Enhanced neutral seawater splitting on less-defective, two-dimensional LaTiO2N photoanodes prepared from layered perovskite BaLa4Ti4O15

Abstract

Perovskite-type LaTiO₂N with a bandgap energy of 2.1 eV is a promising semiconductor for solar (sea)water splitting to produce renewable hydrogen. However, the synthesis of an oxynitride with low defect density to achieve strong charge separation for high solar-to-hydrogen energy conversion is still challenging. Herein, we report less-defective, two-dimensional (2D) LaTiO₂N crystals prepared from layered perovskite BaLa₄Ti₄O₁₅ and their highly improved photoelectrochemical (PEC) seawater-splitting activity at neutral pH. The (111)-type layered perovskite BaLa₄Ti₄O₁₅ has structural similarity to LaTiO₂N and excess Ba species with a strong basicity, thus guaranteeing complete and fast nitridation accompanied by suppressed Ti⁴⁺ reduction. The prepared oxynitride, capable of absorbing visible light up to 610 nm, was highly crystalline and less-defective in both bulk and surface states, which is favorable for efficient photoreactions. Oxynitrides were also prepared from other layered perovskite oxides, namely La₂Ti₂O₇ and La₄Ti₃O₁₂, for comparison. Photoanodes using LaTiO₂N were fabricated by spin-coating and then employed for driving PEC seawater splitting including chloride oxidation at neutral pH. The less-defective LaTiO₂N exhibited a photocurrent density of approximately 2.4 mA cm⁻² at 1.23 V_RHE (4.9 mA cm⁻² at 1.36 V_RHE) in 0.5 M NaCl electrolyte at pH 6.4 under AM 1.5G simulated sunlight, indicating hitherto unreported remarkable activity. This work presents the first demonstration of the complete conversion of layered perovskite BaLa₄Ti₄O₁₅ into perovskite LaTiO₂N as well as the types of starting oxides that reduce the defect density of the resulting oxynitride, thereby improving the hydrogen evolution rate at neutral pH.