<p>Halide perovskite-based heterojunctions have emerged as promising candidates for solar energy conversion and storage due to their unique photophysical properties. However, the current bottleneck lies in the insufficient separation of photogenerated carriers at the interface, primarily due to challenges in the controllable growth of perovskite on the substrate. Herein, we present a growth strategy for depositing lead-free Cs₃Sb₂Br₉ perovskite nanocrystals onto the surface of Co₃O₄ with the assistance of polyacrylic acid (PAA), generating a step-scheme (S-scheme) heterojunction denoted as Co₃O₄–Cs₃Sb₂Br₉. The utilization of PAA as a template can effectively regulate the nucleation and growth of Cs₃Sb₂Br₉, thereby significantly enhancing the charge separation efficiency of the Co₃O₄–Cs₃Sb₂Br₉ heterojunction compared to its counterpart formed without PAA assistance. Under simulated solar light irradiation (100 mW·cm⁻²), the cerium-doped Co₃O₄–Cs₃Sb₂Br₉ heterojunction exhibits excellent photocatalytic CO₂ reduction activity without the need for any sacrificial agent. Specifically, the CO yield reaches up to 700.7&#xa0;μmol·g⁻¹·h⁻¹, marking a 2.8-fold increase over the sample synthesized without PAA mediation. This polymer-assisted in-situ growth strategy should open up a new avenue for designing and developing more efficient photocatalytic materials based on halide perovskites.</p> Graphical abstract <p></p>