Design of Porphyrin-Based 2D Covalent Organic Frameworks and 2D/2D Heterojunctions for Enhanced Photocatalytic Hydrogen Evolution

Abstract Photocatalytic water splitting is a promising approach to sustainable green hydrogen (H2) production by harnessing solar energy. Among emerging photocatalysts, two-dimensional (2D) covalent organic frameworks (COFs) have attracted significant interest because of their crystallinity, stability, and porosity. COFs offer an effective strategy to tune optoelectronic properties by rationally selecting organic linkers, thereby improving water-splitting performance. In this context, porphyrin-based 2D COFs offer an optimal band structure and excellent light-harvesting properties crucial for photocatalytic hydrogen evolution. Herein, we report three porphyrin-based COFs, ZnP-TP, ZnP-BT, and ZnP-TZ, designed by strategically selecting organic linkers. Among them, ZnP-TZ exhibits the highest hydrogen evolution reaction (HER) rate of 34 mmol g–1 h–1. To further enhance HER performance, we fabricated ZnP-TZ COF/g-C3N4 heterojunctions. The optimized heterostructure, g-C3N4/ZnP-TZ (CN-TZ-2) with a 1:2 composition, achieved an HER rate of ∼70 mmol g–1 h–1, twice that of pristine COF. Comprehensive spectroscopic and electrochemical studies revealed a synergistic COF/g-C3N4 interaction that facilitated charge separation and interfacial transfer, suppressed recombination, and enhanced photocatalytic HER. This study highlights the significance of 2D–2D heterostructure materials and provides innovative direction for rational design of efficient photocatalysts for solar-driven hydrogen generation.