General Model for Water Monomer Adsorption on Close-Packed Transition
and Noble Metal Surfaces
A. Michaelides,
1
V. A. Ranea,
2,3
P. L. de Andres,
2
and D. A. King
1
1
Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom
2
Instituto de Ciencia de Materiales (CSIC), Cantoblanco, E-28049 Madrid, Spain
3
Instituto de Investigaciones Fisicoquı
´
micas Teo
´
ricas y Aplicadas (CONICET,UNLP,CICPBA)
Sucursal 4, Casilla de Correo 16 (1900) La Plata, Argentina
(Received 5 December 2002; published 27 May 2003)
Ab initio density functional theory has been used to investigate the adsorption of H
2
O on several
close-packed transition and noble metal surfaces. A remarkably common binding mechanism has been
identified. On every surface H
2
O binds preferentially at an atop adsorption site with the molecular
dipole plane nearly parallel to the surface. This binding mode favors interaction of the H
2
O1b
1
delocalized molecular orbital with surface wave functions.
DOI: 10.1103/PhysRevLett.90.216102 PACS numbers: 68.43.Bc, 68.43.Fg, 82.65.+r
The interaction of H
2
O with metal surfaces is of fun-
damental importance. Particular relevance to heteroge-
neous catalysis and electrochemistry has motivated many
studies [1,2]. However, our atomic level understanding of
H
2
O adsorption systems remains unclear and basic ques-
tions on the binding site and orientation of H
2
O mono-
mers on metal surfaces remain unanswered.
Experimental characterization of H
2
O monomer ad-
sorption is difficult, complicated by facile H
2
O cluster
formation. Cluster formation is problematic because it
masks the true H
2
O-metal interaction, making it difficult
to make any definitive statements about H
2
O-metal bond-
ing [1]. In order to minimize cluster formation, it is
necessary to work with low H
2
O coverages at low tem-
peratures, 100 K. Several experiments have recently
been performed under these conditions. Notable are a
number of scanning tunneling microscopy (STM) studies
on the f111g facets of Pt, Ag, Pd, and Cu [3–6]. However,
to date, it has not been possible with STM to resolve the
internal structure of adsorbed H
2
O molecules. Nor has it
been possible to determine the orientation of the H
2
O
molecule with respect to the surface normal.
The preferred orientation of H
2
O on a surface is im-
portant because it will affect how H
2
O responds to an
applied electrochemical field, how H
2
O dissociates, and
the stability and structure of H
2
O clusters that may form.
Generally it has been assumed that H
2
O adsorbs ‘‘up-
right’’ with the O end down and the OH bonds pointing
away from the surface, since this orientation maximizes
the adsorbate-dipole substrate-image-dipole interactions
[1,6–8]. Several spectroscopic techniques [9–11] and the
electron stimulated desorption ion angular distributions
(ESDIAD) [12,13] approach have been used to probe the
orientation of H
2
O monomers on single crystal surfaces.
However, results are conflicting and ambiguities have
arisen, mainly because of difficulties in discriminating
between H
2
O monomers and clusters.
Despite many theoretical studies in this area a clear
consensus on the nature of H
2
O-metal bonding has not
been arrived at [14–21]. Some predict preferential adsorp-
tion at atop sites while others predict adsorption at higher
coordination sites [16,17]. Further, it is often assumed
that H
2
O sits upright in the plane of the surface normal
[16,18,19]. When this has been explicitly investigated,
however, a range of orientations from upright to nearly
flat lying molecules have been predicted [19–23]. Clearly
a systematic study with a consistent theoretical approach
has the potential to shed new light in this area.
Here we present the results of a density functional
theory (DFT) study of H
2
O monomer adsorption on a
variety of metal substrates. Specifically, adsorption has
been examined on Ruf0001g, Rhf111g, Pdf111g, Ptf111g,
Cuf111g, Agf111g, and unreconstructed Auf111g. From
this database of adsorption systems a common binding
mode is identified. H
2
O monomers bind preferentially at
atop sites and lie nearly flat on the surface.
Total energy calculations within the DFT framework
were performed with the
CASTEP code [24]. Ultrasoft
pseudopotentials were expanded within a plane wave
basis set with a cutoff energy of 340 eV. Exchange and
correlation effects were described by the Perdew-Wang
1991 [25] generalized gradient approximation. Metal sur-
faces were modeled by a periodic array of five or six layer
slabs, separated by a vacuum region equivalent to at least
six layers. A p2 2 unit cell was employed and a single
H
2
O molecule was placed on one side of the slab [26].
