Adso rp tion of Hyd razoic Acid on Pristine Graphyne Sheet:A Com pu tational Study

DEB Jyotirmoy, PAUL Debolina, PEGU David, SARKAR Utpal

Department of Physics, Assam University, Silchar-788011, India.

1 In troduction

In recent years low dimensional structures1–5have gained immense attention due to their potential application in next-generation nanoelectronics6–8. Among these, graphyne9,10is 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 level11, while γ graphyne is a sem iconductor12.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-electronics13–16, optoelectronics17–19, spintronics20, for storing hydrogen21,22, as an electrode in batteries23,24, for the detection of gas molecules25–28and also as an energy storage device29,30.

Interaction of atoms and molecules w ith electromagnetic field modifies its ground as well as excited state reactivity trends in greater extent31–33. Similarly, when confinement of systems takes place its reactivity profile changes significantly compared to unconfined one34–37, which is one of the major reason for a large number of investigations on gas molecule interaction w ith 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 chem ical and biological hazardous elements present in the environment and alsofor the monitoring purposes. It has been also noticed that graphyne material is a promising candidate for designing of the gas sensor. Beheshtian et al.25have 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) molecule38. Some other gas molecules (CO, NH3, formaldehyde etc.), nucleobases and amino acid adsorption on graphyne have been investigated in order to see their sensing capability27,39–41.

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 HN3molecule on human being also results in multi-organ failure and even in many cases, it leads to death. On contact w ith heat, it becomes very dangerous explosive. Thus, some efficient methods should be designed in order to detect HN3molecule 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 Com pu tational m ethod

All the quantum chemical computations have been performed using density functional theory as suggested in SIESTA code42,43. The exchange-correlation functional part of the generalized gradient approximation (GGA) has been represented using Perdew-Burke-Ernzerhof (PBE)44form.Troullier-Martin type norm-conserving pseudo potentials45and 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 ? (1 ? = 0.1 nm) is maintained between the different layers of the graphyne system. Different orientations of the HN3molecule in different positions (on the top of triangular hollow, hexagonal hollow, acetylene linkage etc.) of graphyne have been tested tofind the ground state geometry of the HN3adsorbed graphyne system.

The adsorption energy of hydrazoic acid adsorbed on graphyne sheet can be determined using the relation:Eads= E(Graphyne + molecule) – E(Graphyne) –(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 Grimme46.

Fig.1 Optim ized geometry of (a) pristine graphyne; (b) Hmolecule adsorbed graphyne system (top view); (c) Hmolecu le adsorbed graphyne system (side view).

3 Resu lts and discussion

3.1 Elec tronic s truc tu re

In our present study, we have placed HN3molecule in various adsorption sites (parallelly and perpendicularly) of the molecule w ith 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 w ith our previous finding38.The optim ized structure of pristine and HN3adsorbed graphyne system is shown in Fig.1 and detailed analysis of various parameters such as optimal distance (D), adsorption energy(E ads), energy gap (E g), Mulliken charge transfer (Q) and electric dipole moment (μ) of the minimum energy structure of HN3adsorbed on graphyne sheet are represented in Table 1.Sim ilar to BX3(X = F, Cl and I) molecule38there is also no notable structural deformation observed in graphyne sheet due to the presence of HN3molecule on it. The bond length between the C atoms of intrinsic graphyne remains unaltered not only in our present study of the adsorption of HN3molecule on graphyne sheet but also in the case of several other molecules such as NH339, HCN25etc. on graphyne system.The adsorption energy of HN3adsorbed on graphyne system is found to be ?0.550 eV w ithout 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?0.725 eV. The negative adsorption energy value in both the cases confirms the stability of the HN3adsorbed on graphyne system but the stability of system increases to a certain extent when vdW correction is considered. When NH339, HCN25are adsorbed on hydrogen terminated pristine graphyne, adsorption energy value of NH3, HCN adsorbed graphyne are found to be?0.191, ?0.108 eV respectively, using B3LYP functional and 6-31G(d) basis set. It is already reported40that graphyne can also sense formaldehyde and the adsorption energy is 0.40 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 HN3adsorbed 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 HN3molecule) of graphyne sheet and H atom of HN3molecule. The magnitude of D is 2.884 ? (Table 1) for both, w ith and w ithout vdW correction.The larger value of optimal distance and smaller magnitude of adsorption energy confirms the weak interaction of HN3molecule w ith graphyne sheet. This means weak physical adsorption of HN3molecule has taken place on graphyne system25,28,39. Also, the adsorption energy value of BX3(X = F,Cl and I) molecule adsorbed on graphyne system38is much higher in comparison to HN3 adsorbed graphyne system.

