Adsorption of H 2 O, CO 2, O 2, Ti and Cu on Graphene: A molecular modeling approach

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1 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: Adsorption of H 2 O, CO 2, O 2, Ti and Cu on Graphene: A molecular modeling approach Hilal S Wahab a,*, Salam H. Ali b, Adi M.Abdul Hussein b a Department of Chemistry, College of Sciences, Al-Nahrain University, P.O.Box 64090, Baghdad, Iraq b School of Applied Sciences, University of Technology, Baghdad, Iraq Tel: hswahab@gmail.com; hswahab@yahoo.com Abstract-- In the present work, we investigate the adsorption of small molecules namely, H 2 O, CO 2, O 2 and deposition of Ti and Cu on graphene substrate using molecular modeling calculations. The adsorption of small molecules has very little effect on the electronic properties of graphene. The deposition of metallic atoms presented high molecular doping, i.e., charge transfer and consequently better adsorption energies and stronger dipole moments. Index Term-- Graphene, Molecular calculations, Adsorption, Deposition 1. INTRODUCTION The synthesis of monolayer graphene and the experimental observation of Dirac charge carriers in this model have attracted immense attention and deepened the interest in this two dimensional molecule [1,2]. Since graphene, which is the mother of all known graphitic forms, namely buckyballs, rolled nanotubes and stacked graphite, is distinguished with its unusual structural and electronic flexibility, it could be tailored chemically and structurally in numerous ways; deposition of metal atoms [3] or molecules [4] on top; incorporation of nitrogen or boron in its structure [5] and using different substrates that modify the electronic structure [6]. Furthermore, the main resource for various electrical and optical applications is stemmed due to the interactions between graphene and various chemical molecules [7]. Therefore, this one atom thin 2D material [8] and the deposition and adsorption of different atoms and molecules on its surface have been the subject of numerous experimental [2] and theoretical studies since its discovery in 2004 [9-11]. Wehling et al.[9] have investigated the water adsorption on graphene and the impacts of SiO 2 substrate using computational first principles methodology. Further, Nakamura et al.[10] have studied the variation in the structural and electronic properties of graphene upon oxygen adsorption. In this contribution we employ here a computational method to simulate the adsorption of some small molecules namely water, oxygen and carbon dioxide and further the deposition of titanium and cupper atoms and probe their impacts on the atomic and electronic properties of the pristine graphene. The method is presented briefly in the next section and the obtained results follow. Finally, in the conclusions we summarize our results. 2. COMPUTATIONAL METHOD The quantum chemical model calculations were performed with the MSINDO software package [12, 13].This method is based on a semiempirical simplification of complex molecular matrix elements and permits accurate and efficient molecular orbital (MO), calculations for many-atom configurations [14]. The method has been extensively documented for the first-, second- and third-row main group elements and first-row transition metal elements [12, 13, 15]. The MO calculations were carried out at the level of the self-consistent field (SCF) method and energy was calculated with the structure optimized alongside, with energy convergence criterion below 10-9 Hartree. Additionally, the inner shell electrons are taken into account through the use of Zerner ps eudo potentials [16]. Besides, MSINDO is a reliable method for studies of surface properties and further, reproduces adsorption energy values comparable to higher-level density functional theory (DFT) calculations [17].

