Long-Range Proton Transport for the Water Reaction on Si(001): Study ofHydrogen-Bonded Systems with a Model Liquid-solid Interface

Hiroyuki S. Kato,† Kazuto Akagi,‡ Shinji Tsuneyuki,‡ and Maki Kawai*,†,§

RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan, Department ofPhysics, UniVersity of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan, and Department of AdVanced MaterialsScience, UniVersity of Tokyo, Kashiwa, Chiba 277-8501, Japan

ReceiVed: February 23, 2008; ReVised Manuscript ReceiVed: June 7, 2008

The reaction pathways for water dissociation at a model liquid-solid interface have been investigated by acombination of experimental and theoretical approaches. By scanning tunneling microscopy (STM) and high-resolution electron energy-loss spectroscopy (HREELS), we revealed that the fragments of condensed watermolecules, i.e., OH and H, efficiently terminate the isolated dangling bonds on a precovered Si(001) surface,in comparison with those of the isolated water molecules on the same surface. The most favorable reactionmechanism was predicted by first-principles calculations. At the first stage, the condensed water moleculescreate a new surface OH group at one of the isolated dangling bond sites. Simultaneously, counter fragmentH and surrounding water molecules form a flexible hydronium complex along hydrogen bonds, because thefragment H takes a certain positive charge. Then, another dangling bond is terminated by a H fragment underthe proton relay mechanism via the hydronium complex, in which a very low activation energy is expectedbecause the hydronium complex near the surface is not sufficiently stabilized as in the case of aqueous liquidbut is hindered in shallow potential energy surfaces. Since the spatial hindrance near solid surfaces is a commonproperty, the characteristic proton pathway should appear at various aqueous liquid-solid interfaces andenhance the surface reactions involving proton transfer.

Introduction

The aqueous liquid-solid interfaces, or the interfaces betweena condensed water layer and a solid surface in the microscopicaspect, provide various distinctive reaction fields for bothfundamental and applied chemistry, such as heterogeneouscatalysis, electrochemistry, fuel cells, wet surface modificationprocesses, corrosion, and dissolution.1–4 In the interface reac-tions, the water molecules often behave as a reactant, in which,at the first step, the water molecule is cleaved into OHδ- +Hδ+ (0 e δ e 1) during the creation of new chemical bonds.However, the detailed reaction pathways should not be the sameas those in aqueous solutions because of the existence of thesolid surface or those on the bare surfaces because of theinterference of other condensed water molecules.2–4 The properknowledge of liquid-solid interfaces is required.

In aqueous solutions, proton (H+) transfer is an elementalreaction; in addition, it governs many key processes in chemistryand biochemistry.5,6 In particular, the anomalously highermobility of H+ than other electrolytes has attracted greatscientific attention for a long time. The usual explanation isbased on the sequential H+ relay mechanism along thehydrogen-bonded network of water molecules, the so-calledGrotthuss mechanism.7,8 Briefly, because of the chemical affinitybetween proton and water molecules, the excess proton inaqueous solvents is not intact but forms a hydronium ion,[H(H2O)n]+. By previous ab initio calculations and dynamicalsimulations,9–17 the proton transfer has been described in terms

of fluxional complexes within a concerted reorganization of thesurrounding H2O hydration and the O · · ·O bond lengths,

· · · H3O+ · · · H2O · · · T · · · H5O2

+ · · · T

· · · H2O · · · H3O+ (1)

where H3O+ (oxonium ion; core of Eigen complex18) alternateswith H5O2

+ (Zundel complex19) in thermal fluctuation bycoupling with neighboring H2O along a hydrogen bond, andthus the proton is transported one by one in liquid watermolecules. However, recent acid-base reaction studies withultrafast-laser pump-probe techniques have indicated the exist-ence of at least two types of H+ transport pathways viahydronium ion intermediates.20,21 Thus, even in aqueous solu-tions, detailed paths of proton transport are still under debate.22

The water reactions on various well-defined surfaces havealso been studied with a great deal of effort; most of the studieshave focused on molecular water reactions and their productson bare single-crystal surfaces under vacuum conditions.2–4

Since their results consolidated the fundamental knowledge ofdirect surface-water interactions on a molecular scale, we haveaimed to expand such studies into further investigations bridgingthe gap between bare surface reactions and liquid-solidinterface reactions.

