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.