2 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: For modeling graphene using the supercell technique, we considered a 8x8x1 unit cell containing 128 carbon atoms. The in-plane lattice parameter a = 2.42 Ả for graphene agrees well with other theoretical reported values [7,18]. The adsorption energies, E ads of the molecules on graphene were computed according to the following expression; E ads = E molecule/graphene E pristine graphene E molecule (1) Where E molecule/graphene, E pristine graphene and E molecule are the total energies of the fully relaxed configurations of the molecule/graphene, pristine graphene and molecule, respectively. 3. RESULTS AND DISCUSSION 3.1 Geometry Graphene is composed of carbon atoms arranged in hexagonal structure as presented in Fig. 1a. The structure begins with six carbon atoms, tightly bound together chemically in the shape of what is known as a large assembly of benzene rings that are linked in a sheet of hexagons, i.e.; honeycomb structure. The geometry is made out triangular lattice with a basis of two A-B atoms per unit cell [19] (Fig. 1a). The C-C distance and C-C-C angle have been computed in this work and equal 1.40 Ả and degrees, respectively. These findings are in an excellent accordance with the results of Malard et al. [20], 1.42 Ả and Rosas et al. [21], 1.41 Ả and degrees. The 8x8x1 unit cell which contains 128 carbon atoms has presented a binding energy of a.u. (Table I) which confirms the stability of the modeled geometry. From Fig.2 a, we observe the asymmetric bands of the Highest Occupied Molecular Orbitals (HOMOs) and Lowest Occupied Molecular Orbitals (LUMOs) and further, a broken zone on the minus sign site. Neto et al.[19] have reported, in their study of the electronic properties of graphene, that the plus sign applies to the upper (π*) and the minus sign the lower (π) band, i.e.; HOMO. In addition, they concluded that the electron-hole symmetry was broken and the π and π* bands become asymmetric. Another interesting result is that the broken zone revealed computed gap energy of 3.27 ev (Fig. 2a). This computational finding is in agreement with other reported observations by Rosas et al. [21], 2.95 ev and Martinez et al. [22], 2.8 ev. Nevertheless and in contrary to the findings of Rosas et al. [21], this study has shown zero dipole moment for the modeled graphene (Table I). 3.2 Adsorption of small molecules After ensuring the description of the geometry, the feature of small molecules viz. O 2, CO 2, H 2 O adsorption onto graphene surface was examined. In this simulation the CO 2 and O 2 molecules were initially positioned in a conformation parallel to the graphene surface at and Ả distances, respectively. Upon optimization of the adsorption models, a very slight migration (0.1 Ả) of the considered molecules towards the surface was observed (Figs. 1b and d). This insignificant adsorption with low E ads values (Table I) confirms low susceptibility of these small molecules towards the graphene surface. The adsorption of H 2 O molecule onto the graphene surface via parallel and perpendicular configurations was also examined. From Table I, we learn that the E ads value for the parallel orientation of water is, to some extent, higher than that of perpendicular arrangement and also than that of O 2, CO 2 adsorbates. According to our MSINDO calculations, graphene behaves as a semiconductor material with gap energy of 3.27 ev. This behavior was also reported by other authors [21], for their density functional theory (DFT) calculations. No impacts for the adsorption of the studied small molecules H 2 O, CO 2 and O 2 on the gap energy of graphene were observed, as illustrated in Figs. 2d, e and f, respectively. For further scrutinization, HOMO and LUMO magnitudes have been computed which subsequently verified the insignificant influence of the previously adsorbed molecules (Table II). Our findings are in good agreement with other computational study for Leenaerts et al. [23], who reported that graphene is highly hydrophobic and further, the adsorbed water molecule has very little effect on the electronic structure of graphene. 