Civil Engineering Reference
In-Depth Information
What is the origin of the electrochromism for these oxides? An approxi-
mate answer can be given by arguments based on the crystalline structure,
and a detailed examination of the electrochromic oxides reveals that they
can be represented as (defect) perovskites, rutiles, and having layer/block
structures. All of these structures can be described as comprising 'ubiqui-
tous' MeO
6
octahedra (where Me denotes metal) connected by sharing
common corners and/or common edges. Edge-sharing is related to some
degree of deformation of the octahedra. Only one electrochromic oxide
falls outside this description and exhibits properties with both 'anodic' and
'cathodic' traits: this is vanadium pentoxide (V
2
O
5
) which can be viewed as
built from square pyramidal VO
5
units. The octahedral coordination is very
important for the electronic properties of the electrochromic oxides and
leads to a qualitative model for the optical properties for all of the oxides
mentioned above, as elaborated elsewhere (Granqvist, 1993, 1995).
The detailed mechanisms for the optical absorption in electrochromic
oxides are often poorly understood. Generally speaking, the absorption is
associated with charge transfer, and polaron absorption captures at the
essential features (Granqvist, 1995; Niklasson and Granqvist, 2007). The
electrons inserted together with the ions are localized on metal ions and,
in the specifi c case of tungsten oxide, change some of the W
6+
sites to W
5+
.
Transfer of electrons between sites designated
i
and
j
, say, then can be rep-
resented schematically as W
i
5+
W
j
5+
. This mecha-
nism operates only as long as transitions can take place from a state occupied
by an 'extra' electron to one available to receive that electron, and if the
ion and electron insertion is large enough this is no longer the case so that
'site saturation' (Denesuk and Uhlmann, 1996) becomes signifi cant. Elec-
tron transfer then can occur also according to W
4+
+
W
j
6+
+
photon
→
W
i
6+
+
W
6+
(Berggren
et al.
, 2007). However, these latter kinds of charge transfer do
not dominate since highly reversible electrochemical reactions limit the
permissible insertion levels to those where W
5+
W
5+
and W
4+
↔
↔
W
6+
are prevalent.
Mixed electrochromic oxides can offer a number of advantages, and by
having a large variety of sites available for charge transfer, it is possible to
achieve an increasingly wavelength-independent absorption (i.e., a more
neutral colour). Other advantages of mixed oxides are the possibility to
widen the optical band gap in order to give a higher bleached-state transmit-
tance in nickel-oxide-based (Avendaño
et al.
, 2004) and iridium-oxide-based
(Azens and Granqvist, 2002) fi lms, and to 'dilute' expensive iridium oxide
without major effects on its electrochromism (Backholm and Niklasson,
2008; Harada
et al.
, 2011). Still another advantage is that the coloration effi -
ciency can be increased by mixing suitable oxides, as shown in recent
detailed work on mixed tungsten-nickel oxide fi lms by Green
et al.
(2012).
The ubiquity of the MeO
6
octahedra is important not only for the optical
properties but also for the possibilities to accomplish facile ion insertion
↔
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