DETECTION OF NO 2 ADSORBED ON GRAPHYNE NANOTUBES

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1 DETECTION OF NO 2 ADSORBED ON GRAPHYNE NANOTUBES A.R. KARAMI 1, R. MAJIDI 2 1 Department of Chemistry, Shahid Rajaee Teacher Training University, Lavizan, Tehran, Iran, ar_karami@srttu.edu, arkaramigazafi@gmail.com 2 Department of Physics, Shahid Rajaee Teacher Training University, Lavizan, Tehran, Iran, r.majidi@srttu.edu, royamajidi@gmail.com Received April 16, 2015 We have investigated the possibility of using graphyne nanotubes for NO 2 detection by using density functional theory. The zigzag nanotubes based on α- graphyne are studied. The adsorption distance, adsorption energy, charge transfer, electronic band structures and density of states are calculated. The large adsorption distance and small adsorption energy indicate NO 2 molecule is adsorbed on graphyne nanotubes via physisorption. The zigzag graphyne nanotubes with semiconducting properties become p-type semiconductors with NO 2 adsorption. The sensitivity of the electronic properties of graphyne nanotubes to the presence of NO 2 indicates these nanomaterials are proper for NO 2 detection. Key words: Graphyne nanotube; α-graphyne; NO 2 ; electronic property. 1. INTRODUCTION The discovery of carbon allotropes such as graphene and carbon nanotubes (CNTs) has opened up exciting opportunities to develop promising multifunctional nanomaterials [1, 2]. The unique structural and electronic properties of graphene and CNTs offer great potential applications including nanoelectronics devices, gas storage materials, gas sensors and biosensors [2 4]. Among them, the possibility of using graphene and CNTs as an excellent gas sensor was reported in recent years [5 8]. It was shown that the electronic properties of graphene and CNTs are highly sensitive to gas environment. Upon exposure to O 2, NO, NO 2, CO, CO 2, NH 3, etc. the electrical conductivity of graphene and CNTs is dramatically changed [5 9]. Most recently, new allotropes of carbon named graphyne have been predicted and attracted great attention [10 12]. Graphyne is built by inserting an acetylenic linkage (-C C-) between two bonded carbons in graphene [10 12]. The presence of acetylenic groups in these structures introduces four types of graphyne, so-called α, β, γ, and (6, 6, 12)-graphyne with a rich variety of optical and electronic properties. These properties have been exploited to develop various applications. One of the most promising applications of graphyne is its utilizations as sensor. Graphyne based sensors were demonstrated to detect different molecules such as hydrogen peroxide, formaldehyde and different amino acids [13 15]. The results indicate that graphyne can be a good candidate for use as sensors. Rom. Journ. Phys., Vol. 60, Nos. 9 10, P , Bucharest, 2015

2 2 Detection of NO 2 adsorbed on graphyne nanotubes 1475 Analogous to CNTs which can be considered to be seamless cylinders of graphene sheets, graphyne nanotubes (GNTs) can be constructed following the same approach. In recent years, several theoretical studies have been carried out to investigate the structural and electronic properties of GNTs [16, 17]. It was found that these tubes show even richer variation in electronic properties than do ordinary CNTs. Nevertheless, the good sensor properties of graphene, CNTs and graphyne are already known for some times, the possibility of using GNTs as sensors has received less attention. Hence, we have studied the sensitivity of the electronic properties of GNTs to NO 2 in the present work. 2. COMPUTATIONAL DETAILS The calculations are performed based on the density functional theory (DFT) [18, 19] as implemented in the OpenMX code [20]. For the exchange and correlation functional, the generalized gradient approximation (GGA) is adopted [21]. To choose the cutoff energy, the total energies of GNTs as a function of cutoff energy are calculated. For instance, total energy of (5,0) GNT via cutoff energy is shown in the Figure 1. It is clear that total energy does not change for cutoff energy greater than 100 Ry. We have chosen a large cutoff energy (150 Ry). The charge transfer is calculated based on the Mulliken population analysis [22]. For calculations of the electronic band structures, 51 k-points are considered along the Γ-Z direction. Fig. 1 Total energy of (5,0) GNT via cutoff energy.

