Surface Science 604 (2010) 1666–1673

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Surface Science

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Interaction of water molecules with defective carbonaceous clusters: An abinitio study

Mohamed Oubal a, Sylvain Picaud a,⁎, Marie-Thérèse Rayez b, Jean-Claude Rayez b

a Institut UTINAM — UMR 6213, CNRS/Université de Franche-Comté, 16 route de Gray, F-25030, Besançon Cedex, Franceb Institut des Sciences Moléculaires — UMR 5255, CNRS/Université de Bordeaux 1, 351 cours de la Libération, F-33405 Talence Cedex, France

⁎ Corresponding author. Fax: +33 3 81 66 64 75.E-mail address: sylvain.picaud@univ-fcomte.fr (S. Pi

0039-6028/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.susc.2010.06.011

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 April 2010Accepted 16 June 2010Available online 25 June 2010

Keywords:AdsorptionWaterSootCarbonaceous surfacesDensity functional calculations

First-principle calculations are used to study the interaction of water molecules with carbonaceous clusterscontaining single carbon atom vacancy, similar to those which may be found in soot nanoparticles. It isshown that the dissociative adsorption of one water molecule at the vacancy site may lead to the formationof a “ketone-like” structure which can then act as a nucleation center for additional water molecules. Such amechanism can thus participate in the hydrophilic behavior of soot primary particles although it appears lessfavorable than water nucleation around more hydrophilic sites such as carboxyl or hydroxyl groups.

caud).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

A thorough attention has been paid in the scientific community to amolecular-level understanding of the interactions between water orwater clusters and graphite surfaces, graphene sheets, and carbonnanotubes [1–17]. Water generally adsorbs molecularly with a ratherweak interaction adsorption energy on the pure carbon surfaces,whereas larger interaction energy resulting in the possible dissociationof thewatermolecule has been evidenced at the edges of graphite planes[18], the ends of open carbon nanotubes [19] and near structural defectslike vacancies in graphitic surfaces [20–23]. In addition to pure carbonsurfaces, adsorption of water on functionalized carbons that consist of amore or less irregular stacking of graphitic plates characterized by a highdegree of chemical and morphological heterogeneity, has also attractedattention because of technological and environmental interests [24].

More recently, the role of carbonaceous particles such as soot in ouratmosphere has also been considered as an interesting field ofinvestigation [25,26] because these particles provide surfaces forheterogeneous reactions [27] that may impact on the atmosphericchemistry. In addition, soot may also influence the Earth's climate, eitherdirectly due to its specific optical properties [28], or indirectly by acting asnuclei for ice particles forming artificial cirrus clouds [29]. Indeed,measurements of water adsorption isotherms on laboratory-made sootand on aircraft engine combustor soot recently showed that soot canacquire a substantial amount of water molecules [30,31]. As aconsequence, soot may allow heterogeneous freezing of water and ice

nucleation at much lower pressures than those required for homoge-neous freezing [32], resulting in the formation of condensation trails (theso-called “contrails”) behind planes that may evolve into artificial cirrusclouds which thus add their effect on climate to natural cirrus clouds.

However, the ice-forming activity of soot is still not fully understood.Indeed, in the initial stages soot from an internal combustion engine isexpected to be hydrophobic like other types of graphite-like particles.But a few qualitative laboratory studies have shown that soot maybecome partially hydrated under certain conditions like, for instance,activation by sulphur containing species such as sulphuric acid [33].Furthermore, observations do not rule out the possibility that icenucleation may occur on soot even when no sulphur is present [29,30].In this case, it has been shown that the ice nucleation ability of soot isinfluenced by the size and the surface concentration of functionalgroups which can form hydrogen bonds with water molecules [29,30].As a consequence, understanding the affinity between soot andsurrounding water molecules requires a thorough characterization ofgeometrical and chemical properties of the primary particles constitut-ing soot in realistic conditions [26].

