CHAPTER 15

Electronic Structure andBonding of Water to NobleMetal Surfaces

HIROHITO OGASAWARA* AND ANDERS NILSSON

SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park,CA 94025, USA*Email: hirohito@slac.stanford.edu

15.1 Introduction

Photoelectrochemical (PEC) cells are often designed with semiconductors asphotoelectrodes and noble metals as counter electrodes. The counter reactionon noble metal surface should be fast to avoid any performance limitations. Atthe interface, water interacts both with the noble metal surfaces and viahydrogen (H-) bonding with other water molecules.1,2 Here, we will focus onthe interaction of water on metal surfaces with a detailed discussion about theelectronic structure and the resulting bonding mechanism. We anticipate that inparticular the electronic structure of the water-surface interaction could havesome similarity for water on semiconductor surfaces.The interaction of water on metal surfaces has been the center of an extended

debate during the last decade due to the multitude of bonding and overlayerstructures water assumes on dierent single crystal surfaces. On metal surfaces,water forms two-dimensional hexagonal, or pseudo-hexagonal, H-bond net-works in the rst contact layer,1,2 where the H atoms not involved in the two-dimensional hydrogen-bond network are either directed toward vacuum (H-up)

RSC Energy and Environment Series No. 9

Photoelectrochemical Water Splitting: Materials, Processes and Architectures

Edited by Hans-Joachim Lewerenz and Laurence Peter

r The Royal Society of Chemistry 2013

Published by the Royal Society of Chemistry, www.rsc.org

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or towards the surface (H-down). The detailed structure of the rst contactinglayer at a metal surface will aect barriers to dissociation and surface reactivity,including interaction with additional water layers.3,4

15.2 H-up, H-down and Partially Dissociated WaterLayers

In the traditional structural model proposed by Doering and Madey,5 waterwas considered to bind to metal surfaces exclusively via an oxygen lp orbital.The structure of the internally H-bonded water contact layer on metal surfaceswas consequently considered to be the H-up structure where only every secondwater binds to the metal through the metal-oxygen (M-O) bond (H-up,Figure 15.1) while the other half is displaced towards vacuum and pointing thenon-hydrogen-bonded OH group away from the surface towards vacuum.Recent work utilizing x-ray photoemission spectroscopy (XPS) and x-rayabsorption spectroscopy (XAS) to investigate the structure of the contact layer68

has, however, showed that an H-down layer, where all water molecules in thelayer bind directly to the surface through alternating either M-O or metal-hydrogen (M-HO) bonds, is favored for intact adsorption of water on theseclose-packed metal surfaces. However, the details vary for dierent surfaces interms of the long-range order in the H-up and H-down congurations.9,10

X-ray photoelectron spectroscopy (XPS) is based on the creation of a corehole via ionization and provides a method to study the geometric, electronicand chemical properties of a sample. The XPS binding energies dependvery strongly on the elements involved; even for rst-row elements the core-level binding energies dier on the order of 100 eV making XPS a highly

H-up H-downMixed H2O:OH

Figure 15.1 Illustrations of three proposed structural models for water adsorption onmetal surfaces. All models contain a at-lying O-bonded water (red/lighter) but dier in orientation and chemical nature of the secondmolecule (blue/darker). H-up: Traditional ice-like bilayer with theblue/darker water molecule having one of its Hs pointing toward vacuum,Mixed H2O:OH: partially dissociated water layer with at-lying H2O (red/lighter) and at-lying OH (blue/darker), and H-down: non-O-bondedwater molecule has one of its Hs pointing toward the metal surface.Reprinted from reference 8. Copyright 2010, with permission fromElsevier.

