Constructing a biomimetic TiOF₂@PCN-222-Fe Z-scheme heterojunction using self-assembled <i>L</i>-cysteine for CO₂ visible light photoreduction

Biomimetic synthesis <i>via</i> MOF-based catalysts to construct a Z-scheme heterojunction provides opportunities for enhancement of visible light photocatalytic CO₂ reduction efficiency. Herein, a TiOF₂@PCN-222-Fe heterojunction was designed using the <i>L</i>-cysteine agglomerant to simulate the structure of cytochrome <i>c</i> oxidase. <i>L</i>-cysteine axially coordinated with Fe³⁺ in the ferroporphyrin moiety of PCN-222-Fe through the -SH group and coordinated with Ti⁴⁺ in TiOF₂ through the -COOH group. This heterojunction enhances photocurrent and conductivity, thus improving CO₂ visible light photocatalytic activity. Various characterization methods including EXAFS and theoretical calculations demonstrated that the band bending of TiOF₂ and PCN-222-Fe occurred through <i>L</i>-cysteine coordination and a Z-scheme heterojunction was formed to efficiently separate photogenerated carriers. Thus, TiOF₂ accumulates electrons and acts as the active center of CO₂ reduction, PCN-222-Fe accumulates holes and acts as the active center of water oxidation, and <i>L</i>-cysteine provides the transmission pathway of protons. The introduction of F element in TiOF₂ lowered the valence band, which produced energetic holes that transferred to the valence band of PCN-222-Fe for water oxidation. This strategy provides unique insights into improving the efficiency of CO₂ visible light photoreduction by biomimetic design of Z-scheme heterojunctions.