A lead-free double perovskite Cs2AgBiBr6/BiOBr S-scheme heterojunction with enhanced Bi–Br–Bi coordination for photocatalytic CO2 reduction

Abstract

Constructing S-scheme heterojunctions is a powerful strategy for enhancing photocatalytic CO₂ reduction (PCO₂R) by enabling directional electron transport and promoting charge separation. Herein, we report a lead-free double perovskite-based heterojunction with unique electron transport capabilities, realized by anchoring 2D Cs₂AgBiBr₆ (CABB) nanosheets onto 2D BiOBr (BOB) nanoflowers through robust Bi–Br bonds to form an intimate 2D/2D interfacial structure. Crucially, this interface is chemically bonded via Bi–Br–Bi linkages, which not only establish direct electron transport channels between the [BiBr₆]³⁻ units of CABB and the [Bi₂O₂]²⁺ units of BOB but also synergistically facilitate the formation of an S-scheme heterojunction with a built-in electric field. Driven by this field, the interfacial structure promotes the directional transfer of photogenerated electrons, significantly enhancing charge carrier separation. Consequently, the optimized BOB/CABB heterojunction exhibits superior PCO₂R performance, achieving a CO production rate of 61.1 μmol g⁻¹ h⁻¹ in a gas–solid system with an exceptionally high electron selectivity approaching 100%. Density functional theory (DFT) calculations further corroborate this mechanism, revealing that the interfacial coupling shifts the LUMO of CABB toward the BOB surface to suppress electron recombination, while also enhancing the adsorption of the *COOH intermediate (ICOHP: 0.81 → 0.92) to stabilize the reaction pathway and lowering the thermodynamic barrier (ΔG: 1.99 eV → 1.88 eV) of the rate-limiting step. This work, therefore, introduces a powerful strategy for utilizing Bi–Br–Bi interfacial linkages to rationally design high-performance S-scheme photocatalysts for CO₂ reduction.