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element-specic technique. In the case of water, XPS monitors the bindingenergy of O1s core-level state. The binding energy of the O1s state is aected bythe valence electrons, which in turn are sensitive to the local environment. Wecan therefore expect that the core-levels will be chemically shifted depending onchemical nature (intact or dissociated), adsorption site, distance to the surfaceand molecular orientations. In x-ray absorption spectroscopy (XAS), a coreelectron is resonantly excited into unoccupied atomic or molecular orbitals ator above the Fermi level via a dipole-induced transition.11 XAS provideselement-specic information on the density and the energy level of unoccupiedstates, local atomic structure including molecular orientation, the nature,orientation, and length of chemical bonds (via bonding-antibonding orbitalsplitting12) as well as the chemical nature. Using s- and p- polarized x-ray lighteld, XAS measurements determine the orientation of molecular adsorbatesand the directionality of bonding in an adlayer, including proton orientation,information often unattainable by other spectroscopic probes.11

The work on Pt(111)6 and Ru(0001)7 employed XPS to address the co-ordination of atoms in the molecularly intact monolayer to the surface (seeFigure 15.2) and XAS to determine the orientation and coordination of theinternal OH-groups in water with respect to the surface. While the H-upstructure (Figure 15.1, H-up) would give only 33% surface coordination

Inte

nsity (

arb

. units)

Binding Energy (eV)

279280281

XPS Ru 3d5/2

73 707172

B

S S=1 ML

BS=1/3 ML

B

B

=1 M L

S=1/3 ML

538 536 534 532 530

D2OXPS O 1s

Binding Energy (eV)

Inte

nsity (

arb

. units)

150 K

536 534 532 530

Pt(111)

Ru(0001)

(a) (b)

Figure 15.2 XPS spectra for clean and water-covered Pt(111) and Ru(0001), Theessential results showing low concentration of non-coordinated surfacePt and Ru atoms (a) and non-dissociated water (b) can be seen directly inthe experimental spectra. A qualitative curve tting analysis (black lines)in a is indicated to guide the eye. (a): (Top) (Left) Summed Pt 4f7/2spectra taken at three excitation energies (115, 125 and 135 eV) and(Right) summed Ru 3d5/2 spectra taken at three excitation energies (380,390 and 400 eV) to average out photoelectron diraction eects. Thebulk (B) and uncoordinated surface (S) spectral components are indi-cated. (Bottom) Water coordinated to the surface atoms quenchesintensity of uncoordinated surface atoms (S) and introduces core-levelshifts (new spectral components) compared to the clean surface. Theuncoordinated surface peak (S) is lowered in intensity by more than 60%.Reprinted from reference 8. Copyright 2010, with permission fromElsevier.

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through M-O bonds, the H-down structure (Figure 15.1, H-down) would give67% surface coordination through the combination of M-O andM-HO bonds.The surface coordination number can be experimentally determined byexploiting the surface core-level shift in XPS.6,7,1315 For clean metals, the lowercoordination of atoms at the surface leads to a dierent core-level bindingenergy compared to the bulk.14 On Pt(111), this splitting is 0.4 eV for the Pt 4fphotoemission peak6 (Figure 15.4a, left panel). The introduction of water onPt(111) shifts the Pt 4f surface state towards the bulk value for the atoms thatnow coordinate to water. The change in XPS intensity of the Pt 4f state for theadsorbate system compared to the clean surface indicates that more than 60%of the surface Pt atoms become coordinated to water molecules. This directlyand strongly indicates the H-down layer for Pt(111), where all water moleculesin the contact layer bind to the surface.On Ru(0001), the non-dissociated water contact layer forms a hexagonal

two-dimensional hydrogen-bond network similar to the case of Pt(111). Thesurface coordination number was determined using the surface core level shiftin the Ru 3d photoemission peak7 (Figure 15.2a, right panel), which is sensitiveto changes in local coordination similar to the Pt 4f case. As on Pt(111), morethan 60% of the surface Ru atoms become coordinated, directly showing thatall water molecules in the monolayer bind to atoms at the Ru(0001) surface.Based on the same coordination of the water layer to Ru(0001) as for thewater layer on Pt(111) and near identical O 1s XPS spectra (see Figure 15.2b),an H-down model is suggested also for the non-dissociated water contact layeron Ru(0001).7

544540536532

Photon Energy (eV)

XAS O1s

Inte

nsity (

arb

. units)

Wavenumber/cm12200240026002800

Cu(110)

Pt(111)

ice surface

IR OD R/R=0.005

Figure 15.3 XAS (Reprinted from reference 8. Copyright 2010, with permission fromElsevier.) and IR spectra24 for D2O ice surface and D2O water monolayeron Pt(111) and Cu(110). The light eld oscillates in the direction normalto the surface (p-polarized).