Water reaction studies with well-defined surfaces undervacuum conditions have several advantages for the investigationof condensed water molecules. One is to accurately probe thereactions on a molecular scale by using not only several typesof spectroscopy but also highly spatially resolved microscopy.Therefore, the first-principles calculations based on theseexperiments also provide more reliable considerations. Anotheradvantage is to control the hydrogen bond formation betweenadsorbed water molecules. Since the adsorption energy Ea of

* Corresponding author. E-mail: maki@riken.jp.† RIKEN.‡ Department of Physics, University of Tokyo.§ Department of Advanced Materials Science, University of Tokyo.

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10.1021/jp801598s CCC: $40.75 2008 American Chemical SocietyPublished on Web 07/30/2008

the water molecules on condensed water surfaces is ap-proximately 45 kJ ·mol-1,8 the residence time of impinging watermolecules is expected to be 8 µs ()[ν exp(-Ea/kBT)]-1, wherekB is the Boltzmann constant) at room temperature, T ) 298 K,assuming a typical pre-exponential factor, ν ) 1013 s-1. Thismeans that the condensed water molecules at the surfaces areinstantaneously vaporized under high-vacuum conditions atambient temperature. Therefore, the control of water vaporpressure (impinging rate) and surface temperature (desorptionrate) can provide distinct reaction conditions for either anisolated water regime or a condensed water regime on thesurfaces.4,23,24 Such experiments should clarify the role of thehydrogen-bonded network in the water reactions.

In our previous study, for instance, enhancement of the waterreaction on the SrTiO3(001) surface was observed at the elevatedpartial pressure of g103 Pa at room temperature, in connectionwith the formation of condensed water layers under thoseconditions.24 To ensure well-defined surfaces and to utilizesurface analytical equipment, similarly aimed experiments havebeen performed under ultrahigh vacuum (UHV) at the sub-strate temperature of around 160 K, that is, the thresholdtemperature between condensation and desorption of the ad-sorbed intact water molecules.2,3 So far, only in limitedexperimental and theoretical studies at the ideal Ru(0001),25

Cu(110),26 and MgO(100)27,28 surfaces, a reduced activationbarrier for molecular water dissociation in hydrogen-bondedlayers has been reported and the origin of the activation hasbeen commonly explained in terms of the stability of dissociatedfragments in the coadsorbed water molecules, i.e., mixed OH+ H + H2O layers.

In the present study, we focused on the dissociative waterreaction to the Si(001) surface. The water reaction on the cleanSi(001) surface at room temperature has already been character-ized well by, for example, photoemission spectroscopy,29–34

vibrational spectroscopy,35–39 temperature-programmed desorp-tion (TPD),40,41 scanning tunneling microscopy (STM),42–45 andtheoretical calculation.38,39,46–49 The clean Si(001) surfaceconsists of the (2×1) reconstructed Si-Si dimer rows withdangling bonds at each topmost Si atom. The water moleculesare dissociatively adsorbed on the surface at room temperature,and the OH and H fragments terminate the dangling bonds, asshown in Figure 1. Since these fragments covalently bond tothe dangling bonds, the diffusion of the fragments on the surfacebelow room temperature is completely suppressed.42–45 There-fore, we could spatially identify the reaction sites on the surfaceby STM with the characterization of the products by vibrationalspectroscopy, i.e., high-resolution electron energy-loss spec-

troscopy (HREELS). Consequently, compared with isolatedwater reactions, condensed water reactions on the surface werefound to be an efficient water dissociation process, and thenthe reaction pathways were examined by first-principles calcula-tions based on the density functional theory (DFT). Thus, weexperimentally and theoretically specified the role of thehydrogen-bonded network in water reactions at liquid-solidinterfaces.

