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An in situ reversible assembly strategy is demonstrated for the fabrication of p-n heterojunctions in BiOI/Bi 5 O 7 I composites. This approach allows for the generation of ample interfacial interactions and built-in electric field, resulting in a strong driving force for effective charge separation. These advantages drastically enhance the photocatalytic activity of BiOI/Bi 5 O 7 I composites towards efficient CO 2 reduction and pollutants degradation using visible light. [Display omitted] • In-situ reversible assembly strategy to fabricate BiOI/Bi 5 O 7 I p–n heterojunctions. • BiOI/Bi 5 O 7 I p–n heterojunctions provide ample interfacial interaction. • These advantages enable efficient visible-light photocatalysis for CO 2 reduction. • Theoretical and experimental studies reveal insights on interfacial charge transfer. Constructing efficient p-n heterojunctions holds great potential in the realm of photocatalysis for promoting the sustainable development of the environment and energy industries. However, traditional p-n heterojunctions suffer from limited interfacial interaction which severely restricts the effectiveness of the built-in electric field in boosting charge separation. Herein, we present an in situ reversible assembly approach to fabricate functional p-n junction in BiOI/Bi 5 O 7 I composites at room temperature. By controlling the reaction time or the amount of KI precursor added, we attain the as-designed BiOI@Bi 5 O 7 I and Bi 5 O 7 I@BiOI heterojunctions with large specific surface area and ample interfacial electric field. Subsequent application of the optimized heterojunction material as visible-light photocatalyst achieves efficient photoreduction of CO 2 reactant into CO product (0.46 μmol g−1h−1), even without using any sacrificial agent in the gas–solid reaction system. This catalytic performance is notably ∼ 6.6 times and 15.3 times higher than that of standalone BiOI and Bi 5 O 7 I materials, respectively. In situ XPS, in situ Kelvin probe force microscopy, and theoretical calculations reveal that the built-in electric field induces directional charge transfer to greatly boosts the efficiency of separating photogenerated electron-hole pairs at the interface of BiOI and Bi 5 O 7 I. This study provides valuable insights for the rational design and straightforward fabrication of next-generation, integrated heterostructured photoelectronic materials for efficient energy, environmental, and chemical applications. [ABSTRACT FROM AUTHOR]