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...material described as being electrochromic. By virtue of their numerous applications, both of academic and commercial interest, electrochromic materials are currently attracting great deal of interest. This review provides an introduction to the major classes of electrochromic materials, namely transition metal oxides, Prussian blue systems, viologens, conducting polymers, transition metal and lanthanide coordination complexes and metallopolymers, and metal phthalocyanines. Examples of some new materials and of prototype and commercial electrochromic devices are cited.
Introduction
Electrochromism is the reversible change in optical properties that can occur when a material is electrochemically oxidised (loss of electron(s)) or reduced (gain of electron(s)), and is of great academic and commercial interest. Traditionally, materials have been considered as being electrochromic when they displayed distinct visible colour changes, with the colour change commonly being between a transparent ('bleached') state and a coloured state, or between two coloured states. In cases where more than two redox states are electrochemically available, the electrochromic material may exhibit several colours and be described as polyelectrochromic. However, the working definition of electrochromism has now been extended to include devices for modulation of radiation in the near infrared, thermal infrared and microwave regions, so 'colour' can now mean a response by detectors at these wavelengths, ad not just by the human eye. (1)
Electrochromism enables the darkening of a window at the flick of a switch. This principle has already been demonstrated in electrochromic car rearview mirrors, which automatically darken at night, when a driver is 'dazzled' by the headlights of a car behind. These and many other applications make electrochromic materials highly sought after for commercial use, and a review on electrochromic systems and the prospects for devices has recently been published. (2)
There are a large number of materials that exhibit electrochromism, and a number of excellent recent reviews on various categories of electrochromic materials and their applications have been published, a few of which have been cited. (1-6) The most important classes of compounds which demonstrate this effect are: transition metal oxides, Prussian blue systems, viologens (1,1'-disubstituted-4,4'-bipyridinium salts), conducting polymers, transition metal and lanthanide coordination complexes and metallapolymers, and metal phthalocyanines (Table 1). These classes, and some new examples of materials within each class, will now be detailed. Additionally, examples of applications of the various classes of electrochromic materials will be given.
Transition metal oxides
Electrochromism was first reported in thin films of [WO.sub.3] in 1969, and this material remains the most promising candidate for large-scale uses of electrochromic devices. (3,4,6)
Electrochromism in [WO.sub.3] is conveniently introduced by reference to the simple reaction in equation (1).
[WO.sub.3] + x([M.sup.+] + [e.sup.-]) [right arrow] [M.sub.x][WO.sub.3] (1)
where [M.sup.+] = [H.sup.+], [Li.sup.+], [Na.sup.+], or [K.sup.+], < x [less than or equal to] 1 and [e.sup.-] denoting electrons. Therefore, when a transparent thin film of [WO.sub.3] incorporates electrons and charge-balancing ions, it can be reversibly transformed into a material which is absorbing if the material is heavily disordered and infrared-reflecting if it is sufficiently crystalline. (3) It must, however, be noted that equation (1) is a "gross oversimplification" (3) and the thin films of practical interest are usually hydrous, i.e. contain some hydroxyl groups and incorporated water molecules, and may deviate to some extent from the stated [WO.sub.3] stoichiometry (relative proportions of constituent elements of the compound). (2,3)
Tungsten trioxide, which, as indicated above, is transparent as a thin film, has all tungsten sites in oxidation state [W.sup.VI]. Upon electrochemical reduction, [W.sup.V] sites are generated to give the electrochromic effect. The detailed colouration mechanism is still controversial, but it is generally accepted that the injection and extraction of electrons and metal cations play a key role. More accurately, the reaction can be represented as shown in equation (2).
[WO.sub.3] + x([M.sup.+] + [e.sup.-]) transparent [right arrow] [M.sub.x][W.sup.VI.sub.(1-x)][W.sup.V.sub.x][O.sub.3] blue (2)
At low values of x, the films have an intense blue colour due to intervalence electron transfer between adjacent [W.sup.V] and [W.sup.VI] sites stimulated by absorption of a visible photon. At higher values of x, insertion irreversibly forms a metallic 'bronze' that is red or golden in colour. (6) The thin film microstructure of [WO.sub.3] is heavily dependent on the method of preparation. Gaseous processes include [WO.sub.3] sublimation, RF magnetron sputtering, or oxidation during chemical vapour deposition, whereas solution phase methods include electrodeposition, electrochemical oxidation of tungsten film, the sol-gel route from tungstic acid, or dip coating from colloidal [WO.sub.3]. (7)
Several other thin-film transition metal oxides are also able to switch from a colourless oxidised state to an intensely coloured reduced form, particularly the oxides of molybdenum, vanadium and niobium (equations 3-5).
Mo[O.sub.3] + x([M.sup.+] + [e.sup.-]) transparent [right arrow] [M.sub.x][Mo.sup.VI.sub.(1-x)][Mo.sup.V.sub.x][O.sub.3]blue (3)
[V.sub.2][O.sub.5] + x([M.sup.+] + [e.sup.-]) yellow [right arrow] [M.sub.x][V.sub.2][O.sub.5]pale blue (4)
[Nb.sub.2][O.sub.5] + x([M.sup.+] + [e.sup.-]) [right arrow] transparent [M.sub.x][Nb.sub.2][O.sub.5] pale blue (5)
transparent pale blue
As in the case of [WO.sub.3], this requires simultaneous 'injection' of electrons and small charge-compensating cations, (7) and the more intensely absorbing redox state is produced...
NOTE: All illustrations and photos
have been removed from this article.
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