Built-in electric fields in asymmetric carbon nitride nanotubes for improved solar biomass valorization and hydrogen generation

Abstract

Metal-free photocatalytic oxidation of 5-hydroxymethylfurfural (HMF) coupled with hydrogen (H2) evolution is a promising route for biomass valorization, yet it remains constrained by rapid charge recombination and low selectivity. Here, asymmetric porous carbon nitride nanotubes co-doped with pyrimidine and sulfur are constructed via one-step pyrocondensation to establish an intramolecular donor-acceptor framework. Pyrimidine-doped domains act as electron acceptors, while pristine and sulfur-doped heptazine units serve as electron donors, generating a built-in electric field that enforces directional charge routing. As a result, photogenerated electrons are funneled to pyrimidine-derived N sites for proton reduction, whereas holes are localized at sulfur-doped sites to drive selective Cα-H activation of HMF. This sitedecoupled redox process delivers high anaerobic performance, achieving DFF and H2 production rates of 431 and 446 μmol g -1 h -1 with 95% DFF selectivity. Beyond model conditions, the catalyst operates efficiently under unconcentrated natural sunlight and across a range of biomass-derived alcohols, establishing a general metal-free strategy for solar biomass valorization.