Interaction of Substituted Aromatic Compounds with Graphene

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Langmuir 2009, 25, 210-215

Interaction of Substituted Aromatic Compounds with Graphene Alain Rochefort*,† and James D. Wuest‡ De´partement de ge´nie physique and Regroupement que´be´cois sur les mate´riaux de pointe (RQMP), E´cole Polytechnique de Montre´al, Montre´al, Que´bec H3C 3A7, Canada, and De´partement de chimie, UniVersite´ de Montre´al, Montre´al, Que´bec H3C 3J7, Canada ReceiVed July 16, 2008. ReVised Manuscript ReceiVed September 17, 2008 We have modeled the adsorption of various substituted derivatives of benzene on a graphene sheet, using a firstprinciples density functional theory-local density approximation method. The presence of functional groups can significantly alter the overall magnitude of π-π interactions between the adsorbed molecules and graphene by giving rise to strong medium-range interactions involving π-orbitals of the substituents. When the substituents can simultaneously permit the formation of hydrogen bonds between adsorbed molecules, it is possible to evaluate the relative contributions of hydrogen bonding and π-based interactions to the overall adsorption. Adsorption of individual molecules and hydrogen-bonded aggregates reflects a hierarchical balance of the different interactions that determine the overall energy of adsorption.

Introduction The adsorption of molecules on solid surfaces is a fundamental process that has been studied intensively for many decades. The recent advent of scanning probe microscopy (SPM) has triggered explosive growth of the field and has led to a much deeper understanding of the details of molecular adsorption.1 Of special importance have been studies of adsorption on ordered conducting surfaces such as those of metals and graphite. Particularly informative are studies on such surfaces in which multiple methods of characterization and analysis are used in concert, including (1) direct examination of adsorption by SPM, (2) comparison of the observed 2D patterns with those seen in 3D structures by X-ray crystallography, (3) systematic alteration of the structure of the adsorbate to reveal how the observed 2D pattern changes, and (4) thorough theoretical analysis to help reveal the origin of the observed adsorption. Even when these powerful tools are used in combination, predictions of how a particular compound will be adsorbed remain unreliable, and the relative importance of the many factors that control adsorption is hard to foresee. In recent work,2 we and others have attempted to direct molecular adsorption by using a strategy that has proven effective in engineering 3D crystals. This strategy is based on the use of particular compounds that engage in multiple strong directional interactions, thereby positioning neighboring molecules in predictable ways.3 Such molecules are typically composed of a core of well-defined geometry, attached to functional groups that control intermolecular association by forming directional interactions such as hydrogen bonds. Predicting how these molecules will be adsorbed requires an understanding of (1) how the basic core will interact with surfaces, (2) how the functional groups will modify the behavior of the core and simultaneously bind independently to the surface, * Corresponding author. † ´ Ecole Polytechnique de Montre´al. ‡ Universite´ de Montre´al. (1) For recent reviews, see (a) Barth, J. V. Annu. ReV. Phys. Chem. 2007, 58, 375–407. (b) Otero, R.; Rosei, F.; Besenbacher, F. Annu. ReV. Phys. Chem. 2006, 57, 497–525. (c) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290–4302. (e) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679. (f) Moresco, F. Phys. Rep. 2004, 399, 175–225. (g) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139–150. (h) Samorı´, P.; Rabe, J. P. J. Phys.: Condens. Matter 2002, 14, 9955–9973. (i) Hooks, D. E.; Fritz, T.; Ward, M. D. AdV. Mater. 2001, 13, 227–241. (j) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491–501.

