Density Functional Study of the Adsorption of Methanol and Its Derivatives on Boron Nitride Nanotubes

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1 767 Density Functional Study of the Adsorption of Methanol and Its Derivatives on Boron Nitride Nanotubes Ali Ahmadi Peyghan 1 and Maziar Noei 2, * (1) Young Reserachers and Elite club, Central Tehran Branch, Islamic Azad University, Tehran, Iran. (2) Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran. (Received date: 2 April 213; Accepted date: 1 September 213) ABSTRACT: Changes in the structural and electronic properties of chemically modified boron nitride nanotubes (BNNTs) using methanol and its derivatives including CH 3 CH 2 CH 2 OH, CH 3 CH 2 OH, (ph)ch 2 CH 2 OH, CH 2 COOH and (CN)CH 2 CH 2 OH were investigated using density functional theory calculations. The study results showed that molecules of methanol can be chemically adsorbed on top of a sidewall B atom with an adsorption energy of.67 ev, which is stronger than that of carbon nanotubes. When using different derivatives of methanol, the adsorption energies and charge transfer from the adsorbate to the BNNT depending on the electron-withdrawing or electrondonating capability of the subgroups within the derivatives. Subgroups with strong electron-withdrawing capability generally lead to transfer less charge and smaller adsorption energy. The calculated density of state shows that the electronic properties of the BNNT are only slightly changed by the chemical modification. However, preservation of the electronic properties of BNNTs coupled with the enhanced solubility suggests that chemical modification of BNNTs with either methanol or its derivatives may be an effective way for purification of the BNNTs. 1. INTRODUCTION Ever since the discovery of carbon nanotubes (CNTs) (Iijima 1991), there have been significant research efforts to synthesize nanometer-scale tubular forms of various materials and to study their properties (Türker and Erkoç 22; Beheshtian et al. 211; Ahmadi et al. 212; Kong et al. 212; Peyghan et al. 212; Mukhopadhyay and Behera 213). As one of the isoelectronic structures for CNTs, boron nitride nanotubes (BNNTs) possess geometry and mechanical properties similar to CNTs due to identical σ bonds (sp 2 hybridization), but their electronic properties and chemical activities are different from that of CNTs, which is due to different spatial distributions of π states (Blase et al. 1994; Chopra et al. 199; Xie et al. 212). Unfortunately, like other nanotube materials, poor solubility and difficulties in purifying and processing have hampered the future application of BNNTs (Pal et al. 27). Now, chemically modified (i.e., functionalized) nanotubes are being extensively promoted as one of the solutions to overcome these problems. Despite many theoretical and experimental reports on chemical functionalization of CNTs (Chen et al. 1998; Sun et al. 22; Nakajima 27; Wang et al. 29), BNNT functionalization has largely remained unexplored (Xie et al. 2; Zhi et al. 2). The major issue is due to lack of an easy and effective method to introduce high density of functional groups on BNNTs, *Author to whom all correspondence should be addressed. noeimaziar@gmail.com (M. Noei).

