Dissociation of Water Buried under Ice on Pt(111)

Yigal Lilach,* Martin J. Iedema, and James P. Cowin†

Pacific Northwest National Laboratory, K8-88, Richland, Washington 99354-999, USA(Received 2 June 2006; published 5 January 2007)

Water on Pt(111) is generally thought to be nondissociative. However, by adsorbing a thick ice film[> 150 monolayers (ML)], substantial (� 0:16 to 1 ML) dissociation of the ‘‘buried water’’ occurs forT > 151 K. New temperature-programmed desorption peaks signal the dissociation (after careful iso-thermal predesorption of the overlying ice films). The buried water likely dissociates via the elevatedtemperatures and/or solvation changes experienced under the ice. Dissociation charges the growing icefilm (up to �9 V) due to trapping of �0:007 ML H3O� at the vacuum-ice interface.

DOI: 10.1103/PhysRevLett.98.016105 PACS numbers: 68.43.Vx, 68.65.Ac, 82.30.Rs

Adsorbed water, well studied on metallic surfaces [1–6],is widely thought to not dissociate on Pt(111), while itpartially dissociates on the slightly more reactive Ru(001)surface [6]. This difference is brought about by subtledifferences in the two metals’ adsorption energies and/orkinetic barriers for OH, H, and H2O. Figure 1 shows that if�Hads is several kT’s smaller than the activation energyrequired for dissociation Ea, water will desorb rather thandissociate. To induce water to dissociate on Pt, one might(i) increase the kinetic barrier to desorption, or (ii) increasethe stability of the products relative to reactants. We showthat growing thick layers of ice on Pt(111) above 151 Kdoes both, causing extensive dissociation in the first layerof water. Since Pt(111) is often used as substrate for multi-layer water growth, this dissociation could be important,for example, for H=D exchange studies.

Experimental methods.—Experiments were performedin an ultrahigh vacuum (UHV) chamber (2� 10�10 Torr)with a quadrupole mass spectrometer (QMS) for tempera-ture programmed desorption (TPD) and Auger spectrome-try [7]. A 1 cm diameter Pt(111) sample is T controlled(60–1400 K). H2O or D2O was dosed by a uniform mo-lecular beam doser (with negligible penumbra) for 0 to 200water monolayer (ML) deposits, or by a close-coupled (andgrounded) tube doser. The water films are uniform to�5%.Tube doser dosing times are selectable from 1–300 s (�200 ML= sec typically). The high flux tube doser permitsgrowing multilayer ice up to 30 K higher than in most otherUHV studies. A Kelvin probe (McAllister Tech.) records���t�, via a vibrating gold mesh near the sample. Themesh decreases TPD interferences. The ���t� consists oftwo parts: ���t� � �Vfilm ��int. The �int is the interfa-cial work function change, due to dipoles localized at thePt-ice, at the ice-vacuum interface or in the bulk. �int �1:2 V for a few monolayers of water on Pt(111). Vfilm isthe electrostatic potential difference generated by netcharge within or upon the ice film. Vfilm > 0 reduces��. Water on Pt(111) grows as an aligned, incommensu-rate single crystal for >50 ML and Tgrowth �Tg�> 135 K.We used thick films ( 150 ML), to avoid a roughening

seen below 50 ML [8]. One ML for water refers to onecomplete ‘‘bilayer’’ coverage � 1:06� 1015 waters cm�2.For ions 1 ML � 1:5� 1015 cm�2 (the number of top-layer Pt atoms).

Results.—Water dissociation at the Pt interface, if itoccurs, may result in strongly bound species discerniblevia TPD. D or O preabsorbed on Pt (111) are well known toreact with water to form D3O� or OD, and is apparent inthe TPD’s. TPD’s of �2 ML D2O adsorbed on clean Pt,and Pt predosed with D (� 0:7 ML from�30 L at 150 K)and O (0.1 ML), are shown in Fig. 2(a), and closelyresemble published TPD. On clean Pt the first-ML TPDpeaks at 172 K, the second-ML peak starts at �145 K.Even trace amounts of contaminants shift or modify thesepeaks. Routinely none is seen. Water adsorption withpreadsorbed D shifts the first-ML peak upward 2 K.Adsorption on O=Pt creates a new adsorption state, desorb-ing at�200 K. This peak is associated with recombinativedesorption of dissociated water, residing in a stable mixedlayer of OH� � H2O [4,9]. (Note: the fractional chargestate of surface bound OH (or H) likely varies with con-

FIG. 1. First monolayer water dissociation energy diagram.Adding ice on top of the first ML blocks its desorption (i.e.,bricks). The ice overlayers also can alter the dissociation barrier(Ea) and enthalpy (�Hdiss) to permit dissociation (lower right).