Transmission electronmicroscopy (TEM) studies have evidenced thatsoot emitted by aircraft is made of graphene-like sheets stacked onconcentric spheres of different radii resulting in typical onion-likestructures [34]. Aircraft soot nanoparticles are thus of quasi-sphericalshape, with diameters ranging between 10 and 50 nm [35,36]. Moreover,the carbonaceous nanoparticles constituting soot are likely partiallyoxidized, and they contain a certainnumberof oxygenatomsites [36–38].However, experimental characterizations of aircraft soot are ratherscarce in the literature due to the difficulty of collecting real soot behinda plane. These characterizations also face a number of experimentalchallenges such as the accurate determination of the distribution of

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carbonmicrocrystal sizes and of the density and species of surface groupsand structural defects. As a consequence, computer simulation effortshave been developed during the last decade to understand at amolecularlevel thedetails of the soot formation [39–43] andof thewater adsorptionprocess on soot [44–46].

In a recent series of papers, we usedfirst-principlemethods [47–50],molecular dynamics simulations [51–53] and Grand Canonical MonteCarlo (GCMC) simulations [54–56] to show, at amolecular level, that theinteraction of water with various carbonaceous graphene-like clustersand onion-like nanoparticles modeling those present in soot dependsbothon theprimaryparticle structure (size of thepores) andon the typeand location of the chemical groups that may be present in/on theseprimary particles. To further increase our understanding on the icenucleationproperties of the primary particles of soot,we consider here anewkind of potentially hydrophilic site, namely single atomic vacanciesat the surface of defective carbonaceous clusters. Indeed, this type ofstructural defect may likely occur during the growth of the sootnanoparticles [42] and, if present, can act as a strong trapping site forwater molecules [20–23]. As in our previous papers [47–50], we modelthe surface of soot by small carbonaceous clusters. However, in contrastto the perfect clusters consideredpreviously, these clusters contain hereone carbon atom vacancy. We make use of DFT (Density FunctionalTheory) method to characterize the structure of the correspondingdefective clusters and of the first water molecule adsorbed on it. Then,we characterize the adsorption of additional water molecules on theresulting defective clusters by using either DFT or ONIOM (Our own N-layered Integrated molecular Orbital and Molecular mechanics)methods [57,58], depending on the size of the system considered.Here, we especially focus on the first stages of the water aggregationbecause it is well established that, at least on the surface of activatedcarbons, the formation of small water clusters occurs before all primaryadsorption centers are occupied and that it is the key point for furtherwater condensation [59].

It is worth noting that this study could also be of interest for manyother fields of applications related to water adsorption on defectivegraphite-like substrates and on structural defects on carbon nano-tubes. Indeed, theoretical studies on such systems are scarce and, asfar as we know, they have been limited to the adsorption of a singlewater molecule [20–23]. Moreover, although these studies agree withthe dissociation of the water molecule at the vacancy site, significantdifferences have been found in the calculations of the dissociationenergies depending on the quantummethods used. The present studydevoted to the adsorption of small water aggregates on defectivegraphene-like clusters is thus the first one reporting first-principle

Fig. 1. Geometry of the (a) C52H18 and (b) C80H22 clusters used in our DFT calculations. C andthe carbon atom which is removed to create the atomic vacancy on the defective carbonac

calculations on the water adsorption process at finite coverage ondefective carbonaceous nanoparticles of finite size.

2. Computational details

The defective carbonaceous nanoparticles likely present in soot aremodeled by removing one central carbon atom in two different clustersmade of 18 (model 1) or 30 (model 2) fused benzene rings arranged in asingle atomic layer. The defective particles thus contain either 51(C51H18 cluster, model 1) or 79 carbon atoms (C79H22 cluster, model 2),the edges of these carbonaceous clusters being saturated by hydrogenatoms (Fig. 1). First, DFT calculations based either on the local spindensity approximation (LSDA) [60] or on the hybrid Becke 3-Parameter,Lee, Yang and Parr (B3LYP) [61] exchange-correlation functional areperformed to optimize the structure of the corresponding defectiveparticles (models 1 and 2). In all cases, the polarized 6-31G(d) [62] basisset is used in the DFT calculations. Note that other functionals have alsobeen used in our previous work [50] leading however to similar results.This, together with the results of the present study (see below),indicates that the characterization of the systems considered here doesnot strongly depend on the choice of the functional.