2 768 Ali Ahmadi Peyghan and Maziar Noei/Adsorption Science & Technology Vol. 31 No subsequently making them chemically active, which could allow for further efficient nanotube modification (Dai et al. 211). Both non-covalent and covalent functionalization approaches have been used to functionalize BNNTs. Non-covalent functionalization approaches have advantages over covalent functionalization ones, as they can preserve the intrinsic properties of BNNTs such as high mechanical strengths and conductivity without disrupting the extended π-conjugation systems of BNNTs (Velayudham et al. 21). In a previous study, the surface of BNNTs has been functionalized with amine groups by ammonia plasma irradiation (Ikuno et al. 27). This method renders highly dispersible tubes in the common organic solvent chloroform. In 26, Wu et al. have investigated properties of (8, ) BNNTs chemically modified with NH 3 and four other amino-functional groups on the basis of density functional theory (DFT; Wu et al. 26). However, altering the surface chemistry of BNNTs is still in its early stages, and effective surface functionalization remains a challenging task. In this work, we report the results of a DFT study of functionalized BNNTs modified with methanol (CH 3 OH) and its five other derivatives, namely, CH 3 CH 2 OH, CH 3 CH 2 CH 2 OH, (CN)CH 2 CH 2 OH, (ph)ch 2 CH 2 OH and (COOH)CH 2 COH. 2. COMPUTATIONAL METHODS A zigzag single-walled (, ) BNNT consisting of 4 boron and 4 nitrogen atoms was considered and the end atoms were saturated by hydrogen atoms to avoid the boundary effects. We have performed density-functional calculations using B3LYP as exchange-correlation functional, which has been widely used and proved to be accurate enough for extensive systems including nanostructured materials (Kunsági-Máté and Nie 21; Qian et al. 211; Beheshtian et al. 212). All the calculations were performed using the standard all-electron basis set 6-31G (d) within the GAMESS package (Schmidt et al. 1993). Binding (or adsorption) energy of the adsorbate with the BNNT has been defined as follows: E ad = E adsorbate/bnnt E BNNT E adsorbate + E BSSE (1) where E adsorbate/bnnt is the total energy of an adsorbed molecule on the BNNT, and E BNNT and E adsorbate are the total energies of the pristine BNNT and an adsorbate molecule, respectively. The basis set superposition error (BSSE) has been corrected for all of the interactions. A negative value of E ad corresponds to the exothermic adsorption. To investigate the changes in electronic charge through the BNNTs, the net charge transfer between the adsorbate and BNNT has been calculated using the Mulliken population analysis, which is defined as the charge difference between the adsorbed molecule on the nanotube and an isolated one. 3. RESULTS AND DISCUSSION First, the accuracy of the used method in this work has been evaluated to describe the properties of a methanol molecule. The bond lengths of C O and individual C H as well as the bond angle of the C O H group in the free CH 3 OH are obtained as 1.41 Å, 1.9 Å and 17.6 from our approach, which are in good agreement with previously reported experimental values of 1.43 Å, 1.9 Å and 19. (Remediakis et al. 24), respectively. The optimized structure, geometry parameters and

3 Density Functional Study of Methanol Adsorption onto Boron Nitride Nanotubes 769 density of states (DOS) plot of the pristine BNNT have been shown in Figure 1, where two types of B N bonds can be identified, one with the bond length of 1.44 Å and parallel to the tube axis (the B1N2 bond in Figure 1), and another with the bond length of 1.46 Å, but not in parallel to the tube axis (the B1N3 bond in Figure 1). The results are in fair agreement with those predicted from an early theoretical study (Wu et al. 26). The charge analysis using the Mulliken method indicates that approximately.17e charges are transferred from the boron atom to its adjacent nitrogen atoms within the sidewall, and therefore the B N bonds of the sidewall are partially ionic. The calculated DOS (Figure 1) plot also shows that the tube is a semiconductor with the highest occupied molecular orbital/the lowest unoccupied molecular orbital (HOMO/LUMO) energy gap (E g ) of 3.8 ev. A fragment molecular orbital analysis reveals that the conduction level mainly originates from B atoms, whereas the valence level mainly comes from N atoms. H N B B1 N N E g = Figure 1. Optimized structure of BNNT and its DOS. Bonds lengths are in Angstrom. To find minimum adsorption configurations, the CH 3 OH molecule was initially placed at different positions above a BNNT on top of a sidewall B or N atom from its H or O atom. Full geometrical optimization was then performed with several orientations of the CH 3 OH molecule. To ensure that the most stable configuration is achieved, the initial distance between the molecule and the cluster was adjusted several times from 1. to 3. Å. After carful relax optimization, only three stable (local minimum) complexes of CH 3 OH BNNT were found, which are shown in Figure 2. The calculated adsorption energies are found to be in the range of.14 to.67 ev (Table 1). From a structural perspective, the CH 3 OH molecule was found to be adsorbed with two different orientations on sidewall of the BNNT (i.e. the hydrogen atoms of CH 3 or the oxygen atom close to the surface).

4 77 Ali Ahmadi Peyghan and Maziar Noei/Adsorption Science & Technology Vol. 31 No (P) E g = (Q) E g = (Z) E g = H N B C O Figure 2. Models for the three stable different methanol BNNT complexes and related DOS plots. Distances are in Angstrom.

