Self‐Confined Construction of Facet Heterojunction with Tunable Band Alignment for Enhanced Photocatalytic CO2 Reduction

Abstract Although crystal facet engineering is extensively studied in energy and environmental technologies, the in‐depth understanding of intrinsic mechanisms governing crystal facet heterojunctions in tuning band alignments is still limited. Here, novel Bi 5 O 7 NO 3 crystals exposing tailor 080 facets are synthesized via NH 4 + ‐assisted self‐confined construction. It has been confirmed that NH 4 + ions selectively adsorb on the 141 facets, inversely inducing the growth of desired 080 crystal facets, while the tailored 080 facets facilitate the generation of oxygen vacancies. The controllable concentration of oxygen vacancies, influenced by different exposed facets, can optimize the relative positions of Fermi levels and shift the photoelectron transfer route between the 141 and 080‐OV facets from type‐II to S‐scheme, thus triggering rapid charge transport channels and effectively suppressing electron‐hole recombination. DFT calculation verifies that the energy barrier for the *COOH formation on Bi 5 O 7 NO 3 ‐080‐OV is the lowest, thereby promoting the generation of CO. The well‐designed Bi 5 O 7 NO 3 crystals with the optimal 080/141 facet ratio exhibit a 3. 8‐fold enhancement in photocatalytic CO 2 reduction, compared to traditional Bi 5 O 7 NO 3 dominated by the 141 facets. This work offers new insights into the regulation of band alignments at heterointerfaces and the design of high‐efficiency photocatalysts.