Experimental Setup

The STM and HREELS measurements were performed intwo separate UHV chambers with a base pressure better than 1× 10-8 Pa. For both measurements, B-doped Si(001) sampleswere prepared and cleaned by prolonged annealing at ∼850 K(for g8 h) followed by repeated flashing at g1400 K by directcurrent heating. The cleanliness of the surface in each systemwas confirmed by STM or HREELS, respectively. For watervapor dosing, deionized water (17.8 MΩ cm) was used afterseveral freeze-pump-thaw purification cycles. During dosing,the backfilled water vapor was below 10-6 Pa. The absoluteexposure was defined as the product of the vapor pressure andthe exposure time (10-6 Torr · s ) 1.33 × 10-4 Pa · s ≡ 1 L;Langmuir) without correcting for the ion-gauge sensitivity.

The utilized variable-temperature STM system (VT-STM,Omicron GmbH) is equipped with low-energy electron diffrac-tion (LEED) optics and gas dosers. The Si(001) sample wasclamped onto the sample holder, which was mounted at adetachable cold block bound to the liquid-helium flow refrigera-tor, and cooled to ∼50 K. The sample temperature was obtainedby referring to the sample mount temperature monitored with asilicon diode sensor. In the STM measurements, a mechanicallycut PtIr tip was used to suppress the unstability due to waterreactions at the tip surfaces.

The HREELS system (DELTA0.5, Specs GmbH) is equippedwith a quadrupole mass spectrometer, LEED optics, and gasdosers. The Si(001) sample was mounted at the end of arefrigerator tube and cooled to lower than 90 K with liquid N2.The sample temperature was monitored with a C-type thermo-couple attached to the sample with a tantalum clip. In theHREELS measurements, the vibration spectra shown here weretaken at a sample temperature of 90 K in the specularconfiguration (θi ) θf ) 60° from the surface normal) with aprimary beam energy of 2.5 eV, in which the full-width at half-maximum of the elastic peaks was typically ∼5 meV.

Theoretical Calculations

To elucidate the role of hydrogen bonds in the water reaction,the first-principles calculations based on the density functionaltheory (DFT) were conducted. All the calculations wereperformed with the Tokyo Ab-initio Program Package (TAPP)50

with our own extension on Hitachi SR8000 and SR11000systems of the Super Computer Center at the Institute for SolidState Physics, University of Tokyo.

Total-energy calculations and structure optimizations wereperformed with a plane-wave basis set of 36.0 Ry cutoff energy.An exchange-correlation functional with GGA (PW9151) andthe pseudopotential approach (Troullier-Martins norm-conserv-ing type52 for Si atoms, Vanderbilt’s ultrasoft type53 for H andO atoms) were adopted. The size of the unit cell was 7.72 Å ×15.43 Å × 19.30 Å. This corresponds to a repeated slab modelof the Si(001)-(2×4) surface unit cell with a vacuum regionof 8.8 Å thickness. The slab model consisted of five layers ofsilicon atoms and its back surface was terminated by hydrogenatoms along the direction of Si-Si bonds in crystalline silicon.

Figure 1. Schematic view of a water-reacted Si(001)-(2×1) surface.At room temperature, the impinging water molecules are dissociativelyadsorbed on the surface and the fragment OH and H randomly terminatethe adjacent dangling bonds on intradimer or interdimer sites, as markedby ovals. As a result, isolated dangling bonds remain on the water-saturated surface at room temperature, as marked by light blue opencircles.

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The positions of these hydrogen atoms and silicon atoms in thebottom layer were fixed throughout the calculations. Foursampling k-points in the first Brillouin zone (2×2×1) were usedand the criterion for force convergence was 1.0 × 10-3 Ht/au.

