt/SrTiO₃:Rh, which are one of the best-performing oxynitride and oxide photocatalysts for H₂ evolution, respectively. Z-scheme overall water splitting was accomplished using the H⁺/Ir/K₂LaTa₂O₆N in combination with Cs-modified Pt/WO₃ as an O₂ evolution photocatalyst in the presence of I₃^–/I^–shuttle redox couple.