Adsorption of water on MgO„100…: A singular behavior
C. Girardet,* P. N. M. Hoang, A. Marmier, and S. Picaud
Laboratoire de Physique Mole ´culaire - UMR CNRS 6624, Faculte ´ des Sciences, La Bouloie,
Universite ´ de Franche-Comte ´, 25030 Besanc ¸on Cedex, France
Received 14 January 1998
Interaction-potential minimization and molecular-dynamics simulations show that a perfect MgO100 sur-
face accomodates very stable flat water monolayers differing only by the mutual orientations of the molecules
above the cations, without evidence for significant hydrogen bonding with the substrate and between the H
2
O
molecules. Increasing coverage leads to the occurrence of an upper solid versus liquidlike structure not hy-
drogen bonded to the stable monolayer, which tends to tear up at 300 K. These results are in good agreement
with recent experimental data. S0163-18299803620-0
The fascinating field of water adsorption on various sub-
strates, of fundamental interest for scientists,
1
has gained in
addition a revival of activity this past decade with the devel-
opment of surface technology to determine the conditions
that favor physisorption vs chemisorption of molecular water
or dissociation.
2
The role of defects in initiating the dissocia-
tion of water has been shown to proceed
3,4
via a complicated
pathway implying, for instance, on MgO, energy balance in
the hydroxyl formation. Aside from dissociation, water is
generally known to adsorb molecularly on clean metals or
dielectrics, at least below 300 K. For instance, recent scan-
ning tunneling microscopy studies
5
have demonstrated the
development of ordered two-dimensional 2D ice layers of
water on Pt111 and stable bilayer phases around 130 K
were deduced
6
from helium atom scattering HAS experi-
ments. The scenario generally admitted, proposed on the ba-
sis of molecular-dynamics simulations,
7
is that water forms
icelike hexagonal structures with the oxygens bound to Pt
atoms while the second layer is hydrogen bonded to the first
layer and, beyond, transition to the bulk behavior occurs.
Similar conclusions were drawn for other metals
8–11
with,
however, some differences depending on the substrate geom-
etry and electronic properties, and still more complex results
were obtained on semiconductors regarding the fact that wa-
ter dissociates or molecularly adsorbs.
12–15
Water adsorption on ionic/partially ionic crystals is char-
acterized by the presence of strong electrostatic surface fields
and field gradients which can compete with the lateral bind-
ing between H
2
O molecules and could create a favorable
situation for the formation of stable 2D layers. Helium scat-
tering experiments on UHV cleaved CaF
2
111 surface
showed the occurrence of a p (4 4) phase that was inter-
preted in terms of the formation of water hexamers.
16,17
On
NaCl100, infrared spectroscopy and low-energy electron
diffraction LEED data
18,19
concluded to the presence of an
ordered quasihexagonal c (4 2) bilayer structure and he-
lium scattering data
20
led to the formation of a 11 con-
densed phase for which nearest-neighbor water molecules
are relatively far apart 3.96 Å in the range 80–160 K. Pe-
riodic Hartree-Fock
21
and semiempirical calculations
22,23
showed that the 11 monolayer and the 42 bilayer
phases have a similar binding energy per molecule, and that
the latter should be stable at a larger water density than the
monolayer as a result of the increasing influence of the lat-
eral interactions.
The aim of this paper is to show that the clean 100
surface of MgO behaves as a singular substrate for water
adsorption since the distance a ( a =2.98 Å between oxygen
magnesium sites is close to the oxygen-oxygen distance d
O
between hydrogen bonded molecules (2.67d
O
2.90 Å.
This small mismatch should favor the formation of stable
ordered 2D layers, in contrast to the previous situations. Re-
cent experiments including LEED,
24
HAS,
25
and IR
spectroscopy
26
agree that water forms a very stable p (3
2) phase with molecular density around 6–8 per unit cell
and a denser phase containing 12–16 molecules which dis-
appears at T 180 K due to partial water desorption. The
spectroscopic signature of these two phases is very different
since the dense phase gives broad IR lines whereas the IR
bands of the other phase are quite narrow and less shifted.
Molecular-dynamics MD and Monte Carlo MC
simulations
27
revealed at 300 K a densely packed first layer
with water molecules aligned -17° and +30° with respect to
the surface plane while the other layers appear more diffuse
and become more liquidlike.
Two species of complementary calculations have been
performed here to study the structure of water on MgO100.
First, an energy minimization procedure EM based on a
numerical conjugate-gradient approach at 0 K is applied to
commensurate ( m n ) phases ( m and n integers of a mono-
layer containing one molecule per Mg site after it was dem-
onstrated that larger and smaller coverages are energetically
much less favorable. Then finite-temperature calculations are
conducted using MD simulations for an increasing number
of molecules inside a square 12a 12a simulation patch, i.e.,
from 144 molecules for the monolayer up to 400 molecules
for the multilayering. The substrate and the molecules are
assumed to be rigid.
The interaction potential between the water molecules is
represented by the TIPS2 potential,
28
which has proven to be
very efficient in describing the behavior of liquid water and
ice. The interaction between water and the substrate is the
sum of electrostatic and semiempirical dispersion-repulsion
contributions,
29
which appear to give a good compromise
between accuracy and tractability in MD. The equilibrium
configuration of a single H
2
O admolecule is above the cation
PHYSICAL REVIEW B 15 MAY 1998-I VOLUME 57, NUMBER 19
57 0163-1829/98/5719/119314/$15.00 11 931 © 1998 The American Physical Society