Then, the DFT method is also used to investigate the interaction of afirst water molecule with the two defective carbonaceous clusters, byconsidering that thewatermolecule is locatedabove the carbonvacancyat the beginning of the optimization procedure. Finally, the details of theadsorption process of small water aggregates containing up to sixadditional water molecules are investigated on both small (model 1)and large (model 2) defective carbonaceous clusters. For the smallcluster, all the calculations are also performed by using the DFTmethodat theB3LYP/6-31G(d) of theory,whereas for the larger clusterwemakeuse of the two-layer ONIOM method [57,58] because of the largernumber of atoms in the corresponding system. Using the ONIOMmethod, the central part of the systemthat contains thewatermoleculesand the closest neighboring C atoms (typically the closest four carbonrings) is treated with a high-level of accuracy by using DensityFunctional Theory (DFT) whereas the rest of the system is taken intoaccountwith the semi-empirical PM3 (ParameterizedModel number 3)method [63]. On the basis of our previous works [47,48], the high-levelDFT calculations are performed using the B3LYP (Becke 3-Parameter,Lee, Yang and Parr) exchange-correlation functional [61] and cc-PVDZ(Correlation-Consistent Polarized Valence Double-Zeta) basis set [64].All the calculations are carried out with the Gaussian 03 quantumchemistry package [65] and the stable geometries are calculated witha tight criterion geometry optimization. Moreover, for both small and

H atoms are represented by grey and white balls, respectively. The black circle indicateseous cluster.

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large clusters, once the geometry of the adsorbed water aggregates isoptimized, the final energy is calculated with the larger 6-311++G(2d,2p) basis set (which is a triple-zeta split valence set augmentedwithdiffuse and polarization functions on all atoms) [66] for the DFT layer(i.e., the water molecules+the whole cluster when considering thesmallest cluster, or the water molecules+the nearest surface atomswhen considering the largest cluster), to minimize the basis setsuperposition errors (BSSE) and to allow comparisonwith our previousstudies on oxidized carbonaceous clusters [47–50]. Note that, for clarity,we represent in all the figures of this paper the atoms included in theDFT calculations by small balls,whereas the atoms included in the semi-empirical PM3 layerwhen using the ONIOMmethod are represented bysticks.

3. Results and discussion

3.1. Creation of defective carbonaceous clusters

First, we use DFT calculations (at the B3LYP/6-31G(d) level, asexplained in Section 2) to optimize the structure of the defectivecarbonaceous clusters (C51H18 and C79H22) created by removing onecentral carbon atom and modeling defective surfaces likely present insoot. Note that we create here a defective surface by removing a C atomfrom a perfect carbonaceous surface, because it is the simplest way ofcreating this vacancy from a theoretical point of view, although it is anunlikely process in realistic conditionsbehindplanes becauseof thehighactivation energy of the corresponding process [67]. However, carbonatom vacancies have been evidenced in atomistic simulations of sootformation, coming from incomplete recombination of small sootprecursors (typically CnHm with small n and m values) [42]. In thatsense, the defective clusters optimized here can be considered asrealistic models of defective surfaces that might exist in soot.

The atomic vacancy created here is surrounded by three carbonatoms, each having a dangling sp2 bond corresponding to 3 unpaired σelectrons and an unpaired π electron. The optimization procedure isperformed by considering singlet, triplet and quintet states, and wecheck that our results correspond to almost good eigenstates for the S2

matrix in each situation. This optimization shows that the most stablestructure always corresponds to the triplet case, in accordance withprevious studies on similar systems [44]. For the small C51H18 cluster,two of the three carbon atoms carrying an unpaired electron tend to