and (3) how the formation of interadsorbate interactions will further influence adsorption. Many heats of adsorption have been measured on diverse surfaces, thereby providing a valuable source of experimental data that can be used to assess the intrinsic affinities of different molecules.4 However, these data do not reveal how adsorption occurs, and they do not allow the energy of adsorption to be partitioned quantitatively into individual contributions arising from the effect of the core, the substituents, and their intermolecular interactions. This information is urgently needed to accelerate the development of molecules that can be used to create predictable nanopatterns on surfaces. To provide new insight, we have undertaken a systematic theoretical analysis of the adsorption of representative substituted derivatives of benzene on the (0001) surface of graphite, using a graphene (2) For representative references, see (a) Xiao, W.; Feng, X.; Ruffieux, P.; Gro¨ning, O.; Mu¨llen, K.; Fasel, R. J. Am. Chem. Soc. 2008, 130, 8910–8912. (b) Blunt, M.; Lin, X.; Gimenez-Lopez, M. d. C.; Schro¨der, M.; Champness, N. R.; Beton, P. H. Chem. Commun. 2008, 2304–2306. (c) Li, Y.; Ma, Z.; Qi, G.; Yang, Y.; Zeng, Q.; Fan, X.; Wang, C.; Huang, W. J. Phys. Chem. C 2008, 112, 8649– 8653. (d) Saywell, A.; Magnano, G.; Satterley, C. J.; Perdiga˜o, L. M. A.; Champness, N. R.; Beton, P. H.; O’Shea, J. N. J. Phys. Chem. C 2008, 112, 7706–7709. (e) Otsuki, J.; Arai, Y.; Amano, M.; Sawai, H.; Ohkita, M.; Hayashi, T.; Hara, M. Langmuir 2008, 24, 5650–5653. (f) Zhou, H.; Dang, H.; Yi, J.-H.; Nanci, A.; Rochefort, A.; Wuest, J. D. J. Am. Chem. Soc. 2007, 129, 13774–13775. (g) Nath, K. G.; Ivasenko, O.; MacLeod, J. M.; Miwa, J. A.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. J. Phys. Chem. B 2007, 111. (h) Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40, 16996–17007. (i) Surin, M.; Samorı`, P.; Jouaiti, A.; Kyritsakas, N.; Hosseini, M. W. Angew. Chem., Int. Ed. 2007, 46, 245–249. (j) Lena, S.; Brancolini, G.; Gottarelli, G.; Mariani, P.; Masiero, S.; Venturini, A.; Palermo, V.; Pandoli, O.; Pieraccini, S.; Samorı`, P.; Spada, G. P. Chem. Eur. J. 2007, 13, 3757–3764. (k) Sto¨hr, M.; Wahl, M.; Spillmann, H.; Gade, L. H.; Jung, T. A. Small 2007, 3, 1336–1340. (l) Ruben, M.; Payer, D.; Landa, A.; Comisso, A.; Gattinoni, C.; Lin, N.; Collin, J.-P.; Sauvage, J.-P.; De Vita, A.; Kern, K. J. Am. Chem. Soc. 2006, 128, 15644–15651. (m) Wan, L.-J. Acc. Chem. Res. 2006, 39, 334–342. (n) Pawin, G.; Wong, K. L.; Kwon, K.-Y.; Bartels, L. Science 2006, 313, 961–962. (o) Kampschulte, L.; Lackinger, M.; Maier, A.-K.; Kishore, R. S. K.; Griessl, S.; Schmittel, M.; Heckl, W. M. J. Phys. Chem. B 2006, 110, 10829–10836. (p) Jeong, K. S.; Kim, S. Y.; Shin, U.-S.; Kogej, M.; Hai, N. T. M.; Broekmann, P.; Jeong, N.; Kirchner, B.; Reiher, M.; Schalley, C. A. J. Am. Chem. Soc. 2005, 127, 17672–17685. (q) Tao, F.; Bernasek, S. L. J. Am. Chem. Soc. 2005, 127, 12750–12751. (r) Otero, R.; Scho¨ck, M.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Angew. Chem., Int. Ed. 2005, 44, 2270–2275. (s) Li, Z.; Han, L. J.; Wandlowski, Th. Langmuir 2005, 21, 6915–6928. (t) Gyarfas, B. J.; Wiggins, B.; Zosel, M.; Hipps, K. W. Langmuir 2005, 21, 919. (u) Lu, J.; Lei, S.-b.; Zeng, Q.-d.; Kang, S.-z.; Wang, C.; Wan, L.-j.; Bai, C.-l. J. Phys. Chem. B 2004, 108, 5161–5165. (v) Scho¨nherr, H.; Crego-Calama, M.; Vancso, G. J.; Reinhoudt, D. N. AdV. Mater. 2004, 16, 1416–1420. (3) For reviews, see (a) Wuest, J. D. Chem 2005, 5830–5837. (b) Hosseini, M. W. Acc. Chem. Res. 2005, 38, 313–323. (4) For example, see: Kiselev, A. V. Quart. ReV. (London) 1961, 15, 99–124.