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The question emerges whether the H-down model is valid in general forwater-metal monolayer adsorbate systems, or does the overlayer structure varydepending on surface structure? On the open and corrugated (110) surface ofcopper, it was found that a mixed monolayer with 2/3 ( 15%) of the outer-layer water molecules in H-down conguration, and 1/3 with hydrogenpointing up toward the vacuum. Thus we nd an H-down:H-up ratio of 2:1 forthe water monolayer on Cu(110).16 The spectroscopic indication of a mixedH-up/H-down layer is consistent with the large (78) unit cell for monolayerwater on Cu(110),16 indicating that the hexagonal adlayer is rather distortedwith respect to the open substrate which can be expected to lead to a range ofadsorption sites.The orientation of the uncoordinated OH is conrmed by XAS, which can

selectively probe the local unoccupied orbital structure in dierent directions inthe layer by using s- and p-polarized x-rays, whose x-ray light eld is eitherparallel or orthogonal to the surface. The interaction between water and metalsurface, with the possible formation of M-HO bonds, is probed in the out-of-plane XAS while in-plane XAS is related to the formation of the two-dimensional hydrogen-bond network in the contact layer. There is, indeed, astrong anisotropy in XAS between aligning the E-vector along the surfacenormal (out-of-plane) and parallel to the surface (in-plane) (see reference 17.for the Pt(111) case). If we had the H-up situation, in which non-hydrogenbonded OH group of water, free OH, are present on the surface, we should an

538 536 534 532 530

D2O/H2O

OD/OH

XPS O 1s h=785 eV

Binding Energy (eV)

Inte

nsity (

arb

. units) 150 K

180 K

Reaction Coordinate

H2O(g)

H2O(a)

H(a)+OH(a)

dE

En

erg

y

dissociation desorption

Figure 15.4 XPS for water (H2O andD2O) adsorbed on Ru(0001)7 (left) and schematic

illustration of desorption and dissociation barriers for H2O on Ru(0001)(right). Chemical species on Ru(0001) are identied through chemicalshifts in the O 1s XPS. The formation of hydroxyl at 180 K is characterizedby the appearance of an O 1s XPS peak at ca. 531 eV, whereas that forintact water appears at ca. 532533 eV. The decomposition is observed forH2O but not for D2O due to zero-point energy dierences.Reprinted from reference 8. Copyright 2010, with permission fromElsevier.

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XAS signature of free OH. There are abundant amount of free OH on thesurface of ice water, which gives rise to the peak at 535 eV assignable to orbitalslocalized at free OH groups. Another way to probe the orientation of the non-hydrogen-bonded OH group is via vibrational excitation of the OH vibrationalmode. Several techniques have been utilized to probe the O-H stretch vibrationof adsorbed water: IR absorption, electron energy loss and sum frequencygeneration.16,1820 The ngerprint of non-hydrogen-bonded OH groupspointing towards the vacuum is an isolated high-frequency free OH band at3680 cm1 (2730 cm1 for deuterated water). XAS and IR studies on ammoniaterminated ice surface indicate that these states almost exclusively reside at theice surface.21,22 In Figure 15.3, we compare p-polarized XAS and IR results forthe surface of ice water23 and the water monolayer on Pt(111) andCu(110);6,16,24 the polarization shown is with the light eld oscillating normalto the surface (z-direction), i.e. in the direction of the free OH-groups. OnPt(111), OH groups are no longer uncoordinated resulting in broadened featureand the loss of intensity. The strong feature at 532 eV is attributed to moleculesbinding through oxygen (Pt-O) and the feature at 534 eV to the binding throughhydrogen (Pt-HO), which will be discussed in the next section. On the otherhand, a notable amount of free OH feature remains in XAS for the watermonolayer on Cu(110) with H-down:H-up ratio of 2:1, The IR absorption offree OH is intense for water adsorbed on Cu(110)16 but negligible on bothPt(111),18,25,26 which corroborates with the presence of free OH on the mixedH-down:H-up water layer on Cu(110), but not on the H-down water layer onPt(111) .