Fig. 2. Top (top) and side (bottom)viewsof theoptimizedgeometryof the (a)C51H18and (b)C7basis set. C and H atoms are represented by grey and white balls, respectively. The C–C distanc

form a single bond, and a five-membered ring is consequently stabilizedin the defective cluster. Moreover, a σ dangling bond is localized on thethird atom around the vacancy site, and the presence of this danglingbond togetherwith that of the five-membered ringfinally leads to a lossof planarity for the optimized structure of the C51H18 defective cluster(Fig. 2a) with a small shortening of the first nearest-neighbor distancesaround the vacancy site (1.35 Å instead of 1.42 Å in perfect graphite). Incontrast, the optimized structure for the larger C79H22 cluster does notshow any significant deviation from planarity (Fig. 2b), and the C–Cdistance in the remaining pentagon-like structure is significantly largerthan for the small cluster (1.87 Å instead of 1.56 Å). It thus appears inour calculations that the loss of planarity is a consequence of the smallersize of the C51H18 cluster, which is characterized by a smaller number offused benzene rings around the pentagon-like cycle. Note that verysimilar structures are obtained when using either LSDA or B3LYPfunctionals in the DFT calculations.

These results are in qualitative agreement with results of previousstudies considering carbon vacancies in graphene sheets and in smallgraphite clusters [44,67–69], although small differences in the C–Cbonds and in the deviation from planarity are obtained between thesedifferent studies, dependingon the level of theoryused andon the sizeofthe carbonaceous system considered in the first-principle calculations.

3.2. Adsorption of one water molecule on defective carbonaceous clusters

Although the adsorption of one water molecule at a single vacancysite of a graphite surface and of a graphene sheet has already beendetailed in the literature byusingdifferent theoreticalmethods [20–23],as far as we know it has never been characterized on carbonaceousclusters of finite size such as those considered here for the modeling ofsoot nanoparticles. Because the adsorption of one water molecule isobviously the first step of the water adsorption process, we thus use theDFTmethod (at the B3LYP/6-31G(d) level, see Section 2) to characterizethis adsorption on the C51H18 and C79H22 clusters considered here.However, we do not perform a detailed study of the correspondingreaction path,which is beyond the scope of this studymainly devoted tothe adsorption of large water aggregates.

The adsorption of the first water molecule on the defective cluster isthus studied by locating first this water molecule over the center of thevacancy, at a distance of 4.5 Å from the surface as the starting point ofthe adsorption process, and by decreasing gradually the water–surface

9H22 clusters asobtained fromDFT calculationswith theB3LYP functional and the6-31G(d)e in the pentagon-like structure around the vacancy site is also indicated by an arrow.

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distance. This starting point is used as the energy reference in all thefollowing calculations which are performed by considering the moststable triplet state, only. The distance between the oxygen atom of thewater molecule and the carbon atom bearing the σ dangling bond isconsidered as the reaction coordinate.

Let us first detail the most stable structures (Fig. 3a,c,e) calculated forthe water adsorption above the small C51H18 cluster when the reactioncoordinate is decreased. Note that Fig. 3 also shows the transition statesconnecting theseminima (Fig. 3b and d) and thatwill be detailed further.When thewatermolecule comes close to the surface, a physisorbed stateis evidenced with an adsorption energy equal to−18.4 kJ/mol, in whichthe water molecule points its two hydrogen atoms down to the surface(Fig. 3a). The interaction with the unsaturated cluster is large enough topull the nearest carbon atom slightly out of the surface, with acorresponding O–C distance equal to 2.3 Å.

Approaching water closer to the surface (i.e., at a distance less than1.5 Å) leads to the dissociative chemisorption of the molecule, with asplitting into an H atom chemisorbed on one C atomwith a C–H distanceequal to 1.1 Å, and an OH fragment chemisorbed on a C atom of theunsaturated hexagonal ring, with anO–C bond length equal to 1.36 Å anda C–O–H angle equal to 108° (Fig. 3c). Then, when the water molecule–surface distance is again decreased, a second chemisorbed state isevidenced inwhich thewatermolecule is completelydissociated, its threeatoms occupying all the bonds broken by the creation of the vacancy site(Fig. 3d). This chemisorbed state is thus characterized by the formation oftwoC–Hbonds ina trans configurationwithequal lengthsof about1.08 Å,

Fig. 3. Equilibrium configurations of the different minima ((a), (c), (e)) and transition statesof one water molecule on the defective C51H18 defective cluster. Note that the energy of thesincluded in the DFT calculations.