10.1021/la802284j CCC: $40.75  2009 American Chemical Society Published on Web 11/24/2008

Interaction of Aromatic Compounds with Graphene

sheet as a model. This analysis has yielded a deeper understanding of adsorption, including (1) a dissection of the individual contributions of the benzene core and its substituents and (2) the relative importance of interadsorbate hydrogen bonding and adsorbate-surface interactions in determining how adsorption occurs.

Computational Details Calculations of electronic structure were based on the local density approximation (LDA) of density functional theory (DFT), using the NWChem package.5,6 In the calculations, a model graphene sheet containing 204 carbon atoms and 40 hydrogen atoms was kept fixed, and the C-C and C-H bond distances were set at 1.46 and 1.01 Å, respectively. The basis sets used for carbon and hydrogen atoms in the graphene model were STO-3G and 6-31G*, respectively. For the aromatic substrates, we used the STO-3G basis set for atoms of carbon and 6-31G* for atoms of hydrogen, nitrogen, oxygen, and chlorine. The molecular structures were fully optimized by the quasiNewton method until a gradient convergence factor better than 10-5 hartree/bohr was reached. The bond dissociation energies of the complexes were calculated with respect to the appropriate groundstate species asymptote. During the optimization steps, all species except the graphene sheet were free to move. Covalent bonding of species to graphene can cause structural deformation associated with the transformation of sp2 carbon atoms into sp3 atoms.7 Such drastic changes of hybridization are not expected in the physisorbed-like complexes we have investigated, but minor relaxation may help stabilize the complexes. We acknowledge that our simple single-layer model for graphite may not be adequate for accurately describing interactions when the adsorbate is close to the surface or when the commensurability of large ordered arrays of adsorbates is affected by relatively long-range interactions. On the other hand, our rigid graphene model offers an attractive opportunity to compare the adsorption of molecules on the flat surface of HOPG with that on the curved surface of carbon nanotubes. The LDA potential appears to be an appropriate choice for describing weakly bound systems such as π-stacked complexes or those in which molecules interact with the sp2-like surface of graphite.8-11 In contrast, application of the generalized-gradient approximation (GGA) to two graphene sheets underestimates the small binding energy and gives an interlayer distance that is too large, and similarly skewed results are obtained for two parallel benzene molecules.9 In these systems, LDA gives values that are very close to the experimental results and to high-level quantum chemistry calculations.9 For the specific case of the adsorption of benzene on graphite, which gives a pure π-dispersion complex, the reported binding-energy values for the two most stable adsorption (5) Kendall, R. A.; Apra`, E.; Bernholdt, D. E.; Bylaska, E. J.; Dupuis, M.; Fann, G. I.; Harrison, R. J.; Ju, J.; Nichols, J. A.; Nieplocha, J.; Straatsma, T. P.; Windus, T. L.; Wong, A. T. Comput. Phys. Commun. 2000, 128, 260–283. (6) Straatsma, T. P.; Apra`, E.; Windus, T. L.; Dupuis, M.; Bylaska, E. J.; de Jong, W.; Hirata, S.; Smith, D. M. A.; Hackler, M.; Pollack, L.; Harrison, R.; Nieplocha, J.; Tipparaju, V.; Krishnan, M.; Brown, E.; Cisneros, G.; Fann, G.; Fruchtl, H.; Garza, J.; Hirao, K.; Kendall, R.; Nichols, J.; Tsemekhman, K.; Valiev, M.; Wolinski, K.; Anchell, J.; Bernholdt, D.; Borowski, P.; Clark, T.; Clerc, D.; Dachsel, H.; Deegan, M.; Dyall, K.; Elwood, D.; Glendening, E.; Gutowski, M.; Hess, A.; Jaffe, J.; Johnson, B.; Ju, J.; Kobayashi, R.; Kutteh, R.; Lin, Z.; Littlefield, R.; Long, X.; Meng, B.; Nakajima, T.; Niu, S.; Rosing, M.; Sandrone, G.; Stave, M.; Taylor, H.; Thomas, G.; van Lenthe, J.; Wong, A.; Zhang, Z. NWChem, A Computational Chemistry Package for Parallel Computers, Version 4.5; Pacific Northwest National Laboratory: Richland, WA, 2003. (7) (a) Boukhvalov, D. W.; Katsnelson, M. I. Phys. ReV. B 2008, 78, 085413. (b) Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008, 130, 10697– 10701. (8) Girifalco, L. A.; Hodak, M. Phys. ReV. B 2002, 65, 125404(1)125404(5) (9) Tournus, F.; Charlier, J.-C. Phys. ReV. B 2005, 71, 165421(1)165421(8) (10) Tournus, F.; Latil, S.; Heggie, M. I.; Charlier, J.-C. Phys. ReV. B 2005, 72, 075431(1)-075431(5) (11) Woods, L. M.; Baˇdescu, S¸. C.; Reinecke, T. L. Phys. ReV. B 2007, 75,155415(1) –155415(9).