Each transition state was explored between two metastablestructures by the force inversion method.54 Here, we should becareful when we make a quantitative discussion on a hydrogenbonding system with DFT, because GGA-type functionals asused in this work have a slight tendency to push out thehydrogen atoms along the hydrogen bond. A Mulliken chargeanalysis method based on a plane-wave basis set was appliedto estimate the charge states of water molecules and clusterson a surface.55

Results and Discussion

The water reaction at the clean Si(001)-(2×1)-reconstructedsurface in the UHV environment has already been characterizedwell.29–49 As shown in Figure 1, the water molecules aredissociatively adsorbed on the surface at room temperature, andthe OH and H fragments terminate the surface dangling bonds.Under an equilibrium condition of the impingement rate andthe desorption rate of water molecules, the condensed watercoverage θ (monolayer)56 in that reaction is estimated as afunction of the dosed water vapor pressure p (Pa), i.e., θ ≈(1.3 × 10-3)p at room temperature.24 On the basis of thisestimation, the condensation of water molecules on the surfaceis expected to be negligible at room temperature under UHV;therefore, the impinged water molecules have reacted with thesurface in the isolated water regime under that condition.

The microscopic view of the dissociative adsorption of thewater molecules on Si(001) can be observed by STM.42–45 Figure2 is a series of the observed STM images for the water-reactedSi(001) surface at room temperature. Since the tunnelingprobability depends on both the sample-tip distance and thedensity of states (DOS) in the energy potential biased from theFermi level, the Si dimer rows on the clean Si(001)-(2×1)surface are observed as topographically high stripes in STM(the bright stripes in Figure 2a) reflecting the DOS of danglingbonds on the dimer atoms. With increasing water exposure, thebright dimer rows become fragmented, as in the images in Figure2b,c, because the water fragments bond covalently to the dimerSi atoms and eliminate the dangling bonds. Since the covalentlybonded water fragments do not diffuse over the surface at roomtemperature, the depressed dangling bonds directly indicate thereaction sites of the fragments. Above 0.6 L, the dissociativeadsorption is saturated, and many stable isolated dangling bonds(bright protrusions) remain on the surface, as shown in Figure2d–f. The same water-saturated surface has been reported byAndersohn and Kohler.42 The creation of the isolated danglingbonds is a result of the random termination of two adjacentdangling bonds on interdimers or intradimers,42–45,48,49 as markedby thin oval lines in Figure 1. Moreover, the stabilization ofthe isolated dangling bonds even under further exposureindicates that the two adjacent dangling bonds are necessaryfor dissociative water adsorption in the isolated water regime.

The reaction of condensed water molecules on the samesurface was also examined and the results were compared withthose above. Panels a and b of Figure 3 show the STM imagesbefore and after condensed water reaction, respectively. In theexperiments, we used the Si(001) surface already saturated withthe dissociative water fragments at room temperature, whichwas indeed the same surface as that in the case of Figure 2f.Thus, many isolated dangling bonds remained on the initialsurface, as shown in Figure 3a. To ensure a well-defined surface,

we accomplished water condensation on that surface by coolingthe substrate to 50 K and exposing the surface to 1.5 L of water.By this procedure, ∼0.8 monolayer of water molecules wascondensed on the surface. Then, the sample was detached fromthe cooling block and gradually heated to room temperaturewithin about 20 min. During heating, some condensed watermolecules reacted with the surface, and others were desorbedfrom the surface between 140 and 190 K (the details of thedesorption were measured by the temperature-programmeddesorption technique; not shown here). Consequently, most ofthe isolated dangling bonds disappeared, as shown in Figure3b.