15.3 Competition Between Thermal Dissociation andDesorption

Under certain circumstances, the O-H bond of adsorbed water dissociates.Compilations of studies of water on metal surfaces1,2 show that Ru(0001) andCu(110) are on the border between active and inactive metal surfaces withrespect to dissociation of water. The dissociation of water on these surfaces issupported by the appearance of two dierent O 1s XPS peaks assignable to,respectively, water and hydroxyl, see Figure 15.4 (left). Although a dissociatedlayer is thermodynamically favorable on Ru(0001) and Cu(110), the dissoci-ation must overcome activation barriers. The relative heights among activationbarriers for dierent reaction play in determining the dissociation probability.The structure of water on Ru(0001) was an issue of debate.7,19,20,2734 Feibel-man34 found that a partially dissociated layer consisting of a near-planarhexagonal mixed network of adsorbed water and hydroxyl (Mixed H2O:OH inFigure 15.1). There is, however, an activation barrier that impedes the de-composition of water, as depicted in the schematic potential energy diagram inFigure 15.4 (right). It was found on Ru(0001) that dissociation and desorptionof water occur with very similar barriers, and the probability of dissociationis thus ruled by the balance between desorption and dissociation kinetics.7

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An indication of this delicate balance between dissociation and desorption isfound from the anomalous isotope eect and kinetics in the thermal desorptionspectra of water on Ru(0001).3539 The isotope eect arises from dierences inthe zero-point vibrational energy contribution to the dissociation barrier be-tween H2O and D2O. The dissociation pathway involves elongation of an O-Hor O-D bond. H2O has a 0.1 eV higher zero-point vibrational energy comparedto D2O in the dissociative pathway. The bonding to the surface, on the otherhand, is equivalent for the two isotopes giving similar barriers to desorption.The zero-point contribution directly aects the barrier to dissociation, whichwill be only slightly higher than desorption barrier for H2O but signicantlymore so for D2O. No dissociation is observed on the surfaces of neighboringelements to the right in the periodic table, e.g. Ni(111), Cu(111), Rh(111) andPt(111), for which the barrier to dissociation thus becomes signicantly largerthan the desorption barrier.1,2

15.4 Electronic Structure and Bonding Mechanism

Photoelectron spectroscopy (PES) has been used to probe the valence electronicstructure of water. PES determines the binding energy and character of thedierent occupied molecular orbitals can be determined. In the PES spectra ofgas phase water,40,41 see Figure 15.5, peaks in the valence region were assigned

15 10 5

Binding Energy (eV)

1b23a1

1b1

gas

ice

x0.2

PES h=100eV

Figure 15.5 PES spectrum of gas phase water measured at a photon energy of100 eV40 and PES spectra of ice at photon energies of 100 eV.43

Reprinted with permission from reference 43. Copyright 2005, AmericanInstitute of Physics.

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to 1b2, 3a1 and 1b1 states of water. The highest occupied state (1b2) has a non-bonding character and highly localized on the oxygen atom, as it has a lobepointing away from the two hydrogen atoms. The third lowest occupied state(1b1) has a lobe pointing toward two hydrogen atoms, which bonds the O andH atoms (O-H). Though the second lowest occupied state (3a1) also has an O-Hbonding character, it has a lobe pointing away from two hydrogen atoms,which can be thought as a lp character.X-ray emission spectroscopy (XES) has also been used to determine the