Langmuir, Vol. 25, No. 1, 2009 211 sites (stacked and bridged) range from 0.229 to 0.350 eV for LDA,9,12 whereas GGA suggests that benzene is weakly bonded (0.097 eV) or even unbound.9,13 In fact, neither LDA or GGA gives the true experimental value of the energy of adsorption, which is 0.50 eV.14 It is possible to estimate the DFT energy of pure van der Waals complexes by adding an empirical dispersion energy term to the DFT energy (for example, see ref 15). In another approach, a new functional can be built from a mixture of GGA and LDA functionals, and it also includes a term related to nonlocal correlation energy.16 For interactions stronger than π-dispersion, LDA is well-known to overestimate energies of adsorption.17 However, because we compare the relative stability of different complexes for which the stable structure is correctly given by LDA, we consider LDA to be the most appropriate approach for studying the interaction of aromatic molecules with graphite or with related structures such as carbon nanotubes.

Results and Discussion Intermolecular hydrogen bonding of carboxyl groups (COOH) is a reliable interaction that has been used extensively to engineer crystals in 2D and 3D.18 For this reason, we began our theoretical analysis of the effect of substituents and their interactions by comparing the adsorption of benzene (BZ), benzoic acid (1 ) BZA), isophthalic acid (2 ) IPA), and trimesic acid (3 ) TMA) on graphene.19,20 As revealed in Table 1, the presence of carboxyl groups on the benzene ring dramatically increases binding to graphene. In comparison, the adsorption of benzene itself is weak, and the low adsorption energy (0.197 eV) can be clearly associated with a physisorbed state.