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ditions, regardless of whether it is referred to as OH� orOH).

We looked for similarly modified TPD’s from multilayerice, indicating dissociation. However, the zero order ki-netics of the overlying ice sublimation, while peaking near145 K for thin ice films, moves to progressively higher T asthe water ice film thickens. This obscures TPD informationfrom the last monolayers —the ‘‘buried water.’’ We madethis observable by a three-step approach: (i) Careful iso-thermal desorption (ID) of the thick ice film at T lowenough to prevent desorption of the last 1–2 ML of wateron the Pt. (ii) Drop T to stop the ID, and let the waterbackground decrease and stabilize. (iii) Do a normal TPDof the remaining water.

Figure 2(b) shows the results. Growth of 150 ML ofwater over 20 s at the desired Tg ( � 170 K here) wasfollowed immediately by ID at 155 K while monitoring theQMS signal (Fig. 2 inset). A sharp drop in the QMS signalsignifies that all but the last monolayer on the surface has

desorbed. The subsequent ‘‘normal’’ TPD of the buriedD2O film shows the expected first-ML peak at�170 K, butreduced in size [compared to Fig. 2(a)]. The spectrum isdominated by two new peaks, one at 200 K, similar to thepeak of water on O=Pt�111� (though larger) and a secondpeak at�185 K. Such peaks were never reported before inthe literature for pure water films, to our knowledge. Thecombined TPD area under these two new peaks is roughlydouble that of a full ML of water [the 170 K peak seen inFig. 2(a)]. Clearly a dramatic change has occurred in theburied layer. We attribute these new TPD peaks to therecombination of �1 ML of dissociated water, as we dis-cuss later. Buried water TPD of H2O is also shown inFig. 2(b), with results similar to D2O, only shifteddown by �6 K. The lowest T peak has probably mostlydesorbed since the ID was done at 155 K (as in the D2Ocase), a bit too close to its expected position at 166 K �172–6 K.

As the water dissociation on Pt may be surprising tomany, we took great pains to eliminate possible artifacts(and this in an instrument that has routinely done reliableTPD’s in many studies). To investigate if these new bindingstates could be due to trace O2 contaminating the multi-layer water, we carefully heated to 225 K, to just desorb allthe water film but not the oxygen. Any surface O willremain and will be evident in its effects on the TPD ofsubsequently deposited water as a peak at 200 K. This issimilar to how O was detected in electron irradiated watermultilayers [10]. Figure 2(c) shows no noticeable O, as thewater TPD is identical to TPD on clean Pt. To show that wecan see a new peak if any O was present, we a created aburied water layer starting on top of an O-predosed Ptsurface (� 0:1 ML O). Figure 2(c) shows that the TPDof the buried water is only slightly perturbed. But the extraO remains on the surface, to create a peak at 200 K, for thesubsequent 2 ML water desorption. Thus the absence ofthis peak when no O is dosed, indicates that any O con-tamination is � 0:01 ML O. Similar experiments donewith preadsorbed H (not shown), excluded trace H con-tamination. Similar water dissociation TPD’s were foundeven when we changed water tube dosers (and its watervapor supply) or inserted a new Pt crystal.

To confirm the water dissociation, we looked forchanges in ��: even deep under the ice dissociationshould create easily measurable ��’s. We measured ��created after growing 3000 ML H2O films at each of aseries of T ’s: 140< Tg < 167 K. The ��’s are measuredat Tg, and include any strictly interfacial ��int plus anychange in the net voltage that might develop across the icefilm (� Vfilm). Tg > 140 K avoids a well-characterized,metastable, negative-polarity Vfilm [7] that occur at a lowerT. Figure 3 shows an abrupt change, for Tg 151 K: Vfilm

(presumed to equal ��� here) rises from �0, peaking at�� 9 V for Tg � 162 K. D2O was similar (Fig. 3), exceptthe positive Vfilm commences at �157 K [the same T shiftseen between H2O and D2O TPD peaks in Fig. 2(b)].

FIG. 2. (a) TPD of 2 ML D2O on the indicated surfaces: cleanand predosed with �0:7 ML D or �0:1 ML O. (b) Buried waterTPD for isotope=Tgrowth=Tisothermal as indicated. Inset: isothermaldesorption of 150 ML D2O. (c) Probing for residual O: BuriedD2O TPD to 225 K (Tgrowth � 170 K, Tisothermal � 155 K) onclean Pt (dotted) and O=Pt (dashed), followed by redosing withD2O and TPD again (solid). The heating rate is 1 K= sec for allTPDs.

