Hierarchical Co₉S₈@In_2.77S₄ Heterojunction for Efficient Photocatalytic Reduction of CO₂ to Syngas

Photocatalytic reduction of CO₂ to solar fuels is recognized as a promising route to address environmental and energy issues. However, there exist two challenges of insufficient CO₂ activation and fast charge carrier recombination, impeding this conversion. Herein, a hierarchical Co₉S₈@In_2.77S₄ (CoS@InS) heterojunction is developed by the in situ growth of the In_2.77S₄ nanosheets on the Co₉S₈ nanotubes for efficient photocatalytic reduction of CO₂ to syngas in an aqueous reaction system with [Ru(bpy)₃]Cl₂ serving as a photosensitizer and triethanolamine as a sacrificial agent. In addition to the promoted charge separation and transfer, the strong interfacial electric field formed in this heterojunction tunes the p-band center of In active sites toward the Fermi level. Accordingly, the adsorption of the key intermediate *COOH is enhanced, and the energy barrier of *CO desorption is reduced. Besides, the hierarchical hollow structure enhances light utilization and mass transfer, increases the specific surface area, and provides abundant reaction sites. As a result, the hierarchical CoS@InS heterojunction exhibits superior activity. The optimized heterojunction yields CO and H₂ production rates as high as 83,648 and 28,635 μmol g^-1 h^-1, respectively, with an apparent quantum yield of 5.60% at 450 nm.