Unraveling the Surface Stability and Chemical Reactivity of Aza-Triphenylene Monolayer under O$_2$ and H$_2$O Exposure

TL;DR

This work probes how environmental O$_2$ and H$_2$O affect a novel aza-triphenylene 2D monolayer using first-principles DFT and CINEB to map adsorption and dissociation pathways. It finds that both molecules physisorb weakly but dissociate via substantial energy barriers, forming strong chemisorbed products with significant charge transfer and altered electronic structure (e.g., insulator-to-metal transitions for dissociated states in H$_2$O and O$_2$ cases). The study combines charge density difference and Bader analyses with RDG and PDOS to reveal the mechanistic details of adsorbate–substrate interactions, highlighting the material’s stability for potential device applications under ambient conditions. Overall, the aza-triphenylene monolayer demonstrates resilience to environmental oxidation and hydrogenation, while its electronic properties can be modulated by adsorbate state.

Abstract

Environmental oxidation has a great impact in tuning the physical, chemical and electronic properties of two-dimensional (2D) monolayers which can affect their practical applications in nanoscale engineering devices under ambient conditions. aza-triphenylene is a recently synthesized 2D materials whose practcal applications have not been systematically studied yet. In this study, we report for the first time, the adsorption and dissociation of O$_2$ and H$_2$O molecules on the surface of 2D aza-triphenylene monolayer through first principles calculations in combination with climbing image nudged elastic band (CINEB) method. The results indicates that both the O$_2$ and H$_2$O molecules weakly interact over the monolayer surface with an adsorption energy -0.16 eV and -0.37 eV respectively. In contrast, both the molecules exhibit resistance for dissociation due to the formation of energy barriers. The transition path indicates that molecular oxygen experience two energy barriers (0.16 ev and 1.22 eV) before getting dissociated atomic oxygen. However, the dissociation of H$_2$O requires larger energy barrier (2.3 eV and 0.86 eV) due to breaking of covalent bonds and transfer of hydrogen. The strong chemical adsorption of atomic oxygen and H$^+$/OH$^-$ ions is due to the significant charge transfer from monolayer to the adsorbate as evidenced from the charge density difference and Bader charge analysis. Moreover, the dissociated configuration exhibit a larger band gap as compared to the pristine aza-triphenylene due to the strong hybridization between the p states of carbon and oxygen. our work predicts the robustness of azatriphylene monolayer against oxygen/water exposer thus ensuring their stability for device applications using these materials.

Figures (9)

• Figure 1: (a) Schematic of the crystal structure, (b) band structure and (c) partial density of states of pristine aza-triphenylene monolayer using PBE functional. In figure (a), the color code for the atoms are as follows: brown: C, grey: N, blue: H. Atomic sizes are exaggerated for clarity.
• Figure 2: Schematic representation of (a,b) O$_2$ adsorbed, (c,d) O$_2$ dissociated, (e,f) H$_2$O adsorbed , (g,h) H$_2$O dissociated aza-triphenylene monolayer. The color code for the atoms are as follows: brown: C, grey: N, blue: H, red: O. Atomic sizes are exaggerated for clarity.
• Figure 3: Reaction pathway for O$_2$ with aza-triphenylene: Adsorption and dissociation process of O$_2$ on the aza-triphenylene monolayer surface, including structural configurations at intermediate stages. The colour scheme for atoms corresponds to that utilized in Fig. \ref{['fig:oxygen_water_aza']}
• Figure 4: Reaction pathway for H$_2$O with aza-triphenylene: Adsorption and dissociation process of H$_2$O on the aza-triphenylene monolayer surface, including structural configurations at intermediate stages. The colour scheme for atoms corresponds to that utilized in Fig. \ref{['fig:oxygen_water_aza']}
• Figure 5: Charge density difference of O$_2$ on aza-triphenylene: (a) physisorbed (O$_2$) and (b) dissociated (O + O) configurations. The iso-surface value was set at 0.05 e/Å$^3$. The yellow and cyan colours represent charge accumulation and depletion, respectively. The colour scheme for atoms corresponds to that utilized in Fig. \ref{['fig:oxygen_water_aza']}.