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This 9 V �� change was much bigger than expected,but did occur roughly in the T range that the TPD resultssuggested water dissociation was occurring. Upon raisingT, Vfilm stayed roughly constant [11], and then disappearedwhen the water film desorbed. Detailed low T annealingstudies of the films’ electrical behavior showed [11] thisnew voltage resembled that created in experiments wherewe had intentionally dosed water films with �C’s of posi-tive ions, with a soft-landing ion beam [12]. The amount ofchargeQ one would need to deposit to create this voltage isestimated from Vfilm � Qd=�A"0"�T��, for a film d thick,with "0 the vacuum permittivity and "�T� the T-dependentdielectric constant of water. This gives Q 0:0075 MLneeded (more if they are within the film) to generate the�9 V (with " � 200, 1 ML height � 3:5 �A). Much effortwas put into ruling out spurious ion sources, such asfilaments, ion pumps, or heating wires, as the source ofthe ions [11]. All were less than a few pA, much less thanthe nA to �A of ‘‘stray current’’ needed to generate 9 V.This 9 V change was highly reproducible, over a year’stime, 2 Pt samples, tube doser changes, etc. It was not anartifact.

We showed that this 9 V comes from water dissociationat the Pt surface, and signifies a significant fraction of 1 MLdissociated during film growth: Using different film thick-nesses: (150< �< 6000 ML), Vfilm at Tg � 170 K wasfound to initially be linear in coverage, but then goesasymptotically to a constant. Vfilm was closely fit by8:2 V�1� e��=2200 ML� [11]. This suggests the source ofthe charge comes from the ice-Pt interface, neither fromthe bulk nor from some effect induced in the bulk by theasymmetry of the ice-vacuum interface. To test whetherthis was related to trace H2 (or O2) coadsorbing with thewater (generating H3O� (or OH�) at the Pt surface) wepreadsorbed �0:01 to 0.03 ML of H (or O). This had noeffect on the charging. But larger amounts of surface H orO did change things: A 0.7 ML H predosing increased Vfilm

25%. Substantial O predosing (Fig. 3 triangles, maximumcoverage 0:077 ML) decreased Vfilm an order of magni-tude. Precoating the surface with a thin (100 ML) waterfilm whose Vfilm 0 (Tg � 140 K), followed by 3000 MLat 170 K (where Vfilm is large) gave Vfilm 0.

All can be explained if the positive charging is caused bya substantial (� 2� 0:08 � 0:16) fraction of 1 ML ofH3O�=OH� ions forming via water dissociation at the Ptinterface during the growth under the ice (< 150 ML),with most of the ions remaining at the Pt surface, and asmall amount of H3O� getting trapped at the ice-vacuuminterface. OH is thought to bind to Pt more strongly thanhydronium does, as in the very stable OH-H2O layer [9].We suggest that early in the growth the H3O� at the Pt-iceinterface is in equilibrium with a local minimum at the ice-vacuum interface. Further growth isolates the surface ki-netically. As the film grows thicker, most of the H3O�, nowtrapped at the ice-vacuum interface, is carried along withthe growing interface, as they are more stable on top of theice than getting buried in it. As each new monolayer of iceis added, about 1=2200 of these ice-vacuum interfacialH3O� are left behind in the bulk. Substantial preadsorbed(not trace) H makes substantially more surface H3O�,increasing the amount that is trapped at the ice-vacuuminterface. Substantial preadsorbed O titrates H3O� back towater (2 per O atom), reducing the supply of H3O� (seeRef. [11] for details).

Discussion: When does water dissociate on Pt(111)?—Figure 3 suggests that during growth water dissociates forTg > 151 K (H2O) and >157 K (D2O). Figure 2(b) showsthat films grown at T � 145 K (too low to generate thepositive Vfilm), show high T TPD peaks after 155 Kstripping-ID, indicating water dissociation. This wouldhave had to occur during the ID at 155 K. Not shown ismuch shorter (120 s) stripping-ID at 170 K of 150 MLfilms, which also produces high T TPD features from 170–200 K. ID at 170 K increases slightly the amount of waterdesorbing above 170 K. The minimum ice film thicknessfor dissociation is greater than several ML, and less than orequal to 150 ML.

Why have not others seen this water dissociation?—Others doing TPD of water multilayers would not normallydo our three-step TPD. A normal TPD would have the newhigh T TPD peaks disappearing under the multilayer peak.Also, few UHV studies (none?) grew ice films at T >165 K, lacking any obvious motivation, and needing aspecial high flux source.

Theory has suggested dissociation: Feibelman via den-sity functional theory (DFT) predicts partial dissociationfor 1 ML H2O=Pt�111�, consistent with Pt-ice wetting [13].Jacob et al. [14] predict �E 0 for 50% dissociation, andrelates this to the flat O structures seen in LEED. Mengpredicts partial dissociation [5]. Much theory predicts nodissociation, e.g., Ref. [15].

