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 of 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 astrochemistry. 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.

💡 Research Summary

This paper investigates the potential role of two‑dimensional carbon nitride (2D‑CN) monolayer nanosheets as interstellar dust analogues that can adsorb atomic hydrogen and thereby influence the chemistry of the interstellar medium (ISM). The authors begin by noting that atomic hydrogen accounts for up to 70 % of the neutral gas in the Milky Way and that its interaction with dust grains and icy mantles is a key step in the formation of H₂ and more complex hydrogenated species. While silicates, amorphous carbon, graphene, and polycyclic aromatic hydrocarbons (PAHs) have been extensively studied as catalytic surfaces, nitrogen‑containing 2D materials have received little attention in astrochemistry.

Nine representative 2D‑CN structures are selected: C₂N₁, C₃N₁, C₃N₂, C₃N₄, C₄N₃, C₆N₆, C₆N₈, C₉N₄, and C₉N₇. These vary in carbon‑to‑nitrogen ratio, degree of porosity, and the presence of macropores (≈5–7 Å) surrounded by pyridinic nitrogen atoms. The authors argue that such pores create electron‑rich cavities capable of attracting electron‑deficient species, a property that could be advantageous for surface chemistry under ISM conditions.

Density‑functional theory (DFT) calculations are performed with the Quantum ESPRESSO suite, employing the PBE functional within the generalized gradient approximation and PAW pseudopotentials. A 6 × 6 × 1 Monkhorst‑Pack k‑point mesh and a vacuum slab >15 Å ensure convergence for the quasi‑2D systems. Dispersion corrections (DFT‑D3) are evaluated but found to be negligible for the adsorption energetics.

For each monolayer, the authors systematically sample the potential‑energy surface (PES) of a single hydrogen atom. Lateral positions (x, y) are chosen to represent symmetry‑inequivalent sites: atop carbon, atop nitrogen, bridge (C–C) sites, and hollow (macropore) sites. At each (x, y) the hydrogen atom is displaced along the surface normal (z) in fine steps, while the substrate atoms remain fixed. The resulting three‑dimensional PES is reduced to one‑dimensional energy curves E_int(z) for each site; the equilibrium adsorption height z_eq corresponds to the minimum of the curve, and the adsorption energy is defined as E_ads = E_int(z_eq) − E_surface − E_H. Negative E_ads values indicate exothermic adsorption.

The calculations reveal that bridge and nitrogen sites generally provide the strongest binding, with adsorption energies ranging from –0.45 eV to –0.78 eV. Hollow macropore sites also bind hydrogen, albeit more weakly (–0.30 eV to –0.55 eV). Notably, the C₂N₁ and C₉N₄ sheets, which possess the largest pores surrounded by six pyridinic N atoms, exhibit the deepest minima, suggesting that the combined effect of geometric confinement and local electron density enhances hydrogen affinity. The equilibrium adsorption distances lie between 1.2 Å and 1.8 Å, indicating that physisorption is barrierless at typical cold ISM temperatures (10–20 K).

From an astrochemical perspective, these findings have two major implications. First, the high specific surface area and the presence of energetically favorable adsorption sites mean that 2D‑CN sheets could act as efficient reservoirs for atomic hydrogen, facilitating Langmuir–Hinshelwood diffusion and subsequent H₂ formation on grain surfaces. Second, the nitrogen‑rich, porous architecture may promote the formation of nitrogen‑bearing prebiotic molecules (e.g., pyrimidine, triazine) by providing reactive sites for radical and ion–molecule reactions that are otherwise inefficient on purely carbonaceous grains. The authors therefore propose that 2D‑CN nanosheets could represent a previously overlooked class of interstellar dust components, especially in nitrogen‑rich environments such as dense molecular clouds or circumstellar envelopes.

The paper acknowledges several limitations. Only single‑atom adsorption is considered; real ISM conditions involve a mixture of H, H₂, H⁺, electrons, and UV photons, which could modify adsorption energetics and reaction pathways. Zero‑point energy and entropic contributions are omitted, so the derived adsorption energies represent electronic minima rather than free‑energy values relevant to temperature‑dependent processes. Moreover, the calculations keep the substrate geometry frozen, neglecting possible surface relaxation or reconstruction upon adsorption.

Despite these simplifications, the study provides a concrete computational framework and a set of quantitative adsorption energies that can guide future laboratory synthesis of 2D‑CN analogues, spectroscopic identification (e.g., infrared or rotational transitions), and astrochemical modeling. The authors suggest that experimental work on the synthesis of porous carbon nitride monolayers, combined with surface science techniques (temperature‑programmed desorption, scanning tunneling microscopy), could validate the predicted binding strengths. In parallel, astronomical surveys targeting nitrogen‑rich regions could search for signatures consistent with 2D‑CN dust, potentially revealing a new pathway for hydrogen processing and prebiotic chemistry in space.