The products of water reaction on Si(001) can be character-ized distinctly by HREELS.35–39 Figure 4a shows the obtainedvibration spectrum for the water-saturated surface, in which theclean Si(001)-(2×1) surface at 300 K was exposed to watervapor for 3 L. The spectrum shows the characteristic fragmentvibrations, i.e., the peak of the Si-OH stretching modeoverlapped with the Si-O-H bending mode at 831 cm-1, the

Figure 2. Series of STM images of Si(001)-(2×1) surfaces for variouswater exposures at room temperature: 0 (a), 0.1 (b) 0.3 (c), 0.6 (d), 1.0(e), and 3.0 L (f). All the images were obtained at room temperaturewith the parameters of 35 nm × 35 nm scanning area, -1.2 V samplebias, and 0.2 nA tunneling current under the constant-current mode.The chemisorption of water fragments results in depressed sites in STMtopography (dark sites in the above images). Several depressions inpanel a are mostly due to residual water adsorption during the 3 h waitto reduce thermal drift. The number of depressions increased withincreasing water fragment adsorption, as shown. Even at the saturationabove 1.0 L, stable isolated dangling bonds (bright spots in the images)remained on the surface.

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peak of the Si-H stretching mode at 2097 cm-1, and the peakof the SiO-H stretching mode at 3662 cm-1, as previouslyreported.35–39 In addition, the products after the condensed waterreaction on Si(001) were also examined. Figure 4b shows thevibrational spectrum of the water-reacted surface after theprocedure of water condensation by 3 L exposure at 90 Kfollowed by heating to 300 K. The spectrum is indeed identicalwith that in Figure 4a, except for a slight increase in the intensityof the vibration peaks. This result is clear evidence that thereaction products in the condensed water regime are the sameas those in the isolated water regime, i.e., only the OH and Hfragments terminate the dangling bonds on the Si dimers. Thisis consistent with the STM observation, as shown in Figure 3.

From both the STM and HREELS results, it is clear that theisolated dangling bonds are efficiently terminated by dissociative

water reactions in the condensed water regime. Since thedesorption temperature of condensed water molecules is lowerthan 190 K, the activation barrier in the condensed water reactionshould be very small, i.e., much smaller than 50 kJ ·mol-1,assuming the pre-exponential factor of 1013 s-1 in the first-orderdesorption rate equation. Therefore, we conclude that the useof condensed water provides an efficient reaction path for thedissociative water reaction to the spatially separate adsorptionsites.

To elucidate the role of condensed layers in the water reaction,the first-principles calculations based on DFT were conducted.First, we examined the reaction of a single water molecule witha Si(001) surface including two dangling bonds at diagonalpositions on adjacent Si dimers, as shown in Figure 5, in whichother dangling bonds are terminated with the fragment H atoms.In the initial state (Figure 5a), a single water molecule isstabilized at the site on a remaining dangling bond with theadsorption energy of 70 kJ ·mol-1 (0.73 eV). When one O-Hbond of the adsorbed water molecule is elongated toward anotherdangling bond site, i.e., toward the final state (Figure 5c), alonga minimal reaction coordinate, the activation barrier in thetransition state (Figure 5b) is estimated to be 169 kJ ·mol-1 (1.75eV). Accordingly, the adsorbed water molecule cannot thermallypass the dissociation barrier at room temperature. Even if thesurface is heated, the adsorbed water molecule should desorbfrom the surface before dissociation because of the much smalleradsorption energy. Thus, the water molecule does not proceedto dissociation at isolated dangling bond sites in the single waterregime, as observed in Figure 2.

The reaction of condensed water molecules with the isolateddangling bonds on Si(001) has also been reproduced in thecalculations, using a model surface. Figure 6 shows the stabilizedstructures of condensed water molecules on the surface thatincludes four Si dimers in the unit cell, and the six danglingbonds are terminated with OH or H to reproduce two isolateddangling bonds. When the first water molecule is stabilized onthe surface (Figure 6a), it is positioned on a dangling bond withthe adsorption energy of 75 kJ ·mol-1 (0.78 eV), in the samemanner as in the cases of Figure 5a and a clean Si(001)surface.47–49 A diffusion barrier of the adsorbed water from the

Figure 3. STM images of water-reacted Si(001) surfaces (a) beforeand (b) after the condensed water reaction, where all measurementparameters were the same with those described in Figure 2. The insetsare high-resolution images of typical sufaces (9 nm × 9 nm). The initialsurface (image a) was the same as that in Figure 2f the surface wasexposed to 3 L of water at room temperature. Water condensation wasperformed on that surface at 50 K for 1.5 L exposure, then the reactionwas conducted during heating to room temperature. After this procedure,most of the isolated dangling bonds on the initial surface (bright spotsin panel a) disappeared as shown in panel b.

