Synthesis of mixed-valence Cu phthalocyanine/graphene/g-C3N4 ultrathin heterojunctions as efficient photocatalysts for CO2 reduction

Abstract

Although g-C₃N₄-based heterojunctions show promise for photocatalytic CO₂ reduction, enhancing charge separation and creating sufficient catalytic sites remain challenges. In this study, mixed-valence CuPc/graphene/g-C₃N₄ (H_T-CuPc/G/UCN) ultrathin heterojunctions (∼4.8 nm thickness) with Cu¹⁺/Cu²⁺ species were constructed by H-bond induced assembly and subsequent post-treatment with H₂. The optimized H₁₀₀-CuPc/G/UCN sample showed 36-times higher activity than bare ultrathin g-C₃N₄ (UCN) for photocatalytic CO₂ reduction to CO. Based on atomic force microscopy images, normalized photocurrent action spectra, electron paramagnetic resonances, and in situ diffuse reflectance infrared Fourier transform spectra, it was confirmed that the unexpected photocatalytic activity of H₁₀₀-CuPc/G/UCN was attributed to the dual-functional roles of the introduced graphene: to increase the optimized amount of CuPc and enhance the high-level-energy electrons transferring from UCN to CuPc and the created Cu¹⁺/Cu²⁺ species by H₂ reduction as active catalytic sites for CO₂ reduction. This work provides a comprehensively feasible strategy for preparing rational heterojunctions toward high-efficiency solar-driven CO₂ reduction.