Cu₉S₅/Gel-Derived TiO₂ Composites for Efficient CO₂ Adsorption and Conversion

Engineering phase-selective gel composites presents a promising route to enhance both CO₂ adsorption and conversion efficiency in photocatalytic systems. In this work, Cu₉S₅/TiO₂ gel composites were synthesized via a hydrazine-hydrate-assisted hydrothermal method, using TiO₂ derived from a microwave-assisted sol–gel process. The resulting materials exhibit a porous gel-derived morphology with highly dispersed Cu₉S₅ nanocrystals, as confirmed by XRD, TEM, and XPS analyses. These structural features promote abundant surface-active sites and interfacial contact, enabling efficient CO₂ adsorption. Among all samples, the optimized 0.36Cu₉S₅/TiO₂ composite achieved a methane production rate of 34 μmol·g⁻¹·h⁻¹, with 64.76% CH₄ selectivity and 88.02% electron-based selectivity, significantly outperforming Cu₉S₈/TiO₂ synthesized without hydrazine hydrate. This enhancement is attributed to the dual role of hydrazine: facilitating phase transformation from Cu₉S₈ to Cu₉S₅ and modulating the interfacial electronic environment to favor CO₂ capture and activation. DFT calculations reveal that Cu₉S₅/TiO₂ effectively lowers the energy barriers of critical intermediates (*COOH, *CO, and *CHO), enhancing both CO₂ adsorption strength and subsequent conversion to methane. This work demonstrates a gel-derived composite strategy that couples efficient CO₂ adsorption with selective photocatalytic reduction, offering new design principles for adsorption–conversion hybrid materials.