Figure 4. HREEL spectra of reaction products after water reactionon the Si(001) surface. (a) The vibrational spectrum of the water-saturated surface at 300 K clearly shows the vibrations of surface OHand H groups. (b) The vibrational spectrum after the condensed waterreaction with heating to 300 K. The observed vibrations wereunchanged, except for a slight increase in intensity normalized by theelastic peak height.

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dangling bond site toward neighbor OH to form a hydrogen-bonded state was estimated to be 51 kJ ·mol-1 (0.53 eV).

In this adsorption state, the dangling bond interacts with theoxygen atom of the first water molecule and the electronpopulation of the molecule is slightly changed; the oxygen atomtakes on a little negative charge in contrast to the hydrogenatoms which have a little positive charge, i.e., Hδ+. Then, thesecond water molecule is stabilized between the first watermolecule and a surface hydroxyl, as shown in Figure 6b. Thestabilization seems to be a result of the complex formationwithin the tendency toward OHδ--Hδ+ separation of the firstwater molecule, rather than a hydrogen bond interaction. Indeed,

the distance between oxygen atoms of the first and second watermolecules is 2.46 Å (cf., ∼2.8 Å for bulk water molecules)and the hydrogen atom is almost centered between them. Whilethe distance between oxygen atoms of the second water moleculeand the surface hydroxyl is also short, 2.64 Å, the createdcomplex is regarded as including the surface hydroxyl. Thecalculated adsorption energy of this second water molecule is101 kJ ·mol-1 (1.05 eV), which is twice the expected energy oftwo hydrogen bonds (∼0.25 × 2 eV). This also implies theformation of a complex.

The third water molecule stabilizes in the same manner asthe second water molecule (Figure 6c): the water molecule isinserted between the second water molecule and the surfacehydroxyl, in which the insertion is expected as a nonactivatedprocess if a water molecule is located near the complex (asshown in the Supporting Information, Figure S1). As a result,a large complex consisting of the adsorbed water molecules andthe hydroxyl is formed. The calculated adsorption energy ofthis third water molecule is as high as 90 kJ ·mol-1 (0.93 eV),although the number of intermolecular bonds only increases by

Figure 5. Side and top view pairs of the calculated single-waterdissociation process on Si(001): (a) initial state, (b) transition state,and (c) final state, in which the unit cell was 2×2. The initial danglingbond sites on Si dimers are marked by light blue circles. In the transitionstate, the oxygen atom is artificially fixed near the silicon down-dimersite because of the adopted transition state search algorithm. In fact,the water molecule should be desorbed when we forcibly pull itshydrogen atom toward the target dangling bond. The activation barrierwas estimated to be 169 kJ ·mol-1 (1.75 eV). Each bond length is givenin Å.

Figure 6. Top views of condensed water molecules on water-precovered Si(001): (a) monomer, (b) dimer, (c) trimer, and (d) tetramer,in which the unit cell was 2×4. The initial dangling bond sites on Sidimers are marked by light blue circles. The development of acondensed water cluster promotes the release of proton from the first(left below) water molecule and the formation of a complex. Shorteningof distances between oxygen atoms along the water chain and centeringof each hydrogen atom between them are theoretically observed. Eachbond length is given in Å, and O · · ·O distances along a hydrogen bondare given in parentheses.