and a ketone-like C–O bond of about 1.23 Å. These result are very similarto those found in previous studies on infinite surfaces and based on otherquantum methods [21–23] although an ether-like site has also beenevidenced by Kostov et al. [20] as a possible final state for the waterchemisorption on single atomic vacancy in graphite surface (usingperiodic calculations). Such ether-like site is however found neither herenor in the other related works [21–23]. We thus conclude that theadsorptionof thefirstwatermolecule on thedefective clusters consideredhere ends with the formation of the ketone-like stable site.

We also characterize the transition states connecting the threeminimadetailed above, along the reaction coordinate. The correspondingdiagram, correlating theminimaand the transition states is given in Fig. 4.Although the transition states we found are (first-order) saddle pointsobtainedwhenvarying the reaction coordinate, there is noguarantee thatthis diagram actually correspond to the minimum energy path for thereaction (which would be difficult to find for systems with such a largenumber of degrees of freedom). But, if the diagram shown in Fig. 4 onlygives partial indications on the energy barriers which may exist for thewater chemisorption on the defective clusters considered here, it givesthe correct energyminimum for each of the stable chemisorbed states ofthe water molecule.

Thus, as exhibited in Fig. 4 (black curve), the fully dissociativeadsorption of water on the C51H18 defective cluster (i.e., the formation ofthe ketone-like group) is exothermic by more than 300 kJ/mol (from thereference state), with the intermediate exothermic formation of ahydroxyl-like group (exothermicity of 159 kJ/mol).

((b) and ((d)) calculated when decreasing the reaction coordinate value for adsorptione structures is given in Fig. 4. Black, grey, and white circles represent O, C, and H atoms

Fig. 4. Energyof the differentminima and transition states calculatedwhendecreasing thereaction coordinate value for adsorptionofonewatermoleculeon theC51H18 (black curve)and on the C79H22 (grey curve) defective clusters. All the calculations are performed at theDFT level. The optimized geometries of these minima and transition states are given inFig. 3 for the small C51H18 cluster.

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It is however certainly characterized by quite substantial barriersto dissociation, as indicated by the transition state energy values givenin Fig. 4, and as shown in previous works on infinite surfaces [20–23].It is thus questionablewhether such awater chemisorption eventmayoccur in or just behind an aircraft engine, although these regions arecharacterized by high temperatures and strong turbulences.

Moreover, it should be mentioned that it has been shown in theliterature that O2molecule dissociates in a very strongly exothermicwayat carbon vacancies in graphite-like surfaces without any significantbarrier [21]. As a consequence, oxidation of defective carbonaceousnanoparticles by oxygen molecule behind aircraft does certainlycompete with direct water chemisorption. However, if there is enoughenergy to overcome the first dissociation barrier, water chemisorptionmightalsooccur andwould likely endat thewholedissociated statewiththe formation of a C51H20O defective cluster containing a ketone-likegroup (Fig. 3e). Of course, it should be mentioned that the presentconclusions are based on the thermodynamics of the water chemisorp-tion, only, whereas the actual rate of formation of the ketone-like site atthe surface of soot also depends on the kinetics of the correspondingreaction steps.

Similar results are also foundhere forwater adsorptionon theC79H22

defective cluster (Fig. 4, grey curve), although the overall exothermicityof the chemisorption process is larger than above the smaller C51H18

defective cluster. First of all, by comparing water approach to the twocarbonaceous clusters studied here, it should be emphasized that waterchemisorption is accompanied by structural modifications of theclusters that depend on their size. Indeed both chemically modifiedC51H20O and C79H24O clusters (that contain the ketone-like site formedafter the full water dissociation) are characterized by a rather flatgeometrywhereas only the initial C80H22 cluster is planar. This indicatesthat part of the water dissociation energy is used to flatten the smallcluster. This distortion is accompanied by a much larger increase of theC–C distance of the five-membered ring near the vacancy site on thesmall cluster (from1.56 to2.79 Å) thanon the large cluster (from1.87 to2.81 Å). Moreover, the most stable first chemisorbed state (ΔE=−205 kJ/mol) on the large cluster corresponds to a geometrywhere thefirst dissociated H atom is in a trans position with respect to the OHgroup, whereas it is in a cis position on the small cluster. As aconsequence, the TS2 energy is much lower on the large than on thesmall cluster (Fig. 4) due to a smaller repulsion between this H atomandthe seconddissociatedH atomcoming fromaOHgroupwhich is locatedon the other side of the large cluster (instead of being on the same sideon the small cluster).