5 Density Functional Study of Methanol Adsorption onto Boron Nitride Nanotubes 771 TABLE 1. Calculated Data for BNNT and Different Methanol BNNT Complexes Configuration E ad a E BSSE Q T ( e ) b E HOMO E LUMO E g c E g d BNNT P (1.8%) Q (.%) Z (.7%) In the table, all energies are in electron volt. a Adsorption energy per molecule. b Q T is defined as the total Mulliken charge on the molecules, and positive number means charge transfer from molecule to the BNNT. c HOMO/LUMO energy gap. d Change of HOMO/LUMO energy gap upon adsorption. Overall, two chemisorptions (P and Z) and one physisorption (Q) configurations were found for the CH 3 OH BNNT complex. In the configuration Q, the interaction distances between hydrogen atoms of the CH 3 of methanol and three N atoms of the tube surface are 3.7, 3.23 and 3.3 Å, respectively, along with a small E ad of.14 ev, indicating the physical nature of this interaction. The most stable configuration (Z, Figure 2) is the one in which oxygen atom of the CH 3 OH is close to a sidewall boron atom of the tube accompanied with the adsorption energy of.67 ev, indicating chemical nature of the interaction. In the second most stable configuration (P, Figure 2), a B O bond was formed between oxygen atom of the CH 3 OH molecule and the boron atom of the tube surface with a length of 1.69 Å. Upon adsorption, the adsorbing B atom is slightly pulled out of the tube surface, owing to the hybridization change from sp 2 to sp 3, which has also been confirmed by our natural orbital analysis. Moreover, we have examined the initial configurations in which the C or O atom of the CH 3 OH is close to the N atom of the tube surface. However, it was found that the CH 3 OH reoriented during the full relax optimization, and therefore, its O atom got close to the B atom of the BNNT. It can be described by noting the fact that in BNNT, the HOMO is localized on the N sites, and the LUMO on the B sites. As a result, the HOMO of the CH 3 OH donates electrons preferably to the LUMO centered on B sites of the BNNT. We also have investigated the effect of nanotube diameter on the E ad values. To this end, nanotubes with different chiralities including (6,), (7,) and (8,) were selected and the adsorption process was studied. The obtained values are approximately.6,.63,.6 ev, respectively. It indicates that the E ad is nearly a diameter-independent factor. To investigate the chemical functionalization of exterior surface of the BNNT, the properties of chemically modified BNNT were studied with five alcoholic functional groups, including CH 3 CH 2 OH, CH 3 CH 2 CH 2 OH, (ph)ch 2 CH 2 OH, (CN)CH 2 CH 2 OH and (COOH)CH 2 COH. The most stable structures are shown in Figure 3. We found that all the alcoholic groups can be chemically adsorbed on the BNNT sidewall and covalently bonded with a boron atom, resulting in formation of a B O bond. The B O bond length ranges from 1.67 to 1.74 Å. The corresponding adsorption energies and charge transfer from the five adsorbates to BNNT have been listed in Table 2.

6 772 Ali Ahmadi Peyghan and Maziar Noei/Adsorption Science & Technology Vol. 31 No (a) (b) (c) (d) (e) H N B C O Figure 3. Most stable configurations of chemically modified BNNTs with the adsorbates: (a) CH 3 CH 2 OH, (b) CH 3 CH 2 CH 2 OH, (c) (CN)CH 2 CH 2 OH, (d) (ph)ch 2 CH 2 OH and (e) CH 2 COOH. TABLE 2. Calculated Data for Different Chemical Functionalized BNNT Systems Functional Dipole component a E ad E BSSE moment (D) Q T ( e ) b E HOMO E LUMO c E g d E g CH 3 CH 2 CH 2 OH (4.9%) CH 3 CH 2 OH (.4%) CH 3 OH (.7%) (ph)ch 2 CH 2 OH (1.%) CH 2 COOH (.7%) (CN)CH 2 CH 2 OH (.7%) In the table, all energies are in electron volt. a Adsorption energy per molecule. b Q T is defined as the total Mulliken charge on the molecules, and positive number means charge transfer from molecule to the BNNT. c HOMO/LUMO energy gap d Change of HOMO/LUMO energy gap upon adsorption.

