H2020-MCSA-RISE project N. 734834

Infographics

Molecules | Nanotechnology | Electrochromism | Self-assembly | The colors | Graphene et al | Functionalization | Devices

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Electrochromism

Introduction

Transition metal oxides

Polycyclic aromatic hydrocarbons

Viologens

Conducting polymers

Polythiophenes

Transition metal oxides

Transition metal oxides and hydroxides, such as WO₃, MoO₃, V₂O₅, Nb₂O₅, Ir(OH), Co₃O₄, NiO, are among the first materials where electrochromism was observed. The prototypical example is tungsten trioxide that, in thin film, is transparent. When a small quantity of positive ions, such as Li⁺, coming from an electrolyte, are reversibly incorporated in the film together with electrons coming from an electrode, WO₃ is transformed in a new compound, e.g. Li_xWO₃. In this compound, a fraction x of the W atoms are reduced from the oxidation state W^VI to the oxidation state W^V (x may vary between 0 and 0.3, typically). The reduced film acquires a blue color due to light absorption that produces a transfer of electrons between adjacent W^V and W^VI sites. In contrast with WO₃, Ir, Co and Ni oxides are colored in the oxidized state.

WO3 is a semiconductor. In the crystal, each W atom (blue sphere) bonds to six O atoms. These latter form an open framework of corner-sharing octahedra having W at their center [1]. Alkali atoms can be intercalated into the voids of the structure, at the location of the black dots in the opposite figure, where they donate electrons that start filling the conduction band.

The intercalation of metal cations in an oxide film is a slow process. It means that switching between the transparent and colored states and back takes time. Transition metal oxides are therefore not suitable for display applications, but are good candidates for smart-window technology.

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