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

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1 物理化学学报 Acta Phys. -Chim. Sin. 2018, 34 (5), [Article] doi: /PKU.WHXB Adsorption of Hydrazoic Acid on Pristine Graphyne Sheet: A Computational Study DEB Jyotirmoy, PAUL Debolina, PEGU David, SARKAR Utpal * Department of Physics, Assam University, Silchar , India. 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 ev, which decreases to 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. Key Words: Graphyne; HN3 molecule; DFT; Electronic property; Adsorption; Gas sensor 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 and 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 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, Here we have studied the interaction of hydrazoic acid (HN3) Received: August 2, 2017; Revised: September 19, 2017; Accepted: October 9, 2017; Published online: October 16, * Corresponding author. utpalchemiitkgp@yahoo.com; Tel.: JD is thankful to Department of Science and Technology, New Delhi, India for the INSPIRE Fellowship Award (Grant No. DST/INSPIRE Fellowship/2015/IF150892). Editorial office of Acta Physico-Chimica Sinica

2 538 Acta Physico-Chimica Sinica Vol.34 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 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 Results and discussion 3.1 Electronic structure In our present study, we have placed HN3 molecule in various adsorption sites (parallelly and perpendicularly) of the molecule with respect to graphyne sheet (see Fig.S1 of Supporting Information). Depending on their adsorption energy we have investigated the most stable configuration. The result reflects that parallel orientation is more stable in comparison to perpendicular orientation due to minimum energy in parallel configuration, which well agrees with our previous finding 38. The optimized structure of pristine and HN3 adsorbed graphyne system is shown in Fig.1 and detailed analysis of various parameters such as optimal distance (D), adsorption energy (Eads), energy gap (Eg), Mulliken charge transfer (Q) and electric dipole moment (μ) of the minimum energy structure of HN3 adsorbed on graphyne sheet are represented in Table 1. Similar to BX3 (X = F, Cl and I) molecule 38 there is also no notable structural deformation observed in graphyne sheet due to the presence of HN3 molecule on it. The bond length between the C atoms of intrinsic graphyne remains unaltered not only in our present study of the adsorption of HN3 molecule on graphyne sheet but also in the case of several other molecules such as NH3 39, HCN 25 etc. on graphyne system. The adsorption energy of HN3 adsorbed on graphyne system is found to be ev without considering the van der Waals (vdw) correction and when vdw dispersion correction is taken care of, the adsorption energy of the same system is found to be ev. The negative adsorption energy value in both the cases confirms the stability of the HN3 adsorbed on graphyne system but the stability of system increases to a certain extent when vdw correction is considered. When NH3 39, HCN 25 are adsorbed on hydrogen terminated pristine graphyne, adsorption energy value of NH3, HCN adsorbed graphyne are found to be ev, ev respectively, using B3LYP functional and 6-31G(d) basis set. It is already reported 40 that graphyne can also sense formaldehyde and the adsorption energy is 0.40 Fig.1 Optimized geometry of (a) pristine graphyne; (b) HN3 molecule adsorbed graphyne system (top view); (c) HN3 molecule adsorbed graphyne system (side view).

3 No.5 doi: /PKU.WHXB 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 E ads/ev E g/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 HN 3 adsorbed graphyne ev which is higher than our observed adsorption energy. Since our system possesses lower adsorption energy compared to other small gas molecules such as NH3, HCN and formaldehyde so HN3 adsorbed graphyne system is more stable. Thus, reflects its capability of acting as a sensor. The optimal distance (D) is the minimal distance between the pristine system and the interacting molecule. Here the optimal distance is found between the C atom (nearest to HN3 molecule) of graphyne sheet and H atom of HN3 molecule. The magnitude of D is Å (Table 1) for both, with and without vdw correction. The larger value of optimal distance and smaller magnitude of adsorption energy confirms the weak interaction of HN3 molecule with graphyne sheet. This means weak physical adsorption of HN3 molecule has taken place on graphyne system 25,28,39. Also, the adsorption energy value of BX3 (X = F, Cl and I) molecule adsorbed on graphyne system 38 is much higher in comparison to HN3 adsorbed graphyne system. 3.2 Electronic properties 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. In HCN 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 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 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 Band structure analysis (a) Fig.2 Band structure of HN3 molecule adsorbed graphyne system (a) without vdw correction; (b) with vdw correction. (b)

