Carbon nitride monolayer nanosheets: astrochemical insights into the fate of interstellar hydrogen

Ubiquitously found in the Universe, atomic hydrogen represents up to 70% of the neutral gas composition of the Milky Way. As an adatom, hydrogen can physisorb or chemisorb onto interstellar dust grains and icy mantles, thereby contributing to the formation of H₂ and, potentially, to the synthesis of more complex hydrogenated species. In addition, structures with relatively large specific surface areas - such as silicates, amorphous carbon, graphene sheets, or water ice - host heterogeneous chemistry that is thought to facilitate the emergence of complex organic matter in astrophysical environments. Although the fundamental physical and chemical processes occurring at dust/gas interfaces are well characterized, current understanding of dust properties governing the formation of H₂ and complex molecules remains incomplete. In this context, we introduce graphitic-like two-dimensional carbon nitride monolayer structures (2D-CN) as a putative molecular family of potential relevance to astro-chemistry. The physicochemical and electronic properties of these materials have been extensively examined in recent years for industrial and technological applications. Here, we propose that their importance may likewise extend to interstellar and circumstellar environments. To explore this possibility, we employed density functional theory (DFT) calculations to investigate the characteristics and extent of H adsorption onto C₂N₁, C₃N₁, C₃N₂, C₃N₄, C₄N₃, C₆N₆, C₆N₈, C₉N₄, and C₉N₇ monolayer nanosheets. We identify multiple adsorption sites over C-C bonds, above C and N atoms, and hollow (macropore) locations at which energetically favorable binding of atomic hydrogen could occur in the interstellar medium (ISM). From an astrochemical perspective, these 2D-CN structures, if formed, could therefore contribute to the physicochemical processing and evolution of hydrogen in the ISM. As such, given their structural similarities to prebiotic nitrogen-bearing frameworks (many found in meteoritic samples and organic aerosols), 2D-CN molecules may emerge as promising candidates for exploring the complex interstellar chemistry of astrophysically relevant molecules.