3 1476 A.R. Karami, R. Majidi 3 The zigzag (3,0), (4,0) and (5,0) GNTs based on α-graphyne are considered. The GNTs are placed in a tetragonal simulation box. The length of the simulation box along the tube axis is considered equal to the length of the unit cell of GNTs (12.09 Å). The periodic boundary conditions are applied to simulate an infinitely long GNT. The size of the simulation boxes perpendicular to the tube axis (20 Å) is considered large enough to provide empty spaces which avoids interactions between tubes. As an example, the unit cell of (5,0) GNT in the tetragonal simulation box is shown in Figure 2. The atomic structures of (3,0), (4,0) and (5,0) GNTs are optimized until the residual forces on each atom are below 0.01 ev/å. The C C and C C bonds lengths of the optimized GNTs are presented in Table 1. The axial (parallel to tube axis) and the circumferential (perpendicular to tube axis) bonds are labeled with (a) and (c) in Table 1, respectively. Table 1 Optimized bond lengths (Å) of GNTs (n,m) (3,0) (4,0) (5,0) C C (a) C C (c) C C (a) C C (c) After optimization, one NO 2 molecule is attached to the tube wall in each unit cell. The molecule is placed at different adsorption sites on GNTs. These sites are labeled with in Figure 2. At each adsorption site, the molecule is considered with three orientations respect to the surface of the GNTs: N or O close to the GNTs surface and NO 2 parallel to the GNTs. The nearest distance between GNT and NO 2 is also varied from 1.8 to 4 Å. The adsorption energies of all structures are calculated and compared. The structure with the lowest adsorption energy is chosen as the most stable structure. The adsorption energy is defined as E ads =E GNT+NO2 -(E GNT +E NO2 ) where E NO2 is the total energy of an isolated NO 2 molecule. E GNT+NO2 and E GNT are total energies of GNT with and without a NO 2 molecule adsorbed. The total energy is the sum of kinetic energy, electric part of screened Coulomb energy, difference electron-electron Coulomb energy, neutral atom potential energy, non-local potential energy, exchange-correlation energy and core-core Coulomb energy [20, 23].

4 4 Detection of NO 2 adsorbed on graphyne nanotubes 1477 Fig. 2 Illustration of unit cell of (5,0) GNT. 3. RESULTS AND DISCUSSION First, the most stable adsorption configurations of NO 2 on GNTs are determined. It is found that NO 2 prefers to adsorb on C atom with N atom close to the surface of GNT. Then, we found the minimum adsorption energy by varying the C-N distance. For instance, the most stable structure of NO 2 adsorbed on (5,0) GNT is shown in Fig. 3. The nearest distance between C atom of GNT and N atom of NO 2 is 2.0 Å. The adsorption energies of NO 2 on (3,0), (4,0) and (5,0) GNTs are 0.78, 0.62 and 0.45 ev, respectively. The small adsorption energies and large adsorption distances indicate that the adsorption of NO 2 on the GNTs is physisorption. (a)

5 1478 A.R. Karami, R. Majidi 5 (b) Fig. 3 Illustration of (a) side and (b) top view of NO 2 adsorbed on (5,0) GNT. Electronic band structures and density of states (DOS) of (3,0), (4,0) and (5,0) GNTs are shown in Fig. 4. The results indicate that these GNTs are direct band gap semiconductors. The (3,0), (4,0) and (5,0) GNTs have a band gap of 0.05, 0.34 and 0.35 ev, respectively. These results are in agreement with previous calculations which reports semiconducting properties of these zigzag GNTs [16, 17]. Here, the Fermi energy was taken to be 0 ev.

6 6 Detection of NO 2 adsorbed on graphyne nanotubes 1479 Fig. 4 Electronic band structures and DOS of (3,0), (4,0) and (5,0) GNTs.

7 1480 A.R. Karami, R. Majidi 7 Fig. 5 Electronic band structures and DOS of (3,0), (4,0) and (5,0) GNTs with adsorbed NO 2.

