Improved CO₂ Photoreduction Activity and Selectivity Using an Indirect Z‑Scheme Heterojunction of ZIF-67 and Bi₄O₅Br₂ Nanostructures

Developing highly efficient photocatalysts for CO₂ reduction is critical for addressing global warming and energy supply issues. However, both single-component metal-organic frameworks (MOFs) and conventional semiconductor catalysts face significant challenges, such as severe charge recombination, poor CO₂ adsorption capacity, and low utilization of solar energy in the photocatalytic reduction of CO₂. To address these limitations, the formation of Z-scheme heterojunction photocatalyst has been proved to be an effective way to suppress the recombination of photoinduced carriers. In this work, an indirect Z-scheme heterojunction photocatalytic system, namely, Cu-doped-ZIF-67Co/Bi₄O₅Br₂, is prepared by means of ion-doping and in situ growth strategy. Electrochemical characterization demonstrated the formation of p-type ZIF-67CoCu nanocubes and n-type Bi₄O₅Br₂ nanosheets. The indirect Z-scheme band structure of the heterojunction was well-defined by X-ray photoelectron spectroscopy (XPS), Mott–Schottky test, and ultraviolet photoelectron spectroscopy (UV). The optimal Cu-doping ratio (molar ratio) and composite ratio (mass ratio): 1% ZIF-67CoCu(1:1)/Bi₄O₅Br₂ exhibits excellent photocatalytic CO₂ reduction performance, a remarkable rate of 6469.88 μmol·g^–1·h^–1, and a high CO selectivity of 97%. Compared to individual ZIF-67CoCu and Bi₄O₅Br₂, the performance of reduction of CO₂ to CO is improved by 4.19 and 8.64 times, respectively. Density functional theory calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy results had shown that the existence of Cu in ZIF-67 facilitates the desorption of CO and the reduction process features the key CO* intermediate during the photoreduction of CO₂ to CO on 1% ZIF-67CoCu/Bi₄O₅Br₂. This study provides an exciting example for the design and understanding of the ion-doped MOFs and the semiconductor construction of indirect Z-scheme photocatalytic system for promoting photocatalytic capacity.