4 540 Acta Physico-Chimica Sinica Vol.34 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 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 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 KT b conductivity is proportional to e 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) Density of states The total density of states (DOS) and projected density of states (PDOS) considering with and without vdw correction of each of the constituted atoms of HN3 molecule adsorbed graphyne system showing their individual orbital contribution has been picturized in Fig.3. There is no spin splitting in the system since it shows zero magnetism. So only the up-spin of the system has been plotted. The valence band (VB), as well as the conduction band (CB) in the total DOS, is primarily Eg - contributed by the energy states of C atom. The N and H atoms add some states to the TDOS. The absence of any energy state on the Fermi level (Fig.4) confirms that HN3 molecule adsorbed system is showing semiconducting behavior. The Fig.3(a, b) clearly shows that C atom is dominating the HOMO, LUMO and their adjacent energy states in the TDOS. The VB region of C atom [Fig.3a(ii)] is strictly governed by the pz orbital except 2.7 to 2.0 ev where the px and py orbitals overlap each other and dominate over the contribution of pz orbitals. In the CB region, again the magnitude of pz orbital of C atom is dominating and is spread all over before it vanishes at the extreme part of our considered energy range. But, as we move (a) (b) 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 PDOS of pristine graphyne and HN3 adsorbed graphyne.

5 No.5 doi: /PKU.WHXB farther away from the Fermi level, towards the higher energy range, it is observed that the px and py orbitals start contributing together from 3.4 ev onwards. It may also be noted that in a very small region between 3.5 to 4.1 ev, the contribution from pz orbital of C atom is suppressed by px and py orbitals. Again, the states of px and py orbitals arise from 4.6 ev onwards and dominate in the rest of the region of C atom (where pz vanishes). The s orbital of C atom hardly adds to the DOS. On the other hand, the contribution of the only s orbital of H atom is negligible with two peaks of least magnitude, one at VB and the other at CB region [Fig.3a(iii)]. However, the peak at CB is comparatively of greater amplitude than that of VB. In the case of N atom, the VB and the CB are composed of two peaks, each one from the contribution of px and pz orbitals [Fig.3a(iv)] separately. The highest energy states of px falls between 3.2 to 2.5 ev in the VB region and 3.0 to 3.3 ev in the CB region, while that of pz it lies from 1.5 to 1.4 ev in the VB region and 4.1 to 4.6 ev in the CB region of the N atom. Interestingly, the peak of the pz orbital of N atom is higher than that of px orbital in the VB area and vice versa in case of the CB region. Our study on the DOS analysis of HN3 adsorbed graphyne system is dominated especially by the energy states of the pz orbital of C atom, which well matches with the DOS of pristine graphyne 13,38, where the pz orbital of C atom is dominant. We find that the presence of the dopant (HN3) in graphyne produces less impact on the TDOS. Specifically, the graph of TDOS of the adsorbed system is found to be very similar to that of its pristine counterpart only with an increase in the amplitude. Further, vdw correction produced no effect on the DOS and PDOS of the HN3 adsorbed graphyne system. In order to verify the intrinsic mechanism of the change of band gap, especially the contribution of the band near the Fermi level, the zoom version of pz orbital of contributing atoms of pristine graphyne and HN3 adsorbed graphyne system is presented in Fig.4. For the pristine system, the pz orbital of C atom starts contributed at ev away from the Fermi level. However, when HN3 is adsorbed, the pz orbitals started contributing nearly from the Fermi level, and this is the main reason for lowering the band gap apart from the very small contribution of the pz orbital of N atom which was missing for the pristine system. 4 Conclusions In order to investigate the sensing behavior of graphyne for the detection of HN3 molecule based on first principle calculations with vdw correction, we have studied the adsorption of HN3 molecule on pristine graphyne. The result shows that parallel orientation of HN3 molecule on graphyne sheet is found to be more stable in contrast to transverse orientation and is in good accordance with our previous findings. The negative value of adsorption energy clearly reflects the stability of HN3 adsorbed graphyne system. The smaller magnitude of adsorption energy and higher value of optimal distance signifies that physisorption of HN3 molecule occurs on graphyne system whereas BX3 (X = F, Cl and I) molecule is chemisorbed on it. The band structure analysis indicates that HN3 adsorbed graphyne is also exhibiting semiconducting characteristics like pristine as well as BX3 (X = F, Cl and I) adsorbed graphyne system. The band gap decreases slightly with respect to the pristine system which is also confirming the increase in chemical reactivity profile of the HN3 adsorbed graphyne system. It has been noticed that similar to BCl3, BI3 and other molecules adsorbed graphyne system, HN3 molecule adsorbed graphyne system also behaves like an n-type semiconductor. The pristine graphyne possesses zero dipole moment but a considerable value of dipole moment, even higher than BF3 adsorbed graphyne system, is obtained when the interaction of HN3 molecule occurs with pristine graphyne. The PDOS analysis reveals that pz orbital of C atom is mostly contributed to the total density of states of HN3 adsorbed graphyne system. Finally, the electronic properties of the graphyne system are highly influenced due to the presence of HN3 molecule on it which ensures the possibility to use graphyne system for the identification of hydrazoic acid in the atmosphere. 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