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one. Since the first water molecule is almost dissociated intohydrogen and an hydroxyl group, the complex is also interpretedas a H5O2 cluster strongly bonded with two surface hydroxylgroups. According to the results of Mulliken charge analysis,55

this H5O2 cluster takes on a positive charge of 0.6e. This meansthat we can treat it as a proton complex (H5O2

+).When the fourth water molecule is connected to the complex

from a site on the neighbor silicon dimer, very little activationbarrier is again expected (see the Supporting Information, FigureS2). Of course many complex structures are considered, butthe complex formation seems to be a lower activation processthan the water diffusion on this surface and bond breaking inthe fully hydrogen-bonded water molecules. By the connectionof the fourth water molecule, the first molecule adsorbed at theisolated dangling bond is completely dissociated into a newsurface hydroxyl and hydrogen, as shown in Figure 6d. Thisnew surface hydroxyl group is no longer included in thecomplex, but stabilizes the remaining complex, i.e., H7O3

+, byforming hydrogen bonds with the two other surface hydroxylgroups.

Figure 7a shows the stabilized structure after the adsorptionof the fifth water molecule on the surface shown in Figure 6d.Since the fifth water molecule is also incorporated into thehydroxyl complex, the complex reaches another isolated dan-gling bond site where the dangling bond interacts with ahydrogen atom at the edge of the hydronium complex. Fromthis initial state (Figure 7a), the termination of the seconddangling bond (Figure 7c) easily occurs through a transitionstate (Figure 7b) with a small activation energy of 3 kJ ·mol-1

(0.03 eV). Although the DFT calculation with GGA oftenunderestimates the activation energy, its deviation from theactual value does not exceed a factor of 0.5, according to ourexperience with this system. Therefore, the estimated activationenergy is small enough for the termination of the spatiallyisolated dangling bonds eVen below the desorption temperatureof the condensed water molecules; this result explains well ourobservations.

It is well-known that the formation of hydrogen bonds causesa large red shift of the O-H stretching mode corresponding tothe weakening of the O-H internal bond of the watermolecules.57–60 However, this is insufficient to understand thereason for the small activation energy in the condensed waterregime. At the active sites on the surface, the water moleculesare dissociated into OHδ- and Hδ+ in the condensed water layerbecause of hydrogen-bonding interactions, simultaneously creat-ing the surface hydroxyl at the active site and the hydroniumcomplex with surrounding condensed water, as shown in Figure6; this is the most important step. Once the excess proton isincluded in the hydronium complex, it can be transportedthermally in the condensed water layers in the manner of theproton relay mechanism that enables high mobility.5–8 Whenthe hydronium complex reaches the proton acceptor site, itreleases the proton and then stabilizes as neutral water mol-ecules, as shown in Figure 7. Thus, the long-range protontransport mechanism in a condensed water layer efficientlyenhances the dissociative water reaction to spatially separatedreaction sites, compared to the case of the isolated regime.

Note that the estimated activation energy, i.e., 3 kJ ·mol-1,is lower than the normal value for the excess proton transferin liquid water, i.e., ∼10 kJ ·mol-1.8 This should be causedby interference by the surface. In aqueous liquid, the excessproton forms H3O+ or H5O2

+ depending on the concertedreorganization of the surrounding H2O hydration and theO · · ·O bond lengths in thermal fluctuation.9–17 Therefore,

Figure 7. Side and top view pairs of the calculated condensed-waterdissociation process on Si(001): (a) initial state, (b) transition state and (c)final state, in which the unit cell was 2×4. The initial dangling bond siteson Si dimers are marked by light blue circles. Upon the adsorption of thefifth water molecule, the complex achieves the stable bridging structurebetween two isolated dangling bonds (left lower and right upper sites).The initial state can shift to the transition state with little change in structure.After the proton transfer to the silicon atom, shortening of the distancesbetween oxygen atoms along the water chain and centering of eachhydrogen atom between them disappeared. The activation barrier wasestimated to be 3 kJ ·mol-1 (0.03 eV). Each bond length is given in Å,and O · · ·O distances along a hydrogen bond are given in parentheses.