Increasing the number of COOH groups progressively increases the binding energy of compounds 1-3 to graphene. To evaluate the individual influence of the CdO and OH moieties of COOH on binding, we replaced the OH units of TMA (3) by OCH3 to create triester 4 (TME). As shown in Table 1, this chemical modification has little effect on adsorption. As a result, the enhanced adsorption of benzoic acids relative to benzene itself can be attributed primarily to the presence of the CdO unit within COOH. It is possible to account for this effect by invoking the electron-withdrawing character of the COOH and COOMe groups, which may decrease the π-electron density on the benzene ring, thereby attenuating the magnitude of π-π repulsion and strengthening adsorption. However, this mechanism does not appear operative, because Table 1 reveals that the presence of three strongly electronegative atoms of chlorine in 1,3,5trichlorobenzene (TCB) leads to adsorption on graphene that is even weaker than that of benzene itself. (12) Fisher, A. J.; Blo¨chl, P. E. Phys. ReV. Lett. 1993, 70, 3263–3266. (13) Zhao, J.; Lu, J. P.; Han, J.; Yang, C.-K. Appl. Phys. Lett. 2003, 82, 3746–3748. (14) Zacharia, R.; Ulbricht, H.; Hertel, T. Phys. ReV. B 2004, 69, 155406. (15) Grimme, S. J. Comput. Chem. 2004, 25, 1463–1473. (16) Chakarova-Ka¨ck, S. D.; Schro¨der, E.; Lundqvist, B. I.; Langreth, D. C. Phys. ReV. Lett. 2006, 96, 146107. (17) Salahub, D. R. AdV. Chem. Phys. 1987, 69, 447–520. (18) Herbstein, F. H. In ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D.,Vo¨gtle, F., Eds.; Pergamon: Oxford, UK, 1996; Vol. 6, pp 61-83. (19) For other computational studies of the adsorption of benzene on graphite, see (a) Collignon, B.; Hoang, P. N. M.; Picaud, S.; Liotard, D.; Rayez, M. T.; Rayez, J. C. J. Mol. Struct. THEOCHEM 2006, 772, 1–12. (b) Vernov, A.; Steele, W. A. Langmuir 1991, 7, 3110–3117. (20) For experimental studies of the adsorption of benzoic acids on graphite, see (a) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M.; Flynn, G. W. Langmuir 2005, 21, 4984–4988. (b) Martin, D. S. Surf. Sci. 2003, 536, 15–23.

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Table 1. Energy of Adsorption of Substituted Derivatives of Benzene on Graphene molecule

adsorption energy (eV)

benzene (BZ)a benzoic acid (BZA) isophthalic acid (IPA) trimesic acid (TMA) trimethyl trimesate (TME) 1,3,5-trichlorobenzene (TCB) nitrobenzene (NB) 1,3,5-trinitrobenzene (TNB)

0.197 (0.190) 0.530 0.847 0.976 0.999 0.100 0.421 1.248

a The calculated adsorption energy for benzene is for an on-top site, with the ring centered over a single carbon atom of the surface. The value in parentheses is for the second most stable site (bridge site).

Figure 1. Variation in the energy of adsorption of benzoic acids on graphene as a function of the number of COOH groups. The doubleheaded arrow highlights the deviation from linearity, and the simple arrow marks the calculated position of benzene as a reference.

The model for π-π interactions introduced by Hunter and Sanders in 1990 suggests that decreasing the π-electron density on an aromatic ring should decrease π-π repulsion between the ring and graphene, thereby leading to stronger adsorption.21,22 In fact, however, the adsorption of TCB is weaker, so the Hunter-Sanders model requires that π-σ attractions between TCB and graphene must also decrease. We conclude that the presence of electron-withdrawing substituents on benzene diminishes both repulsive (π-π) and attractive (π-σ) interactions between the aromatic core and graphene. This strongly suggests that the adsorption of benzoic acids and their simple esters on graphite is driven primarily by medium-range interactions between the surface and the COOR groups, and to a lesser extent by long-range π-π interactions between the surface and the aromatic core of the adsorbate. As described below, the attractive mediumrange interactions result primarily from a charge-transfer process caused by exchange and correlation (XC) effects between the COOR moiety and the surface, rather than from an ensemble of electrostatic multipole interactions.23 Variation of the energy of adsorption with the number of COOH groups is graphed in Figure 1. The dashed line represents a linear extrapolation of the adsorption of BZA, taken as a reference, to molecules with two COOH groups (IPA) and three COOH groups (TMA). The figure shows clearly that the energy of adsorption does not depend linearly on the number of COOH groups in the adsorbate. The deviation from linearity is already quite apparent (21) (a) Hunter, C. A. Angew. Chem., Int. Ed. 1993, 32, 1584–1586. (b) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525–5534. (22) For an alternative assessment of π-π interactions, see Grimme, S. Angew. Chem., Int. Ed. 2008, 47, 3430–3434. (23) (a) Schmidt, W. G.; Seino, K.; Preuss, M.; Hermann, A.; Ortmann, F.; Bechstedt, F. Appl. Phys. A: Mater. Sci. Process. 2006, 85, 387–397. (b) Ortmann, F.; Schmidt, W. G.; Bechstedt, F. Phys. ReV. Lett. 2005, 95,186101(1) –186101(4).

