Understanding and Tuning Charge Dynamics in Carbon Nitride/Cobalt(II) Complex Hybrids for Enhanced Photocatalytic CO2 Reduction

Semiconductor/metal complex hybrids are promising photocatalysts for CO2 reduction. A comprehensive mechanism investigation on charge dynamics was carried out for a hybrid g-C3N4/[Co(bpy)3]2+ (bpy = 2,2′-bipyridine) photocatalytic system by using a combination of in situ UV–vis and resonance Raman spectroscopies, electrochemistry, and spectroelectrochemistry. A singly reduced [Co(bpy)2]+ species binding to CO2 was directly identified as an important intermediate. The excessive accumulation of this transformed [CoIII(bpy)2CO2]+ intermediate indicates that the subsequent CO2 reduction reaction is the main rate-limiting process. Built on these findings, the heterosystem was modified to g-C3N4/[Co(dmbpy)3]2+, where dmbpy = 4,4′-dimethyl-2,2′-bipyridine. The electron affinity of the cobalt complex segment and thereby the specific CO2 binding were enhanced. The refined charge dynamics led to 5.4 times enhanced photocatalytic CO2-to-CO conversion, with a selectivity of over 85% and an apparent quantum yield of 1.96% at 400 nm. This study provides an applicable spectroscopic approach to investigate in-depth charge-transfer characteristics and identify the main rate-limiting process, as well as shows the significance of rational design based on mechanism understanding.