3.3 Deposition of metallic atoms We proceed with the study of the metallic titanium (Ti) and cupper (Cu) atoms deposition on single layered graphene surface (Figs. 1f and e, respectively). According to the best of our knowledge, the metallic deposition of Ti and Cu atoms on graphene surface has not previously been reported. Calandra and Mauri [24] have stated that the electronic flexibility promotes the graphene to be tailored chemically for the deposition of metal atoms. Fig. 1 shows some sort of distortion in the carbon polygon upon metallic atoms deposition. The high E ads values (Table I) for Ti ( kj/mol ) and Cu ( kj/mol) which confirm high susceptibility of these atoms towards the graphene surface could be the source of this slight distortion. The HOMO and LUMO energy levels of graphene, graphene+cu and graphene+ti are presented in Table II. From this table one can observe that in the deposition of Ti and Cu onto graphene surface, the interaction of these two metal atoms with HOMO of graphene predominates, and consequently lower negative HOMO values are merged in comparison with the HOMO values of other small molecules adsorption. Accordingly, the former case would require lower activation energy. Based on the above findings, the comparison of the DOSs of these two models in Figs.2b and 2c, with all other adsorption DOSs reveals explicitly lower band gaps of 1.25 and 2.66 ev for Ti and Cu, respectively. Moreover, along the HOMO/LUMO regions of graphene, there is also some titanium and copper character, which is probably due to the

3 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: chemisorptions phenomenon. In contrast, H 2 O, CO 2 and O 2 molecules have presented only a slight, contribution to solely LUMO zone of graphene, which are solely physisorbed. Hence, one can draw a conclusion that the contribution of both Ti and Cu atoms to the HOMO and LUMO bands is another proof of the strong interaction between the graphene surface and metal atoms. This finding is in accordance with the results of Pi and colleagues for transition metals doping on graphene [25]. Currently, a quantitative explanation for the binding of graphene to various metal surfaces is not intensively available in the literature [26]. Nevertheless, the so-called d-band model, developed to elucidate trends in binding of adsorbed molecules on transition metals [27], expects a stronger binding with decreasing occupation of the d orbital. Accordingly, the stronger binding or interaction of graphene with Ti than with Cu is consistent with this hypothesis. Winterlin and Bocquet [27] reported that DFT calculations have revealed that Cu and Ag with their filled d bands hardly interact with the graphene layer, whereas Ni and Co interact strongly. 3.4 Electronic structure The electronic structure of graphene due to the adsorption of small molecules and deposition of metallic atoms is a particularly important issue. It is substantial for understanding the chemical interaction with other species and is even more essential for the physical properties such as electron transport, which turns free standing graphene such a unique material [28,29]. A key issue to investigate the electronic structure is the charge transfer between the substrate (graphene) and adsorbents or deposited species. Giovannette et al. [30] have reported that graphene will be p-doped (n-doped) if the transition metals work function is larger (smaller) than graphene. Recently, DFT calculations predict the n-doped doping of graphene [31]. Leenaerts et al. [2] concluded that there are two different types of charge transfer mechanisms when molecules adsorb on graphene. One is the due to orbital hybridization which occurs for all molecules, but it results in small charge transfer in the case of physisorption processes as it is evidently shown in Table 3 for the small molecules, H 2 O, O 2 and CO 2, adsorption on graphene. Whilst, the second is due to the position of the HOMO and LUMO of the atoms or molecules with respect to the Dirac point of graphene. These charge transfers are relatively large, as it is clearly presented in Table 3 for the deposition of Ti and Cu. The above findings have been ratified due to the relatively high dipole moments of Ti and Cu in comparison to the dipole moments of the small molecules as it is obviously illustrated in Table 1. Based on the above findings, it is therefore, necessary to realize the difference between the different mechanisms when calculating charge transfers to get quantitatively reliable results. For further analysis, one could also observe, from Table 3, the electronic feature of adsorbent/adsorbate complex. The acceptor character of parallel and perpendicular oriented water molecules on graphene is in accordance with the experimental findings of Schedin et al. [32], where they found that the acceptor character is energetically favored on perfect graphene. On contrary, the CO 2 and O 2 act as donor molecules. On the other hand, the deposition of metallic atoms, Ti and Cu, induces the charge transfer from graphene surface into their partially empty 3d orbitals (Table III). Also from Tables I and 3, we observe that the strength of binding and adsorption energies are related to the values of the computed Löwdin charge population changes i.e., charge transfer. The high Löwdin charge population changes in the case of Ti and Cu atoms deposition onto pristine graphene illustrate explicitly the much higher values of adsorption energy than the binding energy of the atoms under consideration. Whilst, the binding and adsorption energy values (Table I) are comparable in the case of H 2 O, O 2 and CO 2, adsorption on graphene. These findings are also in good agreement with theoretical studies of the adsorption of molecules on single wall carbon nano tubes [33]. This suggests that some of the knowledge of adsorption on nanotubes could be transferable to graphene. 