when two COOH groups are present. The deviation may result from increased π-π repulsion as the adsorbate comes closer to the surface, driven by specific interactions involving the COOH groups. As shown in Figure 2, the geometries of adsorption of different benzoic acids on graphene sheets are fully consistent with the notion that π-π repulsion increases as the aromatic core approaches the surface. It is important to emphasize that benzene is physisorbed on graphene, producing a parallel geometry in which the adsorbate is positioned 3.24 Å above the surface. The geometry of adsorbed BZA is significantly different. The aromatic core is now tilted, with the COOH group directed toward the surface. The significant attractive interaction between the COOH group and graphene brings BZA slightly closer to the surface than observed in the case of benzene itself, thereby maximizing overlap. The distances between the two oxygen atoms in COOH and graphene are 2.86 Å (OH) and 3.15 Å (CdO), which are significantly smaller than the equilibrium distance calculated for benzene (3.24 Å). The decreased distance between the adsorbate and graphene increases π-π repulsion, which is minimized by placing the benzene ring in a tilted geometry relative to the surface. Moreover, tilting of the ring follows exactly the asymmetry induced by the binding of OH and CdO to the underlying surface, where the O-C distance between OH and graphene is shorter than that between CO and graphene. The calculated geometry of BZA bound to graphene underscores one of the key conclusions of the Hunter-Sanders model: “It is the properties of the atoms at the points of intermolecular contact rather than the overall molecular properties which are important” in assessing aromatic interactions.21 In the case of IPA (Figure 2B), the energy of adsorption is increased by the additional attractive interaction of a second COOH group with the surface, but it is simultaneously decreased by larger π-π repulsion. Because attractive COOH/graphene interactions are significantly stronger than repulsive π-π interactions between the aromatic core and graphene, IPA remains relatively tightly bound. The opposed effects of attractive COOH interactions and repulsive π-π interactions become even more conspicuous in the case of TMA (Figure 2C). TMA is quite strongly adsorbed on graphene, due to the stabilizing interactions of its three COOH groups, but it becomes highly deformed to minimize π-π repulsion. The deformations involve bending the substituents out of the average plane of the aromatic core, allowing them to approach the surface without simultaneously bringing the core and the surface into closer contact. This is a significant insight, because it shows that the binding of specific functional groups can induce alterations of the geometry of aromatic adsorbates. We have attributed increased adsorption on graphite to medium-range attractive interactions involving the COOH group, which are in competition with long-range π-π interactions between the aromatic core and the underlying surface. The medium-range attractive forces help bring the COOH group near the surface, which improves overlap between the π-electron clouds and facilitates charge transfer, but also increases π-π repulsion. Further analysis of Figure 2 can be performed to identify the following three common structural features: (1) the OH unit tends to lie near the center of an aromatic ring of graphene, and the O-H bond axis is normal to the graphene surface; (2) the oxygen atom in the CdO fragment always sits between two carbon atoms on graphene; and (3) the aromatic core of the adsorbate occupies on-top sites (Figures 2A and 2C) or bridge sites (Figure 2B), which are also the most stable sites of adsorption for benzene (see Table 1). These results strongly

Interaction of Aromatic Compounds with Graphene

Langmuir, Vol. 25, No. 1, 2009 213

Figure 2. Optimized geometries for the adsorption of substituted derivatives of benzene on graphene: (A) BZA, (B) IPA, (C) TMA, and (D) TNB.