7 Density Functional Study of Methanol Adsorption onto Boron Nitride Nanotubes 773 The relative magnitude order of E ad for different functional groups is as follows: CH 3 CH 2 CH 2 OH > CH 3 CH 2 OH > CH 3 OH > (ph)ch 2 CH 2 OH > (COOH)CH 2 COH > (CN)CH 2 CH 2 OH The largest E ad belongs to the adsorbate with the functional group of CH 3 CH 2 CH 2 OH with a value of.7 ev. This phenomenon may be explained by the fact that the CH 2 CH 3 group is an electron-donating functional and can facilitate the electron transfer to the BNNT surface. This trend is also observed in the case of CH 3, having an E ad of.69 ev, and a net charge of.169e is transferred from the functional group to the BNN T. However, the CN group is also well known as an electron-withdrawing functional. In the case of (ph)ch 2 CH 2 OH, the resonance possibilities for a phenyl group allow it to serve as an electron-donating substituent. Although in (ph)ch 2 CH 2 OH, phenyl is an electron donor group with a Q T value of.169e, and the E ad value (.62 ev) is lower than that of CH 3 OH. This trend of charge transfer and change of E ad can be understood on the basis of the steric hindrance caused by the large phenyl group. Unlike R and ph, the COOH and CN are relatively strong electron-withdrawing functional groups. Thus, by replacing H by the COOH and CN groups, the charge transfer to the BNNT should be reduced, thereby weakening the interaction between the alcoholic functional group and the BNNT. Calculated E ad values for these cases are approximately.1 and.6 ev and a net charge of.12e and.12e is transferred from CH 2 COOH and (CN)CH 2 CH 2 OH to the BNNT, respectively. As the electric dipole moment is one of the properties that are traditionally used to discuss and rationalize the structure and reactivity of many chemical systems (Foresman and Frisch 1996), it is of great importance to obtain data of the electronic distribution in a molecule. When the mentioned adsorbates are added in the BNNT, dipole moment of the tube is significantly increased from.42 D in the pristine form to D in the different functionalized BNNT. We think that the increment in solubility mainly originates from the increased dipole moment. Interaction between the adsorbed molecule and BNNT may alter the electronic structure of the tube, which could be reflected by the change in electrical conductance of the tube. Tables 1 and 2 summarize the HOMO, LUMO and HOMO LUMO gaps (E g ) of different functionalized BNNT, indicating that the change is in the range of..7%. To verify the effects of adsorption of the considered molecules on the electronic properties of BNNT, the total electronic DOSs of the molecule BNNT systems were calculated and the results are shown in Figures 3 and 4. It can be found that the E g of the BNNT has no significant change after chemisorption of the alcoholic species and contribution from the functional groups is largely away from the Fermi level. Calculated profiles of HOMO and LUMO for CH 3 OH/BNNT in configuration Z (Figure ) demonstrate that the HOMO and LUMO have still remained on the BNNT surface (in agreement with the energy change). In summary, the chemically modified BNNT is still a semiconductor. Thus, chemical modification of BNNT with alcoholic species can be viewed as some kind of harmless modification. Because the chemical modification of BNNTs with either methanol or alcoholic functional groups results in little change in the electronic properties, more studies are needed to search for other chemical modification strategies to modify the electronic properties of BNNTs using molecular design. By contrast, we believe that the preservation of electronic properties of BNNTs coupled with the enhanced solubility makes the chemical modification of BNNTs with either methanol or its derivatives to be an effective way for purification of the BNNTs.

8 774 Ali Ahmadi Peyghan and Maziar Noei/Adsorption Science & Technology Vol. 31 No (a) (b) E g = 4.6 E g = (c) (d) E g = 4.7 E g = (e) E g = Figure 4. DOS plots for chemically modified BNNTs by (a) CH 3 CH 2 OH, (b) CH 3 CH 2 CH 2 OH, (c) (CN)CH 2 CH 2 OH, (d) (ph)ch 2 CH 2 OH and (e) CH 2 COOH. (a) (b) E LUMO = 2.27 ev E HOMO = 6.34 ev Figure. (a) LUMO and (b) HOMO profiles for the CH 3 OH BNNT complex in configuration Z.

