Interception of Layered LP-N and HLP-N at Ambient Conditions\\ by Confined Template

Dong-Xue Wang(王冬雪)$^{1}$, Jing Fu(付静)$^{2}$, Yi Li(李义)$^{3}$,\\ Zhen Yao(姚震)$^{1\hyperlink{s}{*}}$, Shuang Liu(刘爽)$^{1\hyperlink{s}{*}}$, and Bing-Bing Liu(刘冰冰)$^{1\hyperlink{s}{*}}$

doi:10.1088/0256-307X/41/3/036101

⇦Chinese Physics Letters, 2024, Vol. 41, No. 3, Article code 036101⇨ Interception of Layered LP-N and HLP-N at Ambient Conditions by Confined Template Dong-Xue Wang (王冬雪)1, Jing Fu (付静)2, Yi Li (李义)3, Zhen Yao (姚震)1*, Shuang Liu (刘爽)1*, and Bing-Bing Liu (刘冰冰)1* Affiliations 1State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China 2Basic Science Department, Jilin Institute of Architecture and Technology, Changchun 130000, China 3College of Science, Liaoning University of Technology, Jinzhou 121000, China Received 4 December 2023; accepted manuscript online 26 January 2024; published online 12 March 2024 ^*Corresponding authors. Email: yaozhen@jlu.edu.cn; liu_shuang@jlu.edu.cn; liubb@jlu.edu.cn Citation Text: Wang D X, Fu J, Li Y et al. 2024 Chin. Phys. Lett. 41 036101 Abstract We propose a feasible strategy of intercepting the layered polymeric nitrogen (LP-N) and hexagonal layered polymeric nitrogen (HLP-N) at ambient conditions by using the confinement templates. The stable mechanism of confined LP-N and HLP-N at ambient conditions is revealed, namely the synergistic effect of charge transfer and vdW confinement effect. The influence rule of interlayer spacing on the stability of LP-N is revealed. Most importantly, the nitrogen content and energy density of recoverable LP-N@graphene (70.59%, 8.15 kJ/g), LP-N@h-BN (70.59%, 7.96 kJ/g), HLP-N@graphene (68.97%, 9.31 kJ/g), and HLP-N@h-BN (69.57%, 8.05 kJ/g) refresh the new record for the confinement polynitrogen system.