suggest that benzoic acids, at least as unassociated monomers, are adsorbed on the surface of graphene as commensurate structures. We have extended our theoretical analysis of the influence of electron-withdrawing groups on adsorption by examining the effect of the very strongly electron-attracting NO2 group, which is expected to minimize repulsive and attractive π-π interactions. As shown in Table 1, the energy of adsorption of nitrobenzene (NB) is weaker than that of benzoic acid (BZA). Unlike BZA, which adopts a tilted orientation with respect to the surface, NB is adsorbed in a nearly coparallel geometry, confirming that π-π repulsion is relatively weak. The adsorption of 1,3,5-trinitrobenzene (TNB) follows a similar pattern: It lies flat on graphene, and the energy of adsorption is exactly three times that of NB. This result strongly suggests that π-π interactions do not play a significant role in the adsorption of NB or TNB. This correlation between decreasing π-π interactions and the increasing electronwithdrawing character of substituents is in agreement with studies of the adsorption of substituted derivatives of benzene on carbon nanotubes.24 These studies revealed a Hammett correlation (σp) in which charge transfer between the aromatic substrate and the nanotube increases according to the electron-donating capacity of the substituents (OH > OCH3 > Cl > NO2). To better understand the nature of the medium-range COOH-graphene interaction, we examined the electronic properties of the adsorbed states. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TMA are shown in Figure 3. The HOMO is mostly localized on oxygen atoms and corresponds to lowenergy lone pairs of oxygen. In contrast, the LUMO is delocalized over the entire molecule, but the shape of the contours reveals (24) Star, A.; Han, T.-R.; Gabriel, J.-C. P.; Bradley, K.; Gru¨ner, G. Nano Lett. 2003, 3, 1421–1423.

Figure 3. Isocontour plots of (A) the HOMO and (B) the LUMO of trimesic acid (TMA). Both orbitals are degenerate, and the isovalue varies from -0.05 (blue) to +0.05 (white).

contributions from the π*-orbital associated with the CdO unit and from the benzene ring. Figure 4 shows the energy of the LUMO relative to the Fermi energy (EF) of graphene and the overall net charge on the adsorbates for benzoic acids 1-3. As the number of COOH groups increases, the energy of the LUMO becomes closer to the EF of graphene, and there is an increase in the amount of net charge on the adsorbate contributed by graphene. The net charge strongly suggests that empty orbitals on the adsorbates overlap with filled orbitals localized on graphene, and charge transfer from graphene to the adsorbates

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increases the strength of adsorption. Recent theoretical and experimental results on the adsorption of small molecules such as H2O and NO2 on graphene have revealed an increase in the concentration of holes as charge carriers, supporting the notion of charge transfer from graphene.25 Once molecules 1-3 are adsorbed, their HOMO and LUMO change considerably, with the HOMO still lying well below the EF of graphene, and the LUMO partly overlapping states of graphene below EF. This trend is also observed for other small aromatic molecules we have examined. For example, charge transfer is negligible for 1,3,5-trichlorobenzene (0.00 |e|) and benzene (0.01 |e|), which are weakly adsorbed on graphene, but it is important for the triester of trimesic acid (0.25 |e|) and 1,3,5-trinitrobenzene (0.62 |e|), which are more tightly bound. This trend is fully consistent with the relative positions of the different LUMOs with respect to graphene. We conclude that the magnitude of medium-range interactions is directly related to the capacity of the adsorbate to accommodate additional charge from the graphene surface. For molecules with a low electron affinity, the overall binding is determined largely by long-range π-π interactions. This description is in agreement with previous calculations of a relatively large energy of adsorption (1 eV) of adenine on graphite.23 This was attributed to XC effects operating over a distance of medium range (∼3 Å), with only minor electron transfer. Previous work has established that derivatives of benzoic acid can be designed to form extensively hydrogen-bonded 2D networks, thereby offering a way to create programmed molecular nanopatterns on surfaces. Our calculations confirm that the adsorption of unassociated benzoic acids on graphene is strong, but the molecular structures are predicted to be significantly deformed by competition between attractive COOH-graphene interactions and π-π repulsion. It is therefore important to develop a deeper understanding of how associated benzoic acids are adsorbed on graphite and how intermolecular hydrogen bonding affects the overall energy of adsorption.26 We have therefore analyzed the adsorption on graphene of the hydrogen-bonded dimer of benzoic acid (5 ) BZA2). The calculated energy of dissociation into two molecules of BZA is 1.49 eV in the gas phase, which corresponds to 0.75 eV (17 kcal/mol) per hydrogen bond. The DFT-LDA optimized geometry reveals a planar dimer with very short hydrogen bond distances of 1.42 Å (O---H), showing that the two BZA subunits of BZA2 are tightly associated.

