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J. Phys. Chem. C 2007, 111, 18258-18263
Dissociative Adsorption of Water at Vacancy Defects in Graphite Pepa Cabrera-Sanfelix*,†,‡ and George R. Darling† Surface Science Research Centre, Department of Chemistry, The UniVersity of LiVerpool, LiVerpool L69 3BX, United Kingdom, and Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, San Sebastian 20018, Spain ReceiVed: August 3, 2007; In Final Form: September 20, 2007
We have performed density-functional calculations to investigate the adsorption of H2O on perfect and defected graphite (0001) represented by a single graphene sheet. On the perfect surface, the water physisorbs, as expected, with no significant preference for the adsorption site. At a vacancy site, the interaction is much more significant, with a computed binding energy of ∼210 meV in a weak chemisorption/strong physisorption state. The H2O sits with one H pointing down to a carbon atom, which is pulled out of the plane by ∼0.55 Å. From this physisorption state, dissociative chemisorption will occur after overcoming a barrier of 0.8-0.9 eV (∼0.60.7 eV relative to the gas-phase). The lowest dissociation barrier obtained is ∼0.47 eV along a path largely avoiding the physisorption well. The dissociation paths have an intermediate step, in which the molecule partially dissociates to H and OH. Subsequently, the chemisorbed OH stretches, breaking into O and H atoms chemisorbed on separate C atoms on the vacancy with a total exothermicity of ∼3.21 eV.
1. Introduction The structure and reactivity of water ice films is a topic of intense investigation in many scientific areas from electrochemistry to astrochemistry, with substrates ranging from pure transition metals to oxides and semiconductors.1-3 Water in the astrophysical environments is present in comets and in protoplanetary clouds,4,5 coating dust grains, mainly composed of carbonaceous and silicate particles.6 The chemistry, in particular photochemistry, occurring on the ice-covered dust particles is believed to play an important role in the formation of molecules in outer space.7-9 In addition, water films have received much consideration in the field of atmospheric chemistry, because of their climatic role in stratospheric clouds10-13 and their importance in the catalysis of many reactions occurring at the surface of the tropospheric aerosols.14-19 The heterogeneous nucleation of ice in the atmosphere is produced by water deposition on insoluble nuclei, such as mineral oxides and soot,13,20 generally originating from anthropological industrial pollution and composed of carbonaceous particles.17 Water-carbon interactions are, of course, also important in carbon (coal) gasification.21 More recently, the interaction of water with graphene and carbon nanotubes (CNTs)22-28 has attracted attention, mainly due to the growing technological importance of carbon nanotubes (CNTs)22-28 which, among numerous applications, are expected to be used as nanofluidic devices, such as sensors,29,30 filters,31 channels, and reactors integrated into micro-total analysis systems (µ-TAS).32,33 Graphite is generally considered to be a good surface science model to reproduce the structure and interaction of small molecules with carbonaceous particles and also to give a first estimation of water adsorption on CNTs. However, the clean perfect surface is only a first-order model with a very low * To whom correspondence should be addressed. E-mail: swbcasam@ sc.ehu.es. † The University of Liverpool. ‡ Donostia International Physics Center (DIPC).
reactivity, and is essentially hydrophobic.34 Previous work has considered the binding and reactivity at the edges of graphite planes21 and the ends of open CNTs,35 where the reactivity is noticeably greater. Cluster models have also been used to investigate the binding of water to graphitic surfaces containing carboxyl and hydroxyl groups.36 Very recent work has also examined the interaction of water with defects in graphitic surfaces37 to investigate the reactivity of vacancy and interstitial defects produced by ion irradiation.38,39 Dissociation at structural defects in CNT’s24 has also been examined with infrared spectroscopy and Density Functional Theory (DFT). In particular, Allouche and Ferro37 have presented DFT cluster calculations of water dissociation at a vacancy in a graphite (0001) plane. They found a significant energy barrier of 1.6 eV, with the molecule dissociating to H and OH chemisorbed on two of the carbon atoms adjacent to the vacancy.37 This work also presents the results of a DFT investigation of water adsorption at perfect and vacancy defected graphite (0001) represented by a single graphene sheet. For the perfect graphene, we find only weak physisorption, in agreement with other work.37,40 For the vacancy, however, in contrast to previous work, we find a relatively strongly physisorbed molecule, with a binding energy in excess of 0.2 eV, and much lower dissociation barriers, with the molecule ultimately completely dissociated into O and H atoms bound to the carbons surrounding the vacancy. These findings are presented in the results section, following a description of the computational method. 2. Computational Details All calculations were performed using the Vienna ab initio simulation package (VASP),41-43 employing a plane-wave basis set and implementing DFT within the Perdew-Wang 1991 (PW91) version of the general gradient approximation (GGA).44 The projector augmented wave (PAW) potentials45,46 were used to describe the C, O, and H atoms. The graphene sheet was represented by a 4 × 4 supercell, with periodic boundary
10.1021/jp076241b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007
Dissociative Adsorption of Water
Figure 1. Potential energy curves for the physisorption of a water molecule on the three possible sites (Top, Hollow and Bridge) of a perfect (0001) graphite surface.