DOI:10.1088/0256-307X/41/3/036101 © 2024 Chinese Physics Society Article Text The exploitation of polymeric nitrogen is always an active research topic for their potential applications in energy storage, propellants, and explosives.[1-5] A great deal of polymeric nitrogen structures have been proposed theoretically, such as zero-dimensional (0-D) caged structure (N$_{10}$),[6] one-dimensional (1-D) chain-like structure (CH),[7] two-dimensional (2-D) layered structures [black phosphorus nitrogen (BP-N), layered polymeric nitrogen (LP-N), hexagonal layered polymeric nitrogen (HLP-N), arsenic (A7), layered-boat (LB), $P2_{1}$],[6,8-12] and three-dimensional (3-D) networked structures [cubic gauche nitrogen (cg-N), chaired web, Pnnm, Cccm].[8,13-15] Up to date, only a few of structures have been successfully synthesized at extreme conditions, including the cg-N (110 GPa, 2000 K), LP-N (150 GPa, 3000 K), HLP-N (244 GPa, 3300 K), and BP-N (146 GPa, 2200 K), which can be quenched to 42 GPa, 52 GPa, 66 GPa, and 48 GPa, respectively.[16-19] The released energy in the decomposition of polymeric nitrogen originates from the transformation of single N–N bond ($\sim$ $160$ kJ/mol) or double N=N bond (418 kJ/mol) to the triple N$\equiv$N bond ($\sim$ $954$ kJ/mol). However, the high energy property of single N–N bond and double N=N bond is accompanied with the low kinetic stability, which makes the polymeric nitrogen unquenchable at ambient conditions. In 2023, Wang et al. firstly investigated the stability of cg-N surfaces under different pressures and temperatures. The results show that cg-N decomposes at low pressure, which is caused by the surface instability. They found that the hydrogen saturation of its surface can stabilize the cg-N to 750 K at 0 GPa.[1] This successful interception of cg-N by hydrogen saturation indicates that making a constraint on the surface of polymeric nitrogen can significantly enhance its stability even realize the interception at ambient conditions. This urges us to search for other effective constraint methods to improve the stability of polymeric nitrogen. Excitingly, the nanomaterial with the hollow structure provides an ideal confined space to stabilize the guest molecule.[20-23] In 2008, Abou-Rachid et al. firstly reported that the confined N$_{8}$ chain in carbon nanotube is stable at ambient conditions.[24] Subsequent studies show that N$_{8}$ chain can also be quenched to ambient conditions by other templates, such as graphene matrix,[25] silicon carbide nanotube,[26] boron nitride nanotube,[27] and h-BN matrix.[28] With the enlightenment of successfully capturing of N$_{8}$ chain, the interception of polymeric nitrogen was increasingly directed towards other structures, such as the 2-D layered A7 (A7@h-BN and A7@graphene), the 1-D nanotubed AC3 [AC3@CNT(5,5)] and ZZ4 [ZZ4@CNT(9,0)], and the 0-D caged N$_{10}$ (N$_{10}$@CNT).[29-32] For the confined polymeric nitrogen system, the energy character significantly depends on the nitrogen content due to the fact that the released energy originates from the confined guest. In general, the higher the nitrogen content is, the higher the energy density ($E_{\rm d}$) for the confined polymeric nitrogen system can be obtained. For example, the energy densities of A7@h-BN ($E_{\rm d}=5.40$ kJ/g, N% = 53.84%) and A7@graphene ($E_{\rm d}=5.20$ kJ/g, N% = 53.84%) are larger than those of N$_{10}$@CNT ($E_{\rm d}=3.26$ kJ/g, N% = 23.81%), N$_{8}$@CNT(5,5) ($E_{\rm d}=1.41$ kJ/g, N% = 11.76%), N$_{8}$@CNT(9,0) ($E_{\rm d}=0.98$ kJ/g, N% = 10.00%), and ZZ1'@CNT(9,0) ($E_{\rm d}=1.88$ kJ/g, N% = 14.29%), due to the higher nitrogen content of the formers.[24,29-33] Thus, it is necessary to explore the new confine nitrogen system with the higher nitrogen content. In experiment, the layered LP-N, HLP-N, and BP-N have been successfully synthesized, but failed to stabilize at ambient conditions.[17-19] Different from the A7 phase with the single atomic layer structure, the LP-N, HLP-N, and BP-N are all polyatomic thick-layer structures. When the LP-N, HLP-N, and BP-N are confined into the same 2-D template, the nitrogen content can increase up to 65%–70% in comparison to the A7@h-BN and A7@graphene confinement system (N% = 53.84%). Thus, the energy density of the confined polymeric nitrogen system should be greatly improved if the LP-N, HLP-N, and BP-N are quenched to ambient conditions. In this work, we performed a systematic study of the confined LP-N, HLP-N, and BP-N inside the 2-D template for exploring their potential applications. The research focuses on two main goals: (1) exploring the interceptable conditions of LP-N, HLP-N, and BP-N, and (2) revealing the stable physical mechanism of confinement system. Calculation Method. The calculations of structural relaxation, ab initio molecular dynamic (AIMD) simulation, and electronic properties were performed using the Vienna ab initio simulation package (VASP) based on the first-principles plane-wave pseudopotential density functional theory (DFT).[34] The generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof functional was used for describing the exchange and correlation energy of electrons.[35,36] The cutoff energy for the plane-wave-basis was set as 520 eV. The mesh density of the Monkhorst–Pack sampled Brillouin zone was set to 2$\pi \times 0.03$ Å$^{-1}$ for geometric optimization and electronic structure calculation and to $1 \times 1\times 1$ for AIMD simulations. The energy convergence criteria for relaxation, electronic properties, and AIMD simulation were set as $1 \times 10^{-6}$, $1 \times 10^{-6}$, and $1 \times 10^{-4}$ eV/atom, respectively; in addition the convergence criteria of force for relaxation were set as $5 \times 10^{-2}$ eV/Å. The supercell of the LP-N@h-BN (136 atoms), LP-N@graphene (136 atoms), HLP-N@h-BN (92 atoms), and HLP-N@graphene (116 atoms) in the AIMD simulation are $1 \times 2\times 1$, $1 \times 2\times 1$, $1 \times 1\times 1$, and $1 \times 1\times 1$, respectively. The AIMD simulation was performed using NPT-ensemble (constant number $N$ of atoms, constant pressure $P$, and constant temperature $T$). The Langevin thermostat was used to maintain temperature in AIMD. The AIMD simulation proceeded 10 ps with time step of 1 fs.[37] The vdW interaction was considered by DFT-D3 approximate method in the whole calculation. The VASPKIT was used for generating high symmetry points and post processing.[38-40]

Fig. 1. The unit cell of the LP-N@h-BN (a), LP-N@graphene (b), HLP-N@h-BN (c), HLP-N@graphene (d), BP-N@h-BN (e), and BP-N@graphene (f) viewed from the front and top.