binding energy and character of the dierent occupied molecular orbitals. Oneof the main advantages of XES to PES is that it provides element specicity. Inthe case of water the XES process involves the projection of the valence elec-tronic state onto the oxygen 1s core state of water. Since the spatially localizedcharacter of oxygen 1s orbital on an atom, it provides a tool to probe themolecular orbitals of water selectively and no contribution comes from thesubstrate.In an aqueous electrolyte solution, water makes hydrogen bonds to sur-

rounding water and ions. Though the strength of hydrogen bond is weak,generally B0.25 eV per bond, the valence states of water is altered uponhydrogen bonding. The eect of hydrogen bonding to the valence states isdemonstrated by comparing gas phase and ice water PES results in Figure 15.5.The 3a1 state undergoes the most prominent change of substantial broadening,which have also seen for PES study in liquid water.40,42 This is due to theelectron re-distribution inside the water molecules to minimize the Pauli re-pulsion upon hydrogen bonding.43 PES 40,42 and XES 44,45 studies of the liquidphase have produced similar results.Metal and semiconductor materials have valence and conduction electrons.

The valence electrons are bound to individual atoms, as opposed to conductionelectrons, which move freely within the material. We now consider the role ofthese electrons in the bonding mechanism of water to the metal through XESstudies.X-ray emission spectroscopy (XES) studies on water on Pt(111) showed the

Pt-O and Pt-HO bonding mechanism of water in a site-specic way.6 For XESmeasurements, a core-hole is created in the 1s orbital of water through the x-rayabsorption process. The core-hole will subsequently be lled through the dipoletransition of valence electronic states resulting in emission of x-rays; thereforethe intensity of the x-ray emission peak corresponds to the p component of thevalence electronic states. The s-polarized x-ray emission spectra, where thex-ray light is emitted in the surface plane, correspond to the oxygen 2p com-ponents projected in the surface plane,46 and the electronic states involved inthe two-dimensional hydrogen-bond network of the contact layer are probed.These spectra8 are very similar to those of bulk ice.47,48 The p-polarized x-rayemission spectra, whose x-ray light eld is parallel to the surface normal,corresponds to the oxygen 2p components projected along the surface normaland a component parallel to the surface plane. On Pt(111), water molecules arealternately Pt-O and Pt-HO bonding to the surface. By tuning the excitationphoton energy, we can selectively excite either Pt-O or Pt-HO bonding water,

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projecting the occupied electronic states on the oxygen atom of respectively thePt-O and Pt-HO bonding species (Figure 15.6). The bonding mechanism shownin the insert of Figure 15.6 is proposed based on the analysis of spectral featuresin XES combined with electronic structure calculations.6 The interaction of theO lp orbital (1b1) with the valence electrons in Pt d-orbital form bonding Pt-Ostate and anti-bonding Pt-O* states, which appears in the vicinity the Fermilevel. Here the bond strength is predicted by the d-band model;49 in which thedegree of d-band population and the position of d-band center are important.In the case of water on Pt, a partially unlled nature of Pt 5d-band makes thePt-O* state partially unlled. The depopulation is seen as a peak at 532.5 eV inthe out-of-plane XAS spectrum in Figure 15.7.The closed-shell d10 conguration of Cu surfaces, however, does not aord

this mechanism.50 The decomposition, and comparison with water onCu(110)16 (Figure 15.7, highlights the inuence on the bonding by the d-bandposition with respect to the Fermi level as depicted in the inset. Let us nowconsider the M-O bonding channel. The valence electrons of Cu occupy the 3d

530525520515

051015

Binding Energy(BE)/eV

530525520515

Photon Energy/eV

051015

x4 x4

XES

Int. [arb

. units]

Pt-O

Pt5d Olp

Pt-HO

OH

OH*

Figure 15.6 X-ray emission spectra (p-polarized) from Pt-O and Pt-HO bondingwater showing the occupied orbital structure out-of-plane (pz compon-ents).6 The Pt-O bonding water has an x-ray absorption threshold atlower energy than that for the Pt-HO species bonding through hydrogen.This allows separated XES spectra for the Pt-O and Pt-HO bondingwater to be obtained by using two excitation energies (532 eV and 538 eV)and a subtraction procedure. The inset shows schematic diagrams of thePt-O and Pt-HO bonds.Reprinted from reference 6. Copyright 2002 by the American PhysicalSociety.