3.3. Adsorption of small water aggregates on defective carbonaceousclusters

To characterize the hydrophilicity of the ketone-like clusters(C51H20O and C79H24O) resulting from the dissociation of the firstadsorbed water molecule on the carbon vacancy (see above), weinvestigate here the details of the adsorption process of small wateraggregates containing up to six additional water molecules. Asexplained in Section 2, all the calculations are performed with theDFT method for water adsorption on the small cluster, whereas thetwo-layer ONIOMmethod is used to study the adsorption of water onthe large cluster. Initial positions and orientations of the watermolecules at the beginning of the optimization process are chosen byinformed guesses, assuming that the optimized configuration wouldcorrespond to the formation of a maximum of hydrogen bonds in thesystem under consideration.

Furthermore, for an aggregate containing n water molecules, westart the optimization process either from the most stable geometryfound for the aggregate containing (n−1) adsorbed water molecules,or from the (H2O)n aggregate optimized in the gas phase far from thesurface, to ensure that the optimization process has not ended in alocal minimum.

Themost stable geometries for the small water aggregates above thesmall C51H20O and large C79H24O clusters are shown in Figs. 5 and 6, andthe corresponding adsorption energies per water molecule (ΔEads/n)calculated at the B3LYP/6-311++G(2d,2p) level are given in Table 1.Note that the adsorption energies for an aggregate containing n watermolecules are determined by [50]:

ΔEads ¼ E½CxHyO−ðH2OÞn�−E½CxHyO�−nE½H2O� ð1Þ

where E[H2O] is the energy of the isolated water molecule optimized atthe DFT level, E[CxHyO] is the total energy of the relaxed CxHyO cluster(C51H20O or C79H24O) in the absence of the adsorbed water molecule,and E[CxHyO−(H2O)n] is the energy of the adsorbed system, these twolater energies being calculated using either the DFT method or theONIOM strategy, depending on the size of the cluster.

As shown in Figs. 5 and 6, the most stable geometry for one singlewater molecule adsorbed on the defective clusters is similar onC51H20O and C79H24O clusters. This water molecule is tilted withrespect to the surface and forms one proton donor hydrogen bondwith the O atom of the cluster, with a O–H distance equal to 1.92 Åand a OHO angle equal to 163° above the C51H20O cluster (1.95 Å and158° above the C79H24O cluster). The corresponding adsorptionenergy varies from −15.9 to −12.8 kJ/mol above the C51H20O andC79H24O clusters, respectively, indicating a physisorption process. Thedifference between these two values can be explained by the slightlydifferent water–surface distance above the two clusters. Indeed, thedistance between the O atom of the water molecule and the H atom ofthe CH group at the vacancy site is equal to 2.74 Å above C79H24O,whereas it is slightly larger (2.91 Å) above C51H20O, resulting in asmaller repulsive effect of both the CH group and the underlying π-conjugated system on the smaller carbonaceous cluster.

When increasing the number n of adsorbed water molecules onthe carbonaceous clusters, the equilibrium configurations of the wateraggregates (Figs. 5 and 6) are characterized by the optimization ofthe hydrogen bond network between water molecules and, as aconsequence, the corresponding adsorption energies are governed bythe lateral interactions between water molecules rather than by theinteractions between the water molecules and the surface.Similar configurations and adsorption energies are thus obtained onthe two defective clusters, especially for water aggregates containingmore than 3 water molecules, for which the influence of theunderlying surface appears weaker and weaker when increasing n.As a consequence, the mean adsorption energy ΔEads/n first de-creases with n (or increases in absolute values), from about −13 kJ/

Fig. 5. Optimized geometry for the stable structure of small water aggregates (H2O)n adsorbed on the C51H20O cluster. (a) n=1, (b) n=2, (c) n=3, (d) n=4, (e) n=5, (f) n=6 watermolecules. Black, grey, and white circles represent O, C, and H atoms included in the DFT calculations.