Model. The layer structures (graphene and h-BN) were chosen as the 2-D confinement templates for the following factors: (i) Their adjustable 2-D interlayer spacing could provide sufficient and indispensable space for filling the layer-like polymeric nitrogen (LP-N, HLP-N, and BP-N). (ii) The beneficial mechanical properties, such as high breaking strength ($125 \pm 0.0$ GPa and $70.5 \pm 5.5$), tensile strength (130 GPa for graphene), and Young's modulus ($1.026 \pm 0.022$ TPa and $0.865 \pm 0.073$ TPa), enable them to have the enough tenacity to stabilize the polymeric nitrogen structure. (iii) The chemical inertness makes h-BN hold its structural integrity in a wide range of acids and alkalis. (iv) The high thermal stability, such as high melting point (h-BN: $\sim$ $2873$ K and graphene: 4510 K), makes them applied under extreme high temperature condition.[41-44] As shown in Fig. 1, the confinement models are constructed by filling the LP-N, HLP-N, and BP-N into the space of graphene and h-BN layers. The lattice matches well between the template and confined guest with the mismatch parameters ($|\delta|$) being all smaller in 5%. The initial distances between the confinement template and the N-layer of LP-N@graphene, LP-N@h-BN, HLP-N@graphene, HLP-N@h-BN, BP-N@graphene, and BP-N@h-BN are in-between of 2.2–2.3 Å. The detailed lattice constants of initial models are listed in Table S1 in the Supplemental Material. The nitrogen content of LP-N@graphene, LP-N@h-BN, HLP-N@graphene, HLP-N@h-BN, BP-N@graphene, and BP-N@h-BN are 70.59%, 70.59%, 68.97%, 69.57%, 65.22%, and 65.22%, respectively. Result and Discussion. The relaxed structures of LP-N@graphene, LP-N@h-BN, HLP-N@graphene, HLP-N@h-BN, BP-N@graphene, and BP-N@h-BN at 0 GPa are shown in Fig. S1 in the Supplemental Material. After the structural relaxation, the significant deformation and decomposition occur on the BP-N structure, indicating that it is unstable when being confined inside h-BN and graphene [Figs. S1(e) and S1(f) in the Supplemental Material]. The confined LP-N and HLP-N maintain their layered structures after structural relaxation at 0 GPa [Figs. S1(a)–S1(d)]. Subsequently, the AIMD simulation was performed to further determine the structural stability of confined LP-N and HLP-N at ambient pressure. As shown in Fig. 2, the total energy vibrates near the equilibrium position with small amplitude ($\sim$ $2$ eV) at 300 K and the confined LP-N maintains its structure skeleton after the AIMD simulation, indicating that it is thermal stability at ambient conditions. As the temperature increases, the confined LP-N also maintains its thermal stability at 700 K only with the slightly increased amplitude ($\sim$ $6$ eV) of total energy. For the temperature increasing up to 800 K, the confined LP-N fails to maintain its structural skeleton at $\sim$ $3$ ps. The more serious decomposition occurs at 1000 K. Similarly, the confined LP-N and HLP-N inside graphene are all thermally stable at ambient conditions and maintain their thermal stability up to 700 K and 305 K, respectively (Figs. S2 and S3 in the Supplemental Material). The confined HLP-N inside h-BN is thermally stable at near ambient conditions (270 K) as shown in Fig. S4.

Fig. 2. (a) Energy of the LP-N@h-BN system as a function of time at 300 K, 700 K, 800 K, and 1000 K in AIMDs. (b) Snapshots of the LP-N@h-BN system after the AIMDs at 300 K, 700 K, 800 K, and 1000 K.

Fig. 3. (a) Band structures of LP-N@h-BN, free LP-N, and free h-BN. (b) DOSs of free LP-N, free h-BN, and their hybrid system.

Fig. 4. The electron density difference of LP-N@h-BN (a), LP-N@graphene (b), HLP-N@h-BN (c), and HLP-N@graphene (d), at ambient pressure. The yellow and blue colors represent the increment and reduction of electrons, respectively.