Rochefort and Wuest

Figure 4. Variation in the energy of the formal LUMOs and the degree of charge transfer from graphene to benzoic acids as a function of the number of COOH groups.

smaller than the energy of adsorption for two isolated molecules of BZA (2 × 0.530 eV ) 1.06 eV). The decreased affinity (0.42 eV) can be attributed to increased π-π repulsion, caused by the proximity of the coplanar aromatic cores of adsorbed BZA2 to the underlying surface of graphene. Clearly, the adsorption of benzoic acids on graphene is a complex phenomenon that reflects the affinity of both monomeric and associated forms. It is noteworthy that COOH groups can have important opposing effects on the adsorption of aromatic compounds. In particular, attractive COOH-graphite π-π* interactions favor adsorption, but they simultaneously cause the aromatic core and attached COOH groups to be positioned relative to the underlying surface in ways that make interadsorbate hydrogen bonding somewhat less exothermic than it is in the gas phase. Nevertheless, both experiments and calculations identify benzoic acids as compounds that offer an attractive combination of strong adsorption on graphite with an ability to engage in reliable patterns of interadsorbate hydrogen bonding that are closely similar in geometry and in energy to those observed in other phases. As a result, benzoic acids are particularly suitable for creating predictable nanostructured networks on graphite. Long-range π-π interactions appear able to impose molecular order on adsorbed phases, so further study of the assembly of benzoic acids on graphene will be needed to evaluate the commensurability of extensive networks of associated molecules.

Conclusions

Once adsorbed, BZA2 remains tightly hydrogen-bonded. In contrast to the adsorption of unassociated BZA, where both CO and OH units in the COOH group are bent toward the graphene sheet, the COOH groups in the dimer remain parallel to the underlying surface. Calculations reveal that 2.13 eV is needed to break the adsorbed dimer into two separate adsorbed molecules of BZA. When combined with the energy of association of the dimer in the gas phase (1.49 eV), this gives 0.64 eV for the energy of adsorption of BZA2. This value is (25) (a) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652–655. (b) Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katsnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Nano Lett. 2008, 8, 173–177. (26) For a related theoretical analysis of adsorption and interadsorbate hydrogen bonding involving nucleobases, see: Antony, J.; Grimme, S. Phys. Chem. Chem. Phys. 2008, 10, 2722–2729.

We have shown using a first-principles DFT-LDA method that the presence of functional groups can significantly alter the nature and the magnitude of interactions between small aromatic molecules and graphene. When the substituents can simultaneously permit the formation of hydrogen bonds between adsorbed molecules, it is possible to evaluate the relative contributions of hydrogen bonding and π-based interactions to the overall adsorption. The following three types of interactions help determine the nature of the ultimate assembly, shown in order of increasing importance: (1) long-range repulsive π-π interactions (0.10-0.30 eV), (2) medium-range attractive π-π* interactions (0.30-0.60 eV), and (3) short-range hydrogen bonding (0.60-0.80 eV). The final structure of the adsorbate will reflect the relative contributions of these interactions. Our work highlights the special utility of derivatives of benzoic acids as subunits for the construction of predictably ordered nanopatterns on the surface of graphite. Such compounds offer a highly attractive combination of (1) strong adsorption, driven by attractive COOH-graphite π-π* interactions, and (2) strong

Interaction of Aromatic Compounds with Graphene

directional intermolecular interactions, featuring hydrogenbonded pairing of COOH groups that position adjacent molecules in predictable ways. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministe`re de l′E´ducation du Que´bec, the Canada Foundation for Innovation,

Langmuir, Vol. 25, No. 1, 2009 215

the Canada Research Chairs Program, and the Universite´ de Montre´al for financial support. In addition, we thank the Re´seau que´be´cois de calcul de haute performance (RQCHP) for providing computational resources. LA802284J