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mol (n=1) down to about −30 kJ/mol for an aggregate containing 4water molecules above the large cluster, and from about −16 kJ/mol(n=1) down to about −31 kJ/mol (n=4) above the small cluster

Fig. 6. Optimized geometry for the stable structure of small water aggregates (H2O)n adsorbwater molecules. Black, grey, and white circles represent O, C, and H atoms included in the

(Table 1). Then, an almost constant value around −30 kJ/mol iscalculatedwhen considering five and sixwatermolecules adsorbed onboth small and large clusters, indicating the saturation of the vacancy

ed on the C79H24O cluster. (a) n=1, (b) n=2, (c) n=3, (d) n=4, (e) n=5, (f) n=6DFT cluster. The rest of the system is represented by grey (C) and white (H) sticks.

Table 1Mean adsorption energy per molecule (ΔEads/n) and the corresponding incremental(i.e., stabilization) energy ΔEinc above the face of the C51H20O and C79H24O defectiveclusters for small water aggregates containing up to n=6 water molecules. Theseenergies are given in kJ/mol and they include the (weak) counterpoise corrections tothe basis set superposition error (BSSE corrections). Note that for the small cluster, allthe calculations are performed at the DFT (B3LYP/6-311++G(2d,2p)) level of theory,whereas the ONIOM method is used when considering the larger cluster.

n (H2O) C51H20O cluster C79H24O cluster

ΔEads/n ΔEinc ΔEads/n ΔEinc1 −15.9 −15.9 −12.8 −12.82 −21.7 −27.5 −17.3 −21.83 −23.9 −28.3 −25.7 −42.54 −31.3 −53.5 −30.2 −44.15 −31.4 −31.8 −30.2 −30.26 −29.5 −20.0 −28.3 −18.8

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site, i.e., the additional water molecules do not feel any significantinfluence of this vacancy site.

As in our previous studies [48–50], the saturation of the vacancysite can be characterized by the calculation of the incrementalassociation energy (i.e. the stabilization energy) defined as

ΔEinc ¼ E½CxHyO−ðH2OÞn�−E½H2O�−E½CxHyO−ðH2OÞn−1�; ð2Þ

and corresponding to the energy lost by the system upon the additionof onewatermolecule. Table 1 shows that themaximum energy loss isobtained by adding a fourth molecule on the vacancy site, a situationthat corresponds to the formation of a very stable water tetramer, inwhich the plane containing the four O atoms is nearly parallel to thesurface. This tetramer is weakly tied to the CO group of the surface bytwo CO–HO bonds implying two water molecules, with O–HOdistances equal to about 2.15 Å (on both clusters). A similar bondingis also obtained for larger water aggregates which are also tied to thesurface by two CO–HO bonds. This confirms that such large wateraggregates tend to optimize their internal hydrogen bond networkrather than their hydrogen bonding with the surface.

It is interesting to compare the results obtained here with thosepreviously reported on partially oxidized carbonaceous clusters contain-ing OH, COOH, and epoxide-like groups at their surface [48–50]. For thiscomparison, the adsorption energies of smallwater aggregates adsorbedon three partially oxidized C80H22 clusters are shown in Fig. 7 togetherwith the present results obtained above the defective C79H24O cluster.

Fig. 7. Adsorption energy per molecule (ΔEads/n) calculated for small water aggregatesadsorbed on the defective C79H24O cluster (black circles) and on various oxidized C80H22

clusters containing epoxide-like (grey diamonds), hydroxyl (grey squares) or carboxyl(grey triangles) groups anchored on their face. Note that these latter results are taken fromRefs. [48–50].