4. CONCLUSIONS The current MSINDO calculations investigate the electronic properties of graphene resulting from adsorption of H 2 O, CO 2 and O 2 and deposition of Ti and Cu. The adsorption of the small molecules H 2 O, CO 2 and O 2 have exhibited a comparable binding and adsorption energies, while, the adsorption energies of Ti and Cu deposition onto pristine graphene is approximately an order of magnitude larger than the binding energies of the deposited atoms. Charge transfers between a graphene and the adsorbed molecules are very small, whereas, the deposition of metallic atoms, Ti and Cu, induces the charge transfer from graphene surface into their partially empty 3d orbitals. ACKNOWLEDGEMENTS One of the authors (H.S.Wahab) thanks the IIE / SRF for the partial financial support of the research stay. REFERENCES [1] Y. Zhang, Y. W. Tan, H. L. Stormer, P. Kim, Nature 438, 201 (2005) [2] O. Leenaerts, b. Partoens, F. M. Peeters, Phys. Rev. B 77, (2008) [3] B. Uchoa, C. Y. Lin, A. H. Castro Neto, Phys. Rev. B77, (2008) [4] T. O. Wehling, K. S. Novoselov, S. V. Morozov, E. E. Vdovin, M. I. Katsnelson, A. K. Geim, A. I. 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4 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: [17] H. S. Wahab, A. D. Koutselos, Chem. Phys. 358, 171 (2009) [18] J. Ito, J. Nakamura, A. Natori, J. Appl. Phys. 103, (2008) [19] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K.Geim, The electronic properties of graphene Review modern phys. 81, 109 (2009). [20] L. M. Malard, m. a. Pimenta, G. Dresslhaus, M. S. Dresselhaus, Phys. Reports 473, 51 (2009). [21] J. J. Hernandez ROSAS, R. E. Ramirez Gutierrez, A. Escobedo- Morales, E. C. Anota, J. Mol. Model 17, 1133 (2011). [22] J. I. Martinez, I. Cabria, M. J. Lopez, J. A. Alonso, j. Phys. Chem. C 113, 939 (2009). [23] O. Leenaerts, B. Partoens, F. M. Peeters, Phys. Rev. B 79, (2009) [24] M. Calandra, F. Mauri, Phys. Rev. B 76, (2007). [25] K. Pi, K. M. McCreary, W. Bao, W. Han, Y. F. Chiang, Y. Li, S. W. Tsai, C. N. Lau, R. K. Kawakami, Phys. Rev. B 80, (2009) [26] J. Wintterlin, M. L. Bocquet, Surf. Sci. 603, 1841 (2009). [27] B. Hammer, J. K. Norskov, Adv. Catal. 45, 71 (2000). [28] M. Wilson, Phys. Today Jan., 21 (2006). [29] A. K. Geim, K. S. Novoselov, Nature Mater. 6, 183 (2007). [30] G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J. Van den Brink, P. J. Kelly, Phys. Rev. Lett. 101, (2008). [31] B. Huard, N. Stander, J. A. Sulpizio, D. G. Gordon, Phys. Rev. B 78, (2008). [32] F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, K. S. Novoselov, Nat. Mater. 6, 652 (2007). [33] J. Zhao, A. Buldum, J. Han, J. P. Lu, Nanotechnology 13, 195 (2002). T ABLE I ACOMPUTED ENERGIES FOR THE ADSORBED MOLECULES ONTO THE GRAPHENE SURFACE AET TOTAL ENERGY IN ATOMIC UNITS, A.U. ; ZPE ZERO POINT ENERGY; EB BINDING ENERGY ; Energy Graphene CO 2 O 2 H 2 O Par H 2 O Per Cu Ti E T, a.u ( ) ( ) ( ) ( ) ( ) ZPE, a.u E T, a.u. corrected, (-3.220) E b, a.u E ads, kj/mol DM, Debye

5 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: T ABLE II HOMO-LUMO ENERGIES FOR GRAPHENE AND THE ADSORBED MOLECULES ONTO GRAPHENE SURFACE, IN ATOMIC UNITS Molecule LUMO HOMO Graphene(G) CO 2 + G O 2 + G Par H 2 O + G Per H 2 O + G Cu + G Ti + G T ABLE III Computed Löwdin charge population changes of selected atoms for the graphene and the adsorbed molecules onto graphene surface (atom numbering shown in Fig.1) Atom Graphene CO 2 O 2 H 2 O H 2 O Cu Ti Par Per C C C C C C C C C C C, CO2 O, CO2 O, CO2 O, O O, O O, H2O H, H2O H, H20 Cu Ti

6 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: ( a ) C21 C12 C19 C4 C10 C3 C9 C8 C7 C22 A-B unit cell O C O ( b ) ( c ) H H O O O ( d ) Cu Ti ( e ) ( f ) Fig. 1. Adsorption modes of (b) CO 2; (c) H 2O; (d) O 2; (e) Cu; (f) Ti onto (a) Graphene surface

7 DOS (arbitrary units) International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No: (a) Graphene G Energy, ev Ti + G (b) Cu + G (c) (d) (e) (f) H 2 O + G CO 2 + G O 2 + G Fig. 2. Density of states (DOS) for adsorption modes of (b) Ti;(c) Cu;(d) H 2O;(e) CO 2; (f) O 2 onto (a) Graphene surface.