conditions along the three spatial directions, with a large vacuum gap of 15.7 Å between periodic repeats of the whole plane. This was found to be large enough for all calculations. For computational efficiency, we have restricted the basis set to a 2 × 2 × 1 Monkhorst-Pack k-point sampling and a plane-wave cutoff of 500 eV for all the calculations on the perfect surface. In these computations, all atoms of the system were allowed to fully relax until the residual forces were less than 0.03 eV/Å. For the defected system, the carbon atoms on the cell boundary, distant from the vacancy site, were fixed during relaxation to prevent the entire layer from moving; the rest of the atoms were optimized with the same force criteria. All calculations for the defected system have been performed with the same energy cutoff (500 eV), but a 3 × 3 × 1 Monkhorst-Pack k-point sampling. In both perfect and defected systems, the accuracy of our results have been checked to converge within 10 meV when using different 2 × 2 × 1 and 3 × 3 × 1 Monkhorst-Pack k-point sampling. Similar accuracy is obtained for the energy difference between chemisorbed and physisorbed molecules on changing the cutoff energy from 500 to 800 eV. All calculations for the vacancy were spin-polarized, to account for the spin polarization of isolated vacancies in graphite.47 Adsorption energies, Eads, are calculated from the following:
Eads ) Egraphite + EH2O - EH2O/graphite where Egraphite and EH2O correspond to total energies of the relaxed clean graphite and of the isolated water molecule, respectively; and EH2O/graphite is the energy of the optimized complete system. 3. Results and Discussion In previous DFT calculations, we have determined that water molecules physisorb at 3.5 Å above the perfect (0001) basal plane of graphite, with no preferential physisorption site25 in agreement with other methods.37,40 Figure 1 shows the potential energy as a function of the oxygen surface distance computed by fixing the O atom position while relaxing all other atoms. Although DFT cannot accurately account for dispersion forces dominant in this weak binding, and we cannot place any significance on the physisorption well depth or corrugation, it is clear that there is virtually no surface site dependence in the interaction at the DFT level, with the hollow site showing only
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18259
Figure 2. Binding energy for a water molecule in the deep physisorption state at four possible sites (Vacancy, Top, Hollow and Bridge) around a vacancy defect on graphite. Carbon-carbon distances at the vacancy correspond to the relaxed (0001) defected graphite prior to water adsorption.
marginally weaker repulsive interaction as the molecule is pushed into the surface. We have also investigated the preferred adsorption site of the water molecule on defected graphite. To simulate a vacancy defect, we have used a 4 × 4 unit cell of (0001) graphite and removed one carbon atom, creating a vacancy hole surrounded by three carbon atoms, each with a dangling sp2 bond. The optimized structure is not perfectly symmetric, but distorts slightly as two of the C atoms at the vacancy site approach each other at ∼2.05 Å, leaving the third with a spin-polarized dangling bond.47,48 The possible adsorption sites we have examined at this vacancy are illustrated in Figure 2: A corresponds to the site of the missing carbon atom; B, C, and D characterize the top, hollow, and bridge sites, respectively, around the vacancy. For each adsorption site, the molecule has been initially placed 3.5 Å above the surface, keeping fixed the x and y coordinates of the oxygen atom at the specific adsorption site. In contrast with ideal graphite, where no site preference is evident at the DFT level, as shown in Figure 1, after relaxation, both the physisorption energy and the molecule-surface distance vary significantly depending on the adsorption site (see Figure 2). In general, all binding energies around the vacancy defect are significantly higher than for physisorption on the ideal graphite (0001) surface. This interaction with the unsaturated bonds at the defect is large enough to pull the carbon atoms slightly out of the plane, due to the fairly weak attraction between the positive H and the dangling bond on the C atom. This is not strong enough to significantly change the electronic properties of the vacancy, which remains spin-polarized. In the most favored adsorption configuration, the water is oriented with one hydrogen pointing down to the graphite, with the carbon pulled up by ∼0.55 Å, the shortest H-C bond is only 2.14 Å (see Figure 3). Therefore, on imperfect graphite, one could expect that at ambient conditions, a water molecule would move easily along the clean surface (based on the potential curves in Figure 1) until finding a vacancy where it gets pinned, close to the vacancy site itself. The binding energy at this site is still low compared to binding of a water molecule in ice (>0.6 eV), but such a site could likely provide a nucleus for growth of ice. We have also investigated the possible chemisorption of the water molecule on this vacancy defect by pushing the molecule down to 1.5 Å above the surface and also to 1.0 Å. After geometry optimization (keeping only the carbon atoms on the
18260 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Figure 3. Side and top view of the optimum configuration of a strongly physisorbed H2O molecule at a vacancy. The water molecule sits 3.11 Å above the surface plane, with one H atom bonding to a carbon pulled up by ∼0.55 Å from the graphite plane. The distances between sp1 carbons are also displayed.
