Theoretical Studies on Interaction Between Methanol and Functionalized Single-Walled Carbon Nanotubes

Transcription

1 Commun. Theor. Phys. 55 (2011) Vol. 55, No. 2, February 15, 2011 Theoretical Studies on Interaction Between Methanol and Functionalized Single-Walled Carbon Nanotubes M.D. Ganji, 1, M. Goodarzi, 2 M. Nashtahosseini, 1 and A. Mommadi-nejad 3 1 Department of Chemistry, Islamic Azad University, Ghaemshahr Branch, Mazandaran, Iran 2 Department of Chemistry, Islamic Azad University, Arak Branch, Young Researchs Club, Iran 3 Department of Computer Engineering, Olom Fonon University of Mazandaran, Babol, Iran (Received April 30, 2010; revised manuscript received June 7, 2010) Abstract We study the adsorption of a methanol molecule on single-walled carbon nanotubes (SWCNTs) with various diameters and chiral angles by using the density functional theory based calculations. We find that methanol prefers to be adsorbed physically on the exterior surface of chiral nanotubes in comparison to the armchair and zigzag tubes with binding energy of about 2.76 kcal/mol, which is consistent with recent experimental and theoretical investigation results. We further consider the adsorption of methanol on the exterior surface and edge site of functionalized SWCNTs. The obtained results indicate that the binding energy of methanol is significantly increased for adsorption on the sidewall of functionalized nanotubes. It is also found that the adsorption of methanol at the edge site of both functionalized and pristine SWCNT is remarkably different (chemisoption process) in comparison to the exterior sidewall of the tubes. Furthermore, the electronic structures and Mulliken charge population of the considered complexes at their ground state are discussed within the context. PACS numbers: Pm, Bc, Mb, h, De Key words: Fuel cells, methanol, functionalized SWCNTs, adsorption, ab initio calculations 1 Introduction For the past decade or so, great interest has been dedicated to the interaction and adsorption of different compounds with carbon-based nanostructures, especially on single-wall nanotubes (SWNTs). SWNTs can be thought as fundamental cylindrical structure and these form the building blocks of both multi-wall nanotubes and the ordered arrays of single-wall nanotubes called ropes. However one of the most important applications of SWNTs is adsorption of small molecules which depends on the availability of their internal pore volume that can be varied by subjecting them to different heat treatment processes that open their ends [1 2] and remove functional groups that block pore entry. [3] However purity of SWNTs is also one of the most important parameter that influences the overall adsorptivity of a compound. [1] However the gases adsorption onto SWNTs have become of interest because of its wide potential applications in a lot of fields such as gas sensors and submicroscopic electronic devices. [4 5] Recently, based on this application of SWNTs, several valuable studies have been carried out. [6 7] The adsorption of methanol on SWNTs is one of the most interesting applications of SWNTs because of the methanol characteristics. Methanol naturally is producing in the anaerobic metabolism of varieties of bacterial, which can find ubiquitous in the environment and is an alternate fuel for internal combustion which is known with formula CH 3 OH and often abbreviates MeOH. However, encouraging fuel cells are based on methanol (MeOH) which is safe, renewable, and easily storable. [8] The adsorption and decomposition of MeOH has been studied greatly on metal surfaces, [9] alloys, [10] and metal oxides. [11] On the other hand, several study on methanol adsorption in carbon nanotubes have been done, for instance, Burghaus et al. [12] performed thermal desorption spectroscopy (TDS) technique for the methanol, n-pentane and methanol/npentane co-adsorption on carbon nanotubes (CNTs) supported on silica. They found that only two structures are present in MeOH TDS consistent with the dominance of strong MeOH MeOH hydrogen bonding, although alkane TDS indicates three different adsorption sites (external, groove, internal) in agreement with earlier studies. Ellison et al. [13] based on fourier transform infrared spectroscopy and two-level ONIOM calculations investigated the adsorption of methanol and ethanol on SWCNTs. Their result indicated that methanol does not adsorb onto SWC- NTs at room temperature, despite of the fact that ethanol adsorbs molecularly on SWCNTs. Obviously, computational study such as density functional theory (DFT) methods can be useful in this field of study on account of prevent of time and cost consuming. Note that based on density functional theory at the level of Becke3, Lee Yang Parr (B3LYP) method, the adsorption of methanol on the perfect and defective SWCNTs was investigated by Tang. [14] They used the cluster models in jointing with the density functional theory. They found that methanol Corresponding author, ganji md@yahoo.com c 2011 Chinese Physical Society and IOP Publishing Ltd

