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## Adsorption of Hydrazoic Acid on Pristine Graphyne Sheet: A Computational Study

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 基金资助: JD is thankful to Department of Science and Technology, New Delhi, India for the INSPIRE Fellowship Award (Grant No. DST/INSPIRE Fellowship/2015/IF150892)

Corresponding authors: SARKAR Utpal, Email: utpalchemiitkgp@yahoo.com; Tel.: +91-9401542687

 Fund supported: JDisthankfultoDepartmentofScienceandTechnology,NewDelhi,IndiafortheINSPIREFellowshipAward(GrantNo.DST/INSPIREFellowship/2015/IF150892)

Herein we have investigated the interaction between hydrazoic acid (HN3) and a pristine graphyne system based on density functional theory (DFT) method using generalized gradient approximation. The van der Waals dispersion correction is also considered for predicting the possibility of using the graphyne system for detection of hydrazoic acid. Pristine graphyne has a band gap of 0.453 eV, which decreases to 0.424 eV when HN3 is adsorbed on graphyne. The electrical conductivity of HN3-adsorbed graphyne is greater than that of its pristine counterpart. Charge transfer analysis reveals that the HN3-adsorbed graphyne system behaves as an n-type semiconductor; however, its pristine analogue acts as an intrinsic semiconductor. Pristine graphyne has zero dipole moment; however, its interaction with HN3 increases its dipole moment. The electronic properties of graphyne is significantly influenced by the presence of HN3, leading to the possibility of designing graphyne-based sensors for HN3 detection.

Abstract

Herein we have investigated the interaction between hydrazoic acid (HN3) and a pristine graphyne system based on density functional theory (DFT) method using generalized gradient approximation. The van der Waals dispersion correction is also considered for predicting the possibility of using the graphyne system for detection of hydrazoic acid. Pristine graphyne has a band gap of 0.453 eV, which decreases to 0.424 eV when HN3 is adsorbed on graphyne. The electrical conductivity of HN3-adsorbed graphyne is greater than that of its pristine counterpart. Charge transfer analysis reveals that the HN3-adsorbed graphyne system behaves as an n-type semiconductor; however, its pristine analogue acts as an intrinsic semiconductor. Pristine graphyne has zero dipole moment; however, its interaction with HN3 increases its dipole moment. The electronic properties of graphyne is significantly influenced by the presence of HN3, leading to the possibility of designing graphyne-based sensors for HN3 detection.

Keywords： Graphyne ; HN3 molecule ; DFT ; Electronic property ; Adsorption ; Gas sensor

DEB Jyotirmoy, PAUL Debolina, PEGU David, SARKAR Utpal. Adsorption of Hydrazoic Acid on Pristine Graphyne Sheet: A Computational Study. 物理化学学报[J], 2018, 34(5): 537-542 doi:10.3866/PKU.WHXB201710161

DEB Jyotirmoy, PAUL Debolina, PEGU David, SARKAR Utpal. Adsorption of Hydrazoic Acid on Pristine Graphyne Sheet: A Computational Study. Acta Physico-Chimica Sinica[J], 2018, 34(5): 537-542 doi:10.3866/PKU.WHXB201710161

## 1 Introduction

In recent years low dimensional structures 1-5 have gained immense attention due to their potential application in next-generation nanoelectronics 6-8. Among these, graphyne 9, 10 is a latest proposed allotrope of carbon which is built from double and triple bonded unit of carbon atoms. It has attracted an extensive interest of the scientific society due to its extraordinary properties. Graphynes can be arranged as multiple lattice types, e.g., α, β, γ graphynes, and out of these, α and β graphynes present Dirac cone-like band structure around the Fermi level 11, while γ graphyne is a semiconductor 12. Unlike graphene, γ graphyne has non-zero band gap and this is due to the presence of the acetylenic linkages and non-uniform π bindings. As graphyne and their derivatives possess various versatile characteristics, thus these systems may be strongly recommended for several technological applications such as nano-electronics 13-16, optoelectronics 17-19, spintronics 20, for storing hydrogen 21, 22, as an electrode in batteries 23, 24, for the detection of gas molecules 25-28and also as an energy storage device 29, 30.

