Synthesis of Z-scheme cobalt porphyrin/nitrogen-doped graphene quantum dot heterojunctions for efficient molecule-based photocatalytic oxygen evolution

Abstract

Porphyrin-based photocatalytic oxygen evolution (POE) has exhibited great potential in artificial photosynthesis; however, a few reports exist on efficient nanocatalytic systems due to the fast photoinduced charge recombination. Here, cobalt(III) tetraphenylporphyrin (CoP) nanowires (NWs) are first synthesized at a large-scale via a simple chemical reaction, and transformed in situ into zero-dimensional CoP/NGQDs nanocomposites by adding nitrogen-doped graphene quantum dots (NGQDs) as the template and dopant. Under the visible light irradiation, no detectable POE activity of CoP NWs is observed in an acid heterogeneous system, while the currently highest POE rate on the molecule (350 μmol g⁻¹ h⁻¹) is achieved with CoP/NGQDs as the photocatalyst. Systematic experiments reveal that the NGQDs-limiting self-assembly of CoP molecules along the axial direction and subsequent formation of Z-scheme CoP/NGQDs heterojunctions are crucial for efficient POE. The nanostructures lead to enough exposure of the central Co atom as POE active sites and longer recombination lifetime of the photoinduced charge in CoP/NGQDs (87.8 ps) than that in CoP NWs (32.2 ps) due to the generation of (CoP⁺·NGQDs⁻)* exciplex. This work provides a new strategy to use a nature-like semiconductor molecule as a direct catalyst for artificial photosynthesis.