Ultrathin Cobalt Phthalocyanine/Poly(heptazine imide) Heterojunctions with Co‐N4 Sites for Improved Photocatalytic CO2 Reduction and Electron Kinetics

Ultrathin PHI nanosheets are controllably modified with CoPc for CO₂ photoreduction. The closely contacted interfaces, induced by π–π assembly and electrostatic interaction, endow CoPc/PHI heterojunctions with rapid charge transfer and separation. CoPc, as both electron-accepting platform and catalytic site, contributes to high CO evolution rate (116 µmol g⁻¹ h⁻¹) and superior selectivity (97%) in water without any sacrificial agents.

Abstract

Poly(heptazine imide) (PHI), an emerging substitute for g-C₃N₄ (CN), is a good candidate towards photocatalytic CO₂ reduction, while it still suffers from weak charge separation and low efficiency of electron-induced reduction reaction. Herein, ultrathin PHI nanosheets are synthesized through molten salt method with CN precursors, and subsequently functionalized by assembling cobalt phthalocyanine (CoPc) aggregates via π–π interaction. The optimized CoPc/PHI heterojunction achieves a CO evolution rate of 116 µmol g⁻¹ h⁻¹ with 97% selectivity, exhibiting ≈23 and 15-fold photoactivity improvement compared to CN and PHI, respectively. Experimental and theoretical results reveal that the superior photocatalytic performance is primarily attributed to the photogenerated electrons transfer from PHI to the ligand of CoPc for greatly enhancing charge separation, and then to the single Co-N₄ sites for efficiently catalyzing CO₂ conversion. The high selectivity is derived from the low formation energy barrier of *COOH and rapid CO desorption. The electron transfer efficiency for CO₂ reduction on CoPc/PHI is quantified to be 39.7% by in situ µs-transient absorption spectra, much higher than that of PHI (17.7%), underlining the dual role of CoPc aggregates as electron-accepting platform and catalytic site. This work offers a feasible strategy for designing efficient heterojunctions towards solar fuel production.