2 366 Communications in Theoretical Physics can be adsorbed very weakly on the sidewall of perfect SWCNT, which is in agreement with experiment observation. In this research, we attempt a theoretical study on the adsorption of methanol on functionalized SWCNTs by using DFT-based treatments. 2 Computational Method The calculations were performed in the framework of the self-consistent charge-density-functional-based tightbinding (SCC-DFTB) method.[15] We used the recently developed first-principles package DFTB+.[16] The DFTB+ uses a tabulated set of integrals derived from ab initio DFT calculations,[17] leading to a substantial speedup of the method. Unlike conventional tight-binding method it is possible to produce parameterizations capable of accuracy close to LDA/GGA with minimal adjustable parameters and also transferable between different systems. The basis functions of the DFTB method are also available, allowing the reconstruction of actual wave functions from the calculations. Further details of the method have been fully reviewed for instance in Refs. [16] [19]. In this work the Slater Koster (S K) type parameter set[15] was implemented. The dispersion corrects for the van der Waals interaction have also been considered via the Slater Kirkwood type model.[20] From the well known expression for calculating the molecular binding energies, Eb are obtained for various cases of our study. Eb = ECNT-MOH E CNT EMOH. where ECNT-MOH is the total energy of the adsorbed Vol. 55 methanol on the CNT, ECNT is the total energy of the pristine CNT and EMOH is the total energy of the isolated methanol. 3 Result and Discussion In this study the adsorption of methanol on different types of SWCNTs has been investigated. We first examine the adsorption of methanol on the exterior surface of the armchair (6, 6) structure as typical metallic nanotube and with similar diameter (7.83 A ) zigzag (semiconducting) (10, 0) nanotube (8.14 A ). SWCNT-methanol system is optimized starting from different initial geometrical configurations. In all the cases the hydroxyl ( OH) active site is initially oriented so that the oxygen atom is the closest to the nanotube, and the free electron pair of oxygen is directed perpendicular to the SWNT surface. To examine the interaction between the methanol and considered SWCNTs, four possible configurations were selected for a molecule approaching the outer sidewall of the tubes. In the starting configuration, the oxygen atom situated above the top site directly over a carbon atom (C Top) while, in the second and third configurations, the oxygen is positioned over the centers of two nonequivalent C C bonds, namely, those perpendicular and non-perpendicular to the nanotube axis for armchair nanotube, respectively (or parallel and non-parallel to the axis in the case of zigzag SWCNT). Finally, the oxygen atom is situated over the center of six-member ring of the tube. The respective orientation schemes employed in modeling methanol adsorption are represented in Fig. 1. Fig. 1 (Color online) Model for different adsorption states for methanol on the sidewall of the CNT (6, 6) (a) over a carbon atom, upon the centers of two nonequivalent C C bonds (b) perpendicular and (c) non-perpendicular to the nanotube axis and (d) above the center of a hexagon of carbon atoms. (e) Methanol molecule approaching the surface of the CNT (10, 0) via its OH group.

3 No. 2 Communications in Theoretical Physics 367 The optimized SWCNTs and methanol structures are used for the molecule adsorption. After full structural optimization of the considered systems we find that methanol prefers to be adsorbed on the semiconducting (10, 0) nanotube above the top site directly over a carbon atom, C Top configuration. For the energetically favorable state the calculated binding energy E b and C O equilibrium distance are about 0.11 ev ( 2.63 kcal/mol) and Å, respectively. The relatively far equilibrium methanol-nanotube substrate separation and small binding energy suggest the involvement of only non-covalent interactions in the adsorption. [21 27] Furthermore, the results show that the bond lengths of methanol exhibit only small changes during its binding to the SWCNTs (the lengths of the C O and O H bonds of methanol become longer (increasing from