Light-driven ammonia electrooxidation via carbon nitride-ruthenium molecular interfaces

The homogeneous-heterogeneous catalysis gap remains an unresolved challenge in solar fuels research. Molecular catalysts offer unique selectivity and mechanistic transparency but suffer from poor electrode contact and limited recyclability, while heterogeneous semiconductors provide scalable light harvesting but lack precisely defined active sites. Anchoring molecular ruthenium (Ru) catalysts onto heterogeneous semiconductors, like carbon nitride (C₃N₄), offers a chemically rational strategy to bridge this gap, yielding hybrid photoelectrodes capable of driving ammonia oxidation, a reaction of growing importance as a sustainable hydrogen carrier. Inspired by natural photosynthesis, in which a light-harvesting antenna is spatially coupled to a multielectron catalytic centre, the proposed hybrid system assigns distinct and complementary roles to each component: C₃N₄ absorbs visible light, separates charge carriers, and provides a structurally tunable aromatic surface, while the metal complexes, e.g., RuBda, RuTda, or RuTpyBpy, accept photogenerated holes and drive the demanding six-electron oxidation of ammonia through well-defined coordination chemistry. Two anchoring strategies, covalent amide bond formation exploiting the surface amine groups of C₃N₄, and non-covalent π-π and C-H···π interactions mediated by pyrene-functionalized ligands, are presented as complementary rather than competing routes to the heterointerface, each controlling surface density, electronic coupling, and catalyst stability differently. This perspective article examines how the structural diversity of the C₃N₄ allotropes, spanning semicrystalline polymeric C₃N₄, highly ordered poly(heptazine imide), and high-surface-area amorphous sulfur-doped C₃N₄, offers a tunable platform for optimizing charge carrier dynamics at the hybrid interface. Finally, photoelectrocatalysis is the enabling configuration: Simultaneous illumination and electrochemical bias reduce the thermodynamic penalty, suppress charge recombination, and provide independent control over product selectivity. Despite available materials, precedent reactions, and compelling mechanistic rationale, no study to date has reported photoelectrocatalytic ammonia oxidation at a C₃N₄-Ru hybrid photoelectrode, this gap is the motivation and the central argument of this perspective.