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Carbon Nitride Monolayer Nanosheets: Astrochemical Insights into the Fate of Interstellar Hydrogen

Dubois David, Guichard Pierre, Pasquier Remi

Abstract

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.

Carbon Nitride Monolayer Nanosheets: Astrochemical Insights into the Fate of Interstellar Hydrogen

Abstract

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.
Paper Structure (13 sections, 2 equations, 2 figures, 1 table)

This paper contains 13 sections, 2 equations, 2 figures, 1 table.

Table of Contents

  1. Section
  2. Introduction
  3. 2D Carbon Nitride Nanosheets: Structures, Applications, and Properties
  4. Methods
  5. Results
  6. Astrophysical implications
  7. Conclusions
  8. Author contributions
  9. Conflicts of interest
  10. Data availability
  11. Acknowledgements

Figures (2)

  • Figure 1: Schematic drawings of the unit cells for each of the studied structures.
  • Figure 2: (Upper) Potential energy as a function of the distance from the sheet for a hydrogen atom at a specific sheet position. (Lower) Sheet representation showing the corresponding calculation sites: (a) $\mathrm{C}_2\mathrm{N}_1$, (b) $\mathrm{C}_3\mathrm{N}_4$, (c) $\mathrm{C}_6\mathrm{N}_8$, (d) $\mathrm{C}_3\mathrm{N}_1$, (e) $\mathrm{C}_4\mathrm{N}_3$, (f) $\mathrm{C}_9\mathrm{N}_4$, (g) $\mathrm{C}_3\mathrm{N}_2$, (h) $\mathrm{C}_6\mathrm{N}_6$, (i) $\mathrm{C}_9\mathrm{N}_7$. Black atoms are carbons and the blue atoms are nitrogens. Coloured dots are referring to the specific positions calculated in the sheet.