Chemistry Reference
In-Depth Information
Fig. 7.2 a A voltage pulse of 2 V for 0.5 s was applied to a water molecule imaged as a round
protrusion. The tip was fixed at the set point of I
t
= 0.5 nA and V
s
= 24 mV during the pulse.
b After the pulse the product (OH) appears as paired depressions. The black circle represents the
original position of the parent water molecule. The image size is 47 9 47 Å
2
. c The relative
position of OH species to nearby water molecules which is bonded to a top of Cu atom. The white
grid lines indicate the lattice of the Cu(110) surface, indicating that the adsorption site of OH is
the twofold short-bridge site. The image size is 46 9 30 Å
2
. All images were acquired at
V
s
= 24 mV and I
t
= 0.5 nA
[
19
,
21
,
22
], Ir(110) [
23
] and Cu(110) [
24
] surfaces. (iii) A surface is irradiated by
an electron or UV beam [
25
].
7.2 Results and Discussions
7.2.1 STM-Induced Dissociation of a Water Molecule on Cu(110)
A hydroxyl group can be produced by the dissociation of a water molecule with a
voltage pulse of STM (Fig.
7.2
). First the STM tip is fixed over the water molecule
(the round protrusion at the upper left) in Fig.
7.2
a and then a voltage pulse of 2 V
is applied. After the pulse, the water molecule changes into paired depression
aligned along the [001] direction in Fig.
7.2
b, which is assigned to a hydroxyl
group. The dissociation competes with the hopping of a parent water molecule, and
a hydroxyl is usually produced away from the original position. The dissociation
can be induced with a probability of *50 % by a pulse of 2 V for 0.5 s. An STM-
induced water dissociation was observed on Cu(100) [
26
], Cu(111) [
27
], Ru(0001)
[
28
] and MgO film on a silver surface [
29
]. The dissociation is induced via the
vibrational excitation or electronic excitation of a water molecule. As described in
the previous chapter, water molecules are adsorbed on a top of Cu atoms.
Accordingly, the adsorption site of a hydroxyl group is determined from its
relative location against the nearby water molecules to be a short-bridge site
(Fig.
7.2
b), which is consistent with the previous theoretical predictions [
30
-
32
].