To explore the stable mechanism of confined LP-N and HLP-N, we performed the calculation of electronic property. The band structures and density of states (DOS) of the LP-N@h-BN system, free LP-N, and free h-BN matrix are shown in Figs. 3(a) and 3(b). The LP-N@h-BN, isolated LP-N and h-BN matrix are all insulators, with the band gap of 2.92, 3.2, and 4.25 eV, respectively. For the LP-N@h-BN, the band structure is nearly formed by a superposition of the LP-N and h-BN. The top of the valence band and the bottom of the conduction band originate from the h-BN and LP-N, respectively. The bands originating from the h-BN are slightly changing near Fermi level compared with the isolated one along $\varGamma$–$F$ direction, while the valence and conduction bands originating from LP-N are shifted downwards compared to the free LP-N, which is induced by the weak hybridization interaction between LP-N and h-BN. This is consistent with the presented DOS that the intensity of distribution of LP-N and BP-N in hybrid systems is decreased in comparison to their isolated ones. As the calculated electron density difference shown in Fig. 4(a), the yellow and blue regions represent the capturing and losing of the electrons, respectively. The distributed yellow region on the surface of N atoms on LP-N means that it captures the electrons. The Bader charge transfer analysis shows that each N atom of LP-N structure captures $\sim$ $0.002e$ from h-BN matrix (Table 1). This charge transfer improves the structural stability of LP-N. On the other hand, the vdW interaction with the short range repulsion and long range attraction character plays an important role in this confinement system. This interaction provides an important confinement effect for the LP-N, stabilizing it at ambient conditions. Similarly, the weak hybridization interactions and small charge transfer occur in LP-N@graphene, HLP-N@h-BN, and HLP-N@graphene systems [Figs. S5–S7 in the Supplemental Material and Figs. 4(b)–4(d)]. As discussed above, we know that the stable mechanism of confined LP-N and HLP-N at ambient conditions is due to the synergistic effect of charge transfer and vdW confinement effect. Previous studies argued that the stability of confined nitrogen structures is sensitive to the quantity of charge transfers. Namely, the more charge transfer occurs between the polynitrogen and template will lead to stronger interaction, which results in a higher decomposition temperature of confined polynitrogen.[28-30] Here, it is noted that the confined LP-N, with less charge transfer, is more stable than HLP-N. We deduced that this contradiction is caused by the difference of stability between them. In order to verify our conjecture, we performed a comparison of the enthalpies of LP-N and HLP-N at 0–200 GPa, as shown in Fig. 5(a). The result indicates that LP-N is energetically favorable at the entire pressure region in comparison to the HLP-N. More importantly, the bond length of LP-N is shorter than HLP-N in statistics at ambient pressure, which leads to the greater bond energy and stronger covalent bond of the former, resulting in a higher stability [Figs. 5(b)–5(e)]. For all the reasons discussed above, we know that the stability of themselves is also an important factor for the interception of polynitrogen at ambient conditions.

Table 1. The charge transfer and the stable temperature of LP-N@h-BN, LP-N@graphene, HLP-N@h-BN, and HLP-N@graphene.
Structure	Charge transfer ($e$/N)	Temperature (K)
LP-N@h-BN	0.002	700
LP-N@graphene	0.002	700
HLP-N@h-BN	0.003	270
HLP-N@graphene	0.003	305

Fig. 5. (a) Formation enthalpies of LP-N with respect to HLP-N. (b) Bond length of N–N in LP-N. (c) Electron localization function (ELF) (isovalue = $0.8$) of LP-N. (d) Bond length of N–N in HLP-N. (e) ELF (isovalue = $0.8$) of HLP-N.

Fig. 6. The interlayer spacing between template (graphene and h-BN) and polymeric nitrogen (LP-N and HLP-N) was set as $L$ (a)–(d). The total energies as a function of interlayer spacing between template (graphene and h-BN) and polymeric nitrogen (LP-N and HLP-N) at ambient pressure (e).

According to the above discussion, we know that the confinement effect is important for the stability of polynitrogen. Since the confinement effect depends on the distance between the polynitrogen and template, we studied the influence of confinement spacing on the stability of polynitrogen by performing the calculation of the total energies and AIMD. Firstly, we calculated the total energies with the variable interlayer spacing between template (graphene and h-BN) and polymeric nitrogen (LP-N and HLP-N). As shown in Fig. 6, with the interlayer spacing increasing, the total energies gradually decline, then change gently. The change of total energy with the interlayer spacing is caused by the van der Waals interaction between template (graphene and h-BN) and polymeric nitrogen (LP-N and HLP-N). As is well known, the van der Waals contains both the repulsive force and attractive force. The energy of repulsive force and attractive force are positive and negative, respectively. For the small interlayer spacing, the repulsive force dominates the leading role. As the interlayer spacing increases, both the repulsive force and attractive force exhibit the decreased tendency. However, the repulsive force decreases more steeply. Thus, the total energy exhibits a decreased tendency with the increased interlayer spacing. As the interlayer spacing increases large enough, both the repulsive force and attractive force become very small. Meanwhile, the total energy changes gently. Secondly, we performed the AIMD by selecting the LP-N@h-BN as an example. Seventeen structures with interlayer spacings from 1.598 to 3.198 Å ($\Delta d=0.100$ Å) are constructed to perform structural relaxation and AIMD simulation. As shown in Fig. S8, after the structural relaxation at ambient pressure, the LP-N holds their structures with different confinement spacings. Furthermore, the AIMD simulation was performed to exam their thermal stability at different temperatures. As shown in Fig. 7, the structures with too small (1.598 Å) or too large (3.198 Å) interlayer spacings are not stable at ambient conditions (300 K). This indicates that too small interlayer spacing can produce the large repulsive interaction, which is disadvantage of the structural stability; while too large interlayer spacing can not produce the sufficient confinement effect, also results in the decomposition. It agrees with analysis of total energy against the interlayer spacing, as shown in Fig. 6. For the interlayer spacing converges to the middle region, the decomposition temperatures increase from 300 K to 800 K. Especially, the confined LP-N exhibit highest stability with the interlayer spacings of 2.198 Å and 2.398 Å. Here, we revealed the influence rule of confined spacing on the stability of LP-N and provided the suitable interlayer spacing.