perimeter of the supercell fixed,) the molecule is found to dissociatively chemisorb, as shown in Figure 4. Starting at the higher distance, in Figure 4A, the molecule splits into an H chemisorbed on one C with a C-H bond length of 1.1 Å and an OH chemisorbed on one of the other dangling bonds with an O-C bond length of 1.35 Å. This structure is identical to that obtained by Allouche and Ferro,37 using cluster calculations. This process is exothermic by ∼2.3 eV (∼2.1 eV from the physisorbed state). Starting the molecule even closer to the surface leads to a second dissociative chemisorption state in which the molecule is completely dissociated, occupying all of the bonds broken on formation of the vacancy as shown in Figure 4B. This has a much higher exothermicity of ∼3.42 eV (∼3.21 eV from the physisorption state) and C-H bond lengths of ∼1.07 Å and 1.09 Å, with a C-O bond of 1.24 Å. The saturation of all of the bonds around the vacancy quenches the spin polarization, as observed for a vacancy decorated with H atoms.48 To investigate the reaction path from gas-phase to physisorption to chemisorption, we have moved the molecule in small steps toward the graphite, fixing the height, ZO, but not the lateral
Cabrera-Sanfelix and Darling position of the O atom, optimizing all other atom positions as before. The resulting potential curve is shown in Figure 5, followed by the illustrations of the atomic positions along the chemisorption path in Figure 6. We can see that there is a substantial barrier to dissociation, E1, of ∼0.7 eV (∼0.92 eV starting from the bottom of the physisorption well). Although high, this barrier is lower than that obtained by Allouche and Ferro,37 but corresponds to a different molecular configuration. As shown in Figure 6, at the barrier maximum, the molecule is oriented O-down, pushing the nearest carbon back down into the surface (the nonbonding, i.e., lone-pair, end of the molecule always interacts repulsively with the graphite surface). Immediately after the barrier maximum, the bonding geometry and energy change very rapidly. For ZO ) 1.5 Å, the molecule dissociates (Chemisorption_1 in Figure 6) into the configuration of Figure 4A, i.e., chemisorbed H and OH. This state is found to be metastable relative to the completely dissociated molecule (Chemisorption_2)spushing the O down toward the height of the chemisorbed atom a small barrier E2 ≈ 95 meV must be overcome before the O-H bond breaks. The energy released from E1 to Chemisorption_1 is almost 3 eV, and so it is very unlikely that, in this scenario, the molecule will be able to be trapped in the Chemisorption_1 state, but will rather progress on to the fully dissociated state. To investigate the dissociation path further, we have employed the Nudged Elastic Band (NEB) method.49 Starting at the physisorption minimum, with its H-down orientation, we have considered a path in which the molecule “rolls” over so that the O atom is directed across the vacancy to a different C atom than that to which the H points down in the physisorption state. This did not produce a barrier significantly different from that shown in the path of Figure 6, although at the barrier, the molecule is closer to parallel to the surface, with the H’s pointing downward. Instead, we note that in the physisorption state, the O atom is not favorably positioned with respect to its final position when the molecule is fully dissociated. Starting from the gas-phase for a molecule parallel to the surface, we have recomputed a dissociation path with the oxygen placed directly above the position it occupies after dissociation. The optimized atomic positions for the steps on this path are shown in Figure 7, with the corresponding potential energy curve in Figure 8. We can see that the molecule largely avoids the deep physisorption well. At the barrier maximum, with energy E3 ) 0.47 eV, the orientation is similar to Figure 3, but the O atom
Figure 4. (A) Optimized adsorption geometry for a partially dissociated water molecule. The new covalent bond-lengths are 1.10 Å and 1.35 Å for C-H and C-O, respectively. (B) Chemisorption of the totally dissociated water molecule, with bond lengths of 1.07 Å, 1.09 Å for the C-H bonds and 1.24 Å for the C-O bond. Distances between sp1 carbons atoms are displayed in both panels. The adatoms form separate C-H and C-O bonds, twisted to lie out of the graphite plane.