to Å and from to Å, respectively)). These findings are consistent with both experimental observation [13] and theoretical result at the DFT-B3LYP level of theory. [14] The calculated binding energy for methanol on the (6, 6) CNT is about 2.36 kcal/mol, which is also in good agreement with the obtained results by DFT-B3LYP method. [14] We now investigate the effect of nanotube s chirality on methanol adsorption on the outer surface of SWCNTs. For this propose, similar initial orientations were selected for the methanol approaching the surface of the (8, 4) CNT, which is a semiconducting nanotube with the chiral angle of about 19. Following a similar procedure employed in previous systems, we started by carrying out the optimization process for the molecule adsorption. Our calculation results show that methanol prefers to interact with the C atom of the tube with a binding energy of 2.76 kcal/mol. It can be found from the obtained results that the binding energy of methanol is rather increased for adsorption on the higher chiral angle nanotubes ((8, 4) nanotube in comparison to the (10, 0) nanotube with chiral angle of around 0 ). These binding energies indicate that (8, 4) chiral, (10, 0) zigzag, and (6, 6) armchair have almost the same binding energies to each other and chiral nanotubes possess the most strong physisorption process. Fig. 2 Model for a methanol molecule approaching the sidewall of the CNT (8, 4) above the center of a hexagon of carbon atoms, (a) side-view and (b) front-view. In addition, (7, 0) zigzag SWCNT was used to find the effect of curvature (diameter) on the methanol adsorption process. The calculated binding energy for methanol on the (7, 0) nanotube was determined to be 2.46 kcal/mol, which is weaker physisorption process than the (10, 0) counterpart. Comparing these results with those obtained for the methanol/cnt (10, 0) system, we clearly see that the binding energy of the methanol is increased for adsorption on larger diameter CNTs with low curvature. This is because of the further remove of the carbon atoms from the atoms of the methanol in high curvature CNT in comparison to the corresponding case on a low curvature CNT. It is generally recognized that the pristine surface of CNTs is inert so, it is important to develop proper techniques to improve adhesion through surface modification of CNTs. It is known that nitric acid can create functional groups, such as CHO, COOH and OH on the surface, which can open the closed tips of the tubes and act as nucleation center. [28] In this work, we intend to evaluate the influence of introduced functional groups on the methanol adsorption. Following a similar procedure employed in the previous systems, we start by carrying out the optimization process for the methanol molecule approaching to the introduced functional groups on the sidewall of the (10, 0) SWCNT. The orientation schemes employed in modeling methanol adsorption are shown in Fig. 3. Our calculation results show that methanol prefers to be adsorbed on the functionalized CNT by COOH group with a binding energy of about 8.75 kcal/mol. The calculated binding energies for the CHO and OH functional groups are about 7.03 and 7.68 kcal/mol, respectively. From the present results we find that, the adsorption of methanol on the outer surface of functionalized SWCNT is remarkably different and the calculated binding energies signifying a strong physisoption process for the methanol adsorption. The optimized geometric structures of the considered complexes are represented in Fig. 3 which indicate that the methanol is situated so that the O atom is adjacent to the H atom of all the respected functional groups. We now evaluate the adsorption of methanol at the defective site of the pure open-ended SWCNT. Similar calculations procedure has been performed for the structural optimization of attached molecule to the considered nanotubes. The obtained results indicate that methanol bound stronger to the CNT (10, 0) in comparison to the (8, 4) and (6, 6) CNTs with the binding energies of about , and kcal/mol, respectively. Figure 4 represents the optimized geometric structures of the considered complexes. As it can be seen from the figure, in the case of (10, 0) CNT, a C O bond is formed between the adsorbed methanol and end-cage of the nanotube while, the methanol bounds physically to the defective site of other nanotube counterparts. These results are also in good agreement with the obtained results by using the DFT-B3LYP level of theory. [14] Consequentially, these results exhibit the critical effect of local arrangement of edge carbon atoms on the adsorption of methanol on the open-ended SWCNT.

