Syntheses of asymmetric zinc phthalocyanines as sensitizer of Pt-loaded graphitic carbon nitride for efficient visible/near-IR-light-driven H2 production

Abstract

Zinc phthalocyanine (ZnPc) derivatives with asymmetric (Zn-tri-PcNc-2) or symmetric (Zn-tetrad-Nc) structure, which possess wide spectral response in the visible/near-IR light region, are synthesized and utilized as a sensitizer of graphitic carbon nitride (g-C₃N₄) with 0.5 wt% Pt-loading for photocatalytic H₂ production. The experimental results indicate that Zn-tri-PcNc-2 exhibits much better photosensitization on g-C₃N₄ than Zn-tetrad-Nc under visible/near-IR light although Zn-tetrad-Nc possesses wider and stronger optical absorption property than Zn-tri-PcNc-2. Zn-tri-PcNc-2-Pt/g-C₃N₄ exhibits an average H₂ production rate of 132 μmol h⁻¹, which is much better than that (26.1 μmol h⁻¹) of Zn-tetrad-Nc-Pt/g-C₃N₄ under visible-light (λ ≥ 500 nm) irradiation. Moreover, Zn-tri-PcNc-2-Pt/g-C₃N₄ also shows much higher apparent quantum yield (AQY) than Zn-tetrad-Nc-Pt/g-C₃N₄ under red/near-IR light irradiation. Especially, Zn-tri-PcNc-2-Pt/g-C₃N₄ exhibits impressively higher AQY (1.07%) than that (0.22%) of the Zn-tetrad-Nc-Pt/g-C₃N₄ under 700 nm monochromatic light irradiation. The much better photoactivity of Zn-tri-PcNc-2-Pt/g-C₃N₄ than Zn-tetrad-Nc-Pt/g-C₃N₄ is caused by the asymmetric structure of Zn-tri-PcNc-2, which can result in the electronic orbital directionality of its excited state, much faster photogenerated electron transfer to g-C₃N₄, and higher red/near-IR light utilization efficiency as compared to Zn-tetrad-Nc-Pt/g-C₃N₄. The present results provide an important insight into the effects of molecular structure and optical absorption property of phthalocyanine derivatives on the photoactivity of the dye-sensitized semiconductor, and also guide us to further improve the solar energy conversion efficiency by optimizing the molecular structure and effectively utilizing the visible/near-IR light of sunlight.