Electrochromism is a phenomenon in which a material displays changes in

color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...

opacity Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shie ...

in response to an electrical stimulus. In this way, a smart window made of an electrochromic material can block specific wavelengths of

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

, visible or (near)

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

light. The ability to control the transmittance of near-infrared light can increase the

energy efficiency Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a rat ...

of a building, reducing the amount of energy needed to cool during summer and heat during winter. As the color change is persistent and energy needs only to be applied to effect a change, electrochromic materials are used to control the amount of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

and

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

allowed to pass through a surface, most commonly "smart windows". One popular application is in the

automobile A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, Car seat, seat one to eight people, have four wheels, and mainly transport private transport#Personal transport, peopl ...

industry Industry may refer to: Economics * Industry (economics), a generally categorized branch of economic activity * Industry (manufacturing), a specific branch of economic activity, typically in factories with machinery * The wider industrial sector ...

where it is used to automatically

tint In color theory, a tint is a mixture of a color with white, which increases lightness, while a shade is a mixture with black, which increases darkness. A tone is produced either by mixing a color with gray, or by both tinting and shading. ...

rear-view mirror A rear-view mirror (or rearview mirror) is a, usually plane mirror, flat, mirror in automobiles and other vehicles, designed to allow the driver to see rearward through the vehicle's rear window (rear windshield). In cars, the rear-view mirror ...

s in various

lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. ...

conditions.

Principle

The phenomenon of electrochromism occurs in some transition metal oxides which conduct both

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s and

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s, such as

tungsten trioxide Tungsten(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO3. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid . It is a light ...

(WO₃). These oxides have octahedral structures of oxygen which surround a central metal atom and are joined together at the corners. This arrangement produces a three-dimensional nanoporous structure with "tunnels" between individual octahedral segments. These tunnels allow dissociated ions to pass through the substance when they are motivated by an electric field. Common ions used for this purpose are H⁺ and Li⁺. The electric field is typically induced by two flat, transparent electrodes which sandwich the ion-containing layers. As a voltage is applied across these electrodes, the difference in charge between the two sides causes the ions to penetrate the oxide as the charge-balancing electrons flow between the electrodes. These electrons change the valency of the metal atoms in the oxide, reducing their charge, as in the following example of tungsten trioxide: : + ''n''( + e⁻) → This is a

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

reaction since the electroactive metal accepts electrons from the electrodes, forming a half-cell. Strictly speaking, the ''electrode'' as a chemical unit comprises the flat plate as well as the semiconducting substance in contact with it. However, the term "electrode" often refers to only the flat plate(s), more specifically called the electrode "substrate". Photons that reach the oxide layer can cause an electron to move between two nearby metal ions. The energy provided by the photon causes the movement of an electron which in turn causes optical absorption of the photon. For example, the following process occurs in tungsten oxide for two tungsten ions ''a'' and ''b'': : + + photon → +

Electrochromic materials

Electrochromic materials, also known as ''chromophores'', affect the optical color or opacity of a surface when a voltage is applied. Among the metal oxides,

tungsten oxide Tungsten has several oxidation states, and therefore oxides: * Tungsten(III) oxide * Tungsten(IV) oxide, also known as tungsten dioxide * Tungsten(VI) oxide, also known as tungsten trioxide * Tungsten pentoxide {{Short pages monitor