S-Scheme Co₉S₈@Cd_0.8Zn_0.2S-DETA Hierarchical Nanocages Bearing Organic CO₂ Activators for Photocatalytic Syngas Production

Delicate modulations of CO₂ activation and charge carrier separation/migration are challenging, yet imperative to augment CO₂ photoreduction efficiency. Herein, by supporting diethylenetriamine (DETA)-functionalized Cd_0.8Zn_0.2S nanowires on the exterior surface of hollow Co₉S₈ polyhedrons, hierarchical Co₉S₈@Cd_0.8Zn_0.2S-DETA nanocages are fabricated as an S-scheme photocatalyst for reducing CO₂ and protons to produce syngas (CO and H₂). The amine groups strengthen adsorption and activation of CO₂, while the "nanowire-on-nanocage" hierarchical hollow heterostructure with an S-scheme interface boosts separation and transfer of photoinduced charges. Employing Co(bpy)₃²⁺ as a cocatalyst, the optimal photocatalyst effectively produces CO and H₂ in rates of 70.6 and 18.6 μmu mol h^-1 (i.e., 4673 and 1240 μmu mol g^-1 h^-1), respectively, affording an apparent quantum efficiency of 9.45% at 420 nm, which is the highest value under comparable conditions. Ultraviolet photoelectron spectroscopy, Kelvin probe, and electron spin resonance confirm the S-schematic charge-transfer process in the photocatalyst. The key COOH^* species responsible for CO₂-to-CO reduction is detected by in-situ diffuse reflectance infrared Fourier transform spectroscopy and endorsed by density functional theory calculations, and thus a possible CO₂ reduction mechanism is proposed. © 2023 Wiley-VCH GmbH