Interaction of atoms and molecules with electromagnetic field modifies its ground as well as excited state reactivity trends in greater extent 31-33. Similarly, when confinement of systems takes place its reactivity profile changes significantly compared to unconfined one 34-37, which is one of the major reason for a large number of investigations on gas molecule interaction with various systems. Recent literature survey reveals that detection of various gas molecules present in the atmosphere is now recognized as an emerging field for many of the researchers for the designing of the suitable gas sensor in order to detect chemical and biological hazardous elements present in the environment and also for the monitoring purposes. It has been also noticed that graphyne material is a promising candidate for designing of the gas sensor. Beheshtian et al. 25 have investigated the interaction of HCN on pristine and Si-doped graphynes and their study indicate that the electronic properties of the graphyne system are highly influenced due to the presence of HCN and thus graphyne is a suitable candidate for the detection of HCN. In the year 2016, Deb et al. have reported that adsorption of BX3 (X = F, Cl and I) molecule on graphyne has induced significant changes in the electronic properties of the graphyne system. Thus we have concluded that graphyne based gas sensor can be designed for the detection of BX3 (X = F, Cl and I) molecule 38. Some other gas molecules (CO, NH3, formaldehyde etc.), nucleobases and amino acid adsorption on graphyne have been investigated in order to see their sensing capability 27, 39-41.

Here we have studied the interaction of hydrazoic acid (HN3) on pristine graphyne. We have chosen HN3 molecule as it is a colorless, volatile, highly toxic and explosive molecule. It has a pungent odor and causes various diseases such as headaches, irritation to eyes, nose, throat, skin, respiratory system and mucous membrane. Direct exposures of HN3 molecule on human being also results in multi-organ failure and even in many cases, it leads to death. On contact with heat, it becomes very dangerous explosive. Thus, some efficient methods should be designed in order to detect HN3 molecule present in the environment. In this article, we have investigated the structural properties, electronic properties, band structure and charge transfer analysis of HN3 adsorbed graphyne system.

## 2 Computational method

All the quantum chemical computations have been performed using density functional theory as suggested in SIESTA code 42, 43. The exchange-correlation functional part of the generalized gradient approximation (GGA) has been represented using Perdew-Burke-Ernzerhof (PBE) 44 form. Troullier-Martin type norm-conserving pseudo potentials 45 and double zeta polarized basis set is used for the calculation. The sampling of Brillouin zone is achieved using 11 × 11 × 2 Monkhorst-Pack set of k points and mesh kinetic energy cutoff value was set at 300 Ryd. To avoid any undesirable interactions a vacuum space of 15 Å (1 Å = 0.1 nm) is maintained between the different layers of the graphyne system. Different orientations of the HN3 molecule in different positions (on the top of triangular hollow, hexagonal hollow, acetylene linkage etc.) of graphyne have been tested to find the ground state geometry of the HN3 adsorbed graphyne system.

The adsorption energy of hydrazoic acid adsorbed on graphyne sheet can be determined using the relation:

Eads = E(Graphyne + molecule) – E(Graphyne) – E(molecule)

where, E(Graphyne + molecule), E(Graphyne) and E(molecule) are the energies of hydrazoic acid adsorbed graphyne system, pristine graphyne and hydrazoic acid respectively.

In order to consider the interaction due to van der Waals (vdW) forces, we have also incorporated the van der Waals dispersion correction explicitly by using the empirical correction scheme as proposed by Grimme 46.

### Fig 1

Fig 1   Optimized geometry of (a) pristine graphyne; (b) HN3 molecule adsorbed graphyne system (top view); (c) HN3 molecule adsorbed graphyne system (side view)

Table 1   The optimal distance (D), adsorption energy (Eads), energy gap (Eg), Mulliken charge transfer (Q) and electric dipole moment (μ) of the adsorption of hydrazoic acid on pristine graphyne.

 System D/Å Eads/eV Eg/eV Q/e μ/Debye without vdW with vdW without vdW with vdW without vdW with vdW without vdW with vdW without vdW with vdW HN3 adsorbed graphyne 2.884 2.884 −0.550 −0.725 0.424 0.424 0.078 0.021 0.173 0.173

#### 3.2.1 Charge transfer analysis

The magnitude of Mulliken charge transfer is 0.078e (Table 1) without accounting the vdW correction but the magnitude of charge transfer decreases to 0.021e when vdW interaction is taken into consideration. The result shows that a very small amount of electron transfer has taken place between HN3 molecule and graphyne sheet and the smaller Q value again suggesting the physisorption of HN3 molecule on graphyne sheet 25. This charge transfer occurs from HN3 molecule to pristine graphyne. In our previous study 38, we have shown that BCl3 and BI3 adsorbed graphyne system behaves as an n-type semiconductor, likewise HN3 adsorbed graphyne system also acts as an n-type semiconductor as the number of valence electrons of this system increases significantly. But, the magnitude of charge transfer is much higher when BCl3 and BI3 interact with graphyne system. On the basis of charge transfer, we may remark that although chemisorption of BX3 (X = F, Cl and I) molecule is observed on graphyne sheet, the HN3 molecule, on the other hand, is physisorbed on the pristine graphyne system. InHCN adsorbed graphyne, 0.041e amount of charge calculated using B3LYP functional and 6-31G(d) basis set, is transferred from graphyne to HCN molecule. However, the present study reveals that 0.021e amount of charge is transferred from HN3 molecule to graphyne. So the charge transfer direction of HCN adsorbed graphyne shows a reverse trend with respect to HN3 adsorbed graphyne. Interaction of formaldehyde with graphyne system 40 also shows that formaldehyde is physically adsorbed on graphyne system similar to HN3 adsorbed graphyne system but the charge transfer is quite large when formaldehyde correlates with graphyne system as compare to HN3 interaction with graphyne system.

