Introduction to metal-oxide electrochromes
Metal oxides as thin films feature widely in the literature, in large part owing to their photochemical stability (see Section 6.1.2); by contrast, most, if not all, organic electrochromes may be susceptible to photochemical degradation.
The oxides of the following transition metals are electrochromic: cerium, chromium, cobalt, copper, iridium, iron, manganese, molybdenum, nickel, niobium, palladium, praseodymium, rhodium, ruthenium, tantalum, titanium, tungsten and vanadium. Most of the electrochromic colours derive from intervalence charge-transfer optical transitions, as described in Section 4.4. The intervalence coloured forms of most transition-metal oxide electrochromes are in the range blue or grey through to black; it is much less common for transition-metal oxides to form other colours by intervalence transitions (see Table 6.1).
The oxides of tungsten, molybdenum, iridium and nickel show the most intense electrochromic colour changes. Other metal oxides of lesser colourability are therefore more useful as optically passive, or nearly passive, counter electrodes; see Section 1.4 on ‘secondary electrochromism’.
At least one redox state of each of the oxides IrO2, MoO3, Nb2O5, TiO2, NiO, RhO2 and WO3 can be prepared as an essentially colourless thin film, so allowing the electrochromic transition colourless (clear) ⇄ coloured. This property finds application in on–off or light-intensity modulation roles. Other oxides in Section 6.2 demonstrate electrochromism differently by showing two colours, i.e. switching as colour 1 ⇄ colour 2, one of these colours often being much more intense than the other. Display-device applications can be envisaged for the latter group of electrochromes.