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orbitals more spatially contracted than the 5d orbitals of Pt. Moreover, highdensity of conduction electron in Cu causes the Pauli repulsion with the elec-trons of water, which will inhibit the approach of O lp to the 3d orbitals of Cu.These eects give rise to a smaller splitting between the bonding and anti-bonding states on Cu compared to on Pt as illustrated in Figure 15.7. In theinteraction with the closed-shell water lone pair, both bonding and anti-bonding states become fully occupied leading to Pauli repulsion. Accordingly,no such O-bonding related peak appears in the XAS for Cu surfaces resulting inno net attractive interaction in the M-O bonding channel. The s-electrons aremuch more mobile and can easily move away from the bonded metal atomtowards neighboring atoms to minimize the overall repulsion. This can bedescribed in a simplistic way as that the water lone-pair digs a hole in thes-band50 as shown schematically in Figure 15.8. Since there is now a partialpositive charge on the metal atom, the lone pair orbital will be stabilizedthrough electrostatic interaction, often denoted dative bonding. This providesthe main surface bonding mechanism for water and describes general lone-pair

Cu(110)

544540536532

Pt(111)

Photon Energy (eV)

M-O

Pt5d Olp

Cu3d Olp

Pt5d-Olp

Pt5d-Olp*

Cu3d-Olp

Cu3d-Olp*

EF

EF

Pt-O

Pt-HO

Cu-O

Cu-HO

Pt5d-Olp*

Figure 15.7 X-ray absorption spectrum (p-polarized) for water on Pt(111) (top) andCu(110) (bottom) and computed x-ray absorption spectra for M-O andM-HO bonded water on each surface corresponding to the pz com-ponent.50

Reprinted from reference 8. Copyright 2010, with permission fromElsevier. The inset shows schematic diagrams of the Pt-O and Cu-Obonds.

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interactions on surfaces.50 While no chemisorbed water layer is observed onCu(111),50 it is observed on corrugated Cu(111) in which the depleted density ofthe s-electrons at atomic rows on Cu(110) lowers the energy costs to dig thehole in the s-band.

15.5 Conclusions

The structure of water at noble metal surfaces results from a complex interplayof a number of eects, including the balance between water-water and water-metal bond strength, which will directly aect barriers to dissociation, de-sorption and other catalytic reactions. The localized valence electrons and themobile conduction electrons play important roles in the bonding. Localizedvalence electrons facilitate the covalent bonding to water forming a bondingand anti-bonding combination. The population and position of valence bandare parameter to tune nature of anti-bonding state in the vicinity of the Fermilevel. The Pauli repulsion by the conduction electron hampers the approach ofwater to the valence electrons, which can be reduced by the geometric eects.The mechanistic picture of bonding of water to noble metals is anticipated to beapplicable also to semiconductors in PEC materials.

Acknowledgements

This work was supported by Oce of Basic Energy Sciences, US Departmentof Energy under contract DE-AC02-76SF00515. Portions of this research werecarried out at the Stanford Synchrotron Radiation Lightsource (SSRL), adivision of SLAC National Accelerator Laboratory and an Oce of Scienceuser facility operated by Stanford University for the U.S. Department of En-ergy. The results discussed in this review have naturally been obtained in col-laboration with a large number of extraordinary scientists, and we like to inparticular thank Klas Andersson, Lars GM Pettersson and Theanne Schiros.

References

1. P. Thiel and T. Madey, Surf. Sci. Rep., 1987, 7, 211.2. M. Henderson, Surf. Sci. Rep., 2002, 46, 1.

Olp

s-band

Cu3d

+ dig a hole

Figure 15.8 Schematic illustration of the water O lp orbital digging the hole in themetal sp-band to open for water-metal dative bonding.

