Chemistry Reference
In-Depth Information
complicated to clarify them. Experimental observations and theoretical descriptions
of the H-bond dynamics including quantum effects have been attempted in homo-
geneous systems of gas, liquid, and condensed matter and consistent interpretations
between experiment and theory has been established to some extent in the past
decades. Especially, a water dimer in gas phase has been extensively studied as the
model system because of its simple structure and most of the properties were
determined well [
47
-
50
]; detailed in
Chap. 5
]. However H-bond dynamics in het-
erogeneous systems, like interface, has been unexplored so far, which is potentially
related to the process of catalysis, electrode reaction, and energy productions.
1.2 Previous Studies of Water Adsorption on Metal Surfaces
There are numerous studies about water on surfaces. I briefly take a look back over
studies of water adsorption on metal surfaces in past decades. Here I focus on
studies carried out under ultra-high vacuum (UHV) conditions. The studies in
well-defined environments have provided fundamental knowledge as to water
adsorption on metal surfaces. The adsorption of water molecule on metal surfaces
is related to some practical problems, e.g., corrosion, heterogeneous catalysis,
hydrogen production/storage, and biological sensor. In addition, water-metal
interfaces have been considered as a useful model system to understand the effect
of truncating the three-dimensional H bond structure at the molecular level. Due to
the symmetry similarity to the (0001) plane of ice I
h
phase, single-crystalline
closed-packed metal surfaces have been often employed to examine the growth
and structure of ice film. The formation of H-bonded two-dimensional ice films,
similar in structure to bulk ice, has been established in water adsorption on tran-
sition metal surfaces such as Pt, Ru, Ni, Pd and Rh.
The literature describing water adsorption on metal surfaces has been sum-
marized in several reviews [
51
-
54
]. The early stage studies under UHV conditions
were compiled by Thiel and Madey in 1987 [
51
] and renovated by Henderson in
2002 [
52
], where the structure of water layer and the interaction of water molecule
with metal surfaces were main subjects. They summarized the so-called ''bilayer
model'' of water adsorptions at metal surfaces (Fig.
1.1
), which had been
unmodified over the decades. In this model, water molecules form a network
of interlinked hexamers where each molecule possesses three H bonds to its
neighbors. Half of the water molecule donates both H atoms to the other mole-
cules, whereas remains have either OH pointing towards surface (Fig.
1.1
a) or
vacuum (Fig.
1.1
b). The direction of this remaining OH directly related to the
growth mechanism of ice over metal surfaces. The ordered two-dimensional (2-D)
structures were mainly investigated in earlier experiments using low-energy
electron diffractions (LEED) [
55
-
57
]. Held and Menzel reported the first complete
LEED-IV analysis of an ordered water layer on a Ru(0001) surface [
58
].
Ultraviolet photoelectron spectroscopy (UPS) and vibrational spectroscopy,
i.e., electron energy loss spectroscopy (EELS) and infrared reflection absorption
