Impact of Post-Annealing on the Water Splitting Performance of Polymeric Carbon Nitride: The Role of Hydrogen Bonds

Post-annealing treatments constitute a simple and cost-effective strategy to tailor the structure and photocatalytic performance of polymeric carbon nitride (PCN). In this work, PCNs synthesized from melamine and urea were subjected to post-annealing at 580 °C under air and CO2 atmospheres to elucidate the role of hydrogen bonding, as well as other structural modifications induced by oxidizing atmospheres, on photocatalytic water splitting. Comprehensive structural, chemical, and textural characterization (XRD, FTIR spectroscopy, XPS, SSNMR, HRTEM, BET, TGA, and UV–Vis DRS) reveals that post-annealing induces markedly different effects depending on the precursor. For melamine-derived PCN, the treatment selectively disrupts hydrogen bonds between melon strands without introducing nitrogen vacancies, amorphization, or framework shortening. This structural rearrangement increases surface area, reduces particle size, slightly widens the band gap, and enhances water–framework interactions, resulting in a twofold improvement in the hydrogen evolution rate (HER), reaching ~3300 µmol h−1 g·cat−1 under visible-light irradiation. In contrast, urea-derived PCN undergoes only minor structural modifications, including slight exfoliation and possible nitrogen deficiency, which do not translate into a measurable enhancement of photocatalytic activity. These results demonstrate that selective hydrogen-bond disruption is a key factor governing charge transport and photocatalytic efficiency in PCN. Importantly, the optimized melamine-derived PCN achieves HER values comparable to those of urea-derived PCN while maintaining a substantially higher synthesis yield, highlighting its potential for scalable solar hydrogen production.