Monkhorst-Pack meshes with at least 3 3 1 k-point
sampling within the surface Brillouin zone were used.
Structure optimizations were performed for a variety
of initial orientations of the H
2
O molecule on each sur-
face. These included configurations in which H
2
O was
initially placed in the surface normal with the H atoms
either pointing away from the surface (upright H
2
O)or
towards the surface[27] as well as structures in which
H
2
O was initially parallel to the surface. Atop, bridge,
and threefold sites were studied. From this extensive set
of DFT calculations we find (i) on every surface the
favored adsorption site for H
2
O is the atop site; (ii) at
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this site H
2
O lies nearly parallel to the surface. The tilt
angle () between the molecular dipole plane and the
surface is, on average, 10
, with a minimum value of 6
on Ru and a maximum value of 15
on Cu. Such a
common binding mode for H
2
O on this large variety of
metal surfaces was not anticipated [28].
Figure 1 illustrates this general binding mode for H
2
O
and structural parameters and adsorption energies are
given in Table I. We notice from Table I that H
2
O binds
weakly to all surfaces investigated. The adsorption ener-
gies [29] range from 0.1 to 0.4 eVand are in the sequence:
Au < Ag < Cu < Pd < Pt < Ru < Rh. Bond strengths in
this energy regime place the H
2
O metal bond in the weak
chemisorption/physisorption limit. More importantly,
this energy range straddles the energy of a typical H
bond between H
2
O molecules ( 0:25 eV [1]). An impli-
cation for adsorbed H
2
O clusters is that as one moves
through the above series the relative importance of adsor-
bate-substrate and H bonding interactions is liable to be
reversed. Also shown in Table I are the adsorption ener-
gies of H
2
O at the next most stable site on each surface,
which tends to be the bridge site.
From Table I several other interesting features of H
2
O
adsorption are revealed. First, H
2
O deforms little
upon adsorption: the O-H bonds are slightly elongated
from a calculated gas phase value of 0.97 A
˚
to 0.97–
0.98 A
˚
; and the HOH angle () is expanded by no more
than 2
from a calculated gas phase value of 104
.
Secondly, H
2
O is laterally displaced from the precise
atop site (Oxy), by 0:3
A on Ru, Pt, and Ag.
However the potential energy surfaces for diffusion in
the vicinity of the atop sites are quite smooth. Typically it
costs 0:02 eV to move H
2
O from its equilibrium posi-
tion back to the precise atop site. This is important as it
explains the stability of small H
2
O clusters that form on
Ag [4], Pd [5], and Cu [6] despite apparent mismatches
between the substrate lattice constants and the optimal O-
O separation between H bonded H
2
O molecules. The third
feature is that the metal atom directly beneath H
2
O is
slightly displaced along the surface normal from the
(three) other top layer metal atoms (metal). Finally,
we notice that the largest variation between each adsorp-
tion system is the height of the H
2
O molecule above the
surface: the O-metal bond lengths vary from 2:25
A on
Cu to 3:02
A on Au.
In discussing the preferred adsorption site for H
2
O,
previous studies have argued that H
2
O acts as an electron
donor and the substrate as an electron acceptor, favoring
adsorption on atop sites [1,30]. Furthermore, approximate
rules based on tight-binding arguments also predict atop
adsorption for electron donors under appropriate condi-
tions [31]. Although these ideas may prove to be simplis-
tic, we find that they are indeed consistent with Mulliken
population analyses, which indicate that typically H
2
O
donates 0:1e to the metal. And, as we will show below,
consistent with electron density difference plots, which
reveal that H
2
O mixes with the surface mainly through its
occupied 1b
1
molecular orbitals.
We now examine more closely the orientation of H
2
O at
the atop site. First, rotation about the O-metal bond ( in
Fig. 1) has been examined on Ru, Pd, Pt, and Ag. In
agreement with previous studies, this rotation is essen-
tially unhindered. There tends not to be a clear azimuthal
preference for H
2
O, with different orientations within
0:02 eV of each other. This implies that adsorbed H
2
O
monomers will be randomly distributed about the surface
normal. In addition, it becomes simple for two monomers
adsorbed at adjacent atop sites to reorientate and form a
dimer. With at most a small energy loss, the dimer profits
from H bond formation. Rotation in a plane perpendicular
Θ
Φ
α
FIG. 1 (color online). Top and side views of the typical
structure of a H
2
O monomer adsorbed on a close-packed metal
surface.