#### 3.2.2 Dipole moment

The detailed study of dipole moment leads us to infer that although pristine graphyne has zero dipole moment the interaction of HN3 molecule induces a significant amount of dipole moment into the system. The magnitude of dipole moment in case of HN3 adsorbed graphyne system is found to be 0.173 Debye both for vdW and without vdW correction. The sudden change in dipole moment may be detected with the help of a suitable detector and hence may ensure the possibility of designing as a sensor. This moderate value of dipole moment may be due to the rearrangement of charge carriers between graphyne system and HN3 molecule. On comparing, it has been noticed that the magnitude of dipole moment in case of BF3 adsorbed graphyne 38 is smaller than HN3 adsorbed graphyne whereas, increase in dipole moment are observed for BCl3 and BI3 adsorbed graphyne 38 system. Therefore, based on this comparison, we may conclude that the chance of detection of HN3 molecule is higher than that of BF3 molecule using graphyne as a host material.

#### 3.2.3 Band structure analysis

In this work, we have analyzed the band structure of pristine graphyne when hydrazoic acid is adsorbed on it and presented a comparative discussion with its pristine counterpart. Our analysis and plot of band structure (see Fig. 2) for HN3 adsorbed graphyne system clearly shows that there is no spin splitting taking place for up and down spins and this is certainly because of the zero magnetic moment of the system. Also, it has been observed that the valance band maximum (VBM) and the conduction band minimum (CBM) of the HN3 adsorbed graphyne system are located at the Г point of the hexagonal Brillouin zone and this observation of the VBM and CBM exactly matches with that of pristine graphyne 38. For the pristine graphyne, the difference in energy level between the VBM and CBM is found to be approximately 0.453 eV 13, 38. But just when the adsorption of HN3 molecule takes place on the surface of the pristine graphyne, its band gap gets changed and decreases to 0.424 eV (Table 1) without incorporating vdW correction. There is no change in magnitude of the band gap when vdW interaction is considered. It is known that electrical conductivity is proportional to ${{\rm{e}}^{\frac{{_ - {E_{\rm{g}}}}}{{{K_{\rm{b}}}T}}}}$ where Eg, Kb and T represents band gap, Boltzmann constant and temperature respectively. Since the band gap decreases when pristine graphyne interacts with HN3 molecule, the HN3 adsorbed system possesses larger electrical conductivity than that of the pristine system. The decrease in band gap is also observed in case of formaldehyde interaction with graphyne system 40. However, our previous study on BX3 (X = F, Cl and I) molecule adsorption on graphyne sheet shows that 38, the band gap of the pristine sheet slightly got increased, which is just opposite from the results of our present study of gas adsorption. Again from the literature survey, it is known that systems with low energy gap are less chemically hard and consequently shows relatively low chemical hardness profile 47, 48. Hence from our present work, we can infer that as the band gap of the pristine graphyne decreases on HN3 adsorption on its surface, the resulting system becomes more reactive as compared to the pristine structure but less reactive compare to formaldehyde adsorbed graphyne system 40. This is because the band gap of formaldehyde adsorbed graphyne is minimum compared to our studied molecule. Further, it is a known fact that pristine graphyne is a direct band gap intrinsic semiconductor and it sustains this particular characteristic even when HN3 interacts with its surface. But the adsorption of HN3 molecule on the graphyne system makes it an extrinsic semiconductor (n-type).

### Fig 2

Fig 2   Band structure of HN3 molecule adsorbed graphyne system (a) without vdW correction; (b) with vdW correction.

### Fig 3

Fig 3   Total density of states (DOS) and projected density of states (PDOS) of HN3 molecule adsorbed graphyne system, (a) without vdW correction; (b) with vdW correction.

### Fig 4

Fig 4   PDOS of pristine graphyne and HN3 adsorbed graphyne.

## 4 Conclusions

Supporting Information: available free of charge via the internet at http://www.whxb.pku.edu.cn.

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