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protons are transported one by one in condensed water, asdescribed by eq 1. In contrast, the water-precovered Si(001)surface should provide a different environment. While thesurface OH groups are hydrophilic, the surface H groups arehydrophobic.39,48 Consequently, the concerted reorganizationnear the surface is expected to be insufficient. Even at thefully OH-precovered surface, the stabilization effect ofconcerted reorganization should be less than that in aqueoussolution, because the OH groups are spatially anchored tothe substrate. Therefore, it must be considered that theliquid-solid interfaces provide specific hydronium complexesthat are not as sufficiently stabilized as in the case of aqueousliquid but are hindered in shallow potential energy surfaces.As a result, Figures 6 and 7 show the curious large hydroniumcomplexes consisting of several water molecules, and thusrealize the low activation energy, i.e., 3 kJ ·mol-1, by whichthe rate-determining step for the actual reaction is expectedto be the diffusion and the hydrogen bond rearrangement ofcondensed water molecules to create the large hydroniumcomplexes at around dangling bond sites, rather than theproton transfer itself. It is fact that another reaction path forthe termination of isolated dangling bonds is expected withoutactivation barrier in the complex evolution from Figure 6c,as a restricted case (see the Supporting Information, FigureS3).

It is also considered that such spatial hindrance and thusenhanced proton transfer at the liquid-solid interface shouldbe important not only on inorganic surfaces, as described above,but also for the surfaces of organic soft materials in aqueoussolutions. For instance, bioproteins are folded in solution andcreate many types of functional sites (places) consisting of bothhydrophilic and hydrophobic residues in which programmedhydrogen-bonded networks including water molecules can beformed. In such cases, proton transfer often becomes a key issuefor obtaining efficient functionalities at that site.5,6 Furtherdiversified investigations are required.

Conclusions

The reaction pathways for water dissociation on the Si(001)surface have been investigated by a combination of STM andHREELS measurements, and also DFT calculations. In theisolated water regime, the dissociative water reaction on theSi(001) surface at room temperature causes the terminationof pairs of adjacent dangling bonds by fragment OH and H,so that isolated dangling bonds consequently remain on thesurface. In the condensed water regime, in contrast, theisolated dangling bonds are efficiently terminated even belowroom temperature. The first-principles calculations reproducedwell the differences between isolated and condensed waterreactions with the separated dangling bonds. The activationenergy for the isolated water reaction was estimated to be169 kJ ·mol-1 by using the smallest unit cell. In contrast,the very low activation energy for the condensed waterreaction was estimated. The main cause of the low activationenergy was not only the weakening of the O-H internal bondby hydrogen-bonded interaction, but also the creation of thehydronium complex comprising the fragment proton, Hδ+,with hydrogen-bonded water molecules. Then, the protonrelay mechanism via the hydronium complex provides a highmobility of the fragments in the condensed water layers andreduces the actication barrier for the dissociative waterreactions. In addition, the spatial hindrance of the substrateincreases the flexibility of hydronium complexes because ofthe insufficient concerted reorganization, which must enhance

the proton transport. Since the spatial hindrance near the solidsurface is a common property, similar types of protonpathways should appear at various aqueous liquid-solidinterfaces and enhance the surface reactions involving protontransfer.

Acknowledgment. This work was financially supported inpart by Grants-in-Aid for “Nanoscale Science and TechnologyResearch” and “Hydration Dynamics and Molecular Processes”in RIKEN and for Scientific Research on Priority Areas “SurfaceChemistry of Condensed Molecules” from MEXT, Japan.

Supporting Information Available: “Detailed calculationsof the activation barriers for creating the hydronium complexesshown in Figure 6” and “another reaction path expected fromFigure 6c.” This material is available free of charge via theInternet at http://pubs.acs.org.

References and Notes

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Plenum: New York, 1992, and references cited therein.(6) Hynes, J. T.; Klinman, J. P.; Limbach, H.-H.; Schowen R. L., Eds.

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