TABLE I. Adsorption energies (E
ads
) and optimized structural parameters for H
2
O at its equilibrium (atop) site on several metal
surfaces. metal is the vertical displacement of the atop site metal atom from the other three surface layer metal atoms. Oxy is the
lateral displacement of O from the precise atop site. is the HOH angle and is the H
2
O-surface tilt angle as displayed in Fig. 1.
Also given are the adsorption energies of H
2
O at the nextmost stable site on each surface (E
ads2
).
Surface E
ads
(eV) O-metal (
A)O-H(
A) metal (
A) Oxy (
A) (
) (
) E
ads2
(eV)
Ruf0001g 0.38 2.29 0.98 0:01 0.30 106 6 0.12
Rhf111g 0.42 2.31 0.98 0.06 0.06 106 9 0.15
Pdf111g 0.33 2.28 0.98 0.03 0.18 105 7 0.17
Ptf111g 0.35 2.36 0.98 0.03 0.29 106 7 0.09
Cuf111g 0.24 2.25 0.98 0.07 0.03 106 15 0.19
Agf111g 0.18 2.78 0.97 0.04 0.29 105 9 0.14
Auf111g 0.13 3.02 0.97 0.03 0.06 105 13 0.11
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to the surface [H
2
O tilting in the (1 12) direction, in
Fig. 1] was investigated on Ru, Pd, Pt, and Ag. The total
energy variation with the H
2
O tilt angle is shown in Fig. 2.
Two insights can be gleaned from this. First, the minima
close to 0
confirm that H
2
O lies almost parallel to each
surface. Second, the maxima at 90
reveal that upright
H
2
O molecules are disfavored. Thus, despite assumptions
that H
2
O molecules sit upright when adsorbed, DFT
calculations indicate that this is not the case.
Clearly it is desirable to understand this general ten-
dency of H
2
O to lie nearly parallel to the surface. To this
end we first consider the two higher energy occupied
molecular orbitals of H
2
O, namely, the 3a
1
and 1b
1
orbitals, which are shown in Fig. 3. The 3a
1
orbital is in
the C
2v
symmetry plane of the molecule. The 1b
1
orbital
is orthogonal to this, antisymmetric about a mirror plane
in the molecule. It is plausible, therefore, that when H
2
O
approaches a metal surface an upright H
2
O will favor
interaction through the 3a
1
orbital, whereas a flat H
2
O
will favor interaction through the 1b
1
orbital. An exami-
nation of the electronic structure in these systems
confirms these qualitative assumptions. Figure 3, for
example, displays a partial density of states (PDOS)
plot projected onto the O p orbitals for a relaxed H
2
O
( 7
) and an upright H
2
O ( 90
) on Pt. For each
curve two peaks are visible. A careful examination of the
real space distribution of the individual eigenstates
within each peak reveals that states within the lower
energy peak are mainly 3a
1
d states and states within
the higher energy peak are mainly of 1b
1
d character. A
representative example from each peak, for H
2
O in its
equilibrium structure, is displayed in Fig. 3. The approxi-
mate energies of the 3a
1
and 1b
1
orbitals in the gas phase
are also shown in Fig. 3 [32]. By comparing the energy of
the gas and adsorbed phase peaks and also by inspection
of the individual eigenstates it is found that when H
2
O is
upright (dotted line) on Pt the 3a
1
derived orbitals mix
most strongly with the surface and consequently experi-
ence the greatest stabilization. On the other hand when
H
2
O lies flat (solid line) the 1b
1
derived orbitals undergo
the largest mixing with the surface and experience the
greatest stabilization. However, given that initially the
1b
1
orbital is closer to the Fermi level, orientations that
maximize this interaction will be preferred. Indeed the
crucial role played by the 1b
1
orbital for H
2
O in its
equilibrium structure is clearly seen in the density differ-
ence plot displayed in Fig. 3(b).
Competing with this covalent interaction is the inter-
action between the H
2
O permanent dipole and its image
beneath the surface. To understand the role played by the
electrostatics we have estimated the interaction energies
associated with parallel and perpendicular configurations
of H
2
O. A classical images picture, where the image plane
lies 1
A outside the surface, has been employed [33,34].
