Regulating a Co₃O₄/ZnO heterojunction aerogel with a built-in electric field for enhanced CO₂ photoreduction to solar fuels from DFT insights

P-N-type heterojunctions have the potential to serve as highly efficient photocatalysts for CO₂ reduction, owing to their remarkable carrier separation efficiency, high stability, and strong redox capacity. In this study, a novel P-N Co₃O₄/ZnO heterojunction aerogel photocatalyst was fabricated through a process starting with the propylene oxide ring-opening-induced gelation technique. The resulting Co₃O₄/ZnO aerogel exhibits an interconnected, hierarchical porous structure, which endows it with a particle diameter size at around several tens of nanometers and a large BET-specific surface area, thereby providing abundant exposed active sites. Under simulated solar spectral conditions, the yields of CH₄ and CO can attain 18 μmol g^-1 h^-1 and 14.4 μmol g^-1 h^-1, respectively, in the absence of any sacrificial agent and photosensitizer. These values are 12.0 times and 5.8 times higher than those of the pristine Co₃O₄ aerogel. Based on density functional theory (DFT) calculations, the activation mechanism of CO₂ on the catalyst surface is illustrated. This is confirmed by the elongated CO bond length of the CO₂ molecule from 1.174 and 1.175 Å to 1.376 and 1.259 Å, respectively, after forming the Co₃O₄/ZnO heterojunction, which is further confirmed by the more negative CO₂ adsorption energy. Further research demonstrates that the built-in electric field formed at the heterojunction interface effectively promotes the recombination of electrons in the conduction band of Co₃O₄ with holes in the valence band of ZnO, significantly enhancing the carrier separation efficiency and thereby boosting the photocatalytic reduction activity of CO₂. This work goes beyond providing new strategies for designing efficient CO₂ reduction photocatalysts, extending its impact to advancing the utilization of aerogel materials in the field of photocatalysis.