Poly(heptazine imide) (PHI) is a semi-crystalline carbon nitride, consisting of layered, graphene-like sheets built from heptazine units interconnected through negatively charged imide linkers. This arrangement yields a periodic 2D framework with well-defined ionic pores and long-range structural order. Owing to their crystallinity and ionic character, PHI materials display enhanced charge mobility, chemical robustness, non-toxicity, low cost, and visible-light absorption, while also serving as ideal hosts for stabilizing isolated metal atoms. As such, they have emerged as highly promising semiconductors for energy conversion, artificial photosynthesis1, photodiodes2, microswimmers3, and neuromorphic devices such as photomemristors4.

Given the global demand for sustainable energy solutions, extensive research has been devoted to PHI-based photocatalysts for H2 evolution5. Achieving high H2 production efficiencies, however, requires not only structural refinement but also rational engineering of photogenerated charge carriers. Among the most effective approaches is the introduction of electron-rich heteroelements, such as sulfur or phosphorus, into the PHI lattice, aiming at to narrow the band gap and improve charge-carrier dynamics6-8. Although such strategies have been explored in amorphous carbon nitrides, with only partial success due to their intrinsic structural disorder, no study to date has demonstrated the effective incorporation of heteroelements into the crystalline PHI framework.

We propose a controlled sulfur-doping strategy for PHI aimed at preserving the original crystalline framework and chemical structure while narrowing the band gap and extending the absorption edge toward lower-energy. The approach relies on introducing thioacetamide as the sulfur source during polymerization, together with urea as the primary precursor for the heptazine-based backbone, and a eutectic NaCl/KCl mixture functioning as a structural directing medium to induce PHI crystallization. A series of four materials was prepared: one pristine PHI (S-free) and three sulfur-containing analogues with progressively increasing thioacetamide content. This systematic series allows direct evaluation of how incremental sulfur introduction affects polymerization, dopant incorporation, and optoelectronic properties.

Sulfur incorporation was confirmed by elemental analysis and EDX, which collectively indicate substitution of nitrogen atoms within the PHI lattice by sulfur. This is evidenced by a progressive decrease in nitrogen content concomitant with an increase in sulfur as the thioacetamide amount is raised, suggesting that the heteroatom replace the N in the lattice. The results also reveal an intrinsic upper limit for sulfur incorporation of approximately 0.4 wt%. From a structural standpoint, PXRD, FT-IR, and ss-NMR data show that S-doping levels up to ~0.2 wt% preserve the characteristic semi-crystalline PHI framework. Beyond this threshold, higher sulfur loadings induce increasing amorphous character, with the materials gradually approaching the structural features of melon-like carbon nitride. These observations demonstrate that sulfur incorporation into crystalline PHI is feasible but constrained to a narrow compositional window before long-range order is compromised.

Optoelectronically, S-incorporation exhibit a progressive emergence of Urbach tails, indicative of localized electronic states introduced by structural disorder, likely associated with regions of incomplete condensation containing –NH2 or –SH terminations. Consistently, the optical band gap decreases following the same trend, from 2.74 eV in pristine PHI to 2.02 eV in the most heavily doped sample, enabling absorption close to the red-light region.

All materials were subsequently evaluated for photocatalytic H2 evolution under different irradiance conditions and wavelengths. The highest performance was achieved by the NaK-PHI 1%S sample, reaching approximately 10000 µmol g-1 h-1 under 410 nm irradiation. Moreover, this material sustained activity under green light, producing ~1700 µmol g-1 h-1, an ability not observed for the pristine PHI. Samples with higher sulfur loadings exhibited a progressive decrease in activity, correlating with their reduced crystallinity and increasing similarity to amorphous carbon nitride. Nevertheless, even these more disordered materials maintained appreciable H2 evolution under green-light irradiation.