Tab le 1 The op timal distance (D), adsorption energy (E ads), energy gap (E g), M u lliken charge transfer (Q) and electric dipole moment (μ) ofthe adsorp tion of hydrazoic acid on pristine graphyne.

Fig.2 Band structure of HN3 molecule adsorbed graphyne system (a) w ithout vdW correction; (b) w ith vdW correction.

3.2 Elec tronic p roperties

3.2.1 Charge transfer analysis

The magnitude of Mulliken charge transfer is 0.078e (Table 1) w ithout 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 HN3molecule and graphyne sheet and the smaller Q value again suggesting the physisorption of HN3molecule on graphyne sheet25. This charge transfer occurs from HN3molecule to pristine graphyne. In our previous study38, we have shown that BCl3 and BI3 adsorbed graphyne system behaves as an n-type semiconductor, likew ise HN3adsorbed 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 BCl3and BI3interact w ith 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 HN3molecule, 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 HN3molecule to graphyne. So the charge transfer direction of HCN adsorbed graphyne shows a reverse trend w ith respect to HN3adsorbed graphyne.Interaction of formaldehyde w ith graphyne system40also shows that formaldehyde is physically adsorbed on graphyne system similar to HN3adsorbed graphyne system but the charge transfer is quite large when formaldehyde correlates w ith graphyne system as compare to HN3interaction w ith graphyne system.

3.2.2 Dipole m om ent

The detailed study of dipole moment leads us to infer that although pristine graphyne has zero dipole moment the interaction of HN3molecule induces a significant amount of dipole moment into the system. The magnitude of dipole moment in case of HN3adsorbed graphyne system is found to be 0.173 Debye both for vdW and w ithout vdW correction. The sudden change in dipole moment may be detected w ith 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 HN3molecule. On comparing, it has been noticed that the magnitude of dipole moment in case of BF3adsorbed graphyne38is smaller than HN3 adsorbed graphyne whereas, increase in dipole moment are observed for BCl3and BI3adsorbed graphyne38system. Therefore, based on this comparison, we may conclude that the chance of detection of HN3molecule is higher than that of BF3molecule using graphyne as a host material.

3.2.3 Band structure ana lysis

In this work, we have analyzed the band structure of pristine graphyne when hydrazoic acid is adsorbed on it and presented a comparative discussion w ith its pristine counterpart. Our analysis and plot of band structure (see Fig.2) for HN3adsorbed 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. A lso, it has been observed that the valance band maximum (VBM) and the conduction band m inimum (CBM) of the HN3adsorbed graphyne system are located at the Г point of the hexagonal Brillouin zone and this observation of the VBM and CBM exactly matches w ith that of pristine graphyne38. For the pristine graphyne, the difference in energy level between the VBM and CBM is found to be approximately 0.453 eV13,38.But just when the adsorption of HN3molecule takes place on the surface of the pristine graphyne, its band gap gets changed and decreases to 0.424 eV (Table 1) w ithout 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 tow here E,K and Tgbrepresents band gap, Boltzmann constant and temperature respectively. Since the band gap decreases when pristine graphyne interacts w ith HN3molecule, the HN3adsorbed system possesses larger electrical conductivity than that of the pristine system. The decrease in band gap is also observed in case of formaldehyde interaction w ith graphyne system40.However, our previous study on BX3 (X = F, Cl and I)molecule adsorption on graphyne sheet shows that38, 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 w ith low energy gap are less chem ically hard and consequently shows relatively low chem ical hardness profile47,48. Hence from our present work, we can infer that as the band gap of the pristine graphyne decreases on HN3adsorption on its surface,the resulting system becomes more reactive as compared to the pristine structure but less reactive compare toformaldehyde adsorbed graphyne system40. 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 sem iconductor and it sustains this particular characteristic even when HN3 interacts w ith its surface. But the adsorption of HN3molecule on the graphyne system makes it an extrinsic sem iconductor (n-type).