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3. G. A. Kimmel, N. G. Petrik, Z. Dohnalek and B. D. Kay, Phys. Rev. Lett.,2005, 95, 166102.

4. G. A. Kimmel, N. G. Petrik, Z. Dohnalek and B. D. Kay, J. Chem. Phys.,2007, 126, 114702.

5. D. L. Doering and T. E. Madey, Surf. Sci., 1982, 123, 305.6. H. Ogasawara, B. Brena, D. Nordlund, M. Nyberg, A. Pelmenschikov,

L. G. M. Pettersson and A. Nilsson, Phys. Rev. Lett., 2002, 89, 276102.7. K. Andersson, A. Nikitin, L. G. M. Pettersson, A. Nilsson and

H. Ogasawara, Phys. Rev. Lett., 2004, 93, 196101.8. T. Schiros, K. J. Andersson, L. G. M. Pettersson, A. Nilsson and

H. Ogasawara, J. Electron Spect. Rel. Phen., 2010, 177, 8598.9. A. Hodgson and S. Haq, Surf. Sci. Rep., 2009, 64, 381451.10. J. Carrasco, A. Hodgson and A. Michaelides, Nat. Mater., 2012, 11,

667674.11. J. Stohr, NEXAFS spectroscopy, Springer-Verlag, Berlin Heidelberg,

1992.12. J. Stohr, F. Sette and A. L. Johnson, Phys. Rev. Lett., 1984, 53, 1684.13. O. Bjorneholm, A. Nilsson, H. Tillborg, P. Bennich, A. Sandell,

B. Hernnas, C. Puglia and N. Martensson, Surf. Sci., 1994, 315, L983.14. A. Nilsson, A. Stenborg, H. Tillborg, K. Gunnelin and N. Martensson,

Phys. Rev. B, 1993, 47, 13590.15. N. Martensson and A. Nilsson, J. Electron Spect. Rel. Phen., 1996, 75, 209.16. T. Schiros, S. Haq, H. Ogasawara, O. Takahashi, H. Ostrom,

K. Andersson, L. G. M. Pettersson, A. Hodgson and A. Nilsson, Chem.Phys. Lett., 2006, 429, 415419.

17. T. Schiros, L.-A. Naslund, K. Andersson, J. Gyllenpalm, G. S. Karlberg,M. Odelius, H. Ogasawara, L. G. M. Pettersson and A. Nilsson, J. Phys.Chem. C, 2007, 111, 15003.

18. S. Haq, J. Harnett and A. Hodgson, Surf. Sci., 2002, 505, 171182.19. C. Clay, S. Haq and A. Hodgson, Chem. Phys. Lett., 2004, 388, 8993.20. D. N. Denzler, C. Hess, R. Dudek, S. Wagner, C. Frischkorn, M. Wolf and

G. Ertl, Chem. Phys. Lett., 2003, 376, 618624.21. P. Wernet, D. Nordlund, U. Bergmann, M. Cavalleri, M. Odelius,

H. Ogasawara, L. A. Naslund, T. K. Hirsch, L. Ojamae, P. Glatzel,L. G. M. Pettersson and A. Nilsson, Science, 2004, 304, 995999.

22. H. Ogasawara, N. Horimoto and M. Kawai, J. Chem. Phys., 2000, 112,82298232.

23. D. Nordlund, H. Ogasawara, P. Wernet, M. Nyberg, M. Odelius,L. G. M. Pettersson and A. Nilsson, Chem. Phys. Lett., 2004, 395, 161165.

24. H. Ostrom, A. Nillson, M. Kawai and H. Ogasawara, unpublished.25. H. Ogasawara, J. Yoshinobu and M. Kawai, Chem. Phys. Lett., 1994,

231, 188.26. M. Nakamura, Y. Singaya and M. Ito, Chem. Phys. Lett., 1999, 309, 123.27. D. Menzel, Science, 2002, 295, 58.28. D. N. Denzler, S. Wagner, M. Wolf and G. Ertl, Surf. Sci., 2003,

532535, 113.