This figure clearly evidences that the mean adsorption energy perwater molecule for small water aggregates is very similar on epoxide-like and on ketone-like groups. It is also similar on hydroxyl group withhowever a slightly lower energy. In contrast, much lower adsorptionenergies are calculated for small water aggregates on cluster containingCOOH group irrespective of the number of adsorbed water molecules,indicating that a COOH site is muchmore attractive than the other sitesfor water nucleation. Note, however, that for the largest wateraggregates (n=5,6) the adsorption energy is mainly governed by theoptimization of the lateral interactions betweenwatermolecules and, asa consequence, it is rather similar on the different carbonaceous clustersconsidered in our studies. Nevertheless, some differences are obtainedbetween adsorption on epoxide-like or ketone-like groups on the onehand, and COOH or OH groups on the other hand, because the distancesbetween the water molecules and the underlying C atoms of the clusterare smaller in the first situation, resulting in a larger repulsivecontribution of the carbonaceous surface to the total adsorption energy.In agreement with our previous conclusions [50], this small stericrepulsionof theπ-conjugated systemexplainswhy thewater adsorptionenergies calculated on clusters containing OH and COOH groups areactually lower than on the other oxidized or defective clusters.

Also, it is interesting to mention that the adsorption energy valuescalculated above the different types of functionalized carbonaceousclusters mentioned here agree quite well with experimental valuesestimated for adsorption of water on various activated carbon surfaces[3]. Indeed, the isosteric heat of water adsorption on activated carbonshas been shown to vary typically from −20to −45 kJ/mol whenincreasing the amount of adsorbed water on carbon surfacescharacterized by the presence of oxygen containing organic functionalgroups [70–73].

4. Conclusions

DFT calculations have been performed to investigate the adsorp-tion of watermolecules above the face of large defective carbonaceousclusters modeling surfaces that might be present in soot. The defectsat the surface of the clusters have been modeled by single atomicvacancies on which water molecules have been approached. Theresults of the quantum calculations evidence dissociative chemisorp-tion of the first adsorbed water molecule, resulting in the oxidation ofthe carbonaceous clusters with formation of a ketone-like structure attheir surface. However, although the full water dissociation is anexothermic process, it can probably be achieved only at the expense ofa substantial activation barrier from the initial physisorbed state, asalready evidenced on infinite graphite and graphene surfaces [20–23].Moreover, although the vacancy site at the carbonaceous surface maydissociate a first incoming water molecule, this dissociation results inthe saturation of the vacancy site which then behaves as a ketone-likesite, i.e. as a weak nucleation center for additional water molecules.Indeed, the values of the mean adsorption energy per water moleculeshow that the affinity of such ketone-like site is similar to thatpreviously found on an epoxide-like site [50], andweaker than that ona OH site [48]. It is also much weaker than the affinity between COOHsites and water [47,49].

It should be noted that the present results can be considered asthe first stage toward a more realistic modeling of water condensa-tion around soot. This condensation can be characterized by usingstatistical approaches like the grand canonical Monte Carlo simulationmethod which allows the calculations of the water adsorptionisotherms as a function of the chemical and structural compositionof soot. In a recent series of paper [54–56], we have thus shown therole that chemical functionalities at the surface of soot particles play incondensing water at low pressure. However, it should be noted thatthe accuracy of these statistical approaches mainly depends on theaccuracy of the classical potentials that are used to calculate thewater/surface interaction. A useful way to parametrize these classical

1673M. Oubal et al. / Surface Science 604 (2010) 1666–1673

potentials is to fit their parameters on ab initio results like thosepresented in this paper which can thus be viewed as a preliminary butessential step.

Anyway,wehave shownhere that carbon atomvacancy and ketone-like sites can participate in the hydrophilic behavior of soot primaryparticles, and their presence should be taken into account in furthermodeling of the ice nucleation properties of soot.Moreover, soot being avery complex system, other types of structural and chemical defectsshould also be considered in future studies. Finally, it is worth notingthat the present results can also be of interest for people working onwater adsorption on defects in other types of carbonaceous surfaces.

Acknowledgments

M. Oubal thanks the CNRS and the Région de Franche-Comté forhis PhD grant.

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