4 368 Communications in Theoretical Physics Vol. 55 Fig. 3 Model for a methanol molecule approaching the sidewall of the functionalized CNT (10, 0) by (a) OH, (b) CHO and (c) COOH groups. (d) (f) The optimized geometric structure for the adsorption states of methanol interacting with the considered nanotubes. Fig. 4 The optimized geometric structure for the adsorption states of methanol interacting with edge site of the pure nanotubes (a) CNT (8, 4), (b) CNT (10, 0) and (c) CNT (6, 6). Finally, we consider the adsorption of methanol at the edge site of open-ended functionalized SWCNTs, which expected to be quite different from that on the sidewall of SWCNTs.[14] The calculated binding energies indicate that the adsorption of methanol at the edge site of functionalized CNT by COOH group is remarkably different with the binding energy of about kcal/mol, indicating a strong chemisoption process for the methanol adsorption. Figure 5 shows the optimized geometries for the adsorption of methanol at the edge sites of functionalized SWCNTs. As it can be seen from the figure the OH-bond of methanol is broken and the methanol joint to the edge site of the nanotube. The separation CH3 O and H fragments are well bonded to the suspending edge site of functionalized SWCNT, which leads to the high exothermicity. Similar behavior is observed for the adsorbed methanol on the edge site of the functionalized CNT by OH and CHO groups with binding energies of about and kcal/mol, respectively. From the present obtained results one can conclude that the COOH group can increase the reactivity of SWCNTs while the functionalized nanotubes by OH and CHO groups might be less reactive in comparison to the pristine nanotubes. Further insight with respect to the methanol adsorption on the considered SWCNTs can be gained from total electron density maps of the electronic densities. Figure 6 represents calculated isosurface maps for Methanol/CNT (10, 0)-(COOH) and Methanol/CNT (10, 0)-(OH) complexes. Those are obtained with the Troullier Martine Pseudo Potentials OpenMX[29] computer codes. For adsorbed methanol on the surface of the functionalized CNT (10, 0) by COOH group (Fig. 6(a)), we find that the physically adsorbed methanol, which is far from the tube has

5 No. 2 Communications in Theoretical Physics almost no effect on the electronic charge distribution of C atoms of the tube, and thus no charge transfer between the methanol and CNT orbitals occurs. However, in the case of the methanol interacting with the edge site of func- 369 tionalized CNT (CNT (10, 0)-OH system), (Figs. 6(b)), it is clearly revealed that strong hybridization of the O (and H) atom(s) with nanotube states occurs, resulting in a significant charge transfer in the system. Fig. 5 The optimized geometric structure for the adsorption states of methanol interacting with edge site of the functionalized nanotube by (a) OH, (b) CHO and (c) COOH groups. Fig. 6 Isosurface of the total electron density for (a) a MOH molecule adsorbed on the sidewall of the functionalized CNT by COOH group and (b) a MOH molecule adsorbed at the edge site of the functionalized CNT by OH group where 0.05 was used as an isovalue of total electron density. (c) and (d) represent the calculated orbitals localized at the top most valence band (HOMO) of the considered systems of (a) and (b), respectively. (The absolute values of the isosurfaces of the wavefunctions are 0.02). To evaluate the higher reactivity of edge sites in comparison to the exterior surface of the tubes we also analyze the electronic structures and Mulliken analysis for the considered complexes. Figure 6 represents the highest occupied molecular orbital (HOMO) electron density of the optimized complexes for the Methanol/CNT (10, 0) (COOH) and Methanol/CNT (10, 0) (OH) systems (Figs. 6(c) and 6(d), respectively). From this figure, it is obvious that the HOMO is highly localized on the edge sites of the open-ended nanotube. Due to strong potential of the edge site atoms, the valance charge density is strongly accumulated around the C atoms, resulting in large asymmetry in charge distribution, displaying features of typical ionic bindings. By analyzing the obtained results, it is found that accumulated charges upon the edge site of the nanotube can improve the binding capability because they increase the binding energy of methanol. Further, the electron density is located on both the nanotube and methanol molecules, which leads to a stronger interaction between them in comparison to the adsorbed methanol on the outer surface of the tube (methanol/cnt (10, 0)-COOH complex), in which the electron density

