Civil Engineering Reference
In-Depth Information
different amounts of mobile ions, water molecules, etc. In addition to this com-
plexity, some of the interesting materials—such as NiO—have an electronic
structure that has been debated for decades without any clear picture having been
established. Here, we discuss the coloration of W oxide and Ni oxide; a more
detailed discussion is given elsewhere by Niklasson and Granqvist (
2007
).
The insertion and extraction of hydrogen ions (protons) and electrons can be
written as a simple electrochemical reaction according to
WO
3
þ
H
þ
þ
e
ð
Þ
bleached
$
HWO
3
ð
Þ
colored
;
ð
2
Þ
where the H
+
ion could be replaced by Li
+
and e
-
denotes electrons. In order to
ensure electrochemical reversibility, the amount of H
+
should be limited so that the
colored material is H
x
WO
3
with x \ 0.5 (Berggren and Niklasson
2006
; Berggren
et al.
2007
). The corresponding reaction for Ni oxide is (Avendano et al.
2005
,
2009
)
bleached
$
NiOOH
þ
H
þ
þ
e
Ni
ð
OH
Þ
2
ð
Þ
colored
;
ð
3
Þ
where the reaction is supposed to be confined to grain boundaries.
The electronic band structure of oxides comprised of octahedral units forms a
good starting point for discussing the optical properties of anodic and cathodic
electrochromic oxides. The oxygen 2p bands are separated from the metal d levels,
and the octahedral coordination gives rise to a splitting of the latter band (Goo-
denough
1971
). Inserting and extracting charge leads to a displacement of the
Fermi level, which in the cathodic oxides yields a partial filling of the lower part of
the d band, while for the anodic oxides, this lower part of the d band gets com-
pletely filled so that the band gap between the two portions of the d band produces
optical transparency (Granqvist
1994
,
1995
).
The detailed absorption mechanism can be understood as follows for W oxide:
The electrons inserted along with the ions are localized on tungsten sites, which
means that some W
6+
sites are transformed to W
5+
. Photon absorption can provide
enough energy to transfer this inserted electron on site i to a neighboring site j by
(Schirmer et al.
1977
; Granqvist
1995
; Ederth et al.
2004
)
W
5
þ
i
þ
W
6
þ
j
þ
photon
!
W
6
þ
i
þ
W
5
þ
j
;
ð
4
Þ
which is referred to as intervalence transfer or polaron absorption within chemistry
and physics, respectively. But the transfer is only possible if it takes place from a
site occupied by an electron to an empty site available to receive the electron. This
is not always possible, and if the electron and ion insertion is high, there will not
only be W
5+
$
W
6+
transfer but also W
4+
$
W
6+
and W
4+
$
W
5+
(Berggren and
Niklasson
2006
; Berggren et al.
2007
). Such ''site saturation'' effects, first dis-
cussed by Denesuk and Uhlmann (
1996
), may not be so prevalent in a practical
situation, though, since the permissible intercalation levels for electrochemical
reversibility make W
5+
$
W
6+
overwhelmingly common.