Does this change the water/Pt(111) energy surface?—Figure 1 shows an energy diagram for water dissociation

FIG. 3. The voltage across ice films versus Tgrowth. The positivevoltage appears suddenly starting at T � 151 K (H2O) and157 K (D2O). Triangles: 3500 ML D2O adsorbed at 165 K onPt predosed with O2. O2 dosed at 500 K. (1L � 10�6 torr s andO atom coverages are 0.030, 0.043, 0.054, 0.077 ML).

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on Pt. For water to substantially dissociate, the dissociatedstate (right) would need to be below the adsorbed waterstate (middle) (�Hdiss < 0). That others had not seen waterdissociate had convinced most that the dissociated state isuphill, as drawn at top right. However, dissociation canalso be prevented, if a dissociation barrier Ea is largeenough to makes desorption much faster than dissociation.For 1–2 ML of water on Pt(111) clearly �Hdiss > 0 and/orEa is >� �Hads. When we add the multilayers on top ofthe first monolayer water, they make a large barrier for itsdesorption (the bricks in Fig. 1). This would permit over-coming a large Ea to dissociate (if it exists), using longertimes and/or a higher T. Solvation via the multilayers mayalter the dissociation barrier height to E0a (dotted curve), orchange �Hdiss. Our results show that 0> �H0diss for waterburied under >150 ML ice multilayers.

Stabilization via ionic and hydrogen bonding with un-dissociated water in mixed partial dissociation states:[nH2O�ad� ! �n� 1�H2O�ad� � H�ad� � OH�ad� [5,13–15] ]seems crucial, yet is difficult to calculate. Michaelidesand Hu calculated a dissociation Ea � 0:68 eV and�Hdiss � �0:47 eV [16] to produce nonsolvated, isolated,adsorbed OH and H. Their �Hads, 0.34 eV (similar toexperiment), predicts desorption should precede dissocia-tion in the absence of solvation. Stuve’s group experimen-tally estimated a water to H�ad� binding of 0.1 eV [17]. Chenet al. [18] estimate that H�ad� � 4H2O�ad� ! H9O4�ad�

� is1.17 eV exothermic. On Ru(001) one finds �Hdiss< andEa ��Hads [19,20]. Michaelides et al. [21] predict thatan isolated H2O on Ru(001) has an Ea of 0.8 eV to formisolated OH and H, and �Hdiss 0. Adding solvation via1 ML water, and lateral segregation of H to pure H regions,decreased Ea to 0.5 eV and �Hdiss became exothermic:�0:85 eV. We propose that solvation at the multilayerburied interface makes the H2O=Pt system act more likemonolayer H2O=Ru�001�.

Why is exposed (unburied) dissociated water stable?—Isothermal (155 K) stripping of the multilayers prior to theTPD’s may remove the enthalpy and barrier shifts shownon the right side of Fig. 1, caused by solvation. Nonethelessthe now-exposed ion or water system did not immediatelyrevert to water, as the TPD is altered up to 200 K. Thus thehydrated partially dissociated water may be absolutelystable compared to nondissociated water even withoutthe multilayer ice overlayer. If so, it would mean the lackof dissociation usually seen for 1 ML water adsorptionimplies a larger barrier for dissociation compared to de-sorption. But is that plausible? Reactions of H and O toform water on Pt(111), under constant H2 exposure, showcomplex kinetics, with hydrated OH� regions [5,20,22].The recombination reaction is completed in minutesnear 150 K, well below our 200 K. So how can our (un)-buried layers persist without recombination up to200 K? We have two possible answers. (i) The system

may have formed large regions that are OH�-rich, andothers that are H=H3O� rich. High T is then needed toreunite the species during the TPD. Or (ii) A rather stablehydrated-H3O�-OH� phase of�2 ML total water (ions�undissociated) forms, possibly involving first and secondlayer ions: an ‘‘hydronium-hydroxide’’ hydrate. These hy-drated structures would not occur for <1 ML studies.

Trapping of the monolayer of water in contact with thePt surface by the overlying water, in combination withsolvation effects, permits dissociation. This ‘‘pressurecooker’’ effect may also prove useful in carrying out otherreactions, such as organic reactions, that otherwise can notbe studied under the typical UHV conditions.

We appreciated discussions with Greg Kimmel, MikeHenderson, and Mike White (Pacific Northwest NationalLaboratory). The research used the EnvironmentalMolecular Sciences Laboratory, a national user facilitysponsored by the Department of Energy’s Office ofBiological and Environmental Research. Support is fromDOE/BES Chemical Sciences and DOE/OBER.

*Electronic address: yigalli@yahoo.com†Electronic address: jp.cowin@pnl.gov

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