8 8 Detection of NO 2 adsorbed on graphyne nanotubes 1481 To clarify the effect of NO 2 adsorption on the electronic properties of GNTs, the electronic band structures and DOS of GNTs in the presence of NO 2 are shown in Fig. 5. When NO 2 is adsorbed on GNT, new states and sharp DOS peak are formed in the band gap above the Fermi level. It means GNTs become p-type semiconductors with NO 2 adsorption. This acceptor level comes from the electronic properties intrinsic to NO 2. The NO 2 molecule possesses an unpaired electron, which seems to actively participate in hybridization near the C atom when it is attached to the GNT. We have found that charge is transferred from GNTs to NO 2. The charge transfer from (3,0), (4,0) and (5,0) GNTs to NO 2 are 0.04, 0.03 and 0.03e, respectively. Hence, GNTs show semiconducting properties in the presence of acceptor molecule such as NO 2. Increasing the carrier concentration and conductivity of CNTs upon exposure to NO 2 is reported in the theoretical and experimental studies [9, 24, 25]. 4. CONCLUSIONS We have studied the effect of NO 2 on the electronic properties of GNTs using DFT. The small adsorption energy and large adsorption distance indicate that NO 2 is physisorped on the GNTs. The Mulliken population analysis reveals that charge is transferred from GNTs to NO 2 molecule. We have considered the zigzag (3,0), (4,0) and (5,0) nanotubes based on α-graphyne. These GNTs show semiconducting properties. Upon NO 2 adsorption, the GNTs become p-type semiconductors. The results indicate the electrical sensitivity of GNTs to the presence of NO 2. Acknowledgment. The work was supported by Shahid Rajaee Teacher Training University under contract number REFERENCES 1. M. Katsnelson, M. Iosifovich, Graphene: Carbon in two dimensions, 1 edition, Cambridge University Press, M.J. O Connell, Carbon nanotubes: properties and applications, Taylor and Francis group, CRC press, M.F.L. De Volder, S. H. Tawfick, R. H. Baughman and A. J. Hart, Science 339, 535 (2013). 4. T.K. Das and S. Prusty, Recent advances in applications of graphene, Int. J. Chem. Appl. 4, 39 (2013). 5. J. Zhao, A. Buldum, J. Han and J. P. Lu, Nanotechnology, 13, 195 (2002). 6. O. Leenaerts, B. Partoens and F. Peeters, Phys. Rev. B 77, (2008). 7. S. Jalili, R. Majidi, J. Comput. Theor. Nanosci. 3, 664 (2006). 8. L.H. Nguyen, T.V. Phi, P.Q. Phan, H.N. Vu, C. Nguyen-Duc and F. Fossard, Physica E 37, 54 (2007). 9. J. Kong, N.R. Franklin, C. Chou, M.G. Chaplin, S. Peng, K. Cho and H. Dai, Science 287, 622 (2000).

9 1482 A.R. Karami, R. Majidi A.L. Ivanovskii, Prog. Solid State Chem. 41, 1 (2013). 11. B.G. Kim, H. J. Choi, Phys. Rev. B 86, (2012). 12. D. Malko, C. Neiss, F. Vines, and A. Gorling, Phys. Rev. Lett. 108, (2012). 13. R. Majidi, A. Karami, Physica E 59, 169 (2014). 14. R. Majidi, A.R. Karami, Physica E 54, 177 (2013). 15. R. Majidi, A.R. Karami, Struch. Chem. 26, 5 (2015). 16. V.R. Coluci, D.S. Galvao, R.H. Baughman, J. Chem. Phys. 121 (2004) B. Kang, J. Y. Lee, Carbon 84, 246 (2015). 18. P. Hohenberg and W. Kohn, Phys. Rev 136, 864 (1964). 19. W. Kohn and L.J. Sham, Phys. Rev. B 140, 1133 (1965). 20. T. Ozaki, H. Kino, J. Yu, M. J. Han, N. Kobayashi, M. Ohfuti, F. Ishii, et al. User s manual of OpenMX version J. P. Perdew, K. Burke and M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77, 3865 (1996). 22. R.S. Mulliken, J. Chem. Phys. 23, 1833 (1955). 23. T. Ozaki and H. Kino, Phys Rev. B 72, (2005). 24. H. Chang, J. Do Lee, S.M. Lee and Y.H. Lee. Appl. Phys. Lett. 79, 3863 (2001). 25. J. Kong, N.R. Franklin, C. Chou, M.G. Chaplin, S. Peng, K. Cho and H. Dai, Science 287, 622 (2000).