The Photochromic Mechanism of the Tungsten Trioxide Film

The tungsten trioxide film can be colored by irradiation of light of an appropriate energy. When the tungsten trioxide film is irradiated by light, electrons e- and electron holes h+ are generated, and photogenerated electrons enter the conduction band of the tungsten trioxide. The photo-generated electron holes react with the adsorbed water on the surface of the film to form H+, and then diffuse to the center of tungsten trioxide, thus leads to a difficulty in color change.

The photochromic mechanism of the tungsten trioxide film can be expressed by the following equation:

WO3 + hv → WO3* + e- + h+

h+ + 1/2H2O → H+ + 1/4O2

WO3 + xe- + xH+ → HxWO3

A pure tungsten trioxide film only induces ultraviolet light. Therefore, for the unpretreated tungsten trioxide film, electrons and holes can only be generated by ultraviolet light irradiation. When cathodically polarized, the trapped energy can be generated in the crystal lattice. These levels are located in the forbidden zone and can be irradiated by visible light, causing the color change.

What is photochromism?

Photochromism means that, a compound A can undergo a specific chemical reaction when irradiated with light of a certain wavelength, and generated product B. The mechanism is change in its absorption spectrum due to structural changes, while. Under the action of light or heat at another wavelength, it can return to its original structure. Photochromism is a reversible chemical change.

A compound that undergoes a reversible color change under the action of light, is called a photochromic compound or a photochromic body.

If the compound A is colored, it changes to a colorless compound B after illumination, and B returns to the colored state A by photochemical or thermochemical means, the system is called reverse photochromism.

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