Synergistic effects of heterointerface and surface O vacancies in S-scheme BiO2-x/QL-COF heterojunction for efficient charge transfer: mechanism exploration and DFT calculations

Covalent organic framework (COF) has great potential in the application of photocatalysis, but it still has the defect of easy recombination of photogenerated carriers. Herein, QL-COF was synthesized by carboxylation modification of LZU1 (Synthesis of the COF linked by 4-carboxyquinoline via the Doebner reaction (QL-COF).), and combined with BiO_2-x with oxygen vacancy to construct a novel S-scheme heterojunction BiO_2-x/QL-COF(BQ) by solvothermal method. Furthermore, its charge transfer mechanism has been demonstrated through Density Functional Theory (DFT) calculations. The photocatalytic performance of heterojunction BiO_2-x/QL-COF was evaluated by pollutant degradation, hydrogen production and CO₂ reduction. It was found the degradation rate of RhB over BQ-3 could reach almost 90 %. Moreover, the photocatalytic H₂ evolution rate of BQ-3 was up to 438.7 μmol g⁻¹ h⁻¹, which is 7.6 times and 21 times that of QL-COF and BiO_2-x, respectively. And the evolution rate of photoreduction CO₂ to CO was 686.3 μmol g⁻¹ h⁻¹. The enhancement of its photocatalytic performance may be mainly attributed to: (i) The construction of heterojunction improves the absorption capacity of heterojunction BQ to visible light; (ii) Oxygen vacancies form defect centers to trap photogenerated charges, which inhibit photogenerated carriers recombination, and provide more active sites for CO₂ adsorption; (iii) The electron-withdrawing effect of the carboxyl groups, which can effectively promote the migration of photogenerated carriers.