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Figure 5. Potential energy as a function of the O-surface distance, ZO, for a water molecule approaching the graphite surface at a vacancy site. The molecule first physisorbs, then can dissociate at a barrier of E1 ≈ 0.7 eV (relative to a molecule in the vacuum). Dissociation is first to a configuration with H and OH chemisorbed, followed by complete dissociation on overcoming a small barrier of 95 meV from this state.
Figure 7. Atomic configurations for the reaction path computed using the NEB method for water dissociation at a vacancy in graphite.
Figure 6. Optimized atomic positions for the potential curve in Figure 5, for a water molecule starting 3.9 Å above the surface (top left) to the global minimum chemisorption configuration (bottom right). Along this path (until Zo ) 1.5 Å), the z-coordinate of the oxygen atom has been fixed, the H and surrounding C atoms are fully optimized. For the step labeled E2, the H atom is pulled away from the OH toward its eventual chemisorption site, it is fixed during relaxation and the O atom is optimized.
is almost exactly above the lower H atom, and the graphite is almost perfectly flat (step 5 in Figure 7), i.e., the C atom adjacent to the H is not pulled out from the surface as in Figure 3. After
the barrier, there is again an intermediate with a chemisorbed intact OH, although the reaction path is not sufficiently wellresolved for this chemisorption state to be clearly identified. Considering the potential energy landscape as a whole, at low energies, a molecule approaching from the gas-phase will be steered toward the deep physisorption state in Figure 3. To dissociate from this requires a large input of energy not much below 1 eV, although once dissociated, the reaction is overall exothermic by ∼3.4 eV. A molecule approaching the surface with an energy of ∼0.47 eV can dissociate if it approaches close to the site where the O atom will ultimately chemisorb. Although lower than the barrier in Figure 5, and considerably lower than the barrier estimated from previous work,37 this is still a substantial amount of energy. In dark interstellar clouds, where the temperature is 0.9 eV starting from the physisorption minimum. The dissociation appears to take place in a two-stage process, first partially dissociating to H and OH with an energy release of ∼2.09 eV. Subsequently, the chemisorbed OH stretches breaking into O and H atoms chemisorbed to the separate carbons surrounding the vacancy site. This is the global minimum configuration involving an energy release of ∼3.21 eV. Molecules approaching from the gas-phase can dissociate with less energy (∼0.47 eV) by avoiding the physisorption configuration. Acknowledgment. Financial support from UPV/EHU (Grant No. 9/UPV 00206.215-13639/2001), the Spanish M.E.C. (Grant No. FIS2004-06490-C3-00), the EU network of excellence FP6NoE “NANOQUANTA” (Grant No. 500198-2), and the Basque Government projects “NANOMATERIALES” and “NANOTRON” within the ETORTEK programme is gratefully acknowledged. This work was also supported by the EPSRC on Grant No. GR/S15990. References and Notes (1) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (2) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 5. (3) Verdaguer, A.; Sacha, G. M.; Bluhm, H.; et al. Chem. ReV. 2006, 106, 1478. (4) Sieger, M. T.; Simpson, W. C.; Orlando, T. M. Nature 1998, 394, 554. (5) Westley, M. S.; Baragiola, R. A.; Johnson, R. E.; et al. Nature 1995, 373, 405 (6) Mathis, J. S. Rep. Prog. Phys. 1993, 56, 605 (7) Shi, M.; Baragiola, R. A.; Grosjean, D. E.; et al. J. Geophys. Res.Planets 1995, 100, 26387. (8) Ruffle, D. P.; Herbst, E. Mon. Not. R. Astron. Soc. 2001, 322, 770. (9) Willacy, K.; Langer, W. D. Astrophys. J. 2000, 544, 903.
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