Design and synthesis of a covalent organic framework bridging CdS nanoparticles and a homogeneous cobalt-bipyridine cocatalyst for a highly efficient photocatalytic CO2 reduction

The design and construction of highly efficient photocatalytic CO2 conversion systems are extremely desirable for technological, practical, and economic viability. In this study, a 2,2 '-bipyridine (bpy)-based ketoenamine covalent organic framework (TpBpy; Tp: 1,3,5-triformylphloroglucinol), which can be prepared on a large scale by a facile and environmentally friendly hydrothermal method, was used to promote CO2 photoreduction processes by bridging heterogeneous CdS nanoparticles and a homogeneous [Co(bpy)(3)](2+) cocatalyst. The bpy units played multiple roles in the preparation of TpBpy, formation of strong interactions with CdS, and accommodation of the cocatalyst. In the CO2 reduction process, due to the flexible association/dissociation between the bpy ligand and the cocatalyst, the active [Co(bpy)(x)](+) may separate from heterogeneous CdS/TpBpy and make spaces for other unactive [Co(bpy)(3)](2+) species, thereby maintaining the intrinsic high activity and selectivity of the [Co(bpy)(3)](2+) cocatalyst. The combination of CdS, TpBpy, and [Co(bpy)(3)](2+) shows a strong solar light harvesting ability, a high surface area, a high CO2 adsorption capacity, highly efficient charge carrier transfer at the interface between CdS and TpBpy, and subsequent rapid photoelectron injection into the [Co(bpy)(3)](2+) cocatalyst. These synergistic effects lead to a robust CO production rate of 35.2 mmol g(-1) with 85.0% selectivity over the first four hours of the reaction. Moreover, the quantum efficiencies (AQEs) of the reaction system, with 2 mg of CdS/TpBpy-20%, are 4.75 and 3.65% at 400 and 450 nm, respectively. Finally, the possible mechanism of the photocatalytic CO2 conversion over CdS/TpBpy is proposed and discussed here. This study on the heterostructure and photocatalytic system design might serve as a model for the development of solar-driven CO2 reduction.