Fig.3 Total density of states (DOS) and projected density of states(PDOS) of HN3 molecule adsorbed graphyne system,(a) w ithout vdW correction; (b) w ith vdW correction.

Fig.4 PDOS of pristine graphyne and HN3 adsorbed graphyne.

3.2.4 Density of states

The total density of states (DOS) and projected density of states (PDOS) considering w ith and w ithout vdW correction of each of the constituted atoms of ‘HN3 molecule adsorbed graphyne system’ show ing 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 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 Ferm i level (Fig.4) confirms that HN3molecule adsorbed system is show ing sem iconducting behavior. The Fig.3(a, b)clearly shows that C atom is dom inating 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 pxand pyorbitals overlap each other and dom inate over the contribution of pzorbitals. In the CB region, again the magnitude of pzorbital of C atom is dom inating and is spread all over before it vanishes at the extreme part of our considered energy range. But, as we move farther away from the Fermi level, towards the higher energy range, it is observed that the pxand pyorbitals 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 pzorbital of C atom is suppressed by pxand pyorbitals. Again,the states of px and py orbitals arise from 4.6 eV onwards and dom inate 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 w ith 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 pxand pzorbitals [Fig.3a(iv)]separately. The highest energy states of pxfalls 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 pzit 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 HN3adsorbed graphyne system is dom inated especially by the energy states of the pzorbital of C atom, which well matches w ith the DOS of pristine graphyne13,38, 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 sim ilar to that of its pristine counterpart only w ith 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 pzorbital of contributing atoms of pristine graphyne and HN3adsorbed graphyne system is presented in Fig.4. For the pristine system, the pzorbital of C atom starts contributed at ?0.058 eV away from the Ferm i 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 pzorbital of N atom which was m issing for the pristine system.

4 Conc lusions

In order to investigate the sensing behavior of graphyne for the detection of HN3molecule based on first principle calculations w ith vdW correction, we have studied the adsorption of HN3molecule on pristine graphyne. The result shows that parallel orientation of HN3molecule on graphyne sheet is found to be more stable in contrast to transverse orientation and is in good accordance w ith our previous findings. The negative value of adsorption energy clearly reflects the stability of HN3adsorbed 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 chem isorbed on it. The band structure analysis indicates that HN3adsorbed graphyne is also exhibiting sem iconducting characteristics like pristine as well as BX3(X =F, Cl and I) adsorbed graphyne system. The band gap decreases slightly w ith respect to the pristine system which is also confirming the increase in chemical reactivity profile of the HN3adsorbed graphyne system. It has been noticed that sim ilar to BCl3, BI3and other molecules adsorbed graphyne system,HN3molecule 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 BF3adsorbed graphyne system, is obtained when the interaction of HN3molecule occurs w ith pristine graphyne. The PDOS analysis reveals that pzorbital of C atom is mostly contributed to the total density of states of HN3adsorbed graphyne system. Finally, the electronic properties of the graphyne system are highly influenced due to the presence of HN3molecule on it which ensures the possibility to use graphyne system for the identification of hydrazoic acid in the atmosphere.

Acknow ledgm en t: US would like to acknow ledge the support from Prof. Paul W Ayers, Department of Chemistry,McMaster University, Canada, in various ways and SHARCNET Canada for providing computational facilities for this research work.

Suppo rting In fo rm ation: available free of charge via the internet at http://www.whxb.pku.edu.cn.

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