For a set of three charges, using values from a Mulliken
analysis that produce a dipole moment in agreement with
the experimental value, the perpendicular configuration
is favored over the parallel configuration by 0.05 and
0.02 eV on Pt and Ag, respectively. Thus from a purely
electrostatic perspective there is a preference for H
2
O to
remain upright when adsorbed. However, it is apparent
that this electrostatic desire is small and clearly it is not
decisive. The dominant interaction, and the one that lies at
the origin of the near-parallel configuration, is the cova-
lent one. It is remarkable that this orientation persists on a
wide variety of substrates: the adsorption only moderately
deforms the molecule, yet the interaction is strong enough
to impose a given orientation and even to slightly disturb
the substrate.
Finally it is important to consider how comfortable
this model for H
2
O monomer adsorption sits with existing
0 306090
H
2
O−Surface Angle (
o
)
0
50
100
150
200
Relative Energy (meV)
Ru{0001}
Pt{111}
Ag{111}
Cu{111}
FIG. 2. Relative energy against H
2
O tilt angle () for H
2
O on
several metal surfaces. A tilt angle of 0
corresponds to H
2
O
parallel to the surface, whereas a tilt angle of 90
corresponds
to upright H
2
O in the plane of the surface normal with the O
end down. All points apart from equilibrium structures were
obtained from single point energy optimizations with O at its
equilibrium height above each surface
7.5 6.5 5.5 4.5 3.5
’Flat’
’Up’
E−E (eV)
f
PDOS (arb. units)
(b)
1
1b3a
1
(a)
FIG. 3 (color online). (a) Partial density of states (PDOS)
projected onto the p orbitals of O for H
2
O adsorbed in its
equilibrium (‘‘Flat,’’ 7
) and an upright configuration
(‘‘Up,’’ 90
)onPtf111g. The shape and approximate en-
ergies of the 3a
1
and 1b
1
H
2
O orbitals in the gas phase are
displayed, as are two representative eigenstates from the 3a
1
and 1b
1
resonances for H
2
O adsorbed in its equilibrium struc-
ture. (b) Isosurface of difference electron density for H
2
O on
Ptf111g. This was obtained by subtracting from the adsorption
system the densities of a clean Pt slab and a H
2
O molecule.
Dark (light) regions correspond to a density decrease (increase)
of 3:6 10
2
e
A
3
.
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experimental results. First, although often assumed to sit
at atop sites the only actual characterizations of H
2
O
monomer adsorption are the recent STM study on
Pdf111g [5] and an x-ray absorption fine structure study
on Nif110g [35]. Satisfyingly, both conclude that H
2
O
adsorbs at atop sites. Further, on Nif110g it was shown
that the molecular plane is significantly tilted ( < 70
)
from the surface normal [10,35]. A similar conclusion for
the H
2
O tilt angle was reached from electron-energy-loss
studies of H
2
O monomers at 10 K on Cuf100g and Pdf100g
[9]. In apparent disagreement with this model, however,
are the ESDIAD results for H
2
O on Ruf0001g from which
it was concluded that H
2
O monomers sit upright [12,13].
However, these experiments were performed at 90 K at
coverages of 0.2 monolayers. Subsequent infrared absorp-
tion spectroscopy (IRAS) experiments have shown that
under these conditions on Ruf0001g the dominant surface
species will be H
2
O clusters, probably tetramers, and not
H
2
O monomers [11]. Monomeric H
2
O is only stable on
Ruf0001g below 50 K and the IRAS results provide
evidence that indeed it lies ‘‘nearly parallel’’ to the
surface [11]. Thus it appears that the model for H
2
O
adsorption identified here is not incompatible with ex-
perimental data, rather there are several results in appar-
ent support of it.
In conclusion, a systematic DFT study has identified a
general binding mode for H
2
O on close-packed metal
surfaces. On all surfaces investigated, H
2
O adsorbs pref-
erentially at atop sites and lies nearly parallel to the
surface. This binding mode favors interaction of the
H
2
O1b
1
delocalized molecular orbital with the surface.
This work has been supported by EPSRC and the
Spanish Ministry of Science (PB98-524). A. M. wishes
to thank Gonville and Caius College, Cambridge, for
support. V. A. R. acknowledges support from CONICET
(Argentine).
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