Tailoring Metal-Ion-Doped Carbon Nitrides for Photocatalytic Oxygen Evolution Reaction

Poly(heptazine imides) (PHIs) have emerged as prominent layered carbon nitride-based materials with potential oxygen evolution reaction (OER) catalytic activity owing to their strong VIS light absorption, long excited-state lifetimes, high surface-to-volume ratios, and the possibility of tuning their properties via hosting different metal ions in their pores. A series of metal-ion-doped PHI-M (M = K+, Rb+, Mg2+, Zn2+, Mn2+, and Co2+) were first systematically explored using density functional theory calculations. These simulations led an in-depth understanding of the microscopic OER mechanism in these systems and identified PHI-Co2+ as the best OER catalyst of this family of PHIs, whereas PHI-Mn2+ can be an alternative promising OER catalyst. This level of performance was attributed to a thermodynamically favorable formation of the reaction intermediates as well as its red-shifted absorption in the VIS region involving the population of long-lived states, as revealed by time-dependent density functional theory calculations. We further demonstrated that the electronic properties of the *OH intermediates (Bader population, crystal orbital Hamilton population analysis, and adsorption energies) are reliable descriptors to anticipate the OER activity of this family of PHIs. This rational analysis paved the way toward the prediction of the OER performance of another PHI-M derivative, i.e., PHI-Fe2+. The computationally explored PHI-Fe2+, PHI-Mn2+, and PHI-Co2+ systems were then synthesized alongside PHI-K+, and their photocatalytic OER activities were assessed. These experimental findings confirmed the best photocatalytic OER performance for PHI-Co2+ with an oxygen production of 31.2 μmol·h–1 that is 60 times higher than the pristine g-C3N4 (0.5 μmol·h–1), whereas PHI-Fe2+ and PHI-Mn2+ are seen as alternative OER catalysts with attractive oxygen production of 11.20 and 4.69 μmol·h–1, respectively. Decisively, this joint experimental–computational study reveals PHI-Co2+ to be among the best of the OER catalysts so far reported in the literature including some perovskites.