9 Density Functional Study of Methanol Adsorption onto Boron Nitride Nanotubes CONCLUSION We have investigated structural and electronic properties of BNNTs chemically modified with methanol and its five derivatives using DFT calculations. It was found that NH 3 can be chemically adsorbed on top of the B atom, with a charge transfer from methanol to the BNNT. The relative magnitude order of adsorption energy for the studied adsorbates was found to be as follows: CH 3 CH 2 CH 2 OH > CH 3 CH 2 OH > CH 3 OH> (ph)ch 2 CH 2 OH > CH 2 COOH > (CN)CH 2 CH 2 OH The trend of adsorption energy change can be correlated with the trend of relative electronwithdrawing or electron-donating capability of the functional groups. Overall, the chemical modification of BNNTs with methanol or its derivatives results in little changes in the electronic properties of these tubes. The obtained results suggest that the chemical modification of BNNTs with either methanol or its derivatives may be an effective way for purification of the BNNTs. REFERENCES Ahmadi, A., Hadipour, N.L., Kamfiroozi, M. and Bagheri, Z. (212) Sens. Actuators, B. 161, 12. Beheshtian, J., Kamfiroozi, M., Bagheri, Z. and Ahmadi A. (211) Phys. E 44, 46. Beheshtian, J., Peyghan, A.A. and Bagheri Z. (212) Phys. E 44, Blase, X., Rubio, A., Louie, S.G. and Cohen, M.L. (1994) Europhys. Lett. 28, 33. Chen, J., Hamon, M.A., Hu, H., Chen, Y., Rao, A.M., Eklund, P.C. and Haddon, R.C. (1998) Science 282, 9. Chopra, N.G., Luyken, R.J., Cherrey, K., Crespi, V.H., Cohen, M.L., Louie, S.G. and Zettl A. (199) Science 269, 966. Dai, X.J., Chen, Y., Chen, Z., Lamb, P.R., Li, L.H., du Plessis, J., McCulloch, D.G. and Wang, X. (211) Nanotechnology 22, Foresman, J.B. and Frisch, A.E. (1996) Exploring chemistry with electronic structure methods, Second ed., Gaussian Inc., Pittsburgh, PA. Iijima, S. (1991) Nature 34, 6. Ikuno, T., Sainsbury, T., Okawa, D., Frechet, J.M.J. and Zettl, A. (27) Solid State Commun. 142, 643. Kong, I., Shanks, R., Ahmad S. and Yu, L.J. (212) Adsorpt. Sci. Technol. 9, 463. Kunsági-Máté, S. and Nie, J.C. (21) Surf. Sci. 64, 64. Mukhopadhyay, G. and Behera, H. (213) Adsorpt. Sci. Technol. 1, 39. Nakajima, T. (27) Solid State Sci. 9, 777. Pal, S., Vivekchand, S.R.C., Govindaraj, A. and Rao, C.N.R. (27) J. Mater. Chem. 17, 4. Peyghan, A.A., Omidvar, A., Hadipour, N.L., Bagheri Z. and Kamfiroozi, M. (212) Phys. E 44, 137. Qian, W., Zhang, Y., Wu, Q., He, C., Zhao, Y., Wang, X. and Hu, Z. (211) J. Phys. Chem. C 11, Remediakis, I.N., Abil-Pedersen, F. and Nørskov, J.K. (24) J. Phys. Chem. B 18, 143. Schmidt, M., Baldridge, K.K., Boatz, J.A., Elbert, S.T., Gordon, M.S., Jensen, J.H., Koseki, S., Matsunaga, N., Nguyen, K.A., Su, S., Windus, T.L., Dupuis, M. and Montgomery, J.A. (1993) J. Comput. Chem. 14, Sun, Y.P., Fu, K.F., Lin, Y. and Huang, W.J. (22) Acc. Chem. Res. 3, 196. Türker, L. and Erkoç Ş. (22) J. Mol. Struct.: THEOCHEM 77, 131. Velayudham, S., Lee, C.H., Xie, M., Blair, D., Bauman, N., Yap, Y.K., Green, S.A. and Liu, H. (21) Appl. Mater. Interfaces 2, 14. Wang, X., Zhang, F., Xia, B., Zhu, X., Chen, J., Qiu, S., Zhang, P. and Li, J. (29) Solid State Sci. 11, 6. Wu, X., An, W. and Zeng, X.C. (26) J. Am. Chem. Soc. 128, 121.

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