Fig. 7. The decomposition temperatures versus interlayer spacing for LP-N@h-BN after the AIMD.

Fig. 8. Energy density and nitrogen concentration of the reported confined compounds. The hollow and solid blocks represent the nitrogen content and energy density, respectively.

The stable hybrid systems (LP-N@graphene, LP-N@h-BN, HLP-N@graphene, and HLP-N@h-BN) with the high nitrogen content are typical high energy density materials. Their released energy originates from the decomposing of confined LP-N and HLP-N. The value of energy density is obtained by calculating the enthalpy difference between reactant and product at ambient pressure and 0 K. The reactant is LP-N@template or HLP-N@template, while the product is the $\alpha$-N$_{2}$@template. Due to the fact that the high stability of template and the decomposition of confined LP-N and HLP-N should be step by step, we assume that no significant deformation or decomposition occurs on the template in the product. The effect of the slight deformation of template on the energy density should be very small. Thus, the effect of graphene or h-BN on the energy density is not included in the calculation of energy density. The nitrogen content and energy density of confined LP-N and HLP-N systems are presented in Fig. 8. Other confined systems are presented for a comparison.[24,25,27-33] It is encouraging that the LP-N@graphene (70.59%, 8.15 kJ/g), LP-N@h-BN (70.59%, 7.96 kJ/g), HLP-N@graphene (68.97%, 9.31 kJ/g), and HLP-N@h-BN (69.57%, 8.05 kJ/g) with the highest nitrogen content possess the highest energy density of any known confined polynitrogen compounds. These energy densities are very close to that of cg-N (9.7 kJ/g).[45] In comparison with the conventional explosives, their energy densities are 1.88–2.2 times that of TNT (4.3 kJ/g) and 1.4–1.7 times that of HMX (5.7 kJ/g),[46] making them potential high energy density materials. Here, we propose a feasible strategy of intercepting the LP-N and HLP-N at ambient conditions by using the confinement templates. This work refreshes a new record of nitrogen content and energy density in confinement nitrogen system. It is expected that the mature synthetic methods in experiment can be applied in the near further to realize the goal of intercepting the LP-N and HLP-N by nanostructured confinement method. In summary, we performed a systematic study of intercepting the polyatomic thick-layer structures (LP-N, HLP-N, and BP-N) at ambient conditions by using the 2-D confinement templates. The stability analyses show that the LP-N and HLP-N can be quenched to the ambient conditions, while the BP-N is unstable when being confined inside h-BN and graphene. Further studies indicate that the stable mechanism of confined LP-N and HLP-N at ambient conditions is due to the synergistic effect of charge transfer and vdW confinement effect. The comparative analysis of the decomposition temperature between the LP-N and HLP-N shows that the stability of themselves is also an important factor for their interception at ambient conditions. The influence rule of interlayer spacing on the stability of LP-N is revealed and suitable interlayer spacing is provided. Most importantly, the nitrogen content and energy density of recoverable LP-N@graphene (70.59%, 8.15 kJ/g), LP-N@h-BN (70.59%, 7.96 kJ/g), HLP-N@graphene (68.97%, 9.31 kJ/g), and HLP-N@h-BN (69.57%, 8.05 kJ/g) refresh the new record for the confinement polynitrogen system. Acknowledgements. This work was financially supported by the National Key R&D Program of China (Grant No. 2018YFA0305900), the National Natural Science Foundation of China (Grant Nos. 12174143 and U2032215), and the Natural Science Foundation Project of Liaoning Province (Grant No. 2022-MS-377). 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