6 370 Communications in Theoretical Physics Vol. 55 is localized only on the nanotube. This effect seen in methanol and nanotube complexes can be explained by analyzing the charge transfer between them. Charge analysis shows 0.04e charge transferred from the methanol to the functionalized CNT (10, 0) by COOH group while, about 0.26e are found to have been transferred from the methanol to the CNT in the case of methanol adsorbed on the edge site of the functionalized CNT by OH group. The study of the electronic structures and Mulliken analysis emphasizes that there exists a weak interaction, physisorption, between the methanol and the exterior surface of CNTs while existence of accumulated charges on the edge site of open-ended nanotubes make them good candidate materials for bindings to methanol. 4 Conclusions In summary, we have investigated the interaction of methanol with various types of SWCNTs by using the density functional theory (DFT) calculations. Several possible configurations are selected for a methanol approaching the substrate of nanotubes. The calculated results show that methanol is weakly bound to the outer surface of SWCNTs, consistent with the recent experimental and theoretical investigation at the DFT-B3LYP level of theory. Adsorption energies show that methanol prefers to be physisorbed on the exterior surface of the chiral semiconducting nanotubes, with binding energy of about 2.76 kcal/mol, in comparison to the armchair and zigzag ones. We further consider the adsorption of methanol on the exterior surface of functionalized SWCNTs by OH, CHO and COOH groups as well as at the edge site of both pure and functionalized SWCNTs. The obtained results indicate that the binding energy of methanol is significantly increased for adsorption on the functionalized nanotubes in comparison with the pure one. It is also found that the adsorption of methanol at the edge site of functionalized SWCNT by COOH group is remarkably different from the pure SWCNT as well as functionalized nanotubes by OH and CHO groups. The study of the electronic structures and Mulliken population analysis suggest that for the methanol adsorption at the edge site of nanotubes, significant hybridization between the respective orbitals of the two entities takes place and the adsorption results in strong bonding and charge transfer from methanol toward the nanotube. Acknowledgement The authors gratefully acknowledge support of this work by the Islamic Azad University of Ghaemshahr. References [1] S. Agnihotri, M.J. Rood, and M. Rostam-Abadi, Carbon 43 (2005) [2] A. Fujiwara, K. Ishji, H. Suematsu, H. Kataura, et al., Chem. Phys. Lett. 336 (2001) 205. [3] D.B. Mawhinney, V. Naumenko, A. Kuznetsova, J.T. Yates Jr, et al., Chem. Phys. Lett. 324 (2000) 213. [4] M. Grujicic, G. Cao, and R. Singh, Appl. Surf. Sci. 211 (2003) 166. [5] R. Pati, Y. Zhang, S.K. Nayak, and P.M. Ajayan, Appl. Phys. Lett. 81 (2002) [6] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, and H. Dai, Science 287 (2000) 622. [7] X. Feng, S. Irle, H. Witek, K. Morokuma, R. Vidic, and E. Borguet, J. Am. Chem. Soc. 127 (2005) [8] A.L. Dicks, J. Power Sources 156 (2006) 128. [9] I.E. Wachs, Surf. Sci. 544 (2003) 1. [10] C. Panja, N. Saliba, and B.E. Koel, Surf. Sci. 395 (1998) 248. [11] M.A. Henderson, S. Otero-Tapia, and M.E. Castro, Faraday Discuss. 114 (1999) 313. [12] U. Burghaus, D. Bye, K. Cosert, J. Goering, A. Guerard, E. Kadossov, E. Lee, Y. Nadoyama, N. Richter, E. Schaefer, J. Smith, D. Ulness, and B. Wymore, Chem. Phys. Lett. 442 (2007) 344. [13] M.D. Ellison, S.T. Morris, M.R. Sender, J. Brigham, and N.E. Padgett, J. Phys. Chem. C 111 (2007) [14] Z.R. Tang, Physica B 405 (2010) [15] M. Elstner, D. Porezag, G. Jungnickel, J. Elsner, M. Haugk, Th. Frauenheim, S. Suhai, and G. Seifert, Phys. Rev. B 58 (1998) [16] B. Aradi, B. Hourahine, and Th. Frauenheim, J. Phys. Chem. A 111 (2007) [17] G. Seifert, D. Porezag, and Th. Frauenheim, Int. J. Quantum Chemistry 58 (1996) 185. [18] Th. Frauenheim, G. Seifert, M. Elstner, Z. Hajnal, G. Jungnickel, D. Porezag, S. Suhai, and R. Scholz, Phys. Stat. Sol. 271 (2000) 41. [19] Th. Frauenheim, G. Seifert, M. Elstner, T. Niehaus, C. Kohler, M. Amkreutz, M. Sternberg, Z. Hajnal, A. Di Carlo, and S. Suhai, J. Phys.: Condensed Matter 14 (2002) [20] M. Elstner, P. Hobza, Th. Frauenheim, S. Frauenheim, and E. Kaxiras, J. Chem. Phys. 114 (2001) [21] S. Gowtham, R.H. Scheicher, R. Pandey, S.P. Karna, and R. Ahuj, Nanotechnology 19 (2008) [22] M.D. Ganji, Phys. Lett. A 372 (2008) [23] M.D. Ganji, Nanotechnology 19 (2008) [24] M.R. Gholami and M.D. Ganji, Iran JOC 2 (2009) 5. [25] M.D. Ganji, Phys. E 41 (2009) [26] M.D. Ganji, Diamond Relat. Mater. 18 ( 2009) 662. [27] M.D. Ganji, M. Asghary, and A.A. Najafi, Commun. Theor. Phys. 53 (2010) 987. [28] B.C. Satishkumar, A. Govindaraj, J. Mofokeng, G.N. Subbanna, and C.N.R. Rao, J. Phys. B 29 (1996) [29] DFT OpenMX code is available on the web site