Heptazine-based carbon nitrides are a class of transition metal free semiconductors, which are extensively studied in various photocatalytic processes. The localized nature of the excitons, incomplete exciton conversion into the charge-separated state and the recombination of the charge carriers are the factors that limit performance of the pristine heptazine-based carbon nitrides. Herein, we design heterojunctions composed of poly(triazine imide) intercalated by LiCl and poly(heptazine imide) phases. The donor-acceptor character of the heterojunctions improves the charge separation yield. The heterojunction with the optimal composition of the phases shows an apparent quantum yield of HO formation of 14 % at 365 nm and 7 % at 465 nm, while the HO production rate reaches 7.8 mol L g h (39 mmol L h produced by 5 mg of photocatalyst) in batch and 15.9 mol kg h in flow using a packed-bed photoreactor. The heterojunction undergoes photocharging under anaerobic conditions. The charges stored in the material after irradiation enable O reduction to HO in the dark with the rate 367 µmol g h (1.8 µmol h stored in 5 mg of photocatalyst).

This work was part of the Solar2Chem ITN project funded by the EU. Financial support by the Ministry of Science and Innovation (Severo Ochoa CEX2021–001230-S and RTI2018–89237-CO2–1, both funded by MCIN/AEI/10.13039/501100011033) and Generalitat Valenciana (Prometeo 2021–38) is gratefully acknowledged. This study forms part of the Advanced Materials programme Graphica MFA/2022/023 and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana. J.A. thanks the Spanish Ministry of Science and Innovation for a Ramon y Cajal research associate contract (RYC2021–031006-I funded by MCIN/AEI/10.13039/501100011033 and by “European Union NextGenerationEU/PRTR), and the financial support (PID2022-141099OA-I00 funded by MCIN/AEI/10.13039/501100011033 and, by “European Union NextGenerationEU/PRTR). We gratefully acknowledge The Max Planck Society for financial support.

Peer reviewed