Electronic Structure and Bonding of Water to Noble Metal Surfaces 417

Dow

nloa

ded

by U

nive

rsity

of L

anca

ster o

n 17

/01/

2015

21:

41:4

8.

Publ

ished

on

02 O

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er 2

013

on h

ttp://

pubs

.rsc.

org

| doi:1

0.1039

/97818

497377

39-004

06View Online

29. A. Michaelides, A. Alavi and D. A. King, J. Am. Chem. Soc., 2003,125, 2746.

30. J. Weissenrieder, A. Mikkelsen, J. N. Andersen, P. J. Feibelman andG. Held, Phys. Rev. Lett., 2004, 93, 196102.

31. S. Meng, E. G. Wang, C. Frischkorn, M. Wolf and S. Gao, Chem. Phys.Lett., 2005, 402, 384.

32. G. Materzanini, G. F. Tantardini, P. J. Lindan and P. Saalfrank, Phys.Rev. B, 2005, 71, 155414.

33. N. S. Faradzhev, K. L. Kostov, P. Feulner, T. E. Madey and D. Menzel,Chem. Phys. Lett., 2005, 415, 165.

34. P. J. Feibelman, Science, 2002, 295, 99.35. D. N. Denzler, S. Wagner, M. Wolf and G. Ertl, Surf. Sci., 2003, 544,

348348.36. D. N. Denzler, S. Wagner, M. Wolf and G. Ertl, Surf. Sci., 2003, 532535,

113119.37. W. Homann and C. Benndorf, Surf. Sci., 1997, 377379, 681686.38. G. Held and D. Menzel, Surf. Sci., 1995, 327, 301320.39. P. J. Schmitz, J. A. Polta, S. L. Chang and P. A. Thiel, Surf. Sci. Lett.,

1987, 186, 283284.40. B. Winter, R. Weber, W. Widdra, M. Dittmar, M. Faubel and I. V. Hertel,

J. Phys. Chem. A, 2004, 108, 26252632.41. S. Myneni, Y. Luo, L. A. Naslund, M. Cavalleri, L. Ojamae,

H. Ogasawara, A. Pelmenschikov, W. Ph, P. Vaterlein, C. Heske,Z. Hussain, L. G. M. Pettersson and A. Nilsson, J. Phys.-Condens. Mat.,2002, 14, L213.

42. M. Faubel, B. Steiner and J. P. Toennies, J. Chem. Phys., 1997, 106,90139031.

43. A. Nilsson, H. Ogasawara, M. Cavalleri, D. Nordlund, M. Nyberg,P. Wernet and L. G. M. Pettersson, J. Chem. Phys., 2005, 122, 154505154509.

44. J. H. Guo, Y. Luo, A. Augustsson, J. E. Rubensson, C. Sathe, H. Agren,H. Siegbahn and J. Nordgren, Phys. Rev. Lett., 2002, 89, 137402.

45. S. Kashtanov, A. Augustsson, Y. Luo, J. H. Guo, C. Sathe,J. E. Rubensson, H. Siegbahn, J. Nordgren and H. Agren, Phys. Rev. B,2004, 69, 024201.

46. A. Nilsson and L. G. M. Pettersson, Surf. Sci. Rep., 2004, 55, 49.47. B. Brena, D. Nordlund, M. Odelius, H. Ogasawara, A. Nilsson and

L. G. M. Pettersson, Phys. Rev. Lett., 2004, 93, 148302.48. T. Tokushima, Y. Harada, O. Takahashi, Y. Senba, H. Ohashi,

L. G. M. Pettersson, A. Nilsson and S. Shin, Chem. Phys. Lett., 2008,460, 387.

49. B. Hammer and J. K. Nrskov, Surf. Sci., 1995, 343, 211220.50. T. Schiros, O. Takahashi, K. J. Andersson, H. Ostrom, L. G. M. Pettersson,

A. Nilsson and H. Ogasawara, J. Chem. Phys., 2010, 132, 094701.

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