L-cysteine-bridged Bi₃TiNbO_8.5F@COF-2CN heterojunction for selective photocatalytic CO₂-to-HCOOH conversion

Engineering chemically coupled organic-inorganic heterointerfaces is an effective strategy for promoting directional charge transfer in photocatalytic CO₂ reduction. However, the construction of stable and well-defined interfaces between covalent organic frameworks (COFs) and inorganic semiconductors remains challenging. Herein, we report an L-cysteine-bridged Bi₃TiNbO_8.5F@COF-2CN heterojunction for selective photocatalytic CO₂-to-HCOOH conversion. In this design, L-cysteine acts as an interfacial molecular bridge, anchoring Bi₃TiNbO_8.5F (BNT-F) through BiS coordination bonds while coupling with the carbonyl groups of COF-2CN through hydrogen bonding interactions. Such chemically bridged interface strengthens the electronic coupling between the organic and inorganic components and provides an efficient pathway for interfacial charge migration. Moreover, cyano functionalization and fluorination synergistically optimize the band structure and promote the separation of photogenerated carriers. Driven by the favorable band alignment and built-in interfacial electric field, photogenerated electrons are directed toward BNT-F as the CO₂ reduction center, while holes migrate toward COF-2CN for the oxidation half-reaction. As a result, the optimized BNT-F@COF-2CN heterojunction achieves a HCOOH production rate of 314.7 ± 19.4 μmol g_cat^-1 h^-1 with a selectivity of 96.0%, significantly outperforming the individual components. This work provides a new L-cysteine-mediated molecular bridging strategy for engineering COF-based heterointerfaces, offering a promising approach for efficient and selective photocatalytic CO₂ conversion to HCOOH.