3.1. Tungsten oxide
Tungsten oxide is the most widely studied electrochromic material. As shown in
eqs. (1) and (2) above, when WO 3 is intercalated with protons or lithium ions, its
color changes from transparent or yellow to deep blue due to the formation of a
tungsten bronze. The bronze can also be formed when sodium, potassium or silver
ions are used. As the size of the ions increases, the rate of diffusion decreases,
thereby decreasing the rate of optical modulation [51]. Table 2 shows the rate of
diffusion of various ions into the tungsten oxide coatings.
The rate of diffusion of all ions is sensitive to the microstructure of the coatings.
The data from ref. [54] in table 2 shows that for Li + the diffusion coefficient was
varied by three orders of magnitude by changing the density of the coatings. Table
2 is based on coatings primarily deposited by physical vapor deposition, but sol-gel
technology has been used to prepare both glasses and coatings which have much
higher variations in density. For example, SiO 2 glasses have been made by sol-gel
by varying the composition of the starting materials and processing such that their
densities approach that of SiO 2 made by conventional melt processing [55] and
densities that approach almost zero for aerogels [56]! Not only the density of the
coatings can be varied considerably by sol-gel, but one can even exercise some
control over the pore morphology [57,58].
Another interesting observation in WO 3 is its crystallization behavior. Amor-
phous tungsten oxide crystallizes between 300 and 400°C depending on its prepara-
tion conditions. Polycrystalline tungsten oxide modulates the optical transmission
by reflection (especially in the near infrared region) whereas amorphous tungsten
oxide does that by absorption [59,60]. For window applications where the main
purpose is to produce energy-efficient glazing, crystalline WO 3 may have some
advantages, since energy absorbed in the window can be reradiated into the room.
A low diffusion coefficient of ions in crystalline films can severely affect the kinetic
properties. Table 3 shows the change in diffusion coefficient of protons in coatings,
as they are crystallized by heating to 350 and 400°C. WO 3 crystal size control is
essential; it should be small enough to have good kinetics and large enough to
minimize the electron scattering and maximize the modulation by reflection[ll].
The stability of amorphous WO 3 films is poor in aqueous electrolytes as compared
to crystalline WO 3 [62]. Sol-gel offers an opportunity to control the crystal size
and the microstructure and to reach a balance so that films with both good stability
and fast kinetics can be made.