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

Abstract

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.