Chemistry Reference
In-Depth Information
Fig. 9.8 The calculated
structures for H
2
O-OH
complexes. (upper) Side and
(bottom) top views. a [110]-
complex. b [001]-complex
9.2.2 Structure of Water-Hydroxyl Complexes on Cu(110)
The structure of water-hydroxyl complexes was determined by DFT calculations.
1
Two possible initial geometries are considered; first the water molecule is put on
the same atomic row as the hydroxyl as an initial structure. Second the water
molecule is put on the adjacent atomic row of the hydroxyl. Figure
9.8
shows the
optimized structures. For the first case (Fig.
9.8
a) the structure is stabilized by
0.13 eV compared to the isolated H
2
O and OH on Cu(110), which corresponds to
the energy of H bond. The water molecule is deviated from the on-top site, which
is energetically favorable for a water molecule, to form the optimal H bond with
the OH group. A [110]-complex is assigned to this side-on structure. On the other
hand, for the second case (Fig.
9.8
b) the more stable structure can be obtained,
where the complex is stabilized by 0.44 eV despite the water is fully displaced
from the on-top configuration and both of the water and hydroxyl are bounded at
the adjacent short-bridge sites along the [001] direction. The H bond in this
structure is remarkably strong compared to that of a water dimer on Cu(110) which
has a normal strength of H bond (0.14 eV). Since the adsorption energy of a water
1
DFT calculations were performed using the STATE code [Y. Morikawa et al. Phys. Rev. B 69,
041403 (2004).] The calculations were conducted within the Perdew-Burke-Ernzerhof (PBE)
generalized gradient approximation [J. P. Perdew et al. Phys. Rev. Lett. 77, 3865 (1996).]. PBE
slightly overestimates the binding energies of H
2
O dimer, and slightly underestimates the proton
transfer barrier at a short distance (*2.5 Å), but is sufficiently accurate for the present purpose.
The surface was modeled by a five-layer Cu slab with an H
2
O-OH complex aligned along the
[001] ([1
10]) direction in a 3 9 3(29 4) periodicity, and a 4 9 4 k-point set was used to
sample the Brillouin zone. The adsorbates were put on one side of the slab, and the spurious
electrostatic interaction was eliminated by the effective screening medium method [M. Otani
and O. Sugino, Phys. Rev. B 73, 115407 (2006); I. Hamada et al. ibid. 80, 165411 (2009).].
Adsorbates and the topmost two Cu layers were allowed to relax, while remaining Cu atoms are
fixed at their respective bulk positions.
.
